Surface-potential-modulated piezoresistive effect of core-shell 3C-SiC nanowires

The effect of surface potential on the carrier mobility and piezoresistance of core-shell silicon carbide nanowires (SiC NWs) was investigated to realize small and sensitive SiC-microelectromechanical systems sensors. The p-type cubic crystalline SiC (3C-SiC) NWs were synthesized via the vapor-liquid-solid method and coated with silicon dioxide (SiO₂) or aluminum oxide (Al₂O₃) dielectric shells to form core-shell structured NWs with different surface potentials. Four-point bending devices (FBDs) with a field-effect transistor (FET) configuration integrating a single core-shell 3C-SiC NW as the FET channel were fabricated to apply an additional electric field and strain to the core-shell 3C-SiC NWs. The fixed oxide charge densities of the SiO₂and Al₂O₃shells showed positive and negative values, respectively, which were equivalent to electric fields of the order of several hundred thousand volt per centimeter in absolute values. In the core-shell 3C-SiC NWs with originally low impurity concentrations, the electric field induced by the fixed oxide charge of the shells can determine not only the electrical conduction but also the charge carriers in the NWs. Bending tests using the FBDs showed that the piezoresistive effect of the SiO₂-coated NW was almost the same as that of the as-grown 3C-SiC NW reported previously, regardless of the gate voltage, whereas that of the Al₂O₃-coated NW was considerably enhanced at negative gate voltages. The enhancement of the piezoresistive effect was attributed to the piezo-pinch effect, which was more pronounced in the NW, where the carrier density at the core-shell interface is enhanced by the electric field of the dielectric.

Enhanced Absorption with Graphene-Coated Silicon Carbide Nanowires for Mid-Infrared Nanophotonics

The mid-infrared (MIR) is an exciting spectral range that also hosts useful molecular vibrational fingerprints. There is a growing interest in nanophotonics operating in this spectral range, and recent advances in plasmonic research are aimed at enhancing MIR infrared nanophotonics. In particular, the design of hybrid plasmonic metasurfaces has emerged as a promising route to realize novel MIR applications. Here we demonstrate a hybrid nanostructure combining graphene and silicon carbide to extend the spectral phonon response of silicon carbide and enable absorption and field enhancement of the MIR photon via the excitation and hybridization of surface plasmon polaritons and surface phonon polaritons. We combine experimental methods and finite element simulations to demonstrate enhanced absorption of MIR photons and the broadening of the spectral resonance of graphene-coated silicon carbide nanowires. We also indicate subwavelength confinement of the MIR photons within a thin oxide layer a few nanometers thick, sandwiched between the graphene and silicon carbide. This intermediate shell layer is characteristically obtained using our graphitization approach and acts as a coupling medium between the core and outer shell of the nanowires.

Synthesis of SiC/SiO2 core-shell nanowires with good optical properties on Ni/SiO2/Si substrate via ferrocene pyrolysis at low temperature

In this study, the high-density SiC/SiO₂ core-shell nanowires were synthesized on the nickel coated SiO₂ (100 nm)/Si substrate by chemical vapor deposition (CVD) method with ferrocene precursor at temperature 1000 °C compared to previous studies (1300-1600 °C). The present work provides an efficient strategy for the production of SiC/SiO₂ nanowires with uniform morphology and good optical properties, where the Ni layer plays important roles for this fabrication at low temperature which reduces the decomposition temperature of hydrocarbon gases and improves the growth quality of SiC nanowires. The as-synthesized SiC/SiO₂ nanowires consist of single crystal 3C structures as well as 3C structures with defects along [111] direction. In the photoluminescence (PL) spectrum, the SiC/SiO₂ core-shell nanowires revealed an obvious blueshift. The blueshift is due to the formation of nanoscale silicon carbide polytypism caused by the stacking faults in 3C-SiC and the nanoscale polytypism also caused the transition from indirect to direct bandgap which explains why the stacking faults percentage in SiC confirmed from X-ray diffraction (XRD) is 19%, but ultimately makes the strongest emission intensity. Finally, the PL characteristics are further improved by changing the diameter of the SiC nanowire and etching and an approximate model followed by the vapor-liquid-solid (VLS) mechanism was proposed to explain the possible growth mechanism of the SiC/SiO₂ nanowires.

Hydrogen plasma treatment confers enhanced bioactivity to silicon carbide-based nanowires promoting osteoblast adhesion

Nanomaterials play a pivotal role in modern regenerative medicine and tissue engineering, due to their peculiar physical, optical and biological properties once they are used in the nanometric size. Many evidences are showing the importance of biomaterial micro- and nano-topography on cellular adhesion, proliferation and differentiation, and hence, tissue regeneration. It is well known that nanowires (NWs) can mimic many different tissues as a result of their shape and their surface characteristics, and that surface hydrophilicity affects early protein adsorption and cellular adhesion. Therefore a material able to induce bone regeneration might be obtained by combining optimal surface topography and hydrophilicity. Based on these evidence, we designed silicon carbide (SiC) and core/shell silicon carbide/silicon dioxide (SiC/SiO_x) nanowires with modified wettability in order to analyze cell behavior, using an in-vitro osteoblastic model. First, we synthetized SiC NWs and SiC/SiO_x NWs through a chemical-vapour-deposition (CVD) process, and then we used hydrogen plasma to modify their hydrophilicity. Subsequently we evaluated the four types of NWs in terms of their morphology and contact angle, and we studied their behavior in the presence of MC3T3-E1 murine osteoblasts. Cell metabolic activity, viability, morphology and focal adhesions formation were considered. Morphological data showed different dimensions between SiC and SiC/SiO_x NWs. SiC NWs before the hydrogen plasma treatment showed a very low contact angle, that was absent after the treatment. Osteoblastic cells appeared healthy on all of the samples. Interestingly, both hydrophilic SiC NWs and SiC/SiO_x NWs generated a favorable distribution of focal adhesions around the cell body confirmed also by scanning electron microscopy images. Moreover, osteoblasts grown on hydrogen plasma treated SiC/SiO_x NWs showed an increased metabolic activity testified by a significantly higher cell number. In conclusion, we are here demonstrating that hydrogen plasma treatment of SiC and SiC/SiO_x NWs induce a better osteoblastic cellular adhesion by increasing NWs wettability. We are therefore suggesting that hydrogen plasma treatment of SiC/SiO_x can offer a suitable method to develop scaffolds for bone tissue engineering applications.

Nanowire Electronics: From Nanoscale to Macroscale

Semiconductor nanowires have attracted extensive interest as one of the best-defined classes of nanoscale building blocks for the bottom-up assembly of functional electronic and optoelectronic devices over the past two decades. The article provides a comprehensive review of the continuing efforts in exploring semiconductor nanowires for the assembly of functional nanoscale electronics and macroelectronics. Specifically, we start with a brief overview of the synthetic control of various semiconductor nanowires and nanowire heterostructures with precisely controlled physical dimension, chemical composition, heterostructure interface, and electronic properties to define the material foundation for nanowire electronics. We then summarize a series of assembly strategies developed for creating well-ordered nanowire arrays with controlled spatial position, orientation, and density, which are essential for constructing increasingly complex electronic devices and circuits from synthetic semiconductor nanowires. Next, we review the fundamental electronic properties and various single nanowire transistor concepts. Combining the designable electronic properties and controllable assembly approaches, we then discuss a series of nanoscale devices and integrated circuits assembled from nanowire building blocks, as well as a unique design of solution-processable nanowire thin-film transistors for high-performance large-area flexible electronics. Last, we conclude with a brief perspective on the standing challenges and future opportunities.

Strain engineering of core-shell silicon carbide nanowires for mechanical and piezoresistive characterizations

This study evaluated the mechanical properties and piezoresistivity of core-shell silicon carbide nanowires (C/S-SiCNWs) synthesized by a vapor-liquid-solid technique, which are a promising material for harsh environmental micro electromechanical systems (MEMS) applications. The C/S-SiCNWs were composed of a crystalline cubic (3C) SiC core wrapped by an amorphous silicon dioxide (SiO _x ) shell; however, TEM observations of the NWs showed that hexagonal polytypes (2H, 4H , and 6H) were partially induced in the core by a stacking fault owing to a Shockley partial dislocation. The stress-strain relationship of the C/S-SiCNWs and SiC cores without an SiO _x shell was examined using MEMS-based nanotensile tests. The tensile strengths of the C/S-SiCNWs and SiC cores were 7.0 GPa and 22.4 GPa on average, respectively. The lower strength of the C/S-SiCNWs could be attributed to the SiO _x shell with the surface roughness as the breaking point. The Young's modulus of the C/S-SiCNWs was 247.2 GPa on average, whereas that of the SiC cores had a large value with scatter data ranging from 450 to 580 GPa. The geometrical model of the SiC core based on the transmission electron microscopy observations rationalized this scatter data by the volume content of the stacking fault in the core. The piezoresistive effects of the C/S-SiCNW and SiC core were also evaluated from the I-V characteristics under uniaxial tensile strain. The gauge factor of -30.7 at 0.008 ε for the C/S-SiCNW was approximately two-times larger than that of -15.8 at 0.01 ε for the SiC core. This could be caused by an increase of the surface state density at the SiO _x /SiC interface owing to the positive fixed oxide charge of the SiO _x shell.

Osteoblast adhesion and response mediated by terminal -SH group charge surface of SiOxCy nanowires

Robust cell adhesion is known to be necessary to promote cell colonization of biomaterials and differentiation of progenitors. In this paper, we propose the functionalization of Silicon Oxycarbide (SiO_xC_y) nanowires (NWs) with 3-mercaptopropyltrimethoxysilane (MPTMS), a molecule containing a terminal -SH group. The aim of this functionalization was to develop a surface capable to adsorb proteins and promote cell adhesion, proliferation and a better deposition of extracellular matrix. This functionalization can be used to anchor other structures such as nanoparticles, proteins or aptamers. It was observed that surface functionalization markedly affected the pattern of protein adsorption, as well as the in vitro proliferation of murine osteoblastic cells MC3T3-E1, which was increased on functionalized nanowires (MPTMS-NWs) compared to bare NWs (control) (p < 0.0001) after 48 h. The cells showed a better adhesion on MPTMS-NWs than on bare NWs, as confirmed by immunofluorescence studies on the cytoskeleton, which showed a more homogeneous vinculin distribution. Gene expression analysis showed higher expression levels for alkaline phosphatase and collagen I, putative markers of the osteoblast initial differentiation stage. These results suggest that functionalization of SiO_xC_y nanowires with MPTMS enhances cell growth and the expression of an osteoblastic phenotype, providing a promising strategy to improve the biocompatibility of SiO_xC_y nanowires for biomedical applications.

A novel combined experimental and multiscale theoretical approach to unravel the structure of SiC/SiOx core/shell nanowires for their optimal design

In this work we propose a realistic model of nanometer-thick SiC/SiOx core/shell nanowires (NWs) using a combined first-principles and experimental approach. SiC/SiOx core/shell NWs were first synthesised by a low-cost carbothermal method and their chemical-physical experimental analysis was accomplished by recording X-ray absorption near-edge spectra. In particular, the K-edge absorption lineshapes of C, O, and Si are used to validate our computational model of the SiC/SiOx core/shell NW architectures, obtained by a multiscale approach, including molecular dynamics, tight-binding and density functional simulations. Moreover, we present ab initio calculations of the electronic structure of hydrogenated SiC and SiC/SiOx core/shell NWs, studying the modification induced by several different substitutional defects and impurities into both the surface and the interfacial region between the SiC core and the SiOx shell. We find that on the one hand the electron quantum confinement results in a broadening of the band gap, while hydroxyl surface terminations decrease it. This computational investigation shows that our model of SiC/SiOx core/shell NWs is capable to deliver an accurate interpretation of the recorded X-ray absorption near-edge spectra and proves to be a valuable tool towards the optimal design and application of these nanosystems in actual devices.

Wetting Behavior of Metal-Catalyzed Chemical Vapor Deposition-Grown One-Dimensional Cubic-SiC Nanostructures

Superhydrophobic surfaces can be fabricated by using the self-assembled nanoarchitecture of 3C-SiC one-dimensional (1D) nanostructures as they are capable of forming a dense network of micro-nano air pockets without any help from external sources. Herein, the metal-catalyzed growth of 3C-SiC nanowires/nanorods on Si substrates via vapor-liquid-solid mechanism using five different catalysts, that is, chemically synthesized Au nanoparticles and direct current-sputtered thin films of Au, Cu, Ni, and Ti, is reported. Relatively new or unexplored catalysts such as thin films of Cu and Ti, as well as drop-cast Au nanoparticles, were used. An optimized and separate growth was carried out for each catalyst in an inductively heated horizontal cold-wall atmospheric pressure chemical vapor deposition reactor. An insight into the catalytic growth mechanism of 3C-SiC 1D nanostructures has been presented. All of the bare samples exhibited superhydrophilic behavior, whereas hierarchical Au/Pd nanostructure-coated 3C-SiC nanorod samples grown using Au and Ni thin-film catalysts exhibited hydrophobic and superhydrophobic behavior, respectively. As the better results were obtained for Ni thin-film catalysts in terms of growth density and high water contact angle (WCA ≈ 160°), therefore, the growth temperature, as well as the growth time-dependent wetting behavior, was also studied. It was found that the WCA increased as the growth time and temperature increased because of the increase in the growth density, and it finally reached to an optimum value at the growth temperature of 1200 °C and the growth time of 1 h. Furthermore, their wetting behavior was studied by using a variety of high surface tension (water, milk, tea, and glycerin) and low surface tension (organic liquids such as n-hexane, ethanol, etc.) liquids. High surface tension liquids exhibited superhydrophobic behavior, whereas low surface tension liquids exhibited superhydrophilic behavior. Hence, these fabricated nanostructured surfaces can be exploited for oil-water separation, electrowetting, water harvesting, self-cleaning, lab on a chip, and micro-/nanofluidic device applications.

Ten-gram scale SiC@SiO2 nanowires: high-yield synthesis towards industrialization, in situ growth mechanism and their peculiar photoluminescence and electromagnetic wave absorption properties

SiC@SiO₂ nanowires, as a functional nanocomposite, have attracted widespread attention due to their fascinating performance and broad application prospect.