Pseudopotential Calculations Explain the Structure and Electrical Conductance of Single-Walled Carbon Nanotubes
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STM Manipulation of Single Molecules Provides a Route to Molecular Switching For the past few years, research teams have investigated the electronic properties and the interaction of molecules with metallic surfaces using a scanning tunneling microscope (STM) and the synthesis of molecules for nanoscale experiments. The molecules manipulated by the STM tip must exhibit electronic effects such as switching in order to be useful in the creation of nanoscale devices. An international team of researchers affiliated with the Institut für Experimentalphysik; Freie University, Berlin; Paul Drude Institut für Festkörperelektronik, Hausvogteiplatz, Germany; and CEMES-CNRS, France, has reported results on experimental and theoretical work regarding the STM manipulation of individual porphyrin-based molecules and the related phenomena. As reported in the January 22 issue of Physical Review Letters, the researchers used Cu-TBPP (Cu-tetra-3,5 di-ter-butyl-phenyl porphyrin) molecules evaporated on a Cu(211) surface (prepared in several steps with nanofacets and terraces by Ne sputtering and annealing to 700 K). Performing calculations by the elastic scattering quantum chemistry technique for 770 molecular orbitals, they found that the electrical resistance R of the Cu-TBPP molecules varies drastically by a 90° rotation of one leg of the molecule. Hence, by using STM, it was shown that the internal conformation of the molecule can be modified reversibly, as well as the electric properties of the molecules. Two distinct situations were emphasized, corresponding to two different phases of the molecule: The tunneling resistance through a leg decreases for the flat case, compared with the rotated case, by one order of magnitude. The two situations were called “OFF-state” and “ON-state,” respectively. The researchers also observed that the switching occurs for lateral and vertical manipulation conditions as well. Experimentally, the orientation of the leg can be switched using the STM tip, causing a current variation dependent on the distance between the tip and that molecule that is in very good agreement with theoretical predictions. The conductance differs by two orders of magnitude between the ON and OFF states of the molecule, and the switching process is fully reversible. With this work, the researchers have shown that by means of vertical and lateral manipulation, it is possible to rotate single legs around their σ-bond, in and out of the porphyrin plane, and that the controlled rotation of the legs induced by the STM tip realizes the principle of a conformational molecular switch. CLAUDIU MUNTELE MRS BULLETIN/FEBRUARY 2001
Pseudopotential Calculations Explain the Structure and Electrical Conductance of Single-Walled Carbon Nanotubes Carbon nanotubes have drawn the interest of many researchers because of their extremely strong mechanical properties and adjustable electrical properties. Recently, researchers at the University of California—Berkeley and Seoul National University, South Korea, have carried out first-principles ca
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