M. Yu. Maksimuk

Crystal structure and photoluminescence of single crystals of fullerene-9,9′-trans-bis(telluraxanthenyl) molecular complex: C26H18Te2 · C60 · CS2

Abstract The crystal structure of novel molecular complex of fullerene C60 with 9,9′-trans-bis(telluraxantheny):C26H18Te2 · C60 · CS2 (BTX · C60 · CS2) has been investigated by X-ray structural analysis. Its photoluminescence (PL) and optical reflectivity spectra have been examined. We have found that fullerene C60, donor BTX and carbon disulfide CS2 molecules are situated at the inversion centers (1,0, 1 2 ), ( 1 2 , 1 1 2 ,1), ( 1 2 , 1 2 ,1),( 1 2 , 1 2 , 1 2 ), respectively. The PL spectrum in C60+BTX is shifted by 0.16eV towards lower energies compared to C60. The optical reflectivity spectrum of C60 + BTX is also different from that of C60. The results are explained in terms of a decrease of the singlet exciton energy due to the strong interaction between C60 and BTX molecules. The decrease of PL intensity in the new complex has been found to begin at much lower temperature as compared to the pure C60 crystals.

Synthesis of α-SiC nanocrystals by carbothermal reduction of spherical nanoparticles of amorphous silicon dioxide

The method for carbothermal reduction of spherical particles of amorphous silicon dioxide is developed, and hexagonal α-SiC polytype nanocrystals are synthesized. The prepared samples are characterized by X-ray diffraction, Raman spectroscopy, photoluminescence spectroscopy, and electron microscopy. The silicon carbide nanocrystals prepared have sizes in the range 5–50 nm depending on the diameter of initial silicon dioxide particles. A detailed analysis of the positions of the lines in the Raman spectra, their broadening, and shift makes it possible to reliably establish that the samples under investigation predominantly contain the 6H and 4H silicon carbide polytypes and insignificant amounts of the 2H and 3C phases. The 15R and 21R polytypes in the samples are absent. It is noted that the samples are characterized by a substantial size effect: the luminescence intensity of small silicon carbide nanocrystals is more than three times higher than that of large SiC nanocrystals.

IR-active optical phonons in Pnma-1, Pnma-2 and R3¯c phases of LaMnO3+δ

Infrared-active phonons in LaMnO3+δLaMnO3+δ were studied by means of the reflection and transmission spectroscopy from 50 to 800cm-1 at room temperature. Powder and ceramic samples of the phases of Pnma-2(δ=0.02),Pnma-1(δ=0.08) and R3¯c(δ=0.15) were investigated. In addition, energies of the dipole-active phonons in Pnma-2, Pnma-1 phases were obtained by lattice-dynamics calculations. The transformations of IR-active phonons with the increase in δδ in the sequence of Pnma-2, Pnma-1, R3¯c are discussed.

Optical phonon spectra of PbF2 single crystals

The spectra of dipole-active optical phonons are measured for the cubic and orthorhombic phases of PbF2 single crystals. The frequencies and eigenvectors of normal modes in the Pnma orthorhombic phase are calculated. It is found that the spectrum of the cubic phase exhibits excess vibrational modes of the PbF2 orthorhombic phase.

Photoinduced light absorption by C60 films in the 0.08-4.0-eV spectral range

Spectra of photoinduced light absorption in C60 films at high and low excitations in the temperature range between 15 and 300 K have been measured. In addition to the well-known explanation of photoinduced absorption in terms of optical transitions in the system of photogenerated singlet excitons, triplet excitons, and polarons, changes in the absorption spectrum of the fullerite ground state must be considered. We suggest taking into account the effect of crystal field in explaining the features of the photoinduced absorption spectrum. A feature similar to the inverted luminescence spectrum and ascribed to optical excitation of singlet excitons, which is partially allowed owing to intermolecular interaction, has been detected in spectrum of photoinduced absorption.

Synthesis of a periodic SiC/C nanostructure

Highly porous periodic structures consisting of a three-dimensional replica of pores in the initial opal lattice have been synthesized by high-temperature thermochemical treatment of opal matrices filled with carbon compounds, followed by dissolution of silicon dioxide. It has been shown that the main phases of the composite are carbon and silicon carbide. Based on the X-ray diffraction, Raman, and IR spectroscopy data, it has been assumed that the composite contains fragments of hexagonal diamond. The photoluminescence and optical reflection spectra of the composites have been measured.

Three-dimensional periodic lattice of ZrO2 nanocrystals in transparent silica matrix

The conditions for synthesizing an opal-zirconia-carbon nanocomposite in the form of monolithic (without pores) transparent silica with an ordered distribution of nanocrystals throughout the sample have been determined. Zirconia nanocrystals form a three-dimensional periodic lattice of nanoclusters with sizes ranging from several nanometers to several tens of nanometers. This nanocomposite exhibits properties typical of photonic crystals. The samples have been studied using electron microscopy, X-ray diffraction, photoluminescence, and Raman scattering. The structural state and spatial arrangement of zirconia nanocrystals and carbon clusters in the nanocomposite have been elucidated. Optical transmission and optical reflection photoluminescence spectra of the nanocomposites have been measured.

Growth of ZnO nanocrystals by pulsed laser deposition on sapphire and silicon and the infrared spectra of the nanocrystals

The ZnO nanorods that comprise highly oriented nanorod structures are grown on sapphire and silicon substrates by laser ablation. The nanostructures grown in different conditions are characterized by means of electron microscopy and Fourier infrared reflectance spectroscopy. The contributions of optical phonons and free charge carriers to the infrared spectra of the layers of ZnO nanorods are identified, and the degree of orientation of the ZnO nanorods with respect to the substrate surface is analyzed in relation to the conditions of growth. Softening of optical phonons of ZnO with decreasing the nanorod diameter is observed.

Spectra of optical phonons and low-energy electronic transitions in C60/TMPD and C60/TPA single crystals

Single Crystals of C60/TMPD and C60/TPA have been grown from a chlorobenzene solution. Optical transmission spectra of single crystals of fullerene complexes withN,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) and triphenylamine (TPA) are studied in the spectral range from 600 to 16000 cm−1. Splitting of the intramolecular vibration of C60 is observed at 1428 cm−1, which is likely caused by freezing of the rotation of the C60 molecules due to their interaction with amines. Single crystals of C60/TMPD differ from those of C60/TPA by a decrease in the vibration frequency at 1428 cm−1, vibrations of the C-C bonds of the TMPD molecule, and the redistribution of the forces of the oscillators of the vibrations of the C-N bonds. These peculiarities are interpreted to be the result of partial electron transfer from TMPD to C60 in the C60/TMPD single crystals. The electron transfer in the C60/TPA system is less pronounced.

Oriented ZnO Nanorods and Their IR Reflection Spectra

Oriented ZnO nanorods have been grown on sapphire and silicon substrates by pulsed laser deposition. It is established that IR spectroscopy gives information about the crystal structure of nanorods, their conductivity, and degree of orientation, depending on the growth conditions. A size effect is found: softening of optical ZnO phonons with a decrease in the nanorod diameter.