Driven by applications in chemical sensing, biological imaging and material characterisation, Raman spectroscopies are attracting growing interest from a variety of scientific disciplines. The Raman ...effect originates from the inelastic scattering of light, and it can directly probe vibration/rotational-vibration states in molecules and materials. Despite numerous advantages over infrared spectroscopy, spontaneous Raman scattering is very weak, and consequently, a variety of enhanced Raman spectroscopic techniques have emerged. These techniques include stimulated Raman scattering and coherent anti-Stokes Raman scattering, as well as surface- and tip-enhanced Raman scattering spectroscopies. The present review provides the reader with an understanding of the fundamental physics that govern the Raman effect and its advantages, limitations and applications. The review also highlights the key experimental considerations for implementing the main experimental Raman spectroscopic techniques. The relevant data analysis methods and some of the most recent advances related to the Raman effect are finally presented. This review constitutes a practical introduction to the science of Raman spectroscopy; it also highlights recent and promising directions of future research developments.
Rhenium diselenide (ReSe2) is a layered indirect gap semiconductor for which micromechanical cleavage can produce monolayers consisting of a plane of rhenium atoms with selenium atoms above and ...below. ReSe2 is unusual among the transition-metal dichalcogenides in having a low symmetry; it is triclinic, with four formula units per unit cell, and has the bulk space group P1̅. Experimental studies of Raman scattering in monolayer, few-layer, and bulk ReSe2 show a rich spectrum consisting of up to 16 of the 18 expected lines with good signal strength, pronounced in-plane anisotropy of the intensities, and no evidence of degradation of the sample during typical measurements. No changes in the frequencies of the Raman bands with layer thickness down to one monolayer are observed, but significant changes in relative intensity of the bands allow the determination of crystal orientation and of monolayer regions. Supporting theory includes calculations of the electronic band structure and Brillouin zone center phonon modes of bulk and monolayer ReSe2 as well as the Raman tensors determining the scattering intensity of each mode. It is found that, as for other transition-metal dichalcogenides, Raman scattering provides a powerful diagnostic tool for studying layer thickness and also layer orientation in few-layer ReSe2.
A novel fouling-resistant nanofiltration mixed-matrix membrane was obtained by the incorporation of 2D boron nitride nanosheets (BNNS) in polyethersulfone (PES). The addition of just 0.05 wt% of BNNS ...into the PES matrix led to a 4-fold increase in pure water permeance with a 10% decrease in the rejection of the dye Rose Bengal; up to 95% rejection of humic acid and nearly 100% flux recovery over two cycles in cross-flow fouling tests without the need for chemical cleansing. This performance is attributed to the uniform distribution of the BNNS in the PES matrix, observed via Raman mapping, and the surface chemistry and structure of the BNNS, which hydrophilised the polymer matrix and reduced its surface roughness. The low amount of BNNS filler needed to render the mixed-matrix membrane fouling-resistant opens the way to its use in waste-water treatment applications where organic fouling remains a major challenge.
•Produced first BNNS-PES mixed-matrix nanofiltration membranes.•0.05 wt% of BNNS sufficient to increase permeance 4-fold and reject 95%+ humic acid.•No irreversible fouling after 2 fouling/washing cycles with no chemical cleaning.
Deep-ultraviolet surface-enhanced Raman scattering (UV-SERS) is a promising technique for bioimaging and detection because many biological molecules possess UV absorption lines leading to strongly ...resonant Raman scattering. Here, Al nanovoid substrates are developed by combining nanoimprint lithography of etched polymer/silica opal films with electron beam evaporation, to give a high-performance sensing platform for UV-SERS. Enhancement by more than 3 orders of magnitude in the UV-SERS performance was obtained from the DNA base adenine, matching well the UV plasmonic optical signatures and simulations, demonstrating its suitability for biodetection.
The relationship between structure and properties has been followed for different nanoscale forms of tungsten disulfide (2H-WS2) namely exfoliated monolayer and few-layer nanoplatelets, and ...nanotubes. The similarities and differences between these nanostructured materials have been examined using a combination of optical microscopy, scanning and high-resolution transmission electron microscopy and atomic force microscopy. Photoluminescence and Raman spectroscopy have also been used to distinguish between monolayer and few-layer material. Strain induced phonon shifts have been followed from the changes in the positions of the A1g and E 2 g 1 Raman bands during uniaxial deformation. This has been modelled for monolayer using density functional theory with excellent agreement between the measured and predicted behaviour. It has been found that as the number of WS2 layers increases for few-layer crystals or nanotubes, the A1g mode hardens whereas the E 2 g 1 mode softens. This is believed to be due to the A1g mode, which involves out of plane atomic movements, being constrained by the increasing number of WS2 layers whereas easy sliding reduces stress transfer to the individual layers for the E 2 g 1 mode, involving only in-plane vibrations. This finding has enabled the anomalous phonon shift behaviour in earlier pressure measurements on WS2 to be resolved, as well as similar effects in other transition metal dichalcogenides, such as molybdenum disulfide, to be explained.
Abstract
ReSe
2
and ReS
2
are unusual compounds amongst the layered transition metal dichalcogenides as a result of their low symmetry, with a characteristic in-plane anisotropy due to in-plane ...rhenium ‘chains’. They preserve inversion symmetry independent of the number of layers and, in contrast to more well-known transition metal dichalcogenides, bulk and few-monolayer Re-TMD compounds have been proposed to behave as electronically and vibrational decoupled layers. Here, we probe for the first time the electronic band structure of bulk ReSe
2
by direct nanoscale angle-resolved photoemission spectroscopy. We find a highly anisotropic in- and out-of-plane electronic structure, with the valence band maxima located away from any particular high-symmetry direction. The effective mass doubles its value perpendicular to the Re chains and the interlayer van der Waals coupling generates significant electronic dispersion normal to the layers. Our density functional theory calculations, including spin-orbit effects, are in excellent agreement with these experimental findings.
Abstract
Layered antiferromagnetic materials have recently emerged as an intriguing subset of the two-dimensional family providing a highly accessible regime with prospects for layer-number-dependent ...magnetism. Furthermore, transition metal phosphorus trichalcogenides, MPX
3
(M = transition metal; X = chalcogen) provide a platform on which to investigate fundamental interactions between magnetic and lattice degrees of freedom and further explore the developing fields of spintronics and magnonics. Here, we use a combination of temperature dependent Raman spectroscopy and density functional theory to explore magnetic-ordering-dependent interactions between the manganese spin degree of freedom and lattice vibrations of the non-magnetic sub-lattice via a Kramers-Anderson super-exchange pathway in both bulk, and few-layer, manganese phosphorus triselenide (MnPSe
3
). We observe a nonlinear temperature-dependent shift of phonon modes predominantly associated with the non-magnetic sub-lattice, revealing their non-trivial spin-phonon coupling below the Néel temperature at 74 K, allowing us to extract mode-specific spin-phonon coupling constants.
authoren The lattice parameters and the relative stability of the hypothetical material ReSe have been calculated within the LDA approximation in the NaCl, NiAs, and zincblende structures. The ...zincblende phase appears to remain metallic even when possible strong Coulomb correlations between Re 5d electrons are taken into account by the introduction of a Hubbard U term. Spin‐polarized calculations predict a non‐magnetic phase for U up to 7 eV. Calculations of the phonon dispersion of zincblende ReSe point to an instability with respect to a tetragonal distortion and this raises the interesting possibility that it may also be possible to obtain ReSe experimentally in the PbO (FeSe) structure.
Nanostructures in the PAN-based carbon fibres were investigated by means of the Raman spectroscopy which was performed using a series of different excitation wavelengths. UV-Raman measurements showed ...the predominance of sp2 carbon for the carbon fibres with Young's moduli ranging from 55 to 440 GPa. The excitation wavelength dependence of the spectral line shape implied the presence of the graphitic (sp2 carbon layers) as well as the amorphous carbon-like component (sp2 carbon clusters) in these fibres. An analytical model based upon the Tuinstra-Koenig relation and the rule-of-mixtures concept was proposed to account for the observed behaviour of the peak intensity ratio ID/IG. It was suggested that the carbonisation heat treatment led to the sp2 carbon layers extending rapidly throughout the nanostructure, resulting in the complete loss of the sp2 carbon clusters for the higher-modulus carbon fibres. These new findings together with the proposed analytical model are believed to be essential keys towards the development of new generation carbon fibres.
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