Longitudinal Optical Modes Research Articles

Theoretical and experimental investigations of intermediate bands in ZnS–Mg nanocrystalline thin film photosensor

The electronic structure of ZnS and Mg-doped ZnS thin films were calculated by first principle investigation within the density functional theory. For experimental analysis, the ZnS and Mg–ZnS samples were grown by simple one step, chemical bath deposition on a glass substrates. The electronic structure reveals the systematic formation of the intermediate band through Mg substitution in wurtzite ZnS at zinc site. This result is well correlated with the experimental results. The structural results obtained from X-ray diffraction study showed hexagonal wurtzite crystal structure with average crystallite size 23 and 21 nm for ZnS and Mg–ZnS respectively. It is observed from UV–Vis spectra that the energy band gap decreases with Mg content due to the creation of new intermediate bands in the band gap region. Raman spectra obtained from the pure and Mg–ZnS films exhibit the longitudinal optical phonon modes. Photosensing properties of the as-grown ZnS and Mg–ZnS thin films were studied by I-V measurements, the photosensitivity was found to be 92 and 96% respectively.

Interrelation of vibrational spectra of Ag3Ga5Se9, Ag3Ga5Te9 and Ag3In5Te9 single crystals: A consequence of trivalent cation and anion substitutions

Silver gallium (indium) ternary selenides (tellurides) single crystals were studied through infrared reflection in the frequency range of 80–500cm−1. These spectra presented four infrared-active modes for studied crystals. Spectral dependencies of optical constants were computed from reflectivity spectra. The frequencies of transverse and longitudinal optical modes, damping constants and oscillator strengths were also evaluated. By replacing light selenium anions by heavier tellurium ones in Ag3Ga5Se9 crystal and by substitution of light gallium cations by heavier indium ones in Ag3Ga5Te9 crystal all the bands shift towards low frequencies. The bands observed in IR spectra of studied crystals were assigned to various vibration types (valence and valence-deformation). Crystal structure and atomic composition ratio of the constituent elements in Ag3Ga5Se9, Ag3Ga5Te9 and Ag3In5Te9 crystals were revealed by structural characterization techniques of X-ray diffraction and energy dispersive spectroscopy.

Influence of exciton-phonons coupling on the exciton binding energy in monolayer transition metal dichalcogenides

We theoretically investigate the correction of exciton binding energy arising from the exciton-optical phonon coupling in monolayer transition metal dichalcogenides (TMDs) using the linear operator and Lee-Low-Pines unitary transformation methods. We take into account not only the exciton coupling with intrinsic longitudinal optical phonon modes but also the surface optical phonon modes induced by polar substrates supporting monolayer TMDs. We find that the exciton binding energies are corrected on a large scale due to these exciton-optical phonon couplings. We discuss the dependences of exciton binding energy on the cut-off wave vector of optical phonon modes, the polarization strength of substrate materials, and the distance between polar substrates and TMDs. These results provide potential explanations for the divergence of the exciton binding energy between the experiment and theory in TMDs.

Study of InSb thin films grown on different substrates by the pulsed electrodeposition technique

Stoichiometric InSb thin films were prepared, on four different substrates, using a pulsed electrodeposition technique. The electrochemical bath used for the growth of InSb thin films was made up of a mixture of aqueous solutions of indium trichloride (InCl3), antimony trichloride (SbCl3), citric acid (C6H8O7·H2O) and sodium citrate (Na3C6H5O7·2H2O). Energy dispersive analysis of X rays along with X-ray diffraction (XRD) studies show that the elemental composition and crystalinity of thin films is strongly dependent on the ionic composition of the electrochemical bath. Stoichiometric InSb thin films can be grown on all the four substrates from the same bath, by properly selecting the composition of the bath. XRD studies show that the InSb thin films grown on all the four substrates have preferred orientation along the (111) plane. The presence of sharp Raman peaks of longitudinal optical phonon mode and transverse optical phonon mode in stoichiometric thin films confirm that they are of good crystalinity.

Breakdown of Optical Phonons' Splitting in Two-Dimensional Materials.

We investigate the long-wavelength dispersion of longitudinal and transverse optical phonon modes in polar two-dimensional materials, multilayers, and their heterostructures. Using analytical models and density-functional perturbation theory in a two-dimensional framework, we show that at variance with the three-dimensional case these modes are degenerate at the zone center but the macroscopic electric field associated with the longitudinal-optical modes gives rise to a finite slope at the zone center in their corresponding phonon dispersions. This slope increases linearly with the number of layers and it is determined solely by the Born effective charges of the material and the dielectric properties of the surrounding media. Screening from the environment can greatly reduce the slope splitting between the longitudinal and transverse optical modes and can be seen in the experimentally relevant case of boron nitride-graphene heterostructures. As the phonon momentum increases, the intrinsic screening properties of the two-dimensional material dictate the transition to a momentum-independent splitting similar to that of three-dimensional materials. These considerations are essential to understand electrical transport and optical coupling in two-dimensional systems.

Phonon-Plasmon Coupling and Active Cu Dopants in Indium Arsenide Nanocrystals Studied by Resonance Raman Spectroscopy.

Doping of semiconductor nanocrystals is an emerging tool to control their properties and has recently received increased interest as the means to characterize the impurities and their effect on the electronic characteristics of the nanocrystal evolve. We present a temperature-dependent Raman scattering study of Cu-doped InAs nanocrystals observing changes in the relative scattering intensities of the different modes upon increased dopant concentrations. First, the longitudinal optical (LO) phonon overtone mode is suppressed, indicating weakening of the coupling strength related to the effect of screening by the free electrons. Second, the transverse optical (TO) mode is relatively enhanced compared to the LO mode, which is attributed to the appearance of a coupled phonon-plasmon mode analogous to observations for n-type doped bulk InAs. These signatures indicate that the Cu impurities serve as active dopants and occupy an impurity-related pseudo sub-band akin to the heavy doping limit.

Optical properties of AlGaN nanowires synthesized via ion beam techniques

AlGaN plays a vital role in hetero-structure high electron mobility transistors by employing a two-dimensional electron gas as an electron blocking layer in multi-quantum well light emitting diodes. Nevertheless, the incorporation of Al into GaN for the formation of the AlGaN alloy is limited by the diffusion barrier formed by instant nitridation of Al adatoms by reactive atomic N. The incorporation of Al above the miscibility limit, however, can be achieved by the ion beam technique. The well known ion beam mixing (IBM) technique was carried out with the help of Ar+ irradiation for different fluences. A novel approach was also adopted for the synthesis of AlGaN by the process of post-irradiation diffusion (PID) as a comparative study with the IBM technique. The optical investigations of AlGaN nanowires, synthesized via two different methods of ion beam processing, are reported. The effect of irradiation fluence and post-irradiation annealing temperature on the random alloy formation was studied by the vibrational and photoluminescence (PL) spectroscopic studies. Vibrational studies show one-mode phonon behavior corresponding to the longitudinal optical (LO) mode of A1 symmetry [A1(LO)] for the wurtzite phase of AlGaN nanowires in the random alloy model. A maximum Al atomic percentage of ∼6.3%–6.7% was calculated with the help of band bowing formalism from the Raman spectral analysis for samples synthesized in IBM and PID processes. PL studies show the extent of defects present in these samples.

Phonon-like Hydrogen-Bond Modes in Protic Ionic Liquids

Gigahertz- to terahertz-frequency infrared and Raman spectra contain a wealth of information concerning the structure, intermolecular forces, and dynamics of ionic liquids. However, these spectra generally have a large number of contributions ranging from slow diffusional modes to underdamped librations and intramolecular vibrational modes. This makes it difficult to isolate effects such as the role of Coulombic and hydrogen-bonding interactions. We have applied far-infrared and ultrafast optical Kerr effect spectroscopies on carefully selected ions with a greater or lesser degree of symmetry in order to isolate spectral signals of interest. This has allowed us to demonstrate the presence of longitudinal and transverse optical phonon modes and a great similarity of alkylammonium-based protic ionic liquids to liquid water. The data show that such phonon modes will be present in all ionic liquids, requiring a reinterpretation of their spectra.

Detection of Crystallographic Defects in 3C-SiC by Micro-Raman and Micro-PL Analysis

A new technique with micro-Raman and micro-PL analysis is proposed to detect defects in 3C-SiC epitaxial films. The high-power of an above band-gap laser is used to increase locally the free carriers density in un-doped epitaxial material (n < 1016 cm-3). The electronic plasma couples with the longitudinal optical (LO) Raman mode determining the so-called LOPC effect. The Raman shift of the LO phonon-plasmon-coupled mode (LOPC) changes as the free carriers density is modified. Crystallographic defects induce a modification of the free carriers density determining a change in the Raman shift of LO mode and in the PL emission from the 3C-SiC gap. Thus we suppose that the results observed are connected to crystallographic defects and we propose this technique as a methodology to analyze extended defects in 3C-SiC material because a detailed study of defects in 3C-SiC has not yet been performed.

Features of polarized Raman spectra for homogeneous and non‐homogeneous compressively strained Ge1−ySny alloys

Raman spectra of homogeneous and non‐homogeneous, compressively strained, Ge‐rich Ge1−ySny alloys (with y < 13%), deposited by molecular beam epitaxy (MBE) techniques using various low‐temperature process conditions, were investigated using polarized micro‐Raman spectroscopy. It is demonstrated that collecting Raman spectra using both polarizations ( and ) provides more insight into the structural features of GeSn alloys than Raman spectroscopy using an unpolarized probe beam. Acquiring Raman spectra using polarization offers the advantages of enhancing the behaviour of some modes as well as eliminating the overlap of longitudinal optical (LO) modes with unwanted acoustical phonons from the Ge. Raman spectra of samples, grown at extremely low temperatures and identified by X‐ray diffraction as non‐homogeneous, exhibit a lowering of the Ge–Ge peak position, an increase in linewidth and a decrease of the dichroic ratio. Another type of non‐homogeneity appears when the metastable GeSn separates into two phases after obtaining a Sn content‐dependent thickness. Raman microscopy confirmed that the segregates, obtained as a result of phase separation and Sn diffusion to the surface, are of metallic tin origin. Copyright © 2017 John Wiley & Sons, Ltd.

Dual-mode immunoassay based on shape code and infrared absorption fingerprint signals of silica nanorods.

Silica nanorods were synthesized through a simple one-pot emulsion-droplet-based growth method, in which tetraethylorthosilicate (TEOS) was used as the silica source, ammonia as the catalyst, and polyvinylpyrrolidone (PVP) as the structure-directing agent and stabilizer. By controlling hydrolysis and condensation in the reaction process, we regulated the aspect ratios and the infrared (IR) absorption fingerprint signals (the transverse optical and the longitudinal optical phonon modes) of the silica nanorods. Based on this, a dual-mode immunoassay was performed for detecting model target analyte, human IgG. The shape code of the silica nanorods was used for simple, rapid qualitative, and sensitive semi-quantitative immunoassay by using a conventional optical microscope. The characteristic IR absorption fingerprint signals of the silica nanorods allowed for reliable quantitative immunoassay with good selectivity and high specificity. The detection limit and the linear range were found out to be 0.5pM and 1pM-10nM, respectively. We expect that such dual-mode immunoassay could be applied for the detection of other analytes, such as protein, nucleic acids, bacteria, viruses, explosives, toxins, and so on. Graphical abstract A simple dual-mode immunoassay was performed using the shape code and infrared absorption fingerprint signals of silica nanorods as detection signals.

Anisotropy, phonon modes, and lattice anharmonicity from dielectric function tensor analysis of monoclinic cadmium tungstate

We determine the frequency dependence of four independent CdWO$_4$ Cartesian dielectric function tensor elements by generalized spectroscopic ellipsometry within mid-infrared and far-infrared spectral regions. Single crystal surfaces cut under different angles from a bulk crystal, (010) and (001), are investigated. From the spectral dependencies of the dielectric function tensor and its inverse we determine all long wavelength active transverse and longitudinal optic phonon modes with $A_u$ and $B_u$ symmetry as well as their eigenvectors within the monoclinic lattice. We thereby demonstrate that such information can be obtained completely without physical model line shape analysis in materials with monoclinic symmetry. We then augment the effect of lattice anharmonicity onto our recently described dielectric function tensor model approach for materials with monoclinic and triclinic crystal symmetries [Phys. Rev. B, 125209 (2016)], and we obtain excellent match between all measured and modeled dielectric function tensor elements. All phonon mode frequency and broadening parameters are determined in our model approach. We also perform density functional theory phonon mode calculations, and we compare our results obtained from theory, from direct dielectric function tensor analysis, and from model lineshape analysis, and we find excellent agreement between all approaches. We also discuss and present static and above reststrahlen spectral range dielectric constants. Our data for CdWO$_4$ are in excellent agreement with a recently proposed generalization of the Lyddane-Sachs-Teller relation for materials with low crystal symmetry [Phys. Rev. Lett. 117, 215502 (2016)].

Thermal Conductivity Degradation and Microstructural Damage Characterization in Low-Dose Ion Beam-Irradiated 3C-SiC

This study assesses the impact of low-dose and low-temperature irradiation on the properties of cubic silicon carbide (3C-SiC). 3C-SiC was irradiated with Kr ions to different fluences at 420 K (147 °C). Raman spectroscopy was used to investigate the impact of irradiation-induced defects on vibrational modes and time-domain thermoreflectance (TDTR) was used to measure thermal conductivity. We observe a noticeable reduction in thermal conductivity with increasing fluence. Analysis of Raman spectra reveals the longitudinal optical (LO) and transverse optical (TO) modes with noticeable peak broadening of LO mode with increasing dosage. We also notice a decrease of ratio of peak intensities of LO and TO modes in irradiated samples. We observe a correlation between the thermal conductivity reduction and the decrease in the peak intensity ratio and attribute this to the accumulation of charged vacancy defects.

Virtual gap states induced modifications in charge neutrality level in cadmium oxide thin films

Present manuscript reports on the creation of virtual gap states (ViGS) in sol–gel spin coated cadmium oxide thin films by tin doping and irradiation at different density of electronic excitations (EEs) induced by energetic ions. Effects of ViGS on charge neutrality level (CNL), which affects the modifications in band gap (BG) of such oxide semiconductors are investigated. Studies reveal that there were insignificant changes in the structural and morphological properties of the films. Interestingly, the transmittance spectrum shows insignificant changes in the films irradiated by low density EEs but an overall decrease in transmittance for high density EEs. However, Raman spectroscopy reveals a significant change in longitudinal optical (LO) modes with tin doping which get intensified and they exhibit an insignificant influence upon irradiation. The BG with respect to bulk is observed to be increased by 0.57 eV and 0.83 eV upon tin doping and irradiation, respectively. These variations are attributed to the combined effect of Burstein–Moss shift (BMS) and ViGS, which were created by tin doping and EEs. It is observed that density of ViGS strongly depends on the density of EEs. A schematic band diagram is also developed for demonstrating the observed influence of ViGS on band gap modifications. Thus the reported investigations are very interesting for the in-depth understanding of fundamental interactions and besides their possible potential applications in the development of optoelectronic devices.

Photoluminescence quenching and enhanced spin relaxation in Fe doped ZnO nanoparticles

Cost-effective ultrasonically assisted precipitation method is utilized to synthesize Zinc oxide (ZnO) nanoparticles (NPs) at room temperature and the effect of Iron (Fe) doping on structural, optical and spin relaxation properties also presented. As-synthesized pure and Fe doped ZnO NPs possess a perfect hexagonal growth habit of wurtzite zinc oxide, along the (101) direction of preference. With Fe doping, ‘c/a’ ratio and compressive lattice strain in ZnO NPs are found to reduce and increase, respectively. Raman studies demonstrate that the E1 longitudinal optical (LO) vibrational mode is very weak in pure which remarkably enhanced with Fe doping into ZnO NPs. The direct band gap energy (Eg) of the ZnO NPs has been increased from 3.02 eV to 3.11 eV with Fe doping. A slight red-shift observed with strong green emission band, in photoluminescence spectra, is strongly quenched in 6 wt.% Fe doped ZnO NPs. The field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) reveals spherical shape of ZnO NPs with 60–70 nm, which reduces substantially on Fe doping. The energy dispersive X-ray spectrum and elemental mapping confirms the homogeneous distribution of Fe in ZnO NPs. Moreover, the specific relaxation rate (R2sp = 1/T2) has been measured using Carr-Purcell-Meiboom-Gill (CPMG) method and found to be maximum in 6 wt.% Fe doped ZnO NPs. Further, the correlation of structural, optical and dynamic properties is proposed.

Structural and Thermal Disorder of Solution-Processed CH3NH3PbBr3 Hybrid Perovskite Thin Films.

We extracted the electronic disorder energy of the organic-inorganic lead-halide hybrid perovskite CH3NH3PbBr3 from temperature-dependent absorption data. We showed that the disorder at room temperature is ∼30 meV and is due to strong electron-phonon coupling with the longitudinal-optical mode of energy 16 meV. This mode can be attributed to longitudinal-optical phonons of the inorganic PbBr6 frame; this conclusion highlights the polaronic nature of electronic excitations in CH3NH3PbBr3. We showed that structural disorder is of the same impact as thermal disorder. A temperature-dependence of the exciton binding energy was observed close to the orthorhombic-to-tetragonal phase-transition temperature.

Longitudinal Optical Phonon-Plasmon Coupled Mode in Undoped GaAs/n-Type GaAs Epitaxial Structures Observed by Raman Scattering and Terahertz Time-Domain Spectroscopic Measurements: Difference in Observed Modes and Initial Polarization Effects

We investigated longitudinal optical (LO) phonon-plasmon coupled (LOPC) mode in undoped GaAs/n -type GaAs epitaxial structures using Raman scattering and terahertz time-domain spectroscopic measurements. In the Raman measurement, the LOPC mode, the frequency of which is determined by the doped electron density in the n -type layer, was observed in addition to the LO phonon in the undoped layer. Contrastingly, in terahertz time-domain measurements, the frequency of the observed coherent LOPC mode is dominated by a photogenerated electron density: frequency-tunable LOPC mode. This indicates that the LOPC mode is generated in the undoped layer. Thus, the probing mechanisms for the LOPC mode are different in the Raman and terahertz time-domain measurements. The built-in electric field exists in the undoped layer. This field enlarges the initial polarization of the LO phonon and, as a result, enables to observe the coherent LOPC mode generated in the undoped layer with the use of the terahertz time-domain spectroscopy.

Specific features of tangential modes in Raman scattering spectra of semiconducting single-walled carbon nanotubes with a large diameter

The lattice dynamics and optical properties of individual semiconducting single-walled carbon nanotubes with diameters of greater than 2.0 nm have been investigated using resonant Raman spectroscopy over a wide range of laser excitation wavelengths. Individual suspended semiconducting single-walled carbon nanotubes have been synthesized by the chemical vapor deposition method. The chirality indices (n, m) of the carbon nanotubes have been determined using the electron diffraction method. The dependences of the ratios of absolute intensities of the tangential transverse optical and tangential longitudinal optical modes on the laser excitation energy and on structural parameters (diameter and chiral angle) of the nanotube have been established. The observed dependences are inconsistent with the spectral data obtained previously for structurally identified individual semiconducting single-walled carbon nanotubes with diameters of less than 1.1 nm.

Raman and infrared characterization of gadolinium-doped manganese sulfide

ABSTRACTGadolinium manganese sulfide solid solutions are investigated by infrared and Raman spectroscopy. Longitudinal optical–transverse optical splitting of the manganese–sulfur bond vibration is observed in the vibrational spectra. The Raman spectra contain modes that are prohibited in the crystal structure of the alpha phase of manganese sulfide, which are associated with activation of the phonons from both the Brillouin zone center and its X and L points. The concentration dependence of transverse optical and longitudinal optical modes’ frequencies is calculated within the frame of the modified random-element-isodisplacement model, being in good agreement with the experimental results. Both theory and experiment show that the solid solution under study exhibits a “one-mode” behavior.

Optical properties of lithium niobate and lithium tantalate crystals with impurities and defects

The photoinduced and Raman scattering in lithium niobate and lithium tantalate crystals with impurities and defects have been studied. An exciting laser beam propagated either along the ferroelectric Z axis or perpendicular to it. The conditions for exciting transverse and longitudinal polar optical modes in Raman spectra are established. The regularities of the excitation of Raman spectra in several polarization geometries (X(ZZ)Y, Z(XX, Y Y)Z, Z(XX, Y Y)Z, X(ZX)Y, X(ZX)X and X(ZX)X) have been investigated. Additional (extra) spectral lines are interpreted as a manifestation of a biphonon enhanced by the Fermi resonance and the result of violation of selection rules for pseudoscalar modes of the A2 type due to the reduction of the point symmetry group caused by the presence of impurities and defects in real crystals. The conditions for exciting coherent longitudinal and transverse modes in lithium niobate and lithium tantalate single crystals upon stimulated Raman scattering are analyzed. The temperature evolution of the spectra recorded in the X(ZZ)Y geometry near the ferroelectric phase transition point is explained based on the concept of effective soft mode and analysis of the isofrequency opalescence effect. Strong photoluminescence is found in copper-doped lithium niobate crystals.