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Showing papers on "Raman spectroscopy published in 2014"


Journal ArticleDOI
TL;DR: In this article, an electron beam treatment of commercial TiO2 (p-TiO2) nanoparticles was used to obtain visible light-active nanoparticles, which exhibited a distinct red-shift in the UV-visible absorption spectrum and a much narrower band gap (2.85 eV) due to defects.
Abstract: Visible light-active TiO2 (m-TiO2) nanoparticles were obtained by an electron beam treatment of commercial TiO2 (p-TiO2) nanoparticles. The m-TiO2 nanoparticles exhibited a distinct red-shift in the UV-visible absorption spectrum and a much narrower band gap (2.85 eV) due to defects as confirmed by diffuse reflectance spectroscopy (DRS), photoluminescence (PL), X-ray diffraction, Raman spectroscopy, electron paramagnetic resonance, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and linear scan voltammetry (LSV). The XPS revealed changes in the surface states, composition, Ti4+ to Ti3+ ratio, and oxygen deficiencies in the m-TiO2. The valence band XPS, DRS and PL results were carefully examined to understand the band gap reduction of m-TiO2. The visible light-responsive enhanced photocatalytic activity of m-TiO2 was demonstrated by degrading methylene blue and brilliant blue G. The EIS and LSV in the dark and under visible light irradiation further support the visible light-induced photocatalytic activities of the m-TiO2 due to a decrease in electron transfer resistance and an increase in photocurrent. This study confirms that m-TiO2 can be used effectively as a photocatalyst and photoelectrode material owing to its enhanced visible light-induced photocatalytic activity.

712 citations



Journal ArticleDOI
08 Sep 2014-ACS Nano
TL;DR: The anisotropic Raman response in few-layer phosphorene has enabled us to use an optical method to quickly determine the crystalline orientation without tunneling electron microscopy or scanning tunneling microscopy.
Abstract: Phosphorene is a new family member of two-dimensional materials. We observed strong and highly layer-dependent photoluminescence in few-layer phosphorene (two to five layers). The results confirmed the theoretical prediction that few-layer phosphorene has a direct and layer-sensitive band gap. We also demonstrated that few-layer phosphorene is more sensitive to temperature modulation than graphene and MoS2 in Raman scattering. The anisotropic Raman response in few-layer phosphorene has enabled us to use an optical method to quickly determine the crystalline orientation without tunneling electron microscopy or scanning tunneling microscopy. Our results provide much needed experimental information about the band structures and exciton nature in few-layer phosphorene.

624 citations


Journal ArticleDOI
TL;DR: In this paper, the thickness of monolayer phosphorene is determined by optical contrast spectra combined with atomic force microscopy (AFM), and Raman spectroscopy is used to characterize pristine and plasma-treated samples.
Abstract: There have been continuous efforts to seek novel functional two-dimensional semiconductors with high performance for future applications in nanoelectronics and optoelectronics. In this work, we introduce a successful experimental approach to fabricate monolayer phosphorene by mechanical cleavage and a subsequent Ar+ plasma thinning process. The thickness of phosphorene is unambiguously determined by optical contrast spectra combined with atomic force microscopy (AFM). Raman spectroscopy is used to characterize the pristine and plasma-treated samples. The Raman frequency of the A2g mode stiffens, and the intensity ratio of A2g to A1g modes shows a monotonic discrete increase with the decrease of phosphorene thickness down to a monolayer. All those phenomena can be used to identify the thickness of this novel two-dimensional semiconductor. This work on monolayer phosphorene fabrication and thickness determination will facilitate future research on phosphorene.

565 citations


Journal ArticleDOI
TL;DR: The low-frequency resonant Raman spectrum of methylammonium lead-iodide, a prototypical perovskite for solar cells applications, on mesoporous Al2O3 is reported, which may allow one to further understand the properties of this important class of materials in relation to their full exploitation in solar cells.
Abstract: We report the low-frequency resonant Raman spectrum of methylammonium lead-iodide, a prototypical perovskite for solar cells applications, on mesoporous Al2O3. The measured spectrum assignment is assisted by DFT simulations of the Raman spectra of suitable periodic and model systems. The bands at 62 and 94 cm–1 are assigned respectively to the bending and to the stretching of the Pb–I bonds, and are thus diagnostic modes of the inorganic cage. We also assign the librations of the organic cations at 119 and 154 cm–1. The broad, unstructured 200–400 cm–1 features are assigned to the torsional mode of the methylammonium cations, which we propose as a marker of the orientational disorder of the material. Our study provides the basis to interpret the Raman spectra of organohalide perovskites, which may allow one to further understand the properties of this important class of materials in relation to their full exploitation in solar cells.

548 citations


Journal ArticleDOI
03 Jan 2014-ACS Nano
TL;DR: In this paper, the temperature-dependent Raman spectra of exfoliated, monolayer molybdenum disulfide (MoS2) in the range of 100-320 K were analyzed.
Abstract: Atomically thin molybdenum disulfide (MoS2) offers potential for advanced devices and an alternative to graphene due to its unique electronic and optical properties. The temperature-dependent Raman spectra of exfoliated, monolayer MoS2 in the range of 100–320 K are reported and analyzed. The linear temperature coefficients of the in-plane E2g1 and the out-of-plane A1g modes for both suspended and substrate-supported monolayer MoS2 are measured. These data, when combined with the first-order coefficients from laser power-dependent studies, enable the thermal conductivity to be extracted. The resulting thermal conductivity κ = (34.5 ± 4) W/mK at room temperature agrees well with the first-principles lattice dynamics simulations. However, this value is significantly lower than that of graphene. The results from this work provide important input for the design of MoS2-based devices where thermal management is critical.

528 citations


Journal ArticleDOI
TL;DR: In this paper, the authors quantitatively study the Raman and photoluminescence (PL) emission from single-layer molybdenum disulfide (MoS2) on dielectric (SiO2, hexagonal boron nitride, mica and polymeric dielectrics Gel-Film®) and conducting substrates (Au and few-layer graphene).
Abstract: We quantitatively study the Raman and photoluminescence (PL) emission from single-layer molybdenum disulfide (MoS2) on dielectric (SiO2, hexagonal boron nitride, mica and the polymeric dielectric Gel-Film®) and conducting substrates (Au and few-layer graphene). We find that the substrate can affect the Raman and PL emission in a twofold manner. First, the absorption and emission intensities are strongly modulated by the constructive/destructive interference within the different substrates. Second, the position of the A1g Raman mode peak and the spectral weight between neutral and charged excitons in the PL spectra are modified by the substrate. We attribute this effect to substrate-induced changes in the doping level and in the decay rates of the excitonic transitions. Our results provide a method to quantitatively study the Raman and PL emission from MoS2-based vertical heterostructures and represent the first step in ad hoc tuning the PL emission of 1L MoS2 by selecting the proper substrate.

509 citations


Journal ArticleDOI
TL;DR: In this paper, a successful experimental approach to fabricate monolayer phosphorene by mechanical cleavage and the following Ar+ plasma thinning process was introduced. And the thickness of monolayers was unambiguously determined by optical contrast combined with atomic force microscope (AFM).
Abstract: There have been continuous efforts to seek for novel functional two-dimensional semiconductors with high performance for future applications in nanoelectronics and optoelectronics. In this work, we introduce a successful experimental approach to fabricate monolayer phosphorene by mechanical cleavage and the following Ar+ plasma thinning process. The thickness of phosphorene is unambiguously determined by optical contrast combined with atomic force microscope (AFM). Raman spectroscopy is used to characterize the pristine and plasma-treated samples. The Raman frequency of A2g mode stiffens, and the intensity ratio of A2g to A1g modes shows monotonic discrete increase with the decrease of phosphorene thickness down to monolayer. All those phenomena can be used to identify the thickness of this novel two-dimensional semiconductor efficiently. This work for monolayer phosphorene fabrication and thickness determination will facilitates the research of phosphorene.

508 citations


01 Jan 2014
TL;DR: The temperature-dependent Raman spectra of exfoliated, monolayer MoS2 in the range of 100-320 K are reported and analyzed and the thermal conductivity is significantly lower than that of graphene.
Abstract: Atomically thin molybdenum disulfide (MoS2 )o ffers potential for advanced devices and an alternative to graphene due to its unique electronic and optical properties. The temperature-dependent Raman spectra of exfoliated, monolayerMoS2intherangeof100� 320Karereportedandanalyzed.Thelinear temperature coefficients of the in-plane E2g 1 and the out-of-plane A1g modes for both suspended and substrate-supported monolayer MoS2 are measured. These data,whencombinedwiththe first-ordercoefficientsfromlaserpower-dependent studies, enable the thermal conductivity to be extracted. The resulting thermal conductivity κ=(34.5(4)W/mKatroomtemperatureagreeswellwiththe first- principles lattice dynamics simulations. However, this value is significantly lower than that of graphene. The results from this work provide important input for the design of MoS2-based devices where thermal management is critical.

478 citations


Journal ArticleDOI
TL;DR: H-BN and MoS2 are identified as two different types of 2D materials with potential for use as Raman enhancement substrates and both charge transfer and dipole-dipole coupling may occur, although weaker in magnitude, forMoS2.
Abstract: Realizing Raman enhancement on a flat surface has become increasingly attractive after the discovery of graphene-enhanced Raman scattering (GERS). Two-dimensional (2D) layered materials, exhibiting a flat surface without dangling bonds, were thought to be strong candidates for both fundamental studies of this Raman enhancement effect and its extension to meet practical applications requirements. Here, we study the Raman enhancement effect on graphene, hexagonal boron nitride (h-BN), and molybdenum disulfide (MoS2), by using the copper phthalocyanine (CuPc) molecule as a probe. This molecule can sit on these layered materials in a face-on configuration. However, it is found that the Raman enhancement effect, which is observable on graphene, hBN, and MoS2, has different enhancement factors for the different vibrational modes of CuPc, depending strongly on the surfaces. Higher-frequency phonon modes of CuPc (such as those at 1342, 1452, 1531 cm–1) are enhanced more strongly on graphene than that on h-BN, whi...

435 citations


Journal ArticleDOI
TL;DR: A large area of highly-crystalline MoSe2 atomic layers are synthesized on SiO2/Si, mica and Si substrates using a simple chemical vapour deposition (CVD) method at atmospheric pressure, suggesting that MoSe 2 monolayer is a promising material for photodetection applications.
Abstract: Synthesis of large-area, atomically thin transition metal dichalcogenides (TMDs) on diverse substrates is of central importance for the large-scale fabrication of flexible devices and heterojunction-based devices. In this work, we successfully synthesized a large area of highly-crystalline MoSe2 atomic layers on SiO2/Si, mica and Si substrates using a simple chemical vapour deposition (CVD) method at atmospheric pressure. Atomic force microscopy (AFM) and Raman spectroscopy reveal that the as-grown ultrathin MoSe2 layers change from a single layer to a few layers. Photoluminescence (PL) spectroscopy demonstrates that while the multi-layer MoSe2 shows weak emission peaks, the monolayer has a much stronger emission peak at ∼1.56 eV, indicating the transition from an indirect to a direct bandgap. Transmission electron microscopy (TEM) analysis confirms the single-crystallinity of MoSe2 layers with a hexagonal structure. In addition, the photoresponse performance of photodetectors based on MoSe2 monolayer was studied for the first time. The devices exhibit a rapid response of ∼60 ms and a good photoresponsivity of ∼13 mA/W (using a 532 nm laser at an intensity of 1 mW mm−2 and a bias of 10 V), suggesting that MoSe2 monolayer is a promising material for photodetection applications.

Journal ArticleDOI
29 Sep 2014-ACS Nano
TL;DR: A comprehensive characterization of the properties of tin disulfide (SnS2), an emerging semiconducting metal dichalcogenide, down to the monolayer limit is reported, showing that SnS2 is an indirect bandgap semiconductor over the entire thickness range from bulk to single-layer.
Abstract: Layered metal dichalcogenides have attracted significant interest as a family of single- and few-layer materials that show new physics and are of interest for device applications. Here, we report a comprehensive characterization of the properties of tin disulfide (SnS2), an emerging semiconducting metal dichalcogenide, down to the monolayer limit. Using flakes exfoliated from layered bulk crystals, we establish the characteristics of single- and few-layer SnS2 in optical and atomic force microscopy, Raman spectroscopy and transmission electron microscopy. Band structure measurements in conjunction with ab initio calculations and photoluminescence spectroscopy show that SnS2 is an indirect bandgap semiconductor over the entire thickness range from bulk to single-layer. Field effect transport in SnS2 supported by SiO2/Si suggests predominant scattering by centers at the support interface. Ultrathin transistors show on–off current ratios >106, as well as carrier mobilities up to 230 cm2/(V s), minimal hyster...

Journal ArticleDOI
TL;DR: Results obtained from lithium symmetrical cells suggest that the quality of the LLZO/lithium interface has a significant impact on the device lifetime and surface polishing can effectively remove Li2CO3 and dramatically improve the interfacial properties.
Abstract: Dense LLZO (Al-substituted Li7La3Zr2O12) pellets were processed in controlled atmospheres to investigate the relationships between the surface chemistry and interfacial behavior in lithium cells. Laser induced breakdown spectroscopy (LIBS), scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, synchrotron X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (XAS) studies revealed that Li2CO3 was formed on the surface when LLZO pellets were exposed to air. The distribution and thickness of the Li2CO3 layer were estimated by a combination of bulk and surface sensitive techniques with various probing depths. First-principles thermodynamic calculations confirmed that LLZO has an energetic preference to form Li2CO3 in air. Exposure to air and the subsequent formation of Li2CO3 at the LLZO surface is the source of the high interfacial impedances observed in cells with lithium electrodes. Surface polishing can effectively remove Li2CO3 and dramatically improve the interfacial properties. Polished samples in lithium cells had an area specific resistance (ASR) of only 109 Ω cm(2) for the LLZO/Li interface, the lowest reported value for Al-substituted LLZO. Galvanostatic cycling results obtained from lithium symmetrical cells also suggest that the quality of the LLZO/lithium interface has a significant impact on the device lifetime.

Journal ArticleDOI
TL;DR: High-speed chemical imaging in two- and three-dimensional views of healthy murine liver and pancreas tissues and interfaces between xenograft brain tumours and the surrounding healthy brain matter is demonstrated.
Abstract: An imaging platform based on broadband coherent anti-Stokes Raman scattering (BCARS) has been developed which provides an advantageous combination of speed, sensitivity and spectral breadth. The system utilizes a configuration of laser sources that probes the entire biologically-relevant Raman window (500 cm-1 to 3500 cm-1) with high resolution (< 10 cm-1). It strongly and efficiently stimulates Raman transitions within the typically weak "fingerprint" region using intrapulse 3-colour excitation, and utilizes the nonresonant background (NRB) to heterodyne amplify weak Raman signals. We demonstrate high-speed chemical imaging in two- and three-dimensional views of healthy murine liver and pancreas tissues and interfaces between xenograft brain tumours and the surrounding healthy brain matter.

Journal ArticleDOI
TL;DR: In this paper, the authors quantitatively reproduce existing experimental data and present evidence that the apparent discrepancy between intensity ratios observed experimentally can be explained by the high sensitivity of the Raman-active modes to laser polarization.
Abstract: Raman spectra of MoS2, WS2, and their heterostructures are studied by density functional theory. We quantitatively reproduce existing experimental data and present evidence that the apparent discrepancy between intensity ratios observed experimentally can be explained by the high sensitivity of the Raman-active modes to laser polarization. Furthermore, MoS2/WS2 heterostructures up to four layers are considered in every possible combination and stacking order. Each heterostructure configuration possesses a unique Raman spectrum in both frequency and intensity that can be explained by changes in dielectric screening and interlayer interaction. The results establish a set of guidelines for the practical experimental identification of heterostructure configurations.

Journal ArticleDOI
TL;DR: In this paper, the authors derived an analytical expression for the lowest order nonzero contribution to the surface-enhanced Raman spectrum from a system composed of a molecule adsorbed on a semiconductor nanoparticle.
Abstract: We develop an analytical expression for the lowest order nonzero contribution to the surface-enhanced Raman spectrum from a system composed of a molecule adsorbed on a semiconductor nanoparticle. We consider a combined molecule-semiconductor system and include Herzberg–Teller vibronic coupling of the zero-order Born–Oppenheimer states. This follows a previous derivation for metallic SERS, but instead of a Fermi level, the semiconductor system involves a band gap and we find that the SERS enhancement is maximized at either the conduction or valence band edge. The resulting expression may be regarded as an extension of the Albrecht A-, B-, and C-terms and show that the SERS enhancement is caused by several resonances in the combined system, namely, surface plasmon, exciton, charge-transfer, and molecular resonances. These resonances are coupled by terms in the numerator, which provide strict selection rules that enable us to test the theory and predict the relative intensities of the Raman lines. Furthermor...

Journal ArticleDOI
10 Jul 2014-ACS Nano
TL;DR: In this article, the authors investigated the evolution of the Raman spectrum of defected graphene as a function of doping and found that the intensities of the D and D′ peaks decrease with increasing doping.
Abstract: We investigate the evolution of the Raman spectrum of defected graphene as a function of doping. Polymer electrolyte gating allows us to move the Fermi level up to 0.7 eV, as directly monitored by in situ Hall-effect measurements. For a given number of defects, we find that the intensities of the D and D′ peaks decrease with increasing doping. We assign this to an increased total scattering rate of the photoexcited electrons and holes, due to the doping-dependent strength of electron–electron scattering. We present a general relation between D peak intensity and defects valid for any doping level.

Journal ArticleDOI
TL;DR: In this paper, a simple procedure for fabricating flexible, free-standing, and magnetic film of GO/CNT-Fe3O4 composites by using a one-pot co-precipitation in situ growth route was reported.
Abstract: We report a simple procedure for fabricating flexible, free-standing, and magnetic film of GO/CNT–Fe3O4 composites by using a one-pot co-precipitation in situ growth route. Characterizations including X-ray diffraction, Raman spectroscopy, superconducting quantum interference device magnetometry, scanning electron microscopy and transmission electron microscopy have been carried out to investigate the morphology, crystalline structure and magnetic properties of the composites. The layered structure of the as-prepared composites is porous and superparamagnetic. The GO/CNT–Fe3O4 composites exhibit excellent microwave absorbing properties in the range of 2–18 GHz and are expected to be promising candidates as microwave absorbing materials.

Journal ArticleDOI
05 Nov 2014-ACS Nano
TL;DR: 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.
Abstract: 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 P1. 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 phon...

Journal ArticleDOI
01 Dec 2014-Carbon
TL;DR: In this article, 1-, 2-and 4-layer graphitic carbon nitride (g-C3N4) nanosheets were synthesized in a well-crystallized form by controlling the intercalation time in a simple inter-calation-exfoliation process.

Journal ArticleDOI
10 Sep 2014-ACS Nano
TL;DR: Raman spectroscopy and photoluminescence studies reveal that after a thermal treatment at 300 °C, it is possible to produce van der Waals solids consisting of two interacting transition metal dichalcogenide (TMD) monolayers.
Abstract: Stacking of MoS2 and WSe2 monolayers is conducted by transferring triangular MoS2 monolayers on top of WSe2 monolayers, all grown by chemical vapor deposition (CVD). Raman spectroscopy and photoluminescence (PL) studies reveal that these mechanically stacked monolayers are not closely coupled, but after a thermal treatment at 300 °C, it is possible to produce van der Waals solids consisting of two interacting transition metal dichalcogenide (TMD) monolayers. The layer-number sensitive Raman out-of-plane mode A21g for WSe2 (309 cm–1) is found sensitive to the coupling between two TMD monolayers. The presence of interlayer excitonic emissions and the changes in other intrinsic Raman modes such as E″ for MoS2 at 286 cm–1 and A21g for MoS2 at around 463 cm–1 confirm the enhancement of the interlayer coupling.

Posted Content
TL;DR: A general relation between D peak intensity and defects valid for any doping level is presented, and for a given number of defects, it is found that the intensities of the D and D' peaks decrease with increasing doping.
Abstract: We investigate the evolution of the Raman spectrum of defected graphene as a function of doping. Polymer electrolyte gating allows us to move the Fermi level up to 0.7eV, as monitored by \textit{in-situ} Hall-effect measurements. For a given number of defects, we find that the intensities of the D and D' peaks decrease with increasing doping. We assign this to an increased total scattering rate of the photoexcited electrons and holes, due to the doping-dependent strength of electron-electron scattering. We present a general relation between D peak intensity and defects valid for any doping level

Journal ArticleDOI
TL;DR: In this article, a facial one-pot hydrothermal method was used to load Fe2O3 supported on nitrogen-doped graphene hydrogel and obtain a specific capacitance as high as 618 F g/1 at discharge current density of 0.5 A g/g/1.
Abstract: Fe2O3 supported on nitrogen-doped graphene (Fe2O3/N-rGO) hydrogel was prepared by a facial one-pot hydrothermal method. The efficient Fe2O3 loading and nitrogen doping of graphene was realized with this method. The morphology and structure of the samples were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, thermal gravimetric analysis, Raman spectra, X-ray diffraction, and nitrogen isothermal adsorption–desorption. The chemical environment of the surface composition of the samples was recorded by X-ray photoelectron spectroscopy. The electrochemical performance was tested with a three-electrode system in the aqueous electrolyte of 1 M KOH. The electrochemical measurement demonstrated that Fe2O3/N-rGO shows a specific capacitance as high as 618 F g–1 at a discharge current density of 0.5 A g–1. Even at the current density of 10 A g–1, the specific capacitance is still as high as 350 F g–1. After 5000 cycles, the capacity retention is still maintained at 56.7%.

Journal ArticleDOI
TL;DR: It is shown that coherent anti-Stokes Raman spectroscopy (CARS) can be used to obtain single-molecule detection sensitivity, by exploiting the unique light harvesting properties of plasmonic Fano resonances.
Abstract: The field enhancements arising in plasmonic nanostructures make them ideal as substrates for molecular sensors. In this study, Zhang et al. achieve single molecule sensitivity with Fano resonances in a quadrumer nanostructure and coherent anti-Stokes Raman spectroscopy.

Journal ArticleDOI
27 Mar 2014-ACS Nano
TL;DR: The observations point to strong effects of dimensionality on the phonon properties of MoTe2, which shows a prominent peak of the in-plane E(1)2g mode, with its frequency upshifting with decreasing thickness down to the atomic scale, similar to other dichalcogenides.
Abstract: Two-dimensional layered crystals could show phonon properties that are markedly distinct from those of their bulk counterparts, because of the loss of periodicities along the c-axis directions. Here we investigate the phonon properties of bulk and atomically thin α-MoTe2 using Raman spectroscopy. The Raman spectrum of α-MoTe2 shows a prominent peak of the in-plane E12g mode, with its frequency upshifting with decreasing thickness down to the atomic scale, similar to other dichalcogenides. Furthermore, we find large enhancement of the Raman scattering from the out-of-plane B12g mode in the atomically thin layers. The B12g mode is Raman inactive in the bulk, but is observed to become active in the few-layer films. The intensity ratio of the B12g to E12g peaks evolves significantly with decreasing thickness, in contrast with other dichalcogenides. Our observations point to strong effects of dimensionality on the phonon properties of MoTe2.

01 Jan 2014
TL;DR: In this paper, the phonon dispersions and Raman-active modes of semiconducting transition metal dichalcogenides (STMDs) were calculated for WSe2, MoSe2, W S 2 and MoS2.
Abstract: Although the main Raman features of semiconducting transition metal dichalcogenides are well known for the monolayer and bulk, there are important differences exhibited by few layered systems which have not been fully addressed. WSe2 samples were synthesized and ab-initio calculations carried out. We calculated phonon dispersions and Raman-active modes in layered systems: WSe2, MoSe2 ,W S 2 and MoS2 ranging from monolayers to five-layers and the bulk. First, we confirmed that as the number of layers increase, the E9 ,E 0 and E2g modes shift to lower frequencies, and the A91 and A1g modes shift to higher frequencies. Second, new high frequency first order A91 and A1g modes appear, explaining recently reported experimental data for WSe2, MoSe2 and MoS2. Third, splitting of modes around A91 and A1g is found which explains those observed in MoSe2. Finally, exterior and interior layers possess different vibrational frequencies. Therefore, it is now possible to precisely identify few-layered STMD.

Journal ArticleDOI
TL;DR: The Raman spectra of carbonaceous material (CM) from 19 metasediment samples collected from six widely separated areas of Southwest Japan and metamorphosed at temperatures from 165 to 655°C show systematic changes with metamorphic temperature that can be classified into four types: low-grade CM (c. 150-280°C), medium-grade graphite, high grade graphite and well-crystallized graphite (> c. 650°C) as discussed by the authors.
Abstract: The Raman spectra of carbonaceous material (CM) from 19 metasediment samples collected from six widely separated areas of Southwest Japan and metamorphosed at temperatures from 165 to 655°C show systematic changes with metamorphic temperature that can be classified into four types: low-grade CM (c. 150–280°C), medium-grade CM (c. 280–400°C), high-grade CM (c. 400–650°C), and well-crystallized graphite (> c. 650°C). The Raman spectra of low-grade CM exhibit features typical of amorphous carbon, in which several disordered bands (D-band) appear in the first-order region. In the Raman spectra of medium-grade CM, the graphite band (G-band) can be recognized and several abrupt changes occur in the trends for several band parameters. The observed changes indicate that CM starts to transform from amorphous carbon to crystallized graphite at around 280°C, and this transformation continues until 400°C. The G-band becomes the most prominent peak at high-grade CM suggesting that the CM structure is close to that of well-crystallized graphite. In the highest temperature sample of 655°C, the Raman spectra of CM show a strong G-band with almost no recognizable D-band, implying the CM grain is well-crystallized graphite. In the Raman spectra of low- to medium-grade CM, comparisons of several band parameters with the known metamorphic temperature show inverse correlations between metamorphic temperature and the full width at half maximum (FWHM) of the D1- and D2-bands. These correlations are calibrated as new Raman CM geothermometers, applicable in the range of c. 150–400°C. Details of the methodology for peak decomposition of Raman spectra from the low to medium temperature range are also discussed with the aim of establishing a robust and user-friendly geothermometer.

Journal ArticleDOI
TL;DR: In this article, a facile vapor diffusion method in combination with calcination at 550 °C was used to synthesize novel CoFe2O4 hollow sphere/graphene composites.
Abstract: Novel CoFe2O4 hollow sphere/graphene composites were synthesized by a facile vapor diffusion method in combination with calcination at 550 °C. The structure and morphology of as-prepared hybrid materials were characterized by electron microscopy, X-ray diffractometry, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Uniform CoFe2O4 hollow spheres with a diameter of about 500 nm and a shell thickness of approximately 50 nm were homogeneously distributed on graphene sheets. The electromagnetic parameters were measured using a vector network analyzer. A minimum reflection loss of −18.5 dB was observed at 12.9 GHz for the CoFe2O4 hollow sphere/graphene composites with a thickness of 2 mm, and the effective absorption frequency ranged from 11.3 to 15.0 GHz. The CoFe2O4 hollow sphere/graphene composites exhibited better microwave absorbing performance than the CoFe2O4 hollow spheres. A possible formation mechanism for CoFe2O4 hollow sphere/graphene composites was proposed.

Journal ArticleDOI
TL;DR: In this article, a complete analysis of all Raman active modes of Cu2ZnSnS4 measuring with six different excitation wavelengths from near infrared to ultraviolet was presented.
Abstract: This work presents a complete analysis of all Raman active modes of Cu2ZnSnS4 measuring with six different excitation wavelengths from near infrared to ultraviolet. Simultaneous fitting of spectra allowed identification of 18 peaks from device grade layers with composition close to stoichiometry that are attributed to the 27 optical modes theoretically expected for this crystalline structure, including detection of 5 peaks not observed previously, but theoretically predicted. Resonance effects are assumed to explain the observed increase in intensity of weak modes for near infrared and ultraviolet excitations. These results are particularly relevant for experimental discrimination of Raman modes related to secondary phases.

Journal ArticleDOI
TL;DR: In this article, an electron beam irradiation (30 kGy and 90 kGy) approach was used to narrow the band gap of the pristine CeO2 nanostructure (p-CeO2) to enhance their visible light activity through defect engineering.
Abstract: This work reports an electron beam irradiation (30 kGy and 90 kGy) approach to narrow the band gap of the pristine CeO2 nanostructure (p-CeO2) to enhance their visible light activity through defect engineering. This was confirmed by diffuse reflectance spectroscopy, photoluminescence, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller, electrochemical impedance spectroscopy, and linear scan voltammetry. XPS revealed changes in the surface states, composition, Ce4+ to Ce3+ ratio, and other defects in the modified CeO2 nanostructures (m-CeO2). The m-CeO2 exhibits excellent photocatalytic activities by degrading 4-nitrophenol and methylene blue in the presence of visible light (λ > 400 nm) compared to the p-CeO2. The optical, photocatalytic, and photoelectrochemical studies and proposed mechanism further support the enhanced visible light photocatalytic activities of the m-CeO2. This study confirmed that defect-induced band gap engineered m-CeO2 could be use...