FI-STM Investigation of Fullerenes Adsorbed on the Semiconductor and Metal Surfaces(STM-C_<60>)

Our systematic investigation using field-ion scanning tunneling microscopy (FI-STM) is reviewed for the adsorption and film growth of fullerenes (C_, C_, C_, and C_) and metallofullerenes (Sc@C_/Sc_2@C_, Sc_2@C_, Y@C_, and Gd@C_) on the Si(100)-2x1, Si(111)-7x7, GaAs(001)-2x4-As, Cu(111)1x1, and Ag(111)1x1 surfaces. The fullerene molecules are stable on the Si(100)-2x1 and Si(111)-7x7 surfaces at room temperature because of the strong bonding to the substrate dangling bonds. As a result, unique intramolecular structures of C_ molecule are observed. The STM images of the C_, Sc@C_/Sc_2@C_, and Sc_2@C_ molecules are used to draw information on the molecular structures. In contrast to the Si surfaces, the fullerene molecules are mobile on the terrace of the metal surfaces and initially segregate to the step. A well-ordered two dimensional overlayer forms with a close-packed arrangement upon annealing the fullerene covered surfaces. On the Cu(111) surface, the commensurate (4x4) phase forms for the cases of the C_, C_ and C_C_ adsorption, indicating a strong interaction between the Cu substrate and fullerenes. Beautiful intramolecular structures of the C_ and C_ molecules are observed and are interpreted as the local mappings of the electron density of states. One-dimensional cluster formation of C_, Y@C_, and Gd@C_ on the step of the Cu(111)-1x1 surface is analyzed and the preferential dimer formation is only observed for the case of Y@C_, which is attributed to the interaction between the unpaired electrons of the Y@C_ molecules. On the Ag(111) surface, the C_ and Sc_2@C_ monolayer films show several phases that have an almost identical nearest neighbour distance but are rotated from each other. Among the systems studied, C_ on the GaAs(001)-2x4-As surface shows a unique highly-strained fcc(110)-oriented multi-layer film growth, while all other systems show the well-known fcc(111)-oriented multi-layer growth

Field Ion Microscopy of C_<60> Molecules(APFIM/FIM)

Field ion microscopy of C_ on the tungsten substrate showed that (1) the field ion image of the C_ adsorbate reflects the LUMO (lowest unoccupied molecular orbital)-induced local density of states of the C_, similar to the recent STM image of the C60 on the Cu(111)1 x 1 surface and that (2) the field evaporation occurs as a C_ molecule, in glaring contrast to the recent report by Ohmae et al

Stripe charge ordering in SrO-terminated SrTiO3(001) surfaces

The local electronic structure of the SrO-terminated SrTiO3(001) surface was explored using scanning tunneling microscopy. At low bias voltages in the empty states, a unidirectional structure with a periodicity of 3 unit cells, superimposed on a c(2 x 2) reconstructed structure, was found to develop along the crystallographic a axis. This structure indicates a charge-ordered stripe induced by carrier doping from oxygen vacancies in the SrO and the subsurface TiO2 planes. In the filled states, localized deep in-gap states were observed in addition to large energy gaps in the tunneling spectra. This result represents inelastic tunneling due to significant electron-lattice interaction associated with unidirectional lattice distortion in the SrO-terminated surface.Comment: 6 pages, 5 figures, accepted for publication in PR

Atomically Resolved Surface Structure of SrTiO3(001) Thin Films Grown in Step-Flow Mode by Pulsed Laser Deposition

The surface structure of SrTiO3(001) thin films homoepitaxially grown by PLD in step-flow mode was characterized using low temperature STM. It was found that one-dimensional (1D) TiOx-based nanostructures were formed on the thin film surface and their density increased with increasing thin film thickness. Most of the 1D nanostructures disappeared after a post-deposition annealing, indicating that this structure is metastable due to the nonequilibrium growth mode. The resulting surface after annealing exhibited similar features to that of a thinner film, having a domain structure with (2x1) and (1x2) reconstructions, but with fewer oxygen-vacancy-type defects. These results imply that the step-flow growth is likely to produce TiOx-rich surface and Ti deficiencies in the film. By the post-deposition annealing, the rich TiOx would diffuse from the surface into the film to compensate defects associated with Ti vacancies and oxygen vacancies, resulting in the stable surface structure with fewer oxygen vacancies. Thus, STM measurements can provide us with a microscopic picture of surface stoichiometry of thin films originating in the dynamics of the growth process, and can present a new approach for designing functional oxide films.Comment: 12 pages, 4 figure

Stability of Sb line structures on Si(001)

Structure and stability of Sb-dimer linear chains on the Si(001) surface are studied by means of ab initio quantum-mechanical molecular dynamics using pseudopotentials. It is confirmed that the model comprising a double core of seven-membered rings of silicon for Bi/Si(001) nanolines is indeed one of the most stable structures energetically, and it also explains Sb/Si nanolines. Moreover, it is clear that stability of the odd-membered-ring (5-7-5) structure will decrease as the group-V adatom changes from Bi to Sb, and disappear for As/Si due to the size effect

Orientational Ordering of C_<60> Adsorbed on the Cu(111)1x1 Surface Studied by the FI-STM

The adsorption of C_ molecules on the Cu(111)1x1 surface has been investigated by field ion-scanning tunneling microscopy (FI-STM). At the initial stage adsorption, C_ molecules are mobile on the terrace at room temperature and segregate to the steps to form linear chains. With increasing coverage, two-dimensional islands form with a close-packed configuration. Upon monolayer adsorption, highly ordered Cu(111)-(4x4)-C_ overlayer forms. Bias voltage-dependent STM images of the individual C_ molecule show unique intramolecular structures. The adsorption geometry is determined by analyzing the STM images and the intramolecular structures are interpreted as the local density of the states of C_ interacting with the substrate

Ferromagnetism in a Hubbard model for an atomic quantum wire: a realization of flat-band magnetism from even-membered rings

We have examined a Hubbard model on a chain of squares, which was proposed by Yajima et al as a model of an atomic quantum wire As/Si(100), to show that the flat-band ferromagnetism according to a kind of Mielke-Tasaki mechanism should be realized for an appropriate band filling in such a non-frustrated lattice. Reflecting the fact that the flat band is not a bottom one, the ferromagnetism vanishes, rather than intensified, as the Hubbard U is increased. The exact diagonalization method is used to show that the critical value of U is in a realistic range. We also discussed the robustness of the magnetism against the degradation of the flatness of the band.Comment: misleading terms and expressions are corrected, 4 pages, RevTex, 5 figures in Postscript, to be published in Phys. Rev. B (rapid communication

Scanning Tunneling Microscopy of the GaAs(001) Surface Reconstructions(STM-GaAs)

The atomic structure of the GaAs(001) surface has been disputed since molecular beam epitaxy (MBE) technique was developed in the earlier nineteen sixties. The invention of scanning tunneling microscopy (STM) with its real-space atom-resolution capability, has revolutionized the situation. This paper reviews the STM investigations of the principal reconstructions found on the GaAs(001) surface, As-rich 2x4 and 2x6, Ga-rich 4x2 and 4x6. These studies, together with advanced theoretical analyses, have finally resulted in establishment of a unified structural model for various reconstructions, with which we can explain most of the observations and long-standing controversies about the atomic structures and surface stoichiometries

Initial Stage of Molecular Adsorption on Si(100) and H-terminated Si(100) Investigated by UHV-STM(STM-Si(001))

We have investigated the initial stage of adsorption of a conjugated aromatic compound, 1, 4-bis[β-pyridyl-(2)-vinyl]benzene (P2VB), on the clean Si(100)-2×1 surface and the hydrogen terminated Si(100)-2×1-H surface by ultra-high-vacuum (UHV) scanning tunneling microscopy (STM). We found adsorbed molecules cannot migrate on the chemically active Si(100)-2×1 surface, while they can migrate on the chemically inactive hydrogen terminated Si(100)-2×1-H surface until they are trapped to hydrogen-missing dangling bonds. On the clean Si(100)-2×1, we observed four different adsorption directions. An individual molecule appears as two or three bright spots, the brightness and distance between bright spots varying for different cases. Through structural analysis and bias-voltage-dependent STM images, we conclude that the electronic states of Si dimers modulated by the adsorbed molecules are observed instead of the molecules themselves. A simple estimation by considering only the molecular size and shape reproduces the distribution of four different kinds of adsorption structures we observed