I. Kh. Akopyan

Luminescence of mercuric iodide crystals

Low-temperature (4.2–130 K) photoluminescence spectra of HgI2 crystals have been measured in the 540–700 nm region. An analysis of the characteristics (intensity vs temperature and excitation power relations, afterglow times, excitation spectra) of the 560, 620, and 635 nm emission bands suggests the following assignments: the 560 nm band is due to radiative annihilation of excitons bound to mercury vacancies, and the “red” emission originates from recombination of free (620 nm) and donor-localized (635 nm) electrons with a hole-filled acceptor level. The energies of the corresponding donor and acceptor levels have been estimated. New emission bands at 540, 545, and 575 nm have been discovered, and their origin discussed.

Specific features of the CuI nanocrystal structure in photochromic glasses

The absorption spectra of CuI nanocrystals with an average radii of 5.3 and 6.2 nm in photochromic glass matrix have been studied in the temperature range 38–300 K. It has been established that the copper iodide nanocrystals, like copper chloride and copper bromide nanocrystals of the same sizes, predominantly crystallize in a nonequilibrium form different from the stable low-temperature cubic modification. The structure of metastable modifications of copper halide nanocrystals in photochromic glasses has been discussed.

Structure of copper halide nanocrystals in photochromic glasses

The crystal structure of CuCl nanocrystals with effective radii ranging from 2.4 to 18.5 nm in photochromic glasses heat treated under different conditions is investigated using exciton spectroscopy, x-ray powder diffraction, and small-angle x-ray scattering. It is revealed that the size dependence of the energy of the Z3 exciton exhibits an anomalous behavior. The results obtained suggest that small-sized CuCl nanocrystals can crystallize in a modification different from the stable cubic modification.

Size-quantization effects in the optical spectra of PbI2 and HgI2 nanocrystals

Size-quantization effects (short-wavelength shift of the structure and broadening) in low-temperature absorption spectra of PbI2 and HgI2 nanocrystals embedded in porous sodium borosilicate glass matrix with pores ranging in size from 2 to 50 nm have been observed and studied. The nanocrystal size was evaluated in the strong quantization approximation. It is shown that the size does not depend on the pore diameter of the matrices used, but is determined by the concentration of the solution introduced into the pores. The absorption and luminescence spectra of the systems thus obtained are shown to evolve as the substance emerges from the pores out onto the surface. The dynamics of formation and the phase composition of the PbI2 and HgI2 crystals on the surface are studied.

Determination of the heat of superionic Ag2HgI4 and Cu2HgI4 crystals formation in the course of the solid state reactions

Abstract The values of the heat of formation of Ag 2 HgI 4 and Cu 2 HgI 4 crystals in the course of solid state chemical reactions 2AgI + HgI 2 Ag 2 HgI 4 and 2CuI + HgI 2 Cu 2 HgI 4 (5.5 kJ/mol and 7.6 kJ/mol, respectively) have been determined experimentally by differential scanning calorimetry method.

Manifestation of oxygen desorption in photoluminescence spectra of ZnO

The influence of oxygen desorption on the photoluminescence of zinc oxide has been investigated. Photodesorption and thermal desorption of oxygen have been controlled using mass spectrometry. It has been found that the removal of oxygen from the ZnO surface leads to a manifold increase in the exciton photoluminescence intensity. The process has a reversible character. The mechanism of increase in the exciton luminescence intensity of ZnO during the oxygen desorption has been discussed.

Photoluminescence Spectra of thin Zno films grown by ALD technology

The photoluminescence of ZnO films grown by atomic layer deposition (ALD) on silicon substrates has been investigated. A new broad photoluminescence band has been revealed in the exciton region of the spectrum. The properties of the band in the spectra of the films with different crystallographic orientations of substrates have been studied in a wide temperature range at different excitation levels. A model describing the origin of the new band has been proposed.

Photostimulated growth of whiskers in AgI-type superionic crystals

The growth of whiskers inside and on the surface of superionic crystals (AgI, CuI, RbAg4I5) is considered. The crystals are exposed to radiations with different spectral compositions at temperatures above and below the temperature of the superionic phase transition. The chemical composition, structure, and properties of whiskers are studied with optical microscopy, scanning electron microscopy, transmission electron microscopy, and photoluminescence. The mechanisms of photostimulated growth of whiskers in semiconductors with a high ionic conductivity are discussed, and the role of the “molten” cation sublattice during nucleation and growth of whiskers is considered.

Metastable modifications in mercury diiodide nanocrystals

The formation of the HgI2 crystalline phase in pores of various porous matrices and on their surfaces was studied. Based on the analysis of the evolution of the HgI2 optical spectra and electron and atomicforce microscopy data, it was shown that the formation of microcrystals of the stable red α modification is preceded by the formation and growth of particles of metastable yellow (β′) and orange modifications. The absorption edge of the β′ phase at T = 77 K is formed by excitonic transitions with an energy of ∼2.7 eV. The Raman spectra of the β′ phase were compared to the spectra of known metastable yellow modifications.

Stabilized High-Temperature Hexagonal Phase in Copper Halide Nanocrystals

Low-temperature (T = 4.2–77 K) absorption spectra of CuCl and CuBr nanocrystals in photochromic glass matrixes are studied. A fine structure of exciton absorption bands (Z3 band for CuCl and Z12 band for CuBr) is discovered and studied as a function of nanocrystal size. It is suggested that the high-energy part of the absorption band is due to the high-temperature hexagonal β phase being stabilized in very small samples; a transition to the stable cubic phase with increasing nanocrystal size is demonstrated.