The SERS and TERS effects obtained by gold droplets on top of Si nanowires

We show that hemispherical gold droplets on top of silicon nanowires when grown by the vapor-liquid-solid (VLS) mechanism, can produce a significant enhancement of Raman scattered signals. Signal enhancement for a few or even just single gold droplets is demonstrated by analyzing the enhanced Raman signature of malachite green molecules. For this experiment, trenches (approximately 800 nm wide) were etched in a silicon-on-insulator (SOI) wafer along <110> crystallographic directions that constitute sidewalls ({110} surfaces) suitable for the growth of silicon nanowires in <111> directions with the intention that the gold droplets on the silicon nanowires can meet somewhere in the trench when growth time is carefully selected. Another way to realize gold nanostructures in close vicinity is to attach a silicon nanowire with a gold droplet onto an atomic force microscopy (AFM) tip and to bring this tip toward another gold-coated AFM tip where malachite green molecules were deposited prior to the measurements. In both experiments, signal enhancement of characteristic Raman bands of malachite green molecules was observed. This indicates that silicon nanowires with gold droplets atop can act as efficient probes for tip-enhanced Raman spectroscopy (TERS). In our article, we show that a nanowire TERS probe can be fabricated by welding nanowires with gold droplets to AFM tips in a scanning electron microscope (SEM). TERS tips made from nanowires could improve the spatial resolution of Raman spectroscopy so that measurements on the nanometer scale are possible.     

A simple hydrothermal route to GaOOH intersecting-rods with regular diamond-like cross section

GaOOH crystals were synthesized by a simple hydrothermal method at 90-200 °C for 18 h. The products were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscope (HRTEM). The rods synthesized at 110-150 °C exhibited intersecting morphology and regular diamond-like cross section with a diagonal length ratio of 2:1, and those synthesized at 180-200 °C exhibited nunchakus-like architecture. The formation of diamond-like cross section was due to homogeneous growth of lattice planes, namely {1 1 0}, along [4 1 0], [4 1¯ 0], [4¯ 1 0], and [4¯ 1¯ 0] directions of orthorhombic GaOOH. Moreover, the formation of the trunk was due to the growth along [0 0 1] direction.     

磁控溅射法可控制备有序碲纳米线阵列

采用磁控溅射法在较低基底温度下(200 ℃)制备了有序碲纳米线阵列, 并利用X射线衍射、扫描电镜和透射电镜对所制备薄膜进行了相、形貌和微观结构分析。结果表明, 所制备的纳米线阵列由单晶碲纳米线组成, 单根碲纳米线具有针状形貌, 并沿[101]晶向生长, 平均直径和长度分别为100 nm和1 μm。氩气压力和基底温度均对碲纳米线阵列的形成具有重要影响, 以平衡碲原子沿[101]晶向和(101)晶面方向的扩散和生长。提出了碲纳米线阵列的生长机制, 包括吸附、结合、成核和生长等过程。     

Growth and optical properties of sallow-like hierarchical SnO2 nanostructures

Novel self-organized hierarchical SnO2 nanostructures have been successfully prepared by vapor phase transport with the assistance of a stainless-steel grid at 950 degrees C. Scanning electron microscopy shows that the synthesized product displays interesting sallow-like morphology, in which numerous secondary branches (beak-like nanowires) are grown randomly around the main stems (microwires). Transmission electron microscopy analysis indicates that the branches grow along a direction of [100] and the beak is formed with the growth direction switching to [110]. A room temperature photoluminescence spectrum of the present SnO2 nanostructures shows a strong emission at 572 and 604 nm(-1).     

Ti单晶[0001]位向循环变形的SEM观察

用区域熔炼高纯Ｔｉ经应变退火获得单晶，定向切割出的［０００１］位向单晶试样在ＳＥＭ上观察循环变形时表面形貌变化。结果表明，循环应变几乎全部是｛１０１２｝，｛１１２２｝和｛１１２４｝孪晶组成的循环孪晶带的贡献，疲劳裂纹在孪晶带部沿｛１０１２｝孪晶和｛１１２２｝孪晶效界萌生。     

The effects of gallium droplets on the morphologies and structures of alpha-Fe2O3 nanostructures grown on iron substrates

Alpha-Fe2O3 nanowires and nanobelts were grown by the thermal oxidation of iron substrates with or without gallium droplets in the air. The nanowires and nanobelts show a bicrystal structure with the growth direction uniformly along [110]. The morphological and structural properties of the as-grown alpha-Fe2O3 nanostructures are described and the growth condition dependence of the alpha-Fe2O3 nanostructures is shown. The transformation from nanowires to nanobelts occurs with the increase of growth temperature and addition of gallium. In addition, the growth evolution is investigated with reference to the Fe surface diffusion and supersaturation.     

Microwave-hydrothermal synthesis and structural characterization of PX-phase single-crystalline PbTiO3 nanowires by electron microscopy

PX-phase PbTiO₃ (PT) nanowires were synthesized by microwave-hydrothermal process, and their microstructures were characterized by electron microscopy. The PX-phase PT nanowires exhibit acicular morphology with diameter sizes of 20-80 nm and length over 1 µm. They tend to grow into a regular structure with parallel arrangement along their long axis in the [001] direction. Selected area electron diffraction patterns demonstrate the PX-phase PT nanowires with a 3-fold modulated periodicity along the [110] direction and a 4-fold modulated periodicity in the [001] direction. These results were also confirmed by the high-resolution transmission electron microscopy images.     

Reaction enhanced wetting of quartz by silicon droplets and its instabilities

A miniature reaction cell was developed in order to study the influence of gas phase components on reactive wetting and dewetting processes. Small silicon droplets on fused silica can be observed in situ at 4000 frames per second under reaction conditions by a microscope equipped with a high speed camera. Additionally, ex-situ investigations of etch profiles originating from the reactive wetting process are conducted by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Under certain reactive conditions large dynamic spreading modes exhibiting a new type of droplet instability are observed. Some spread-out droplets suddenly disrupt and decay into a ring of smaller droplets. Each of these then spreads again and disrupts into a circular array of even smaller droplets forming a ring-shaped structure. Whole cascades of decay can be traced by means of the etch profiles found on the substrate. The results are discussed within a simple thermodynamic model that relates the changes in the oxygen chemical potential to the changes of solid–liquid interface tension.     

Significant enhancement of hole mobility in [110] silicon nanowires compared to electrons and bulk silicon

Utilizing sp3d5s* tight-binding band structure and wave functions for electrons and holes we show that acoustic phonon limited hole mobility in [110] grown silicon nanowires (SiNWs) is greater than electron mobility. The room temperature acoustically limited hole mobility for the SiNWs considered can be as high as 2500 cm2/V s, which is nearly three times larger than the bulk acoustically limited silicon hole mobility. It is also shown that the electron and hole mobility for [110] grown SiNWs exceed those of similar diameter [100] SiNWs, with nearly 2 orders of magnitude difference for hole mobility. Since small diameter SiNWs have been seen to grow primarily along the [110] direction, results strongly suggest that these SiNWs may be useful in future electronics. Our results are also relevant to recent experiments measuring SiNW mobility.     

A quantitative metallographic assessment of the evolution of porosity during processing and creep in single crystal Ni‐base super alloys

The present work reviews previous research on the evolution of porosity. It presents new results from a detailed study on the evolution of porosity during casting, heat treatment and creep of a single crystal Ni‐base superalloy subjected to uniaxial tensile creep at 1050 °C and 160 MPa in [001] and [110] directions. A quantitative metallographic study was performed on carefully polished metallographic cross sections, monitoring sampling fields of 4500 × 1000 µm² using the back scatter contrast of an analytical scanning electron microscope; evolutions of pore sizes and pore form factors were analyzed and all important details which were previously revealed in a synchrotron study could be reproduced. In addition, it was observed that micro cracks form at larger cast pores. They interlink and thus initiate final rupture. The [110] tensile creep tests showed lower rupture strains than the [001] experiments. In agreement with earlier work, this can be rationalized on the basis of aligned porosity along primary dendrites.     

Orientation and temperature dependence of the tensile behavior of GaN nanowires: an atomistic study

Gallium nitride (GaN) is a high-temperature semiconductor material of considerable interest. It emits brilliant light and has been considered as a key material for the next generation of high frequency and high power transistors that are capable of operating at high temperatures. Due to its anisotropic and polar nature, GaN exhibits direction-dependent properties. Growth directions along [001], [1?10] and [110] directions have all been synthesized experimentally. In this work, molecular dynamics simulations are carried out to characterize the mechanical properties of GaN nanowires with different orientations at different temperatures. The simulation results reveal that the nanowires with different growth orientations exhibit distinct deformation behavior under tensile loading. The nanowires exhibit ductility at high deformation temperatures and brittleness at lower temperature. The brittle to ductile transition (BDT) was observed in the nanowires grown along the [001] direction. The nanowires grown along the [110] direction slip in the {010} planes, whereas the nanowires grown along the [1?10] direction fracture in a cleavage manner under tensile loading.     

Preferred growth orientation and microsegregation behaviors of eutectic in a nickel-based single-crystal superalloy

Abstract

A nickel-based single-crystal superalloy was employed to investigate the preferred growth orientation behavior of the (γ + γ′) eutectic and the effect of these orientations on the segregation behavior. A novel solidification model for the eutectic island was proposed. At the beginning of the eutectic island’s crystallization, the core directly formed from the liquid by the eutectic reaction, and then preferably grew along [100] direction. The crystallization of the eutectic along [110] always lagged behind that in [100] direction. The eutectic growth in [100] direction terminated on impinging the edge of the dendrites or another eutectic island. The end of the eutectic island’s solidification terminates due to the encroachment of the eutectic liquid/solid interface at the dendrites or another eutectic island in [110] direction. The distribution of the alloying elements depended on the crystalline axis. The degree of the alloying elements’ segregation was lower along [100] than [110] direction with increasing distance from the eutectic island’s center.     

Measurement of the bending strength of vapor-liquid-solid grown silicon nanowires

The fracture strength of silicon nanowires grown on a [111] silicon substrate by the vapor-liquid-solid process was measured. The nanowires, with diameters between 100 and 200 nm and a typical length of 2 microm, were subjected to bending tests using an atomic force microscopy setup inside a scanning electron microscope. The average strength calculated from the maximum nanowire deflection before fracture was around 12 GPa, which is 6% of the Young's modulus of silicon along the nanowire direction. This value is close to the theoretical fracture strength, which indicates that surface or volume defects, if present, play only a minor role in fracture initiation.     

Growth of cubic and hexagonal InN nanorods

InN nanorods with polyhedral ends were grown by evaporating indium in the flow of ammonia gas on Si substrates coated with Au catalyst. The samples were characterized by scanning electron microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). XRD shows that the samples contain both cubic and hexagonal phases. From the TEM results, it can be found that the cubic nanorods grow along [010] direction with a lattice constant of 0.497 nm, while the hexagonal nanorods grow along [112¯0] direction. The growth mechanism of the obtained nanostructures is discussed.     

Surface acoustic waves in a gallium arsenide-conducting layerstructure

The electron attenuation and SAW velocity versus the surface conductivity and magnetic field induction for the structure consisting of the (001) cut anisotropic GaAs substrate and an isotropic conducting layer was calculated. The SAW propagated along the [110] piezoactive direction distinguished by the greatest electromechanical coupling coefficient     

Preferred growth orientation and microsegregation behaviors of eutectic in a nickel-based single-crystal superalloy

A nickel-based single-crystal superalloy was employed to investigate the preferred growth orientation behavior of the (γ + γ′) eutectic and the effect of these orientations on the segregation behavior. A novel solidification model for the eutectic island was proposed. At the beginning of the eutectic island’s crystallization, the core directly formed from the liquid by the eutectic reaction, and then preferably grew along [100] direction. The crystallization of the eutectic along [110] always lagged behind that in [100] direction. The eutectic growth in [100] direction terminated on impinging the edge of the dendrites or another eutectic island. The end of the eutectic island’s solidification terminates due to the encroachment of the eutectic liquid/solid interface at the dendrites or another eutectic island in [110] direction. The distribution of the alloying elements depended on the crystalline axis. The degree of the alloying elements’ segregation was lower along [100] than [110] direction with increasing distance from the eutectic island’s center.     

InAs自组织量子点(线)的制备和表征

在InP(001)基衬底上用分子束外延方法生长了InAs纳米结构材料，通过改变生长方式，得到了InAs量子点和量子线。根据扫描电镜和透射电镜观测结果的分析，认为衬底旋转时浸润层三角形状的台阶为InAs量子线的成核提供了优先条件，停止衬底旋转时InAlAs缓冲层沿[11^-0]方向分布的台阶促使InAs优先形成量子点。讨论了量子点和量子线的形成机理。     

High-resolution elemental maps for three directions of Mg2Si phase in Al-Mg-Si alloy

Elemental maps of the Mg and Si sub-lattices of the Mg₂Si phase in an Al-1.0mass% Mg₂Si alloy were produced using an energy-filtering transmission electron microscope (EFTEM). Low magnification elemental maps were obtained using both low and high energy loss edges, and the intensities of the high energy loss edges were sufficiently high to allow the Mg₂Si phase to be observed at high magnification. High-resolution core-loss images of Mg and Si-K edges were taken parallel to [001], [111] and [110] of the Mg₂Si phase. In the [110] direction, Mg and Si atoms were successfully identified as sub-lattices. The Mg atoms formed a 0.39 nm diamond network, whereas the Si atoms formed a 0.32 nm by 0.22 nm rectangular network. This result is in good agreement with the projected potential of the Mg₂Si phase in the [110] direction. This is the first report of magnesium and silicon atoms in the Mg₂Si phase being successfully identified at the atomic level by EFTEM.     

Synthetic control of large-area,ordered Fe nanotubes and their nanotube-core/alumina-sheath nanocables

Fe nanotubes with different outer diameters (55 nm, 80 nm, 200 nm) have been successfully fabricated into the anodic alumina membranes (AAMs) by direct-current electrochemical deposition technique, and their nanotube-core/alumina-sheath nanocables were further realized with a simple chemical etching method. X-ray diffraction, scanning electron microscopy and transmission electron microscopy analyses show that the Fe nanotube core is single crystalline along [110] crystal direction, and the sheath is an amorphous alumina. Such a new nanostructure may be suitable for two-dimensional ferromagnetic structural materials and the diameter of the nanocables can be modulated by controlling the experimental conditions, and the formation mechanism was explored. Our experimental results provide a useful way for fabricating low-cost and large-scale metal nanocables, which are of important applications in modern nanoelectronics.     

Hydrothermal fabrication of copper sulfide nanocones and nanobelts

Novel copper sulfide nanocones and nanobelts have been hydrothermally fabricated at 140 °C for 24 h with acrylamide and sodium dodecyl benzene sulfonate (SDBS) as surfactants, respectively. X-ray diffraction (XRD) pattern indicates that the as-prepared samples are the pure hexagonal phase CuS. XPS spectra show that the ratio of Cu/S is about 1:1.084. The morphology was characterized by scanning electron microscopy (SEM) and field emission scanning electron microscopy (FE-SEM) techniques. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images reveal that nanocones and nanobelts grow along the [110] axis. The surfactant is found to be critical to the morphologies of the products. The possible formation mechanism is also discussed.