Silicon is a poor light emitter, unless reduced to nanoscale dimensions. Recently, synthesis of Si quantum dots and quantum rods has been demonstrated. This presentation will cover the synthesis of bright Si quantum dots with widely controlled diameter and bright Si quantum rods made using a colloidal approach. A synthesis of colloidal silicon (Si) nanocrystals, or quantum dots, with widely tunable average diameter, from less than 3 nm up to 90 nm and peak photoluminescence (PL) from visible wavelengths to the bulk band gap of Si at 1100 nm has been developed that relies on the high temperature (>1100C) decomposition of hydrogen silsesquioxane (HSQ). The Si quantum dots are crystalline with tunable size and uniform size distribution, and can be hydrosilylated with alkenes for stable and bright photoluminescence (PL). H-terminated Si nanocrystals isolated from the HSQ-derived product can also undergo room temperature hydrosilylation with bifunctional alkenes with distal polar moieties—ethyl-, methyl-ester or carboxylic acids—without the aid of light or added catalyst to yield Si nanocrystals with bright PL, dispersable in polar solvents, including water. In this case, a reaction mechanism is proposed in which ester or carboxylic acid groups facilitate direct nucleophilic attack of the highly curved Si surface of the nanocrystals by the alkene. Fluorescent Si nanorods can also be synthesized. In this case, the nanorods are produced by a ligand-controlled solution-liquid-solid (SLS) growth using tin (Sn) nanocrystal seeds. The nanorods are hydrosilylated with organic ligands to stabilize the PL and prevent oxidation with PL quantum yields of 4-5%.

The combination of nanoparticles with well-established thin-film technologies is a promising way to profit from the advantages of both technological approaches: Thin-film technologies based e.g. on silicon or organics on the one hand have proven to provide excellent devices on very large scales with extraordinary high material quality at low process temperatures. Nanoparticles on the other hand can be processed prior to device fabrication and thus open up the possibility to use higher temperatures than compatible with cheap substrate materials such as glass and plastic. Additionally the short diffusion length of defects towards the surface of nanoparticles is beneficial since defects can be be passivated there. The approach of combining materials with large absorption and suitable bandgaps like CuO, Cu2O or Cu2S prepared as nanoparticles with a thin-film matrix will benefit from both technologies.
One major challenge regarding the inclusion of nanoparticles is their dispersion for a well-controlled deposition. Mono-dispersion in solution is usually achieved by stabilizing the particles with a shell of organic ligands. This shell acts as a source of impurities and can lead to the generation of an insulating layer during the inclusion into matrix material and has hence to be removed prior to device fabrication. Plasma mitigated processes can be used to remove ligands and passivate the surface of nanoparticle absorbers.
In the present paper we present results on the plasma mediated ligand removal and surface passivation of copper sulfide and copper oxide nanoparticles. Radio frequency at 13.56 MHz is applied to generate plasmas in Argon and Hydrogen gases. Plasma parameters like gas pressure and in coupled power have been varied.
The influence of plasma parameters on the morphology of the particles has been studied using scanning electron microscopy. The treatment conditions were optimized to prevent any sintering or change of the shape of the particles.
Additionally, the removal of attached ligands is studied by measuring their specific infrared absorption with fourier transform infrared spectroscopy. Using optimized conditions the organic shell that was initially present around the nanoparticles was removed.
The opto-electronic quality of the particles before and after ligand removal and passivation was studied using photoluminescence spectroscopy. To study the influence of plasma treatments on the total absorption of nanoparticle layers photothermal deflection spectroscopy was applied. With this method the removal of ligands was verified by a reduced absorption in the near IR.
Solar cell devices based on an amorphous silicon p-i-n structure with nanoparticles embedded into the intrinsic layer have been prepared. The influence of the plasma treatment on solar cell performance has been studied.

Silicon quantum dots functioning as luminescent downshifters have the potential to become a simple means for enhancing silicon solar cell efficiencies. As a result of their indirect band gap, silicon quantum dots absorb high energy photons—light that cannot be effectively utilized by the solar cell—and emit the photons at lower energy, in the spectral range where the cell is more efficient. As successful luminescent downshifters, silicon quantum dots must possess high photon conversion efficiencies, or photoluminescent quantum yields; this has been achieved in the past by passivating the surface of silicon quantum dots produced in a non-thermal plasma with organic ligands through a post-synthesis, thermally-activated hydrosilylation process. This method, however, produces quantum dots with quantum yields that degrade upon exposure to ultraviolet light. This is a significant obstacle as it indicates that sunlight can diminish the effectiveness of the luminescent downshifting system, rendering its implementation impractical. To overcome this obstacle, silicon quantum dots are produced in a process known as photochemical hydrosilylation, where organic ligands are attached onto the quantum dot&’s surface using ultraviolet light, instead of heat. This work will show that not only do the resulting quantum dots possess equally high quantum yields, but also stability against ultraviolet irradiation. Furthermore, as a gentler technique, it offers the possibility of a gas-phase quantum dot synthesis, passivation, and deposition process, eliminating the use of solvents.
Acknowledgement: This work was supported primarily by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-0819885. Part of this work was carried out in the College of Science and Engineering Characterization Facility, University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC program.

Semiconductor quantum dots (QDs) constitute very promising candidates as light emitters for numerous applications in the field of biotechnology, such as cell labeling or other bioimaging techniques. For such applications, semiconductor QDs represent an attractive alternative to classic organic fluorophores as they exhibit a far superior photostability by several orders of magnitude and a higher brightness thanks to large absorption cross-sections. Within this family of materials, core-shell heterostructures such as CdSe/CdS QDs are especially of interest. In particular CdSe/CdS QDs with relatively thick CdS shells offer several properties essential to biolabeling, including high photoluminescence quantum yields, low blinking behavior and robustness towards aggressive environments. We recently developed a new, fast and very efficient method for the synthesis of such QDs, denoted ‘flash&’ CdSe/CdS, which can feature up to 20 monolayers of CdS after no more than 3 minutes of synthesis. These ‘flash&’ CdSe/CdS QDs show state-of-the-art optical properties (sharp emission spectra, high photoluminescence quantum yields, low blinking behavior), and the CdS shell thickness can be easily controlled thanks to the full chemical yield of the reaction. These QDs were encapsulated in silica nanoparticles through a water-in-oil microemulsion process, a technique that allows a high control on the morphology of the resulting QD@SiO2 nanoparticles. All the nanoparticles contain one single QD located in its center and the thickness of the silica shell can be varied from only a few nanometers up to several tens of nanometers. The silica matrix provided the QDs with enhanced colloidal stability in polar solvents, but also enhanced photo-physical and chemical stability under irradiation. More importantly, QD@SiO2 nanoparticles based on ‘flash&’ CdSe/CdS QDs fully retain their photoluminescence quantum yield even after more than a year of storage in water, whereas QD@SiO2 nanoparticles based on ‘classical&’ SILAR grown core-shell QDs typically lose their luminescence after a few weeks or even days. Thereafter, these ‘flash&’ CdSe/CdS@SiO2 nanoparticles have proven to be very promising nanoprobes for bioimaging techniques. Indeed, the rapid uptake of high levels of these nanoparticles by live cells was evidenced by confocal fluorescence microscopy. Furthermore, thanks to the high stability of their optical properties but also to their low toxicity after silica encapsulation, these nanoparticles are particularly appropriate for long term cell labeling and tracking. Thus, in this contribution we will report from the synthesis and characterization of these ‘flash&’ CdSe/CdS@SiO2, all the way to the study of their toxicity and their application to cell labeling.

Tina Zhang*, Mary Ann Go+, Vincent Daria+ and Antonio Tricoli*
*Nanotechnology Research Laboratory, Research School of Engineering,
College of Engineering and Computer Sciences, Australia National University,
Canberra, Australia
+John Curtin School of Medical Research, The Australian National University,
Canberra, ACT, Australia
Highly crystalline, pure TiO2 nanoparticles of tunable size were synthesized by scalable flame spray pyrolysis of organometallic precursor[1] resulting in up to 86wt% Anatase content. A novel protocol is presented for the controlled functionalization of these particles with fluorescein isothiocyanate (FITC), an important biomedical dye. The dissociation of water into hydrogen and an active hydroxyl group is first promoted at the particle surface via UV exposure at room temperature. The adsorbed hydroxyl group is subsequently reacted with the carboxylic group of amino acid L-Lysine to form covalently functionalized, amine-capped nanoparticles. The pH, stoichiometry and time of reaction were set to control the final surface density of active amine groups and degree of polylysine formation. Functionalization in excess L-lysine pH > 7 yielded large agglomerates (~ 3mu;m) of nanoparticles inside a polypeptide matrix of 5 - 20 nm thickness, while reducing the pH to 1.5 at the same reaction stoichiometry reduced the maximal agglomerate size to 1 mu;m and organic layer became irresolvable by TEM inspection. Under stoichiometric and sub-stoichiometric conditions, the agglomerate size reached 300 nm, indistinguishable from the hard agglomeration of the nanoparticles due to sintering during flame spray synthesis, while retaining active amine groups for further reaction. Subsequent reaction with FITC in aqueous solution at room temperature produced fluorescent nanoparticles with a surface coverage of up to 0.3 dye molecules per Ti-atom. The dye attachment proved sufficiently strong and able to survive ultra-sonication and low temperature drying. The resulting particles were suitable for use as neuron imaging tools, showing high fluorescence and sufficient dispersion to flow through 0.5 mu;m needles. Functionalized particles were suspended in intracellular fluid (pH 7.25), containing K-methylsulfate, KCl, HEPES, NaCl, Mg-ATP, Na-GTP, and Alexa Fluor 594, and injected into neurons of 350 mu;m thick coronal slices from 14-day-old C57BL/6 mouse. The wide band gap (3.2 eV) semiconductor characteristic of TiO2 was exploited for simultaneous imaging and in cell photo catalysis. The effect of inducing photocatalytic reactions in the neuron by electron-hole separation in the TiO2 nanoparticles was investigated. Additionally, the cytotoxicity of these novel nanocomposites was investigated showing good biocompatibility.
1.Tricoli, A., M. Righettoni, and S.E. Pratsinis, Minimal cross-sensitivity to humidity during ethanol detection by SnO2-TiO2 solid solutions. Nanotechnology, 2009. 20(31): p. 315502.

We report synthesis and optical properties of PbSe/PbS core-shell, PbSe/CdSe/PbS, and PbS/CdS/PbS core-barrier-shell colloidal quantum dots (QDs). Engineering barrier layers between inner core and outer shell of these structures are monitored by TEM and spectroscopic signatures, and their band structures are simulated using effective mass model. While electrons are delocalized over entire QD structures, holes are confined in either core or shell depending on their geometries. This is also affected by core and shell materials, where PbSe/CdSe/PbS causes more asymmetric distribution of excitons while PbS/CdS/PbS generates more symmetric distribution of excitons by stronger coupling between core and shell. Lifetime, quantum yield, and photoluminescence excitation spectroscopy suggest how to engineer core/barrier/shell nanostructures to control exciton dynamics, which will impact on optoelectric applications such as photovoltaics and photodetectors.

A systematic study of the temperature dependence of Cd-cation replacement in PbSe and PbS quantum dots reveals energetic and dynamical insights into this important process for creating stable infrared-active quantum dots. Through appropriate choice of conditions, we demonstrate the synthesis of PbSe/CdSe and PbS/CdS core/shell quantum dots featuring very thick shells (up to 2.5 nm) and discrete core-shell interfaces without alloying. In addition to core-based infrared emission, PbSe/CdSe quantum dots with shells thicker than ~2 nm exhibit measurable visible emission. Static and ultrafast time-resolved photoluminescence studies are interpreted using a simple effective-mass calculations to suggest that the visible emission can be ascribed to 1S-electron to 2S-hole transition that arises as a result of slow intraband cooling of holes excited in the quantum dots shell. Finally, nonlinear optical studies of dual-emitting PbSe/CdSe quantum dots reveals surprisingly efficient two-photon up-conversion, with effective two-photon cross sections 1-2 orders of magnitude greater than those observed in pure CdSe quantum dots.

Cationic exchange (CE) has become a widely used method for the formation of heteronanocrystals (HNC). It proved possible for example to create multiple dot-in-rod structures featuring a series of Ag2S discs or PbS dots inside a CdS rod.1 Moreover, it has been used most extensively to from PbX/CdX (X=S,Se,Te) dot-in-dots. Even if CE is often implemented, little is known about the actual mechanism of the reaction. Recently, a mechanism based on vacancy diffusion was put forward to account for the strong tendency of the CE process to from {111} interfaces in PbSe/CdSe HCN.2 This however raises the question as to how these vacancies are formed.
Here, we analyze the formation of PbS/CdS core/shell HCN by CE starting from PbS NC synthesized using two different methods, the main difference between both being the amount of excess surface Pb. For 6.8 nm PbS NC, with the Abel synthesis3 (PbS-1), a Pb:S ratio of 1.45 was found, which amounts to a full surface coverage by excess Pb, while with the Cademartiri synthesis4 (PbS-2), an excess of only 1.28 is obtained, indicating that these NC still have vacant excess Pb2+ sites on their surface.
In the case of PbS-1 NC, the extend of the CE reaction follows a trend that is very much in line with the expectations. Thicker CdS shells are obtained for longer exchange times, higher Cd:Pb ratios in the exchange bath and higher temperatures. Nevertheless, even using the most favorable exchange conditions, shell thicknesses of only 1 nm at the most are obtained. Opposite from this, in the case of PbS-2, there is a remarkable dependence on reaction time and Cd:Pb ratio. First, even at room temperature, a CdS shell with a thickness of asymp;0.2 nm is obtained within seconds, which only grows thicker at elevated temperature. Second, the exchange proceeds further for lower Cd:Pb ratios. In fact, at Cd:Pb ratios of 0.5 and 0.25, part of the PbS NC can be completely transformed into CdS, while at high Cd:Pb ratios, similar results as with PbS-1 particles are obtained, i.e., 1 nm thick shells.
These observations show that unoccupied excess Pb2+ sites at the PbS NC surface enhance CE reaction by providing (1) adsorption sites for dissolved Cd2+ species and (2) vacancies for the transfer of Pb2+ from the inside of the NC to the surface. Especially the latter point is well demonstrated by the anomalous dependence of exchange rate on the Cd:Pb ratio. When the amount of Cd is too low, obviously too little Cd is provided for CE to happen. However, when the amount of Cd is too high, adsorbed Cd2+ will block all empty surface sites, thus hampering the transfer of Pb2+ cations. By stressing the importance of the surface reaction, this result provides an essential missing link in the mechanistic understanding of CE reactions and may thus help to achieve better control over these processes and the resulting HCN.
1 JACS, 2012, 134 (12), 5484
2 Chem. Mater., 2012, 24 (2), 294
3 Chem. Mater. 2008, 20, 3794
4 J. Phys. Chem. B 2006, 110, 671

In semiconductors, heterostructure architectures enable modulation of electrical and optical properties in devices. However in nanoparticles (NPs), methods to create 2D quantum well heterostructures in spherical particles are not well-developed. Here, we demonstrate dual interface formation in NPs through cation exchange, creating epitaxial heterostructures within isotropic, spherical NPs. To achieve this, we perform a cation exchange reaction on copper sulfide NPs, converting them into a hexagonal zinc sulfide (ZnS). Interfaces symmetrically form between the {100} planes of copper sulfide and the {001} planes of ZnS from opposing sides of the NPs due to the similar sulfur sublattice shared by copper sulfide and ZnS. By controlling the reaction time, the copper sulfide region can be confined to a 2D layer 1 nm to 10 nm in width in the center of the NP forming a 2D quantum well-type structure. Upon full conversion, the ZnS nanoparticles become single crystalline and preserve the original shape and size of the initial copper sulfide nanoparticles. Additionally, cation exchange from copper sulfide to hexagonal cadmium sulfide (CdS) yields similar interface formation. However, the interface curvature for the copper sulfide-CdS NPs differs from that of the copper sulfide-ZnS NPs due to opposite interfacial strains. The smaller lattice constant of ZnS induces concave interfaces between copper sulfide and ZnS whereas the larger lattice constant of CdS creates convex interfaces between copper sulfide and CdS. These heterostructured NPs exhibit surface plasmon resonance due to the intrinsically high carrier concentration of the copper sulfide phase. The surface plasmon resonance can be tuned over a broad range of wavelengths from 1270 nm to 1800 nm by controlling the thickness of the 2D copper sulfide layer confined between either ZnS or CdS. These NPs with heterostructures might show unique charge transfer and photoluminescence since they build type-II energy band alignments. Our interface formation using chemical transformations may provide a new approach to creating heterostructures in semiconductor nanomaterials.

In binary semiconductor nanocrystals (e.g. CdSe, PbS) the optical and electronic properties can be precisely tuned by controlling the size. In ternary compounds like CuInS2, on the other hand, changing the stoichiometry adds a further degree of freedom, which enables to switch from p-type (Cu-rich) to n-type (In-rich) behavior. However, widely applied synthesis methods relying on the use of dodecanethiol (DDT) as the sulfur source and surface ligand do not allow for the independent control of composition and size of the nanocrystals. We carried out an in situ X-ray study of the nucleation and growth of CuInS2 nanocrystals using synchrotron radiation. SAXS measurements during the pre-nucleation stage indicate the auto-assembly of the precursors in a lamellar phase. The formation of this phase has direct influence on the growth kinetics of the nanocrystals. In any case the reaction proceeds through an initial copper-rich phase, while indium is subsequently incorporated into the nanocrystals.
Ligand exchange with small organic or inorganic ligands is a general strategy for reducing the barrier for charge transfer and transport imposed by initial insulating surface ligands. We present conductivity measurements on assemblies of CuInS2 nanocrystals as a function of surface ligands, showing a dramatic increase in conductivity when replacing DDT with tetrafluoroborate or sulfide ligands. As an alternative, we propose a novel synthesis method, which directly yields CuInS2 nanocrystals capped with very short ligands.
Finally, the use of CuInS2 nanocrystals in hybrid solar cells combined with P3HT and PCBM will be discussed. As compared to neat P3HT:PCBM devices a remarkable improvement of the efficiency is observed for P3HT:PCBM:CuInS2 blends of 1:1:0.5 wgt ratio. By means of light-induced electron spin resonance measurements we identify enhanced exciton dissociation and charge carrier generation to be at the origin of this behavior.

Current synthetic techniques for III-V colloidal quantum dots (QDs) lag substantially behind those for other similar materials such as CdSe. Colloidal III-V QDs are attractive as visible emitters for lighting and display technology (InP) or stable infrared fluorophores for deep tissue in-vivo imaging (InAs). However, these materials have proven more challenging to synthesize with the narrow size distributions that are readily obtained using II-VI and IV-VI materials such as CdSe and PbSe. The superior size distributions of II-VI and IV-VI materials can be explained by the size focusing that occurs as molecular precursors react and add to growing nanocrystals. In contrast, previous studies showed that the III-V precursors react quickly, and particles grow in the absence of molecular precursors.
Here we supplement previous reports of mechanistic studies and novel precursors for III-V synthesis with the development of additional group-V precursors for both InP and InAs that have reaction rate constants that span several orders of magnitude. We performed a systematic of the effect of rate constant on III-V QD growth. This systematic study illuminates differences between the effect of precursor reaction rates on InP growth and InAs growth.

Semiconductor colloidal nanocrystals (NCs) are promising materials in many research fields including biomedicine, biosensing, or optoelectronics.1,2 During the past years, an exceptional research effort has been accomplished in order to develop well-defined methods to obtain and characterize highly monodisperse and efficient NCs. However, a better knowledge about the nucleation events determining the final structure and properties of these NCs is still missing. This is of high importance to understand the final crystal structure and size and shape of the nanostructures, since the stable nuclei for the formation of these NCs is not necessarily identical to the small clusters formed in the first nucleation stages. Likely, these initial structures form other intermediate products that evolve to form the final NCs. Parameters such as initial precursors ratio, temperature and capping surface ligands are known to strongly influence the nucleation and growth pathways and thus, their influence in the nucleation process should be deeply studied.
CdTe represents a model system to study the early nucleation events and the influence of different reaction parameters, such as ligand type and concentration, since the energy difference between the cubic (zinc blende) and hexagonal (wurtzite) crystal structures is large enough to promote (by the hot injection method) the nucleation in the cubic phase followed by a growth in the wurtzite phase. As a result, cubic-wurtzite tetrapod CdTe NCs are observed. 3
The aim of this work is to study the nucleation events on CdTe NCs, with special attention to the influence of different reaction parameters, such as the nature and concentration of different phosphonic capping ligands. Optical measurements as well as X-ray diffraction experiments have been used to study the structure and optical properties of the CdTe NCs. Moreover, with the aim to investigate the dynamics and kinetics of NCs formation, and to overcome the limited time-resolution of these conventional techniques, we are developing a continuous flow reactor, which allows studying different reaction times. Its combination with either optical lasers or X-ray beam, allows to deeply study the first nucleation stages of the mentioned model system. From this knowledge, more complex systems could be studied.
1Gill, R.; Zayats, M.; Willner, I. Angew. Chem. Int. Ed. 2008, 7602-7625.
2Talapin, D.V.; Lee, J.S.; Kovalenko, M.V.; Shevchenko, E.V. Chem. Rev. 2010, 389-458.
3Manna, L.; Milliron D.J.; Meinsel, A.; Scher, E.C.; Alivistaos, A.P. Nature Materials, 2003, 382-385.

Lead sulfide (PbS) nanocrystals have a size-dependent, tunable bandgap in the infrared region, making them particularly interesting for photovoltaics, photodetectors, and infrared communication. In most of these applications it is beneficial to have monodisperse nanocrystals- whether to provide a flat energy landscape for charge transport or to ensure narrow absorbance / emission spectra. To date, the synthetic recipes for PbS nanocrystals have not been able to achieve the same degree of monodispersity as nanocrystals of CdSe or PbSe. In this work, we find that by controlling the stoichiometry of our lead and sulfur precursor species we can synthesize monodisperse PbS nanocrystals (size distribution standard deviation < 5% of mean diameter) over a wide range of sizes. Specifically, we employ a large excess of Pb precursor to achieve monodisperse nanocrystals from 4.5 - 8.0 nm in diameter, corresponding to absorbance peaks from 1000 - 1800 nm, or 1.25 - 0.70 eV. We hypothesize that the large Pb-to-S ratio delays the onset of Ostwald ripening, as indicated by the time evolution of the absorption linewidth for varying Pb concentrations. We observe absorbance peak half-width at half-max (HWHM) values as small as 20 meV, and obtain evidence that such numbers reflect an ensemble that is almost entirely homogeneously broadened. This degree of monodispersity enables accurate measurements of interparticle spacing and the formation of well-ordered nanocrystal superlattices. Moreover, the nanocrystals are air-stable, exhibiting no change in absorbance features for over three months.

The design and synthesis of functional II-VI and III-V colloidal semiconducting nanocrystals (NCs) have been extensively studied due to the ability to control their photophysical properties and their potential for replacement of traditional materials in many applications including optoelectronics and biological labeling. The properties of NCs, including their spectroscopic, reactivity, stability, and processability properties, can be controlled by changing their size, shape, and surface chemistry. For optoelectronic applications, it is desirable to use capping ligands that facilitate charge transfer across the NC interface, while maintaining the miscibility and solution processability of the material. Specifically, conjugated thiols have been shown to effectively quench CdSe photoluminescence and exhibit shorter photoluminescence lifetimes than that of TOPO, a common insulating capping ligand [1]. The functionalization of colloidal semiconducting nanocrystals has traditionally been limited to ligand exchange processes that often require a large excess of the desired capping ligand and long reaction times at elevated temperatures for often incomplete native ligand displacement. We have found it difficult to synthesize true hybrid materials via the functionalization of colloidal semiconducting nanocrystals with semiconducting polymers according to conventional methods, and as a consequence of these trials, we have recently developed a new method for a one-pot, in-situ functionalization of CdSe nanocrystals using aryl functionalized thiol ligands with a cleavable phosphonate moiety [2]. Semiconducting polymer ligand attachment via this novel method has been verified by NMR and XPS, and the spectroscopic properties of the resulting hybrid materials have also been investigated by steady state and time-resolved spectroscopies. This is a simple and versatile procedure that allows quantum dot functionalization where traditional ligand exchange processes prove to be difficult or unsuccessful.
[1] I. Liu, et al. J. Mater. Chem. 18, 675 (2008)
[2] K. A. Mazzio, et al. Chem. Commun. 49, 1321 (2013)

The continuous and increasing discussion regarding possibilities for energy harvesting, power efficiency and sustainability has led to an intensive (re)search for further development and optimization of energy conversion and storage. In many cases, utilization of nanomaterials is a promising way to improve efficiency and enhance the fields of application. This covers multiple areas such as photovoltaics, battery systems, and thermoelectrics. All of these applications are dealing with materials exhibiting specific optical and/or electronic properties and commonly, processing either as dry powders or as dispersion is required. As a matter of fact, nanoparticles are particularly suitable as they provide specific properties and can be easily transformed into dispersions for further processing.
The synthesis and processing of group IV nanoparticles from gas phase synthesis is studied with respect their practicability in the above mentioned fields of application. Microwave plasma synthesis is used as method of choice as it provides steep temperature gradients and electrostatic separation during particle nucleation and growth. Moreover, short residence time within the reaction zone enables for kinetic control and surprinsingly high dopant concentrations. This leads to the formation of spherical, highly crystalline and soft-agglomerated materials with controlled size and composition. The formation of specific silicon and germanium nanoparticles with adjustable particle size and dopant concentration will be discussed and few examples will be shown concerning their applicability in photovoltaics, thermoelectrics, and lithium ion batteries.

The ability to form crystalline group IV nanoparticles makes plasma synthesis an attractive production mechanism. While the formation of silicon nanoparticles in nonthermal plasmas is well known, the heating mechanism leading to their crystallization is poorly understood. Temperatures that are significantly higher than the gas temperature are required for crystallization of these materials to occur. The nanoparticle heating mechanism therefore remains one of the poorly understood aspects of the plasma synthesis technique.
In this study, we investigate the crystallization of nanoparticles using a tandem plasma configuration, characterizing both the nanoparticles and the plasma to obtain a comprehensive understanding of nanoparticle heating in the plasma. In situ measurement of nanoparticle temperature during plasma processes is difficult, but the nanoparticles themselves can serve as “thermometers”, as their crystallinity and surface will change depending on the heating they experience in the plasma.
Amorphous silicon nanoparticles with diameters of 3, 4 or 5 nm are formed in a low-power nonthermal upstream plasma, and injected directly into a second separate downstream plasma. This approach allows for the decoupling of nanoparticle synthesis and heating. Crystallization of the amorphous silicon nanoparticles is investigated as a function of the power used to maintain the second plasma. From ex-situ characterization of the nanoparticles using x-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM) we discovered a size-dependent threshold power for complete crystallization of the particles.
A combination of comprehensive plasma characterization with a computational nanoparticle heating model reveals the underlying physics leading to crystallization. Here we found that the nanoparticles reach temperatures as high as 750 to 850 K in the secondary plasma, which is well above the gas temperature and sufficient for complete nanoparticle crystallization. While we demonstrate this method of predicting nanoparticle temperature using silicon, the approach can be applied broadly to other plasma-synthesized nanomaterials.
This work was supported in part by the DOE Plasma Science Center for Predictive Control of Plasma Kinetics. Partial support by the NSF/DOE Partnership in Basic Plasma Science through grants DOE/DE- SC0002391 and NSF CBET-0903842 is acknowledged, which provided support for NK, MM and URK. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program.

Galvanic replacement reactions provide a simple and versatile route for producing hollow nanostructures with controllable pore structures and compositions. However, these reactions have previously been limited to the chemical transformation of metallic nanostructures. We extended galvanic replacement reaction to transformation of metal oxide nanocrystals and demonstrated that the reaction can be general in oxide systems. When manganese oxide (Mn3O4) nanocrystals were reacted with iron(II) perchlorate, hollow box-shaped nanocrystals of Mn3O4/γ-Fe2O3 ("nanoboxes") were produced. These nanoboxes ultimately transformed into hollow cagelike nanocrystals of γ-Fe2O3 ("nanocages"). The formation mechanism obeyed pin-hole corrosion in which localized dissolution of Mn3O4 occurred simultaneously with surface precipitation of γ-Fe2O3 through galvanic replacement reaction. The generality of this approach was demonstrated with other metal pairs, including Co3O4/SnO2 and Mn3O4/SnO2. Our approach is different from typical galvanic replacement reactions in terms of redox couple reaction between multivalent metallic ions, which can produce multimetallic ionic nanocrystals. Typical galvanic replacement reactions that have been applied for metallic systems used simple metal ions as reactant, producing metallic nanocrystals. The transformation enables precise control over the composition of metal species in hollow multimetallic oxide nanocrystals in low temperature (~90 C). Furthermore, the reaction imposed oxide nanocrystals porosity in facile and scalable manner. Because of their nonequilibrium compositions and hollow structures, these nanoboxes and nanocages exhibited good performance as anode materials for lithium ion batteries.

Owing to their exceptional physical properties, such as large Stokes shifts and efficient multiple exciton generation, PbSe nanorods are attractive for optoelectronic devices. Recently, single-crystal, homogeneous, PbSe nanorods, with aspect ratios from 1.5 to 12, were synthesized using a one pot, catalyst-free, solution synthesis method.^1,2 In this work, we demonstrate that the quantity of water in this reaction, added either intentionally or present as a contaminate, has a dramatic effect on the PbSe nanorod morphology and yield.³ If the amount of water in the reaction is not controlled, the small amount present in the nanorod precursors is enough to create large batch-to-batch irreproducibility. Therefore, it is critical to control the quantity of water in the reaction in order to control the nanorod morphology. When the precursors are carefully dried and water is intentionally added back into the reaction at concentrations from 0 to 204 mM, the nanorod aspect ratio can be precisely controlled from 1.1 to 10 and the yield from 1 to 14%.
We have found that water indirectly affects the nanorod morphology and yield by reacting with the tris(diethylamino)phosphine used in the reaction to form bis(diethylamido)phosphorus acid. Synthesizing bis(diethylamido)phosphorus acid by the hydrolysis of tris(diethylamino)phosphine and independently adding it back into the nanorod reaction, we have identified it as responsible for both the nanorod aspect ratio and yield variations. Furthermore, we have found that excess oleic acid in the reaction can also create bis(diethylamido)phosphorus acid from tris(diethylamino)phosphine. When both excess oleic acid and water are removed, the reaction slows and highly uniform, non-branching, nanorods are formed.
1. Koh, W.-k.; Bartnik, A.C.; Wise, F.W.; Murray, C.B. J. Am. Chem. Soc. 2010, 132, 3909.
2. Padilha, L.A.; Stewart, J.T.; Sandberg, R.L.; Bae, W.K.; Koh, W.-k.; Pietryga, J.M.; Klimov, V.I. Nano Lett. 2013,
13, 1092.
3. Boercker, J.E.; Foos, E.E.; Placencia, D.; Tischler, J.G. J. Am. Chem. Soc. 2013, DOI: 10.1021/ja404576j

Multicomponent copper chalcogenide-based, namely CuInS/Se2 (CIS), CuInGaS/Se2 (CIGS), Cu2ZnSnS4 (CZTS) and Cu2Zn(Sn1minus;xGex)S4 (CZTGS), have recently attracted a great deal of attention as direct band gap materials for the fabrication of high-efficiency photovoltaic devices. In particular, these compound (I-III-VI2 and I2-II-IV-VI4) semiconductor material possess high optical absorption coefficients (105 cm-1), high energy conversion efficiencies, offer good photostability against long-term radiation and cause less environmental problems due to their relatively low toxicity. Solution based methods are an attractive alternative to vacuum deposition processes due to their simplicity and cost effectiveness. We have developed a facile, low-cost method to synthesize these compound semiconductor nanocrystal with excellent monodispersity in shape, controlled stoichiometry and crystal phase by methods of colloidal chemistry. The as-synthesized nanocrystal are stabilized in the metastable phase (wurtzite) by optimizing the reaction condition such as ligand selection, precursor reactivity and reaction temperature.The crystal structure, shape and composition of the as-synthesized nanocrystal were investigated with transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray analysis (EDX).

Doping is extremely important for controlling the electronic conductivity of bulk semiconductors. However, very few examples exist where impurities that have been incorporated into colloidal semiconductor nanocrystals affect their electronic properties. Here we will discuss the challenges in this area and recent progress. In particular, we will describe an approach to lightly dope semiconductor nanocrystals with a controllable amount of electronic impurities. The physical characterization of these materials then shows that the addition of even one impurity per nanocrystal has a dramatic effect on their optical properties. Furthermore, studies of the electrical transport through films of these nanocrystals show complex behavior in the Fermi level as a function of dopant concentration. The results demonstrate that dopant behavior in nanocrystals is not as simple as one might expect. Thus, these experiments begin to reveal the properties of a new class of nanocrystal materials that may be important for future nanocrystal devices.

We introduce stoichiometric imbalance in lead chalcogenide nanocrystal (NC) thin film solids by thermal evaporation of excess lead or selenium atoms. Hall effect, capacitance-voltage, and field-effect transistor (FET) measurements show that the Fermi level and charge carrier type, concentration, and mobility in NC thin films may be precisely controlled through their stoichiometry. Unlike as-prepared NC films which may be unpredictably and unintentionally doped, enriching NC films in Pb or Se transforms lead chalcogenide NC thin film behavior into n-type and p-type, respectively. By controlling stoichiometry, we engineered the characteristics of lead-chalcogenide NC electronic and optoelectronic devices. In FETs, introducing excess Pb in PbSe NC semiconductor channels enhances electron mobilities upto 10 cm2/Vs. In Schottky solar cells, depositing excess Se increases the power conversion efficiency up to 3.9%. Finally, we extend methods to introduce stoichiometric imbalance using a solution-based, post-synthetic ionic layer deposition method, provide a simple, low-cost route to similarly high-performance devices.

Controlling the morphology and doping levels of nanocrystals can enable the observation of shape-dependent magnetic or optoelectronic properties as well as the emergence of new nanoscale physical phenomena. To date, the development of precise colloidal nanocrystal shape control strategies has focused on mostly-covalent materials such as CdSe, and a recipe to controllably extend these techniques to more ionic II-VI compounds is still lacking. Zinc oxide (ZnO) is an ionic wide band gap semiconductor with many optoelectronic applications, but morphological tunability and controlled doping of ZnO nanocrystals are both necessary conditions to their utilization in optoelectronic devices. Here we show for the first time a modular synthetic approach that exploits temporal control of individual reactant concentrations to achieve unprecedented morphological tunability of zinc oxide nanorods. We created this scheme by separating each active component of the synthesis into orthogonal precursor solutions and using an automated colloidal synthesis robot. We demonstrate that this unique approach to colloidal synthesis enables interrogation of reaction conditions that are not typically accessible using typical colloidal approaches and we expect that this synthetic design methodology will be generally applied to other functional ionic materials systems. We show exquisite control of the nanocrystal morphology and aspect ratios - the nanorods have diameters 6-12 nm, lengths 40-110 nm, and precisely controlled shapes. Finally, the flexibility of this approach enables us to controllably dope the nanocrystals with aluminum (Al3+) cations and achieve tunable IR absorption characteristics.

Cation exchange is a reversible chemical reaction widely used to create new materials by replacing one type of cation with another, usually provided from solution. We propose an atomistic model describing the mechanism of cation exchange by heterovalent cations in semiconductor nanocrystals based on theoretical investigations of silver impurities in CdSe. The model makes use of a small set of results obtained for silver-doped CdSe at zero temperature from density-functional theory, which were then used in dynamical simulations performed at finite temperature over time scales far beyond the reach of DFT. Our simulations span a wide range of Ag concentrations, from the light-doping regime well into the cation exchange regime, and are consistent with many experimentally reported aspects of both phenomena. Thus our model provides a single conceptual framework in which cation exchange and light doping can be understood as two endpoints. A striking and unexpected finding of our simulations is that the Coulomb interaction plays a central, but changing, role as the Ag concentration varies from light doping to cation exchange. For example, if the Coulomb interaction is artificially suppressed then cation exchange does not occur. In addition, our simulations suggest that Coulomb effects are also responsible for two unusual observations by Sahu et al. in silver-doped CdSe nanocrystals: 1) upon increasing the silver concentration, its dopant character changes from n-type to p-type, and 2) at low silver concentrations, the brightness of the band-edge photoemission is strongly enhanced compared to undoped CdSe.

A barrier to the integration of nanocrystals (NC) into devices lies on the relatively poor electrical performance of nanoparticulate films. Thus, achieving effective doping and strong electronic connectivity across NCs stand as major challenges, with dopant segregation, confinement and incomplete screening usually held responsible for such difficulties. Routes to overcome these problems may involve shaping the chemistry of the NC surface to reduce barriers for inter-NC charge transfer, or to modify the screening properties of the NCs to tune electrical levels. We have carried out density functional calculations on doped NCs and NC-networks made of group-IV and II-VI species to understand how surface chemistries can improve both the electronic activation of dopants and the electronic connectivity across NC thin films. We found that specific molecules adsorbed on NC surfaces may lead to adequate NC/molecule redox alignment, and become strong contenders to enhance the electronic resonance between adjacent NCs. Particularly interesting molecules are tetracyanoquinodimethane (TCNQ) derivatives that were found to introduce acceptor states close to the conduction band of Si-NC superlattices. These states are localized at the interstitialcies of the superlattice, thus enhancing the likelihood of excited electrons to hop between adjacent NCs. Core-shell structuring also stands as an alternative route to improve the electrical performance of NC solids. We found that heterostructured core-shell NCs can induce a charge transfer of carriers from a doped core towards the shell, bringing carriers to regions in space which are more likely to overlap with neighboring NCs. While this effect is weak when the misalignment across the core-shell density of states is small (ex. n-type doped SiGe core-shell NCs), we anticipate a strong shell localization of holes in p-doped SiGe and CdSe-CdTe NCs.

Colloidal dispersions of semiconductor nanocrystals (NCs) can be used as “inks” for low-cost, solution-based deposition of films for optoelectronic applications. A common feature of colloidal NCs is capping of the surface with organic ligands which sterically or electrostatically stabilize NCs in solution. However, long organic ligands hinder charge carrier transport of films fabricated from these NCs. Processes for ligand removal or exchange in solution or during film formation have been employed. Unfortunately, these techniques cannot be applied to Si, which is the most environmentally friendly and an extensively used semiconductor, because Si forms covalent bonds with organic ligands. We have recently developed a novel method to stabilize Si-NCs in solution without organic ligand passivation.[1] The strategy to attain solution dispersibility is heavy doping of B and P simultaneously on the surface of Si-NCs. Si-NCs with B and P doped shells are prepared by the following process. First, Si and phosphosilicate and borosilicate glasses are simultaneously sputter-deposited. The films are then annealed to grow B and P codoped Si-NCs in silicate glass matrices. By dissolving the matrices in hydrofluoric acid solution, Si-NCs with heavily B and P doped shells are extracted. The shell induces negative potential on the surface, which prevents agglomeration of NCs in polar solvents due to the electrostatic repulsion. One of the purposes of this work is to study the structure of heavily impurity-doped shells. X-ray photoelectron spectroscopy reveals existence of large amounts of non-oxidized P and B on the surface even after air-exposure of NCs for a long period. In Raman spectra, besides the TO phonon mode of Si crystal, signals assigned to B-P local modes in Si are observed. Combining these data with those of chemical reactivity of codoped Si-NCs in different kinds of acid, the structure of the shells is discussed. Another purpose of this work is to control the HOMO-LUMO gap of colloidal Si-NCs in an extended range by doping. The most important property of codoped Si-NCs is that a part of B and P are doped in the NCs core and electrically active. The HOMO-LUMO gap is thus determined by acceptor and donor states. As a result, codoped Si-NCs exhibit PL at energies much lower than those of undoped Si-NCs with comparable sizes. We show that the PL energy of codoped colloidal Si-NCs is tunable in a very wide range (0.85-1.8 eV) by controlling the size. Remarkably, the PL tunable range is extended below the bulk Si bandgap (1.12 eV). The third objective of this work is the development of NC films from the colloids. The films produced by drop casting the colloids exhibit stable PL in air and the spectra are almost identical to those of the colloids. The films are highly conductive (10^-6 S/cm) without any treatments after drop-casting. [1] J. Phys. Chem. C 116, 17969 (2012), J. Phys. Chem. C 117, 6807 (2013), J. Phys. Chem. C 117, 11850 (2013).

To obtain electronic or thermoelectric functionality from ink-deposited films of group-IV nanoparticles, a precise control of both n- and p-type doping is essential.
For the case where the films are subjected to post-deposition sintering or annealing to enhance their performance, we presented an extremely flexible and controllable method to dope the resulting films with most group-III and -V elements [1]. In contrast to conventional doping during the gas-phase synthesis of the nanoparticles, no toxic precursor gases or liquids are necessary. Rather, films of undoped silicon and germanium nanoparticles are subjected to a liquid containing the dopants as, e.g., phosphoric acid or gallium dissolved in 2% HNO₃. Oxide removal from the nanoparticles can be combined with this doping treatment by admixing traces of HF. Short-pulse laser-sintering of the dried films forms mesoporous semiconducting layers with electronically active dopant concentrations of up to 10²⁰ cm^-3. Here we use macroscopic in-plane conductivity and thermopower measurements, micro-Raman spectroscopy and laser-ablation inductively-coupled plasma mass spectrometry measurements to quantify the doping efficiency and present a detailed study of the influence of process parameters such as immersion time, temperature or nanoparticle diameter on the efficiency of laser-assisted wet-chemical doping. In the case of the model system arsenic in germanium, adsorption of arsenic species to the germanium nanoparticle surface is found to be the kinetically governing step in this wet-chemical doping process. Since grain growth is a by-product of laser-sintering, attempts are presented to limit grain growth using other annealing methods while simultaneously keeping doping and electrical performance high.
[1] B. Stoib et al., physica status solidi (a) 210, 153 (2013)

Doping has been an important means to tune the properties of silicon nanocrystals (Si NCs).1-3 It has been recently realized that the hyperdoping of Si NCs well above the solubility limit may enable the localized surface plasmon resonance (LSPR) of Si NCs.4,5 This leads to great interest in the investigation of hyperdoped Si NCs. We will show that Si NCs can be hyperdoped with both B and P in nonthermal plasma. The intriguing mechanism for the hyperdoping is actually related to kinetics, in which the probability of collision between a Si NC and a B/P atom, the binding energy of Si-B/P at the NC surface and impurity-induced stress play important roles. The differences between B and P in doping efficiency and dopant distribution can both be understood based on the kinetic model.
[1] M. Fujii, “Optical properties of intrinsic and shallow impurity-doped silicon nanocrystals,” in Silicon Nanocrystals: Fundamentals, Synthesis and Applications, L. Pavesi and R. Turan, Eds., pp. 43-68, Wiley-VCH, Weinheim, Germany, 2010.
[2] X. D. Pi, “Doping silicon nanocrystals with boron and phosphorus,” Journal of Nanomaterials 2012, 912903 (2012).
[3] R. N. Pereira, A. J. Almeida, A. R. Stegner, M. S. Brandt and H. Wiggers, “Exchange-coupled donor dimers in nanocrystal quantum dots,” Physical Review Letters 108, 126806 (2012).
[4] D. J. Rowe, J. S. Jeong, K. A. Mkhoyan, and U. R. Kortshagen, “Phosphorus-doped silicon nanocrystals exhibiting mid-infrared localized surface plasmon resonance,” Nano Letters 13, 1317 (2013).
[5] X. D. Pi and C. Delerue, “Tight-binding calculations of the optical response of optimally P-doped Si nanocrystals: a model for localized surface plasmon resonance,” Physical Review Letters 111, 177402 (2013).

We summarize recent advances in making solar cells based on solution-synthesized, solution-processed, inorganic nanoparticles that offer quantum-size-effect-tuned bandgaps for multi-junction cells.

Next generation photovoltaic technology has shifted toward solution-processable materials due to the inherent reduction in fabrication costs and freedom of deposition. Inorganic semiconductor nanomaterials can be synthesized and solution processed to form thin-film absorber layers for solar cells. Devices based on these materials are typically built on top of transparent conducting oxides (e.g. ITO, FTO) and then completed with an evaporated top metal contact. Under these pristine conditions, only the active layers are truly solution processed. Here, we report on a top-to-bottom solution processed solar cell by utilizing precursor solutions for each layer of a working solar cell device on non-conductive glass substrates. Cadmium chalcogenide nanocrystals are known for their solution processability and utility in device fabrication after a CdCl2 annealing treatment to promote both ligand removal and grain growth. Likewise, solution precursors for ITO and Au can be similarly annealed to produce high-quality films for use as electrodes in solar cells to replace conventional methods. Preliminary data show that under simulated one sun, all-solution processed devices produced an open-circuit voltage (Voc) of 580+24 mV, a short-circuit current (Jsc) of 1.61+0.4 mAcm-2, and an efficiency (eta;) of 0.53% while pristine contacts gave Voc=408+18 mV, Jsc=13.1+1.0 mAcm-2, and eta;= 2.1%. The solution processed contacts are characterized by SEM, UV/Vis, XRD, and four-point probe methods, and these results correlated with the overall PV device performance obtained from dark and light JV characterization.

Colloidal quantum dot solar cells (CQDSC) based on lead sulfide (PbS) have drawn much attention due to their ability to deliver a power efficiency of 8%. PbS quantum dots (QDs) exhibit a low and tunable band gap with a high absorption coefficient, thus establishing these QDs as a promising candidate for colloidal quantum dot (CQD) solar cells. CQDSCs consist of a compact/mesoporous TiO2 layer on a transparent conducting oxide (TCO), followed by a PbS QD layer, and finally a thermally evaporated metal which serves as a back contact. In these devices, it is important to control the band gap as well the band position of the QDs to efficiently inject electrons into TiO2 and holes into the metal electrode. However, in the QD layer, both the electron and hole mobility suffers due to the high band gap organic passivating layer around the QDs. The similar energy levels present within the PbS QDs also increase the probability of electron-hole recombination.
In this work, we have carried out experimental and theoretical studies of the effect of surface ligands on band positions in colloidal QDs. We have tuned the band alignment of the PbS QDs through the dipole moment of the passivating ligand. The dipole moment creates an induced electric field, which significantly alters the electronic properties of the individual QDs. The variation of the band gap and ionization potential of the same size PbS QDs treated with ligands consisting of differently functionalized thiophenol molecules were experimentally measured by absorption and photoelectron spectroscopy in air (PESA). Our results show that the ionization potential of PbS QDs, having a diameter of 3 nm, can be tuned from -5.5 eV to -4.8 eV by changing the ligand from nitrothiophenol to methylthiophenol, whereas the band gap varies from 1.25 eV to 1.37 eV, respectively. DFT calculations were performed to evaluate the dipole moment of the ligands, and a comparison of the calculated and experimental results shows that the ionization potential varies linearly with the dipole moment of the ligand. CQDSCs were fabricated and tested using the ligand-modified quantum dots. We have also considered another approach to tune the band gap as well as the band alignment by tuning the composition of ternary PbSe(x)S(1-x). The results show a correlation between band alignment and solar cell performance metrics. We will discuss how the charge mobility and total power conversion efficiency of the solar cell is affected by band alignment.

Quantum dot solar cells from colloidal, inorganic nanocrystals are an emerging PV technology with high potential and have reached a 7.0% independently-verified power conversion efficiency employing a heterojunction device architecture using a wide-band gap metal oxide (ZnO or TiO2), ~400 nm thick 1.3 eV PbS quantum dot layer, and a thin hole-transport layer, MoOx. Further improvement in device efficiency will require understanding of interfacial energetics and kinetics and how photogenerated carriers are collected, either through drift or diffusion currents. A collaborative research endeavor involving institutions of the Center for Advanced Solar Photophysics EFRC was conducted to measure spatially resolved potentials and electric fields across the device stack using scanning Kelvin probe microscopy (SKPM). SKPM uses a conductive AFM probe to directly map surface potentials of a sample (or in our case, a working device cross-section). As the tip scans over the surface, a feedback system nullifies the contact potential difference between tip and sample by applying a DC voltage to the tip, thus providing a spatially resolved work function map with 10 mV energy resolution. Scanning of cross-sectioned ZnO/PbS heterojunction solar cells as a function of applied bias and illumination was accomplished to obtain operating potential and electric field profiles to characterize interfacial energy band alignments and drift/diffusion regions across the device. Initial studies focused on mapping device cross-sections with varying thicknesses of the PbS layer to measure the width of a depletion region formed between ZnO and PbS. It was determined that the depletion width is independent of applied bias and PbS layer thickness, which are not well described by the traditional p-n model applied to the ZnO/PbS heterojunction. Furthermore, capacitance voltage measurements on ZnO/PbS cross-sections show a bias-independent capacitance, emphasizing the lack of formation of a depletion region. Our results are more readily explained by an n-i-n-like energy band alignment, analogous to a p-i-n amorphous silicon solar cell. Here, the intrinsic PbS layer is sandwiched between two n-type semiconductors, ZnO and MoOx, with MoOx possessing a deeper work function than ZnO, where the work function offset between the contacts produces an electric field across the light absorbing PbS QD layer. Replacement of the PbS layer with intrinsic amorphous Si shows similar potential profiles throughout the stack, thus providing strong evidence for the true operating mechanism for these solar cells. Trends in SKPM profiles and current-voltage characteristics were similar for both these structures using either PbS or amorphous silicon as the active layer. These results provide key insight into the optimization of future devices, whereby selection of contacts are crucial to create a strong electric field within the PbS layer for better charge extraction of photogenerated carriers.

Nanocrystal quantum dots (NQDs) have been extensively investigated lately with a hope for realizing next-generation (low-cost, high-efficiency) solar cells. A noticeable photoconversion efficiency of nanocrystal quantum dot solar cells has been achieved, more than 7 % (Nat. Nanotech. 2012) and carrier collection efficiencies over 100 % was demonstrated in working solar cell (Science 2011). Achieving higher efficiencies with enhanced stability in NQD solar cells require understanding and controlling over the surface of NQDs. In this presentation, our recent efforts on a fundamental aspects of efficient nanocrystal quantum dots-based photovoltaics focusing on the surface chemistry of NQDs will be discussed. Shape originated size-dependent stability, defect controlled fabrication processes, and chemical approaches of controlling the carriers will be presented.

The physical properties of a semiconductor are determined both by the properties of the intrinsic material and by the presence of impurities and defects. Control of these defects, which reside either inside or on the surface (abrupt termination of surface bonds) of the semiconductor, determines the performance of semiconductor devices. This is particularly important for quantum dot (QD) solar cells, where the presence of surface defects induces recombination of photogenerated carriers and degrade three important parameters that determine the power conversion efficiency (eta;) of the cell: short-circuit current (J_SC), open-circuit voltage (V_OC) and fill factor (FF).
Here we describe a method to improve the performance of lead sulfide (PbS) QD solar cells through exposure of the QD film to a solution containing metal salts. The halide ions, such as chloride ion, passivate surface lead sites whereas alkali metal ions passivate surface chalcogen sites (or mend Pb vacancies). The simultaneous introduction of both positive and negative ion maintains charge neutrality of QDs. The QD films are exposed to a metal salt solution prior to the ligand exchange procedure, where metal cations and halide anions with small ionic radius have high probability of reaching the QD surface to eliminate surface recombination sites. Compared to the control device fabricated using only the ligand exchange procedure, devices with metal salt treatment show an increased J_SC and FF, accompanied by a distinct reduction in a crossover between light and dark J-V characteristics.

Colloidal quantum dot solar cells are promising due to the rapid increase in efficiency over the past years. Most of those improvements can be attributed to enhanced control of the quantum dot surface and film deposition. Meanwhile, the metal oxide component has received considerably less attention. By doping zinc oxide we show that the metal oxide can be equally important. Nitrogen doping reduces the carrier concentration in ZnO by up to two orders of magnitude, and we show that this reduction can be used to supress interfacial recombination in ZnO/PbS solar cells. Furthermore we show that magnesium doping raises the ZnO acceptor levels. Using large-bandgap PbSe quantum dots as the active layer in a ZnO/PbSe solar cell, we could dramatically increase the open-circuit voltage by magnesium doping ZnO. We identify that the sub-bandgap states of ZnO pose a fundamental challenge in designing colloidal quantum dot solar cells and that metal oxides with a cleaner bandgap will lead to more efficient devices.

Tomohiro Nozaki*, Yi Ding, and Ryan Gresback
Department of Mechanical Sciences and Engineering, Tokyo Institute of Technology
2-12-1 Ookayama, Meguro-ku, Tokyo Japan 152-8550.
*Email: [email protected]
Silicon nanocrystals (SiNCs) have drawn keen attention as it has several novel properties mainly resulting from the size-dependent quantum confinement effect such as tunability of optical emission and absorption features. This makes it attractive for device applications including solar cells, transistors, light emitting diodes. Different forms of silicon nanocrystals have been developed to meet the requirements of the vast application fields. Among them, freestanding silicon nanocrystals can form inks allowing for solution processing, which is seen as a promising route to reducing the cost of the semiconductor device manufacturing.
In this paper, highly crystalline SiNCs with mean size of smaller than 6 nm were synthesized from chlorinated silicon precursor (SiCl4) using flow-type, non-thermal VHF (70 MHz) plasma reactor [1,2]. SiNCs, as an electron acceptor, were blended with p-type organic semiconductor materials (P3HT or PTB7) using appropriate solvent, producing stable SiNCs containing inks [3-4]. After spin casting on ITO substrate and evaporation of metal contact, it forms bulk heterojunction (BHJ) type organic/inorganic hybrid solar cells with isolated SiNCs embedded in polymer matrix [5]. Performance was evaluated under AM1.5 solar simulator, resulting short circuit current of 10 mA/cm2, open circuit voltage of 0.55 V, and power conversion efficiency of 2% by now. The electronic band structure and its tunability by size, surface termination; -Cl (as produced), -H (HF treatment), -O (controlled oxidation), and controlled phosphorus light doping is the key to maximizing the power conversion capability of the device. Examining the performance of those devices would offer valuable insights into the electronic band structure of SiNCs and their tunability. Successful fabrication of highly efficient and inexpensive solar cell with SiNCs is the important goal to meet the future energy demand.
[1] L. Mangolini, E. Thimsen, and U. Kortshagen: Nano Lett. 5 (2005) 655.
[2] R. Gresback, T. Nozaki, and K. Okazaki: Nanotechnology 22 (2011) 305605.
[3] R. Gresback, Y. Murakami, Y. Ding, R. Yamada, K. Okazaki and T Nozaki: Langumuir, 29(6), 1802-1807, 2013.
[4] C. Y. Liu, Z. C. Holman, and U. R. Kortshagen: Nano Lett. 9 (2009) 449.
[5] Y Ding, R Gresback, R Yamada, K Okazaki, T Nozaki: Jpn Jounal of Applied Physics, 52, in press, 2013.

The use of inorganic nanoparticles as acceptor materials in photovoltaic devices has seen promising increases in efficiency over recent years, with current records using P3HT as a donor polymer rivaling traditional all organic systems. Using nanoparticles as a replacement for organic acceptors has a variety of advantages including increased mobility, chemical stability, cost of synthesis and the ability of the acceptor to also absorb light alongside the donor polymer. The ability to absorb light is key to increasing photocurrents generated by photovoltaics by covering more of the solar spectrum but also we have recently shown the hole transfer process from the inorganic component to polymer to be more efficient than the converse electron transfer process.(1)
One of the major challenges with these so called hybrid solar cells is the difficulty with processing both the inorganic and organic components from a common solvent. This problem is traditionally overcome by the use of solublising capping ligands bound to the surface of the nanoparticles. These ligands allow the materials to be solution processed together, but they also directly inhibit charge generation. We reported an in-situ method for the production of the inorganic component within the polymer after co-deposition by utilising the decomposition of metal xanthate complexes.(2) This allows for a one-pot ligand free generation of hybrid heterojunctions and has been shown to give superior charge generation and higher power conversion efficiency in photovoltaic devices when compared to an equivalent ex-situ capped nanoparticle system.(3)
As well as a ligand free heterojunction the use of xanthate precursors also opens up the possibility of synthesising a variety of sulfide materials that have not previously been fully explored, including several relatively non-toxic materials in comparison to the current materials of choice, mainly cadmium sulfide. We present the synthesis of non-toxic heterojunctions of bismuth sulfide and antimony sulfide for use in hybrid photovoltaics. The control of the morphology of these heterojunctions is studied by tuning of the xanthate precusors and through varying processing parameters. These heterojunctions are characterised using a variety of techniques including SEM, TEM and XRD and spectroscopic studies of charge photogeneration using transient absorption spectroscopy (TAS) are used along with the fabrication of photovoltaic devices.
(1) Dowland, S. A.; Reynolds, L. X.; MacLachlan, A.; Cappel, U. B.; Haque, S. A. Journal of Materials Chemistry A 2013, 1, 13896.
(2) Leventis, H. C.; King, S. P.; Sudlow, A.; Hill, M. S.; Molloy, K. C.; Haque, S. A. Nano Letters 2010, 10, 1253.
(3) Reynolds, L. X.; Lutz, T.; Dowland, S.; MacLachlan, A.; King, S.; Haque, S. A. Nanoscale 2012, 4, 1561.

Colloidal quantum dots (QD), also known as semiconductor nanocrystals, are a promising material platform for solution-processable optoelectronic devices, such as solar cells, light-emitting diodes, thermoelectric modules, and flexible electronics. While the diffusion of singlet and triplet excitons in organic semiconductors has been studied extensively, exciton diffusion in colloidal QD solids remains largely unexplored. In this talk, I will detail my group&’s efforts to obtain a deeper understanding of excitonic energy transport in colloidal QD materials. These studies include spectrally-resolved transient photoluminescence spectroscopy, transient photoluminescence quenching, time-resolved optical imaging, and kinetic Monte Carlo simulation. We find that surface chemistry and energetic disorder play a central role in regulating exciton transport, and apply this understanding to control exciton movement in colloidal QD assemblies.

Colloidal nanostructures are under intense research for a number of optoelectronic applications including photovoltaics, light emitting diodes, lasers, photodetectors, etc. Recently, quasi-1D nanorods have excited much interest due to their enhanced multiple exciting generation[1] and ability to suppress Auger recombination.[2] 1D semiconductors also exhibit polarization sensitive properties such as polarized emissions and pronounced anisotropic absorption. This is often assigned to either quantum confinement effects or the influence of dielectric contrast. However, there have been no studies of semiconductor nanorods in the strongly confined regime.
We present measurements of the degree of absorption anisotropy in PbSe nanorods as a function of aspect ratio.[3] Our degenerate ultrafast pump-probe spectroscopic technique exploits the polarization memory effect to measure anisotropic absorption in randomly oriented ensembles of nanorods. We measure the absorption cross-sections of the 1S excitonic absorpotion in PbSe nanorods for parallel and perpendicular relative pump-probe polarizations, and compute the anisotropy in the absorption cross-section. We explore nanorod aspect ratios between 1 and 10. We find that the absorption anisotropy increases upon elongation from aspect ratio 1, where absorption is isotropic, to aspect ratio 4, above which the anisotropy reaches a near constant value near 0.3. Though the nanorod dimensions place them in the strong confinement regime, our observations can be described classically by considering the effect of dielectric contrast on the local fields on an ellipsoidal nanostructure. These observations are influenced by orientational averaging in the random solution ensembles. Based on our classical electrodynamics model, we predict absorption anisotropy >0.9 for aligned nanorods. Further we predict an absorption cross-section >10x what would be observed in nanocrystals of the same volume for a film of aligned nanorods with aspect ratio >4. These results show that the polarization sensitivity of the lowest exciton absorption feature in PbSe nanorods is dominated by dielectric contrast and not quantum confinement. Our results imply that the dielectric constant between semiconductor nanorods and the surrounding medium can be used to influence the optoelectronic properties of nanorods, including phonon modes,[4] multiple exciton generation, and Auger recombination, which all depend on Coulombic interaction and may be sensitive to local fields.
1 - P.D. Cunningham, et al. “Enhanced Multiple Exciton Generation in Quasi-One-Dimensional Semiconductors” Nano Lett 11 3476 (2011)
2 - L.A. Padilha, et al. “Aspect Ratio Dependence of Auger Recombination and Carrier Multiplication in PbSe Nanorods” Nano Lett 13 1092 (2013)
3 - P.D. Cunningham, et al. “Anisotropic Absorption in PbSe Nanorods” submitted to ACS Nano
4 - B.-R. Hyun, et al. “Far-Infrared Absorption of PbSe Nanorods” Nano Lett 11 2786 (2011)

We investigate the size-dependence of the photo-induced electronic transitions in colloidal β-SiC quantum dots (QDs), by analyzing their absorption and emission spectra. β-SiC QDs are synthesized by photo-assisted electrochemical corrosion of bulk powders. We separate fractions through centrifugation and sieving, then perform detailed size analysis of the QD suspensions using transmission electron microscopy (TEM) and dynamic light scattering (DLS).
Our results confirm quantum confinement in β-SiC quantum dots. We observe a correlation between particle size and absorption edge, as well as between particle size and position of the emission spectrum. Large QDs exhibit absorption edges slightly above the bulk value of 2.2 eV, while small QDs exhibit a clear blue shift of the absorption edge, which increases up to 3.5 eV. Ultra-small QDs exhibit additional absorption edges with an onset at 4 eV shifting to 6 eV for the smallest QDs. Hence our experiments detail these features, which had been predicted in previous theoretical studies.
Further, to utilize the potential of these QDs in sensing and biological applications, we have developed simple protocols for reliably tailoring their surfaces with different functional groups. We begin by covalently attaching primary amines as the base. The amine terminations are then converted to amine/carboxylate (-NH2/COOH-), amine/phosphonate (-NH2/PO2CH3), and amine/thiolate (-NH2/SH). TEM (aggregation), fluoresceamine assay test and FTIR confirm the success of the surface modification scheme.

Crystalline silicon nanoparticles (SiNPs) are under intense investigation as they combine the unique features of Si at the nanoscale (e.g. wavelength tunable light emission [1], multiple exciton generation [2], and doping [3]) with the versatile and inexpensive device fabrication associated with nanoparticle (NP) processing [4]. Due to the small dimensions, the large surface shell and species chemically/physically attached to the NP, interface- and surface-related phenomena eventually dominate the optical response of SiNPs.
Here, temperature-dependent steady-state and time-resolved photoluminescence (PL) and absolute quantum yield (QY) are used to study gas-phase grown SiNPs (3.0nm) surface-functionalized with 1-dodecene [4]. Before functionalization, the SiNPs (H-terminated) display a single PL component (E1) at 1.67eV, resulting from recombination of photogenerated electrons and holes at the crystalline core with QY~0.03. After functionalization, the emission spectra reveal E1 with a significant enhancement of the QY to ~0.24. Both the H-terminated and organic-functionalized SiNPs were exposed to air, yielding the formation of a native oxide shell (~0.3 nm), resulting in major changes in the emission. The E1 component is observed after oxidation for both types of SiNPs. Another PL component (E2) at ~2.25 and ~1.91eV is detected after oxidation of the H-terminated and 1-dodecene-functionalized NPs, respectively. From steady-state and time-resolved PL we conclude that E2 results from donor-acceptor recombination within oxide shell states. Selective excitation spectroscopy showed that E2 is excited through core states of the NPs, proving that core-to-shell energy/charge transfer takes place. The QY of the surface-oxidized NPs (~0.07) is higher than that of H-terminated NPs, whereas the oxidized 1-dodecene-functionalized NPs display the same QY (~0.24) as measured before oxidation. For surface-oxidized NPs without organic-functionalization, the oxide shell affects the core-related E1 component by hindering inter-NP charge transfer, resulting in a higher QY for surface-oxidized SiNPs when compared to H-terminated NPs. For the organic-capped NPs the QY is independent of the presence of an oxide shell and is substantially higher than that of non-functionalized NPs, suggesting that 1-dodecene promotes a more efficient surface passivation even after natural oxidation. From low-temperature time-resolved PL, the radiative transition probabilities as a function of surface type and air exposure will be determined, and energy diagrams depicting the more probable energy/charge transfer paths will be drawn.
1. A Gupta, MT Swihart, H Wiggers, Adv. Funct. Mater. 19, 696 (2009)
2. MC Beard, KP Knutsen, P Yu, J.M. Luther, Q Song, WK Metzger, RJ Ellingson, AJ Nozik, Nano Lett. 7, 2506 (2007)
3. AR Stegner, RN Pereira, R Lechner, K Klein, H Wiggers, M Stutzmann, MS Brandt, Phys. Rev. B 80, 165326 (2009)
4. U Kortshagen, J. Phys. D: Appl. Phys. 42, 113001 (2009)

Silicon nanocrystals have a great interest due to their excellent size-tunable optical and electronic properties which made them a promising material especially for photovoltaic and optoelectronic applications. Size dependency of Si nanocrystal plays an important role on transport properties of charge carriers which is a critical on the performance of device. Therefore, understanding the origin of carrier dynamics in Si nanocrystals on ultrafast time scales following optical injection and its dependence on nanocrystal size distribution is extremely important for designing devices with maximum efficiency.
In this respect, we report the direct imaging of carrier dynamics of Si nanocrystals by time-resolved photoemission electron microscopy (TR-PEEM) using femtosecond laser pulses. The pump-probe method with a femtosecond temporal resolution is used for time evolution of photogenerated electrons and then the PEEM which has a 40 nm spatial resolution captures the dynamics of system. We performed the first ultrafast temporal and ultrasmall spatial resolved measurements to observe carrier lifetime in Si nanocrystals deposited on a Si substrate. Si naocrystals were prepared via very high frequency (VHF) chemical vapour deposition (CVD) systems. The size of the fabricated Si nanorcystals are ranges from 3 nm to 20 nm controlled by both the flux of Silane and the plasma power.
The relaxation time of photoexcited electrons analyzed by the direct imaging from TR-PEEM is different for each Si nanocrystals with nanometer size in the timescale of femtosecond. In particular we clearly observed different relaxation times in sub-ps to ps ranges from different size of isolated nanocrystals and also from clusters which then enable us to determine the influence of size on carrier relaxations. Moreover, our analyses demonstrate that the TR-PEEM is an effective tool for direct imaging of the relaxation time of photocarriers in nano-scale range with a fs resolution.

A theory of photo- and dark-band conductivities in semiconductor supercrystals consisting of nanocrystals is developed by assuming scattering by structural defects in the supercrystals [1]. A new proposed mechanism of photoexcitation, which is triggered by an efficient Auger ionization of charged nanocrystals, provides explanation for the measured photocurrent being 2minus;3 orders of magnitude larger than the dark current. For dark conductivity, the metalminus;insulator transitions and temperature dependence of mobility in the metal phase are considered.
[1] A. Shabaev, Al. L. Efros, and A. L. Efros, Nano Lett. 13, 5454 (2013)

Trions are the simplest form of multicarrier excitonic states in semiconductor nanocrystal quantum dots (QDs) that comprises a neutral exciton (X⁰) and either an extra electron (negative trion, X^-) or a hole (positive trion, X⁺). The presence of the extra charge carrier induces significant changes in both radiative and nonradiative decay dynamics in QDs. An increase in in the number of possible recombination channels in trions leads to the increase in radiative decay rates of both X^- and X⁺+ ideally by a factor of two compared to X⁰. Furthermore, the presence of an extra carrier also opens a nonradiative channel via the Auger process, which is orders of magnitude faster than radiative decay, and therefore, becomes a dominant decay mechanism of charged excitons. It is well known that very low quantum yield (QY) of charged excitons is responsible for photoluminescence (PL) intermittency often observed in single-dot PL measurements. Nonradiative loss by trion Auger recombination also complicates a number of applications of QDs, especially in lasers and light-emitting diodes (LEDs). In particular, our recent study (Bae, et al., Nature Comm. 2013) has revealed that charged states resulting from imbalance between electron and hole injection can have a deleterious effect on LED performance and specifically lead to the drop in the external quantum efficiency (EQE) which progressively decreases with increasing the driving current. The latter effect is usually referred to as EQE “droop” or “roll-off”.
Here, we present systematic spectroscopic studies of nonradiative Auger recombination of X^- and X⁺ for three QD samples that differ in chemical composition and/or energy-band structure. We found that Auger decay rates of X^- and X⁺ are similar for core-only PbSe QDs that feature a mirror symmetry between the conduction and valence bands. This symmetry between X^- and X⁺ decay channels can be distorted in CdSe/ZnS QDs. In these nanostructures, the spectral density of the valence band states is much higher than that of the conduction band states, which facilitates the X⁺ decay pathway. For example, in QDs with a large band gap (E_g = 2.34 eV) the lifetime of X⁺ is twice as short as that of X^- . The asymmetry of Auger decay rates between X^- and X⁺ is further enhanced in thick-shell CdSe/CdS QDs. In these structures, the hole is confined within the core, while the electron is delocalized over the entire QD, which leads to an additional reduction in the X^- Auger decay rate. In single-dot spectroscopic studies of thick-shell QDs, we are able to identify PL bands of neutral excitons and both types of trions (X^- and X⁺). We found that, while X⁺ PL shows a short lifetime (1-2 ns) and a low QY (< 5 %), X^- PL features an extended lifetime (up to ~10 ns) and a high QY (up to ~ 60% of X⁰ QY) due to strong suppression of X^- Auger decay channel.

Magnetic anisotropy critically conditions the utility of magnetic nanocrystals (NCs) in new nanomagnetism technologies [1-3]. In this study, we investigated the electron magnetic resonance of ensembles of randomly distributed NCs of CdSe capped with a shell of organic ligands [4]. We measured the magnetic anisotropy in different random ensembles of CdSe NCs by recording magnetic resonance spectra for various orientations of the external magnetic field. We observe amplitudes of angular variation of resonant magnetic field of up to 350 mT. Moreover, the observed angular dependencies of resonant field are different for apparently similar CdSe NC ensembles, and the largest magnetic anisotropies observed are close to values reported for ensembles of magnetic nanoparticles organized in well-defined anisotropic forms [5,6]. Since the NCs in our ensembles are randomly distributed and randomly oriented, the observation of large magnetic anisotropies is rather unexpected. We show that our observations can be quantitatively explained if we consider the magnetic dipole-dipole interaction between dipoles that are randomly distributed over the surface of each NC in the ensemble. The surface dipoles may originate from dangling bonds on surface sites that are not passivated by ligands [7]. In our model, we consider a small number of dipoles on the surface of each NC that depends on the coverage of the NC surface with ligands, which in turn follows a normal distribution. From our calculations, we find that for different random ensembles of NCs the strength of the magnetic anisotropy induced by dipole-dipole interactions may take values spanning four orders of magnitude, depending on the specific arrangement of the NCs in the ensemble and the specific distribution of the surface dipoles in each NC. This huge variability may justify the observed disparity of magnetic anisotropies in our random ensembles of CdSe NCs, and may also provide insight into some conflicting results reported for the magnetic behavior of expectedly similar nanomaterials [7,8].
[1] C. Stamm, F. Marty, A. Vaterlaus, V. Weich, S. Egger, U. Maier, U. Ramsperger, and D. Pescia, Science 282, 449 (1998)
[2] S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser. Science 287, 1989 (2000)
[3] R. P. Cowburn, and M. E. Welland, Science 287, 1466 (2000)
[4] A. Sahu, M. S. Kang, A. Kompch, C. Notthoff, A. W. Wills, D. Deng, M. Winterer, C. D. Frisbie, and D. J. Norris, Nano Lett. 12, 2587 (2012)
[5] S. Tomita, K. Akamatsu, H. Shinkai, S. Ikeda, H. Nawafune, C. Mitsumata, T. Kashiwagi, and M. Hagiwara, Phys. Rev. B 71, 180414(R) (2005)
[6] D. S. Schmool, and M. Shmalzl, J. Non.-Cryst. Solids 353, 738 (2007)
[7] S. Neeleshwar, C. L. Chen, C. B. Tsai, Y. Y. Chen, C. C. Chen, S. G. Shuy, and M. S. Seehra, Phys. Rev. B, 71, 201307(R) (2005)
[8] R. W. Meulenberg, J. R. I. Lee, S. K. McCall, K. M. Hanif, D. Haskel, J. C. Lang, L. J. Terminello, and T. van Buuren, J. Am. Chem. Soc. 131, 6888 (2009)

The tunability of band gaps to the energies of the solar spectrum has brought immense attention to the research and development of quantum dots (nanocrystals). Surface passivation using organic ligands to enhance stability and fluorescence quantum yield has been a major focus in the area. The electronic structure of such spacers is important for self-assembly but must also play a role as charge-carriers between dots. The structural versatility of quantum dots that arises from considering different atomic composition, lattice structure, size, shape and ligand-spacers have brought a considerable divergence among the transport properties of dots.
Lead chalcogenide nanocrystals is one popular choice of study for their easy synthesis and large Bohr exciton radius; their electronic structure is also interestingly different from other II-VI chalcogenides such as CdSe, CdTe and ZnSe.[1] We have previously studied the role of shape on the electronic structure and charge transport of bare and HS—ligated PbSe nanocrystals with octahedral and cube-octahedral shape.[2] Using density functional theory-based methods, we extend this study to explore the electronic modification that results from choosing other unconventional ligands, such as phthalocyanines. We calculate the HOMO-LUMO gap and band edges of ligands as determinants of charge transport characteristics. We also study the absorption spectra of dots both with and without ligands using time-dependent density functional theory. The red shift caused by the presence of ligands has a direct dependence on their binding energy to the dot. Our extensive calculations show how these changes in electronic properties scale as we move from consideration of a single quantum dot to a superlattice array and to bulk materials.
1. An, J. M.; Franceschetti, A.; Dudiy, S. V.; Zunger, A. The peculiar electronic structure of PbSe quantum dots. Nano Letters 2006, 6, 2728-2735.
2. Kaushik, A. P.; Lukose, B.; Clancy, P. The role of shape on electronic structure and charge transport in faceted PbSe nanocrystals. Submitted to ACS Nano 2013.

Semiconductor nanocrystals exhibit a size-tunable, discrete electronic density of states. As such, carrier energy level spacings can significantly exceed phonon energies, suggesting the possibility of substantially slowed intraband carrier relaxation due to a phonon bottleneck. Despite theoretical predictions, intraband excitonic relaxation occurs on sub-to-single picosecond timescales in semiconductor nanocrystals, similar to bulk-phase semiconductors. A complete picture of intraband carrier relaxation in semiconductor nanocrystals has remained elusive. Direct measurements of phonon dynamics during intraband relaxation present inherent challenges, as the typical subpicosecond carrier relaxation lifetime and small phonon energies obviate the utility of transient spontaneous Raman spectroscopy.
Here, we report the use of femtosecond stimulated Raman spectroscopy (FSRS) to measure lattice dynamics in semiconductor nanocrystals and characterize longitudinal optical (LO) phonon production during confinement-enhanced, ultrafast intraband relaxation. We find that LO phonon populations are maximal at 600 fs following photoexcitation, and that these timescales are independent of particle size. We also note evidence of LO-to-acoustic phonon down-conversion and excited-state LO phonon mode softening. These results suggest that hole relaxation is primarily facilitated by size-independent hole-phonon coupling and enable quantitative studies of the dissipation of excitonic energy into vibrational modes.

Silicon nanocrystals were synthesized by baking of hydrogen silsesquioxane (HSQ) and characterized on a single particle level in the prepared powder form and from the solution by spin-coating. Temperature-dependent measurements revealed narrowing of the emission linewidth with decreasing temperature and time traces featured blinking between two states: ON and OFF. The full width at half maximum (FHWM) at room temperature for a single particle turned out to be around 30 meV, which is several times narrower than FWHM for silicon oxide passivated nanoparticles (100-150 meV). The effect of surrounding matrix on homogeneous linewidth is discussed using Lamb&’s theory description of stretching phonon modes for free-standing and oxide-encapsulated nanoparticles. Ensemble quantum yield measurements resulted in the value of 35% for dodecyne passivated solutions of such nanoparticles. It is shown that the quantum yield can be enhanced by micro-plasma treatment possibly due to de-clustering and improved surface passivation of the nanocrystals.

Zinc oxide (ZnO) which has been widely investigated has many outstanding properties, including its wide direct band gap and high-exciton binding energy. In this study, two crystal morphologies of ZnO nano-powders (nano-particle and nano-rod) are synthesized via chemical precipitation method. Their crystal structure, surface area, band gap, and photocatalytic efficiency would be studied. The ZnO nano-particle and nano-rod have a polycrystalline hexagonal structure with the wurtzite-type. The ZnO nano-particle has a particle size around 20~60 nm, while nano-rod shows a needle-shape morphology and a particle size around 20 nm*125 nm. The band gap of ZnO nano-particle and nano-rod is 3.22 eV and 3.20 eV, respectively, which is measured by the UV-Vis spectrometer. The BET result, measured from the surface area and porosity analyzer, shows that the specific surface area of the ZnO nano-particle and nano-rod is 27.65 m^2/g and 27.17 m^2/g, respectively. According to the XRD patterns, the full width at half maximum (FWHM) of the diffraction peaks of the nano-particle are broader than those of nano-rod, which means the crystallinity of nano-rod is better than nano-particle. It is also observed that both morphologies of ZnO nano-crystals exhibit good photocatalytic activity under UV-light illumination. The photodegradation on methylene blue is ~95% within first 170 and 105 min for ZnO nano-particle and nano-rod, respectively. Hence, ZnO nano-powders is a superior photocatalyst, effective for the clean removal of organic dyes, and maybe suitable for wastewater treatment applications.

In recent years, quantum dot-sensitized solar cells (QDSSCs) have emerged as promising low-cost alternatives to existing photovoltaic technologies, in an effort to meet the demand for clean energy. Here, we investigate the bimolecular photo-induced electron transfer (PET) dynamics between oleate-coated PbS QDs as an electron donor, and phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor using femtosecond (fs) broadband transient absorption (TA) spectroscopy and steady-state photoluminescence quenching. Upon optical excitation with fs pulses, the ultrafast electron injection and charge separation at the quantum dot/electron acceptor interface were examined as a function of the QD size and the acceptor concentration. The dependence of electron transfer on the QD size was investigated for QDs with band gaps of 880 nm (1.41 eV), 1080 nm (1.15 eV), 1310 nm (0.95 eV), and 1560 nm (0.79 eV), revealing inevitably that quantum confinement is the key element for efficient electron injection and charge separation processes. More specifically, only small-sized PbS QDs reveal strong photoluminescence quenching, indicating a dominant photo-induced charge transfer process between the donor-acceptor species. This observation can be explained on the basis of energy level alignment of the QD-PCBM adduct. More importantly, we monitored the charge injection and separation in real-time by observing the evolution of the spectroscopic signature bands for the formation of PCBM anionic radical, and we found that the electron injection from the excited PbS QDs to PCBM occurs within 200 fs for large band-gap QDs. These new insights into the photo-induced interfacial electron transfer in semiconductor QDs may serve as guidelines for the design of suitable composites to optimize the charge-separated state for solar-energy conversion applications.

Silicon quantum dots (SiQD) have been researched as potential materials for use in solar cells, photovoltaic devices, organic light emitting diodes, luminescent markers for biological imaging, and even ink-jet printable displays. Passivation of SiQDs with organic ligands can be used to protect the dots from oxidation, improve solubility, and stabilize luminescence by tuning their electronic band gap. Passivating the dots with silyl ethers is not a new process, but the state-of-the-art method requires the use of corrosive chlorinating agents to produce chlorosilane precursors, ultimately resulting in the production of toxic and corrosive HCl gas. The method here utilizes a palladium catalyst for the direct conversion of silyl hydrides to silyl ethers. The mechanism can produce both Si-O-C bonds and the more hydrolytically stable Si-O-Si bonds while creating a milder by-product, hydrogen gas. Twelve one-pot synthesis reactions were completed on various model compounds to the SiQDs and analyzed via FT-IR, GCMS, and NMR, producing a viable set of reaction conditions for the SiQDs themselves. Upon reaction with 1-octanol and a 10% Pd/C catalyst, the resulting quantum dot mixture turned a golden yellow color, increased the solubility of the dots, and produced bright emission under UV light, suggesting passivation. FT-IR spectroscopy revealed the disappearance of the original silyl hydride groups, also suggesting a successful reaction. This preliminary evidence indicates that this method is effective in quantum dot passivation, providing an easy and safe means of creating silyl ether passivated SiQDs.

Colloid CdSe nanocrystals have been suggested as an appropriate material for high efficient solar cells [1]. Besides quantum dots, which have been a topic of research for several years, other morphologies have been synthesized. Nanoplatelets have shown to preserve qualities of quantum dots like a tunable bandgap, while on the other hand, the flat architecture offers new possibilities e.g. to build layered systems. In semiconductor quantum dots electron relaxation by emission of single phonons is not possible, since the optical phonon energy is much smaller than the energy spacing between the lowest discrete conduction band levels that can easily be in the range of several hundred meV. This in turn, might allow a dramatic increase in the lifetime of hot electrons and is very attractive, when thinking of an efficient hot carrier solar cell. However, it was shown, that the relaxation is nevertheless fast and dominated by an Auger like process where the electron relaxes by transferring energy to the hole. Using time-resolved two-photon photoemission spectroscopy (2PPE), we have shown that, by replacing the original capping molecules with hole scavengers, the life time is strongly increased as the Auger like process is suppressed [2]. We also found evidence that the dominating process occurs via states between the discrete conduction band states, probably related to the surface or capping. 2PPE allows to measure the temporal evolution of the photoinduced electron distribution simultaneously energy and time resolved. In contrast to transient absorption spectroscopy or photoluminescence, here the measured signal is exclusively related to the electron distribution and is not influenced by the holes, as no dipole transitions are probed. As CdSe nanoplatelets are 2-dimensional and not 0-dimensional like quantum dots, the electronic band structure is most likely comparable to semiconductor quantum wells, with a continuum of states in the electronic conduction band. In the presented study, we compare measurements on CdSe quantum dots to 2PPE measurements on CdSe nanoplatelets. We observe that the relaxation in the nanoplatelets is not influenced by the ligand, and is furthermore much faster than in those quantum dots where the hole had been trapped to suppress the Auger relaxation pathway. Low temperature experiments reveal that the relaxation process is temperature dependent, and shows a clearly faster relaxation at lower temperatures. This is in contrast to previous measurements on quantum dots, where the relaxation process was shown to be only slightly influenced by the temperature [3] and supports the assumption, that electron relaxation in CdSe nanoplatelets is dominated by electron-phonon scattering.
[1] A.. Nozik, Physica E 14, 115 (2002).
[2] P. Sippel, W. Albrecht, D. Mitoraj, R. Eichberger, T. Hannappel, and D. Vanmaekelbergh, Nano Lett. 13, 1655 (2013).
[3] P. Guyot-Sionnest, M. Shim, C. Matranga, and M. Hines, Phys. Rev. B 60, R2181 (1999).

Calcium sulfide (CaS) nanoparticles are cadmium free fluorescent nanostructures with potential applications in nanomedicine and photovoltaic cells. We report on the synthesis and optical properties of CaS nanoparticles prepared by various methods, including (1) the reaction of Ca(CH3CO2)2 and Na2S in DMSO, (2) the reaction of Ca(CH3CO2)2 and DMSO in a microwave and (3) dissolving bulk quantities of CaS in DMSO. The absorption spectra of CaS prepared from these methods consists of a well-defined peak in the UV and a long wavelength tail that extends above 700 nm. Emission bands centered around 500 nm with a long wavelength tail that extends above 600 nm are observed upon excitation at 405 nm. STM measurements reveal the formation of CaS nanoparticles with an average diameter of (3.2 + 0.7) nm. The direct and indirect band gaps are estimated to be (0.403 + 0.003) eV and (4.135 + 0.006) eV. Theoretical calculations on small CaS clusters are used to establish the physical properties of calcium sulfide nanoclusters, including the optical absorption spectra. Unique to CaS nanostructures is the absorption of light at wavelengths longer that in the bulk material instead of the blue shift associated with quantum confinement effects in semiconductor. Indeed, the strong absorption bands in the visible region of the spectra of the CaS nanostructures do not have a counterpart in the gas or solid phases. The optical absorption spectra are proposed to have a significant contribution from indirect transitions which are discussed in terms of the dispersion of the phonon frequency.

ZnO, an n-type wide bandgap semiconductor, is frequently used as electron transport materials in organic photovoltaic (OPV) devices and as acceptors in hybrid photovoltaic (HPV) devices. Various groups have shown that variations in ZnO work function or bandgap can significantly impact device performance, but efforts to understand the precise correlation between ZnO electronic properties and OPV/HPV device physics have been complicated by the fact work function and bandgap were not be controlled independently. Here, we present a solution method to synthesize ZnO nanoparticles with independent control of work function and bandgap. We adapt microwave-assisted heating of Zn(acac) in butanol in the presence of two different additives, diethanolamine (DEA) and water. From UV-vis spectroscopy, work function measurement, and transmission electron microscopy, we demonstrate that DEA modifies the ZnO crystals from nanorods to spherical particles, improves the dispersion of ZnO nanoparticles, while lowers the work function from 4.4 to 3.9 eV. In contrast, for a given DEA concentration, reducing water content increases bandgap from 3.3 to 3.6 eV through quantum confinement without affecting work function. ZnO work function was found to influence the performance of OPV devices using ZnO:DEA nanoparticles as the electron transport layer on P3HT:ICBA inverted OPV devices, whereas ZnO bandgap was found to affect the performance of P3HT/ZnO HPV devices. The ZnO surface was further treated with surfactants, and the effect on the charge collection efficiency for both OPV and HPV devices will be discussed.

Colloidal Si nanocrystals (Si-NCs) have been attracting significant attention as precursors for printable optoelectronic devices and for biological applications. In Si-NCs, the bandgap energy can be controlled from the bulk bandgap (1.12 eV) to a visible range with decreasing the size. However, some of the applications, e.g., solar cells, demand smaller bandgap for effective utilization of the solar energy. Another problem of Si-NCs is the small absorption cross-section in the visible and near infrared range due to the indirect bandgap nature of the energy band structure. Formation of Si_1-xGe_x alloy NCs (Si_1-xGe_x-NCs) is a promising approach to enhance the absorption cross-section. However, research on Si_1-xGe_x-NCs is still very limited [1,2].
In this work, we develop a new type of colloidal Si_1-xGe_x-NCs by applying a method we have developed for the formation of Si-NCs dispersible in polar solvents without organic capping [3,4]. The method is simultaneously doping B and P to NCs. Doped B and P atoms form a high B and P concentration layer on the surface of Si-NCs. The high B and P concentration shell induces negative potential on the surface and makes Si-NCs dispersible in polar solvents due to the electrostatic repulsion.
We apply the same strategy to Si_1-xGe_x-NCs and develop solution dispersible all-inorganic Si_1-xGe_x-NCs. In order to produce colloidal Si_1-xGe_x-NCs, we first prepare B and P codoped Si_1-xGe_x-NCs in silicate matrices by simultaneously sputter-depositing silicon, germanium, and borosilicate and phosphosilicate glasses and annealing the films. By removing silicate matrices by hydrofluoric acid etching, B and P codoped Si_1-xGe_x-NCs are extracted. We control the average diameter from 3.4 to 5.7 nm and the Ge composition (x in Si_1-xGe_x) from 0 to 0.48. In the size and composition ranges, the codoped Si_1-xGe_x-NCs are dispersed in methanol without any functionalization processes and very clear yellowish to brownish solutions are obtained. X-ray photoelectron spectroscopy and inductively coupled plasma atomic emission spectroscopy reveal that high B and P concentration layers are formed on the surface of Si_1-xGe_x-NCs. The mechanism of solution dispersibility is thus the same as that of codoped Si-NCs. The optical bandgap is controlled by the size and the composition, and the absorption cross-section is enhanced compared to that of Si-NCs having the same bandgap energy [5]. The photoluminescence energy is also controlled by the size and composition in a wide range (1.43-0.98 eV). [1] F. Erogbogbo et al., ACS Nano5, 7950 (2011). [2] S. D. Barry et al., Chem. Mater.23, 5096 (2011). [3] H. Sugimoto et al., J. Phys. Chem. C117, 6807 (2013). [4] H. Sugimoto et al., J. Phys. Chem. C117, 11850 (2013). [5] R. Gresback et al., Langmuir29, 1802 (2013).

The synthesis of colloidal nanocrystals of chalcopyrites (such as CuInS₂) and kesterites (such as Cu(Zn,Sn)S₄) has attracted considerable attention during the last years, since they can be used as precursors for the formation of absorber layers of thin film solar cells. One of the central processing steps in this context is the formulation of printable inks containing these nanocrystal precursors, which requires a detailed knowledge of their surface chemistry. Clearly, this surface chemistry will be closely related to the synthesis method used for the formation of the nanocrystals. In the case of CuInS₂ and Cu(Zn,Sn)S₄ nanocrystals for thin film photovoltaics, this often involves a reaction in the presence of oleylamine (OLA).[1] Here, we report on a full characterization of the surface chemistry of CuInS₂ nanocrystals synthesized using this approach by means of advanced NMR spectroscopy.
As a first observation, we find that OLA ligands bound to CuInS₂ nanocrystals show no exchange with a free ligand pool at room temperature. A similar behaviour is typically found with X-type coordinating ligands such as carboxylates or phosphonates.[2] However, it contrasts with the dynamic stabilization of CdSe, CdTe and ZnO nanocrystals by amines, which is typically interpreted as the result of a weak, L-type coordination. To clarify the binding of oleylamine to CuInS₂, we performed in-situ heating experiments where the NMR sample is heated up to 130 °C and 1D ¹H NMR spectra are recorded in steps of 10 °C. In the case of CdSe stabilized by X-type oleate ligands in a well-purified, apolar dispersion, this heating has no effect on the ligand shell. Opposite from this, we observe a progressive release of OLA upon heating and an, albeit slow yet clearly discernible, re-adsorption upon cooling. Hence, it follows that the OLA/CuInS₂ bond can be broken and formed in apolar dispersions without the addition of excess free ligands - a prerequisite in the case of oleate or phosponate ligands. We thus conclude that OLA coordinates CuInS₂ nanocrystals as an L-type ligand, yet with a strong activation energy for desorption and adsorption.
The proposed surface chemistry is further elaborated in a series of exchange experiments towards thiols and carboxylic acids that are monitored with NMR spectroscopy. In line with the difficult desorption of OLA, the exchanges requires harsh conditions but give satisfying results. As it appears, exchange of oleylamine is possible and is completely reversible. Interestingly, there are some variations in the total ligand density when changing to thiols or carboxylic acids, which can be attributed to the complex surface chemistry of the CuInS₂ nanoparticles.
[1] Panthani, M.G., et al., J. Am. Chem. Soc. 2008, 130(49), 16770-16777
[2] Hens, Z., and Martins, J.C., Chem. Mat. 2013, 25(8), 1211-1221

Demand for sensitive and selective chemical sensors has been on the rise due to their vast practical uses in industrial and residential settings. Nanoscale metal oxides (nanoparticles, nanowires, nanoarrays, etc.) have been intensively investigated, over the past decade, in the application of gas sensors due to their intrinsic properties, such as tunable electrical and surface properties, and morphologies. Furthermore, there are a multitude of factors which affect the sensitivity and selectivity of chemical sensors, but the most important parameters focused on by researchers are material properties. Surface modification of metals oxide nanostructures using metal catalysts is one of the efficient routes to enhance both sensitivity and selectivity of the subsequent chemical sensor. Specifically, the use of a chemistry method eliminates the shadow effect, associated with physical deposition methods, and can create a more homogeneous coating. In this presentation, we chemically attached Pt metal catalyst on the surface of the In2O3 nanoparticles and nanowires to investigate their effective sensitivities. The In2O3 nanoparticles and nanowires were synthesized through the thermal decomposition of metal-organic precursors. Chemical coating of Pt on the as-synthesized In2O3 nanstructures was achieved by refluxing ethylene glycol, chloroplatinic acid and In2O3. The synthesized nanocrystals were characterized using transmission electron microscopy (TEM), electron energy dispersive X-ray (EDS), and X-ray diffraction (XRD. Both physically and chemically modified sensors were tested under oxidizing and reducing environments. These nanostructures can be translated onto flexible substrates for applications on variable surfaces, and the results will be discussed in detail in the presentation.

In the field of colloidal semiconductor nanocrystals or quantum dots, overcoating initial core nanocrystals with an inorganic shell has become a standard procedure to passivate trap states at the nanocrystal surface.
Moreover, depending on the alignment of the energy bands between core and shell, a considerable tuning of the opto-electronic properties of the resulting core/shell QDs is possible. In general, three different localization regimes are considered depending on the offsets between the bulk bandgaps, conveniently described as Type 1, Type 1.5 and Type 2. Type 1 regime corresponds to a confinement of e and h inside the core with a consequent passivation of the surface, while in the Type 2 case the staggered energy level alignment results in the spatial separation of e and h on different sides of the heterojunction. Type 1.5 exhibits an intermediate behavior.
The most widely used core/shell systems active in the near infrared emitting core/shell system are PbX/CdX QDs (with X=S, Se, Te), which can be conveniently made by means of a cationic exchange reaction. Although they have a different crystal structure, PbX (rock-salt) and CdX (zinc blende) share a common anion sublattice with little mismatch. Although this approach has been widely used, only few studies have addressed the optical properties of PbX/CdX core/shell quantum dots.
Here we report on the optical properties of PbS/CdS core/shell QDs.[1] Using elemental analysis, we find that the PbS sizing curve can be used for an accurate estimation of the PbS core diameter in PbS/CdS core/shell QDs. Moreover, the intrinsic absorption coefficient, predicted using the Maxwellminus;Garnett model coincides with that of the experimental samples. Therefore, we can use mu;400,th as a function of Vs/Vtot to calculate the volume fraction or the concentration of dispersed PbS/CdS core/shell QDs. We show that PbS/CdS core/shell QDs can have a photoluminescence quantum yield of 80% or more and an average lifetime of 1minus;3 mu;s. The oscillator strength of the PbS/CdS band gap transition, calculated from either the spectrum of the intrinsic absorption coefficient or the radiative lifetime, agrees with that of PbS QDs as well as with numbers calculated for PbS QDs using tight binding calculations. We thus conclude that the electronic structure of PbS and PbS/CdS QDs are highly similar, meaning that PbS/ CdS core/shell QDs exhibit a type 1 band alignment.
[1] Justo el al. J. Phys. Chem. C 2013 , 117 (39), 20171

The majority of successful synthetic routes to fabricate monodisperse II-VI and IV-VI group nanocrystals (NCs) employ long chain hydrocarbon molecules [1]. After synthesis, the bulky molecules form a dense ligand shell around the NCs which is covalently bound via functional groups such as -COOH, -NH₂, or -SH. The ligands suppress the agglomeration of the NCs and offer an efficient protection against oxidation [2]. Despite the benefits of the ligand shell, this also creates an electrically insulating barrier that leads to a significant reduction of the electric conductivity of NC thin films. To overcome this burden, several postproduction treatments, such as exchanging the long ligands by shorter ones, attaching small ions, or even fully removing the ligands, have been proposed for CdX and PbX (X=S, Se, Te) NCs [1,3,4]. In this work, we compared the efficacy of different ligand exchange/removal methods on semiconducting copper sulfide NCs (Cu₂S NCs) passivated with dedecanethiol molecules. In contrast to the more traditional chalcogenide NCs, the Cu₂S NCs consist solely of both abundant and non-toxic elements. The direct energy band gap of Cu_2-xS NCs (1 < x < 2) has been shown to increase from 1.2 eV to 2 eV with increasing x [5]. We systematically studied the exchange of the dodecanethiol ligands by the small molecule ethanedithiol and by S^2- ions as well as the removal of ligands with hydrazine treatment. The enhancement of the charge transport properties due to these chemical treatments was comprehensively evaluated by studying NC thin film field-effect transistors. The ligand exchange/removal was structurally monitored by infrared spectroscopy and x-ray photoelectron spectroscopy and the changes in the films&’ morphology were studied using scanning electron microscopy.
[1] H. Zhang, B. Hu, L. Sun, R. Hovden, F. W. Wise, D. A. Muller, R. D. Robinson, Nano Lett. 11, 5356-5361 (2011)
[2] X. Peng, L. Manna, W. Yang, J. Wickham, E. Scher, A. Kadavanich, A. P. Alivisatos, Nature 404, 59-61 (2000)
[3] J. M. Luther, M. Law, Q. Song, C. L. Perkins, M. C. Beard, A. J. Nozik, ACS Nano. 2, 271-280 (2008)
[4] D. V. Talapin, C. B. Murray, Science 310, 86-89 (2005)
[5] Y. Zhao, H. Pan, Y. Lou, X. Qiu, J. Zhu, C. Burda, J. Am. Chem. Soc. 131, 4253-4261 (2009)

Nanocrystalline germanium (Ge) and silicon (Si), as well as the GeSi alloy are promising materials for the application in printable electronics. Dispersed in organic liquids and deposited as thin functional film the nanocrystals (NCs) can be utilized in electronics, optoelectronics or photovoltaics. The synthesis of freestanding NCs is challenging and can for example be achieved in a nonthermal plasma reactor. We report on our progress in synthesizing Si, Ge and GeSi NCs in a reactor system utilizing two consecutive hot-wall reactors.
The setup allows to produce Si nanoparticles (NPs), GeNPs, GeSi alloy NPs and anisotropic hybrid particles from Ge and Si (Ge@Si NPs). The first reactor stage (HWR I) can be used for SiNP synthesis via monosilane (SiH4) pyrolysis in Argon carrier gas. The size of the resulting spherical particles can precisely be controlled maintaining a small geometric standard deviation (GSD). Furthermore, the shape of the SiNPs can be influenced solely by proper choice of the governing process parameters. After the first reactor stage the aerosol is quenched by additional argon. Prior to the second reactor stage (HWR II) GeH4 can be fed into the system. As a consequence, Ge growth at the NPs from the first stage (e.g. Si NPs) takes place resulting in Ge@Si NPs. At the reactor exit the aerosol is quenched by nitrogen. The particles are collected with a membrane filter or sampled via a low pressure impactor by deposition on TEM grids or silicon wafers.
Ge@Si NPs are synthesized at 600 to 700 °C. It is observed that at lower temperature the Ge grows in several patches, while at higher temperatures predominantly one patch is observed. This is explained by differences in surface diffusion. At higher temperatures higher diffusion leads to decreasing surface supersaturation of Ge on the SiNP surface in a zone around a Ge patch, thus supressing nucleation of further patches. At lower temperatures surface diffusion is lower and several patches can nucleate. Furthermore stress strain patterns evolving due to the lattice mismatch between Si and Ge are identified as influencing factors in patch number and positioning.
GeSi alloy particles of variable composition are produced by feeding SiH4 and monogermane (GeH4) into a reactor stage simultaneously and varying their ratio. The resulting particles are spherical and monocrystalline. Their mean size varies between 22 and 46 nm (depending on composition) with a GSD of around 1.34. XRD, EDX and Raman spectroscopy are used to determine their composition.
The synthesis of SiNPs and GeNPs will shortly be reviewed. The progress on the synthesis of the germanium-silicon hybrid particles will be demonstrated regarding their morphology, composition and growth. Furthermore the tailoring of GeSi alloy nanoparticle morphology and composition will be demonstrated.
This work was supported by the German Research Council (DFG) and the Cluster of Excellence “Engineering of Advanced Materials” (EAM).

Indium containing III-V semiconductor nanocrystals, or quantum dots (QDs) offer exciting applications based on their tunable photoluminescent properties in the visible (indium phosphide, InP) and in the infrared (indium arsenide, InAs). However, the current generation of III-V compounds cannot compete with the narrow size distributions, high quantum yields and photostabilities of state of the art II-VI QDs, which prevents them from being employed in in-vivo biomedical imaging or as downshifting fluorophores in novel displays.
While the nucleation and growth mechanisms of II-VI compounds such as cadmium selenide (CdSe) seem to be well understood, we still lack insight into the fundamental basics of III-V nanocrystal formation, which hinders the synthesis of high-quality nanocrystals. Recent publications have shown that the conversion rate of the molecular precursor can exhibit a strong influence on the resulting size and size distribution of the nanomaterial.
We present a theoretical discussion of the thermochemistry of group-V precursors with the general structure (R3E)3V (R = H, Me, Et; E = Si, Ge, Sn; V = P, As, Sb) for the early intermediates during QD nucleation. The results are discussed in close connection to experimentally obtained size distributions and kinetic data for a variety of phosphorus and arsenic precursors. We relate changes in molecular and electronic structure of the precursor to the properties of the resulting QD ensemble. Furthermore we modeled the presence of proton donors during QD growth and investigated the resulting protonolysis pathways as a competing side reaction to QD growth. Together, our data allow insight into the black box of QD nucleation and growth.

The peroxo-based synthesis has been applied to obtain TiO₂ in different phases, since rutile in acidic environment until titanates in soft basic conditions. Also in different sizes, shapes and with different surface features.¹ However, the phase formation from peroxotitanium complex (PTC) remains unclear. It is well known that the crystallization of PTC is pH sensitive. Since the PTC preparation requires ammonium solution, pH control during crystallization becomes hard. In this sense, the synthesis of a solid precursor from PTC that can be redispersed in water without high pH variation is the best way to get a controllable synthesis environment.² This synthesis method could be successfully explored regarding the photocatalytic properties of the products obtained in different pH values.² However, the crystallization pathway, which may be related to the final products features, needs to be clarified. This work aim to study the evolution of the precursor during its hydrothermal treatment. The photocatalytic properties of the as synthesized materials to Rhodamine B (RhB) dye photodegradation were also studied. The samples were collected during different times (20 to 200 min) in the specific heating ramp. X-ray diffraction analysis showed that, in the condition applied in this work, anatase formation happened only in temperature higher than 150°C, probably near to 200°C. Before anatase formation, samples presented crystalline structure similar to the precursor. However, a small peak related to anatase phase could be detected in sample obtained in 150°C. Other samples presented pure anatase phase. Crystallite size obtained by Scherrer&’s equation didn&’t showed remarkable difference between anatase samples, even after 2h/200°C. It occurred probably because of the low solubility of anatase phase, which may prevent growth by Ostwald Ripening. SEM images showed uniform spherical anatase nanoparticles with 15nm in diameter. The anatase phase is probably obtained after a precursor&’s solid transformation, according to HRTEM analysis. Rhodamine B photodegradation under UV radiation was studied and, surprisingly, sample obtained at 150°C presented almost the same photoactivity of anatase samples, despite the fact that this sample is not crystalline anatase.
Acknowledgments: This work was supported by CNPq and FAPESP.
References
[1] NAG, M. et al, Controlling Phase, Crystallinity, and Morphology of Titania Nanoparticles with Peroxotitanium Complex: Experimental and Theoretical Insights. J. Phys. Chem. Lett. v. 1, p. 2881-2885, 2010.
[2] DE MENDONCcedil;A, V. R.: RIBEIRO, C. Influence of TiO₂ morphological parameters in dye photodegradation: A comparative study in peroxo-based synthesis. Applied Catalysis B: Environmental. v.105, p. 298-395, 2011.

PbS nanocrystal quantum dots (NQDs) are very promising for applications in solution- processed photovoltaic device because of their large bulk exciton Bohr radius (20 nm) and size-tunability over the entire near-infrared wavelength region. Analysis of surface is important due to large surface to volume ratio. Nuclear magnetic resonance(NMR) spectroscopy is used to analyze the surface chemistry of NQDs.(1,2) When studying NQDs with NMR, we focus on the ligands that passivate PbS NQDs. When studying PbS NQDs capped with oleate, we focus especially on the vinyl group resonance at around 5.5 ppm. We present measurement and assignment of the size-dependent NMR spectrum in PbS NQDs. Our results help to study surface of PbS NQDs.
1) Hassinen et al., J. Am. Chem. Soc. 2012, 134, 20705
2) Cass et al., Anal. Chem. 2013, 85, 6974

Whereas tremendous progress has been made in creation, manipulation, detection, and application of optical activities generated from chiral materials, the opposite approach, effects of circularly polarized light (CPL) on molecules and nano-organizaion, has not been understood yet. We demonstrated how CPL can influence on racemic mixture of CdTe nanoparticles (NPs). For better understanding of the self-assembly, we studied electrokinetics and chemical mechanisms by calculations of the pair-potentials between the NPs using extended Derjaguin-Landau-Verwey-Overbeek theory (E-DLVO). This study can open new synthetic methods to chiral nanostructures, and open the door to understanding life&’s homochirality.

Defects engineering on ZnO is a paramount issue in the high-efficiency photocatalysis for the next generation. ZnO with both donor and acceptor defects is expected to be a superior photocatalyst due to the effective charge separation in the photodegradation process. Here, graphene oxide (GO) was employed as a catalyst for defect formation in ZnO lattice in a simple solvothermal reaction. ZnO/reduced graphene oxide (RGO) nanocomposites with many zinc and oxygen vacancies exhibit significant photocatalytic activity. Photoluminescence and electron paramagnetic resonance (EPR) measurements of ZnO/RGO powder indicate that the zinc vacancies (g=1.958) and oxygen vacancies (g=2.003) were generated on ZnO surface. In which, zinc vacancies generate partially occupied states that were close to the valence band, and oxygen vacancies create local states near the conduction band. The increasing O2 partial pressure during GO reduction had impacts on the paramagnetic defect formation in ZnO lattice and the possible mechanism was discussed in detail. Furthermore, EPR measurements of ZnO/RGO suspension confirm that the presence of both zinc and oxygen vacancies in ZnO/RGO cause the effective charge separation in the photodegradation of methyl orange. The photo-induced charges, holes and electrons were trapped by VZn acceptor and VO donor, respectively. The scavenger-assisted photocatalytic reactions further show that h+ and hydroxyl radicals enhanced the photoactivity of ZnO/RGO as the photocatalyst. 1
Keywords: Photocatalysis; Defect; Graphene; Graphene oxide; ZnO
1. Chen, T.-T.; Chang, I. C.; Yang, M.-H.; Chiu, H.-T.; Lee, C.-Y., The exceptional photo-catalytic activity of ZnO/RGO composite via metal and oxygen vacancies. Applied Catalysis B: Environmental 2013, 142-143 (0), 442-449.

Lead chalcogenide nanocrystals (NCs) have proven to produce efficient photoactive films for solar cell applications. One of their particularly exciting properties is the possibility of multiple exciton generation (MEG), i.e. the generation of multiple charge carrier pairs from a single absorbed photon. Devices containing these types of NCs are therefore considered as one avenue to overcome the Shockley Queisser limit on device efficiency. Current research focuses on the application of PbS and PbSe NCs. However, it has been predicted that PbTe NCs would show the highest MEG efficiencies within the Pb chalcogenides [1].
In the work presented here we demonstrate the first PbTe NC solar cell. Employing a p-n junction between TiO2 and PbTe NCs in a photovoltaic device, we show power conversion efficiencies of up to 2.3%. We demonstrate the importance of CdCl2 or ZnCl2 treatment in achieving working devices, and analyse the effect of these treatments on the chemical composition of the NC films and on the stability of the devices in the presence of oxygen.
[1]. Murphy, J. E.; Beard, M. C.; Norman, A. G.; Ahrenkiel, S. P.; Johnson, J. C.; Yu, P.; Micacute;icacute;, O. I.; Ellingson, R. J.; Nozik, A. J., PbTe Colloidal Nanocrystals: Synthesis, Characterization, and Multiple Exciton Generation., J. Am. Chem. Soc. 2006, 128, 3241-7.

In this work, we demonstrate fabrication of ion doped ZnSe QDs(d-dots) using aqueous phase synthesis method. With proper functionalization, the high quality d-dots shows great potential for biological applications. First, the d-dots with different emission wavelength were surface modified with bio-function molecules for targeted imaging of in vitro cancer cells with multichannel ability. After that, the d-dot was further developed as a vector for intracellular delivery of cancer drug, doxorubicin, for cancer cell treatment. In vitro cellular experiment performed on Panc-1 cells showed great treatment effects of the d-dot based vectors. More importantly, cell viability test showed that these heavy metal free d-dots are highly biocompatible. According to these results, we envision that these aqueous phase synthesized d-dots could be developed as a powerful tool for biological imaging and cancer therapy.
Key words: ZnSe, doped quantum dots, aqueous synthesis, bioimaging, drug delivery

Semiconductor colloidal quantum-dots (CQD) have attracted much attention due to their distinctive optical properties such as wide spectral responses and tunable absorption spectra with simple size control. These properties, together with the energy level controls with ligands and advantage of solution processing, have motivated the recent investigation of CQD-based solar cells, which have seen rapid growth in power conversion efficiency in just the last five years, to a current record of more than 8%. However, a deep understanding of the charge transport phenomena occurring in quantum-dot active layers is still lacking. While experimental efforts to design more favorable device architectures have been explored, including ligand engineering and junction optimization, direct assessment of quantum dot active layers with respect to their detailed configuration is less common because the connection between the CQD layer design and the photovoltaic device performance is challenging to characterize.
Analysis of charge transport in various designs of quantum dot layers based on computational approaches can reveal the role of CQD layer modulations, since detailed information such as the electric field distribution and carrier recombination rates in individual quantum dots in complex designs cannot be obtained easily from experiments. In addition, the transport properties of CQD films, such as dot-to-dot transport rates, effects of energy-level distributions and the role of trap states, can be understood in a systematic manner computationally.
In this work, we use a computational approach based on a hopping/tunneling transport model to simulate the charge transport in quantum dot layers as a function of energy levels and density of states of individual quantum dots, arrangement of dots, and the effects of external electric fields. Tuning these design parameters, we investigate the key factors governing the IV characteristics of CQD layers. Furthermore, the effects of CQD layer design on photovoltaic performance are estimated, which elucidates effective strategies for both the CQD-preparation and the layer deposition in order to achieve higher efficiency CQD solar cells.

In advancing photovoltaic technologies, quantum dot (QD) based solar cells has shown great promise in reducing the cost and enhancing the efficiency of light-harvesting systems compared to single crystal silicon cells - the efficiency of which is limited by the Shockley Queisser limit. In depth knowledge of exciton dynamics is required for design of the cells with optimum efficiencies. The quantum confinement effect in QDs enables bandgap tuning which has been widely adapted to facilitate exciton generation and separation, therefore harnessing more photons in a single device. However, the precise control of band-edge alignment between the QDs and the electron transporting material (typically a wide band gap semiconductor like ZnO or TiO2) is critical in enhancing the performance of the device by regulating charge generation, separation and collection.
Here we present the study of diameter and composition dependence of the surface potential of single Lead and Cadmium Chalcogenide quantum dots and their composite configurations using Kelvin probe force microscopy and determined the energy-level arrangement at nanoscale, charge injecting configuration and the PbS/CdS composition for optimum photoconversion efficiency. Our findings are also consistent with QD solar cell performance using the same configurations.

Oxide semiconductors have a lot of attentions as materials for highly selective and sensitive gas sensor because of their specific electrical and structural features. In n-type metal oxide semiconductor materials such as ZnO, WO₃, SnO₂, TiO₂ and In₂O₃, effective electron depletion layer in oxide semiconductors play an important role to get the highly sensitive and selective gas detection. In order to improve the performance of gas sensor, oxide semiconductor is designed in the nano-scale and/or attached using catalytic additive. Unfortunately, gas sensor with n-type oxide semiconductor isn&’t possible to distinguish the analyte gases with humidity in the atmosphere. Moreover, the humidity has a bad effect in the sensor response, reproducibility of sensor signal, and response/recovery speeds.
In this study, we fabricate the hierarchical Sb:SnO₂-ZnO structures using nanoimprint lithography and hydrothermal growth. At first, Sb:SnO₂-ZnO nanoparticle dispersed UV imprint resists, including the Sb:SnO₂ and ZnO nanoparticles, alcohol, monomer and UV initiator, are prepared with different weight ratio. Then, micro-scale Sb:SnO₂-ZnO patterns fabricated onto the substrate using UV nanoimprint lithography of the resists. In order to remove the polymer, patterned substrate is heated up to 500 °C. After annealing, ZnO nanorods are grown onto the patterned substrate by hydrothermal method. Subsequently, gold electrodes are deposited on the substrate.
The hierarchical Sb:SnO₂-ZnO structures are analyzed using scanning electron microscopy, x-ray diffraction, and gas dectecting analyzer. Besides, the hierarchical structures are also measured there selectivity and sensitivity for some kinds of specific gas such as H₂S, toluene, acetone and etc, which compared with different weight ratio.

Copper oxide and copper sulfide nanoparticles show great promise for application as absorber materials in thin-film solar cells due to their suitable band gaps accompanied by high absorption coefficients. Since the synthesis of nanoparticles can be separated from the device fabrication, high temperatures that are not compatible with thin-film technology can be applied to synthesize and modify the nanoparticles in order to improve their quality prior to device fabrication. In addition to direct sythesis of copper sulfide sulfurization of copper oxide is an option which also opens up routes for preparation of heterojunctions.
We have sulfurized commercially available CuO nanoparticles by wet-chemical reaction with Na2S(aq.) or by H2S plasma treatment s that are. The CuO nanoparticle layers are prepared on quartz glass substrates via spin-coating from aqueous dispersions. To investigate the influence of the sulfurization conditions and of the subsequent thermal annealing on the structural and electronic properties of the nanoparticles, Raman, photoluminescence (PL), and optical absorption by photothermal deflection spectroscopy (PDS) are applied, supplemented by investigations of the microstructure with TEM and XRD.
The as-prepared nanoparticles are not uniform and exhibit a large variation in size and shape. According to TEM, sizes around 30 nm as well as larger particles and agglomerates have been found. Confirmed by Raman spectra and XRD diffraction patterns, the as-prepared nanoparticles are found to be pure CuO phase. The PL spectra obtained at room temperature exhibit a peak at about 1.3 eV and an onset of the emission at about 1.6 eV, which is in good agreement with the band gap deduced from the PDS spectrum.
The sulfurization of the nanoparticles after immersion into Na2S(aq.) or after H2S plasma treatment is confirmed by Raman and XRD. The Raman spectra reveal the presence of CuS but no PL is detected. Subsequent annealing of the sulfurized nanoparticles in N2 atmosphere either in an oven or by laser irradiation partially transforms the CuS into the Cu2S phase. After annealing, Cu2S band edge emission is observed in the PL spectra at about 1.2eV. Additionally, the presence of Cu2S is confirmed by XRD. The observed PL at room temperature indicates high quality material suitable as absorber material in solar cells.

Copper indium sulfide/ zinc sulfide (CuInS2/ZnS) core/shell quantum dots (QDs) are an important class of nanomaterials for optoelectronic, photovoltaic and photoluminescence applications. They consist of low toxicity materials and show long fluorescence lifetimes, which opens up great potential in biological imaging applications. It is particularly important to develop reproducible synthetic methods for these nanomaterials in order to maintain small sizes with high quantum yields. CuInS2 core QDs have been shelled with ZnS at various temperatures from 90-210°C for times ranging from a few minutes to a couple of hours to examine the role of both thermodynamics and kinetics on the shell growth. Their absorbance, photoluminescence, and excitation spectra were measured, as well as their fluorescence lifetime decays and TEM images. Temperature was found to have a significant effect on the photoluminescence intensity and lifetime; higher temperatures usually result in higher intensities and longer lifetimes. The effect of shelling time played less of a role indicating more thermodynamic than kinetic control. Using HR-TEM and EDX-TEM, it was observed that shelling initially results from cation exchange, leading to negligible size increase upon shelling but a significant increase in fluorescence. Only after a second round of injection does a ZnS shell 'grow' onto the particles, thereby increasing the particle size, and is accompanied by a further increase in fluorescence. HR-TEM also provides evidence of a hexagonal lattice structure in core-shell particles. Overall, the strong effect of temperature during shelling indicates a strong thermodynamic control of shelling albeit via a more complex mechanism than the more prototypical CdSe-based quantum dots.

Crystalline quantum dots (QDs) are currently investigated for next generation applications in photovoltaics, photo detectors and bio-imaging. For over a decade, there has been a growing interest in developing the synthesis of Group IV nanocrystals (Ge, Si) and exploration of their properties at nanoscale. They are relatively non-toxic when compared to the more popular lead and cadmium based QDs. Tunable properties in nanocrystals are typically achieved by size/shape control and surface modifications. Doping of nanocrystals by impurity atoms is another novel approach to tune properties of nanocrystals. The dopants can affect the band gap and Fermi energy levels leading to improved conductivity and tunable electronic properties.
In this work, we have systematically explored the doping of Ge nanocrystals with antimony (Sb) as an n-type dopant by a microwave assisted synthesis.. Antimony triiodide (SbI3) was used as a precursor for Sb along with germanium diiodide (GeI2) in a previously optimized microwave synthesis for Ge nanocrystals. A series of Sb-doped Ge nanocrystals have been prepared with varying (1%, 2%, 3%, 5%) Sb content to study the effect of doping on their band gap, crystallinity, conductivity and optical properties. X-ray diffraction patterns indicate an increasing crystallite size with increasing SbI3 concentration. Sb-Sb stretching modes were observed in the Raman spectra and energy dispersive spectroscopy (EDS) confirmed the presence of Sb in the samples. Consistent with observations made from PXRD patterns, selected area electron diffraction (SAED) images indicated an increase of the d-spacing with increasing amount of Sb. STEM-EDS characterization of these samples is underway to characterize presence of Sb in individual nanocrystals. The synthesis and complete characterization of these Sb-doped Ge nanocrystals will be presented.

Quantum confinement effects lead to size and shape tunable properties in nanocrystals. Popular methods of size control involve the use of coordinating ligands, systematic variation in reaction conditions (temperature, heating time, precursor concentration) and multiple introduction of precursors. Utilizing precursors in two different oxidation states, a novel approach first reported by Neale and coworkers, we previously reported a size controlled microwave synthesis of Germanium nanocrystals. In our efforts to expand on that work, we explored the formation of GeSi nanocrystals using Ge(II) and Si(IV) iodides as precursors. While no signs of Si reduction was observed, the addition of Si(IV) iodides was observed to significantly affect the crystallinity and size of the resulting Ge nanocrystals. A systematic study confirmed that Ge nanocrystal size could be controlled by careful variation of SiI4 quantity. We hypothesized that in-situ oxidation of Ge(II) species to Ge(IV) possibly leads to this increase in nanocrystal size. Purposeful addition of calculated quantities of iodine to oxidize the Ge(II) precursor resulted in the successful validation of our hypothesis. The nanocrystals prepared by this method have been observed to be highly crystalline in nature. The synthesis, characterization and our attempts to understand the mechanism will be part of the presentation.

Inorganic semiconductor nanoparticles are of significant interest for applications that rely on their size dependent properties. Particles with a narrow size distribution are most desired for applications in optoelectronic devices, and precise knowledge of the optical and electronic bandgap is crucial to study properties such as electron transfer. UV-vis spectroscopy is commonly employed to determine the optical bandgap of nanoparticles, but to determine the absolute energy levels of the valence band (VB) and conduction band (CB) other methods must be used. Cyclic voltammetry is a simple, thorough way to study the electronic gap of a nanoparticle ensemble. The poster presented here focuses on the solution-based, microwave synthesis of Ge nanoparticles characterized by cyclic voltammetry to investigate the bandgap energy levels of the particles as a function of size, crystallinity, and surface ligands. Germanium is an n-type material so the oxidation of the particles and corresponding valence band energy is the investigated. The optical gap does follow the expected trend for quantum confinement, however there is little difference in the VB energy determined by CV, suggesting the bandgap increase on the nano-scale is the result of CB energy changes.

Colloidal nanocrystalline semiconductor particles, known as quantum dots (QDs), have been considered as new semiconductor materials for next generation electronic and opto-electronic device applications. In particular, there have been active attempts to incorporate QDs into light emitting diodes, lasers, solar cells, and photodetectors owing to the following properties of QDs: a high absorption coefficient, a high quantum yield, a tunable bandgap, and compatibility with low-cost solution processing. Diverse approaches have been carried out to improve the electronic properties of QD thin films.
Electronic states inside the bandgap, known as electronic traps, are one of the well-known limiting factors for device performance in traditional semiconductor devices. Since the surface of QDs is believed to be the main source of electronic traps, researchers have implemented different chemical treatments with various ligands. Analyzing the effect of chemical treatments on the electronic states of QDs in a thin film geometry or device platform provides a measure on improving the performance of QD devices. In this work, we demonstrate how drive-level capacitance profiling (DLCP) [1] methods can be used to quantify trap levels and density of traps in QD thin films.
We have tested Schottky diodes with PbS QD thin films treated with a variety of ligands to quantify the density of trap states and trap depth. These results will be discussed in the context of different surface ligand treatments and deep-trap characterization.
[1] J. T. Heath et al., J. Appl. Phys., 95(3), 2004, 1000-1010

New semiconductor materials, manufactured by low cost, solution-based deposition of colloidally synthesized semiconductor nanocrystals (NCs) are of ever growing interest as the absorptive layer in third generation solar cells.
It is well established in traditional semiconductor research that device performance is limited by the presence of electronic states within the band gap. These states act as electronic traps and - even in very small concentrations (<< 1ppm) - can alter the electrical properties of the material. The significance of trap states in devices incorporating NC solids is gaining recognition; however, systematic and quantitative understanding is still lacking.
Here, we demonstrate a variety of optoelectronic techniques to quantify the density of trap states in NC solids and relate our findings to solar cell performance [1]. Specifically, we implement deep level transient spectroscopy (DLTS) [2], thermal admittance spectroscopy (TAS), and Fourier transform photocurrent spectroscopy (FTPS) on PbS NC solids [1]. We use these techniques to study the impact of (1) oxygen- and water vapor- induced degradation of NC solids and (2) different ligand-, salt-, and molecular-based passivation schemes on the number density of traps. These results enable us to understand the fundamentals behind transport in nanocrystal solids and the potential of chemical treatments for improving device performance.
[1] D. Bozyigit, S. Volk, O. Yarema, and V. Wood, Nano Letters (2013).
[2] D. Bozyigit, M. Jakob, O. Yarema, and V. Wood, ACS Applied Materials & Interfaces (2013).

Recently, there have been exciting advances in the performance of colloidal semiconductor nanocrystal devices with demonstrations of excellent field-effect transistors, solar cells, light-emitting diodes and flexible, integrated circuitry. However, the air and solvent sensitivity of inherently high surface-area, nanocrystal semiconductor materials has typically limited their fabrication to nitrogen gloveboxes and to dry processes. Here, we introduce in-situ a chemical agent, namely indium metal, into the device electrodes that is thermally triggered to diffuse and repair trap states created upon air and solvent exposure, recovering the high-performance device behavior. We exemplify the advantage of the recovery process using conventional photolithography and atomic layer deposition, to realize large-area and flexible nanocrystal field-effect transistors that operate in air stably over months. This recovery process opens up a wide-range of conventional and unconventional semiconductor processes for the low-cost, large-area fabrication of nanocrystal device technologies.

Colloidal quantum dots (QD) are promising materials for applications in solution-processing devices such as solar cells and light-emitting diodes due to spectral tunability through quantum confinement effects (1-4). Additionally, the chemistry of capping ligands on the surface of colloidal QDs and high surface-volume ratio strongly affects their physical and chemical properties. Kinetic processes in colloidal solutions lead to an existence of deviations from an ideal structure of each individual QD. Such deviations can create trap states in the electronic structure of QDs (5-7).
In the present work, we systematically investigate various defects that may exist as deviations from the ideal QD with ideal passivation. We show that the electronic structure of QDs is very stable to a majority of intrinsic defects and only few defects are responsible for mid-gap-states (MGS) in the electronic structure. Furthermore, we investigate where the MGS show up in the electronic structure. This work provides a reason of why, despite of high probability of various defects, the MGS are located only at certain energies while the colloidal QDs exhibit an open band gap.
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3. Talapin D. V., Murray C. B., Science 310, 86-89 (2005).
4. Gur, I., Fromer, N. A., Geier, M. L. & Alivisatos, A. P. Science 310, 462-465 (2005).
5. Tang J, et al. Nature Materials 10, 765-771 (2011).
6. Nagpal P., Klimov V. Nature Commun. 2, 486 (2011).
7. Diaconescu B., et al. Phys. Rev. Let. 110, 127406 (2013).

The narrow bulk band gap and large exciton Bohr radius of germanium (Ge) make it an attractive material for optoelectronics utilizing band-gap-tunable photoluminescence (PL). However, realization of PL due to quantum confinement remains scarcely reported. Instead, PL is often observed from surface trap states and is independent of nanocrystal (NC) size. Here, we demonstrate tunable band gap PL by chemically passivating the Ge NC surface. The exchange of native Geminus;Cl surface groups with alkyl groups using Grignard reagents leads to the first instance of tunable band gap emission from free-standing Ge NCs synthesized in the gas phase. Ge NCs between 4.8 and 10.2 nm in diameter exhibit near-infrared emission featuring spectral line widths that are at least a factor of 2 narrower than any previous report.
J. Phys. Chem. Lett. 2013, 4, 3392minus;3396, dx.doi.org/10.1021/jz401576b

Semiconductor nanocrystals are known as artificial atoms with quantum confined electronic states. However, their charge transfer and transport properties cannot be sufficiently explained within the simple “particle in a box” picture. The capping ligands on the surface of the quantum dots can induce mid-gap states and/or tune the energy level of the quantum dots, which have dramatic effects on their electronic and optoelectronic device performance. Here we present direct measurements of the density of electronic states (DOS) of PbS nanocrystals using Kelvin probe force microscopy and scanning tunneling spectroscopy, which are correlated with field-effect transport and ultrafast charge transfer properties. This correlation allows us to decipher the charge transfer and transport pathway of the quantum dot solids.

Two-dimensional (2D) semiconductor crystals with a thickness much smaller than their lateral dimensions are one of the key elements of modern microelectronic and optoelectronic. Recently, ultrathin semiconductor layers have been obtained or synthesized in a free-standing form, so that the ultra-thin layers can be manipulated without their substrate[1]. We will present the synthesis of colloidal atomically flat, fluorescent chalcogenide nanoplatelets with a thickness that is controlled between 2 and 7 monolayers with atomic precision[2]. These nano-platelets can be extended laterally into nano-sheets up to the micron-scale[3]. As their spherical counter parts, the quantum dots, they can be grown into 2D core/shell structures which are the first demonstration of colloidal multiple quantum wells[4]. They can also be grown into atomically flat anisotropic hetero-structures that behave as exciton concentrator. We will discuss the physical properties and the spectroscopy of this novel generation of 2D systems that have quantum yield that can reach 80% at room temperature[5], and radiative fluorescence lifetime as short as 300ps[6]. The auger recombination in these structures will be discussed and compared to the non-blinking core-thick shell spherical QDs we have recently synthesized and characterized[7]. In the last part of the talk, we will show that these nano-platelets can be used for various applications ranging from bio-imaging, to field effect transistors.
1. Bouet, C. et al. Flat Chem Mater 25, 1262-1271, (2013).
2. Ithurria, S. et al. J Am Chem Soc 130, 16504-16505, (2008).
3. Bouet, C. et al. Chem Mater 25, 639-645, (2013).
4. Mahler, B. et al. J Am Chem Soc 134, 18591-18598, (2012).
5. Tessier, M. D. et al. Nano Letters 13, 3321-3328, (2013).
6. Ithurria, S. et al. Nat Mater 10, 936-941, (2011).
7. Javaux, C. et al. Nat. Nanotechnol. 8, 206-212, (2013).

Near-field light-matter interactions can be exploited in photonic structures. However, the placement of nano-sized active materials, such as colloidal quantum dots (QDs), at specific near-field locations with the required sub-diffraction resolution remains a challenge. Here, we report the controlled placement of QDs in plasmonic hotspots using an electrohydrodynamic printing technique (EHD NanoDrip) capable of producing features well below the diffraction limit. QD integration is achieved by ejecting attoliter, QD-containing droplets from an electrified nozzle and guiding them to the desired near-field hotspot. The carrier liquid then evaporates leaving QD deposits of the desired width, thickness, and shape. In contrast to traditional wetting-based techniques, this process allows the QDs (which are active) to be structured independently from the passive plasmonic substrate. This novel patterning flexibility is used to design hybrid QD-plasmonic circuits that exhibit excitation, propagation and out-coupling of surface plasmon polaritons - essential functions of active plasmonics.

Si nanoparticle films are increasing in interest recently for their high surface area to volume ratio, quantum properties, and re-crystallization potential. Promising applications of Si nanoparticle films include Li ion battery electrode, spontaneous hydrogen generation, photon downshifting for solid state lighting and solar cells, quantum energy devices, and re-crystallized microcrystalline Si. The latter is the present focus area. Flat panel displays with microcrystalline Si thin film transistors (TFTs) can achieve higher display brightness and faster dynamic response while consuming less power compared to amorphous Si TFTs. Microcrystalline Si can be used as the bottom sub-cell of multi-junction thin film Si solar cells to absorb low energy (red) light and can improve the stability and efficiency of the solar cell. The work objectives include (1) developing a stable Si nanoparticle suspension, (2) depositing high quality Si nanoparticle-based films using the electrophoretic deposition method, and (3) re-crystallizing Si nanoparticle-based film using a rapid, large area and low temperature method to produce microcrystalline Si thin films on low cost substrates such as glass. It is well known that high quality nanoparticle-based films require stable particle suspensions. However, no known stable suspensions for Si nanoparticles exist. A non-aqueous solution was developed without the use of salts or surfactants to subsequently achieve high quality electronic films after re-crystallization. A high stability (hours) of 120 nm Si particles was achieved. Si nanoparticles were electrophoretic deposited into films with high uniformity and density. Photonic curing is a suitable method to selectively crystallize light absorbing thin films without exceeding the substrate melting temperature. Preliminary photonic curing attempts by a Novacentrix PulseForge system showed promise as Si nanoparticles were able to be re-crystallized into 1 to 10 micron droplets as confirmed with Raman spectroscopy, though dewetting of the crystallized Si from the ITO substrate did occur.

The processing of inorganic semiconducting thin films from nanoparticle dispersions has been proven for a range of materials, most prominently metal oxides like ZnO, and led to the demonstration of thin-film transistors with remarkable performance. Despite these achievements, solution processing of silicon would deliver additional degrees of freedom in device formation and optimization like doping capability, p- and n-type materials for CMOS application, or moisture insensibility. However, handling of silicon nanoparticles is difficult due to surface oxidation when exposed to ambient air. Thus, only few examples with layer formation in vacuum, under inert ambient, or employing elaborate post-processing have been reported for the preparation of electrically functional thin films from silicon nanoparticles.
We present a novel route for preparation of silicon nanoparticles (SiNP) with defined surfaces by wet chemistry under ambient air conditions. A non-polar solvent is brought into contact with DI water containing the initially oxidized SiNPs. The surface oxide is removed upon addition of HF to the aqueous phase and the surface energy lets the SiNPs transfer to the non-polar phase. From the latter the particles can easily be collected. The technique allows for simple, ambient-air dispersion formulation and processing of silicon nanoparticles into thin films that can compete in terms of electrical performance with layers fabricated utilizing more elaborate schemes.
FTIR analysis shows that silicon nanoparticles prepared by the proposed scheme are free of surface oxide and entirely terminated by hydrogen. Deposition by spin and drop casting delivers well percolated, rugged thin films of up to several µm in thickness. Upon moderate thermal treatment at 300 °C for 15 min, the layers show current enhancement for several orders of magnitude when compared to dispersion processed nanoparticle films without oxide removal and respective surface termination. Using oxidant-free solvents, the electrical behavior is not determined by the time between deposition and annealing as well as the storage time in air. This suggests that a most likely steric stabilization of the inter-particle contacts can be achieved.
Thin films derived from dispersions of oxide-free nanoparticles in butanol and chloroform show field effects resulting in on/off current ratios of approximately one order of magnitude in a bottom-gate coplanar thin-film transistor setup.

Thermoelectric generators (TEGs) convert heat flows directly into useable electrical energy. Therefore combustion processes could greatly benefit from attached thermoelectric generators, since the contribution of those could directly increase the over-all process efficiency by several percentages. Thus, thermoelectric heat-to-electricity conversion is discussed as one main player for waste heat recovery, but good converter materials are still desperately sought for. Criteria are not only a competitive thermoelectric figure of merit and hence conversion efficiency but also abundance of the raw material, toxicity, scalability of the synthesis processes. Nanoparticles offer the chance to fulfill these challenging needs. The availability of doped semiconductor nanoparticles in large quantities enables the fabrication of nanostructured bulk thermoelectric materials which have a superior thermoelectric figure of merit compared to the single crystalline counterpart due to a reduced lattice thermal conductivity. As a consequence of the improvements of the material's figure of merit, nowadays, also materials are considered for thermoelectric heat-to-electricity conversion which have a poor material's figure of merit in their single crystalline form, but competitive performance as an optimized nanocrystalline material. Nanocrystalline silicon is an example for this development.
This talk will provide an overview about thermoelectric materials from semiconducting nanoparticles. Own work concentrates on silicon and silicon-germanium alloy nanoparticles from a scalable gas phase synthesis which are further processed to a nanocrystalline bulk by a current assisted sintering technique. Thermoelectric transport properties of the material as well as the technological route towards test demonstrators will be discussed.

Printed logics encompass a large fraction of activities in the field of printed electronics and attracting increasing interest in recent years. While traditionally, organic semiconductor based field-effect transistors (FET&’s) have been studied, solution-processed FETs from inorganic materials (mostly, inexpensive and non-toxic metal oxides) have been introduced relatively recently. Generally, two distinct ways for the realization of printable FET&’s from inorganic materials are demonstrated in the literature. On one hand, solution-processed inorganic oxide semiconductors are prepared from metal precursors where a satisfactory performance can only be attained with process temperatures far beyond the glass transition temperature of commonly-used, inexpensive polymer substrates. On the other hand, devices made from inorganic nanoparticles have shown to suffer from inefficient gating due to the large roughness of nanoparticulate films at the dielectric interface. Here, we show that a new concept of electrolyte gating can address some of the above mentioned challenges. The advantage of electrolyte gating for the printed and usually rough channel morphologies will be demonstrated for both the porous nanoparticulate channel or the precursor based dense channel devices. While the first approach, for the first time, reduces the process temperature of solution processed oxide FETs down to room temperature, the second approach yields unprecedented device mobility values resulting from atomically smooth interface between the rough oxide semiconductor films and the electrolytic dielectric. Furthermore, it will be shown that the speed of the electrolyte gated FETs may not limited by the ionic conductivity of the composite solid polymer electrolyte (CSPE) used in our study, it rather the printing resolution that would determine the channel length and along with the maximum attainable speed in printed and electrolyte gated transistors.
1. B. Nasr, D. Wang, R. Kruk, H. Rösner, H. Hahn, S. Dasgupta, Adv. Funct. Mater. 23 (2013) 1750
2. S. Dasgupta, G. Stoesser, N. Schweikert, R. Hahn, S. Dehm, R. Kruk, H. Hahn, Adv. Funct. Mater. 22 (2012) 4909
3. S. Dasgupta, R. Kruk, N. Mechau, H. Hahn, ACS Nano 5 (2011) 9628

Luminescent concentration of sunlight onto solar cells allows the manipulation and concentration of both direct and diffuse light for photovoltaic energy conversion. In addition, luminescent concentrators downconvert incident sunlight to spectral regions that are efficiently captured by the photovoltaic device. Previous luminescent concentrators have produced concentration factors on the order of 10, far lower than the thermodynamic limit. Here we show, through theory and experiment, a path to the high concentration regime. We utilize CdSe/CdS seeded nanorods as a tunable lumophore, as they are resistant to photobleaching, have high luminescent quantum yields, and have tunable absorption and emission spectra. These nanorods are compatible with free-radical initiated polymers, facilitating ease of integration into large-area devices. Transfer-printed thin-film (30 um) crystalline Si solar cells with small area (0.15 mm^2) are surrounded by a 210 um thick quartz and polymer waveguide with embedded nanorod lumophores, allowing the study of luminescent concentrators with 10,000 times the area of the solar cell. The optimum loading fraction of nanorods into the polymer is 0.03 vol%, as agglomeration leads to scattering of light at higher loadings. By tuning the size of the CdS rod with respect to the CdSe seed, reabsorption by the nanorod at luminescence wavelengths is suppressed. Increasing the nanorod size from 4 x 8 nm to 9 x 80 nm increases photon propagation distances inside the waveguide by nearly an order of magnitude, reaching propagation distances 400 times the thickness of the waveguide. At long distances, the reduced reabsorption overcomes the diminished luminescent quantum yield of the larger nanorods, which is shown to originate from their longer radiative lifetimes. A first-principles Monte-Carlo ray tracing model accurately reproduces the behavior of the luminescent concentrator and is used to study the effects of light trapping on the performance of the luminescent concentrator. While high luminescent efficiency, large Stokes shift, low reabsorption, and efficient light trapping of the emitted photons all increase performance individually, large concentration factors are possible only if all are present simultaneously. We utilize a 1D Bragg reflector top surface in our model to selectively reflect and trap emitted photons, achieving angle- and wavelength-averaged reflectivity of over 99% for nanorods. Trapping the luminesced light is facilitated by the narrow emission linewidths of the nanorods (25 nm) in contrast to organic dyes (often 100 nm), for which the same design strategy results in reflectivity of only 95%. This linewidth-enabled light trapping allows nanorod-based luminescent concentrators to outperform their organic counterparts in the limit of extreme trapping. We find that concentration factors exceeding 100 could be achieved with the right combination of nanorod lumophore and light-trapping.

Solar cells based on colloidal lead chalcogenide quantum dots (QDs) offer the potential to achieve high efficiency by utilizing near-infrared photons. In addition to a tunable bandgap that covers the optimal range for single and multi-junction solar cells, their solution-processability makes QDs a promising candidate for large-area, flexible photovoltaic devices. However, fabrication of state-of-the-art devices generally involves high-temperature annealing of the window layer or inert-atmosphere processing conditions, both of which increase manufacturing complexities and cost. To realize efficient QD solar cells with simple fabrication procedures, further understanding of materials properties and improvement to device design are required.
Here we demonstrate air-stable, room-temperature solution-processed PbS/ZnO solar cells with a certified record efficiency of 8.55% by optimizing band alignment effects. PbS QDs treated with different ligands show shifts in band edge energies, Fermi levels, and photo-responses, enabling the use of QDs with different treatments to serve complementary functions in photovoltaic devices. The air stability is also found to depend to a greater extent on the interface and band alignments between QDs and contacts than on the bulk QD layer itself. By a rational device structure design, a significant improvement in both open-circuit voltage and short-circuit current density is achieved as well as excellent air stability. The performance of unencapsulated devices fabricated in air under room-light remains unchanged for over 30 days. This strategy opens a new route to realizing efficient, air-stable, low-cost solution-processed solar cells

As colloidal quantum dot (CQD) optoelectronic devices continue to improve, interest grows in the scaled-up and automated synthesis of high-quality materials. Unfortunately, all reports of record-performance CQD photovoltaics have been based on small-scale batch syntheses.We report on the dual-temperature-stage flow reactor synthesis of high-quality PbS colloidal quantum dots (CQDs), and compare their quality to the conventional batch synthesis, and single-stage reactor synthesis. Separate stages for nucleation and growth allow a high degree of control over the monodispersity of the CQDs. Also, We use a kinetic model to explain and optimize the nucleation and growth processes in the reactor. High photoluminescence quantum yield (50 %) and narrow full width-half max values were achieved for the optimized dual-stage flow-synthesized CQDs. Solar cells fabricated from the flow-synthesized PbS CQDs achieve on-par performance with that of batch synthesized CQDs at a power conversion efficiency of 4.1%. The dual-stage flow reactor approach, with its versatility and rapid screening of multiple parameters, combined with its efficient materials utilization, offers an attractive path to automated synthesis of CQDs for photovoltaics and, more broadly, active optoelectronics.

Colloidal semiconductor nanocrystals (NCs) have received considerable attention for a variety of optoelectronic applications due to their high photoluminescence efficiency and broad spectral tunability. Semiconductor NCs can be processed from solution and integrated into hybrid light-emitting devices that use organic semiconductors as charge transport layers. While electroluminescence from group II-VI and III-V NCs has been well studied, emission from group IV NCs including silicon has not been as extensively characterized. This talk will examine electroluminescence from plasma-synthesized silicon nanocrystals (SiNCs) where the nanocrystal surface is passivated with ligands of 1-dodecene. We will discuss the relevant device design issues that permit high efficiency to be realized. The role of surface ligand coverage will also be considered in the realization of optimum device performance. We find that in properly optimized devices, hybrid nanocrystal-organic light-emitting devices based on SiNCs are capable of realizing forward-emitted external quantum efficiencies approaching 10% for electroluminescence peaked in the infrared portion of the electromagnetic spectrum.

Colloidal quantum dots (CQDs) offer promise in flexible electronics, light sensing, and energy conversion. These applications rely on the formation of rectifying junctions, requiring the creation of high-quality CQD solids that are controllably n-type (electron-rich) or p-type (hole-rich). Unfortunately, n-type semiconductors made using soft matter are notoriously prone to oxidation - indeed unencapsulated n-CQDs lose their n-type character within minutes of exposure to air. Here we report the first high-performance, air-stable n-type CQD solids that do not require encapsulation. We employ density functional theory to identify inorganic surface passivation agents that bind strongly to repel oxidative attack. We find that - only when a novel materials processing strategy is employed that wards off strong protic attack from polar solvents - a subclass of halide ligands can, when incorporated at nanoparticle surfaces, provide n-type character, air stability, and excellent photovoltaic performance simultaneously. Only by creating an air-stable n-type CQD solid were we able to build the first inverted quantum junction device, one that begins with the new n-type active layer and finishes with application of air-processed p-type CQD solids. We report as a result the highest current density from any CQD solar cell; and the highest power conversion efficiency from a quantum junction solar cell. The work paves the way for new families of optoelectronic devices that leverage air-stable quantum-tuned materials.
Reference:
Z. Ning, O. Voznyy, J. Pan, J. Xu, S. Hoogland, V. Adinolfi, K. W. Kemp, J. Minor, H. Dong, L. Rollny, A. Labelle, G. Carey, B. Sutherland, O. M. Bakr, E. H. Sargent, submitted to Nature Material, under review.

Recently, the colloidal quantum dot (QD) semiconductors have attracted many attentions due to the excellent optical properties such as the tunable color emission by controlling the size of quantum dot and the low process cost using the solution process. Especially, several quantum dot (QD) materials made with compound semiconductor have been a strong candidate for the optical & electrical devices such as the light emitting diodes (LEDs). So, many groups have developed QD-LEDs to improve the device performances using various colloidal QDs compositions and the stable and efficient carrier transport layers. But, because Cd-based QDs have showed the best performance until now, we need to develop the eco-friendly QDs for the commercial applications.
In this work, we will show the organic-inorganic hybrid light emitting diode using Cd-free quantum dots with inverted structure. Our eco-friendly QD-LEDs were fabricated using multishell InP-core QDs, which were synthesized by convenient heating up method. The electroluminescent (EL) properties of Cd-free QD-LEDs can be improved by optimization of device structure for the balance of electron and hole charge injections in the stacked multilayer structures or designing of core/shell QD structure to facilitate efficient EL. When we used the nano-particle inorganic charge transport layers with the proper energy levels, we could control the injection barrier, charge blocking layers and process conditions. When we used the oxide nano-particles with large bandgap as the electron transport layer, the device showed the luminescence of > 1,000 cd/m2 and the current efficiency of > 3 cd/A. When we used the additional charge injection layers, we could observe the improved emission characteristics. So, if we develop new combinations of layers for QD-LEDs, we can drastically enhance the electrical and optical characteristics of the QD-LEDs.

Semiconductor quantum dots are promising candidates for light-emitters in integrated photonics and single-photon sources in quantum communications. In those applications, electrical control over light emission of individual nanocrystals with sub-diffraction spatial resolution will allow for nanoscale device footprint and ultralow-power operation. We demonstrate electrical switching of photoluminescence of semiconductor nanocrystals using electrostatic gating of graphene. Our device consists of submonolayer lead sulfide colloidal nanocrystals on top of gated graphene with a thin (~3 nm) dielectric layer between them. For the “off” state, the photoexcitation of a nanocrystal is transferred to graphene by Förster resonant energy transfer, quenching the light emission. During the “on” state, we apply a gate bias to graphene to change the Fermi level and, consequently, open the optical bandgap of graphene by Pauli blocking effects. The graphene becomes transparent for the luminescence wavelength of the nanocrystal, switching on the light of nanocrystals. Using lead sulfide nanoparticles emitting at the wavelength of 1400 nm, we demonstrate the modulation of photoluminescence up to 40% during the on/off switching.

Colloidal semiconductor quantum dots (QDs) are attractive building blocks for solar photovoltaics (PV). In this talk, I will provide an overview our ongoing efforts to design lead salt QD thin film absorbers for next-generation PV. Basic requirements for QD absorber layers include efficient light absorption, charge separation, charge transport, and long-term stability. I will first discuss several methods used to make conductive QD films by solution deposition and ligand exchange. Studies of carrier mobility as a function of basic film parameters such as inter-QD spacing, QD size, and QD size distribution have led to a better understanding of charge transport within highly disordered QD films. Efforts to improve carrier mobility by enhancing inter-dot electronic coupling, passivating surface states, and implementing rudimentary doping will be highlighted. Engineering the inter-QD matrix to produce QD/inorganic or QD/organic nanocomposites is introduced as a promising way to optimize coupling, remove surface states, and achieve long-term environmental stability for high-performance, robust QD films. To obtain large photocurrent from QD solar cells, it is critical to increase the minority carrier diffusion length to rival the optical absorption length, possibly by harnessing band-like transport through extended electronic states. The relative roles of superlattice order, energy disorder, and surface states in this regard will be summarized. The talk will conclude with comments on the prospects for controlled doping and rational p-n junction formation in QD systems.

Silicon thin-film transistors (TFTs), amorphous or in several crystallized morphologies, are the industry standard for large area, e.g. display, and integrated applications. The replacement of gas phase processing along with the required subtractive patterning by solution processing, i.e. printing, holds out the prospect for cost-effective and scalable materials deposition. Dispersed nanoparticle systems allow the separation of high-temperature synthesis from device layer processing.
We presented electrical characteristics of solution-processed silicon nanoparticle (SiNP) thin films that are processed in a closed inert environment [1]. Particle powder derived from pyrolysis of silane in a hot-wall reactor [2] is transferred without contact to air to an inert glove-box system, where ink formulation, thin-film formation, electrical characterization and post processing are carried out on bottom-gate bottom-contact TFT-type test substrates [1]. This approach is extended to germanium nanoparticles (GeNPs) and silicon-germanium alloy nanoparticles (SiGeNPs) in the present work.
With respect to formulation and deposition, SiNPs, GeNPs, and homogeneous SiGeNPs show substantially similar behavior. All nanoparticle powders are dispersable in acetone and/or ethanol and can be utilized for spin coating of ultrathin films. Dispersions containing 1 wt.-% of nanoparticles or below form percolation networks of single-to-few particles in thickness and an optimum ratio of on currents to off currents is reached at a distinct number of deposition steps.
The Source-Drain current characteristics of the SiNP thin films is of a space-charge-limited-current (SCLC) type and is insensitive to thermal annealing up to 450 °C. The response to gate voltage (VG) variations yields field-induced switching effects with significant hysteresis that can be attributed to surface charge states that are filled/emptied during VG cycling. With increasing Ge content of the nanoparticles, the SCLC behavior of the output characteristics is less pronounced with nearly ohmic behavior for pure GeNPs after annealing. In GeNP and SiNP systems, the surface charging effects / hysteresis can be suppressed by surface coating with polymeric or inorganic dielectrics with respective effect on the overall electrical performance [1]. In coated GeNP thin films a clear off state over a broad gate voltage range can be achieved.
In annealing experiments, GeNPs have rendered highly sensitive, as expressed by enhanced interaction with metallic contacts (T> 400°C) or etching (T>500°C) in contrast to SiNPs which are almost insensitive to annealing up to 1000°C. To overcome the narrow process window for GeNPs we extend our experiments on furnace and photonic annealing to SiGeNPs which allow for the exploration of custom-tailored processing.
[1] Weis et al., Small, Vol 7, Iss. 20 (2011)
[2] Koermer et al., Journal of Aerosol Science, Vol 41, Iss. 11 (2010)

The incorporation of semiconducting nanocrystals in electronic devices, in particular thin films of II-VI or IV-VI materials, has been the subject of many studies in the recent years [1,2]. However, up to now, only few works employ elemental semiconductors like silicon nanocrystals (Si-NCs) [3-6]. Recently, a significant enhancement of the electron conduction in Si-NC thin films has been reported by simply doping the films with a small amount of tetrafluorotetracyanoquinodimethane (F4-TCNQ) [7]. According to this work, F4-TCNQ molecules provide empty electronic states, which are close to the lowest unoccupied states of the NCs, within the free space between the Si-NCs. This enables a more efficient charge transport in the Si-NC network. Moreover, it was suggested that the energies of the molecule-induced states depend on the molecule&’s electron affinity, meaning that other molecules may induce energetically more favorable states than F4-TCNQ. To test this hypothesis, we carried out a comprehensive study of the electrical activation of Si-NC thin films by doping with different molecules such as tetracyanoquinodimethane (TCNQ) and some of its fluorinated derivatives like F2-TCNQ and F4-TCNQ. We established the concentration range of each of these molecules that optimizes their electrical activation of the films. For a deeper understanding of the physics governing the molecule-induced electrical activation, films made of p- and n-type Si-NCs were also studied. According to theory [7], the empty electronic states introduced by the small molecules are solely accessible to electrons and consequently are not expected to lead to an enhancement of hole conduction.
[1] D. V. Talapin, C. B. Murray, Science 310, 86-89 (2005)
[2] D. S. Chung, J.-S. Lee, J. Huang, A. Nag, S. Ithurria, D. V. Talapin, Nano Lett. 12, 1813-1820 (2012)
[3] A. R. Stegner, R. N. Pereira, K. Klein, R. Lechner, R. Dietmueller, M. S. Brandt, M. Stutzmann, H. Wiggers, Phys. Rev. Lett. 100, 026803 (2008)
[4] S. Weis, R. Körmer, M. P. M. Jank, M. Lemberger, M. Otto, H. Ryssel, W. Peukert, L. Frey, Small 7, 2853-2857 (2011)
[5] R. N. Pereira, S. Niesar, W. B. You, A. F. da Cunha, N. Erhard, A. R. Stegner, H. Wiggers, M.-G. Willinger, M. Stutzmann, M. S. Brandt, J. Phys. Chem. C 115, 20120-20127 (2011)
[6] Z. C. Holman, C.-Y. Liu, U. R. Kortshagen, Nano Lett. 10, 2661-2666 (2010)
[7] R. N. Pereira, J. Coutinho, S. Niesar, T. A. Oliveira, H. Wiggers, M. J. Rayson, P. R. Briddon, M. S. Brandt, M. Stutzmann, submitted, (2013)

Manipulating inter-particle coupling in semiconductor quantum dot ensembles has become the key to improved electronic devices like solar cells, photodetectors and transistors.[1-2] Where most attempts have focused on reducing the tunnel barrier width by ligand exchange with small molecules, first examples have appeared which utilize appropriate molecular orbitals of larger ligands to reduce the tunnel barrier height.[3,4] Often, this involves intermixing of inorganic nanoparticles with conductive polymers or organic semiconductors to form hybrid nanomaterials which are prone to phase segregation, mitigating the potentially synergetic effects of the material combination.[5,6]
We show how tetrathiafulvalene derivatives can be used to functionalize PbS nanoparticles by covalently binding to the surface, replacing the former insulating ligand shell and forming a structurally stable, all-semiconducting hybrid nanomaterial. We introduce ambient pressure x-ray photoelectron spectroscopy to precisely determine the absolute position of the hole ground state in PbS and rationally tune the nanoparticle size to match with the ligand&’s HOMO. Our atomistic calculations predict a near-resonant alignment of the hole levels at the nanoparticle-ligand interface and thus a negligible barrier height to hole transport in thin films of this hybrid material. This is confirmed by field-effect transistor measurements showing a high preference for hole transport and unusually large mobilities considering the wide barrier width.
References:
[1] Tang, J.; Liu, H.; Zhitomirsky, D.; Hoogland, S.; Wang, X.; Furukawa, M.; Levina, L.; Sargent, E. H. Nano Lett. 2012, 12, 4889-4894.
[2] ) Lee, J.-S.; Kovalenko, M. V.; Huang, J.; Chung, D. S.; Talapin, D. V. Nat. Nanotechnol. 2011, 6, 348-352.
[3] Luther, J. M.; Law, M.; Song, Q.; Perkins, C. L.; Beard, M. C.; Nozik. ACS Nano 2008, 2, 271-280.
[4] Kovalenko, M. V.; Scheele, M.; Talapin, D. V. Science 2009, 324, 1417-1420.
[5] Stalder, R.; Xie, D.; Zhou, R.; Xue, J.; Reynolds, J. R.; Schanze, K. S. Chem. Mater. 2012, 24, 3143-3152.
[6] Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425-2427.

Dense semiconductor nanocrystal (NC) arrays represent interesting composite systems, in which the unique properties of individual nanocrystals are combined with collective interaction driven effects to produce novel material properties. If not doped NC arrays are insulating. There are two main routes to make them conducting: making a field effect transistor (FET) or doping the array bulk. This talk offers theories for both these routes, emphasizing a critical role of S-P gap in the confinement electron spectrum in small spherical NC. In the FET part of this talk we show that source-drain conductance G is determined by the first layer of NC. When number of electrons in the first layer reaches two, conductance G should vanish. Because of S-P gap additional electrons go to the second layer and provide small contribution to G until their electrostatic potential compensates S-P gap and filling of the first layer resumes. In the second part of this talk we examine a simple model of a bulk doped semiconductor NCs array. We show that in sufficiently small NCs, the fluctuations in donor number from one NC to another provide disorder sufficient to produce charging of some NCs, as electrons are driven to vacate P-shell. This S-P-gap-driven charging produces a disordered Coulomb landscape throughout the array and leads to VRH at low temperatures. We explain transition from activated transport to variable range hopping (VRH) in the space of temperature, doping level, and NC diameter.

Inorganic nanocrystals (NCs) are of great interest for their tunable electric, optical, and magnetic properties. Traditional colloidal synthesis produces NCs capped with long hydrocarbon chains which hinder their applications in electronic and optoelectronic devices such as transistors, LEDs, solar cells, and photodetectors. Our group has developed the concept of inorganic ligands that include molecular metal chalcogenide complexes (MCC) and metal-free chalcogenide ligands. These small inorganic molecules displace organic ligands and provide stabilization of NCs in colloidal solutions while facilitating electronic communication in layers of close-packed NCs. As the field progresses, it is important to understand the effect of the inorganic ligands on the properties of individual NCs and NC arrays. Here, we report on the temperature-dependent Hall effect characteristics of NC arrays prepared from colloidal InAs NCs capped with MCC ligands (In2Se42- and Cu7S4-). We show that the measurements of the Hall effect is a powerful route to explore fundamental properties of NC solids. This study revealed high Hall mobility over 16 cm2/Vs in solution-cast 5.3 nm InAs NC films capped with the copper sulfide MCC ligands. We also showed that the doping in the NC solids can be controlled by the nature of the MCC ligands. The comparative study of the temperature dependent Hall and field-effect mobility values provided valuable insights on the charge transport mechanism and pointed to the transition from weak to strong coupling regime in all-inorganic InAs NC solids.

Semiconductor nanocrystals (NCs) have great potentials for thin-film optoelectronics, such as solar cell and light emitting diodes due to their size-tunable electronic properties and solution processability. Among these NC materials, silicon has attracted substantial interest because of its high abundance and low toxicity. Significant progress has been made in development of synthetic methods to prepare high quality Si NCs, achievement in controllable doping as well as in demonstration of outstanding performance in optoelectronic devices. Since many applications rely on electrical conduction through films of NCs, understanding fundamentals of the electrical transport in NCs is necessary to improve device performance. We investigate the electrical transport in thin films of hydrosilylated Si NCs, in terms of studying temperature and electrical field dependent conductivity. A vertical two-terminal structure was employed in this study, and the electrical conductivities in these films were examined at variable temperatures (300 ~ 10 K). At low bias, electron transport follows nearest neighbor hopping model, and the activation energy attributed to the charging energy is primarily provided by thermal energy. At high bias, the conduction is field dominated. The external electrical field affords electrons sufficient energy to overcome hopping barrier, leading to temperature-independent conductivities above a critical electric field ~ 8 × 10⁷ V/m. This is the first thorough electrical conduction study in hydrosilylated Si NC films and we believe that this study provides fundamental understanding of charge transport in thin films of freestanding Si NCs capped with organic ligands.
This work was supported primarily by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-0819885. Part of this work was carried out in the College of Science and Engineering Characterization Facility, University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC program.

Semiconductor quantum dots are considered a promising material class with the potential of highly tunable and novel optoelectronic properties. Recent research efforts have shown that quantum dots, assembled in well ordered 1D, 2D and 3D geometries have the potential to funnel excitons via Foerster Resonance Energy Transfer through the nano crystal composite. Understanding the inter quantum dot coupling and the spatial extend of exciton diffusion is key to design material for the deliberate control of energy transport through them. In this regard, we study Foerster Resonance Energy Coupling between CdSe quantum dots in well defined 2D assembly via template assisted deposition and using ligand exchange to precisely control the inter quantum dot distance. We then examine the extent of radiative coupling between quantum dots using confocal fluorescence hyperspectral imaging. We spatially map out the degree of the coupling between quantum dots by exciting the quantum dots at a known location and collect fluorescence at various distances relative to the excitation. We observe spatial diffusion on the order of 100 nm in the Foerster Resonance Energy regime (quantum dot spacing <5 nm) as well as in the radiatvie coupling regime where the quantum dots are separated beyond more than 40 nm. The results presented here provide insight into the spatial extend of exiton diffusion through quantum dot assemblies.

A common method to vary quantum dot (QD) solubility, biocompatibility, photoluminescence and colloidal stability involves using multifunctional coordinating ligands. Tuning these properties for specific applications may be improved with a better microscopic understanding of how the ligands interact with the QD surface. It is known that using bidentate ligands improves colloidal stability, but we have recently shown that they can be worse at inhibiting non-specific adsorption of thiolated target molecules compared to monodentate ligands (Takeuchi et al (2013) Nano Lett., 13, 4746-4752). Here, we have extended this study to investigate the role of the ligand coordination geometry and size in how these target molecules act as energy acceptors by studying the ensemble and single particle fluorescence properties. We uncover complexities in connecting the ensemble and single particle FRET efficiencies as a result of heterogeneities in ligand binding and fluorescence properties of the QDs at the microscopic level.