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.

Understanding the role of membrane cholesterol upon Epstein Barr virus infection in astroglial cells.

Background: EBV infection has long been postulated to trigger multiple sclerosis (MS) and anti-EBV antibodies showed a consistent presence in MS patients. Previous reports from our group have shown that the EBV infects different brain cells. Entry of the virus in neuronal cells is assisted by several host factors including membrane cholesterol. By using an inhibitor, methyl-ß-cyclodextrin (MßCD), we evaluated the role of membrane cholesterol in EBV infection and pathogenesis. Methodology: The membrane cholesterol depleted cells were infected with EBV and its latent genes expression were assessed. Further, EBV-mediated downstream signalling molecules namely STAT3, RIP, NF-kB and TNF-α levels was checked at protein level along with spatial (periphery and nucleus) and temporal changes in biomolecular fingerprints with Raman microspectroscopy (RS). Results: Upon treatment with MßCD, lmp1 and lmp2a suggested significant downregulation compared to EBV infection. Downstream molecules like STAT3 and RIP, exhibited a decrease in protein levels temporally upon exposure to MßCD while NF-kB levels were found to be increased. Further, the intensity of the Raman spectra exhibited an increase in triglycerides and fatty acids in the cytoplasm of EBV-infected LN-229 cells compared to MßCD+EBV. Likewise, the Raman peak width of cholesterol, lipid and fatty acids were found to be reduced in EBV-infected samples indicates elevation in the cholesterol specific moieties. In contrast, an opposite pattern was observed in the nucleus. Moreover, the ingenuity pathway analysis revealed protein molecules such as VLDLR, MBP and APP that are associated with altered profile of cholesterol, fatty acids and triglycerides with infection-related CNS disorders. Conclusion: Taken together, our results underline the important role of membrane cholesterol over EBV entry/pathogenesis in astroglia cells which further trigger/exacerbate virus-associated neuropathologies. These results likely to aid into the prognosis of neurological disease like MS.

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.

Early biomolecular changes in brain microvascular endothelial cells under Epstein-Barr virus influence: a Raman microspectroscopic investigation.

The brain microvascular endothelial cells (ECs) play an important role in protecting the brain from hazardous pathogens. However, some viral pathogens can smartly modulate the endothelial pathways to gain entry inside the brain. Further, these viruses can cause endothelial dysfunction which could develop serious neurological ailments. Epstein-Barr virus (EBV), an oncogenic virus, has also been linked to various neurological disorders. The virus primarily infects epithelial and B cells, however, it also has a tendency to infect ECs and cause endothelial activation. However, the impact of EBV influence on ECs is still underexplored. Studying the early events of virus-mediated cellular modulation could help in understanding the virus' infection strategy or aftermath. Raman microspectroscopy has been widely utilized in biomedical sciences to decipher cellular changes. To understand the EBV-influenced EC modulation by studying intracellular biomolecular changes at early time points, we utilized the Raman microspectroscopy tool. We treated the ECs with EBV and acquired the Raman spectra at different time points (2, 4, 6, 12, 24 and 36 h) and different sites (nucleus and periphery) to check changes in Raman intensities associated with specific biomolecules. In the EBV-treated cells, the status of various biomolecules in terms of Raman intensities was observed to be altered compared with uninfected cells. Specifically, the cholesterol, polysaccharide, nucleotides, nucleic acid and proline moieties were altered at different time points. We also investigated the possible correlation between these molecules using molecular network analysis and observed various associated factors. These factors could be influenced by EBV to alter the associated biomolecular levels. Our study paves the pathway to study EBV infection in human brain microvascular ECs and highlights specific biomolecular alterations, which can be focused for further mechanistic investigations.

Nonlinear Temperature-Dependent Phonon Decay in Heavily Doped Silicon: Predominant Interferon-Mediated Cold Phonon Annihilation.

A nonlinear Fano interaction has been reported here which is manifest in terms of a parabolic temperature-dependent phonon decay process observable in terms of a Raman spectral parameter. Temperature-dependent Raman spectroscopic studies have been carried out on heavily and moderately doped crystalline silicon to investigate the behavior of anharmonic phonon decay in semiconductor systems where Fano interactions are present inherently. Systematic study reveals that in heavily doped systems an interferon-mediated decay route exists for cold phonons present at lower temperatures (<475 K) where Fano coupling is stronger and dominates over the typical multiple-phonon decay process. On the other hand, the anharmonic phonon decay remains the predominant process at higher temperatures irrespective of the doping level. Temperature-dependent phonon self-energy has been calculated using experimentally observed Raman line-shape parameters to validate the fact that the nonlinear decay of phonons through interferon mediation is a thermodynamically favorable process at low temperatures.

Comparative Account of Biomolecular Changes Post Epstein Barr Virus Infection of the Neuronal and Glial Cells Using Raman Microspectroscopy.

Raman microspectroscopy is a vibrational spectroscopy technique used for investigating molecular fingerprints of a wide range of liquid or solid samples. The technique can be efficiently utilized to understand the virus-mediated cellular changes and could provide valuable insights into specific biomolecular alterations. The Epstein Barr virus (EBV) has been associated with various types of cancers as well as neurodegenerative diseases. However, EBV-mediated neurological ailments are yet underexplored in terms of biomolecular changes in neuronal and glial cells (astrocytes and microglia). In continuation of our earlier exploration of EBV-influenced glial cells, we tried to decipher biomolecular changes in EBV-infected neuronal cells using Raman microspectroscopy. Additionally, we compared the consecutive biomolecular changes observed in neuronal cells with both the glial cells. We observed that EBV infection gets differentially regulated in the neuronal cells, astrocytes, and microglia. The viral entry and initiation of infection-mediated cellular modulation could start as soon as 2 h post infection but may regulate a distinct biomolecular milieu in different time intervals. Similar to the early timespan, the 24-36 h interval could also be important for EBV to manipulate neuronal as well as glial cells as depicted from elevated biomolecular activities. At these time intervals, some common biomolecules such as proline, glucose, lactic acid, nucleotides, or cholesterol were observed in the cells. However, at these time intervals, some distinct biomolecules were also observed in each cell, such as collagen, lipid, and protein stretches in the neuronal nucleus (2-4 h); tyrosine and RNA in the astrocyte nucleus (2-4 h nucleus); and fatty acids in the microglia nucleus (24-36 h). The observed biomolecular entities could ultimately play pivotal roles in the viral usurpation of cells. We also provided insights into whether these biomolecular changes can be correlated to each other and mediate virus-associated manifestations which can be linked to neurological complications. Our study aids in the understanding of EBV-mediated biomolecular changes in the various compartments of the central nervous system.

Enhanced interfacial charge transfer on strain-induced 2D-1D/MoS2-TiO2heterostructures for electrochemical and photocatalytic applications.

Two-dimensional (2D)/one dimensionsal (1D)-MoS2/TiO2heterostructures have proven to be potent for photocatalytic applications. Enhancement of a heterostructure's photocatalytic activity may be influenced by the accumulation of strain at the interface, which affects the interfacial interaction. Keeping this in mind, the present paper reports strain-accumulated interfacial modification of 2D/1D-MoS2/TiO2heterostructures for the enhancement of photocatalytic activity. Two different synthesis methods, namely the hydrothermal and chemical vapor deposition (CVD) methods, are used for the growth of MoS2on TiO2nanostructures. Micro-Raman spectroscopy reveals that strain is accumulated at the interface of the growth of the MoS2over the TiO2nanostructures. It is further revealed that the MoS2/TiO2heterostructure synthesized by the CVD method induces compressive strain. Also, the heterostructure synthesized by the hydrothermal method induces tensile strain that modifies the charge separation at the interface, which is further confirmed by x-ray photoelectron spectroscopy (XPS). Moreover, ultraviolet photoelectron spectroscopy (UPS) reveals upward band-bending in the MoS2/TiO2heterostructure synthesized by the hydrothermal method. Similarly, the heterostructure synthesized by the CVD method shows downward band-bending that leads to improved charge separation at the interface. The modified interfaces of the heterostructures are further studied for electrochemical measurements using cyclic voltammetry (CV) and photocatalytic activity by degradation of a model compound.

Ultrafast carrier dynamics in a monolayer MoS2at carrier densities well above Mott density.

Due to the growing interest in monolayer (ML) molybdenum disulfide (MoS2) in several optoelectronic applications like lasers, detectors, sensors, it is important to understand the ultrafast behavior of the excited carriers in this material. In this article, a comprehensive study of the charge carrier dynamics of a monolayer MoS2flake has been studied using transient transmission technique near A-exciton under high excitation densities well above the Mott density. Fluence dependent studies has been carried out to understand the origin of the processes which modifies its optical response under excitation. The dissociation of excitons leads to an observed fast bandgap renormalization. At later times when large number of carriers relax the remaining carriers forms excitons leading to a bleaching effect.

Raman Spectroscopy as a Simple yet Effective Analytical Tool for Determining Fermi Energy and Temperature Dependent Fermi Shift in Silicon.

The Fermi energy is known to be dependent on doping and temperature, but finding its value and corresponding thermal Fermi shift experimentally is not only difficult but is virtually impossible if one attempts their simultaneous determination. We report that temperature dependent Raman spectromicroscopy solves the purpose easily and proves to be a powerful technique to determine the position and temperature associated Fermi shift in an extrinsic semiconductor as demonstrated for silicon in the present study. The typical asymmetrically broadened Raman spectral line-shape from sufficiently doped n- and p-type silicon contains the information about the Fermi level position through its known association with the Fano coupling strength. Thus, Raman line-shape parameters, the terms quantify the Fano-coupling, have been used as experimental observables to reveal the value of the Fermi energy and consequent thermal Fermi shift. A simple formula has been developed based on existing established theoretical frameworks that can be used to calculate the position of the Fermi level. The proposed Raman spectroscopy-based formulation applies well for n- and p-type silicon. The calculated Fermi level position and its temperature dependent variation are consistent with the existing reports.

Bifunctional Application of Viologen-MoS2-CNT/Polythiophene Device as Electrochromic Diode and Half-Wave Rectifier.

A dual purpose solid state electrochromic diode has been fabricated using polythiophene (P3HT) and ethyl Viologen (EV), predoped with multiwalled carbon nanotubes (MWCNTs) and MoS2. The device has been designed by considering two important aspects, first, the complementary redox activity of P3HT and EV and second, the electron holding properties of MoS2 and MWCNTs. The latter is found to enhance the electrochromic performance of the solid state device. On the other hand, the complementary redox nature gives the asymmetric diodic I-V characteristic to the device which has been exploited to use the electrochromic device for rectification application. The MoS2 nanoflower and MWCNTs are synthesized by one-step hydrothermal and pyrolysis techniques and well characterized by scanning electron microscopy (SEM), X-ray analysis (XRD), and Raman spectroscopy. Electrochromic properties of the device have been studied in detail to reveal an improvement in device performance in terms of faster speed and high coloration efficiency and color contrast. In situ bias-dependent Raman spectroscopy has been performed to understand the operation mechanism of the electrochromic diode which reveals (bi-)polaron formation as a result of dynamic doping eventually leading to color change. A half-wave rectifier has been realized from the electrochromic diode which rectifies an AC voltage of frequency 1 Hz or less making it suitable for low frequency operation. The study opens a new possibility to design and fabricate multipurpose frequency selective electrochromic rectifiers.

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.

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.

Atypical Green Luminescence from Raw Cassia Siamea Extract: A Comparison with Red Emitting Tinospora Cordifolia.

Optical and electrochemical properties from Cassia and Giloy leaves' raw extract have been studied, and they show similar properties as UV absorber but different emission properties, under UV excitation, even though they appear the same in natural light. Giloy and Cassia extracts show red and green luminescence, respectively, under UV excitation. Like the appearance, their redox properties are also similar, which shows that both can act as antioxidants. Raman spectroscopy and excitation wavelength dependent photoluminescence data have been compared. The difference in relative emission intensities have been explained based on the presence of corresponding color centers in different ratios in the two leaves.

Tracking Dynamic Doping in a Solid-State Electrochromic Device: Raman Microscopy Validates the Switching Mechanism.

Solid-state electrochromic devices often need appropriate characterization to establish the real working mechanism for optimization and diagnosis. Raman mapping has been used here to track "dynamic doping", an important concept in organic electronics and in polythiophene-based solid-state electrochromic devices to understand and validate the mechanism of bias-induced redox-driven color switching. The proposed method demonstrates the live formation and movement of polarons which is best suited for in situ solid-state Raman spectroelectrochemistry. A 2-fold approach has been adopted here for this (1) by fabricating a working device in cross bar geometry followed by in situ spectroscopy to demonstrate the device functioning and (2) by carrying out Raman mapping from a device in custom-designed thin-film-transistor-like geometry to track and actually "see" the mechanism spectroscopically.

Deconvoluting Diffuse Reflectance Spectra for Retrieving Nanostructures' Size Details: An Easy and Efficient Approach.

A new model has been reported here to estimate the mean size and size distribution in nanostructured materials by utilizing a simple and economic diffuse reflectance spectroscopy through spectral line-shape analysis. In the proposed model, a theoretical line shape has been derived by taking into account a size distribution function, which represents a variation in absorption coefficient as a function of size, which in turn depends on the band gap and thus on the excitation photon energy. A fitting of the experimental absorption spectra with the derived line-shape function yields the mean crystallite size and size distribution. The size and size distribution have been successfully estimated from two different silicon nanostructured samples, prepared by metal induced etching. The model has been validated by comparing the estimated values with the sizes estimated using Raman spectroscopy, which is a well-known technique. The two results are not only consistent with each other but are also found to be consistent with the electron microscopy's results, revealing that a technique as simple and as economic as diffuse reflectance spectroscopy can be used to estimate size distribution. In addition, the proposed model can also be used to investigate the homogeneity in the size distribution in a nanostructured sample.

Quantifying the Short-Range Order in Amorphous Silicon by Raman Scattering.

Quantification of the short-range order in amorphous silicon has been formulized using Raman scattering by taking into account established frameworks for studying the spectral line-shape and size dependent Raman peak shift. A theoretical line-shape function has been proposed for representing the observed Raman scattering spectrum from amorphous-Si-based on modified phonon confinement model framework. While analyzing modified phonon confinement model, the term "confinement size" used in the context of nanocrystalline Si was found analogous to the short-range order distance in a-Si thus enabling one to quantify the same using Raman scattering. Additionally, an empirical formula has been proposed using bond polarizability model for estimating the short-range order making one capable to quantify the distance of short-range order by looking at the Raman peak position alone. Both the proposals have been validated using three different data sets reported by three different research groups from a-Si samples prepared by three different methods making the analysis universal.