I. Sildos

Raman scattering in nanosized nickel oxide NiO

Magnetic ordering in nanosized (100 and 1500 nm) nickel oxide NiO powders, prepared by the plasma synthesis method, was studied using Raman scattering spectroscopy in a wide range of temperatures from 10 to 300 K. It was observed that the intensity of two- magnon band decreases rapidly for smaller crystallites size. This effect is attributed to a decrease of antiferromagnetic spin correlations and leads to the antiferromagnetic-to- paramagnetic phase transition .

Nitrogen and Luminescent Nitrogen‐Vacancy Defects in Detonation Nanodiamond

An efficient method to investigate the microstructure and spatial distribution of nitrogen and nitrogen-vacancy (N-V) defects in detonation nanodiamond (DND) with primary particle sizes ranging from approximately 3 to 50 nm is presented. Detailed analysis reveals atomic nitrogen concentrations as high as 3 at% in 50% of diamond primary particles with sizes smaller than 6 nm. A non-uniform distribution of nitrogen within larger primary DND particles is also presented, indicating a preference for location within the defective central part or at twin boundaries. A photoluminescence (PL) spectrum with well-pronounced zero-phonon lines related to the N-V centers is demonstrated for the first time for electron-irradiated and annealed DND particles at continuous laser excitation. Combined Raman and PL analysis of DND crystallites dispersed on a Si substrate leads to the conclusion that the observed N-V luminescence originates from primary particles with sizes exceeding 30 nm. These findings demonstrate that by manipulation of the size/nitrogen content in DND there are prospects for mass production of nanodiamond photoemitters based on bright and stable luminescence from nitrogen-related defects.

Spectroscopic study of nanocrystalline TiO2 thin films grown by atomic layer deposition

Abstract Photoluminescence characteristics of nanocrystalline TiO 2 films grown by atomic layer deposition were studied. In dependence of growth conditions the films consisted of anatase, rutile and TiO 2 -II phases. A broad band and two sharp peaks were observed in the photoluminescence spectra measured under continuous-wave Ar + laser excitation at temperatures 5–165 K. At 5 K the maximum of the broad band was at 2.24 eV in the anatase films, and at 2.37–2.40 eV in the rutile and TiO 2 -II films. The intensity of sharp lines peaking at 3.31 and 3.37 eV depended on the crystal structure of the films and increased significantly after X-ray irradiation. The temperature dependence and decay times of different emission bands were also investigated. The data obtained allowed a defect-trapped-exciton interpretation of the sharp peaks although the free-exciton origin of the 3.31 eV peak could still be argued. The broad-band emission at 2.24–2.40 eV was obviously due to self-trapped exciton recombination.

Structural study of TiO2 thin films by micro-Raman spectroscopy

The Raman spectroscopy method was used for structural characterization of TiO2 thin films prepared by atomic layer deposition (ALD) and pulsed laser deposition (PLD) on fused silica and single-crystal silicon and sapphire substrates. Using ALD, anatase thin films were grown on silica and silicon substrates at temperatures 125–425 °C. At higher deposition temperatures, mixed anatase and rutile phases grew on these substrates. Post-growth annealing resulted in anatase-to-rutile phase transitions at 750 °C in the case of pure anatase films. The films that contained chlorine residues and were amorphous in their as-grown stage transformed into anatase phase at 400 °C and retained this phase even after annealing at 900 °C. On single crystal sapphire substrates, phase-pure rutile films were obtained by ALD at 425 °C and higher temperatures without additional annealing. Thin films that predominantly contained brookite phase were grown by PLD on silica substrates using rutile as a starting material.

Polarisation dependent Raman study of single-crystal nickel oxide

The magnetic domain structure and Raman scattering have been studied in NiO single-crystals with three different (100), (110) and (111) orientations. Twin-domain structure was observed in NiO(100) and NiO(110) single-crystals using cross-polarized optical microscopy. We found that the ratio of the two-magnon (at 1500 cm−1) to the two-phonon (2LO, at 1100 cm−1) Raman bands intensity is sensitive in a particular way to the type of the twin-domain pattern.

Self-trapped exciton emission in crystalline anatase

A comparative spectroscopic investigation of self-trapped exciton (STE) emission in anatase single crystals and thin films was carried out at 5 K. In the single crystals, a strongly polarized STE emission with an electrical vector perpendicular to the c-axis was observed. Influence of annealing on the spectral characterictics of samples was tested.

Polarized luminescence of CsPbBr3 nanocrystals (quantum dots) in CsBr:Pb single crystal

Abstract Polarization of the free exciton luminescence of CsPbBr 3 -type semiconducting nanocrystals of perovskite structure dispersed in CsBr:Pb crystal has been observed for the first time. The polarization spectra, temperature and angular dependences of the degree of polarization as well as the dependence of polarization characteristics on size and shape of CsPbBr 3 nanocrystals have been studied. The conclusions have been made about the orientation of anisotropic CsPbBr 3 nanocrystals with respect to CsBr crystal axes, the origin of optical transitions responsible for the exciton emission as well as about the mechanism of the production of the nanocrystals in the process of thermally stimulated diffusion of Pb 2+ ions in CsBr:Pb crystal lattice.

Spectral narrowing of self-trapped exciton emission in anatase thin films

Self-trapped excition (STE) emission of TiO2 (anatase) thin films grown by atomic layer deposition technique on single crystal α-Al2O3 (0 1 2) substrates were studied at temperatures 5–120 K. An in-plane preferential orientation of anatase crystallites was detected in the films by using the dependence of the emission polarisation on the observation angle. The STE emission recorded in the direction perpendicular to the film surface had a wide spectrum with the maximum at approximately 2.3 eV. The spectrum recorded from a cleaved edge in the direction nearly parallel to the film surface, showed two sharp peaks (spectral width as low as 0.03 eV) on the top of the broad STE emission band. The beam divergence determined at the wavelengths of these peaks was 6–10°. The effect was interpreted as a constructive interference of the emission leaving the film at the angle that is close to the angle of total internal reflection.

Spectroscopic and crystal field studies of YAlO(3) single crystals doped with Mn ions.

Detailed analysis of the spectroscopic properties of the YAlO(3) crystals doped with manganese ions has been performed. The exchange charge model of the crystal field was used to calculate the crystal field parameters and energy levels of the Mn(4+) and Mn(5+) ions in YAlO(3). It was shown that both ions contribute to the formation of the absorption spectra. The calculated energy levels are in good agreement with the main observed absorption peaks. Comparison of the Racah parameters B for both ions in YAlO(3) with those for free ions shows a significant role played by the covalent effects, especially for Mn(5+).

Structural and Magnetic Properties of Nickel Oxide Nanopowders

Structure and magnetic properties of nickel oxide (NiO) nanopowders have been studied by X-ray/neutron diffraction, SQUID magnetometer, and micro-Raman spectroscopy. Our diffraction data indicate that at room temperature all NiO powders are antiferromagnetically ordered and have a rhombohedral (R-3m) phase. The SQUID magnetometry and Raman spectroscopy measurements support the presence of the antiferromagnetic ordering.