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1
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Raman Scattering from Acoustic Phonons Confined in Spherical Nanoparticles

100%
Kostić R.
Acta Physica Polonica A
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2009
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vol. 116
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issue 1
62-64
EN
Low-frequency Raman scattering from small spherical particles is analyzed. Frequencies of vibrational modes are calculated in elastic continuum approximation, which considers one nanoparticle as homogeneous elastic sphere. Parameters of this model are transverse (v_T) and longitudinal (v_L) sound velocities of material, i.e. elastic properties of bulk material. Frequencies of vibrational modes are scaled as function of mentioned bulk parameters for symmetric l=0 and quadrupolar l=2 spheroidal modes, in the case of stress-free boundary conditions. Calculated values are compared with the low-frequency Raman experimental results from literature (Ge, Si, CdS, CdSe, CeO_2, ...). These calculated relations can be practically used to examine nanoparticles of any bulk material. We presented also a procedure how to establish v_L and v_T of material from low-frequency Raman spectra and dimension d of particles.
2
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Resonant Raman Spectra of CdTe/ZnTe Self Assembled Quantum Dots

80%
Romčević N., Romčević M., Kostić R., Stojanović D., Abolmasov B., Karczewski G., Galazka R.
Acta Physica Polonica A
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2009
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vol. 116
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issue 1
88-90
EN
In this paper we present Raman scattering and photoluminescence spectra measured on CdTe/ZnTe self assembled quantum dots. The photoluminescence spectrum has shown two main emission peaks, both connected with existing of quantum dots. One presents direct deexcitation to ground state and the other is optical phonon (οmega = 207.3 cm^{-1})-assisted deexcitation. The registered multiphonon emission process depends on temperature. At low temperature phonon spectra shows line at 450 cm^{-1} (TA + 2LO in ZnTe) and 595 cm^{-1} (2LO + TO(Γ) in ZnTe).
3
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Low-Frequency Raman Spectroscopy of Pure and La-Doped TiO_2 Nanopowders Synthesized by Sol-Gel Method

80%
Šćepanović M., Aškrabić S., Grujić-Brojčin M., Golubović A., Dohčević-Mitrović Z., Kremenović A., Popović Z.
Acta Physica Polonica A
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2009
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vol. 116
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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.
4
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The Effect of Substrate on Vibrational Properties of Single-Layer MoS₂

80%
Gołasa K., Molas M., Nogajewski K., Grzeszczyk M., Zinkiewicz M., Potemski M., Babiński A.
Acta Physica Polonica A
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2016
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vol. 130
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issue 5
1172-1175
EN
We report on the Raman scattering from single-layer molybdenum disulfide (MoS₂) deposited on various substrates: Si/SiO₂, hexagonal boron nitride (h-BN), sapphire, as well as suspended. Room temperature Raman scattering spectra are investigated under both resonant (632.8 nm) and non-resonant (514.5 nm) excitations. A rather weak influence of the substrate on the Raman scattering signal is observed. The most pronounced, although still small, is the effect of h-BN, which manifests itself in the change of energy positions of the E' and A'₁ Raman modes of single-layer MoS₂. We interpret this modification as originating from van der Waals interaction between the MoS₂ and h-BN layers.
5
Content available remote

High Pressure Raman Spectroscopy in Carbon Nanotubes

80%
Kourouklis G., Arvanitidis J., Christofilos D., Ves S.
Acta Physica Polonica A
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2009
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vol. 116
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issue 1
13-18
EN
This work focuses on the high pressure Raman study of carbon nanostuctures comprising of single- or double-wall carbon nanotubes. The detailed examination of the Raman peaks, especially those attributed to the radial breathing modes of the carbon nanotubes, as a function of pressure provides a wealth of information concerning the pressure response of individual nanotubes as well as the internal-external tube (intratube) interactions. The radial breathing modes of both the internal and the external tubes in double-wall carbon nanotubes show reduced pressure slope values as compared to the corresponding in single-wall carbon nanotubes. The reduced slopes for the internal tubes reflect the pressure screening effect inside the external tubes, while those for the external tubes their structural reinforcement due to the encapsulation of smaller diameter tubes in their interior. Moreover, the magnitude of the pressure screening effect depends strongly on the intratube spacing and thus on the intratube interaction. All the experimental observations are well reproduced qualitatively by means of theoretical calculations based on a simple phenomenological model.
6
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Anharmonicity Effects in Nanocrystals Studied by Raman Scattering Spectroscopy

80%
Dohčević-Mitrović Z., Popović Z., Šćepanović M.
Acta Physica Polonica A
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2009
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vol. 116
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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.
7
Content available remote

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

80%
Grujić-Brojčin M., Šćepanović M., Dohcević-Mitrović Z., Popović Z.
Acta Physica Polonica A
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2009
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vol. 116
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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.
8
Content available remote

Optical Properties of Molybdenum Disulfide (MoS_2)

80%
Gołasa K., Grzeszczyk M., Korona K., Bożek R., Binder J., Szczytko J., Wysmołek A., Babiński A.
Acta Physica Polonica A
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2013
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vol. 124
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issue 5
849-851
EN
Research of a monolayer and a bulk MoS_2 is reported. The room temperature Raman spectra of the natural MoS_2 crystals for the both resonant (632.8 nm) and the non-resonant (532 nm) excitation are presented. The apparent differences observed in the spectra from the bulk and the one monolayer MoS_2 are discussed. In particular, the feature due to a first order scattering involving the LA(M) phonon in the resonance Raman spectrum of the one monolayer MoS_2 was observed and explained in terms of the disorder in the natural crystal. The disorder is also documented by the line-shape of the room-temperature photoluminescence spectra observed from both the bulk and the one monolayer MoS_2.
9
Content available remote

Phonon Signatures of a Quantum Impurity with Induced Electron Pairing

70%
Barański J., Domański T.
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vol. 126
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issue 4a
A-73-A-76
EN
We analyze the effective spectrum of a vibrating molecule coupled between one conducting and another superconducting electrode. The proximity effect induces electron pairing which is manifested by the subgap quasiparticle peaks (the Andreev states) whose broadening depends on a hybridization with the conducting electrode. On the other hand, the electron-phonon interaction leads to a multilevel structure with the polaronic states separated by the phonon energy. We inspect a combined effect of both these (polaronic and induced pairing) phenomena.
10
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Depth-Selective Study of Surface Crystallization in NANOPERM-Type Alloys

61%
Miglierini M., Hatala T., Bujdoš M.
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vol. 126
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issue 1
56-57
EN
Evolution of structure and magnetic order is studied in the ^{57}Fe_{81}Mo_{8}Cu_{1}B_{10} alloy by conversion electron and conversion X-ray ^{57}Fe Mössbauer spectrometry. Surface features exhibit differences at both sides of ribbon-shaped samples with respect to the screening depth. In the as-quenched state, traces of magnetite (in addition to bcc-Fe) are revealed only at the wheel side of the ribbon to the depth of about 200 nm. It is present also after annealing up to 510 °C, then it disappears. In deeper regions (down to 1 μm), magnetite is less abundant. At the air side, only bcc-Fe is detected, though the progress of crystallization is more pronounced here than at the wheel side. Vibration properties of the bulk of amorphous and nanocrystalline samples are obtained from experiments of nuclear inelastic scattering of synchrotron radiation using partial densities of phonon states.
11
Content available remote

Molecular Beam Epitaxy Growth for Quantum Cascade Lasers

51%
Kosiel K., Szerling A., Kubacka-Traczyk J., Karbownik P., Pruszyńska-Karbownik E., Bugajski M.
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EN
The fabrication of quantum cascade lasers emitting at 9 μm is reported. The devices operated in pulsed mode at up to 260 K. The peak powers recorded at 77 K were over 1 W and the slope efficiency η ≈ 0.5-0.6 W/A per uncoated facet. This has been achieved by the use of GaAs/Al_{0.45}Ga_{0.55}As heterostructure, with the "anticrossed-diagonal" design. Double plasmon planar confinement with Al-free waveguide has been used to minimize absorption losses. The double trench lasers were fabricated using standard processing technology, i.e., wet etching and Si_{3}N_{4} for electrical insulation. The quantum cascade laser structures have been grown by molecular beam epitaxy, with Riber Compact 21 T reactor. The stringent requirements - placed particularly on the epitaxial technology - and the influence of technological conditions on the device structure properties were presented and discussed in depth.
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