S. S. Kosolobov

Surface enhanced Raman scattering of light by ZnO nanostructures

Raman scattering (including nonresonant, resonant, and surface enhanced scattering) of light by optical and surface phonons of ZnO nanocrystals and nanorods has been investigated. It has been found that the nonresonant and resonant Raman scattering spectra of the nanostructures exhibit typical vibrational modes, E2(high) and A1(LO), respectively, which are allowed by the selection rules. The deposition of silver nanoclusters on the surface of nanostructures leads either to an abrupt increase in the intensity (by a factor of 103) of Raman scattering of light by surface optical phonons or to the appearance of new surface modes, which indicates the observation of the phenomenon of surface enhanced Raman light scattering. It has been demonstrated that the frequencies of surface optical phonon modes of the studied nanostructures are in good agreement with the theoretical values obtained from calculations performed within the effective dielectric function model.

Precise surface measurements at the nanoscale

Availability of self-assembly effects occurring at the atomically clean Si(1 1 1) surface during high temperature anneals in an ultrahigh vacuum chamber for fabrication of a precise calibrator at nanoscale measurements is discussed. These effects provide formation of ordered monatomic step arrays assembled by step bunches divided by almost singular surface areas with widely spaced monatomic steps suitable for calibration of atomic force microscopes. The monatomic step height at the Si(1 1 1) surface and its replication by the native oxide layer was attested by the high-resolution transmission electron microscopy followed by Digital Micrograph analysis and found to be equal to interplanar spacing (0.314 nm) in the volume of Si crystal with ±0.001 nm of accuracy. Excellent replication of the monatomic step height by oxide film covering the Si surface makes available precise AFM calibration at the nanoscale at ambient conditions. The averaged step height measured by AFM scanning of 1 × 1 µm2 is found to be 0.314 ± 0.003 nm (~1% of uncertainty). However, when the scan area becomes bigger than 2 × 2 µm2, the height measurement uncertainty increases sharply 15 times (0.310 ± 0.034 nm). We assume that this is due to differences between piezo element calibrations at small and large scan areas. The height measurement uncertainty for step bunches with well-defined quantity of steps (28) even at a large scan area (18 × 18 µm2) turns out to be 0.3%.

Origin of temperature oscillations of nickel catalyst occurring in methane oxidation

The catalytic oxidation of methane over a nickel foil has been investigated in a wide range of methane: oxygen molar ratios at a constant reactor temperature of 680°C. Under oxygen-deficient conditions, the reaction takes place in the self-oscillating mode. Stable self-sustained oscillations of the methane oxidation rate are observed in the CH4: O2 molar ratio range from 2: 1 to 19: 1. This process is accompanied by catalyst temperature oscillations, whose amplitude reaches 80°C. It has been demonstrated by scanning electron microscopy that the origination of the self-sustained oscillations is accompanied by morphological changes in the catalyst surface. Under the action of the reaction medium, a porous layer 5–10 µm in thickness forms on the nickel foil surface. The mechanism of methane oxidation over nickel that accounts for the onset of the self-sustained oscillations is discussed. This mechanism is based on periodical nickel oxidation and reduction. When nickel is in the high-activity state, it is mainly in metallic form, and the passage of nickel into its low-activity state is accompanied by the formation of a nickel oxide layer on the foil surface. The reduction of this nickel oxide causes a periodic decrease in the catalyst temperature. The total and partial oxidations of methane on the reduced surface of the nickel foil raise the catalyst temperature. Under oxygen-deficient conditions, a carbon deposit builds up on the catalyst surface, and the combustion of this deposit complicates the catalyst temperature oscillation profile.

In situ study of the interaction of oxygen with the Si(111) surface by ultrahigh-vacuum reflection electron microscopy

Reactions of gases with an atomically clean silicon surface were examined by ultrahigh-vacuum reflection electron microscopy. The initial stages of interaction of oxygen with the Si(111) surface were studied in the temperature range from 500 to 900°C. Motion of monatomic steps to the upper terraces during etching by oxygen at high temperatures was visualized. The conditions for the formation of surface vacancies were determined. The dependence of the step velocity on the width of adjacent terraces was measured. Oscillations of the intensity of the electron beam reflected specularly from the Si surface were observed during the etching of silicon by molecular oxygen, which proceeded by a two-dimensional-island mechanism. The activation energy for the diffusion of surface vacancies, which are formed owing to the interaction of oxygen with silicon, was estimated to be 1.35±0.15 eV.

Electrophoretic deposition of CdS colloidal nanoparticles onto an amorphous silicon membrane

A method for producing nanoporous silicon membranes suspended over the microchannels of a silicon matrix is proposed. Solid concentrates of CdS nanoparticles are produced by electrophoretic deposition onto this thin-film carrier. The results of complex studies that confirm the formation of a Si-CdS composite in the regions of ion-current flow through the porous membrane are reported. It is shown that the gradual accumulation of semiconductor nanoparticles encapsulated in thioglycolic-acid organic shells occurs during electrophoresis. It is inferred that a similar technological approach is applicable also to biological substances, whose dosed concentrates can be used in biomedical investigations.

The role of Euler buckling instability in the fabrication of nanoelectromechanical systems on the basis of GaAs/AlGaAs heterostructures

Mechanical stresses are investigated in suspended nanowires made on the basis of GaAs/AlGaAs heterostructures. Though there are no intentionally introduced stressor layers in the heterostructure, the nanowires are subject to Euler buckling instability. In the wide nanowires, the out-of-plane buckling is observed at length significantly smaller (3 times) than the theoretically estimated critical value, while in the narrow nanowires, the experimentally measured critical length of the in-plane buckling coincides with the theoretical estimation. The possible reasons for the obtained discrepancy are considered. The observed peculiarities should be taken into account in the fabrication of nanomechanical and nanoelectromechanical systems.

Precise measurements of nanostructure parameters

The precision of measurements performed by atomic-force microscopy (AFM) and high-resolution electron microscopy (HREM) for solving problems of metrology and diagnostics of solid nanostructures is discussed. The HREM-measured height of a monatomic step on a Si(111) surface covered by a thin natural oxide film is demonstrated to be 0.314 ± 0.001 nm. The same accuracy is ensured by AFM measurements through controlling the Si surface relief with heating in ultra-high vacuum on specially created test objects with the distance between the steps being approximately 2 µm. It is shown that the geometric phase method can be used to quantify the strains in the crystal lattice of strained heterostructures on the basis of HREM images with accuracy to 10−4%, and in situ irradiation by electrons in HREM measurements can be used to visualize ordered clusterization of vacancies and self-interstitial atoms in {113} planes in Si samples.

Instability of the Distribution of Atomic Steps on Si(111) upon Submonolayer Gold Adsorption at High Temperatures

The gold adsorption effect on the distribution of monatomic steps on the (111) silicon surface is studied in situ by ultrahigh vacuum reflection electron microscopy at temperatures of 850–1260°C. A new effect of the instability of silicon surface morphology has been detected. This effect leads to the redistribution of regular steps (RSs) to step bunches (STs) and vice versa on a surface covered with a gold submonolayer. For the crystal heated by directly passing an electric current, the behavior of the RS ⇔ SB morphological transitions on the silicon surface is investigated as a function of the gold coverage and the direction of the heating current. Thus, isothermal annealing at 900°C is accompanied by the following transitions on the silicon surface with predeposited 0.75 monolayer gold coverage: RS (0.72) ⇒ SB (0.42) ⇒ RS (0.24) ⇒ SB (0.07) RS ⇒ (0). The numbers given in parentheses are estimated values of the critical gold coverage measured in the monolayers at which the morphological transitions are observed. A change in the direction of the electric current used to heat the crystal leads to the reversible changes RS ⇒ SB and SB ⇒ RS at the same values of the critical gold coverage.

A simple approach to prepare molecularly imprinted polymers from nylon-6

A convenient and simple approach for the preparation of molecularly imprinted polymers (MIPs) based on polyamide (nylon‐6) was developed. The polymer matrix formation occurred during the transition of nylon from dissolved to solid state in the presence of template molecules in the initial solution. 2,2,2‐Trifluoroethanol (TFE) was chosen as a main solvent for the polyamide. It provides a high solubility of nylon and does not significantly change the structure of biopolymers. The alteration of the polymer matrix structure after the addition of different types of porogens in the nylon/TFE solution was investigated. The structured polymers in the form of films and microparticles were prepared in the chosen optimal conditions. Different model biomolecular templates (of low‐ and high‐molecular weight) were used for the preparation of MIPs, which were shown to specifically recognize these molecules upon binding experiments. The binding of the template molecules to MIPs was monitored using spectrophotometric, radioisotopic, or fluorometric detection. The selectivity coefficients of the MIPs were estimated to be 1.4–4.6 depending on the type of templates and conditions of the polymer matrix formation. Copyright

High-precision nanoscale length measurement

Modern lithographical methods used to create linear measures for nanometer-range dimensions and the main factors which limit the applications of such gages have been analyzed in the paper. Prospects for developing high-precision measures based on an atomically structured crystalline surface (containing monoatomic steps) whose parameters are bound to the crystallographic parameters of the crystal (traceable to the length measure) are shown. A method which can be used to create such measures based on controlling the surface morphology of monocrystalline silicon at an atomic level due to the effects of self-organization arising at the atomically clean surface as a result of annealing in ultrahigh vacuum is proposed. A description of the set of high-precision gages of vertical dimensions STEPP-IFP-1 in a size range of 0.31–31 nm with an error in the whole interval of gages of less than 0.05 nm is presented. The set of high-precision gages after carrying out state testing is included into the state registry of measuring means as measuring type no. 48115-11 (Federal Agency on Technical Regulating and Metrology order no. 6290 of October 31, 2011).