Yiying Wu

Thermal conductivity of individual silicon nanowires

The thermal conductivities of individual single crystalline intrinsic Si nanowires with diameters of 22, 37, 56, and 115 nm were measured using a microfabricated suspended device over a temperature range of 20–320 K. Although the nanowires had well-defined crystalline order, the thermal conductivity observed was more than two orders of magnitude lower than the bulk value. The strong diameter dependence of thermal conductivity in nanowires was ascribed to the increased phonon-boundary scattering and possible phonon spectrum modification.

NixCo3−xO4 Nanowire Arrays for Electrocatalytic Oxygen Evolution

Oxygen evolution reaction (OER) is coupled with a number of important cathodic processes, for instance water splitting for hydrogen production. An effective electrocatalyst can reduce the overpotential and thus enhance the energy efficiency. Therefore extensive research efforts have been invested in developing inexpensive and efficient OER electrocatalysts that have sufficient stability in prolonged exposure to oxidizing conditions. Co3O4 has been demonstrated to have good efficiency and corrosion stability for the OER. It also has much lower cost than RuO2or IrO2-based catalysts. In prior reports of Co3O4, the electrodes were thin films or particle agglomerates bound together by polymers. Comparatively speaking, selfstanding nanowire (NW) arrays should have the advantages of efficient mass transfer and large surface area. To the best of our knowledge, there are still no OER studies of Co3O4 NW arrays. Moreover, despite the fact that Ni doping has been demonstrated to enhance the electrocatalytic efficiency of Co3O4, [5] there is no report on NixCo3 xO4 NWs yet. In this Communication, we report the first synthesis of mesoporous NixCo3 xO4 NW arrays and their electrocatalytic performance in OER. NW arrays grown directly on conductive substrates have several structural advantages: the open space between NWs can facilitate the diffusion of active species; the large surface areas associated with NWs and their mesoporous structures accelerate the surface reaction; and the direct contact of NWs to the underneath conductive substrate ensures each NW to participate in the reaction and also allows their direct use in the electrochemical cells. The NW arrays were grown on Ti foils in an aqueous solution containing Co(NO3)2, via the ammonia-evaporation-induced growth. Different amounts of Ni(NO3)2 were added to the solution in our efforts to tune the Ni-doping level. It is interesting to notice that different Ni(NO3)2 concentrations result in different NW surface roughness as shown in the scanning electron microscopy (SEM) images in Figure 1. Pure Co3O4 NWs are about 400 nm in diameter and 15–20mm in length (Fig. 1A and 1B). They have relatively smooth surfaces. As we increased the ratio of starting Ni(NO3)2 to Co(NO3)2 precursors to 0.5:1.0 and 1.0:1.0, while keeping the total concentration of metal salts constant at 0.2 M, the corresponding NW products (denoted as NCO-1 and NCO-2 respectively) became thicker and rougher (Fig. 1C–F). The spinel crystal structure was maintained after doping, and no

Very high breakdown voltage and large transconductance realized on GaN heterojunction field effect transistors

We report record high breakdown voltages up to 340 and 230 V realized on unintentionally doped (1.5 μm gate length) and Si doped (1 μm gate length) AlGaN/GaN modulation doped field effect transistors (MODFETs), respectively. The devices also have large transconductances up to 140 mS/mm and a full channel current of 150–400 mA/mm. The Si doped MODFET sample demonstrated a very high room temperature mobility of 1500 cm2/Vs. With these specifications, GaN field effect transistors as microwave power devices are practical.

High Al-content AlGaN/GaN MODFETs for ultrahigh performance

The use of an AlGaN layer with high Al mole-fraction is proposed to increase the equivalent figures of merit of the AlGaN/GaN MODFET structure. It is shown that the room temperature mobility has little degradation with increasing Al mole-fraction up to 50%. 0.7-/spl mu/m gate-length Al/sub 0.5/Ga/sub 0.5/N/GaN MODFETs by optical lithography exhibit a current density of 1 A/mm and three-terminal breakdown voltages up to 200 V. These devices on sapphire substrates without thermal management also show CW power densities of 2.84 and 2.57 W/mm at 8 and 10 GHz, respectively, representing a marked performance improvement for GaN-based FETs.

Thermal conductivity of Si/SiGe superlattice nanowires

The thermal conductivities of individual single crystalline Si/SiGe superlattice nanowires with diameters of 58 and 83 nm were measured over a temperature range from 20 to 320 K. The observed thermal conductivity shows similar temperature dependence as that of two-dimensional Si/SiGe superlattice films. Comparison with the thermal conductivity data of intrinsic Si nanowires suggests that alloy scattering of phonons in the Si–Ge segments is the dominant scattering mechanism in these superlattice nanowires. In addition, boundary scattering also contributes to thermal conductivity reduction.

Influence of sapphire nitridation on properties of gallium nitride grown by metalorganic chemical vapor deposition

The properties of 1.2 μm thick GaN films were found to be significantly influenced by the duration of exposing the sapphire substrate to ammonia prior to the GaN growth initiation. The different nitridation schemes of sapphire strongly affect the dislocation structure of GaN films resulting in a decrease of the dislocation density from 2×1010 to 4×108 cm−2 for shorter NH3 preflow times. Room‐ and low‐temperature electron transport characteristics of these films are specifically affected by the dislocation structure. A 300 K electron mobility as high as 592 cm2/V s was obtained for a short ammonia preflow whereas a long nitridation caused the mobility to drop to 149 cm2/V s. Additionally, the photoluminescence quality deteriorates for samples with a long sapphire nitridation time.

Nanoscale design to enable the revolution in renewable energy

The creation of a sustainable energy generation, storage, and distribution infrastructure represents a global grand challenge that requires massive transnational investments in the research and development of energy technologies that will provide the amount of energy needed on a sufficient scale and timeframe with minimal impact on the environment and have limited economic and societal disruption during implementation. In this opinion paper, we focus on an important set of solar, thermal, and electrochemical energy conversion, storage, and conservation technologies specifically related to recent and prospective advances in nanoscale science and technology that offer high potential in addressing the energy challenge. We approach this task from a two-fold perspective: analyzing the fundamental physicochemical principles and engineering aspects of these energy technologies and identifying unique opportunities enabled by nanoscale design of materials, processes, and systems in order to improve performance and reduce costs. Our principal goal is to establish a roadmap for research and development activities in nanoscale science and technology that would significantly advance and accelerate the implementation of renewable energy technologies. In all cases we make specific recommendations for research needs in the near-term (2–5 years), mid-term (5–10 years) and long-term (>10 years), as well as projecting a timeline for maturation of each technological solution. We also identify a number of priority themes in basic energy science that cut across the entire spectrum of energy conversion, storage, and conservation technologies. We anticipate that the conclusions and recommendations herein will be of use not only to the technical community, but also to policy makers and the broader public, occasionally with an admitted emphasis on the US perspective.

Photoelectrochemical Study of the Band Structure of Zn2SnO4 Prepared by the Hydrothermal Method

It is fundamentally interesting to study the photoelectrochemical properties of complex oxides for applications in photovoltaics and photocatalysis. In this paper, we study the band gap (E(g)) and energetics of the conduction band (CB) and valence band (VB) for films of zinc stannate (Zn(2)SnO(4)) nanoparticles (ca. 25 nm) of the inverse-spinel structure prepared by the hydrothermal method. UV-vis spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), electrochemistry, and photoelectrochemistry were used to study the films. The fundamental E(g) for Zn(2)SnO(4) is proposed to be 3.6-3.7 eV with a direct-forbidden transition. The position of the CB was approximated from the flat band potential, E(fb), measured by the photocurrent onset potential. In aqueous and nonaqueous solutions the E(fb) of n-Zn(2)SnO(4) was found to be more positive than TiO(2) anatase in the electrochemical scale. In aqueous solutions E(fb) of Zn(2)SnO(4) was found to follow a 59 mV/pH slope with E(fb) extrapolated at pH 0 of 0.08 V vs NHE. In acetonitrile solutions that simulate the electrolyte for dye-sensitized solar cells (DSCs) the E(fb) of Zn(2)SnO(4) was found to be strongly dependent on electrolyte composition and more positive than TiO(2) vs the I(-)/I(3)(-) couple. The reverse trend observed for the open-circuit voltage in certain DSC electrolytes is explained in terms of the higher rates of electron-triiodide recombination of TiO(2) despite the lower position of the Zn(2)SnO(4) CB in the vacuum scale.

Melting and Welding Semiconductor Nanowires in Nanotubes

and deformations are obtained after removal of the silica matrix using aqueous HF. Our study indicates that mesoporous silicas such as C16MCM-41, C22MCM-41, and SBA-15 can act as good templates for the synthesis of Pd nanowires. Moreover, a significantly depressed melting point of the Pd nanowires is observed around 300 C. To the best of our knowledge, this is the first report on the synthesis and thermal behavior of Pd nanowires of less than 10 nm diameter. Along with our matrix-assisted process, in particular, our low-temperature CVI approach is attractive because it can be carried out under conditions mild enough to avoid any disruption of both the desired material and the template structure. The combined matrix-assisted and CVI process can be extended not only to other sizes of nanowires but also to multidimensional structures using appropriate host architectures.

Bias dependent microwave performance of AlGaN/GaN MODFET's up to 100 V

1 /spl mu/m gate-length AlGaN/GaN modulation doped field effect transistors (MODFETs) have been fabricated on an insulating GaN buffer layer for better carrier confinement. These devices demonstrate simultaneously high current levels (>500 mA/mm), excellent pinch-off and high gate-drain breakdown voltages (220 V for 3 /spl mu/m gate-drain spacing). In contrast to their GaAs counterparts, the current-gain cutoff frequency of the AlGaN/GaN devices shows little degradation at high drain voltage biases. A power-gain cutoff frequency of 19 GHz is obtained at 100 V. ACW power density of 1.57 W/mm at 4 Ghz is also achieved when biased at 28 V and 205 mA/mm.