Behavior Of Optical Phonons Research Articles

Effect of laser heating on partial decomposition of ZnCo2O4 nanoparticles: Raman study

We conducted a comprehensive investigation into the nanostructuring of the ZnO/CoO mixture by laser heating and optical attributes of partial decomposition of the obtained two-phase system represented by ZnO and ZnCo2O4. Starting mixtures were obtained across a broad range of dopant, CoO, concentrations, spanning from 5 % to 90 % of CoO. The samples were methodically prepared using the coprecipitation method and subjected to calcination at 600 °C. Laser-induced heating experiments were conducted at nine distinct laser powers. The characterization of these samples was accomplished through the utilization of SEM, XRD, and Raman spectroscopy. XRD analysis unveiled the presence of ZnO and ZnCo2O4 phases. Concurrently, we systematically monitored nanostructuring effects caused by laser-induced heating, the influence of partial decomposition on the behavior of surface optical phonons (SOP), and phase transitions in the samples with varying dopant concentrations during the performed experiment. New phases, including Zn1-xCoxO, ZnyCo3-yO4, CoO, and even the Co3O4 phase, were unveiled. The Raman spectra obtained distinctly indicate the presence of surface optical phonons (SOP), emphasizing the existence of the ZnO phase. The alterations in the behavior of surface optical phonon (SOP) modes were meticulously examined by laser-induced heating nanostructuring effects where it became evident that there was a discernible loss of these modes with an increase in dopant concentration and laser power. This detailed study sheds light on the intricate interplay between dopant concentration, laser power nanostructuring, partial decomposition, and the evolution of phase transformations and surface optical phonon modes in the examined samples.

Observation of the nonanalytic behavior of optical phonons in monolayer hexagonal boron nitride

Phonon splitting of the longitudinal and transverse optical modes (LO-TO splitting), a ubiquitous phenomenon in three-dimensional polar materials, will break down in two-dimensional (2D) polar systems. Theoretical predictions propose that the LO phonon in 2D polar monolayers becomes degenerate with the TO phonon, displaying a distinctive “V-shaped” nonanalytic behavior near the center of the Brillouin zone. However, the full experimental verification of these nonanalytic behaviors has been lacking. Here, using monolayer hexagonal boron nitride (h-BN) as a prototypical example, we report the comprehensive and direct experimental verification of the nonanalytic behavior of LO phonons by inelastic electron scattering spectroscopy. Interestingly, the slope of the LO phonon in our measurements is lower than the theoretically predicted value for a freestanding monolayer due to the screening of the Cu foil substrate. This enables the phonon polaritons in monolayer h-BN/Cu foil to exhibit ultra-slow group velocity (~5 × 10−6c, c is the speed of light) and ultra-high confinement (~ 4000 times smaller wavelength than that of light). These exotic behaviors of the optical phonons in h-BN presents promising prospects for future optoelectronic applications.

Stimulated Raman blockade in coherent phonon generation

We theoretically investigate the behavior of coherent optical phonons in an electron–phonon coupled system at moderate laser intensity: below GW/cm2. We calculate the coherent phonons from impulsive stimulated Raman scattering (ISRS) and impulsive absorption (IA) mechanisms with a nonperturbative model. The generation efficiency of coherent phonons in single-pulse excitation exhibits gradual nonlinear saturation with increasing optical intensity in a nonperturbative regime. The ISRS mechanism exhibits substantial saturation at relatively low pulse intensity, whereas the IA mechanism remains roughly linear. We further examine the influence of double-pulse excitation on quantum path interference. The ISRS mechanism exhibits a pronounced saturation effect near 0 fs of pump–pump delay, whereas the IA process exhibits negligible saturation behavior. These results reproduce well experimental data obtained by double-pulse excitation and support our proposal that ISRS is the primary quantum path for the observed coherent phonons in gallium arsenide.

Probing the thermally driven response of Raman-active phonon modes in sapphire single crystals by in situ Raman spectroscopy

Given the practical application of sapphire as a promising transparent crystal in extreme environments, understanding the fundamental physical response of its optical properties to temperature is urgently needed. Herein, the vibrational characteristics of Raman-active modes of the strain-free sapphire and the strained sapphire substrate under the AlN epilayer were systematically investigated by Raman scattering measurements in the temperature range of 95–870 K. The observed nonlinear temperature-dependent frequency redshift for all phonon modes over the entire temperature range can be fitted with some empirical formulas. Interestingly, the frequency of these phonon modes varies linearly with temperature at high temperatures and can be explained by first-order temperature coefficients. The linear temperature coefficients of A1g,1 modes for strain-free sapphire and strained sapphire determined at high temperatures are −0.02224 cm−1/K and −0.02189 cm−1/K, respectively. The strong temperature dependence in sapphire is expected to portray the local temperature distribution of sapphire-based devices by in situ Raman spectroscopy as a non-contact thermal probe. Furthermore, the frequency downshift and linewidth broadening observed in strain-free sapphire can be well quantified by a theoretical model involving thermal expansion and anharmonic decay. The epilayer-substrate interaction, however, needs to be additionally considered in strained sapphire. This theoretical analysis process can also be radically extended to other emerging ceramic and semiconductor materials. Our quantitative analysis may pave the way for a deeper understanding of the behavior of optical phonons in sapphire under extra perturbation effects.

Impact of interfacial thickness on Raman intensity profiles and phonon anisotropy in short-period (AlSb)n/(GaSb)m superlattices

Comprehensive simulations of phonon dispersions ωj(q→SL) and atomic displacements are reported for short-period (AlSb)n/(GaSb)m superlattices (SLs) using a modified linear-chain model. A bond-polarizability methodology is employed for exploring the impact of meticulously included interfacial thickness Δ (≡1–3 monolayers) on the Raman intensity profiles of graded SLs in the optical phonon region. Results of ωj(q→SL) are also presented by exploiting a realistic rigid-ion-model (RIM) and considering short-range, long-range Coulomb interactions and SL symmetry. Besides the anisotropic behavior of optical phonons, the RIM provided evidence of the acoustic-mode anticrossing, minigap formation, confinement, and interface modes. Controlling the vibrational traits by altering a number of monolayers (n, m) in SLs can provide excellent opportunities for improving the electrical and thermal properties of Sb-based materials for engineering various electronic device structures.

Anomalous Structural Evolution and Glassy Lattice in Mixed-Halide Hybrid Perovskites.

Hybrid halide perovskites have emerged as highly promising photovoltaic materials because of their exceptional optoelectronic properties, which are often optimized via compositional engineering like mixing halides. It is well established that hybrid perovskites undergo a series of structural phase transitions as temperature varies. In this work, the authors find that phase transitions are substantially suppressed in mixed-halide hybrid perovskite single crystals of MAPbI3-x Brx (MA = CH3 NH3 + and x= 1 or 2) using a complementary suite of diffraction and spectroscopic techniques. Furthermore, as a general behavior, multiple crystallographic phases coexist in mixed-halide perovskites over a wide temperature range, and a slightly distorted monoclinic phase, hitherto unreported for hybrid perovskites, is dominant at temperatures above 100 K. The anomalous structural evolution is correlated with the glassy behavior of organic cations and optical phonons in mixed-halide perovskites. This work demonstrates the complex interplay between composition engineering and lattice dynamics in hybrid perovskites, shedding new light on their unique properties.

Absence of temperature-dependent phonon anomalies in Sr2IrO4 and Sr3Ir2O7

Following previous works reporting an anomalous behavior of several zone-center optical phonons across the magnetic transition of square lattice iridates ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ and ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$, we have investigated the lattice dynamics as a function of momentum in these materials by means of high-resolution inelastic x-ray scattering. The observed phonon energies and scattering intensities across the Brillouin zone are in excellent agreement with ab initio lattice dynamical calculations based on nonmagnetic density-functional-perturbation theory. Our results do not evidence any renormalization of the phonons at finite momentum across the magnetic transition of ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$. The only anomalous behavior was detected for the in-plane polarized longitudinal-acoustic phonon branch in ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$, which anomalously softens towards low temperatures and might be related to anisotropic negative thermal expansion in this compound. No anomalies related to potential charge ordering were observed.

Temperature-Dependent Phonon Shifts in van der Waals Crystals.

In recent years, there has been an explosive increase in the research on van der Waals (vdW) crystals because of their great potential applications in many optoelectronic devices. It is necessary to determine their temperature-dependent lattice vibration characteristics because their thermal and electrical transport are closely related to the anharmonic phonon effect, which will affect the performance of the devices. We review the temperature-dependent Raman spectroscopy of vdW crystals, systematically introduce the thermal behavior of optical phonons, and summarize their shift with temperature. Upon analyzing the theoretical models and summarizing the reported experimental data, it is found that the phonon shifts of vdW crystals have a "quasi-linear" relationship with temperature, which is widely described with first-order temperature (FOT) coefficients obtained through a linear fit. Thus, subsequently, the phonon shifts of monolayer materials, different-thickness crystals, suspended and supported samples, in-plane and out-of-plane modes in the same vdW materials, as well as heterostructures and alloys are discussed through comparative analysis of FOT coefficients.

Spin–phonon interaction and magnetoelastic coupling associated with unusual cluster-glass states in YFe0.9Cr0.1O3

The magnetic, optical phonon behavior of the polycrystalline YFe0.9Cr0.1O3 (YFC-10), prepared by solid-state reaction route, had been studied via static and ac magnetization measurements and Raman spectroscopic techniques respectively. The results indicated that YFe0.9Cr0.1O3 undergoes unusual cluster-glass transitions at low temperatures viz. Tf1 (SG1) = 108 K (Freez.1) and Tf2 (SG2) = 30 K (Freez.2) coexisting with the long-range weak ferromagnetic (WFM) state. Raman spectroscopic (RS) measurements revealed 12 distinct modes of vibrations at all temperatures with 83–433 K. Interestingly, the freezing transition at Tf1 is found to be accompanied by significant phonon renormalization. From the detailed analysis of the line-shape function of the Raman modes, the anomalous T-variation of the peak position and the linewidths suggests the existence of spin-phonon interaction in the present solid solution. Moreover the temperature dependent synchrotron X-ray diffraction studies between 10 and 300 K revealed the anomalous thermal variation of the lattice parameters and microscopic structural parameters such as super-exchange angles and (Fe/Cr)O6 octahedral bond lengths and bond angles at both the freezing transitions. Overall, the system exhibits the strong magnetoelastic and spin phonon coupling over a broad thermal regime viz. from Liquid. N2 to temperatures above room temperature (RT = 300 K) which makes YFe0.9Cr0.1O3 reliable for application purpose.

Mn-intercalated MoSe2 under pressure: Electronic structure and vibrational characterization of a dilute magnetic semiconductor.

Intercalation offers a promising way to alter the physical properties of two-dimensional (2D) layered materials. Here, we investigate the electronic and vibrational properties of 2D layered MoSe2 intercalated with atomic manganese at ambient and high pressure up to 7 GPa by Raman scattering and electronic structure calculations. The behavior of optical phonons is studied experimentally with a diamond anvil cell and computationally through density functional theory calculations. Experiment and theory show excellent agreement in optical phonon behavior. The previously Raman inactive A2u mode is activated and enhanced with intercalation and pressure, and a new Raman mode appears upon decompression, indicating a possible onset of a localized structural transition, involving the bonding or trapping of the intercalant in 2D layered materials. Density functional theory calculations reveal a shift of the Fermi level into the conduction band and spin polarization in MnxMoSe2 that increases at low Mn concentrations and low pressure. Our results suggest that intercalation and pressurization of van der Waals materials may allow one to obtain dilute magnetic semiconductors with controllable properties, providing a viable route for the development of new materials for spintronic applications.

Optical phonons of SnSe(1\u2212x)Sx layered semiconductor alloys

The evolution of the optical phonons in layered semiconductor alloys SnSe(1–x)Sx is studied as a function of the composition by using polarized Raman spectroscopy with six different excitation wavelengths (784.8, 632.8, 532, 514.5, 488, and 441.6 nm). The polarization dependences of the phonon modes are compared with transmission electron diffraction measurements to determine the crystallographic orientation of the samples. Some of the Raman modes show significant variation in their polarization behavior depending on the excitation wavelengths. It is established that the maximum intensity direction of the Ag2 mode of SnSe(1−x)Sx (0 ≤ x ≤ 1) does not depend on the excitation wavelength and corresponds to the armchair direction. It is additionally found that the lower-frequency Raman modes of Ag1, Ag2 and B3g1 in the alloys show the typical one-mode behavior of optical phonons, whereas the higher-frequency modes of B3g2, Ag3 and Ag4 show two-mode behavior.

Optical phonon behavior and magnetism of columbite Zn0.8Co0.2Nb2O6**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0403704), the National Natural Science Foundation of China (Grant Nos. 11304113, 11474127, and 11574112), and the Fundamental Research Funds for the Central Universities of China.

Columbite Zn0.8Co0.2Nb2O6 crystals were grown by optical floating zone methods. The x-ray diffraction (XRD) was used to check the structure information of the grown Zn0.8Co0.2Nb2O6 crystal. The room temperature and temperature-dependent Raman spectra were tested to investigate the optical phonon behaviors of columbite Zn0.8Co0.2Nb2O6, which exhibited a temperature stable property. The magnetics properties of Zn0.8Co0.2Nb2O6, measured by a physical property measurement system (PPMS), were also presented in this work.

Pressure-dependent phase transition of 2D layered silicon telluride (Si2Te3) and manganese intercalated silicon telluride

Two-dimensional (2D) layered silicon telluride (Si2Te3) nanocrystals were compressed to 12 GPa using diamond anvil cell techniques. Optical measurements show a color change from transparent red to opaque black indicating a semiconductor-to-metal phase transition. Raman scattering was used to observe the stiffening of the crystal lattice and subsequent phase behavior. A possible phase transition was observed at 9.5 ± 0.5 GPa evidenced by the disappearance of the A1g stretching mode. Si2Te3 was intercalated with elemental manganese to ∼ 1 at.%. Intercalation lowers the pressure of the proposed phase transition to 7.5 ± 1 GPa. Raman modes show both phonon stiffening and phonon softening, suggesting negative linear compressibility. These results provide fundamental insight into the high-pressure optical phonon behavior of silicon telluride and illuminate how a specific electron-donating intercalant can chemically alter pressure-dependent optical phonon behavior.

Growth and properties of spinel structure Zn1.8Co0.2TiO4 single crystals by the optical floating zone method.

The spinel structure Zn1.8Co0.2TiO4 single crystals with 5 mm diameter and 30 mm length were successfully grown by an optical floating zone method. The as-grown crystals were characterized by X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS). Some Zn2+ ions at tetrahedral and octahedral sites should be replaced by doped transition metal Co2+ ions. The temperature-dependent Raman spectra of spinel Zn1.8Co0.2TiO4 crystals were also described. The optical phonon behaviors of Zn1.8Co0.2TiO4 are stable within the temperature range. The magnetic properties of Zn1.8Co0.2TiO4 were investigated by using Physical Property Measurement System.

Electronic, magnetic, thermoelectric and lattice dynamical properties of full heusler alloy Mn2RhSi: DFT study

The manganese based heusler compounds are important class of materials having remarkable ferrimagnetic and thermoelectric properties. Among them, Mn2RhSi which crystallizes in both Hg2CuTi and Cu2MnAl type structure have undergone detailed investigation for its electronic, magnetic and mechanical properties in Hg2CuTi type structure. However, no studies have been performed on Cu2MnAl type structure of Mn2RhSi. In this study we have performed a detailed comparative study on electronic, magnetic, mechanical, thermoelectric and lattice dynamical behavior of both types of Mn2RhSi structures. The higher value of thermal conductivity reduces the thermo electric figure of merit in the material and has large value of Seebeck coefficient making these alloys as a potential candidate for thermoelectric device application. Our results show high value of spin polarization ratio for Mn2RhSi which can increase the efficiency of spintronics devices which further have direct application in magnetic random access memory device. The heusler compound are least studied for their lattice dynamical behavior. In our work, we perform detail investigation on lattice dynamical behavior of both type structures of Mn2RhSi. Our calculations revealed the anomalous behavior of optical phonons in Cu2MnAl type structure which was attributed to the presence of peak right at the Fermi level in electronic density of states whereas phonon dispersion curves of Hg2CuTi type structure confirms its dynamical stability.

Wavevector and polarization resolved analysis of phonon scattering from embedded nanoparticles

Embedded nanocrystals are capable of dramatically reducing the thermal conductivity of alloy semiconductors through increased phonon scattering, but many aspects of thermal wavelength phonon interactions with embedded nanoparticles remain understudied. Here, the wavevector- and polarization-resolved capabilities of the Frequency Domain Perfectly Matched Layer (FDPML) computational technique are exploited to study several fundamentally important phonon scattering problems across the entire Brillouin zone. We compare the atomistic predictions of FDPML against continuum mechanics approaches for spherically embedded particles. For long to mid-wavelength phonons, reasonable agreement is found with continuum theories that consider the Mie regime accurately, while commonly used “patching” theories which empirically connect the Rayleigh scattering to the geometric limit are shown to have poor agreement with more rigorous approaches. Next, the scattering cross section of optical phonons from nanoparticles is explored for the first time. We show that the scattering behavior of optical phonons is fundamentally different than their acoustic counterparts in that long wavelength optical phonons exhibit a scattering cross section nearly independent of wavelength, which we interpret as being due to zone folding of short wavelength acoustic modes from the geometric scattering regime into long wavelength optical modes. Finally, we study the scattering cross section of nanoparticles exhibiting atomic interdiffusion at the matrix-particle interface, where we find that interdiffusion both suppresses Mie oscillations and substantially increases the scattering cross section at short wavelength, compared to a solid nanoparticle with the same number of impurity atoms.

Optical Phonon Behaviors of Photocharged Nanocrystals: Effects of Free Charge Carriers.

For semiconductor nanocrystals (NCs), the precise knowledge of phonons in the presence of free carriers is important for understanding their electronic and photonic properties in device applications. With Raman spectroscopy, this study investigates the effects of free charge carriers on optical phonon behaviors of NCs. The adoption of the photocharging method allows us to introduce free charge carriers into NCs without inducing other side effects. In the photocharged ZnO NCs, lower longitudinal optical (LO) phonon frequencies and weaker LO overtones relative to the fundamentals were found, which was explained by the screening and band-filling effects caused by the induced free carriers. The free carrier effects on optical phonon behaviors of NCs, usually neglected in previous studies, should be taken into consideration when discussing the electronic and photonic properties of NC-based devices.

The Nature of Metastable AA' Graphite: Low Dimensional Nano- and Single-Crystalline Forms.

Over the history of carbon, it is generally acknowledged that Bernal AB stacking of the sp2 carbon layers is the unique crystalline form of graphite. The universal graphite structure is synthesized at 2,600~3,000 °C and exhibits a micro-polycrystalline feature. In this paper, we provide evidence for a metastable form of graphite with an AA’ structure. The non-Bernal AA’ allotrope of graphite is synthesized by the thermal- and plasma-treatment of graphene nanopowders at ~1,500 °C. The formation of AA’ bilayer graphene nuclei facilitates the preferred texture growth and results in single-crystal AA’ graphite in the form of nanoribbons (1D) or microplates (2D) of a few nm in thickness. Kinetically controlled AA’ graphite exhibits unique nano- and single-crystalline feature and shows quasi-linear behavior near the K-point of the electronic band structure resulting in anomalous optical and acoustic phonon behavior.

Optical phonons, OH vibrations, and structural modifications of phlogopite at high temperatures: An in-situ infrared spectroscopic study

The thermal behavior of optical phonons and OH vibrations of phlogopite (a trioctahedral mica) was examined at temperatures up to 1000 K using in situ infrared spectroscopy. The results showed that with increasing temperature, O–K bands in phlogopite exhibited a relatively strong variation in frequency in a manner similar to those in muscovite. The work revealed that different types of OH bands (fundamentals and combinations) have very different thermal behavior or temperature dependence, and their absorption coefficients are commonly not constant on heating. OH combination bands that are associated with summation processes of multi-phonon interactions commonly show a decrease in their intensities on heating, but in contrast combination bands due to difference processes generally exhibit an increase. This means that temperature dependencies of their absorption coefficients need to be considered when using the Beer-Lambert law to determine or estimate OH contents or hydrogen concentrations at high temperatures. The results showed a structural anomaly associated with a discontinuity in the temperature derivative of the wavenumber of Al–O and Si–O vibrations and O–H stretching near 600 K. However, framework-related phonon modes in the FIR and MIR regions do not suggest a break of the original monoclinic structural symmetry in the investigated temperature region. The complex changes are attributed to temperature-induced alteration of local configuration involving TO4 tetrahedra and a possible change of the orientation of OH dipoles, in addition to a previously reported distortion of MO6 octahedra. Increasing temperature to 1000 K also causes partial dehydroxylation, as evidenced by the disappearance of the OH band near 3623 cm−1 and the decrease in OH band height and area of other OH bands. The study did not record the formation of H2O inside phlogopite as a result of partial dehydroxylation. The work offers new data and findings that have important implications in understanding the complex structural modifications and the behavior of phonon modes and the thermal stability of hydroxyls on approaching the dehydroxylation, as well as the way hydrogen is released from micas at high temperatures. Our data also show that phologpite becomes less transparent with increasing temperature suggesting a change of radiative properties and ability to transmit heat, which could be of interest for modeling thermal-transmission in crustal rocks.

Polarized and spatially resolved Raman scattering from composition-graded wurtzite InGaAs nanowires

We report Raman scattering from wurtzite single-crystalline InGaAs nanowires (NWs) to probe optical phonon behaviors associated with spatial grading in alloy composition along the NW length. Polarized Raman spectra revealed several optical phonons and their scattering symmetries: (i) InAs-like A1(LO) and A1(TO) phonons and (ii) GaAs-like A1(LO), A1(TO), and E2(high) phonons. In addition, strong anisotropic behavior was observed in the Raman tensor elements of the A1(TO) phonon mode. Interestingly, a spatial mapping of the GaAs-like A1(TO) phonon along the NW length direction showed a systematic increase in energy from the NW top (~255 cm−1) to the midpoint (~263 cm−1), indicating an increase in the Ga mole fraction from about 0.5 to about 0.8. Further toward the NW bottom, the GaAs-like A1(TO) phonon energy saturated to the peak value at about 264 cm−1. In the upper half of the NW, the phonon linewidths broadened significantly due to the spatial grading in In/Ga composition along the NW length. When the composition grading was negligible in the bottom half of the NW, the spectral widths were considerably narrowed. The GaAs-like E2(high) phonon showed similar variations in both energy and spectral width along the NW length.