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Showing papers on "Raman spectroscopy published in 2016"


Journal ArticleDOI
TL;DR: A review of the plasmon-enhanced Raman spectroscopy (PERS) field can be found in this paper, where a new generation of hotspots that are generated from hybrid structures combining PERS-active nanostructures and probe materials are discussed.
Abstract: Since 2000, there has been an explosion of activity in the field of plasmon-enhanced Raman spectroscopy (PERS), including surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). In this Review, we explore the mechanism of PERS and discuss PERS hotspots — nanoscale regions with a strongly enhanced local electromagnetic field — that allow trace-molecule detection, biomolecule analysis and surface characterization of various materials. In particular, we discuss a new generation of hotspots that are generated from hybrid structures combining PERS-active nanostructures and probe materials, which feature a strong local electromagnetic field on the surface of the probe material. Enhancement of surface Raman signals up to five orders of magnitude can be obtained from materials that are weakly SERS active or SERS inactive. We provide a detailed overview of future research directions in the field of PERS, focusing on new PERS-active nanomaterials and nanostructures and the broad application prospect for materials science and technology. Assisted by rationally designed novel plasmonic nanostructures, surface-enhanced Raman spectroscopy has presented a new generation of analytical tools (that is, tip-enhanced Raman spectroscopy and shell-isolated nanoparticle-enhanced Raman spectroscopy) with an extremely high surface sensitivity, spatial resolution and broad application for materials science and technology.

1,158 citations


Journal ArticleDOI
TL;DR: O2 removal is charge compensated by the formation of localized electron holes on O atoms coordinated by Mn(4+) and Li(+) ions, which serve to promote the localization, and not the formation, of true O2(2-) species.
Abstract: During the charging and discharging of lithium-ion-battery cathodes through the de- and reintercalation of lithium ions, electroneutrality is maintained by transition-metal redox chemistry, which limits the charge that can be stored. However, for some transition-metal oxides this limit can be broken and oxygen loss and/or oxygen redox reactions have been proposed to explain the phenomenon. We present operando mass spectrometry of (18)O-labelled Li1.2[Ni0.13(2+)Co0.13(3+)Mn0.54(4+)]O2, which demonstrates that oxygen is extracted from the lattice on charging a Li1.2[Ni0.13(2+)Co0.13(3+)Mn0.54(4+)]O2 cathode, although we detected no O2 evolution. Combined soft X-ray absorption spectroscopy, resonant inelastic X-ray scattering spectroscopy, X-ray absorption near edge structure spectroscopy and Raman spectroscopy demonstrates that, in addition to oxygen loss, Li(+) removal is charge compensated by the formation of localized electron holes on O atoms coordinated by Mn(4+) and Li(+) ions, which serve to promote the localization, and not the formation, of true O2(2-) (peroxide, O-O ~1.45 A) species. The quantity of charge compensated by oxygen removal and by the formation of electron holes on the O atoms is estimated, and for the case described here the latter dominates.

824 citations


Journal ArticleDOI
TL;DR: A robust approach for sample preparation, instrumentation, acquisition parameters and data processing is explored and it is expected that a typical Raman experiment can be performed by a nonspecialist user to generate high-quality data for biological materials analysis.
Abstract: Raman spectroscopy can be used to measure the chemical composition of a sample, which can in turn be used to extract biological information. Many materials have characteristic Raman spectra, which means that Raman spectroscopy has proven to be an effective analytical approach in geology, semiconductor, materials and polymer science fields. The application of Raman spectroscopy and microscopy within biology is rapidly increasing because it can provide chemical and compositional information, but it does not typically suffer from interference from water molecules. Analysis does not conventionally require extensive sample preparation; biochemical and structural information can usually be obtained without labeling. In this protocol, we aim to standardize and bring together multiple experimental approaches from key leaders in the field for obtaining Raman spectra using a microspectrometer. As examples of the range of biological samples that can be analyzed, we provide instructions for acquiring Raman spectra, maps and images for fresh plant tissue, formalin-fixed and fresh frozen mammalian tissue, fixed cells and biofluids. We explore a robust approach for sample preparation, instrumentation, acquisition parameters and data processing. By using this approach, we expect that a typical Raman experiment can be performed by a nonspecialist user to generate high-quality data for biological materials analysis.

814 citations


Journal ArticleDOI
TL;DR: A further size division within the smaller microplastics fraction into 500–50 μm (rapid and reliable analysis by FTIR imaging) and into 50–1 μm (detailed and more time-consuming analysis by Raman imaging) is proposed.
Abstract: The contamination of aquatic ecosystems with microplastics has recently been reported through many studies, and negative impacts on the aquatic biota have been described. For the chemical identification of microplastics, mainly Fourier transform infrared (FTIR) and Raman spectroscopy are used. But up to now, a critical comparison and validation of both spectroscopic methods with respect to microplastics analysis is missing. To close this knowledge gap, we investigated environmental samples by both Raman and FTIR spectroscopy. Firstly, particles and fibres >500 μm extracted from beach sediment samples were analysed by Raman and FTIR microspectroscopic single measurements. Our results illustrate that both methods are in principle suitable to identify microplastics from the environment. However, in some cases, especially for coloured particles, a combination of both spectroscopic methods is necessary for a complete and reliable characterisation of the chemical composition. Secondly, a marine sample containing particles <400 μm was investigated by Raman imaging and FTIR transmission imaging. The results were compared regarding number, size and type of detectable microplastics as well as spectra quality, measurement time and handling. We show that FTIR imaging leads to significant underestimation (about 35 %) of microplastics compared to Raman imaging, especially in the size range <20 μm. However, the measurement time of Raman imaging is considerably higher compared to FTIR imaging. In summary, we propose a further size division within the smaller microplastics fraction into 500–50 μm (rapid and reliable analysis by FTIR imaging) and into 50–1 μm (detailed and more time-consuming analysis by Raman imaging).

522 citations


Journal ArticleDOI
TL;DR: In this paper, X-ray diffraction, cyclic voltammetry, chronoamperometry, and in situ Raman spectroscopy were used to investigate the electrochemical oxygen evolution reaction (OER) on Cu, Cu2O, Cu(OH)2, and CuO catalysts.
Abstract: Scanning electron microscopy, X-ray diffraction, cyclic voltammetry, chronoamperometry, in situ Raman spectroscopy, and X-ray absorption near-edge structure spectroscopy (XANES) were used to investigate the electrochemical oxygen evolution reaction (OER) on Cu, Cu2O, Cu(OH)2, and CuO catalysts. Aqueous 0.1 M KOH was used as the electrolyte. All four catalysts were oxidized or converted to CuO and Cu(OH)2 during a slow anodic sweep of cyclic voltammetry and exhibited similar activities for the OER. A Raman peak at 603 cm–1 appeared for all the four samples at OER-relevant potentials, ≥1.62 V vs RHE. This peak was identified as the Cu–O stretching vibration band of a CuIII oxide, a metastable species whose existence is dependent on the applied potential. Since this frequency matches well with that from a NaCuIIIO2 standard, we suggest that the chemical composition of the CuIII oxide is CuO2–-like. The four catalysts, in stark contrast, did not oxidize the same way during direct chronoamperometry measurement...

501 citations


Journal ArticleDOI
TL;DR: The results indicate that the hybrid nanocomposites with enhanced microwave absorption properties and lightweight have a promising future in decreasing electromagnetic wave irradiation.
Abstract: Hybrid nanocomposites with enhanced microwave absorption properties have been designed by growing CuS nanoflakes on magnetically decorated graphene, and the effect of special nanostructures on microwave absorption properties has been investigated. The structure of the nanocomposites was characterized by Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), N2 adsorption–desorption, and vibrating sample magnetometer (VSM). The influence of cetyltrimethylammonium bromide (CTAB) on the morphology of CuS nanoflakes was also investigated. A possible formation process of the nanocomposites and the mechanism of microwave absorption were explained in detail. As an absorber, the nanocomposites with a filler loading of 20 wt % exhibited enhanced microwave absorption properties due to the special nanostructures, extra void space, and synergistic effect...

423 citations


Journal ArticleDOI
21 Nov 2016-ACS Nano
TL;DR: The two key factors, i.e., surface functional group-involved bonding/debonding-induced pseudocapacitance, and ion exchange-featured charge storage, simultaneously contribute to the superior capacitance of Ti3C2Tx MXene in acidic electrolytes.
Abstract: MXenes represent an emerging family of conductive two-dimensional materials. Their representative, Ti3C2Tx, has been recognized as an outstanding member in the field of electrochemical energy storage. However, an in-depth understanding of fundamental processes responsible for the superior capacitance of Ti3C2Tx MXene in acidic electrolytes is lacking. Here, to understand the mechanism of capacitance in Ti3C2Tx MXene, we studied electrochemically the charge/discharge processes of Ti3C2Tx electrodes in sulfate ion-containing aqueous electrolytes with three different cations, coupled with in situ Raman spectroscopy. It is demonstrated that hydronium in the H2SO4 electrolyte bonds with the terminal O in the negative electrode upon discharging while debonding occurs upon charging. Correspondingly, the reversible bonding/debonding changes the valence state of Ti element in the MXene, giving rise to the pseudocapacitance in the acidic electrolyte. In stark contrast, only electric double layer capacitance is reco...

418 citations


Journal ArticleDOI
23 Feb 2016-ACS Nano
TL;DR: The monolayers used for building of heterostructures by van der Waals stacking could be considered as the candidates for artificial 2D materials with unusual ferroelectric and magnetic properties.
Abstract: 2D semiconducting metal phosphorus trichalcogenides, particularly the bulk crystals of MPS3 (M = Fe, Mn, Ni, Cd and Zn) sulfides and MPSe3 (M = Fe and Mn) selenides, have been synthesized, crystallized and exfoliated into monolayers. The Raman spectra of monolayer FePS3 and 3-layer FePSe3 show the strong intralayer vibrations and structural stability of the atomically thin layers under ambient condition. The band gaps can be adjusted by element choices in the range of 1.3–3.5 eV. The wide-range band gaps suggest their optoelectronic applications in a broad wavelength range. The calculated cleavage energies of MPS3 are smaller than that of graphite. Therefore, the monolayers used for building of heterostructures by van der Waals stacking could be considered as the candidates for artificial 2D materials with unusual ferroelectric and magnetic properties.

372 citations



Journal ArticleDOI
TL;DR: It is shown that once understood, the anisotropic optical absorption appears to be a reliable and simple way to identify the crystalline orientation of BP, which cannot be determined from Raman spectroscopy without the explicit consideration of excitation wavelength and flake thickness, as commonly used previously.
Abstract: Orthorhombic black phosphorus (BP) and other layered materials, such as gallium telluride (GaTe) and tin selenide (SnSe), stand out among two-dimensional (2D) materials owing to their anisotropic in-plane structure. This anisotropy adds a new dimension to the properties of 2D materials and stimulates the development of angle-resolved photonics and electronics. However, understanding the effect of anisotropy has remained unsatisfactory to date, as shown by a number of inconsistencies in the recent literature. We use angle-resolved absorption and Raman spectroscopies to investigate the role of anisotropy on the electron–photon and electron–phonon interactions in BP. We highlight, both experimentally and theoretically, a nontrivial dependence between anisotropy and flake thickness and photon and phonon energies. We show that once understood, the anisotropic optical absorption appears to be a reliable and simple way to identify the crystalline orientation of BP, which cannot be determined from Raman spectrosc...

329 citations


Journal ArticleDOI
25 Mar 2016-ACS Nano
TL;DR: It is shown that irradiation causes partial removal of sulfur and the dependence of the Raman peak shifts with S vacancy density is quantitatively correlated with vacancy concentration, as rationalized by first-principles density functional theory calculations.
Abstract: We report how the presence of electron-beam-induced sulfur vacancies affects first-order Raman modes and correlate the effects with the evolution of the in situ transmission-electron microscopy two-terminal conductivity of monolayer MoS2 under electron irradiation. We observe a red-shift in the E' Raman peak and a less pronounced blue-shift in the A'1 peak with increasing electron dose. Using energy-dispersive X-ray spectroscopy and selected-area electron diffraction, we show that irradiation causes partial removal of sulfur and correlate the dependence of the Raman peak shifts with S vacancy density (a few %). This allows us to quantitatively correlate the frequency shifts with vacancy concentration, as rationalized by first-principles density functional theory calculations. In situ device current measurements show an exponential decrease in channel current upon irradiation. Our analysis demonstrates that the observed frequency shifts are intrinsic properties of the defective systems and that Raman spectroscopy can be used as a quantitative diagnostic tool to characterize MoS2-based transport channels.

Journal ArticleDOI
TL;DR: Reliable synthetic and measurement strategies for plasmonically coupled nanostructures with ∼1 nm gap, in which both the nanogap size and the position of a Raman-active molecule in the gap can be controlled with nanometer/sub-nanometer-level precision, can address important issues regarding the synthesis and optical properties of pl asmonic nanostructure, including structural and signal reproducibility.
Abstract: ConspectusPlasmonic coupling-based electromagnetic field localization and enhancement are becoming increasingly important in chemistry, nanoscience, materials science, physics, and engineering over the past decade, generating a number of new concepts and applications. Among the plasmonically coupled nanostructures, metal nanostructures with nanogaps have been of special interest due to their ultrastrong electromagnetic fields and controllable optical properties that can be useful for a variety of signal enhancements such as surface-enhanced Raman scattering (SERS). The Raman scattering process is highly inefficient, with a very small cross-section, and Raman signals are often poorly reproducible, meaning that very strong, controllable SERS is needed to obtain reliable Raman signals with metallic nanostructures and thus open up new avenues for a variety of Raman-based applications. More specifically, plasmonically coupled metallic nanostructures with ultrasmall (∼1 nm or smaller) nanogaps can generate very...

Journal ArticleDOI
Xin Zhang1, Qing-Hai Tan1, Jiang-Bin Wu1, Wei Shi1, Ping-Heng Tan1 
TL;DR: A unified method based on symmetry analysis and polarization measurements to assign the observed Raman modes and characterize the crystal structure of different types of LMs is introduced, and the recent advances on Raman spectroscopy in the characterization of anisotropic LMs, such as black phosphorus and rhenium diselenide are reviewed.
Abstract: Two-dimensional layered materials, such as graphene and transition metal dichalcogenides (TMDs), have been under intensive investigation. The rapid progress of research on graphene and TMDs is now stimulating the exploration of different types of layered materials (LMs). Raman spectroscopy has shown its great potential in the characterization of layer numbers, interlayer coupling and layer-stacking configurations and will benefit the future explorations of other LMs. Lattice vibrations or Raman spectra of many LMs in bulk have been discussed since the 1960s. However, different results were obtained because of differences or limitations in the Raman instruments at early stages. The developments of modern Raman spectroscopy now allow us to revisit the Raman spectra of these LMs under the same experimental conditions. Moreover, to the best of our knowledge, there were limitations in detailed reviews on the Raman spectra of these different LMs. Here, we provide a review on Raman spectra of various LMs, including semiconductors, topological insulators, insulators, semi-metals and superconductors. We firstly introduce a unified method based on symmetry analysis and polarization measurements to assign the observed Raman modes and characterize the crystal structure of different types of LMs. Then, we revisit and update the positions and assignments of vibration modes by re-measuring the Raman spectra of different types of LMs and by comparing our results to those reported in previous papers. We apply the recent advances on the interlayer vibrations of graphene and TMDs to these various LMs and obtain their shear modulus. The observation of the shear modes of LMs in bulk facilitates an accurate and fast characterization of layer numbers during preparation processes in the future by a robust layer-number dependency on the frequencies of the shear modes. We also summarize the recent advances on the layer-stacking dependence on the intensities of interlayer shear vibrations. Finally, we review the recent advances on Raman spectroscopy in the characterization of anisotropic LMs, such as black phosphorus and rhenium diselenide. We believe that this review will benefit the future research studies on the fundamental physics and potential applications of these various LMs, particularly when they are reduced down to monolayers or multilayers.

Journal ArticleDOI
TL;DR: An overview of recent static and time-resolved vibrational spectroscopic studies of liquid water from ambient conditions to the supercooled state, as well as of crystalline and amorphous ice forms, reveals a coherent picture of water dynamics and energetics.
Abstract: We present an overview of recent static and time-resolved vibrational spectroscopic studies of liquid water from ambient conditions to the supercooled state, as well as of crystalline and amorphous ice forms. The structure and dynamics of the complex hydrogen-bond network formed by water molecules in the bulk and interphases are discussed, as well as the dissipation mechanism of vibrational energy throughout this network. A broad range of water investigations are addressed, from conventional infrared and Raman spectroscopy to femtosecond pump–probe, photon-echo, optical Kerr effect, sum-frequency generation, and two-dimensional infrared spectroscopic studies. Additionally, we discuss novel approaches, such as two-dimensional sum-frequency generation, three-dimensional infrared, and two-dimensional Raman terahertz spectroscopy. By comparison of the complementary aspects probed by various linear and nonlinear spectroscopic techniques, a coherent picture of water dynamics and energetics emerges. Furthermore,...

Journal ArticleDOI
01 Apr 2016-Carbon
TL;DR: In this paper, the effects of various vacancy and substitution defects in a coronene parent molecule have been systematically analyzed using density functional theory (DFT), and the impacts of these defects are best understood in terms of a reduced symmetry as compared to a “parent” coronene molecule.

Journal ArticleDOI
15 Jan 2016-Sensors
TL;DR: The data resulting from the characterizations of graphite oxide and graphene oxide have made it possible to interpret the obtained results from the point of view of physicochemical changes occurring in these structures.
Abstract: The paper presents the results of investigations on resistance structures based on graphite oxide (GRO) and graphene oxide (rGO). The subject matter of the investigations was thaw the sensitivity of the tested structures was affected by hydrogen, nitrogen dioxide and carbon dioxide. The experiments were performed at a temperature range from 30 °C to 150 °C in two carrier gases: nitrogen and synthetic air. The measurements were also aimed at characterization of the graphite oxide and graphene oxide. In our measurements we used (among others) techniques such as: Atomic Force Microscopy (AFM); Scanning Electron Microscopy (SEM); Raman Spectroscopy (RS); Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray Photoelectron Microscopy (XPS). The data resulting from the characterizations of graphite oxide and graphene oxide have made it possible to interpret the obtained results from the point of view of physicochemical changes occurring in these structures.

Journal ArticleDOI
03 Aug 2016-ACS Nano
TL;DR: The appealing physical properties demonstrated in this study clearly identify ReSe2 as a highly anisotropic 2D material for exotic electronic and optoelectronic applications.
Abstract: Atomically thin 2D-layered transition-metal dichalcogenides have been studied extensively in recent years because of their intriguing physical properties and promising applications in nanoelectronic devices. Among them, ReSe2 is an emerging material that exhibits a stable distorted 1T phase and strong in-plane anisotropy due to its reduced crystal symmetry. Here, the anisotropic nature of ReSe2 is revealed by Raman spectroscopy under linearly polarized excitations in which different vibration modes exhibit pronounced periodic variations in intensity. Utilizing high-quality ReSe2 nanosheets, top-gate ReSe2 field-effect transistors were built that show an excellent on/off current ratio exceeding 107 and a well-developed current saturation in the current–voltage characteristics at room temperature. Importantly, the successful synthesis of ReSe2 directly onto hexagonal boron nitride substrates has effectively improved the electron motility over 500 times and the hole mobility over 100 times at low temperature...

Journal ArticleDOI
TL;DR: In this article, the authors present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K.
Abstract: We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations. Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm−1. Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamic effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C–N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity. We show that optical rather than acoustic phonon scattering is likely to prevail at room temperature in these materials.

Journal ArticleDOI
TL;DR: The results implied that the fluorescent N-CDs showed less cytotoxicity, further which was successfully applied as a staining probe for the confocal imaging of MDCK and HeLa cells.
Abstract: A fast and facile microwave approach for the synthesis of fluorescent nitrogen-doped carbon dots (N-CDs) is reported. The N-CDs were hydrothermally synthesized using l -ascorbic acid (AA) and β-alanine (BA) as the carbon precursor and the nitrogen dopant, respectively. The morphology of synthesized N-CDs was characterized by high resolution transmission electron microscopy (HR-TEM) and the elemental composition was analyzed using elemental mapping method. The crystallinity and graphitation of N-CDs were examined by X-ray diffraction (XRD) and Raman spectroscopy. The doping of nitrogen over the carbon dots (CDs) was revealed by attenuated total reflection conjunction with Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photo electron spectroscopy (XPS). The optical properties of synthesized N-CDs were examined by UV–Visible (UV–Vis) and fluorescence spectroscopy. The synthesized N-CDs emit strong blue fluorescence at 401 nm under excitation of 325 nm. The excitation dependent emission property of synthesized N-CDs was exposed from fluorescence results. The quantum yield of synthesized N-CDs is about 14% against the reference quinine sulfate. The cytotoxicity of synthesized N-CDs on Madin-Darby Canine Kidney (MDCK) and HeLa cells were evaluated through Cell Counting Kit-8 (CCK-8) cytotoxicity assay. The results implied that the fluorescent N-CDs showed less cytotoxicity, further which was successfully applied as a staining probe for the confocal imaging of MDCK and HeLa cells.

Journal ArticleDOI
TL;DR: The results suggest that the unripe fruit extract of peach is an ideal candidate for the preparation of N-CDs, which exhibited a high fluorescence quantum yield and low cytotoxicity, and can be used as fluorescence imaging probes.

Journal ArticleDOI
TL;DR: In this article, the authors exfoliated CrSiTe3, a bulk ferromagnetic semiconductor, to mono-and few-layer 2D crystals onto a Si/SiO2 substrate.
Abstract: Finite range ferromagnetism and antiferromagnetism in two-dimensional (2D) systems within an isotropic Heisenberg model at non-zero temperature were originally proposed to be impossible. However, recent theoretical studies using an Ising model have shown that 2D magnetic crystals can exhibit magnetism. Experimental verification of existing 2D magnetic crystals in this system has remained exploratory. In this work we exfoliated CrSiTe3, a bulk ferromagnetic semiconductor, to mono- and few-layer 2D crystals onto a Si/SiO2 substrate. Raman spectra indicate good stability and high quality of the exfoliated flakes, consistent with the computed phonon spectra of 2D CrSiTe3, giving strong evidence for the existence of 2D CrSiTe3 crystals. When the thickness of the CrSiTe3 crystals is reduced to a few layers, we observed a clear change in resistivity at 80–120 K, consistent with theoretical calculations of the Curie temperature (Tc) of ∼80 K for the magnetic ordering of 2D CrSiTe3 crystals. The ferromagnetic mono- and few-layer 2D CrSiTe3 indicated here should enable numerous applications in nano-spintronics.


Journal ArticleDOI
21 Jun 2016-ACS Nano
TL;DR: The interlayer coupling for the WSe2/WS2 bilayer heterojunctions indicated by substantial PL quenching, enhanced absorption, and rapid charge transfer was found to be insensitive to the relative twist angle, indicating that stamping provides a robust approach to realize reliable optoelectronics.
Abstract: van der Waals (vdW) heterostructures are promising building blocks for future ultrathin electronics. Fabricating vdW heterostructures by stamping monolayers at arbitrary angles provides an additional range of flexibility to tailor the resulting properties than could be expected by direct growth. Here, we report fabrication and comprehensive characterizations of WSe2/WS2 bilayer heterojunctions with various twist angles that were synthesized by artificially stacking monolayers of WS2 and WSe2 grown by chemical vapor deposition. After annealing the WSe2/WS2 bilayers, Raman spectroscopy reveals interlayer coupling with the appearance of a mode at 309.4 cm–1 that is sensitive to the number of WSe2 layers. This interlayer coupling is associated with substantial quenching of the intralayer photoluminescence. In addition, microabsorption spectroscopy of WSe2/WS2 bilayers revealed spectral broadening and shifts as well as a net ∼10% enhancement in integrated absorption strength across the visible spectrum with re...

Journal ArticleDOI
TL;DR: The first successful isolation of monolayer and few-layer samples of the compound nickel phosphorus trisulfide (NiPS3) by mechanical exfoliation is reported, providing a spectral fingerprint for distinct layer numbers, allowing a sensitive and convenient means for layer number determination.
Abstract: The range of mechanically cleavable Van der Waals crystals covers materials with diverse physical and chemical properties. However, very few of these materials exhibit magnetism or magnetic order, and thus the provision of cleavable magnetic compounds would supply invaluable building blocks for the design of heterostructures assembled from Van der Waals crystals. Here we report the first successful isolation of monolayer and few-layer samples of the compound nickel phosphorus trisulfide (NiPS3) by mechanical exfoliation. This material belongs to the class of transition metal phosphorus trisulfides (MPS3), several of which exhibit antiferromagnetic order at low temperature, and which have not been reported in the form of ultrathin sheets so far. We establish layer numbers by optical bright field microscopy and atomic force microscopy, and perform a detailed Raman spectroscopic characterization of bilayer and thicker NiPS3 flakes. Raman spectral features are strong functions of excitation wavelength and sample thickness, highlighting the important role of interlayer coupling. Furthermore, our observations provide a spectral fingerprint for distinct layer numbers, allowing us to establish a sensitive and convenient means for layer number determination.

Journal ArticleDOI
TL;DR: This work reanalyse the Raman spectra of graphenes and shows that traditional methods rely upon an apparent G peak which is in fact a superposition of the G and D’ peaks, which has important implications for purification techniques because, once GO is reduced beyond a critical threshold, further reduction offers limited gain in conductivity.
Abstract: Raman spectroscopy is among the primary techniques for the characterisation of graphene materials, as it provides insights into the quality of measured graphenes including their structure and conductivity as well as the presence of dopants. However, our ability to draw conclusions based on such spectra is limited by a lack of understanding regarding the origins of the peaks. Consequently, traditional characterisation techniques, which estimate the quality of the graphene material using the intensity ratio between the D and the G peaks, are unreliable for both GO and rGO. Herein we reanalyse the Raman spectra of graphenes and show that traditional methods rely upon an apparent G peak which is in fact a superposition of the G and D’ peaks. We use this understanding to develop a new Raman characterisation method for graphenes that considers the D’ peak by using its overtone the 2D’. We demonstrate the superiority and consistency of this method for calculating the oxygen content of graphenes, and use the relationship between the D’ peak and graphene quality to define three regimes. This has important implications for purification techniques because, once GO is reduced beyond a critical threshold, further reduction offers limited gain in conductivity.

Journal ArticleDOI
TL;DR: The optomechanical approach recovers known results, provides a quantitative framework for the calculation of cross-sections, and enables the design of novel systems that leverage dynamical backaction to achieve additional, mode-selective enhancements.
Abstract: The exceptional enhancement of Raman scattering by localized plasmonic resonances in the near field of metallic nanoparticles, surfaces or tips (SERS, TERS) has enabled spectroscopic fingerprinting down to the single molecule level The conventional explanation attributes the enhancement to the subwavelength confinement of the electromagnetic field near nanoantennas Here, we introduce a new model that also accounts for the dynamical nature of the plasmon-molecule interaction We thereby reveal an enhancement mechanism not considered before: dynamical backaction amplification of molecular vibrations We first map the system onto the canonical Hamiltonian of cavity optomechanics, in which the molecular vibration and the plasmon are parametrically coupled We express the vacuum optomechanical coupling rate for individual molecules in plasmonic 'hot-spots' in terms of the vibrational mode's Raman activity and find it to be orders of magnitude larger than for microfabricated optomechanical systems Remarkably, the frequency of commonly studied molecular vibrations can be comparable to or larger than the plasmon's decay rate Together, these considerations predict that an excitation laser blue-detuned from the plasmon resonance can parametrically amplify the molecular vibration, leading to a nonlinear enhancement of Raman emission that is not predicted by the conventional theory Our optomechanical approach recovers known results, provides a quantitative framework for the calculation of cross-sections, and enables the design of novel systems that leverage dynamical backaction to achieve additional, mode-selective enhancements It also provides a quantum mechanical framework to analyse plasmon-vibrational interactions in terms of molecular quantum optomechanics

Journal ArticleDOI
TL;DR: Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm-1, and it is found that MAPbI3 shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity.
Abstract: We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations.1 Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm-1. Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamics effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C-N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity. We show that optical rather than acoustic phonon scattering is likely to prevail at room temperature in these materials.

Journal ArticleDOI
TL;DR: A multi-peak deconvolution system and spectral band assignment were proposed based on the chemical and structural modifications identified by X-ray photoelectron spectroscopy, thermogravimetry,X-ray diffraction, specific surface areas and the comparative analysis of the first and second order regions of the Raman spectra.
Abstract: Raman spectroscopy is highly sensitive to the morphology and electronic structures of graphitic materials, but a convenient interpretation model has been lacking for multiwalled carbon nanotubes (MWCNTs), in particular for the discrimination of spectral changes induced by covalent functionalization. The present work describes a systematic investigation of the Raman analysis of covalently functionalized MWCNTs by diazonium chemistry and oxidation methodologies, with typically different mechanisms and reaction sites. A multi-peak deconvolution system and spectral band assignment were proposed based on the chemical and structural modifications identified by X-ray photoelectron spectroscopy, thermogravimetry, X-ray diffraction, specific surface areas and the comparative analysis of the first and second order regions of the Raman spectra. Diazonium functionalization takes place mainly in the π-system of the external sidewall, while oxidation occurs on defects and leads to structure burning. This allowed us to distinguish between spectral features related to aromaticity disruptions within the sidewalls and spectral features related to changes within the inner tubes. The model was validated extending the studies to the functionalization of MWCNTs by the Bingel reaction.

Journal ArticleDOI
07 Jun 2016-Analyst
TL;DR: This review focuses on important developments in Raman spectroscopy that have evolved into the exciting technique of live-cell Raman microscopy and highlights some of the most recent and significant applications to cell biology.
Abstract: One of the most exciting developments in Raman spectroscopy in the last decade has been its application to cells and tissues for diagnostic and pharmaceutical applications, and in particular its use in the analysis of cellular dynamics. Raman spectroscopy is rapidly advancing as a cell imaging method that overcomes many of the limitations of current techniques and is earning its place as a routine tool in cell biology. In this review we focus on important developments in Raman spectroscopy that have evolved into the exciting technique of live-cell Raman microscopy and highlight some of the most recent and significant applications to cell biology.

Journal ArticleDOI
TL;DR: In this paper, a simple hydrothermal method has been developed to prepare Ti3C2Tx from Ti3AlC2 as a high-performance electrode material for supercapacitors.
Abstract: In this study, a simple hydrothermal method has been developed to prepare Ti3C2Tx from Ti3AlC2 as a high-performance electrode material for supercapacitors. This method is environmentally friendly and has a low level of danger. The morphology and structure of the Ti3C2Tx can be controlled by hydrothermal reaction time, temperature and NH4F amounts. The prepared Ti3C2Tx was characterized by X-ray diffraction, field emission scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and Brunauer-Emmet-Teller. The results show that the prepared Ti3C2Tx is terminated by O, OH, and F groups. The electrochemical properties of the Ti3C2Tx sample exhibit specific capacitance up to 141 Fcm−3 in 3 M KOH aqueous electrolyte, and even after 1000 cycles, no significant degradation of the volumetric capacitance was observed. These results indicate that the Ti3C2Tx material prepared by this hydrothermal method can be used in high performance supercapacitors.