O Oleg Kurnosikov

Cleaved thin-film probes for scanning tunneling microscopy

We introduce an alternative type of probe for scanning tunneling microscopy (STM). Instead of using a needle-like tip made from a piece of metallic wire, a sharp-edged cleaved insulating substrate, which is initially covered by a thin conductive film, is used. The sharp tip is formed at the intersection of the two cleaved sides. Using this approach a variety of materials for STM probes can be used, and functionalization of STM probes is possible. The working principle of different probes made of metallic (Pt, Co, and CoB), indium-tin oxide, as well as Cu/Pt and Co/Pt multilayer films are demonstrated by STM imaging of clean Cu(001) and Cu(111) surfaces as well as the epitaxial Co clusters on Cu(111).

Diamond deposition on modified silicon substrates: Making diamond atomic force microscopy tips for nanofriction experiments

Fine-crystalline diamond particles are grown on standard Si atomic force microscopy tips, using hot filament-assisted chemical vapor deposition. To optimize the conditions for diamond deposition, first a series of experiments is carried out using silicon substrates covered by point-topped pyramids as obtained by wet chemical etching. The apexes and the edges of the silicon pyramids provide favorable sites for diamond nucleation and growth. The investigation of the deposited polycrystallites is done by means of optical microscopy, scanning electron microscopy and micro-Raman spectroscopy. The resulting diamond-terminated tips are tested in ultra high vacuum using contact-mode atomic force microscope on a stepped surface of sapphire showing high stability, sharpness, and hardness.

Ordering of organic molecules on passivated reactive substrates: PTCDA on O-p(2x2)-Ni(1 1 1)

The ordering of an organic molecular layer on a ferromagnetic substrate is studied using scanning tunnelling microscopy. Highly ordered layers of perylene-tetracarboxylic-dianhydride (PTCDA) were prepared by vacuum sublimation on an oxygen precovered Ni(1 1 1) surface. The structure of thin layers of PTCDA deposited at room temperature was investigated as a function of growth rates and thickness. It is demonstrated that oxygen passivation reduces the reactivity sufficiently to lead to well-ordered overlayers of PTCDA. For thin films grown at low deposition rates, a herringbone-like structure has been observed. This structure is consistently observed in the islands with typically 100 nm in diameter and 1–2 ML thickness. Depositing thicker films at higher deposition rates results in polycrystalline islands. Within the polycrystalline islands two distinct stripe-like phases are observed in domains with a lateral size of typically 10 nm. The potential impact of these results for organo-metallic spintronic devices is addressed.

Direct observation of local hot electron transport through Al2O3 tunnel junctions

A modified ballistic electron emission microscopy (BEEM) technique using local transport of hot electrons through a buried interface, was successfully applied to study the Al2O3 barrier in the Co/Al2O3/Ru tunnel junction. This technique enabled us to straightforwardly measure an effective barrier height of 1.7 eV and to observe the rise of the barrier height due to continuous current injection into a single point of the junction attributed to charging effects and/or degradation of the barrier structure. Scanning over an area of 510 nm×510 nm showed a spatial inhomogenity of the barrier resulting in different dependencies of the BEEM current on the energy of the injected electrons.

Multi-channel Andreev reflection in Co-W nanocontacts fabricated using focused electron/ion beam induced deposition.

We report multi-channel electron transport in nano-contacts fabricated using focused electron beam induced deposited (FEBID) cobalt and focused ion beam induced deposited (FIBID) tungsten. Anomalous Andreev reflection (AR) effect is observed to which the conventional Blonder-Tinkham-Klapwijk (BTK) fit cannot be applied. In specific, we have observed multiple number of shoulders near the AR peak, whose origin is unknown in literature. We explain this effect based on a simple model that takes into account the material properties of the FIBID grown W superconductor, as well as the specific interface properties that are an outcome of using FEBID/FIBID as a fabrication technique. We show that numerical calculations using the BTK approximation based on the consideration of multiple channels generate similar shoulders as we observed in the AR experiments. Electrical measurements and x-ray photoemission spectroscopy carried out on FIBID W deposits puts additional evidence towards multi-channel current transport occuring at the interface of the nanocontacts.

Kinetic simulation of the 3D growth of subsurface impurity nanoclusters during cobalt deposition onto a copper surface

A physical model of the 3D growth of a subsurface cobalt cluster during the epitaxy of cobalt atoms on a copper substrate is developed. Time dependences are established for the cobalt cluster radius and height.

Computer simulation of the vertical growth of subsurface cobalt nanoclusters in gold

The vertical growth of nanodimensional cobalt clusters buried under the surface of a gold substrate has been studied using computer simulation methods with allowance for the interdiffusion of Au and Co atoms and the fields of elastic stresses generated by cobalt clusters in the gold matrix. The temporal variation of the concentrations of Co and Au atoms in the near-surface layers is described and the characteristic times of gold replacement by cobalt in layers above the buried clusters are determined. The results of simulations are compared to the available experimental data.

Interface spin–flip scattering model for point contact Andreev reflection

Point contact Andreev reflection measurements show a correlation between measured spin polarization and the interface scattering parameter Z extracted from fits of the modified Blonder–Tinkham–Klapwijk model to the conductance–voltage curves of superconductor/ferromagnet point contacts. We present a simple spin–flip scattering model which identifies Z2 as the effective scattering parameter and explains the observed exponential decay of the spin polarization.

Theoretical analysis of the formation of impurity precipitates in nanocavities I. Thermodynamic analysis

The formation of the impurity phase in materials containing nanopores is investigated theoretically. The formation of impurity clusters on the inner surface of pores is studied using the thermodynamic approach. The most advantageous states of metal impurities in silicon are determined, and comparison with available experimental data is carried out. The possibility of the formation of cobalt nanoclusters in subsurface cavities in copper is substantiated theoretically.

Surface superstructures of ordered layers of Al2O3 on Ni3Al(001)

Atomically ordered Al2O3 obtained by thermal oxidation of the Ni3Al(001) surface was studied using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). LEED analysis has shown that oxidation at 800 and 1100 K forms two different structural phases of Al2O3. Detailed STM studies of both phases also reveal remarkable differences in surface pattern structure and arrangement of the domains. We have shown that well-ordered Al2O3 layers are, in principle, full of dislocations. However, the dislocations, which are responsible for surface superstructures, are well ordered, forming either stripes in the low-temperature phase, or patches in the high-temperature phase. Interacting with the single-crystalline substrate, the nanopatches form their own lattice of superarrangement, introducing linelike or more complex superdislocations.