Presence Of Energy Levels Research Articles

Light emission of flame-generated TiO2 nanoparticles: Effect of IR laser irradiation

The aim of this work is the analysis of the evolution of titania nanoparticles based on their light emission spectra. Measurements are performed during the synthesis of the nanoparticles in a flame spray, which is irradiated by a pulsed IR laser. Due to the relatively high energy band gap of titania of about 3 eV, the observed substantial light absorption at the laser wavelength of 1.064 μm, i.e. within the region of titania transparency, makes our study novel and interesting.Laser-induced light emission at different laser fluences is compared with spontaneous light emission from titania nanoparticles in flame. For signal processing, Wien plot is introduced to obtain the particle temperature. A peculiar feature of the temperature difference of the irradiated and nonirradiated particles versus laser fluence is obtained. Three well-defined regions of this fluence curve are observed and discussed with a particular attention on the plateau-like behavior at very low laser fluence, followed by a linear increase. An anomalously high titania absorption is inferred in the linear region.The increase of the Urbach energy is speculated to account for the change in the optical properties of titania nanoparticles. It is related to structural defects, and, therefore, the presence of energy levels in the forbidden band due to the particular synthesis condition and/or induced by laser irradiation.

Electron transport and thermoelectric properties of ZnO ceramics doped with Fe

Currently, special attention is paid to the search for new ceramic materials based on wide-gap oxides, as well as to the study of their structure and properties with a view to their application in various areas of electronic and optoelectronic industry.Conventional double-step ceramic technology has been used to obtain samples in this experiment. After compacting at the pressure of 6 GPa of ZnO and ZnO-FexOy powders in different weight relations, the samples were subjected to the procedure of synthesis at 1173 K for 2 h and then to the annealing at 1473 K for 3 h on air.The samples structure was investigated by the Scanning Electron Microscopy (SEM), Energy-dispersive X-ray Spectroscopy (EDX), X-ray Diffraction (XRD) and Raman spectroscopy methods. Temperature dependences of resistivity, magnetoresistance, Hall and Seebeck effects were experimentally studied in the range from 4 to 700 K.As the experiments have shown, the size of grains in the obtained ceramic samples after synthesis was in submicron range. An XRD study showed the saving of the wurtzite structure in Zn1-δFeδO solid solutions where 0.66 < δ < 0.81 at. % regardless of the type of the doping agent. At the same time, the replacement of zinc by iron atoms led to the contraction of the ZnO lattice.All the samples studied (ZnO and Zn1-δFeδO) demonstrate n-type conductivity. The temperature dependencies of resistivity have shown two specific features: the presence of energy level about 0.35 eV below the conduction band bottom for the doped ceramic samples (unknown in literature) and conductance with the changing activation energy at temperatures below 200 K for the undoped ZnO ceramic samples. Seebeck coefficient increased by100–150% with doping due to growth of electron concentration. Some model concepts about scattering mechanisms and reasons of Seebeck effect enhancement have been developed.

63,65Cu NQR Spectra and Spin–Lattice Relaxation in Thermoelectric CuAlO2

The 63,65Cu nuclear quadrupole resonance spectra and spin–lattice relaxation rate (1/T1) have been measured in the semiconductor compound CuAlO2. The value of the nuclei quadrupole interaction constant QCC = 56.24(6) MHz (T = 298 K) has been obtained. The broad maximum has been found in the temperature dependence of 1/T1 in the low-temperature region (below 276 K). This maximum can be associated with the presence of energy levels in the forbidden band. The activation energy has been estimated in CuAlO2 [EA = 45(2) meV], assuming the activation character of the mobility of holes.

Photoluminescence Features and Mechanoluminescence Mechanism Inherent in Composite Materials Based on a Photopolymerizing Resin and Finely Dispersed Powders of SrAl2O4:(Eu2+,Dy3+) and Sr4Al14O25:(Eu2+,Dy3+,B) luminophores

Photoluminescence spectra and kinetics have been studied for finely dispersed powders of such luminophores as SrAl2O4:(Eu2+,Dy3+) and Sr4Al14O25:(Eu2+,Dy3+,B). Several lasers with different wavelengths (λ = 355, 404, 440 and 530 nm) were used for the excitation of photoluminescence. The photoluminescence of the materials is revealed to occur in a long-wave spectral range from 550 to 750 μm, which indicates the presence of energy levels, the radiative transitions between which form the radiation spectrum. The structure of the powders has been studied using a scanning electron microscope. It is established that the powder consists of microparticles (granules) up to 100 μm in size with a grain size up to 25 μm. According to the results of studies on the mechanoluminescence of SrAl2O4:(Eu2+,Dy3+) and Sr4Al14O25:(Eu2+,Dy3+,B) powders, it is concluded that the mechanoluminescence is excited via an activation of traps when they interact with the electric fields of moving grain-boundary dislocations in the course of intergranular slipping during the deformation of microparticles under an impact load.

Heavy light tetraquarks from Lattice QCD

We present preliminary results from a lattice calculation of tetraquark states in the charm and bottom sector of the type udbb, usbb, udcc and scbb. These calculations are performed on Nf = 2 + 1 + 1 MILC ensembles with lattice spacing of a = 0:12 fm and a = 0:06 fm. A relativistic action with overlap fermions is employed for the light and charm quarks while a non-relativistic action with non-perturbatively improved coefficients is used in the bottom sector. Preliminary results provide a clear indication of presence of energy levels below the relevant thresholds of different tetraquark states in the double bottom sector while a scattering state is observed in the charm sector.

Comparison of TSC measurements performed on PbI2 samples prepared in different ways

AbstractThe role of preparation and growth on the presence of energy levels in the band gap of lead iodide crystallized material has been investigated by means of thermally stimulated current measurements. The most significant difference is concerned with the defect introduced by the lead vacancy: the associated energy levels are always present when the material has experienced a molten state at any step of its preparation. Conversely, it does not appear or at a very low concentration in samples fabricated at low temperature from chemically synthesized powder. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Size and surface effects in porous silicon studied by X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) at the Si-K absorption edge has been applied to study the influence of size and chemical-physical surface effects on the optical properties of porous silicon (PS). The comparison between XAS spectra obtained by monitoring both total electron yield (TEY) and photoluminescence yield (PLY) has allowed us to discriminate between light emitting and non-emitting sites in PS. Thanks to partial PLY-XAS measurements and their site selectivity, we have been able to detect the presence of energy levels inside the band gap of the Si nanocrystallites. The origin of these radiative states is related to the surface passivation, whose influence becomes particularly significant at decreasing sizes.