Full-text resources of PSJD and other databases are now available in the new Library of Science.
Visit https://bibliotekanauki.pl
Preferences help
Polish version English version Language
enabled [disable] Abstract
Number of results

Results found: 12

Number of results on page
first rewind previous Page / 1 next fast forward last

Search results

help Sort By:

help Limit search:
first rewind previous Page / 1 next fast forward last
1
Content available remote

Anharmonicity Effects in Nanocrystals Studied by Raman Scattering Spectroscopy

100%
Dohčević-Mitrović Z., Popović Z., Šćepanović M.
Acta Physica Polonica A
|
2009
|
vol. 116
|
issue 1
36-41
EN
Phonon-phonon interactions were investigated in various nanocrystalline powders like anatase TiO_{2-δ}, pure CeO_{2-δ} and ceria doped with Nd(Gd) analyzing temperature dependent Raman spectra of these systems. Phonon confinement model based on size, inhomogeneous strain and anharmonic effects was used to properly describe the evident changes present in the Raman spectra of pure and doped ceria nanocrystalline samples. In small particles of pure and doped ceria nanocrystals, when size effects have minor impact on Raman modes, four phonon anharmonic processes prevail under the three-phonon ones. When nanopowdered particles are grown enough size effects provoke changes of the anharmonic interactions when three-phonon coupling prevails over the four-phonon anharmonic processes. In nanocrystalline anatase TiO_2 evident blueshift of the most prominent E_g Raman mode probably originates from dominant four-phonon anharmonic interactions.
2
Content available remote

Use of Phonon Confinement Model in Simulation οf Raman Spectra of Nanostructured Materials

100%
Grujić-Brojčin M., Šćepanović M., Dohcević-Mitrović Z., Popović Z.
Acta Physica Polonica A
|
2009
|
vol. 116
|
issue 1
51-54
EN
The simulation of the Raman spectra of nanostructured materials, where the effects of frequency shift and asymmetric broadening of the Raman modes play an important role, can be very useful in systematic characterization of these materials. Use of phonon confinement model for calculating Raman spectra of different nanomaterials is considered both from the viewpoint of different confinement function and the confinement strength, as well as the dimensionality of the confinement model. The phonon dispersion relations and the choice of their approximation are also studied. The influence of particle size distribution on the shape of the calculated spectra is discussed and contributions of Gaussian and asymmetric Gaussian distribution are compared. The effects of average and inhomogeneous strain on the behavior of simulated Raman spectra are also discussed. The results of the phonon confinement model are compared to the experimental spectra of CeO_2 and anatase TiO_2 nanopowders.
3
Content available remote

Imperfect Cloaking Devices Based on Metamaterials

88%
Isić G., Gajić R., Novaković B., Popović Z., Hingerl K.
Acta Physica Polonica A
|
2007
|
vol. 112
|
issue 5
1083-1088
EN
Cloaking devices designed using the coordinate transform approach were shown to be realizable, at least in principle, within the realm of electromagnetic metamaterials. In this paper we investigate the strictness of conditions imposed on the parameters of metamaterial cloaks by calculating the degree of wave scattering when those parameters have variations with respect to theoretically ideal values. A simple idea is used to obtain analytic results for the case of the nonideal two-dimensional cloaking cylinder. Also, results of realistic finite element simulations of the Helmholtz equation are presented and it is found that they are in excellent agreement with the analytic results.
4
Content available remote

Optical Characterization of Laser-Synthesized Anatase TiO_{2} Nanopowders by Spectroscopic Ellipsometry and Photoluminescence Measurements

88%
Šćepanović M., Grujić-Brojčin M., Mirić M., Dohčević-Mitrović Z., Popović Z.
Acta Physica Polonica A
|
2009
|
vol. 116
|
issue 4
603-606
EN
Nanosized titania (TiO_{2}) is synthesized by laser-induced pyrolysis using TiCl_{4} as a liquid precursor. X-ray diffraction and Raman scattering confirmed anatase structure of TiO_{2} nanocrystals. The dielectric function ε(ω) of TiO_{2} nanopowders has been determined by spectroscopic ellipsometry in the energy range from 1.5 to 6 eV at room temperature. The features observed in ε(ω) have been fitted to analytical line shapes by using the second derivatives of experimental spectra. The energies corresponding to different interband electronic transitions have been determined. Photoluminescence measurements have been carried out in vacuum for T = 20 K and T = 300 K. Under laser irradiation with sub-band gap photon energy, anatase nanocrystals have displayed strong visible photoluminescence emission. In this broad photoluminescence band different variations of line shape and position with excitation energy and temperature are observed for nanopowders with different crystallite size, pointing out to the various electronic transitions mediated by defect levels within the band gap.
5
Content available remote

Characterization of La-Doped TiO_2 Nanopowders by Raman Spectroscopy

76%
Šćepanović M., Aškrabić S., Berec V., Golubović A., Dohčević-Mitrović Z., Kremenović A., Popović Z.
Acta Physica Polonica A
|
2009
|
vol. 115
|
issue 4
771-774
EN
Titanium dioxide (TiO_2) nanopowders doped with 0.65, 1, 2, 3 and 4 wt.% of lanthanum ions (La^{3+}) were synthesized by sol-gel technology. Dependence of structural and morphological characteristics of nanopowders on La^{3+} content and synthesis conditions is investigated by the Raman spectroscopy. Very intensive modes observed in the Raman spectra of all nanopowder samples are assigned to anatase phase of TiO_2. Additional Raman modes of extremely low intensity can be related to the presence of certain amount of highly disordered brookite phase in nanopowders. Dependence of the intensity ratio of the Raman modes which originate from anatase and brookite on doping conditions is specially analyzed. In order to estimate the variation of nanocrystallite size with dopant content, shift and asymmetrical broadening of the most intensive E_g Raman mode of anatase are analyzed by phonon confinement model. The obtained results are compared with the results of X-ray diffraction spectroscopy. Special attention is dedicated to the changes in the Raman spectra of pure and La-doped TiO_2 nanopowders observed after high temperature treatment.
6
Content available remote

Hydrothermal Synthesis of CeO_{2} and Ce_{0.9}Fe_{0.1}O_{2} Nanocrystals

76%
Radović M., Dohčević-Mitrović Z., Golubović A., Matović B., Šćepanović M., Popović Z.
Acta Physica Polonica A
|
2009
|
vol. 116
|
issue 4
614-617
EN
Pure and 10 mol% Fe^{3+} doped CeO_2 nanocrystals were synthesized by hydrothermal method using two different basic solutions (NH_4OH and NaOH). All the samples were calcinated at 140°C and 200°C. The characterization of crystalline structure, vibrational and optical properties was performed using X-ray diffraction, Raman spectroscopy and spectroscopic ellipsometry. The obtained results showed that the Fe-doped samples are solid solutions with different size of nanocrystals, very dependent on the synthesis temperature and type of basic solution. The Raman measurements demonstrated electron molecular vibrational coupling and increase of oxygen vacancy concentration whereas doping provokes a small decrease of optical absorption edge in comparison with pure ceria.
7
Content available remote

Low-Frequency Raman Spectroscopy of Pure and La-Doped TiO_2 Nanopowders Synthesized by Sol-Gel Method

76%
Šćepanović M., Aškrabić S., Grujić-Brojčin M., Golubović A., Dohčević-Mitrović Z., Kremenović A., Popović Z.
Acta Physica Polonica A
|
2009
|
vol. 116
|
issue 1
99-102
EN
Pure and La-doped titania (TiO_2) nanopowders are synthesized by sol-gel technology. The crystallite sizes determined by X-ray diffraction measurements range from 10 to 15 nm. Dependence of structural and morphological characteristics of nanopowders on synthesis conditions and La^{3+} content is investigated by the Raman spectroscopy. Very intensive modes observed in the Raman spectra of all nanopowder samples are assigned to anatase phase of TiO_2. Additional Raman modes of extremely low intensity can be related to the presence of a small amount of brookite amorphous phase in nanopowders, which is in accordance with the results of X-ray diffraction analysis. The particle size distribution in TiO_2 nanopowders was estimated from the low frequency Raman spectra, using the fact that the phonon modes in nanosized TiO_2 observed in the low frequency region (ω <40 cm^{-1}) can be well described by the elastic continuum model, assuming that nanoparticles are of perfect spherical shape and isotropic. The nanosized particle distribution obtained by this method is used for the calculation of the frequency and shape of the most intensive E_g Raman mode in anatase TiO_2 by the phonon confinement model. The calculated broadening of this mode, associated with the particle size distribution, coincides well with the characteristics of E_g mode observed in measured Raman spectra of TiO_2 nanopowders. This confirms the Raman spectroscopy method as a powerful tool for determination of particle size distribution in nanosized materials.
8
Content available remote

Characterization of ZnSe Nanolayers by Spectroscopic Ellipsometry

76%
Šćepanović M., Grujić-Brojčin M., Nesheva D., Levi Z., Bineva I., Popović Z.
Acta Physica Polonica A
|
2009
|
vol. 116
|
issue 4
708-711
EN
Single layers of ZnSe with thicknesses of 30, 40, 50, 70 and 100 nm are deposited at room substrate temperature by thermal evaporation of ZnSe powder in vacuum. The layers surface morphology has been investigated by atomic force microscopy. Structural characterization by the Raman scattering measurement revealed the existence of randomly oriented crystalline ZnSe particles in all layers, and the presence of amorphous phase in layers thinner than 100 nm. The ellipsometric measurements were performed in the range from 1.5 to 5 eV at room temperature in air. To interpret the experimental results, the Bruggeman effective medium approximation of dielectric function of ZnSe layers has been used, representing the layers as different mixtures of crystalline ZnSe (c-ZnSe), amorphous ZnSe (a-ZnSe), and voids. The assumption of polycrystalline ZnSe layers modeled as mixture of porous c-ZnSe (with volume fraction of voids ≈ 0.17) and a-ZnSe gives the best fit of ellipsometric experimental data. Single layer thicknesses similar to those expected from preparation conditions have been obtained by this fitting procedure. It has been also found that decrease in the layer thickness causes an increase of the volume fraction of a-ZnSe. Thus, c-ZnSe/a-ZnSe ratio, porosity and layer thickness obtained by spectroscopic ellipsometry, provides useful information about crystallinity and micro-/nanostructure of ZnSe nanolayers.
9
Content available remote

The Effects of Nonstoichiometry on Optical Properties of Oxide Nanopowders

76%
Šćepanović M., Grujić-Brojčin M., Dohčević-Mitrović Z., Vojisavljević K., Srećković T., Popović Z.
Acta Physica Polonica A
|
2007
|
vol. 112
|
issue 5
1013-1018
EN
In this paper we illustrate the change of optical properties of mechanically activated wurtzite ZnO powder and laser synthesized anatase TiO_2 nanopowder due to the nonstoichiometry caused by mechanical activation and/or laser irradiation in vacuum. Both of the investigated materials are widely used in optoelectronics and the examination of their optical properties under different preparation and environmental conditions is of great practical interest.
10
Content available remote

Optical Characterization of Graphene and Highly Oriented Pyrolytic Graphite

76%
Kostić R., Mirić M., Radić T., Radović M., Gajić R., Popović Z.
Acta Physica Polonica A
|
2009
|
vol. 116
|
issue 4
718-721
EN
We present a symmetry analysis of allowed infrared and Raman modes in graphene and highly oriented pyrolytic graphite. Surface structure for highly oriented pyrolytic graphite is examined using atomic force microscopy. As experimental tools, we used infrared spectroscopic ellipsometry in order to investigate the pseudodielectric function of highly oriented pyrolytic graphite in the mid-infrared range (500-7000 cm^{-1}) and Raman spectroscopy to investigate the influence of layers number decrease. As a result, we propose a method for an experimental verification of graphene.
11
Content available remote

Raman Scattering from ZnSe Nanolayers

76%
Nesheva D., Šćepanović M., Aškrabić S., Levi Z., Bineva I., Popović Z.
Acta Physica Polonica A
|
2009
|
vol. 116
|
issue 1
75-77
EN
A series of ZnSe single layers having thickness between 30 nm and 1 μm was deposited on c-Si and glass substrates at room substrate temperature. Thermal evaporation of ZnSe powder in high vacuum has been applied. Moreover, SiO_x/ZnSe periodic multilayers prepared by the same deposition technique and having ZnSe layer thickness of 2 and 4 nm have been studied. Raman spectra were measured at 295 K, using the 442 nm line of a He-Cd laser as well as different lines of the Ar^+ or Ar^+/Kr^+ lasers. The observed Raman features have been related to multiple optical phonon (1LO to 4LO) light scattering and connected with the existence of randomly oriented crystalline ZnSe grains in both ZnSe single layers and ZnSe layers of the multilayers. Relatively large line width ( ≈ 15 cm^{-1}) of the 1LO band has been observed and related to lattice distortion in the crystalline grains and existence of amorphous phase in the layers thinner than 100 nm. The Raman spectra measured on both ZnSe single layers and SiO_x/ZnSe multilayers using the 488 nm line with a gradually increased laser beam power indicate an increased crystallinity at high irradiation levels.
12
Content available remote

Effect of Fe^{2+} (Fe^{3+}) Doping on Structural Properties οf CeO_2 Nanocrystals

76%
Radović M., Dohčević-Mitrović Z., Paunović N., Šćepanović M., Matović B., Popović Z.
Acta Physica Polonica A
|
2009
|
vol. 116
|
issue 1
84-87
EN
We have measured the Raman scattering and magnetization of pure and Fe^{2+}(Fe^{3+}) doped CeO_2 nanopowders at room temperature. The Raman scattering spectra revealed the existence of CeO_2 fluorite cubic structure for all investigated samples. The Raman active mode at about 600 cm^{-1}, seen in all samples, can be ascribed to the CeO_2 intrinsic oxygen vacancies. Additional Raman modes at 720 cm^{-1}, 1320 cm^{-1} and 1600 cm^{-1}, which appear in the spectra of doped samples, can be assigned to maghemite (γ-Fe_2O_3) cation deficient structure, to 2ω_{LO} IR-allowed overtone and two magnon structure, respectively. This implies that our powders are composed of mixed valence states and have defective structure. Presence of oxygen defect states and magnetic ions can be responsible for the observed ferromagnetism at room temperature in both pure and Fe doped samples.
first rewind previous Page / 1 next fast forward last
JavaScript is turned off in your web browser. Turn it on to take full advantage of this site, then refresh the page.