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).
Associate Professor, Mechanical Engineering (2014 - Present)
Assistant Professor, Mechanical Engineering (2007 - 2014)
Honors & Awards
Presidential Early Career Award for Scientists and Engineers, Presidential Early Career Awards (2009)
Young Investigator Program, ONR (2008)
Young Faculty Award, DARPA (2008)
Terman Faculty Award, Stanford University (2007)
Bernard Lewis Fellowship, The Combustion Institute (2004)
Amelia Earhart Fellowship, Zonta International Foundation (2003)
One of the Pioneers on the TR35 Global list, MIT Technology Review (2013)
3M Nontenured Faculty Grant Award, 3M (2013)
BS, Tsinghua University, Thermal Engineering (2000)
PhD, Princeton, Mechanical and Aerospace Engineering (2006)
- Energy Systems I: Thermodynamics
ME 370A (Aut)
- High Temperature Gasdynamics Laboratory Research Project Seminar
ME 390A (Aut)
- Nanomaterials Synthesis and Applications for Mechanical Engineers
ME 373 (Win)
Independent Studies (8)
- Engineering Problems
ME 391 (Aut, Win, Spr, Sum)
- Engineering Problems and Experimental Investigation
ME 191 (Aut, Win, Spr, Sum)
- Experimental Investigation of Engineering Problems
ME 392 (Aut, Win, Spr, Sum)
- Honors Research
ME 191H (Aut, Win, Spr, Sum)
- Ph.D. Research
MATSCI 300 (Aut, Win, Spr, Sum)
- Ph.D. Teaching Experience
ME 491 (Aut, Win, Spr, Sum)
- Practical Training
ME 299A (Aut, Win, Spr, Sum)
- Practical Training
ME 299B (Aut, Win, Spr, Sum)
- Engineering Problems
Prior Year Courses
- Engineering Thermodynamics
ENGR 30 (Spr)
- Nanomaterials Synthesis and Applications for Mechanical Engineers
ME 373 (Win)
- Engineering Thermodynamics
ENGR 30 (Spr)
- Nanomaterials Synthesis and Applications for Mechanical Engineers
ME 373 (Win)
- Engineering Thermodynamics
ENGR 30 (Spr)
- Nanomaterials Synthesis and Applications for Mechanical Engineers
ME 373 (Win)
- Engineering Thermodynamics
Experimental and Computational Study of Non-premixed Ignition of Dimethyl Ether in Counterflow
Proceedings of the Combustion Institute
View details for DOI 10.1016/j.proci.2004.08.241
- Methanol Photo-Oxidation on Rutile TiO2 Nanowires: Probing Reaction Pathways on Complex Materials JOURNAL OF PHYSICAL CHEMISTRY C 2017; 121 (18): 9910-9919
- Electrochemical generation of sulfur vacancies in the basal plane of MoS2 for hydrogen evolution NATURE COMMUNICATIONS 2017; 8
- Enhancing ignition and combustion of micron-sized aluminum by adding porous silicon PROCEEDINGS OF THE COMBUSTION INSTITUTE 2017; 36 (2): 2317-2324
- Molybdenum disulfide catalyzed tungsten oxide for on-chip acetone sensing APPLIED PHYSICS LETTERS 2016; 109 (13)
- One-Step Hydrothermal Deposition of Ni:FeOOH onto Photoanodes for Enhanced Water Oxidation ACS ENERGY LETTERS 2016; 1 (3): 624-632
- High-Performance Ultrathin BiVO4 Photoanode on Textured Polydimethylsiloxane Substrates for Solar Water Splitting ACS ENERGY LETTERS 2016; 1 (1): 68-75
Kinetic Study of Hydrogen Evolution Reaction over Strained MoS2 with Sulfur Vacancies Using Scanning Electrochemical Microscopy
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
2016; 138 (15): 5123-5129
Molybdenum disulfide (MoS2), with its active edge sites, is a proposed alternative to platinum for catalyzing the hydrogen evolution reaction (HER). Recently, the inert basal plane of MoS2 was successfully activated and optimized with excellent intrinsic HER activity by creating and further straining sulfur (S) vacancies. Nevertheless, little is known about the HER kinetics of those S vacancies and the additional effects from elastic tensile strain. Herein, scanning electrochemical microscopy was used to determine the HER kinetic data for both unstrained S vacancies (formal potential Ev0 = −0.53 VAg/AgCl, electron-transfer coefficient αv = 0.4, electron-transfer rate constant kv0 = 2.3 × 10(–4) cm/s) and strained S vacancies (Esv0= −0.53 VAg/AgCl, αsv = 0.4, ksv0 = 1.0 × 10(–3) cm/s) on the basal plane of MoS2 monolayers, and the strained S vacancy has an electron-transfer rate 4 times higher than that of the unstrained S vacancy. This study provides a general platform for measuring the kinetics of two-dimensional material-based catalysts.
View details for DOI 10.1021/jacs.6b01377
View details for Web of Science ID 000374812100021
View details for PubMedID 26997198
Quasi-ballistic Electronic Thermal Conduction in Metal Inverse Opals.
2016; 16 (4): 2754-2761
Porous metals are used in interfacial transport applications that leverage the combination of electrical and/or thermal conductivity and the large available surface area. As nanomaterials push toward smaller pore sizes to increase the total surface area and reduce diffusion length scales, electron conduction within the metal scaffold becomes suppressed due to increased surface scattering. Here we observe the transition from diffusive to quasi-ballistic thermal conduction using metal inverse opals (IOs), which are metal films that contain a periodic arrangement of interconnected spherical pores. As the material dimensions are reduced from ∼230 nm to ∼23 nm, the thermal conductivity of copper IOs is reduced by more than 57% due to the increase in surface scattering. In contrast, nickel IOs exhibit diffusive-like conduction and have a constant thermal conductivity over this size regime. The quasi-ballistic nature of electron transport at these length scales is modeled considering the inverse opal geometry, surface scattering, and grain boundaries. Understanding the characteristics of electron conduction at the nanoscale is essential to minimizing the total resistance of porous metals for interfacial transport applications, such as the total electrical resistance of battery electrodes and the total thermal resistance of microscale heat exchangers.
View details for DOI 10.1021/acs.nanolett.6b00468
View details for PubMedID 26986050
- Enhancing Low-Bias Performance of Hematite Photoanodes for Solar Water Splitting by Simultaneous Reduction of Bulk, Interface, and Surface Recombination Pathways ADVANCED ENERGY MATERIALS 2016; 6 (4)
Activating and optimizing MoS2 basal planes for hydrogen evolution through the formation of strained sulphur vacancies
2016; 15 (1): 48-?
As a promising non-precious catalyst for the hydrogen evolution reaction (HER; refs ,,,,), molybdenum disulphide (MoS2) is known to contain active edge sites and an inert basal plane. Activating the MoS2 basal plane could further enhance its HER activity but is not often a strategy for doing so. Herein, we report the first activation and optimization of the basal plane of monolayer 2H-MoS2 for HER by introducing sulphur (S) vacancies and strain. Our theoretical and experimental results show that the S-vacancies are new catalytic sites in the basal plane, where gap states around the Fermi level allow hydrogen to bind directly to exposed Mo atoms. The hydrogen adsorption free energy (ΔGH) can be further manipulated by straining the surface with S-vacancies, which fine-tunes the catalytic activity. Proper combinations of S-vacancy and strain yield the optimal ΔGH = 0 eV, which allows us to achieve the highest intrinsic HER activity among molybdenum-sulphide-based catalysts.
View details for DOI 10.1038/NMAT4465
View details for Web of Science ID 000366690600019
View details for PubMedID 26552057
General Characterization Methods for Photoelectrochemical Cells for Solar Water Splitting
2015; 8 (19): 3192-3203
Photoelectrochemical (PEC) water splitting is a very promising technology that converts water into clean hydrogen fuel and oxygen by using solar light. However, the characterization methods for PEC cells are diverse and a systematic introduction to characterization methods for PEC cells has rarely been attempted. Unlike most other review articles that focus mainly on the material used for the working electrodes of PEC cells, this review introduces general characterization methods for PEC cells, including their basic configurations and methods for characterizing their performance under various conditions, regardless of the materials used. Detailed experimental operation procedures with theoretical information are provided for each characterization method. The PEC research area is rapidly expanding and more researchers are beginning to devote themselves to related work. Therefore, the content of this Minireview can provide entry-level knowledge to beginners in the area of PEC, which might accelerate progress in this area.
View details for DOI 10.1002/cssc.201500075
View details for Web of Science ID 000362729800001
View details for PubMedID 26365789
Highly Efficient Solar Water Splitting from Transferred TiO2 Nanotube Arrays.
2015; 15 (9): 5709-5715
We report a synergistic effect of flame and chemical reduction methods to maximize the efficiency of solar water splitting in transferred TiO2 nanotube (TNT) arrays on a transparent conducting oxide (TCO) substrate. The flame reduction method (>1000 °C) leads to few oxygen vacancies in the anatase TNT arrays, but it exhibits unique advantages for excellent interfacial characteristics between transferred TNT arrays and TCO substrates, which subsequently induce a cathodic on-set potential shift and sharp photocurrent evolution. By contrast, the employed chemical reduction method for TNT arrays/TCO gives rise to an abrupt increase in photocurrent density, which results from the efficient formation of oxygen vacancies in the anatase TiO2 phase, but a decrease in charge transport efficiency with increasing chemical reduction time. We show that flame reduction followed by chemical reduction could significantly improve the saturation photocurrent density and interfacial property of TNT arrays/TCO photoanodes simultaneously without mechanical fracture via the synergistic effects of coreducing methods.
View details for DOI 10.1021/acs.nanolett.5b01406
View details for PubMedID 26261876
- Enhancing Catalytic CO Oxidation over Co3O4 Nanowires by Substituting Co2+ with Cu2+ ACS CATALYSIS 2015; 5 (8): 4485-4491
- Optoelectronic crystal of artificial atoms in strain-textured molybdenum disulphide NATURE COMMUNICATIONS 2015; 6
Interwoven Three-Dimensional Architecture of Cobalt Oxide Nanobrush-Graphene@NixCo2x(OH)(6x) for High-Performance Supercapacitors
2015; 15 (3): 2037-2044
Development of pseudocapacitor electrode materials with high comprehensive electrochemical performance, such as high capacitance, superior reversibility, excellent stability, and good rate capability at the high mass loading level, still is a tremendous challenge. To our knowledge, few works could successfully achieve the above comprehensive electrochemical performance simultaneously. Here we design and synthesize one interwoven three-dimensional (3D) architecture of cobalt oxide nanobrush-graphene@Ni(x)Co(2x)(OH)(6x) (CNG@NCH) electrode with high comprehensive electrochemical performance: high specific capacitance (2550 F g(-1) and 5.1 F cm(-2)), good rate capability (82.98% capacitance retention at 20 A g(-1) vs 1 A g(-1)), superior reversibility, and cycling stability (92.70% capacitance retention after 5000 cycles at 20 A g(-1)), which successfully overcomes the tremendous challenge for pseudocapacitor electrode materials. The asymmetric supercapacitor of CNG@NCH//reduced-graphene-oxide-film exhibits good rate capability (74.85% capacitance retention at 10 A g(-1) vs 0.5 A g(-1)) and high energy density (78.75 Wh kg(-1) at a power density of 473 W kg(-1)). The design of this interwoven 3D frame architecture can offer a new and appropriate idea for obtaining high comprehensive performance electrode materials in the energy storage field.
View details for DOI 10.1021/nl504901p
View details for Web of Science ID 000351188000090
View details for PubMedID 25710223
- Laminar flame speeds, counterflow ignition, and kinetic modeling of the butene isomers PROCEEDINGS OF THE COMBUSTION INSTITUTE 2015; 35: 309-316
- Fabrication of Nanowire Electronics on Nonconventional Substrates by Water-Assisted Transfer Printing Method MICRO- AND NANOTECHNOLOGY SENSORS, SYSTEMS, AND APPLICATIONS VII 2015; 9467
- Titanium incorporation into hematite photoelectrodes: theoretical considerations and experimental observations ENERGY & ENVIRONMENTAL SCIENCE 2014; 7 (10): 3100-3121
- Transfer Printing Methods for Flexible Thin Film Solar Cells: Basic Concepts and Working Principles ACS NANO 2014; 8 (9): 8746-8756
Sol-flame synthesis of cobalt-doped TiO2 nanowires with enhanced electrocatalytic activity for oxygen evolution reaction.
Physical chemistry chemical physics
2014; 16 (24): 12299-12306
Doping nanowires (NWs) is of crucial importance for a range of applications due to the unique properties arising from both impurities' incorporation and nanoscale dimensions. However, existing doping methods face the challenge of simultaneous control over the morphology, crystallinity, dopant distribution and concentration at the nanometer scale. Here, we present a controllable and reliable method, which combines versatile solution phase chemistry and rapid flame annealing process (sol-flame), to dope TiO2 NWs with cobalt (Co). The sol-flame doping method not only preserves the morphology and crystallinity of the TiO2 NWs, but also allows fine control over the Co dopant profile by varying the concentration of Co precursor solution. Characterizations of the TiO2:Co NWs show that Co dopants exhibit 2+ oxidation state and substitutionally occupy Ti sites in the TiO2 lattice. The Co dopant concentration significantly affects the oxygen evolution reaction (OER) activity of TiO2:Co NWs, and the TiO2:Co NWs with 12 at% of Co on the surface show the highest OER activity with a 0.76 V reduction of the overpotential with respect to undoped TiO2 NWs. This enhancement of OER activity for TiO2:Co NWs is attributed to both improved surface charge transfer kinetics and increased bulk conductivity.
View details for DOI 10.1039/c4cp01748j
View details for PubMedID 24820239
Simultaneously Efficient Light Absorption and Charge Separation in WO3/BiVO4 Core/Shell Nanowire Photoanode for Photoelectrochemical Water Oxidation.
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
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.
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
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
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 2013; 8
Sol-Flame Synthesis: A General Strategy To Decorate Nanowires with Metal Oxide/Noble Metal Nanoparticles
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 2013; 102 (4)
- Flame synthesis of WO3 nanotubes and nanowires for efficient photoelectrochemical water-splitting PROCEEDINGS OF THE COMBUSTION INSTITUTE 2013; 34: 2187-2195
Codoping titanium dioxide nanowires with tungsten and carbon for enhanced photoelectrochemical performance.
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
Morphological control of heterostructured nanowires synthesized by sol-flame method.
Nanoscale research letters
2013; 8 (1): 347-?
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 PubMedID 23924299
- Sol-flame synthesis of hybrid metal oxide nanowires PROCEEDINGS OF THE COMBUSTION INSTITUTE 2013; 34: 2179-2186
- Flame synthesis of 1-D complex metal oxide nanomaterials PROCEEDINGS OF THE COMBUSTION INSTITUTE 2013; 34: 2229-2236
Peel-and-Stick: Fabricating Thin Film Solar Cell on Universal Substrates
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
View details for PubMedCentralID PMC3533453
Shrinking and Growing: Grain Boundary Density Reduction for Efficient Polysilicon Thin-Film Solar Cells
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
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
View details for PubMedCentralID PMC3494563
- Copper Ion Enhanced Synthesis of Nanostructured Cobalt Oxide Catalyst for Oxidation of Methane CHEMCATCHEM 2012; 4 (10): 1551-1554
Fabrication of Flexible and Vertical Silicon Nanowire Electronics
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
2012; 32 (3): 256-256
View details for Web of Science ID 000307529700014
- Flash ignition of Al nanoparticles: Mechanism and applications COMBUSTION AND FLAME 2011; 158 (12): 2544-2548
Branched TiO2 Nanorods for Photoelectrochemical Hydrogen Production
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
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
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
2011; 11 (6): 2390-2395
Single crystal γ-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 μm/minute. Because of their superior crystallinity, these FVD γ-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
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
2011; 11 (2): 872-877
We report an atmospheric, catalyst-free, rapid flame synthesis technique for growing single, branched, and flower-like α-MoO(3) nanobelt arrays on diverse substrates. The growth rate, morphology, and surface coverage density of the α-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 2011; 33: 1909-1915
- Methane oxidation over catalytic copper oxides nanowires PROCEEDINGS OF THE COMBUSTION INSTITUTE 2011; 33: 3169-3175
- Flame synthesis of tungsten oxide nanostructures on diverse substrates PROCEEDINGS OF THE COMBUSTION INSTITUTE 2011; 33: 1891-1898
- Orientation-Controlled Alignment of Axially Modulated pn Silicon Nanowires NANO LETTERS 2010; 10 (12): 5116-5122
Plasma-Enhanced Catalytic CuO Nanowires for CO Oxidation
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
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
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
View details for PubMedCentralID PMC2890492
Direct Growth of Nanowire Logic Gates and Photovoltaic Devices
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
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
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
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
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
2007; 449 (7164): 885-U8
Solar cells are attractive candidates for clean and renewable power; with miniaturization, they might also serve as integrated power sources for nanoelectronic systems. The use of nanostructures or nanostructured materials represents a general approach to reduce both cost and size and to improve efficiency in photovoltaics. Nanoparticles, nanorods and nanowires have been used to improve charge collection efficiency in polymer-blend and dye-sensitized solar cells, to demonstrate carrier multiplication, and to enable low-temperature processing of photovoltaic devices. Moreover, recent theoretical studies have indicated that coaxial nanowire structures could improve carrier collection and overall efficiency with respect to single-crystal bulk semiconductors of the same materials. However, solar cells based on hybrid nanoarchitectures suffer from relatively low efficiencies and poor stabilities. In addition, previous studies have not yet addressed their use as photovoltaic power elements in nanoelectronics. Here we report the realization of p-type/intrinsic/n-type (p-i-n) coaxial silicon nanowire solar cells. Under one solar equivalent (1-sun) illumination, the p-i-n silicon nanowire elements yield a maximum power output of up to 200 pW per nanowire device and an apparent energy conversion efficiency of up to 3.4 per cent, with stable and improved efficiencies achievable at high-flux illuminations. Furthermore, we show that individual and interconnected silicon nanowire photovoltaic elements can serve as robust power sources to drive functional nanoelectronic sensors and logic gates. These coaxial silicon nanowire photovoltaic elements provide a new nanoscale test bed for studies of photoinduced energy/charge transport and artificial photosynthesis, and might find general usage as elements for powering ultralow-power electronics and diverse nanosystems.
View details for DOI 10.1038/nature06181
View details for Web of Science ID 000250230600042
View details for PubMedID 17943126
- Experimental counterflow ignition temperatures and reaction mechanisms of 1,3-butadiene PROCEEDINGS OF THE COMBUSTION INSTITUTE 2007; 31: 367-375
Thermochemical and kinetic analyses on oxidation of isobutenyl radical and 2-hydroperoxymethyl-2-propenyl radical
JOURNAL OF PHYSICAL CHEMISTRY A
2005; 109 (40): 9044-9053
In recognition of the importance of the isobutene oxidation reaction in the preignition chemistry associated with engine knock, the thermochemistry, chemical reaction pathways, and reaction kinetics of the isobutenyl radical oxidation at low to intermediate temperature range were computationally studied, focusing on both the first and the second O2 addition to the isobutenyl radical. The geometries of reactants, important intermediates, transition states, and products in the isobutenyl radical oxidation system were optimized at the B3LYP/6-311G(d,p) and MP2(full)/6-31G(d) levels, and the thermochemical properties were determined on the basis of ab initio, density functional theory, and statistical mechanics. Enthalpies of formation for several important intermediates were calculated using isodesmic reactions at the DFT and the CBS-QB3 levels. The kinetic analysis of the first O2 addition to the isobutenyl radical was performed using enthalpies at the CBS-QB3 and G3(MP2) levels. The reaction forms a chemically activated isobutenyl peroxy adduct which can be stabilized, dissociate back to reactants, cyclize to cyclic peroxide-alkyl radicals, and isomerize to the 2-hydroperoxymethyl-2-propenyl radical that further undergoes another O2 addition. The reaction channels for isomerization and cyclization and further dissociation on this second O2 addition were analyzed using enthalpies at the DFT level with energy corrections based on similar reaction channels for the first O2 addition. The high-pressure limit rate constants for each reaction channel were determined as functions of temperature by the canonical transition state theory for further kinetic model development.
View details for DOI 10.1021/jp058116a
View details for Web of Science ID 000232482400014
View details for PubMedID 16332010
- Nonpremixed ignition of H-2/air in a mixing layer with a vortex PROCEEDINGS OF THE COMBUSTION INSTITUTE 2005; 30: 415-421
- Experimental and computational study of nonpremixed ignition of dimethyl ether in counterflow PROCEEDINGS OF THE COMBUSTION INSTITUTE 2005; 30: 1101-1109
- Experimental determination of counterflow ignition temperatures and laminar flame speeds of C-2-C-3 hydrocarbons at atmospheric and elevated pressures PROCEEDINGS OF THE COMBUSTION INSTITUTE 2005; 30: 193-200
- Ignition of premixed hydrogen/air by heated counterflow under reduced and elevated pressures COMBUSTION AND FLAME 2004; 136 (1-2): 168-179
Ignition of premixed hydrogen/air by heated counterflow
PROCEEDINGS OF THE COMBUSTION INSTITUTE
2002; 29: 1637-1643
View details for Web of Science ID 000182866500028