Single-walled Silicon Nanotubes Research Articles

Effect of vacancy defect on the structural and electrical properties of single-walled silicon nanotube

The effect of vacancy defects on zigzag and armchair single-walled silicon nanotubes (SiNTs) has been investigated using the self-consistent charge density functional tight-binding (SCC-DFTB) method. The findings indicate that the introduction of defects leads to the inability of the smaller-diameter tube to uphold its tubular structure and cluster. Conversely, the larger-diameter tube undergoes an elliptical transformation, yet retains tubular characteristics and hexagonal structures displaying specific symmetry. The double vacancy defect alters the atomic arrangement on the side opposite the defect. The stability of single-walled SiNTs is related to defect type and chiral index. In addition, the single vacancy defect increases the binding energy and stability of the zigzag tube, while causing a rapid decrease in stability of the armchair tube (7,7). The spin polarization phenomenon manifests within the defect tube, resulting in the splitting of the initial degenerate energy band and a significant decrease in its degeneracy. The existence of electronic interaction, results in a prominent manifestation of vacancy-induced magnetism in armchair and zigzag defect tubes. This phenomenon gives rise to localized spin-up bands situated below the Fermi level, as well as localized spin-down bands positioned above the Fermi level. In the energy band structure of most nanotubes, there are conduction bands or valence bands passing through the Fermi level, leading to a transition from semiconductor to semimetal or metal properties.

Influence of Stone-Wales defects on the structural and electronic properties of single-walled silicon nanotubes by SCC-DFTB calculations

In this work, using the self-consistent charge density functional tight-binding (SCC-DFTB) approach, we explored the effects of Stone-Wales (SW) defects on the structure and electronic properties of perfect single-walled silicon nanotubes(SWSiNTs), including zigzag (n1,0) (8≤n1≤16), armchair (n2,n2) (5≤n2≤10), and chiral (2n3,n3) (3≤n3≤8). We found that the presence of SW defects leads to changes in bond length and bond angle, resulting in structural changes at the defect site. However, nanotubes still exhibit tubular and hexagonal structures. Also, the defect structure of zigzag and armchair SW-II defective tubes is symmetrical, which means that the bond length of the Si-Si bond is equivalent to that of its symmetric bond. Besides, after introducing the SW-II defect, zigzag and chiral tubes become more stable, but the stability of armchair tubes with small diameter is reduced. The introduction of SW-Ⅰ and SW-Ⅱ defects transforms armchair SiNTs from semiconductor into semi-metallic properties. Moreover, for a perfect structure, the charge moves from inside out along the radial direction of the tube and the amount of charge is distributed in an axisymmetric manner. After introducing SW defects, the range of charge transfer is expanded, SW-Ⅱ defective structures appear inward deviation, and the atoms near the defect change from regular arrangement to inward aggregation. This work may provide a new way to adjust the electronic properties of SWSiNTs and open up wider applications in nanoelectronic devices.

Structures and electronic properties of single-walled silicon nanotubes (6,m) under external electric field by SCC-DFTB calculations

In this work, using the self-consistent charge density functional tight-binding(SCC-DFTB) approach, we studied essential properties of SWSiNTs (6,m) and effects of perpendicular electronic field and strengths on electronic properties. After atomic relaxation, the structure of the tube changes obviously and different degrees of warping appear. The wall buckling distance is determined by the distance between outward and inward maximum values of the atom. The deviation of atomic radial displacement obviously affects the amount of atomic charge transfer, which increases with the increase of degree of buckling structure. The three primitive tubes are direct band gap semiconductors, among which the armchair tube has the highest binding energy and the most stable structure. As the applied electric field strength increases, the energy separation between adjacent subbands increases, causing a semiconductor-metal transition. The effect of electric field on the atomic charge redistribution in the tube is positively correlated with the warpage of the tube.

The possible structure and electronic structure of zigzag silicon nanotubes doped with group V elements

We performed spin-polarized density functional theory (DFT) calculations to study the stable structures and electronic structure of X-doped (X = N, P, As, Sb and Bi) single-walled silicon nanotubes (SiNTs). Intrinsic SiNTs with small diameters can exist stably and maintain a smooth surface structure. However, due to the addition of impurities, small-diameter nanotubes cannot maintain a stable tube structure. In addition to Bi, the doping of group five elements can effectively improve the stability of (14, 0) SiNTs. Moreover, the addition of group V elements makes SiNTs produce impurity levels at the Fermi level, thereby reducing the band gap. The doping of P, As, Sb, and Bi even led to the transformation of SiNTs from semiconductors to metals, and the energy level shift appears in N-doped SiNTs.

Longitudinal, Transverse, and Torsional Free Vibrational and Mechanical Behavior of Silicon Nanotubes Using an Atomistic Model

Abstract Silicon nanotubes (SiNTs) have been successfully synthesized recently. Despite the wide potential applications of SiNTs, their mechanical properties are rarely reported. In this study, as the first step, the mechanical behavior of clamped-free single-walled silicon nanotubes (SWSiNTs) for both armchair and zigzag structures is investigated by using molecular dynamics (MD) simulations while the inter-atomic forces are described by the Tersoff-Brenner many-body potential energy function. Meanwhile, the results of the total strain energy are used to establish an expression for predicting Young’s modulus of the nanotubes. Afterward, the free vibrational analysis including torsional, longitudinal and transverse vibrations behavior of SWSiNTs with different diameters and lengths are investigated to report the corresponding fundamental frequencies as a significant design parameter. In this study, also, an important stress-strain parameter ratio is defined for vibration analyses named dynamic Young’s modulus. Thereafter, the natural frequencies that are obtained using the current atomistic model are successfully compared with those evaluated by the continuum mechanics model. It is concluded that the utilized approach can predict the frequencies with reasonable accuracy. Furthermore, the effects of geometry on the natural frequencies for both armchair and zigzag structures are examined. The obtained results provide valuable insights into the vibrational behavior of silicon-based nanotubes.

The electron properties of infinite length single-walled silicon nanotubes are studied by density functional theory

In this article, we aim to use LGTO-PBC-DFT calculation method with PBEPBE/6–31G(d) basis sets and PBC model to optimize geometric structure, obtain electron characteristics and band gaps of (n, m) chiral (6 ≤ n ≤ 16, 2 ≤ m ≤ 8); (n, 0) zigzag (3 ≤ n ≤ 15) and (n, n) armchair (2 ≤ n ≤ 8) pure infinite length single-walled Silicon nanotubes. We have obtained that the HOCO-LUCO gaps decrease with the radii of the SWSiNTs increase by using the PBEPBE functional and 6–31G(d) basis set. most of the infinite length SWSiNTs which we studied are narrow band gaps semiconductors. perhaps the surface structure is the most important factor affecting the band gap of the nanotubes. There have been a research on the infinite silicon nanotubes and with the increase of the tube radius, an indirectdirect band gap transition has been revealed. For all the armchair SWSiNTs, the infinite zigzag SiNTs (n,0), (3 ≤ n ≤ 9) and the infinite chiral SWSiNTs (6, 2), (9, 3), (10, 5), (12, 6) are semiconductors with indirect band gaps. While for the infinite zigzag SiNTs (n, 0) 15 ≥ n ≥ 10, and the infinite chiral SWSiNT (12, 4) are semiconductors with direct gaps at Ｘ point, the direct gaps open at Ｘ point. It is possible that direct band gap will become potential building blocks for electronic and optoelectronic devices.

The electron properties of infinite length single-walled silicon nanotubes are studied by density functional theory

In this article, we aim to use LGTO-PBC-DFT calculation method with PBEPBE/6–31G(d) basis sets and PBC model to optimize geometric structure, obtain electron characteristics and band gaps of (n, m) chiral (6 ≤ n ≤ 16, 2 ≤ m ≤ 8); (n, 0) zigzag (3 ≤ n ≤ 15) and (n, n) armchair (2 ≤ n ≤ 8) pure infinite length single-walled Silicon nanotubes.We have obtained that the HOCO-LUCO gaps decrease with the radii of the SWSiNTs increase by using the PBEPBE functional and 6–31G(d) basis set. Most of the infinite length SWSiNTs which we studied are narrow band gaps semiconductors. Perhaps the surface structure is the most important factor affecting the band gap of the nanotubes. There have been a research on the infinite silicon nanotubes and with the increase of the tube radius, an indirectdirect band gap transition has been revealed.For all the armchair SWSiNTs, the infinite zigzag SiNTs (n,0), (3 ≤ n ≤ 9) and the infinite chiral SWSiNTs (6, 2), (9, 3), (10, 5), (12, 6) are semiconductors with indirect band gaps. While for the infinite zigzag SiNTs (n, 0) 15 ≥ n ≥ 10, and the infinite chiral SWSiNT (12, 4) are semiconductors with direct gaps at Ｘ point, the direct gaps open at Ｘ point. It is possible that direct band gap will become potential building blocks for electronic and optoelectronic devices.

Beyond carbon nanotubes: adsorptions on and electronic structures of silicon nanotubes.

In this paper, we have reviewed some of the recent theoretical studies on the electronic and structural properties of silicon nanotubes from single-walled to double-walled nanostructures, primarily focusing on the studies performed by the present authors. The studies so far have not indicated any metallic behavior in both single-walled and double-walled silicon nanotubes. Atomic and molecular adsorptions of elements including hydrogen, oxygen and alkali metals on single-walled silicon nanotubes are also reviewed and new results presented in detail. A systematic study of molecular adsorption and co-adsorptions of hydrogen and oxygen molecules in zigzag silicon nanotube (SiNT) has been performed using hybrid density functional theory. For adsorption of two hydrogen molecules in SiNT (10, 0), the original diatomic molecular structure was maintained after adsorption. The most preferred final site for hydrogen molecules is the on-top site. For adsorption of two oxygen molecules, the most preferred sites are bridge sites, which are the parallel or zigzag bridge sites. Complete dissociation, partial dissociation and non-dissociation were observed for adsorption of two oxygen molecules. Peroxide structure and Si-O-O structures have also been observed in adsorption of two oxygen molecules with smaller adsorption energies rather than complete dissociation. For the co-adsorption of one hydrogen molecule and one oxygen molecule, the hydrogen molecule is slightly polarized, and a suppression effect on HOMO-LUMO gap was observed.

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.

Hydrogen storage on silicon, carbon, and silicon carbide nanotubes: A combined quantum mechanics and grand canonical Monte Carlo simulation study

Grand canonical Monte Carlo (GCMC) simulation combined with ab initio quantum mechanics calculations were employed to study hydrogen storage in homogeneous armchair open-ended single walled silicon nanotubes (SWSiNTs), single walled carbon nanotubes (SWCNTs), and single walled silicon carbide nanotubes (SWSiCNTs) in triangular arrays. Two different groups of nanotubes were studied: the first were (12,12) SiNTs, (19,19) CNTs, and (15,15) SiCNTs and the second were (7,7) SiNTs, (11,11) CNTs, and (9,9) SiCNTs with the diameters of ∼26 and ∼15 Å for the first and second groups, respectively. The simulations were carried out for different thermodynamic states. The potential energy functions (PEFs) were calculated using ab initio quantum mechanics and then fitted with (12,6) Lennard-Jones (LJ) potential model as a bridge between first principles calculations and GCMC simulations. The absolute, excess, and delivery adsorption isotherms of hydrogen were calculated for two groups of nanotubes. The isosteric heat of adsorption and the radial distribution functions (RDFs) for the adsorbed molecules on different nanotubes were also computed. Different isotherms were fitted with the simulation adsorption data and the model parameters were correlated. According to the results, the hydrogen uptake values in (19,19) CNT array exceeded the US DOE (Department of Energy) target of 6.0 wt% (FY 2010) at 77 K and 1.0 and 2.0 MPa for absolute and excess uptakes, respectively. The results also show that SiNTs and SiCNTs are not more useful materials compared with corresponding CNTs for hydrogen storage.

First-Principles Study on Chemical Bonding Characteristics between Cr and Single-Walled Silicon Nanotubes

The adsorption energy and structural properties of Cr doped armchair (5, 5) single-walled silicon or Carbon nanotubes are investigated in detail by the first-principles theory. It is found that Cr atom above on hole position is most energetically favorable for SWSiNTs, which means that Cr atom is prone to absorb on Silicon nanotubes than Carbon nanotubes. Structural analyses suggest that Cr adsorption in silicon nanotubes induces the dehybridization of mixed sp2-sp3hybrid orbital and the subsequent formation of sp3-like orbital. That enhances the adsorption energy of silicon nanotubes.

Natural gas storage on silicon, carbon, and silicon carbide nanotubes: a combined quantum mechanics and grand canonical Monte Carlo simulation study

Grand canonical Monte Carlo (GCMC) simulation combined with ab initio quantum mechanics calculations were employed to study methane storage in homogeneous armchair open-ended single-walled silicon nanotubes (SWSiNTs), single-walled carbon nanotubes (SWCNTs), and single-walled silicon carbide nanotubes (SWSiCNTs) in triangular arrays. Two different groups of nanotubes were studied: the first were (12,12) SiNTs, (19,19) CNTs, and (15,15) SiCNTs and the second were (7,7) SiNTs, (11,11) CNTs, and (9,9) SiCNTs with the diameters of 26 and 15 A for the first and second groups, respectively. The simulations were carried out at different thermodynamic states. The potential energy functions were calculated using ab initio quantum mechanics and then fitted with (12,6) Lennard–Jones potential model as a bridge between first-principles calculations and GCMC simulations. The absolute, excess, and delivery adsorption isotherms of methane were calculated for two groups of nanotubes. The specific surface area and the isosteric heat of adsorption were computed. The radial distribution functions for the adsorbed molecules on different nanotubes were also calculated. Different isotherm models were fitted with the simulation adsorption data. According to the results, the excess uptake value of methane adsorption in (11,11) CNT array exceeded the US Department of Energy target (180 V/V at 298 K and 35 bar). The results also indicate that SiNTs and SiCNTs are not desirable materials compared with corresponding CNTs for natural gas storage.

Carbon/Silicon Heterojunction Formed by Inserting Carbon Nanotubes into Silicon Nanotubes: Molecular Dynamics Simulations

Using molecular dynamics (MD) simulations, we report a carbon/silicon (C/Si) heterojunction formed by inserting carbon nanotubes (CNTs) into silicon nanotubes (SiNTs). Due to the weak mechanical property of the SiNTs, insertion of CNTs into them can not only reinforce their mechanical stabilities but also form multiwalled C/Si nanotube heterojunctions. The driving force of the coaxial assembly is primarily the intertube van der Waals (vdW) interactions. The coaxial self-assembly process is strongly tube size dependent, and the intertube distance (Δd) for a successful assembly between the two type nanotubes is around 3.5 Å. Simulations suggest possible bottom-up self-assembly routes for fabrication of novel nanomachines and nanodevices in nanomechanical systems. This study also suggests that the possibility of synthesizing SiNTs with fewer walls, even single-walled SiNT in aid of CNTs.

Structure-dependent optical properties of single-walled silicon nanotubes

The electron excitations of Single-Walled Silicon Nanotubes (SWSiNTs), with sp(2) and sp(3) hybridization, were studied using the localized-density-matrix (LDM) method with INDO/S parameters. Strong anisotropic characteristics of the dynamic polarizabilities were found for all the nanotubes. The transitional intensity along the tubular axis is much larger than that perpendicular to the axis for all the nanotubes. The optical gaps of sp(3)-hybridized infinitely-long pentagonal SWSiNTs are near 3.0 eV and 4.7 eV owing to σ-σ* transitions along the direction of the tubular axis. The optical gaps of sp(2)-hybridized infinitely-long armchair SWSiNTs along the tube axis direction are about 0.7 eV and 2.4 eV for Si(3,3) SWSiNTs and 0.7 eV and 2.7 eV for Si(4,4) SWSiNTs. The former peak at 0.7 eV originated from π-π* electron transitions and the latter peak at 2.4 eV or 2.7 eV originated from σ-σ* electron transitions. Meanwhile, the intensities of π-π* electron transitions are stronger than those of σ-σ* electron transitions in SWSiNTs. The low sp(2) transition energy derived from the weak overlap of unpaired p(z) orbitals of silicon atoms. Moreover, the electronic excitations of zigzag SWSiNTs are similar to those of armchair structures. This indicates that sp(2)-hybridized silicon nanotubes possess much greater potential for application in optical fields.

Investigation of hydrogen and methane adsorption/separation on silicon nanotubes: a hierarchical multiscale method from quantum mechanics to molecular simulation

A combination of ab initio quantum mechanical (QM) calculations and canonical Monte Carlo (CMC) simulations are employed to investigate possible usage of single-walled silicon nanotubes (SWSiNTs) as a novel media for hydrogen and methane adsorption as well as their separation from each other. By fitting the force field, a Morse potential model is selected as an efficient potential to describe the binding energies between both hydrogen-SiNTs and methane-SiNTs obtained from ab initio calculations. Then CMC simulations are performed to evaluate the adsorption and separation behaviors of H2 and CH4 on the three different sizes of SiNTs including (5, 5), (7, 7), and (9, 9) SiNTs at ambient temperatures and pressures from 1 up to 10 MPa. As a comparison, the adsorption and separation of H2 and CH4 on the (8, 8) CNTs which are isodiameter with (5, 5) SiNTs are also simulated. Results are indicative of remarkable enhancement of H2 and CH4 adsorption capacity on the SiNTs compared to the CNTs, which arise from stronger van der Waals (VDW) attractions. In the case of methane adsorption on SiNTs, the stored volumetric energy exceeds the goal of the US Freedom CAR Partnership by 2010, which can not be achieved by methane compression at such low pressures. Moreover, simulation results indicate that SiNTs preferentially adsorb methane relative to hydrogen in their equimolar mixture, which results in efficient separation of these gases from each other at 293 K.

Facial dissociations of water molecules on the outside and inside of armchair single-walled silicon nanotubes: theoretical predictions from multilayer quantum chemical calculations

The possible reaction pathways of dissociative adsorption of a single water molecule on the sidewalls of armchair (n, n) (n = 4–10) single-walled silicon nanotubes (SWSiNT) have been investigated by the multilayer models. Both the simplified fragment embedding and ONIOM calculations were carried out to study the diameter dependence of reactivity for the dissociation of water on SWSiNTs. The active fragments with different cluster sizes, such as Si16H10, Si30H16, and Si10mH4m (m = 4–10), were used for the multilayer calculations. The employment of the medium-sized Si30H16 cluster is able to reach a good balance between the computational efficiency and accuracy for the large-sized reaction system. In comparison with those full B3LYP/LANL2DZ calculations for Si(4,4) and Si(5,5) nanotubes, the approximate multilayered models can give reasonable predictions on the optimized geometries, activation energies, and exothermic energies with significant reduction in computational cost. The external complexes of the dissociative adsorption of H2O on SWSiNTs were predicted to be more stable than those internal complexes. The convex surfaces of SWSiNTs were also more reactive to H2O with the smaller activation barrier energies (10–13 kcal/mol) than those (15–22 kcal/mol) on the concave side. Both the activation barriers and exothermic energies of dissociative adsorptions of H2O on the internal (external) sidewalls of armchair SWSiNT were found to be insensitive to the tube curvature. The passivation of the outer surface and the removal of water molecules may be crucial for the experimental preparation of the single-walled silicon nanotubes.

Energy band and band-gap properties of deformed single-walled silicon nanotubes

The fantastic physical properties of single-walled silicon nanotubes (SWSiNTs) under mechanical strain make them promising materials for fabricating nanoscale electronic devices or transducers. Here we investigate the energy band and band-gap properties of the SWSiNTs calculated from the tight-binding model approximation. The results show that the band-gap properties are very sensitive to the deformation degree and the helicity of the SWSiNTs. The results can be employed to guide the design of nanoelectronic devices based on silicon nanotubes.

Silicon nanotubes with distinct bond lengths

In this paper, we extend both the rolled-up and the polyhedral models for single-walled silicon nanotubes with equal bond lengths to models having distinct bond lengths. The silicon nanotubes considered here are assumed to be formed by sp3 hybridization with different bond lengths so that the nanotube lattice is assumed to comprise only skew rhombi. Beginning with the three postulates that all bonds lying on the same helix are equal, all adjacent bond angles are equal, and all atoms are equidistant from a common axis of symmetry, we derive exact formulae for the polyhedral geometric parameters such as chiral angles, bond angles, radius and unit cell length. The polyhedral model presented here with distinct bond lengths includes both the rolled-up model with distinct bond lengths which arises from the first term of an asymptotic expansion, and an existing polyhedral model of the authors which assumes equal bond lengths. Finally, some molecular dynamics simulations are undertaken for comparison with the geometric model. These simulations start with equal bond lengths and then stabilize in such a way that two distinct bond lengths emerge.

Investigation of Silicon Model Nanotubes as Potential Candidate Nanomaterials for Efficient Hydrogen Storage: A Combined Ab Initio/Grand Canonical Monte Carlo Simulation Study

Grand canonical Monte Carlo (GCMC) simulations combined with ab initio QM calculations were employed to study the adsorption capacity of H2 in single-walled silicon nanotubes (SWSiNTs) of a hypothetical armchair structural model. The interaction energy of H2 with a graphite-like sheet from the surface of a single SiNT obtained from the QM calculations was fitted to an accurate potential function used to simulate the system. This theoretical approach is also used in SWCNTs of similar characteristics and at the same thermodynamic states. The GCMC simulation of NT bundles with H2 showed enhancement of H2 adsorptivity of SiNTs, as compared with CNTs. Concretely, the (14, 14) SWSiNTs present remarkable percentage improvement of 100, 70, 44, and 25% in the gravimetric (weight percent) adsorption of H2 at 293 K and 0.1, 1.0, 5.0, and 10.0 MPa, respectively, as compared with isodiameter (22, 22) CNTs. This is attributed to the stronger attractive interaction of H2 with SiNTs as compared to CNTs, found from the fi...

Hydrogen adsorption on hexagonal silicon nanotubes

We present a first-principles study of geometrical structure and energetics of hydrogen adsorbed on hexagonal single-walled silicon nanotubes (SiNTs). The adsorption behaviors of hydrogen molecules on SiNTs are investigated. The binding energies for the most stable physisorbed configurations are calculated to be less than 0.1 eV. The energy barriers are also investigated for dissociation of H 2 molecules. Finally, we consider encapsulation of H 2 molecules in SiNTs. The possibility of SiNTs as hydrogen storage materials is discussed.