Far-infrared Reflectivity Spectra Research Articles

Optical phonon behaviors in ZnGeP2 single crystals from temperature dependent far-infrared reflectance spectra

Lattice vibration behaviors of ZnGeP2 single crystals derived from the far-infrared (FIR) reflectance spectra in the temperature range of 4–300K are reported in this study. Dielectric permittivity dispersion is obtained for the polarization along the a- and c-axes of a single crystal. Phonon modes are described by the Lorentz oscillator model. Temperature dependence of the parameters of Lorentz oscillator model and their influence on the imaginary dielectric permittivity dispersion is discussed. Optical phonon behaviors are ascribed to thermal expansion of the lattice.

Far-infrared spectroscopic study of CeO2 nanocrystals

We present the far-infrared reflectivity spectra of 5 nm-sized pure and copper-doped Ce1−xCuxO2−y (x = 0; 0.01 and 0.10) nanocrystals measured at room temperature in the 50–650 cm−1 spectral range. Reflectivity spectra were analyzed using the factorized form of the dielectric function, which includes the phonon and the free carriers contribution. Four oscillators with TO energies of approximately 135, 280, 370, and 490 cm−1 were included in the fitting procedure. These oscillators represent local maxima of the CeO2 phonon density of states, which is also calculated using the density functional theory. The lowest energy oscillator represents TA(L)/TA(X) phonon states, which become infrared-active Eu modes at the L and X points of the Brillouin zone (BZ). The second oscillator originates from TO(Γ) phonon states. The oscillator at ~400 cm−1 originates from Raman mode phonon states, which at the L point of BZ also becomes infrared-active Eu mode. The last oscillator describes phonons with dominantly LO(Γ) infrared mode character. The appearance of phonon density of states related oscillators, instead of single F2uinfrared-active mode in the far-infrared reflectivity spectra, is a consequence of the nanosized dimension of the CeO2 particles. The best fit spectra are obtained using the generalized Bruggeman model for inhomogeneous media, which takes into account the nanocrystal volume fraction and the pore shape.

Temperature dependent Raman scattering and far-infrared reflectance spectra of MgO modified Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 ceramics: A composition effect

Lattice dynamics and phase transition of MgO modified Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 (PZTN-x wt. % MgO, x = 0, 0.1, 0.2, 0.5) ceramics have been investigated by far-infrared (FIR) reflectance in the temperature range of 5.5–300 K and Raman spectra between 77 and 300 K, respectively. With the aid of above complementary methods, the structure of all ceramics was defined as low-temperature ferroelectric rhombohedral phase [FR(LT)] at room temperature. The FIR dielectric functions were extracted from the multi-Lorentz oscillator dispersion model. The lowest frequency phonon mode, which is related to Pb-BO3 (B = Zr, Ti, Nb) vibration, mainly dominates the FIR dielectric response. With increasing MgO composition, the dielectric constants ε(0) at room temperature are estimated to 85.4, 73.4, 73.9, and 41.9, respectively. The decreasing trend can be due to the doubly ionized oxygen vacancies induced by Mg substitution for B-site. The order-disorder phase transition located around 120 K can be clearly clarified from temperature evolution of phonon frequency, damping, and intensity. It decreases slightly with increasing MgO composition, which influence the distortion due to the broken correlation chains and local permanent dipoles creation. Moreover, the transformation from antiferroelectric orthorhombic AO to [FR(LT)] phase has been observed around 250 K, which is associated with the antiferroelectric displacement of Pb atoms along 〈110〉 and coupled rotations of the corner-connected oxygen octahedral. Furthermore, the transition from [FR(LT)] to [FR(HT)] (high-temperature ferroelectric rhombohedral phase) was identified around 290 K for MgO-doped PZTN ceramics. It arises from the shift of cation (Pb and Zr/Ti/Nb/Mg ions) along the 〈111〉 direction and the transition temperature slightly decreases compared to the pure ceramic.

Far-infrared spectroscopy of CdTe1−xSex(In): Phonon properties

The far-infrared reflectivity spectra of CdTe0.97Se0.03 and CdTe0.97Se0.03(In) single crystals were measured at different temperatures. The analysis of the far-infrared spectra was carried out by a fitting procedure based on the dielectric function which includes spatial distribution of free carriers as well as their influence on the plasmon–phonon interaction. We found that the long wavelength optical phonon modes of CdTe1−xSex mixed crystals exhibit a two-mode behavior. The local In mode at about 160cm−1 is observed. In both sample, a surface layer with a low concentration of free carriers (depleted region) are formed.

Optical phonon behaviors and unstable polar mode in transparent conducting Ba1−xLaxSnO3films from temperature dependent far-infrared reflectance spectra

We report lattice vibration behaviors of transparent conducting Ba1−xLaxSnO3 films derived from the far-infrared reflectance spectra in the temperature range of 105–300 K. Three main phonon modes can be observed at room temperature and the frequencies approximately decrease with increasing La content. A polar mode TO1 appears at about 137 cm−1 and becomes unstable with decreasing temperature. On cooling, the TO1 mode vanishes at 270, 240, 180 and 210 K for the films with x = 0.00, 0.02, 0.04 and 0.06 and appears again with further decreasing of the temperature for the La content of 0.00 and 0.02. It can be explained by the reversal of the TO1 mode effective charge, which originates from a drastic change of the O2/3 eigendisplacement. The content dependent phonon behaviors are mainly caused by octahedral tilting with substituting La for Ba.

Characterization of partially inverse spinel ZnFe2O4 with high saturation magnetization synthesized via soft mechanochemically assisted route

ZnFe2O4 was prepared by a soft mechanochemical route from two starting combinations of powders: (1) Zn(OH)2/α-Fe2O3 and (2) Zn(OH)2/Fe(OH)3 mixed in a planetary ball mill. The mechanochemical treatment provoked reaction leading to the formation of the ZnFe2O4 spinel phase that was monitored by XRD, TEM, IR and Raman spectroscopy. The spinel phase was first observed after 4h of milling and its formation was completed after 18h in both the cases of starting precursors. The synthesized ZnFe2O4 has a nanocrystalline structure with a crystallite size of about 20.3 and 17.6nm, for the cases (1) and (2), respectively. In the far-infrared reflectivity spectra are seen four active modes. Raman spectra suggest an existence of mixed spinel structure in the obtained nanosamples. In order to confirm phase formation and cation arrangement, Mössbauer measurements were done. Estimated degree of inversion is about 0.58 for both starting mixtures. The magnetic properties of the prepared ZnFe2O4 powders were also studied. The results show that the samples have a typical superparamagnetic-like behavior at room temperature. Higher values of magnetization in the case of samples obtained with starting mixture (2) suggest somewhat higher degree of cation inversion.

Galvanomagnetic and optical properties of chromium doped PbTe

Behavior of PbTe samples doped with different amount of Cr, prepared by two methods of synthesis is analyzed using galvanomagnetic measurements and far-infrared spectroscopy. The far-infrared reflectivity spectra of PbTe(Cr) alloys were measured in the 50–650cm−1 spectral range at different temperatures and analyzed using a numerical fitting procedure based on the plasmon–phonon interaction model. The two local Cr modes at about 260cm−1 and 480cm−1 together with the strong plasmon–phonon coupling are observed. The persistent photoconductivity is registered. Using results of galvanomagnetic measurements the carrier concentrations is determined and hence we obtained the Fermi level temperature dependence.

Infrared vortex-state electrodynamics in type-II superconducting thin films

The vortex-state electrodynamics of s-wave superconductors has been studied by infrared spectroscopy. Far-infrared transmission and reflection spectra of superconducting NbTiN and NbN thin films were measured in a magnetic field perpendicular to the film surface, and the optical conductivity was extracted. The data show clear reduction of superconducting signature. We consider the vortex state as a two-component effective medium of normal cores embedded in a BCS superconductor. The spectral features are well explained by the Maxwell-Garnett theory. Our analysis supports the presence of magnetic-field-induced pair-breaking effects in the superconducting component outside of the vortex cores.

Optical properties of CdS thin films

The properties of CdS thin films were investigated by applying atomic force microscopy (AFM), Raman and far-infrared spectroscopy. CdS thin films were prepared by using the thermal evaporation technique under base pressure 2×10−5torr. The quality of these films was investigated by AFM spectroscopy. We applied Raman scattering and far-infrared spectroscopy to investigate the optical properties of CdS thin films, and reveal the existence of Surface Optical Phonon (SOP) mode at 297cm−1. Effective permittivity of mixture was modeled by Maxwell–Garnet approximation. In the analysis of the far-infrared reflection spectra, a numerical model for calculating the reflectivity coefficient for a system which includes films and substrate was applied.

Far-infrared reflection spectra of SmS polycrystals

The reflection spectra of polycrystalline SmS samples have been measured in the far-infrared region within a broad temperature interval (293–80 K). Such measurements have been demonstrated to be efficient in the determination of the gradient in free electron concentration, an important aspect for the estimation of the magnitude of the thermovoltaic effect in a sample.

Soft-mode-driven lattice instabilities in Cs2HgCl4 crystal: phenomenological treatment and far-infrared spectroscopy of the structurally modulated phases

This paper reports a comprehensive phenomenological description and experimental infrared (IR) investigations of the soft-mode-driven lattice instabilities into various commensurately and incommensurately modulated phases of Cs2HgCl4 crystals. Our theoretical analysis shows that the lattice instabilities along the a and c crystallographic directions are related to low-frequency transverse optical (TO) phonon branches of Σ2 and Λ3 symmetry, respectively, which merge together in the center of the Brillouin zone at the point of B3g symmetry. As the temperature decreases both branches fall down, leading first to the direct condensation of the soft TO Σ2 mode in the symmetric Σ direction (k ∥ a*). On the other hand, coupling of the TO and transverse acoustic (TA) modes of Λ3 symmetry causes, at somewhat lower temperatures, a series of frozen modulated commensurate and incommensurate states developing along the symmetric Λ direction (k ∥ c*). Polarized far-infrared (FIR) reflectivity spectra (15–600 cm−1) of Cs2HgCl4 crystals were measured in a broad temperature region, 10–297 K. Despite a rich sequence of structurally modulated phases existing above 163 K we observed rather moderate temperature evolution of IR spectra where only a few new modes of different polarizations have been activated. However, the commensurately modulated phases occurring below 163 K made an essential impact on the spectra of all three polarizations. The process of activation of both the Raman- and the IR-active phonons in the structurally modulated phases is subjected to the phenomenological analysis.

Decrypting the cyclotron effect in graphite using Kerr rotation spectroscopy

We measure the far-infrared magneto-optical Kerr rotation and reflectivity spectra of graphite and achieve a highly accurate unified microscopic description of all data in a broad range of magnetic fields by taking rigorously the c-axis band dispersion and the trigonal warping into account. We find that the second- and the forth-order cyclotron harmonics are optically almost as strong as the fundamental resonance even at high fields. They must play, therefore, a major role in magneto-optical and magneto-plasmonic applications based on Bernal stacked graphite and multilayer graphene.

Far-infrared spectroscopy of Cd1−xMnxS quantum dots

Far-infrared reflectivity spectra, in spectral region 50–600cm−1 and temperature region 80–300K, of CdS and Cd1−xMnxS nanocrystals embedded in hexametaphosphate, are presented. The analysis of the far-infrared experimental reflectivity spectrum was made by the fitting procedure. The Maxwell–Garnet effective medium theory is used for modeling an effective dielectric function as well as to separate respond from nanoparticles.To analyze spectra in CdS ωTO(≈238cm−1) to ωLO(≈305cm−1) spectral region, quantized dipolar modes in a spherical isotropic material quantum dot (QD) are considered in the framework of a continuum model. As to the mechanical boundary conditions rigid sphere is concerned. Experimental far-infrared reflectivity spectra of our samples are in general agreement with the predictions of this model.Experimentally registered and through fitting procedure located features for CdS nanoparticles in spectral region below CdS ωTO at: ≈102cm−1, ≈135cm−1, ≈170cm−1 and ≈210cm−1 are associated to defect induced modes, especially to defects located near the surface of CdS QD. In region over CdS ωLO, modes are identified as multiphonons. In Cd1−xMnxS QD spectra new reflectivity peaks are at: ≈85cm−1, ≈110cm−1 and ≈180cm−1 in spectral region below CdS ωTO, ≈270cm−1 inside CdS ωTO–ωLO spectral region and ≈356cm−1 and ≈376cm−1 in region over CdS ωLO. First two registered modes are associated to both mass and force constant defects at the surface, and rest four modes are consequence of MnS phases present in the sample.

Evidence of charge delocalization in Ce1−xFex2+(3+)O2−ynanocrystals(x=0, 0.06, 0.12)

We have measured Raman scattering spectra of pure and iron-doped Ce${}_{1\ensuremath{-}x}$Fe${}_{x}$${}^{2+(3+)}$O${}_{2\ensuremath{-}y}$ ($x=0$, 0.06, and 0.12) nanocrystals. According to the x-ray diffraction study, Fe doping produces contraction of the CeO${}_{2}$ unit cell, leading to the Raman mode hardening. Contrary to expectation, the ${F}_{2g}$ Raman mode exhibits softening and broadening by changing the valence state of Fe dopant, as a consequence of the electron-molecular vibration coupling. This finding supports the assumption that additional charge in highly oxygen-deficient pure and Fe-doped CeO${}_{2\ensuremath{-}y}$ samples are not only localized at Ce${}^{3+}$ ions but also delocalized onto Ce(Fe)-O(V${}_{O}$)-Ce(Fe) orbitals. Delocalization of electrons from Ce${}^{3+}$ ions causes insulator-to-metal transition in highly oxygen-deficient nanoceria. The far-infrared reflectivity spectrum of nanoceria shows metalliclike reflectivity, which in the low-frequency region is well fitted with the Hagen-Rubens approximation for metals. Photoluminescence measurements revealed the existence of a defect-related band in the energy gap of CeO${}_{2\ensuremath{-}y}$. The electron-molecular vibration (phonon) coupling constants $\ensuremath{\lambda}$ and density of electron states at the Fermi level per spin and molecule $N(0)$ were determined within the framework of Allen's theory. The proposal of an energy band structure of nanoceria is also presented.

Spectroscopy characterization of MnSe nanoclasters randomly distributed in HgMnTe single crystal

New semimagnetic semiconductor Hg 0.91Mn 0.09Te-MnSe mixture, grown by Bridgman method, was investigate by Atomic Force Microscopy, X-ray and far-infrared spectroscopy, in order to obtain the necessary feedback information to determine the optimal conditions for crystal growth. The investigated mixture consists of MnSe nanoclasters randomly distributed in Hg 0.91Mn 0.09Te matrix. Far-infrared reflectivity spectrum of the Hg 0.91Mn 0.09Te-MnSe mixture was measured in the 80–500 cm −1 range at a room temperature. The analysis of the far-infrared spectrum was made by the fitting procedure. The Maxwell-Garnett effective medium theory is used for modeling an effective dielectric function, and on that way calculating both electron and phonon response.

Infrared reflectance and transmission spectra in II-VI alloys and superlattices

Room temperature measurements of the far-infrared (FIR) reflectance spectra are reported for the polar optical phonons in a series of bulk Cd${}_{x}$Zn${}_{1\ensuremath{-}x}$Te (0 \ensuremath{\le} $x$ \ensuremath{\le} 1) and CdSe${}_{x}$Te${}_{1\ensuremath{-}x}$ (0 $x$ \ensuremath{\le} 0.35) crystals grown by Bridgman technique. The composition-dependent spectra exhibited many fundamental aspects of lattice vibrations, while the results of long-wavelength optical modes for the end member binary compounds displayed phonon values in good agreement with the existing inelastic neutron scattering and/or Raman spectroscopy data. Using a standard methodology of multilayer optics with effective-medium dielectric tensors, we simulated the FIR transmission and reflectivity spectra at oblique incidence (Berreman's effect) in many II-VI free-standing thin films, epilayers and superlattices. Berreman's approach provided us with a strong basis for analyzing the infrared experimental data and offered an effective means of estimating the zone-center transverse and longitudinal optical (${\ensuremath{\omega}}_{\mathrm{TO}}$, ${\ensuremath{\omega}}_{\mathrm{LO}}$) phonon frequencies in polar-semiconductor thin films and heterostructure materials of increasing technological importance.

Micro Far-Infrared Reflectivity of CaNb2O6 Single Crystal Fibers Grown by the Laser-Heated Pedestal Growth Technique

CaNb2O6 single crystal fibers were grown by the laser-heated pedestal growth technique, directly from the starting reagents. Optically transparent fibers were obtained in the form of rods with elliptical cross-section, free from cracks, impurities, and secondary phases, with an average diameter of 0.4 mm and about 20 mm of length. The fibers grew within the orthorhombic Pbcn columbite structure, with the growth axis nearly parallel to the crystallographic a-direction. The parameters b and c were parallel to the shorter and larger ellipsis axes. A special setup using a microscope was developed to obtain the far-infrared reflectivity spectra of these micrometer-sized fibers, allowing the identification and assignment of 34 of the 38 polar phonons foreseen for the material. From these phonons, the intrinsic dielectric constant (<εr> = 18.2) and quality factor (<Qu × f > of 185 THz) could be estimated, showing the potential of the material for applications in microwave circuitry. These results, along with previous polarized Raman data (Cryst. Growth Des. 2010, 10, 1569), allow us to present a comprehensive set of optical phonon modes and to discuss the potential use of designed CaNb2O6 microcrystals in compact optical devices.

Far Infrared Optical Properties of Bulk Wurtzite Zinc Oxide Semiconductor

Polarized far infrared (FIR) reflectance technique was applied to study the optical properties of a bulk wurtzite zinc oxide (ZnO) single crystal. Room temperature polarized FIR reflectance spectra were taken at various angles of incidence, from 20° to 70°. The theoretical polarized FIR reflectance spectra were simulated based on the anisotropic dielectric function model. Good agreement was achieved between the experimental and the theoretical FIR reflectance spectra. Through this work, a complete set of reststrahlen parameters of a bulk wurtzite ZnO at the Brillouin zone centre was obtained. Additionally, other FIR optical properties such as the real and the imaginary parts of the complex dielectric function, real and imaginary parts of the refractive index, the absorption coefficient and the reciprocal of the absorption coefficient were also obtained by using numerical calculation.

Effect of Sn addition on the optical gap and far-infrared reflectivity spectra of amorphous Sb–Se films

This paper reports the spectroscopic studies on the thermally evaporated Sn x Sb 20Se 80 − x (2 ≤ x ≤ 8) films. Optical gap and band tailing parameter decreases except for x = 8, where it increases. The red shift in peak reflectivity spectrum in the interband transition region (200–800 nm) has been observed. The position and shape of the reflectivity bands (80–378 cm − 1 ) indicate the change in the local structure of the films. The Kramer–Kronig transformations have been used to evaluate the dielectric constants from the far-infrared reflectivity spectrum. Splitting of the longitudinal optic and transverse optic mode characterizes the partial ionic character for Sn–Sb–Se system. The relationship between the optical properties and structure has been revealed.

Far-Infrared−Ultraviolet Dielectric Function, Lattice Vibration, and Photoluminescence Properties of Diluted Magnetic Semiconductor Sn1−xMnxO2/c-Sapphire Nanocrystalline Films

Optical properties of Sn1−xMnxO2 (x from 0.0 to 0.15) nanocrystalline films grown on c-plane sapphire substrates have been investigated at room temperature by ultraviolet-infrared transmittance, far-infrared reflectance, and photoluminescence spectra. The X-ray diffraction analysis indicate that the films are of tetragonal rutile structure except for 5% Mn doping, in which the slight orthorhombic phase appears due to the presence of defects and strain. The dielectric functions are successfully determined from 0.025 to 6.5 eV using the Adachi and Lorentz multioscillator dispersion models in the high and low photon energy regions, respectively. The fundamental absorption edge is found to shift toward a lower energy side with increasing Mn composition. The refractive index of pure SnO2 film is estimated to be the lowest among the Sn1−xMnxO2 system. On the other hand, the low Eu transverse optical (TO) phonon frequencies slightly increase with the Mn composition. However, the highest Eu(TO) and A2u(TO) vibration modes present an opposite change trend. Compared with SnO2 single crystal, four corresponding longitudinal optical (LO) phonon frequencies decrease for the films owing to the variation of the lattice constants and destruction of the crystal symmetry. Photoluminescence spectra of doped SnO2 films show the remarkable intensity changes and a blue-shift trend compared to pure SnO2 film. Moreover, a novel emission peak of about 1.56 eV associated with the Mn dopant can be observed. It can be concluded that the Mn incorporation effects are the main contributions because the replacement of Sn with Mn ion can induce the 2p−3d hybridization and result in the electronic band structure modification of the Sn1−xMnxO2 films.