Symposium Organizers
Anna Fontcuberta i Morral, Ecole Polytechnique Federale de Lausanne, Switzerland
Esther Alarcon-Llado, AMOLF
Sudha Mokkapati, Australian National University
Carl Thompson, Massachusetts Institute of Technology
Symposium Support
Journal of Physics D | IOP Publishing
NM7.1: Piezoelectronic Nanowire Devices
Session Chairs
Esther Alarcon-Llado
Anna Fontcuberta i Morral
Tuesday PM, April 18, 2017
PCC West, 100 Level, Room 105 A
11:30 AM - *NM7.1.01
Metal Oxide Nanosurfaces and Hetero-Interfaces for Energy Harvesting and Sensing Applications
Sanjay Mathur 1
1 Department of Chemistry, University of Cologne, Cologne Germany
Show AbstractMetal oxide nanostructures with hetero-contacts and phase boundaries offer unique platform for designing materials architectures for sensing applications. Besides the size and surface effects, the modulation of electronic behaviour due to junction properties leads to modified surface states that promote selective detection of analytes. The growing possibilities of engineering nanostructures in various compositions (pure, doped, composites, heterostructures) and forms (particles, tubes, wires, films) has intensified the research on the integration of different functional material units in a single architecture to obtain new sensing materials. In addition, new concepts of enhancing charge transduction by surface functionalization and use of pre-concentrator systems are promising strategies to promote specific chemical interactions, however the challenge related to reproducible synthesis and device integration of nanomaterials persist.
A novel sensing concept was developed based on the integration and correlation of complementary functionalities originating from multiple junctions in a singular nanostructure to palliate the current issues in gas sensor technologies such as low power consumption, low operating temperature and cost effective production. In this work, the gas sensing and solar energy harvesting abilities of metal oxide semiconductors were utilized to deliver a self-sustained gas sensing signal without any external power sources. The generality of the concept was demonstrated by extending the new sensing approach to other nanomaterial based systems such as thin-film based heterojunctions and core-shell radial heterojunctions. This talk will present how chemically grown and designed nanoparticles, nanowires and nanocomposites of different metals and metal oxides open up new vistas of material properties, which can be transformed into advanced material technologies.
12:00 PM - NM7.1.02
Voltage-Current Characteristics of a Piezoelectric Semiconducting Nanowire under Dynamic Axial Tension Deformations
Shuaiqi Fan 1 , Yuantai Hu 2
1 Department of Mechanics, Huazhong University of Science and Technology, Wuhan, Hubei, China, 2 Department of Mechanics, Huazhong University of Science and Technology, Wuhan, Hubei, China
Show AbstractThe electromechanical coupling fields and carrier motion in a piezoelectric semiconducting ZnO nanowire under dynamic axial tension are studied in this report. The contact characteristics of a metal-semiconductor interface were illustrated in detail and the electric potential distribution was deduced by utilizing piezoelectric governing equation and the Gauss’s law of electrostatics. Then, the governing equation of carrier concentration in the nanowire was derived from the current balance conduction. As a result, the electric potential distribution under dynamic axial tension in a ZnO nanowire was solved and the corresponding voltage-current characteristics were obtained. The result shows that the voltage-current characteristics can be adjusted through an axial force or elastic wave.
12:15 PM - NM7.1.03
All Oxide VO2 Nanowire Bimorph Actuators
Helmut Karl 1
1 , University of Augsburg, Augsburg Germany
Show AbstractVO2 undergoes a first order metal-insulator phase transition (MIT) at 341 K, it is accompanied by a sudden and drastic decrease in electrical resistivity and optical transmittance in the near infrared spectral region. Especially it is also marked by a structural phase transition with extremely large, anisotropic and phase dependent length change in the percentage range at the MIT. The VO2 transforms from either the insulating monoclinic M1- or mechanical stress stabilized monoclinic M2- to the tetragonal metallic rutile R-phase. In this work we present a new type of all oxide VO2 nanowire bimorph actuator by side argon ion implantation into single-clamped VO2 nanowires, which introduces crystal lattice defects along a side of the VO2 nanowire leading to a deactivation of the MIT in the ion implanted volume fraction of the nanowire. The first order MIT in perfectly crystalline VO2 nanowires is characterized by a large temperature hysteresis, which worsens actor efficiency and performance. It will be shown that ion implantation induced lattice defects seed the MIT in the crystal defect free portion of the VO2 nanowire and thus suppress very effectively the temperature hysteresis in comparison to iridium side coated perfectly crystalline VO2 nanowire bimorphs. In addition, we show that mechanical strain intentionally built-in during implantation with the nanowire kept either in the M1 or M2 phase allows directing the phase transition from the M1 via the M2 to the R phase and allows to produce a bidirectional bending response during a single temperature ramp.
12:30 PM - *NM7.1.04
Piezotronics and Piezo-Phototronics of Nanowires
Zhong Lin Wang 1 2
1 , Georgia Institute of Technology, Atlanta, Georgia, United States, 2 , Beijing Institute of Nanoenergy and Nanosystems, CAS, Beijing China
Show AbstractPiezoelectricity, a phenomenon known for centuries, is an effect that is about the production of electrical potential in a substance as the pressure on it changes. For wurtzite structures such as ZnO, GaN, InN and ZnS, due to the polarization of ions in a crystal that has non-central symmetry, a piezoelectric potential (piezopotential) is created in the crystal by applying a stress. The effect of piezopotential to the transport behavior of charge carriers is significant due to their multiple functionalities of piezoelectricity, semiconductor and photon excitation. By utilizing the advantages offered by these properties, a few new fields have been created. Electronics fabricated by using inner-crystal piezopotential as a “gate” voltage to tune/control the charge transport behavior is named piezotronics, with applications in strain/force/pressure triggered/controlled electronic devices, sensors and logic units. This effect was also extended to 2D materials such as MoS2. Piezo-phototronic effect is a result of three-way coupling among piezoelectricity, photonic excitation and semiconductor transport, which allows tuning and controlling of electro-optical processes by strain induced piezopotential. The objective of this talk is to introduce the fundamentals of piezotronics and piezo-phototronics and to give an updated progress about their applications in energy science (LED, solar) and sensors (photon detector and human-CMOS interfacing).
.Z. Wu, X.N. Wen, Z.L. Wang “Pixel-addressable matrix of vertical-nanowire piezotronic transistors for active/adaptive tactile imaging”, Science, 340 (2013) 952-957.
.F. Pan, L. Dong, G. Zhu, S. Niu, R.M. Yu, Q. Yang, Y. Liu, Z.L. Wang* “Micrometer-resolution electroluminescence parallel-imaging of pressure distribution using piezoelectric nanowire-LED array”, Nature Photonics, 7 (2013) 752-758.
Z.L. Wang “Piezopotential Gated Nanowire Devices: Piezotronics and Piezo-phototronics”, Nano Today, 5 (2010) 540-552.
[4] Q. Yang, W.H. Wang, S. Xu and Z.L. Wang* “Enhancing light emission of ZnO microwire-based diodes by piezo-phototronic effect”, Nano Letters, 11 (2011) 4012–4017.
[5] W.Z. Wu, L. Wang, Y.L. Li, F. Zhang, L. Lin, S. Niu, D. Chenet, X. Zhang, Y. Hao, T.F. Heinz, J. Hone, and Z.L. Wang “Piezoelectricity of single-atomic-layer MoS2 for energy conversion and piezotronics", Nature, 514 (2014) 470-474.
[6] W.Z. Wu and Z.L. Wang “Piezotronics and piezo-phototronics for smart adaptive electronics and optoelectronics”, Nature Review Materials, 1 (2016) 16031 doi:10.1038/natrevmats.2016.31.
NM7.2: Pushing the Frontier of Efficient Light Emitting Devices with Nanowire Structures
Session Chairs
Sudha Mokkapati
Carl Thompson
Tuesday PM, April 18, 2017
PCC West, 100 Level, Room 105 A
2:45 PM - *NM7.2.01
Flexible Optoelectronic Devices Based on Nitride Nanowires Embedded in Polymer Films
Maria Tchernycheva 1 , Nan Guan 1 , Xing Dai 1 , Hezhi Zhang 1 , Valerio Piazza 1 , Elie Lefeuvre 1 , Francois Julien 1 , Nicolas Jamond 2 , L. Lu 1 , Martina Morassi 1 2 , Noelle Gogneau 2 , Jean-Christophe Harmand 2 , Ludovic Largeau 2 , Martin Foldyna 3 , Joel Eymery 4 , Christophe Durand 4
1 C2N-Orsay, UMR 9001 CNRS, University of Paris-Sud, Orsay France, 2 C2N-Marcoussis, UMR 9001 CNRS, Route de Nozay, Marcoussis France, 3 LPICM-CNRS, Ecole Polytechnique, Palaiseau France, 4 , CEA/CNRS/Université Joseph Fourier, Grenoble France
Show AbstractWe propose a method to combine high flexibility of polymer films with high quantum efficiency provided by nitride nanowires to demonstrate flexible inorganic devices. The lift-off and transfer procedure enables the assembly of free-standing layers of nanowire materials with different bandgaps without any constraint related to lattice-matching or growth conditions compatibility. This concept therefore allows for a large design freedom and modularity since it enables combination of materials with very different physical and chemical properties, which cannot be achieved by monolithic growth. In this presentation I will first review the state-of-the-art in the domain of flexible optoelectronic devices based on inorganic nanowires and then I will present our recent work on nitride nanowire based light emitters [1,2], photodetectors [3] and energy harvesting devices [4].
Xing Dai, Agnes Messanvi, Hezhi Zhang, Christophe Durand, Joël Eymery, Catherine Bougerol, François H Julien, Maria Tchernycheva « Flexible Light-Emitting Diodes Based on Vertical Nitride Nan-owires» Nano Letters 15 (10), 6958-6964 (2015).
Nan Guan, Xing Dai, Agnès Messanvi, Hezhi Zhang, Jianchang Yan, Eric Gautier, Catherine Bougerol, François H. Julien, Christophe Durand, Joël Emery and Maria Tchernycheva, “Flexible White Light Emit-ting Diodes Based on Nitride Nanowires and Nanophosphors”, ACS Photonics 2016, 3, 597−603.
Hezhi Zhang, Xing Dai, Nan Guan, Agnes Messanvi, Vladimir Neplokh, Valerio Piazza, Martin Vallo, Catherine Bougerol, François H. Julien, Andrey Babichev, Nicolas Cavassilas, M. Foldyna, Christophe Durand, Joël Eymery and Maria Tchernycheva, “Flexible photodiodes based on nitride core/shell p-n junction nanowires”, ACS Appl. Mater. Interfaces, 2016, 8 (39), pp 26198–26206.
N. Jamond, P. Chretien, F. Houze, L. Lu, L. Largeau, O. Mauguin, L. Travers, J.-C. Harmand, F. Glas, E. Lefeuvre, M. Tchernycheva, N. Gogneau, “Piezo-generator integrating a vertical array of GaN nanowires”, Nanotechnology 27, 325403 (2016).
3:15 PM - NM7.2.02
Simultaneously Enhancing Light Emission and Suppressing Efficiency Droop in GaN Microwire-Based UV LED by Piezo-Phototronic Effect
Xingfu Wang 1 , Ruomeng Yu 1 , Zhong Lin Wang 1
1 , Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractGroup III-nitride-based ultraviolet (UV) light-emitting diodes (LEDs) have gained considerable interest due to a wide range of applications such as high-density optical data storage, water treatment, sterilization of medical equipment, and biological and cellular imaging. Here, the piezo-phototronic effect is utilized to simultaneously enhance the quantum efficiency and suppress the efficiency droop in GaN microwire (MW)-based p-n junction UV LED. By applying -0.12% compressive strain along the direction perpendicular to the p-n junction interface, the quantum efficiency of GaN-based UV LED is enhanced by over 600%. An obvious efficiency droop phenomenon is observed in the fabricated GaN-based LEDs as increasing injection current under strain-free condition. As the current density increase to 83.3 A cm-2, the relative external quantum efficiency (EQE) drops by 46.6% of the maximum value occurss at 10 A cm-2. This drop value of relative efficiency is effectively reduced from 46.6% to 7.5% and corresponding droop onset current density is increased from 10 A cm-2 to 26 A cm-2 when piezo-phototronic effect is introduced by applying -0.12% compressive strain. The physical mechanisms of the piezo-phototronic effect on the performances of GaN-based LED are carefully proposed. Performances optimization of the GaN MW-based LEDs is attributed to the formation of an electron potential dip within depletion region of the p-n junction and energy band tilt across the bulk region, which can effect trap electron at junction region and enhance the carriers injection. Corresponding piezo-potential distributions across the p-n junction under externally applied compressive strain are calculated via finite element analysis (FEA) to confirm the proposed working mechanisms. This study presents in-depth understandings about the piezo-phototronic effect in p-n homo-/hetero-junctions and offers an unconventional path for the development of high efficiency and high powered III-Nitride UV LEDs/LDs.
3:30 PM - *NM7.2.03
3D Nano-Architectures on Si Platforms for Opto-Electronic Device Concepts such as Solarcells, Light Emitting Devices and Sensors
S.H. Christiansen 1
1 , Helmholtz Zentrum Berlin, Berlin Germany
Show AbstractThe successful integration of new, 3D nano-architectures into devices such as sensors, solarcells, light emitting diodes (LEDs) a.o. on a silicon (Si) platform requires new approaches towards fabrication and characterization. The present paper will show device concepts relying on Si nanowires (NWs) or inverted, cone-shaped, 1D Si structures (SiNCs) which permit the concentration and confinement of visible (VIS) and near infrared (NIR) light in whispering gallery modes (WGMs). Fabrication concepts based on nano-lithography and dry etching and optical property optimization based on numerical simulations (finite difference time domain - FDTD) will be presented. Individual as well as ensembles of these 1D nano-structures will be integrated in pn-junction solar cells (all-inorganic or organic-inorganic hybrids) and optical sensors. Design rules for optimized absorption or light emission (VIS-NIR) will be derived. Moreover, novel carrier selective, nano-material based contacting schemes will be presented. Among those, composites containing silver nanowires and graphene are demonstrated. Materials and device characterization will rely on advanced correlated electron microscopy and optical spectroscopy (CORMIC) containing electron beam induced current (EBIC) measurements, I-V characterization with and without illumination (with tunable power and wavelength) inside a scanning electron microscope (SEM), cathode-, photo- luminescence as well as in-SEM micro-Raman spectroscopy.
NM7.3: Nanowire Architectures for Light Management in Photovoltaics
Session Chairs
Sudha Mokkapati
Carl Thompson
Tuesday PM, April 18, 2017
PCC West, 100 Level, Room 105 A
4:30 PM - *NM7.3.01
Resonantly Excited Semiconductor Wire Motifs in Photovoltaic, Photoelectrochemical and Thermoelectric Devices
Harry Atwater 1 , Kelly Mauser 1
1 , California Institute of Technology, Pasadena, California, United States
Show AbstractNear-unity, broadband, angle-insensitive and polarization-independent absorption is possible in sparse InP nanowire arrays, embedded in flexible polymer sheets via geometric control of waveguide modes in two wire motifs: i) arrays of tapered wires and ii) arrays of nanowires with varying radii. Sparse arrays of these structures exhibit enhanced absorption due to strong coupling into the 1st order azimuthal waveguide modes of individual nanowires; wire radius thus controls the spectral region of the absorption enhancement. Whereas arrays of cylindrical wire with uniform radius exhibit narrowband absorption, arrays of tapered wires and arrays with multiple wire radii expand this spectral region and achieve broadband absorption enhancement. Using a top-down lithographic pattern/etch fabrication method, we create sparse tapered and multi-radii InP nanowire arrays and demonstrate optical absorption that is broadband (450-900 nm), angle-insensitive and near-unity (>90%) in arrays with roughly 100 nm planar equivalent thickness of InP. We will discuss applications of these highly absorbing sparse nanowire arrays in photodetectors, solar cells, and photoelectrochemical devices.
We also demonstrate subwavelength thermoelectric nanowire arrays that exhibit resonant spectrally selective absorption, and which create large enough localized temperature gradients to generate easily measureable thermoelectric voltages. We show that such thermoelectric nanowire arrays are tunable and are capable of highly wavelength specific detection, with high responsivity and response times of nearly 3 kHz, by combining resonant absorption and thermoelectric junctions within a single structure, yielding a bandgap-independent photodetection mechanism. We report results for both resonant nanophotonic bismuth telluride – antimony telluride wire arrays and chromel–alumel wire arrays as examples of a broad class of nanophotonic thermoelectric structures useful for fast, low-cost and robust optoelectronic applications such as non-bandgap-limited hyperspectral and broad-band photodetectors.
5:00 PM - NM7.3.02
Broadband and Omnidirectional Anti-Reflectivity of Hierarchically Structured Silicon
Anna Hiszpanski 1 , Juan J. Diaz Leon 1 , Tiziana Bond 1 , Joshua Kuntz 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractHierarchically structuring silicon has been shown to significantly reduce its reflectivity over a broad range of the solar spectrum and angles of incidence, thereby increasing silicon solar cells’ absorption efficiency and overall power conversion efficiency. But optimizing the hierarchical structures for desired performance and fabricating well-controlled nanometer-scale structures atop micrometer-scale structures are ongoing challenges. To meet this first challenge, we performed COMSOL simulations to determine how the dimensions and periodicity of hierarchical structures affect the specular and diffuse reflectivity of silicon across the UV, visible, and IR wavelength regimes. These studies have yielded rules of thumb for the design of hierarchical structures depending on the desired spectral response.
Furthermore, we validated our simulations by fabricating our targeted hierarchical structures. Our fabrication approach used only wet-chemistry techniques, making it inherently simple and scalable. To control the nanometer-scale features, we devised a block copolymer-patterning scheme, which enabled us to reproducibly make sub-50 nm features and addresses the second challenge previously described of fabricating well-controlled nanometer-scale structures atop textured surfaces. With these tools, we were able to pattern 4” wafers with micrometer-scale pyramids having either nanowires or a thick nanoporous silicon layer on top. Our best structures reduced silicon’s total reflectivity from 34% to below 1.5% across the UV, visible, and IR regimes at near-normal incidence and maintained this reduced reflectivity even up to a 70° angle of incidence. Incorporated into solar cells, these hierarchical silicon structures should increase light absorption by 50% compared to their planar counterparts.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
5:15 PM - NM7.3.03
Metamaterial Absorber for Efficient Perovskite Solar Cell
Omar Abdelraouf 1 , Nageh Allam 1
1 Energy Materials Laboratory (EML), Department of Physics, School of Sciences and Engineering, The American University in Cairo, Cairo Egypt
Show AbstractSince 2009, organic halide perovskite material CH3NH3PbI3 became a promising material for low cost photovoltaic solar cell. Recorded perovskite efficiency reached up to 19.6% in 2016. Although these exponential developments in perovskite solar cell efficiency, there are a huge room for enhancing overall efficiency much more using metamaterial absorber surfaces. Metamaterial surfaces could localize and guide sun light within active layer of solar cell for maximum efficiency.
In this work, we studied effect of depositing silver metamaterial wire grating on upper surface of active layer. Many nanowires cross section shapes and dimensions were simulated. Results showed big enhancements in sun light absorption and increasing in overall efficiency of perovskite solar cell. To reduce the cost of used materials, we replaced silver and used low cost transition metal nitride such as titanium nitride (TiN). TiN is a suitable alternative as it has similar plasmonic properties similar to noble metal such as gold. All simulations done in three dimensional optical models using finite element method software, optical refractive index of materials taken from literature.
5:30 PM - *NM7.3.04
Calculating and Measuring the Thermodynamic Limits and Losses in Nanophotonic Solar Cells
Erik Garnett 1
1 Center For Nanophotonics, FOM Institute AMOLF, Amsterdam Netherlands
Show AbstractNanophotonic structures have been suggested as a viable option for breaking the Shockley-Queisser limit. Here we will discuss the benefits in both the ideal case - where materials are operating at the radiative limit - and the more realistic case where there is substantial non-radiative recombination. In particular, we will show that the nanophotonic "concentration" effect is very different from macroscopic concentration and cannot be used to break the Shockley-Queisser limit, even for perfect materials. However, nanophotonic solar cells can break the conventional limit by altering the angular emission pattern. This is especially interesting because in nanostructures such directional emission is not necessarily coupled to a reduction in light outcoupling efficiency as in planar solar cells, and therefore can lead to efficiencies above the Shockley-Queisser limit even in materials with substantial non-radiative recombination. In the second half of the talk, we will discuss a new characterization technique - integrating sphere microscopy - that allows us to measure absorption cross section, internal quantum efficiency and photoluminescence quantum yield in single nanowire solar cells with diffraction-limited spatial resolution. Using these metrics, we calculate the thermodynamic limit and losses in a single InP nanowire solar cell and compare the performance to that of state-of-the-art technology.
Symposium Organizers
Anna Fontcuberta i Morral, Ecole Polytechnique Federale de Lausanne, Switzerland
Esther Alarcon-Llado, AMOLF
Sudha Mokkapati, Australian National University
Carl Thompson, Massachusetts Institute of Technology
Symposium Support
Journal of Physics D | IOP Publishing
NM7.4: Electrical Properties in Nano-Photovoltaics
Session Chairs
Esther Alarcon-Llado
Anna Fontcuberta i Morral
Wednesday AM, April 19, 2017
PCC West, 100 Level, Room 105 A
9:30 AM - NM7.4.02
Surface Passivation and Quantum Efficiency Enhancement of InP Nanowires by ALD Al2O3 with POx Interlayer
Lachlan Black 1 , Alessandro Cavalli 1 , Marcel Verheijen 1 2 , Jos Haverkort 1 , Erik Bakkers 1 , Erwin Kessels 1
1 , Eindhoven University of Technology, Eindhoven Netherlands, 2 , Philips Innovation Services, Eindhoven Netherlands
Show AbstractIII/V semiconductor nanowires possess various properties which are advantageous for solar energy conversion applications, including effective light-trapping, enhancement of conversion efficiency through optical concentration, and efficient use of scarce materials. InP in particular is well suited for photovoltaic applications due to its close-to-ideal bandgap of ~1.34 eV and relatively good surface properties, and InP nanowire solar cells have achieved the highest reported conversion efficiencies of any nanowire photovoltaic device. However, the performance of such devices remains limited by non-radiative surface recombination, the importance of which is enhanced by the high nanowire surface-to-volume ratio. Despite some decades of work on this material, there currently exists no well-established stable means of passivating InP surfaces. Indeed, deposition of standard dielectric layers typically results in strong depassivation relative to the native surface. In this contribution, we will report on our recent work on passivating InP nanowire and planar surfaces using atomic-layer-deposited (ALD) dielectric layers. ALD is uniquely suited to this application due to its high conformality, thickness control, and ability to form high-quality layers at low deposition temperatures. We demonstrate successful passivation of InP surfaces using a deposited dielectric stack consisting of a phosphorous oxide (POx) interlayer together with an Al2O3 capping layer, formed at room temperature using plasma-assisted ALD. The POx layer passivates the InP surface, while the Al2O3 capping layer provides chemical and thermal stability. Deposition at room temperature, which is enabled by the use of a plasma, is found to be critical to achieving the best level of InP surface passivation. Time-resolved and steady-state photoluminescence (PL) measurements are used to characterise the quality of the passivation. We demonstrate a 3 times increase in PL decay lifetime (from 1.8 to 5.4 ns), and a factor of 10 increase in steady-state PL intensity at room temperature for POx/Al2O3-passivated undoped wurtzite InP nanowires formed by selective-area epitaxy compared to the as-grown wires. Comparison of low-temperature (10K) steady-state PL intensity (in which non-radiative recombination is effectively suppressed) with room temperature measurements suggests a similar ~10 times enhancement of the 1 sun internal PL quantum efficiency to ~1.5% at room temperature, yielding an implied open circuit voltage of ~1050 mV compared to ~990 mV for the as-grown wires. Qualitatively similar trends are observed for planar InP surfaces of various orientation and doping. The passivated surface is found to be more thermally stable than the native surface, significantly expanding the temperature window for InP device processing. These results represent the first clear demonstration of InP surface passivation by dielectric layers of appreciable thickness.
9:45 AM - NM7.4.03
Hybrid Free-Standing Metal-Semiconductor Nanowire Arrays for Semi-Transparent Organic Solar Cells
Yuyi Feng 1 , Kwang-Dae Kim 1 , Paul Kim 2 , Yoonseok Park 3 , Clay Nemitz 4 , Karl Leo 3 , James Dorman 5 , Jonas Weickert 1 , Lukas Schmidt-Mende 1
1 Physics, University of Konstanz, Konstanz Germany, 2 , Yale University, New Haven, New Jersey, United States, 3 , Dresden University of Technology, Dresden Germany, 4 , University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States, 5 , Louisiana State University, Baton Rouge, Louisiana, United States
Show AbstractFree-standing nanowire arrays have received increasing attention for a variety of applications such as photovoltaics, surface-enhanced Raman scattering (SERS), biosensing, plasmonics, and photocatalysis, due to their unique electrical and optical properties which are different from those in the bulk state. In particular, for organic and hybrid solar cells, an ideal architecture is proposed to consist of vertical arrays of silver-semiconductor core-shell nanostructures, since they provide efficient light harvesting and fast charge collection pathways.
Here we present a novel industrially-applicable method for fabricating uniform free-standing silver nanowire arrays on ITO glass, using anodic aluminum oxide (AAO) template assisted pulsed electrochemical deposition [1]. Moving forward, the silver nanowires are further quasi-conformally coated with a ZnO shell, and then those Ag-ZnO core-shell nanowires are integrated into semi-transparent bulk heterojunction organic solar cells. In-depth investigation of the device physics in terms of external quantum efficiency, charge collection, and recombination indicate a clearer general design route for ultimately achieving highly efficient and low-cost solar cells.
Reference:
[1] Y. Feng, K. Kim, C. Nemitz, P. Kim, T. Pfadler, M. Gerigk, S. Polarz, J. Dorman, J. Weickert, L. Schmidt-Mende. 2016. J. Electrochem. Soc. 163:6
10:00 AM - *NM7.4.01
Exploring Electromechanical Properties of III-V Nanowires for Energy Applications
Sohini Kar-Narayan 1
1 , University of Cambridge, Cambridge United Kingdom
Show AbstractThe increasing demand for portable, low-power, flexible and sustainable electronics for applications in wearable devices and self-powered sensors has spurred interest in the development of efficient piezoelectric nanogenerators (PENGs), where mechanical energy from ambient vibrations can be transformed into electrical energy to power autonomous devices. The most efficient piezoelectric materials tend to be ceramic in nature and often contain lead, such as lead zirconate titanate (PZT), however, in an effort to reduce the use of toxic materials, and even developing bio-compatible PENGs, other materials have been studied, such as ZnO nanostructures and nanowires (NWs), due to their relative ease of fabrication, and wurtzite (WZ) crystalline structure that gives rise to piezoelectric properties. In comparison, the piezoelectric properties of III-V semiconductor NWs with the required piezoelectric WZ structure, have hardly been studied, and more interestingly, the mere existence of piezoelectric properties in III-V NWs is far from trivial. Indeed there are no direct reports on the piezoelectric properties of single III-V NWs. The study of III-V based piezoelectrics is attractive considering the vast knowledge base related to these materials in terms of processing, and in particular in the possible combinations with optical properties in wavelengths spanning the visible spectrum. In this talk, I will present our recent work on developing novel scanning probe microscopy-based techniques that enables the direct characterization of the considerably less-explored but technologically relevant piezoelectric properties of III-V NWs. Piezo-response force microscopy (PFM) and quantitative nanomechanical mapping (QNM) will be discussed in this context as powerful tools to study and characterise the electromechanical properties of III-V nanowires, and the extraction of relevant materials properties for PENG design.
10:30 AM - NM7.4.05
Characterization of Silicon Nanowires with Infrared Near-Field Optical Microscopy
Earl Ritchie 1 , David Hill 1 , Tucker Mastin 1 , James Cahoon 1 , Joanna Atkin 1
1 Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Show AbstractSemiconducting nanowires have been proposed as materials platforms for the energy solutions of the future. In particular, silicon nanowires have been studied as photovolataic and photoelectrochemical materials for clean energy conversion. In order to realize these applications, careful design and characterization of optical and electrical properties, such as VOC, dopant concentration, and p-n junction abruptness, is needed. We report the use of mid-infrared (IR) scattering-type scanning near-field optical microscopy (s-SNOM) as a non-destructive optical method to extract quantitative, nanoscale mapping of free-carriers with high sensitivity in axially-doped silicon nanowires (SiNWs). Using this technique, we can detect local changes in the electrically-active doping concentration from the free-carrier absorption in both n-type and p-type doped SiNWs. The high spatial resolution (< 20 nm) allows us to directly measure transition abruptness arising from B and P dopants in single and multi-junction SiNWs. Combined with finite element analysis, this provides insight into the junction properties arising from the reservoir effect. This is especially valuable in boron-doped p-type SiNWs, for which nanometer-scale information on the junction properties is difficult to obtain without intensive processing. We are also able to extract quantitative information on the local carrier concentration and correlate nanoscale variations in doping and structure along the length of a SiNW, as well as detect changes in carrier density arising from fluctuations in growth conditions. The combination of s-SNOM with electrical techniques has the potential for enabling extraction of detailed, quantitative information about local conductivity properties in nanostructured functional materials and devices.
NM7.5: Nanowires for Next Generation Batteries
Session Chairs
Esther Alarcon-Llado
Anna Fontcuberta i Morral
Wednesday PM, April 19, 2017
PCC West, 100 Level, Room 105 A
11:15 AM - *NM7.5.01
Synthesis and In Situ Scanning Electron Microscopy Investigations of Semiconductor Nanowires for Smart Material and Energy Storage Applications
Steven Boles 1
1 , Hong Kong Polytech University, Kowloon Hong Kong
Show AbstractNanowires are often considered to be a core component for the next generation of electronic devices. However, the unique geometry and size-scale of nanowires makes them particularly attractive for discovering and exploring fundamental material properties which are not exclusively inherent to nanowires, but rather are also exhibited in micro- and bulk-specimens. In this work, silicon and other semiconductor nanowires are used a platform for in situ scanning electron microscopy investigations of energy-related materials and applications. In particular, single-crystal silicon nanowires grown using the vapor-liquid-solid technique provide new insights into the electrochemical alloying process which occurs inside lithium-ion batteries. Using our platform, potentiostatic control of lithiation can yield information about the nature of transformation in silicon-based anodes. Furthermore, these same nanowires can be mechanically investigated to determine their basic mechanical properties, such as elastic modulus, fracture strength and creep behavior. Findings from these investigations will be discussed. In situ mechanical studies can also be coupled with real-time electrical measurements. Coupled electro-mechanical testing of vanadium dioxide nanowires confirms the stress-induced phase change in this correlated electron material. The change between the two metal phases of the material is clearly coupled with a change in the material resistivity, which may be of interest for new, smart material applications. Lastly, new investigations and research directions based on the testing platform and methods previously described will be presented.
11:45 AM - NM7.5.02
Silicon-Germanium Alloy Nanowires as Potential High Capacity Lithium-Ion Battery Anodes
Killian Stokes 2 1 , Kevin Ryan 2 1
2 , Bernal Institute, Limerick Ireland, 1 , University of Limerick, Limerick Ireland
Show AbstractSi and Ge nanowires (NWs) have been identified as potential next generation anode materials for lithium ion batteries (LIBs) due to their high capacities of 3579 mAh/g and 1384 mAh/g respectively.1’ 2, 3 Ge has shown excellent capacity retention and rate characteristics but the high cost of Ge is a limiting factor in its implementation. Despite the low cost and natural abundance of Si, as well as its higher theoretical capacities, there are concerns over the long-term cyclability and rate performance of Si NWs in LIBs. The ability to combine both the greater stabilities of Ge with the high capacities offered by Si is very appealing for the future of LIB technologies.
Here, we report the formation of high-performance and high-capacity Li-ion battery anodes from very high-density Si-Ge alloy NW arrays grown directly from stainless steel current collectors. The synthesis uses a low-cost solvent vapour growth (SVG) system to grow Si-Ge alloy NWs of controllable ratios by the simultaneous delivery of Si and Ge monomers to the Sn catalyst. By systematically altering the amount of each precursor and carefully balancing precursor reactivity, the amount of expensive Ge precursor required can be minimized while still allowing for a wide range of alloy compositions to be synthesized (5:1 to 1:1 Ge:Si atomic %). This direct formation of Si-Ge alloy represents an efficient processing route for acquiring anodes possessing the benefits of Si (high capacity, lower cost) and Ge (improved rate performance and capacity retention). We show by ex-situ high-resolution transmission electron microscopy (HRTEM) and high-resolution scanning electron microscopy (HRSEM) studies that the excellent capacity retention could be attributed to the formation of a porous network after repeated charge and discharge cycling. Once formed, this network is highly stable, allowing for excellent capacities of greater than 1400 mAh/g to be retained after 100 cycles.
1. Kennedy, T.; Brandon, M.; Ryan, K.M. Adv. Mater. 2016, 28, 5969-5704.
2. Kennedy, T.; Mullane, E.; Geaney, H.; Osiak, M.; O’Dwyer, C.; Ryan, K. M. Nano Lett. 2014, 14, (2), 716-723.
3. Mullane, E.; Kennedy, T.; Geaney, H.; Ryan, K. M. ACS Appl. Mater. Interfaces 2014, 6, (21), 18800-18807.
12:00 PM - NM7.5.03
Optimizing Performance of Ge Nanowire Based Li-Ion Full-Cells
Hugh Geaney 1 , Kevin Ryan 1
1 , University of Limerick, Limerick Ireland
Show AbstractGe nanowires (NWs) have been highlighted as a promising anode material for future demanding Li-ion battery applications due to their stability over extended charge/discharge cycles and impressive high-rate performance. The majority of reports to date have focussed on testing Ge based anodes in half-cell geometries (i.e. vs a Li reference and counter electrode). In half-cells, Ge NWs have been shown to be capable of stable operation over 1000s of charge/discharge cycles due to the formation of a stable Ge network which prevents pulverisation of the active material.1, 2
Despite the promise of Ge anodes, reports on Ge based full-cells are very limited. To fully examine the potential of Ge anodes for real-world applications, full-cell testing against suitable cathode materials are urgently required. Here we present the operation of full cells consisting of Ge anodes paired with LCO cathodes. The cells show good capacity retention with an average discharge voltage of 3.5 V. Various parameters such as operating potential window, anode/cathode capacity matching, anode pre-conditioning to circumvent initial irreversible capacity losses, charge/discharge rate asymmetry and the effect of electrolyte additives are extensively probed to maximize the performance of the full cells. Failure mechanisms for the Ge anodes in full-cells and half-cells are compared, further emphasising the importance of extensive full-cell testing for promising anode materials.
1. T. Kennedy, E. Mullane, H. Geaney, M. Osiak, C. O’Dwyer & K. M. Ryan, Nano Lett., 2014, 14, 716-723.
2. E. Mullane, T. Kennedy, H. Geaney & K. M. Ryan, ACS Appl. Mater. Interfaces, 2014, 6, 18800-18807.
12:30 PM - *NM7.5.05
Nanowires for Enabling Fundamental In Situ Investigations and New Materials Architectures in Electrochemical Energy Systems
Matthew McDowell 1 2
1 George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractOver the past decade, a wide variety of nanostructured materials have been developed for improving the performance of electrochemical devices for energy storage and conversion (e.g., batteries and solar fuels systems). Along with this progress, the use of nanomaterials has also enabled fundamental in situ investigation of dynamic electrochemical reaction processes at the atomic-to-nanoscale, which is critical knowledge for further advancing device performance. Nanowires in particular have been critical elements in the development of in situ transmission electron microscopy (TEM) experiments for probing nanoscale reaction mechanisms in high-capacity, next-generation lithium battery materials. This talk will discuss fundamental insights from in situ TEM experiments into the reaction mechanisms of silicon nanowires, which are a promising, high-capacity anode material for lithium-ion batteries. The alloying reaction of lithium with silicon nanowires is shown to be strongly influenced by materials interfaces and crystallinity. In addition, the use of nanowire geometries in in situ experiments has also enabled the investigation of different reaction mechanisms in other nanomaterials for lithium and sodium-based batteries. Finally, the importance of hierarchical nanowire architectures for solar fuels devices will also be presented.
NM7.6: Pushing the Frontiers of Thermoelectric Energy Conversion
Session Chairs
Sudha Mokkapati
Carl Thompson
Wednesday PM, April 19, 2017
PCC West, 100 Level, Room 105 A
2:45 PM - *NM7.6.01
Investigating the Thermoelectric Properties of Semiconductor Nanowires
Ilaria Zardo 1
1 Departement Physics, Universitat Basel, Basel Switzerland
Show AbstractA large part of used energy is lost as waste heat. Thermoelectric materials offer the possibility to convert a temperature gradient into electrical energy, thus providing a mechanism to recover some of this energy. However, conventional thermoelectric materials suffer from low efficiency and/or high cost. The TE efficiency depends on the dimensionless figure of merit, zT=(σS^2)/κ T, with σ electrical conductivity, κ thermal conductivity, S the Seebeck coefficient, and T temperature. The low efficiency is caused by the interdependency of the parameters in the figure of merit zT. To overcome this problem nanostructuring has been proposed to independently enhance the thermoelectric properties [1]. Nanowires (NWs) are ideal candidates for exploring the effects of low dimensionality in TE applications, due to the theoretically predicted alteration of the density of states (thereby enhancement of the Seebeck coefficient) and thermal conductivity suppression, owing to an enhancement of the phonon scattering. To fully exploit these effects a full understanding of the influence of nanostructuring on thermal and electrical behavior is required. Recently considerable theoretical progress has been made, however experimental verification is still lacking. Indeed, the development of reliable technique that performs a complete assessment of the TE properties remains an experimental challenge. This is mainly due to the unknown thermal contact resistance [2].
The TE properties of single nanowires can be investigated using suspended the thermal bridge method. We recently investigated the diameter dependence of the thermal conductivity of InAs nanowires [3]. Furthermore, we have determined the zT of InSb NWs using a novel method based on the hybridization of spatially resolved Raman spectroscopy and transport measurements [4]. We have provided experimental evidence of the crucial role played by thermal contact resistance in determining the thermal conductivity using two distinct measurements methods of the thermal conductivity [5].
[1] L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47 16631 (1993)
[2] A. Soudi, R. Dawson and Y. Gu, ACS Nano 5 255 (2010); D. Liu et al. Nano Lett. 14 806 (2014)
[3] M. Y. Swinkels, et al. Nanotechnology 26, 385401 (2015)
[4] S. Yazji, et al. Nano Research 8, 4048 (2015)
[5] S. Yazji, et al. Semicond. Sci. and Technol. 31, 064001 (2016)
3:15 PM - NM7.6.02
Enhanced Thermoelectric Properties of PEDOT Nanowires for Printed Devices
Verena Schendel 1 , Silas Aslan 1 , Andre Gall 1 , Matthias Hecht 1 , Frederik Lessmann 1 , Ulrich Lemmer 1
1 Light Technology Institute, Karlsruhe Institute of Technology, Karlsruhe Germany
Show AbstractOrganic semiconductors based on conjugated systems have been successfully utilised for organic electronic devices ranging from photovoltaics to flexible OLED displays. However, the field of thermoelectrics - materials that convert a temperature difference into electrical energy- has been dominated over the last decades by inorganics, which are mainly based on low abundant and toxic compounds such as Bi2Te3. This has been mostly attributed to the poor figure of merit (ZT) of organics in comparison to their inorganic counterparts and has been seen by the community as the show-stopper for their ultimate use.
New material designs and synthetic strategies based on nanostructured polymers have boosted ZT substantially and by these means have resurrected interest in organic thermoelectrics and devices.
Here, we show the enhanced thermoelectric properties of PEDOT nanowires in comparison to their bulk counterparts and demonstrate – for the first time- their successful integration in printed thermoelectric generators.
NM7.7: Thermoelectric Nanowires and Devices
Session Chairs
Sudha Mokkapati
Carl Thompson
Wednesday PM, April 19, 2017
PCC West, 100 Level, Room 105 A
4:30 PM - *NM7.7.01
Photothermoelectric Energy Harvesting and Light Detection in Heterostructure Nanowires
Heiner Linke 1 2
1 NanoLund and Solid State Physics, Lund University, Lund Sweden, 2 School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, New South Wales, Australia
Show AbstractIn photo-thermoelectrics, incoming energy from light is converted into heat and then into electricity using the thermoelectric effect. A key advantage of using this effect is the possibility to directly harvest heat from photo-excited, non-equilibrium carriers (electrons and/or holes), thereby eliminating the phonon-mediated heat leaks that typically limit the efficiency of traditional thermoelectrics. Indeed, the carrier temperature of photogenerated carriers in nanowires has been observed to be substantially higher than that of the lattice, allowing for high, local temperature differentials. We demonstrate this approach by using heterostructure InAs/InP nanowires, employing an InP barrier as a thermionic energy filter. Under illumination of the full wire we demonstrate a phototermoelectric effect with high open-circuit voltage, consistent with a large carrier temperature difference across the thermionic barrier, and novel functionality in photodetection.
S. Limpert et al., submitted (2016)
5:00 PM - NM7.7.02
Remote Doping-Based Approach to Thermoelectric Performance Enhancement in N-Type Flexible Nanocomposites
Hyeunhwan An 1 , Matthew Pusko 1 , Dale Karas 1 , Jaeyun Moon 1
1 , University of Nevada, Las Vegas, Las Vegas, Nevada, United States
Show AbstractThermoelectric (TE) materials, which can generate electricity from heat or vice versa, have great potential for waste heat recovery and solid-state heating/cooling applications. The eco-friendly TE power generation could be widely used as a novel energy harvesting system with high efficiency energy conversion. However, most TE devices are characterized by relatively high cost and the poor processability of the inorganic TE materials has retarded their application development. In order to employ cost-effective manufacturing technologies, such as printing techniques, flexible TE materials with high thermoelectric figure of merit (ZT) are required [1]. One promising strategy to achieve high ZT is remote doping, resulting in increasing carrier mobility [2]. Herein, we proposed the remote-doping approach for improving the TE properties of n-type flexible TE material consisting of the in situ synthesis of Bi2Te3 nanowires matrix with single-walled carbon nanotubes (SWCNTs).
The nanocomposites of n-type Bi2Te3 nanowires and SWCNTs with varied compositions have been prepared through in situ synthesis. The in-situ synthesis is beneficial to the uniform distribution of the CNTs in the matrix, the cleaner interface and the stronger interfacial bonding within the composites in comparison with ex-situ one [3]. The remote doping is that charge carriers are spatially separated from their parent impurity atoms to reduce the influence of impurity scattering and thereby increase the charge carrier mobility [2]. In this material system, SWCNTs provide carrier transport path helping to increase electrical conductivity. The dispersion of SWCNTs form new interfaces with the Bi2Te3 nanowires matrix and introduce defects to cause heat-carrying phonons scattering, resulting in reducing the lattice thermal conductivity. In addition, it can also simultaneously enhance the Seebeck coefficient due to electron energy filtering effect [4]. Along with in situ synthesis, the post thermal treatment was employed not only to form tight interface bonding but also to adjust Bi2Te3 composition close to the stoichiometric composition, because the TE properties of Bi2Te3 are strongly dependent on its stoichiometry [5]. In consequence, the power factor of the post-annealed composites was significantly enhanced. The in-situ synthesis and post-treatment effects on Bi2Te3 nanowires and SWCNTs composites were structurally characterized by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM). The interface bonding of the composite was studied through X-ray Photoelectron Spectroscopy (XPS). Understanding these phenomena is necessary to improve thermoelectric efficiency of n-type flexible TE devices.
5:15 PM - NM7.7.03
Manipulating Electrical and Thermal Transport in Nanowire Based Nanocomposite through Doping for Thermoelectric Applications
Yue Wu 1
1 , Iowa State University, Ames, Iowa, United States
Show AbstractIn this talk, we will discuss our work on doping p-type BixSb2-xTe3 nanowire based bulk nanocomposites to shift the operation temperature to over 500K with a much improved ZT over 1. Our success in increasing the efficiency of p-type BixSb2-xTe3 at higher temperatures can be largely explained by the increase in the optical band gap, according to the Burstein-Moss shift, due to doping, thus delaying the onset of bipolar conduction.
5:30 PM - *NM7.7.04
Silicon-Based Nanowires for Fully Integrated Micro Thermo-Electric Generators
Gerard Gadea 1 , Alex Morata 1 , J.D. Santos 1 , Carlos Calaza 2 , Marc Salleras 2 , Inci Donmez 2 , Luis Fonseca 2 , Albert Tarancon 1
1 , Institut de Recerca en Energia de Catalunya (IREC), Barcelona Spain, 2 IMB-CNM (CSIC), Carrer dels Til-lers s/n, Campus UAB, Bellaterra Spain
Show AbstractSilicon-based nanowires are promising candidates for being functional thermoelectric materials for energy harvesting since they combine the low cost, high availability and easy integration of silicon with a great enhancement of their figure of merit conferred by nanostructuring.
In this work, Si and SiGe nanowires were integrated in planar micromachined structures to exploit their good thermoelectric properties. Vertically aligned silicon-based nanowires were grown by Vapour-Liquid-Solid synthesis in Chemical-Vapour-Deposition reactors (CVD-VLS) from gold seeds selectively deposited by galvanic displacement. This bottom-up approach allows fabricating doped Si and SiGe epitaxial nanowires with controlled properties (diameter, length, doping level and alloy composition) fully integrated in silicon microdevices, i.e. monolithically integrated with low contact resistances. Moreover, a comprehensive thermoelectrical characterization was carried out for obtaining the figure of merit of single and arrays of Si and SiGe nanowires.
NM7.8: Poster Session
Session Chairs
Thursday AM, April 20, 2017
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - NM7.8.02
Surface Effects in Polytypic Gallium Arsenide Nanowires
Natasa Vulic 1 2 , Dmitry Mikulik 2 , Gozde Tutuncuoglu 2 , Heidi Potts 2 , Stephen Goodnick 1 , Anna Fontcuberta i Morral 2
1 School of Electrical, Computer, and Energy Engineering, Arizona State University, Phoenix, Arizona, United States, 2 Institut des Matériaux, Ecole Polytechnique Fédérale de Lausanne, Lausanne Switzerland
Show AbstractArrays of GaAs nanowires (NWs) are promising candidates for solar cell devices due to their improved optical and electrical properties over planar devices. The NW geometry increases light absorption for the same quantity of material, while the potential for radial p-n junctions reduces the need for long diffusion lengths (i.e. high purity materials) [1-2]. While the carrier losses in the bulk are reduced with NW geometry, the high surface-to-volume ratio makes surface carrier recombination a dominant loss mechanism in non-passivated NWs [3], strongly impacting the expected efficiencies in NW solar cells [4]. Polytypic nature of NWs grown by molecular beam epitaxy (MBE) creates additional challenges with respect to the surface recombination. During NW growth, bulk defects readily propagate to the surface of the NW; poor quality of the bulk thus increases recombination centers at the surface. In addition, carrier losses are further enhanced due to the type II band alignment in the regions of alternating zinc-blende/wurtzite crystal phases. As a result, study of carrier dynamics and surface passivation in these NWs is especially needed.
The goal of this study is to evaluate the efficiency of surface passivation in self-catalyzed GaAs NWs with photoluminescence (PL) spectroscopy, by correlating time-integrated PL (TIPL) and time-resolved PL (TRPL) measurements for NWs with different surface passivation methods. Individual NWs are removed from an array of GaAs NWs grown by catalyst-free MBE and dispersed onto a polished Si wafer. We perform TRPL to assess the relaxation rate and potentially the presence of multiple decay channels for different transitions in GaAs NWs: bandgap of GaAs, defect bands, and indirect transitions in polytypic regions. In parallel, we perform power series TIPL to understand which transitions are dominant in intensity. The two methods complete each other: the lack of information about radiative/non-radiative processes in TRPL is complemented by looking at the radiative population variations in the TIPL signals. We measure three sets of samples with GaAs cores: AlGaAs capped, Al2O3 capped, and uncapped NWs. Preliminary results indicate superior surface passivation of AlGaAs capped NWs compared to atomic layer deposited (ALD) Al2O3 coating. Suppressing surface recombination has allowed us to observe the presence of multiple radiative decay channels in GaAs NWs: a fast radiative decay channel at the bandgap of GaAs (1.52 eV) and a slow radiative decay channel in the red-shifted region (1.48 eV). To further investigate the surface effects, we correlate power-series TIPL for the three sets of samples (i.e. saturation of electronic transitions) with the rate of radiative decay at the bandgap of GaAs.
[1] E.C. Garnett et al, Annu. Rev. Mater. Res., 2011, 41, 269
[2] P. Krogstrup et al, Nature Photonics, 2013, 7, 306
[3] O. Demichel et al., Appl. Phys. Lett., 2010, 97, 201907
[4] S. Yu et at., J. Photon. Energy., 2012, 2(1), 028002
9:00 PM - NM7.8.03
Gas Depletion Effects on Preferential Growth Directions of Aluminum-Catalyzed Silicon Nanowires
Mel Hainey 1 , Ke Wang 1 , Joan Redwing 1
1 , The Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractSilicon nanowires grown in the <110> direction are of scientific interest because they have been predicted to show improved hole mobilities compared to <111> wires. However, uniform growth of these wires has been difficult. Recently, the growth of aluminum-catalzyed <110> wires was discussed as a function of growth temperature. Growth temperatures above the Al-Si eutectic (573°C) lead to <111> wire growth, while growth temperatures below the eutectic promoted <110> growth. This temperature dependence was believed to occur due to the change in silicon supersaturation in the Al catalyst droplet. At sub-eutectic temperatures, Si solubility in Al is only 1.5%, while above the eutectic the solubility increases to 12.5%. Droplet supersaturation was previously reported to control growth direction during germanium nanowire growth, and provides a good explanation for the observed transition between <110> and <111> growth directions.
However, growth temperature alone provides an incomplete explanation for how droplet supersaturation determines growth direction, as aluminum-catalyzed <111> wire growth from Si(111) substrates has been previously reported at sub-eutectic temperatures. The role of silane partial pressure, particularly as it changes across the reactor due to decomposition, has not been previously examine. To study the effect of changing silane partial pressures, 3 cm long samples were loaded into the reactor covering both upstream, high partial-pressure regions, and downstream, low-partial pressure regions. LPCVD growth was performed on Si(110) samples with 10 nm Al films following conditions previously used for <110> wire growth. Growths were performed at 300 and 500 Torr in a hydrogen ambient with a silane partial pressure of 2.5 Torr and a total flow rate of 100 sccm. A 60 minute pre-anneal at 550°C was performed prior to a 30 minute growth at the same temperature. Subsequent post-growth characterization using cross-section SEM was used to confirm growth directions of the nanowires.
After growth, an abrupt transition in growth regime from upstream to downstream regions of the sample can be observed visually, with the upstream region appearing much darker than the downstream region. Cross-section SEM reveals that the wires in the upstream region are almost entirely <111> oriented and present in high densities. In the downstream region, wires are primarily <110> oriented and present in lower densities. Remarkably, similar behavior is observed when the Si(110) substreates are replaced with Si(100) substrates. In downstream regions of the samples, vertical wire growth from the Si(100) substrates can be directly observed, with TEM confirming the <100> growth direction. These results suggest that the silane partial pressure is decreasing along the reactor as it reacts, as expected for a horizontal flow LPCVD process, and are low enough to promote the low droplet supersaturations necessary for <110> and <100> wire growth.
9:00 PM - NM7.8.04
Morphology Dependent Optical Properties of ZnO/SiNWs Nanocomposites
Vitaly Bondarenko 1 , Bruno Azeredo 2 , Stanislau Niauzorau 1 , Aliaksandr Sharstniou 1
1 Micro- and Nanoelectronics, Belarusian State University of Informatics and Radioelectronics, Minsk Belarus, 2 , Arizona State University, Phoenix, Arizona, United States
Show AbstractZinc oxide/silicon nanowires (ZnO/SiNWs) nanocomposites are a promising material for solar cells as they combine the low-reflectivity of SiNWs and wide-band gap of ZnO to effectively harvest the solar spectrum. This paper presents a study of the anti-reflective properties of nanocomposites based on SiNWs synthesized by metal-assisted chemical etching (MACE) and electrochemically deposited ZnO nanoparticles. The reflectance of ZnO/SiNWs nanocomposites were measured before and after an annealing treatment (at 750 °C for 30 min), and compared it to a reference sample of SiNWs. It was found that SiNWs demonstrated an average total reflectance of 0.03-0.05 % in the wavelength range from 350 to 1100 nm. The total reflectance increased to 10-15 % for as-synthesized nanocomposites and decreased to 0.2-0.5 % for annealed nanocomposites. This increase and reduction in reflectance is attributed to (a) a high defect density originally present in the as-synthesized ZnO crystal lattice and (b) an increase in the ZnO crystalline quality during the annealing process, respectively. These trends remained true for SiNWs of different lengths and doping levels. Thus, the total reflectance of ZnO/SiNWs nanocomposites are tied to the morphology of ZnO grains formed alongside the SiNWs array. The reflectance of the ZnO/SiNWs nanocomposites (~ 0.5 %) are comparable with recent studies in this field.
9:00 PM - NM7.8.05
Synthesis and Characterization of Colloidal CsPbX3 (X = Cl, Br, I) Nanowires
Dandan Zhang 1 , Yiming Yang 1 , Yehonadav Bekenstein 1 , Yi Yu 1 , Samuel Eaton 1 , Natalie Gibson 1 , Andrew Wong 1 , Letian Dou 1 , Stephen Leone 1 , A. Paul Alivisatos 1 , Peidong Yang 1
1 , University of California Berkeley, Berkeley, California, United States
Show AbstractSemiconductor nanowires (NWs), geometrically combining the nano-scale radial dimension with micro-scale lengths, are promising building blocks for various applications in electronics, optoelectronics, sensing, and energy harvesting at the nanoscale.
Halide perovskite (PVSK) is a class of important optoelectronic materials that have attracted significant interested in the past few years, because of their remarkable charge transport properties, high defect tolerance, broad chemical tunability, and facial solution processability. The interesting material properties, and the lack of the shape-controlled synthesis of its one-dimensional geometry, as well as the incomplete understanding of its structure-property relation, motivated us to pursue the synthetic strategies for halide PVSK NWs, and to study their properties and applications.
This talk will focus on our recent progress on the synthesis, and characterization of CsPbX3 (X = Cl, Br, I) nanowires. CsPbX3 nanowires with difference sizes and a wide range of alloy compositions were successfully synthesized through a colloidal method combined with anion-exchange reactions. The bright photoluminescence can be tuned over nearly the entire visible spectrum. Furthermore, extensive optical, electronic, and thermoelectric characterization has been done on those samples, showing the NWs are with high photoluminescence quantum yield (PLQY), low defect density, and low thermal conductivity. Finally, in order to address the concern of the stability and lead leakage issues for the actual application of the halide PVSK, a polymer-PVSK composite has been demonstrated with enhanced water and light stability, and no detected lead leakage in the environment.
9:00 PM - NM7.8.06
On the Physical Properties of Radial ZnO-core/ZnS-Shell Nanostructures Deposited at Atmospheric Pressure Using DEZn, N2O and DTBS
Jyh-Rong Gong 1 , Ho-Ching Ni 1
1 , National Chung Hsiang University, Taichung, Choose a State or Province, Taiwan
Show AbstractRadial ZnO-core/ZnS-shell nanostructures were deposited at atmospheric pressure by atomic layer deposition (ALD) using diethylzinc (DEZn), nitrous oxide (N2O) and ditert-butylsulfide (DTBS) precursors. The structural and optical properties of radial ZnO-core/ZnS-shell nanostructures were characterized using θ-2θX-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), and photoluminescence spectroscopy (PL).
Both ZnO-core and ZnS-shell of the ALD-deposited radial ZnO-core/ZnS-shell nanostructure were found wurtzite in nature based upon the results of HRTEM investigations. Moire fringes were identified in the radial ZnO-core/ZnS-shell nanowire structure with an observed period being in good agreement with the lattice mismatch between ZnO-core and ZnS-shell of the pseudomorphic ZnO-core/ZnS-shell nanostructure.
Strong neutral donor excitonic emissions (DoX) along with related longitudinal optical phonon (LO) replicas of ZnO cores were observed in the 10K PL of the ALD-deposited radial ZnO-core/ZnS-shell nanostructures. It appears that the ALD-deposited radial ZnO-core/ZnS-shell nanostructures exhibit high structural and optical quality.
9:00 PM - NM7.8.07
Solution Synthesis of Colloidal RbPbI3 Orthorhombic Perovskite Nanowires
Da-Hye Lim 1 , Parthiban Ramasamy 1 , Min-Sang Lee 2 , Jong-Soo Lee 1
1 , DGIST, Deagu Korea (the Republic of), 2 , Ecolumy, Deagu Korea (the Republic of)
Show AbstractSince the study on metal halide perovskites materials, metal halide perovskites have been generating enormous interest among researchers due to their excellent properties such as facile band gap tunability, high fluorescence quantum yield, long carrier lifetimes as well as low non-radiative recombination rates. Because of these properties, metal halide perovskites have been intensively studied as a promising class of materials in various applications such as solar cells, light-emitting diodes (LEDs), lasers, X-ray detectors and photodetectors. Especially, hybrid organic-inorganic lead perovskite based solar cells which is commonly fabricated using methylammonium lead iodide (CH3NH3PbI3), have been demonstrated rapid improvement on power conversion efficiencies from 3% to over 22%. However, weak stability in air because of disassociating organic-iodide and lead iodide cause serious problem on commercialization.
Therefore, to improve capricious hybrid composition, all-inorganic cesium lead halide perovskite nanocrystals have attracted considerable attention as an alternative material. Replacing organic cation to Cesium cation exhibit brighter photoluminescence with emission tunable over the entire visible region, high absorption coefficient and facile band gap engineering with exchanging halide anion. Currently, new perovskite compositions have been introduced with les toxicity such as Cs3Bi2I9, CsBi3I10, CsSnX3 and Rb3Sb2I9. Smaller radii rubidium embedded hybrid organic-inorganic perovskite solar cells have been recently reported and the authors suggest rubidium could help to improve stability.
Here we present solution colloidal synthesis of rubidium lead iodide (RbPbI3) nanowires. We studied their crystal structure by measuring X-ray diffraction. It indicates orthorhombic structure for rubidium lead iodide nanowires. In addition, these nanowires exhibit strong absorption around 420 nm. Moreover, by changing reaction temperature and time, we can control the length and thickness of nanowires. In addition, photo-responsive behavior is observed for the RbPbI3 nanowires based device. This indicating their promising potential as photoelectrical applications such as solar cells and photodetectors.
9:00 PM - NM7.8.08
Indium Selenide (In2Se3) Nanowires Synthesis and Characterization
Ya Chu Hsu 1 , Chiu-Yen Wang 1
1 , National Taiwan University of Science and Technology, Taipei Taiwan
Show AbstractIn2Se3 nanowires (NWs) were synthesized in a quartz-tube furnace system with two-temperature zones by using a VLS mechanism. Here, we discussed the two methods to grow the NWs. First, trying to grow the NWs by controlling the temperature of precursor, temperature of growth area, pressure, carrier gas, thickness of Au film, position of substrate and growth time of NWs. However, we found the major problem, which is the larger diameter of the In2Se3 NWs. In order to grow the proper NWs of the diameter for fabricating the device, by controlling the different conditions to grow the proper size of the nanowires. Second, before operating the tube furnace processes, the silicon (100) substrates coated with thickness 2 nm gold film doing the rapid thermal processing (RTP) first, which the annealing temperature is 600 oC (100 oC/S). Sum up with the above conditions, NWs in different conditions can be grown verity of configuration or distribution. When the temperature of precursor is 850 oC, temperature of growth area is 600 oC, pressure is 1 Torr, and carrier gas is 25 sccm, we can get the proper length and diameter of the NW. In2Se3 NWs were characterized by field-emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) the morphology and crystal structure. In the future, we can grow the more suitable In2Se3 NWs to increase the diversity of the research on electrical application.
9:00 PM - NM7.8.09
Vapor Phase Synthesis and Optoelectrical Properties of Structurally Tailored Organic-Inorganic Perovskites for Nanowires Transistors
Dong Ruoting 1
1 , City University of Hong Kong, Hong Kong Hong Kong
Show AbstractSemiconductor nanowires have received considerable attention in the past decade driven by both unprecedented physics derived from the quantum size effect and strong isotropy and advanced applications as potential building blocks for nanoscale electronics and optoelectronic devices. Organic-inorganic hybrid perovskites, the structural analogues of calcium titanium oxide crystal, have been widely studied due to their unique structure of alternating stacking sheet of organic and inorganic components. Importantly, the excellent optical and electrical transport properties of these organic-inorganic perovskites make them suitable for various technological devices, such as light-emitting diodes, lasers, photo-detectors, and field-effect transistors. Herein, we present the vapor phase synthesis, structural, and optoelectronic properties of semiconducting CH3NH3PbI3 (MAPbI3) perovskite nanowires. The intriguing properties make MAPbI3 nanowires suitable for the applications in electronics, optoelectronics, and etc.
9:00 PM - NM7.8.10
Spectroscopic Characterization by Super Resolution Transient Absorption Microscopy
Eric Massaro 1 , Andrew Hill 2 , Casey Kennedy 1 , Erik Grumstrup 1 2
1 Chemistry Biochemistry Department, Montana State University, Bozeman, Montana, United States, 2 Materials Science Program, Montana State University, Bozeman, Montana, United States
Show AbstractThe electronic environment of complex disordered materials is heavily influenced by microscale structural variation and defects. Due to the limited spatial resolution of bulk spectroscopic characterization techniques they are unable to deconvolve the spectroscopic contributions of the various heterogeneities. In order to more accurately determine the structure function relationship in complex materials we have developed Structured Pump-Probe Microscopy (SPPM). SPPM incorporates a structured excitation field along with a diffraction limited probe pulse in order to collect additional high spatial frequency information that would typically be filtered by the optical system. Both modeling and experimental data show a nearly two-fold increase in one dimensional spatial resolution is possible across a wide range of wavelengths. Along with the increased imaging resolution, SPPM also retains the spectroscopic capabilities of traditional pump-probe microscopy. Continued development of SPPM promises to produce a useful tool for the characterization of sub-wavelength materials systems.
9:00 PM - NM7.8.11
Epitaxy of GaN Nanowires on Versatile Substrates
Vishnuvarthan Kumaresan 1 3 , Ludovic Largeau 1 3 , Ali Madouri 1 3 , Frank Glas 1 3 , Hezhi Zhang 2 3 , Fabrice Oehler 1 3 , Antonella Cavanna 1 3 , Andre Babichev 2 4 3 , Laurent Travers 1 3 , Noelle Gogneau 1 3 , Maria Tchernycheva 2 3 , Jean-Christophe Harmand 1 3
1 , CNRS, Marcoussis France, 3 , University Paris Saclay, Palaiseau France, 2 , University Paris-Sud, Orsay France, 4 , ITMO University, Saint Petersburg Russian Federation
Show AbstractNitride nanowires (NWs) are today actively explored as an active material for a large number of optoelectronic devices (light emitting diodes, photodetectors, solar cells). The NWs are usually grown on bulk crystalline substrates (Si, sapphire) but these substrates impose their properties which may not be adapted to the device functionality (low electrical or thermal conductivity, opacity, weigh, rigidity, cost,..). In this study, we will present our last results dealing with the growth, structural and optical properties of GaN NWs grown plasma assisted molecular beam epitaxy (PA-MBE) on fused silica [1] and graphene [2].
Glass substrates can be produced in large areas and are widely available. Amorphous substrates could bring new flexibility or functionality as well as cheaper production costs, especially for devices such as LEDs and solar cells for which the glass transparency could be effective. We have shown that high quality GaN NWs can be grown on thermal or fused silica. The NWs are N-polar, they have a random in-plane orientation but their axial orientation is remarkably aligned with the substrate normal. No extended defects are present at the GaN-silica interface. Photoluminescence (PL) and Transmission Electron Microscopy (TEM) studies indicate that inversion domains boundaries (IDBs) are less likely to form in NWs grown on fused silica than on Si (111). The weaker bonding of the NWs on fused silica should be very well suited to an eventual transfer process : peeling off the NWs from the silica must be easier than from any crystalline substrate. The growth of near vertical GaN NWs on silica without degradation of their structural and optical properties opens new routes for the growth of GaN NWs on foreign substrates.
Graphene is transparent, flexible and it has high thermal and electrical conductances and it can be synthesized at low cost on large areas. Furthermore, graphene films are easily transferable to almost any carrier substrate, including amorphous and/or flexible materials. We have identified that CVD-grown graphene is excellent for GaN NW growth. Epitaxy is demonstrated on large and small graphene flakes transferred onto amorphous SiOx carrier layers. In our growth conditions, NW growth is highly selective; NWs nucleate only on graphene layers and not on the surrounding silica. The nanowires grow vertically along their c-axis and we observed a unique epitaxial relationship with the <10-10> directions of the wurtzite GaN lattice normal to the directions of the carbon zigzag chains. We proposed that the two materials are strained to accommodate a -3.1% in-plane misfit insuring a certain lattice coincidence. Finally, the optical quality is comparable to that of GaN NWs grown on crystalline Si substrate. This successful epitaxial growth of GaN NWs on graphene seems particularly promising for the development of flexible devices.
[1] Kumaresan, et al. Nanotechnology, 269501 (27) 2016
[2] Kumaresan, et al. Nano Letters, 4895 (16) 2016
9:00 PM - NM7.8.12
New Insight into the Effect of Annealing on the Photoelectrochemical Performance of Rutile Single-Crystalline TiO2 Nanorods Arrays
Chao Huang 1 , Juncao Bian 1 , Ruiqin Zhang 1
1 , City University of Hong Kong, Shatin Hong Kong
Show AbstractTitanium dioxide nanorods arrays (TiO2 NRAs) have been widely studied as photoanode materials in a photoelectrochemical (PEC) cell but are severely limited by their poor performance without further thermal treatment. Herein, we systematically investigated the effect of annealing treatment on the optical and electronic properties, and PEC performance of the rutile single-crystalline TiO2 NRAs. The hydrothermal method prepared rutile TiO2 NRAs are separately annealed in nitrogen (N2) and oxygen (O2) gas flows to trigger a synergistic surface modification. A maximum photocurrent density of 1.38 mA/cm2 was recorded from O2 annealed TiO2 NRAs, far exceeding that of pristine TiO2 NRAs (0.05 mA/cm2). X-ray photoelectron spectroscopy (XPS) were applied to study the changes of surface states of TiO2 NRAs after annealing. The largest depletion region width was calculated from the Mott-Schottky curves to be 34.6 nm from O2 annealed TiO2 NRAs. These observed results demonstrate that dramatic enchantment of PEC performance is attributed to the removal of surface adsorbed Cl and the enlargement of depletion layer width. This work may be beneficial for the optimization of TiO2 NRAs for potential applications.
9:00 PM - NM7.8.13
Three-Dimensional Cobalt Phosphide Nanowire Arrays for Flexible Solid-State Asymmetric Supercapacitors
Zhi Zheng 1 , Michael Retana 1 , Ramona Luna 1 , Weilie Zhou 1
1 Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana, United States
Show AbstractFlexible energy storage devices have received tremendous interest recently due to the increasing demand for sustainable and renewable energy in modern electronic industry. In this study, three-dimensional cobalt phosphide nanowire arrays on carbon cloth were synthesized by a simple two-step hydrothermal method. Owing to the unique nanostructures, it exhibits good performance such as high capacitance and high rate capability when utilized as supercapacitor electrodes. Moreover, the solid-state flexible asymmetric supercapacitor based on cobalt phosphide electrode demonstrates excellent performance such as high energy density and power density. In addition, the solid-state supercapacitor devices show remarkable cycle stability with the capacitance retention of more than 80% after 5000 cycles. This work demonstrates an example of cobalt phosphide for supercapacitor electrode application as well as a promising candidate for next-generation energy storage material.
9:00 PM - NM7.8.14
Electron Beam at Low Acceleration Voltage does not Damage Carbon Nanotube and Graphene
Jae Hong Choi 1 , Chang Young Lee 1
1 , Ulsan National Institute of Science and Technology (UNIST), Ulsan Korea (the Republic of)
Show AbstractScanning electron microscope (SEM), offering both convenience and sub-nm lateral resolution, is a principal tool for studying nanomaterials below the Abbe’s diffraction limit. Imaging soft nanomaterials, including single walled carbon nanotubes (SWNTs) and graphene, is usually performed at low acceleration voltage. Imaging carbon nanomaterials in SEM, however, is known to increase the disorder mode (D-mode) in their Raman spectra. Earlier studies attributed the phenomenon to electron beam (e-beam)-induced structural damages to the materials under investigations, which can be recovered by thermal treatment. For more than a decade, researchers have accepted this conclusion without further verification. Here, we perform single-tube measurements and clearly demonstrate that the e-beam-induced D-mode in carbon nanomaterials can be restored chemically even at room temperature, and the e-beam-induced deposition (EBID) of hydrocarbon is not the main cause of the D-mode enhancement. For both SWNTs and graphene, e-beam-induced D-mode can be suppressed by removing amorphous carbon via heat treatment. These results strongly suggest that the e-beam-induced D-mode increase originates solely from the irradiated amorphous carbon, and not from the e-beam-induced structural damage to the carbon nanomaterials. Our study suggests that the notion that has long been accepted by the community needs to be reconsidered, and thus helps to study nanoscale phenomena in nanomaterials minimizing any potential ambiguities.
9:00 PM - NM7.8.15
Preferential Transport of Cations along the Exterior of Single Walled Carbon Nanotubes Assisted by Cation- π Interaction
Yun-tae Kim 1 , Chang Young Lee 1
1 , Ulsan National Institute of Science and Technology, Ulsan Korea (the Republic of)
Show AbstractA nanoscale conduit that preferentially transports a selected charge provides a unique opportunity of studying charge-specific chemistry in 1D space, single-molecule transport and detection, and novel charge storage devices. The π-electron-rich surface of single walled carbon nanotube (SWNT), being theoretically predicted to strongly interact with cations, potentially allows charge-specific transport of ions. Here we demonstrate via surface analyses the first experimental evidence of cation-selective ionic transport along the exterior of SWNTs under electric field, while anions slowly counter-diffuse along the substrate adjacent to the nanotubes. The counter-flow of spatially separated charges at different fluxes causes ionomigration of water molecules toward cathode, thereby pumping micro-droplets of electrolytes along the nanotubes near anode, followed by their sequential migration toward cathode. The micro-droplets containing hygroscopic NaCl not only help optically visualize individual nanotubes for an extended period of time, but serve as micro-lenses that enhance Raman scattering of nanotubes by up to two orders of magnitude. Our study on the relatively-overlooked exterior of SWNTs presents how the cation-π interaction leads to spatiotemporal non-uniformity during transport of ions, a 1D analog of ionomigration observed in biological systems and bulk ionic crystals.
9:00 PM - NM7.8.17
Characterization of GaAsP NWs and GaSb TPV Solar Cells Grown on Si Substrate
Sarfraz Ali 1 , Sergey Rybchenko 1 , Yunyan Zhang 2 , Huiyun Liu 2 , Stephanie Haywood 1
1 , University of Hull, Hull United Kingdom, 2 , University College London, London United Kingdom
Show AbstractIntegration of direct band gap III-V materials onto and cost effective substrates like Si can be highly advantageous. This offers high absorption coefficients, high carrier mobilities and coverage over a broad range of solar spectrum for efficient application in photovoltaic devices. However, this integration has always been challenging because of the large lattice mismatch and difference in thermal expansion coefficients. In recent years, one dimensional semiconductor nanowire structures have gained significant attention as a means to integrate III-V materials onto Si substrates. It is now also possible to grow abrupt hetero-interfaces with fewer defects despite a large lattice mismatch between the substrate and the bulk materials. Strain between III-V bulk material and substrate is relieved by using different growth techniques like - step graded buffer growth, inverse growth, etc. However, strain in NWs relieved naturally due to their small interfacial area. Single NW GaAsP solar cells grown on Si have already demonstrated efficiency of more than 10% (Holm et.al, 2013). Developing less defective NW structures is considered to be a key step in reaching the efficiency values closer to the theoretical limit of 43% two-junction III-V nanowire on Si (LaPierre, 2011).
Novel GaSb/Si TPV solar cells and GaAsP NWs, grown by our collaborators in UCL (Prof. Huiyun Liu’s group), are characterised using TEM, photoluminescence and Raman spectroscopy. Cross sectional TEM analysis of GaSb/Si TPV cells reveals dislocations at the interface. Similarly, a high density of twin and stacking faults appear in our initial TEM results of GaAsP Nanowires. Visible strain relaxation patterns at NWs/Si substrate interface are also observed in the cross sectional TEM images. We present the results of these structural studies and correlate them with optical spectroscopy data.
References:
1. Holm, J.V., Jørgensen, H.I., Krogstrup, P., Nygård, J., Liu, H. and Aagesen, M., 2013. Surface-passivated GaAsP single-nanowire solar cells exceeding 10% efficiency grown on silicon. Nature communications, 4, p.1498
2. LaPierre, R.R., 2011. Theoretical conversion efficiency of a two-junction III-V nanowire on Si solar cell. Journal of applied physics, 110(1), p.014310.
9:00 PM - NM7.8.18
Structural and Optical Studies of Doped GaAsP Nanowires Grown by Aerotaxy
Sudhakar Sivakumar 1 2 , Wondwosen Metaferia 1 2 , Axel Persson 1 3 , Kilian Mergenthaler 1 2 , Olof Hultin 1 2 , Reine Wallenberg 1 3 , Mats-Erik Pistol 1 2 , Knut Deppert 1 2 , Lars Samuelson 1 2 , Martin.H Magnusson 1 2
1 NanoLund, Lund University, Lund Sweden, 2 Solid State Physics, Lund University, Lund University, Lund Sweden, 3 nCHREM/Centre for Analysis and Synthesis, Lund University, Lund Sweden
Show AbstractDue to their direct band gap and excellent electrical and optical properties, III-V semiconductor NWs have attracted considerable attention for their potential use in various applications, e.g., high-efficiency solar cells [1], energy storage [2], and lasers [3]. GaAsP offers a wide tunability of the energy band gap from near infrared (Eg = 1.42 eV) to visible regions (Eg = 2.3 eV), that makes it an important ternary alloy for optoelectronic applications. We have realized the growth of GaAsP and GaAs NWs with energy bandgap in the range of 1.42 to 1.9 eV [4], by Aerotaxy- a low cost, efficient and continuous gas phase III-V NW mass production technique [5]. In Aerotaxy, an aerosol of size-selected Au catalyst nanoparticles in N2 is mixed with MOCVD precursors (TMGa, AsH3, PH3, DEZn, TESn) in a flow-through reactor at atmospheric pressure, whereby nanowires are produced continuously in high concentrations and at a high growth rate of about 1 µm/s.
Here, we present structural and optical studies of GaAsP NWs and discuss the effect of growth conditions on structural and optical qualities. From HR-TEM studies, the GaAsP NWs are found to exist only in the zincblende crystal phase, in contrast to MOVPE-grown nanowires which often include wurtzite segments. From low temperature PL, the phosphorus content in the NWs was estimated and the incorporation rate ratios of As/P is determined to be ~4 at a growth temperature of 550 °C. Zn and Sn doping of GaAsP NWs for p- and n- type respectively was conducted and the effects of doping on the structural and optical properties of the NWs were investigated.
[1] I. Åberg et al., IEEE J. Photovoltaics, 6 (1), (2016)
[2] L. Mai et al., Chem. Rev., 114 (23), (2014)11828
[3] D. Saxena., Nat Photonic 7(12), (2013), 963
[4] W. Metaferia et al., Nano Lett. 16, (2016), 5701
[5] M. Heurlin et al., Nature. 492, (2012), 90
9:00 PM - NM7.8.19
Novel Morphologies of Bi2S3 Synthesized by Microwave Heating without Complexing Agents
Evelyn B. Diaz-Cruz 1 , Claudia Martinez-Alonso 2 , Omar Castelo 1 , Alejandro Baray 1 , Concepcion Arenas 1 , Hailin Zhao Hu 1
1 , UNAM, Temixco, Morelos Mexico, 2 Facultad de Química, Universidad Autonoma de Queretaro, Queretaro, Queretaro, Mexico
Show AbstractBi2S3 is a direct semiconductor with a band gap of 1.3 eV. It belongs to the family of metal chalcogenides of type A2B3 ( A = As, Sb, and Bi; B = S, Se, and Te), whose importance in applications such as photovoltaics and thermoelectric is well recognized. On the other hand, the application of microwave (MW) heating in synthesis of materials is a fast growing research area due to its advantages, in comparison to conventional heating methods, of rapid volumetric heating, high reaction rate and selectivity that can reduce reaction time by orders of magnitude and increase yield of products. In recent years, various methods with MW have been developed to synthesize Bi2S3 with different sources of S and Bi, solvents, complexing agents, temperature and reaction time to obtain Bi2S3 with different morphologies. However, most of them used some kind of complexing agents during the synthesis, which could leave trace of impurity that affects the performance of final devices. In this work, Bi2S3 of different morphologies were obtained with bismuth chloride or bismuth nitrate without any complexing agent by the MW irradiation method , Thiourea was used as sulfur source, and distilled water, dimethylformamide (DMF) or ethylene glycol (EG), was employed as solvent. The reaction temperature was varied beween 100 and 170 °C. Different morphologies of Bi2S3 products were obtained by varying the type of the bismuth source and the solvent, as well as the solution pH in the case of water as the solvent. The crystal phase, morphology, binding energy and optical properties of the as-synthesized Bi2S3 products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectra (XPS), UV–vis diffuse reflection spectroscopy (UV–vis DRS), respectively. The obtained products gave an orthorhombic phase of Bi2S3 (PDF: 17-320). With BiCl3 as the bismuth source and alkaline aqueous solution, a morphology of cabbage roses of 1 – 2 µm in diameter was obtained. By change the solvent, from basic water to DMF, the morphology of the products was of flower of 0.5 – 1 µm in diameter. The most interesting fact was that with EG as the solvent, Bi2S3 nanowires could be obtained with 1.12 – 4.4 µm in length. With Bismuth nitrate as bismuth source, however, all the products showed a morphology of flowers of different sizes: 3 – 6 µm in diameter with water, 1.5 – 3.0 µm with EG, and 2.8 – 5.0 µm with DMF. The asymmetrical nanowire morphology of MW synthesized Bi2S3 may encourage their application in hybrid solar cells.
9:00 PM - NM7.8.20
Growth of Nanostructured CdS on a CdS Film by Microwave Assisted Heating for Hybrid Solar Cells Applications
Alejandro Baray 1 , Claudia Martinez-Alonso 2 , Evelyn B. Diaz-Cruz 1 , Omar Castelo 1 , Concepcion Arenas 3 , Hailin Zhao Hu 1
1 , UNAM, Temixco, Morelos Mexico, 2 Facultad de Quimica, Universidad Autonoma de Queretaro, Queretaro, Querétaro, Mexico, 3 , Escuela Nacional de Estudios Superiores, Leon, Guanajuato, Mexico
Show AbstractCadmium sulfide (CdS) is an inorganic semiconductor with appropriate properties to work as electron acceptor material in hybrid solar cells (HSCs) coupled with organic semiconductors as electron donor materials. In our group, HSCs has been reported with a power conversion efficiency (PCE) of 0.44% with a CdS compact layer (CdS-f) and poly(3-hexylthiophene) (P3HT) as photoactive layer. The mainly reason of the low PCE was the poor Jsc value (0.3 mA/cm2) due to the little interfacial contact area between CdS and P3HT films. To increase Jsc, microwave synthesized CdS nanoparticles has been deposited above CdS-f in HSCs, getting a PCE of 0.55% with Jsc value of 2.1 mA/cm2, but not good enough because of the low connectivity between nanoparticles of CdS. The highlight of this work is the in-situ growth of CdS nanowires by microwave assisted heating on CdS-f in order to form a CdS sponge (CdS-s) and use it in the photoactive layer, increasing the contact area and the connectivity between both materials and, consequently, improving the solar cell performance. In this work, CdS-f was formed on transparent conductive glass Indium–Tin Oxide (ITO) by chemical bath deposition at 80 °C with thicknesses of about 100 nm. The CdS-s was growth submerging the CdS-f substrates in the chemical solution that was collocated in a microwave reactor. The reaction parameters were varied to form a good deposition of CdS-s on CdS-f. The obtained layers were analyzed by X-ray diffraction, photoresponse measurements, SEM and UV-Vis spectra to study their structural, electrical, morphological and optical properties. HSCs were fabricated incorporating a P3HT film by spin coating method on top of CdS and using carbon paint/gold as top contacts. Also HSCs were fabricated with only CdS-f, that is to say, no CdS-s was included in order to use it like reference cell. The role of the CdS-s layer on the photovoltaic performance of solar cells was studied by analyzing the I-V curves and external quantum efficiency spectra. According to I-V curves, the growth of CdS-s layer on CdS-f has a significantly effect on the Jsc value as wells as the fill factor. At the same time, the external quantum efficiency (EQE) measurements showed an increase in the CdS absorption range due to the presence of CdS-s. It is concluded that the in-situ deposition of nanostructured CdS on thin films of the same compound is a good strategy to improve the photovoltaic performance of HSCs.
9:00 PM - NM7.8.21
Optical and Electrical Properties of Random Ag NW@ ZnO Core-Shell Structure Based Transparent and Conductive Thin Film
Fen Qin 1 , Ziye Xiong 1 , Hyunwoo Shim 1 , Po-shun Huang 1 , Jung-Kun Lee 1 , Gill Sang Han 1
1 , University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show AbstractWe present simulation and experimental results ofor optical and electric properties of the random Ag nanowire (Ag NW) networks covered with ZnO thin shells. The special structures of our Ag NW@ZnO core-shell networks allow for use in transparent and conducting (TC) films in photoelectronic devices with good transparency (> 85%), high electrical conductivity (<20Ω/sq) as well as excellent heat resistance (>450°C) and humidity resistance. For simulation, The randomly oriented Ag NW networks are first generated by Java with a designed aspect ratio and a surface coverage ratio. Then, this mesh is used as import the geometric information into FDTD, Lumerical for Finite-Difference Time-Domain Method (FDTD) and Electrodynamics simulations. With the help of 3-D Maxwell FDTD simulations, Simulation results show that a change in the aspect ratio and the shell thickness has a significant impact on optical and electric performance of othe Ag NW network. Calculation parameters for high ptimal light transmittance of visible light and ssion low resistance guide the experimental design of the Ag NW networkcan be calculated by tuning the absorption peaks into red light zone applying different aspect ratio and shell thickness. Experimentally, the Ag NW@ ZnO structure is prepared using a simple sol-gel method. The aspect ratio of Ag NW and the thickness of ZnO shell for the optimum TC application are obtained by controlling reaction temperature/time and precursor concentration. is controlled by reaction temperature and time, while the thickness of ZnO is controlled by the concentration of precursors. Temperature-dependent Hall effect measurements indicates that the carrier concentrations for AgNW @ZnO films are higher than the pure Ag NW networks, due to lower Schottky barrier at , which reveals that the electrons donated from Ag NW are capable of overcoming the low Schottky/Ohmic barrier between Ag-ZnO interfaces and further transfer into ZnO shells. By this mechanism, the resistance ivity of Ag NW-NW junctions can be effectively reduced (since Rsh(junctions) >> Rsh(body)). AlsoIn sum, Ag NW@ZnO films networks have tunable absorption peaks, lower Hazeness, compared to ZnO/Ag NW/ ZnO films and efficientand good current collection behavior, all of which can contribute to lead to better photoelectronic performancesimproving the performance of Ag NW base TC.
9:00 PM - NM7.8.22
Passivation of Silicon Nanopillar fabricated by Each Different Metal Adhesion Layer of Metal Assisted Chemical Etching
Sangpyeong Kim 1 , Stuart Bowden 1 , Christiana Honsberg 1
1 , Arizona State University, Tempe, Arizona, United States
Show AbstractWe present fabrication and passivation of nanopillar on silicon wafer. Si nanostructure is fabricated for improving of photovoltaic performance. In the photovoltaic field, plenty of fabrication methods are reported [1-3]. Si nanopillars are fabricated using silica nanospheres lithography (SNL) with metal assisted chemical etching (MACE) [4]. For silica nanospheres lithography, silica nanospheres are mixed with DMF solution and taken sonication process for releasing cluster of nanospheres. After sonication, nanospheres are coated on Si wafer using spinner. The coated nanospheres are optimized size by RIE (Reactive Ion Etch) process. After size modification, we deposit metal adhesion layer such as Ni, Cr or Ti with Au using Electron beam evaporator for MACE process. Through MACE process, we have easy structure height modification by control of etching time. After whole fabrication, we take sample cleaning using RCA, piranha and HF solution. Additionally, we take sample surface treatment for high performance passivation. The surface treatment is chemical surface etching by wet oxidation with HF, HNA and KOH. The wet oxidation process is using mixed solution from sulfuric acid and Hydrogen peroxide of 1:1 ratio. Nanopillar surface is oxidized by this solution and then cleaned by 1% HF solution. In the HNA treatment, we use diluted HNA solution which it is mixed HNA and DI water of 1:3 ratio. Last surface treatment is KOH treatment. We use 1% KOH at room temperature. We expect the surface treatments can improve sample performance by removing metal surface contamination. All Si nanopillar samples are taken Al2O3 passivation using thermal atomic layer deposition (ALD). After all of them, we compare and analyze the results of passivation each different metal adhesion samples and different surface treatments.
References
[1] K. Peng, Y. Xu, Y. Wu, Y. J. Yan, S. T. Lee and J. Zhu, "Aligned single-crystalline Si nanowire arrays for photovoltaic applications," Small, vol. 1, pp. 1062-1067, 2005.
[2] H. Fang, X. Li, S. Song, Y. Xu, J. Zhu, "Fabrication of slantingly-aligned silicon nanowire arrays for solar cell application," Nanotechnology, vol. 19,pp. 255703, 2008.
[3] E. C. Garnett, P. Yang, "Silicon nanowire radial p-n junction solar cells," Journal of the American Chemical Society, vol. 130, pp. 9224-9225, 2008.
[4] J.-Y. Choi, T. L. Alford and C. B. Honsberg, "Fabrication of Periodic Silicon Nanopillars in a Two-Dimensional Hexagonal Array with Enhanced Control on Structural Dimension and Period," Langmuir, vol.31, pp. 4018-4023, 2016
9:00 PM - NM7.8.23
Experimental and Theoretical Detection of Axial Strain in InAs/InSb Heterostructured Nanowires
Atanu Patra 1 , Monodeep Chakraborty 1 , Anushree Roy 1
1 Physics, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India
Show AbstractA strain in a desired direction of InAs-InSb semiconductor nanowires (NWs) helps to achieve topological inversion in the electronic bandstructure. Therefore, the presence of strain in the heterostructured NWs find a great importance.
It is generally believed that the interfacial strain relaxes within few tens of nanometers across the interface of InAs/InSb NWs [1]. We have exploited sensitivity of confocal Raman spectroscopy to find the evolution of strain along the axis of InAs/InSb NW [2]. We find a gradual change in transverse mode of optical phonon of InSb along the NW axis. The observed Raman shift is attributed to the residual strain in the crystal structure, which sustains upto the tip of the NW. The measured strain is small to be detected by electron diffraction measurements, the commonly used technique to measure the strain in a crystal structure. In addition, we employ a first principle investigation with InAs/InSb supercell simulating the experimental ambiance to unearth the origin of non-vanishing residual strain. We believe that our results will introduce a new away for better understanding of strain relaxation mechanism in heterostructure NWs.
References
[1] M. d. Mata, C. Magen, P. Caroff and J. Arbiol, Nano Lett. 14, 6614 (2014).
[2] A. Patra, J. K. Panda, A. Roy, M. Gemmi, J. David, D. Ercolani and L. Sorba, Appl. Phys. Lett. 107, 093103 (2015).
Symposium Organizers
Anna Fontcuberta i Morral, Ecole Polytechnique Federale de Lausanne, Switzerland
Esther Alarcon-Llado, AMOLF
Sudha Mokkapati, Australian National University
Carl Thompson, Massachusetts Institute of Technology
Symposium Support
Journal of Physics D | IOP Publishing
NM7.9: New Fabrication Methods
of Nano‐PV Devices
Session Chairs
Esther Alarcon-Llado
Anna Fontcuberta i Morral
Thursday AM, April 20, 2017
PCC West, 100 Level, Room 105 A
10:00 AM - *NM7.9.02
Single and Tandem Radial Junction Solar Cells Built on Silicon Nanowires Produced by Plasma-Assisted VLS
Pere Roca i Cabarrocas 1 , Misra Soumyadeep 1 , Ileana Florea 1 , Martin Foldyna 1 , Yu Linwei 2
1 , LPICM, CNRS, Ecole Polytechnique, Palaiseau France, 2 , School of Electronics Science and Engineering, Nanjing China
Show Abstract
Silicon nanowires (SiNWs) provide an effective research platform for developing a new generation of low-cost and high efficiency solar cells, owing to their enhanced light trapping and anti-reflection effects. By decoupling the light absorption and carrier collection directions, SiNW solar cells permit to use very thin intrinsic layers for PIN radial junctions. By optimizing the density of radial junctions we have so far demonstrated an initial efficiency of 9.2 % with just 100 nm thick intrinsic hydrogenated amorphous (a-Si:H) layer [1]. However, higher conversion efficiency is required to revive the Si-thin film technology. This can be obtained via tandem and triple junction solar cells which have been predicted to be able to lead to conversion efficiencies over 20 % [2]. While this approach has been successfully applied to planar devices, its application in radial junction solar cells brings more advantages and also more challenges. Indeed, besides few modelling studies [3], the practical implementation of tandem radial junction solar cells has not been addressed so far. Here we review our previous work on plasma-assisted VLS growth of silicon nanowrires and their application to radial junction PIN solar cells and focus on new results concerning the successful realization of tandem radial junction solar cells.
1. S. Misra, L. Yu, M. Foldyna, P. Roca i Cabarrocas. IEEE JPV, 5, 40 (2015).
2. M. Konagai, Jpn. J. of Appl. Phys. 50, 030001 (2011).
3. S. Qian, S. Misra, J. Lu, Z. Yu, L. Yu, J. Xu, J. Wang, L. Xu, Y. Shi, K. Chen, and P. Roca i Cabarrocas. Appl. Phys. Lett. 107, 043902 (2015).
10:30 AM - NM7.9.05
From Single NW to Large Scale NW Array Solar Cell Development through Conductive AFM Analysis
Dmitry Mikulik 1 , Pablo Romero-Gomez 1 , Maria Ricci 2 , Gozde Tutuncuoglu 1 , Federico Matteini 1 , Jelena Vukajlovic-Plestina 1 , Natasa Vulic 3 , Martin Friedl 1 , Esther Alarcon-Llado 4 , Benjamin Dwir 5 , Alok Rudra 5 , Elyahou Kapon 5 , Anna Fontcuberta i Morral 1
1 Laboratory of Semiconductor Materials, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Vaud, Switzerland, 2 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 3 Solar Power Labs, Arizona State University, Phoenix, Arizona, United States, 4 Center for Nanophotonics, FOM Institute AMOLF, Amsterdam Netherlands, 5 Laboratory of the Physics of Nanostructures, Ecole Polytechnique Fédérale de Lausanne, Lausanne Switzerland
Show AbstractSemiconductor nanowires (NWs) are promising candidates for photovoltaic applications due to their superior light absorption properties with respect to their thin film counterparts [1-2]. So far, the highest efficiencies have been achieved using the top-down fabrication approaches, resulting in several limitations of the device such as small solar cell area and high cost [3]. In contrast, the major challenge for bottom-up NW array solar cells is achieving perfect uniformity of NWs over the whole surface area as the individual NWs p-n junctions are connected in parallel. As a result, the electrical properties of the full device are limited by the lower performance of any single NW in the array [4].
To make progress on the full device efficiency, it is thus necessary to get accurate information on the electrical characteristics of single NWs in the array. This would enable us to determine the factors that significantly affect the performance of the full device. So far, to gain this information, single NWs have been first detached from the substrate and then singularly contacted to perform electrical measurements [5]. This process often requires an arduous and long nanofabrication process.
Conductive atomic force microscopy (C-AFM) is a powerful current-sensing technique for the characterization of conductivity variations in resistive samples. C-AFM can simultaneously map surface topography and current distribution of samples by applying a constant voltage between the scanning conductive tip and the sample surface. The main advantage of C-AFM is the possibility to access local conductivity information down to the nanoscale and, at the same time, explore significantly large areas of the sample surface during the scanning process. This enables single wire resolution but on a statistically significant number of NWs. In addition, C-AFM provides a noninvasive method to characterize photovoltaic properties of the ensemble of NWs, allowing further fabrication of the device.
Here, we demonstrate the advantage of using C-AFM for addressing the individual electrical properties of GaAs NW p-n junctions in the array and comparing different bottom-up growth methods on Si and GaAs substrates. The self–assemble vapor-liquid-solid (VLS) method in MBE results in large area NW growth, however, due to a random distribution, the uniformity of the NWs is relatively poor. In contrast, a pre-defined hole pattern allows us to fabricate highly uniform NW arrays with fixed pitch size by either MBE or MOCVD method.
References:
[1] P. Krogstrup et al, Nature Photonics, 2013, 7, 306
[2] E. Garnett et al, Nano Lett., 2010, 10 (3), 1082
[3] Y. Cui et al, EU PVSEC 2016
[4] Y. Su et al, Nature Nanotechnology, 2016, 11, 609
[5] C. Colombo et al, Appl. Phys. Lett., 2009, 94, 173108
10:45 AM - NM7.9.06
Fabrication of CIGS Nanowire-Based Solar Cell by Electrodeposition
Muntaser Al-Mansoori 1 , Mariam Mansouri 1 , Lina Orabi 1 , Boo Hyun An 1 , Jisung Lee 1 , Daniel Choi 1
1 Department of Mechanical and Materials Engineering, Masdar Institute of Science and Technology, Abu Dhabi United Arab Emirates
Show AbstractIn recent years, nanomaterial-based solar cells attracted people’s attention for its clean and renewable properties. One-dimensional nanostructures can provide attractive architectures for solar energy applications, not only because of the unique physical properties that can be seen from the nanometer-scale structures, but also because of geometrical impacts that affect the performance of solar cells. CIGS nanowires are promising materials for solar cell applications due to the large light abortion coefficient (around ~105 cm−1), and wide adjustable bandgap range (1.04 to 1.72 eV). In this study, we present a feasible approach for fabrication of CuIn(1−x)GaxSe2 (CIGS) nanowire based solar cell by taking advantage of size-effect which increase the effective pn junction size per cell. This electrochemistry-based process includes one-step electrodeposition technique to grow CIGS nanowires into the porous anodized aluminum oxide (AAO) template, which is a cost-effective alternative process to vacuum-based deposition process. Composition of Cu1In0.55Ga0.45Se2 nanowires, determined by energy dispersive spectroscopy (EDS), was achieved through a manipulation of the applied potential and composition of electrolytes. X-ray diffraction analysis showed that crystallinity of CIGS nanowires were improved by annealing. We fabricated the solar cell structure by etching AAO structure after growing highly ordered vertical array of CIGS nanowires as p-type. Chemical bath deposition was followed to deposit a smooth CdS layer as n-type. Thin-film of PEDOT:PSS was deposited by spin coating on top of CdS to act as a transparent and conductive layer. Photocurrent measurements on the CIGS nanowire-based solar cell are in progress.
NM7.10: Nanowire Strategies for
Efficient Photodetection
Session Chairs
Esther Alarcon-Llado
Anna Fontcuberta i Morral
Thursday PM, April 20, 2017
PCC West, 100 Level, Room 105 A
11:30 AM - NM7.10.01
Performance Boosting of Flexible ZnO UV Sensors with Rational Designed Absorbing Antireflection Layer and Humectant Encapsulation
Heng Zhang 1 , Youfan Hu 1 , Zongpeng Wang 2 , Lianmao Peng 1
1 Department of Electronics, Peking University, Beijing China, 2 , Peking University, Beijing China
Show AbstractNanostructured antireflection (AR) layer is widely introduced in photovoltaic devices for efficiency enhancement by increasing light absorption. However, to construct these nanostructures, techniques such as electron-beam lithography, interference lithography, and reactive-ion etching have to be adopted in the top-down fabrication process, which greatly increases the devices’ cost and the fabrication process’s complexity. Most recently, solution-grown ZnO nanorods arrays (NRAs) provide a new bottom-up method to construct the moth-eye nanostructure in large areas. Considering light absorption efficiency is also a big issue in the realization of high sensitive photosensors, adopting the absorbing AR layer design with easy growth process in ZnO UV sensors is very promising, but the related work has not been investigated yet. Besides sensitivity, the slow response and recovery speed is considered as an intrinsic drawback in ZnO UV sensors as the slow oxygen molecular adsorption and desorption are involved in the photoresponse process. There are more work needs to be done to recheck the underlying mechanism, and find a new way to solve the problem essentially. In this work, flexible ZnO thin film UV sensors with three orders of magnitude improvement in sensitivity and two orders of magnitude acceleration in speed are realized via light absorption efficiency enhancement and surface encapsulation. Devices are constructed on polyethylene (PET) substrate incorporating morphology controlled ZnO nanorods arrays (NRAs) to realize rational designed absorbing antireflection (AR) layers. Experimental and finite-difference time-domain (FDTD) simulation results reveal that by adjusting the morphology of ZnO NRAs, the light reflectance at the wavelength of 365 nm is suppressed to below 1% and the absorptance reaches 99% by effectively trapping incident photons in it. As a result, the sensitivity of the UV sensor is increased by three orders of magnitude, reaching 109000. Moreover, a mechanism of competitive chemisorption between O2 and H2O at oxygen vacancy sites is proposed and a new approach of humectant encapsulation is demonstrated for speed acceleration. Two orders of magnitude speed enhancement in reset time is achieved by polyethylene glycol (PEG) encapsulation. After the UV sensor suffering a total of 3000 bending cycles, the decay of the responsivity is within 20%, indicating a good mechanical stability. All these results not only demonstrate a simple and effective approach to fabricate high sensitive and fast response flexible ZnO UV sensors, but also provide meaningful references for performance boosting of photoelectronic devices based on other oxide semiconductors.
11:45 AM - *NM7.10.02
Silicon-Germanium-Tin Nanowires—Growth, Structure and Device Properties
Simone Assali 1 , Anis Attiaoui 1 , Samik Mukherjee 1 , Matthieu Fortin-Deschenes 1 , Etienne Bouthillier 1 , Oussama Moutanabbir 1
1 , Ecole Polytechnque de Montreal, Montreal, Quebec, Canada
Show AbstractCompound semiconductor alloys have been successfully used for a precise and simultaneous control of lattice parameters and bandgap structures bringing to existence a variety of functional heterostructures and low-dimensional systems. Extending this paradigm to group IV semiconductors will be a true breakthrough that will pave the way to creating an entirely new class of silicon-compatible clean energy conversion, optoelectronic, and photonic devices. With this perspective, germanium-tin (GeSn) and germanium-silicon-tin (GeSiSn) alloys have recently been the subject of extensive investigations as new material systems to independently engineer lattice parameter and bandgap energy and directness. The ability to incorporate Sn atoms into silicon and germanium at concentrations about one order of magnitude higher than the equilibrium solubility is at the core of these emerging potential technologies. In this presentation, we will address the epitaxial growth and stability of these metastable semiconductors nanowires. We will also discuss the optical and electronic properties as well as the nature of the atomic order in Sn-rich GeSiSn. The strategies to integrate these nanowires in fabrication of photovoltaic and thermoelectric devices will be presented. Finally, based on in situ electron microscopy, we will show real time studies of their thermal behavior which reveal unprecedented insights into the key phenomena that govern the thermal decomposition and phase separation in these materials. These studies define the temperature window for the processing of GeSiSn-based devices. Combining the unique properties of GeSiSn with the flexibility in design and fabrication offered by nanowires creates a wealth of opportunities to implement innovative devices.
12:15 PM - NM7.10.03
Extreme IR Light Absorption in Group IV-SiGeSn Core-Shell Nanowires
Anis Attiaoui 1 , Simone Assali 1 , Oussama Moutanabbir 1
1 , Ecole Polytechnique Montreal, Montreal, Quebec, Canada
Show AbstractThe group IV GeSiSn ternary alloy semiconductors are attracting a great deal of attention due to their unique electronic and optical properties. This CMOS-compatible family of semiconductors allow an independent control of both lattice parameter and band gap and its compatibility to technology. These adventages can enable a wide spectrum of silicon-based applications including photovoltaic cells, photodetectors, meta-materials and thermal emitters. With this perspective, we present a thorough investigations of the effect of Sn on light absorption for GeSiSn based heterostructures and nanowires. In the present work, we demonstrate that Sn-containing group IV (Ge1-x-ySixSny) core-shell nanowire (CSNW) structures are effective in enhancing Near-Infrared (NIR) light absorption in Si-based structures. We also demonstrate that this additional Sn-containing shell layers enhance the optical properties of germanium based-nanowires (GeNWs) as well. First, we measured the optical properties of Ge1-ySny and Ge1-x-ySixSny at different Sn composition ranging from 0 to 12% for Ge1-ySny and (x, y) = (12%, 4%), (8%, 6%) and (4%, 12%) for Ge1-x-ySixSny. Next, we incorporated the optical properties into Mie-Lorentz scattering formalism, adapted to a core-shell cylindrical geometry, to evaluate light scattering and absorption of the CSNW structures. We then continue with the optimization of the absorption efficiencies by engineering the best possible match between the absorption spectrum of the wires and visible range (0.4-0.75 μm) as well as the Near-Infrared (NIR) range (0.75-1.4 μm). In addition, as a figure of merit of absorption efficiency, we analyze the photocurrent of different group IV Ge1-x-ySixSny based films and nanowires structures: Si/Ge1-x-ySixSny CSNWs, Ge/Ge1-x-ySixSny CSNWs. Thus, we demonstrate, in Si/Ge0.88Sn0.12 NW heterostructures, a ~14-fold increase in photocurrent as compared to bare Si. We also demonstrated an extreme enhancement of light absorption for the Si/GeSn system relative to SiNW where between 800 and 1000 nm, the absorption efficiency reaches 103, whereas, in the near-infrared region the absorption enhancement is four order of magnitude higher than that of the SiNW. These results show that SiGeSn NWs can be versatile building blocks for efficient silicon-compatible optoelectronics.
12:30 PM - *NM7.10.04
Design of Nanowire Quantum-Well Infrared Photodetectors for Intersubband Absorption
Dingkun Ren 1
1 , University of California, Los Angeles, Los Angeles, California, United States
Show AbstractIntersubband transition between sub-bands in quantum wells (QWs) serve as a key aspect for device engineering of mid-wave infrared and long-wave infrared optical devices, such as quantum-cascade lasers/detectors and quantum-well infrared photodetectors (QWIPs). In order to achieve high operating temperature, non-radiative carrier scattering by longitudinal optical phonon needs to be suppressed. However, in-plane momentum in typical QWs-based optical devices is continuous, and therefore electrons are easily depopulated from upper bands. Nevertheless, forming those quantized structures in vertically grown nanowires offers a solution, because lateral confinement of energy states would overcome electronphonon scattering and enhance effective carrier lifetime. To date, there have not been any major breakthroughs in demonstrating intersubband transition in nanowires over 3 µm. The main challenge is the growth of axial quantized structures by precisely controlling group-III or group-V intermixing at heterogeneous interfaces. Fortunately, InAs/InAsP heterostructures can be potentially implemented to obtain quantized states, and resonance-tunneling diodes based on InAs well and InP barriers in nanowires have already been demonstrated.
In this work, we design a structure of nanowire QWIPs based on InAs/InAs0.7P0.3 heterostructures with optimized sub-band energies and metallic gratings for further intersubband spectral response measurement. Growth of high uniform and high vertical yield nanowire arrays with such heterostructures has been successfully demonstrated by our group using catalyst-free selective-area epitaxy by metal-organic chemical vapor deposition (MOCVD). The overall study involves quantum modeling and FDTD optical simulation to solve (1) sub-band energies with different width of QWs and doping levels, (2) intersubband absorption coefficient, and (3) mode of surface plasmon. In the first step, the band-structure is solved selfconsistently with Poisson equation to study sub-band energy, band-bending, Fermi-level, and nonparabolicity of conduction band. We use 8-band k●p model instead of effective-mass approximation for band-structure in order to include the coupling between conduction and valence bands in narrow band-gap materials. Next, oscillation strength is calculated between sub-bands, and absorption coefficient is obtained by adding up all possible transitions along in-plane k. Here, we simply pick up a broadening factor Γ in meV to estimate Lorentzian distribution. The calculated absorption coefficient is then imported into the material database of FDTD simulator. The last step is to match surface plasmon modes to the peaks of absorption spectrum. Note that the metallic coupler is aimed to excite surface plasmon and enhance Zpolarized electric-field for selection rules in QWs.
NM7.11: Nanowire Strategies for Efficient Solar to Fuel Conversion
Session Chairs
Sudha Mokkapati
Carl Thompson
Thursday PM, April 20, 2017
PCC West, 100 Level, Room 105 A
2:30 PM - *NM7.11.01
CO2 + H2O + Sunlight = Chemical Fuels + O2
Peidong Yang 1
1 Department of Chemistry and Department of Materials Science Engineering, University of California, Berkeley, Berkeley, California, United States
Show AbstractSolar-to-chemical (STC) production using a fully integrated system is an attractive goal, but to-date there has yet to be a system that can demonstrate the required efficiency, durability, or be manufactured at a reasonable cost. One can learn a great deal from the natural photosynthesis where the conversion of carbon dioxide and water to carbohydrates is routinely carried out at a highly coordinated system level. There are several key features worth mentioning in these systems: spatial and directional arrangement of the light-harvesting components, charge separation and transport, as well as the desired chemical conversion at catalytic sites in compartmentalized spaces. In order to design an efficient artificial photosynthetic materials system, at the level of the individual components: better catalysts need to be developed, new light-absorbing semiconductor materials will need to be discovered, architectures will need to be designed for effective capture and conversion of sunlight, and more importantly, processes need to be developed for the efficient coupling and integration of the components into a complete artificial photosynthetic system. In this talk I will begin by discussing the challenges associated with fixing CO2 through traditional chemical catalytic means, contrasted with the advantages and strategies that biology employs through enzymatic catalysts to produce more complex molecules at higher selectivity and efficiency. I then discuss a number of different photosynthetic biohybrid systems (PBS) architectures from the last few years, and the numerous strategies to interface biotic and abiotic components. Each demonstrates the advantages of PBSs in converting sunlight, H2O and CO2 into food, fuels, pharmaceuticals, and materials. Finally, I will outline the future of this field, opportunities for improvement, and its role in sustainable living here on Earth, and beyond.
3:00 PM - NM7.11.02
Mixing Cu Nanowires with ZnO Nanowires as Highly Stable Catalysts for Methanol Synthesis and Steam Reforming
Jia Xu 1 , Yiwei Yu 2 , Jingyue Liu 2
1 School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona, United States, 2 Department of Physics, Arizona State University, Tempe, Arizona, United States
Show AbstractZinc oxide supported Cu catalysts are used industrially for methanol synthesis and reforming reactions. A critical issue for the Cu based catalysts is that Cu nanoparticles (NPs) sinter easily during catalytic reactions, especially at high reaction temperatures. The synergistic effect of Cu and ZnO may also play a major role in determining the catalytic performance of the Cu/ZnO systems [1-2]. In order to understand the synergy between the Cu and Zn/ZnO during the methanol synthesis and reforming reactions and to alleviate the sintering processes of the Cu based catalysts, we recently fabricated novel model catalysts which consist of only well-defined Cu and ZnO nanowires (NWs). The pentagonal Cu NWs expose primarily the {100} family of surfaces and the hexagonal ZnO NWs consist of primarily the {10-10} surfaces. The diameters of both the Cu and ZnO NWs can be controlled to tune the Cu surface area and the contact areas between the Cu and the ZnO NWs. Catalytic tests of the mixed Cu/ZnO NWs catalyst revealed that the catalyst was stable for methanol steam reforming, even at reaction temperatures as high as 420°C. For comparison purposes, Cu nanocubes and NPs have also been synthesized and will be tested for methanol synthesis and reforming reactions. Detailed atomic scale structural characterization of the fabricated Cu/ZnO NW catalysts, their stability, and the synthesis-structure-performance relationships will be discussed [3].
[1] R. Burch, S. E. Golunski, M. S. Spencer. J. Chem. Soc. Faraday Trans. 86, 2683 (1990).
[2] M. Behrens et al. Science 336, 893 (2012).
[3] Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. This work was partially supported by the College of Liberal Arts and Sciences of Arizona State University. The authors gratefully acknowledge the use of facilities within the LeRoy Eyring Center for Solid State Science at Arizona State University.
3:15 PM - NM7.11.03
Three-Dimensional Array of Highly Conductive SnO2 Nanowires Arrays as a Current Collector of CdSe/CdS/TiO2 Cascade Heterojunction Photoelectrochemical Cells
Salim Caliskan 1 , Lingtao Jiang 1 , Gill Sang Han 1 , Jung-Kun Lee 1
1 Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show AbstractThe photoelectrochemical (PEC) cell is one of the most promising devices to convert solar energy into hydrogen via water splitting. For photoelectrochemical (PEC) hydrogen production, low charge collection efficiency of a photoelectrode is a common problem for many metal oxide photocatalysts. This largely limits the PEC performance. Loading of semiconductors on top of porous and electrically conductive scaffolds is one strategy to address this issue of the low charge collection. Conductive scaffolds need to have large surface area to deposit a enough amount of semiconductor capable of absorbing most of the incident light. Additionally, the scaffolds are required to facilitate electrolyte transport, scatter most of incident lighting, provide short conductive paths and be inexpensively fabricated.
Here, we report the growth of well defined antimony-doped tin oxide (ATO) and tin oxide (SnO2) nanowires array (NWs) on different substrates and their performance as the transparent conducting electrode of CdSe/CdS/TiO2 Cascade Heterojunction the photoelectrochemical cells. The ATO and TO NWs with an average thickness of ~15 μm are firstly grown on commercial FTO/glass and metal mesh by vapor-liquid-solid method. Subsequently, the TiO2 and CdSe/CdS shell layers are deposited by an atomic layer deposition (ALD) and a electrodeposition method, respectively. Then, the microstructure, crystal structure, optical properties and electrochemical performance of the resultant photoelectrodes are systematically investigated using multiple characterization tools.
4:00 PM - *NM7.11.04
Nanowires, Nanoplates and Nanofilms of Two-Dimensional Layered Materials
Yi Cui 1 2 , Hongtao Yuan 1 2
1 Department of Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California, United States
Show AbstractTwo-dimensional (2D) layered materials host many interesting physical and chemical phenomena. Their nanowires, nanoplates and nanofilms represent novel candidates to host those phenomena. Here we present our study on chemistry and physics of 2D layered nanowires and other nanostructures. First, we have synthesized a range of morphologies and their heterostructures. Second, we have developed a new method of zero-valent intercalation which allows unprecedented high levels of various metal intercalants inserted into the van der Waals gaps. Third, we have fabricated single nanostructure electrical transport devices and demonstrate novel interesting electronic properties and their application in water disinfection.
4:30 PM - NM7.11.05
Silicon Nanowires for Solar-to-Fuel Conversion
Yude Su 1 , Chong Liu 1 2 , Sarah Brittman 1 2 , Jinyao Tang 1 2 , Anthony Fu 1 2 , Nikolay Kornienko 1 2 , Qiao Kong 1 , Hao Zhang 1 , Peidong Yang 1 2 3
1 Department of Chemistry, University of California, Berkeley, Berkeley, California, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States
Show AbstractSemiconductor nanowires are considered a promising candidate for solar-to-fuel conversion applications because of their unique properties, such as the high surface area and strong light trapping effect. Nanowires made of silicon, one of the most abundant semiconducting materials, have been widely investigated for photoelectrochemical H2 evolution and CO2 reduction. Recently, Yang and co-workers interfaced silicon nanowires with the biological CO2-reducing catalyst and realized solar-powered CO2 reduction to acetate. However, the mechanism of the electron transfer from solid-state materials to bacterium is still elusive. Here, we use electrochemical methods to probe the extracellular electron transfer from silicon nanowire to bacterium, S. Ovata. Specifically, with the help of Tafel plot and electrochemical impedance spectroscopy, we found that the kinetics of the heterogeneous electron transfer would switch at a certain overpotential, providing insight into the competition between the direct and indirect electron transfer pathways. Moreover, although considerable research effort has focused on studying wire arrays, the inhomogeneity in the geometry, doping, defects and catalyst loading present in such arrays can obscure the link between these properties and the photoelectrochemical performance of the wires, and correlating performance with the specific properties of individual wires is difficult because of ensemble averaging. Recently, we developed a single-nanowire-based photoelectrode platform that can be used to reliably probe the current–voltage (I–V) characteristics of individual nanowires. We find that the photovoltage output of ensemble array samples can be limited by poorly performing individual wires, which highlights the importance of improving nanowire homogeneity within an array. Furthermore, the platform allows the flux of photogenerated electrons to be quantified as a function of the lengths and diameters of individual nanowires. Such characterization of the photogenerated carrier flux at the semiconductor/electrolyte interface is essential for designing nanowire photoelectrodes that match the activity of their loaded electrocatalysts.
4:45 PM - *NM7.11.07
Efficient Solar Cells and Water Reduction with Nanowires
Dick van Dam 1 , Yingchao Cui 1 , Anthony Standing 1 2 , Simone Assali 1 , Lu Gao 3 , Marcel Verheijen 1 5 , Peter Notten 3 4 , Jos Haverkort 1 , Erik Bakkers 1 6
1 Department of Applied Physics, Eindhoven University of Technology, Eindhoven Netherlands, 2 , BioSolar Cells, Wageningen Netherlands, 3 Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, Eindhoven Netherlands, 5 , Philips Innovation Services Eindhoven, Eindhoven Netherlands, 4 , Forschungszentrum Julich, Julich Germany, 6 Kavli Institute of Nanoscience Delft, Delft University of Technology, Delft Netherlands
Show AbstractSemiconducting nanowires have several appealing advantages for the fabrication of efficient solar cells and for the reduction of water. In this talk I will discuss recent results from our group on nanowire solar cells. By optimizing the in- and outcoupling of light we can enhance both the short circuit current and the open circuit potential with respect to bulk geometry, reaching an efficiency of 17.8%. Similar structures can be used for water reduction. By tuning the dopant profile, carrier recombination can be suppressed resulting in unprecedented photocathodic power-saved efficiency of 15.8%.
5:15 PM - NM7.11.08
High Efficiency and Highly Stable Photocatalytic Overall Water Splitting on III-Nitride Nanowire Arrays
Mohammad Chowdhury 1 , Xiangjiu Guan 2 , Aagnik Pant 1 , Xianghua Kong 3 , Md Golam Kibria 4 , Hong Guo 3 , Franz Himpsel 5 , Lionel Vayssieres 2 , Zetian Mi 1 6
1 Department of Electrical and Computer Engineering, McGill University, Montreal, Quebec, Canada, 2 International Research Center for Renewable Energy, Xi'an Jiaotong University, Xi'an, Shaanxi China, 3 Department of Physics, McGill University, Montreal, Quebec, Canada, 4 Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, Canada, 5 Department of Physics, University of Wisconsin Madison, Madison, Wisconsin, United States, 6 Department of Electrical Engineering and Computer Science, Centre for Photonics and Multiscale Nanomaterials, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractSolar water splitting is considered to be one of the most viable approach of renewable energy production and environmental remediation by means of harvesting abundant solar energy and storing in the clean chemical energy form. In this context, III-nitride nanowire structures, especially Ga(In)N exhibits distinct photo-physical and photo-chemical/-electrochemical properties. Our recent studies have shown that one-dimensional GaN nanowires allow control over the crystal plane and surface band bending promoting either reduction or oxidation reaction. Moreover, we have recently identified that careful optimization of the surface charge properties through controlled dopant (Mg) incorporation into III-nitride nanowire arrays, provides enhanced photocatalytic activity under both UV (p-GaN, internal quantum efficiency, IQE ~51%) and visible light irradiation (p-GaN/p-InGaN, IQE ∼69% and STH ~1.8%). However, prior to the progress towards large-scale and commercially viable solar water splitting system, it is necessary to ensure significant longevity of the device in harsh photocatalytic environment.
Through surface modification for efficient charge carrier separation, a solar to hydrogen conversion efficiency (STH) of ~2.7% has been derived on MBE grown InGaN/GaN nanowire structures and maintained for an unprecedented duration of ~600 hours till today in neutral pH overall water splitting reaction. To find out the underlying reasons of extended stability, we investigate the structural and electronic arrangement down to atomic level. Our systematic investigation reveals the key factors for efficient and stable water splitting on p-GaN nanowire-arrays which include engineered optimum surface band bending through controlled Mg-dopant incorporation, and N-termination of the top (polar) and side (nonpolar) surfaces by using a Ga seeding layer and growing under N-rich conditions, respectively. Such a highly specific configuration leads to negatively-charged surfaces which, together with the optimized Mg-dopant incorporation, reduce the energy barrier for injecting holes into the electrolyte and generating H2 from water oxidation reaction. Furthermore, the N-terminated surfaces protect the GaN nanowires against attack by the electrolyte (oxidation and photo-corrosion) and thus, the engineered optimum surface band bending can be maintained during the photocatalytic overall solar water splitting reaction. STEM analysis also reveals that the nonpolar surfaces of MBE grown GaN nanowires under nitrogen rich condition are N-terminated. The extended stability of MBE grown III-N nanowire arrays can further be correlated with ab initio materials modeling which reveals that the N-terminated wurtzite GaN m-plane surfaces are energetically more stable than the conventional m-plane surface with equal number of Ga and N atom coverage. Work is currently in progress to demonstrate photocatalytic overall water splitting with efficiency ~5-10% and with long-term stability.
5:30 PM - NM7.11.09
An InGaN Nanowire/Si Tandem Photoanode for High-Efficiency Photoelectrochemical Water Splitting
Srinivas Vanka 1 , Sheng Chu 1 , Yichen Wang 1 , Zetian Mi 1 2
1 , McGill, Montreal, Quebec, Canada, 2 EECS, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractIII-nitride semiconductors are one of the most promising materials to realize high efficiency photoelectrodes: their energy bandgap can be varied across nearly the entire solar spectrum by changing the alloy compositions and the band edge positions straddle water oxidation and reduction potentials under deep visible light irradiation. To date, however, the previously reported InGaN photoelectrodes generally exhibit very low photocurrent densities, due to the presence of extensive defects, dislocations, and indium phase separation. We have recently demonstrated that In-rich InGaN nanowires with nearly homogeneous indium distribution can be achieved by plasma-assisted molecular beam epitaxy1. Moreover, we have further investigated their monolithic integration with a Si solar cell wafer to realize a double-junction photoanode. Such a tandem photoelectrode consists of a top InGaN junction (Eg ~1.75 eV) and bottom Si junction (Eg ~1.1 eV) to achieve current matching conditions under AM1.5G one-sun illumiation2. An n++/p++ Si tunnel junction is incorporated at the InGaN/Si heterointerface. Compared to conventional III-V tunnel junction designs, the use of a Si tunnel junction minimizes parasitic absorption, due to the indirect bandgap nature of Si. Due to the presence of an upward band bending for n-type InGaN nanowires, photo-generated holes can readily diffuse to the nanowire/electrolyte interface to drive oxidation reaction.Photo-generated electrons, on the other hand, migrate toward to the InGaN/Si hetero-interface, which recombine with photo-generated holes from the underlying Si solar cell through the p++/n++ Si tunnel junction. Water oxidation cocatalyst and passivation techniques play an important role to achieve efficient and stable water splitting3. With the incorporation of Ir2O3 co-catalyst particles on InGaN nanowire surface, we have demonstrated solar water splitting on InGaN/Si double-band photoanode with a maximum current density of 8 mA/cm2 in 0.5 M H2SO4 and a solar-to-hydrogen conversion efficiency ~3% under AM1.5G one-sun illumination. Work is currently in progress to achieve long-term stable operation by surface passivation4 and by engineering the surface to be nitrogen-rich to protect against oxidation and photo-corrosion5.
References:
1. Fan, S.; Woo, S. Y.; Vanka, S.; Botton, G. A.; Mi, Z. APL Mater. 2016, 4, (7).
2. Fan, S.; Shih, I.; Mi, Z. Adv. Energy Mater. 2016, 1600952-n/a.
3. Antolini, E. ACS Catal. 2014, 4, (5), 1426-1440.
4. Yang, X.; Liu, R.; Du, C.; Dai, P.; Zheng, Z.; Wang, D. ACS Appl. Mater. Interfaces 2014, 6, (15), 12005-11.
5. Kibria, M. G.; Qiao, R.; Yang, W.; Boukahil, I.; Kong, X.; Chowdhury, F. A.; Trudeau, M. L.; Ji, W.; Guo, H.; Himpsel, F. J.; Vayssieres, L.; Mi, Z. Adv. Mater. 2016, 28, (38), 8388-8397.