Ab Initio Study of Si Doped Carbon Nanotubes: Electronic and Structural Properties
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Ab Initio Study of Si Doped Carbon Nanotubes: Electronic and Structural Properties
A. Fazzio1,2, R. J. Baierle3, Solange B. Fagan2, Ronaldo Mota2, and Antônio J. R. da Silva1 1 Instituto de Física, Universidade de São Paulo, Caixa Postal 66318, 05315-970, São Paulo, Brazil 2 Departamento de Física, Universidade Federal de Santa Maria, 97105-900, Santa Maria, RS, Brazil 3 Departamento de Ciências Naturais e Exatas, Centro Universitário Franciscano, 97010-032, Santa Maria, RS, Brazil
ABSTRACT
We report the electronic and structural properties of silicon doped carbon nanotubes using first principles calculations based on the density-functional theory. In the doped metallic nanotube a resonant state appears about 0.7 eV above the Fermi level and for the semiconductor tube the Si introduces an empty level at approximately 0.6 eV above the top of the valence band. INTRODUCTION
The new structural form of carbon discovered by Iijima in the early 1990 [1] has attracted the attention of several research groups due to their interesting properties from both a fundamental physics as well as the possible application viewpoint. Single-walled nanotubes, depending on their chirality and diameter, are either a one-dimensional metal or semiconductor [2]. The presence of semiconductor and metallic one-dimensional structures in the same family of materials opens up exciting possibilities of new phenomena and novel device structures. In the recent literature there are reports of several groups that have succeeded in synthesizing doped carbon tubes, using different techniques as arc discharge [3] and gas-phase pyrolisis [4]. The doped nanotubes can exibit dramatic changes with respect to the undoped material. Recently Ray et al. [5] have reported the synthesis of silicon doped heterofullerenes in the carbon-rich limit, where the photofragmentation spectra provide a clear evidence that such clusters have Si atoms located in the fullerene network. These results are corroborated by ab inito calculations [6]. In this paper we present a study about isovalent substitutional impurities in carbon nanotubes. In particular, we focus on the electronic and structural properties of silicon as the substitutional atom. This doping, due to the different sizes between carbon and silicon atoms will affect locally the lattice. As is well known, silicon, as opposed to carbon, does not have the sp2 configuration as the most stable one [7]. As a consequence there will also be, besides the simple volume relaxation, a lattice distortion that may introduce a new state in the gap, leading to the possibility that, for instance, it can be a hole or electron trap working like an amphoteric deep center. Moreover, from a structural point of view, the lone orbital due to the Si-impurity could be a center to trap other atoms or molecules. It is important to point out that there are experimental realizations of Si doping on fullerenes [8,9], which suggests that it is quite possible to also dope carbon nanotubes with Si. In the following we present an ab initio calcula
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