High resolution EELS on individual carbon nanotubes by monochromated TEM

Abstract

Single‐walled carbon nanotube (SWNT) has been known to exhibit a wide range of electronic properties upon its atomic arrangement [1,2]. However it is still difficult to directly correlate the distinct electronic properties with its atomic structure from an individual carbon nanotube. Here, we successfully demonstrate highly localized electronic properties of individual carbon nanotubes with precise atomic structures by means of transmission electron microscopy (TEM) consisting of a monochromator. We have used a JEOL TEM (TripleC#2) equipped with a Schottky field emission gun, a double Wien filter monochromator and delta correctors. The energy resolution is adjustable from 30 to 200 meV by choosing the energy selecting slits in the dispersion plane of the monochromator. We have performed the electron energy loss spectroscopy (EELS) on individual freestanding SWNTs with the scanning TEM (STEM) mode at 60 kV. The target SWNTs are also imaged by both STEM/TEM modes to fully assign their atomic structures. Figure 1(a) presents a TEM image of two closely aligned SWNTs (inset) and their C K‐edge (C1s) spectra. The chirality of thicker SWNT (top) and thinner one (bottom) are assigned as (9, 7) and (6, 5), respectively. Each spectrum has several sub‐peaks on the π* response and exhibits completely different features. The line shape analysis [3] suggests that the π* response of (6, 5) tube consists of four sub‐peaks related to the van Hove singularity (vHs) (1s →E 1 *, E 2 *, E 3 * and E 4 *) and a broad π* resonance (Fig. 1(b)). The position of the sub‐peaks fairly consists with the vHs in the shifted ab‐initio DOS [3] (inset in Fig. 1(b)). The relative intensities are possibly influenced by core‐hole effects. The valence‐loss spectra taken from the same SWNTs also exhibit the peaks related to the vHs (E i → E i * (i=1,2,…)). However it is difficult to distinguish the two closely aligned SWNTs from the valence‐loss spectra because the large delocalization mixes the peaks for both SWNTs. This means that the core‐loss spectra has a much higher special resolution and reflects more localized electronic structures. Then, we have experimentally investigated how the core‐loss spectra changes corresponding to nonperiodic structures. Figure 2 shows STEM images of a typical hybrid SWNT and its C K‐edge (C1s) spectra. We have confirmed from TEM image (not shown here) that the hybrid SWNT involves a serial junction between the thinner (11, 1) semiconducting part and the thicker (10, 10) metallic part. The C K‐edges (i to x) presented in Figs. 2(b) and 2(c) are obtained when the electron probe is scanned across the broken lines (i to x) in Fig. 2(a). The π* peaks in the junction part (iv to vii) are different form the spectra for either (10, 10) and (11, 1) and show new peaks (black arrows in Fig. 2(c)) reflecting the distinct electronic structures. Such a highly localized measurement of electronic properties for individual carbon nanotubes has never been realized by any other methods.