Hydrogenated Silicon Nanowires Research Articles

Reduction of Cr(VI) to Cr(III) using silicon nanowire arrays under visible light irradiation

We report an efficient visible light-induced reduction of hexavalent chromium Cr(VI) to trivalent Cr(III) by direct illumination of an aqueous solution of potassium dichromate (K2Cr2O7) in the presence of hydrogenated silicon nanowires (H-SiNWs) or silicon nanowires decorated with copper nanoparticles (Cu NPs-SiNWs) as photocatalyst. The SiNW arrays investigated in this study were prepared by chemical etching of crystalline silicon in HF/AgNO3 aqueous solution. The Cu NPs were deposited on SiNW arrays via electroless deposition technique. Visible light irradiation of an aqueous solution of K2Cr2O7 (10−4M) in presence of H-SiNWs showed that these substrates were not efficient for Cr(VI) reduction. The reduction efficiency achieved was less than 10% after 120min irradiation at λ>420nm. Addition of organic acids such as citric or adipic acid in the solution accelerated Cr(VI) reduction in a concentration-dependent manner. Interestingly, Cu NPs-SiNWs was found to be a very efficient interface for the reduction of Cr(VI) to Cr(III) in absence of organic acids. Almost a full reduction of Cr(VI) was achieved by direct visible light irradiation for 140min using this photocatalyst.

Anomalous Effect on the Phononic Thermal Conductivity of Silicene Nanoribbon by Hydrogenation

Silicene is a two-dimensional (2D) allotrope of silicon known to have a lower thermal conductivity than graphene; thus, more suitable for thermoelectric applications. This paper investigates the effect of hydrogenation on the thermal conductivity of silicene nanoribbon (SiNR) using equilibrium molecular dynamics (EMD) simulations. The simulations were carried out in Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) using a modified Tersoff potential that considers both Si-Si and Si-H interactions. The thermal conductivity of fully hydrogenated silicene nanoribbon (H-SiNR), also known as silicane nanoribbon, was found to be higher than that of pristine SiNR in all the temperatures and dimensions considered here. This anomalous enhancement in the thermal conductivity is similar to that found in hydrogenated silicon nanowires (H-SiNWs). A mechanism for this anomalous effect has been proposed relating the hydrogenation of SiNR with the stiffening and increase of the acoustic out-of-plane flexural (ZA) phonon modes. Also, for both SiNR and H-SiNR, the thermal conductivities generally increase as the dimensions are increased while they generally decrease as the temperatures are increased, in agreement to other reports.

Anomalous effect of hydrogenation on phonon thermal conductivity in thin silicon nanowires

Using equilibrium molecular-dynamics simulations, we investigate the phonon thermal conductivity of hydrogenated silicon nanowires (H-SiNWs). In contrast to the usual thermal conductivity reduction observed upon surface passivation due to the enhancement of phonon-surface scattering in nanostructures, we observe increased thermal conductivity upon the surface hydrogenation of thin SiNWs as compared to pure SiNWs. We show that such an anomalous effect arises mainly from the hydrogenation-induced specific changes in the lattice structures and phonon modes of surface silicon atoms.

First-Principles Study of Silicon Nanowire Approaching the Bulk Limit

First-principles density functional theory calculations on hydrogenated silicon nanowires (SiNWs) with diameters up to 7.3 nm are carried out for comparing to experimentally relevant SiNWs and evaluating its radial doping profiles. We show that the direct band gap nature of both the small diameter (110) and (100) SiNWs fades when the diameter reaches beyond about 4 nm, where the difference of direct and indirect band gaps are close, within the experimental measurement uncertainty of ±0.1 eV, suggesting the diameter size where the gap nature transition starts. In addition, we reveal that core-surface boron (B) codoped SiNW forms more preferably at large diameter than that of the surface-surface codoped one, attributing to the lower energy configuration raised by the core B dopant at large diameter SiNW. More importantly, the diameter for such a preferential transition increases as the doping concentration decreases. Our results rationalize photoluminescent measurements and radial doping distributions of SiNWs.

Theoretical study of the elasticity of ultra-thin finite silicon nanowires. I. Semiempirical model considerations

We present a theoretical framework towards the development of a hybrid theoretical technique for the mixed ab initio and semiempirical calculation of the elastic modulus of ultra thin hydrogenated silicon nanowires. The present approach achieves accurate results at low computational cost by combining ab initio and semiempirical theoretical techniques. Here we describe the basic steps of such a theoretical scheme which combines continuum models like Timoshenko and Euler-Bernoulli beam theory with ab initio and semiempirical methods. Our current modified beam-method-like technique is implemented through accurate real space density functional theory, using the PBE functional, which calibrates semiempirical results from AM1. The present results for the Young's modulus of 110] NWs range from 148-156GPa, comparable to results in the literature obtained by methods of very high computational cost.

Anisotropy in stability and Young’s modulus of hydrogenated silicon nanowires

Abstract To discern the anisotropy of silicon nanowires (SiNWs) grown in different directions, the binding energy, heat of formation, and Young’s modulus of hydrogen-passivated SiNWs with various diameters and crystallographic orientations were calculated using all-electron density functional theory. In the size range studied, nanowires grown in the [1 1 0] direction are most stable while those in the [1 0 0] direction are energetically least favorable. Similar trend was observed in the computed Young’s modulus. With the same radius, the nanowire along the [1 1 0] direction possesses the highest Young’s modulus, while the [1 0 0] wire has the lowest value.

Stress-induced band gap tuning in ⟨112⟩ silicon nanowires

We show via density functional calculations that the electronic band structures of ⟨112⟩-oriented hydrogenated silicon nanowires (SiNWs) could be significantly altered by axial stresses. In particular, an axial compression could cause an indirect-to-direct band gap transition in ⟨112⟩ SiNWs. As direct energy band may induce strong light-emission properties of Si, the possibility of indirect-to-direct band transition via axial stress has fundamental implications in exploiting SiNWs for optoelectronic applications.

Structural properties and magic structures in hydrogenated finite and infinite silicon nanowires

Unusual effects such as bending and “canting,” related with the stability, have been identified by ab initio real-space calculations for hydrogenated silicon nanowires. We have examined in detail the electronic and structural properties of finite and infinite nanowires as a function of length (and width) and have developed stability and bending rules, demonstrating that “magic” wires do not bend. Reconstructed 2×1 nanowires are practically as stable as the magic ones. Our calculations are in good agreement with the experimental data of Ma et al. [Science 299, 1874 (2003).].

Theoretical investigation of silicon nanowires: Methodology, geometry, surface modification, and electrical conductivity using a multiscale approach

The structural and electronic properties of hydrogenated silicon nanowires (SiNWs) oriented in ⟨100⟩, ⟨110⟩, ⟨111⟩, and ⟨112⟩ directions are investigated systematically using a multiscale approach: geometry optimization is done by a semiempirical method and electronic band structure is calculated by density-functional theory with Gaussian basis set. The calculated band gaps agree very well with the available experimental data. We propose that the multiscale approach is an accurate and effective way for calculating the structural and electronic properties of SiNWs with diameter up to $3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. Besides, we found that surface modification of the SiNWs using the hydroxyl and fluoro groups can strongly reduce the band gap by as much as $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and more interestingly, alter the gap nature. On the contrary, the ⟨110⟩ SiNWs exhibiting different patterns at the cross section do not show significant difference in their band gaps $(<0.09\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, indicating that the electronic structures of SiNWs are much more sensitive to surface modification than the change of cross section. Moreover, SiNWs of different orientations exhibit different degrees of band gap reduction upon surface modification, in which the ⟨110⟩ SiNW demonstrates the highest sensitivity. The result indicates that SiNWs oriented in ⟨110⟩ direction are the better candidate for sensor application. On the contrary, the ⟨111⟩ SiNW is found to be very tight to structural change with diameter and the most reluctant to surface modification, showing that it is structurally stable and rather inert. In addition, from the $I\text{\ensuremath{-}}V$ curves of the SiNWs with surfaces modified with hydroxyl groups, which are calculated by the nonequilibrium Green's function theory, we found that the electrical conductivity of the SiNWs is highly chemical sensitive. Besides, the phenomenon of negative differential resistance is observed in the $I\text{\ensuremath{-}}V$ curves.

Electronic properties of hydrogenated silicon nanowires with surface defects

Our ab initio results indicate that some defects, such as atomic vacancies, on the surface of hydrogenated silicon nanowires in the ⟨001⟩ orientation can preserve the direct character of the gap and enhance oscillator strength of the first direct transition. This finding extends a possible scenario of the efficient photoluminescence from porous silicon recently published [D. B. Migas, J. Appl. Phys. 98, 054310 (2005)].

Effects of Morphology and Doping on the Electronic and Structural Properties of Hydrogenated Silicon Nanowires

We study the electronic and atomic structures of hydrogenated silicon nanowires (SiNWs) by changing the mean diameter, morphology, and orientation using state-of-the-art density functional calculations. Most of the SiNWs are found to have large and direct band gaps, which make them very interesting for silicon-based nano-optoelectronic devices and lasers. The band gap increases with decreasing diameter in all cases because of quantum confinement, but the scaling is dependent on the morphology of the SiNWs. For thin [112] SiNW, the calculated band gap agrees well with the recent experiments. Variation in hydrogen concentration is used to explore the sensing capabilities of different surface morphologies and the associated surface reconstructions. Further studies on p- or n-doping show bulklike modifications in the electronic structure with several advantages that can be used to design nanoscale devices of SiNWs.

Effects of morphology on the electronic and photoluminescence properties of hydrogenated silicon nanowires

By means of first-principles calculations we show that hydrogenated silicon nanowires in the ⟨001⟩ orientation can have a direct band gap with appreciable oscillator strength of the first direct transition if their surface is characterized by dimers displaying the SiH species. The breaking of some dimers (the SiH2 species) seems to help in stabilizing the direct nature of the gap of such nanostructures with increasing diameters. We also discuss how our findings can explain the efficient photoluminescence from porous silicon.