Quantum confined semiconductor nanocrystals have been widely investigated as light harvesting and charge separation components in photovoltaic and photocatalytic devices. The efficiency of these semiconductor nanocrystal-based devices depends on many processes, including light harvesting, carrier relaxation, exciton localization and transport, charge separation and charge recombination. The competition between these processes determines the overall solar energy conversion (solar to electricity or fuel) efficiency. Semiconductor nanorod heterostructures, combining two or more materials, offer unique opportunities to control their charge separation properties by tailoring their compositions, dimensions and spatial arrangment. Further integration of catalysts (heterogeneous or homogeneous) to these materials form multifunctional nanoheterostructures. Using CdSe/CdS/Pt, dot-in-rod nanorods(NRs) with Pt tips, as a model system, we are examining the mechanism of long-lived charge separation and H₂ generation in the presence of sacrificial electron donor. The rates of exciton localization, electron transfer, hole transfer and charge recombination are directly monitored by transient absorption and time-resolved fluorescence spectroscopy. In this talk, we will discuss the mechanism of exciton dissociation, the dependence of the rates of elementary charge transfer processes on the dimension (size and length) and band alignment in these materials, and how these rates affect the overall H2 generation efficiency.

Efficient utilization of solar energy via innovative materials technology is expected to lead to a fundamental solution to the problems of energy shortage and environmental pollutions. Especially, design and construction of new nanostructured photoactive materials for chemical conversion and environmental remediation using earth-abundant materials are highly desirable.
This talk will introduce the latest research activities in our group^1-13), focusing on our challenges on the scientific and technological possibilities of nano-photocatalytic materials for solar fuel conversion as well as the environmental remediation offered by nanoarchitectonics of metal/semiconductor materials. Efforts to explore suitable materials and to optimize their energy band configurations for specific applications, and the design and fabrication of advanced nano-metal/semiconductor composite materials in the framework of nanotechnology will be introduced, taking examples of recent research progress on LSPR induced charge transfer and/or photothermal effect of nanometals on solar energy conversion. The current theoretical understanding of key aspects of photoactive materials, as well as the crucial issues that should be addressed in future research activities will also be introduced and discussed.
References:
1) Z. Yi, J. Ye, N. Kikugawa, T. Kako, et al., Nature Mater. 9, 559-564 (2010).
2) S. Ouyang and J. Ye, J. Am. Chem. Soc. 133, 7757-7763 (2011).
3) Y. Bi, S. Ouyang, N. Umezawa, J. Cao, J. Ye, J. Am. Chem. Soc. 133, 6490-6492 (2011).
4) H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Adv. Mater., 24(2), 229-251 (2012).
5) S. Ouyang, H. Tong, N. Umezawa, J. Cao, P. Li, Y. Bi, Y. Zhang, J. Ye, J. Am. Chem. Soc., 134, 1974minus;1977 (2012).
6) G. Xi, S. Ouyang, P. Li, J.Ye, et al., Angew Chem Int. Ed., 51, 2395 -2399 (2012).
7) J. Guo, S. Ouyang, P. Li, Y. Zhang, T. Kako, and J. Ye, Appl. Catal. B: Environ., 134-135, 286-292 (2013).
8) H. Zhou, J. Guo, P. Li, J. Ye, et al., Scientific Reports, 3, 1667(2013).
9) L. Liu, S. Ouyang, J. Ye, Angew. Chem. Int. Ed., 52, 6689-6693 (2013).
10) Kun Chang, Zongwei Mei, Tao Wang, Qing Kang, Jinhua Ye, et al., ACS Nano, 8(7), 7078-7087 (2014).
11) Tao Wang, Xianguang Meng, Jinhua Ye, et al., Nano Energy, 9, 50-60 (2014).
12) X. Meng, Tao Wang, Lequan Liu, Jinhua Ye, et al., Angew. Chem Int. Ed., Angew. Chem. Int. Ed., 53, 11478-11482 (2014).
13) Lequan Liu, Hideki Abe, Jinhua Ye, et al., Adv. Funct. Mater. 24, 7754-7762 (2014).

Since functionalities of nano-scale materials are closely related to the size, size control over nanomaterials is critical for their applications in the field of clean and renewable energy. Size-controlled strategy is limited to traditional seeding method that uses preformed seeds to well separate nucleation and growth step. Herein, we report a fast in situ photocatalytic seeding approach based on zinc (II) porphyrin (ZnP) under white light irradiation. This light-controlled seeding allows to manipulating the size and size uniformity of nanomaterials simply by varying exposure time. We chose platinum as an example to demonstrate this new in situ seeding method. Compared with commercial platinum black, resultant nearly mono-disperse platinum nanodendrites exhibit much enhanced electrocatalytic activity to both oxygen reduction and methanol oxidation due to the branched structure and the high electrochemically active surface area (44.0 m²/g) of the platinum nanodendrites that is 2.2 times of that of commercial platinum black (20. 4 m²/g). This ZnP-based photocatalytic seeding approach provides an alternative route to precisely control the size and size uniformity of metal and alloy nanomaterials with white light.

Zinc oxide is known as photocatalytically active material for water splitting. It fulfills functional requirements in terms of electronic structure and electrochemical properties. Its use as nanosized particles stabilized by small ligands in organic solution enables versatile strategies for material integration into functional devices. Several methods such as spincoating, dropcasting, self-assembly, covalent as well as electrostatic attachments are available meeting the exact requirements of the setup. Further advantages are a large surface to volume ratio which supports efficient charge-carrier dynamics and the prevalent surface states which may also improve electron-hole separation.
The photocatalytic activity of pure zinc oxide is inevitably associated with the presence of the UV light, which has often unfavorable side effects (material degradation, corrosion etc.) Coupling a plasmonic gold particle to the zinc oxide nanostructures will help to overcome this limitation. The plasmonic oscillation of small gold particles (~ 10 nm) is excited by photons in the visible range, which can populate the conduction band of zinc oxide leading to catalytic activity.
In our work we present a facile two-step synthesis of tailored nanosized gold-zinc oxide hybrids. In the first step zinc oxide nanopyramids are generated via a simple basic hydrolysis using oleic acid as a stabilizing agent. In the second step a photocatalytic reaction yields gold nanoparticle formation exclusively on the tip of the pyramid. The mechanism of the gold deposition will be discussed in the presentation in detail.
Furthermore, the nanohybrids are characterized optically and electrochemically to receive detailed information on their electronic structure which determines the photocatalytic properties. The application of the gold-zinc oxide nanopyramids in water splitting is discussed using the obtained results from cyclic voltammetry and electrochemical impedance spectroscopy.

Photocatalysis is a practical methodology for degradation of organic pollutant for environmental application and hydrogen generation for renewable power source application. Although in most cases the light used for photocatalysis research and application is still artificial light sources at present, the solar light is the real target for for practical photocatalysis. However, up to now, most available photocatalysts are UV and visible or UV-visible light active semiconductor with broad band gap, and there is only a few methods or photocatalyst can realize NIR photocatalystic activity. Therefore, design or find NIR light active photocatalysts and building UV-Vis-NIR full solar light photocatalysis system is great challenge for full utilization of solar energy in environmental and clean energy fields.
Recent years, several methods have been suggested to convers NIR light to visible or UV light and realized NIR light photocatalysis by light conversion nanoparticle-semiconductor hybrid photocatalysts. Most recently, our group proposed to use some semiconductor with impurity energy level, and semiconductor with narrow band gap to achieve NIR light photocatalysis. Based on this mechanism, some oxides, such as Ag₂O and Bi₂WO₆, and some cholcogenides, such as WS₂ and In₂S₃, were discovered to possess high NIR light photocatalytic activity, and some of them possess high photocatalytic properties in UV-Vis-NIR full solar spectrum photocatalytic property.
In this talk, we will summary the principle for design full solar light spectrum photocatalysis systems, or finding new NIR photocatalyst. Several new NIR and full solar spectrum light photocatalysts will be introduced. In addition, the application of full solar light photocatalysts will be reviewed.
References
Gang Wang, Baibiao Huang, Xiangchao Ma, Zeyan Wang, Xiaoyan Qin, Xiaoyang Zhang, Ying Dai, Myung-Hwan Whangbo, Angew. Chem. Int. Ed. 2013, 52, 1 - 5
Jian Tian, Yuanhua Sang, Guangwei Yu, Huaidong Jiang, Xiaoning Mu, Hong LIU, Advanced Materials, 2013, 25, 5075-5080
Jian Tian, Zhenhuan Zhao, Anil Kumar, Robert I. Boughton, Hong Liu, Chem. Soc. Rev., 2014,43, 6920-693

Photoelectrodes based on iron oxide (III) were prepared in a simple and efficient way. The α-Fe₂O₃ was produced from combustion synthesis in the presence of different fuels: citric acid, alanine, glycine and urea as fuels. Also, all the oxides formed were thermically treated at 800^oC for 1 hour. The synthesized materials were structurally and morphologically characterized through X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetry (TG). Its band-gap energies (E_g) were calculated through Wood-Tauc&’s method using diffuse reflectance spectra. The synthesis proved to be efficient on producing the desired material resulting in 100% of the hematite (α-Fe₂O₃) for all the fuels studied in this manuscript. However, morphology and band-gap energies shown to be dependent of the fuel. The reducing power of each fuel investigated was crucial to the available surface area of α-Fe₂O₃ materials formed. Moreover, 5-layered electrodes were prepared through the Layer-by-Layer (LBL) method, and were electrochemically characterized through its voltammetric profiles on the presence and absence of visible light. The efficiency of the thin films was verified through the kinetic of dye discoloration using methylene blue (MB) or rhodamine (RhB), considering the electrode's electroactive area. The photoelectrodes displayed significant photoelectrocatalytical responses for dye discoloration, the dye solutions were composed of 1.0x10^-5 mol L^-1 of the dye and 0.10 mol L^-1 Na₂SO₄ as support electrolyte. The electrode produced with alanine as fuel has shown to be the most efficient one on photoelectrocatalysis, with a discoloration constant (k_obs) of 5.34x10^-3 mol L^-1 min^-1 cm^-2 for methylene blue and 2.41x10^-3 mol^-1 L min^-1 cm^-2 for rhodamine B. The complete conversion to α-Fe₂O₃ at thermal treatment, as well as the particle size were crucial for the efficiency photoeletrocatalytic of photoelectrodes produced in the present manuscript.

Semiconducting chalcogenide films can be produced via photoelectrochemical deposition with nanostructured morphologies that are a function of the instantaneous illumination during growth. Films spontaneously adopt an ordered, highly anisotropic lamellar pattern during electrodeposition under illumination with polarized light. Feature size and pitch was controlled by varying the illumination wavelength, feature orientation by the polarization vector and the growth direction by the incident light vector. Illumination with the sum of several discrete sources provides further control over morphology and enables the synthesis of structures inaccessible with a single source. Illumination with two beams of differing wavelengths, polarizations and/or intensities resulted in structures that were either intermediates of the corresponding single source morphologies or co-expressions of the two. An example of the latter, illumination with two narrowband sources with differing wavelengths and orthogonal polarizations resulted in cross-hatched structures wherein the periodicity in the two perpendicular dimensions as well as the feature height could be controlled independently. The periodicity and orientation of the intermediate patterns were assessed quantitatively and related to the source composition. Also, the limits wherein intermediate patterns were generated was assessed. All electrodeposition was performed under ambient conditions with fully oxidized precursors in aqueous solution. No photomask, patterned substrate nor chemical templating agent was utilized. The deposition substrate did not display photoactivity. The same general morphology was observed for a range of varying materials. Growth was modeled using a Monte Carlo simulations based on computation of the light-matter interactions in the films.

CO₂ is the major greenhouse gas emitted through humankind activities. Forests and non-agricultural land act as a natural sink for CO₂ removal from the atmosphere. Nevertheless, the amount of emitted CO₂ is significantly larger than the capacity of these natural sinks. This leads to global warming and malignant impacts on human body. One of the means to decrease the amount of generated CO₂ is to end using fossil fuels. However, coal-fired power plants remain to be the largest source of electricity generation in 2014. Electricity generation and transportation hold the largest share in green-house gas (such as CO₂) emission. Hence, new systems and strategies are crucial to remove CO₂. In this work, we are presenting novel TiO₂ nanoparticles, synthesized via a facile solution-phase method, which show a significant visible light absorption. A thorough photoemission spectroscopy analysis outlined the energy structure of the materials which shows a sub-bandgap absorption in visible range due to the presence of intragap states. The origin of intragap states was investigated in a greater detail using various characterization techniques. An in-depth chemical composition study of the developed material using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) indicated that the synthesized material is considerably un-doped. Further structural study using transmission electron microscopy (TEM) showed that distances between visible lattice fringes are matched with ordered crystalline phases of TiO₂. The core emission study using XPS revealed that the oxygen vacancy defects in the structure--i.e. likely due the synthesis--are responsible for intragap states formation. This study offers a novel inexpensive material and method for photoreduction of CO₂ and generation of solar fuels.

The development of new color switching systems that can reversibly change color in response to external stimuli, such as light or heat, has attracted a great deal of attentions for their important applications in sensing devices, display and signage technologies, rewritable media, and security features. Here we discuss a new color switching system based on reversible redox reaction that could be initiated by photocatalytic response of TiO₂ nanocrystals. With the assistance of TiO₂ nanocrystal-based photocatalysts, UV light irradiation can rapidly reduce the imaging materials and result in obvious color change, while the recoloration can be achieved by re-oxidizing the system with the assistance of visible light irradiation or heating. The excellent performance of the new color switching system promises their potential applications as attractive rewritable media to meet our society&’s increasing needs for sustainability and environmental conservation.

Silver halides represent a class of semiconductor materials intensively investigated for photographic research in last century although much less attention has been paid in the past decade due to the surge of digit technology. On the other hand, their unique optical responses and surface chemistries make them to be a class of promising materials for solar energy conversion. In this presentation, a number of hybrid systems composed of silver halide and noble metal nanoparticles will be discussed in terms of synthesis and their applications in efficiently photocatalytic decomposition of organic pollutants, hydrogen generation from water splitting, etc.
This work was performed at the Center for Nanoscale Materials, a U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under Contract No. DE-AC02-06CH11357.

Solar fuels have a tremendous potential to reduce the world&’s dependence on the declining reserves of fossil fuels, therefore, not only providing a solution to environmental problems but also geopolitical ones. Various solar fuels, in particular hydrogen, are viewed as an alternative to replace fossil fuels. They propose an interesting pathway to dispatchable renewable energy capture: Solar fuels are generated when sunlight is available, can easily be stored and transported, and finally converted to kinetic, thermal, and electric energy on demand. Solar fuels solve the two major disadvantages of solar power: Sunlight of low power density and with time variance is transformed into compact and deployable fuels. Solar fuels take advantage of the existing infrastructure already used for conventional fossil fuels.
In contrast to conventional solar power capture, we propose an entirely novel approach to solar fuel generation: Instead of electrochemically driving chemical reactions as attempted by artificial photosynthesis, we thermally overcome the activation energy. In contrast to the conventional solarthermal approach, we avoid the heating of the entire reactor and, thus, the associated high-energy losses, by locally at nanoscale creating a thermally excited environment and by in-situ light absorption and catalytic reaction. The objective of this proposed project is to take advantage of localized surface plasmon resonance (LSPR) occurring on plasmonic nanostructures to thermally drive catalytic reactions that normally require elevated temperatures to overcome the reaction activation energy.
A first experimental study has demonstrated the basic feasibility of performing methanol steam reforming in a cold environment by plasmon-assisted catalysis. Polystyrene spheres of 430 nm diameter were coated with a 200-nm thick layer of Au as plasmonic nanostructures. Catalytic CuO/Al₂O₃/ZnO nanoparticles (15 nm diameter) were then deposited on top of the plasmonic substrate and immersed in liquid water-methanol. When solar irradiance of 1 sun (1000 W/m²) was focused with 13x concentration onto the sample, significant generation of gas bubbles was visible for hours. A gas production rate in the order of 10 ml/min per m² of irradiated area was estimated. During all tests, the bulk liquid remained at a temperature very close to ambient (below 30°C). Reference tests with identical plasmonic substrates (without catalyst) and with catalyst only (without plasmonic structures) did not show any gas bubble generation, indicating the effective interaction between catalytic and plasmonic structures. The gas bubbles accumulated at the top of the cell without condensation or other loss, showing that they consist of gas, not merely evaporated water and methanol. Gas chromatography analysis of the accumulated gas confirmed the product gas composition of approximately 25% CO₂ and 75% H₂, as expected for methanol steam reforming.

Acquiring solar fuels is considered as one of strategies to solve the challenge of energy shortage and environment pollution. Photocatalysis is a very promising technology to convert solar energy into chemical energy by splitting water for hydrogen, and reducing carbon dioxide for methane, methanol, etc. The central task in realizing the practical application of photocatalysis is to develop highly efficient photocatalysts. The characteristics of surface atomic structure and electronic structure in semiconductor photocatalysts intrinsically determine the absorption range, absorbance, and redox power of photoexcited electrons and holes as well as their bulk diffusion and surface separation capability, which consequently contribute to the reactivity of photocatalysts. Therefore, designing and constructing favorable surface atomic structure and electronic structure of photocatalysts for photocatalysis plays an indispensable role in overcoming the bottleneck of photocatalysis. Our effort mainly focuses on four aspects to reach such a goal, namely faceting photocatalysts, doping photocatalysts, heterostructuring of photocatalysts, and exploring unknown photocatalysts. Recent representative results on engineering the compositions and components to increase photocatalytic activity will be introduced in this talk.
Reference
[1] H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H. M. Cheng, G. Q. Lu, Nature 2008, 453, 638.
[2] G. Liu, H. G. Yang, G. Q. Lu, H. M. Cheng, and et al. Chemical Reviews 2014, DOI:10.1021/cr400621z
[3] G. Liu, L. C. Yin, J. Pan, F. Li, L. Wen, C. Zhen, H. M. Cheng, submitted.
[4] W. Jiao, Y. P. Xie, R. Z. Chen, C. Zhen, G. Liu,* X. L. Ma, H. M. Cheng, Chem Commun 2013, 49, 11770.
[5] G. Liu, L.-C. Yin, J. Q. Wang, P. Niu, C. Zhen, Y. P. Xie, H.-M. Cheng, Energy Environ Sci 2012, 5, 9603.
[6] Y. P. Xie, Z. B. Yu, G. Liu, X. L. Ma, H. M. Cheng, Energy Environ Sci 2014, 7, 1895.
[7] Y. Q. Yang, C. H. Sun, L. Z. Wang, Z. Liu, G. Liu, H. M. Cheng, Adv Energy Mater 2014, 4, 1400057.
[8] C. Zhen, J. C. Yu, G. Liu, H. M. Cheng, Chemical Communications 2014, 50, 10416.
[9] P. Niu, L. C. Yin, Y. Q. Yang, G. Liu, H. M. Cheng, Advanced Materials, In press.

Photocatalysis has been widely applied to environmental pollution removal, solar hydrogen generation, photocatalytic water splitting, etc. ZnO is one of the possible candidates for photocatalysis. However, its large bandgap limits its optical absorption in the UV region, and thus limits its overall photocatalytic effeciency as UV only accounts for less than 5% of the solar spectrum. Here, we would like to demonstrate that hydrogenated ZnO nanoparticles can absorb light in the near-infrared region, and has displayed much enhanced photocatalytic performance over the photodecomposition of methylene blue.

Among various photocatalysts, p-type Cu₂O with a direct-band of 2.0 eV has been widely examined by many groups on applications for water splitting, solar cell, gas sensor, and so on. However, the rapid recombination of photo-induced electron-hole pairs and the instability of Cu₂O under light illumination seriously limit its photocatalytic performance. It had been reported that the valence and conduction bands of Cu₂O are higher than those of TiO₂, which thermo-dynamically favors the transfer of excited electrons and holes between them. Most previous studies of Cu₂O/TiO₂ heterojuntion photocatalysts were based on TiO₂ and a small amount of Cu₂O deposited onto TiO₂ surface to provide visible light absorption. Some other researchers synthesized Cu₂O/TiO₂ core-shell structure or TiO₂/Cu₂O layered film structure to protect Cu₂O from corrosion. However, the main visible light absorber in this composite photocatalyst system was the narrow-band-gap semiconductor Cu₂O. Thus, photocatalysts based on TiO₂ with just a small amount of Cu₂O could not achieve high photoactivity under visible light illumination. For Cu₂O/TiO₂ core-shell structure or TiO₂/Cu₂O layered film structure, the Cu₂O core or the underlying Cu₂O film could not directly contact with water or pollutants in water and the holes accumulation will occur in Cu₂O which may cause further photocorrosion on Cu₂O.
In this work, a novel Cu₂O/TiO₂ composite photocatalyst structure of Cu₂O nano-spheres (Cu₂O-NS) decorated with TiO₂ nano-islands (TiO₂-NI) was synthesized successfully. In this Cu₂O/TiO₂ composite photocatalyst, Cu₂O nano-spheres served as the main component for light absorption so it demonstrated excellent absorption capability in the visible light region. TiO₂ nano-islands formed heterojunctions of good contact with Cu₂O nano-spheres, beneficial to the photo-excited electron transfer between them. The transfer of photo-excited electrons from Cu₂O to TiO₂ effectively separated the electron-hole pairs and subsequently enhanced their photocatalytic performance under visible light illumination. The partial coverage of TiO₂ nano-islands made it possible for photo-generated holes to react with water to produce bull;OHs or directly react with organic pollutants/microorganisms in water to avoid the accumulation of holes on Cu₂O. Without the accumulated holes, the photochemical stability of Cu₂O was enhanced during the photocatalysis process. Furthermore, a post-illumination catalytic "memory" was also observed for this composite photocatalyst, which demonstrated a disinfection effect on E. coli bacteria in the dark after the visible light was shut off. This interesting post-illumination catalytic "memory" could be attributed to the transfer of photo-excited electrons from Cu₂O to TiO₂ under visible light illumination, which could be trapped by TiO₂ nano-islands and then be released in the dark to produce radicals for the demonstrated catalytic "memory".

Silver nanostructures are attractive for a range of photo-based applications owing to their fasinating surface plasmon resonance properties superior to other noble metals. In fact, silver nanorods are probbaly the only plasmonic nanostructures that could be synthesized with very strong resonance in the UV and NIR regions, with essentially no absorption in the entire visible region. This type of nanostructures are potentially useful in the fabrication of condutive electrodes for display, light management, and smart window applications. In this talk, I will discuss a range of new develoments with regard to the controlled syntheses of silver nanostructures, including the facile syntheses of silver nanocubes with edge lengths below 15 nm, silver nanowires with diameters below 20 nm, silver nanorods with no plasmon resonance in the visible region., and silver nanocrysatls with concave surfaces. I will also discuss some of their properties and prototype applications.

Carbon dots (CDs) and graphene quantum dots (GQDs) have attracted significant interest due to their luminescence properties and diverse applications. We demonstrate that CDs surface-functionalized with hydrocarbon chains - i.e. amphiphilic CDs - constitute powerful labeling agents of membranes, both in model systems (vesicles) as well as in actual bacterial and mammalian cells. Specifically, the carbon nanoparticles seamlessly incorporate in vesicle bilayers and cell membranes, and their multicolor luminescence can be readily exploited for membrane studies using fluorescence spectroscopy and microscopy. We present application of amphiphilic CD labeling for microscopic visualization and analysis of membrane interactions of amyloid peptides, bacterial sensing, and imaging of cellular processes, such as cell division.
Reference: “Membrane analysis with amphiphilic carbon dots”, Nandi et al, ChemComm 2014, 50 (71), 10299 - 10302

The template-mediated synthesis of noble metal nanostructures has proven to be one of the most effective routes for directing growth pathways toward a desired endpoint. Such syntheses have given rise to an impressive collection of complex nanostructures with asymmetric, core-shell and hollowed morphologies that offer numerous functionalities based on tunable plasmonic resonances and intense near-fields. Nearly all such routes have, however, been restricted to syntheses performed on solution-dispersed templates. Creating photoactive surfaces from such structures is, therefore, reliant on the controlled dispersal and attachment of colloidal nanostructures onto a substrate. Here, we present an alternative strategy reliant on the use of substrate-immobilized nanostructures formed through vapor phase assembly processes as templates for solution-based reactions. Specific advantages of using such templates include the ability to form periodic arrays, dictate the crystallographic orientation of the template through epitaxy and, since nanostructure agglomeration is negated by substrate-immobilization, allow for the synthesis of surfactant-free structures. Demonstrated are three modalities directed toward additive processes where material is epitaxially deposited on the template, subtractive processes where the template is consumed and multistage processes which combine the additive and subtractive strategies. Additive processes resulting from the reduction of Ag onto Au templates using ascorbic acid reveal, depending upon the reaction kinetics, the formation of periodic arrays of either Au-Ag bimetallic heterodimers or Au@Ag core-shell structures.¹ Subtractive processes, reliant on galvanic replacement reactions performed on sacrificial Ag templates, yield arrays of hollow nanostructures with either a nanoshell or nanocage geometry.² Multistage processes which first reduce Ag onto a Au template and then subject the Ag to a galvanic replacement reaction give rise to a nanostructure which is confined within a cage. While this multistage process duplicates the strategy used to obtain solution-dispersed nanorattles, it is unique in that motion is prohibited due to the substrate-immobilization applied to both the cage and the confined structure. It, therefore, presents the intriguing opportunity to define particle-cage combinations with tunable gaps. Collectively this work demonstrates that an impressive colloidal chemistry and its associated functionalities can brought to the substrate platform to form organized surfaces of photoactive materials.
¹ K. D. Gilroy, R. A. Hughes, S. Neretina, Kinetically Controlled Nucleation of Silver on Surfactant-Free Gold Seeds, J. Am. Chem. Soc.2014, DOI: 10.1021/ja5081635.
² K. D. Gilroy, A. Sundar, P. Farzinpour, R. A. Hughes, S. Neretina, Mechanistic Study of Substrate-Based Galvanic Replacement Reactions, Nano Res.2014, 7, 365-379.

As a new category of fluorophores, silver nanodots attract huge attention owing to their small size, high brightness and excellent photostability. However, the exact structures and transition between different silver nanodots remain unclear. Red and near-IR silver nanodots under loose single-stranded DNA protection can be selectively oxidized to various nanodots with blue, green or yellow emissions by reactive oxygen species. The blue emitter especially showed excellent chemical and photophysical stability. The HPLC-MS indicated that the blue was an oxidized species, in line with the excellent stability in oxidizing environments. The kinetic data indicated that the intermediates from oxidation of the red and other non-emissive species led to the formation of the blue. Such a feature enables us to develop ratiometric luminescence probes and oxidant-resistant imaging agents. In addition to probes via direct spectral response of the AgNDs to various ROS, silver nanodots were, for the first time, formulated to detect any analyte with excellent selectivity and the limit of detection when coupled to glucose oxidase and some necessary strong binding agents. Our results will not only greatly benefit the optical imaging in harsh conditions, but also dramatically extended the scope of analytic application with silver nanodots.
1. Choi S, Dickson RM, Yu J. Developing luminescent silver nanodots for biological applications. Chem. Soc. Rev. 2012, 41(5): 1867-1891.
2. Choi S, Yu J, Patel SA, Tzeng Y-L, Dickson RM. Tailoring silver nanodots for intracellular staining. Photochem. Photobiol.Sci. 2011, 10(1): 109-115.
3. a) Choi S, Park S, Lee K, Yu J. Oxidant-resistant imaging and
ratiometric luminescence detection by selective oxidation of silver nanodots. Chem. Commun. 2013, 49(93): 10908-10910. b) Park S, Choi S, Yu J. DNA-encapsulated silver nanodots as ratiometric luminescent probes for hypochlorite
detection. Nanoscale Res. Lett. 2014, 9, 129.

Conformational changes control the regulatory functions of proteins in cellular processes. Techniques and tools to characterize protein structural transitions, both in-vitro and in-cell are required to progress our knowledge of protein function and mis-function towards the development of new treatments for diseases including Alzheimer&’s, type II diabetes and many cancers. We present conformational change detection, both in vitro and for the first time within living cells, of the ubiquitous protein fibronectin using high sensitive local surface plasmon resonant (LSPR) nanoplates. The nanoplates used are solution phase Au-edge-coated triangular silver nanoplates (AuTSNP) which exhibit some of the highest refractive index sensitivity values recorded to date.^1,2 Here they are applied for straight-forward and versatile sensing of un-labeled protein conformational transitions of fibronectin which plays an important role in cellular adhesion and migration processes, embryogenesis, wound healing, and is implicated in metastasis, tumour development and carcinoma.
Using the LSPR spectral shifts of AuTSNP nanosensors we distinguish between compact and extended conformations of Fn. The transition of Fn from extended to compact and visa versa is readily tracked in real-time. Changes in Fn size due to cleavage by cathepsin B (CTSB) are detected. Over expression of CTSB and increased digestion of extra cellular matrix proteins is associated with tumour mestastasis. We demonstrate a straightforward versatile serum assay for CTSB, based on the LSPR detection of Fn cleavage. Fn conformational changes during cell binding and matrix assembly are detected where LSPR spectral shifts are used to monitor the progression of Fn diffusely bound to cells in a compact state to form a Fn fibril matrix in which Fn displays a highly extended conformation. The versatility of the AuTSNP nanosensors and their in-cell applicability presents them as a powerful new tool for in-vivo protein conformation signaturing.
D. E. Charles, D. Aherne, M. Gara, D. M. Ledwith, Y. K. Gun&’ko, J. M. Kelly, W. J. Blau, M. E. Brennan-Fournet, ACS Nano 2010, 4, 55-64.
Yi Zhang, Denise E. Charles, Deirdre M. Ledwith, Damian Aherne, Stephen Cunningham, Muriel Voisin, Werner J. Blau, Yurii K. Gun'ko, John M. Kelly and Margaret E. Brennan-Fournet, RSC Adv., 2014,4, 29022-29031

Naturally occurring system responses such as folding and unfolding in self-assembled DNA bundles show that natural designs are hierarchical, with structures and property on multiple scales arising through interactions of subunits or building blocks. Mimicking these designs in the fabrication of active materials requires a clear picture of the energy landscape that governs local interactions such as hydrogen bonding, van der Waals interactions, dipole-dipole interactions, and capillary forces, which will provide correct thermodynamic end points as well as facile kinetics for precise control of hierarchical structure leading to responsive functions. To date, fabrication of active and responsive nanostructures has been conducted at ambient pressure and largely relied on these specific chemical or physical interactions. Here we show, using stress-induced assembly (SIA) that we recently demonstrated as an artificial actuator, we can emulate the natural folding and unfolding processes to: explore the energy landscapes that govern local interactions; design new classes of active materials with structures and functions that are not attainable for current nanomaterials; and investigate new properties resulting from folding and unfolding processes. We show that under a hydrostatic pressure field, the unit cell dimension of ordered 3D nanoparticle arrays can be manipulated to reversibly shrink and swell during compression and release of pressure. This allows for precise tuning of interparticle symmetry and spacing - ideal for controlled investigation of distance-dependent energy couplings and collective chemical and physical properties such as surface plasmon resonances. Moreover, beyond a threshold pressure nanoparticles are forced into contact and sinter, forming new classes of chemically and mechanically stable 1-3D nanostructures that cannot be manufactured by current top-down or bottom-up methods. Depending on the orientation of the initial nanoparticle arrays, 1-3D ordered nanostructures (Au, Ag, CdSe, etc) including nanorods, nanowires, nanosheets, and nanoporous networks can be fabricated. The SIA method mimics embossing and imprinting manufacturing processes and opens exciting new avenues for the study of responsive behaviors of active materials during compression (folding) and pressure release (unfolding). Stress-dependent control over the structure of nanoparticle or building block arrays provides a unique and robust system for the understanding of collective chemical and physical characteristics of nanocrystal superlattices.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

The versatile features, such as tunable band gap and high thermal and electrical conductivity, of nanoparticles have made them promising materials in the preparation of thin film nanocomposites for a range of organic electronic devices. Solution based processing of nanoparticles such as fullerenes, carbon nanotubes and quantum dots is a cost-effective technique to generate thin films of composite materials for the above-mentioned applications. The most critical synthesis parameters for these processes are the choice of solvent, processing temperature and concentration of nanoparticles. The scientific literature lacks detailed study of the effect of these parameters on the morphology of the thin nanocomposite films comprising distributed crystalline and amorphous domains of constituent species. The structure of the thin film is ultimately determined by the nanoparticle agglomerates that grow in the solvent during solution processing. To understand the fundamental physics behind agglomeration of nanoparticles, we performed molecular dynamics (MD) simulations of a range of binary systems that comprised fullerene derivatives-solvents, fullerene derivatives-polymer, carbon nanotubes-solvents in our previous studies. The simulations provided us fundamental insights that help in selecting favorable processing parameters, such as solvents and relative concentration of nanoparticles and solvents. However, conclusive prediction can only be attained by analyzing systems of length scales relevant to the specific device, which is not accessible to MD simulations. In an effort to bridge this existing gap, we developed a novel multi-scale model, based on MD and kinetic Monte Carlo (kMC), to study the growth of nanoparticle agglomerates.
We simulated a well-studied system containing fullerenes in an electrolyte solution, NaCl, to validate the model. The model included two steps: 1) perform coarse-grained molecular dynamics (CGMD) simulation to determine the relevant events and their corresponding rate constants, and 2) conduct kMC simulations using the estimated rate constants to track the growth of fullerene agglomerates with time. Under the simulated conditions, the agglomeration process evolves from a diffusion limited cluster-cluster aggregation (DLCA) regime to percolating clusters in transition and finally to a gelation regime. The fractal dimension of the aggregates was ~1.9 for the DLCA regime, which is in good agreement with existing data in the literature. The fractal dimension of the percolating clusters was ~2.5, while that for the gelation regime was calculated to be ~3, and occurs when the radius of gyration of fullerene clusters is 5 times larger than that in DLCA aggregation regime. Overall the data from our multi-scale numerical model showed good agreement with existing theory of colloidal particle growth. Future work will be geared towards determining the synthesis parameters for solution processing of a range of nanocomposite thin films.

DNA programmable assembly technique provides unprecedented control over the assembly of nanoparticles into superlattices with deliberately tailorable compositions, crystal symmetries, lattice parameters, and crystal habits. In principle, such bottom-up approaches can be used to assemble interesting photonic structures, including ones containing quantum dots and metal nanoparticles. 3D dielectric photonic crystals have well-established enhanced light-matter interactions via high Q factors. Their plasmonic counterparts based on arrays of nanoparticles, however, have not been experimentally well-explored due to a lack of available synthetic routes for preparing them. However, such structures should facilitate these interactions based on the small mode volumes associated with plasmonic polarization. Herein, we report strong light-plasmon interactions within 3D plasmonic photonic crystals that have lattice constants and nanoparticle diameters which can be independently controlled in the deep sub-wavelength size regime by utilizing a DNA programmable assembly technique. The strong coupling within such crystals is probed with backscattering spectra, and the mode splitting (0.10 and 0.24 eV) is defined based on dispersion diagrams. Numerical simulations predict that the crystal photonic modes (Fabry-Perot modes) can be enhanced by coating the crystals with a silver layer, achieving moderate Q factors (~10²) over the visible and NIR spectrum.
This opens up exciting possibilities for tuning the interaction between light and highly organized collections of particles at the nanoscale for applications ranging from lasers, to quantum electrodynamics to biosensing.

Photoactive mesoscopic particles can be drawn together and stabilized by electrodynamic interparticle forces in an optical field, resulting in optical matter. Here we show that the phase gradient of an extended optical field can drive assembly of Ag nanoparticle arrays in solution and allow tailoring the interparticle interaction and the structures (optical matter) they form. The phase gradient force induces strain in optically bound Ag nanoparticle chains, causing structural transitions from 1-D chains to 2-D lattices, and even to amorphous structures. The structures and structural changes make an analogy between optical matter and traditional materials that could show similar behavior under pressure, and, more importantly, reflect the fact that a rich phase diagram of optical matter exists and awaits further exploration. Understanding and tuning phase-related optical potentials will facilitate the design and fabrication of optical materials with novel properties.

Single-walled carbon nanotubes have strong infrared absorption and high charge carrier mobility that can be uniquely exploited to make infrared photodetectors with unprecedented performance. Despite their potential, the performance of SWNT-based infrared photodetectors has been limited due to strong exciton binding energy, presence of metallic SWNTs that quench excitons, and poor charge carrier collection. In this presentation, we present a novel SWNT/C₆₀ phototransistor composed of a bilayer of purely semiconducting SWNTs and C₆₀ with channel lengths on the order of 0.5-1 microns. The use of purely semiconducting SWNTs minimized exciton quenching, while the short channel lengths increased charge carrier collection efficiency. The addition of C₆₀ effectively dissociated the excitons and trapped the photoinduced electrons, increasing the hole concentration in the SWNT film and causing schottky barrier thinning at the contacts. Such improvements and modifications yielded current responsivity and specific detectivity of 97.5 A/W and 1.17 x 10(9) cmHz^1/2W^-1 at 1 kHz, respectively, under a low bias of -0.5 V, which are respectively, four and one order of magnitude higher than the highest SWNT-based planar photoconductors and bolometers reported. We have also achieved photoconductive gain of ~10(4) and response time of several milliseconds. Finally, we have demonstrated our device&’s IR imaging capability using an array of photodetectors, and its applicability as flexible/lightweight IR detector. This work puts SWNT photodetectors in a highly competitive position compared to other high-performance photodetectors.

Organic/inorganic hybrid glasses doped with rear-earth metal ions have been widely investigated for a laser amplifier application. We prepared a hybrid glassy host by inserting fluoroalkyl-chains. Subsequently, Er⁺³/CdSe nanophases were doped into the fluorinated hybrid glass. Mixing efficiency of the photoactive dopants was examined including solubility, phase separations, dopant levels, and homogeneity. There was no significant phase separation. A photo luminescent spectroscopy of the hybrid glass doped with Er⁺³/CdSe was carried out. Usually, laser amplifiers based on non-fluorinated hosts often fail to produce high lasing performance, partially due to a strong absorption from the OH-group at 1540 nm. The low solubility of dopants and the small absorption cross-section of rear-earth metal ions also limit the performance of amplifying. In contrast, the hybrid glass doped with Er⁺³/CdSe nanoparticles show an increase on photoluminescence intensity, especially around 1540 nm. The OH-absorption at 1540 nm was reduced in the highly fluorinated glass. Presence of CdSe nanoparticles, by virtue of their low phonon energy, also appears to significantly influence the nature of the surrounding chemical environment of Er⁺³ ions in the hybrid glass, resulting in increase of photo luminescent intensity.

Nanocrystalline Silicon (nc-Si), which is composed of Silicon nanocrystals in an amorphous Si (a-Si) matrix, is a thin film material of considerable interest for optoelectronic, display, and photovoltaic applications, due to its promising efficiency at low cost. Amorphous Si and crystalline Si (c-Si) have been extensively studied and their optoelectronic properties have been characterized. Amorphous Si degrades overtime in the sun and c-Si has low absorption. It is important now to understand how these two phases interact in nanocrystalline Si in order to develop solar cells that are durable with high absorption.
A deposition technique using plasma enhanced chemical vapor deposition (PECVD) permits the deposition of amorphous Si and Si nanocrystals to be decoupled, allowing for the size and density of Si nanocrystals to be tuned and deposited in quantum-confined regimes. Band edge absorption data is crucial in understanding and optimizing these new materials; however, the low absorption coefficient at these energies makes it difficult to obtain these data with standard reflection and transmission measurements. An alternative is to use, a highly sensitive technique called, photothermal deflection spectroscopy (PDS). PDS gains its sensitivity by measuring the small changes in temperature of a sample when optical energy is absorbed by it. It takes advantage of a mirage effect created by the thermal gradient setup in the surrounding environment of the material as it absorbs light. A measure of the nc-Si thin films quality was recorded through the use of electron paramagnetic resonance (EPR), which allows for studying the nature and the density of unpaired electrons in the material. The technique excites paramagnetic species by applying a microwave beam and varying a magnetic field, inducing the Zeeman effect in the material.
For the experiment, Using PECVD, a series of four NCSi thin films with varying amounts of Si nanocrystals was deposited. The absorption spectra of these films were obtained using PDS and, it was found that the nc-Si thin film with the lowest crystallinity had an absorption spectrum resembling that of (a-Si) and the sample with the highest crystallinity had an absorption spectrum slightly higher than thin films made of purely Si nanocrystal. The difference in absorption is believed to be a result of the a-Si present in the nc-Si thin film. EPR found that the material had a number of defects that were believed to be Si dangling bonds on the surface of the nanocrystals. In conclusion, we implemented a novel approach to acquire absorption spectra for mixed two phase nc-Si thin films and found that the absorption can be tuned based on the amount of Si nanocrystals deposited in the thin film.

Based on the utilization of near infrared (NIR) light, the full solar spectra active photocatalysts have been hot topic in recent years. The strategy with light energy conversion by the up-conversion materials is the most accepted way for the utilization of NIR light[1,2]. Following by the special band structure alignment, band structure modification, some more photocatalysts have been reported possessing the NIR light active photocatalysis [3,4]. However, it is surprise that the semiconductors with intrinsic narrow band is rarely studied for their NIR light active photocatalysis. Although the narrow band gap of the materials is the necessary condition for the NIR light active photocatalysis, most materials with narrow band gap or impurity level in gap do not have NIR light photocatalytic activity. Winnow the NIR light photocatalytic active materials from semiconductors with narrow band gap should be a very important topic in this research area. After trying a lot of different materials, we found that WS₂ nanosheets possess very strong NIR light photocatalytic property. Under the NIR light irradiation, the photo-generated electrons and holes are identified in WS₂ nanosheets instead of energy conversion corresponding to the thermo-effect. Surprisingly, WS₂ nanosheets also has strong UV and visible light photocatalytic activity, indicating it is a rare and valuable full solar spectrum photocatalyst. The results could draw attention back to the semiconductors with narrow band gap for the seeking of the NIR light active photocatalyst, which implies a convenient path for the expanding utilization of solar light.
References
[1] Jiang, Q.J;. Yu, J.G; Jaroniec, M.; Chem. Soc. Rev., 2012, 41, 782.
[2] Tong, H.; Quyang, S.;. Bi, Y; Umezawa, N.; Oshikiri, M.; Ye, J.; Adv. Mater., 2012, 24, 229-251.
[3] Pak, C.; Woo, J. Y.; Lee, K.; Kim, W. D.; Yoo, Y.; Lee, D. C.; J. Phys. Chem. C, 2012, 116, 25407-25414;
[4] Wang, G.; Huang, B. B.; Ma, X. C.; Wang, Z. Y.; Qin, X. Y.; Zhang, X. Y.; Dai, Y.; Whangbo, M-H; Angew. Chem. Int. Ed., 2013, 52, 4810-4813

TiO₂ nanoparticles are known to be the best candidate for photo-catalytic applications. They draw attention due to their extraordinary photo-catalytic activity while being non-toxicity, high available at a low price. Photocatalytic particles are utilized in various applications such as self-cleaning surfaces, water and air purification, anti-fogging surfaces and photovoltaics. During past several years many studies have been done to immobilize TiO₂ nanoparticles onto textile materials to obtain self-cleaning, anti-bacterial and uv-protective products [1]. Processing of textile products with TiO₂ nanoparticles is not a complex process but insufficient binding efficiency between certain fibers and TiO₂ nanoparticles is the major problem which is related to the stability and durability of nano-composite system during exploitation. Furthermore, TiO₂ is only active under the UV light without any surface improvements. Recently SiO₂ nanoparticles are being used to improve the attachment properties and stability of TiO₂ nanoparticles. SiO₂ is an ideal binder since it does not decompose due to the strong photocatalytic activity of TiO_2. Earlier studies have shown the discoloration of stains was more efficient for TiO₂-SiO₂ coated samples than TiO₂ coated ones [2].
In this study a novel surface modification technique was developed to improve the stability and surface attachment of TiO₂/SiO₂/polymer branch composite nanoparticles. The first objective of this study was to synthesize a binder with functional groups to increase the binding efficiency of TiO₂ nanoparticles to the textile surfaces. The second objective was to improve the photocatalytic activity within the visible light region by doping the composite system with ions like Ag⁺, Au⁺⁺ ions. It has been demonstrated that better surface durability and photocatalytic functionality is achieved with the new composite titania based nanoparticles.
References:
1.Radetic M., Functionalization of textile materials with TiO₂ nanoparticles, Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 16 (2013) 62- 76
2.Yuranova T., Mosteo R. Bandare J., Laub D., Kiwi J., Self-cleaning cotton textiles surfaces modified by photoactive SiO₂/TiO₂ coating, Journal of Molecular Catalysis A: Chemical, 244(2006) 160-167

Titanium dioxide (TiO₂) has attracted intensive research interests during the past decades due to its wide application in photocatalysis, photoelectrochemical cell, solar cells, and smart coating.[1]Controlled synthesis of titanium dioxide (TiO₂) with both high surface areas and high energy facets is technologically important for its application.[2] However, due to its large band gap (3.0 eV for rutile and 3.2 eV for anatase), it only responds to UV radiation, which is less than 5% of sunlight. In addition, the fast recombination of photo-generation electron-hole pairs and hot carrier diffusion hinder the performance of TiO₂ and its application as an efficient photoelectrocatalytic material. Many efforts have been made to make TiO₂ with visible-light response. For example, doped TiO₂ with metal or nonmetal ions exhibited a broad visible light absorption and great performance. Recently, reduced TiO₂(TiO_2-x), incorporating Ti³⁺ and/or oxygen vacancies in TiO₂, has emerged as an effective route to obtain visible-light photoactivity.[3] Herein, we simultaneously control the morphology and defect composition of titanium dioxide (TiO_2-x),which exhibits enhanced photocatalytic activity. We demonstrate a simple and facile solvothermal strategy for preparing highly active and stable TiO_2-x rutile nanorods with exposed (110) facets and tunable oxygen vacancy.
References:
[1] H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H. M. Cheng and G. Q. Lu, Nature, 2008, 453, 638-641.
[2] Z. Zhao, Z. Sun, H. Zhao, M. Zheng, P. Du, J. Zhao and H. Fan, Journal of Materials Chemistry, 2012, 22, 21965.
[3] Chen, X., Liu, L., Yu, P. Y. ,Mao, S. S.,Science, 2011,331,746-750

Quantum dots (QDs) of p-type lead chalcogenide are excellent light absorbers for high efficiency solar cells. Recently, a layer of wide bandgap semiconductor such as ZnO or TiO₂ has been inserted between the QD film and a transparent conducting oxide (TCO) film to form a depletion region, and its built-in potential enhances exciton dissociation and directional electron flow. In the planar heterojunction QD solar cells, a tradeoff between light absorption and carrier extraction is an important issue and better device structures are needed for QD-based photovoltaics to compete with the conventional technologies. Here, we present the formation of a depleted bulk heterojunction (DBH) for a PbS-TiO₂ thin film in F-doped SnO₂ (FTO)/TiO₂/PbS (QD)/Au solar cells, and a consequent increase in cell performance by thermal annealing. Our study was focused on the detailed structure of the depletion layer at the interface between QD layer and wide bandgap semiconductor film after thermal annealing. We observed that the average inter-particle distance decreases and necking between adjacent QDs occurs after thermal treatment. This partial connection between QDs enhances the carrier transport by increasing the probability of carrier hopping and decreasing the chance of carrier trapping. More importantly, we found that the QDs diffuse into the TiO₂ mesoporous film to form a DBH structure. This increases the TiO₂/PbS interface area, resulting in more efficient carrier transfer from PbS QDs to TiO₂ nanoparticles. Annealing also improved the crystallinity of the PbS QDs, introduced a necking structure between adjacent QDs, and decreased the bandgap of PbS QDs. All of these changes made during the thermal annealing contribute to a dramatic increase in the short circuit current density with a minimal change in the open circuit voltage, leading to high performance PbS QD solar cells.

Ta₃N₅ is a promising photocatalyst for solar water splitting, absorbing light up to ~600 nm and having band positions that straddle the redox potentials of H⁺/H₂ and O₂/H₂O. Elucidating structure-performance relationships is essential to develop functional materials, but little effort has been directed toward the synthesis of structurally defined Ta₃N₅ particles. This oversight can be account for the difficulty in preparing shape-defined Ta₂O₅ precursor which is typically used to convert to Ta₃N₅. Here, aerosol-assisted molten salt synthesis (AMSS) is used to prepare Ta₃N₅ crystals with defined shapes. Specifically, Ta₂O₅ plates and rhombohedra are synthesized by AMSS and subsequently converted to Ta₃N₅ via ammonolysis, with shape preservation. The Ta₂O₅ phase is amorphous as prepared, with crystal growth limited to the nanoscale by the spatial and temporal confinement provided by the aerosol droplet. This features rapid conversion to Ta₃N₅, at lower temperature and ammonia flow rate than most of the reported study in which crystalline Ta₂O₅ is used for conversion to Ta₃N₅. The generation of amorphous Ta₂O₅ as nanoplate was unexpected given the orthorhombic crystal system assumed for low-temperature form of Ta₂O₅, with control studies providing insight into the underlying of crystal growth mechanism. The synthesis of shape-controlled Ta₃N₅ highlights AMSS as a facile approach to materials with new crystal habits and enables important structure-properties to be elucidated from shape-controlled nanocystals.

ZnO particles with rod and plate configurations were synthesized using a solvothermal method using zinc acetate and zinc chloride, respectively. The surface of the as-synthesized ZnO rods and plates were characterized using various analysis tools (XRD, XPS, photoluminescence, FE-SEM, HR-TEM, BET, and UV-Vis) and their photocatalytic activities were examined for six different redox reactions. The surface areas and bandgaps of the two ZnO samples were nearly identical; however, XPS and photoluminescence (PL) studies showed that the rods and the plates have relatively pronounced oxygen vacancy and oxygen interstitial contributions, respectively. ZnO rods were found to be active for the decomposition of methylene blue and phenol, the production of OH radicals, and the generation of photocurrents, all of which are associated with single-electron transfer reactions. On the other hand, ZnO plates were more effective for the production of molecular hydrogen and hydrogen peroxide, both of which are initiated by two-electron transfer reactions. These single vs. multiple charge transfers are discussed with regard to the roles of oxygen vacancies and oxygen interstitials, which are located near the conduction and the valence bands, respectively.

Important to the development of dye-sensitized solar cells (DSSC) is the longevity and photo-conversion efficiency of the dye. To improve cost effectiveness dyes of superior thermal and chemical stability are desirable to extend device lifetimes. One particularly promising class of dye sensitizer is phthalocyanines (Pcs), which exhibit high molar absorptivity and spectral tunability. A well-recognized problem regarding Pcs is there inherent tendency to aggregate in stacked geometries. In the application of thin films, the aggregation beyond monolayer coverage is often detrimental to maintaining desired photo-physical properties of the surface coating. By modifying the molecular periphery of Pcs with bulky perfluoroalkyl substitutions, it is anticipated that molecular aggregation could be mitigated. The intrinsic chemical and thermal stability of C-F bonds compared with C-H bonds in the parent Pc molecule can also enhance the environmental performance of the dye while the ability to incorporate various metal and metalloid ions to tune photo-physical properties allows them to be tuned to specific spectral sensitivity. We examined a series of peripherally substituted phthalocyanines for the effects of substituent groups on surface adhesion and aggregation over TiO₂ surfaces. Using molecular dynamics (MD) and first principles modeling, the effects of bulky substituents on the molecular periphery were investigated for a series of zinc phthalocyanines with both symmetric and asymmetric substitution of peripheral hydrogen atoms with fluorine and isopropyl fluoroalkyl groups. The results provide a rational basis to design novel DSSC dye materials that mitigate deleterious molecular aggregation in thin film architectures.

Bismuth vanadate (BiVO₄) films was synthesized by solution combustion synthesis with different fuels (alanine, glycine and urea) and deposited by dip-coating technique with 3 layers and several drying temperature between layers (50 0C, 100 0C and 150 0C). Optical, morphological, photoelectrochemical and photoelectrocatalytic properties were investigated by X-ray diffraction (XRD), diffuse reflectance spectrometry (DRS), scanning electron microscopy (SEM) and photoelectrochemical analysis. The monoclinic structure in BiVO₄ films via dip-coting was confirmed by XRD, which showed that the drying temperature between layers of 50 °C was more efficient to adhesion the precursor gel on the ITO conductor substrate and it presented higher crystallinity, mainly for films synthetized with the glycine fuel. The lowest values of band-gap energy and the smaller particle size observed by SEM were found in films with drying temperature between layers of 50 0C, making possible a more advantageous material for use solar source for discoloration of organic pollutants, such as dyes. The photoelectrocatalytic activity of BiVO₄ films was evaluated by discoloration of methylene blue and methyl orange dyes. Photoelectrochemical discoloration of dyes by BiVO₄ films shows up as a very efficient process, mainly with the film synthesized with drying temperature between layer of 50 0C, which it shown higher crystallinity, smaller particles size and lower band-gap energy.

Nanotechnology has attracted the interest of many research groups around the world due to its vast potential for applications in various industrial sectors and the impact it can give to the technological and economic development. In this context. the nanoparticles have been studied due to their special properties, where two important materials are titanium dioxide (TiO2) and zinc sul#64257;de (ZnS). Nanoparticles of ZnS have special properties such as photoelectric, photoluminescence and electroluminescence. Titanium dioxide in its anatase structure is the most common material used in heterogeneous photocatalysis due to its properties, such as activation by visible light, chemical stability over a wide pH range, low cost, low toxicity, water-insolubility, easy handling and the energy gap of 3.0 to 3.2 eV, which is associated with a high photocatalytic activity. Being semiconductor type materials, they have the electronic structure characterized by a #64257;lled valence band and an empty conduction band, allowing them to act as photocatalyst materials. Nanoparticles are prepared by some methods, which can be divided into two categories: chemical methods and physical methods. In this work ZnS nanoparticles were obtained by microwave-hydrothermal synthesis (MHS) method and solid-liquid chemical reactions (SLCR) and the TiO2 nanoparticles used were the commercially available Degussa (Evonik) P25 and nanoparticles synthetized by MHS. Both Zns and TiO2 were also synthetized and supported on carbon #64257;ber by the same methods for further evaluation of their characteristics and properties. To increase the surface area and roughness of the carbon #64257;bers and the number of functional groups on the carbon surfaces, they were treated by a chemical method using 65% HNO3 at 100 ° C under different ‘exposure times. The treatment times were 0, 10, 20 and 30 min, time range that did not completely degrade the fibers. The pure nanoparticles and the nanocomposites carbon #64257;ber-ZnS and carbon #64257;ber-TiO2 were characterized by X-ray diffraction, high resolution scanning electron microscopy, Raman spectroscopy and energy-dispersive X-ray spectroscopy. Preliminary results showed that proper adhesion of the nanoparticles to the carbon #64257;bers surfaces successfully occurred. To test the photocatalytic activity of the nanoparticlesand the nanocomposites, phototocatalisys degradation experiments were performed in aqueous solutions containing the dye Rhodamine B under irradiation of visible light and static and dynamic #64258;ow conditions. The photodegradation performance of both nanocomposites were evaluated under UV-visible spectroscopy and according to the kinetic studies, the nanoparticles themselves and the carbon #64257;bers doped with TiO2 and ZnS under visible light are ef#64257;clent to photodegradate the Rodamine B, what indicates that both nanocomposites can be ef#64257;cient to photodegradate hazardous materials. Acknowledgements to CAPES, CNPq and FAPERGS for the financial support.

Nanoparticles (NPs) of Indium Antimonide (InSb), varying from 10nm to 200nm in size, were synthesized using a vapor phase synthesis technique known as Inert Gas Condensation Sputtering (IGCS). These NPs were directly deposited over 1 inch-square (111) p-type Silicon (Si) wafer, glass cover slip and Sodium Chloride (NaCl) substrates. The Wide-Angle X-ray Scattering spectra obtained for these NPs showed (111) and (220) diffraction peaks that revealed the crystalline behavior of these NPs exhibiting a cubic symmetry. The 1:1 composition ratio of In:Sb was confirmed by the Energy Dispersive X-Ray Spectroscopy studies. The Raman spectra of these NPs exhibited a peak at 186.7cm-1, which corresponds to the LO modes of phonon vibration in InSb. NPs will be size selected during the synthesis process. Their size dependent band gap, measured using Fourier Transform Infrared (FTIR) spectroscopy, will be presented. Optoelectronic characterization of these NPs for Mid-IR photo detector application will also be discussed. To determine the photoelectric properties, wavelength dependent photo-responsivity will be measured, in addition to their dark current and conductivity.
Along with InSb NPs, 1:1 InSb thin films, 50nm to 250nm thick, were synthesized using D. C. Sputtering and were deposited on (111) p-type Si wafer, glass cover slip and NaCl substrates. The Raman spectra of the 100nm thick InSb thin film showed a single peak at 177.8cm-1, which corresponds to the TO modes of phonon vibration in InSb. The band-gap of this InSb thin film, estimated using FTIR spectroscopy, was found to be 0.207eV. The conductivity of these varied thickness films was measured to be in the range of 0.8-5.5 (ohm-cm)-1 at 298K. These films will also be characterized for Mid-IR photo detector application. Optoelectronic features of the InSb NPs synthesized using ICGS will be compared to that of these thin films.
Hence the use of IGCS, a vapor phase technique, for synthesis of IR sensitive nanoparticles will be demonstrated. The potential of InSb nanoparticles synthesized using IGCS for Mid-IR Photodetectors will be explored. It will also be demonstrate that these quantum dot devices can be tuned dependent on the particle size.

Dye-sensitized solar cells (DSSCs) have attracted widespread interest as a promising photovoltaic technology due to their low cost and high conversion efficiencies. Extensive research efforts have led to the development numerous ruthenium complexes, organometallic dyes, and metal-free organic sensitizers. One particularly promising class of solar cell dye material is phthalocyanines (Pcs) which exhibit exceedingly high molar absorptivity and spectral tunability through structural modification. Phthalocyanine dyes are extraordinary robust and unique in their light harvesting ability in the far-red and near-infrared spectral region. Herein, we present the results of computational investigations of the photophysical properties a new class of modified zinc phthalocyanines (ZnPc) as potential sensitizer materials on TiO₂ surfaces. The Pc molecules investigated include a perfluoro-Pc (F₁₆ZnPc), in which all labile carbon-hydrogen bonds are replaced with carbon-fluorine bonds, as well as a series of modified Pcs containing bulky perfluoro substitutions on the molecular periphery predicted to mitigate the effects of molecular aggregation. Time dependent density functional theory (TDDFT) and semi-empirical computational methods were used to predict Pc-TiO₂ binding, electronic structure, and charge injection dynamics. The Pc sensitizers investigated exhibit charge injection rates ranging from 5 fs to 40 ps and excited-state lifetimes in the 1 - 10 ns range. These results indicate that this class of DSSC dye may offer enhanced environmental performance (chemical stability) while still providing efficient charge injection rates.

The synthesis of hybrid nanostructures combining semiconductors and graphene are being attracted increasing attention because of their excellent optoelectronic properties for photovoltaics applications. The previous device reports to date have relied on ligand exchange in the solution phase or halide treatment during solid-state ligand exchanges. Here we report a facile and simple solution method to synthesize PbS colloidal quantum dot capped by graphene quantum dots. We develop a quasi core shell structure of PbS quantum dots using graphene quantum dots with an uniform dispersion and size of dots and fabricate colloidal quantum dot solar cell based on this new structure. The device characterization and density functional theory showed that graphene quantum dot can increase the current density and improve the efficiency of the device due to the surface passivation of PbS quantum dot, decreasing of the deep trap states and the distance between the dots. We achieve solar power conversion efficiencies of 4.1% using PbS-Graphene quasi core shell structure.

The band gap energy of the TiO2 photocatalytic is high at 3.2 eV. Ultraviolet (UV) light irradiation (<388 nm) is required for the photocatalytic application. The lowering the band gap energy of TiO2 and enlarging light-absorbing area are effective ways to enhance the efficiency of photocatalytic activity. Furthermore, the morphology and crystal structure of nanosized TiO2 considerably influences its photocatalytic behavior. In this study, sodium titanate nanorods were formed using an alkali-treatment and were heat treated at different temperatures. The photoelectrochemical properties of sodium titanate nanorods was measured as a function of heat treatment temperature.
The nanorods were prepared on the surface of Ti disk with a diameter of 15 mm and a thickness of 3 mm. Ti disk was immersed in 5 M NaOH aqueous solution at a temperature of 60#8451; for 24 h. Morphology of sodium titanate nanorods was observed using FE-SEM. Crystal structure of sodium titanate nanorods was analyzed using X-ray diffractometer. Photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) was used to evaluate photoelectrochemical properties of sodium titanate nanorods.
The thin amorphous sodium titanate layer was formed during alkali-treatment. The sodium titanate layer was changed to nanorods after heat treatment at a temperature of 700#8451;. The thickness and length of sodium titanate nanorods obtained at 700#8451; were around 100 nm and 1 #13211;, respectively. The crystal structure of sodium titanate was identified with Na2Ti6O13. Above 900#8451;, the morphology of nanorods changed to agglomerated shape and the thickness of nanorods increased to 1 #13211;. The lowest value of PL was obtained at a temperature of 700#8451;, while non-alkali treated specimen showed the highest value of PL. EIS revealed that polarization resistance at interface between sodium titanate nanorods and electrolyte was increased with increasing heat treatment temperature.

Fabrication of membrane electrode assemblies (MEA) of proton exchange membrane fuel cells (PEMFC) with traditional spray-painting and brush-painting easily causes up to 60% of Pt nanoparticles without three-phase boundary (Nafion-proton channel, carbon-electron channel, and nanopores-gas channel), and thus a low Pt utilization efficiency Pt.^[1-2] It is essential to construct the three-phase boundary and to improve the Pt efficiency. In this regard, self-assembly and ion-exchange/deposition approach has been investigated, leading to the construction of the proton channel for Pt nanoparticles. However, these approaches cannot build up the electron and gas channel, together with a resultant low Pt loading (2.8-14 mu; g_Pt/cm²) that cannot meet the requirement of a large number of active sites for electrode reactions like oxygen reduction reaction. ^[3-5]
In this study, we aim to construct the three-phase boundary of Pt nano through a light-driven route. Initially, metalloporphyrin photocatalysts self-assemble upon Nafion membrane, then irradiate with light to make Pt selectively nucleate in the vicinity of the photocatalysts on the membrane, which leads to the construction of Pt-Nafion membrane interface for each Pt nucleation center, the proton channel. Next, with the employment of certain types of surfactants, Pt nucleation centers grow into nanoporous continuous Pt thin-film, supplying the gas channel. In addition, the continuous Pt thin-film is electrically conductive, offering the electron channel. The loading of Pt on Nafion membrane can be easily tuned by changing the concentration of Pt precursor. Finally, gas diffusion layers are hot-pressed with the Pt-loaded membrane to obtain MEA. The proposed investigation is expected to build up the three-phase boundary for each Pt nanoparticle, and thus significantly enhancing the utilization efficiency of Pt.
References
(1) David A. Bussian, James R. O&’Dea, Horia Metiu, and Steven K. Buratto. Nanoscale Current imaging of the conducting channels in proton exchange membrane Fuel Cells. Nano Letters, 2007 7(2): 227 232.
(2) G. Sasikumara, J.W. Ihma, H.Ryu. Dependence of optimum Na#64257;on content in catalyst layer on platinum loading. J. Power Sources, 2004 132: 11-17.
(3) Shan Zhu, Suli Wang, Luhua Jiang, Zhangxun Xia, Hai Sun, Gongquan Sun. High Pt utilization catalyst prepared by ion exchange method for direct methanol fuel cells. Hydrogen Energy, 2012 37: 14543-14548.
(4)Mu Pan, Haolin Tang, San Ping Jiang and Zengcai Liu. Fabrication and performance of polymer electrolyte fuel cells by self-assembly of Pt nanoparticles. J. Electrochem. Soc., 2005 6(152): 1081-1088.
(5) Siguo Chen, Zidong Wei, Hua Li and Li Li. High Pt utilization PEMFC electrode obtained by alternative ion-exchange/electrodeposition. Chem. Comm., 2010 46: 8782-8784.

Lanthanide-doped nanoparticles exhibit near-infrared-to-visible upconversion (UC) luminescence that could enable improved solar energy harvesting, high-density optical memory, and deep-tissue, background-free biological imaging. However, the use of these materials is generally hindered by their low quantum efficiency, a quantity largely dictated by the radiative rates of the emitting species. In this work, we demonstrate a novel approach to improving UC efficiencies: engineering the crystal field environments of the lanthanide ions to enhance their radiative decay rate. Increasing the asymmetry of the crystal fields experienced by the lanthanide ions can increase the probability of the f-to-f electronic transitions necessary for emission of upconverted photons, ultimately boosting the quantum efficiency of the process. We show the viability of this approach using hydrostatic compression as a means of engineering the crystal fields before reproducing the experimentally-determined optimum degree of lattice contraction in an equilibrium fashion via substitutional doping.
We consider colloidally synthesized nanoparticles of cubic-phase NaYF₄ co-doped with trivalent Yb and Er ions. We employ a diamond anvil cell to subject the particles to hydrostatic pressures as high as 25 GPa, compressing the NaYF₄ host lattice and thereby increasing the crystal field interactions between the host and dopant ions. Exciting with 980-nm light, we monitor UC intensity and excited-state lifetimes of the NaYF₄:Yb,Er particles. At low pressures (< 5 GPa), we observe an approximately twofold enhancement to UC emission alongside a concurrent lifetime reduction, together evidencing a successful radiative rate increase. We also perform in situ x-ray diffraction (XRD) measurements using synchrotron radiation to quantify structural changes to the lattice. These XRD data reveal that the aforementioned increase in quantum yield is achieved with only a 1% reduction in lattice parameter. To reproduce and exceed this modification in native materials, we synthesize cubic-phase NaLu_xY_1-xF₄:Yb,Er particles, where Y³⁺ is systematically replaced by optically inactive Lu³⁺ to contract the host matrix. We also synthesize NaGd_xY_1-x:Yb,Er particles to explore the effects of expanding the host lattice. XRD confirms phase purity and shows that the lattice constant of the nanoparticles can be varied from 5.458 to 5.572 Å (approximately ± 1% compared to pure NaYF₄). Optical spectroscopy is used to monitor the numbers of near-infrared photons absorbed and upconverted photons emitted, ultimately enabling correlation of absolute UC quantum yield and host lattice constant. This approach is also employed using hexagonal-phase host lattices, which are known to yield the most efficient near-infrared-to-visible UC. Our results constitute a proof-of-concept of crystal field engineering using heteroatoms as “structural dopants” to influence and improve the UC efficiency of lanthanide-based materials.

Due to the size- and shape-dependent properties, nanomaterials such as nanoparticles have been successfully fabricated for multi-dimensional (D) ordered assemblies for the development of ‘artificial solids&’ (e.g., metamaterials) with potential applications in nanoelectronics and nanophotonics. To date, synthetic approaches and property engineering of nanoparticle arrays have primarily focused on chemical methods. Here we report a simple physical method to engineer nanoparticle assemblies and to fabricate new nanomaterial architectures based on stress-induced phase transformation and sintering of spherical nanoparticle superlattices. We show that under a hydrostatic pressure field, the unit cell dimension of a 3D ordered nanoparticle arrays can be manipulated to shrink and swell reversibly, allowing fine-tuning of interparticle separation to interrogate optical coupling of nanoparticles arrays. Moreover, beyond a threshold pressure, nanoparticles are forced to contact and sinter, forming new classes of chemically and mechanically stable 1-3D nanostructures that cannot be manufactured by current top-down or bottom-up methods. Depending on the mesophase of the initial nanoparticle arrays, 1-3D ordered nanostructures (Au, Ag, etc.) including nanorod, nanowire, nanosheet, and nanoporous network can be fabricated. Exerting stress-dependent control over the structure and property provides a unique and robust system to understand collective chemical and physical characteristics of nanomaterials. This method mimics embossing and imprinting manufacturing processes and opens exciting new avenues for large-scale fabrication of novel active nanomaterials during applying and releasing of stress.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Recently hybrid nanocomposites consisting of graphene-nanomaterial heterostructures have emerged as perfect candidates for the fabrication of optoelectronic devices, because such nanostructure play a very significant role to improve the device performance. In this work, we use a novel and facile technique, a simple solution method, for the synthesis of ZnO-graphene nanocomposte and report on a significant power conversion efficiency improvement of PbS colloidal quantum dots solar cells from 4% to 5.4% due to using ZnO-Graphene nanocomposite as electron transfer layer. The device characterization tests showd that the device based on ZnO-Graphene can increase the current density and as a result the efficiency due to improving of the carriers collection. Furthermore, a strong quenching of CQD photoluminescence was observed at sim;1100 nm upon the addition of the graphene, which is pronouncedly correlated with the increase of the IPCE of the cell. This work highlights that ZnO-Graphene layer can help for a superfast electron tunnel for optoelectronic devices.

Hematite (α-Fe₂O₃) nanoparticles (NPs) show great potential for application as absorber material in multi-junction solar cells due to their suitable band gap accompanied by high absorption coefficients. Additionally, hematite is a highly promising photoanode material for photoelectrochemical solar water splitting due to its chemical stability, band structure and abundance. However, the pure hematite phase is required for both applications.
In this study we investigated the temperature dependent electronic transport properties of hematite NP layers. A successively increasing electrical conductivity for annealing temperatures above 360 K under vacuum condition is observed. We interpret the increased conductivity as percolating transport through magnetite shells surrounding the hematite NPs, i.e. the results indicate a continuous material conversion of hematite into magnetite (Fe₃O₄). This phase transition is confirmed by Raman spectroscopy for annealing temperatures above 600 K. The known conversion temperature of hematite as bulk material into magnetite is around 1000 K under vacuum annealing. The results therefore indicate a significantly reduced conversion temperature of the NP surface region as compared to the NP bulk.
Due to the continuous conversion of the NP surface into the higher conductive magnetite phase, we are able to tune the conductivity of the NP layer over seven orders of magnitude. Electrons are identified as the majority charge carriers by thermoelectric power measurements. The size and shape of the NP&’s remains unchanged during the annealing process as confirmed by Scanning Electron Microscopy. Using synthetic air during the annealing process, we are also able to reverse the phase transition and the conductivity drops close to its initial value.
The present study shows the importance of controlling temperature and atmosphere when hematite is processed, since the structural phase of iron oxide can change on a nanometer scale.

The photovoltaic (PV) solar cell technology based on polycrystalline thin films like CuIn_xGa_1-x Se₂, (CIGS) and CdTe have shown commercially a great interest within the last decade due to their higher conversion efficiencies over 20%. Recently, Cu₂ZnSnS₄ (CZTS) has attracted great attention as an alternative absorber material that replaces In, Ga, and Se of CIGS with Zn, Sn, and S which are less expensive, nontoxic, earth-abundant elements. CZTS has an optimum band gap (1.4-1.5 eV) and large absorption coefficient (>10⁴ cm^-1). However, most synthesis studies of CZTS have been focused on thin films and nanoparticles. Little information is available for the synthesis of CZTS nanostrutures such as nanowires and nanowhiskers. Recent theoretical calculations showed that radial PV nanorod solar cells are capable of enhancing significantly solar conversion efficiency because of their short collection length for excited carriers. In the present work, we report the direct chemical bath deposition of CZTS nanowires and nanowhiskers based on studying the role of hexamethylenetetramine ((CH₂)₆N₄, HMTA) as a complex agent. HMTA is a non-ionic, heterocyclic tertiary amine derivative and used as shape inducing molecule especially for ZnO nanowires growth. CZTS nanowires and nanowhiskers were synthesized using HMTA whereas uniform CZTS thin films were formed using NH₃ as a complex agent. CZTS nanowires were observed at lower HMTA concentration (0.1 M). As increasing HMTA concentration to 2 M, closely-packed CZTS nanowhiskers were synthesized on a substrate. With adding NH₃ at 0.1 M HMTA, these nanostructures were demolished and continuous particles were deposited on substrates. We will further discuss the role of HMTA on CZTS nanostructure formation.

Three-dimensional (3D), tubular-structured monolayer graphene networks were hybridized with TiO₂ nanoparticular layer for futuristic and robust electrode applications. A continuous form of 3D graphene with good carrier mobility provides a direct pathway for electrons to the current collector for a photoanode in dye-sensitized solar cells. This characteristic feature, coupled with its energy level, ensures an enhanced charge collection efficiency. Particular attention was paid to the graphene surface functionalization and the effective loading of TiO₂ nanoparticles to improve the light harvesting and minimize electron recombination for a photoanode. The optimal hybrid structure resulted in a 15% enhanced energy conversion efficiency, compared to the TiO₂-based analog without graphene. The impedance spectra confirmed that the increase in photovoltaic performance was mainly driven by the efficient charge collection through the 3D, tubular-structured monolayer graphene. This new electrode prototype can serve as a perfect complement to conventional TiO₂ nanostructures.

We have successfully fabricated a highly continuous thin film of CdTe quantum dots on Silicon and GaAs substrates. Highly water soluble CdTe quantum dots were synthesized using regular one pot synthesis wet-chemical reflux / hydrothermal method and also using same wet-chemical method in microwave. The CdTe quantum dots were highly luminescent, cost-effective and simple to make in both cases. But using microwave we were able to reduce time of preparation from hours to minutes. Also, our data suggests that quality of CdTe QDs were highly improved along with good photoluminescence, absorption and size distribution. TEM results have shown highly crystalline and spherical individual dots. SEM result shows a uniform thin film on Si and GaAs. We chose orange luminescent quantum dots for solar cell fabrication. We have successfully demonstrated photovoltaic behavior from the I-V curve measurements. We also compared the photovoltaic activity of GaAs and Si based solar cell devices. Preliminary results show that GaAs-CdTe QD based solar cells have superior performance.

We describe the synthesis, characterization, and electrochemical properties of a series of PCBM dyads containing 4-fluoro-α-cyanostilbene units for potential usage in organic photovoltaic devices. The PCBM dyads are fully characterized by NMR, MALDI-TOF, UV-vis absorption, FT-IR and cyclic voltammetry. It is found that the presence of 4-fluoro-α-cyanostilbenes affects the cyclic voltammetry and absorption spectrum very little. The PCBM modified-functionalized fullerenes significantly influences on the electrochemical and photophysical properties, resulting in up-shifted LUMO and wide absorption compared to PCBM. Preliminary results are low in comparison with the optimized high-performance PSCs, it is believed that the efficiency would be improved through successful device optimization of P3HT/PCBM-F dyad cells.

Organic photovoltaic cells from a Bulk-heterojunction (BHJ) active layer which based on phase-separated blends of donor and acceptor (fullerene derivative) have been development during the past two decades.[1-3] Recently, the solution-processed small molecule based BHJ solar cells exhibits comparable conversion efficiency around 7-9% to the polymer solar cells with optimized nanomorphology and device structures.[4-6] The small molecule materials have several attractive characteristics such as relatively simple synthesis and purification steps, and mono-dispersity which give rise to improved device reproducibility.
In this work, we have been demonstrated the results of several positive effects that arise from the addition of graphene quantum dots (GQDs) to solution-processed small molecule bulk-heterojunction (SM-BHJ) solar cells fabricated from a p-DTS(FBTTh₂)₂/[6,6]-phenyl C₇₁ butyric acid methyl-ester (PC₇₁BM). The device with an optimized ratio of GQDs exhibits increased current density and fill factor which correlate an 10% improved external quantum efficiency (EQE) and induce a favorable SM-BHJ morphology. Additionally, the multiple scattering of the GQDs in the SM-BHJ leads to longer optical path lengths according to the analysis of diffuse reflectance spectra and UV/Vis absorption spectra. The GQDs inserted SM-BHJ film with optimized concentration exhibits decreased charge transport resistance significantly by impedance measurements with effective charge extraction at the device which contribute to 15% enhancement of power conversion efficiency (PCE). The combination of those factors: improved light absorption and effective charge conductor lead to a high efficiency solution processed small molecule solar cells.
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(3) D. H. Wang, J. K. Kim, J. H. Seo, I. Park, B. H. Hong, J. H. Park, A. J. Heeger, Angew. Chem. Int. Ed. 2013, 52, 2874.
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We studied surface photovoltage (SPV) transients in nanoporous silicon and silicon nanotubes. The nanoporous silicon samples were grown by anodization of n-type silicon wafers and magnesiothermic reduction of biogenic silica plant extracts followed by acid treatment whereas silicon nanotubes were fabricated by sacrificial template method. The SPV experiments were performed in nitrogen and in vacuum environments. It was found that the nanoporous silicon grown by both the techniques was sensitive to the ambient conditions. For the nanoporous silicon grown by the anodization method, the SPV transients obtained in nitrogen gas environment revealed multiple components corresponding to processes occurring on different time scales during both “light-on” and “light-off” events. The transients measured in vacuum contained only a single component for both types of events. In the case of nanoporous silicon obtained by plant extracts, we observed the “light-on” and “light-off” transients with a single component only regardless of the ambient conditions. However, the surface of these samples was found to change from the accumulated n-type to the depleted n-type when the nitrogen gas environment was changed to vacuum. The silicon nanotubes also revealed SPV transients with multiple components in both nitrogen and in vacuum with a significant difference in the behavior of the slow component during the “light-on” events. The “light-off” processes on the other hand produced only a single transient component. We model the observed phenomena by considering either charge transfer from/to the surface adsorbates or a modification of surface electronic structure by the gas species.

We report the synthesis of Cu₂SnS₃ (CTS) nanostructures and for the first time its incorporation into an inorganic-organic hybrid device to enhance the photoresponse under AM 1.5 G solar illumination. The nanostructures were structurally and optically characterized. From X-ray diffraction (XRD) and Transmission electron microscopy (TEM) the CTS nanocrystals were found to be tetragonal. Both spheres and flower like structures of CTS were obtained as seen from Scanning electron microscopy (SEM). A band gap of 1.23 eV was obtained from absorption studies. Two devices have been studied, P3HT: PCBM = 1: 1 and CTS: P3HT: PCBM = 8:1:1. The photocurrent increased from a value of 2.33 mA at dark to 2.5 mA for the P3HT-PCBM blend to 3.36 mA for CTS: P3HT: PCBM = 8:1:1 device. The responsivity, sensitivity, external quantum efficiency and specific detectivity increased from 18.81 mA/W, 1.07, 4.25% and 6.88 × 10⁸ Jones respectively for P3HT:PCBM sample to 189.97 mA/W, 1.44, 42.9% and 6.95 × 10⁹ Jones for CTS: P3HT: PCBM = 8:1:1 sample at 1V bias and 1 Sun illumination intensity. The time dependent photoresponse was stable over different ON-OFF cycles. From the fit to the rise and decay curves, the rise and decay time constants were obtained. The diode parameters were calculated and the ideality factor, barrier height, reverse saturation current and shunt resistance were found to vary from 1.46, 89.73 meV, 1.23 × 10^-7 A and 26.33 mOmega; for P3HT: PCBM sample to 1.61, 70.82 meV, 2.56 × 10^-7 A and 36 mOmega; for CTS: P3HT: PCBM = 8:1:1 sample at 1V bias and 1 Sun illumination intensity.

Since the small-sized metallic nanoparticles (NPs) show high absorption ratio, small-sized NPs to obtain high efficiency light trapping structure in photovoltaic devices is particularly necessary. Based on Yiming and al. works, we successfully restrained the formation of small-sized Ag NPs made via a chemical process and a rapid thermal annealing, which is driven by the film fluctuation and rupture mechanism of dewetting theory. Relatively, the increase of short-circuit current density was achieved when Ag NPs fabricated by rapid thermal annealing were used as light trapping structure of solar cells. This study validates the necessity of fabrication of small-sized NPs experimentally, which is helpful for obtaining high efficiency light trapping structure and understanding the metallic film annealing mechanism.

Solar energy has great potential to provide all of the energy needs of the society, if only it could be harvested, stored and distributed efficiently. Photoactive materials which absorb solar photons and store the energy as photoisomers are promising to achieve these goals. Fulvalene tetracarbonyl diruthenium, FvRu2(CO)4, which was initially reported in 1997, is a good example of a such a compound [1]. When FvRu2(CO)4 absorbs a photon of 350 nm wavelength (~3.55 eV photon energy), it gets excited to a higher energy photoisomer, which is a metastable state with a reverse transition barrier of 1.3 eV. About 30% of the stored photon energy is recovered when the compound relaxes back to its ground state conformation by an external stimulus like heat. The objective of this research is to replace ruthenium, which is a rare element, with magnetic alternatives both to increase the stored energy through ferromagnetic interactions and to control the conformational change by magnetic means, rather than by heat. Elements chosen to create the ferromagnetic interaction are iron (Fe), cobalt (Co) and nickel (Ni). Antiferromagnetic interaction with other elements was reported in a prior paper [2]. Storage properties of fulvalene with the chosen elements are investigated by quantum mechanical calculations. The method used is DFT with B3LYP functional and Pople type basis sets augmented with polarization functions. Atomic models consist of the fulvalene molecule incorporated with two ferromagnetic atoms, namely FvFe2, FvCo2 and FvNi2. Test case for the calculations is the original compound FvRu2(CO)4. Calculated properties are stored energy of 1.4 eV, forward transition barrier of 3.0 eV and reverse transition barrier of 1.6 eV. In the case of the proposed ferromagnetic compounds, first optimal spin state is determined by calculating energy of the ground state as a function of spin, s=1-3. Energy minimum is observed at s=1 for FvCo2 and s=2 for FvFe2 and FvNi2. Next, photoisomers and transition pathways are calculated at the spin value pertaining to the energy minimum. Calculated properties are: Stored energy: 1.2 eV for FvFe2, 2.0 eV for FvCo2 and 2.2 eV for FvNi2 Forward transition barrier: 3.0 eV for FvFe2, 3.2 eV for FvCo2 and 3.1 eV for FvNi2 Reverse transition barrier: 1.8 eV for FvFe2, 1.2 eV for FvCo2 and 0.9 eV for FvNi2 Results obtained for FvCo2 and FvNi2 look promising to store a greater fraction of the incident photon energy as compared to the benchmark FvRu2(CO)4. Calculated values of the forward transition barrier correspond to blue wavelengths of the solar spectrum. Reverse transition barrier is most favorable in the case of FvNi2. [1] R. Boese, et al., “Photochemistry of fulvalene tetracarbonyl diruthenium and its derivatives: efficient light energy storage devices,” J. Am. Chem. Soc. 1997, 119, pp 6757-6773 [2] H. Gokturk, "Fulvalene incorporated with transition metals," MRS Spring Meeting, April 2013

Colloidal Quantum Dots (QDs) are seen as ideal candidates to replace current phosphors in LEDs for general lighting as they could significantly enhance the overall LED efficiency due to their narrow-band emission. CdSe QDs represent the current workhorse, and have been well studied and developed for such applications. Nowadays, several procedures are available to obtain CdSe core/shell QDs with quantum yield (QY) exceeding 90%. However, Cd based quantum-dot phosphors are undesirable due to their high toxicity. As an alternative, InP based QDs are widely recognized as the most promising Cd-free phosphor, since they can offer a compatible, emission range to that of Cd-based QDs.² For that reason, the preparation of high-quality InP QDs has been actively pursued in the framework of electroluminescence or down-conversion light-emitting applications, nevertheless, their full potential has not been demonstrated yet.
This contribution deals with the fabrication of highly luminescent core/shell In based QDs. To achieve this we grew a shell of wide band gap materials (i.e. GaP and ZnSeS) on the bare InP core. Our system consists of a first shell of GaP and of an outer alloy shell of ZnSeS.
First GaP acts as a buffer layer to completely passivate the surface defect of InP cores and notably increases the PL efficiency of the cores (form 5 % up to 50%). By monitoring the QY dependency on the GaP shell thickness we observed that the optimal thickness is 3 monolayers. We observed that the intermediate GaP shell has a key role for improving the efficiency of the final QDs. Secondly, by carefully engineering the composition of the ZnSeS shell we could reveal the optimal conditions to fabricate In based core/shell QDs with QY up to 75%, emitting in a range of 500-600 nm. Currently, the stability of these systems is investigated at Philips research.
Additionally, we are also exploring alternative route to enhanced the processability of the In based QDs by replacing the organic ligands with inorganic ligands (i.e. S^2-).⁴ This all-inorganic QDs, could hold the promise to yield a high photoluminescence quantum efficiency, tunable and narrow-band emission and long-term operational stability.
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Particle size or ‘quantum-confinement&’ effects have been used for decades to tune semiconductor optical and electronic properties. More recently, however, particle size control as the primary means for properties control has been succeeded by nanoscale hetero-structuring.¹ In this case, the nanosized particle is further modified to include internal, nanoscale interfaces, generally defined by compositional variations that induce additional changes to semiconductor properties. These changes can entail enhancements to the size-induced properties as well as unexpected or ‘emergent&’ behaviors. Common structural motifs include enveloping a spherical semiconductor nanocrystal, i.e., a quantum dot, within a shell of a different composition, or converting a semiconductor nanowire into a superlattice nanowire by introducing compositional variations along the nanowire length.
In this talk, I will discuss how solution-phase synthesis techniques can be used to create these structures with precisely ‘engineered&’ complexity and, in many cases, for realizing relevant and novel functionality. Most notably, I will discuss our experiences with so-called ‘giant&’ quantum dots, which due to their structure exhibit a range of unique behaviors, including non-blinking and non-photobleaching behavior with both visible- and near-infrared-emitting examples,^2-4 as well as remarkably efficient ‘multi-exciton&’ emission as a result of suppressed non-radiative Auger recombination.⁵ By controlling nanoscale architectures to achieve tailored optoelectronic properties, we show that it is possible to realize a range of light-emission applications from ‘building blocks&’ for solid-state devices for enhanced energy efficiency^6,7 to bright and ultrastable aqueous-phase molecular probes in biology.⁸ I will further show that even these arguably advanced nano-heterostructures can be further improved upon, where InP-based core/shell/shell quantum emitters yield dual-color stable emission,⁹ while giant quantum dots enveloped in a plasmonic shell afford both medically relevant ‘theranostic&’ functionality and novel photophysics. Finally, I will discuss how automating the synthesis of complex heterostructured nanocrystals will make realizing practical applications more feasible.
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9. M. R. Buck, A. M. Dennis et al. In preparation2014.

Cadmium selenide (CdSe) aqueous quantum dots (AQDs) have been synthesized using an environmentally friendly, aqueous method at room temperature. While traditional quantum dots (QDs) synthesized by hot injection method using organic solvent generally have edge-state emission with narrow photoluminescent (PL) peaks, the AQDs tend to have trap-state emissions with broad peaks probably due to the excess Cd ions on the QD surface creating selenium vacancy trap sites. A Pb coating on QD surface was achieved in solution by forming CdSe/Pb core-shell QDs due to the lower solubility of Pb compared to Cd. The excess Cd ions on the CdSe QD surface prevent the direct contact of Pb with Se and thus no PbSe was formed. It was found that the UV absorption edge and PL peak of CdSe blue-shifted due to the Pb coating. Furthermore the Pb coating doubled the PL intensity and quantum yield of the CdSe AQDs likely due to the creation of more mid-gap defect states as recombination centers. Our study provides a unique way to increase the PL properties of trap-state emission.

Semiconductor quantum dots (QDs) are intriguing light-harvesters and excited-state charge donors for solar energy conversion, due to their size-dependent optical properties and band-edge potentials, high oscillator strengths, and the possibilities of hot-carrier extraction and multiexciton generation. However, excited-state relaxation mechanisms of QDs are complex and multiexponential, which in turn complicates the mechanisms of photoinduced charge-transfer processes at QD-containing interfaces. In this regard, excited-state charge-transfer process of QDs differ fundamentally from than those of molecular chromophores.
This presentation will highlight our recent time-resolved spectroscopic experiments to elucidate the mechanisms and time scales of excited-state electron transfer at QD-molecule-TiO₂ interfaces, with particular emphasis on the relative rate constants and quantum yields of electron transfer from band-edge vs. trap states of CdSe QDs. Such mechanistic questions are of fundamental interest. Moreover, extraction of charge carriers from band-edge states of QDs, prior to their relaxation into trap states, is necessary to minimize deleterious losses of potential energy associated with charge trapping. Our results from time-resolved emission spectroscopy¹ and time-correlated single photon counting reveal that electrons are transferred from both band-edge and trap states of CdSe QDs to TiO₂ on relatively fast (<10^-8 s) and slow (> 10^-8 s) time scales. Additionally, the relative quantum yield of electron transfer from trap states decreases as the trap-state distribution is shifted to lower energies, suggesting a driving-force dependence of the rate constant for electron transfer. These results highlight the key role of trapped carriers in interfacial charge-transfer processes of QDs and the influence of the energies and densities of trap states on the efficiencies of such processes.
Ongoing time-correlated single-photon-counting experiments, in which charge-transfer-induced dynamic quenching of band-edge and trap-state emission from QDs are measured simultaneously, are providing enhanced insight into mechanisms and time scales of photoinduced electron transfer at QD-molecule-TiO₂ interfaces as a function of interfacial properties. Results from these experiments will be presented.
(1) Sellers, D.G.; Watson, D.F. "Probing the Energetic Distribution of Injected Electrons at Quantum Dot-Linker-TiO₂ Interfaces." J. Phys. Chem. C2012,116, 19215-19224.

Quantum dots (QDs) hold tremendous potential for low-cost processing and fabrication of optoelectronic devices. In addition to their size- tunable electronic structure and high photo-stability, their ability to support multiple optical excitations is being explored as a strategy for developing more efficient solar cells and brighter LEDs. Metal nanoparticles are able to collect and focus light over very small length scales by forming surface plasmon oscillations of electron density. When assembled with resonant QDs they have been shown to enhance photoemission and absorption rates.
Using a newly developed multi-pulse time-resolved fluorescence technique we are able to separate the recombination dynamics of multiply excited QDs from the dynamics of singly excited QDs in an ensemble solution. This allows us to selectively determine the effect of plasmonic structures on the emission and absorption rates of multi-excitons and other multiply excited species. In this talk we will demonstrate the technique and show how multi-excited states can be generated in reasonably high yields in CdSe QDs in the presence of nanostructured gold, but not at all when no gold is present.
Furthermore, to study the factors that control this interaction we have developed two systems that enable the distance between QDs and gold nanoparticles to be controlled. We will illustrate these systems and discuss their implications for future light harvesting technologies.

Inherent structural heterogeneity of colloidally synthesized quantum dots (QDs) has a marked effect on the ensemble optical properties. This structural heterogeneity results in QD-to-QD variance in photoluminescence (PL) transience, radiative decay rate, biexciton quantum yield, and PL spectra. Additionally, it is known that a “dark” fraction of QDs - made up of structures that do not exhibit radiative decay - exists within the overall QD population. With this in mind, we have developed a procedure to systematically correlate the fine atomic structure and photoluminescence properties of individual QDs. Furthermore, we have been able to perform scanning transmission electron microscopy with energy dispersive spectroscopy mapping (STEM-EDS) on the same individual QDs for which we have previously acquired PL data. This achievement allows us to link specific QD PL behavior with variances in the elemental composition of nanostructures, e.g. with the precise location and orientation of the core within the core/shell heterostructure, on a single QD basis. We have used our method to study the ramifications of surface and compositional heterogeneity in a large population of QDs; correlation of distinctive structural attributes with single QD transient photoluminescence allows us to pinpoint nonradiative defects that can then be targeted and eliminated via informed synthesis. Most recently, we have begun examination of giant thick-shell nanocrystal quantum dots (g-nQDs) with the specific aim of understanding the nature and influence of a fraction of permanently nonradiative, or “dark,” g-nQDs. Our characterization is being used in conjunction with the Hollingsworth laboratory to inform development of new synthetic procedures for elimination of this dark fraction, with the intent of increasing the observed ensemble photoluminescence quantum yield of g-nQDs to a value near-unity.

Nanoparticle assemblies that can change their optoelectronic properties in response to an external stimulus could have applications ranging from computing and energy harvesting to biodiagnostics. To this end, we study the optical properties of assemblies of DNA-linked plasmon resonant metal nanoparticles modified with azobenzene photoswitches. The azobenzene modification allows the hybridization state of the DNA to be controlled with UV irradiation. We use this ability to reversibly control the assembly and interparticle distances in DNA-linked particle structures. We study the optical properties of these structures and the optical properties of the photoswitches in the DNA/nanoparticle environment, and explore examples of how these materials can be used for sensing and diagnostic applications.

One- and zero-dimensional organic/inorganic heterostructure materials have been attracting considerable attention in materials science because of their outstanding optical and electrical properties and high tailorability in terms of composition, structure, and morphology. We discuss the synthetic methods available for preparing heterostructures incorporating diverse components; the functionality of the heterostructure materials; and their potential applications in the fields of electronics, optics, biology, and catalysis. The future development of such heterostructure materials will require deeper understanding of organic-inorganic or organic-organic interfaces on the nanoscale, collective phenomena, and interparticle coupling.
References
(1) Huibiao Liu, Jialiang Xu, Yongjun Li, Yuliang Li, Aggregate Nanostructures of Organic Molecular Materials, Acc. Chem. Res.2010, 43, 1496.
(2) Haiyan Zheng, Yongjun Li, Huibiao Liu, Xiaodong Yin, Yuliang Li, Construction of heterostructure materials toward functionality, Chem. Soc. Rev. 2011, 40, 4506.

Nanostructuring organic polymers and organic/inorganic hybrid materials and control of blend morphologies at the molecular level have become the prerequisites for modern electronic devices. To achieve all-around high performance, multiple organic and inorganic entities, each designed for specific functions, are commonly incorporated into a single device. Current state-of-the-art approaches to morphology control in these multi-component systems typically involve physical blending and optimization via thermal/solvent annealing. Such trial-and-error approaches are however highly system dependent, lack controllability on the molecular level and generally lead to morphologies at only thermodynamically meta-stable states.
We present herein a modular bottom-up approach to construct device favorable nanostructures consisting of multiple components with precise spatial placement and designated functionalities. Our method is built upon supramolecular chemistry through cooperation of several orthogonal non-covalent interactions including block copolymer self-assembly, conjugated polymer crystallization, fullerene aggregation and specific non-covalent interactions. Specifically, we have assembled poly(3-hexylthiophene), a low bandgap Pt-containing polymer, CdSe quantum dots and fullerene derivatives into core-shell type composite nanofibers. Such nanostructures provide broad light absorption and cascade energy level alignment, as well as control of thin film morphologies at both the macroscopic and microscopic levels.
References:
(1) “Nano-Structuring Polymer/Fullerene Composites through the Interplay of Conjugated Polymer Crystallization, Block Copolymer Self-Assembly and Complementary Hydrogen Bonding Interactions.” Li, F.; Yager, K. G.; Dawson, N. M.; Jiang, Y.-B.; Malloy, K. J.; Qin, Y.* Polym. Chem. 2015, Advance Article, DOI: 10.1039/C4PY00934G.
(2) “Stable and Controllable Polymer/Fullerene Composite Nanofibers through Cooperative Noncovalent Interactions for Organic Photovoltaics.” Li, F.; Yager, K. G.; Dawson, N. M.; Jiang, Y.-B.; Malloy, K. J.; Qin, Y.* Chem. Mater. 2014, 26, 3747-3756.

Colloidal semiconducting quantum dots (QDs) are solution-processable, size-tunable and efficient emitters. For optoelectronic applications, such as solar cells, light-emitting diodes (LEDs), and QD lasers it is crucial to understand and control their charge transport, recombination and emission dynamics, which are influenced by trap-states. These trap states can be filled and thus deactivated when operating devices at very high carrier densities. While carrier densities in light-emitting diodes are generally low, very high charge densities (up to 1 charge per QD) can be achieved in light-emitting field-effect transistor (LEFET). Here, we demonstrate the first quantum dot LEFET. This is accomplished by using electrolyte-gated PbS QD thin films. They exhibit near-infrared electroluminescence from a confined region within the channel with identical size-dependent electroluminescence and photoluminescence spectra, which proves true ambipolar transport through the S_h and S_e states of the PbS QDs. In contrast to QD LEDs, external quantum efficiencies in LEFETs continuously increase with current density. This effect correlates with the unexpected increase in photoluminescence quantum yield and longer average lifetimes at higher electron and hole concentrations. We attribute the initially low emission efficiencies to non-radiative losses via electron and hole traps. At higher carrier densities trap states are deactivated and emission starts to be dominated by trions with fast radiative decay rates.

Mesoporous nanocrystal based thin films synthesized by block copolymer templating of ligand stripped nanocrystals and followed by thermal treatment to remove the polymer template is a robust method used to make high surface area structures from a variety of nanocrystal building blocks. However, the use of thermal processing to remove the polymer template can have detrimental effects on other material properties such as grain size and crystal structure, which affect key size dependent properties such as the band gap. Here we present a new method to form mesoporous films of semiconducting nanocrystals that avoids thermal processing. In our method, nanocrystals are first assembled with a diblock-copolymer template to form an organic-inorganic composite, and the resulting films are soaked in a solution of small molecule cross linking agents to lock the nanocrystals into a robust structure. The polymer template is gently dissolved out of the film leaving behind a porous film of nanocrystals. These films show disordered but homogeneous porosity as observed by scanning electron microscopy and ellipsometric porosimetry, and maintain the initial nanocrystal size, as determined by X-ray diffraction and ultraviolet-visible absorption spectroscopy. The combination of tunable porosity, high surface area and precisely controlled optical properties make these materials interesting for a variety of optoelectronic applications such as photovoltaics and photocatalysis.

Template-directed synthetic method has some obvious advantages, e.g., easy fabrication, low cost, high throughput, and various compositions of materials. On the other hand, although the properties of nanomaterials are frequently superior to those of their bulk counterparts, translating the unique characteristics of individual nanoscale components into macroscopic functional devices still remains a challenge.
We independently developed a facile hydrothermal method for synthesizing ultrathin Te nanowires (TeNWs) by using conventional chemicals. Through systematical study in past several years, we have demonstrated that the ultrathin TeNWs can be employed as a versatile templating material for fabricating a series of high-quality 1D nanostructures by taking the unique advantages of TeNWs, i.e., large-scale synthesis, high processability and high reactivity. Importantly, the obtained 1D products inherit the dimensional (high aspect ratio) and mechanical (high flexibility) features of original TeNWs templates, and thus allow it possible for constructing macroscopic architectures by using them as nanoscale building blocks. A family of 1D nanostructures that covers a wide range of materials, including noble metals, metal oxides, semiconductors, carbon, polymers, their binary and multiple hybrids can be prepared through this multiplex templating hydrothermal process starting from TeNWs. A series of macroscopic assemblies of 1D nanostructures based on these well-defined nanowires/nanocables, including free-standing membranes, films, hydrogels, and aerogels, can be fabricated, showing enormous application potential in diverse fields, such as liquid filtration and separation, catalysis, electrocatalysis, electronic devices, super adsorbent, elastomeric conductors, and polymer-based nanocomposites.

Top-down manufacturing of functionalized nanoparticles offer several distinct advantages over bottom-up approaches, including improved scalability, elimination or combination of synthetic steps, and adaptability to a wide range of core materials and surface functional groups. Two of the most common top-down nanoparticle manufacturing techniques are mechanochemistry (reactive high energy ball milling) and sonochemistry (reactive cavitation erosion). The former is a relatively well-established technique, the latter an emerging technique. Both have been used to form sub-10 nm silicon core nanoparticles with covalently-bound, short-chain hydrocarbon surface functional groups. The current state of knowledge relative to particle size distribution, impurity levels, surface functionalization, and optoelectronic properties for each technique is describe. The relative benefits and drawbacks of each techniques are enumerated, leading to identification of the key research topics that are currently the focus of work in each area.

The intercalation of fluorescent molecule HPTS (8-hydroxy-pyrene-1,3,6-trisulphonate) and surfactant OS (1-octane sulfonic acid sodium) into LGdH:Eu produces a kind of organic-inorganic photofunctional composite material exhibiting versatile luminescence properties. The fluorescent acid molecule and anionic surfactant are intercalated into the Eu³⁺-doped layered gadolinium hydroxide (LGdH:Eu) result in. The XRD pattern shows that the composite has a d_basal value of 2.01 nm, with series (00l) reflections at 2.01, 1.00, 0.67, 0.50, and 0.40 nm. When dispersed in formamide (FM), the composite was found to be facilely exfoliated and the as-formed colloidal suspension reveals strong blue emission (448 nm), being different from the dual fluorescence (453 nm for blue emission and 520 nm for green emission) of free HTPS in FM. In water/FM system, the composite possibly exists as a swollen state which prefers to green luminescence (517 nm). In delaminated state, a synergistic effect (electrostatic interactions between positively-charged LGdH:Eu layers and anionic HPTS, hydrogen bonding between HPTS and FM, and energy transfer of Eu³⁺ and HPTS) may contribute to the blue luminenscence. The facile delamination of LRHs composites with fluorescent molecules provides a bene#64257;cial pathway for assembly of promising blue luminous film materials.

Graphene quantum dots (GQDs) are emerging fluorescent carbon nanoparticles. Due to the quantum confinement effect of sp²-carbon domain and the edge effect, GQDs are strong light absorbers and emit bright fluorescence. GQDs were prepared by chemically oxidizing carbon nano-onions. The as-produced GQDs (ox-GQDs) have oxygen-related chemical functionalities at the edge sites. The ox-GQDs have the lateral dimension of 3-5 nm and the thickness of a single or few graphene layers. Reduced GQDs (re-GQDs) were prepared by treating ox-GQDs with NaBH₄. While ox-GQDs exhibited green emission, re-GQDs showed blue emission. The green emission of ox-GQDs was attributed to the emission from lower energy, emissive trap sites associated with oxygen functionalities. The fluorescence images of isolated GQDs excited at different excitation energies revealed the significant inhomogeneity of lower energy trap sites from particle to particle. Single particle fluorescence spectroscopic studies were conducted for ox-GQDs and re-GQDs. Both ox-GQD and re-GQD particles exhibited discrete fluorescence intensity fluctuations jumping up and down between different intensity levels with the durations of hundreds of milliseconds to seconds. Interestingly, a higher percentage of re-GQD particles showed multi-level fluorescence fluctuations while ox-GQD particles dominantly showed single-step fluorescence fluctuations. The multi-level fluctuation behaviors imply the presence of multi-domains in GQDs where each chromophoric domain randomly absorbs excitation light. The different fluctuation behavior between ox-GQDs and re-GQDs reflects their different emission pathway.

Stable aqueous emulsions of conjugated polymer nanoparticles (CPN) have been synthesized by cross-coupling reactions of fluorene monomers with a wide range of co-monomers. Highly fluorescent dispersions were obtained and the nanoparticle shape (spheres or rods), emission color (blue, green and red) and size (10-300 nm) can be tailored by the choice of synthetic route and co-monomer.¹ The charge and chemical functionality on the surface of the nanoparticle can be controlled by the nature of the side groups attached to the polymer backbone² and the nanoparticles can be cross-linked through the main chain or the side chains.³ The physical and optical properties of the particles will be discussed and the properties compared to those of molecular dyes and quantum dots. Application of the nanoparticles in cellular imaging will be explored.
1. D. Muenmart, A. B. Foster, A. Harvey, M.-T. Chen, O. Navarro, V. Promarak, M. C. McCairn, J. M. Behrendt, and M. L. Turner, Macromolecules, 2014, 47, 6531-6539.
2. J. M. Behrendt, Y. Wang, H. Willcock, L. Wall, M. C. McCairn, R. K. O'Reilly, and M. L. Turner, Polym. Chem., 2013, 4, 1333.
3. J. M. Behrendt, A. B. Foster, M. C. McCairn, H. Willcock, R. K. O'Reilly, and M. L. Turner, J. Mater. Chem. C, 2013, 1, 3297-3304.

Charge transfer (CT) complexes are of growing interest because of their unique solid-state physical properties. In particular, with well-defined architecture, charge transfer complexes are showing prominent merits over their bulk counterparts for applications in electrical and optical memory devices, sensors, and magnetic devices1. In the present studies, we have described the design and fabrication of low-dimensional CT complexes nanoscale aggregates by novel techniques, such as organic vapor solid phase reaction, organic liquid solid phase reaction and self-assembly. Interestingly, we have demonstrated the ability to tune the morphologies, size and dimensions of CT complexes nanomaterials by controlling experimental conditions. The excellent electrical properties were observed in the as-grown CT complexes nanoscale aggregates. The results demonstrate that CT complex is a kind of great potential field emission cathode materials. The dimension- and size-dependent effect of field emission properties of CT complexes nanostructures are investigated and controlled.
References:
(1) Liu, H. B.; Zhao, Q.; Li, Y. L.; Liu, Y.; Lu, F. S.; Zhuang, J. P.; Wang, S.; Jiang, L.; Zhu, D. B.; Yu, D. P., Chi, L. F. J. Am. Chem. Soc. 2005, 127, 1120
(2) Wang, K.; Qian, X. M. Zhang, L.; Li, Y. J.; Liu, H. B. ACS Appl. Mater. Interfaces 2013, 5, 5825

Quantum confinement is a foundational nanoscience concept that allows tuning electronic properties of quantum dots. Core-shell quantum dots are promising nanoparticles and found applications as light-emitting optoelectronic devices and biomarkers [1] due to their robustness and tunability of both core and shell. The fluorescent quantum yield of these quantum dots can achieve 100% [2] even at room temperature [3]. However, to understand many phenomena of carrier dynamics, photoluminescence efficient and Auger effects, fine electronic structures of the exciton are needed. Here, using large scale electronic structure calculations based on charge patching method, we have investigated the exciton binding energy, band alignment between core and shell, charge separation between electron and hole. We will discuss how these can be tuned by changing the core/shell dimensions.
1. Shirasaki Y., Supran G., Bawendi M., Bulovicacute; V. Nature Photonics 7, 13 (2013).
2. Javaux C. et al, Nature Nanotech. 8, 206 (2013)
3. Achieved in Alivisatos lab (2014).

Graphene quantum dots (GQDs) are an emerging new material. They are composed of small graphene sheets (3-20 nm wide and 1-2 nm thick) where their edges are covalently linked to chemical functionalities. Due to quantum confinement effect and edge effect, GQDs strongly absorb light and are highly fluorescent. Applications of GQDs include bioimaging, biosensing, metal-ion sensing, light emitting diodes, photovoltaic devices and supercapacitors. Various methods have been reported for the synthesis of GQDs, but can roughly be classified into two categories: top-down method and bottom up method. In a top-down approach, bulk graphitic materials are cut into small pieces by chemical or physical treatments, whereas a bottom up method is associated with the carbonization process of small molecules such as glucose or citric acids into GQDs. Despite of the various methods to synthesize GQDs, GQD fluorescence was usually limited to visible regions. Most of the reported GQDs emit either blue (450 nm) or green (530 nm) fluorescence. Ultraviolet emission of GQDs was very rarely reported and was only limited to GQDs synthesized by bottom-up method. Here we present a top-down synthesis of GQDs, using nanodiamond derived carbon nano-onions (N-CNOs) as a precursor. We successfully synthesized UV- and blue- emitting GQDs. The two GQDs were separated and purified by a simple dialysis. Transmission electron microscopic (TEM), atomic force microscopic (AFM), and X-ray photoelectron spectroscopic (XPS) characterizations were used to probe the morphology and element composition of the two GQDs. Metal-ion sensing capabilities of the two GQDs were also compared. Fluorescence intensity of blue emitting GQDs was much more sensitive to the presence of metal ions than the UV emitting GQDs. Our work has implications in two aspects. Firstly, this is the only report of synthesizing UV emitting GQDs by a top down method. The synthesis of UV emitting GQDs could be important for photovoltaic applications because they can utilize UV region of sunlight. Secondly, this is the first report to relate metal sensing capabilities of GQDs to the energetics (i.e., emission color) and surface functionality of GQDs. Our results may bring insights to the fluorescence quenching mechanism of GQDs by metal ions. Understanding of the quenching mechanism will help design metal ion sensors with high selectivity and sensitivity.

Solar thermal fuels make use of molecules that undergo reversible photo-isomerization to store solar energy and convert it into thermal energy [Kanai, Y., et al. Angew. Chem. Int. Ed.49, 8926 (2010), Kucharski, T. J., et al.Energy Environ. Sci.4, 4449 (2011)]. Because solar thermal fuels produce no emissions and can store and convert energy within one material, they are an attractive option for a renewable alternative energy source. However, finding a proper material that exhibits high energy density, high energy conversion efficiency and long energy storage lifetime, and can be produced at low cost, remains a challenge . A recent proposal is a nanotemplate-photoisomer hybrid system, e.g. functionalized azobenzene, a well-known photoisomer molecule, attached onto carbon nanostructure templates such as carbon nanotubes, graphene, pentacene or alkene chains. Such structures have been suggested and tested as candidate solar thermal fuel materials with high energy density and long storage time [Kolpak, A. M., et al. Nano Lett.11, 3156-3162 (2011)., Kolpak, A. M., et al.J. Chem. Phys.138, 034303 (2013)., Kucharski, T. J., et al.Nat. Chem.6, 441-447 (2014)]. In this study, we further investigated how to achieve high energy conversion efficiency with nanotemplate-azobenzene hybrid systems. In particular, we aim to maximize the overlap of the absorption spectrum of the trans-isomer (stable state) with the solar spectrum while simultaneously minimizing that of cis-isomer (metastable state) with the solar spectrum. The more the overlap of solar spectrum and absorption spectrum of trans-isomer exceeds that of cis-isomer, the higher the molar fraction of cis-isomer at the photostationary state where forward and backward photoisomerization reaction are in steady state. The molar fraction of cis-isomer at the photostationary state determines maximum energy harvesting capacity of solar thermal fuels and hence needs to be maximized. Using time-dependent density functional theory (TDDFT), we calculated absorption spectra of different systems: one azobenzene molecule attached to different sites of nanotemplates, and azobenzene molecules in different relative positions. Through these calculations, we analyzed the effect of template-photoisomer interactions and that of photoisomer-photoisomer interaction on absorption properties of the system and from that knowledge, we tried to design the best nanotemplate-photoisomer hybrid structure for solar thermal fuels, which can achieve highest cis-isomer molar fraction at the photostationary state.

Surface enhanced Raman spectroscopy (SERS) relies on substrates with nanometer-scale curvature in order to concentrate and amplify the incident electromagnetic field to increase the spectroscopic signature of Raman scattering. Raman spectroscopy is widely used in sensing applications such as detecting pharmaceutical ingredients, explosives, narcotics, because of its high degree of selectivity and specificity. Large enhancements in Raman scattering signals can be achieved by creating “hot spots” between plasmonic nanostructures, which localize and amplify the incident illumination. Here, we report a new, scalable method for fabricating high-performance SERS substrates based on self-assembly of nanostructured block copolymer thin films. Due to the high spatial density and extremely high field strengths of substrate hot spots, these substrate are capable of enhancing Raman scattering signals from target molecules by more than 10 billion times. We will describe the process of fabricating these remarkable diagnostic tools, which are ~cm² area substrates composed of an extremely high density (~ 10¹¹ /cm²) of hexagonally-arranged Au or Ag nanoparticles positioned atop ~ 70nm tall silicon nanopillars. Key to the substrate performance is the sub-5 nm separation between particles, which we control with nm level precision. By systematically varying the gap between nanoparticles, we demonstrate that both the high hotspot density and sub 5nm hot spot gap are necessary to achieve the highest degree of enhancement of the Raman signal. The enormous enhancements provided by these substrates make possible the detection of single molecules

Hybrid Si/nanocrystal structures have attracted considerable attention as candidates for practical photovoltaic devices. Previous work has shown very efficient excitonic non-radiative (NRET) and radiative (RET) energy transfer from single nanocrystal monolayers to various Si structures. The next step towards practical utilization of such structures is the development of densely packed nanocrystal multilayer assemblies able to efficiently absorb incoming sun flux. Conventional deposition techniques (i.e. drop casting and spin coating) do not adequately allow fabrication of multi-functional assemblies such as size-gradient multilayer structures for directed energy flow and, often, lead to a reduced quantum yield (QY) due to the unintentional formation of trap states known to scavenge charge carriers.
Here, we discuss the preparation and characterization of densely packed nanocrystal quantum dot (NQD) multilayer structures on various Si substrates. Assemblies consisting of 1-10 monolayers (MLs) of CdSe NQDs were prepared on thick, thermally grown SiO#8322; and on oxide-free Si surfaces functionalized with self-assembled monolayers that prevent oxidation and formation of surface states. By slowly evaporating a NQD dispersion in toluene, a self-assembled NQD monolayer was formed at an air/liquid interface and then deposited on the underlying substrate. For multilayer samples, this process was repeated until the desired number of layers was deposited.
We confirm the presence of densely packed NQD layers using various characterization methods (SEM, AFM, ellipsometry). Using micro photoluminescence (PL) and UV-Vis measurements, we observe a linear dependence of the PL emission and the absorbance on the number of NQD MLs deposited, which is indicative of a steady quantum yield. Using time-resolved PL measurements, we observe a gradual increase in the measured average NQD PL lifetime on Si surfaces with an increase in the number of NQD MLs. Such behavior indicates a slowly reducing efficiency of energy transfer vs. the distance to the substrate as confirmed by modeling. We directly measured QYs of the multilayer structures and confirmed that it did not change appreciably from its value for a single NQD monolayer. This confirms our assumption that such a multilayer deposition method does not introduce any additional interface trapping centers and can be utilized to create densely packed NQD solids. Size-gradient NQD solids could be applicable for the implementation of the energy harvesting schemes based on controlled energy flow.

Cadmium Selenide (CdSe), a well-known direct band gap semiconductor which absorbs a wide range of the visible and UV spectrum, is a promising material for applications in light-emitting diodes (LEDS), photodetectors, solar cells, biomedical imaging and field-effect transistors (FETs). Two dimensional (2D) geometry of CdSe such as nanoplatelates (NPLs) and nanosheets (NSs) has drawn extensively attention due to their unique properties associated with its morphology. Ultrathin CdSe NSs and NPLs have been successfully synthesized using solution-based route; however, there are difficult to produce a large lateral size in the range of micrometer scale.
We present the use of cation exchange to synthesize micrometer-sized single-crystal thin CdSe (NSs) difficulty to produce directly through solution-based route. Starting from hexagonal-phased CuSe NSs as a template, CdSe NSs are obtained by cation exchange of copper to cadmium. This exchange reaction preserves well the 2D morphology of the starting NSs and also retains their hexagonal crystal structure. Resulting CdSe NSs have a lateral size up to 6 µm and an average of thickness approximately 5 nm. Such a large lateral dimensional size of these NSs opens opportunities for single sheet optical property studies and also ease enables fabrication of optical and electronic devices.

The field of optogenetics relies on visible light to excite channelrhodopsins and invoke action potentials, a powerful method of modulating brain activity. However visible light undergoes a substantial propagation loss as it travels through brain tissue, therefore, deep tissue stimulation can only be performed using invasive fiber-optic light delivery. By optimizing the two and three-photon upconversion of infrared photons to blue in NaYF₄ nanoparticles doped with Tm and Yb, we have opened the door to non-invasive, highly localized optogenetics studies deep within tissue. Utilizing the low attenuation of near-IR light, our nanoparticles allow for deep optical penetration into the brain, which then excite opsin-expressing neurons locally without being impeded by blue light&’s high tissue absorption. This technology opens many new possibilities for optogenetic experiments in live, non-hindered animals.

Carbon nandots (C-dots), a class of paracrystalline carbon nanostructures, have been the subject of extensive research because of their excellent biocompatibility, innocuousness, and photostability. They are known to have a structure that some sp² carbon clusters are embedded in a heterogeneous sp³ carbon matrix, presumably due to relatively “cold” reaction temperature, definitely insufficient to give a pure sp² carbon structure or carbon crystal. There have been a number of synthetic methods to prepare C-dots; for instance, arc discharge, laser ablation, electrochemical oxidation, solvothermal carbonization, wet oxidation, microwave synthesis, and so on. These methods have offered highly fluorescent C-dots, but their coloration, defined as the diversity in colors of photoluminescent light, is limited. Firstly, the color of photoluminescent light is generally confined in a blue wavelength region. Second, the spectral width of photoluminescent light is very broad, or the “purity” of the color is rather inferior to other fluorescent molecules or nanoparticles. Herein, we demonstrate spectrally narrow multicolor (yellow to red) photoluminescence of C-dots through the surface functionalization with para-substituted anilines. We study that surface functional groups play a very major role in the photoluminescence of C-dots using transient absorption spectroscopy. These C-dots are then bioconjugated with proteins for target specific in vivo imaging, where they are found to have many advantages such as excellent brightness, photoluminescent stability, and almost zero toxicity.

The present work describes a study of PbSe and PbSe/CdSe colloidal quantum dots (CQDs), the latter being produced by cation exchange of Pb²⁺ for Cd²⁺ ions. The cation exchange starts on preferred crystallographic facets and results in either non-concentric CdSe shells or Pb_xCd_1-xSe alloyed-shell layers. The obtained heterostuctures are referred to as PbSe/CdSe "thin shell" CQDs. The CQDs were examined by applying structural and optical characterization methods. The structural analysis indicates formation of alloyed shell. The time-resolved photoluminescence and optical spectra recorded at various temperatures show extention of the band-edge exciton lifetime about a factor of two relative to the parent PbSe CQDs. These results are consistent with effective mass based calculations, which unveil quasi type II charactersistics of the mentioned CQDs. Moreover, by utilizing effective mass-based calculations, we show that non-monothonic shift of band-edge energy gap is a result of non-concentric CdSe shell.

Nanomedicine is fostering significant advances in the development of platforms for early detection and treatment of diseases. Nanoparticles (NPs) like quantum dots (QDs) exhibit size-dependent optical properties for light-driven technologies, which might become important in Bio-imaging, Sensing and Photo-dynamic Therapy (PDT) applications. Although the synthesis of quantum dots using organometallic resources in organic solvents has been reported in the literature, advances on the direct synthesis of water-stable nanocrystals, as required for biomedical applications, are scarce. The present research addresses the synthesis of water-stable Cd-based QDs via a Microwave-Assisted synthesis approach using cadmium sulfate salt, and thioglycolic acid as Cd- and S- precursors, respectively. Selenide ions were available by reductive leaching of metallic Selenium in Sodium bisulfite solution. The size control and the tunability of the optical properties were achieved by a suitable control of the reaction temperature (in the 140°C- 190°C range) and reaction time (10 minutes - 40 minutes). X-ray diffraction analyses suggested the development of a CdSe/S face cubic centered structure; the broadening of the diffraction peaks indicated the presence of very small nanocrystals in the samples. The average crystallite size was estimated at 5.50 nm ± 1.17 and 3.72 nm ± 0.04 nm, for nanoparticles synthesized at 180°C after 40 minutes or 10 minutes of reaction, respectively. HRTEM images confirmed the crystalline nature and the small size of the synthesized nanocrystals. In turn, the exciton was red-shifted from 461 to 549 nm when the reaction temperature was prolonged from 140°C to 190 °C, suggesting the crystal growth. The corresponding band gap values were approximately 2.2 eV, confirming the quantum confinement effect (bulk value 1.74eV). This red shift was also evidenced in PL measurements where the main emission peak was shifted from 507 nm to 564 nm when the samples were excited at 420 nm. A narrow size-tunable emission also was supported by the full width at half maximum (~ 45 nm) for the synthesized nanocrystals. The singlet oxygen generation capability of as-synthesized QDs was also investigated. The correlation between the particle size and the generation of singlet oxygen will be presented and discussed.

Photoluminescent carbon dots (CDs) have attracted much intention due to their potentials of cellular imaging and therapeutic monitoring. The CDs with broad photoluminescence (PL) spectra act as good multicolor cell imaging probes as well as load highly aromatic anti-cancer drug such as doxorubicin (Dox) via strong π-staking interactions, which makes it a promising material as a drug carrier for treatment of disease. For efficient delivery of Dox into nucleus, we develop zwitterionic CDs passivated by b-alanine. Efficacy of drug delivery is enhanced by cellular internalization of carrier. The zwitterionic CDs enter negatively charged cancer cell membrane as well as nucleus, as demonstrated by time-dependent internalization. Additionally, Dox loaded on the CD has superior antitumor efficacy compared to free Dox in vitro and in vivo, due to enhanced nuclear delivery. Therefore, the CDs with good biocompatibility, low cytotoxicity, excellent solubility, and stable photoluminescence are promising multifunctional platform that are capable of expediting and sensing the delivery of drugs in a simple and efficient manner.

Nanorods are of great interest for both fundamental reserch and technical applications due to several unique electrical and optical properties, such as linearly polarized emission, exciton storage, electroluminescence, and efficient 1D electrical transport. In this paper, we present the conductive and photo-electrical properties of CdS nanorods prepared by the seeded growth approach. The nanorods were assembled into boe-tie electrode with 100nm gap size by AC dielectrophoresis. We can control the quantity of trapped nanorods between the electrodes by regulate the frequency, trapping time. Scanning electron microscopy inspection confirmed the trapping of nanorods in our devices with a success rate of 90 %. Electrical characterization and photo-induced conductivity showed stable and reproducible conductive behavior for the nanorods. This devices showed high device performance indicating the prospective application of CdS nanorods for optoelectronic devices, electro-modulators, and photosensors.

Lanthanide-doped upconversion nanoparticles have generated a large amount of interests in the areas of information technology, biotechnology, energy, and photonics. Despite the attractions, further progress in using upconversion processes has been largely hindered because upconversion nanoparticles are typically sensitized by Yb³⁺ ions that only respond to narrowband NIR excitation centered at 980 nm. The absorption of 980 nm light by the water component in biological samples usually limits deep tissue imaging and induces potential thermal damages to cells and tissues. Herein, we describe a general solution to these problems by constructing a new class of materials that display tunable upconversion emissions spanning from UV to the visible spectral region by NIR excitation in the medical spectral window (700-900 nm). We employed a core-shell-shell nanoparticle platform to integrate and coordinate incompatible optical processes for rational photon energy upconversion. Appropriate lanthanide ions are confined in separate layers of the nanoparticle for performing independent light harvesting, photon energy upconverting, and optical tuning, respectively. Interlayer energy exchange inter-actions are mediated by arrays of lanthanide migrator ions that eventually lead to well-defined energy cascades and eventually unusual photon upconversion processes. Tunable upconversion emissions spanning from UV to visible spectral region have been realized by single wavelength excitation at 808 nm through use of core-shell nanostructure in combination with energy migration. In addition to providing a platform for enhancing our understanding and control of lanthanide luminescence, these studies also pave the way for constructing novel upconverison nanoparticles that outperform existing biomarkers for therapeutic and diagnostic imaging applications by enabling a better way of excitation.

Photoluminescent graphene quantum dots (GQDs) have received enormous attention not only because of their unique chemical, electronic and optical properties but also their wide range of applications in bioimaging,[1] photocatalysts,[2] and ion detection[3]. Here, nitrogen doped GQDs (N-GQDs) and sulphur/nitrogen co-doped GQDs (S, N-GQDs) can be produced inexpensively by one-step hydrothermal reaction. The highest PL QY (94%) of N-GQDs was obtained using citric acid (CA) as a carbon source and ethylene diamine (EDA) as a N source. The obtained N- GQDs exhibit an excitation-independent blue emission with single exponential lifetime decay.[4] Then blue, green and yellow emission N-GQDs was easily obtained from water, DMF and solvent free, respectively. The PL spectra confirm that the N-GQDs clearly show excitation-independent emission, and single exponential life-time decay, indicating a single emission center for each sample. That gives a great opportunity for bio-imaging applications to avoid the autofluorescence and increase penetration depth of excitation light. After doping with S and N, the absorption band of the as-prepared S, N-GQDs can extend into the visible region (~ 650 nm), which makes the S, N-GQD/TiO₂ composites are prepared and show remarkable photocatalytic activity by degradation of rhodamine B compared with pure TiO₂, implying their considerable potential for application in environmental protection and energy conversion.[2] Interestingly, in the next work we found a facile solvothermal route to synthesize S, N co-doped graphene quantum dots (S, N-GQDs) with unique optical properties. It was observed that three absorption bands at 338, 467, 557 nm, and their photoluminescent spectra displayed excitation-wavelengths-independent three primary colors emissions. The obtained S, N-GQDs provides an attractive means of effectively making them used both in visible light photocatalytic again bioimaging.
References
[1] Zheng, M.; Liu, S.; Li, J.; Qu, D.; Zhao, H.; Guan, X.; Hu, X.; Xie, Z.; Jing, X.; Sun, Z. Integrating oxaliplatin with highly luminescent carbon dots: An unprecedented theranostic agent for personalized medicine. Advanced Materials 2014,26, 3554-3560.
[2] Qu, D.; Zheng, M.; Du, P.; Zhou, Y.; Zhang, L.; Li, D.; Tan, H.; Zhao, Z.; Xie, Z.; Sun, Z. Highly luminescent s, n co-doped graphene quantum dots with broad visible absorption bands for visible light photocatalysts. Nanoscale 2013,5, 12272-12277.
[3] Zheng, M.; Xie, Z.; Qu, D.; Li, D.; Du, P.; Jing, X.; Sun, Z. On-off-on fluorescent carbon dot nanosensor for recognition of chromium(vi) and ascorbic acid based on the inner filter effect. ACS Appl. Mater. Interfaces 2013,5, 13242-13247.
[4] Qu, D.; Zheng, M.; Zhang, L.; Zhao, H.; Xie, Z.; Jing, X.; Haddad, R. E.; Fan, H.; Sun, Z. Formation mechanism and optimization of highly luminescent n-doped graphene quantum dots. Sci. Rep. 2014,4, srep05294.

The conjecture of ZnSnP₂ semiconductor nanowires as promising inexpensive solar absorber and the nanowire preparation method was discussed in this study. Single-phase crystalline ZnSnP₂ nanowires have been prepared via simple chemical-vapor-deposition (CVD) of Zn and SnP₃ powder in a custom-built tube furnace reactor, allowing the vapors to react with thermally evaporated Sn islands on 40 mm² Si(111) substrates. Powder X-ray diffraction data and Raman scattering of the nanowire films, along with single-nanowire analysis using high-resolution transmission electron microscope (HRTEM) indicate that the as-prepared ZnSnP₂ nanowires possess single-crystalline sphalerite structure, as opposed to the antisite defect-free chalcopyrite structure. Nonetheless, the nanowires show homogenous elemental distribution of Zn, Sn, and P, with chemical composition close to the 1:1:2 stoichiometric ratio. Photoelectrochemical measurements in aqueous electrolyte show the as-prepared ZnSnP₂ nanowires exhibit stable cathodic photoresponse under white light illumination. Tapered nanowires that lead to formation of Zn₃P₂ nanowires dominate at extended growth duration, implying the Sn islands serve both as the vapor-liquid-solid (VLS) growth seed and sole contributor of Sn. Approaches to minimize consumption of Sn VLS seeds will be presented to ensure production of long nanowire with consistent diameter. This study presents a benign and straightforward approach to prepare single-phase Zn-IV-P₂ nanowires suitable for energy conversion applications.

More than a century ago, Ciamician first envisioned the use of sunlight as an abundant and renewable source of energy for performing chemical synthesis under mild and green conditions.¹ In the past several years, there has seen a resurging interest in the role of visible light driven photocatalysis in synthetic chemistry thanks to the development of transition metal chromophores.² Heterogeneous photocatalysts can be easily removed and recycled after use. They can also be readily incorporated into continuous flow systems, which offer facile automation and predictable scale-up. Π-conjugated porous polymers with hierarchical pore structures were synthesized by Suzuki-Miyaura and Sonogashira cross-coupling reactions via high internal phase emulsion (HIPE) polymerization. Combining the photoactive π-electron backbone and the interconnected pore structure, these porous polymers were employed as visible light driven heterogeneous photocatalysts for various photocatalytic redox reactions, such as the highly selective photo-oxidation of organic sulfides into the corresponding sulfoxides³ and photo-reductive dehalogenation of halo-ketones⁴ under visible light irradiation at room temperature. The pore size can be tuned by the incorporation and removal of a thermally cleavable tert-butyl carboxylate (Boc) functional group in the polymer network. After the removal of Boc group, the Brunauer-Emmett-Teller (BET) surface areas can be increased up to eight times while the porous skeleton remains intact. A continuous flow system, where the monolithic conjugated porous polymers were packed in a transparent glass column reactor, was employed for the photocatalytic investigations. It was shown that the product can be obtained continuously in a quantitative manner and the catalyst can be easily recycled by simple filtration and reused for further reactions. Moreover, it was also demonstrated that π-conjugated porous polymers can also initiate the free radical polymerization of methyl methacrylate (MMA) under sustainable and environmentally benign reaction conditions such as a household energy saving fluorescent light bulb.⁵ The influence of the HOMO/LUMO levels of the conjugated porous polymers on their photocatalytic efficiency was also studied.
Literature:
(1) Ciamician, G. Science1912, 36, 385-394.
(2) (a) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem Rev2013, 113, 5322-5363(b) Schultz, D. M.; Yoon, T. P. Science2014, 343, 1239176.
(3) Wang, Z. J.; Ghasimi, S.; Landfester, K.; Zhang, K. A. I. Chem Commun2014, 50, 8177-8180.
(4) Wang, Z. J; Ghasimi, S.; Landfester, K.; Zhang, K.A.I. J Mater Chem A2014.
(5) Wang, Z. J.; Landfester, K.; Zhang, K. A. I. Polym Chem2014, 5, 3559-3562.

Monodispersed 200 nm-sized ZnO sphere (ZnO SP) with porous structure emanating from 10 nm zinc oxide nanoparticles composing ZnO SP were synthesized by dissolving zinc acetate dihydrate in diethylene glycol at 160 ^oC. The prepared ZnO SP were employed for the fabrication of Au-loaded ZnO (Au-ZnO) composite materials, exhibiting high photocatalytic activity in decomposing salicylic acid under UV-visible light irradiation. It is deduced that its high catalytic activity of Au-ZnO SP originates from the photo-induced charge separation by transferring electrons from the conduction band (CB) of ZnO to Au, since the CB level of ZnO (-0.51 V vs. NHE, at pH 7) is located more negative side than that of Au (+0.6 V vs. NHE). The evidences for the charge separation were provided by monitoring the amount of ^.OH radical with bare ZnO SP and Au-ZnO SP produced in the solution which readily react with 1,4-terephthalic acid (TA) generating 2-hydroxy terephthalic acid (TAOH) that shows unique fluorescence peak at 426 nm.

Ultrasmall CdSe nanocrystals are a unique material that emit broad spectrum white light, due to multiple, surface-related emitting states. These surface states are sensitive, and changes in the nanocrystals surrounding environment can change the emission spectrum. This material has been studied for applications in lighting and spectroscopy, but it has not been investigated as a sensor. Thermal and mechanical responses to external impulses have previously been analyzed at microscopic levels by using high definition digital thermography and scanning laser vibrometry, however to completely understand the overall response to external impulses, the material must be examined spectroscopically. In this experiment we encapsulate white-light emitting CdSe nanocrystals in the composite material, and using an integrating sphere, record the changes in the emission spectrum that arise from stress applied to the composite. The material response to multiple loading conditions provides insight into the material&’s ability to monitor and detect damage due to external impulses and loads.

The nanostructured materials have been extensively studied, not only because of the new properties and their possible technological applications, but also by the search for a better understanding of the physical and chemical processes that cause these changes. Compared to semiconductor materials, studies of the structure at the nanometer scale has received considerable interest due to the quantum size effect that these materials present. nanocrystalline semiconductors have intermediate electronic properties between those of molecular structure and macrocrystalline solid, presenting a wide range of applications. The materials based on zinc oxide (ZnO) are of great interest for industry of optoelectronic devices such as solar cells, gas sensors, transparent conducting oxides, heat reflecting mirrors among other applications. The versatile morphological settings of ZnO have proven important to explore new phenomena and materials at the nanoscale. In this work, samples of ZnO were synthesized by chemical routes such as the polymeric precursor, co- precipitation and hydrothermal assisted by microwaves methods. Images obtained by scanning electron microscopy show that the ZnO samples exhibit different morphologies depending on the synthesis method, besides having particle size in the nanometer range. An emission predominantly in green for ZnO is observed, and verified dependence of luminescent properties based on morphology of the nanoparticles, attributed to chemical defects such as interstitial zinc, oxygen vacancies or zinc vacancies.

Bulk I-III-VI₂ semiconductors (e.g., CuInS₂, CuInSe₂) have been actively explored as photovoltaic materials and exhibit the highest efficiencies for thin-film solar cells. Quantum dots composed of these materials can allow for greater flexibility in fabrication via, e.g., ink-jet printing or spray deposition and also performance enhancements due to beneficial effects enabled by quantum confinement such as spectral tuning of a band gap for the realization of multijunction solar cells and carrier multiplication for boosting a photocurrent. Although critical for high performance optoelectronics, neither the polarity nor mobility of charge transport in films of CuInSe₂ or CuInS₂ quantum dots has previously been investigated. Here we report field-effect transistors made with CuInSe_xS_2-x quantum dots that exhibit p-type behavior with hole mobilities as high as 0.03 cm²/Vs after surface modification. With appropriate treatments of the quantum dot surfaces, we demonstrate ambipolar and n-type transport with carrier mobilities of ~ 0.02 cm²/Vs. The ability to finely tune charge transport properties of films of CuInSe_xS_2-x quantum dots demonstrates the large promise of these materials as versatile building blocks for a wide range of optoelectronic devices including solution-processed p-n junction solar cells and NIR photodetectors.

Controlling nanocrystal (NC) size provides us with the ability to tune the electronic fine-structure in semiconductor NCs as a convenient tool. Because of this band gap tunability, it is possible to construct cascaded NC layers with a designed band gap gradient where the NC diameters monotonically change. Previously, such layered NC films were demonstrated in the pioneering work of Franzl et al.^[1] This previous work has been critical for show-casing the benefits of cascaded exciton transfer. Also such layered NC structures were previously reported to be used in photovoltaic device, for solar conversion.^[2]
In this work, we present the first account of exciton funneling in photosensitive nanocrystal skin (PNS) platforms that rely on photovoltage buildup readout for light-sensing applications, leading to significant improvement in the device performance. Large area (> 40 cm²) and semi-transparent PNS is demonstrated based on photogenerated potential buildup. With only a single NC layer in the device structure, noise generation is significantly reduced, which enhances the device sensitivity. Thus, although increasing the NC layer&’s thickness results in increased optical absorption, in the absence of any applied bias across the device, photogenerated excitons may not dissociate and sufficient charge may not accumulate. Here we show highly photosensitive gradient nanocrystal skins enabled by the exciton funneling across the layers of NC film. To explore this proposition, we fabricated devices that rely on accumulating dissociated excitons after photogenerating and funneling. With our systematic characterizations, we demonstrated how funneling excitons along a gradually decreasing band gap gradient of the cascaded NC monolayers enhances photosensitivity of the device platform. We observed substantial improvements in the photosensitivity over a broadband spectral range (350-600 nm), with an approximately twofold enhancement factor along the entire operating wavelength range. To better understand and prove the presence of the exciton transfer process in the cascaded NC skins, we also conducted time-resolved fluorescence measurements at the donor and acceptor wavelengths. The results confirmed the exciton transfer process in the NC gradient structure.^[3]
We believe that these findings have significant implications for the future design of photosensing NC platforms and for moving toward next generation large-surface light-sensing NCs devices, including smart transparent windows, light-sensitive walls and facades of smart building and other large-area optical detection systems using NCs.
[1] T. Franzl, T. A. Klar, S. Schietinger, A. L. Rogach, J. Feldmann, Nano Lett.2004, 4, 1599-1603.
[2] I. J. Kramer, L. Levina, R. Debnath, D. Zhitomirsky, E. H. Sargent, Nano Lett.2011, 11, 3701-3706.
[3] S. Akhavan, A. F. Cihan, B. Bozok, and H. V. Demir, Small2014, 12, 2470-2475.

Plasmonic nanoparticles are well-known for their strongly size- and shape-dependent optical properties. Besides being highly relevant for applications such as theranostics, these properties can also be seen as tools for studying the formation mechanism of the particles themselves. In this contribution, we describe the use of optical spectroscopies alongside electron microscopy in order to elucidate the reaction mechanism and crystal growth characteristics for the synthesis of ultrathin silver patches on silica nanospheres. This approach, based on the electroless reduction of silver via the Tollens reaction was recently demonstrated by our group to be a promising scalable and versatile method for the fabrication of so-called patchy particles. In the present work, particular attention was paid to the role of ammonia, a base which both complexes silver ions and interacts with the silica surface. Through systematic experiments we demonstrated that the latter interactions, in the form of hydrogen bonds, provide a mechanism for silver adsorption and surface diffusion on the silica, processes which promote the heterogeneous nucleation and surface conformal growth of the metal patches. We also showed that the concentration of ammonia has a profound effect on the crystal growth regime of the silver patches. Notably, structures ranging from integration limited cup-like patches to diffusion limited dendritic patches could be produced. With the aid of measurements and finite element method electrodynamic simulations, the very distinctive optical properties of these different morphologies could be identified. Subsequently, in situ linear and ex situ non-linear (second harmonic scattering) optical spectroscopies were carried out. The former allowed us to study the reaction kinetics for different patch morphologies by tracking their growth in real time. The latter provided qualitative information on the influence of the core particle curvature on patch growth. Our results suggest that for smaller core particles, the higher curvature leads to the appearance of morphological defects, presumably resulting from the higher elastic stress imposed on the thin spreading patch. Our work appears to indicate that substrate curvature offers significant untapped potential for tailoring novel plasmonic nanostructures via interface-mediated growth processes.

Organic polymer- and small molecule-based solar cells have reached impressive power conversion efficiencies during the past years, however, the precise design and spatial control of the nanoscale morphology within photoactive materials remain a major challenge.
Covalent organic frameworks (COFs) offer a strategy to position molecular semiconductors within a rigid network in a highly controlled and predictable manner.¹ The π-stacked columns of layered 2D COFs enable electronic interactions between the COF sheets, thereby providing a path for exciton and charge carrier migration.² Frameworks comprising two electronically separated subunits can form highly defined interdigitated donor—acceptor heterojunctions, which can drive the photogeneration of free charge carriers.
Here, we report the first example of a photovoltaic device that utilizes exclusively a crystalline organic framework with an inherent type II heterojunction as active layer. The novel boronate ester linked COF comprises ordered columns of electronically separated donor and acceptor moieties. Oriented films of this COF were applied in the construction of a photovoltaic device in which the COF itself provides the photoactive junction. The interdigitated heterojunction of this COF was found to promote charge separation upon photoexcitation of either building block. We found that the charge carrier collection yield can be further enhanced under reverse bias, enabling a peak external quantum efficiency of above 30% and showing the potential of this novel concept. Based on the degree of morphological precision that can be achieved with COFs and the enormous diversity of molecular building blocks, these materials show great potential as model systems for organic heterojunctions and might ultimately provide an alternative to the current bulk heterojunctions.
[1] Cote, A. P., Benin, A. I.; Ockwig, N. W., O'Keeffe, M., Matzger, A. J., Yaghi, O. M., Science 2005,310 (5751), 1166-1170.
[2] Dogru M., Handloser M., Auras F., Kunz T., Medina D., Hartschuh A., Knochel P., Bein T., Angew. Chem. Int. Ed. 2013, 52, 2920-2924.

Faceted space filling colloidal polyhedral have been predicted in thermodynamic simulations to self-organized into diverse liquid crystals, plastic crystals, degenerate crystals as well as a range of crystallographic groups. These arrangements hold uncharted slab photonic band gaps and negative refraction properties inaccessible from the commonly available spherical colloidal building blocks. Such materials have been desired for applications in sensing, solar cells, optoelectronics and photonics. In addition, doping sphere-based photonic crystals with rare earths, lanthanides, organic dyes and quantum dots has established properties such as suppression or enhancement of spontaneous emission and photonic crystal lasing due to multiple reflections and enhanced density of states at the band gap edges. Only a handful of experimental reports address nonspherical colloidal building blocks with an active optical functionality at the single particle level.
This talk presents fullerene microcrystals as a new materials platform, suitable for ‘active&’ light emitting elements in photonic crystals. We report the synthesis of shape diverse and monodisperse microcrystal solvates using co-solvent precipitation with C₆₀ and C₇₀ fullerene-methylbeneze solutions in 2-propanol as a poor solvent. Monodisperse nonspherical colloids with diverse morphology including hexagonal platelets and rods with facets, hollow ends, or ‘flower-like&’ crosssections, are grown based on the concentration and injection volume of the fullerene solution. The solvated microcrystals display enhanced photoluminescence over pristine fullerenes. The self-organization of fullerene microcrystals into photonic crystal arrangements under physical confinement will also be discussed.

Liquid crystals as functional soft materials have been widely used for potential applications in various functional devices. The nanostructured dopant particles, oxides and quantum dots markedly improved their performance and opened path for future novel FLC device applications. Recently, photo-functional liquid crystal materials doped with variety of nanomaterials exhibiting photoluminescence (PL), electroluminescence, have been intensively developed. PL behavior of these materials can be easily tuned by altering the size/shape and concentration of dopants. In current study, we have dispersed varied concentration of nano sized Nd₂O₃ suspension in FLC (Zli3654) which lead to significant changes in photoluminescence behavior of nanocolloids. PL spectra of Nd₂O₃ nanoparticles recorded at 248 nm and 292 nm excitation wavelengths with best response at 248 nm show series of emission peaks on broad absorption band with maxima at ~ 345-397 nm (UV), ~ 425-475 nm (blue), ~ 545-560 nm (green) and ~ 613 nm (red) wavelength regions. The broad emission band in the UV region appears due to the singly ionized oxygen vacancies in Nd₂O₃ by the radiative recombination of photo-generated hole with an electron occupying the oxygen vacancy. The existence of blue, green and red emissions confirm the presence of surface defects in the lattice. PL emission of Nd₂O₃ nanoparticles (NPs):FLC composites recorded at an excitation wavelength where Nd₂O₃ NPs show intense emission, i.e. 248 nm, exhibits an enhancement effect with gradually increasing the concentration of NPs upto 8 wt%. On the other hand, emission spectra at 303, 323, 333, 343 nm excitation wavelength show quenching of all the emission bands in FLC-NPs nanocomposites which is explained in terms of the change in population of Nd⁺³ ions at different excited levels. Detailed mechanisms involved in the changes of the PL spectrum of FLC-Nd₂O₃ NPs nanocomposites with varying NPs concentration and excitation wavelengths will be discussed at the time of presentation.

A simple and effective surface enhanced Raman scattering (SERS) based DNA sensor utilizing toehold-mediated DNA displacement reaction as a target-capturing scheme has been demonstrated for the detection of Avellino Corneal Dystrophy which is caused by a single point mutation of BIGH3 gene. For a SERS substrate, Au-Ag bimetallic nanodendrites able to provide large numbers of hot spots and surface area were electrochemically synthesized and adopted in this study. The incorporation of both Ag and Au was to simultaneously secure demanding sensitivity and stability of the substrate and an optimal composition satisfying the need was determined. A double-strand sensing element composed of ‘a probe DNA (pDNA)&’ complementary to ‘a target DNA (tDNA)&’ and ‘indicator DNA tagged with a Raman reporter (iDNA-R)&’ was conjugated on the optimized substrate. The conjugation made the reporter molecule close to the surface and induced improved generation of Raman signal. A tDNA released the pre-hybridized iDNA from pDNA via toehold-mediated replacement, which was confirmed by increase of percent intensity change with decrease of Raman signal according to the concentrations of tDNA. The concentration correlation was good and linear in the concentration range from 200 fM to 20 nM, and the achieved limit of detection (LOD) was 62.5 fM, which is superior to that of previously reported toehold-mediated DNA displacement reaction sensing schemes using other tagging methods such as fluorescence and electrochemistry. This study provides simple and effective diagnostic SERS platforms for more sensitive and stable detection of various DNA point mutations, leading to possible diagnosis of point mutation-related diseases including Avellino corneal dystrophy reported here. [This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korean Government (2013R1A1A3008484 and 2014S1A2A2027641)]

Architectural design is essential to achieve ideal chemical and physical properties of nanomaterials. In this study, we used the epitaxial growth of submonolayer Ag-Pd alloy on the surface of Ag nanocubes to develop a novel class of Ag-Pd nanocubes with both high catalytic and strong SERS activity. The SERS properties of Ag-Pd nanocubes with different Ag-Pd alloy coverage were systematically investigated using 1, 4-benzenedithiol (1, 4-BDT) as Raman probe molecules. Our results suggested that the SERS activity of Ag-Pd nanocubes was slightly decreased with the increase of Ag-Pd alloy coverage but it was still comparable to that of Ag nanocubes. We further used the Ag-Pd alloy nanocubes to monitor the reduction of 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP) by in-situ SERS detection. We demonstrated that the Ag-Pd nanocubes were capable of tracking the Pd-catalyzed reaction in real time by detecting SERS of the reactant, intermediate, and final products.

Detection of metal ions is crucial for diagnosis of some human diseases, so thousands of assays have developed for this purpose. Excessive intake of silver ions (Ag⁺) causes argyria in humans, so silver ion should be detected with high sensitivity, specificity and rapidity. Silver ion detection methods that use inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectroscopy have been developed, but they need highly-sophisticated instruments and a complex preparation process. Various organic compounds, quantum dots, nanoparticles, polymers and DNA have been used as chemosensors. Although these methods show high sensitivity for silver ion detection, they have drawbacks such as laborious synthesis of reagents, complex detection system, and limited duration of storage.
Water-soluble DNA-templated Ag nanoclusters (DNA-AgNCs) fabricated by simple mixing and reduction process can be used in an imaging or sensing probe that exploits their DNA sequence-dependent fluorogenic property. N3 atoms and the carboxyl group in the pyrimidine ring of cytosine (Cyt) and the N atoms in the purine ring of guanine (Gua) are the most probable binding sites for silver ion during DNA-AgNCs formation, though the detailed mechanism is not yet known. Especially a cytosine 12-mer (Cyt₁₂) is the most frequently used DNA template for AgNCs formation.
Recently, we found that a fluorescence switch property of DNA-AgNCs was triggered by silver ion. Herein, we report a simple method for switching the fluorescence of DNA-AgNCs from red to green, and also investigate the changing process via fluorescence spectroscopy, circular dichroism spectroscopy, DNA hybridization assay and mass spectrometry. From upon analytical studies, silver ion induce a dimeric structure of Cyt₁₂-AgNCs by forming a bridge between two Cyt₁₂-AgNCs, this change cause the fluorescence of Cyt₁₂-AgNCs to change from red to green. Using this silver ion-triggered fluorescence switch, we successfully detected silver ion at concentrations as low as 10 nM. Furthermore, as a real system, Silmazine, which is dermatological burn ointment having silver sulfadiazine, is utilized as silver ion quantification product. The silver ion detection method using this fluorescence switch has high selectivity, high sensitivity, and short response time, can be used successfully even in the presence of other metal ions.

Silver nanoplates (AgNPs) are highly anisotropic two-dimensional nanostructures that exhibit unique localized surface plasmon resonance (LSPR) behavior which can be taken advantage of in optical applications. The LSPR behavior for AgNPs highly depend on the size and morphology of the nanostructures. We report a seed-mediated growth process that controls the reduction of Ag⁺ ions for selective deposition on the corners of AgNPs which lead to the sharpening of corners and the subsequent red-shifting of the AgNPs&’ LSPR behavior. In order to understand the growth process, we investigated the roles of seed concentration, initial Ag⁺ concentration, and PVP concentration in the AgNP growth characteristics. Our results suggest a mechanism which is initiated by (1) the reduction of the AgNO₃ precursor to generate small-sized multiple-twinned random particles of Ag and followed by the subsequent (2) selective transfer of Ag from the random particles to the corners pre-added AgNPs via Ostwald ripening. The resultant AgNPs with fixed corners (which are much red-shifted from the original seed AgNPs) are mixed with independently generated multiple-twinned random particles (which have LSPR peaks in the blue region) to generate an optical filter that can simultaneously absorb light in the wavelengths of 400 nm and >700 nm. This study could serve as a groundwork for optimizing the growth behavior of AgNPs and the tuning of its LSPR behavior which may lead to various optical applications.

Nanocomposite hybrid films were prepared by depositing tungstate on PEO-PPO-PEO (EPE) templates under acidic conditions by a one-pot self-assembly sol-gel method. Uniform and transparent films based on this precursor were fabricated and easily manipulated using a facile casting method. Upon irradiation by sunlight, the film exhibits a rapid photochromic response that was reversible at room temperature. UV-Vis analysis revealed that folding of the -CH₂-O- chains in the polymer and the larger tungstate clusters increased the red shift for the adsorption of visible light. The mechanism underlying the effect of hybridization of polyethylene oxide (PEO) on the enhanced photochromic effect was characterized by NMR, Raman and FTIR. The degree of folding of the -CH₂-O- polymer chains was influenced by the presence of acid, which increased the oxygen coordination of WO₆ and WO₄ in the tungstate clusters. The characterized W-O bonding peaks shifted upon EPE incorporation, consistent with changes in oxygen coordination within the material. This weak coordination between tungstate and -CH₂-O- improved the intervalence charge transfer (IVCT). In addition, the lengthened lifetime of photo electron-hole separation is attributed to PT network&’s electron trapping. The trapped electron in different type of amorphous PT building block network by weak sunlight excitation resulted in a various efficient coloration. More importantly, the existance of hole scavenger further prevents recombination of electron and hole, so that the long-lived photoelectron could provide sustainable reactive oxygen species (ROS). The slow released and reversible H₂O₂ delivery attribute to induce apoptosis of A375 melanoma cells.

Quantum dots (QDs) are semiconductor nanocrystallites with size-dependent multiple quantum-confined states which are being explored for utilizing broadband radiation. While DNA has been used for the self-assembly of nanocrystals, it has not been investigated for the formation of simultaneous conduction pathways for transporting multiple energy charges or excitons. These exciton shelves can be formed by coupling the conduction band, valence band, and hot-carriers states in QDs with different HOMO-LUMO levels of DNA nucleobases, resulting from varying degrees of conjugation in the nucleobases. Here, we present studies on the electronic density of states (DOS) in naturally occurring nucleobases (guanine, thymine, cytosine, and adenine), which energetically couple with quantized states in semiconductor QDs. Using scanning tunneling spectroscopy of single nanoparticle-DNA constructs, we demonstrate composite DOS of chemically-coupled DNA oligonucleotides and cadmium chalcogenide QDs (CdS, CdSe, CdTe). While perfectly aligned QD-DNA states lead to exciton-shelves for multiple energy charge transport and energy transfer, mismatched energy levels in QD-DNA hybrids introduce intra-bandgap states which can lead to charge trapping and recombination. Using single nanoparticle and ensemble device measurements we also show successful extraction and transport of both bandedge and high-energy charge carriers, and energy transport using excitons. These results can have important implications for the development of a new class of nano-bio electronics and biological transducers.

Carbon nanomaterials have attracted extensive attention due to its nontoxic, biocompatible properties. Carbon nanodots (C-dots) are one of the most promising candidate of fluorescent nanoparticles, as their excellent low-toxicity and eco-friendly characteristics are similar to other popular carbon nanomaterials including carbon nanotube, fullerene, and graphene. Moreover, it is easy to prepare large-scale C-dots due to abundance of raw material resources. Because of these inherent advantages, C-dots have been widely used in the fields of biosensors, bioimaging, optoelectronic, photocatalysis, etc. Recent studies reported heteroatom doped C-dots can improve the property of nanomaterials. In order to enhance the intrinsic properties of C-dots, such as their local and surface chemical activities, electronic properties and photoluminescent properties, numerical preparation strategies has been developed. However, sulfur-doped(S-doped)C-dots with highly photoluminescent (quantum yield > 50%) have not been reported in the literature. In this study, sodium citrate and sodium thiosulfate were firstly adopted as precursors for the preparation of C-dots doped with S-doped alone with high photoluminescent (quantum yield>64%). The S-doped C-dots prepared under optimum experimental conditions were characterized by transmission electron microscopy (TEM), fluorescence spectroscopy, X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectroscopy (FTIR). It is found the the S-doped C-dots have spherical shape with an average size of 4.6 nm and were well separated from each other. The fluorescent properties of S-doped C-dots are stable in various solutions, making them a promising material used in biological field. Furthermore, the as-prepared S-doped C-dots could be used as probes without any modification for the fluorescence turn-off detection of Fe³⁺ in the range of 1 - 500 mu;M and the limit of detection is 0.1 mu;M.

Photodynamic therapy (PDT) is an alternative to traditional cancer treatments. This approach involves the use of photosensitizing (PS) agents and their interaction with light. As a consequence, cytotoxic reactive oxygen species (ROS) are generated that, in turn will destroy tumors. On the other hand, ZnO is a biocompatible, nontoxic, and biodegradable material with the capability to generate ROS, specifically singlet oxygen (SO), which makes this material a promising candidate for 2-photon PDT. Doping ZnO with Li species is expected to induce defects in the host oxide structure that favored the formation of trap states that should affect the electronic transitions related to the generation of SO. The present work reports the effect of the level of Li-doping on the ZnO structure and its capability to generate SO. Li-doped ZnO nanoparticles were synthesized under size-controlled conditions using a modified version of the polyol route. The X-Ray diffraction measurements confirmed the development of well-crystallized ZnO Wurtzite; the average crystallite sizes ranged between 13.3nm and 14.2 nm, with an increase in Li content. The corresponding band gap energy values, estimated from UV-vis spectroscopy measurements, decreased from 3.33 to 3.25 eV. The Photoluminescence spectroscopy (PL) measurements of Li-ZnO revealed the presence of emission peaks centered on 363nm, 390nm, and 556 nm; these emission peaks correspond to the exciton emission, transition of shallow donor levels near of the conduction band to valence band such as interstitial Zn, and oxygen vacancies, respectively. The observed increase of the emission intensity of the 390 nm emission peak, relative to the intensity of the main emission peak at 363nm, was attributed to the promote of trap states due to interstitial Zn or Li-incorporation into the host oxide lattice. SO measurements evidenced the enhancing effect of the Li concentration on the capability of the doped ZnO to generate this species. This Li - dependence of SO generation can be attributed to the enhancement of the concentration of trap states in the host ZnO, as suggested by PL measurements. Accordingly, Li-ZnO would become cytotoxic to cancer cells via photoinduced ROS generation and enables this nanomaterial to be considered a potential direct PS agent for cancer treatment by the 2-photon PDT approach.

The evolution of semiconductor electronics and optoelectronics began with the controlled doping of impurity atoms into semiconductor crystals, which opened the way to engineer their charge transport properties. On the other hand, unwanted impurities or defects can hinder transport and limit device performance. In this work, we studied the origin of defects in PbS quantum dots (QDs), and performed controlled doping of the QD films. We analyzed the energy level alignment of these QDs with metal oxides, and that of the p and n type QDs. The energy diagram of different junctions are correlated with the optoelectronic performance of p-n and p-i-n junction diodes, designed for applications as solar cells and photodetectors.

The self-organization of colloidal nanoparticles in superstructures is a well-proven method of a fabrication of new materials with desirable properties. The colloidal quantum dots (QDs), such as lead sulfide (PbS), can be utilized in such superstructures as building blocks. PbS QDs possess unique optical properties, such as large extinction coefficient in infrared (IR) region of spectrum, small and equal masses of charge carriers, multi-exciton generation. The materials based on PbS QD superstructures are very promising for utilizing them in IR-photovoltaics, such as biosensors and solar cells. In this work we investigated the structures, obtained by self-organization of PbS QDs of different sizes on mica slides.
The test samples of QD superstructures were prepared by a slow evaporation method of a saturated QD stock solution in tetrachlormethane (TCM), which leads to the formation of ordered QD superstructures. To investigate morphology and crystal symmetry of obtained structures formed from QDs of different sizes, a Small Angle X-ray Scattering (SAXS) technique is used. Optical properties of the test samples are investigated using a spectrophotometer Shimadzu UV3600 and purpose-built IR spectrofluorimeter with opportunity to obtain a steady-state and time-resolved photoluminescence spectra.
The analysis of X-ray data shows that self-organized superstructures possess QD ordering similar to an ordering of atoms in real atomic crystals. SAXS patterns from test samples contain the set of narrow peaks, corresponding to the scattering from the supercrystal planes. By indexing of the diffraction peaks in SAXS patterns it is obtained that QD supercrystals have primitive orthorhombic crystal lattice. Absorption and photoluminescence bands of superstructures are broadened as compared to that for QD in TCM. The data on the ordered superstructures from PbS QDs are of considerable interest for the design and further fabrication of novel materials for the photovoltaic devices with improved properties in IR-region of spectrum.

PbS quantum dots (QDs) have been extensively studied last decade for the development of new generation of photovoltaics. A special attention in this area is paid to creation of cascade or blend QD-based structures, consisting of QDs of different size with, respectively, different band gaps. Optical and photoelectric properties of such structures are affected by energy and charge transfer processes.
In the present work we have analyzed an energy transfer process in blends of PbS QDs of different size. Colloidal QDs with mean diameters of 3.5, 4.6, and 6.1 nm were infiltrated into a porous matrix forming corresponding pairs and a triple blend. Transient and steady-state photoluminescence (PL) analysis performed at 300 and 77 K let us thoroughly consider different pathways of the energy transfer process.
We found that in the triple blend all QDs are coupled by successive transfer of energy from smaller to larger dots, which leads to merger of PL decay curves for QDs of different size, caused by increase of PL decay times of acceptors. Middle-size QDs serve as both donors and acceptors of energy, and their PL is also enhanced. At the same time, an analysis of the energy transfer in QD pairs showed that energy transfer process differently affects the acceptor PL decay times for different pairs. An explanation of this phenomenon lies in complicated low-energy electronic structure of PbS QDs. Namely, different energy states, including the long-lived in-gap state, are involved into the energy transfer process. When energy transfer occurs via the in-gap state of donor-QD, PL decay time of acceptor-QD undergoes drastic increase. Hence, successive energy transfer from the in-gap state of smaller dots to the larger dots in the triple blend leads to merger of their PL decay curves.
This feature of the energy transfer process in the triple blend may be useful for creating of graded band PbS QD-based solar cells. In accordance to this model, an energy transfer will increase PL decay times of larger QDs, which initially have much faster decay rates, and will promote more efficient separation of charge carriers in the photovoltaic cell.

Hybrid structures based on nanocarbons and semiconductor nanocrystals is a fast growing area in modern nanoscale science [1-4]. These structures combine effective light absorption of nanocrystals and high charge mobility in nanocarbons - the properties are attractive for photovoltaic and optoelectronic devices.
Photoinduced increasing of graphene conductivity was shown in presence of quantum dots (spherical shaped nanocrystals) compare with graphene alone [1-3]. Often charge transfer from quantum dots to graphene is regarded as the major mechanism of photoinduced conductivity increasing [3]. On the other hand, unlike molecules and quantum dots - point dipoles in the most of approximation - graphene is two-dimensional nanostructure. Therefore the effective long-range energy transfer implements in molecule/graphene or quantum dot/graphene hybrid structures inducing generation of charge carrier and increasing photoconductivity of graphene [4]. The first or the second mechanism will prevail depends on hybrid structures design.
The purpose of our work was to investigate mechanism of conductivity amplification in hybrid structures based on graphene and colloidal quantum dots. Structures were formed by successive graphene and quantum dots films deposition on substrate by Langmuir-Blodgett technique. Dependence of photoelectrical and luminescent properties of structures on quantum dot film thickness and energy band gap was investigated. It was found that increasing thickness of quantum dots layer leads to stronger amplification of samples conductivity. Band gap narrowing works in the same way. We suppose that charge transfer from quantum dots placed directly on graphene surface to graphene leads to increasing number of charge carrier in graphene and faint growing of conductivity. Nonradiative energy transfer occurs from upper layers of quantum dots also and induced generation of charge carrier and conductivity gain.
Present results allow improving photoresponse of photovoltaic and optoelectronic devices based on graphene and nanocrystals.
[1] Zhao J. et al. Improving the photovoltaic performance of cadmium sulfide quantum dots-sensitized solar cell by graphene/titania photoanode //Electrochimica Acta. - 2013. - V. 96. - pp. 110-116.
[2] Guo C. X. et al. Layered graphene/quantum dots for photovoltaic devices //Angewandte Chemie International Edition. - 2010. - V. 49. - #8470;. 17. - pp. 3014-3017.
[3] Lin Y. et al. Dramatically enhanced photoresponse of reduced graphene oxide with linker-free anchored CdSe nanoparticles //ACS nano. - 2010. - V. 4. - #8470;. 6. - pp. 3033-3038.
[4] Dai L. Layered graphene/quantum dots: nanoassemblies for highly efficient solar cells //ChemSusChem. - 2010. - V. 3. - #8470;. 7. - pp. 797-799.

Nanostructured thin films represent an exciting new class of materials. Electronic and optoelectronic devices with active layers consisting II-VI semiconductor thin films are objects of growing scientific and technological interest due to high electron mobilities compared to amorphous silicon. Transparency in the visible spectrum, low temperature processing and compatibility with large area make CdSe, CdS and Al: ZnO thin films good candidates for flexible substrate applications. We report fabrication of highly continuous and smooth CdSe semiconductor films containing self-assembled CdSe nanocrystals (NCs). High quality mono-dispersed CdSe NCs are synthesized and using them we have developed a novel technique to enhance the electron coupling between NCs by reducing the inter-particle distance, which showed four orders of magnitude of photocurrent in the pn-device. High performance thin film transistors (TFT) are also fabricated using CdS as an active layer prepared by chemical bath deposition method (CBD). Replacement of conventional SiO₂ gate dielectric with ZrO₂ (high-k) grown by Atomic layer deposition significantly low leakage current as well as remarkable lower operating voltages. Thin film solar cells based on CIGS absorber layer and CdS buffer layer have been a subject of intensive investigations for many years. Solar cells containing multi-junctions perform better than their single-junction consideration must be given to study the optimal growth kinetics each layer. As an alternative vacuum deposition, electrodeposition is an excellent technique for large area processing, low cost, and high material yield. A systematic approach of CIGS device fabrication and detailed study of cell performance is carried out. Acknowledgements: This work is supported by the NSF-CREST (CNBMD) Grant No. HRD 1036494, and partially by DoD (CEAND) Grant No. W911NF-11-1-0209 (US Army Research Office).

The remarkable efficiency, functional robustness, and scalability of rigorously organized photoactive molecules found in biological systems provide compelling inspiration for the design of next generation photoactive technologies. Mimicking the molecular scale assembly and integration of these components in synthetic, systems, however, remains a daunting challenge. We explore here light-absorbing Metal-Organic Frameworks (MOFs) as highly porous, supramolecular crystalline materials capable of organizing molecular building blocks with atomic resolution, while providing pathways to tailored chemical interactions, optical absorption, and electronic properties through variations in MOF chemistry and structure. Moreover, the porosity and rich chemical functionality of these materials makes them attractive, functional templates for the organized integration of donor and acceptor materials within the MOF architecture. Guided by periodic Density Functional Theory calculations, we have synthesized and characterized a family of MOFs aimed at optimizing nanoscale ordering and energy-transfer processes in dye sensitized solar cells (DSSCs). We specifically describe the synthesis and integration of these photoactive MOFs into DSSCs. Subsequent characterization using techniques such as UV-Vis spectroscopy, electrochemical impedance spectroscopy, electron microscopy, and cyclic voltammetry reveals how supramolecular structure and chemistry influence the performance of these materials as optical absorbers and charge transfer mediators in active devices. Exploiting these chemically and structurally versatile supramolecular materials promises a new avenue to the rigorous molecular organization needed for next generation photoactive materials development.
Sandia National Laboratories is a multi program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Semiconductor quantum dots (QDs) fabricated via colloidal chemistry have recently emerged as a versatile platform for the realization of low-cost, solution-processed luminescent solar concentrators (LSCs). LSCs are photon photon-management devices that represent a cost-effective alternative to optics-based solar concentration systems. In addition to enhancing the performance of stand-alone solar cells, LSCs can help integrate PV devices into existing or newly constructed buildings, in the form of, for example, semi-transparent windows that would transform the energy-passive house faccedil;ades into large-area energy generation units. A typical LSCs consists of a plastic optical waveguides doped with fluorophores or glass slabs coated with active layers of emissive materials. Direct as well as diffused sunlight, which penetrates the matrix, is absorbed by the fluorophores and then re-emitted at a longer wavelengths. The luminescence, guided by total internal reflection, propagates towards a PV cell placed at the edge of the waveguide where it is converted into electricity. Since the LSC area exposed to sunlight is much greater than the area of the PV itself, the use of this approach can greatly increase the flux of radiation incident onto the device and thus boost both the photocurrent and the photovoltage. An additional increase in the power output can be obtained by matching the emission wavelength of LSC emitters to the spectral peak of the PV efficiency of a given device. QDs are attractive for LSCs but their small energy separation between the emission line and the band-edge absorption peak (Stokes-shift) results in re-absorption losses that hinder the realization of large-area devices. In this talk, I will show that using "Stokes-shift-engineered" CdSe/CdS QDs with giant shells (giant-QDs) it is possible to realize LSCs without re-absorption losses for device dimensions up to tens of centimeters. Monte-Carlo simulations show 100-fold increase in LSC efficiency using giant-QD with respect to core-only QDs. We demonstrate the feasibility of this approach by using high-optical-quality QD-polymethylmethacrylate (PMMA) nanocomposites fabricated by a modified industrial method that allows to preserve the light-emitting properties of giant-QDs upon incorporation into PMMA. The study of these LSCs yield optical efficiencies >10% and demonstrate a significant promise of Stokes-shift-engineered QDs for large-area LSCs fabricated via industrial methods. Furthermore, I will show that the approach to Stock-shift engineering is general and can be extended to smaller band gap materials for achieving a better match with the absorption spectrum of traditional Si-based PV cells and the spectrum of solar radiation. Further, the procedure for QD incorporation into a high-quality PMMA matrix is also not QD-material specific and can be directly applied to colloidal nanocrystals of various compositions and shapes. [Nature Photonics, DOI: 10.1038/NPHOTON.2014.54]

Organic photovoltaic cells (OPVs) represent an exciting class of renewable energy technology, and are under intensive investigation in both academic institutions and industrial companies due to their potential to enable mass production of flexible and cost-effective devices through roll-to-roll techniques. The proper choice of interface materials is a must for highly efficient and stable OPV devices and has become a significant part of the OPV research today. Interface materials are either non-conducting, semiconducting or conducting layers which not only provide selective contacts for carriers of one sort, but can also determine the polarity of OPV devices, affect the open-circuit voltage, and act as optical spacers or protective layers.
Owing to their unique two-dimensional structure, and functionalization-induced tunable electronic structures, graphene and its derivatives have been used as a new class of efficient interface materials in OPVs. Highly efficient and stable OPVs have been fabricated with graphene and its derivatives as interface materials. In this talk, we will summarize recent progress in this emerging research field. And then present our rational concepts for the design and development of the graphene-based interface materials for high performance OPVs.

Presented here is a device characterization method correlating electron beam-induced current (EBIC) and high resolution energy dispersive spectroscopy (EDS). The device of interest is a PbS quantum dot (QD) depleted-heterojunction architecture, which has been studied first at the macroscopic level (efficiency), and then at the nanoscale (EBIC, EDS). Through these methods, features that are not visible by efficiency testing alone becomes detectable; it was discovered that the PbS QDs intermix with the TiO₂ layer creating a decaying “p-n junction” that decreases as a function of distance into the TiO₂. EBIC assisted in determining the p-n junction location as well as alluding to the aforementioned mixing; the latter was confirmed using high resolution EDS. Defects were engineered to further correlate efficiency with EBIC, where the line profiles based off of the electronic maps suggested a change in the behavior in the device. Utilizing these techniques, a better understanding of this device can be achieved as well as improvements to the architecture can be made. A comparatively increased efficiency device was fabricated based upon the results. This technique can also be used for the characterization of other solid-state photovoltaics.

Excitonic solar cells including dye-sensitized solar cells, quantum dot-sensitized solar cells, bulk heterojunction organic photovoltaics, are built upon nanostructures of various functional materials. Nanostructures are essential for the high power conversion efficiency, for example, in dye-sensitized solar cells and quantum dot-sensitized solar cells, mesoporous photoanodes made of nanoparticles offer large specific surface area for loading a large amount of dyes or quantum dots so as to capture a sufficient fraction of photons. However, the large surface area of such nanostructures also provide easy pathways for charge recombination, and surface defects and connections between adjacent nanoparticles may retard effective charge injection and charge transport, leading to a loss of power conversion efficiency. Surface facets and chemistry may also affect the conformal coating and adhesion of dye molecules and polymer layers. In this presentation, I will present and discuss our recent work on the design and control of (1) nanostructures and surface chemistry of photoanodes for quantum dot - sensitized solar cells and (2) nonstoichiometric composition, doping, and allignment of quantum dots in quantum dot-sensitized solar cells, and (3) morphologies and chemistry of cathodic buffer layers for inverted polymer solar cells. Our research has demonstrated that the power conversion efficiency can be significantly enhanced with excellent device stability when both nanostructures and interface chemistry are properly controlled.

Due to their potential applications to high efficiency and light-weight photovoltaic devices, chalcopyrite-type semiconductors have received a lot of interest in recent years. Among these semiconductors, CuIn_yGa_1-ySe₂ (CIGS) nanocrystals are of particular importance because its bandgap can be controlled from 1 eV to 1.7 eV by adjusting the ratio of indium and gallium contained in the material. CIGS belongs to the family of I-III-VI₂ semiconducting materials of the tetragonal chalcopyrite structure. Since the material has very prominent absorption properties, the sunlight to electricity conversion efficiency as high as 20.3% has been obtained with CIGS that has an energy bandgap around 1.13 eV. However, the laborious process used to obtain these high efficient CIGS solar cells is costly and requires precise multiple element co-evaporation under high vacuum and high temperature conditions. Furthermore, the required post-deposition treatments involve toxic gases such as H₂Se. Overall, room temperature and non-vacuum thin film deposition techniques using CIGS nanoparticles or microparticles as the starting material have been realized as most cost effective techniques for commercially scalable CIGS high efficiency solar panels.
In order to use the cost effective methods to fabricate high efficient and light-weight CIGS solar cells, developing synthesis techniques to produce high quality CIGS nanoparticles are crucial. We have used a solvothermal method to synthesize CIGS nanoparticles and nanorods, in which only simple, inexpensive, and low temperature processes are utilized. Briefly, the starting materials include cupric chloride dihydrate, elemental Indium beads, elemental gallium and selenium powders, which are loaded into a 50 ml Teflon-coated autoclave reactor. Ethylenediamine is used as the solvent. To accommodate with the organic nature of the solvent and in order to remove oxygen from the reactions, argon gas is introduced into the furnace during the reaction period. The reaction period is from 24-48 hours. To understand the effect of reaction temperature on particle size, shape and stoicheometric composition, the temperature is controlled from 180°C-280°C to incorporate the presence of solid indium and selenium as well as liquid indium and selenium. CIGS nanocrystals with different shapes have been grown under various reaction environments. After the CIGS nanocrystals are grown, several characterizations have been conducted as well. We use high resolution scanning electron microscope (SEM) to investigate the effect of temperature on the size and shape of CIGS nanoparticles. We have also used X-ray diffraction (XRD) technique to study the symmetry and lattice constants of the nanocrystals.
During the conference, I will present our research results of CIGS nanocrystal and nanorod growth, SEM images of GIGS nanocrystals and nanorods, and characterizations of XRD and optical absorbance.

Cost effective solar cells with high efficiency based on abundant, non-toxic material is the long term target of present research and development. Copper Sulfide (Cu₂S) with a suitable band gap of 1.2eV and high absorption coefficient is an appropriate candidate for an absorber in single or multijunction solar cells.
Copper sulfide nanoparticles (NP) were synthesized in ionic liquid using a microwave reactor. TEM reveals an average diameter of about 70nm and the NP are partially agglomerated. XRD measurements show that the pristine NP are present in multiple phases which are predominantly djurleite (Cu_1.96S) as well as high digenite (Cu_1.8S) and tetragonal chalcocite (Cu₂S). The absorption characteristics deduced from Photothermal Deflection Spectroscopy (PDS) measurements indicate a high free carrier concentration. Taking the free carrier absorption into account the absorption characteristic is in good agreement with the absorption properties of djurleite bulk material. PL spectra of the NP at room temperature exhibit a broad PL peak centered at 1.23eV with FWHM of 360meV which is interpreted as a superposition of the emission of different copper sulfide phases.
To modify the material, NP where annealed at 370°C and 450°C in vacuum and argon atmosphere, respectively. Annealing reduces the free carrier absorption significantly. Also, the fundamental absorption of the NP changes and exhibits an increase of the absorption between 1.2eV and 1.3eV which is in good agreement with the absorption coefficient of chalcocite bulk material. Consequently, the initial multi-phase material is transformed to chalcocite. After the annealing the PL intensity is increased by about one order of magnitude accompanied by a decrease of the FWHM to 160meV. The intensive narrower emission is attributed to chalcocite band edge emission indicating a high quality material.
Pristine NP were embedded in 50nm amorphous silicon (a-Si:H) via a PECVD process to investigate the suitability of the material with a-Si:H and with the deposition process in terms of compatibility and stability. For energies <1.3eV the absorption characteristics of the embedded NP points to a transformation into the chalcocite phase. For energies >1.5eV the absorption is dominated by a-Si:H. The PL of the embedded NP is significantly increased and similar to the PL of the annealed NP. Since the deposition temperature is about 220°C, the improvement of the PL efficiency is unlikely to arise just by the temperature treatment during the deposition. Therefore, we expect the H-plasma and the a-Si:H to be responsible for a decreased defect concentration, e.g. by surface passivation or saturation of unsaturated bonds of the NP.
The intensive band edge emission of the annealed and embedded copper sulfide NP indicates the suitability of the material as absorber in photovoltaic applications.

Nanocrystalline boron doped diamond films were grown by a microwave plasma enhanced chemical vapor deposition.[1,2] Electrochemical impedance spectroscopy provided the flatband potentials and concentrations of acceptors, which relate to the B-concentrations from the neutron depth profiling. Electrochemical cleaning of the surface from sp² carbon impurities was demonstrated by Raman spectroscopy.[1] The quality of nanocrystalline diamond electrodes can be thus analyzed in detail. Spectral sensitization of the nanodiamond surface was carried out by anchoring of dyes like 4-(bis-{4-[5-(2,2-dicyano-vinyl)-thiophene-2-yl]-phenyl}-amino)-benzoic acid (P1 from Dyenamo AB). The target device is a nanodiamond-based p-type dye-sensitized solar cell, which is an alternative of the well-known n-type dye-sensitized solar cell based on titania photoanode. In a two-step procedure, polyethyleneimine is adsorbed on hydrogenated diamond surface and subsequently modified with P1. The sensitized diamond exhibits stable cathodic photocurrents under visible light illumination in aqueous electrolyte solution with dimethylviologen serving as electron mediator. In spite of the simplicity of the surface sensitization protocol, the photoelectrochemical performance is similar or better compared to that of other sensitized diamond electrodes which were reported in previous studies (2008-14).[3]
Acknowledgment. This work was supported by the Grant Agency of the Czech Republic (contract No. 13-31783S).
References
[1] Z. Vlckova-Zivcova, O. Frank, V. Petrak, H. Tarabkova, J. Vacik, M. Nesladek, L. Kavan, Electrochim. Acta, 18 (2013) 518.
[2] P. Ascheulov, J. Sebera, A. Kovalenko, V. Petrak, F. Fendrych, M. Nesladek, A. Taylor, Z. Vlckova-Zivcova, O. Frank, L. Kavan, M. Dracinsky, P. Hubik, J. Vacik, I. Kraus, I. Kratochvilová, Eur. Phys. J. B, 86 (2013) 443.
[3] S. W. Yeap, D. Bevk, X. Liu, H. Krysova, A. Pasquarelli, D. Vanderzande, L. Lutsen, L. Kavan, M. Fahlman, W. Maes, K. Haenen, RCS Adv., 4 (2014) 42044.

Recently, iron pyrite has come under scrutiny for failure to live up to its photovoltaic potential. While pyrite has a promising bandgap of 0.95 eV and high absorption coefficient, recent reports have shown pyrite to have a p-type inversion layer on the surface, regardless of material thickness, that can account for the low open circuit voltage in pyrite-based solar cells.¹ This inversion layer comes from the inherent decrease of Fe-S coordination at the surface. Secondly, pyrite shows low thermal stability; non-pyrite phases appear at temperatures below 500 ^oC. An alternative material, Fe₂GeS₄, has been suggested in the literature² that retains the desirable pyrite qualities - earth abundant materials, non-toxic elements, and high absorption coefficient - but avoids the thermal and structural instabilities present in iron pyrite. In order to keep costs low when manufacturing photovoltaics using this material, we are interested in solution processing Fe₂GeS₄ in nanoscale morphologies for eventual large area printing. While the material properties have been described for a sputtered thin film of Fe₂GeS₄, the nanoscale form is not as fully characterized or tested in prototype cells. Using XRD and TGA, we found that the thermal stability is lower for nanomaterial Fe₂GeS₄ than sputtered thin films, and via kelvin probe measurements that Fe₂GeS₄ has a relatively high work function. Informed by these material properties, we fabricated a number of prototype photovoltaic cells, finding that a bulk-heterojunction like cell is a promising test structure.
This project is funded through the NASA Space Technology Research Fellowship.
1. Limpinsel, M. et al. An Inversion Layer at the Surface of n-type Iron Pyrite. Energy Environ. Sci. (2014). doi:10.1039/c3ee43169j
2. Yu, L. et al. Iron Chalcogenide Photovoltaic Absorbers. Adv. Energy Mater.1, 748-753 (2011).

We developed an optically controllable organic field transistor (OFET) by employing photochromic diarylethene molecules for a channel layer. The drain current was reversibly changed by alternating UV and VIS light irradiation; a closed ring isomer produced by UV light allowed transistor operation, meanwhile an open ring isomer produced by VIS light showed no drain current. As a result, a considerably high on/off ratio of 10² (10⁴ %) was achieved. These results indicate that this compound has two distinct properties: organic semiconductor and photochromism. We clarified that the drain current modulation can be attributed to the transformation in the π-conjugation system accompanied by photoisomerization, i.e., a phase transition between semiconductor-insulator was induced by light irradiation. Our finding demonstrates the potential to achieve high-performance opto-electrical organic devices including optical sensors, optical memory and photoswitching transistors.

Polymer/inorganic nanostructure composite materials are widely used, e.g. in solar cells, catalysis, chem/bio sensing and flexible electronics. Such applications do not necessarily require regular spacing between the nanoparticles, but rather it is important to either achieve a homogeneous blend of the two components, avoid phase separation and aggregation, or a precise spatial distribution of one within the other, e.g. along air-solid interfaces in porous catalyst materials. The negative impact of poor spatial control within polymer/nanostructure composites has been demonstrated for various applications.
Typically 3D composite materials of polymers and inorganic nanostructures are synthesized using empirically determined parameters to optimize a product&’s properties, without elucidating or developing predictive methodologies that address the mechanism of nanoparticle assembly or distribution in these compounds and subsequently their properties. They are synthesized by blending, electrostatically-binding or tethering nanostructures to existing polymer chains.
This presentation introduces a method to predictively produce composite materials with extreme homogeneity based on uniquely functionalized nanostructures. Using polymerizable surfactant molecules allows the disguise of the inorganic materials (similar to the approach viruses use to penetrate cell walls) and leads to a well-blended composite device. While this novel method is applicable to incorporate metal oxide as well as metal nanostructures into polymers, we here focus on the example of PMMA/ZnO films to demonstrate the compositional homogeneity of the material, as well as the tunability of optical properties.

Conventional infrared (IR) photodetectors has a limited application because of the high costs of epitaxial grown inorganic semiconductors, only suitable for small area applications. Recently, organic and/or solution-processed colloidal inorganic quantum dot (QD) materials for IR photodetectors have received a lot of attention because of their compatibility with low-cost large-area manufacturing. We demonstrated high gain and low dark current solution processed colloidal PbS quantum dots (QDs) infrared (IR) PIN photodetectors with IR sensitivity up to 1500 nm. The low dark current is due to the P-I-N structure with both electron and hole blockers. The high gain in our IR photodiodes is due to the enhancement of electron tunneling injection through the electron blocker (EB) under IR illumination resulting from a distorted electron blocking barrier in the presence of photo-generated holes trapped in the electron blocker. We further found that the trap states in the EB layer are generated by the Ag atoms penetrated in the EB layer during the thermal evaporation process. The resulting photodetectors have a high detectivity value of detectivity value of 7E13 Jones, which is even higher than that of a commercial InGaAs photodiode

We designed a plasmonic gap bar nanoantenna, which is in resonances with the emission band of silica-coated giant-CdSe/CdS quantum dots. This antenna is composed of two parallel gold nano-bars with 40 nm gap fabricated on top of a 20 nm thick gold film and glass substrate. Utilizing two-step e-beam lithography process, we have fabricated this antenna and integrated the single silica-coated giant quantum dots into the gap. The enhanced emission rate and emission polarization have been studied for quantum dots placed at different positions along the gap bar antennas. The use of silica shell suppressed the energy transfer quenching from giant quantum dots to metal. Therefore, the detected lifetime shortening by a factor of 8 is purely resulted from the enhancement of decay rate. Experimental measurements also show that the photoluminescence intensity with polarization perpendicular to the bar can be ~ 9 times stronger than along the bar, leading to a linear polarization degree of 0.8. Strong modulation of emission spectra has also been observed. Numerical simulations indicate that the strong linear polarization and the spectral modulation could be due to the emission coupling with the plasmonic waveguide modes.

Inorganic quantum dot (QD) nanoparticles have distinctive optical properties, including high photostability and quantum yield, as well as narrow symmetrical emission peaks that render them very valuable as probes for fluorescence-based detection methods. The versatility in QDs&’ emission wavelengths (from UV to IR) attributable to variations in their composition and size makes them intrinsically suitable for multiplexing in immunological diagnostics, cell trafficking analysis or cell identification via multiple markers. Realizing this potential requires the stable and specific functionalization of QDs with biological molecules, such as antibodies or other cell membrane-binding units, like lectins. In our group, a stable surface chemistry strategy has been developed to enable the passage of inorganic QDs to aqueous media and their further functionalization with bioactive molecules. The work here presented focus on the various biofunctionalization strategies currently under investigation and on their promising applications in immunohistochemistry, cell identification and cell receptor trafficking analysis. Antibody immobilization to CdSe/CdS/ZnS QD nanoparticles is carried out following oriented and cleavable linker approaches. The biofunctional nanoparticles are characterized regarding total protein content, biological recognition specificity and stability in biological media. Cell-biofunctional nanoparticle interactions are examined to determine the extent of non-specific interactions and the effects on cell metabolic activity under cell incubation conditions. Relevant applications of these nanoparticles illustrate on the advantages of biofunctional QDs as fluorescent nanoprobes for the inspection of biological features and processes.

Herein we present a totally polymeric one dimensional photonic crystal material processed from a photoactive block copolymer. The minority component of this block copolymer can be selectively degraded by electromagnetic radiation and then solvated by acetic acid whilst ensconced in a matrix capable of plastic deformation. The resulting material presents periodical order at the mesoscale that confers photonics properties to the polymeric structure. [1] We show that the formation of standing waves in the starting film is responsible for the periodicity observed in the multilayers, while the interplay with the structural ordering present in the segregated phase accounts for the fine details of the observed nanostructure.[2] Our analysis is supported both by experimental evidence, obtained from structural and optical characterization, and theoretical modeling of the spatial profile of light patterns, calculated using analytical and numerical tools. Several predictions based on this hypothesis are experimentally confirmed and the origin of all previous effects that remained unexplained is clarified under this assumption. These results constitute, as far as we know, the first demonstration of bulk patterning of block copolymers by the spatial modulation of induced photochemical effects at the few tens of nanometers length.[3] This method opens the opportunity to create a total polymeric porous structure with applications in optoelectronics and photonics.
[1] P. Zavala-Rivera, K. Channon, V. Nguyen, E. Sivaniah, D. Kabra, R. H. Friend, S. K. Nataraj, Shaheen A. Al-Muhtaseb, A. Hexemer, M. E. Calvo, H. Miguez. Nature Mater. 11, 2012, 53 - 57
[2] M. Anaya, M. E. Calvo, J. M. Luque Raigoacute;n, H. Míguez . J. Amer. Chem. Soc. 135, 2013, 7803-7806
[3] M.E. Calvo et al. submitted

Atomic layer deposition (ALD) has been widely used to fabricating ultra-thin films, but the atomic-level control is of increasing interest for its ability to tune the material properties of films during deposition process.
As an irreplaceable tool for high precision fabrication, multiple outer energy sources have been introduced into ALD system, such as plasma, ozone to further tune the material properties. It shows merits especially in freedom of precursor choice and reduced substrate temperatures.
Here we present a brand new technique, variable electric field-assisted atomic layer deposition (E-PEALD), which integrated with plasma, DC bias voltage and in-situ doping modules. Not only the structure defect and the crystal orientation, but also the electrical properties could be modulated by controlling the motion of precursor ions through changing the DC bias voltage. This E-PEALD approach would be a promising research filed for ALD application.

Combining unique properties of the semiconductor TiO₂ and carbon-based nano-materials including carbon nanotubes (CNTs) and graphenes leads to enhanced performance for many important applications such as photo-catalysis, solar cells, water splitting and nano-electronics. However, controlled deposition of thin conformal oxide films on CNTs and graphenes using atomic layer deposition (ALD) is still challenging as the early stages of nucleation and subsequent growth are not yet well understood. Here we adopted ALD to grow TiO₂ on multiwall nanotubes (MW-CNTs) and single layer graphenes (SLGs), and advanced transmission electron microscopy (TEM) to characterize the structure and chemistry of the ensemble [1,2]. For MW-CNTs, the effect of deposition conditions, including temperature (120°C-240°C), number of ALD cycles (20-750), and surface pre-treatment of the MW-CNTs with oxygen plasma, on morphology and crystallinity of TiO₂ has been systematically studied through high-resolution TEM imaging and electron energy loss spectroscopy (EELS). By tuning the deposition conditions, controllable nucleation and growth of TiO₂ on CNT can be achieved, resulting in various morphology and crystallinity. Coalesced anatase TiO₂ conforming onto the MW-CNTs can be achieved with a growth temperature at 200°C. Aberration-corrected TEM imaging at 120 keV on an atomic scale was able to reveal the structure of a reduced TiO_x at the interface at the early stage of nucleation at 200°C. For SLG, aberration-corrected TEM imaging operated at 80 keV combined with exit-wave reconstruction (EWR) using a focal series allows an atomic-scale study of TiO₂ nucleation on the SLG after 20 ALD cycles. Consequently, we can compare the nucleation and growth of TiO₂ on the two different carbon-based nano-materials.
[1] Zhang Y, Utke I, Michler J, Ilari G, Rossell MD, Erni R. Growth and characterization of CNT-TiO₂ heterostructures. Beilstein Journal of Nanotechnology. 2014;5:946-55.
[2] Zhang Y, Guerra-Nuñez C, Utke I, Michler J, Rossell MD, Erni R. Revealing early stage of nucleation and growth of ALD-deposited TiO₂ on MW-CNT. To be submitted.

ZnO film is considered a candidate material for various applications. Both improving the crystal quality and controlling the preferred orientations of ZnO films is necessary to obtain desirable properties. Producing ZnO films with high crystal quality could open up the possibility of producing ultraviolet light emitters [1], high-power transistors [2] and other optoelectronic devices. Recent attention has been focused on atomic layer deposition (ALD) as the preferred method to grow ZnO films [3-6]. Researches have been extensively investigated the influences of the substrate pretreatment [3], deposition temperature [4] and the purging or pulsing time [5] on properties of the obtained ZnO films for this technique. In 2002, Liu et al. revealed that higher quality epitaxial films could be obtained with a proper magnitude of the electric field assisted ALD [6]. However, the constant electric field is limited to the various precursors and surface chemical-reactions.
Here, we proposed a variable electric field-assisted ALD (E-ALD) technique and showed the correlation between electric field directions and structures of the obtained ZnO films. X-ray diffraction (XRD) and XPS spectra indicate that not only the crystal orientation, but also the structure defect could be modulated by changing the electric field directions during the precursor pulses. ZnO films with c-axis preferred orientation and the least oxygen vacancy defect were obtained when the holder electric polarities were positive and negative during the DEZn and H₂O pulses, respectively. This E-ALD approach would open a new way of achieving films with controlled electrical properties, preferred orientations and crystal qualities.
References
[1]Y. F. Li, B. Yao, R. Deng, B. H. Li, Z. Z. Zhang, C. X. Shan, D. X. Zhao, D. Z. Shen, A comparative study on electroluminescence from ZnO-based double heterojunction light emitting diodes grown on different lattice mismatch substrates, J. Alloys Compd. 575 (2013) 233-238.
[2]J. Yang, J. K. Park, S. Kim, W. Choi, S. Lee, H. Kim, Atomic-layer-deposited ZnO thin-film transistors with various gate dielectrics, Phys. Status Solidi A 209 (2012) 2087-2090.
[3]J. Lim, K. Shin, H. Kim, C. Lee, “Enhancement of ZnO nucleation in ZnO epitaxy by atomic layer epitaxy,” Thin solid film, 475, (2005) 256-261.
[4]N. Y. Yuan, S. Y. Wang, C. B. Tan, X. Q. Wang, G. G. Chen, J. N. Ding, “The influence of deposition temperature on growth mode, optical and mechanical properties of ZnO films prepared by the ALD method,” J. Cryst. Growth, 366, (2013) 43-46.
[5]P. Genevée, F. Donsanti, G. Renou, D. Lincot, “Study of the aluminum doping of zinc oxide films prepared by atomic layer deposition at low temperature,” Appl. Surf. Sci., 264, (2013) 464-469.
[6]C. H. Liu, R. P. H. Chang, Theoretical and experimental study of impact of electric field on the atomic layer epitaxy of ZnO on α-Al₂O₃ surface, J. Chem. Phys. 116 (2002) 8139-8143.

Luminescent materials find many applications in different fields such as phosphors for lighting, displays, X-ray monitoring and imaging, scintillators, bio-imaging and therapy. The development of syntheses and processing methods able to form new nanostructures and geometries seem particularly attracting because of the unique optical properties exhibited by materials at the nanoscale. The employment of nanoparticles in optical applications is an ideal strategy for minimizing the detrimental scattering of the emitted light, associated with the discrete form of the material, but a higher control over structure and emission maxima would be a crucial advantage in order to extend the possibilities to tailor materials for the final application.
Hafnium dioxide is a wide band-gap semiconductor which possesses outstanding properties in terms of very high thermal stability, remarkable chemical inertness, high density, and UV-visible transparency.
Here we report a strategy for the production of nearly spherical HfO₂ ultrasmall nano-dots obtained by a nonaqueous synthesis where the multifunctional role of rare earth doping is evidenced. On one hand, rare earth dopant ions activate the visible luminescence of the nanocrystals. On the other hand the incorporation of trivalent ions is shown to be suitable for the room temperature stabilization of the cubic polymorph of HfO₂, with potentially great benefits in the realization of new polycrystalline optical ceramics for scintillator applications with reduced birefringence.
Moreover the stabilization of colloidal suspensions of these nanoparticles may represent a suitable tool for the realization of photostable, Cd-free, highly viable and biocompatible dyes for bio-imaging and therapy.
It was also possible to incorporate the colloids into passive hosts such as amorphous polymers and oxides in the attempt to realize optical components where the very high workability of the host cooperates with the outstanding photostability of inorganic nano-phosphors. These systems might be suitable for the realization of new low-cost/solution-processed radiation detectors and scintillators, as well as for luminescent solar concentrators expressly designed for the photoconversion in harsh conditions of high energy radiation, from the UV to the X-rays range.

Raman scattering in graphene can be significantly enhanced through the coupling of metal nanostructures. Typically this is performed by the deposition of metal on graphene or by placing graphene on a surface with metallic nanostructures. Many of these approaches require patterning of graphene or the underlying substrate. Graphene-metal-nanostructure hybrids realized in this manner are promising as substrates for surface-enhanced Raman spectroscopy (SERS) for the detection of small amounts of analyte molecules. One of the major limitations in such cases is the inability to measure the Raman enhancement at the same location as a function of variation in particle size and / or density. Moreover, there is no direct possibility of tuning the strength of enhancement once the structures are prepared. Here we demonstrate the fabrication of graphene gold-nanoparticle (AuNP) hybrids by direct electrodeposition of gold on to graphene sheets. The size or density of the nanoparticles can be continuously increased through appropriate choice of electrochemical parameters. [1] This allows for a systematic investigation of Raman enhancement as a function of size and density of the nanoparticles at the same location. We observe that the Raman intensity can be significantly enhanced by varying predominantly the density of the nanoparticles. Most importantly we clearly show that the strength of Raman enhancement varies as a function of the frequency of the vibrational mode (D, G or 2D of graphene) and may be correlated with the plasmonic fingerprint of the attached AuNPs. On top of this electromagnetic enhancement, an additional chemical contribution can be deciphered through charge transfer from AuNPs on to graphene. Subsequently, we show that these devices can be efficiently utilized as SERS substrates for the detection of specifically bound non-resonant analyte molecules. The possibility to tune the degree of Raman enhancement paves way for a platform to design and engineer SERS substrates to optimize the detection of trace amounts of chemical species and biomolecules. Finally, we show that the same platform can be used for the identification of chemical functionalities present on graphene.
[1] L. Zuccaro, K. Kern, K. Balasubramanian, Adv. Funct. Mater. 24, 6348 (2014).