Carbon‐Based Composite as an Efficient and Stable Metal‐Free Electrocatalyst

Silicon carbide (SiC) encapsulated with graphitized nanodiamond (GD) sheaths (SiC‐GD) as a core–shell nanocrystal (NC) is synthesized with atomic H post‐treatment of vertically aligned carbon nanotubes, which are unzipped and converted into graphene nanoribbons (GNRs) and preserve their vertically aligned integrity. The SiC‐GD core–shell NCs anchor on the GNRs framework and form SiC‐GD@GNRs composite. The nucleation and growth mechanisms for single crystal nanodiamond without diamond seed are discussed. The SiC‐GD@GNRs composite as a metal‐free electrocatalyst exhibits a high activity in the hydrogen evolution reaction, with a small onset overpotential of 8 mV, Tafel slope of 54 mV dec^?1, and exchange current density @ 200 mV of 77.4 mV, as well as long term stability in acid medium. The superior electrocatalytic performances of SiC‐GD@GNRs are ascribed to a high dispersion of SiC‐GD NCs on the GNRs support and a high stability of the GD and GNRs in acid solution.     

Well‐Aligned ZnO Nanowire Arrays Fabricated on Silicon Substrates

Arrays of well‐aligned single‐crystal zinc oxide (ZnO) nanowires of uniform diameter and length have been synthesized on a (100) silicon substrate via a simple horizontal double‐tube system using chemical vapor transport and condensation method. X‐ray diffraction and transmission electron microscopy (TEM) characterizations showed that the as‐grown nanowires had the single‐crystal hexagonal wurtzite structure with detectable defects and a <0002> growth direction. Raman spectra revealed phonon confinement effect when compared with those of ZnO bulk powder, nanoribbons, and nanoparticles. Photoluminescence exhibited strong ultraviolet emission at 3.29 eV under 355 nm excitation and green emission at 2.21 eV under 514.5 nm excitation. No catalyst particles were found at the tip of the nanowires, suggesting that the growth mechanism followed a self‐catalyzed and saturated vapor–liquid–solid (VLS) model. Self‐alignment of nanowires was attributed to the local balance and steady state of vapor flow at the substrate. The growth technique would be of particular interest for direct integration in the current silicon‐technology‐based optoelectronic devices.     

Stabilization of PbS Nanocrystals by Bovine Serum Albumin in its Native and Denatured States

PbS nanocrystals (NCs) are synthesized in aqueous phase within a temperature range of 40–80 °C in the presence of native and denatured states of bovine serum albumen (BSA) as the capping/stabilizing agent. The NCs are characterized with the help of field‐emission scanning electron microscopy, high‐resolution transmission electron microscopy, X‐ray diffraction, and energy‐dispersive X‐ray analysis. At 40 °C, large ball‐shaped NCs (145 ± 37 nm) with small surface protrusions are formed when 1 × 10^?4 g mL^?1 BSA is used. As the reaction temperature is increased towards 80 °C, the size of NCs decreases and they acquire somewhat cubic geometries (49.1 ± 7.0 nm) due to a change in the capping behavior of BSA between its native and denatured states. The native and denatured states of BSA are simultaneously studied by fluorescence spectroscopy using tryptophan emission, and pH measurements with respect to time and temperature. Gel electrophoresis is used to determine the polarity of the BSA capped NCs. Only the small sized NCs conjugated with relatively larger amounts of BSA show a displacement towards the positively charged electrode in comparison to larger NCs with lower amounts of BSA capping. It was concluded that the denatured state of BSA is more effective in controlling the crystal growth of PbS than its native state especially in the low concentration range.     

热丝化学气相沉积法低温制备纳米晶态碳化硅薄膜

采用热丝化学气相沉积(HFCVD)技术以甲烷(CH4)和硅烷(SiH4)作为源反应气体在Si(111)衬底上合成了纳米晶态SiC薄膜。通过X射线衍射(XRD)、扫描电镜(SEM)、高分辨透射电镜(HRTEM)以及光致发光(PL)检测技术对薄膜的晶体结构、表面形貌和PL特性进行了分析和表征。结果表明，在较低的衬底温度下所沉积的薄膜是由镶嵌于非晶SiC网络中的晶态纳米SiC构成。纳米晶粒平均尺寸约为6nm。室温下用HeCr激光激发样品，观到薄膜发出波长位于400～550nm范围内可见光辐射。     

TPA Immobilization on Iron Oxide Nanocubes and Localized Magnetic Hyperthermia Accelerate Blood Clot Lysis

The low specificity and high risk of intracranial hemorrhage associated with currently approved thrombolytic therapies limit their efficacy in recanalizing occluded vessels. Here, a nanoscale thrombolytic agent is demonstrated by immobilizing tissue plasminogen activator molecules (tPA) over 20 nm clustered iron oxide nanocubes (NCs). The resulting nanoconstructs (tPA–NCs) are capable of dissolving clots via both direct interaction of tPA with the fibrin network (chemical lysis) and localized hyperthermia upon stimulation of superparamagnetic NCs with alternating magnetic fields (AMFs) (mechanical lysis). In vitro, as compared to free tPA, the proposed nanoconstructs demonstrate a ≈100‐fold increase in dissolution rate, possibly because of a more intimate interaction of tPA with the fibrin network. The clot dissolution rate is further enhanced (≈10‐fold) by mild, localized heating resulting from the exposure of tPA–NCs to AMF. Intravital microscopy experiments demonstrate blood vessel reperfusion within a few minutes post tail vein injection of tPA–NCs. The proposed nanoconstructs also exhibit high transverse relaxivity (>400 × 10^–3 m ^?1 s^?1) for magnetic resonance imaging. The multifunctional properties and the 3 orders of magnitude enhancement in clot dissolution make tPA–NCs a promising nano‐theranosis agent in thrombotic disease.     

Ultra-short pulsed laser deposition and patterning of SiC thin films for MEMS fabrication

A Ti:Sapphire (IR 800-nm) femtosecond (fs) pulsed laser was used to ablate a sputtering grade of silicon carbide (SiC) in an ultra-high vacuum chamber. The laser-induced plasma species were then driven and grown to form 3C-SiC films of about 1 μm thick on single crystal silicon wafers at 20 °C (room temperature) and 500 °C. Scanning electron microscopy, atomic force microscopy, X-ray photoelectron microscopy, X-ray diffraction and nanoindentation were used to characterize the structure, composition, thickness and properties of the SiC films. Results of the femtosecond-pulse laser deposited (fs-PLD) films were compared with those obtained by atmospheric pressure chemical vapor deposition (APCVD) and nanosecond-pulse laser (excimer laser at 248-nm) deposition (ns-PLD). The distinctive features of fs-PLD films are their extremely smooth surfaces, stoichiometry, amorphous structure and low defect density compared to APCVD films, along with better film quality and higher growth rates than ns-PLD films. In addition to film growth studies, a SiC microgripper (to grab 20-μm-sized objects) was micromachined by use of the fs-pulsed laser to demonstrate the utility of ultra-short PLD in SiC-device fabrication.     

A microstructural comparison of the initial growth of AlN and GaN layers on basal plane sapphire and sic substrates by low pressure metalorganic chemical vapor deposition

The initial growth by low pressure metalorganic chemical vapor deposition and subsequent thermal annealing of A1N and GaN epitaxial layers on SiC and sapphire substrates is examined using high resolution transmission electron microscopy and atomic force microscopy. Growth under low pressure conditions on sapphire substrates is significantly different from that reported for conventional (atmospheric pressure) conditions. Smooth, single crystal A1N and GaN layers were deposited on sapphire in the initial low temperature (600°C) growth step. Interfacial bonding and not lattice mismatch was found to be the determin ing factor for obtaining good crystallinity for the epitaxial layers as indicated by the growth results on SiC substrates.     

Vertically Aligned Hybrid Core/Shell Semiconductor Nanowires for Photonics Applications

A family of 1D organic/inorganic core/shell materials formed by an inner organic nanowire (ONW) conformally covered with an inorganic wide band gap semiconductor (ZnO or TiO₂) layer is presented. The developed procedure is a two‐steps vacuum methodology involving the formation of supported single crystal small‐molecule nanowires by physical vapor deposition and plasma enhance chemical vapor deposition (PECVD) of the inorganic shell. Critical characteristics of the last technique are the possibilities of low temperature and remote configuration deposition. Additionally, an initial step has to be included in order to create nucleation centers for the growth of the ONWs. The procedure and its general character in terms of the variability in organic core and inorganic shells composition and the applicability of the technique to different substrates are presented. The formation of the inorganic shell with no damage of the organic core single‐crystalline structure is demonstrated by high resolution transmission electron microscopy. The vertical alignment of the hybrid nanostructure is achieved thanks to the interaction of the 1D organic nanostructured surfaces and the glow discharge during the deposition of the inorganic shell by PECVD. The optical properties of these core/shell NWs are studied by fluorescence spectroscopy and microscopy, and their application as nanoscale waveguides in the 550–750 nm range addressed.     

A Raman Spectroscopy Study of Individual SiC Nanowires

Raman spectroscopy of a single 40 nm 3C‐SiC nanowire (NW) has been achieved at room temperature with the use of surface‐enhanced Raman scattering (SERS). The structure used to enhance the Raman scattering process is based on a tungsten tip covered by a thin gold layer; a NW is attached to the apex of this tip. The specific dimensions of the SiC NWs (diameters are a few tens of nanometers and lengths are a few micrometers) allow us to study several parts of an individual NW according to the lateral resolution of the Raman microspectrometer. High‐resolution transmission electron microscopy (HRTEM) images show both atomic arrangements in the SiC NWs with growth predominantly in the [111] direction and abundant structural defects. Effort has been focused on the correlation between the Raman spectroscopic profiles and the structural deformations. The Fano interference features of the sharp phonon lines have been used to evaluate the free carrier concentration.     

Effects of substrate surface preparation on chemical vapor deposition growth of 4H-SiC epitaxial layers

The effects of chemical mechanical polish (CMP) and pre-growth oxidation and etching of vicinal 4H−SiC substrates on the quality of epitaxial films have been investigated. Samples with and without a collodial silica CMP and oxidation/etch treatment were studied with optical microscopy, cross section transmission electron microscopy (TEM) and atomic force microscopy (AFM) before and after chemical vapor deposition. Evidence of polishing damage was evident prior to growth in all samples without CMP treatment. Oxidation and etching appeared to generate defects by preferential etching of bulk defects such as dislocations and low angle grain boundaries. Evidence of the polishing damage remained after chemical vapor deposition (CVD) growth on the samples without CMP and the defect density was worse for the oxidized samples compared to the unoxidized ones. The unoxidized CMP wafer had the lowest defect density and rms roughness of the samples studied.     

Laser‐Ablation Growth and Optical Properties of Wide and Long Single‐Crystal SnO2 Ribbons

Wide and long ribbons of single‐crystalline SnO₂ have been achieved via laser ablation of a SnO₂ target. Transmission electron microscopy (TEM) shows the as‐grown SnO₂ ribbons are structurally perfect and uniform, with widths of 300–500 nm, thicknesses of 30–50 nm (width‐to‐thickness ratio of ～ 10), and lengths ranging from several hundreds of micrometers to the order of millimeters. X‐ray diffraction (XRD) pattern and energy‐dispersive X‐ray spectroscopy (EDS) spectral analysis indicate that the ribbons have the phase structure and chemical composition of the rutile form of SnO₂. Selected‐area electron diffraction (SAED) patterns and high‐resolution TEM images reveal that the ribbons are single crystals and grow along the [100] crystal direction. Photoluminescence measurements show that the synthesized SnO₂ ribbons have one strong emission band at ～ 605 nm and a red‐shift of ～ 30 nm, as compared to standard SnO₂ powder, which may be attributed to crystal defects and residual strains accommodated during the growth of the ribbons.     

无坑洞n-3C-SiC/p-Si(100)的LPCVD外延生长及其异质结构特性

在MBE/CVD高真空系统上,利用低压化学气相淀积(LPCVD)方法在直径为50mm的单晶Si(100)衬底上生长出了高取向无坑洞的晶态立方相碳化硅(3C-SiC)外延材料,利用反射高能电子衍射(RHEED)和扫描电镜(SEM)技术详细研究了Si衬底的碳化过程和碳化层的表面形貌,获得了制备无坑洞3C-SiC/Si的优化碳化条件,采用霍尔(Hall)测试等技术研究了外延材料的电学特性,研究了n-3C-SiC/p-Si异质结的I-V、C-V特性及I-V特性对温度的依赖关系.室温下n-3C-SiC/p-Si异质结二极管的最大反向击穿电压达到220V,该n-3C-SiC/p-Si异质结构可用于制备宽带隙发射极SiC/Si HBTs器件.     

High Electron Mobility and Ambipolar Charge Transport in Binary Blends of Donor and Acceptor Conjugated Polymers

High electron mobility and ambipolar charge transport are observed in phase‐separated binary blends of n‐type poly(benzobisimidazobenzophenanthroline) (BBL) with p‐type polymer semiconductors, poly[(thiophene‐2,5‐diyl)‐alt‐(2,3‐diheptylquinoxaline‐5,8‐diyl)] (PTHQx) and poly(10‐hexylphenoxazine‐3,7‐diyl‐alt‐3‐hexyl‐2,5‐thiophene) (POT). Atomic force microscopy (AFM) and transmission electron microscopy (TEM) show phase‐separated domains of 50–300 nm in the binary blend thin films. The TEM images and electron diffraction of BBL/PTHQx blends show the growth of single‐crystalline phases of PTHQx within the BBL matrix. A relatively high electron mobility (1.0 × 10^–3 cm² V^–1 s^–1) that is constant over a wide blend‐composition range is observed in the PTHQx blend field‐effect transistors (FETs). Ambipolar charge transport is observed in both blend systems at a very high concentration of the p‐type semiconductor (≥90 wt % PTHQx or ≥80 wt % POT). Ambipolar charge transport is exemplified by an electron mobility of 1.4 × 10^–5 cm² V^–1 s^–1 and a hole mobility of 1.0 × 10^–4 cm² V^–1 s^–1 observed in the 98 wt % PTHQx blend FETs. These results show that ambipolar charge transport and the associated carrier mobilities in blends of conjugated polymer semiconductors have a complex dependence on the blend composition and the phase‐separated morphology.     

Inkjet Printing of Luminescent CdTe Nanocrystal–Polymer Composites

Inkjet printing is used to produce well‐defined patterns of dots (with diameters of ca. 120 μm) that are composed of luminescent CdTe nanocrystals (NCs) embedded within a poly(vinylalcohol) (PVA) matrix. Addition of ethylene glycol (1–2 vol %) to the aqueous solution of CdTe NCs suppresses the well‐known ring‐formation effect in inkjet printing leading to exceptionally uniform dots. Atomic force microscopy characterization reveals that in the CdTe NC films the particle–particle interaction could be prevented using inert PVA as a matrix. Combinatorial libraries of CdTe NC–PVA composites with variable NC sizes and polymer/NC ratios are prepared using inkjet printing. These libraries are subsequently characterized using a UV/fluorescence plate reader to determine their luminescent properties. Energy transfer from green‐light‐emitting to red‐light‐emitting CdTe NCs in the composite containing green‐ (2.6 nm diameter) and red‐emitting (3.5 nm diameter) NCs are demonstrated.     

热丝法制备硅纳晶颗粒及其发光性质

利用热丝化学气相沉积法,在真空腔中通过选择设定不同加热功率的热丝热解硅烷制备出一系列不同颗粒尺寸的硅纳米颗粒样品。透射电镜分析表明硅纳米颗粒具有结晶良好的纳晶结构。通过自然氧化钝化硅纳米颗粒表面,消除表面悬键的影响,使得硅颗粒具有高效的光致发光特性,发光范围在红光到近红外区,发光特性与量子限域效应相吻合。发光寿命测量表明所制备的硅颗粒的发光寿命在微秒量级,并且通过比较不同硅颗粒分布样品中相同发光波长衰减寿命的差异,揭示了所制备样品中硅纳米颗粒之间存在耦合作用。     

A Simple Method for the Growth of Very Smooth and Ultra-Thin GaSb Films on GaAs (111) Substrate by MOCVD

We present a simple thermal treatment with the antimony source for the metal–organic chemical vapor deposition of thin GaSb films on GaAs (111) substrates for the first time. The properties of the as-grown GaSb films are systematically analyzed by scanning electron microscopy, atomic force microscopy, x-ray diffraction, photo-luminescence (PL) and Hall measurement. It is found that the as-grown GaSb films by the proposed method can be as thin as 35 nm and have a very smooth surface with the root mean square roughness as small as 0.777 nm. Meanwhile, the grown GaSb films also have high crystalline quality, of which the full width at half maximum of the rocking-curve is as small as 218 arcsec. Moreover, the good optical quality of the GaSb films has been demonstrated by the low-temperature PL. This work provides a simple and feasible buffer-free strategy for the growth of high-quality GaSb films directly on GaAs substrates and the strategy may also be applicable to the growth on other substrates and the hetero-growth of other materials.     

A Vapor–Liquid–Solid Mechanism for Growing 3C‐SiC Single‐Domain Layers on 6H‐SiC(0001)

Growing device‐quality 3C‐SiC monocrystalline material is still an issue despite two decades of work dedicated to the subject. Using silicon as the substrate generates too many defects in the layers, owing to lattice mismatch, while it is very difficult to control the initial nucleation on an α‐SiC substrate so that 60° rotated domains are randomly formed. Herein, the elaboration of mono‐orientated 3C‐SiC layers on a 6H‐SiC(0001) on‐axis, Si face substrate using a vapor–liquid–solid mechanism is reported. This non‐conventional approach for growing monocrystalline layers involves feeding a Ge–Si melt by a propane flux at a temperature ranging from 1250 to 1550 °C. We show that, by using this technique, the 3C‐SiC material is almost always obtained on an hexagonal substrate, even if the crystal seed is oriented 8° off‐axis. Using on‐axis 6H‐SiC seeds and optimal growth conditions results in the reproducible deposition of single‐domain 3C‐SiC layers. A mechanism is proposed to clarify some aspects of this process.     

Hexagonal Boron Nitride Thin Film for Flexible Resistive Memory Applications

Hexagonal boron nitride (hBN), which is a 2D layered dielectric material, sometimes referred as “white graphene” due to its structural similarity with graphene, has attracted much attention due to its fascinating physical properties. Here, for the first time the use of chemical vapor deposition ‐grown hBN films to fabricate ultrathin (≈3 nm) flexible hBN‐based resistive switching memory device is reported, and the switching mechanism through conductive atomic force microscopy and ex situ transmission electron microscopy is studied. The hBN‐based resistive memory exhibits reproducible switching endurance, long retention time, and the capability to operate under extreme bending conditions. Contrary to the conventional electrochemical metallization theory, the conductive filament is found to commence its growth from the anode to cathode. This work provides an important step for broadening and deepening the understanding on the switching mechanism in filament‐based resistive memories and propels the 2D material application in the resistive memory in future computing systems.     

Metalorganic vapor phase epitaxial growth and structural characterization of self-assembled InAs nanometer-sized Islands on InP(001)

Self-assembled InAs islands were grown by metalorganic vapor phase epitaxy on InP(001) and characterized by atomic force microscopy and transmission electron microscopy. The growth temperature (450–600°C), the InAs deposition time (3–12 s, using a growth rate of ～2.3Å/s), and the growth interruption time (8–240 s) were varied systematically in order to investigate the effect of thermodynamic and kinetic factors on the structural properties of InAs/InP and InP/InAs/InP structures. It is found that the structural properties of islands vary widely with the growth conditions, ranging from very small (4–5 nm height, ～30–60 nm in diameter) coherent islands at low temperature (450–500°C) to large (～350 nm wide) plastically relaxed islands at high temperature (600°C). For a given deposition time, the height of the coherent islands increases markedly with the growth temperature while their diameter shows only a moderate increase. The growth interruption time also affects the formation and the evolution of islands, which clearly shows that these processes are kinetically limited. Coherent islands with structural properties suitable for use in optoelectronic devices are obtained from ～2.4–4.8 monolayer thick InAs layers using a growth temperature of 500°C and a 30 s interruption time.     

超长六角多型纳米碳化硅晶须的光学性质

采用改进的PECVD技术首次制备出六角多型碳化硅纳米晶须。高分辨率电镜观察其直径在2~6 nm之间。长度为0.3 mm~6 mm。拉曼光谱表明它是六角多型(4 H)纳米碳化硅晶须。紫外光激发出现高强度蓝光发射。随着激发强度的增加,在397 nm处的蓝光强度即刻上升,并溢出,其宽度急剧减小,可能是蓝色激光。