V. E. Asadchikov

Structure and properties of “nematically ordered” aerogels

The microstructure, specific area, and mechanical properties of various samples of “nematically ordered” aerogels whose strands are almost parallel to each other at macroscopic distances have been studied. The strong anisotropy of such aerogels distinguishes them from standard aerogels, which are synthesized by solgel technology, and opens new possibilities for physical experiments.

Model approach to solving the inverse problem of X-ray reflectometry and its application to the study of the internal structure of hafnium oxide films

The key features of the inverse problem of X-ray reflectometry (i.e., the reconstruction of the depth profile of the dielectric constant using an experimental angular dependence of reflectivity) are discussed and essential factors leading to the ambiguity of its solution are analyzed. A simple approach to studying the internal structure of HfO2 films, which is based on the application of a physically reasonable model, is considered. The principles for constructing a film model and the criteria for choosing a minimal number of fitting parameters are discussed. It is shown that the ambiguity of the solution to the inverse problem is retained even for the simplest single-film models. Approaches allowing one to pick out the most realistic solution from several variants are discussed.

Condensation of silica nanoparticles on a phospholipid membrane

The structure of the transient layer at the interface between air and the aqueous solution of silica nanoparticles with the size distribution of particles that has been determined from small-angle scattering has been studied by the X-ray reflectometry method. The reconstructed depth profile of the polarizability of the substance indicates the presence of a structure consisting of several layers of nanoparticles with the thickness that is more than twice as large as the thickness of the previously described structure. The adsorption of 1,2-distearoyl-sn-glycero-3-phosphocholine molecules at the hydrosol/air interface is accompanied by the condensation of anion silica nanoparticles at the interface. This phenomenon can be qualitatively explained by the formation of the positive surface potential due to the penetration and accumulation of Na+ cations in the phospholipid membrane.

Structural contribution to the roughness of supersmooth crystal surface

Technological advances in processing crystals (Si, sapphire α-Al2O3, SiC, GaN, LiNbO3, SrTiO3, etc.) of substrate materials and X-ray optics elements make it possible to obtain supersmooth surfaces with a periodicity characteristic of the crystal structure. These periodic structures are formed by atomically smooth terraces and steps of nano- and subnanometer sizes, respectively. A model surface with such nanostructures is proposed, and the relations between its roughness parameters and the height of atomic steps are determined. The roughness parameters calculated from these relations almost coincide with the experimental atomic force microscopy (AFM) data obtained from 1 × 1 and 10 × 10 μm areas on the surface of sapphire plates with steps. The minimum roughness parameters for vicinal crystal surfaces, which are due to the structural contribution, are calculated based on the approach proposed. A comparative analysis of the relief and roughness parameters of sapphire plate surfaces with different degrees of polishing is performed. A size effect is established: the relief height distribution changes from stochastic to regular with a decrease in the surface roughness.

Characterization of single-crystal sapphire substrates by X-ray methods and atomic force microscopy

The possibility of characterizing a number of practically important parameters of sapphire substrates by X-ray methods is substantiated. These parameters include wafer bending, traces of an incompletely removed damaged layer that formed as a result of mechanical treatment (scratches and marks), surface roughness, damaged layer thickness, and the specific features of the substrate real structure. The features of the real structure of single-crystal sapphire substrates were investigated by nondestructive methods of double-crystal X-ray diffraction and plane-wave X-ray topography. The surface relief of the substrates was investigated by atomic force microscopy and X-ray scattering. The use of supplementing analytical methods yields the most complete information about the structural inhomogeneities and state of crystal surface, which is extremely important for optimizing the technology of substrate preparation for epitaxy.

Laboratory X-ray microtomographs with the use of monochromatic radiation

The possibility and expediency of designing X-ray microtomographs based on X-ray diffractometers have been analyzed. Some biomedical objects have been investigated. It has been demonstrated that it is possible to achieve a resolution of the order 10 μm with a field of view of the order of 20 mm without recourse to magnifying X-ray optical elements.

On the formation of a macroscopically flat phospholipid membrane on a hydrosol substrate

The dependence of the structure of a phospholipid layer (DSPC and SOPC) adsorbed on a hydrosol substrate on the concentration of NaOH in a solution of 5-nm silica particles has been studied by X-ray reflectrometry with the use of synchrotron radiation. Profiles of the electron density (polarizability) have been reconstructed from the experimental data within a model-independent approach. According to these profiles, the thickness of the lipid film can vary from a monolayer (~35 Å) to several bilayers (~450 Å). At the volume concentration of NaOH of ~0.5 mol/L, the film on the hydrosol surface is a macroscopically flat phospholipid membrane (bilayer) with a thickness of ~60 Å and with areas of (45 ± 2) and (49 ± 3) Å2 per DSPC and SOPC molecule, respectively.

Kinetics of the formation of a phospholipid multilayer on a silica sol surface

The ordering of a multilayer consisting of DSPC bilayers on a silica sol substrate is studied within the model-independent approach to the reconstruction of profiles of the electron density from X-ray reflectometry data. It is found that the electroporation of bilayers in the field of anion silica nanoparticles significantly accelerates the process of their saturation with Na and H2O, which explains both a relatively small time of formation of the structure of the multilayer of (1 − 7) × 10 s and 13% excess of the electron density in it. A bilayer of phospholipid molecules is considered as the simplest model of a cell membrane [1-5]. We previously observed the crystallization of a multilayer of phospholipid bilayers whose thickness is given by the Debye screening length ΛD in the bulk of a hydrosol substrate on the surface of the aqueous solution of amorphous silicon dioxide nanoparticles [6, 7] (Fig. 1). In this work, the ordering of the multilayer is studied within the model-independent approach to the reconstruction of profiles of the electron density from X-ray reflectometry data without any a priori assumptions on the structure of the multilayer [8-12]. According to our data, the characteristic time of formation of the structure of the surface is (1 − 7) × 10 s; after that, the lipid film can be considered as a two-dimensional organic crystal with a quite high degree of perfection tikhonov@kapitza.ras.ruThe ordering of a multilayer consisting of DSPC bilayers on a silica sol substrate is studied within the modelindependent approach to the reconstruction of profiles of the electron density from X-ray reflectometry data. It is found that the electroporation of bilayers in the field of anion silica nanoparticles significantly accelerates the process of their saturation with Na+ and H2O, which explains both a relatively small time of formation of the structure of the multilayer of 1–7×105 s and ~13% excess of the electron density in it.

Upgraded X-ray topography and microtomography beamline at the Kurchatov synchrotron radiation source

An upgraded X-ray Topography and Microtomography (XRT-MT) station is described, the parameters of the optical schemes and detectors are given, and the experimental possibilities of the station are analyzed. Examples of tomographic reconstructions are reported which demonstrate spatial resolutions of 2.5 and 10 μm at fields of view of 2.5 and 10 mm, respectively.

Effective regularized algebraic reconstruction technique for computed tomography

A new fast version of the reconstruction algorithm for computed tomography based on the simultaneous algebraic reconstruction technique (SART) is proposed. The algorithm iteration is asymptotically accelerated using the fast Hough transform from O(n3) to O(n2logn). Similarly to the algebraic reconstruction technique (RegART), which was proposed by us previously, the regularization operator is applied after each iteration. A bilateral filter plays the role of this operator. The algorithm behavior is investigated using the model experiment.