Adsorption of pure C7p and its mixture with C«on the Cu(111)1X 1 surface was investigated by using scanning tunneling microscopy. The pure C7p monolayer was found to form the 4X4-commensurate superlattice, similar to the case of pure C6p after a brief annealing at approximately 320 C. The individual C7p molecules take the upright position with the long axis perpendicular to the surface. Introducing C6p s in the C7p layer allows some of the C7p molecules to take the lying-down position with the long axis parallel to the (110) direction of the surface. By comparing with theoretical simulations, we propose that C7p molecules in the C7p-C6p mixture cease their internal rotation and are ratcheted with one of the hexagonal rings around the equator facing down the Cu(111) surface.Since the discovery of C6Q and larger fullerenes, ' and their synthesis in a macroscopic quantity, efforts have been made to try to image the molecular structures with various kinds of methods, including scanning tunneling microscopylspectroscopy (STMiSTS). However, due to the rapid intrinsic rotation of individual C6Q s in the bulk phase, only a few have succeeded in imaging their molecular structures by adsorbing C6Q molecules on Au, Si(100), and Si(111) surfaces.Most recently, it was found that the Cu(111) surface serves as a good substrate to observe the intramolecular structures of C6Q, since the small lattice misfit (-2%) between the nearest-neighbor (NN) distance for C6o in bulk phase (10.0 A) and four times the NN distance for the Cu(111)1X1 surface (10.2 A) and a reasonable strong interaction help the C60's ratcheted at the threefold hollow site and force them to cease their rotation.The observed bias voltage dependence of the STM images for the case of C6o/Cu (111)4 X 4 convincingly demonstrated that the STM images indeed represent the local electronic structures, in good agreement with those calculated by Kawazoe et al. using the local-density approximation. C7Q is the second most abundant fullerene, and is known to take an ellipsoidal shape. STM studies of C7Q adsorption and thin-film growth on the GaAs(110) (Ref. 10) and Au(111) (Ref. 11) surface confirm its ellipsoidal structure and form close-packed arrangements. The NN distance of C7o in bulk phases (both fcc and hcp) is measured to be 10.6 A, ' which is 4% larger than four times the Cu-Cu NN distance (10.2 A) on the Cu(111)1X 1 surface. Therefore, it is likely that pure C7Q s also form the 4 X 4 reconstruction on the Cu(111) surface, by taking the upright position with one of the pentagon rings facing the Cu surface and their long axes perpendicular to the surface. Under this configuration, the intermolecular dis-0 tance can be 10.2 A, identical to that for pure C6Q adsorption. When C6Q is coadsorbed with C7Q molecules on the Cu(111) surface, it may be possible to find a situation where some of the C7Q molecules take a lying-down position with the long axis parallel to the surface, due to readjustment of the surface stress and the adsorbateadsorbate interactions. These speculat...
The electronic states of silicon donors in a wide gap semiconductor, β-Ga2O3(100), have been studied using low-temperature scanning tunneling microscopy. We observe one-dimensional rows along [010], as expected from the crystal structure. In addition, substitutional Si donors are identified up to the fourth subsurface layer with clear spectroscopic features at the bottom of the conduction band. The decay length of each subsurface Si donor is systematically measured, and reasonably agrees with a picture of the Si donor in bulk β-Ga2O3. These results strongly suggest that Si impurities are shallow donors and responsible for the high electrical conductivity of β-Ga2O3.
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