Bela Shanina

Specific features of the EPR spectra of KTaO3: Mn nanopowders

The electron paramagnetic resonance spectra of KTaO3: Mn nanocrystalline powders in the temperature range from 77 to 620 K have been measured and studied for the first time. The change observed in the spectra has been investigated as a function of the doping level. The doping regions in which Mn2+ ions are individual paramagnetic impurities have been established, as well as the regions where the dipole-dipole and exchange interactions of these ions begin to occur. The spin-Hamiltonian constants for the spectrum of non-interacting individual Mn2+ ions have been determined as follows: g = 2.0022, D = 0.0170 cm−1, and A = 85 × 10−4 cm−1. A significant decrease in the axial constant D in the KTaO3: Mn nanopowder, as compared to the single crystal, has been explained by the remoteness of the charge compensator from the paramagnetic ion and by the influence of the surface of the nanoparticle. It has been assumed that the Mn2+ ions are located near the surface and do not penetrate deep into the crystallites.

Electrically-detected electron paramagnetic resonance of point centers in 6H-SiC nanostructures

The results of investigation of electrically-detected electron paramagnetic resonance (EDEPR) and classical electron paramagnetic resonance (EPR) (X-band) for the identification of shallow and deep boron centers, NVSi defects, and isolated silicon vacancies (VSi), which are formed directly during the preparation of planar nanostructures under conditions of silicon-vacancy injection at the SiO2/n-6H-SiC interface without any subsequent irradiation, are presented. The prepared sandwich nanostructures are an ultra-narrow p-type quantum well, confined by δ barriers heavily doped with boron on an n-6H-SiC surface, which are self-ordered during pyrolytic-oxide deposition and subsequent short-time boron diffusion. The EDEPR data of point centers in sandwich nanostructures, prepared within the framework of Hall geometry, are recorded by measuring the field dependences of the magnetoresistance without an external cavity, microwave source and detector, due to the presence of microcavities embedded in the quantum-well plane and microwave generation under conditions of a stabilized source-drain current from δ barriers containing dipole boron centers. The obtained EDEPR spectra of the shallow and deep boron centers are analyzed using the data of EPR studies in 6H-SiC bulk crystals [10]. The EDEPR spectrum of the isolated silicon vacancy reveals both the negatively charged state VSi− (S = 3/2) and the neutral state in hexagonal (VSi(h)) and quasicubic (VSi(k1, k2)) states (S = 1). In turn, NVSi defects are detected not only by the EDEPR method at 77 K, but also through the use of a Bruker ELEXSYS E580 EPP spectrometer at 9.7 GHz, in a temperature range of 5–40 K. The EDEPR and EPR spectra recorded on the same sandwich nanostructure are virtually identical and correspond to the center in the triplet state with spin S = 1. The EPR spectrum, which is a lowintensity line doublet with a splitting value equal to ΔB = 237.6 mT, is observed in the background of the EPR spectrum from donors of nitrogen, the concentration of which in the n-6H-SiC initial sample was 5 × 1018 cm−3, whereas nitrogen donor centers are not revealed in the EDEPR spectrum because of total occupation by silicon vacancies inside the 6H-SiC sandwich nanostructure.

EPR Study of the Nitrogen Containing Defect Center Created in Self-Assembled 6H SiC Nanostructure

Triplet center with spin state S = 1 is detected in the EPR spectrum of the self-assembled 6H SiC nanostructure obtained by non-equilibrium boron diffusion into the n-type 6H SiC epitaxial layer (EL) under conditions of the controlled injection of the silicon vacancies at the temperature of T = 900°C. From the analysis of the angular dependences of the EPR spectrum and the numerical diagonalization of the spin Hamiltonian, the value of the zero-field splitting constant D and g-factor are found to be D = 1140•10-4см-1 and gpar = 1.9700, gper = 1.9964. Based on the hyperfine (hf) structure of the defect originating from the hf interaction with one 14N nuclei, the large value of the zero-field splitting constant D and technological conditions of the boron diffusion into the n-type 6H SiC EL, the triplet center is tentatively assigned to the defect center consisting of nitrogen atom and silicon vacancy.

On magnetism of nickel clusters in nanoporous carbon

The structural and magnetometric characteristics of a nanoporous carbon with nickel clusters in nanopores have been studied. The main attention is focused on the fact of substantial decrease in the nickel magnetization as nickel has a close contact to carbon as compared to similar parameter of free nickel microparticles with almost the same sizes. A theoretical model of a periodic cluster consisting of 16 carbon atoms and one nickel atom in a micropore surrounded by carbon atoms has been built. The electron density in such a cluster and its magnetism were found by computer calculations. It is shown that the magnetic moment of a nickel atom in the cluster is substantially smaller than that of a free nickel atom. Based on the results, it is concluded that the decrease in the magnetization of nickel clusters in the carbon environment observed experimentally is due to the adsorption interaction of the nickel and carbon electron subsystems.

Luminescent and Optically Detected Magnetic Resonance Studies of CdS/PVA Nanocomposite

A series of solid nanocomposites containing CdS nanoparticles in polymeric matrix with varied conditions on the interface particle/polymer was fabricated and studied by photoluminescence (PL) and optically detected magnetic resonance (ODMR) methods. The results revealed interface-related features in both PL and ODMR spectra. The revealed paramagnetic centers are concluded to be involved in the processes of photo-excited carriers relaxation.

Silicon vacancy-related centers in non-irradiated 6H-SiC nanostructure

We present the first findings of the silicon vacancy related centers identified in the non-irradiated 6H-SiC nanostructure using the electron spin resonance (ESR) and electrically-detected (ED) ESR technique. This planar 6H-SiC nanostructure represents the ultra-narrow p-type quantum well confined by the δ-barriers heavily doped with boron on the surface of the n-type 6H-SiC(0001) wafer. The new EDESR technique by measuring the only magnetoresistance of the 6H-SiC nanostructure under the high frequency generation from the δ-barriers appears to allow the identification of the isolated silicon vacancy centers as well as the triplet center with spin state S = 1. The same triplet center that is characterized by the large value of the zero-field splitting constant D and anisotropic g-factor is revealed by the ESR (X-band) method. The hyperfine (HF) lines in the ESR and EDESR spectra originating from the HF interaction with the 14N nucleus seem to attribute this triplet center to the N-VSi defect.

Photoluminescence Spectroscopy of Neutron-Irradiated Cubic SiC Crystals

Photoluminescence (PL) spectroscopy has been used to characterize neutron-irradiated cubic silicon carbide crystals. The effects of thermal annealing (600-1100OC) on the PL bands have been studied. Several PL bands consisting of a sharp line and its phonon replicas have been observed in the 9-80 K temperature range. Certain of them like the D1 spectrum doublet with 1.975 eV and 1.977 eV zero-phonon lines (ZPL) at 9 K and the L2 spectrum with ZPL at 1.121 eV were reported previously for ion-implanted and electron irradiated 3C-SiC crystals, respectively. Besides, some new bands with ZPL at 2.027, 1.594, 0.989 and 0.844 eV and a broad band at 1.360 eV have been found. A correlation of PL and EPR spectra intensities of these neutron-irradiated and annealed cubic SiC crystals is briefly discussed.

An EPR Study of Defects in Neutron-Irradiated Cubic SiC Crystals

EPR spectroscopy has been used to characterize neutron-irradiated cubic SiC samples after thermal annealing in the 200-1100OC temperature range. Three new paramagnetic defects named Ky6, Ky7 and Ky8 have been revealed. Based on the present results, these defects have been tentatively attributed to the negatively charged carbon vacancy-carbon antisite pair, negatively charged divacancy and neutral carbon <100> split interstitial, respectively. Furthermore, the finding of practically isotropic hyperfine splitting for EPR lines of the T6 center confirms its assignment as a carbon vacancy-interstitial pair.