Electronic Transport in Quantum Confined Systems

The experimental creation and annihilation of defects on single-walled carbon nanotubes (SWCNT) with the tip of a scanning tunneling microscopeare reported. The technique used to manipulate the wall structure of a nanotube at the atomic scale consists of a voltage ramp applied atconstant tunneling current between the tip and the nanotube adsorbed on a gold substrate. While topographic images show an interferencepattern at the defect position, spatially resolved tunneling spectroscopy reveals the presence of localized states in the band gap of thenanotube. Removal of the defect by the same procedure demonstrates the reversibility of the process. Such a precise control in the localmodification of the nanotube wall opens up new opportunities to tailor SWCNT electronic properties at will. Owing to their unique aspect ratio, single-walled carbonnanotubes (SWCNTs) have attracted considerable attentionin the study of transport across one-dimensional systems.1The way they are wrapped not only determines theirgeometric properties but also their electronic structure:semiconductive or metallic. Such properties allow SWCNTsto be used as functional devices like transistors.2 Recentstudies have focused on the engineering of SWCNT proper-ties such as the exploitation of intramolecular junctions3 orthe modification of their band gap by the insertion ofimpurities: ions4 or molecules.5 Atomic defects are alsoinvestigated as good candidates for tailoring the electronicproperties of SWCNTs; for example, Lee et al. observed deepand shallow levels associated with vacancy-adatom andpentagon-heptagon defects,6 and Park et al. modulated theelectron transmission probability of a defect by an electricfield.7 Until now, several types of defects, like Stone-Walesdefects,8 vacancies,9 ad-dimers,10 or H-C complex11 couldbe introduced on SWCNTs by electron or ion sputtering.9,11,12But the incorporation of defects in the nanotube generallyrelies on macroscopic processes, which inherently involvethe creation of several defects in the same nanotube with alarge dispersion on their spatial distribution. Therefore noprecise control of the creation or even removal of defects ina single nanotube has been achieved so far.Because of the nanometer size of SWCNTs, scanningtunneling microscopy (STM) is a quite suitable techniquefor the investigation of the SWCNTs properties.13 It has beenwidely used to study their electronic structure as well asidentify and characterize individual defects on their surface.14More remarkably, it can also be used to manipulate SWCNTby locally cutting the tube for example.15 But to date thistechnique has not yet been proved to tailor the properties ofSWCNTs without destroying them.In this paper, we describe a method to selectively modifythe electronic properties of semiconductor SWNTs by thecreation and destruction of point defects on their surface withan STM tip. The fabrication of the defects is detected fromthe measurements of the tip height variation when the samplevoltage is increased to a few volts maintaining the tunnelingcurrent constant. The investigation of the defect nature bySTM topographic images shows the formation of interferencepatterns on the nanotube wall, and spatially tunnelingspectroscopic measurements reveal that this pattern is related* Corresponding author. Website: http://dora.ims.tsukuba.ac.jp.† University of Tsukuba.‡ Nagoya University.§ NTT Corporation.| IEMN (CNRS, UMR 8520).BATCH: nl12a52 USER: ckt29 DIV: @xyv04/data1/CLS_pj/GRP_nl/JOB_i12/DIV_nl071845c DATE: November 11, 2007