Armchair Silicon Research Articles

Effect of edge dual-hydrogenation on electronic and magnetic properties of armchair silicon carbide nanoribbons

Using the first-principles calculations, we investigate the electronic and magnetic properties of armchair silicon carbide nanoribbon (aSiCNR) with different combinations of edge dual-hydrogenation. The dual-hydrogenation on the boundary Si or C atom changes it from sp2 hybridization to sp3 hybridization, which will have an important role on the stability of aSiCNR. Only the full dual-hydrogenation on the one edge or two edges don't change the band structure and magnetic moment of aSiCNR. However, the other different combinations of edge dual-hydrogenation can result in aSiCNR exhibiting metallic, semiconductor, and half-metallic properties under non-magnetism state, ferromagnetic and anti-ferromagnetic states. These results may present a new avenue for band engineering of aSiCNR, as well as valuable suggestions for the practical application of SiC based nanomaterials in spintronics and multifunctional nanodevices.

Variation in properties of double-walled zigzag and armchair silicon nanotubes depending on SW defects and applied electric fields by SCC-DFTB method

The geometric structure and electrical properties of the zigzag DWSiNT (8,0)@(12,0) and armchair DWSiNT (5,5)@(7,7) perfect tubes with the same diameter are simulated by self-consistent charge density functional tight binding method (SCC-DFTB). We considered the defective tubes by introducing three types of Stone-Wales (SW) defects, and further investigated the impact of both strong and weak electric fields on these defective tubes. Calculations demonstrate that the atomic arrangement regularity, degree of buckling, stability, energy gap, and charge distribution strongly depend on the type of perfect tubes. Following the introduction of the SW defects, the symmetry of the tubes disappeared, and there is a obvious atomic aggregation occurring at the defects. The armchair tubes demonstrate a higher stability compared to zigzag tubes and all exhibit metallic properties. SWⅡ and SWⅢ defects in zigzag tubes result in a transition from semiconductor to semi-metallic properties. The number of charge transfer increases, and the degree of atomic aggregation is more dispersed. The effect of applied electric field strength on perfect zigzag tubes is more obvious. The weak field has minimally impacted on the stability, and the strong field makes the stability improved to some extent. The presence of a strong field caused the energy gap values of (8,0)@(12,0) tube decrease sharply, and all tubes are transformed into semi-metal or metal. The weak electric field has a more pronounced regulation of band gap within the range of 0 V/nm - 0.3 V/nm on the inner tube. The consequences of this investigation can certainly be helpful in future experimental studies.

Studies of the hydrogen energy storage potentials of Fe- and Al-doped silicon carbide nanotubes (SiCNTs) by optical adsorption spectra analysis

This work investigated the hydrogen adsorption potentials of the Fe-doped (magnetic) and Al-doped (nonmagnetic) armchair silicon carbide nanotubes (SiCNTs) as candidates for hydrogen storage materials. Calculations of the electronic transport properties of the investigated systems were performed using the popular density functional theory as implemented in quantum ESPRESSO while corrections to quasi-particle energies were performed using Yambo codes within many-body perturbation theory. The obtained results of the structural properties revealed that pristine single-walled SiCNT was found to be more stable due to its higher binding energy of −393.85 eV while less stability was recorded with Al-doped SiCNT having binding energy of −393.81 eV. Furthermore, the stability of the Fe-doped SiC nanotube was found to be normal between bond lengths 1.73 Å to 1.79 Å, indicating its great potential to accommodate more Fe dopants. In terms of electronic properties, the Fe-doped SiC nanotube demonstrated half-metallic and half-monometallic properties. Ferromagnetic features were also observed for the Fe-doped SWSiCNT when Fe replaced the Si atoms, while antiferromagnetic features were observed when Fe replaced the C atoms. The results obtained from optical spectra analysis indicated that Fe-doped SiCNT adsorbed hydrogen only at 1.8 eV which is the lower part of the visible spectrum while Al-doped SiCNT adsorbed hydrogen at energy levels 0 eV, 2.8 eV and 7.1 eV which extended from visible to UV regions. Therefore, Al-doped SiCNT is recommended as a better candidate for hydrogen storage under ambient conditions.

Armchair silicon carbide nanoribbon for potential anode material in lithium-ion batteries (LIBs).

In this work, we have investigated the electrochemical characteristics of armchair silicon carbide nanoribbon (ASiCNR) for its potential deployment as 2D lithium-ion battery anode material. Density functional theory approach is used to calculate the adsorption energy, storage capacity, and open circuit voltage of ASiCNR for LIB. Adsorption of Li atoms introduces the new energy bands which cross the Fermi level; this results in semiconductor to metallic transition of ASiCNR. It indicates the strong interaction of Li atoms towards the ASiCNR. When adsorption of Li atoms increases one by one, the adsorption energy (E[Formula: see text]) per Li atoms increases gradually. When all favourable sites are adsorbed by Li atoms E[Formula: see text] reached its maximum value and it results in maximum storage capacity of 818 mAhg[Formula: see text] and open circuit voltage of 1.15 V. Diffusion barrier of Li atoms for the substrate is 0.42 eV. Our computational results suggest that ASiCNR can be used as an anode material for Li-ion batteries, and it provides the theoretical background for the future study on ASiCNR and other Li storage structures.

Ab-initio analysis of Li adsorption on boron doped armchair silicon carbide nanoribbon for lithium ion batteries (LIBs)

It is well known that the electrochemical storage capacity of anode materials can be modified by the doping of heteroatom. Here, first-principles approach is used to investigate the adsorption energy (Eads), open circuit voltage (OCV), and storage capacity of boron co-doped armchair silicon carbide anode (B-ASiCNR) material for Li ion battery. When B atoms are doped at the center of the armchair silicon carbide nanoribbon (ASiCNR), new energy states emerge and it results in Fermi-Level shift towards the valence band. The binding strength of a single Li atom is significantly higher in B-ASiCNR because of B atom doping. The open circuit voltage decreases from 3.76 V to 2.34 V when Li atoms adsorbed from first hexagonal position to all hole position of the substrate. Calculations reveal that the storage capacity of B-ASiCNR reached to the maximum value of 836 mAhg−1 when all hole positions are adsorbed by Li atoms. Calculated Eads, OCV, storage capacity suggest that the B-ASiCNR can be used as future anode materials for Li ion battery.

Investigation of structural and electronics properties of boron co-doped silicon carbide nanoribbons

In this analysis, we investigated the structural and electronic properties of boron-doped armchair silicon carbide nanoribbons (B-doped ASiCNRs). By making use ofdensity functional theory (DFT) approachthe binding energy (BE), density of state (DOS), energy band structure, and bond length are computed. Calculation shows that B-doped ASiCNRs have strong structural stability as compared to ASiCNR. Before doping, the energy band-gap of armchair silicon carbide nanoribbons is relatively higher than that of the boron-doped armchair silicon carbide nanoribbons. It is found that when boron (B) atoms are doped in the center, the Fermi level shifts towards the valence band. This indicates that the doping of B atoms results in emerging of new energy bands which transform semiconductor to the P-type nature of B-doped ASiCNRs. Our findings give a theoretical framework for adjusting the electronic characteristics of B-doped ASiCNRs. It can be used in future nanoelectronic devices, etc.

Perfect spin filtering of T-shaped device based on the zigzag silicon carbide nanoribbons

How to generate stable and pure spin-polarized current in nanodevice based on low-dimensional magnetic materials and systems has attracted extensive attention. In this work, using the non-equilibrium Green’s function (NEGF) combined with density-functional theory (DFT), we predict highly conductive transport and perfect spin filtering of the novel T-shaped device, which is constructed by a finite armchair silicon carbide nanoribbon (ASiCNR) connected to one edge of an infinite zigzag silicon carbide nanoribbon (ZSiCNR). This particular performance is due to the fact that there are always two edge states contributing to electron transport around the Fermi level, no matter whether the ZSiCNR is in the ferromagnetic or anti-ferromagnetic state. As a consequence, the T-shaped device would ideally yield 100% spin-polarized current with one spin channel blocked by the ASiCNR part. Moreover, it is found that such blocking effect is independent of the shape of the ASiCNR part. Based on the above analysis, it is easily predicted that such pure spin-polarized current could also be obtained in the device constructed by ZSiCNRs, as long as one pristine edge of ZSiCNRs is defected by doping, absorbing, vacancies and so on, which opens new possibility of two dimensional silicon carbide in spintronic device applications.

Electronic and optical properties of armchair silicon carbide nanotubes from first principles

We calculate the electronic band gap and optical spectra of armchair (n, n) silicon carbide nanotubes (n = 9–12) using density functional theory (DFT) combined with Non equilibrium Green's function (NEGF) method as implemented in the SIESTA code. Our simulation suggests that the armchair SiCNTs are indirect-gap semiconductors and SiCNTs (11, 11) have the largest electronic band gap. The z component of the refractive index of SiCNTs (9, 9) reaches its zenith at energies below 2.5 eV. Moreover, the optical absorption takes place in the energy range ∼2−5 eV for the z polarization. The simulated armchair SiCNTs with larger diameter have the smaller first optical band gap as well.

Separation of cyanide from an aqueous solution using armchair silicon carbide nanotubes: insights from molecular dynamics simulations

Separation of cyanide, as a model contaminant, from aqueous solution was investigated using molecular dynamics simulations.

Molecular dynamics simulation of salt rejection through silicon carbide nanotubes as a nanostructure membrane

Salt rejection phenomenon was investigated using armchair silicon carbide (SiC) nanotubes under applied electric fields. The systems included the (7,7) and (8,8) SiC nanotubes surrounded by silicon nitride membrane immersed in a 0.4mol/L aqueous solution of sodium chloride. Results of molecular dynamics (MD) simulations for selective separation of Na+ and Cl− ions showed that the (7,7) SiC nanotube is suitable for separation of cations and the (8,8) SiC nanotube can be used for separating anions. The water desalination by SiC nanotubes was demonstrated by potential of mean force for Na+ and Cl− ions in each SiC nanotube. Furthermore, the ionic current, ion residence time, and the radial distribution functions of species were measured to evaluate the properties of the system. Based on the results of this research, the studied SiC nanotubes can be recommended as a nanostructure model for water desalination.

Layer effects on electronic structures of multi-walled armchair silicon carbide nanotubes

The electronic structures of triple-, quadruple- and quintuple-walled armchair silicon carbide nanotubes (SiCNTs) are investigated using first-principle calculations based on dispersion-corrected density functional theory. Band shifts narrow the band gaps of multi-walled SiCNTs and form significant coupling in different layers of the nanotubes, which originate from the differences in the work functions and band gaps of the individual layers. With the increase of the layer number of the multi-walled SiCNT, the similarity in the electronic structures of the two outer layers is increased and the influence of the band shifts is weakened. Therefore, the electronic properties of SiCNTs formed with more than three layers are largely independent of their layer number.

Separation of nitrate ion from water using silicon carbide nanotubes as a membrane: Insights from molecular dynamics simulation

Molecular dynamics (MD) simulations were performed to investigate the separation of nitrate ion as contaminant from aqueous solution by means of armchair silicon carbide (SiC) nanotubes. The (7,7) and (8,8) SiC nanotubes embedded in a silicon nitride (SiN) membrane as a membrane immersed in an aqueous solution of NaNO3. An external electric field was applied to the system along the z axis of the simulation cell. In order to investigate these systems, we calculated the ion current, the radial distribution function of water-nanotube and ion-water, the retention time of ions, water density, the hydrogen bond of water, and the autocorrelation function of the hydrogen bond. The results showed that the considered armchair SiC nanotubes can be used for separating nitrate ion from water. The results revealed that nitrate ions were separated successfully using the (8,8) SiC nanotube, though these ions did not pass through the (7,7) SiC nanotube.

The structural evolution process and the electronic properties of armchair silicon nanotubes

The structural evolution process of the capped armchair single- and double-walled SiNTs grown from silicon clusters was investigated using the DFT method. The evolution process was described quantitatively by monitoring change of the geometry structures. The initial structural configuration of the single- and double-walled SiNTs was determined by optimizing structure of the small silicon clusters. The evolution process of the SWSiNTs is through forming tubular clusters with a global reconstruction from structure of the double-rings. Then, it elongates through the layer-by-layer growth process with local reconstructions. Eventually, the infinite SiNTs can be constructed with corresponding repeat unit, designed by the periodic characteristics on the basis of tubular clusters. Eventually, All of the SiNTs have a narrow band gap. From calculation of band structure, the band gap which occurs oscillations and gradually decreases with increase of the diameter, length, and the number of walls.

Optical properties of F- and H-terminated armchair silicon nanoribbons**Project supported by the Henan Provincial Joint Funds of the National Natural Science Foundation of China (Grant Nos. U1404608 and U1304612), the Science and Technology Key Projects of Henan Province, China (Grant No. 1502102210124), and the Special Fund for Theoretical Science Foundation of Nanyang Normal University, China (Grant No. ZX2013018).

The optical properties of F- and H-terminated silicon nanoribbons with armchair edges (F- and H- terminated ASiNRs) are compared by using the first-principles within the density function theory (DFT) framework. The results show that compared with for H-terminated 7-ASiNR, the dielectric function for the F-terminated 7-ASiNR has a red shift, which is mainly attributed to the narrower band gap because of the σ–π mixing effect of F–Si bonds in F-terminated 7-ASiNR. The peaks in the energy loss spectra for both systems represent the characteristics associated with the plasma resonance, which correspond to the trailing edges in the reflection spectra. These properties show that the different terminated atoms in 7-ASiNRs affect mainly the optical properties in the low energy range. Because of the rich optical properties, the 7-ASiNR could be a potential candidate for photoelectric nanodevice.

Electronic Structures and Optoelectronic Properties of C/Ge-doped Silicon Nanotubes

The electronic structures and optoelectronic properties of C/Ge-doped single-walled armchair silicon nanotubes were determined by using first-principles calculations in the framework of density-functional theory. In particular, the calculated results indicate that both pure and C/Ge-doped silicon nanotubes display a direct band gap. The band gap is decreased firstly and then increased for the silicon nanotubes doped with C and Ge, respectively. Moreover, the upper of valence band is mainly contributed by the Si-3p states and the lower of conduction band is primarily occupied by the Si-3p states. The state dielectric constant is improved and the optical absorption shows a red-shifted for the C-doped silicon nanotubes, whereas the state dielectric constant is reduced and the optical absorption shows a blue-shifted for the Ge-doped silicon nanotubes. The results provide an useful theoretical guidance for the applications of single-walled armchair silicon nanotubes in optical detectors.

The electronic structure and optical properties of P-doped silicon nanotubes

We perform first-principles calculations in the framework of density-functional theory to determine the effects of P doping on the electronic structure and optical properties of single-walled armchair silicon nanotubes. The calculated results indicate that the band-gap of single-walled armchair silicon nanotubes changes from indirect to direct one, with the P element doped. The top of valence band is determined mainly by the Si-3p electrons, and the bottom of conduction band is occupied by the Si-3p electrons and Si-3s electrons. Moreover, the band gap of single-walled armchair silicon nanotubes decreases and the optical absorption is red-shifted, with the P element doped. The results provide useful theoretical guidance for the applications of silicon nanotubes in optical detectors.

Structural and electronic properties of SiC nanotubes filled with Cu nanowires: A first-principles study

The structural and electronic properties of Cun (n=5,9,13) nanowires encapsulated in armchair (8,8) silicon carbon nanotubes (SiCNTs) are investigated using the first principles calculations within the generalized-gradient approximation. We find that the formation processes of these systems are all exothermic. The initial shapes (quadratic–prismatic Cu wire and cylindrical (8,8) SiCNT) are preserved without any visible changes after optimization for the Cu5@(8,8) and Cu9@(8,8) combined systems, but a quadratic-like cross-section shape is formed for the outer nanotube of the Cu13@(8,8) combined system due to the stronger interaction between nanowire and nanotube. The electrons for Si and C atoms in outer SiC sheath affect the electron conductance of the encapsulated metallic nanowire in the Cu13@(8,8) combined system. But in the Cu5@(8,8) and Cu9@(8,8) combined systems, the conduction electrons are distributed only on the copper atoms, so electron transport will occur only through the inner Cu nanowires and the outer inert SiCNTs only function as insulating cable sheaths. Considering the maximal metal filling ratio in nanotube, we know that the Cu9@(8,8) combined system is top-priority in the ultra-large-scale integration (ULSI) circuits and micro-electromechanical systems (MEMS) devices that demand steady transport of electrons.

Molecular hydrogen and oxygen interactions with armchair Si nanotubes

Molecular hydrogen and oxygen adsorptions on a (6, 6) armchair silicon nanotube have been studied by optimizing the distances of the admolecules from both inside and outside the tube. Full geometry and spin optimizations have been performed without any symmetry constraints with an all electron 3-21G* basis set and the B3LYP functional. The molecule is originally placed perpendicular or parallel to the tube axis. Hydrogen adsorption with the molecular axis aligned parallel to the surface of the nanotube is less favorable. Hydrogen molecule does not dissociate while oxygen molecule dissociates after optimization. The on-top site is the only preferred site for hydrogen molecule with an adsorption energy of 3.71 eV and an optimized distance of 3.31 for external adsorption whereas the on-top site is the most preferred site with adsorption energy of 3.69 eV for internal adsorption. For oxygen, the molecule dissociates and the most preferred sites are the two bridge sites with an adsorption energy of 9.64 eV, the optimized distance being 1.65/1.68 A when it is adsorbed from the outside of the tube. When oxygen molecule is originally placed at on-top site it will hold as a molecule after adsorption with a slightly increased bond length. For the internal adsorption of oxygen, the molecules also dissociate in most cases and the zigzag bridge site is the most preferred site. After molecular adsorption for both hydrogen and oxygen, the buckling of the nanotubes increased. Frustration effects in the nanotube due to molecular adsorption are also noted.

Existence and stability of co-axial and meshed double-walled armchair silicon nanotubes

A systematic study of armchair double-walled Si nanotubes (DWNT) (n, n)@(m, m) (3≤n≤6; 7≤m≤12) using the finite cluster approximation is presented. The geometries of the tubes have been spin optimized with an all electron 3–21G⁎ basis set and the B3LYP functional. Analysis of the electronic structure properties of these tubes has also been performed with a larger basis set. The study indicates that the stabilities of the Si nanotubes are of the same order as those of single-walled Si nanotubes. It should be possible to experimentally synthesis both single-walled and double-walled Si nanotubes. The binding energy per atom or the cohesive energy of the nanotubes depends not only on the number of atoms but also on the coupling of the constituent single-walled nanotubes. Nanotubes with small interlayer separations, called meshed tubes, do not hold the coaxial cylindrical structure after optimization. The SiNTs (n, n)@(n+3, n+3) are found to have large formation energies and binding energies per atom. For example, (3, 3)@(6, 6), (4, 4)@(7, 7), (5, 5)@(8, 8), and (6, 6)@(9, 9) all have large binding energies per atom, around 3.7eV/atom. All Si nanotubes are found to be semiconductors. However, the band gap, in general, is observed to decrease from single walled nanotubes to double walled nanotubes.

The structural and electronic properties of silicon nanoribbons on Ag(110): A first principles study

Abstract We have investigated the structures of silicon nanoribbons on Ag(110) using first principles calculations. The armchair silicon nanoribbons (ASiNRs) and zigzag silicon nanoribbons (ZSiNRs) with different widths are analyzed. The formation energy study shows that the ASiNRs with the width of 16 A are the most stable structures. These ASiNRs have the structural parameters same as experimental ones. The simulated scanning tunneling microscope (STM) images of these ASiNRs also agree well with the experimental results. Thus, these ASiNRs are supposed to be the nanoribbons grown in experiment. The electronic structures shows that the ASiNRs are metallic, which is in agreement with the experiments.