Browsing by Author "Petkova T."
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Item Batio3 Structure as a Function of the Preparation Method(2020-01-01) Nicheva D.; Harizanova R.; Ilcheva V.; Mihailova I.; Petkova T.; Petkov P.Barium titanate is synthesized by sol-gel and hydrothermal methods. The sol-gel technique allows the preparation of amorphous powders which are subsequently subjected to appropriate time-temperature programs and barium titanate is precipitated. The allotropic modification of the barium titanate from the obtained sol-gel powder depends on the temperature applied. It is cubic for heat treatment at 900 °C for 4 h, while in case of crystallization at 1100 °C for 3 h the tetragonal modification occurs as witnessed by X-ray diffraction analyses. Barium titanate is also successfully prepared by the hydrothermal method which results in the crystallization of the cubic allotrope as observed by X-ray diffraction. The microstructure of the formed barium titanate powders is imaged by scanning electron microscopy and reveals the formation of large fraction of crystals which are polydispersed for the samples prepared via sol-gel method and tend to agglomerate in case of hydrothermal synthesis. The infrared spectroscopy investigation of the powders obtained by both synthesis methods shows the presence of the absorption peak characteristic for the barium titanate phase in the range 540–580 cm−1.Item Compositional dependence of the optical properties of silver containing As2Se3 thin films(2012-01-01) Ilcheva V.; Petkov P.; Petkova T.; Boev V.; Monchev B.We obtained amorphous thin films by evaporation and condensation in three-component systems based on As, Se and Ag. The aim was to investigate the influence of the third component on the thin film structure and optical properties. The refractive index and the film thickness were determined from the upper and lower envelopes of the optical transmission spectra measured in the spectral range 400 - 2500 nm. The absorption coefficient (α) was determined after extrapolating the values of n in the region of strong absorption (where α ≥ 104 cm-1); its spectral distribution is discussed. The dispersion of the refractive index was analyzed in terms of the single-oscillator Wemple-DiDomenico model. The shift of the optical absorption edge was described using the non-direct transition model proposed by Tauc. The optical band gap was calculated from the Tauc plot ahv B(Egopt - hv)2; the compositional dependence of the optical gap is also discussed. © Published under licence by IOP Publishing Ltd.Item Influence of the preparation method on the As-Se-AgI thin films behaviour(2008-05-01) Hineva T.; Petkova T.; Petkov P.; Mikli V.; Socol G.; Mihailescu C.; Mihailescu I.Bulk glasses of the (As2Se3)1-x (AgI) x and (AsSe)1-x(AgI)x systems, where x 5, 10, 15 up to 35 mol.% have been prepared by the melt-quenched technique. The thin films have been deposited by means of vacuum thermal evaporation (VTE) and pulsed laser deposition (PLD). The XRD investigation reveals a generally amorphous structure; small peaks are only observed in the samples with the highest AgI. The film compositions have been determined by EDS (energy dispersive X-ray microanalysis). WDS (wavelength dispersive spectroscopy) studies have shown that the films do not contain oxygen within the accuracy of the method (1 %). The films are dense with smooth surface as revealed by using scanning electron microscopy (SEM) and atomic force microscopy (AFM). © 2008 IOP Publishing Ltd.Item Mechanical behaviour of thin As-S-AgI films for sensor applications(2008-05-01) Kolev K.; Petkova T.; Popov C.; Petkov P.The mechanical behaviour of chalcogenide thin films is of great importance for a variety of applications. The requirements for homogeneity, uniformity, surface smoothness, excellent adhesive strength and minimal residual stress are decisive for the technological performance of thin films in the field of optical and sensor devices. Thin films from the system have been prepared by vacuum thermal evaporation from the corresponding bulk glasses. The film structure and surface morphology have been studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The investigated chalcogenide films are amorphous as revealed by XRD, featureless and with smooth surfaces viewed from the AFM pictures. The stress measurements of the thin films deposited on silicon cantilevers have been carried out by cantilever bending technique. The results obtained have been discussed with respect to the film composition and structure. The change of the stress from tensile to compressive registered in the films with AgI content higher than 15 mol. % could be explained with structural transformations. © 2008 IOP Publishing Ltd.Item Molybdenum-Doped ZnO Thin Films Obtained by Spray Pyrolysis(2024-05-01) Bancheva-Koleva P.; Zhelev V.; Petkov P.; Petkova T.A batch of ZnO thin films, pure and doped with molybdenum (up to 2 mol %), were prepared using the spray pyrolysis technique on glass and silicon substrates. The effect of molybdenum concentration on the morphology, structure and optical properties of the films was investigated. X-ray diffraction (XRD) results show a wurtzite polycrystalline crystal structure. The average crystallite size increases from 30 to 80 nm with increasing molybdenum content. Scanning electron microscopy (SEM) images demonstrate a smooth and homogeneous surface with densely spaced nanocrystalline grains. The number of nuclei increases, growing over the entire surface of the substrate with uniform grains, when the molybdenum concentration is increased to 2 mol %. The estimated root mean square (RMS) roughness values for the undoped and doped with 1 mol % and 2 mol % of ZnO thin films, defined by atomic force microscopy (AFM), are 6.12, 23.54 and 23.83 nm, respectively. The increase in Mo concentration contributes to the increase in film transmittance.Item Optical behavior of (GeS1.5)1-x(AgI)x glasses(2013-01-01) Monchev B.; Petkova T.; Petkov P.; Ilcheva V.In this research we report deposition of thin films from the already synthesized chalcohalide (GeS1.5)1-x(AgI)x glasses, where x=5, 10, 20 mol.%. The bulk samples used and the corresponding layers are checked by XRD analysis which reveals only a diluted halo on the respective diffractograms. Some basic optical parameters of the investigated Ge-SAgI thin films are estimated and their relation with materials' composition is determined. The influence of the third component on the optical absorption is discussed. The optical energy gap (Eg) is determined from the Tauc plot h =B(EgTauc - h )2 and Eg 04 is found from the relationship a=f(hv ). The Eg values calculated by both methods reveal slight increase with silver iodide addition probably due to structural changes.© 2013 The Authors. Published by Elsevier B.V.Item Optical properties of thermally evaporated (As2Se 3)100-xAgx thin films(2013-01-01) Ilcheva V.; Petkov P.; Boev V.; Petkova T.We obtained amorphous thin films by evaporation and condensation in three component systems based on As, Se and Ag. The aim was to investigate the influence of the third component on the thin film structure and optical properties. The refractive index and the film thickness were determined from the upper and lower envelopes of the optical transmission spectra, measured in the spectral range 400 - 2500 nm. The absorption coefficient (α) was determined after extrapolation the values of n in the region of strong absorption (whereα ≥104 cm-1) and its spectral distribution was discussed. The dispersion of the refractive index was analyzed in terms of single-oscillator Wemple-DiDomenico model. The shift of the optical absorption edge was described using the non-direct transition model, proposed by Tauc. Optical band gap was calculated from the Tauc plot αhν =B(EgTauc - hν )2 and the compositional dependence of the optical gap was discussed. In order to realize photoinduced changes in the films, they were illuminated by He-Ne laser and their transmission spectra were measured before and after illumination. The shift of the absorption edge of as - deposited and illuminated films was investigated and discussed as a function of the films composition. © 2013 The Authors.Item Photoinduced changes in As-Se-Ag amorphous films(2008-05-01) Ilcheva V.; Boev V.; Roussev D.; Petkov P.; Petkova T.; Sharlandjiev P.; Nasarova D.Thin films from the As-Se-Ag system have been prepared on glass and silicon substrates by thermal vacuum evaporation from previously fabricated bulk glassy samples. The amorphous state of the samples has been proved by X-ray diffraction. Some optical properties of the amorphous As-Se-Ag thin films have been studied in relation to the Ag concentration in the sample. In order to investigate the photoinduced changes due to irradiation by with He-Ne laser light, transmission spectra of the thin films have been measured before and after irradiation. The optical characteristics as a function of the composition have been studied. © 2008 IOP Publishing Ltd.Item Structure of GeSe4-In and GeSe5-In glasses(2010-10-13) Kaban I.; Jóvári P.; Petkova T.; Petkov P.; Stoilova A.; Hoyer W.; Beuneu B.(Ge0.2Se0.8)100-xInx and (Ge0.17Se0.83)100-xInx (x = 0, 5, 10, 15 at.%) chalcogenide glasses have been studied with high-energy x-ray diffraction, neutron diffraction and extended x-ray absorption spectroscopy at Ge, Se and In K-edges. The experimental data were modelled simultaneously with the reverse Monte Carlo simulation method. GeSe4/2 tetrahedra are shown to be the main structural units in the binary and ternary glasses investigated. Indium bonds to the excess Se atoms in the ternary Ge-Se-In glasses. While the majority of In atoms have three Se neighbours, some In atoms may be tetrahedrally coordinated by Se. © 2010 IOP Publishing Ltd.