Raman Line Shape Research Articles

Shedding light on evolution of Raman line shape with probing laser power: Light-induced perturbation in electron-phonon coupling.

The laser power mediated changes in the Raman line shape have been considered in terms of interference between discrete phonon states ρ and the electronic continuum states ϰ contributed by Urbach tail states. The laser-induced effects are treated in terms of the increase in the surface temperature and thereby the scaling of electronic disorder, i.e., Urbach energy, which can further contribute to the electron-phonon interactions. Therefore, the visualization of this effect is attempted analytically as a perturbation term in the Hamiltonian, which clearly accounts for the observed changes with laser power. This has been investigated based on the experimental results of laser power dependent Raman spectra of bulk EuFeO3 and silicon nanowires, which are found to provide convincing interpretations.

Effect of dimensionality on the excitation wavelength dependence of the Fano-Raman line-shape: a brief review.

The already existing heterogeneity in nanomaterials makes it an intriguing yet complex system to study size effect vis-à-vis other external perturbations and thereby local modifications at the nanoscale, thus demanding an improved tool and analysis for the choice of study. The analysis of existential subtle perturbations and interactions in a wide class of materials using Raman spectromicroscopy has proved to be of utmost importance, and various phenomena such as quantum confinement and its interplay with Fano resonance have already been investigated in nanomaterials, including the role of various perturbations such as temperature, pressure, doping, bias, and excitation wavelength on Raman spectral line shape parameters. Amongst different perturbations that cause a change in the spectral profile of Fano resonance, the gray area of wavelength dependence of Fano Raman line shape profiles has been least analysed in the literature. Moreover, the true signature of Fano resonance in nanoscaled systems, which is the wavelength dependence of Fano interaction, remains the least discussed. This review summarises the wavelength dependent correlation of Fano resonance and its effect on the Raman spectral line-shape parameters in some bulk materials, nanomaterials, and molecular systems involving heavily doped p-type crystalline silicon, 2-D MoS2, graphene, WS2, single walled carbon nanotubes, etc. A brief overview of Fano resonance in metamaterials and photonic systems is also provided.

Tuning the electron–phonon interaction via exploring the interrelation between Urbach energy and fano-type asymmetric raman line shape in GO-hBN nanocomposites

Hexagonal boron nitride (hBN), having an in-plane hexagonal structure in the sp2 arrangement of atoms, proclaims structural similarity with graphene with only a small lattice mismatch. Despite having nearly identical atomic arrangements and exhibiting almost identical properties, the electronic structures of the two materials are fundamentally different. Considering the aforementioned condition, a new hybrid material with enhanced properties can be evolved by combining both materials. This experiment involves liquid phase exfoliation of hBN and two-dimensional nanocomposites of GO-hBN with varying hBN and graphene oxide (GO) ratios. The optical and vibrational studies conducted using UV–vis absorption and Raman spectroscopic analysis report the tuning of electron–phonon interaction (EPI) in the GO-hBN nanocomposite as a function of GO content (%). This interaction depends on disorder-induced electronic and vibrational modifications addressed by Urbach energy (E u ) and asymmetry parameter (q), respectively. The EPI contribution to the induced disorders estimated from UV–vis absorption spectra is represented as EPI strength (E e–p ) and its impact observed in Raman phonon modes is quantified as an asymmetry parameter (q). The inverse of the asymmetry parameter is related to E e–p , as E e–p ∼ 1/|q|. Here in this article, a linear relationship has been established between E u and the proportional parameter (k), where k is determined as the ratio of the intensity of specific Raman mode (I) and q 2, explaining the disorders’ effect on Raman line shape. Thus a correlation between Urbach energy and the asymmetry parameter of Raman mode confirms the tuning of EPI with GO content (%) in GO-hBN nanocomposite.

Systematic Assessment of Phonon and Optical Characteristics for Gas-Source Molecular Beam Epitaxy-Grown InP1−xSbx/n-InAs Epifilms

Experimental and theoretical assessments of phonon and optical characteristics are methodically accomplished for comprehending the vibrational, structural, and electronic behavior of InP1−xSbx/n-InAs samples grown by Gas-Source Molecular Beam Epitaxy. While the polarization-dependent Raman scattering measurements revealed InP-like doublet covering optical modes (ωLOInP~350 cm−1, ωTOInP~304 cm−1) and phonons activated by disorders and impurities, a single unresolved InSb-like broadband is detected near ~195 cm−1. In InP1−xSbx, although no local vibrational (InSb:P; x → 1) and gap modes (InP:Sb; x → 0) are observed, the Raman line shapes exhibited large separation between the optical phonons of its binary counterparts, showing features similar to the phonon density of states, confirming “two-mode-behavior”. Despite the earlier suggestions of large miscibility gaps in InP1−xSbx epilayers for x between 0.02 and 0.97, our photoluminescence (PL) results of energy gaps insinuated achieving high-quality single-phase epilayers with x ~ 0.3 in the miscibility gap. Complete sets of model dielectric functions (MDFs) are obtained for simulating the optical constants of binary InP, InSb, and ternary InP1−xSbx alloys in the photon energy (0 ≤ E ≤ 6 eV) region. Detailed MDF analyses of refractive indices, extinction coefficients, absorption and reflectance spectra have exhibited results in good agreement with the spectroscopic ellipsometry data. For InP0.67Sb0.33 alloy, our calculated lowest energy bandgap E0 ~ 0.46 eV has validated the existing first-principles calculation and PL data. We feel that our results on Raman scattering, PL measurements, and simulations of optical constants provide valuable information for the vibrational and optical traits of InP1−xSbx/n-InAs epilayers and can be extended to many other technologically important materials.

Temperature Dependence of Non-Condon Effects in Two-Dimensional Vibrational Spectroscopy of Water.

Non-Condon effects in vibrational spectroscopy refers to the dependence of a molecule's vibrational transition dipole and polarizability on the coordinates of the surrounding environment. Earlier studies have shown that such effects can be pronounced for hydrogen-bonded systems like liquid water. Here, we present a theoretical study of two-dimensional vibrational spectroscopy under the non-Condon and Condon approximations at varying temperatures. We have performed calculations of both two-dimensional infrared and two-dimensional vibrational Raman spectra to gain insights into the temperature dependence of non-Condon effects in nonlinear vibrational spectroscopy. The two-dimensional spectra are calculated for the OH vibration of interest in the isotopic dilution limit where the coupling between the oscillators is ignored. Generally, both the infrared and Raman line shapes undergo red shifts with decrease in temperature due to strengthening of hydrogen bonds and decrease in the fraction of OH modes with weaker or no hydrogen bonds. The infrared line shape is further red-shifted under the non-Condon effects at a given temperature, while the Raman line shape does not show any such red shift due to non-Condon effects. The spectral dynamics becomes slower on decrease of temperature due to slower hydrogen bond relaxation and, for a given temperature, the spectral diffusion occurs at a faster rate upon inclusion of non-Condon effects. The time scales of spectral diffusion extracted from different metrics agree well with each other and also with experiments. The changes in the spectrum due to non-Condon effects are found to be more significant at lower temperatures.

Energy dispersive anti-anharmonic effect in a Fano intervened semiconductor: revealed through temperature and wavelength-dependent Raman scattering.

It is always interesting to understand how the interplay between two perturbations, affects any physical process and gets manifested in a semiconductor. Temperature- and wavelength-dependent Raman Spectromicroscopy was performed on heavily-doped Si to reveal an unusual anti-anharmonic effect. Additionally, the energy dispersive behaviour of Fano coupling strength was also studied and its possible interrelation with the observed anti-anharmonic effect was explored. A systematic study revealed that at the different excitation wavelengths, the strength of the Fano interaction was different, where the involved electron-phonon (Fano-Fano-interferon) bound states were counted together with different energies. By understanding how the interplay manifests in terms of the Raman line shape, a method to calculate the Fano-interferon dissociation energy was developed. The slope of the Raman linewidth at different excitation wavelengths with temperature showed a negative temperature coefficient and sign reversal on decreasing the doping concentration. A wavelength-dependent empirical relation is proposed to calculate the required thermal energy, required to dissociate the electron-phonon bound state.

Analysis of synthesized doped vertical silicon nanowire arrays for effective sensing of nitrogen dioxide: As gas sensors

In the present study, the controllable fabrication of silicon nanowires (Si NWs) with vertical alignment was accomplished using metal assisted chemical etching (MACE). The different characteristics, such as structural, morphological, chemical, optical, and dielectric properties were analyzed using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), Raman spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy (UV-DRS), and LCR [inductance (L), capacitance (C), and resistance (R)] meter (volume of the gas-sensing chamber is 650 mm3). It was revealed from the morphological study i.e., from the FESEM that p-type Si NWs are smaller in size than n-type Si NWs which is attributable to the energy band gap. The optical band gap (Eg) is observed to increase from 1.64 to 1.89 eV with the decreasing of the crystallite size and the optical reflection spectra of the Si NWs show a shift toward a lower wavelength (blue shift). Moreover, Raman spectra verified the red-shifted, asymmetrically broadened Raman line-shapes, which provides information about the size confinement effect in Si NWs. The MACE approach is excellent for synthesizing nanowire structures for use in gas-sensing applications due to its flexibility. The sensitivity of synthesized Si NWs was tested for NO2 gas. The sensor method is unique based on the testing of the device in the presence of a test gas because the use of the gas-sensing setup has the potential to measure the change in resistance by varying frequency, temperature, and time.

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.

Convolutional neural network-based retrieval of Raman signals from CARS spectra

We report the studies on the automatic extraction of the Raman signal from coherent anti-Stokes Raman scattering (CARS) spectra by using a convolutional neural network (CNN) model. The model architecture is adapted from literature and retrained with synthetic and semi-synthetic data. The synthesized CARS spectra better approximate the experimental CARS spectra. The retrained model accurately predicts spectral lines throughout the spectral range, even with minute intensities, which demonstrates the potential of the model. Further, the extracted Raman line-shapes are in good agreement with the original ones, with an RMS error of less than 7% on average and have shown correlation coefficients of more than 0.9. Finally, this approach has a strong potential in accurately estimating Raman signals from complex CARS data for various applications.

Fano-Type Wavelength-Dependent Asymmetric Raman Line Shapes from MoS2 Nanoflakes.

Excitation wavelength-dependent Raman spectroscopy has been carried out to study electron-phonon interaction (Fano resonance) in multi-layered bulk 2H-MoS2 nano-flakes. The electron-phonon coupling is proposed to be caused due to interaction between energy of an excitonic quasi-electronic continuum and the discrete one phonon, first-order Raman modes of MoS2. It is proposed that an asymmetrically broadened Raman line shape obtained by 633 nm laser excitation is due to electron-phonon interaction whose electronic continuum is provided by the well-known A and B excitons. Typical wavelength-dependent Raman line shape has been observed, which validates and quantifies the Fano interaction present in the samples. The experimentally obtained Raman scattering data show very good agreement with the theoretical Fano-Raman line-shape functions and help in estimating the coupling strength. Values of the electron-phonon interaction parameter obtained, through line-shape fitting, for the two excitation wavelengths have been compared and shown to have generic Fano-type dependence on the excitation wavelength.

Positive and negative signal and line shape in stimulated Raman spectroscopy: Resonance femtosecond Raman spectra of diphenylbutadiene.

Resonance stimulated Raman signal and line shape are evaluated analytically under common electronic/vibrational dephasing and exponential Raman/probe pulse, exp(-|t|/τ). Generally, the signal from a particular state includes contributions from higher and lower electronic states. Thus, with S0 → S1 actinic excitation, the Raman signal consists of 15 Feynman diagrams entering with different signs. The negative signindicates vibrational coherences in S1 or higher Sn, whereas the positive signreveals coherences in S0 or Sn via S1 → Sn → Sm (n < m) coupling. The signal complexity is in contrast to spontaneous Raman with its single diagram only. The results are applied to femtosecond stimulated Raman spectra of trans-trans, cis-trans (ct), and cis-cis (cc) 1,4-diphenyl-1,3-butadiene, the ct and cc being reported for the first time. Upon actinic excitation, the Stokes spectra show negative bands from S1 or Sn. When approaching higher resonances Sn → Sm, some Raman bands switch their signfrom negative to positive, thus, indicating new coherences in Sn. The results are discussed, and the measured Raman spectra are compared to the computed quantum-chemical spectra.

(INVITED) Stimulated Raman lineshapes in the large light–matter interaction limit

Stimulated Raman scattering (SRS) represents a powerful tool for accessing the vibrational properties of molecular compounds or solid state systems. From a spectroscopic perspective, SRS is able to capture Raman spectra free from incoherent background processes and typically ensures a signal enhancement of several orders of magnitude with respect to its spontaneous counterpart. Since its discovery in 1962, SRS has been applied to develop technological applications, such as Raman-based lasers, frequency shifters for pulsed sources and Raman amplifiers. For the full exploitation of their potential, however, it is crucial to have an accurate description of the SRS processes under the large gain regime. Here, by taking as an example the stimulated Raman spectrum of a model solvent, namely liquid cyclohexane, we discuss how the spectral profiles and the lineshapes of Raman excitations critically depend on the pump excitation regime. In particular, we show that in the large light–matter interaction limit the Raman gain undergoes an exponential increase (decrease) in the red (blue) side of the spectrum, with the Raman linewidths that appear sharpened (broadened).

Investigating the correlation between the Urbach energy and asymmetry parameter of the Raman mode in semiconductors

A quantitative correlation between the Urbach energy $({E}_{u})$ and asymmetry parameter $q$ of Raman spectra has been derived. For this purpose, the effect of electronic disorder $(\ensuremath{\sim}{E}_{u})$ on the possible interference between the electronic continuum states and discrete phononic states is reinvestigated. The equation of the form $\frac{I}{{q}^{2}}\ensuremath{\propto}\ensuremath{\xi}{E}_{u}\ifmmode\pm\else\textpm\fi{}\ensuremath{\lambda}$ ($I$: intensity of phonon mode $;\phantom{\rule{0.28em}{0ex}}\ensuremath{\xi},\ensuremath{\lambda}:$ some material-dependent parameters) has been obtained and verified experimentally on different semiconductors. The experimental results reveal an offset which has been understood as intrinsic or disorder-induced contribution to electron-phonon interactions in the system. The obtained equations quantitatively explain the changes in the Raman line shape due to disorder induced by atomic substitutions, temperature, etc., and provide understanding of the superposition between electronic and phononic states and of electron-phonon coupling in semiconductors with disorder.

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.

Effect of some physical perturbations and their interplay on Raman spectral line shapes in silicon: A brief review

AbstractRaman spectroscopy is a proven versatile tool for characterization of materials spanning almost all areas of science because of its ability to non‐invasively extract information about materials. This technique is able to detect any perturbation in a system that can affect the phonons. A detailed discussion on various factors that affect the Raman line shape for a material has been summarized here by taking the example of silicon. Methods to identify the actual reason(s) behind the observed Raman spectral line shape have also been briefly discussed. Raman line shape obtained from silicon nanostructures when analyzed closely along with their bulk counterparts, reveals important information about the quantum confinement in such systems characterized by the Bohr's exciton radius. Raman line‐shape parameters are analyzed closely to understand the influence of any perturbation like quantum confinement, heavy doping, temperature rise, pressure, excitation wavelength, electron–phonon interaction, and so on. Current review briefly deals with the origin of asymmetric Raman line shapes in (nano‐) silicon due to various physical perturbations and their interplays, which becomes the origin of different line shapes. Advantages of using Raman microscopy in analyzing subtler physical processes taking place in a semiconductor have also been underlined.

Insights into hyperbolic phonon polaritons in h−BN using Raman scattering from encapsulated transition metal dichalcogenide layers

Alternative techniques for probing hyperbolic phonon polaritons (HPPs) in two-dimensional materials will support the development of the emerging technologies in this field. Previous reports have shown that it is possible for $\mathrm{W}{\mathrm{Se}}_{2}$ monolayers in contact with the hexagonal boron nitride ($h\text{\ensuremath{-}}\mathrm{BN}$) to generate HPPs in the $h\text{\ensuremath{-}}\mathrm{BN}$ via Raman scattering. In this paper, we set out our results on HPP Raman scattering induced in $h\text{\ensuremath{-}}\mathrm{BN}$ by $\mathrm{W}{\mathrm{Se}}_{2}$ and $\mathrm{Mo}{\mathrm{Se}}_{2}$ monolayers including multiple resonances at which the Raman scattering is enhanced. Analysis of the observed Raman line shapes demonstrates that Raman scattering allows HPPs with wave vectors with magnitudes significantly in excess of $15\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--1}$ to be probed. We present evidence that the Raman scattering can probe HPPs with frequencies less than the expected lower bound on the reststrahlen band, suggesting new HPP physics still waits to be discovered.

Predicting Raman line shapes from amorphous silicon clusters for estimating short‐range order

AbstractA theoretical model obtained based on direct matrix method and bond polarization model (DMM‐BPM) has been used to predict Raman scattering line shape for amorphous silicon (a‐Si). These line shapes can describe the observed nonunique peak position from these a‐Si crystallites, unlike crystalline Si, that varies between 470 and 485 cm−1. The model has been validated using three different sets of Raman scattering data by fitting it with the proposed model. The Raman spectra are obtained from a‐Si samples prepared by three different methods by three different research groups reported earlier. Clear consonance with the experimentally observed and theoretical Raman line shape was obtained. The model has also been used to estimate the distance up to which the crystallinity remains intact in a‐Si. The used model has been compared with the existing phonon confinement model, and a discrepancy (in the latter) therein has been discussed. Rationale behind adopting this model has been explained using the constraints related to the Raman selection rule, their relaxation, or breakdown in appropriate materials' regimes. A handy empirical relation between Raman peak position and amorphous cluster size has been proposed.

Pseudo-Anomalous Size-Dependent Electron–Phonon Interaction in Graded Energy Band: Solving the Fano Paradox

Quantum size effects on interferons (electron-phonon bound states), confined in fractal silicon (Si) nanostructures (NSs), have been studied by using Raman spectromicroscopy. A paradoxical size dependence of Fano parameters, estimated from Raman spectra, has been observed as a consequence of longitudinal variation of nanocrystallite size along the Si wires leading to local variations in the dopants' density which actually starts governing the Fano coupling, thus liberating the interferons to exhibit the typical quantum size effect. These interferons are more dominated by the effective reduction in dopants' density rather than the quantum confinement effect. Detailed experimental and theoretical Raman line shape analyses have been performed to solve the paradox by establishing that the increasing size effect actually is accompanied by receding Fano coupling due to the weakened electronic continuum. The latter has been validated by observing a consequent variation in the Raman signal from dopants which was found to be consistent with the above conclusion.

My Life in Changing Times: New Ideas and New Techniques.

I describe some of the science that I have been involved in during the last 60 years and the changes in equipment that made it possible. Starting with an interest in spectroscopy and measurement of NMR parameters, I moved to work on theoretical aspects of spin systems and infrared and Raman line shapes. This morphed into using the new technique of computer simulation to study such problems. The last half of my working life has concentrated on the application of computer simulation to a number of problems culminating in pioneering investigations of the behavior of ionic liquids.

Doping-Induced Combined Fano and Phonon Confinement Effect in La-Doped CeO2: Raman Spectroscopy Analysis

The effect of La-doping on the crystallographic structure and the Raman line-shape of CeO2 has been investigated. X-ray diffraction analysis suggests that the lattice parameter and the Ce/La–O bond...