Nitrogen doping of single walled carbon nanotubes by low energy ${\text{N}}_2^{+}$ ion implantation

Abstract

Nitrogen doping in single walled carbon nanotubes by 300 eV ${\text{N}}_2^{+}$ ion implantation has been studied with X-ray photoelectron spectroscopy. We investigated the nitrogen doping concentration in the range of 1.5–11.3 at.% and post-irradiation annealing up to 1000 °C. We found that nitrogen atoms can be substitutionally inserted into the perfect sp² hexagonal network, or bind to two sp² carbon neighbors in a pyridine-like configuration, or be connected to three or four sp³ carbon atoms in a reconstructed double vacancy site and that the substitutional doping is the most stable bonding against high temperature annealing.

Introduction

Since the discovery in the early 1990s, carbon nanotubes have attracted ever increasing interest due to their extraordinary mechanical and electronic properties [1]. A single walled carbon nanotube (SWCNT) can be regarded as rolled up of a graphene sheet into a cylinder and can be either metallic or semiconducting depending on the wrapping angle and diameter. Many SWCNTs can be packed together into bundles in which they are weakly bonded in a triangular lattice. These tubular objects can be as long as many μm or even up to some mm so that they are typical one dimensional systems. The remarkable properties of these materials open up unprecedented great prospective for possible technological applications.

Parallel to the tremendous efforts in understanding their fundamental physics, attempting the fabrication of these materials with desired characteristics and on a larger production scale, and researching for potential applications in nano-scale electronic devices, the possibility of incorporating hetero dopant atoms into the carbon graphitic network has also been extensively explored. These studies are mainly aimed to tailor the electronic properties of carbon nanotubes in a controllable fashion. In this regard, boron and nitrogen are the natural candidates because of their similar atomic sizes as that of C and of their electron acceptor and donor characters, respectively [2], [3], [4].

Nitrogen doped carbon nanotubes have been synthesized using a variety of techniques including chemical vapor deposition [5], [6], [7], [8], [9], [10], pyrolysis [11], [12], [13], [14], [15], [16], and arc-discharge [17], [18]. Most of these CNTs are multi walled with a typical bamboo like morphology [7], [8], [9], [11], [12], [13], [14], [17] and relatively large diameters [19]. Nitrogen doping of SWCNTs is relatively difficult and only recently some groups have reported to succeed in arc-discharge procedure [18]. A quite different route of post growth doping through forced nitrogen incorporation by ion implantation in SWCNTs has also been proven to be quite efficient [20], [21], [22].

In this paper we present a study on 300 eV ${\text{N}}_2^{+}$ ion irradiation onto SWCNTs using X-ray photoelectron spectroscopy (XPS). We investigated the nitrogen doping concentration in the range of 1.5–11.3 at.% and post-irradiation annealing up to 1000 °C. Our results show that nitrogen atoms can substitutionally bind to three sp² C in a perfect graphene network, or to two sp² C in a pyridine-like configuration, or to three or four sp³ C in a reconstructed vacancy site. The latter one results from knocking off two C atoms and replacing with a N. These defective configurations are energetically less stable than the substitutional bonding which is the dominant one after high temperature annealing.