Showing papers on "Raman spectroscopy published in 2018"

Raman spectroscopy of graphene-based materials and its applications in related devices.

Abstract: Graphene-based materials exhibit remarkable electronic, optical, and mechanical properties, which has resulted in both high scientific interest and huge potential for a variety of applications. Furthermore, the family of graphene-based materials is growing because of developments in preparation methods. Raman spectroscopy is a versatile tool to identify and characterize the chemical and physical properties of these materials, both at the laboratory and mass-production scale. This technique is so important that most of the papers published concerning these materials contain at least one Raman spectrum. Thus, here, we systematically review the developments in Raman spectroscopy of graphene-based materials from both fundamental research and practical (i.e., device applications) perspectives. We describe the essential Raman scattering processes of the entire first- and second-order modes in intrinsic graphene. Furthermore, the shear, layer-breathing, G and 2D modes of multilayer graphene with different stacking orders are discussed. Techniques to determine the number of graphene layers, to probe resonance Raman spectra of monolayer and multilayer graphenes and to obtain Raman images of graphene-based materials are also presented. The extensive capabilities of Raman spectroscopy for the investigation of the fundamental properties of graphene under external perturbations are described, which have also been extended to other graphene-based materials, such as graphene quantum dots, carbon dots, graphene oxide, nanoribbons, chemical vapor deposition-grown and SiC epitaxially grown graphene flakes, composites, and graphene-based van der Waals heterostructures. These fundamental properties have been used to probe the states, effects, and mechanisms of graphene materials present in the related heterostructures and devices. We hope that this review will be beneficial in all the aspects of graphene investigations, from basic research to material synthesis and device applications.

A Simple Route to Porous Graphene from Carbon Nanodots for Supercapacitor Applications

Abstract: A facile method to convert biomolecule-based carbon nanodots (CNDs) into high-surface-area 3D-graphene networks with excellent electrochemical properties is presented. Initially, CNDs are synthesized by microwave-assisted thermolysis of citric acid and urea according to previously published protocols. Next, the CNDs are annealed up to 400 °C in a tube furnace in an oxygen-free environment. Finally, films of the thermolyzed CNDs are converted into open porous 3D turbostratic graphene (3D-ts-graphene) networks by irradiation with an infrared laser. Based upon characterizations using scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and Raman spectroscopy, a feasible reaction mechanism for both the thermolysis of the CNDs and the subsequent laser conversion into 3D-ts-graphene is presented. The 3D-ts-graphene networks show excellent morphological properties, such as a hierarchical porous structure and a high surface area, as well as promising electrochemical properties. For example, nearly ideal capacitive behavior with a volumetric capacitance of 27.5 mF L-1 is achieved at a current density of 560 A L-1 , which corresponds to an energy density of 24.1 mWh L-1 at a power density of 711 W L-1 . Remarkable is the extremely fast charge-discharge cycling rate with a time constant of 3.44 ms.

Strain and grain size of TiO2 nanoparticles from TEM, Raman spectroscopy and XRD: The revisiting of the Williamson-Hall plot method

Abstract: The Williamson-Hall (W-H) equation, which has been used to obtain relative crystallite sizes and strains between samples since 1962, is revisited. A modified W-H equation is derived which takes into account the Scherrer equation, first published in 1918, (which traditionally gives more absolute crystallite size prediction) and strain prediction from Raman spectra. It is found that W-H crystallite sizes are on average 2.11 ± 0.01 times smaller than the sizes from Scherrer equation. Furthermore the strain from the W-H plots when compared to strain obtained from Raman spectral red-shifts yield factors whose values depend on the phases in the materials – whether anatase, rutile or brookite. Two main phases are identified in the annealing temperatures (350 °C–700 °C) chosen herein – anatase and brookite. A transition temperature of 550 °C has been found for nano-TiO2 to irreversibly transform from brookite to anatase by plotting the Raman peak shifts against the annealing temperatures. The W-H underestimation on the strain in the brookite phase gives W-H/Raman factor of 3.10 ± 0.05 whereas for the anatase phase, one gets 2.46 ± 0.03. The new βtot2cos2θ-sinθ plot and when fitted with a polynomial yield less strain but much better matching with experimental TEM crystallite sizes and the agglomerates than both the traditional Williamson-Hall and the Scherrer methods. There is greater improvement in the model when linearized – that is the βtotcos2θ-sinθ plot rather than the βtot2cos2θ-sinθ plot.

Simultaneous photoreduction of Uranium(VI) and photooxidation of Arsenic(III) in aqueous solution over g-C 3 N 4 /TiO 2 heterostructured catalysts under simulated sunlight irradiation

Abstract: In the present work, the hererostructured catalysts of g-C3N4/TiO2 were synthesized and well characterized by XRD, SEM, TEM, Raman, UV–vis diffuse reflectance spectra, PL, Mott-Schottky, and XPS. Simultaneous photoreduction of Uranium(VI) and photooxidation of Arsenic(III) was firstly achieved over the g-C3N4/TiO2 catalysts. And the experimental results show that the removal rate of U(VI) decreases with the increase of As(III) concentration, whereas the photooxidation rate of As(III) to As(V) increases with the increase of As(III) concentration. Noteworthily, the photoreduction of U(VI) to U(IV) and photooxidation of As(III) to As(V) was confirmed by XPS analysis in time-scale. The experimental results of free radical capture and quantitative test indicate that holes, hydroxyl radical and superoxide radical are the major active species for photooxidation of As(III), while U(VI) was reduced to U(IV) by the photogenerated electrons. Furthermore, a possible mechanism was proposed to well explain the improved photocatalytic performance of the g-C3N4/TiO2 and the competitive relationship between photoreduction of U(VI) and photooxidation of As(III). The present work develops a heterostructured catalyst for potential application to the simultaneous removal of U(VI) and As(III), and makes clear the effect of photooxidation of As(III) on photoreduction of U(VI) for the first time.

Comparison of Raman and Fourier Transform Infrared Spectroscopy for the Quantification of Microplastics in the Aquatic Environment

Abstract: Microplastics (MPs, 500 μm were visually sorted and manually analyzed by μ-Raman and attenuated total reflection (ATR)-FTIR spectroscopy. Microplastics ≤500 μm were concentrated on gold-coated filters and analyzed by automated single-particle exploration coupled to μ-Raman (ASPEx-μ-Raman) and FTIR imaging (reflection mode). The number of identified MPs >500 μm was slightly higher for μ-Raman (+23%) than ATR-FTIR analysis. Concerning MPs ≤500 μm, ASPEx-μ-Raman quantified two-times higher MP numbers but required a four-times higher analysis time compared to FTIR imaging. Because ASPEx-μ-Raman revealed far higher MP concentrations (38–2621 particles m–3) compared to the results of previous water studies (0–55...

Characterization of Graphite Oxide and Reduced Graphene Oxide Obtained from Different Graphite Precursors and Oxidized by Different Methods Using Raman Spectroscopy

Abstract: In this paper, the influences of the graphite precursor and the oxidation method on the resulting reduced graphene oxide (especially its composition and morphology) are shown. Three types of graphite were used to prepare samples for analysis, and each of the precursors was oxidized by two different methods (all samples were reduced by the same method of thermal reduction). Each obtained graphite oxide and reduced graphene oxide was analysed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (RS).

Gold Nanoparticle Plasmonic Superlattices as Surface-Enhanced Raman Spectroscopy Substrates

Abstract: Metal colloids are of great interest in the field of nanophotonics, mainly due to their morphology-dependent optical properties, but also because they are high-quality building blocks for complex plasmonic architectures. Close-packed colloidal supercrystals not only serve for investigating the rich plasmonic resonances arising in strongly coupled arrangements but also enable tailoring the optical response, on both the nano- and the macroscale. Bridging these vastly different length scales at reasonable fabrication costs has remained fundamentally challenging, but is essential for applications in sensing, photovoltaics or optoelectronics, among other fields. We present here a scalable approach to engineer plasmonic supercrystal arrays, based on the template-assisted assembly of gold nanospheres with topographically patterned polydimethylsiloxane molds. Regular square arrays of hexagonally packed supercrystals were achieved, reaching periodicities down to 400 nm and feature sizes around 200 nm, over areas up to 0.5 cm2. These two-dimensional supercrystals exhibit well-defined collective plasmon modes that can be tuned from the visible through the near-infrared by simple variation of the lattice parameter. We present electromagnetic modeling of the physical origin of the underlying hybrid modes and demonstrate the application of superlattice arrays as surface-enhanced Raman scattering (SERS) spectroscopy substrates which can be tailored for a specific probe laser. We therefore investigated the influence of the lattice parameter, local degree of order, and cluster architecture to identify the optimal configuration for highly efficient SERS of a nonresonant Raman probe with 785 nm excitation.

UV assisted ultrasensitive trace NO2 gas sensing based on few-layer MoS2 nanosheet–ZnO nanowire heterojunctions at room temperature

Abstract: Nitrogen dioxide (NO2) is a hazardous gas species that could impose a great threat on environmental protection and human health even at very low doses. Thus, it is of great importance to selectively detect trace NO2 gas below the ppm level. This has been a serious challenge so far, especially in the presence of other interfering gases. Herein, we report ultrasensitive, room-temperature and UV light-assisted NO2 gas sensing based on few-layer MoS2 nanosheet/ZnO nanowire composites serving as the sensing layer. A series of characterization techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), Raman, energy-dispersive X-ray spectroscopy (EDS), UV-Visual and ultraviolet photoelectron spectroscopy (UPS), were employed to explore the componential and structural properties of the obtained sensitive materials. Initially, the as-prepared MoS2/ZnO composites showed a tiny response (40%) and an incomplete recovery to 10 ppm NO2 gas in dark condition. While under UV illumination, the sensing response attained 8.4 and 188, toward 50 ppb and 200 ppb NO2 with a full recovery. Meanwhile, a sensitivity of 0.93 ppb−1, a detection limit of 50 ppq (10−15), and excellent repeatability and selectivity were also achieved. The experimental results were better than most previous work on NO2 detection to the best of our knowledge. Two main aspects are responsible for the outstanding performance. One is that a mass of photo-excited electron–hole pairs participated in the reaction with NO2 molecules under UV illumination. The other lies in numerous p–n MoS2/ZnO nanojunctions, favorable for the extension of the depletion region and the separation of the charge carrier. Additionally, long-term stability, as well as the effect of film thickness and carrier gas species on sensor performance, were simply investigated. We combined p–n nanojunctions with the UV illumination method, providing an alternative strategy to realize room-temperature operation and high sensitivity in the field of gas sensors.

Applications of Raman spectroscopy in cancer diagnosis.

Abstract: Novel approaches toward understanding the evolution of disease can lead to the discovery of biomarkers that will enable better management of disease progression and improve prognostic evaluation. Raman spectroscopy is a promising investigative and diagnostic tool that can assist in uncovering the molecular basis of disease and provide objective, quantifiable molecular information for diagnosis and treatment evaluation. This technique probes molecular vibrations/rotations associated with chemical bonds in a sample to obtain information on molecular structure, composition, and intermolecular interactions. Raman scattering occurs when light interacts with a molecular vibration/rotation and a change in polarizability takes place during molecular motion. This results in light being scattered at an optical frequency shifted (up or down) from the incident light. By monitoring the intensity profile of the inelastically scattered light as a function of frequency, the unique spectroscopic fingerprint of a tissue sample is obtained. Since each sample has a unique composition, the spectroscopic profile arising from Raman-active functional groups of nucleic acids, proteins, lipids, and carbohydrates allows for the evaluation, characterization, and discrimination of tissue type. This review provides an overview of the theory of Raman spectroscopy, instrumentation used for measurement, and variation of Raman spectroscopic techniques for clinical applications in cancer, including detection of brain, ovarian, breast, prostate, and pancreatic cancers and circulating tumor cells.

Tl4CdI6 Nanostructures: Facile Sonochemical Synthesis and Photocatalytic Activity for Removal of Organic Dyes

Abstract: In the present study, Tl4CdI6 nanostructures were synthesized via a facile sonochemical method. The effect of molar ratio of TlI to CdI2, reaction time, power of sonication, and the capping agents was investigated on morphology, size, and purity of the products. The as-prepared nanomaterials were characterized by X-ray diffraction, X-ray energy dispersive spectroscopy, field emission scanning, transmission electron microscopy, and Raman spectroscopy. The optical property of Tl4CdI6 nanoparticles was investigated by ultraviolet–visible spectroscopy (UV–vis), and the band gap was estimated about 2.82 eV. The photocatalytic behaviors of the nanoparticles were investigated by removal and degradation of different organic dyes under the UV irradiation. The results indicated a highest degradation for acid black 1 of 85.7% in 110 min. This sample was selected as an optimum sample for photocatalytic application.

Structural, Magnetic, and Catalytic Evaluation of Spinel Co, Ni, and Co-Ni Ferrite Nanoparticles Fabricated by Low-Temperature Solution Combustion Process.

Abstract: Here, we present the low-temperature (∼600 °C) solution combustion method for the fabrication of CoFe2O4, NiFe2O4, and Co0.5Ni0.5Fe2O4 nanoparticles (NPs) of 12–64 nm range in pure cubic spinel structure, by adjusting the oxidant (nitrate ions)/reductant (glycine) ratio in the reaction mixture. Although nitrate ions/glycine (N/G) ratios of 3 and 6 were used for the synthesis, phase-pure NPs could be obtained only for the N/G ratio of 6. For the N/G ratio 3, certain amount of Ni2+ cations was reduced to metallic nickel. The NH3 gas generated during the thermal decomposition of the amino acid (glycine, H2NCH2COOH) induced the reduction reaction. X-ray diffraction (XRD), Raman spectroscopy, vibrating sample magnetometry, and X-ray photoelectron spectroscopy techniques were utilized to characterize the synthesized materials. XRD analyses of the samples indicate that the Co0.5Ni0.5Fe2O4 NPs have lattice parameter larger than that of NiFe2O4, but smaller than that of CoFe2O4 NPs. Although the saturation magneti...

Interfacial charge transfer in oxygen deficient TiO2-graphene quantum dot hybrid and its influence on the enhanced visible light photocatalysis

Abstract: The present work focuses on understanding the heterojunction formation of graphene quantum dots (GQDs) and oxygen deficient TiO2 nanoparticle hybrid system and its enhanced photocatalytic activity under visible light illumination. We explain the formation of TiO2-GQD heterojunction through the bonding between oxygen vacancy sites in TiO2 and in-plane oxygen functional (epoxy) groups in GQDs possibly via C O Ti bonds. Our FTIR, XPS and Raman results lend support to the proposed mechanism of heterojunction formation. In the TiO2/GQD hybrid, the Raman Eg(1) peak of anatase TiO2 is blue shifted indicating the strong interaction between the GQD and TiO2. The heterojunction formation was simulated through the density functional theory (DFT) calculation to obtain the optical spectrum on the hybrid between oxygen deficient TiO2 and oxygen functionalized GQDs. Interestingly, the calculated results for the hybrid structure show strong optical absorption in the visible to near infrared region, which is in close agreement with the experimental results. The TiO2-GQD heterojunction exhibits enhanced photocatalytic degradation (97%) of MB due to the facile interfacial charge separation, as revealed from the steady state and time resolved photoluminescence studies. Interestingly, the photoluminescence intensity of the TiO2-GQD heterojunction was partially quenched indicating the electron transfer from GQDs to TiO2. The degradation rate constant (first order) for TiO2-GQD hybrid is 5.2 times higher than that of the TiO2. Free radical scavenger test revealed that OH radical played a major role in MB degradation as compared to O 2 ° − radical. These results are significant for the development of metal free catalysts based on carbon nano-materials for ensuing optoelectronic, energy and environmental applications.

Giant Electron-Phonon Coupling and Deep Conduction Band Resonance in Metal Halide Double Perovskite.

Abstract: The room-temperature charge carrier mobility and excitation–emission properties of metal halide perovskites are governed by their electronic band structures and intrinsic lattice phonon scattering mechanisms. Establishing how charge carriers interact within this scenario will have far-reaching consequences for developing high-efficiency materials for optoelectronic applications. Herein we evaluate the charge carrier scattering properties and conduction band environment of the double perovskite Cs2AgBiBr6 via a combinatorial approach; single crystal X-ray diffraction, optical excitation and temperature-dependent emission spectroscopy, resonant and nonresonant Raman scattering, further supported by first-principles calculations. We identify deep conduction band energy levels and that scattering from longitudinal optical phonons—via the Frohlich interaction—dominates electron scattering at room temperature, manifesting within the nominally nonresonant Raman spectrum as multiphonon processes up to the fourth ...

Challenges in application of Raman spectroscopy to biology and materials

Abstract: Raman spectroscopy has become an essential tool for chemists, physicists, biologists and materials scientists. In this article, we present the challenges in unravelling the molecule-specific Raman spectral signatures of different biomolecules like proteins, nucleic acids, lipids and carbohydrates based on the review of our work and the current trends in these areas. We also show how Raman spectroscopy can be used to probe the secondary and tertiary structural changes occurring during thermal denaturation of protein and lysozyme as well as more complex biological systems like bacteria. Complex biological systems like tissues, cells, blood serum etc. are also made up of such biomolecules. Using mice liver and blood serum, it is shown that different tissues yield their unique signature Raman spectra, owing to a difference in the relative composition of the biomolecules. Additionally, recent progress in Raman spectroscopy for diagnosing a multitude of diseases ranging from cancer to infection is also presented. The second part of this article focuses on applications of Raman spectroscopy to materials. As a first example, Raman spectroscopy of a melt cast explosives formulation was carried out to monitor the changes in the peaks which indicates the potential of this technique for remote process monitoring. The second example presents various modern methods of Raman spectroscopy such as spatially offset Raman spectroscopy (SORS), reflection, transmission and universal multiple angle Raman spectroscopy (UMARS) to study layered materials. Studies on chemicals/layered materials hidden in non-metallic containers using the above variants are presented. Using suitable examples, it is shown how a specific excitation or collection geometry can yield different information about the location of materials. Additionally, it is shown that UMARS imaging can also be used as an effective tool to obtain layer specific information of materials located at depths beyond a few centimeters.

Cu-Doped TiO2: Visible Light Assisted Photocatalytic Antimicrobial Activity

Abstract: Surface contamination by microbes is a major public health concern. A damp environment is one of potential sources for microbe proliferation. Smart photocatalytic coatings on building surfaces using semiconductors like titania (TiO2) can effectively curb this growing threat. Metal-doped titania in anatase phase has been proven as a promising candidate for energy and environmental applications. In this present work, the antimicrobial efficacy of copper (Cu)-doped TiO2 (Cu-TiO2) was evaluated against Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive) under visible light irradiation. Doping of a minute fraction of Cu (0.5 mol %) in TiO2 was carried out via sol-gel technique. Cu-TiO2 further calcined at various temperatures (in the range of 500–700 °C) to evaluate the thermal stability of TiO2 anatase phase. The physico-chemical properties of the samples were characterized through X-ray diffraction (XRD), Raman spectroscopy, X-ray photo-electron spectroscopy (XPS) and UV–visible spectroscopy techniques. XRD results revealed that the anatase phase of TiO2 was maintained well, up to 650 °C, by the Cu dopant. UV–vis results suggested that the visible light absorption property of Cu-TiO2 was enhanced and the band gap is reduced to 2.8 eV. Density functional theory (DFT) studies emphasize the introduction of Cu+ and Cu2+ ions by replacing Ti4+ ions in the TiO2 lattice, creating oxygen vacancies. These further promoted the photocatalytic efficiency. A significantly high bacterial inactivation (99.9999%) was attained in 30 min of visible light irradiation by Cu-TiO2.

Thickness-Tunable Synthesis of Ultrathin Type-II Dirac Semimetal PtTe2 Single Crystals and Their Thickness-Dependent Electronic Properties.

Abstract: The recent discovery of topological semimetals has stimulated extensive research interest due to their unique electronic properties and novel transport properties related to a chiral anomaly. However, the studies to date are largely limited to bulk crystals and exfoliated flakes. Here, we report the controllable synthesis of ultrathin two-dimensional (2D) platinum telluride (PtTe2) nanosheets with tunable thickness and investigate the thickness-dependent electronic properties. We show that PtTe2 nanosheets can be readily grown, using a chemical vapor deposition approach, with a hexagonal or triangular geometry and a lateral dimension of up to 80 μm, and the thickness of the nanosheets can be systematically tailored from over 20 to 1.8 nm by reducing the growth temperature or increasing the flow rate of the carrier gas. X-ray-diffraction, transmission-electron microscopy, and electron-diffraction studies confirm that the resulting 2D nanosheets are high-quality single crystals. Raman spectroscopic studies ...

Facile One-Pot Synthesis of Graphene Oxide by Sonication Assisted Mechanochemical Approach and Its Surface Chemistry.

Abstract: Facile one pot synthesis of graphene oxide (GO) by sonication assisted mechanochemical approach has been reported here The amalgamation of ultrasonication and mechanical stirring has assisted the synthesis of GO in a short time duration of only 4 hours with good reaction yield The structural characterization of GO was performed by X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectroscopy (FTIR), UV-Visible spectroscopy and Raman spectroscopy Atomic force microscopic (AFM) analysis manifested the flake like morphology of GO with average sheet thickness ~15 nm AFM also provides important information about the surface roughness Transmission electron microscope (TEM) analysis gave clear visualization of well exfoliated structure of GO in the form of thin flakes The field emission scanning electron microscope (FESEM) analysis revealed a crimpling surface morphology of GO The average size of GO flake as revealed through various morphological as well as light scattering techniques was around 3 μm Moreover, important surface chemistry of the synthesized GO was well ascertained through contact angle analysis, AFM analysis and zeta potential analysis

Raman Spectroscopy, Photocatalytic Degradation, and Stabilization of Atomically Thin Chromium Tri-iodide.

Abstract: As a 2D ferromagnetic semiconductor with magnetic ordering, atomically thin chromium tri-iodide is the latest addition to the family of two-dimensional (2D) materials. However, realistic exploration of CrI3-based devices and heterostructures is challenging due to its extreme instability under ambient conditions. Here, we present Raman characterization of CrI3 and demonstrate that the main degradation pathway of CrI3 is the photocatalytic substitution of iodine by water. While simple encapsulation by Al2O3, PMMA, and hexagonal BN (hBN) only leads to modest reduction in degradation rate, minimizing light exposure markedly improves stability, and CrI3 sheets sandwiched between hBN layers are air-stable for >10 days. By monitoring the transfer characteristics of the CrI3/graphene heterostructure over the course of degradation, we show that the aquachromium solution hole-dopes graphene.

Moiré Phonons in Twisted Bilayer MoS 2

Abstract: The material choice, layer thickness, and twist angle widely enrich the family of van der Waals heterostructures (vdWHs), providing multiple degrees of freedom to engineer their optical and electronic properties. The moire patterns in vdWHs create a periodic potential for electrons and excitons to yield many interesting phenomena, such as Hofstadter butterfly spectrum and moire excitons. Here, in the as-grown/transferred twisted bilayer MoS2 (tBLMs), one of the simplest prototypes of vdWHs, we show that the periodic potentials of moire patterns also modify the properties of phonons of its monolayer MoS2 constituent to generate Raman modes related to moire phonons. These Raman modes correspond to zone-center phonons in tBLMs, which are folded from the off-center phonons in monolayer MoS2. However, the folded phonons related to crystallographic superlattices are not observed in the Raman spectra. By varying the twist angle, the moire phonons of tBLM can be exploited to map the phonon dispersions of the monolayer constituent. The lattice dynamics of the moire phonons are modulated by the patterned interlayer coupling resulting from periodic potential of moire patterns, as confirmed by density functional theory calculations. The Raman intensity related to moire phonons in all tBLMs are strongly enhanced when the excitation energy approaches the C exciton energy. This study can be extended to various vdWHs to deeply understand their Raman spectra, moire phonons, lattice dynamics, excitonic effects, and interlayer coupling.

Degradation of indometacin by simulated sunlight activated CDs-loaded BiPO4 photocatalyst: Roles of oxidative species

Abstract: In this study, novel carbon dots/BiPO4 (CDBP) photocatalytic complexes were successfully synthesized via a facile hydrothermal-calcination synthesis strategy. The physicochemical properties of the synthesized samples were studied by X-ray diffraction (XRD), UV–vis diffuse reflectance spectra (DRS), Fourier infrared spectrometer (FT-IR), Raman spectrometer, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), photoluminescence26spectra (PL), electrochemical workstation, etc. The activities of the CDBP were evaluated through the photocatalytic degradation of Indometacin(IDM) in an aqueous solution under simulated sunlight irradiation. With increasing concentrations of carbon dots (CDs), the photocatalytic activity of the CDBP initially increased, and then decreased. A CDs content of 3.0 wt% shows 12 times higher photocatalytic activity than that of pristine BiPO4. Reactive oxidative species, particularly O2 − and h+, were the two critical reactive oxidative species to mediator immediate the photocatalytic degradation of IDM. A notable sign of 5, 5-dimethyl-1-pyrrolidone-N-oxyl(DMPOX) was observed through electron spin resonance spectroscopy(EPR) with CDBP as the photocatalyst, which indicated higher oxidability than pristine BiPO4 under simulated sunlight irradiation. This enhanced photocatalytic activity might due to high-efficiency charge separation, unique up-converted PL properties, as well as the bandgap narrowing of the CDs. Moreover, the byproducts of IDM were detected by HPLC–MS/MS and GC–MS, and the probable pathways were deduced. The acute toxicity at three trophic levels initially increased slowly and then decreased rapidly as the IDM dechlorination and total organic carbon(TOC) decreased during photocatalytic degradation.

Low-temperature synthesis of 2D MoS2 on a plastic substrate for a flexible gas sensor

Abstract: The efficient synthesis of two-dimensional molybdenum disulfide (2D MoS2) at low temperatures is essential for use in flexible devices. In this study, 2D MoS2 was grown directly at a low temperature of 200 °C on both hard (SiO2) and soft substrates (polyimide (PI)) using chemical vapor deposition (CVD) with Mo(CO)6 and H2S. We investigated the effect of the growth temperature and Mo concentration on the layered growth by Raman spectroscopy and microscopy. 2D MoS2 was grown by using low Mo concentration at a low temperature. Through optical microscopy, Raman spectroscopy, X-ray photoemission spectroscopy, photoluminescence, and transmission electron microscopy measurements, MoS2 produced by low-temperature CVD was determined to possess a layered structure with good uniformity, stoichiometry, and a controllable number of layers. Furthermore, we demonstrated the realization of a 2D MoS2-based flexible gas sensor on a PI substrate without any transfer processes, with competitive sensor performance and mechanical durability at room temperature. This fabrication process has potential for burgeoning flexible and wearable nanotechnology applications.

Angstrom-Size Defect Creation and Ionic Transport through Pores in Single-Layer MoS2

Abstract: Atomic-defect engineering in thin membranes provides opportunities for ionic and molecular filtration and analysis. While molecular-dynamics (MD) calculations have been used to model conductance through atomic vacancies, corresponding experiments are lacking. We create sub-nanometer vacancies in suspended single-layer molybdenum disulfide (MoS2) via Ga+ ion irradiation, producing membranes containing ∼300 to 1200 pores with average and maximum diameters of ∼0.5 and ∼1 nm, respectively. Vacancies exhibit missing Mo and S atoms, as shown by aberration-corrected scanning transmission electron microscopy (AC-STEM). The longitudinal acoustic band and defect-related photoluminescence were observed in Raman and photoluminescence spectroscopy, respectively. As the irradiation dose is increased, the median vacancy area remains roughly constant, while the number of vacancies (pores) increases. Ionic current versus voltage is nonlinear and conductance is comparable to that of ∼1 nm diameter single MoS2 pores, provin...

Study of short range structure of amorphous Silica from PDF using Ag radiation in laboratory XRD system, RAMAN and NEXAFS

Abstract: At present synchrotron and neutron sources are the preferred choices for the Pair Distribution Function (PDF) analysis, but there is a need to explore PDF in a laboratory XRD system for quick feedback about the short range structure of the amorphous materials. Present work considered both crystalline (quartz) and amorphous silica to study the structural differences in silica by PDF analysis using Ag radiations in laboratory XRD. The structural information about short range ordering of the oxygen (O) atoms around silicon (Si) atoms as obtained by the PDF were compared with the results as obtained by Near Edge X-ray Absorption Fine Structure (NEXAFS) and RAMAN experiments. The PDF studies showed that the amorphous silica possessed short range periodicity within the basic unit of (SiO4)4− tetrahedra with a Si O & O O distance are of about 1.622 A and 2.713 A while the short range as well as long range ordered structure present in quartz with Si O & O O distance are 1.562 A and 2.661 A respectively. Raman spectra showed some asymmetry in amorphous silica which corresponds to the defects present in the lattice and thus forming the n-fold ring structure with Si and O resulting in the wide variation of bridging bond angle Si O Si in amorphous silica. NEXAFS studies revealed the structure of amorphous silica and quartz in the intermediate range (3–5 A) at the Si L and O K edges. The structural information about short range ordering of the O around Si atoms as obtained by these methods were found to be in good match with the results as obtained by PDF, suggesting this technique may be used as a screening tool for routine PDF studies of amorphous materials.

Synthesis, photoluminescence and Magnetic properties of iron oxide (α-Fe2O3) nanoparticles through precipitation or hydrothermal methods

Abstract: In this work the iron oxide (α-Fe 2O3 ) nanoparticles are synthesized using two different methods: precipitation and hydrothermal. Size, structural, optical and magnetic properties were determined and compared using X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Fourier Transform Infra-Red (FT-IR), Raman spectroscopy , Differential Thermal Analysis (DTA), Thermogravimetric Analysis (TGA), Ultraviolet–Visible (UV–Vis) analysis, Superconducting QUantum Interference Device (SQUID) magnetometer and Photoluminescence (PL). XRD data further revealed a rhombohedral (hexagonal) structure with the space group (R-3c) and showed an average size of 21 nm for hydrothermal samples and 33 nm for precipitation samples which concorded with TEM and SEM images. FT-IR confirms the phase purity of the nanoparticles synthesized. The Raman spectroscopy was used not only to prove that we have synthesized pure α-Fe 2O3 but also to identify their phonon modes. The TGA showed three mass losses, whereas DTA resulted in three endothermic peaks. The decrease in the particle size of hematite of 33 nm for precipitation samples to 21 nm for hydrothermal samples is responsible for increasing the optical band gap of 1.94–2.10 eV where, the relation between them is inverse relationship. The products exhibited the attractive magnetic properties with good saturation magnetization , which were examined by a SQUID magnetometer. Photoluminescence measurements showed a strong emission band at 450 nm. Pure hematite prepared by hydrothermal method has smallest size, best crystallinity , highest band gap and best value of saturation magnetization compared to the hematite elaborated by the precipitation method.

Raman spectroscopy in black phosphorus

Abstract: In this article, we review the current status of Raman spectroscopy in orthorhombic black phosphorus (BP) (for simplicity, henceforth, referred to as BP). BP is a layered semiconductor crystal that recently regained interest because it can be exfoliated down to the single layer, thus exhibiting 2-D properties. First, we briefly review the crystalline structure and the phonon dispersion relations in BP. Then, the symmetries of the Raman-active modes are discussed in the light of group theory, and the scattering configurations for observing the different phonon modes are presented. Polarized Raman spectroscopy results are discussed and reveal unusual angular dependence features, which can be ascribed to the linear dichroism and to the complex nature of the electron–phonon matrix elements. Edge phonon effects originated from rearrangements of the atomic terminations are also discussed. Subsequently, Raman modes that emerge from interlayer interaction and that are only visible in the few-layer regime are presented and discussed. Finally, we outline new perspectives to BP Raman spectroscopy in directions that remain partially or totally unexplored and that can provide potentially important outcomes to problems such as defects, oxidation, doping, strain, stacking order and other BP-like 2-D materials. Copyright © 2017 John Wiley & Sons, Ltd.

Raman enhancement on ultra-clean graphene quantum dots produced by quasi-equilibrium plasma-enhanced chemical vapor deposition.

Abstract: Graphene is regarded as a potential surface-enhanced Raman spectroscopy (SERS) substrate. However, the application of graphene quantum dots (GQDs) has had limited success due to material quality. Here, we develop a quasi-equilibrium plasma-enhanced chemical vapor deposition method to produce high-quality ultra-clean GQDs with sizes down to 2 nm directly on SiO2/Si, which are used as SERS substrates. The enhancement factor, which depends on the GQD size, is higher than conventional graphene sheets with sensitivity down to 1 × 10−9 mol L−1 rhodamine. This is attributed to the high-quality GQDs with atomically clean surfaces and large number of edges, as well as the enhanced charge transfer between molecules and GQDs with appropriate diameters due to the existence of Van Hove singularities in the electronic density of states. This work demonstrates a sensitive SERS substrate, and is valuable for applications of GQDs in graphene-based photonics and optoelectronics. Surface-enhanced Raman spectroscopy (SERS) is a promising technology for sensitive optical sensors, generally using rough metal films. Here, Liu et al. synthesize high-quality graphene quantum dot films which offer a large SERS enhancement due to a strong light-matter interaction with Van Hove singularities.

Preparation of Sb nanoparticles in molten salt and their potassium storage performance and mechanism

Abstract: Sb nanoparticles with a size of 55 nm are fabricated via the reduction of SbCl3 by metallic Al in the molten salt of SbCl3 at 80 °C. In situ XRD patterns and ex situ Raman spectra show that the potassium storage mechanism is an alloying-type with the formation of a cubic K3Sb phase when fully potassiated and an amorphous phase when fully depotassiated. As an anode for potassium-ion batteries, Sb nanoparticles coated with graphene could deliver a reversible capacity of 381 mA h g-1 at 100 mA g-1, and maintain a capacity of 210 mA h g-1 at 500 mA g-1 for 200 cycles.