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661A. Kargar, S. S. Partokia, M. Niu, P. Allameh, M. Yang, S. May, J. Cheung. K. Sun, K. Xu, and D. Wang, "Solution-grown 3D Cu2O networks for efficient solar water splitting," Nanotechnology, 2014, accepted.

Abstract: We report a facile and large-scale solution fabrication of cuprous oxide (Cu2O) nanowires/nanorods and 3D porous Cu2O networks and their application as photocathodes for efficient solar water splitting. The growth mechanism and structural characterization of 3D porous Cu2O network are studied in detail. The photocathodic performance of Cu2O electrodes prepared under different growth conditions is investigated in a pH neutral medium. The porous Cu2O network photocathodes exhibit large photocurrent, high spectral photoresponse, and incident photon-to-current efficiency, compared to Cu2O nanowire/nanorod photoelectrodes. The photoelectrochemical stability of the 3D Cu2O network is significantly improved by applying a metal oxide multi-layer protection. 

651K. Sun, S. Shen, J. S. Cheung, X. Pang, N. Park, J. Zhou, Y. Hu, Z. Sun, S. Y. Noh, C. T. Riley, P. K. L. Yu, S. Jin, and D. Wang, "Si photoanode protected by a metal modified ITO layer with ultrathin NiOx for solar water oxidation," Phys. Chem. Chem. Phys., 2014, 16, 4612-4625. (pdf) (SI)

We report an ultrathin NiOx catalyzed Si np+ junction photoanode fora stable and efficient solar driven oxygen evolution reaction (OER) in water. A stable semi-transparent ITO/Au/ITO hole conducting oxide layer, sandwiched between the OER catalyst and the Si photoanode, is used to protect the Si from corrosion in an alkaline working environment, enhance the hole transportation, and provide a pre-activation contact to the NiOx catalyst. The NiOx catalyzed Si photoanode generates a photocurrent of 1.98 mA/cm2 at the equilibrium water oxidation potential (E0ER = 0.415 V vs. NHE in 1 M NaOH solution). A thermodynamic solar-to-oxygen conversion efficiency (SOCE) of 0.07% under 0.51-sun illumination is observed. The successful development of a low cost, high efficient, and stable photoelectrochemical electrode based on earth abundant elements is essential for the realization of a large-scale practical solar fuel conversion.

641Y. Jing, X. Bao, W. Wei, K. Sun, C. Li, D. Aplin, Y. Ding, Z. L. Wang, Y. Bando, and D. Wang, "Catalyst-free Heteroepitaxial MOCVD Growth of InAs Nanowires on Si Substrates," J. Phys. Chem. C, 2014, 118, 1696-1705. (pdf)

We report the systematic study of catalyst-free syntheses of InAs nanowires on Si substrates with various growth parameters and surface treatments. Nanowire morphology and crystal structure were studied using scanning electron microscopy and transmission electron microscopy. High-resolution cross sectional transmission electron microscopy studies reveal heteroepitaxy of InAs [111] nanowires on Si (111) substrate with clean and sharp interface. Single nanowire field-effect transistor measurements of InAs nanowires under optimal growth conditions indicate a typical electron concentration of 1018-1019 cm-3 and mobility of around 1,000 cm2/V.s. III/V on Si devices with InAs nanowire array on p-Si show a broadband photodetection up to wavelength of 3.5 um.


631A. Kargar, Y. Jing, S. J. Kim, C. T. Riley, X. Pan, and D. Wang, "ZnO/CuO Heterojunction Branched Nanowires for Photoelectrochemical Hydrogen Generation," ACS Nano, 2013, 7, 11112-1112. (pdf) (SI)

Abstract: We report a facile and large-scale fabrication of three-dimensional (3D) ZnO/CuO heterojunction branched nanowires (b-NWs) and their application as photocathodes for photoelectrochemical (PEC) solar hydrogen production in a neutral medium. Using simple, cost-effective thermal oxidation and hydrothermal growth methods, ZnO/CuO b-NWs are grown on copper film or mesh substrates with various ZnO and CuO NWs sizes and densities. The ZnO/CuO b-NWs are characterized in detail using high-resolution scanning and transmission electron microscopies exhibiting single-crystalline defect-free b-NWs with smooth and clean surfaces. The correlation between electrode currents and different NWs sizes and densities are studied in which b-NWs with longer and denser CuO NW cores show higher photocathodic current due to enhanced reaction surface area. The ZnO/CuO b-NW photoelectrodes exhibit broadband photoresponse from UV to near IR region, and higher photocathodic current than the ZnO-coated CuO (core/shell) NWs due to improved surface area and enhanced gas evolution. Significant improvement in the photocathodic current is observed when ZnO/CuO b-NWs are grown on copper mesh compared to copper film. The achieved results offer very useful guidelines in designing b-NWs mesh photoelectrodes for high-efficiency, low-cost, and flexible PEC cells using cheap, earth-abundant materials for clean solar hydrogen generation at large scales.

621A. Kargar, K. Sun, Y. Jing, C. Choi, H. Jeong, G. Y. Jung, S. Jin, and D. Wang, "3D Branched Nanowire Photoelectrochemical Electrodes for Efficient Solar Water Splitting," ACS Nano, 2013, 7, 9407-9415. (pdf) (SI)

Abstract: We report the systematic study of 3D ZnO/Si branched nanowire (b-NW) photoelectrodes and their application in solar water splitting. We focus our study on the correlation between the electrode design and structures (including Si NW doping, dimension of the trunk Si and branch ZnO NWs, and b-NW pitch size), and their photoelectrochemical (PEC) performances (efficiency and stability) in neutral condition. Specifically, we show that for b-NW electrodes with lightly-doped p-Si NW core, larger ZnO NW branches and longer Si NW cores give higher photocathodic current, while for b-NWs with heavily-doped p-Si NW trunks smaller ZnO NWs and shorter Si NWs provide higher photoanodic current. Interestingly, photocurrent turn-on potential decreases with longer p-Si NW trunks and larger ZnO NW branches resulting in a significant photocathodic turn-on potential shift of ~600 mV for the optimized ZnO/p-Si b-NWs compared to that of the bare p-Si NWs. A photocathode energy conversion efficiency of greater than 2% at -1 V versus Pt counter electrode and in neutral solution is achieved for the optimized ZnO/p-Si b-NW electrodes. The PEC performances or incident photon-to-current efficiency are further improved using Si NW cores with smaller pitch size. The photoelectrode stability is dramatically improved by coating a thin TiO2 protection layer using atomic-layer deposition method. These results provide very useful guidelines in designing photoelectrodes for selective solar water oxidation/reduction and overall spontaneous solar fuel generation using low cost earth-abundant materials for practical clean solar fuel production. 

611N. Park, K. Sun, Z. Sun, Y. Jing, and D. Wang, "High Efficiency NiO/ZnO Heterojunction UV Photodiode by Sol-Gel Processing," Journal of Materials Chemistry C, 2013, 1, 7333-7338. (pdf) (SI)

We studied the thin film heterojunction photodiode made of nickel oxide (NiO) and zinc oxide (ZnO) deposited by low cost energy-efficient sol-gel spin coating. The highly visible-transparent heterojunction photodiode with smooth interface gives rise to a good photoresponse and quantum efficiency under the ultra-violet (UV) light illumination. With an applied reverse bias of 5 V, very impressive peak photo responsivity of 21.8 A/W and external quantum efficiency (EQE) of 88% at an incident light wavelength of 310 nm were accomplished. 

601S. Vishniakou, B. W. Lewis, X. Niu, A. Kargar, K. Sun, M. Kalajian, N. Park, M. Yang, Y. Jing, P. Brochu, Z. Sun, C. Li, T. Nguyen, Q. Pei, and D. Wang, "Tactile Feedback Display with Spatial and Temporal Resolutions", Scientific Reports, 2013, 3, 1-6. (pdf) (SI)

Work highlighted by ScienceDaily, Calit2, UCSD, UC News, CNN Money, Science World Report, etc

Watch videos of the work: Video 1, Video 2

We report the electronic recording of the touch contact and pressure using an active matrix pressure sensor array made of transparent zinc oxide thin-film transistors and tactile feedback display using an array of diaphragm actuators made of an interpenetrating polymer elastomer network. Digital replay, editing and manipulation of the recorded touch events were demonstrated with both spatial and temporal resolutions. Analog reproduction of the force is also shown possible using the polymer actuators, despite of the high driving voltage. The ability to record, store, edit, and replay touch information adds an additional dimension to digital technologies and extends the capabilities of modern information exchange with the potential to revolutionize physical learning, social networking, e-commerce, robotics, gaming, medical and military applications.

591A. Kargar, K. Sun, S. J. Kim, D. Lu, Y. Jing, Z. Liu, X. Pan, and D. Wang, "Three-dimensional ZnO/Si broom-like nanowire heterostructures as photoelectrochemical anodes for solar energy conversion", Phys. Status Solidi A, 2013, 1-8. (pdf) (SI) (Most accessed papers)

Abstract: We report a low-cost solution fabrication of threedimensional ZnO/Si broom-like nanowire (“nanobroom”) heterostructures, consisting of Si nanowire (NW) “backbones” and ZnO nanowire “stalls”, and their application as photoelectrochemical anodes for solar water splitting and energy conversion. The nanobroom morphology and atomic structure are characterized using the scanning, transmission, and scanning transmission electron microscopy. Both Si nanowire backbones and ZnO nanowire stalls are defect-free and single-crystalline, and their surfaces are smooth. The optical absorption and photocurrents from nanobroom array electrodes with different Si and ZnO nanowire dimensions are studied. The longer Si NW backbones and smaller ZnO NW stalls lead to better light absorption and larger photoanodic current. The ZnO/Si nanobrooms show much higher photoanodic current than the bare Si NWs due to the effective Si/ZnO junction and increased surface area. The nanobroom electrode stability is also investigated and using a thin TiO2 coating layer protecting the NWs against dissolution, long-term stability is obtained without any change in shape and morphology of nanobrooms. Finally, the effect of catalyst to improve the oxygen evolution reaction at the electrode surface is studied resulting in large enhancement in photoanodic current and significant reduction in anodic turn-on potential. This study reveals the promise of the use of simply fabricated and low-cost three-dimensional heterostructured nanowire photoelectrodes for clean solar energy harvesting and conversion.


581A. Kargar, K. Sun, Y. Jing, C. Choi. H. Jeong, Y. Zhou, K. Madsen, P. Naughton, S. Jin, G. Y. Jung, and D. Wang, "Tailoring n‑ZnO/p-Si Branched Nanowire Heterostructures for Selective Photoelectrochemical Water Oxidation or Reduction", Nano Lett., 2013, 13, 3017−3022. (pdf) (SI)

We report the fabrication of three-dimensional (3D) branched nanowire (NW) heterostructures, consisting of periodically ordered vertical Si NW trunks and ZnO NW branches, and their application for solar water splitting. The branched NW photoelectrodes show orders of magnitudes higher photocurrent compared to the bare Si NW electrodes. More interestingly, selective photoelectrochemical cathodic or anodic behavior resulting in either solar water oxidation or reduction was achieved by tuning the doping concentration of the p-type Si NW core. Specifically, n-ZnO/p-Si branched NW array electrodes with lightly doped core show broadband absorption from UV to near IR region and photocathodic water reduction, while n-ZnO/p+-Si branched NW arrays show photoanodic water oxidation with photoresponse only to UV light. The photoelectrochemical stability for over 24 h under constant light illumination and fixed biasing potential was achieved by coating the branched NW array with thin layers of TiO2 and Pt. These studies not only reveal the promise of 3D branched NW photoelectrodes for high efficiency solar energy harvesting and conversion to clean chemical fuels, but also developing understanding enabling rational design of high efficiency robust photocathodes and photoanodes from low-cost and earthabundant materials allowing practical applications in clean renewable energy.

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571K. Sun, X. Pang, S. Shen, X. Qian, J. S. Cheung, and D. Wang, "Metal Oxide Composite Enabled Nanotextured Si Photoanode for Efficient Solar Driven Water Oxidation", Nano Lett., 2013, 13, 2064−2072. (pdf) (SI)                         

Abstract: We present a study of a transition metal oxide
composite modified n-Si photoanode for efficient and stable water oxidation. This sputter-coated composite functions as a protective coating to prevent Si from photodecomposition, a Schottky heterojunction, a hole conducting layer for efficient charge separation and transportation, and an electrocatalyst to reduce the reaction overpotential. The formation of mixed-valence oxides composed of Ni and Ru effectively modifies the optical, electrical, and catalytic properties of the coating material, as well as the interfaces with Si. The successful application of this oxide composite on nanotextured Si demonstrates improved conversion efficiency due to enhanced catalytic activity, minimized reflection, and increased surface reaction sites. Although the coated nanotextured Si shows a noticeable degradation from 500 cycles of operation, the oxide composite provides a simple method to enable unstable photoanode materials for solar fuel conversion.

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561S. Y. Noh , K. Sun, C. Choi, M. Niu, K. Xu, M. Yang, S. Jin, and D. Wang, Branched TiO2/Si Nanostructures for Enhanced Photoelectrochemical Water splitting”, Nano Energy, 2013, 2, 351–360. (pdf) (SI) (Cover page)

Abstract: We report a successful fabrication of hierarchical three-dimensionally branched TiO2/Si nanowire arrays and their application as the photoelectro chemical electrode for solar water splitting. The branched TiO2/Si nanowire arrays improved the photoelectrochemical (PEC) performance compared to TiO2 thin film-coated Si nanowire arrays because of the substantially increased surface area for electrochemical reactions and enhanced charge transfer kinetics. Wavelength-dependent photocurrent response of the branched nanowire array indicates a strong response in the ultraviolet region (<400 nm), while a negligible photocurrent is observed under visible illumination, primarily caused by the high overpotential loss of n-Si photoanode and energy band configuration of the TiO2/Si heterojunction. The working mechanism based on recombination at this heterogeneous n–n junction is proposed. This study provides insights on the fundamental understanding and potential optimizations of nanoscale hierarchical 3D structured devices for renewable energy applications.

nanoenergy 2012



551T. Yang, K. Sun, X. Liu, W.Wei, T. Yu, G. Xiong, D. Wang and C. Yong, Zinc Oxide Nanowire as an Electron-Extraction Layer for Broadband Polymer Photodetectors with an Inverted Device Structure”, Journal of Physcical Chemistry C, 2012, 116, 13650–13653. (pdf)

Abstract: We report the fabrication of high performance broadband polymer photodetector based on small bandgap conjugated polymers with an inverted device structure, where electrons and holes are collected on ITO and metal contact with high working functions. High quality wide bandgap vertically aligned ZnO nanowire array offers an enhanced surface area and is used as the cathode in this device for effectively extracting electrons and blocking holes from the active polymer layer. The room temperature detectivity of polymer PDs with such an inverted device structure is greater than 1010 Jones with the spectral response from 400 nm to 1450 nm. Our results define a promising pathway for fabrication of high sensitivity polymer PDs with an inverted device structure using ZnO nanowire array cathode buffer layer for a wide range of applications.

541K. Sun, N. Park, Z. Sun, J. Zhou, J. Wang, X. Pang, S. Shen, S. Noh, Y. Jing, S. Jin, P. Yu, and D. Wang, Nickel oxide functionalized silicon for efficient photooxidation of water”, Energy and Environmental Science, 2012, 5, 7872-7877. (pdf)
Abstract: We report a nickel oxide (NiOx) thin film, from a cost-effective sol-gel process, coated n-type silicon (n-Si) as a photoanode for efficient photo-oxidation of water under neutral pH condition. The NiOx thin film has three functions: (i) serves as a protection layer to improve the chemical stability of Si photoelectrode, (ii) acts as an oxygen evolution catalyst, and (iii) provides junction photovoltage to further reduce overpotential. The oxygen evolution onset potential is reduced to below the thermodynamic water oxidation level and oxygen evolution was observed at low overpotentials. Our results demonstrate the fabrication of robust photoelectrodes from low-cost NiOx and Si, which enable a practical solar water oxidation with high efficiency.
ees 2012

531K. Sun, K. Madsen, P. Andersen, W. Bao, Z. Sun, and D. Wang, “Metal and metal oxide nanowire co-catalyzed Si photocathode ”, Nanotechnology 2012, 23, 194013. (pdf)
Abstract: We report a systematic study of Si|ZnO and Si|ZnO|metal photocathodes for effective photoelectrochemical cells and hydrogen generation. Both ZnO nanocrystalline thin films and vertical nanowire arrays were studied. Si|ZnO electrodes showed increased cathodic photocurrents due to improved charge separation by the formation of a p/n junction, and Si|ZnO:Al (n+-ZnO) and Si|ZnO(N2) (thin films prepared in N2/Ar gas) lead to a further increase in cathodic photocurrents. Si|ZnONW (nanowire array) photocathodes dramatically increased the photocurrents and thus photoelectrochemical conversion efficiency due to the enhanced light absorption and enlarged surface area. The ZnO film thickness and ZnO nanowire length were important to the enhancements. A thin metal coating on ZnO showed increased photocurrent due to a catalyzed hydrogen evolution reaction and Ni metal showed comparable catalytic activities to those of Pt and Pd. Moreover, photoelectrochemical instability of Si|ZnO electrodes was minimized by metal co-catalysts. Our results indicate that the metal and ZnO on p-type Si serve as co-catalysts for photoelectrochemical water splitting, which can provide a possible low-cost and scalable method to fabricate high efficiency photocathodes for practical applications in clean solar energy harvesting.
IOP 2012

521K. Sun, Y. Jing, C. Li, X. Zhang, R, Aguinaldo, A. Kargar, K. Madsen, K. Banu, Y. Zhou, Y. Bando, Z. Liu, and D. Wang, 3D branched nanowire heterojunction photoelectrodes for high-efficiency solar water splitting and H2 generation”, Nanoscale 2012, 4, 1515-1521. (pdf)

Abstract: We report the fabrication of a three dimensional branched ZnO/Si heterojunction nanowire array by a two-step, wafer-scale, low-cost, solution etching/growth method and its use as photoelectrode in a photoelectrochemical cell for high efficiency solar powered water splitting. Specifically, we
demonstrate that the branched nanowire heterojunction photoelectrode offers improved light absorption, increased photocurrent generation due to the effective charge separation in Si nanowire backbones and ZnO nanowire branching, and enhanced gas evolution kinetics because of the dramatically increased surface area and decreased radius of curvature. The branching nanowire heterostructures offer direct functional integration of different materials for high efficiency water photoelectrolysis and scalable photoelectrodes for clean hydrogen fuel generation. issue cover (most read articles) (News highlight)



511H. Jeong, K. S. Kim, Y. H. Kim, H. Jeong, H. Song, K. H. Lee, M. S. Jeong, D. Wang, and G. Y. Jung, "A crossbar-type high sensitivity ultraviolet photodetector array based on a one hole–one nanorod configuration via nanoimprint lithography", Nanotechnology, 22, 275310. (pdf)

Abstract: Single crystalline vertical ZnO nanorods were grown in a one hole–one rod configuration using a hydrothermal method with a patterned polymer template generated by nanoimprint lithography, allowing precise control over the position and density of the ZnO nanorods. An 8 × 8 ZnO nanorod-based ultraviolet photodetector array is demonstrated, in which a well-confined number of ZnO nanorods are sandwiched between crossbar-type platinum and indium tin oxide electrodes (e.g. 16 nanorods in a 2 × 2 µm2 area). A high photocurrent/dark current ratio of 3 × 103 at a reverse bias of 1.5 V under UV illumination at room temperature, a responsivity of 4381.4 A W − 1 at 365 nm, and an ultraviolet-to-visible rejection ratio of 83 are obtained and maintained, irrespective of pixel size. A uniform photoresponse is achieved in each of the pixels, indicating the scalability with this technique for fabricating an integrated UV photodetector array circuit.


501W. Park, G. Jo, W.-K. Hong, J. Yoon, M. Choe, S. Lee, Y. Ji, G. Kim, Y. H. Kahng, K. Lee, D. Wang and T. Lee, "Enhancement in the photodetection of ZnO nanowires by introducing surface-roughness-induced traps", Nanotechnology 2011, 22, 205204 (pdf)

Abstract: We investigated the enhanced photoresponse of ZnO nanowire transistors that was introduced with surface-roughness-induced traps by a simple chemical treatment with isopropyl alcohol (IPA). The enhanced photoresponse of IPA-treated ZnO nanowire devices is attributed to an increase in adsorbed oxygen on IPA-induced surface traps. The results of this study revealed that IPA-treated ZnO nanowire devices displayed higher photocurrent gains and faster photoswitching speed than transistors containing unmodified ZnO nanowires. Thus, chemical treatment with IPA can be a useful method for improving the photoresponse of ZnO nanowire devices.

491K. Sun, W. Wei, Y. Jing, Yong Ding, Zhong Lin Wang, and D. Wang Crystalline ZnO Thin Film by Hydrothermal Growth ”, ChemCommun, 2011, 47, 7776 - 7778. (pdf)
Abstract:In this work, we have successfully synthesis ZnO crystal thin film with a high quality from hydrothermal reaction on sapphire substrate. Growth mechanism is clarified based on extensive XTEM study. In addition, electrical and optical properties of the crystal thin film can be controlled by impurities are characterized. The synthesis technique provides fairly high quality and cost-effective substrate for optoelectronic and renewable energy applications.
ZnO thin film

481K. Sun, A. Kargar, N. Park, K. N. Madsen, P. W. Naughton, T. Bright, Y. Jing, and D. Wang Compound Semiconductor Nanowire Solar Cells ”, IEEE JSTQE 2011, 17 (4), 1033-1049.  (pdf)
Abstract:There have been many recent developments in compound semiconductor nanowire photovoltaic devices. Of these, advances in nanowire synthesis and performance enable nanowires to be implemented for efficient and low cost solar energy harvesting devices. On the other hand, many challenges in device fabrication must be resolved in order for nanowires to assure a role at the forefront of solar cell technology.


471K. Sun, Y. Jing, N. Park, C. Li, Y. Bando, and D. Wang, Solution Process of Large Scale High Sensitivity ZnO/Si Hierarchical Nano-heterostructure Photodetectors”, Journal of American Chemical Society 2010, 132, 15465. (pdf)
Abstract: This communication reports a low-cost solution fabrication of wafer-scale ZnO/Si branched nanowire heterostructures and their high photodetection sensitivity of ON/OFF ratio larger than 250 and a peak photo responsivity of 12.8 mA/W at 900 nm. This reported unique 3D branched nanowire structures offer generic approach for the integration of new functional materials for photodetection and photovoltaic applications.

461C. Soci, A. Zhang, X. Bao, H. Kim, Y. Lo and D. Wang, Nanowire photodetector”, Journal of Nanoscience and Nanotechnology 10, 1430 2010. (pdf)
Abstract: The use of nanowires and nanowire structures as photodetectors is an emerging research topic. Despite the large amount of reports on nanowire photoresponse that appeared in the literature over the last decade, the mechanism leading to high photosensitivity and photoconductive gain in high aspect ratio nanostructures has been elucidated only recently. Novel device architectures integrated in single nanowire devices are also being actively studied and developed. In this article, the general nanowire photodetector concepts are reviewed, together with a detailed description of the physical phenomena occurring in nanowire photoconductors and phototransistors, with some examples from recent experimental results obtained in our groups. An outlook on future directions toward the use of semiconductor nanowire photoconductors as intrachip interconnects, single-photon detectors, and image sensors, is also given.



451 W. Wei, X. Bao, C. Soci, Y. Ding, Z. Wang and D. Wang, Direct Heteroepitaxy of Vertical InAs Nanowires on Si Substrates for Broad Band Photovoltaics and Photodetection”, Nano Letters 9, 2926 2009. (pdf)

Catalyst-free, direct heteroepitaxial growth of vertical InAs nanowires on Si(111) substrates was accomplished over a large area by metal−organic chemical vapor deposition. Nanowires showed very uniform diameters and a zinc blende crystal structure. The heterojunctions formed at the interface between the n-type InAs nanowires and the p-type Si substrate were exploited to fabricate vertical array photodiode devices which showed an excellent rectification ratio and low reverse leakage current. Temperature-dependent current transport across the heterojunctions was studied theoretically and experimentally in the dark and under AM 1.5 illumination.

441 S. A. Dayeh, C. Soci, X. Bao, and D. Wang, Advances in the synthesis of InAs and GaAs nanowires for electronic applications”, Nano Today 4, 347 2009. (pdf)

New materials and device concepts are in great demand for continual (opto)electronic device scaling and performance enhancement. Arsenide III-V semiconductor nanowires promise novel device architectures and superior (opto)electronic properties. Recent insights into the growth and optimal control over the InAs and GaAs nanowire morphology and distinguished key physical aspects in their growth are discussed. Direct correlation of individual nanowire crystal structure with their electronic transport properties is also presented.

431 S. Raychaudhuri, S. A. Dayeh, D. Wang, and E. T. Yu, Precise Semiconductor Nanowire Placement Through Dielectrophoresis”, Nano Letters 9, 2260 2009. (pdf)

We demonstrate the ability to precisely control the alignment and placement of large numbers of InAs nanowires from solution onto very narrow, prepatterned electrodes using dielectrophoresis. An understanding of dielectrophoretic behavior associated with such electrode geometries is essential to development of approaches for assembly of intricate nanowire systems. The influence of signal frequency and electrode design on nanowire manipulation and placement is examined. Signal frequencies in the range of 10 MHz are found to yield high percentages of aligned nanowires on electrodes with dimensions similar to that of the nanowire. Strategies for further improvement of nanowire alignment are suggested and analyzed.

421 S. A. Dayeh, E. T. Yu, and D. Wang, “Surface Diffusion and Substrate−Nanowire Adatom Exchange in InAs Nanowire Growth”, Nano Letters 9, 1967 2009.(pdf)

We report new fundamental insights into InAs nanowire (NW) nucleation and evolution on InAs (111)B surfaces using organometallic vapor phase epitaxy and present the first experimental demonstration of two distinct NW growth regimes, defined by the direction of substrate−NW adatom exchange, that lead to nonlinear growth rates. We show that the NW elongation rate and morphology in these two growth regimes are governed by the relative difference between the In adatom diffusion lengths on the growth substrate surface and on the NW sidewalls, resulting in strong growth rate dependence on the NW length. These results indicate that surface solid−phase diffusion of In adatoms is a key process in InAs NW growth, which is also supported by diameter-dependent growth rates. These developments enable rational growth of axial and radial NW heterostructures.

411 S.A. Dayeh, D. Susac, K.L. Kavanagh, E.T. Yu, and D. Wang, “Structural and Room-Temperature Transport Properties of Zinc Blende and Wurtzite InAs Nanowires”, Advanced Functional Materials 19, 2102 2009.  (pdf)

Here, direct correlation between the microstructure of InAs nanowires (NWs) and their electronic transport behavior at room temperature is reported. Pure zinc blende (ZB) InAs NWs grown on SiO2/Si substrates are characterized by a rotational twin along their growth-direction axis while wurtzite (WZ) InAs NWs grown on InAs (111)B substrates have numerous stacking faults
perpendicular to their growth-direction axis with small ZB segments. In transport measurements on back-gate field-effect transistors (FETs) fabricated from both types of NWs, significantly distinct subthreshold characteristics are observed (Ion/Ioff2 for ZB NWs and 104 for WZ NWs) despite only a slight difference in their transport coefficients. This difference is attributed to spontaneous polarization charges at the WZ/ZB interfaces, which suppress carrier accumulation at the NW surface, thus enabling full depletion of the WZ NW FET channel. More>>>

401 J. M. Law, S. A. Dayeh, D. Wang, E. T. Yu, “Scanning capacitance characterization of potential screening in InAs nanowire devices”, Journal of Applied Physics 105, 014306 2009. (pdf)

We have used scanning capacitance microscopy and spectroscopy to examine the effects of micron-scale metal contacts, typically present in nanowire-based electronic devices, on carrier modulation and electrostatic behavior in InAs semiconductor nanowires. We observe a pronounced dependence of scanning capacitance images and spectra on distance between the scanning capacitance probe tip and nanowire contact up to distances of 3–4 µm. Based on the comparison of these data with results of finite-element electromagnetic simulations, we interpret these results as a consequence of electrostatic screening of the tip-nanowire potential difference by the large metal contact. More>>>

391 S. A. Dayeh, E. T. Yu, and D. Wang, “Transport Coefficients of InAs Nanowires as a Function of Diameter”, Small 5, 77 2009.  (pdf)
Herein, we present a comprehensive method to extract the transport coefficients in NWs as functions of their diameter and vertical (gate) and lateral (drain) fields, thus enabling consistent comparison due to their field dependence. This extraction technique extends earlier studies on the variation of transport properties in NWs as functions of their diameter, which was attributed to either 1) only changes in carrier concentration (Si NWs) or 2) only changes in carrier mobility (GaN NWs). One-dimensional (1D) Schro¨ dinger– Poisson self-consistent solutions for a material structure similar to that used in our experiments, but with different InAs thicknesses, was performed to validate our experimental observations and analysis.





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381 A. Zhang, S. You, C. Soci, Y. Liu, D. Wang, and Y.-H.Lo, “Silicon Nanowire Detectors Showing Phototransistive Gain”, Applied Physics Letters 93, 121110 2008. (pdf)

Nanowire photodetectors are shown to function as phototransistors with high sensitivity. Due to small lateral dimensions, a nanowire detector can have low dark current while showing large phototransistive gain. Planar and vertical silicon nanowire photodetectors fabricated in a top-down approach using an etching process show a phototransistive gain above 35 000 at low light intensities. Simulations show that incident light can be waveguided into vertical nanowires resulting in up to 40 times greater external quantum efficiency above their physical fill factor. Vertical silicon nanowire phototransistors formed by etching are attractive for low light level detection and for integration with silicon electronics.

371 S.A. Dayeh, P. Chen, Y. Jing, E.T. Yu, S.S. Lau, and D. Wang, “Integration of Vertical InAs Nanowire Arrays on Insulator-on-Silicon for Electrical Isolation”, Applied Physics Letters 93, 203109, 2008. (pdf)

Vertical and electrically isolated InAs nanowires (NWs) are integrated with Si in a technique that bypasses structural defects and transport barriers at the Si–III–V NW interface. Smart-cut® technique is used to transfer a thin InAs layer onto SiO2/Si and is subsequently used for ordered organometallic vapor phase epitaxy of InAs NWs. The InAs layer in the regions between the InAs NWs is etched resulting in ordered, vertical, and electrically isolated InAs NW arrays. This transfer and fabrication technique enables heteroepitaxy of three dimensional III–V structures on Si and allows the realization of vertical devices with unprecedented control over their architectures.

361 S.A. Dayeh, D. Susac, K.L. Kavanagh, E.T. Yu and D. Wang, “Field Dependent Transport Properties in InAs Nanowire Field Effect Transistor”, Nano Letter 8, 3114, 2007. (pdf)

We present detailed studies of the field dependent transport properties of InAs nanowire field-effect transistors. Transconductance dependence on both vertical and lateral fields is discussed. Velocity-field plots are constructed from a large set of output and transfer curves that show negative differential conductance behavior and marked mobility degradation at high injection fields. Two dimensional electrothermal simulations at current densities similar to those measured in the InAs NWFET devices indicate that a significant temperature rise occurs in the channel due to enhanced phonon scattering that leads to the observed mobility degradation. Scanning transmission electron microscopy measurements on devices operated at high current densities reveal arsenic vaporization and crystal deformation in the subject nanowires.

351 C. Soci, X. Bao, D.P.R. Aplin, and D. Wang, “A Systematic Study on the Growth of GaAs Nanowires by Metal Organic Chemical Vapor Depositon”, Nano Letter 8, 4275, 2008. (pdf)

The epitaxial growth of GaAs nanowires (NWs) on GaAs(111)B substrates by metal−organic chemical vapor deposition has been systematically investigated as a function of relevant growth parameters, namely, temperature, arsine (AsH3) and trimethyl-gallium (TMGa) flow rates, growth time, and gold nanoparticle catalyst size. When growing in excess As conditions (V/III molar ratios greater than four), the NW growth rate is independent of AsH3 concentration, while it is linearly dependent on TMGa concentration, and it is thermally activated. The NW morphology is primarily affected by the growth temperature, with very uniform NWs growing at around 400 °C and severely tapered NWs growing above 500 °C. A simple phenomenological expression that allows prediction of the NW growth rate over a wide range of growth parameters has been derived. The growth rate dependence on the seed nanoparticle size has also been investigated, which reveals valuable information on the role of catalyst supersaturation and Ga surface diffusion in the growth mechanism. The NW growth rate is found to be almost independent of Au nanoparticle size down to diameters of 20 nm over a wide range of temperatures and TMGa and AsH3 molar flows. More>>>

341 X. Bao, C. Soci, D. Susac, J, Bratvold, D.P.R. Aplin, W, Wei, C.Y. Chen, S.A. Dayeh, K.L. Kavanagh, and D. Wang, “Heteroepitaxial Growth of Vertical GaAs Nanowires on Si(111) Substrates by Metal Organic Chemical Vapor Deposition”, Nano Letters 8, 3755, 2008. (pdf)

Epitaxial growth of vertical GaAs nanowires on Si (111) substrates is demonstrated by metal−organic chemical vapor deposition via a vapor−liquid−solid growth mechanism. Systematic experiments indicate that substrate pretreatment, pregrowth alloying temperature, and growth temperature are all crucial to vertical epitaxial growth. Nanowire growth rate and morphology can be well controlled by the growth temperature, the metal−organic precursor molar fraction, and the molar V/III ratio. The as-grown GaAs nanowires have a predominantly zinc-blende crystal structure along a 111 direction. Crystallographic {111} stacking faults found perpendicular to the growth axis could be almost eliminated via growth at high V/III ratio and low temperature. Single nanowire field effect transistors based on unintentionally doped GaAs nanowires were fabricated and found to display a strong effect of surface states on their transport properties.




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331 S.A. Dayeh, E.T. Yu and D. Wang, “Excess Indium and Substrate Effects on the Growth of InAs Nanowires”, Small 3, 1683, 2007. (pdf)

In this study, we examine nucleation and growth of InAs NWs from excess In with and without Au nanoparticles. Au nanoparticles on SiO2/Si substrates were found to facilitate AsH3 pyrolysis[18] but not to be necessary to nucleate NWs, while excess In, supplied from either the input group III precursor source or the III–V substrate, was found to nucleate
InAs NWs. We show that due to the catalytic effect of Au nanoparticles on substrate decomposition, NW growth in a closed chemical vapor deposition (CVD) tube is possible without any additional source other than the growth substrate itself.

32 S.A. Dayeh, E.T. Yu and D. Wang, “Growth of InAs Nanowire on SiO2 substrates: Nucleation, Evolution, and Role of Au Nanoparticles”, Journal of Physical Chemistry C 111, 13331 (2007). (pdf)

We have studied the nucleation and growth of InAs nanowires (NWs) on SiO2/Si substrates by organometallic vapor-phase epitaxy (OMVPE). Through systematic characterization of InAs NW morphology as a function of V/III precursor ratio, precursor flow rates, growth temperature, growth time, and the presence/absence of Au nanoparticles, a number of significant insights into InAs NW growth using OMVPE have been developed. Specifically, we have found that (i) the growth of InAs NWs can be initiated from a single indium (In) droplet, (ii) Au nanoparticles (NPs) enhance group V precursor (AsH3) pyrolysis but are not necessary to nucleate growth, (iii) growth of InAs NWs on SiO2 substrates occurs in the kinetically limited vapor−liquid−solid (VLS) growth regime, (iv) InAs NWs on SiO2 films decompose at elevated temperatures even under significant AsH3 overpressure, and (v) the V/III ratio is the growth-rate-limiting factor in the VLS growth of the InAs nanowires. Many of these findings on InAs NW growth can be generalized to and provide very useful information for rational synthesis of other III−V compound semiconductor NWs.

31 S.A. Dayeh, C. Soci, E.T. Yu and D. Wang, “Transport Properties of InAs Nanowire Field Effect Transistors: Effects of Surface States”, J. Vac. Sci. Tech. B 25, 1432 (2007).  (pdf)

It is shown that interface trap states have pronounced effects on carrier transport and parameter extraction from top-gated InAs nanowire field effect transistors (NWFETs). Due to slow surface state charging and discharging, the NWFET characteristics are time dependent with time constants as long as ~45 s. This is also manifested in a time-dependent extrinsic transconductance that severely affects carrier mobility and carrier density determination from conventional three-terminal current-voltage characteristics. Slow gate voltage sweep rates result in charge balance between carrier capture and emission from interface states and lead to reduced hysteresis in the transfer curves. The gate transconductance is thus increased and intrinsic NW transport parameters can be isolated. In the InAs NWFETs, a carrier mobility value of ~16 000 cm2/V s was obtained from the transfer curves at slow sweep rates, which is significantly higher than ~1000 cm2/V s obtained at fast sweep rates. A circuit model that takes into account the reduction in the extrinsic transconductance is used to estimate an interface state capacitance to be ~2 µF/cm−2, a significant value that can lead to underestimation of carrier mobility.

Selected for the August 13 2007 Issue of Virtual Journal of Nanoscale Science & Technology

30 X. Zhou, S.A. Dayeh, E.T. Yu and D.Wang “Analysis of Carrier Modulation in InAs Nanowires by Scanning Gate Microscopy”, J. Vac. Sci. Tech. B 25, 1427, 2007. (pdf)

The authors have used scanning gate microscopy combined with numerical simulations to analyze local carrier and current modulation effects in InAs semiconductor nanowires grown by metal-organic chemical vapor deposition. Measurements of current flow in the nanowire as a function of probe tip position, at both high and low drain bias, reveal that carrier and current modulation is strongest when the probe tip is near the source and drain nanowire contacts, and decreases at greater tip-contact distances. The measured transconductance is approximately 80% greater near the source contact for high drain bias condition and 120% greater near the drain contact for low drain bias condition, respectively, than at the center of the nanowire. Numerical simulations for different tip positions relative to the metal contact confirm that carrier modulation should be stronger when the tip is closer to the source or drain contact than at the center of the wire, consistent with the experimental measurements.

Selected for the May 2007 Issue of Virtual Journal of Nanoscale Science & Technology

29 S.A. Dayeh, E.T. Yu and D. Wang, “III-V Nanowire Growth Mechanism: V/III Ratio and Temperature Effects”, Nano Letters 7, 2486, 2007. (pdf)

We have studied the dependence of Au-assisted InAs nanowire (NW) growth on InAs(111)B substrates as a function of substrate temperature and input V/III precursor ratio using organometallic vapor-phase epitaxy. Temperature-dependent growth was observed within certain temperature windows that are highly dependent on input V/III ratios. This dependence was found to be a direct consequence of the drop in NW nucleation and growth rate with increasing V/III ratio at a constant growth temperature due to depletion of indium at the NW growth sites. The growth rate was found to be determined by the local V/III ratio, which is dependent on the input precursor flow rates, growth temperature, and substrate decomposition. These studies advance understanding of the key processes involved in III−V NW growth, support the general validity of the vapor−liquid−solid growth mechanism for III−V NWs, and improve rational control over their growth morphology.

28 X. Zhou, S.A. Dayeh, D. Wang and E.T. Yu, “Scanning Gate Microscopy of InAs Nanowires”, Applied Physics Letters 90, 233118, 2007.  (pdf)

Scanning gate microscopy, in which a conductive probe tip in an atomic force microscope is employed as a local, nanoscale top gate contact, has been used to characterize local carrier and current modulation effects in a 45 nm diameter InAs semiconductor nanowire grown by metal organic chemical vapor deposition. Measurement of current flow in the nanowire as a function of tip position reveals that for both positive and negative tip bias voltages, carrier and current modulation is strongest when the probe tip is near the source and drain nanowire contacts, reaching a global maximum approximately 100–200 nm distant from the source contact and a secondary maximum a similar distance from the drain contact and decreasing at greater tip-contact distances. This effect is explained, with verification by numerical simulation, as a consequence of the capacitance between the tip and the source and drain contacts as a function of tip location. Measurement of transconductance as a function of tip position reveals that the transconductance is approximately 80%–90% greater near the source contact than at the center of the nanowire.

27 S.A Dayeh, C. Soci, P.K.L. Yu, E.T. Yu and D. Wang, “Influence of Surface States on the Extraction of Transport Parameters from InAs Nanowire Field Effect Transistors”, Applied Physics Letters 90, 162112 2007(pdf)

The capacitive effects of interface trap states in top-gated InAs nanowire field effect transistors and their influence on the experimental extraction of transport parameters are discussed. Time resolved transfer characteristics exhibit transient behavior indicating surface state trapping and detrapping with long characteristic time constants of 45 s. Varying gate voltage sweep rate results in a time-dependent extrinsic transconductance; a reduced gate voltage sweep rate leads to a charge neutral interface, reduced interface state capacitance, higher measured transconductance, and minimal hysteresis. These results demonstrate that measurements with a charge neutralized or passivated surface are key to extract intrinsic nanowire transport parameters.

26C. Soci, A. Zhang, B. Xiang, S.A. Dayeh, D.P.R. Aplin, J. Park, X.Y. Bao, Y.H. Lo and D. Wang, “ZnO Nanowire UV Photodetectors with High Internal Gain”, Nano Letters 7, 1003 2007. (pdf)

ZnO nanowire (NW) visible-blind UV photodetectors with internal photoconductive gain as high as G 108 have been fabricated and characterized. The photoconduction mechanism in these devices has been elucidated by means of time-resolved measurements spanning a wide temporal domain, from 10-9 to 102 s, revealing the coexistence of fast (τ 20 ns) and slow (τ 10 s) components of the carrier relaxation dynamics. The extremely high photoconductive gain is attributed to the presence of oxygen-related hole-trap states at the NW surface, which prevents charge-carrier recombination and prolongs the photocarrier lifetime, as evidenced by the sensitivity of the photocurrrent to ambient conditions. Surprisingly, this mechanism appears to be effective even at the shortest time scale investigated of t < 1 ns. Despite the slow relaxation time, the extremely high internal gain of ZnO NW photodetectors results in gain-bandwidth products (GB) higher than 10 GHz. The high gain and low power consumption of NW photodetectors promise a new generation of phototransistors for applications such as sensing, imaging, and intrachip optical interconnects.


Highlighted by Azonano, Scientific Frontline, Photonics Spectra, Daily Science News, First Science, Medical Technology Buisness, SOFTPEDIA, etc…Listed #9 on the top 20 most cited articles and “Hottest Articles”published in Nano Letters in 2007

25 B. Xiang, P. Wei, X. Zhang, S.A. Dayeh, D. P. R. Aplin, C. Soci, D. Yu, and D. Wang, “Rational Synthesis of p-Type Zinc Oxide Nanowire Arrays Using Simple Chemical Vapor Deposition”, Nano Letters 7, 323 2007.(pdf)

We report, for the first time, the synthesis of the high-quality p-type ZnO NWs using a simple chemical vapor deposition method, where phosphorus pentoxide has been used as the dopant source. Single-crystal phosphorus doped ZnO NWs have their growth axis along the 001 direction and form perfect vertical arrays on a-sapphire. P-type doping was confirmed by photoluminescence measurements at various temperatures and by studying the electrical transport in single NWs field-effect transistors. Comparisons of the low-temperature PL of unintentionally doped ZnO (n-type), as-grown phosphorus-doped ZnO, and annealed phosphorus-doped ZnO NWs show clear differences related to the presence of intragap donor and acceptor states. The electrical transport measurements of phosphorus-doped NW FETs indicate a transition from n-type to p-type conduction upon annealing at high temperature, in good agreement with the PL results. The synthesis of p-type ZnO NWs enables novel complementary ZnO NW devices and opens up enormous opportunities for nanoscale electronics, optoelectronics, and medicines.

Highlighted by Laser Focus World, Semiconductor Today, PHYSORG, Photonics Spectra, LEDs Magazine, Scientific Frontline, EE Times, Compound SEMI Online, etc…Listed #12 on the top 20 most cited articles published in Nano Letters in 2007

24 S.A. Dayeh, D.P.R. Aplin, X. Zhou, P. K. L. Yu, E.T. Yu, and D. Wang, “High Electron Mobility Indium Arsenide Nanowire Field Effect Transistors”, Small 3, 326 2007. (pdf)

Single-crystal InAs nanowires (NWs) are synthesized using metal-organic chemical vapor deposition (MOCVD) and fabricated into NW field-effect transistors (NWFETs) on a SiO2/n+-Si substrate with a global n+-Si back-gate and sputtered SiOx/Au underlap top-gate. For top-gate NWFETs, we have developed a model that allows accurate estimation of characteristic NW parameters, including carrier field-effect mobility and carrier concentration by taking into account series and leakage resistances, interface state capacitance, and top-gate geometry. Both the back-gate and the top-gate NWFETs exhibit room-temperature field-effect mobility as high as 6580 cm2 V-1 s-1, which is the lower-bound value without interface-capacitance correction, and is the highest mobility reported to date in any semiconductor NW. small06




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23 L. Wang, D. Wang, P.M. Asbeck, "A Numerical Schrödinger–Poisson Solver for Radially Symmetric Nanowire Core–shell Structures",  Solid State Electronics 50, 1732 2006. (pdf)

22 X. Zhou, S. A. Dayeh, D. Aplin, D. Wang, and E. T. Yu, “Direct observation of ballistic and drift carrier transport regimes in InAs nanowires”, Applied Physics Letters 89, 053113 2006. (pdf)

21 X. Zhou, S. A. Dayeh, D. Aplin, D. Wang, and E. T. Yu, “Scanned electrical probe characterization of carrier transport behavior in InAs Nanowires”, J. Vac. Sci. Technol. B. 24, 2036 2006. (pdf)


20 F. Zhang, R. Barrowcliff, G. Stecker, W. pan, D. Wang, and S.-T. Hsu, “Synthesis of Metallic Iridium Oxide Nanowires via Metal Organic Chemical Vapor Deposition”, Japanese Journal of Applied Physics 44, L398 2005. (pdf)