Electron scattering in a multiwall carbon nanotube bend junction studied by scanning tunneling microscopy L. Tapasztó, 1, * P. Nemes-Incze, 2 Z. Osváth, 1 Al. Darabont, 2 Ph. Lambin, 3 and L. P. Biró 1 1 Research Institute for Technical Physics and Materials Science, H-1525 Budapest, P.O. Box 49, Hungary 2 Faculty of Physics, “Babes-Bolyai” University, Str M. Kogalniceanu No 1, R-3400 Cluj-Napoca, Romania 3 Facultes Universitaire Notre Dame de la Paix, 61 Rue de Bruxelles, B-5000 Namur, Belgium Received 16 December 2005; revised manuscript received 7 October 2006; published 13 December 2006 The atomic resolution scanning tunneling microscopy investigation of a multiwall carbon nanotube bend junction is reported. Atomic resolution images taken at the junction region revealed position-dependent modu- lation of the electronic density of states, with a period larger than but commensurate to the underlying atomic lattice, attributed to the scattering of electrons on defect sites present in the junction region. We propose an interference model, suitable to interpret the experimentally observed electron density patterns by considering electronic states near the bands crossing points involved in the scattering processes. The model predicts that complex charge density oscillations present near defects are tunable by varying the applied bias potential. DOI: 10.1103/PhysRevB.74.235422 PACS numbers: 73.22.-f, 73.63.Fg I. INTRODUCTION The unique relationship between the atomic structure and electronic properties of carbon nanotubes has been the focus of attention since their discovery. 1,2 The presence of defects in nanostructured materials is expected to substantially alter their electronic behavior. Often the modifications, induced by native or artificially created defects, confer on the nanotubes the necessary functionality for realization of different types of electronic devices; a carbon nanotube bend junction is an eloquent example of such a device. 3–5 Transport measure- ments on carbon nanotubes containing defects indicate that the defect sites not only act as scattering centers for electrons but also behave as gate tunable barriers. 6,7 This specific be- havior has its origins in the quantum coherence phenomena, leading to Fermi energy-dependent, long-range interference effects. 6,8,9 Scanning tunneling microscopy STM is a method that can directly reveal interference patterns in the electron den- sity distribution of solid surfaces. Such interference patterns, known as Friedel oscillations present near different types of defects, were observed by STM on both metal 10 and doped semiconductor 11 surfaces. On graphite surfaces, similar os- cillations are known as  3-type superstructures 12 due to the special correlation between the oscillations and the underly- ing atomic structure of the hexagonal lattice. Electron-wave interference patterns were also observed by STM on single-walled carbon nanotubes SWCNTs con- taining defect sites 9,13 near tube caps 14 and intramolecular junctions joining nanotubes with different helicities. 15 The phenomenon was applied to directly probe the one- dimensional 1D energy band dispersion near the Fermi level in metallic SWCNTs. 9 Recently  3-type superstructures have been observed by STM measurements on ion-irradiated multiwalled carbon nanotubes MWCNTs. 16 Two kinds of effects induced by the presence of the de- fects in carbon nanotubes CNTs directly perceptible by STM measurements have been revealed: 1 Short-range 1 nm modifications in the local den- sity of states LDOS at the defect site are due to localized states and can be directly related to the atomic structure of the defect. They manifest themselves as hillocklike protru- sions in STM images. 2 Beside these very local modifications, defects also me- diate a redistribution of electron density on a larger scale, which is primarily determined by the available scattered electron states. 17,18 These long-range effects appear as super- structure patterns in STM images. Mizes and Foster 19 demonstrated theoretically that elec- tronic superstructures observed by STM on graphite are in- deed the consequence of the interference between electron waves scattered by defects and those characterized by regular electron wave functions of graphite. In order to describe the long-range interference effects, a simple but effective interference model was constructed by Shedd and Russell, 20 suitable to reproduce the experimen- tally observed superstructure patterns. Their results were confirmed by tight binding 18 and first-principles 21 calcula- tions and applied successfully in interpretation of various experimental results, 18,22 being able to describe most of the simple superstructure patterns observed experimentally. However, experiments often reveal more complex patterns of coexisting superstructures. 12,17,20 STM is well known for its ability to image individual wave functions of CNTs. 23,24 In this paper we report on how STM can be used to collect information, through long-range interference effects, about the defect-scattered electronic states which can seriously affect the transport characteristics of the defective carbon nanotubes. We show that STM im- ages of coexisting superstructures are signatures of coherent scattering of electrons by defects, between the few electron states available in CNTs in the vicinity of bands crossing points K points of the Brillouin zone. Fermi energy- dependent scattering of electrons in nanotubes is also dis- cussed. We believe that the detailed understanding of defect- induced long-range modifications on electronic behavior of nanotubes is essential in the development of novel molecular electronic devices based on carbon nanotubes. PHYSICAL REVIEW B 74, 235422 2006 1098-0121/2006/7423/2354226 ©2006 The American Physical Society 235422-1