STM studies of single-walled carbon nanotubes

This topical review summarizes scanning tunneling microscopy investigations of the electronic properties of single-walled carbon nanotubes (SWNTs). First, the unique relationship between the atomic structure and electronic properties of SWNTs is reviewed. Second, the one-dimensional electronic band structure of SWNTs is introduced, and a comparison of tunneling spectroscopy to tightbinding calculations is presented. Third, localized structures in nanotubes, such as intramolecular junctions and ends, are addressed. Fourth, quantum size effects in SWNTs are discussed. Lastly, the effects of external perturbations, specifically magnetic impurities, on the low energy properties of metallic SWNTs are discussed. Prospects for future research directions are considered. Carbon nanotubes have attracted considerable attention from both scientific and technological communities. This interest is not surprising in light of their promise to exhibit exceptional physical properties that may be exploited in areas from advanced composites to nanoelectronics [1–3]. Experimental studies have shown that carbon nanotubes are the stiffest known material [4, 5] and buckle elastically under large bending or compressive strains [5,6]. These mechanical characteristics suggest that nanotubes have potential for superstrong composites, and could be unique force transducers to the molecular world. Moreover, the remarkable electronic properties of carbon nanotubes offer great intellectual challenges and the potential for novel applications. For example, theoretical calculations first predicted that single-walled carbon nanotubes (SWNTs) could exhibit either metallic or semiconducting behavior depending only on diameter and helicity [7–9]. This ability to display fundamentally distinct electronic properties without changing the local bonding, which was experimentally demonstrated through atomically resolved scanning tunneling microscopy (STM) measurements [10, 11], sets nanotubes apart from all other nanowire materials [12, 13]. Scanning tunneling microscopy and spectroscopy (STS) have been exploited to interrogate the electronic properties of individual carbon nanotubes and SWNTs on the surfaces of ropes. This review will describe a broad range of STM studies, drawn primarily from the authors’ 1 Author to whom any correspondence should be addressed. 0953-8984/02/060145+23$30.00 © 2002 IOP Publishing Ltd Printed in the UK R145