Scanning Tunneling Microscopy and Atomic Force Microscopy Investigation of Organic Tetracyanoquinodimethane Thin Films
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Scanning tunneling microscopy and atomic force microscopy investigation of organic tetracyanoquinodimethane thin films H. J. Gao, H. X. Zhang, Z. Q. Xue, and S. J. Panga) Department of Electronics, Peking University, Beijing 100871, China and Beijing Laboratory of Vacuum Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, China (Received 6 February 1996; accepted 22 January 1997)
Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) investigation of tetracyanoquinodimethane (TCNQ) and the related C60 -TCNQ thin films is presented. Periodic molecular chains of the TCNQ on highly oriented pyrolytic graphite (HOPG) substrates were imaged, which demonstrated that the crystalline (001) plane was parallel to the substrate. For the C60 -TCNQ thin films, we found that there were grains on the film surface. STM images within the grain revealed that the well-ordered rows and terraces, and the parallel rows in different grains were generally not in the same orientation. Moreover, the grain boundary was also observed. In addition, AFM was employed to modify the organic TCNQ film surface for the application of this type of materials to information recording and storage at the nanometer scale. The nanometer holes were successfully created on the TCNQ thin film by the AFM.
Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been successfully employed in imaging the surface topography of some polymer thin films at nanometer (or atomic) scale resolution level in the last decade.1–4 Meanwhile, polymers are increasingly being used in a wide range of applications in electronics and optics because of their special structures and properties.5–8 Tetracyanoquinodimethane (TCNQ), if combined with tetrathiafulvalene (TTF) and special metal such as Ag and Cu, is imbued with special properties such as electrical conductivity and optical bistability. Consequently, it has been intensively investigated in the past years. The C60 molecule, a unique form from nature, has extensively excited many scientists’ interest since 1985.9,10 The structure of the molecule is the truncated icosahedron typified by a soccer ball, with the carbon atoms at the 60 vertices of the pentagons and hexagons that make up the near-spherical surface. Researchers have studied the C60 crystal and its electronic structure as well as its applications in superconductivity, nonlinear optics, and other scientific fields. To apply these thin films into integrated devices, we have studied, first, the preparation of organic polymer thin films by an ionizedcluster-beam (ICB) method in a high vacuum system. In the paper, we present the surface structures of TCNQ and C60 -TCNQ thin films by STM. Modification of the surface of the TCNQ thin films on mica substrates by AFM is also described, which was carried out to ex-
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Author to whom all correspondence should be addressed: Professor S. J. Pang, Beijing Laboratory of Vacuum Physics, Chinese Academy of Sciences, P.O. Box 2724, Beij
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