Professor Vasyl Stadnyk is a scientific supervisor of the state budget theme FZ-53F named “New materials for functional electronics on the basis of semiconductor and dielectric crystals of А₄ВХ₆ and А₂ВХ₄ groups” (01.01.2017 – 31.12.2019) and a co-head of the theme with the State Fund for Fundamental Research F76/77-2018 (Fz-5-18) named “Synthesis of a new class of crystalline materials of А₂ВХ₄ group and study of the prospects of their effective application in electromagnetic radiation controlling devices” (30.08.2017 – 31.12.2017; 01.09.2018 – 30.11.2018).

The state budget theme “New materials for functional electronics on the basis of semiconductor and dielectric crystals of А₄ВХ₆ and А₂ВХ₄ groups”:

Aim of the work: to develop the technological fundamentals of synthesis of nanostructured materials based on А₂ВХ₄ and А₂ВХ₆ crystals, including those having birefringence sign inversion and high anisotropy of physical characteristics; to study unit cell parameters, space group symmetry, transmission, reflection and absorption spectra; temperature and spectral changes in refractive indices, birefringences, optical activity and dichroism.

The work is supposed to investigate the structural transformations, the impact of the structural elements replacement on the optical properties as well as the impact of external factors (pressure, temperature, electric and magnetic fields, radiation) on the changing of the electronic properties of bulk crystals and nanocrystals.

The crystals of LiNH₄SO₄ (LAS) in α- and β-modification, Rb₂ZnCl₄ (RTCZ) crystals with incommensurate phase, as well as In_xTl_1–хІ substitutional solid solutions were synthesized. The influence of uniaxial stresses on the spectral and temperature changes of the refractive indices n_i of RTCZ crystals was investigated. It has been established that in the studied spectral and temperature ranges, the dispersion n_i(λ) of mechanically free and stressed crystals is normal. The baric changes of n_i have been studied and crystal optics parameters of mechanically deformed crystals calculated. The band-energy structure of LAS crystals is calculated. As the quantitative measure of the dispersion of electronic levels the effective mass of charge carriers obtained for the top of the valence band and the bottom of the conduction band was considered. Using the calculated full and partial densities of states, the genesis of the calculated bands is established.

Results of the investigation of infrared reflection spectra in the 700–1700 cm^–1 region wave numbers are presented of mechanically free and clamped by the uniaxial pressure LiNH₄SO₄ crystal. Evaluated and discussed are frequencies of longitudinal ν_LO and transverse ν_ТO oscillations, damping constant g and oscillator strength f. Using the Kramers–Kronig dispersion relations for reflection spectra, baric changes in spectral dependences of optical constants were obtained and analyzed. Calculations of the band-energy structure of LiNH₄SO₄ crystals of two modifications were performed using density functional theory. Dispersion of electronic levels E(k) for the points of high symmetry of the Brillouin zone was constructed using various functionals for describing the exchange-correlation interaction of electrons. Width of the bandgap is estimated.

18 papers in journals with impact factor, 10 abstracts in the materials of the international conferences indexed in Scopus, and 3 patents of Ukraine for invention and useful models were published, as well as  the  thesis by M. V. Kovalenko entitled “Electronic energy structure, opto-spectral and sensory properties of nanostructures based on ZnO” (a Supervisor was a Docent, Candidate of Physical and Mathematical Sciences O. V. Bovhyra, specialty 01.04.10 – physics of semiconductors and insulators, 2017); A. I. Kashuba entitled “Transformation of electronic energy structures and optical parameters of substitutional solid solutions of halides indium and thallium” (a Supervisor was a Professor, Doctor of Physical and Mathematical Sciences A. V. Franiv, specialty 01.04.10 – physics of semiconductors and insulators, 2018); V. B. Stakhura entitled “The optoelectronic properties of uniaxially clamped incommensurate modulated Rb₂ZnCl₄ crystals” (a Supervisor was a Docent, Candidate of Physical and Mathematical Sciences V. Yu. Kurlyak, specialty 01.04.10 – physics of semiconductors and insulators, 2018); M. Ya. Rudysh entitled “Optical-electronic parameters of lithium ammonium sulfate crystals under the action of uniaxial pressures” (a Supervisor was a Professor, Doctor of Physical and Mathematical Sciences V. Yo. Stadnyk, specialty 01.04.10 – physics of semiconductors and insulators, 2018) were defended.

The theme with the State Fund for Fundamental Research “Synthesis of a new class of crystalline materials of А₂ВХ₄ group and study of the prospects of their effective application in electromagnetic radiation controlling devices”:

The crystals of LiNH₄SO₄ (LAS), K_1.75(NH₄)_0.25SO₄ (PAS) and (NH₄)₂SO₄ (AS) of good optical quality were synthesized, their spatial symmetry groups and parameters of elementary cells were determined; dispersion and temperature dependences of refractive indices n_і(λ) and birefringence Δn_і(λ) of these crystals were studied; for PAS crystal at a wavelength λ=500 nm at temperatures T₀₁≈541 K and T₀₂≈589 K two isotropic points are found which correspond to the equality of there fractive indices n_z=n_y and n_z=n_x; the temperature-spectral-baric diagrams of the isotropic state of the K_1.75(NH₄)_0.25SO₄ crystal, which allow to find the correlation between the deformations of the optical indicatrix due to temperature changes, uniaxial compression, or wavelength change have been constructed. The dispersion of refractive indices n(λ) of LiNH₄SO₄ crystals in two modifications is investigated using Obreimov method for the three crystal-physical directions in the optical spectral region. The refractive indices of the studied crystals are of a significant anisotropy. The presence of birefringence-sign-inversion points for α-modification is found in the long-wavelength region of the spectrum, for β – in the short-wavelength region. It was proposed to use LiNH₄SO₄ and K_1.75(NH₄)_0.25SO₄ crystals as new crystals with anisotropic point in accessible temperature and spectral ranges.

Based on the results, 5 articles were published in journals with impact factor and one article is prepared for publication; 5 abstracts in the materials of international conferences indexed by Scopus are published.

Scientific interests and researches of the Department staff are:

• refractive parameters of mechanically free and uniaxially stressed crystalline ferroics;
• phase transformations in crystalline dielectrics and physics of incommensurate phases;
• kinetic effects in semiconductor single crystals, thermodynamics of formation and electronic properties of nano- and submicron clusters in semiconductors;
• spectral-luminescent properties of wide-bandgap dielectric crystals, radiation-sensitive properties of scintillation materials and luminophors;
• optical and electrical properties of conducting polymers;
• luminescent and kinetic parameters of crystals and calculation of their electronic energy structure;
• high precision polarimetry, computerization of optical experiment;
• optical, luminescent and electrical properties of single crystal materials based on halides and oxides of metals.