Yanfeng Zhang

Bio: Yanfeng Zhang is an academic researcher from Lawrence Berkeley National Laboratory. The author has contributed to research in topics: Chemical vapor deposition & Nanowire. The author has an hindex of 6, co-authored 7 publications receiving 4076 citations.

Low-temperature wafer-scale production of ZnO nanowire arrays.

Abstract: Since the first report of ultraviolet lasing from ZnO nanowires, substantial effort has been devoted to the development of synthetic methodologies for one-dimensional ZnO nanostructures. Among the various techniques described in the literature, evaporation and condensation processes are favored for their simplicity and high-quality products, but these gas-phase approaches generally require economically prohibitive temperatures of 800–900 8C. Despite recent MOCVD schemes that reduced the deposition temperature to 450 8C by using organometallic zinc precursors, the commercial potential of gas-phase-grown ZnO nanowires remains constrained by the expensive and/or insulating (for example, Al2O3) substrates required for oriented growth, as well as the size and cost of the vapor deposition systems. A low-temperature, large-scale, and versatile synthetic process is needed before ZnO nanowire arrays find realistic applications in solar energy conversion, light emission, and other promising areas. Solution approaches to ZnO nanowires are appealing because of their low growth temperatures and good potential for scale-up. In this regard, Vayssieres et al. developed a hydrothermal process for producing arrays of ZnO microrods and nanorods on conducting glass substrates at 95 8C. Recently, a seeded growth process was used to make helical ZnO rods and columns at a similar temperature. Here we expand on these synthetic methods to produce homogeneous and dense arrays of ZnO nanowires that can be grown on arbitrary substrates under mild aqueous conditions. We present data for arrays on four-inch (ca. 10 cm) silicon wafers and two-inch plastic substrates, which demonstrate the ease of commercial scale-up. The simple two-step procedure yields oriented nanowire films with the largest surface area yet reported for nanowire arrays. The growth process ensures that a majority of the nanowires in the array are in direct contact with the substrate and provide a continuous pathway for carrier transport, an important feature for future electronic devices based on these materials. Well-aligned ZnO nanowire arrays were grown using a simple two-step process. In the first step, ZnO nanocrystals (5–10 nm in diameter) were spin-cast several times onto a four-inch Si(100) wafer to form a 50–200-nm thick film of crystal seeds. Between coatings, the wafer was annealed at 150 8C to ensure particle adhesion to the wafer surface. The ZnO nanocrystals were prepared according to the method of Pacholski. A NaOH solution in methanol (0.03m) was added slowly to a solution of zinc acetate dihydrate (0.01m) in methanol at 60 8C and stirred for two hours. The resulting nanoparticles are spherical and stable for at least two weeks in solution. After uniformly coating the silicon wafer with ZnO nanocrystals, hydrothermal ZnO growth was carried out by suspending the wafer upside-down in an open crystallizing dish filled with an aqueous solution of zinc nitrate hydrate (0.025m) and methenamine or diethylenetriamine (0.025m) at 90 8C. Reaction times spanned from 0.5 to 6 h. The wafer was then removed from solution, rinsed with deionized water, and dried. A field-emission scanning electron microscope (FESEM) was used to examine the morphology of the nanowire array across the entire wafer, while single nanowires were characterized by transmission electron microscopy (TEM). Nanowire crystallinity and growth direction were analyzed by X-ray diffraction and electron diffraction techniques. SEM images taken of several four-inch samples showed that the entire wafer was coated with a highly uniform and densely packed array of ZnO nanowires (Figure 1). X-ray diffraction (not shown) gave a wurtzite ZnO pattern with an enhanced (002) peak resulting from the vertical orientation of the nanowires. A typical synthesis (1.5 h) yielded wires with diameters ranging between 40–80 nm and lengths of 1.5–2 mm.

Single-crystal gallium nitride nanotubes.

Abstract: Since the discovery of carbon nanotubes in 1991 (ref. 1), there have been significant research efforts to synthesize nanometre-scale tubular forms of various solids. The formation of tubular nanostructure generally requires a layered or anisotropic crystal structure. There are reports of nanotubes made from silica, alumina, silicon and metals that do not have a layered crystal structure; they are synthesized by using carbon nanotubes and porous membranes as templates, or by thin-film rolling. These nanotubes, however, are either amorphous, polycrystalline or exist only in ultrahigh vacuum. The growth of single-crystal semiconductor hollow nanotubes would be advantageous in potential nanoscale electronics, optoelectronics and biochemical-sensing applications. Here we report an 'epitaxial casting' approach for the synthesis of single-crystal GaN nanotubes with inner diameters of 30-200 nm and wall thicknesses of 5-50 nm. Hexagonal ZnO nanowires were used as templates for the epitaxial overgrowth of thin GaN layers in a chemical vapour deposition system. The ZnO nanowire templates were subsequently removed by thermal reduction and evaporation, resulting in ordered arrays of GaN nanotubes on the substrates. This templating process should be applicable to many other semiconductor systems.

Crystallographic alignment of high-density gallium nitride nanowire arrays

Abstract: Single-crystalline, one-dimensional semiconductor nanostructures are considered to be one of the critical building blocks for nanoscale optoelectronics1 Elucidation of the vapour–liquid–solid growth mechanism2 has already enabled precise control over nanowire position and size1,3,4,5,6,7,8, yet to date, no reports have demonstrated the ability to choose from different crystallographic growth directions of a nanowire array Control over the nanowire growth direction is extremely desirable, in that anisotropic parameters such as thermal and electrical conductivity, index of refraction, piezoelectric polarization, and bandgap may be used to tune the physical properties of nanowires made from a given material Here we demonstrate the use of metal–organic chemical vapour deposition (MOCVD) and appropriate substrate selection to control the crystallographic growth directions of high-density arrays of gallium nitride nanowires with distinct geometric and physical properties Epitaxial growth of wurtzite gallium nitride on (100) γ-LiAlO2 and (111) MgO single-crystal substrates resulted in the selective growth of nanowires in the orthogonal [110] and [001] directions, exhibiting triangular and hexagonal cross-sections and drastically different optical emission The MOCVD process is entirely compatible with the current GaN thin-film technology, which would lead to easy scale-up and device integration

Metalorganic Chemical Vapor Deposition Route to GaN Nanowires with Triangular Cross Sections

Abstract: High-quality gallium nitride nanowires have been synthesized via metal-initiated metalorganic chemical vapor deposition for the first time. Excellent substrate coverage was observed for wires prepared on silicon, c-plane, and a-plane sapphire substrates. The wires were formed via the vapor−liquid−solid mechanism with gold, iron, or nickel as growth initiators and were found to have widths of 15-200 nm. Transmission electron microscopy confirmed that the wires were single-crystalline and were oriented predominantly along the [210] or [110] direction. Wires growing along the [210] orientation were found to have triangular cross-sections. Transport measurements confirmed that the wires were n-type and had electron mobilities of ∼65 cm2/V·s. Photoluminescence measurements showed band edge emission at 3.35 eV (at 5 K), with a marked absence of low-energy emission from impurity defects.

Ultrafast Carrier Dynamics in Single ZnO Nanowire and Nanoribbon Lasers

Abstract: Time-resolved second-harmonic generation (TRSHG) and transient photoluminescence (PL) spectroscopy are utilized to probe the ultrafast creation and subsequent relaxation of excited carriers immediately following band-gap excitation in single ZnO nanowire and nanoribbon lasers. The TRSHG signal consists of a 1−5 ps recovery present only during strong lasing and a 10−80 ps intensity-dependent component. The transient PL response from single structures exhibits an 80 ps decay component independent of pump power (free exciton PL), and a < 10 ps power-dependent component (stimulated emission) that shifts to earlier delay by ca. 10 ps at high pump fluence.

Nanowire dye-sensitized solar cells

Abstract: Excitonic solar cells1—including organic, hybrid organic–inorganic and dye-sensitized cells (DSCs)—are promising devices for inexpensive, large-scale solar energy conversion. The DSC is currently the most efficient2 and stable3 excitonic photocell. Central to this device is a thick nanoparticle film that provides a large surface area for the adsorption of light-harvesting molecules. However, nanoparticle DSCs rely on trap-limited diffusion for electron transport, a slow mechanism that can limit device efficiency, especially at longer wavelengths. Here we introduce a version of the dye-sensitized cell in which the traditional nanoparticle film is replaced by a dense array of oriented, crystalline ZnO nanowires. The nanowire anode is synthesized by mild aqueous chemistry and features a surface area up to one-fifth as large as a nanoparticle cell. The direct electrical pathways provided by the nanowires ensure the rapid collection of carriers generated throughout the device, and a full Sun efficiency of 1.5% is demonstrated, limited primarily by the surface area of the nanowire array.

Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy

Abstract: Recent years have seen a renewed interest in the harvesting and conversion of solar energy. Among various technologies, the direct conversion of solar to chemical energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclusively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise. Here we review recent progress in using plasmonic metallic nanostructures in the field of photocatalysis. We focus on plasmon-enhanced water splitting on composite photocatalysts containing semiconductor and plasmonic-metal building blocks, and recently reported plasmon-mediated photocatalytic reactions on plasmonic nanostructures of noble metals. We also discuss the areas where major advancements are needed to move the field of plasmon-mediated photocatalysis forward.

Growth of oriented single-crystalline rutile TiO(2) nanorods on transparent conducting substrates for dye-sensitized solar cells.

Abstract: Dye-sensitized solar cells (DSSCs) made from oriented, one-dimensional semiconductor nanostructures such as nanorods, nanowires, and nanotubes are receiving attention because direct connection of the point of photogeneration with the collection electrode using such structures may improve the cell performance. Specifically, oriented single-crystalline TiO2 nanorods or nanowires on a transparent conductive substrate would be most desirable, but achieving these structures has been limited by the availability of synthetic techniques. In this study, a facile, hydrothermal method was developed for the first time to grow oriented, single-crystalline rutile TiO2 nanorod films on transparent conductive fluorine-doped tin oxide (FTO) substrates. The diameter, length, and density of the nanorods could be varied by changing the growth parameters, such as growth time, growth temperature, initial reactant concentration, acidity, and additives. The epitaxial relation between the FTO substrate and rutile TiO2 with a smal...

Conjugated Polymer Photovoltaic Cells

Abstract: Conjugated polymers are attractive semiconductors for photovoltaic cells because they are strong absorbers and can be deposited on flexible substrates at low cost. Cells made with a single polymer and two electrodes tend to be inefficient because the photogenerated excitons are usually not split by the built-in electric field, which arises from differences in the electrode work functions. The efficiency can be increased by splitting the excitons at an interface between two semiconductors with offset energy levels. Power conversion efficiencies of almost 4% have been achieved by blending polymers with electron-accepting materials such as C60 derivatives, cadmium selenide, and titanium dioxide. We predict that efficiencies higher than 10% can be achieved by optimizing the cell's architecture to promote efficient exciton splitting and charge transport and by reducing the band gap of the polymer so that a larger fraction of the solar spectrum can be absorbed.

Optical properties of ZnO nanostructures.

Abstract: We present a review of current research on the optical properties of ZnO nanostructures. We provide a brief introduction to different fabrication methods for various ZnO nanostructures and some general guidelines on how fabrication parameters (temperature, vapor-phase versus solution-phase deposition, etc.) affect their properties. A detailed discussion of photoluminescence, both in the UV region and in the visible spectral range, is provided. In addition, different gain (excitonic versus electron hole plasma) and feedback (random lasing versus individual nanostructures functioning as Fabry-Perot resonators) mechanisms for achieving stimulated emission are described. The factors affecting the achievement of stimulated emission are discussed, and the results of time-resolved studies of stimulated emission are summarized. Then, results of nonlinear optical studies, such as second-harmonic generation, are presented. Optical properties of doped ZnO nanostructures are also discussed, along with a concluding outlook for research into the optical properties of ZnO.