Adsorption and thermal decomposition of C60 on Co/Si(111)-7 × 7

We present a study on the adsorption and thermal decomposition of C₆₀ on Co covered Si(111)-7 × 7 using scanning tunneling microscopy and X-ray photoelectron spectroscopy. Co-induced magic clusters grown on Si(111)-7 × 7 are identified as a possible adsorption site where 51 ± 3% of C₆₀ molecules adsorb at room temperature. On Co/Si(111)-7 × 7, C₆₀ molecules start to decompose at 450 °C, and are completely dissociated to form SiC by 720 °C. This temperature is significantly lower than 910 °C at which C₆₀ completely dissociates on clean Si(111)-7 × 7. This is a possible low temperature method for growing crystalline SiC films using C₆₀ as a precursor molecule.     

Ge(111) 7 × 7 surface structure induced by Sn

When tin was deposited by 0.3–0.8 monolayer thick on Ge(111) clean surface at room temperature and annealed at 345°C, 7 × 7 superlattice structure appeared on the surface. Features of the diffraction pattern of the 7 × 7 structure are strikingly similar to those of the diffraction pattern of Si(111) 7 × 7 surface structure, which suggests that atomic arrangements in the two 7 × 7 structures resemble each other very well.     

The interaction of H2S with Si(111) 7 × 7 surfaces by energy loss and Auger electron spectroscopies

Low energy electron loss spectroscopy (ELS) and Auger electron spectroscopy (AES) have been applied for the studies of the interaction of H₂S molecules with Si(111)7 × 7 surfaces. The observations are consistent with the interpretation that the room temperature non-dissociative adsorption state of H₂S molecules changes substantially after annealing at 550°C, resulting in the desorption of hydrogen and the covalent bond formation between silicon and sulfur atoms. The silicon disulfide films formed on Si(111) surfaces have been identified by the characteristic loss peaks in comparison with those of silicon dioxide.     

Comparison of the Si(111) (7 × 7) and (1×1)surfaces

High-resolution electron energy-loss spectroscopy and monochromatic low-energy electron diffraction have been applied to the study of the Si(111)(7 × 7) surface and the thermally-quenched Si(111) (1 × 1) surface. For the (1 × 1) surface, the inelastic continuum, observed for the (7 × 7) surface, due to the Drude absorption of electrons in the dangling-bond surface states is not existent, which indicates that the surface-state band associated with the dangling-bonds of the (1 × 1) surface is insulating. The observed electronic transitions indicate that the (7 × 7) and (1 × 1) surfaces have similar local band geometries and that they differ only in long-range order. The (1 × 1) surface is considered to have a disordered structure. The defect model is favored for the (7 × 7) structure.     

Core level photoemission evidence of frustrated surface molecules: a germ of disorder at the (111) surface of C60 before the order-disorder surface phase transition

Using high resolution core level photoemission, we investigated the disordering transition of the fullerene molecules at the (111) surface of C (60) films. The experimental evidence of a two-step mechanism for the rotational disordering of surface fullerene molecules is provided. The data are consistent with a recent model in which the rotational degrees of freedom of one molecule, out of the four inequivalent C (60) molecules of the low temperature (2x2) surface unit cell, melt about 100 K before the bulk phase transition.     

Angle-resolved ultraviolet photoemission study of Si(111) 7 × 7 and 1 × 1 surfaces

Angle-resolved ultraviolet photoelectron spectra of Si(111) 7 × 7 and 1 × 1 surfaces have been measured as a function of temperature from ambient temperature to ≈ 1120°C. Both the Si(111) 7 × 7 and 1 × 1 surfaces show obvious surface metallic edge at all the temperatures. A middle peak among three surface-state peaks observed for ambient-temperature 7 × 7 surface has been found to disappear for high-temperature 7 × 7 surface. In going from high-T 7 × 7 surface to high-T 1 × 1 surface, no essential changes in the surface-state peaks have been found to occur.     

Photon-stimulated desorption of methanol on Si(111)7 × 7 and Si(100)2 × 1 at the C 1s and O 1s thresholds

Photostimulated desorption experiments have been performed on deuterated methanol adsorbed on Si(111)7 × 7 and Si(100)2 × 1 at the C 1s and O 1s thresholds. D⁺ and the masses of the series CD⁺_x are produced in the photofragmentation process in both energy ranges. A comparison has been made with the photofragmentation spectra of methanol in the gas phase and two different desorption mechanisms have been hypothesized for the desorption of D⁺ and higher masses from the silicon surfaces at the C 1s threshold.     

Electronic structures of chemisorption systems on Si(111) surface I. Si(111)7 × 7/H,Cl

Electronic structures of chemisorption on Si(111)/H,C1 are investigated by the first principle DV-Xα cluster method. The calculations are carried out for chemisorption on different sites, based on the Si₁₃H₁₅ cluster, and the effect of surface vacancy and buckling on the electronic structure is examined in detail. The present calculation shows that the Si₁₃H₁₅ surface cluster reproduces very well the more sophisticated band calculation for the Si(111) surface. It is concluded that the vacancy model with chemisorbed atoms at appropriate sites is reasonable to interpret the observed UPS of Si(111) 7 × 7/H,C1. The charge transfer between the substrate atom and the adatom depends strongly both on the chemisorption sites and on the electronegativitv difference.     

Structural analogy between GeSi(111)-5 × 5 and Si(111)-7 × 7 surfaces

Low energy electron diffraction (LEED) patterns for the GeSi(111)-5 × 5 surface are reported and compared to those for the Si(111)-7 × 7 surface. Parallels between the observed LEED patterns are explained by a structural analogy between GeSi(111)-5 × 5 and Si(111)-7 × 7 surfaces. Both the (5 × 5) and (7 × 7) patterns are shown to be consistent with structural models of the triangle-dimer type previously proposed for Si(111)-7 × 7 surface.     

Symmetrization of the trimer model for Si(111) 7×7

A symmetrized extension of the trimer model for the Si(111) 7×7 structure is presented. It exhibits new features such as sixfold coordinated Si atoms which are not unusual in Si chemistry but have not been considered in previous structural models. Atomic coordinates are optimized by a Keating-type strain energy minimization and the structure factor for transmission electron diffraction is calculated.     

Partially dissociative adsorption of water vapor on the Si(111) (7×7) surface

High-resolution vibrational electron energy-loss spectra have been measured of the Si(111) (7×7) surface exposed to water vapor at 300 K. Direct experimental evidence is given that water adsorption on the Si(111) (7×7) surface is partially dissociative.     

Catalytic dehydrogenation and thermal chemistry of cyclohexene and 1-methyl-1,4-cyclohexadiene on Si(111)7 × 7

Recently, we reported a thermal desorption study on the evolution of an intense mass 78 profile for the room-temperature exposure of cyclohexene to Si(111)7 × 7 surface, which was believed to give rise to the formation of benzene by a surface dehydrogenation reaction. Because mass 78 was also found to be the base ion in the gas-phase cracking patterns of both 1,3- and 1,4-cyclohexadiene, the dehydrogenation of cyclohexene on clean, sputtered and oxidized Si(111)7 × 7 surfaces has been re-examined in order to determine the origin of the intense mass 78 desorption profile; i.e. whether it was in fact due to the evolution of benzene or cyclohexadiene, or both. Moreover, a similar dehydrogenation reaction giving rise to toluene desorption between 350 and 600 K has been observed for the room-temperature exposure of 1-methyl-1,4-cyclohexadiene to clean and sputtered Si(111)7 × 7 surfaces. The effects of methyl substitution on the reactivity of these cyclic olefins towards Si(111)7 × 7 can be inferred from these studies. Furthermore, the catalytic activity of Si(111)7 × 7 was found to be enhanced significantly by extending the thermal desorption cycles to a higher temperature of 925 K. The dehydrogenation of these olefins on Si(111)7 × 7 also gave rise to a unique 7 × 1 low energy electron diffraction pattern. Possible factors that may play a role in any proposed model for the dehydrogenation reaction are discussed. Finally, evidence of other surface reactions including cyclohexene hydrogénation to cyclohexane will also be presented.     

Observation of methoxy species on the Si(111)(7 × 7) surface — A vibrational study

Chemisorption of methanol on the Si(111)(7 × 7) surface has been studied at ～ 300 K using high-resolution electron energy loss spectroscopy. Methanol reacts with the Si(111) surface to form the stable surface species SiOCH₃ and SiH. The methoxy species (CH₃O) is bonded to the Si surface with a covalent bond formed between its oxygen end and the dangling bond of the Si(111) surface atom. A structural model for methanol chemisorbed on the Si(111) surface is proposed.     

Pits and adatoms at the interface of 1-ML C84/Ag （111）

We prepare a well-defined C84 monolayer on the surface of Ag （111） and study the geometric structure by scanning tunneling microscopy （STM）. The C84 molecules form a nearly close-packed incommensurate R30° lattice. The lattice is long-distance ordered with numerous local disorders. The monolayer exhibits complex bright/dim contrast; the largest height difference between the molecules can be greater than 0.4 nm. Annealing the monolayer at 380 ℃ can desorb part of the molecules, but more than sixty percent molecules stay on the Ag （111） surface even after the sample has been annealed at 650 ℃. Our analyses reveal that the 7-atom pits form beneath many molecules. Some other molecules sit at the 1-atom pits. Ag adatoms （those removed substrate atoms, accompanying the pit formation） play a very important role in this system. The adatoms can either stabilize or destabilize the monolayer, depending on the distribution manner of the adatoms at the interface. The distribution manner is determined by the co-play of the following factors： the dimension of the interstitial regions of the C84 overlayer, the number of the adatoms, and the long-distance migration of part adatoms.     

Investigations of C60 molecules deposited on Si(111) by noncontact atomic force microscopy

We demonstrated the high resolution imaging of the organic molecules using noncontact atomic force microscopy in ultrahigh vacuum. The sample was C₆₀ molecules deposited on the Si(111)-7×7 reconstructed surface. When the thickness of the C₆₀ film was submonolayer, we could image some isolated C₆₀ molecules and the reconstructed Si surface simultaneously. However, the imaging was highly unstable not only because of the large structure but also due to the large difference between the interaction forces on the molecules and on the Si surface. On the other hand, when the thickness of the C₆₀ molecules was almost monolayer, individual molecules could be stably imaged.     

Low temperature scanning force microscopy of the Si(111)-(7x7) surface

A low temperature scanning force microscope (SFM) operating in a dynamic mode in ultrahigh vacuum was used to study the Si(111)- (7x7) surface at 7.2 K. Not only the twelve adatoms but also the six rest atoms of the unit cell are clearly resolved for the first time with SFM. In addition, the first measurements of the short range chemical bonding forces above specific atomic sites are presented. The data are in good agreement with first principles computations and indicate that the nearest atoms in the tip and sample relax significantly when the tip is within a few A of the surface.     

Scanning force microscopy on the Si(111)7×7 surface reconstruction

Scanning force microscopy images of the Si(111)7×7 surface reconstruction are presented which are taken in the contact mode in ultrahigh vacuum. Topographic and lateral force data are acquired simultaneously. A special treatment of the sensing tip with PTFE helps to overcome the strong adhesion and wear effects that normally occur on this particular surface.     

Surface phase transitions and defect-phonon interactions: The Si(111)-(2 × 1) to (7 × 7) to (1 × 1) transitions

A thermodynamic equation is derived, showing the relation between defect-phonon interactions, the free energy of formation of defects and surface disordering or melting. When applied to Si(111), the theory shows that the (2 × 1) to (7 × 7) transition temperature is deduced from the (7 × 7) to (1 × 1) temperature via the relative concentrations of dangling bonds on the (2 × 1) and (7 × 7) surfaces.     

Structure of Si(111)−7 × 7

A model of the Si(111)?7 × 7 surface atomic arrangement is put forward on the basis of results already established for Si(111) and Si(100) surfaces. The unit mesh contains a triangular double-layer island with 21 first-layer atoms. The island is laterally expanded and is bounded by [1̄1̄2] steps with second-layer edge atoms forming asymmetric dimers. It is shown that salient features of low energy electron diffraction (LEED) patterns for Si(111)?7 × 7 can be explained by the model. The LEED patterns are interpreted qualitatively by a double-diffraction mechanism involving forward diffraction in the selvedge. It is shown that the patterns contain characteristic formations of fractional-order spots attributable to the dimers at the island boundaries. The best agreement with observed patterns is obtained with the following parameter values: dimer bond length 2.5 ± 0.2 Å, island lateral expansion 3 ± 2%. Some of the implications of the model for the chemical reactivity and electronic properties of the Si(111)?7 × 7 surface are discussed.     

A superstructural 2D-phase diagram for Ga on the Si(111)- 7x7 system

The structural modifications of an Si(111)- 7x7 reconstructed surface and the evolution of growth induced by Ga adsorption in the submonolayer regime at various substrate temperatures ranging from room temperature (RT) to 600 °C, with a low Ga flux rate of 0.1 ML/min (1 ML∼6.8×10¹⁴ atoms/cm²) have been studied in-situ in Ultra High Vacuum (UHV) using Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED) and Electron Energy Loss Spectroscopy (EELS) as characterization probes. Ga grows in the Stranski-Krastanov (SK) growth mode for temperatures ranging from RT to 350 °C, where 3D-islands form after one and two flat monolayers of Ga adsorption, while for higher temperatures ranging from 450 to 550 °C, Ga grows in the Volmer-Weber (VW) growth mode. A comprehensive 2D-phase diagram for this Ga/Si(111) system for adsorption, which provides pathways to attain the observed superstructural phases, viz., √3x√3-R30°, 6.3x6.3, 6.3√3x6.3√3-R30° and 11x11, has been investigated. The characteristic EELS spectrum for each superstructural phase is also reported in this study.