Petra Reinke

C60 thin film growth on graphite: Coexistence of spherical and fractal-dendritic islands

The initial growth stage of C(60) thin film on graphite substrate has been investigated by scanning tunneling microscopy in ultrahigh vacuum at room temperature. The C(60) layer grows in a quasi-layer-by-layer mode and forms round, monolayer high islands on the graphite surface. The islands are confined by terraces on the graphite surface and the mobility of C(60) fullerenes across steps is low in all layers. The second and all subsequent layers adopt a fractal-dendritic shape, which was confirmed by calculating the fractal dimension (D=1.74 prior to island coalescence) and is in agreement with a diffusion limited aggregation. The profound differences between the growth of C(60) layers on graphite (first layer) and on C(60) surfaces (second and higher layers) are caused by the restriction of the C(60) mobility on the highly corrugated fullerene surfaces. The orientation of the fractal islands follows the hexagonal symmetry of the densely packed (111) surface of the fullerene lattice, which introduces a bias in the direction of molecule movement. The differences in surface topography on the nanoscale determine the mode of film growth in this van der Waals bonded system.

Hydrogen-plasma etching of ion beam deposited c-BN films: An in situ investigation of the surface with electron spectroscopy

In the present study nanocrystalline c-BN films deposited with a mass selected ion beam were subjected to a hydrogen plasma or atomic hydrogen produced by the hot filament method. Film composition and electronic properties of the surface were subsequently analyzed in situ by photoelectron spectroscopy in the x-ray and ultraviolet regime, and by electron energy loss spectroscopy. The sp2-bonded surface layer, which is native to ion beam deposited c-BN films, is rapidly removed by hydrogen plasma etching and the almost phase pure c-BN layer uncovered. Continuation of hydrogen plasma exposure leads to a removal of the c-BN layer at an etch rate of about 0.65 nm/min until finally the sp2-BN nucleation layer is detected. No preferential etching of either B or N is observed and an equal concentration of the constituents is maintained throughout the experiment. The large atomic hydrogen flux from the plasma is held responsible for the efficient etching, which could not be achieved with the hot filament method. T...

Gold cluster formation on a fullerene surface.

The growth of Au clusters on a fullerene thin film was investigated by in situ photoelectron spectroscopy in the ultraviolet (UPS) and x-ray (XPS) regime. Due to its highly corrugated surface fullerene films provide a wide range of bonding sites which could be exploited as molecular templates and serve to create a cluster superstructure. To gain insight into the fullerene-Au interaction two types of experiments were performed: (i) the deposition of Au on a fullerene surface, and (ii) the deposition of fullerenes on a Au surface. In both experiments an island growth mode is observed. The deposition of submonolayer amounts of C60 onto a gold film showed that the main interaction of the two species is due to chemisorption of the first C60 monolayer. In addition a constant band bending in the fullerene film is detected, but the UPS valence-band spectra show that there is no charge transfer from the Au to the C60 lowest unoccupied molecular orbital. In the reverse experiment, the cluster growth of Au on the corrugated C60 surface, the analysis of the Au core level does not reveal a specific bonding or nucleation site for Au atoms and clusters. This is in contrast to observations with Si clusters, which prefer to reside in the troughs between the fullerene molecules. The Au clusters grow continually from a size of about 55 atoms for the early stages of growth up to 150 atoms for the deposition of a nominal coverage of 1.5 nm. These data are derived from an analysis of the d-band splitting and the Au 4f core-level shift due to delayed photohole relaxation. The thermal stability of the Au-clusters-covered fullerene film was investigated by annealing in situ up to temperatures of 650 degrees C. For temperatures up to 450 degrees C a continuous growth of the clusters is detected, which is accompanied by a slight drop in Au concentration in the range of XPS for annealing temperatures higher than 350 degrees C. This may be due to a ripening of the clusters. The presence of Au apparently delays fullerene sublimation. The film shows a very good thermal stability and even after annealing at 650 degrees C there is still a fullerene film detectable in the photoelectron spectroscopy spectra.

Interaction of diamond with water: an in situ XANES investigation

The interaction of water and oxygen with a polycrystalline diamond film was investigated using XANES ( X -ray absorption near edge structure) spectroscopy. A novel reactor design allowed to access pressure ranges up to 4.0 mbar and simultaneous recording of the gas phase and sample spectra at the C-K and O-K edges, respectively. The diamond surface is inert with respect to the interaction with water in the investigated te m- perature below 500°C. A defect (sp 2 carbon) rich surface layer, is created through ion irradiation (Ar + , 2 keV), and the ratio of the */ * reso- nance was used as a measure of the extent of damage. A partial removal of the damage is observed in the first few minutes of water exposure, but a steady-state concentration of defects remains even after prolonged exposure periods. The resultant steady -state damage is nearly independent of the water pressure. The reaction of the damaged surface with water leads also to the atta chment of oxidic groups, mostly in the form of carbonyl groups, to the surface. The carbonyl groups seem to be located mostly at the surface and result directly from the reaction with dangling bonds or sp 2 -carbon defects with water. The irradiation with oxygen ions leads likewise to an overall r eduction in damage, the chemcial etching dominates the creation of new defects through ion impact.

Nanoscale probing of electronic band gap and topography of VO2 thin film surfaces by scanning tunneling microscopy

The metal-insulator transition (MIT) in vanadium dioxide in the vicinity of room temperature makes it one of the most interesting materials for novel switching device applications. It is therefore essential to have a fundamental understanding of the VO2 surface when it is incorporated into multilayer structures or nanodevices. This study focuses on the surface modification of VO2 in response to the thermal treatment during phase transition. Vacuum annealing at temperatures in the vicinity of the MIT triggers a partial reduction in the surface, and thus initiates a chemical phase transition. Scanning tunneling microscopy and spectroscopy are used to investigate the electronic properties and surface structure of the VO2 thin film on (0001) sapphire substrates. Band gap maps with a high spatial resolution and single point spectroscopy I-V curves are measured as the sample is cycled through the MIT, and thus provide a direct observation of the surface phase transition at the nanoscale. The VO2 surface exhibit...

Gold nanoclusters on amorphous carbon synthesized by ion-beam deposition

Gold clusters have been deposited by a monoenergetic, mass-selected ion beam with low energies (20–350eV) on amorphous carbon substrates in order to minimize the influence of the surface crystallinity and the ion-induced structural changes. Gold has been used as a model system, due to the poor reactivity with carbon, to study the ion-energy dependence, the temporal evolution, and the influence of the temperature on the cluster distribution. The cluster size is very sensitive to the energy and the mean size strongly decreases from 4 to less than 1nm as the ion energy increases. We can also note that the size distribution becomes broader. For impact energies below 100eV, surface processes dominate the cluster nucleation and growth. If higher energies are used, an increasing number of ions is implanted below the surface and different processes control the cluster formation. When the energy increases above 350eV, the cluster size drastically drops below 5nm. The samples are analyzed with different methods suc...

C60 bonding to graphite and boron nitride surfaces

The present study focuses on the interaction of C60 with the surfaces of highly oriented pyrolitic graphite (HOPG) and sp2-bonded boron nitride (BN). The nanocrystalline BN film was deposited by mass selected ion beams and features an sp2-bonded surface layer, which covers a cubic phase BN film. The first part of the experiment is the sequential deposition of C60, which is monitored by photoelectron spectroscopy in the x-ray (XPS) and ultraviolet (UPS) regime. The growth of the C60 layer on HOPG is close to a layer-by-layer growth mode, but on the BN surface island growth is favored. No charge transfer or chemical reaction (e.g., carbide formation) between the fullerene layer, and the underlying substrate is observed in either case. In the second part of the experiment the samples are heated at a rate of 10 K/min while simultaneously recording the UPS VB spectra. The complete desorption of C60 from the HOPG surface occurs in a small temperature interval between 510–530 K. For the sp2 BN surface the majori...

Ge1−xMnx heteroepitaxial quantum dots: Growth, morphology, and magnetism

Heteroepitaxial Ge1-xMnx quantum dots (QDs) were grown on Si (001) by molecular beam epitaxial co-deposition, with x = 0 to 0.10, in order to explore the interaction between Mn content, surface morphological evolution, and magnetism. Morphological evolution typical of the Ge/Si (001) system was observed, where the effect of Mn on surface morphology is surprisingly minimal at low Mn content, with no obvious surface morphological indicators of second phase formation. As the Mn content increases, secondary phase formation becomes evident, appearing to heterogeneously nucleate on or within Ge QDs. Still higher Mn concentrations lead to extensive second phase formation interspersed with an array of Ge QDs. Although ferromagnetism up to 220 K is observed, likely arising from intermetallic precipitates, there is no clear evidence for room-temperature ferromagnetism associated with a dilute magnetic solution phase.

The metal-insulator transition in vanadium dioxide: A view at bulk and surface contributions for thin films and the effect of annealing

Vanadium dioxide is investigated as potential oxide barrier in spin switches, and in order to incorporate VO2 layers in complex multilayer devices, it is necessary to understand the relation between bulk and surface/interface properties. Highly oriented VO2 thin films were grown on (0001) sapphire single crystal substrates with reactive bias target ion beam deposition. In the analysis of the VO2 films, bulk-sensitive methods [x-ray diffraction (XRD) and transport measurements] and surface sensitive techniques [photoelectron spectroscopy (PES) and scanning tunneling microscopy and spectroscopy] were employed. The samples were subjected to heating cycles with annealing temperatures of up to 425 and 525K. Prior to annealing the VO2 films exhibit the transition from the monoclinic to the tetragonal phase with the concurrent change in conductivity by more than a factor of 103 and their phase purity is confirmed by XRD. Annealing to 425K and thus cycling across the metal-insulator transition (MIT) temperature h...

Alternative Route to Silicene Synthesis via Surface Reconstruction on h-MoSi2 Crystallites

Silicene is a two-dimensional material with a Dirac-type band structure and it is particularly attractive due to its potential for integration with Si-based technology. The primary focus has been to grow single silicene layers and understand how the electronic structure is affected by the substrate and the phase transition between low- and high-buckling configurations. Typically, silicene is synthesized by depositing monolayer amounts of silicon onto a heated Ag(111) surface; however, other growth substrates such as Ir(111) and ZrB2 have been studied recently. We present a novel route for silicene synthesis via a high-temperature surface reconstruction of hexagonal-MoSi2 nanocrystallites. The h-MoSi2 crystallites are formed by annealing of thin Mo-layers on Si(100)-(2 × 1) and their crystallographic orientation is controlled via an epitaxial relation with the Si-substrate. The (0001) plane of h-MoSi2 is comprised of Si-hexagons with a Mo atom residing in the center. Annealing above approximately 650 °C causes the (0001) plane to undergo a surface reconstruction process leaving a honeycomb pattern on the surface of these crystallites as shown by scanning tunneling microscopy. We define this surface layer as a silicene-like reconstruction (SLR), and a detailed geometric analysis of our structure yields a perfect match with the (√3 × √3)R30° silicene superstructure in a low-buckled configuration (ABA̅). Scanning tunneling spectroscopy data of the SLR, Si(001)-(2 × 1) and h-MoSi2 surfaces agree with this interpretation. The formation of this structure on a transition metal silicide opens up the opportunity for integration into Si-based devices without the necessity for a transfer scheme.