Metallic single-walled silicon nanotubes

Abstract

Atomistic computer-simulation evidences are presented for the possible existence of one-dimensional silicon nanostructures: the square, pentagonal, and hexagonal single-walled silicon nanotubes (SWSNTs). The local geometric structure of the SWSNTs differs from the local tetrahedral structure of cubic diamond silicon, although the coordination number of atoms of the SWSNTs is still fourfold. Ab initio calculations show that the SWSNTs are locally stable in vacuum and have zero band gap, suggesting that the SWSNTs are possibly metals rather than wide-gap semiconductors.L ow-dimensional nanostructures are known to have properties that can be markedly different from their bulk counterparts. For example, as shown in a recent experiment (1), Ho and coworkers demonstrated atom-by-atom evolution of electronic band structure of 1D gold chain. For semiconductor nanostructures, when the carriers (electrons and holes) are confined to dimensions less than their de Bröglie wavelength (typically a few nanometers), quantum-mechanical size effects can emerge. The carrier confinement at the nanoscale can be in 0D (quantum dots), 1D (quantum wires), or 2D (quantum wells) (2). Indeed, the current interest in fabricating low-dimensional semiconductor nanostructures, coined as band-structure engineering, has largely relied on novel quantum-mechanical size effects.The crystalline structure of 3D bulk silicon is cubic diamond, similar to that of carbon diamond. However, unlike the carbon counterpart (3), a 1D single-walled silicon nanotube (SWSNT) has not been found in nature yet, largely because silicon prefers sp 3 bonds rather than sp 2 bonds (4). Indeed, silicon has been viewed as nontubular solid rather than tubular solid, similar to carbon and boron nitride. The cubic diamond silicon is known as a semiconductor with an energy band gap of 1.17 eV (1 eV ϭ 1.602 ϫ 10 Ϫ19 J). At high pressures, however, the cubic-diamondstructured silicon can undergo a phase transformation to highly coordinated (sixfold or above) metallic phases such as the ␤-tin and hexagonal closed-packed structures (5). To date, experimentally produced 1D-like nanostructures of silicon [e.g., porous silicon (6) and silicon nanowires (7-12)] all assume either the cubic diamond crystalline structure or the local structure of amorphous silicon. A commonly reported electronic property of the 1D silicon nanostructures is that they all have wider band gap than the cubic diamond silicon. Here we present atomistic computer-simulation evidences of three thinnest SWSNTs: the square, pentagonal, and hexagonal silicon nanotubes. The local geometric structure of these SWSNTs differs from that of cubic diamond silicon and the surface-passivated 1D silicon nanowires. It also differs from that of the carbon-nanotube-like ''hypothetical'' SWSNTs (4, 13-15), because the latter types of SWSNTs are composed of both sp 3 and sp 2 bonds. Moreover, ab initio quantum-mechanical calculations show that the present SWSNTs exhibit an entirely opposite trend in the band-gap change compared wi...

Structure and thermophysical properties of single-wall Si nanotubes

“…This agrees with the ab initio prediction that metallic Si structures have a bond order larger than 4 [3].…”

“…Despite the similarity in the atomic structure between Si and C, Si nanotubes with a similar structure to CNTs or bulk crystal Si (c-Si) have not been found. Recently, single-wall Si nanotubes (SWSNTs) with stable structure of different cross section shapes: rectangular, pentagonal and hexagonal have been predicted using atomistic simulations [3]. The local geometric structure of SWSNTs is very different from that of bulk Si and surface-passivated 1D Si nanowires.…”

Structures and energetics of silicon nanotubes from molecular dynamics and density functional theory

“…10 Different trial geometries with 5, 6, 10, 11, and 12 rows of pentagons or hexagons allow for different possible periodicities of the final structure. To obtain well-relaxed NTs, the trial structures are cyclically compressed and expanded using MD at two temperatures ͑300 and 600 K͒.…”

“…5,6 The several hollow and nonhollow structures proposed and theoretically characterized in recent years include fullerene-based structures 5 and hexagonal wires, 7 polycrystals with fivefold symmetry, 8 carbon NT structure, 9 and square, pentagonal, and hexagonal single-walled NTs. 10 These structures were proposed based on intuition or the behavior of similar materials, and some are high-energy configurations unlikely to be observed experimentally. To address this limitation, we propose a computational approach to predict the atomic configuration of nanostructures and apply it to Si NTs; we find various hollow, low-energy structures with similar stability but disparate Young's moduli.…”

First-principles investigation of pentagonal and hexagonal core-shell silicon nanowires with various core compositions

“…Moreover, contact metals might account as an alternative to traditional doped source-drain device structures where one suffers from fundamental problems such as high leakage current and parasitic resistance arise from the sub-100 nm range scaling. On the contrary, silicon nanowire, 17 or nanotube [18][19][20][21][22] itself might be incorporated as nanocontact structure. Although the stability of various silicon nanotubes ͑SiNTs͒ has been verified from computational studies, [18][19][20][21][22] and energetics of SiNTs, for example dependence of strain energy on the tube diameter and chirality, is studied in detail, 23 it is very difficult to realize them experimentally because of sp 3 hybridization tendency of silicon in SiNTs.…”