V. Petrova

Growth of Semiconducting Graphene on Palladium

We report in situ scanning tunneling microscopy studies of graphene growth on Pd(111) during ethylene deposition at temperatures between 723 and 1023 K. We observe the formation of monolayer graphene islands, 200-2000 A in size, bounded by Pd surface steps. Surprisingly, the topographic image contrast from graphene islands reverses with tunneling bias, suggesting a semiconducting behavior. Scanning tunneling spectroscopy measurements confirm that the graphene islands are semiconducting, with a band gap of 0.3 +/- 0.1 eV. On the basis of density functional theory calculations, we suggest that the opening of a band gap is due to the strong interaction between graphene and the Pd substrate. Our findings point to the possibility of preparing semiconducting graphene layers for future carbon-based nanoelectronic devices via direct deposition onto strongly interacting substrates.

Moiré superstructures of graphene on faceted nickel islands.

Using scanning tunneling microscopy and spectroscopy, in combination with density functional theory calculations, we investigated the morphology and electronic structure of monolayer graphene grown on the (111) and (110) facets of three-dimensional nickel islands on highly oriented pyrolytic graphite substrate. We observed graphene domains exhibiting hexagonal and striped moiré patterns with periodicities of 22 and 12 Å, respectively, on (111) and (110) facets of the Ni islands. Graphene domains are also observed to grow, as single crystals, across adjacent facets and over facet boundaries. Scanning tunneling spectroscopy data indicate that the graphene layers are metallic on both Ni(111) and Ni(110), in agreement with the calculations. We attribute this behavior to a strong hybridization between the d-bands on Ni and the π-bands of carbon. Our findings point to the possibility of preparing large-area epitaxial graphene layers even on polycrystalline Ni substrates.

TiN(001) and TiN(111) island coarsening kinetics: in-situ scanning tunneling microscopy studies

Abstract In-situ high-temperature scanning tunneling microscopy was used to follow the coarsening (Ostwald ripening) and decay kinetics of two-dimensional TiN islands on atomically-flat TiN(001) and TiN(111) terraces at 750–950°C. The rate-limiting mechanism for island decay was found to be adatom surface-diffusion on (001) and attachment/detachment at step edges on (111) surfaces. We have modeled island decay kinetics based upon the Gibbs–Thomson and steady-state diffusion equations to obtain a 001-step edge energy per unit length of 0.23±0.05 eV/A with an activation energy of 3.4±0.3 eV for adatom formation and diffusion on TiN(001). The activation energy for adatom formation and attachment/detachment on TiN(111) is 3.5±0.3 eV.

IN-SITU HIGH-TEMPERATURE SCANNING-TUNNELING-MICROSCOPY STUDIES OF TWO-DIMENSIONAL ISLAND-DECAY KINETICS ON ATOMICALLY SMOOTH TiN(001)

In-situ high-temperature scanning tunneling microscopy was used to follow the coarsening (Ostwald ripening) and decay kinetics of single and multiple two-dimensional TiN islands on atomically flat TiN(001) terraces and in single-atom deep vacancy pits at temperatures of 750–950°C. The rate-limiting mechanism for island decay was found to be surface diffusion rather than adatom attachment/detachment at island edges. We have modeled island-decay kinetics based upon the Gibbs–Thomson and steady state diffusion equations to obtain a step-edge energy per unit length of 0.23±0.05 eV/A and an activation energy for adatom formation and diffusion of 3.4±0.3 eV.

TiN surface dynamics: role of surface and bulk mass transport processes

Transition‐metal nitrides, such as TiN, have a wide variety of applications as hard, wear‐resistant coatings, as diffusion barriers, and as scratch‐resistant and anti‐reflective coatings in optics. Understanding the surface morphological and microstructural evolution of these materials is crucial for improving the performance of devices. Studies of surface step dynamics enable determination of the rate‐limiting mechanisms, corresponding surface mass transport parameters, and step energies. However, most models describing these phenomena are limited in application to simple elemental metal and semiconductor surfaces. Here, we summarize recent progress toward elucidating the interplay of surface and bulk diffusion processes on morphological evolution of compound surfaces. Specifically, we analyze the coarsening/decay kinetics of two‐ and three‐dimensional TiN(111) islands and the effect of surface‐terminated dislocations on TiN(111) steps.

YBCO/LSMO and LSMO/YBCO double-layer deposition by off-axis magnetron sputtering and strain effects

We report here on ferromagnetic/superconductor FM/SC and SC/FM double layers deposited without a buffer layer between FM and SC. Thin films of La0.7Sr0.3MnO3 (LSMO) are used for the FM, and YBa2Cu3O7-x (YBCO) for the SC. Both films can grow crystalline on SrTiO3 (STO) or LaAlO3 (LAO) substrates in high temperature and in oxygen atmosphere conditions. Magnetic and transport measurements and X-ray diffraction analysis are used to characterise film quality and properties. It is shown that off-axis magnetron sputtering can be used for high-quality double layer deposition. A prerequisite for this is the suppression of the interdiffusion process. This is achieved by lowering the deposition temperature and shortening the deposition time for the top layer. Lattice relaxation for LSMO films deposited on LAO substrates is seen. It is demonstrated that post-deposition annealing or additional top layer deposition enlarges the relaxed part of the lower LSMO film. LSMO films are smooth and free of imperfections and the stress is partly relieved by the formation of misfit dislocations. For YBCO films, SEM, XRD, EDX and magnetisation characterisations show that film lattice relaxation starts when single-element-oxide crystals start to grow into the film. Many experiments give evidence of a critical thickness of about 30nm of a strained layer, after which the top part of the YBCO film relaxes. Despite the small compositional deviation due to the interdiffusion, the YBCO/LSMO double films demonstrate high enough transport and magnetic properties to allow their application in the investigation of the injection of spin-polarised quasiparticles from FM to SC film. The critical current for YBCO film is Jc∼0.7 × 106A/cm2 if it is a bottom layer, and Jc∼1.7 × 106A/cm2 if YBCO is a top layer, whilst our optimal single-layer YBCO films have Jc∼(2.5-5) × 106A/cm2 at 77 K.

Role of ethylene on surface oxidation of TiO2(110)

Using in situ high-temperature (700-1000 K) scanning tunneling microscopy (STM), we studied the influence of ethylene on the surface dynamics of oxygen-deficient, rutile-structured TiO2(110). STM images were acquired during annealing the sample as a function of time, oxygen and ethylene pressures, and temperature. With increasing oxygen pressure and/or decreasing temperature, TiO2(110) surface mass increased, consistent with previous results. Interestingly, annealing the sample in ethylene with traces of oxygen also results in the growth of TiO2 at higher rates than those observed during annealing in pure oxygen. Our results indicate that ethylene promotes oxidation of TiO2(110).