Analogous mechanical behaviors in [Formula: see text] and [Formula: see text] directions of Cu nanowires under tension and compression at a high strain rate

This study analyzes the plastic deformation on the atomic scale of Cu nanowires (NWs) with [Formula: see text] and [Formula: see text] orientations during uniaxial tension and compression, using a molecular dynamic simulation. The maximum local stress (MLS) method is employed to evaluate mechanical behavior during deformation. Following yielding, the flow stress strongly depends on the variation in the degree of orientation caused by twinning. Both the tension of the [Formula: see text] NW and the compression of the [Formula: see text] NW cause twin deformation and consequent geometrical softening. In contrast, the compression of the [Formula: see text] NW and the tension of the [Formula: see text] NW form twin bands and cause geometrical hardening. These behaviors result in the stress-strain curves that reveal the pseudo-skew-symmetry characteristic. With respect to the difference between the critical resolved shear stress (τ(c)) associated with the distinct orientations, τ(c) depends strongly on the surface critical resolved stress (τ(sc)). Under tension, τ(sc) depends on the degree of lattice distortion. A larger lattice distortion (pre-tensile stress) corresponds to higher τ(sc). However, under compression, a geometrical factor can be used to describe the difference in τ(sc) between the different orientations. A larger geometrical factor corresponds to a larger τ(sc).     

Pristine semiconducting [110] silicon nanowires

We report results of ab initio calculations on silicon nanowires oriented along the [110] direction and show for the first time that these pristine silicon nanowires are indirect band gap semiconductors. The nanowires have bulk Si core and are bounded by two (100) and two (110) planes in lateral directions. The (100) planes are atomically reconstructed with dimerization in a manner similar to the (100) surface of bulk Si but the dimer arrays are perpendicular to each other on the two (100) planes. An interesting consequence of surface reconstruction is the possibility of polytypism in thicker nanowires. We discuss its effects on the electronic structure. These findings could have important implications for the use of silicon nanowires in nanoscale devices as experimentally [110] nanowires have been found to grow preferentially in the small diameter range.     

Strain-driven electronic band structure modulation of si nanowires

One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.     

Magic structures of h-passivated 110 silicon nanowires

We report a genetic algorithm approach combined with ab initio calculations to determine the structure of hydrogenated 110 Si nanowires. As the number of atoms per length increases, we find that the cross section of the nanowire evolves from chains of six-atom rings to fused pairs of such chains to hexagons bounded by {001} and {111} facets. Our calculations predict that hexagonal wires become stable starting at about 1.2 nm diameter, which is consistent with recent experimental reports of nanowires with diameters of about 3 nm.     

Photomodulated rayleigh scattering of single semiconductor nanowires: probing electronic band structure

The internal electronic structures of single semiconductor nanowires can be resolved using photomodulated Rayleigh scattering spectroscopy. The Rayleigh scattering from semiconductor nanowires is strongly polarization sensitive which allows a nearly background-free method for detecting only the light that is scattered from a single nanowire. While the Rayleigh scattering efficiency from a semiconductor nanowire depends on the dielectric contrast, it is relatively featureless as a function of energy. However, if the nanowire is photomodulated using a second pump laser beam, the internal electronic structure can be resolved with extremely high signal-to-noise and spectral resolution. The photomodulated Rayleigh scattering spectra can be understood theoretically as a first derivative of the scattering efficiency that results from a modulation of the band gap and depends sensitively on the nanowire diameter. Fits to spectral lineshapes provide both the band structure and the diameter of individual GaAs and InP nanowires under investigation.     

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.     

镍间隙掺杂硅纳米线电子结构的研究

采用基于密度泛函理论的第一性原理的方法,对[100]方向镍间隙掺杂硅纳米线结构的稳定性和电子性质进行了计算。计算结果表明Ni原子更喜欢占据硅纳米线内部六角形间隙位置;掺杂体系费米能级附近的电子态密度来源于Ni3d态电子的贡献;同时发现不同构型的Ni掺杂硅纳米线,其带隙不同,且与未掺杂硅纳米线相比,带隙普遍减小。     

Controlled synthesis of millimeter-long silicon nanowires with uniform electronic properties

We report the nanocluster-catalyzed growth of ultralong and highly uniform single-crystalline silicon nanowires (SiNWs) with millimeter-scale lengths and aspect ratios up to approximately 100 000. The average SiNW growth rate using disilane (Si 2H 6) at 400 degrees C was 31 mum/min, while the growth rate determined for silane (SiH 4) reactant under similar growth conditions was 130 times lower. Transmission electron microscopy studies of millimeter-long SiNWs with diameters of 20-80 nm show that the nanowires grow preferentially along the 110 direction independent of diameter. In addition, ultralong SiNWs were used as building blocks to fabricate one-dimensional arrays of field-effect transistors (FETs) consisting of approximately 100 independent devices per nanowire. Significantly, electrical transport measurements demonstrated that the millimeter-long SiNWs had uniform electrical properties along the entire length of wires, and each device can behave as a reliable FET with an on-state current, threshold voltage, and transconductance values (average +/-1 standard deviation) of 1.8 +/- 0.3 muA, 6.0 +/- 1.1 V, 210 +/- 60 nS, respectively. Electronically uniform millimeter-long SiNWs were also functionalized with monoclonal antibody receptors and used to demonstrate multiplexed detection of cancer marker proteins with a single nanowire. The synthesis of structurally and electronically uniform ultralong SiNWs may open up new opportunities for integrated nanoelectronics and could serve as unique building blocks linking integrated structures from the nanometer through millimeter length scales.     

NaCl multi-layer islands grown on Au(111)-([Formula: see text]) probed by scanning tunneling microscopy

The growth of multi-layer NaCl islands on Au(111)-([Formula: see text]) surfaces was investigated using scanning tunneling microscopy (STM). We observed that the aspect of the NaCl islands drastically differs depending on the tunneling conditions. It is therefore possible to observe the layers forming an NaCl island or to image the gold reconstruction below the first NaCl layer. Atomically resolved STM images obtained on the first NaCl layer demonstrate that NaCl grows as an epitaxial crystalline film on Au(111)-([Formula: see text]). STM images also suggest that some NaCl layers can be non-crystalline.     

Highly resolved non-contact atomic force microscopy images of the Sn/Si(111)-([Formula: see text]) surface

The Sn/Si(111)-([Formula: see text]) surface is observed by using non-contact atomic force microscopy (NC-AFM) at room temperature. The images at relatively far tip-surface distances show four protrusions in each ([Formula: see text]) unit cell, which are similar to previously reported scanning tunnelling microscopy (STM) images. On the other hand, it is found that, at closer tip-surface distances, eight protrusions are clearly resolved, which indicates that the spatial resolution of NC-AFM is higher than that of STM as far as imaging this surface is concerned. Our high-resolution NC-AFM images are in good agreement with a recently proposed model based on 13 Sn atoms per unit cell.     

Surface charge sensitivity of silicon nanowires: size dependence

Silicon nanowires of different widths were fabricated in silicon on insulator (SOI) material using conventional process technology combined with electron-beam lithography. The aim was to analyze the size dependence of the sensitivity of such nanowires for biomolecule detection and for other sensor applications. Results from electrical characterization of the nanowires show a threshold voltage increasing with decreasing width. When immersed in an acidic buffer solution, smaller nanowires exhibit large conductance changes while larger wires remain unaffected. This behavior is also reflected in detected threshold shifts between buffer solutions of different pH, and we find that nanowires of width >150 nm are virtually insensitive to the buffer pH. The increased sensitivity for smaller sizes is ascribed to the larger surface/volume ratio for smaller wires exposing the channel to a more effective control by the local environment, similar to a surrounded gate transistor structure. Computer simulations confirm this behavior and show that sensing can be extended even down to the single charge level.     

Ab initio study of electronic structures of InAs and GaSb nanowires along various crystallographic orientations

InAs and GaSb nanowires oriented along different crystallographic axes—the [0 0 1], [1 0 1] and [1 1 1] directions of zinc-blende structure—have been studied utilizing a first-principles derived nonlocal screened atomic pseudopotential theory, to investigate the band structure, polarization ratio and effective masses of these semiconductor nanowires and their dependences on the wire lateral size and axis orientation. The band energy dispersion over entire Brillouin zone and orbital energy are determined and found to exhibit different characteristics for three types of wires. There is an explicit dispersion hump in the conduction bands of [0 0 1] nanowires with two larger diameters and [1 0 1] nanowires with the smallest diameter considered. Moreover, the [1 1 1] nanowires are shown to exhibit very different orbital energy for the maximum valence state at the zone-boundary point, compared with [0 0 1] and [1 0 1] nanowires. These differences present significant and detailed insight for experimental determination of the band structure in InAs and GaSb nanowires. Furthermore, we study the polarization ratio of these nanowires for different orientations. Our calculation results indicate that, for the same lateral size, the [1 1 1] nanowires give extraordinarily higher polarization ratio compared to nanowires along the other two directions, and at the same time have larger band-edge photoluminescence transition intensity. Consequently, the [1 1 1] nanowires are predicted to be better suitable for optoelectronic applications. We also significantly find that polarization ratio and transition intensity displays different varying trend of dependence on lateral size of nanowires. Specially, the calculated polarization ratio is shown to increase with the decreasing size, which is opposite to the behavior displayed by the optical transition intensity. The predicted polarization ratios of [1 0 1] and [1 1 1] nanowires for 10.6 Å diameter approach the limit of 100%. In addition, the electron and hole masses for InAs and GaSb nanowires with different crystallographic axes have been calculated. For the [1 0 1] and [1 1 1] oriented nanowires, the hole masses are predicted to be around 0.1–0.2 m₀, which are notably smaller than the values (∼0.5 m₀) along the same direction for their bulk counterparts. Thus, we demonstrates an inspired possibility of obtaining a high hole mobility in nanowires that is not available in bulk. The small hole mobility is interpreted as to be associated with the strong electronic band mixing in nanowires.     

Vapor-liquid-Solid synthesis of [010]-oriented Sb2Se3 nanowires

In this article we report for the first time the synthesis of Sb2Se3 nanowires using a physical vapor-liquid-solid (VLS) process. We used microcrystals of Antimony as solid catalytic material and molten Selenium to generate the vapor source. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) images show that as-obtained Sb2Se3 nanowires have diameters in the range between 20 nm and 2 microm and lengths up to 30 microm. Fringes in TEM imaging reveals that Sb2Se3 nanowires are oriented along the [010] crystallographic direction. This orientation is being reported for the first time.     

Structural and electronic properties of lead nanowires: Ab-initio study

Ab-initio self-consistent study has been performed to analyze the stability of lead nanowires in its six stable configurations like linear, zigzag, triangular, ladder, square and dumbbell. In the present study, the lowest energy structures have been analyzed under the revised Perdew-Burke-Ernzerhof (revPBE) parameterization of generalized gradient approximation (GGA) potential. The two-atom zigzag shaped atomic configuration with highest binding energy and lowest total energy has been confirmed as the most stable structure out of the six atomic configurations. The electronic band structure and density of states have been discussed in detail with a remarkable observation in case of three-atom triangular lead nanowire having a very small band gap while other configurations are found to be metallic. Bulk modulus, pressure derivatives and lattice parameters for different lead nanowires have also been computed and discussed.     

Electrophoretic deposition [EPD] applied to reaction joining of silicon carbide and silicon nitride ceramics

Electrophoretic Deposition (EPD) was used to deposit a mixture of SiC or Si₃N₄ filler and reactive carbon (graphite and carbon black) particles onto various SiC or Si₃N₄ parts in preparation for reaction bonding. The particles had gained a surface charge when mixed into an organic liquid consisting of 90 w % acetone + 10 w % n-butyl amine to form a slurry. The charged particles then moved when placed under the influence of an electric field to form a green deposit on the ceramic parts. The green parts were then dried and subsequently joined using a reaction bonding method. In this reaction bonding, molten Si moves into the joint via capillary action and then dissolves carbon and precipitates additional SiC. An optimum mixture of SiC filler to C powder ratio of 0.64 was identified. Residual un-reacted or free Si was minimized as a result of selecting powders with well-characterized particle size distributions and mixing them in batch formulas generated as part of the research. Image analysis of resulting microstructures indicated residual free Si content as low as 7.0 vol % could be realized. Seven volume percent compares favorably with the lowest free Si levels available in experimental samples of bulk siliconized (reaction-bonded) SiC manufactured using conventional reaction-bonding techniques. The joints retained the residual silicon over a large number of high-temperature thermal cycles (cycling from below to above the melting point of silicon). Comparisons to commercial reaction-bonded SiC indicated the majority of residual silicon of the joint was retained in closed porosity. This infers that parts made with these joints might be successfully utilized at very high temperatures. It was demonstrated that the EPD technique could be applied to butt, lap, and scarf type joints, including the capability to fill large gaps or undercut sections between parts to be joined. The overall results indicate that EPD, combined with reaction bonding, should allow for the fabrication of large complex structures manufactured from smaller components consisting of silicon carbide or silicon nitride.     

New developments on FRAMs: [3D] structures and all-perovskite FETs

A discussion is presented of new directions in ferroelectric random access memories (FRAMs) and ferroelectric capacitors for dynamic random access memories (DRAMs), emphasizing [3D] structures and new materials, as well as ferroelectric gates and new mechanisms of domain wall motion in nano-scale geometries.     

Forest of ultra thin silicon nanowires: realization of temperature and catalyst size

One of the most important progresses in the field of nano science and technology was partially due to the high surface to volume ratio of quasi one-dimensional silicon nanowires (SiNWs) with various applications in biological and chemical sensors, optoelectronic devices, catalysis, Li ion batteries and solar cells. In this study we have prepared a uniform forest of ultrathin SiNWs using plasma enhanced chemical vapor deposition method. Uniformly distributed SiNWs were obtained based on an Au layer containing gold nano-seeds with the average diameters ranging from 10 to 40 nm at various temperatures. The physicochemical properties of SiNWs were characterized using field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction (XRD), photoluminescence (PL) and high-resolution transmission electron microscopy. Microscopic assessments revealed that crystalline-amorphous core–shell SiNWs with different diameters and lengths ranging from 35 to 130 nm and ~?0.7 to 1.9 µm are formed during the vapor–liquid–solid mechanism, respectively. The XRD spectra show that the main lattice directions are Si(111), Si(220) and Si(311) which confirm crystalline structure of synthesized NWs. The PL spectrum reveal two distinct emission peaks at wavelengths of about 480 nm (blue range) and 690 nm (red range) as sharp and a broad peak, respectively.