O.A. Utas

Stepwise self-assembly of C60 mediated by atomic scale moiré magnifiers

Self-assembly of atoms or molecules on a crystal surface is considered one of the most promising methods to create molecular devices. Here we report a stepwise self-assembly of C₆₀ molecules into islands with unusual shapes and preferred sizes on a gold-indium-covered Si(111) surface. Specifically, 19-mer islands prefer a non-compact boomerang shape, whereas hexagonal 37-mer islands exhibit extraordinarily enhanced stability and abundance. The stepwise self-assembly is mediated by the moiré interference between an island with its underlying lattice, which essentially maps out the adsorption-energy landscape of a C₆₀ on different positions of the surface with a lateral magnification factor and dictates the probability for the subsequent attachment of C₆₀ to an islands periphery. Our discovery suggests a new method for exploiting the moiré interference to dynamically assist the self-assembly of particles and provides an unexplored tactic of engineering atomic scale moiré magnifiers to facilitate the growth of monodispersed mesoscopic structures.

A cooling system for samples for studying their surfaces in ultrahigh vacuum

The device described is designed to cool samples with liquid nitrogen to T=−165°C during low-energy electron diffraction (LEED) studies at an ultra-high-vacuum MALTIPROBE Compact system (Omicron) and versions thereof equipped with a scanning tunneling microscope and an LEED system. The efficiency of the system is demonstrated using the example of a low-temperature 2×1 ↔ c(4×2) phase transition on a (100)-oriented silicon surface observed using the LEED technique.

Magnetoresistive properties of nanostructured magnetic metals, manganites, and magnetic semiconductors

We consider methods for controlling magnetoresistive parameters of magnetic metal superlattices, manganites, and magnetic semiconductors. By reducing the thickness of ferromagnetic layers in superlattices (e.g., Fe layers in Fe/Cr superlattices), it is possible to form superparamagnetic clustered–layered nanostructures with a magnetoresistance weakly depending on the direction of the external magnetic field, which is very important for applications of such type of materials. Producing Mn vacancies and additionally annealing lanthanum manganites in the oxygen atmosphere, it is possible to increase their magnetoresistance by more than four orders of magnitude. By changing the thickness of p–n junction in the structure of ferromagnetic semiconductors, their magnetoresistance can be increased by 2–3 orders of magnitude.

Molecular simulations of C60 self-assembly on metal-adsorbed Si(111) surfaces

The authors have proposed a simulation procedure for the evaluation of energetics of C60 islands on crystalline surfaces that allows questions relating to shape, size, and orientation of the islands to be addressed. Simulation consists of placing a patch of close-packed C60 array of a given shape and size on a surface potential relief and finding energy minima by variation of island position and orientation. Upon appropriate adjustment of the surface potential relief, simulations reproduce well all the main results of the scanning tunneling microscopy observations. For C60 islands forming on In-adsorbed Si(111)3×3-Au and pristine Si(111)3×3-Ag surfaces, the optimal surface relief shows up as a periodic array of cosine-shaped peaks. The proposed approach provides a hint for understanding the driving mechanisms of C60 self-assembly, and, in principle, it can be applied to other adsorbate-substrate systems.

Increase in the density of β-FeSi2 nanoclusters on a Si(111) surface by means of Si(111) √3 × √3R30°-B reconstruction

The formation of iron disilicide (β-FeSi2) nanoclusters as a result of solid-state epitaxy at T = 500–700°C and an iron coverage of 0.05–0.5 monolayer on a boron-modified Si(111)√3 × √3 R30° surface has been studied by scanning tunneling microscopy. It is established that the number density of β-FeSi2 nanoclusters on the Si(111) √3 × √3 R30°-B surface significantly exceeds the density of silicide clusters formed on the atomically clean Si(111) surface with a 7 × 7 reconstruction for the analogous iron coverages and annealing temperatures. At the same time, the density of point defects and clusters possessing metallic conductivity on the Si(111) √3 × √3 R30°-B surface is several orders of magnitude lower than on the Si(111)7 × 7 surface treated under identical conditions.

Electrical conductivity of surface phases on silicon

Electrical conductivity of surface phases on silicon have been studied in ultrahigh vacuum at room temperature by four-point probe method. It has been shown that surface conductance of silicon substrate strongly depends on crystal and electronic structure of surface phases, surface morphology and density of atoms involved in surface phases formation. The atom adsorption, structural and morphological transformation lead to changing of long-order structure of surface phases and consequently in decreasing of electrical conductivity. It is shown that surface phases as new ultrathin 2D material presents additional conducting channel on silicon substrate and is believed to be promising in microelectronics technology.