Study of the cap structure of (3,3), (4,4) and (5,5)-SWCNTs: Application of the sphere-in-contact model

Constantinos D Zeinalipour-Yazdi,Eriketi Z Loizidou

doi:10.1016/j.carbon.2017.01.074

Constantinos D Zeinalipour-Yazdi, Eriketi Z Loizidou

Open Access

https://doi.org/10.1016/j.carbon.2017.01.074

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Abstract

We have applied the sphere-in-contact model supported by hybrid Density Functional Theory (DFT) calculations to elucidate the cap geometry of the sub-nanometer in dimension (3,3), (4,4) and (5,5) single-wall carbon-nanotubes (SWCNTs). Our approach predicts certain cap-geometries that do not comprise of the commonly known for their stability combination of pentagonal and hexagonal carbon rings but also tetragonal, trigonal and all-pentagonal structures. Based on hybrid-DFT calculations carbon atoms in these new cap geometries have similar stability to carbon found in other fullerene-like capped zig-zag and arm-chair nanotubes (i.e., (5,5), (6,6), (9,0) and (10,0)) that are known to be stable and synthetically accessible. We find that the cap structure of the (3,3)-CNTs is a pointy carbon geometry comprised of six pentagonal rings with a single carbon atom at the tip apex. In this tip geometry the carbon atom at the tip apex does not have the usual sp2 or sp3 geometry but an unusual trigonal pyramidal configuration. DFT calculations of the molecular orbitals and density-of-states of the tip show that this tip structure apart from being stable can be used in scanning probe microscopies such as STM for very high resolution imaging.

Highlights

Scanning probe microscopy (SPM) instruments have been very popular in research since the discovery of the Scanning Tunneling Microscope (STM) in the 80's
We explore some of the electronic properties of tubular and capped single-wall carbon-nanotubes (SWCNTs) in order to show that they exhibit certain features, which make them suitable as STM probes
The cap geometry of this (4,4)-SWCNT is such that four orbitals have protruding orbitals in the tip-to-surface vacuum gap, which could result in a multiple-tip effect in STM

Summary

Introduction

Scanning probe microscopy (SPM) instruments have been very popular in research since the discovery of the Scanning Tunneling Microscope (STM) in the 80's. There has been an increased amount of research over the last three decades to increase the resolution of these instruments from several nanometers to sub-nanometer resolution, either by vibrational isolation of the SPM head or by modification of the tip composition and structure. Carbon nanotubes (CNT) attached to commonly used tips in STM, such as W or Pt/Ir probes, have been suggested as a way to improve the resolution of topographic imaging to sub-nanometer resolution. In atomic-force microscopy (AFM) they enable high resolution measurements and manipulation when in tapping-mode [1]. The large aspect ratio, high mechanical strength and chemical stability of CNT modified SPM probes makes them suitable for measurements where sub-nanometer resolution is necessary [2]. The CNTmodified STM tips have been shown to be superior than the typical W-tips used for measuring the square profile of gratings [3]

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