Raman Line Shape Research Articles

Sensitivity of coherent anti-Stokes Raman lineshape to time asymmetry of laser pulses.

We show that coherent anti-Stokes Raman lineshapes do not follow known spectral profiles if the time asymmetry of realistic laser pulses is taken into account. Examples are given for nanosecond and picosecond laser pulses commonly employed in frequency-resolved coherent anti-Stokes Raman scattering. More remarkably, the analysis suggests an effect of line narrowing in comparison to the customary approach, based primarily on the Voigt lineshape.

Controllable single-mode random laser using stimulated Raman gain.

The present study demonstrates numerically a novel approach to perform a controllable single-mode random laser, using stimulated Raman gain. Due to the narrow linewidth of the Raman line shape, only one of the modes of the passive random system can lase. The robust control on the emission spectrum was achieved through the selection of any desired quasi-modes, by adjusting the pump wavelength in order to place the center of Raman line shape on desired quasi-modes. This approach was proved using a developed nonlinear transfer matrix method for a 1D Raman random system. The proposed method includes the Raman gain saturation and the frequency-dependent index of refraction.

Raman random laser in one-dimensional system

The probability of occurrence of random cavities’ resonance frequency in the Stokes bandwidth of the Raman active medium can cause Raman laser oscillation in the random structures. Due to the small bandwidth of Raman line shape, the probability of the simultaneous appearance of random cavity resonance frequencies in the cascade Stokes line shapes is small. As a result, the cascade Raman laser oscillation effects on the saturation behavior are negligible. The nonlinear transmission matrix method is employed to determine the statistical behavior of a Raman random laser. Our results showed that the statistical behavior depends on the position of the Stokes line shape relative to the center of the stopband and the edge of the passband of the corresponding nondisorder system.

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...

Qualitative Evolution of Asymmetric Raman Line-Shape for NanoStructures

A step-by-step evolution of an asymmetric Raman line-shape function from a Lorentzian line-shape is presented here for low dimensional semiconductors. The evolution reported here is based on the phonon confinement model which is successfully used in literature to explain the asymmetric Raman line-shape from semiconductor nano-structures. Physical significance of different terms in the theoretical asymmetric Raman line-shape has been explained here. Better understanding of theoretical reasoning behind each term allows one to use the theoretical Raman line-shape without going into details of theory from first principle. This will enable one to empirically derive a theoretical Raman line-shape function for any material if information about its phonon dispersion, size dependence etc is known.

Probing substrate influence on graphene by analyzing Raman lineshapes

We provide a new approach to identify the substrate influence on graphene surface. Distinguishing the substrate influences or the doping effects of charged impurities on graphene can be realized by optically probing the graphene surfaces, included the suspended and supported graphene. In this work, the line scan of Raman spectroscopy was performed across the graphene surface on the ordered square hole. Then, the bandwidths of G-band and 2D-band were fitted into the Voigt profile, a convolution of Gaussian and Lorentzian profiles. The bandwidths of Lorentzian parts were kept as constant whether it is the suspended and supported graphene. For the Gaussian part, the suspended graphene exhibits much greater Gaussian bandwidths than those of the supported graphene. It reveals that the doping effect on supported graphene is stronger than that of suspended graphene. Compared with the previous studies, we also used the peak positions of G bands, and I2D/IG ratios to confirm that our method really works. For the suspended graphene, the peak positions of G band are downshifted with respect to supported graphene, and the I2D/IG ratios of suspended graphene are larger than those of supported graphene. With data fitting into Voigt profile, one can find out the information behind the lineshapes.

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).

Size-dependent photoluminescence in silicon nanostructures: quantum confinement effect

Visible photoluminescence (PL) from laser-etched silicon nanostructures has been analysed. A systematic size dependence study of PL from silicon nanostructures has been performed. The PL from these structures is attributed to the quantum confinement effect. Different quantum confinement models have been used for PL and Raman lineshape fitting to calculate the mean size and size distribution of silicon nanostructures and the results are comparatively studied. Calculated values of oscillator strength and radiative lifetime show that PL is due to radiative recombination of confined excitons.

Self-assembled Ge islands and nanocrystals by RF magnetron sputtering and rapid thermal processing: The role of annealing temperature

Germanium (Ge) nanostructures were fabricated through the rapid thermal processing of radio frequency sputtered Ge on silicon (Si). The substrates were unheated during the growth, resulting in a post-growth Ge deposited layer that was 250nm thick with a surface that had no evidence of nanostructure formation. The samples subsequently underwent rapid annealing from 400°C to 800°C for 15s. Dramatic nanostructuring was observed at the surface following annealing above 600°C. Scanning electron microscopy showed that as annealing temperatures increased, the Ge layer evolved into circular islands. The sizes of the Ge islands increased and the islands became more uniform and dense at 700°C. Viable Ge nanocrystals were obtained at 600°C and 700°C given the increased annealing temperatures that improved crystallinity. The calculated Raman line shape based on the phonon confinement model agreed well with the experimental spectrum which constituted crystalline Ge nanostructures with an estimated size of 2.65–3.5nm. In addition, no Ge–Si intermixing was observed at the interface. High resolution X-ray diffraction revealed the tetragonal Ge phases and the samples were of polycrystalline structures. The annealing temperature also enhanced photo currents of the fabricated metal semiconductor metal photodetector. These results suggest that 700°C for 15s is an optimum annealing temperature for the production of viable crystalline Ge nanostructures.

Surface Reconstruction of Electrochemically Etched Nano-structured Solar Cell

Inexpensive silicon solar cell utilizes nanotechnology offers great possibility to enhance the conversion efficiency and reduce the maintenance requirements as well. Moreover, the large surface area of the nanostructured solar cell enables this cell to reduce the number of the required cells. The surface of ordinary solar cell has been reconstructed by electrochemical etching in suitable cell. It is found that the etching time and current density have optimum values to obtain higher conversion efficiency. Effects of these two parameters on the optoelectronic properties of the solar cell have been studied. Spectroscopic methods involving Raman and photoluminescence emission were carried out on the reconstructed surface of the solar cell.Fitting of Raman line shape with theoretical quantum confinement model implies that nanostructure of mean value of 6 nm was established. While the surface morphology investigation reveal synthesize of a pore-like structure separated by thin walls consisting silicon nanostructures.

First-principles studies of the vibrational properties of amorphous carbon nitrides

Raman spectra of amorphous carbon nitride films (a-C:N) resemble those of typical amorphous carbon (a-C), and no specific features in the spectra are shown due to N doping. The present work provides a correlation between the microstructure and vibrational properties of a-C:N films from first principles. The six periodic model structures of 64 atoms with various mass densities and nitrogen contents are generated by the liquid-quench method using Car—Parinello molecular dynamics. By using Raman coupling tensors calculated with the finite electric field method, Raman spectra are obtained. The calculated results show that the vibrations of C=N could directly contribute to the Raman spectrum. The similarity of the Raman line shapes of N-doped and N-free amorphous carbons is due to the overlapping of C=N and C=C vibration bands. In addition, the origin of characteristic Raman peaks is also given.

Raman line shape studies of hydrogen cryosolutions

The pure rotational and rotation-vibration Raman transitions in H2 dissolved in liquid argon (LAr, T=87 K) and nitrogen (LN2, T=80 K) are studied at different hydrogen concentrations. Whereas the rotational S0(J) (J=0, 1) shapes of H2-LAr can be accurately fitted by Lorentzian profiles, the wings of the same H2-LN2 lines demonstrate a distinct sub-Lorentzian behaviour. The difference between these patterns is attributed to the change of the time scale of anisotropic interactions which perturb the rotational motion of H2. They should be slower for the H2-LN2 system in which the long-range quadrupole-quadrupole interactions develop. The latter dominate in the formation of the symmetric H2-LN2 S1(J) line shapes. By contrast, the same H2-LAr lines are mainly monitored by the vibrational broadening and their shapes are distinctly asymmetric.

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.

Disorder-activated Raman spectra of cubic rocksalt-type Li(1−x)/2Ga(1−x)/2MxO (M = Mg, Zn) alloys

Disorder-activated phonon behaviors of the cubic rocksalt-type semiconductor alloy Li(1−x)/2Ga(1−x)/2MxO (M = Mg, Zn) prepared by high-pressure and high-temperature method has been studied by Raman scattering analysis. The LO Raman phonon in Li(1−x)/2Ga(1−x)/2MxO was found to exhibit a distinct two-mode behavior. The compositional dependence of Raman frequency, peak-width, and intensity has been discussed. A model associated with a composite mode of the Brillouin zone center and edge phonons combined with phonon dispersion curves determined by first-principle calculations were employed to explain the asymmetric broadening of the LO phonon mode. The broadening and asymmetric Raman line-shape in Li(1−x)/2Ga(1−x)/2MxO can be interpreted as a composite mode of the softening Brillouin zone center mode and the Brillouin zone edge mode.

Melt Homogenization and Self‐Organization in Chalcogenides‐Part I

An FT‐Raman profiling method is developed to monitor growth of structural homogeneity of binary GexSe100‐x melts in real time (tR) non‐invasively as starting materials are reacted over days. Raman spectra of quenched melts were acquired along a one inch long column of a sample in a quartz tube. In the first step of reaction, tR &lt; 2 days, Ge‐rich crystalline‐ and glassy‐ phases form and coexist with Se‐rich glasses. In the second step, tR extending up to 7 days, local structures characteristic of melts/glasses form, and steadily homogenize as Ge and Se atoms diffuse. The process terminates when all Raman lineshapes taken along the length of a sample coalesce into unique profile. Several factors contribute to the long reaction time tR for melts to homogenize, including liquid density difference of Ge and Se, diffusion controlled nanoscale mixing of melts, batch dryness, batch sizes, and laser spot size. Physical properties of such homogeneous GexSe100‐x glasses are found to be quite different from their inhomogeneous counterparts realized after the first step of reaction. Slow homogenization of chalcogenides melts may occur generally. Variations in physical properties of chalcogenide glasses possessing nominally the same composition may have their origin in structural heterogeneity and purity.

Melt Homogenization and Self‐Organization in Chalcogenides‐Part II

The compositional variation of the non‐reversing enthalpy at Tg, ΔHnr(x), in GexSe100−x glasses decreases abruptly by an order of magnitude as x increases to xc(1) = 19.5(5)%, the rigidity transition, and then remains minuscule till x increases to xc(2) = 26.0(5)%, when the term abruptly increases by an order of magnitude as glasses become stressed‐rigid. The rigid but unstressed networks formed in between these two transitions represent the Intermediate Phase (IP). The square‐well like variation of ΔHnr(x), also known as the reversibility window develops sloping walls, then a triangular shape and eventually disappears as glasses of increasing heterogeneity are studied. The ΔHnr term ages over weeks outside the IP but not inside the IP. Raman line shapes of as‐quenched melts are quite similar to those of Tg‐cycled glasses for compositions in the IP, but not outside the IP– an optical analog of the thermal reversibility window. Variations of Molar volumes, display a global minimum in the IP and a pronounced increase outside that phase. Physical behavior of dry and homogeneous chalcogenide glasses that leads to sharp elastic and chemical phase transitions remains to be understood theoretically. The physics of network may be even more interesting than hitherto recognized.

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.

Probing Phonons in Nonpolar Semiconducting Nanowires with Raman Spectroscopy

We present recent developments in Raman probe of confined optical and acoustic phonons in nonpolar semiconducting nanowires, with emphasis on Si and Ge. First, a review of the theoretical spatial correlation phenomenological model widely used to explain the downshift and asymmetric broadening to lower energies observed in the Raman profile is given. Second, we discuss the influence of local inhomogeneous laser heating and its interplay with phonon confinement on Si and Ge Raman line shape. Finally, acoustic phonon confinement, its effect on thermal conductivity, and factors that lead to phonon damping are discussed in light of their broad implications on nanodevice fabrication.

Low-frequency Raman scattering from silicon nanostructures

Low-frequency Raman scattering due to acoustic phonons is studied for silicon nanostructures. The lineshapes of the first-order Raman active modes exhibit asymmetry. A tail is observed toward low frequency and high frequency for the optic mode and acoustic mode, respectively. The Raman lineshapes of these modes are determined by a Gaussian envelope function convoluted with the vibrational density of states. The observed blueshift of the acoustic mode with reducing size of the nanostructures can be explained by the relaxation of the wavevector selection rule (q = 0), which is used in the phonon confinement model for positive-slope (dω/dq &amp;gt; 0) phonon dispersion. Because the acoustic and optical phonon branches have high positive and moderate negative slopes, respectively, around the “Gamma”-point in the phonon dispersion, a larger Raman shift of the acoustic mode to a higher frequency is observed in comparison with the shift to a lower frequency of the optic mode for a given nanostructure size.