Transverse Optical Phonon Frequencies Research Articles

The electron–phonon scattering and charge transport of the two-dimensional (2D) polar h-BX(X = P, As, Sb) monolayers

In this paper, the electron–phonon scattering and phonon-limited transport properties of the two-dimensional polar h-BX(X = P, As, Sb) have been studied through first-principles calculations in combination with Boltzmann transport theory. The electron–phonon scattering in these three systems is systematically assessed. Remarkably, intravalley scattering and intervalley scattering are separately investigated, of which the contribution to total scattering is found to be relatively comparable. The carrier mobility is determined over a broad range of carrier concentrations. The results indicate that h-BX (BP, BAs, BSb) simultaneously possess ultrahigh electron mobilities (4097 cm2 V−1 s−1, 4141 cm2 V−1 s−1, 12 215 cm2 V−1 s−1) and hole mobilities (7563 cm2 V−1 s−1, 7606 cm2 V−1 s−1, 22 282 cm2 V−1 s−1) at room temperature as compared to the most known two-dimensional (2D) materials. Additionally, it is discovered that compressive strain can induce a further increase in carrier mobility. The exceptional charge transport properties exhibited by these 2D semiconductors are attributed to the small effective masses in combination with the significant suppression of scattering due to high optical longitudinal optical- and transverse optical-phonon frequencies. This is the first time that we have provided a systematic interpretation of the reason for the exceptional charge transport properties exhibited by the 2D h-BX(X = P, As, Sb) semiconductors. Our finding can provide a theoretical perspective regarding the search for 2D materials with the high carrier mobility.

A Theoretical Estimation of Optical, Vibrational and Structural Properties of II–VI Quaternary Alloy Zn0.5Cd0.5SySe1–y

We present a theoretical estimation of optical, vibrational, and structural properties of II–VI semiconducting quaternary alloy Zn0.5Cd0.5SySe1−y for 0 < y < 1 giving total 10 compositions. The estimation of refractive index, elastic constants, bulk modulus, and vibrational frequencies are performed using the important input parameters provided by the empirical pseudopotential method. In this method, the bandgaps are computed, and the alloying effects are modeled through the modified virtual crystal approximation. We have computed the static refractive index, static and high-frequency dielectric constants, longitudinal and transverse optical phonon frequencies, elastic constants, bulk modulus, and cohesive energy for 10 compositions of the alloy. The results are compared to other experimental and theoretical values wherever available.

TO1 soft mode in Fe doped β-Ga2O3 studied by terahertz time-domain spectroscopy

The transmission spectra of terahertz (THz) waves through Fe doped β-Ga2O3 in the range of 1–6 THz by air-plasma-based THz time-domain spectroscopy (THz-TDS) at room temperature and their temperature dependence between 90 and 500 K in the range of 0.7 THz-1.8 THz have been measured. We estimated the complex dielectric constants by fitting the spectra with the classical damped oscillator dispersion model and obtained the anisotropic lowest transverse optical (TO1) phonon frequencies. The values at room temperature agreed with the reported values, and no significant temperature dependence was observed.

Ultrafast nonlinear phonon response of few-layer hexagonal boron nitride

Ultrafast optical-phonon dynamics and the related nonlinear lattice response of few-layer hexagonal boron nitride ($h$-BN) are elucidated in a combined experimental and theoretical study. The lifetimes of transverse optical (TO) phonons and interlayer modes are 1.2 and 20 ps. In-plane TO phonon displacements are shown to couple to interlayer shear and breathing motions. In the nonlinear phonon response, a transient reduction of the TO phonon frequency arises from this anharmonic coupling, which is a particular property of few-layer $h$-BN.

Fano Resonance Enabled Infrared Nano-Imaging of Local Strain in Bilayer GrapheneSupported by the National Key Research and Development Program of China (Grant No. 2016YFA0302001) and the National Natural Science Foundation of China (Grant Nos. 11774224, 12074244, 11521404, and 61701394). Z.S. acknowledges support from the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, and additional

Detection of local strain at the nanometer scale with high sensitivity remains challenging. Here we report near-field infrared nano-imaging of local strains in bilayer graphene by probing strain-induced shifts of phonon frequency. As a non-polar crystal, intrinsic bilayer graphene possesses little infrared response at its transverse optical phonon frequency. The reported optical detection of local strain is enabled by applying a vertical electrical field that breaks the symmetry of the two graphene layers and introduces finite electrical dipole moment to graphene phonon. The activated phonon further interacts with continuum electronic transitions, and generates a strong Fano resonance. The resulted Fano resonance features a very sharp near-field infrared scattering peak, which leads to an extraordinary sensitivity of ∼ 0.002% for the strain detection. Our results demonstrate the first nano-scale near-field Fano resonance, provide a new way to probe local strains with high sensitivity in non-polar crystals, and open exciting possibilities for studying strain-induced rich phenomena.

Critical Coupling and Perfect Absorption Using α‐MoO3 Multilayers in the Mid‐Infrared

Abstractα‐Phase molybdenum trioxide(α‐MoO3) is a biaxial van der Waals semiconductor that can naturally support anisotropic phonon polaritons with elliptic and hyperbolic in‐plane dispersion. α‐MoO3 can meet critical coupling (CC) condition for both transverse magnetic (TM) and transverse electric (TE) polarizations along the x and y directions and exhibit high absorption in the mid‐infrared region due to strong phonon polaritons. In this study, an absorber composed of few‐layer α‐MoO3 and one‐dimensional photonic crystal separated by a cavity is proposed. Perfect absorption for TE and TM polarized light is achieved at the longitudinal and transverse optical phonon frequencies, respectively. The underlying mechanism of perfect absorption is investigated by using coupled mode theory and simulating electric field and power dissipation profiles. The results prove that CC is responsible for the perfect absorption. Moreover, the influence of the thickness of α‐MoO3 layer, width of cavity between α‐MoO3 and photonic crystal, and angle of incident light on the CC and absorption performance are examined. Accordingly, the rules for realizing perfect absorption at different operation frequencies are established. Overall, this study provides useful insights on designing a perfect absorber based on α‐MoO3 for detection and sensing applications in the mid‐infrared region.

Dielectric and vibrational properties of CdTe studied by first-principles and terahertz time-domain spectroscopy

The optical phonon frequency and damping factor of Cadmium telluride (CdTe) have been investigated by using both the first-principles calculation and terahertz time-domain spectroscopy (TDS) system. The temperature dependence of transverse and longitudinal optical phonon frequency obtained by using the quasi-harmonic approximation is consistent with experimental results at the lower temperature range. Finally, the damping factors estimated from the three phonon contributions deviate from the extracted experimental data in the higher temperature range, which indicates the scattering process of four phonon plays an important role in anharmonic phenomena.

Elastic constants and optical phonon frequencies of BX (X= P, As, and Sb) semiconductors: Semi-empirical prediction

Based on some simple empirical formulas established by Adachi in, Properties of group-IV, III-V and II-VI semiconductors, John Wiley & Sons, Chichester (2005), and the experimental lattice constants reported in the literature; the present work aims to predict the elastic constants and some other significant properties of cubic zinc-blende boron compounds (BP, BAs and BSb). The obtained values of C12and C44 are in general good agreement with other data of the literature, while C11 and B are slightly lower. The zone-center longitudinal optical (LO) and transverse optical (TO) phonon frequencies are also obtained. The LO and TO phonon frequencies of BP compound were found at 866.6and 834.5 cm–1, respectively; these of BAs were found at 731.3 and 727.1 cm–1, respectively; while for the BSb narrow-gap semiconducting compound were found at around 598.3and 586.2 cm–1, respectively. These two later values are in general slightly lower than the calculated values, and the observed Raman spectroscopy values reported in the literature.Â

Optical response and structural properties of Fe-doped Pb(Zr0.52Ti0.48)O3 nanopowders

In this paper, the Pb(Zr0.52Ti0.48)1−xFexO3 (FePZT, x = 0.05) nanopowders were prepared by dry and wet sol–gel methods in the morphotropic phase boundary (MPB) region. The effect of Fe concentration on the structural, morphological, and optical properties of PZT nanopowders was investigated using X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), Fourier-transform infrared (FTIR), and Ultraviolet–Visible (UV–Vis) analysis. XRD results showed that the nanopowders have a perovskite structure with the tetragonal phase. The lattice parameters and average crystallite size of samples decreased from 33 to 21 nm with increasing Fe incorporation due to the substitution of Fe atoms instead of Ti and Zr atoms. FESEM images showed that all average diameters of the nanopowders decreased with the Fe concentration. The optical properties of the pure and FePZT nanopowders such as longitudinal optical (LO) and transverse optical (TO) phonon frequencies, refractive index, extinction coefficient, and the real-imaginary parts of dielectric function were examined by the Kramers–Kronig model. As a result, the TO and refractive index of nanopowders are increased by substituting Ti and Zr with Fe atoms due to their different ionic radius. Also, while the crystallite size increases from 20.95 to 33.52 nm, the LO–TO splitting increases too. The optical band-gap values of the pure and FePZT nanopowders were estimated using UV–Vis spectroscopy and Kubelka–Munk model. The band-gap increased from 3.52 to 3.60 eV with a decrease in the crystal size of nanopowders.

Temperature Dependence of the Vibrational Mode of Pb1 –xSnxTe Films Grown by MBE on the GaAs/CdTe Hybrid Substrate

Reflectance spectra of thin Pb1 –xSnxTe films (~60 nm) with x = 0.25, 0.53, and 0.59, grown by MBE on hybrid GaAs/CdTe substrates, are measured in the frequency range of 20–5500 cm–1 and the temperature range of 5–300 K. The temperature dependence of transverse optical phonon frequencies and structural phase transition temperatures are determined based on a dispersion analysis. An increase in the plasma frequency with decreasing temperature in the range 300‒77 K in all films is detected.

Effect of metal nanoparticles on polariton dispersion in low dimensional systems

The polariton dispersion of a metal nanocomposite-based system is studied for various low dimensional systems. The polariton dispersion of a novel LiNbO3 piezo-electric superlattice is studied for different periods. Al metal nanoparticles are doped in the novel superlattice system and the polariton dispersion is studied for various filling factors. It is found that when percentage of Al metal nanoparticle increases, the frequency of the upper mode increases and also the gap shifts towards longer wave vector region. The polariton dispersions at the brillouin Zone center and at the edge of LiNbO3/LiTaO3 Quantum Well, Quantum Wire and Quantum Dot superlattice systems are studied and compared. Additional modes of propagation are obtained in the dispersion of Al nanoparticle doped low dimensional systems. Some of which reflect the metal behavior of Al particles and some other modes of propagation are found to be degenerate at transverse optical phonon frequencies of LiNbO3/LiTaO3 and at interfacial frequency modes. It is found that the polaritonic gap shift towards lower wave vector region with filling factor. This type of tuning of polaritonic gap can be exploited in the fabrication of novel optical devices especially in the field of communication.

First- and second-order photon–phonon interactions and optical parameters of ZnTe crystal: a broadband terahertz time-domain spectroscopy study

We performed broadband terahertz (THz) time-domain spectroscopy measurements to study the photon–phonon interactions and frequency-resolved optical parameters of bulk ZnTe. Using laser-induced plasma as the source of THz radiation and a GaP bulk wafer as the sensor crystal, a broadband THz response of bulk ZnTe wafer with thickness of 0.1 millimeter within the frequency range from 0.1 to 7.5 THz was obtained. Comparing the measured photon absorption spectrum with phonon dispersion curves obtained from ab initio calculations, we identified the first- and second-order photon–phonon interactions of ZnTe crystal. The frequency-resolved optical parameters including the complex refractive index, dielectric response and conductivity of ZnTe crystal within the frequency range between 0.1 and 7.0 THz were derived from the measured THz transmitted field. By fitting the frequency dependent dielectric functions with the Lorentz oscillator model, a static permittivity of 10.5 along with a transverse optical phonon frequency of 5.9 THz and a phonon lifetime of 1.25 picosecond were obtained for ZnTe crystal.

Raman spectroscopic determination of hole concentration in undoped GaAsBi

Raman spectra of undoped GaAs1−xBix (0 < x < 0.037) grown on GaAs by molecular beam epitaxy were investigated. With an increase of Bi component, we find that the longitudinal optical phonon-hole-plasmon-coupled (LOPC) mode first appears in the vicinity of the unscreened longitudinal optical (ULO) phonon frequency, and then shifts towards the transverse optical (TO) phonon frequency. A new vibrational mode (∼287 cm−1) between the TO and the ULO phonons was verified by use of low temperature polarized Raman measurement and the corresponding scattering intensities are found to be linearly proportional to the composition of Bi in GaAsBi. The hole concentrations determined by using the LOPC/ULO Raman intensities ratio increase from ∼6.5 × 1016 to ∼2.8 × 1017 cm−3 with increased Bi content and the measured results are in agreement with Hall measurements. Furthermore, the influence of excitation laser power on the estimation of the hole densities is discussed with the help of power dependent Raman spectroscopy.

Temperature dependence of the optical and lattice vibration properties in gallium arsenide

The temperature dependence of the band gaps, refractive index, high-frequency and static dielectric constants and optical phonon frequencies in GaAs has been computed by using a pseudopotential approach. Our findings for all features of interest at given temperatures yielded values that agree with those previously reported. The variation of the features being studied here versus temperature showed a monotonic behavior. The transverse optical (TO) and longitudinal optical (LO) phonon frequencies are shifted towards low frequencies when the temperature is raised from 0 to 600 K. The change in the LO-TO splittings by raising temperature reflects the change in the ionicity character of the material under investigation.

Effect of particle size on the optical properties of lead zirconate titanate nanopowders

AbstractNanopowder samples of lead zirconate titanate (Pb1.1Zr0.52Ti0.48O3 or PZT) were prepared by the sol‐gel method with controlled pH values. The samples were characterized using FTIR spectroscopy, XRD, FE‐SEM, and TEM techniques. Most of the peaks in the XRD pattern were related to the coexistence of tetragonal‐rhombohedral phases and confirmed the formation of PZT with a perovskite structure. Also, the crystallite size of PZT nanopowders was in a range of 17‐28 nm. FTIR spectroscopy revealed a longitudinal optical (LO) and transverse optical (TO) phonon modes corresponding to the stretching vibration of Ti‐O and Zr‐O bonds. The influence of pH values on the LO and TO phonon modes, LO‐TO splitting, refractive index n(ω), extinction coefficient k(ω), and the real ɛ1(ω) and imaginary ɛ2(ω) parts of dielectric function was discussed. These properties were investigated in the mid‐infrared region (450‐750 cm−1). The energy loss function Im[−1/(ɛ)] of PZT nanopowders was obtained by Kramers‐Kronig dispersion relations. The TO phonon frequency decreases with increasing crystallite size of the PZT samples. This effect does not happen at pH 8 to pH 9. As the crystallite size increased from 17.26 nm (at pH 5) to 27.25 nm (at pH 7), the LO‐TO splitting increased as well. This result showed that the optimum pH for absorption of IR radiation and optical application was at pH 7.

Optical phonon modes and polaron related parameters in GaxIn1−xP

Based on a pseudopotential approach under the virtual crystal approximation that includes the effect of compositional disorder, the optical lattice vibration frequencies and polaron related parameters in zinc-blende GaxIn1−xP have been studied. Our findings showed generally reasonably good accord with data in the literature. Other case, our results are predictions. The composition dependence of longitudinal optical (LO) and transverse optical (TO) phonon modes, LO-TO splittings, Frӧhlich coupling parameter, Debye temperature of LO phonon frequency, and polaron effective mass has been analyzed and discussed. While a non-monotonic behavior has been noticed for the LO and TO phonon frequencies versus Ga concentration x, a monotonic behavior has been observed for the rest of the features of interest. The information derived from this investigation may be useful for optoelectronic technological applications.

Coordinated EDX and micro‐Raman analysis of presolar silicon carbide: A novel, nondestructive method to identify rare subgroup SiC

AbstractWe report the development of a novel method to nondestructively identify presolar silicon carbide (SiC) grains with high initial 26Al/27Al ratios (&gt;0.01) and extreme 13C‐enrichments (12C/13C ≤ 10) by backscattered electron‐energy dispersive X‐ray (EDX) and micro‐Raman analyses. Our survey of a large number of presolar SiC demonstrates that (1) ~80% of core‐collapse supernova and putative nova SiC can be identified by quantitative EDX and Raman analyses with &gt;70% confidence; (2) ~90% of presolar SiC are predominantly 3C‐SiC, as indicated by their Raman transverse optical (TO) peak position and width; (3) presolar 3C‐SiC with 12C/13C ≤ 10 show lower Raman TO phonon frequencies compared to mainstream 3C‐SiC. The downward shifted phonon frequencies of the 13C‐enriched SiC with concomitant peak broadening are a natural consequence of isotope substitution. 13C‐enriched SiC can therefore be identified by micro‐Raman analysis; (4) larger shifts in the Raman TO peak position and width indicate deviations from the ideal 3C structure, including rare polytypes. Coordinated transmission electron microscopy analysis of one X and one mainstream SiC grain found them to be of 6H and 15R polytypes, respectively; (5) our correlated Raman and NanoSIMS study of mainstream SiC shows that high nitrogen content is a dominant factor in causing mainstream SiC Raman peak broadening without significant peak shifts; and (6) we found that the SiC condensation conditions in different stellar sites are astonishingly similar, except for X grains, which often condensed more rapidly and at higher atmospheric densities and temperatures, resulting in a higher fraction of grains with much downward shifted and broadened Raman TO peaks.

Effect of polytypism on the long and short range crystal structure of InAs nanostructures: An EXAFS and Raman spectroscopy study

The authors present a study on growth mechanism and crystalline structure of zinc blende (ZB)/wurtzite (WZ) polytypic InAs nanostrutures grown on Si-substrate using x-ray diffraction, Raman spectroscopy, and extended x-ray absorption fine structure (EXAFS) spectroscopy. A significant change in the bond length is observed in the short range crystal structure as an effect of the interfacial strain, and the structure exhibits disorder due to twin defect/stacking fault formation at the homointerfaces of ZB/WZ segments in the polytypic crystalline structure, which is related to the ZB:WZ phase ratio. The average ratio of ZB and WZ crystal phase present in the nanostructures is estimated through EXAFS technique. This study opens up an alternate pathway for quantitative estimation of crystal phases over a large area of polytypic samples with distribution in size as well as morphology. Raman spectroscopy study uses E2H and transverse-optical phonon modes to identify the WZ and ZB structural phases of these polytypic nanostructures. It reveals that the interfacial strain shifts the transverse and longitudinal optical phonon frequency significantly and the frequency shifts are proportional to the concentration ratio of ZB and WZ crystal phases. The ZB:WZ ratio derived from EXAFS technique is correlated with Raman spectroscopy results. The long range crystalline structure of the nanostructures is also explored using x-ray diffraction technique, and the observations are corroborating with EXAFS and Raman spectroscopy results. The present study not only elucidate in-depth understanding of the local structure of polytypic InAs but also suggests that careful engineering of interfacial strain through controlled growth can further improve the performance of the polytypic ZB/WZ homointerface based optoelectronic devices.

Lattice Vibration and Polaron Properties of InSb Under Pressure

The optical phonon modes and polaron properties in InSb at zero and under hydrostatic pressure are investigated using a pseudopotential approach. Our findings are compared to data in the literature, where possible, and found to be in reasonable agreement. The pressure dependence of longitudinal and transverse optical phonon frequencies, electron effective mass, Debye temperature of the longitudinal optical phonon frequency and polaron effective mass has been examined and discussed. All features of interest are found to vary monotonically upon compression. The results regarding polaron properties under pressure may be seen as the first predictions.

Strained rocksalt ScN: ab initio studies of electronic structure and lattice-dynamical properties

Generalized gradient approximation calculations are performed to determine electronic and lattice vibration properties of unstrained and strained rocksalt scandium nitride (ScN). In agreement with earlier studies, our results reveal that unstrained rocksalt ScN is an indirect gap semiconductor from the valence band at Γ point to the conduction band at the X point. The effect of strain is found to have a large variation of energy band gaps and optical phonon frequencies. A large longitudinal and transverse optical phonon frequencies splitting is noted for the material of interest suggesting thus a remarkable dynamic ionicity of rocksalt ScN which varies significantly when ScN is strained. This can lead to engineering novel behaviors and holds promise for new technological applications.