Raman Line Shape Analysis Research Articles

Photoexcited carrier dynamics in semi-insulating 4H-SiC by Raman spectroscopy

The photoexcited carrier dynamics of high-purity (HPSI) and vanadium-doped semi-insulating (VDSI) 4H-SiC irradiated by lasers with different wavelengths and powers were investigated. Raman spectra were measured at room temperature and the photoexcited carrier concentrations were extracted from the Raman line shape analysis of longitudinal optical phonon–plasmon coupled mode. It was found that the longitudinal optical (LO) peaks of HPSI and VDSI did not shift with laser power variations, due to a low concentration of photoexcited carriers, when a 532- nm laser was used. However, when a 355- nm laser was adapted, the relationship between the photoexcited carrier concentrations and the laser power was found to be nonlinear because of the dominance of trap-assisted Auger (TAA) recombination. The coefficient of TAA recombination was laser power–dependent. The proposed carrier dynamic model deepens the understanding of the physical mechanism of semi-insulating SiC irradiated by nanosecond laser and provides an insight into the interpretation of experimental phenomena related to laser energy in optoelectronic devices.

Investigation of Be doped GaSb with varying hole concentrations using single magnetic field Hall measurement and Raman spectroscopy

Raman scattering results on Beryllium doped Gallium antimonide (GaSb) epi-layers with varying hole concentrations (3 × 1017 cm−3 to 5 × 1019 cm−3) are presented in this study. The hole concentrations are determined from single magnetic field Hall measurements and corrected taking into consideration two band conduction. The Raman spectra show four distinct features: Longitudinal optical (LO) phonon mode from the surface accumulation/depletion region, disorder activated forbidden transverse optical (TO) phonon mode, LO phonon coupled Plasmon (LOPC) mode and the uncoupled LO phonon mode from the bulk region. A detailed Raman line-shape analysis using deformation potential and electro-optic contribution has been carried out to reproduce the experimental results. The effect of plasmon damping on the LO phonon coupled Plasmon (LOPC) mode is discussed. Free hole concentrations are derived from the Raman line-shape analysis and compared with that from the Hall measurements.

Low-Temperature Chemical Vapor Deposition Growth of MoS2 Nanodots and Their Raman and Photoluminescence Profiles

We report on the growth of an ordered array of MoS2 nanodots (lateral sizes in the range of ∼100–250 nm) by a thermal chemical vapor deposition (CVD) method directly onto SiO2 substrates at a relatively low substrate temperature (510–560°C). The temperature-dependent growth and evolution of MoS2 nanodots and the local environment of sulfur-induced structural defects and impurities were systematically investigated by field emission scanning electron microscopy, micro-Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) techniques. At the substrate temperature of 560°C, we observed mostly few-layer MoS2, and at 510°C, multilayer MoS2 growth, as confirmed from the Raman line shape analysis. With reduced substrate temperature, the density of MoS2 nanodots decreases, and layer thickness increases. Raman studies show characteristic Raman modes of the crystalline MoS2 layer, along with two new Raman modes centered at ∼346 and ∼361 cm−1, which are associated with MoO2 and MoO3 phases, respectively. Room temperature photoluminescence (PL) studies revealed strong visible PL from MoS2 layers, which is strongly blue-shifted from the bulk MoS2 flakes. The strong visible emission centered at ∼ 658 nm signifies a free excitonic transition in the direct gap of single-layer MoS2. Position-dependent PL profiles show excellent uniformity of the MoS2 layers for samples grown at 540 and 560°C. These results are significant for the low-temperature CVD growth of a few-layer MoS2 dots with direct bandgap photoluminescence on a flexible substrate.

Assessing the structural properties of graphitic and non-graphitic carbons by Raman spectroscopy

We study the transformation from molecular to crystalline of (non-)graphitic carbons synthesized from organic precursors by heat-treatment. Easy assessment of structural properties resulting from heat-treatment protocols is mandatory for industrial process monitoring. We demonstrate that Raman spectroscopy, in particular, the Raman lineshape analysis of G and D mode, offers quick assessment of the average sheet size of such carbons. We validate this method by performing Raman, WAXS and EPR measurements of series of resin and pitch-based carbons synthesized. The crystallite sizes of the WAXS analysis for the individual samples are related to corresponding positions and linewidths of G and D Raman modes and show excellent agreement between experiment and modelling from large sizes down to 4 nm. The theoretical master curves are independent of the precursor used in the synthesis, in contrast to models for the intensity ratio of D and G band versus size. We show that the latter are not universally valid and differ for each class of precursors. For sizes below 4 nm, our lineshape model fails as it is based on the bandstructure and phonon dispersions of ideal graphene. Thus, 4 nm corresponds to the fundamental transition from molecular to crystalline character for non-graphitic carbons.

Appearance of Fröhlich-like phonon mode and defect dynamics in La3+-doped ceria

The underlying physics which explains the role of cationic and anionic defect dynamics in determining the catalytic activity and ionic conductivity of aliovalent ion-doped ceria is complex and not yet fully understood. To address this issue, we have investigated the structural evolution of LaxCe1-xO2–δ, where x = 0.1, 0.2, 0.3, 0.4, and 0.5. Rietveld refinements of the X-ray diffraction patterns reveal that the crystal structure belongs to a disordered fluorite phase for all samples. We find a systematic increase in the lattice parameter along with a gradual decrease in oxygen site occupancy with the increase in La3+ doping concentration. In Raman spectra of doped compounds, other than well-known crystalline F2g and defect related modes, we observe the presence of an additional Raman mode. The Raman line-shape analysis indicates the non-resonant Fröhlich character of this mode. Temperature dependent Raman measurements demonstrate unique characteristics of this Raman peak. In addition, from the change in relative intensity ratio of the Raman modes related to different types of defect states, we show that vacancies cannot be considered as isolated defects beyond a critical doping level. We propose that the evolution of Raman intensities of defect related modes and Fröhlich mode with the doping level can be used as a marker to determine the role of electron-phonon coupling and anion vacancies in the catalytic activity of doped ceria. Furthermore, by studying the photocatalytic measurements for La3+ doped ceria compounds, we demonstrate that the anion vacancies do not always play a direct role in controlling functional properties.

Substrate Dependent Water Splitting with Ultrathin α-Fe2O3 Electrodes

Thin films of hematite (α-Fe2O3) were deposited by atomic layer deposition (ALD), and the effects of metal oxide underlayers on the photocatalytic water oxidation performance were investigated. It was found that a Ga2O3 underlayer dramatically enhances the water oxidation performance of the thinnest hematite films. The performance enhancement is attributed to the increased crystallinity of the ultrathin films induced by the oxide underlayers. The degree of crystallinity was examined by Raman line shape analysis of the characteristic hematite phonon modes. It was found that multiple metal oxide underlayers, including Nb2O5, ITO, and WO3, increase the film crystallinity compared to hematite deposited on bare FTO. The increased crystallite size was also clearly evident from the high resolution SEM images. The degree of crystallinity was found to correlate with absorbance and the photocatalytic water oxidation performance. These findings shed light on the origin of the dead layer at the interface of the FTO s...

The influence of growth temperatures on the characteristics of GaN nanowires: The Raman study

The study of Raman scattering of GaN nanowires under different growth temperatures (750°C to 1000°C) is presented. GaN nanowires grown at 950°C possessed the highest crystallinity. The Raman measurement illustrated E2(high) mode experienced a blueshift at lower temperature and redshift with the rising growth temperatures. This was related to the presence of stress/strain and the considerable oxygen content in the nanowires. By using Raman line shape analysis, the carrier concentration and mobility at different growth temperatures were determined which varied in the range of 4.0×1016cm−3 to 5.27×1017cm−3 and 158.2cm2/V s to 376.2cm2/V s, respectively. A comparative analysis of GaN thin film and nanowires revealed that the decreasing dimension of nanowires caused the broadening and lower frequency shift of Raman spectrum.

Mode-Specific Reorganization Energies and Ultrafast Solvation Dynamics of Tryptophan from Raman Line-Shape Analysis

Tryptophan is widely used as an intrinsic fluorophore for studies of protein structure and dynamics. Its fluorescence is known to have two decay components with lifetimes of 0.5 and 3.1 ns. In this work we measure the ultrafast dynamics of Tryptophan at <100 fs through measurements and modeling of the Raman excitation profiles with time-dependent wave packet propagation theory. We use a Brownian oscillator model to simulate the water-tryptophan interaction. Upon photoexcitation to the higher singlet electronic state (Bb) the structure of tryptophan is distorted to an overall expansion of the pyrrole and benzene rings. The total reorganization energy for Trp in water is estimated to be 2169 cm−1 with a 1230 cm−1 contribution from the inertial response of water. The value of reorganization energy of water corresponding to the fast response is found to be higher than that obtained upon excitation to the La state by previous studies that used computational simulations. The long-time dynamics of Trp manifests as a conformational heterogeneity at shorter times and contributes to inhomogeneous broadening of the Raman profiles (315 cm−1).

Investigation of spatial disorder in graphite by Raman lineshape analysis

Disorder in graphite is studied as a function of phonon softening of Raman active modes. A comprehensive analysis of disorder is discussed here using the G and D modes of the graphite. Two-dimensional disorder is manifested in the correlation length of the sp2 hybridization in the graphitic plane. It is characterized here by lineshape analysis of Raman activated G and D modes. Phonon softening of the G mode is almost insensitive to disorder. It is more asymmetric on the lower energy side with increasing disorder. Phonon softening and line broadening of the D mode have high sensitivity to disorder in polycrystalline graphite. Electron–phonon coupling is responsible for phonon softening and line broadening of the D mode, which is double-resonant Raman scattering involving disorder, electron, and TO (tansverse optical) phonon. Increasing disorder allows TO phonons of higher wavevector and lower energy during double-resonant Raman scattering.

Vertically aligned indium doped zinc oxide nanorods for the application of nanostructured anodes by radio frequency magnetron sputtering

Vertically aligned indium doped zinc oxide (IZO) nanorods (NRs) have been grown without any external catalyst on an indium tin oxide (ITO) coated glass substrate by radio frequency magnetron sputtering. Wurtzite IZO NRs distributed homogeneously on the substrate with a preferential orientation along the (002) crystallographic plane. The indium atoms are found to be spatially incorporated into the ZnO lattice along the axial direction by the migration of adatoms from the ITO coated glass substrate at the growth temperature of 550 °C. The doping concentration and mobility of the NRs are determined to be 1.3 × 1017 cm−3 and 68 cm2 V−1 s−1 by the Raman line shape analysis of the longitudinal optical phonon–plasmon coupled mode. The maximum optical transmittance of 83% in the visible region could be an ideal system for nanotextured anode and photovoltaic applications.

Defect modulated Raman response of KC8 single crystals

AbstractIn situ experiments of graphite intercalation with potassium have been performed under high vacuum (HV) conditions. A detailed Raman lineshape analysis of the G‐line was applied to determine the intrinsic response of the stage one KC8 compound. Four defect‐modulated graphite single crystals were analyzed. We observe five vibrational modes (∼560, 1260, 1510, 1547, and 1563 cm−1). From a Breit–Wigner–Fano (BWF) lineshape analysis we elucidate the contribution of each vibrational mode observed. The intensity of the z‐axis mode (CZ) around ∼560 cm−1 varies depending on the defect modulated nominal KC8 crystal chosen. We prove that the intrinsic G‐line of KC8 is at 1510 cm−1, and it is strongly dependent on the actual defect content in the sample, which has important implications for the electron phonon coupling (EPC) responsible of superconductivity in this system.

Effect of growth temperature on the morphology and phonon properties of InAs nanowires on Si substrates

Catalyst-free, vertical array of InAs nanowires (NWs) are grown on Si (111) substrate using MOCVD technique. The as-grown InAs NWs show a zinc-blende crystal structure along a < 111 > direction. It is found that both the density and length of InAs NWs decrease with increasing growth temperatures, while the diameter increases with increasing growth temperature, suggesting that the catalyst-free growth of InAs NWs is governed by the nucleation kinetics. The longitudinal optical and transverse optical (TO) mode of InAs NWs present a phonon frequency slightly lower than those of InAs bulk materials, which are speculated to be caused by the defects in the NWs. A surface optical mode is also observed for the InAs NWs, which shifts to lower wave-numbers when the diameter of NWs is decreased, in agreement with the theory prediction. The carrier concentration is extracted to be 2.25 × 1017 cm-3 from the Raman line shape analysis. A splitting of TO modes is also observed.PACS: 62.23.Hj; 81.07.Gf; 63.22.Gh; 61.46.Km

Raman line shape analysis as a mean characterizing molecular glass-forming liquids

Raman scattering spectra in glass forming toluene were studied in the temperature range 50–323 K with the goal of extracting information about homogeneous, inhomogeneous and orientational broadening. It was found that the temperature dependence of inhomogeneous line width allows one to depict two peculiar temperatures: T A and T g, where T g is the glass transition temperature and T A is the temperature of transition from an Arrhenius-like to a non-Arrhenius behavior for the α-relaxation time dependence on temperature, τ α( T). Temperature dependence of the orientational phase loss time τ OPL was found to correspond well to τ α at T > T A and continues approximately Arrhenius behavior for lower temperature in contrast to τ α( T). Also, a comparative analysis of homogeneous broadening of polarized and depolarized lines was done, which provided an estimation of the orientational broadening γ NL( T). The found γ NL( T) decreases linearly as the temperature decreases and goes to zero at T ~ T c, where T c is the critical temperature in framework of the mode-coupling theory (note that T c is close to other peculiar temperatures T B and T β, but we did not intent to distinguish among them in the present work). Thus, it was shown that the Raman line shape analysis in molecular glass forming materials allows one to extract peculiar temperatures: T A, T g, and, probably, T c. The test of the possibility to use a probe molecule for the Raman line shape analysis has revealed that the extracted data for probe molecule lines do not characterize the host matrices, at least in the low-viscous state ( T > T A).

Hydrogen bonding in different pyrimidine–methanol clusters probed by polarized Raman spectroscopy and DFT calculations

AbstractWe report on the hydrogen bonding between pyrimidine (Pd) and methanol (M) as H‐donor in this study. Hydrogen bonds between pyrimidine and methanol molecules as well as those between different methanol molecules significantly influence the spectral features at high dilution. The ring‐breathing mode ν1 of the reference system Pd was chosen as a marker band to probe the degree of hydrogen bonding. Polarized Raman spectra in the region 970–1020 cm−1 for binary mixtures of (pyrimidine + methanol) at 28 different mole fractions were recorded. A Raman line shape analysis of the isotropic Raman line profiles at all concentrations revealed three distinct spectral components at mole fractions of Pd below 0.75. The three components are attributed to three distinct groups of species: ‘free Pd’ (pd), ‘Pd with low methanol content’ (pd1) and ‘Pd with high‐methanol content’ (pd2). The two latter species differ considerably in the pattern and the strengths of the hydrogen bonds. The results of density functional theory calculations on structures and vibrational spectra of neat Pd and eight Pd/M complexes with varying methanol content support our interpretations of the experimental results. A nice spectra–structure correlation for the different cluster subgroups was obtained, similar to earlier results obtained for Pd and water. Apart from N···H and O···H hydrogen bonds between pyrimidine and methanol, O···H hydrogen bonds formed among the methanol molecules in the cluster at high methanol content also play a crucial role in the interpretation of the experimental results. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies

We studied structural, optical and magnetic properties of high-quality 5 and 15% Co-doped ZnO films grown by plasma-assisted molecular beam epitaxy (MBE) on (0001)-sapphire substrates. Magnetic force microscopy (MFM) and magnetic measurements with SQUID magnetometer show clear ferromagnetic behavior of the films up to room temperature whereas they are antiferromagnetic below 200 K approximately. Temperature dependence of the carrier mobility was determined using Raman line shape analysis of the longitudinal-optical-phonon-plasmon coupled modes. It shows that the microscopic mechanism for ferromagnetic ordering is coupling mediated by free electrons between spins of Co atoms. These results bring insight into a subtle interplay between charge carriers and magnetism in MBE-grown Zn(1-x)CoxO films.

Raman analysis of longitudinal optical phonon-plasmon coupled modes of aligned ZnO nanorods

The electronic properties of vertically aligned ZnO nanorods have been investigated using micro-Raman spectroscopy. The concentration and mobility of the charge carriers were determined via Raman line shape analysis using longitudinal-optical-phonon-plasmon coupled mode. The local laser heating and the stress effects have been considered when analyzing the Raman spectra. The mobility and carrier concentration of the aligned ZnO nanorods are 84.8cm2∕Vs and 3.8×1017cm−3, respectively. As a comparison, the mobility and carrier concentration of the undoped bulk ZnO were also obtained from the Raman line shape analysis. The mobility of the aligned ZnO nanorods is about 20% lower than that of the undoped bulk ZnO, which can be attributed to enhanced surface scattering due to the reduction in dimension.

G ′ band Raman lineshape analysis in graphitic foams

In this work we extend the study of the use of Raman spectroscopy to characterize graphitic foams by analyzing the dependence of the G ′ band lineshape on both the excitation laser energy and on the topology of the studied area. The G ′ band lineshape was found to be strongly correlated with the presence of stacking faults. The contribution of the TO + LA combination mode at ∼ 2450 cm −1 to the evaluated skewness includes an excitation energy dependence which needs to be considered. We analyze how to take this effect into consideration, thus obtaining a good evaluation of the dependence of the skewness of the G ′ band on the presence of stacking faults. We also discuss the correlation between the presence of stacking faults and of in plane defects in graphitic foams.

Transition from single-molecule to cooperative dynamics in a simple glass former: Raman line-shape analysis

Parameters for orientational and inhomogeneous broadening are found from the line shape analysis of the Raman spectrum of the glass former alpha-picoline during cooling from low-viscous to glassy state. The orientational phase loss time tau(OPL), extracted from the analysis, coincides with the alpha relaxation time at T > T(A), where T(A) is the temperature of transition from an Arrhenius-like to a non-Arrhenius behavior for the alpha-relaxation time dependence on temperature. At lower temperatures tau(OPL) (T) continues the Arrhenius behavior, in contrast to the alpha-relaxation time. The width of inhomogeneous broadening of the Raman line decreases noticeably as temperature increases in the temperature range T(g) < T < T(A), approaching to zero at T approximately T(A). The findings evidence the transition of molecular dynamics from individual to cooperative at T = T(A).

Isotopic dilution, self-association, and Raman non-coincidence in the binary system (CH3)2CO+(CD3)2CO reinvestigated by polarized Raman measurement and ab initio calculations

Abstract Raman spectra of the binary system (CH 3 ) 2 CO (acetone)+(CD 3 ) 2 CO (acetone-d 6 ) were reinvestigated employing a more precise and sensitive scanning multichannel detection scheme. The I ∥ and I ⊥ components of the Raman scattered radiation were recorded in the CO stretching region, 1600–1800 cm −1 as well as in the C–D and C–H stretching regions, 2000–2300 and 2800–3100 cm −1 , respectively, in different mixtures with the varying mole fractions of the reference system. A careful Raman line shape analysis yielded after fit Δ ν nc due to Raman non-coincidence effect (NCE) in acetone and acetone-d 6 as 4.87±0.01 and 4.49±0.01 cm −1 , respectively. The optimized geometries and wavenumbers of the neat acetone molecule and its self-associated structure were calculated using ab initio method at the MP2 level using 6-31++G(d,p) basis set. The influence of self-association and NCE together were examined. A systematic study of Raman line shape analysis led to a hitherto unexplored aspect, the wavenumber shift and linewidth variation of the ν 1 (C–H) stretching mode of acetone upon isotopic dilution. The linewidth variation with the mole fraction of the reference system shows an excellent agreement with the corresponding linewidths derived from the T 2 values obtained by three color fs-CARS study reported earlier.

Raman line-shape analysis of nano-structural evolution in cation-ordered ZrTiO 4-based dielectrics

It is known that the microwave dielectric characteristics of ZrTiO 4 are greatly influenced by processing conditions such as the degree of Sn-modification and the cooling rate after sintering. The effects of these processing variables on the nano-structural characteristics of ZrTiO 4-based dielectrics were studied by analyzing the Raman line-shape parameters using the phonon-confinement model. It was shown that the nano-structural shape of the cation-ordered domains underwent a sequential change from thin-slab form to platelet shape, and finally to spherical shape with increasing Sn-content or cooling rate.