Professor Zheng received her Ph.D. in Mechanical & Aerospace Engineering from Princeton University (2006), B.S. in Thermal Engineering from Tsinghua University (2000). Prior to joining Stanford in 2007, Professor Zheng did her postdoctoral work in the Department of Chemistry and Chemical Biology at Harvard University. Professor Zheng is a member of MRS, ACS and combustion institute. Professor Zheng received the TR35 Award from the MIT Technology Review (2013), one of the 100 Leading Global Thinkers by the Foreign Policy Magazine (2013), 3M Nontenured Faculty Grant Award (2013), the Presidential Early Career Award (PECASE) from the white house (2009), Young Investigator Awards from the ONR (2008), DARPA (2008), Terman Fellowship from Stanford (2007), and Bernard Lewis Fellowship from the Combustion Institute (2004).

Academic Appointments

Administrative Appointments

  • Assistant Professor, Mechanical Engineering (2007 - Present)

Honors & Awards

  • 3M Nontenured Faculty Grant Award, 3M (2013)
  • One of the Pioneers on the TR35 Global list, MIT Technology Review (2013)
  • Amelia Earhart Fellowship, Zonta International Foundation (2003)
  • Bernard Lewis Fellowship, The Combustion Institute (2004)
  • Terman Faculty Award, Stanford University (2007)
  • Young Faculty Award, DARPA (2008)
  • Young Investigator Program, ONR (2008)
  • Presidential Early Career Award for Scientists and Engineers, Presidential Early Career Awards (2009)

Professional Education

  • BS, Tsinghua University, Thermal Engineering (2000)
  • PhD, Princeton, Mechanical and Aerospace Engineering (2006)

2013-14 Courses

Postdoctoral Advisees

Journal Articles

  • Simultaneously Efficient Light Absorption and Charge Separation in WO3/BiVO4 Core/Shell Nanowire Photoanode for Photoelectrochemical Water Oxidation. Nano letters Rao, P. M., Cai, L., Liu, C., Cho, I. S., Lee, C. H., Weisse, J. M., Yang, P., Zheng, X. 2014; 14 (2): 1099-1105


    We report a scalably synthesized WO3/BiVO4 core/shell nanowire photoanode in which BiVO4 is the primary light-absorber and WO3 acts as an electron conductor. These core/shell nanowires achieve the highest product of light absorption and charge separation efficiencies among BiVO4-based photoanodes to date and, even without an added catalyst, produce a photocurrent of 3.1 mA/cm(2) under simulated sunlight and an incident photon-to-current conversion efficiency of ∼60% at 300-450 nm, both at a potential of 1.23 V versus RHE.

    View details for DOI 10.1021/nl500022z

    View details for PubMedID 24437363

  • Rapid and Controllable Flame Reduction of TiO2 Nanowires for Enhanced Solar Water-Splitting NANO LETTERS Cho, I. S., Logar, M., Lee, C. H., Cai, L., Prinz, F. B., Zheng, X. 2014; 14 (1): 24-31


    We report a new flame reduction method to generate controllable amount of oxygen vacancies in TiO2 nanowires that leads to nearly three times improvement in the photoelectrochemical (PEC) water-splitting performance. The flame reduction method has unique advantages of a high temperature (>1000 °C), ultrafast heating rate, tunable reduction environment, and open-atmosphere operation, so it enables rapid formation of oxygen vacancies (less than one minute) without damaging the nanowire morphology and crystallinity and is even applicable to various metal oxides. Significantly, we show that flame reduction greatly improves the saturation photocurrent densities of TiO2 nanowires (2.7 times higher), α-Fe2O3 nanowires (9.4 times higher), ZnO nanowires (2.0 times higher), and BiVO4 thin film (4.3 times higher) in comparison to untreated control samples for PEC water-splitting applications.

    View details for DOI 10.1021/nl4026902

    View details for Web of Science ID 000329586700005

    View details for PubMedID 24295287

  • Flash ignition of freestanding porous silicon films: effects of film thickness and porosity. Nano letters Ohkura, Y., Weisse, J. M., Cai, L., Zheng, X. 2013; 13 (11): 5528-5533


    We report the first successful xenon flash ignition of freestanding porous Si films in air. The minimum flash ignition energy (Emin) first decreases and then increases with increasing the porous Si film thickness due to the competition between light absorption and heat loss. The Emin is lower for higher porosity film because high porosity reduces both the heat capacity and the thermal conductivity, facilitating the temperature rise. These results are important for initiating controlled porous Si combustion and preventing their unwanted combustion for safety reasons.

    View details for DOI 10.1021/nl403114g

    View details for PubMedID 24175629

  • Peel-and-Stick: Mechanism Study for Efficient Fabrication of Flexible/Transparent Thin-film Electronics SCIENTIFIC REPORTS Lee, C. H., Kim, J., Zou, C., Cho, I. S., Weisse, J. M., Nemeth, W., Wang, Q., van Duin, A. C., Kim, T., Zheng, X. 2013; 3


    Peel-and-stick process, or water-assisted transfer printing (WTP), represents an emerging process for transferring fully fabricated thin-film electronic devices with high yield and fidelity from a SiO2/Si wafer to various non-Si based substrates, including papers, plastics and polymers. This study illustrates that the fundamental working principle of the peel-and-stick process is based on the water-assisted subcritical debonding, for which water reduces the critical adhesion energy of metal-SiO2 interface by 70 ~ 80%, leading to clean and high quality transfer of thin-film electronic devices. Water-assisted subcritical debonding is applicable for a range of metal-SiO2 interfaces, enabling the peel-and-stick process as a general and tunable method for fabricating flexible/transparent thin-film electronic devices.

    View details for DOI 10.1038/srep02917

    View details for Web of Science ID 000325469500004

    View details for PubMedID 24108063

  • Electroassisted Transfer of Vertical Silicon Wire Arrays Using a Sacrificial Porous Silicon Layer NANO LETTERS Weisse, J. M., Lee, C. H., Kim, D. R., Cai, L., Rao, P. M., Zheng, X. 2013; 13 (9): 4362-4368


    An electroassisted method is developed to transfer silicon (Si) wire arrays from the Si wafers on which they are grown to other substrates while maintaining their original properties and vertical alignment. First, electroassisted etching is used to form a sacrificial porous Si layer underneath the Si wires. Second, the porous Si layer is separated from the Si wafer by electropolishing, enabling the separation and transfer of the Si wires. The method is further expanded to develop a current-induced metal-assisted chemical etching technique for the facile and rapid synthesis of Si nanowires with axially modulated porosity.

    View details for DOI 10.1021/nl4021705

    View details for Web of Science ID 000330158900063

    View details for PubMedID 23919596

  • Morphological control of heterostructured nanowires synthesized by sol-flame method NANOSCALE RESEARCH LETTERS Luo, R., Cho, I. S., Feng, Y., Cai, L., Rao, P. M., Zheng, X. 2013; 8


    Heterostructured nanowires, such as core/shell nanowires and nanoparticle-decorated nanowires, are versatile building blocks for a wide range of applications because they integrate dissimilar materials at the nanometer scale to achieve unique functionalities. The sol-flame method is a new, rapid, low-cost, versatile, and scalable method for the synthesis of heterostructured nanowires, in which arrays of nanowires are decorated with other materials in the form of shells or chains of nanoparticles. In a typical sol-flame synthesis, nanowires are dip-coated with a solution containing precursors of the materials to be decorated, then dried in air, and subsequently heated in the post-flame region of a flame at high temperature (over 900°C) for only a few seconds. Here, we report the effects of the precursor solution on the final morphology of the heterostructured nanowire using Co3O4 decorated CuO nanowires as a model system. When a volatile cobalt salt precursor is used with sufficient residual solvent, both solvent and cobalt precursor evaporate during the flame annealing step, leading to the formation of Co3O4 nanoparticle chains by a gas-solid transition. The length of the nanoparticle chains is mainly controlled by the temperature of combustion of the solvent. On the other hand, when a non-volatile cobalt salt precursor is used, only the solvent evaporates and the cobalt salt is converted to nanoparticles by a liquid-solid transition, forming a conformal Co3O4 shell. This study facilitates the use of the sol-flame method for synthesizing heterostructured nanowires with controlled morphologies to satisfy the needs of diverse applications.

    View details for DOI 10.1186/1556-276X-8-347

    View details for Web of Science ID 000323330600001

    View details for PubMedID 23924299

  • Sol-Flame Synthesis: A General Strategy To Decorate Nanowires with Metal Oxide/Noble Metal Nanoparticles NANO LETTERS Peng, Y., Cho, I. S., Rao, P. M., Cai, L., Zheng, X. 2013; 13 (3): 855-860


    The hybrid structure of nanoparticle-decorated nanowires (NP@NW) combines the merits of large specific surface areas for NPs and anisotropic properties for NWs and is a desirable structure for applications including batteries, dye-sensitized solar cells, photoelectrochemical water splitting, and catalysis. Here, we report a novel sol-flame method to synthesize the NP@NW hybrid structure with two unique characteristics: (1) large loading of NPs per NW with the morphology of NP chains fanning radially from the NW core and (2) intimate contact between NPs and NWs. Both features are advantageous for the above applications that involve both surface reactions and charge transport processes. Moreover, the sol-flame method is simple and general, with which we have successfully decorated various NWs with binary/ternary metal oxide and even noble metal NPs. The unique aspects of the sol-flame method arise from the ultrafast heating rate and the high temperature of flame, which enables rapid solvent evaporation and combustion, and the combustion gaseous products blow out NPs as they nucleate, forming the NP chains around NWs.

    View details for DOI 10.1021/nl300060b

    View details for Web of Science ID 000316243800001

    View details for PubMedID 22494023

  • Reducing minimum flash ignition energy of Al microparticles by addition of WO3 nanoparticles APPLIED PHYSICS LETTERS Ohkura, Y., Rao, P. M., Cho, I. S., Zheng, X. 2013; 102 (4)

    View details for DOI 10.1063/1.4790152

    View details for Web of Science ID 000314723600076

  • Electro-Assisted Transfer of Vertical Silicon Wire Arrays Using a Sacrificial Porous Silicon Layer Nano Letters Weisse, J., M., Lee, C., H., Kim, D., R., Cai, L., Rao, P., M., Zheng, X., L. 2013; 9 (13): 4362-4368
  • Flame synthesis of WO3 nanotubes and nanowires for efficient photoelectrochemical water-splitting PROCEEDINGS OF THE COMBUSTION INSTITUTE Rao, P. M., Cho, I. S., Zheng, X. 2013; 34: 2187-2195
  • Sol-flame synthesis of hybrid metal oxide nanowires PROCEEDINGS OF THE COMBUSTION INSTITUTE Feng, Y., Cho, I. S., Cai, L., Rao, P. M., Zheng, X. 2013; 34: 2179-2186
  • Flame synthesis of 1-D complex metal oxide nanomaterials PROCEEDINGS OF THE COMBUSTION INSTITUTE Cai, L., Rao, P. M., Feng, Y., Zheng, X. 2013; 34: 2229-2236
  • Codoping titanium dioxide nanowires with tungsten and carbon for enhanced photoelectrochemical performance. Nature communications Cho, I. S., Lee, C. H., Feng, Y., Logar, M., Rao, P. M., Cai, L., Kim, D. R., Sinclair, R., Zheng, X. 2013; 4: 1723-?


    Recent density-functional theory calculations suggest that codoping TiO2 with donor-acceptor pairs is more effective than monodoping for improving photoelectrochemical water-splitting performance because codoping can reduce charge recombination, improve material quality, enhance light absorption and increase solubility limits of dopants. Here we report a novel ex-situ method to codope TiO2 with tungsten and carbon (W, C) by sequentially annealing W-precursor-coated TiO2 nanowires in flame and carbon monoxide gas. The unique advantages of flame annealing are that the high temperature (>1,000 °C) and fast heating rate of flame enable rapid diffusion of W into TiO2 without damaging the nanowire morphology and crystallinity. This is the first experimental demonstration that codoped TiO2:(W, C) nanowires outperform monodoped TiO2:W and TiO2:C and double the saturation photocurrent of undoped TiO2 for photoelectrochemical water splitting. Such significant performance enhancement originates from a greatly improved electrical conductivity and activity for oxygen-evolution reaction due to the synergistic effects of codoping.

    View details for DOI 10.1038/ncomms2729

    View details for PubMedID 23591890

  • Rapid and Controllable Flame Reduction of TiO2 Nanowires for Enhanced Solar Water-Splitting Nano Letters, ASAP Cho, I., S., Logar, M., Lee, C., H., Cai, L., L., Prinz, F., B., Zheng, X., L. 2013

    View details for DOI 10.1021/nl4026902

  • Peel-and-Stick: Fabricating Thin Film Solar Cell on Universal Substrates SCIENTIFIC REPORTS Lee, C. H., Kim, D. R., Cho, I. S., William, N., Wang, Q., Zheng, X. 2012; 2


    Fabrication of thin-film solar cells (TFSCs) on substrates other than Si and glass has been challenging because these nonconventional substrates are not suitable for the current TFSC fabrication processes due to poor surface flatness and low tolerance to high temperature and chemical processing. Here, we report a new peel-and-stick process that circumvents these fabrication challenges by peeling off the fully fabricated TFSCs from the original Si wafer and attaching TFSCs to virtually any substrates regardless of materials, flatness and rigidness. With the peel-and-stick process, we integrated hydrogenated amorphous silicon (a-Si:H) TFSCs on paper, plastics, cell phone and building windows while maintaining the original 7.5% efficiency. The new peel-and-stick process enables further reduction of the cost and weight for TFSCs and endows TFSCs with flexibility and attachability for broader application areas. We believe that the peel-and-stick process can be applied to thin film electronics as well.

    View details for DOI 10.1038/srep01000

    View details for Web of Science ID 000312530900001

    View details for PubMedID 23277871

  • Shrinking and Growing: Grain Boundary Density Reduction for Efficient Polysilicon Thin-Film Solar Cells NANO LETTERS Kim, D. R., Lee, C. H., Weisse, J. M., Cho, I. S., Zheng, X. 2012; 12 (12): 6485-6491


    Polycrystalline Si (poly-Si) thin-film, due to its low Si consumption, low substrate cost, and good stability, is an attractive candidate for cost-effective solar cells, but the as-deposited poly-Si typically has a columnar structure with grain boundaries in between, severely limiting the efficiency of the poly-Si. Here, we report a micropillar poly-Si solar cell that utilizes the columnar structure of the as-deposited poly-Si grains. We first formed submicrometer diameter poly-Si pillars, smaller than the initial grain sizes, and used these pillars as the seeds for the subsequent epitaxial growth of Si, which effectively reduces grain boundary density in the final poly-Si crystal. In addition, the vertically aligned micropillar arrays form radial p-n junctions that further mitigate the grain boundary recombination losses by improving the light absorption and charge-carrier collection efficiencies. Consequently, the maximum efficiency of micropillar poly-Si thin-film solar cells is 6.4%, that is, ?1.5 times higher than that of the planar cells.

    View details for DOI 10.1021/nl3041492

    View details for Web of Science ID 000312122100073

    View details for PubMedID 23167740

  • Thermal conductivity in porous silicon nanowire arrays NANOSCALE RESEARCH LETTERS Weisse, J. M., Marconnet, A. M., Kim, D. R., Rao, P. M., Panzer, M. A., Goodson, K. E., Zheng, X. 2012; 7


    The nanoscale features in silicon nanowires (SiNWs) can suppress phonon propagation and strongly reduce their thermal conductivities compared to the bulk value. This work measures the thermal conductivity along the axial direction of SiNW arrays with varying nanowire diameters, doping concentrations, surface roughness, and internal porosities using nanosecond transient thermoreflectance. For SiNWs with diameters larger than the phonon mean free path, porosity substantially reduces the thermal conductivity, yielding thermal conductivities as low as 1 W/m/K in highly porous SiNWs. However, when the SiNW diameter is below the phonon mean free path, both the internal porosity and the diameter significantly contribute to phonon scattering and lead to reduced thermal conductivity of the SiNWs.

    View details for DOI 10.1186/1556-276X-7-554

    View details for Web of Science ID 000311320400001

    View details for PubMedID 23039084

  • Copper Ion Enhanced Synthesis of Nanostructured Cobalt Oxide Catalyst for Oxidation of Methane CHEMCATCHEM Feng, Y., Zheng, X. 2012; 4 (10): 1551-1554
  • Fabrication of Flexible and Vertical Silicon Nanowire Electronics NANO LETTERS Weisse, J. M., Lee, C. H., Kim, D. R., Zheng, X. 2012; 12 (6): 3339-3343


    Vertical silicon nanowire (SiNW) array devices directly connected on both sides to metallic contacts were fabricated on various non-Si-based substrates (e.g., glass, plastics, and metal foils) in order to fully exploit the nanomaterial properties for final applications. The devices were realized with uniform length Ag-assisted electroless etched SiNW arrays that were detached from their fabrication substrate, typically Si wafers, reattached to arbitrary substrates, and formed with metallic contacts on both sides of the NW array. Electrical characterization of the SiNW array devices exhibits good current-voltage characteristics consistent with the SiNW morphology.

    View details for DOI 10.1021/nl301659m

    View details for Web of Science ID 000305106400114

    View details for PubMedID 22594496

  • Nanowire electronics that can be shaped to fit any surface and attach to any material developed at Stanford SENSOR REVIEW Zheng, X., Bergeron, L. 2012; 32 (3): 256-256
  • Flash ignition of Al nanoparticles: Mechanism and applications COMBUSTION AND FLAME Ohkura, Y., Rao, P. M., Zheng, X. 2011; 158 (12): 2544-2548
  • Branched TiO2 Nanorods for Photoelectrochemical Hydrogen Production NANO LETTERS Cho, I. S., Chen, Z., Forman, A. J., Kim, D. R., Rao, P. M., Jaramillo, T. F., Zheng, X. 2011; 11 (11): 4978-4984


    We report a hierarchically branched TiO(2) nanorod structure that serves as a model architecture for efficient photoelectrochemical devices as it simultaneously offers a large contact area with the electrolyte, excellent light-trapping characteristics, and a highly conductive pathway for charge carrier collection. Under Xenon lamp illumination (UV spectrum matched to AM 1.5G, 88 mW/cm(2) total power density), the branched TiO(2) nanorod array produces a photocurrent density of 0.83 mA/cm(2) at 0.8 V versus reversible hydrogen electrode (RHE). The incident photon-to-current conversion efficiency reaches 67% at 380 nm with an applied bias of 0.6 V versus RHE, nearly two times higher than the bare nanorods without branches. The branches improve efficiency by means of (i) improved charge separation and transport within the branches due to their small diameters, and (ii) a 4-fold increase in surface area which facilitates the hole transfer at the TiO(2)/electrolyte interface.

    View details for DOI 10.1021/nl2029392

    View details for Web of Science ID 000296674700082

    View details for PubMedID 21999403

  • Fabrication of Nanowire Electronics on Nonconventional Substrates by Water-Assisted Transfer Printing Method NANO LETTERS Lee, C. H., Kim, D. R., Zheng, X. 2011; 11 (8): 3435-3439


    We report a simple, versatile, and wafer-scale water-assisted transfer printing method (WTP) that enables the transfer of nanowire devices onto diverse nonconventional substrates that were not easily accessible before, such as paper, plastics, tapes, glass, polydimethylsiloxane (PDMS), aluminum foil, and ultrathin polymer substrates. The WTP method relies on the phenomenon of water penetrating into the interface between Ni and SiO(2). The transfer yield is nearly 100%, and the transferred devices, including NW resistors, diodes, and field effect transistors, maintain their original geometries and electronic properties with high fidelity.

    View details for DOI 10.1021/nl201901z

    View details for Web of Science ID 000293665600065

    View details for PubMedID 21696196

  • Hybrid Si Microwire and Planar Solar Cells: Passivation and Characterization NANO LETTERS Kim, D. R., Lee, C. H., Rao, P. M., Cho, I. S., Zheng, X. 2011; 11 (7): 2704-2708


    We report an efficient hybrid Si microwire (radial junction) and planar solar cell with a maximum efficiency of 11.0% under AM 1.5G illumination. The maximum efficiency of the hybrid cell is improved from 7.2% to 11.0% by passivating the top surface and p-n junction with thin a-SiN:H and intrinsic poly-Si films, respectively, and is higher than that of planar cells of the identical layers due to increased light absorption and improved charge-carrier collections in both wires and planar components.

    View details for DOI 10.1021/nl2009636

    View details for Web of Science ID 000292849400024

    View details for PubMedID 21609002

  • Unique Magnetic Properties of Single Crystal gamma-Fe2O3 Nanowires Synthesized by Flame Vapor Deposition NANO LETTERS Rao, P. M., Zheng, X. 2011; 11 (6): 2390-2395


    Single crystal ?-Fe(2)O(3) nanowires with 40-60 nm diameters were grown for the first time by single-step atmospheric flame vapor deposition (FVD) with axial growth rates up to 5 ?m/minute. Because of their superior crystallinity, these FVD ?-Fe(2)O(3) nanowires are single magnetic domains with room temperature coercivities of 200 Oe and saturation magnetizations of 68 emu/g.

    View details for DOI 10.1021/nl2007533

    View details for Web of Science ID 000291322600034

    View details for PubMedID 21563788

  • Vertical Transfer of Uniform Silicon Nanowire Arrays via Crack Formation NANO LETTERS Weisse, J. M., Kim, D. R., Lee, C. H., Zheng, X. 2011; 11 (3): 1300-1305


    Vertical transfer of silicon nanowire (SiNW) arrays with uniform length onto adhesive substrates was realized by the assistance of creating a horizontal crack throughout SiNWs. The crack is formed by adding a water soaking step between consecutive Ag-assisted electroless etching processes of Si. The crack formation is related to the delamination, redistribution, and reattachment of the Ag film during the water soaking and subsequent wet etching steps. Moreover, the crack facilitates embedding SiNWs inside polymers.

    View details for DOI 10.1021/nl104362e

    View details for Web of Science ID 000288061500068

    View details for PubMedID 21322602

  • Morphology-Controlled Flame Synthesis of Single, Branched, and Flower-like alpha-MoO3 Nanobelt Arrays NANO LETTERS Cai, L., Rao, P. M., Zheng, X. 2011; 11 (2): 872-877


    We report an atmospheric, catalyst-free, rapid flame synthesis technique for growing single, branched, and flower-like ?-MoO(3) nanobelt arrays on diverse substrates. The growth rate, morphology, and surface coverage density of the ?-MoO(3) nanobelts were controlled by varying the flame equivalence ratio, the source temperature, the growth substrate temperature, and the material and morphology of the growth substrate. This flame synthesis technique is a promising, alternative way to synthesize one-dimensional metal oxide nanostructures in general.

    View details for DOI 10.1021/nl104270u

    View details for Web of Science ID 000287049100095

    View details for PubMedID 21261293

  • Synthesis and ignition of energetic CuO/Al core/shell nanowires PROCEEDINGS OF THE COMBUSTION INSTITUTE Ohkura, Y., Liu, S., Rao, P. M., Zheng, X. 2011; 33: 1909-1915
  • Methane oxidation over catalytic copper oxides nanowires PROCEEDINGS OF THE COMBUSTION INSTITUTE Feng, Y., Rao, P. M., Kim, D. R., Zheng, X. 2011; 33: 3169-3175
  • Flame synthesis of tungsten oxide nanostructures on diverse substrates PROCEEDINGS OF THE COMBUSTION INSTITUTE Rao, P. M., Zheng, X. 2011; 33: 1891-1898
  • Orientation-Controlled Alignment of Axially Modulated pn Silicon Nanowires NANO LETTERS Lee, C. H., Kim, D. R., Zheng, X. 2010; 10 (12): 5116-5122

    View details for DOI 10.1021/nl103630c

    View details for Web of Science ID 000284990900058

  • Plasma-Enhanced Catalytic CuO Nanowires for CO Oxidation NANO LETTERS Feng, Y., Zheng, X. 2010; 10 (11): 4762-4766


    We report the first experimental study of catalytic CO oxidation over copper oxide (CuO) nanowires (NWs) grown directly on copper meshes. The catalytic activity of CuO NWs is significantly improved by a brief argon or hydrogen radio frequency plasma treatment. The plasma enhancement effect comes from the generation of grain boundaries and the reduction of Cu(II) to the more active oxidation state Cu(I) according to our TEM, XPS, and kinetic study.

    View details for DOI 10.1021/nl1034545

    View details for Web of Science ID 000283907600079

    View details for PubMedID 20964283

  • Characterization of the wettability of thin nanostructured films in the presence of evaporation JOURNAL OF COLLOID AND INTERFACE SCIENCE Rogacs, A., Steinbrenner, J. E., Rowlette, J. A., Weisse, J. M., Zheng, X. L., Goodson, K. E. 2010; 349 (1): 354-360


    Vapor chambers using conventional porous membrane wicks offer limited heat transfer rates for a given thickness. This limitation can be addressed through wick nanostructuring, which promises high capillary pressures and precise control of the local porosity. This work develops a measurement technique for the wettability of nanostructured wicks based on optical imaging. Feasibility is demonstrated on a hydrophilic silicon nanowire array (SiNW) synthesized using the Vapor-Liquid-Solid (VLS) growth mechanism followed by surface plasma treatment. The wettability is determined by comparing the time-dependent liquid interface rise with a model that accounts for capillary, viscous, and gravitational forces and for evaporation. This model is demonstrated to be useful in extracting internal contact angle from thin ( approximately 10microm) porous films.

    View details for DOI 10.1016/j.jcis.2010.05.063

    View details for Web of Science ID 000279966700045

    View details for PubMedID 20579656

  • Fabricating nanowire devices on diverse substrates by simple transfer-printing methods PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Lee, C. H., Kim, D. R., Zheng, X. 2010; 107 (22): 9950-9955


    The fabrication of nanowire (NW) devices on diverse substrates is necessary for applications such as flexible electronics, conformable sensors, and transparent solar cells. Although NWs have been fabricated on plastic and glass by lithographic methods, the choice of device substrates is severely limited by the lithographic process temperature and substrate properties. Here we report three new transfer-printing methods for fabricating NW devices on diverse substrates including polydimethylsiloxane, Petri dishes, Kapton tapes, thermal release tapes, and many types of adhesive tapes. These transfer-printing methods rely on the differences in adhesion to transfer NWs, metal films, and devices from weakly adhesive donor substrates to more strongly adhesive receiver substrates. Electrical characterization of fabricated NW devices shows that reliable ohmic contacts are formed between NWs and electrodes. Moreover, we demonstrated that Si NW devices fabricated by the transfer-printing methods are robust piezoresistive stress sensors and temperature sensors with reliable performance.

    View details for DOI 10.1073/pnas.0914031107

    View details for Web of Science ID 000278246000010

    View details for PubMedID 20479263

  • Direct Growth of Nanowire Logic Gates and Photovoltaic Devices NANO LETTERS Kim, D. R., Lee, C. H., Zheng, X. 2010; 10 (3): 1050-1054


    Bottom-up nanowires are useful building blocks for functional devices because of their controllable physical and chemical properties. However, assembling nanowires into large-scale integrated systems remains a critical challenge that becomes even more daunting when different nanowires need to be simultaneously assembled in close proximity to one another. Herein, we report a new method to directly grow nanowire devices consisting of different nanowires. The method is based on the epitaxial growth of nanowires from the sidewalls of electrodes and on the matching of electrode design with synthesis conditions to electrically connect different nanowires during growth. Specifically, the method was used to grow silicon nanowire-based AND and OR diode logic gates with excellent rectifying behaviors, and photovoltaic elements in parallel and in series, with tunable power output.

    View details for DOI 10.1021/nl100011z

    View details for Web of Science ID 000275278200053

    View details for PubMedID 20178355

  • Rapid Catalyst-Free Flame Synthesis of Dense, Aligned alpha-Fe2O3 Nanoflake and CuO Nanoneedle Arrays NANO LETTERS Rao, P. M., Zheng, X. 2009; 9 (8): 3001-3006


    This paper describes a simple and yet rapid flame synthesis method to produce one-dimensional metal oxide nanostructures by directly oxidizing metals in the postflame region of a flat flame. Single and bicrystal alpha-Fe(2)O(3) nanoflakes and CuO nanoneedles were grown in the postflame region by a solid diffusion mechanism and were aligned perpendicularly to the substrate with a surface coverage density of 10 nanostructures per square micrometer. The alpha-Fe(2)O(3) nanoflakes reached lengths exceeding 20 microm after only 20 min of growth. This rapid growth rate is attributed to a large initial heating rate of the metal substrate in the flame and to the presence of water vapor and carbon dioxide in the gas phase that together generate thin and porous oxide layers that greatly enhance the diffusion of the deficient metal to the nanostructure growth site and enable growth at higher temperatures than previously demonstrated.

    View details for DOI 10.1021/nl901426t

    View details for Web of Science ID 000268797200034

    View details for PubMedID 19588968

  • Probing Flow Velocity with Silicon Nanowire Sensors NANO LETTERS Kim, D. R., Lee, C. H., Zheng, X. 2009; 9 (5): 1984-1988


    We report our experimental efforts to quantify the impact of fluidic and ionic transport on the conductance level of silicon nanowire (SiNW) sensors configured as field effect transistors (FETs). Specifically, the conductance of SiNW FETs placed in a microfluidic channel was observed to change linearly with the flow velocity of electrolytic solutions. The direction of conductance change depends on the doping type of the SiNWs and their location inside the microfluidic channel, and the magnitude of the conductance change varies with the ionic strength and compositions of the electrolytic solution. Our quantitative analysis suggests that the flow velocity sensing is a consequence of the streaming potential that is generated by the movement of counterions inside the electrical double layer (EDL) of the silica substrate. The streaming potential, which varies with the flow velocity and the ionic properties of the electrolytic solution, acts in the same way as the charged analytes in affecting the conductance of SiNWs by changing the surface potential. This study highlights the importance of considering the ionic transport in analyzing and optimizing nanowire FET sensors, which can significantly change the conductance of NWs. Moreover, SiNWs were demonstrated for the first time to be able to detect the streaming potential, the flow velocity and the ionic strength, opening up their new application potentials in microfluidics.

    View details for DOI 10.1021/nl900238a

    View details for Web of Science ID 000266157100045

    View details for PubMedID 19331420

  • Single and Tandem Axial p-i-n Nanowire Photovoltaic Devices NANO LETTERS Kempa, T. J., Tian, B., Kim, D. R., Hu, J., Zheng, X., Lieber, C. M. 2008; 8 (10): 3456-3460


    Nanowires represent a promising class of materials for exploring new concepts in solar energy conversion. Here we report the first experimental realization of axial modulation-doped p-i-n and tandem p-i-n(+) -p(+)-i-n silicon nanowire (SiNW) photovoltaic elements. Scanning electron microscopy images of selectively etched nanowires demonstrate excellent synthetic control over doping and lengths of distinct regions in the diode structures. Current-voltage (I-V) characteristics reveal clear and reproducible diode characteristics for the p-i-n and p-n SiNW devices. Under simulated one-sun solar conditions (AM 1.5G), optimized p-i-n SiNW devices exhibited an open circuit voltage (Voc) of 0.29 V, a maximum short-circuit current density of 3.5 mA/cm(2), and a maximum efficiency of 0.5%. The response of the short-circuit current versus Voc under varying illumination intensities shows that the diode quality factor is improved from n=1.78 to n=1.28 by insertion of the i-type SiNW segment. The temperature dependence of Voc scales as -2.97 mV/K and extrapolates to the crystalline Si band gap at 0 K, which is in excellent agreement with bulk properties. Finally, a novel single SiNW tandem solar cell consisting of synthetic integration of two photovoltaic elements with an overall p-i-n(+) -p(+)-i-n structure was prepared and shown to exhibit a Voc that is on average 57% larger than that of the single p-i-n device. Fundamental studies of such well-defined nanowire photovoltaics will enable their intrinsic performance limits to be defined.

    View details for DOI 10.1021/nl8023438

    View details for Web of Science ID 000259906800070

    View details for PubMedID 18763836

  • Numerical Characterization and Optimization of the Microfluidics for Nanowire Biosensors NANO LETTERS Kim, D. R., Zheng, X. 2008; 8 (10): 3233-3237


    The present study aims to enhance the analyte transport to the surface of nanowires (NWs) through optimizing the sensing configuration and the flow patterns inside the microfluidic channel, and hence to reduce the response time of NW biosensors. Specifically, numerical simulations were carried out to quantitatively investigate the effects of the fundamental surface reaction, convection, and diffusion processes on the sensing performance. Although speeding up all these processes will reduce the sensing response time, enhancing the diffusional transport was found to be most effective. Moreover, the response time of NW biosensors is inversely proportional to the local concentration of the analyte in the vicinity of the NWs, which suggests that the sensing response time can be significantly reduced by replenishing the local analyte rapidly. Therefore, the following three optimization strategies were proposed and their effects on the time response of NWs were characterized systematically: device substrate passivation, microfluidic channel modification, and suspending NWs. The combination of these three optimization methods was demonstrated to be able to reduce the response time of NW biosensors by more than 1 order of magnitude.

    View details for DOI 10.1021/nl801559m

    View details for Web of Science ID 000259906800029

    View details for PubMedID 18788786

  • Coaxial silicon nanowires as solar cells and nanoelectronic power sources Nature Tian, B., Zheng, X., L., Kempa, T., J., Fang, Y., Yu, N., Yu, G. 2007; 449: 885-890
  • Thermochemical and Kinetic Analyses on Oxidation of Isobutenyl Radical and 2-Hydroperoxymethyl-2-Propenyl Radical J. Phys. Chem. A Zheng, X., L., Sun, H., J., Law, C., K. 2005; 109: 9044-9054
  • Ignition of Premixed Hydrogen/Air by Heated Counterflow under Reduced and Elevated Pressures Combust. Flame Zheng, X., L., Law, C., K. 2004; 1-2 (136): 168-179

Conference Proceedings

  • Experimental Counterflow Ignition Temperatures and Reaction Mechanisms of 1, 3-Butadiene Zheng, X., L., Lu, T., L., Law, C., K. 2007
  • Experimental Determination of Counterflow Ignition Temperatures and Laminar Flame Speeds of C2-C3 Hydrocarbons at Atmospheric and Elevated pressures Jomaas, G., Zheng, X., L., Zhu, D., L., Law, C., K. 2005
  • Non-premixed Ignition of H2/Air in a Mixing Layer with a Vortex Zheng, X., L., Yuan, J., Law, C., K. 2005
  • Experimental and Computational Study of Non-premixed Ignition of Dimethyl Ether in Counterflow Zheng, X., L., Lu, T., F., Law, C., K., Westbrook, C., K., Curran, H., J. 2005
  • Ignition of Premixed Hydrogen/Air by Heated Counterflow Zheng, X., L., Blouch, J., D., Zhu, D., L., Kreutz, T., G., Law, C., K. 2002