Calculation of scanning tunnelling microscopy images for Kr/graphite system

The scanning-tunnelling-microscopy (STM) images of Kr atoms adsorbed on a monolayer graphite sheet (Kr/graphite system) are calculated using the first-principle total-energy electronic structure calculations within the density functional theory in the local density approximation. The results obtained agree well with the observations. It is found that the optimal site of the adsorbed Kr atom is at the top of the centre of the carbon hexagon, and its equilibrium distance from monolayer graphite surface is about 0.335nm. It is shown that the hybridization of C 2p electronic states (π-electronic states) and Kr 4p and 5s electronic states is the main origin of the Fermi-level local density of state.     

Substrate-step-induced effects on the growth of CaF2 on Si (111)

The growth of CaF₂ on vicinal Si (111) surfaces was studied by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) for the temperature range from 300 to 750 °C. In the submonolayer range a transition from terrace to step nucleation is observed for increasing temperature. The first monolayer grows in the step-flow growth mode since second layer islands are not nucleated before completion of the wetting layer. For the subsequent growth of CaF₂ on the CaF interface layer, substrate-induced steps do not act as preferential nucleation sites, but rather as growth barriers. Thus CaF₂ films grow very inhomogeneously at high temperatures. At lower deposition temperatures, the film homogeneity increases due to the increased (homogeneous) nucleation rate on terraces. The lattice mismatch leads to (lateral) relaxation of thicker CaF₂ film close to substrate steps. In addition, CaF₂ self-decoration effects are caused by the relaxed regions close to the film steps at temperatures above 500 °C. Received: 7 August 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002     

Structural change of radiation defects in graphite crystals induced by STM probing

It was found that STM (scanning tunneling microscopy) images of defects in highly oriented pyrolytic graphite introduced by bombardment of 400 eV Ar⁺ ions in ultra-high vacuum exhibit substantial changes in the course of STM probing. Detailed examination of abrupt changes in the tunneling current measured at defect sites during voltage scans shows that the primary cause of the defect-image change was found to be neither the injected current nor the injected power but the absolute value of the voltage applied between the probe tip and the sample. We propose that an electric polarization induced force attracting the sample surface toward the probe tip widens the layer spacing of the graphite surface, leading to an acceleration of the lateral diffusion of interstitial atoms introduced by the ion irradiation, which results in a change in the defect structures and the accompanying electronic structures sensible in the STMimaging. Received: 14 June 2001 / Accepted: 7 September 2001 / Published online: 20 December 2001     

“Sputtered-atom assembly” of carbon nanostructures during ion bombardment

We report the observation of a carbon nanostructure grown on a graphite surface by Ar⁺ ion bombardment. We demonstrate experimentally that, in view of transmission electron microscopy (TEM)-based evidence, some of these carbon nanostructures emerged the whiskerlike protrusion and/or the sputtered-surface, suggesting a new growth model for nanocarbon, distinctly different from that found in arc-discharge and many other methods. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 13 July 2000     

Clusters on surfaces: high-resolution spectroscopy at low temperatures

With controlled growth in nanometer-sized pits we produced silver and gold clusters on a graphite surface. We give a summary of the preparation method and discuss the scanning tunneling imaging and the crystalline orientation of the clusters. The electronic structure of the clusters was studied by an in-situ combination of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). For both techniques we obtained an energy resolution in the range of 10 meV employing low sample temperatures. Dynamic final-state effects together with averaging over a cluster-size distribution result in characteristic spectral shapes in UPS, which can be understood referring to STS data taken on individual clusters. Finally, directions for future experiments are pointed out. Received: 13 April 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Multilayer model of interlayer spacing in graphite intercalation compounds

The elastic deformation of host layers is calculated by constructing a model where the host layers, regarded as continuous two-dimensional elastic sheets, are infinitely stacked and compressible intercalants, represented by harmonic springs, are intercalated into them. If we treat the intercalants as being rigid relative to the soft host layers in the stage-1 graphite intercalation compound, Li_xC₆ (0≤x≤1), the calculated average gallery spacing agrees well with the experiments, without using any adjustable parameter. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 23 August 2000     

Observation of coexistence of 1D and 2D nanostructures in cobalt dipyrromethene trimer complexes adsorbed on a graphite surface

We report the direct observation of 1D and 2D nanostructures of cobalt dipyrromethene trimer complexes adsorbed on a highly oriented pyrolytic graphite surface using scanning tunneling microscopy (STM). STM images showed two types of ordered structures coexisting on the surface: long 1D molecular chains isolated on the terraces, and 2D hexagonal patterns confined by a 1D chain and/or a graphite step edge. These 1D and 2D structures are attributed to ‘edge-on’ and ‘face-on’ complex alignments on the surface, respectively. In both configurations, substrate-mediated molecule-molecule interactions may play a significant role in stabilizing the nanostructures.     

Infrared absorption spectra of 4H silicon carbide

We performed infrared absorption measurements on 4H-SiC samples with polarization E∥c and E⊥c at 8, 85 and 300 K. From the strong temperature dependence of the absorption lines, electronic transitions are separated from vibronic transitions. The electronic transition lines between 300 and 500 cm^-1 are assigned to the shallow nitrogen donor. It is found that the electronic transitions of the shallow nitrogen donor are polarization dependent. Zeeman spectroscopy was performed to study the influence of the magnetic field on the electronic transitions up to 15 T. The results show no linear Zeeman splitting and only a diamagnetic shift. This is consistent with the effective mass tensor of three different diagonal components in 4H-SiC. Received: 25 November 1999 / Accepted: 20 April 2000 / Published online: 2 August 2000     

Electronic stopping in ion–fullerene collisions

The electronic friction experienced by a multiply charged ion interacting with the valence electrons of a single fullerene is an important aspect of the collision dynamics. It manifests itself in a considerable loss of projectile kinetic energy transferred to the target, resulting in excitation. The latter mainly leads to direct ionization and multifragmentation and can be recognized in specific patterns of the fragmentation spectra. These fingerprints can be used to quantify electronic stopping and to identify its typical properties as known from particle–solid interactions, such as the oscillation of the electronic stopping with the projectile atomic number Z. These essentially many-body effects can therefore be studied in a well-defined system of finite size. Received: 4 April 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Local electronic edge states of graphene layer deposited on Ir(1 1 1) surface studied by STM/CITS

Scanning tunnelling microscopy and current imaging tunnelling spectroscopy were used to observe electronic structure of the edges of monolayer graphite film deposited on the Ir(1 1 1) surface. The electronic structure derived from the tunnelling spectra revealed peak in electron local density of states very close to the Fermi level. This electronic state was interpreted in terms of localised edge state caused by the topology of the π electrons networks typical for the zig-zag edges. The observed maximum of local density of states at about 0.2 eV above the Fermi level was ascribed to the presence of resonant state caused by the appearance of disclinations centres in the vicinity of the graphite edges.     

Phase and population decay of image-potential states – the influence of defects

Quantum-phase and population decay of image-potential states have been investigated by two-photon photoemission with femtosecond time resolution. The influence of steps and defects on quasielastic and inelastic scattering processes is illustrated for a vicinal Cu(119) surface and diluted adsorbate layers of CO and Cu on Cu(001). Received: 19 April 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Measuring the implantation depth of silver clusters in graphite

Scanning tunnelling microscopy (STM) and molecular dynamics (MD) simulations have been used to investigate the implantation of Ag₇ ^- clusters into the graphite surface. An experimental measure of the implantation depth of individual clusters is gained via thermal oxidation of the bombarded graphite surfaces. This process results in etching of the cluster-induced defects to form etch pits which grow laterally whilst retaining the depth of the implanted cluster. STM imaging of the etch pits reveals the distribution of implantation depths for deposition energies of 2 keV and 5 keV. Molecular dynamics simulations for clusters of 5 keV energy show that the implantation depth for Ag₇ ^- is largely independent of the impact site on the graphite surface and the cluster orientation. The implantation depth found by MD lies at the upper edge of the experimental depth distribution. Received 30 November 2000     

Epitaxial growth of metals with high Ehrlich–Schwoebel barriers and the effect of surfactants

Interlayer diffusion in epitaxial systems with a high energy barrier at the atomic steps – the so-called Ehrlich–Schwoebel (ES) barrier – is strongly reduced. As a consequence of this, a continuous accumulation of roughness takes place during growth. This undesirable effect can be corrected by using surfactant agents. We have studied the influence of the ES barrier on the preparation of epitaxial films on Cu(111), and the surfactant effect of a monolayer of Pb. Received: 21 April 1999 / Accepted: 17 August 1999 / Published online: 6 October 1999     

Theoretical study of small silicon clusters on a graphite layer

In a series of recent experiments, the HOMO-LUMO energy gaps of small Si clusters deposited on a graphite substrate have been determined by Scanning Tunneling Microscopy (STM). The values obtained were found to be substantially smaller than the energy gaps of corresponding passivated clusters. This work considers dimensional reduction as a possible mechanism for a sizeable energy gap narrowing by the example of the system Si₅. The impact of the graphite substrate on the deposited species is investigated in the framework of a pseudocluster model. Received 30 November 2000     

Real-time imaging of K atoms on graphite: interactions and diffusion

Scanning tunneling microscopy (STM) at liquid helium temperature is used to image potassium adsorbed on graphite at low coverage (≈0.02 monolayer). Single atoms appear as protrusions on STM topographs. A statistical analysis of the position of the atoms demonstrates repulsion between adsorbates, which is quantified by comparison with molecular dynamics simulations. This gives access to the dipole moment of a single adsorbate, found to be 10.5±1 D. Time-lapse imaging shows that long-range order is broken by thermally activated diffusion, with a 30 meV barrier to hopping between graphite lattice sites.     

STM analysis of triosmium carbonyl cluster adsorption at HOPG

The study of metallic carbonyl clusters as precursors in tailoring the heterogeneous metal catalysts has been of great importance. The catalytic nature of the adsorbed clusters in thin film form depends on the chemical properties of the substrate used. The metal-support interaction will determine various properties such as the surface morphology, adsorption features and the structural orientations. We report a scanning tunneling microscopy (STM) study of an osmium carbonyl cluster (Os₃(CO)₁₁(NCCH₃)) adsorbed on highly oriented pyrolytic graphite (HOPG). STM measurements showed that the osmium carbonyl cluster interacts with HOPG in such a way that it adsorbs on the basal plane showing regular lattice structure, whereas the axial planes of the HOPG surface shows no ordered structure. The regular cluster lattice structure of the carbonyl cluster on the basal plane of the graphite has lattice parameters of a=1.4 nm and b=1.5 nm. We believe that the regular orientation of the cluster indicates a monolayer adsorption of the cluster on the graphite basal planes. Scanning tunneling spectroscopy (STS) measurements also indicated an insulating behavior for the cluster molecules on HOPG, with a significant energy gap value of ca. 300 mV. The cluster interaction at the active sites, i.e. axial planes of the graphite, was also observed by in situ STM measurements.     

Pulsed-laser-induced transformation path of graphite to diamond via an intermediate rhombohedral graphite

The new phase transformation of hexagonal graphite to cubic diamond was experimentally produced without catalyst, using a high-power pulsed laser. Interestingly, by the X-ray diffraction spectra, it was proved that this transition was not direct, but through an intermediate rhombohedral phase. Furthermore, it is important that the rhombohedral phase, as the theoretical transformation path of hexagonal graphite to cubic diamond, was first truly substantiated by our experimental results. The transformation mechanism was suggested that diamond with hexagonal structure was obtained by the direct transforming of hexagonal graphite to hexagonal diamond, and diamond with cubic structure was formed by the indirect transforming, i.e., hexagonal graphite to rhombohedral graphite to cubic diamond. Received: 7 February 2000 / Accepted: 28 March 2000 / Published online: 9 November 2000     

Y-branching of single walled carbon nanotubes

Y-branching was observed by scanning tunnelling microscopy (STM) in single wall carbon nanotubes grown by thermal decomposition of C₆₀ fullerene in the presence of transition metals. These novel carbon nanostructures may play an important role in carbon-based nanoelectronics. Received: 18 November 1999 / Accepted: 20 January 2000 / Published online: 8 March 2000     

The adsorption of iron phthalocyanine on graphite: A scanning tunnelling microscopy study

Different adsorption phases of iron phthalocyanine (FePc) on highly oriented pyrolitic graphite (HOPG) have been characterized by scanning tunnelling microscopy (STM). Evaporation of FePc onto the graphite (0 0 0 1) surface, kept at room temperature, results in the formation of three-dimensional molecular islands.After annealing to 400 °C different two-dimensional features are identified, depending on the initial coverage. At low doses, domains with well defined boundaries have been observed, within which molecules tend to organise in chains. At higher coverage, islands exhibiting well-ordered densely-packed square or hexagonal molecular arrangement have been resolved. For the adsorption structures corresponding to one monolayer islands our results show that the molecules adsorb with the molecular plane parallel to the surface. The high resolution STM images allow us to resolve the orientation of single molecules and subsequently we suggest that the molecular monolayer is stabilized by van der Waals interactions. The characterization of the observed Moiré contrast and a comparison with other similar systems underlines the importance of the central metal in the molecule-molecule and molecule-substrate interactions, which govern the molecular adsorption geometry.