11098
2008, 112, 11098–11101 Published on Web 07/04/2008
Atom-Resolved Imaging of Carbon Hexagons of Carbon Nanotubes Hongwei Zhu,*,† Kazutomo Suenaga,‡ Jinquan Wei,† Kunlin Wang,† and Dehai Wu† Department of Mechanical Engineering, Tsinghua UniVersity, Beijing 100084, PR China, and National Institute of AdVanced Industrial Science and Technology, Tsukuba 305-8565, Japan ReceiVed: May 18, 2008; ReVised Manuscript ReceiVed: June 06, 2008
Atomic configurations of individual single-walled and double-walled carbon nanotubes have been obtained by high-resolution transmission electron microscopy with atomic sensitivity. A structural reconstruction is carried out by Fourier-filtered analysis of Moire´ patterns, and it is now possible to acquire the carbon honeycomb lattice images through all of the periphery of individual nanotubes. This visualization technique provides supplementary access in nanoscale characterizations by combining with scanning tunneling microscopy. Introduction
Experimental Section
Considerable effort has been made to investigate the atomresolved configurations of nanomaterials because their properties strongly depend on the way in which the atoms are organized on the atomic scale. This study is of great interest 1–9,12 in the case of carbon nanotubes (CNTs) because of their unique 1D structure made up of a section of the graphene lattice that has been wrapped in a cylinder. They exhibit more remarkable electronic, optical, and mechanical properties than do other nanomaterials. Besides scanning tunneling microscopy (STM),1,2 some progress in transmission electron microscopy (TEM) combined with an electron diffraction technique has been shown to be feasible in determining tube chirality.3–5 Direct observation techniques have been employed to image the hexagonal networks of CNTs and other nanotubes (e.g., boron-nitride nanotubes) by high-resolution TEM (HRTEM) image analysis with image simulations and processing.6,10,11 However, among the techniques mentioned above, except STM, there is still no direct atomic-resolution imaging technique to “see” the carbon hexagons and 3D conformation of a tube. A specially fabricated HRTEM imaging technique was applied in situ to observe defect formation in single graphene layers thanks to a desired contrast transfer function (CTF) that can selectively visualize the zigzag chains with atomic-scale sensitivity.7 This technique was further optimized to obtain both the lattice images and chiralities of single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs).8,9,12 The tube chirality has been determined to high accuracy by the systematic analysis of optical diffraction and image simulation.12 On the basis of this, we have proposed a rational approach to visualize the nucleation points of SWNTs on the atomic scale to study the growth mechanism of nanotubes.13 Here we report nearly perfect imaging of honeycomb carbon hexagons in individual SWNTs and DWNTs. A quasi-3D reconstruction is performed on a SWNT to show the potential application of this visualization technique in nanoscale characterizations.
SWNTs and DWNTs used here were grown directly on a silicon dioxide (SiO2) film-coated TEM copper mesh grid using cobalt as a catalyst by a conventional chemical vapor deposition method.12,13 The specimens were examined in a JEOL JEM2010F HRTEM (operated at 120 kV). Raw atomic-resolution TEM images of individual nanotubes were acquired using a GATAN 794 CCD camera. Both image acquisition and simulation were carried out under the optimized conditions with a spherical aberration of Cs ) 0.45 mm, a semiangle of divergence R ) 1 mrad, and a defocus of ∆f ) -32 nm.7
* Corresponding author. E-mail:
[email protected]. † Tsinghua University. ‡ National Institute of Advanced Industrial Science and Technology.
10.1021/jp804385a CCC: $40.75
Results and Discussion The intact HRTEM image of a nanotube and corresponding optical diffraction (Fourier transformation) involves all of the information related to d, R, and tube tilting angle β (Figure 1a). One of the examined nanotube is shown in Figure 1b; for example, the honeycomb lattice formed by the 6-fold carbon rings can be recognized in the Moire´ pattern because of the enhanced contrast induced by the specially fabricated CTF7 though they are not resolved very clearly. Its optical diffraction (Figure 1c) shows unambiguous characteristics of a chiral tube. The first maxima of the first-order lines are reminiscent of the spot of the graphene, and they define two distorted hexagons (1 and 2) from the top and bottom of the examined tube, respectively. Figure 1d is a reconstructed image after inverse Fourier processing, showing a well-resolved graphitic lattice and carbon hexagons. The lattice spacings are slightly different from those in a planar graphene, which is attributed to the curve surface of the nanotube wall. A detailed analysis of optical diffraction12 has provided strong evidence for the atomic conformation of the examined tube, which is unambiguously assigned to be (23, 20) with an inclination of β ) 0°. The diameter and index angle measured here are so-called apparent values that weakly depend on electron defocus and tube size. The true diameter (d) and the index angle (R) can be extracted from the original HRTEM image (Figure 1b) and its optical diffraction (Figure 1c) with errors of
