Hydrogen-passivated Silicon Nanowires Research Articles

Interstitial sodium and lithium doping effects on the electronic and mechanical properties of silicon nanowires: a DFT study.

In this work, we present a theoretical study of the electronic band structure and the Young's modulus of hydrogen-passivated silicon nanowires (H-SiNWs), grown along the [110] crystallographic direction, as a function of the concentration of interstitial sodium (Na) and lithium (Li) atoms. The study is performed using the supercell scheme and the density functional theory (DFT), within the local density approximation (LDA). The results show that the presence of Na or Li atoms closes the former semiconducting band gap of the H-SiNWs and shifts the Fermi energy into the conduction band. The transition from semiconductor to metal occurs as soon as a single Na or Li atom is added to the nanowire and the number of occupied states near the Fermi level is larger for the H-SiNWs with Li atoms in comparison with those nanowires with the same concentration of Na atoms. The calculated formation energies reveal that the system becomes less stable when the concentration of Na and Li atoms augments. Moreover, the obtained binding energies indicate that Si-Li and Si-Na bonds are formed. It is worth mentioning that the binding energies of H-SiNWs with interstitial Li atoms are larger than those corresponding to the H-SiNWs with interstitial Na atoms. On the other hand, the Young's moduli of H-SiNWs with Na atoms are lower than those of pure H-SiNWs and their values diminish when the concentration of Na atoms increases. In contrast, Young's moduli of H-SiNWs present a non-monotonic behavior as a function of the concentration of interstitial Li atoms and for the largest studied concentration the nanowire fractures. These results give insight into the changes that electronic and mechanical properties of H-SiNWs suffer during the charge-discharge process, which should be taken into account in the design of electrodes of Na or Li-ion batteries.

Lithiation effects on the structural and electronic properties of Si nanowires as a potential anode material

The need for better energy-storage materials has attracted much attention to the development of Li-ion battery electrodes. Si nanowires have been considered as alternative electrodes, however the effects of Li on their electronic band gap and mechanical properties have been scarcely studied. In this work, a density functional study of the electronic and mechanical properties of hydrogen passivated silicon nanowires (H-SiNWs) grown along the [001] direction is presented. The Li atoms are gradually inserted at interstitial positions or replacing surface H atoms. The results show that, for surface-lithiated H-SiNWs, the semiconducting band gap decreases when the concentration of Li atoms increases; whereas the H-SiNWs become metallic even with the addition of only one interstitial Li atom. The formation energy diminishes with the concentration of Li atoms for surface-lithiated H-SiNWs, whereas the contrary behavior is found in the interstitial-lithiated H-SiNWs. Furthermore, for the surface-lithiation case, the Li binding energy reveals the existence of SiLi bonds, whereas for the interstitial-lithiation case, the Li binding energy increases when the Li grows up to a critical concentration, where some SiSi bonds break. Finally, for the case of surface-lithiation, the Young's modulus (Y) increases with the concentration of Li, whereas for the interstitial-lithiation case, Y suffers a sudden diminution at a certain Li concentration due to the large internal mechanical stresses within the nanowire structure. These results should be considered when regarding H-SiNWs as potential electrodes in Li-ion battery anodes.

Theoretical study of the mechanical and electronic properties of [111]-Si nanowires with interstitial lithium

In this work, we present a density functional study of the Young’s modulus and electronic properties of hydrogen passivated silicon nanowires (H-SiNWs) grown along [111] crystallographic direction as function of concentration of interstitial lithium (Li) atoms. The study is performed using the supercell scheme, within the local density approximation implemented in the SIESTA code. The results show that the presence of Li closes the known semiconductor band gap of the H-SiNWs showing a like metallic behavior even when just one Li atom is placed in the nanowire structure. The participation of the Li atoms in the electronic density of states is almost constant in the valence and conduction bands. The formation energy analysis show how the system loses energetic stability when the concentration of Li grows, while the binding energy per Li atom suggests the formation of Si–Li bonds. On the other hand, the Young’s modulus of the silicon nanowires (SiNWs) is higher than that of the H-SiNW and lower than the bulk value. Moreover, the Young’s modulus is almost constant independently of the Li concentration. This result indicates that the H-SiNWs support the internal stress due to the addition of Li atoms and could offer a better useful life as electrodes in Li-ion batteries. The results of this work help to understand how the electronic and mechanical properties of H-SiNWs change during the charge/discharge process and the possibility to incorporate them as electrodes in Li batteries.

Carbon monoxide sensing properties of B-, Al- and Ga-doped Si nanowires

Silicon nanowires (SiNWs) are considered as potential chemical sensors due to their large surface-to-volume ratio and their possible integration into arrays for nanotechnological applications. Detection of harmful gases like CO has been experimentally demonstrated, however, the influence of doping on the sensing capacity of SiNWs has not yet been reported. For this work, we theoretically studied the surface adsorption of a CO molecule on hydrogen-passivated SiNWs grown along the [111] crystallographic direction and compared it with the adsorption of other molecules such as NO, and O2. Three nanowire diameters and three dopant elements (B, Al and Ga) were considered, and calculations were done within the density functional theory framework. The results indicate that CO molecules are more strongly adsorbed on the doped SiNW than on the pristine SiNW. The following trend was observed for the CO adsorption energies: EA[B-doped] > EA[Al-doped] > EA[Ga-doped] > EA[undoped], for all diameters. The electronic charge transfers between the SiNWs and the adsorbed CO were estimated by using a Voronoi population analysis. The CO adsorbed onto the undoped SiNWs has an electron-acceptor character, while the CO adsorbed onto the B-, Al-, and Ga-doped SiNWs exhibits an electron-donor character. Comparing these results with the ones obtained for the NO and O2 adsorption, the larger CO adsorption energy on B-doped SiNWs indicates their good selectivity towards CO. These results suggest that SiNW-based sensors of toxic gases could represent a clear and advantageous application of nanotechnology in the improvement of human quality of life.

Band-gap engineering of halogenated silicon nanowires through molecular doping.

In this work, we address the effects of molecular doping on the electronic properties of fluorinated and chlorinated silicon nanowires (SiNWs), in comparison with those corresponding to hydrogen-passivated SiNWs. Adsorption of n-type dopant molecules on hydrogenated and halogenated SiNWs and their chemisorption energies, formation energies, and electronic band gap are studied by using density functional theory calculations. The results show that there are considerable charge transfers and strong covalent interactions between the dopant molecules and the SiNWs. Moreover, the results show that the energy band gap of SiNWs changes due to chemical surface doping and it can be further tuned by surface passivation. We conclude that a molecular based ex-situ doping, where molecules are adsorbed on the surface of the SiNW, can be an alternative path to conventional doping. Graphical abstract Molecular doping of halogenated silicon nanowires.

Functionalization of silicon nanowires by conductive and non-conductive polymers

The work reports on the development of hybrid devices based on silicon nanowires (SiNW) with polymers and the difference obtained when using conductive and non-conductive polymers. SiNW have attracted much attention due to their importance in understanding the fundamental properties at low dimensionality as well as their potential application in nanoscale devices as in field effect transistors, chemical or biological sensors, battery electrodes and photovoltaics. SiNW arrays were formed using metal assisted chemical etching method. This process is simple, fast and allows obtaining a wide range of silicon nanostructures. Hydrogen-passivated SiNW surfaces show relatively poor stability. Surface modification with organic species confers the desired stability and enhances the surface properties. For this reason, this work proposes a covalent grafting of organic material onto SiNW surface. We have chosen a non-conductive polymer polyvinylpyrrolidone (PVP) and conductive polymers polythiophene (PTh) and polypyrrole (PPy), in order to evaluate the electric effect of the polymers on the obtained materials. The hybrid structures were elaborated by the polymerization of the corresponding conjugated monomers by electrochemical route; this electropolymerization offers several advantages such as simplicity and rapidity. SiNW functionalization by conductive polymers has shown to have a huge effect on the electrical mobility. Hybrid surface morphologies were characterized by scanning electron microscopy (SEM), infrared spectroscopy (FTIR-ATR) and contact angle measurements.

Enhanced multiple exciton generation in amorphous silicon nanowires and films

ABSTRACTMultiple exciton generation (MEG) in nanometer-sized hydrogen-passivated silicon nanowires (NWs), and quasi two-dimensional nanofilms depends strongly on the degree of the core structural disorder as shown by the perturbative many-body quantum mechanics calculations based on the density functional theory simulations. Working to the second order in the electron–photon coupling and in the screened Coulomb interaction, we calculate quantum efficiency (QE), the average number of excitons created by a single absorbed photon, in the Si29H36 quantum dots (QDs) with crystalline and amorphous core structures, simple cubic three-dimensional arrays constructed from these QDs, crystalline and amorphous NWs, and quasi two-dimensional silicon nanofilms, also both crystalline and amorphous. Efficient MEG with QE ranging from 1.3 up to 1.8 at the photon energy of about 3Eg, where Eg is the electronic gap, is predicted in these nanoparticles except for the crystalline NW and crystalline film where QE ≃ 1. MEG in the amorphous nanoparticles is enhanced by the electron localisation due to structural disorder. Combined with the lower gaps, the nanometer-sized amorphous silicon NWs and films are predicted to have effective carrier multiplication within the solar spectrum range.

Electronic Properties of Pristine and Se Doped [001] Silicon Nanowires: An Ab Initio Study

We report the results of first-principles density functional theory calculations of the properties of pristine and Se doped hydrogen passivated silicon nanowires oriented along the [001] direction. We show that the effect of confinement in the orthogonal directions is to increase the bandgap as the diameter of the nanowire is progressively reduced, as expected from the simple quantum confinement model. Moreover, we observe that the introduction of surface reconstruction before H passivation can lead to an indirect bandgap due to a shift of the valence band maximum away from the convergence point. We also study the formation energy and hyperfine parameters of Se located in different positions in a nanowire with diameter of 0.8 nm. Our results confirms a tendency to surface segregation for Se atoms, probably driven by deformations close to the nanowire surface. These deformations cause also a steep decrease in the hyperfine contact term of Se in surface positions compared to the situation when Se is placed near the axis of the nanowire. The latter values of the hyperfine contact term are instead much greater than the corresponding bulk values.

Computing Raman and infrared wavenumbers of nanostructures: application to silicon nanowires

The characterization of nanostructures with spectroscopic methods is a fundamental tool in nanoscience. For novel nanostructures, the interpretation of spectral features is a challenging task. To address this issue, we present the “Symmetry‐Filtered Molecular Dynamics (SFMD)” method to calculate Raman and infrared wavenumbers from molecular dynamics (MD) simulations, employing only the symmetry of the atomic structure. Explicit and expensive calculations of the electric polarizability or the dipole moment are not required. Therefore, our method can be easily used with any standard MD software. On the basis of the density functional tight‐binding method for the MD simulations, we apply our method to bulk silicon and small‐diameter hydrogen‐passivated silicon nanowires. For bulk silicon, we study the wavenumber shift of the Raman peak with temperature and obtain results that are in good agreement with experiments. We further show that thermal lattice expansion is a minor effect (22%) and that temperature‐driven anharmonic effects (78%) are the main contributions to that wavenumber shift. By analyzing the bond lengths of different silicon nanowires, we found that surface stress manifests as a 0.37% shortening of bonds only in the outermost silicon layer. We further analyzed the diameter‐dependent wavenumber shift of a Raman peak in silicon nanowires. We found that the main contribution to the wavenumber shift comes from the phonon confinement effect and surface stress leads to an additional shift of 9–22%. Our results indicate that our method is able to produce quantitative results that can be compared with experiments. We propose our method to be used for the understanding of Raman and infrared spectra of novel bulk and nanostructures. Copyright © 2012 John Wiley & Sons, Ltd.

Strain-Modulated Half-Metallic Properties of Carbon-Doped Silicon Nanowires with Single Surface Dangling Bonds

The electronic and magnetic properties of hydrogen-passivated silicon nanowires (SiNWs) doped with an interstitial carbon atom are investigated by spin polarized first-principles calculations. It i...

Water adsorption on hydrogen-passivated silicon nanowires from density functional theory with dispersion correction

Density functional theory with semiempirical dispersion correction has been applied to investigate water monomer adsorption onto the surface of hydrogen-passivated silicon nanowires (h-SiNWs) of different shapes and growth orientations. We find that water adsorption is in the weak physisorption limit, leading to a hydrophobic surface. The adsorption site and orientation are determined by electrostatic interactions involving both the surface and interior atoms of the h-SiNWs and are not affected qualitatively by including the dispersion correction. The barrier for water diffusing on the surface of h-SiNWs depends sensitively on the nanowire facet and growth orientation. The barrier for water rotation is small, which may facilitate water cluster formation on the h-SiNWs.

Site-selected doping in silicon nanowires by an external electric field

The properties of dopant-related defects in silicon nanowires are key characteristics in semiconductive devices. Our first-principles calculations predicted that the preferred doping sites of B and P atoms in hydrogen-passivated silicon nanowires have opposite distribution behavior under electric field, suggesting a steady intrinsic p-n junction can be spontaneously formed in (B and P) codoped silicon nanowires.

Prediction of surface passivation doping of silicon nanowires with phosphorus

We report a prediction of enhanced surface passivation doping effect in silicon nanowires (SiNWs) by phosphorus adsorption based on first-principles calculations. Recent theoretical and experimental studies all showed that hydrogen-passivated SiNWs present typical p-type characteristic due to charge transfer between the surface passivant and the SiNW core. Here, we show that a phosphorus-passivated SiNW with a moderate diameter facilitates improved hole generation in the core and efficient separation of electron and hole, which may provide a practical avenue for fabricating low cost solar cells with high efficiency.

Impacts of size and cross-sectional shape on surface lattice constant and electron effective mass of silicon nanowires

We investigate the surface lattice and electronic structure of [110] oriented hydrogen-passivated silicon nanowires (SiNWs) of different cross-sectional shapes by using the first-principles tight-binding method. Remarkable quantum confinement effects are observed on the surface lattice constant and the electron effective mass. Moreover, with the same cross-sectional area, the triangular-SiNW has more obvious size dependence than rectangular-SiNW. The significant size and cross-sectional shape effects are explained by the concept of surface-to-volume ratio. Our results demonstrate that due to the smaller electron effective mass, the rectangular-SiNW has obvious advantage over triangular-SiNW in application in higher on current in SiNW transistor.

A Universal Expression of Band Gap for Silicon Nanowires of Different Cross-Section Geometries

We use the first-principles tight binding method to investigate the electronic structure and band gap of [110] oriented hydrogen-passivated silicon nanowires (SiNWs) of different cross-sectional geometries. A quantitative universal band gap expression for [110] SiNWs is obtained, which shows a linear dependence of band gap on the surface area to volume ratio (SVR), and it is independent of the cross-sectional geometry. In contrast to the ambiguity in the definition of the SINW transverse dimension, using of SVR has the advantage to readily predict band gap for SiNWs with any cross-sectional shapes. Our results demonstrate that the SVR is an ideal "gauge" to describe the band gap of SiNWs.

Formation of Dopant-Pair Defects and Doping Efficiency in B- and P-Doped Silicon Nanowires

First-principles calculations are performed to investigate the stability of dopant-related defects and the dependence of doping efficiency on wire diameter and orientation in hydrogen-passivated silicon nanowires doped with B and P dopants. As the diameter decreases below a critical value, it is energetically more favorable for donor atoms to form donor-pair defects, which consist of two donors separated at the nearest-neighbor distance. While donor-pair defects are unstable in bulk Si, the stability of these defects is greatly enhanced because of the confinement effect in nanostructures, which leads to the increase of band gap and thereby the shallow level of a substitutional donor. As donor-pair defects are electrically inactive defects, the doping efficiency is expected to be suppressed in small-diameter wires, regardless of the presence of surface or interface dangling-bond defects which were previously proposed to be the compensating defects. In the case of B dopants, the formation of pair defects is unfavorable against shallow acceptor levels, in contrast to n-type dopants, without affecting the doping efficiency.

Strain induced change of bandgap and effective mass in silicon nanowires

This work computationally investigates the electromechanical properties of hydrogen passivated silicon nanowires under uniaxial tensile strain. It has been observed that bandgap changes can be as large as 60 and 100 meV per 1% axial strain for [100] and [110] nanowires, respectively. This rate of change in the bandgap is independent of nanowire size and depends only on the growth direction. More importantly, the nature of the bandgap can reversibly change from indirect to direct as a function of strain. It is also observed that for larger diameter nanowires, the indirect-to-direct transition occurs at smaller compressive strain.

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.

Size and orientation dependence in the electronic properties of silicon nanowires

By using first-principles pseudopotential methods, we have studied the electronic properties of hydrogen-passivated silicon nanowires along the [100], [110], and [111] directions with diameter up to $3.4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. It is found that as the diameter decreases, the energy band gaps are distinctly enlarged due to the confinement effect. The valence-band maximum moves down while the conduction-band minimum moves up compared with the bulk. By using the many-body perturbation theory within the $GW$ approximation, we have also investigated the self-energy correction to the energy band gaps. Our calculational results show that, although the band gap values strongly depend on both the diameter and orientation, the $GW$ corrections are mainly dependent on diameter and less sensitive to the growth orientation. The effective mass as a function of diameter is also discussed.

Effect of growth orientation and surface roughness on electron transport in silicon nanowires

We report a study of the effect of growth orientation and surface roughness on electron transport in small-diameter hydrogen passivated silicon nanowires (NWs). We employ a nonequilibrium Green's function technique within an $s{p}^{3}{d}^{5}{s}^{*}$ tight-binding approximation to show that band structure strongly affects current-voltage characteristics of ideal NWs, leading to current falloff at high drain bias for certain growth orientations. Surface roughness suppresses small bias conductance and current, and leads to a nonmonotonic dependence on gate bias. We also find that surface roughness results in a decrease of current with the length of a NW. The rate of the decrease depends on the growth directions and diameter. As the diameter of a NW becomes smaller, a transition of electron transport to Anderson localization regime may occur even at room temperature.