Ultraviolet Raman Spectroscopy Research Articles

Polarity Inversion of SiC(0001) during the Al Doped PVT Growth

We found that the polarity of the 4H-SiC is reversed from Si-face to C-face by high Al doping during the physical vapor transport (PVT) growth. KOH etching and deep ultraviolet (DUV) Raman spectroscopy were used to confirm the polarity of the grown crystals. The results show the polarity inversion is occurred in the samples grown on Si-face SiC with using Al doped SiC source material.

Post-synthesis of hierarchically structured Ti-β zeolites and their epoxidation catalytic performance

Hierarchically structured Ti-β zeolites were prepared using a two-step post-synthesis approach. The physicochemical properties of the resultant Ti-mβ-Sx materials were determined using various techniques, including X-ray diffraction, scanning electron microscopy, inductively coupled plasma atomic emission spectroscopy, transmission electron microscopy, and ultraviolet-visible and ultraviolet-Raman spectroscopies. The porous structures of the synthesized hierarchical Ti-β zeolites were well preserved. Their catalytic performance was compared with that of pure microporous Ti-β materials using alkene epoxidation as a model reaction. It was found that although comparable catalytic activities were obtained for both types of catalyst in the epoxidation of small molecules, e.g., cyclohexene, the synthesized hierarchical Ti-β gave an enhanced catalytic performance in the epoxidation of bulky 1-dodecene.

Thermal Decomposition of the Fullerene Precursor C60H21F9 Deposited on Graphite

Specially fluorinated polycyclic aromatic hydrocarbons (F-PAHs) are of interest as precursors for transition metal catalyzed CVD growth of chiral-index pure single-walled carbon nanotubes as well as for the rational synthesis of fullerenes. Laser desorption/ionization of a prototypical F-PAH has recently been shown to lead to C60 via a sequence of regioselective intramolecular cyclodehydrofluorination steps: C60H21F9 → C60H20F8 + HF → C60H19F7 + HF ... → C60 (Kabdulov et al. Chem.–Eur. J. 2013, 19, 17262). We have studied the thermal stability of solid C60H21F9 films on graphite under UHV conditions toward exploring the extent to which such intramolecular dehydrofluorination can also occur on a hot chemically inert surface and to what extent intermolecular interactions influence such transformation processes. C60H21F9 films were probed in situ by ultraviolet photoionization, X-ray ionization, Raman spectroscopy, and thermal desorption mass spectrometry, as well as by ex situ atomic force microscopy. Heating multilayer films results first in C60H21F9 emission from the bulk (peaked at ∼630 K) followed at higher temperatures by desorption from the interface region (in the range 750–850 K). Sublimation from the interface region is also associated with some on-surface cyclo-dehydrofluorination as indicated by C60H21–nF9–n, n = 1, 2, 3 emission. C60 was not observed in the desorbed material suggesting that complete cage closure cannot be achieved on HOPG. Furthermore, C60H21F9 deposits cannot be fully removed from HOPG. Instead, competing on-surface polycondensation of reactive intermediates yields a fluorinated carbon phase, which remains stable up to at least ∼1000 K. To complement these studies we have also used mass selective ion beam soft-landing to probe the desorption properties of monodispersed films consisting of mass-selected C60H21–nF9–n fragments, n = 1, 2.

Formation of carbon nanospheres via ultrashort pulse laser irradiation of methane

Irradiation of methane with intense, spatially- and temporally-shaped femtosecond laser pulses forms a spatially confined microplasma that produces carbon nanospheres. The morphology and composition of the nanospheres are characterized by transmission electron microscopy (TEM), ultraviolet (UV) Raman spectroscopy, and infrared spectroscopy (IR). Increasing the pressure of methane from 6.7 to 133.3 kPa results in a decrease of the median diameter of the spheres from ∼500 nm to 85 nm. At pressures of 101.3 kPa and higher, particles with non-spherical morphologies are observed in TEM analysis. The morphology of the nanospheres is determined to be amorphous, containing both sp2 and sp3 hybridized carbon atoms, based on the presence of the carbon D and T peaks in the UV Raman spectrum. The red shift of the G peak and a high fluorescence background in the Raman spectrum indicates that the hydrogen content of the spheres is at least 30%. The presence of hydrogen is supported by the IR spectrum, which shows the presence of C–H bonds in the nanosphere product.

Synthesis of phosphorescent ceramic pigment BaAl1.7B0.3O4 doped with Eu2+ and Dy3+

The optimal formulation and processing conditions were investigated for the preparation of the phosphorescent ceramic pigment BaAl1.7B0.3O4 doped with Eu2+ and Dy3+ using the polymeric precursor method or Pechini method. The properties of the synthesized pigment were analyzed by X-ray diffraction, excitation by ultraviolet light, Raman spectroscopy, photoluminescence, and luminescence decay. The hexagonal barium aluminate phase BaAl2O4 could be identified in all doped compounds. The photoluminescence spectra analysis indicated that phosphorescence efficiency increased with higher Eu2+concentrations, and that the pigments emitted green light in the visible range of 430–500nm corresponding to the 4f6–5d →4f7 of Eu2+ transition with curves of luminescence decay between 160 and 790s. We conclude that these techniques and analyses are suitable to produce a high quality phosphorescent pigment suitable for use in the ceramic industry.

Characterization of graphene films grown on CuNi foil substrates

Abstract The electronic properties of graphene films depend on the number of atomic layers and the stacking sequence between the layers. One method of growing graphene films that are more than one atomic layer thick is by chemical vapor deposition on metal substrates that have non-negligible carbon solubility. This allows precipitation of carbon from the bulk during the cooling phase of the growth process. In this study, graphene films were grown on foil substrates composed of a CuNi alloy with a nominal bulk composition of 90:10 by weight. To determine the average thickness of the graphene films, angle-resolved X-ray photoelectron spectroscopy was used. For films grown at 1050 °C for 5, 25, and 50 min on the CuNi substrates, thicknesses of 1.06 ± 0.14, 1.19 ± 0.13, and 1.87 ± 0.13 monolayers were measured, respectively. Scanning electron microscopy was used to measure the growth morphology of the graphene films and provided a method of confirming the coverages determined by the analysis of the photoemission data. Ultra-violet Raman spectroscopy measurements were also performed on the graphene films, and it was found that the G-peak intensity increases and the frequency decreases with graphene thickness.

Mechanism of seeding in hydrothermal synthesis of zeolite Beta with organic structure-directing agent-free gel

The organic structure-directing agent-free synthesis of zeolite Beta was carried out using several zeolite Beta seeds that differed in SiO2/Al2O3 ratio and crystal size. The synthesis was studied using X-ray diffraction, X-ray fluorescence, scanning electron microscopy, transmission electron microscopy, ultraviolet-Raman spectroscopy, infrared spectroscopy, and N2 physisorption. Synthesis was successful using different zeolite Beta seeds including pure silica seeds. During the induction period, the seeds underwent dissolution. The SiO2/Al2O3 ratio and crystal size, pretreatment (calcination), and seed addition time had a significant influence on seed dissolution behavior, crystallization process, and product. Morphological studies revealed that the seed residues produced by dissolution (except for pure silica) resulted in the formation of “immobilized” surface nuclei, which allowed for the dense growth of fresh small zeolite Beta crystals. The dissolved small seed fragments yielded dispersed nuclei, which formed relatively scattered small zeolite Beta crystals thought to be the main nuclei source of the pure silica seed. It is suggested that the use of an appropriately high SiO2/Al2O3 ratio, small size, and precalcined zeolite Beta seed is helpful for the synthesis of highly crystalline and pure zeolite Beta from the organic structure-directing agent-free gel.

Reexamination of cell contents in Pennsylvanian spores and pollen grains using Raman spectroscopy

Permineralization offers exceptional structural preservation to the level of cellular components, many of which are found within fossilized plant specimens. In some cases, structures within plant cells, particularly those found within spores and pollen grains, have been interpreted as nuclei. Although these structures have been studied morphologically and ultrastructurally, little is known about their composition and origin. This study uses petrographic thin sections and ultraviolet (UV) Raman spectroscopy to reexamine cellular inclusions within three types of Pennsylvanian spores and pollen grains with the intent to interpret the molecular composition and therefore, the identity of these structures. Thin sections are examined using Raman spectroscopy, which indicates the presence of disordered carbonaceous material and calcite. There is no significant difference in the chemical composition between the intracellular inclusions and surrounding areas within each specimen. This study represents one of the first applications of Raman spectroscopy in which the internal cell contents of fossil plant taxa are analyzed. Results do not support the presence of nuclei, although further examination is recommended to elucidate the origin and identity of the intracellular inclusions.

Structure evolution and properties of TiAlCN/VCN coatings deposited by reactive HIPIMS

2.5μm thick TiAlCN/VCN coatings were deposited by a reactive high power impulse magnetron sputtering (HIPIMS) process. Cross-sectional TEM showed gradual evolution of the structure of the coating with thickness. The initial structure is a nanoscale multilayer with sharp interlayer interfaces. This transforms to nanocomposite of TiAlCN and VCN nanocrystalline grains surrounded by a C-rich tissue phase and finally changes to an amorphous carbon rich Me-C phase. In contrast deposition in similar conditions using standard magnetron sputtering produces a well-defined nanoscale multilayer structure. Depth profiling by AES showed that the carbon content in the HIPIMS coating gradually increased from 25% at the coating substrate interface to 70% at the top thus supporting the TEM observations.Energy-resolved mass spectrometry revealed that HIPIMS plasma is a factor of 10 richer in C1+ ions, and therefore more reactive, as compared to the plasma generated by standard magnetron discharge at the same conditions. The peculiar structure evolution in HIPIMS is discussed in relation to target poisoning effect and carbon outward diffusion during coating growth.Highly abrasive AlSi9Cu1 alloy was dry machined using TiAlCN/VCN coated 25mm diameter end mills to investigate the coating-work piece material interaction. Green (532nm excitation) and ultraviolet (325nm excitation) Raman spectroscopy was employed to identify the phase composition of the built up material on the cutting edge and chip (swarf) surfaces produced during machining. These analyses revealed formation of lubricious Magnèli phases namely V2O5 and graphitic carbon as well as highly abrasive SiO2 and mixed (AlSi)O thus shedding light on the wear processes and coating tribological behavior during machining.

Vibrational Spectroscopic Investigations of Commercial Nafion 112 and Related Fluorinated Ionomer Materials

Fourier transform infrared (FTIR) and ultraviolet (UV) Raman spectroscopy methods were applied to analyze commercial Nafion and structurally related fluorinated ionomer membranes. Nafion, a commercial perfluorosulfonic acid material, has been the focus of extensive study, as it is utilized in a wide range of areas, including energy production and chemical sensing. However, issues with water management and temperature stability have motivated the search for other types of ionomer. As such, structurally related fluorinated membranes have been developed in attempts to optimize essential properties, such as water transport and proton conductivity. Structural differences in these materials were examined by vibrational spectroscopy. Raman studies were performed using an UV source to allow the enhancement of the Raman signal and reduce fluorescence versus a visible source. Characteristics derived from the spectra will be discussed, and comparisons will be made between the two vibrational methods.

Fabrication And Characterization Of Efficient Hybrid Photocatalysts Based On Titania And Graphene For Acid Orange Seven Dye Degradation Under UV Irradiation

Simple and proficient methodology has been proposed for the preparation of hybrid photocatalyst based on titanium dioxide (TiO2)-graphene (GR) nanocomposite for acid orange 7 (AO7) dye degradation under UV irradiation. High Resolution Transmission Electron Microscopy (HRTEM) and Scanning Electron Microscopy (SEM) studies revealed that TiO2 nanoparticles were uniformly dispersed on GR surface. TiO2-GR hybrid nanocomposite has also been characterized by Ultraviolet Diffusive Reflectance Spectroscopy (UV-DRS), Raman spectroscopy and X-ray diffraction (XRD) studies. Electrochemical Impedance spectroscopy (EIS) measurement revealed that the incorporation of GR with TiO2 nanoparticles significantly enhanced the electrical conductivity.The peak intensity of PL spectra of GR containing catalysts are lower than that of pristine TiO2, indicating that the electron–hole recombination rate of self-trapped excitations in TiO2 is reduced by the introduction of GR. The photocatalytic degradation measurements demonstrated that the TiO2-GR composites exhibited an enhanced photocatalytic activity for AO7 degradationunder UV irradiation compared to pure TiO2. This may due to greater adsorptivity ofdyes, extended light absorption and increased charge separation efficiency due to excellent electricalproperties of graphene and the large surface contact between graphene and TiO2 nanoparticles. Therefore, the TiO2-GR composites can be widely used as a ternary composite photocatalyst for treating the organic contaminant in the field of environmental protection.

Dispersion and characterization of arc discharge single-walled carbon nanotubes – towards conducting transparent films

This study addresses a combination of a well-developed and mild dispersion method and high-quality arc discharge single-walled carbon nanotubes (SWCNTs) as starting materials. Thus, we advance in fabrication of transparent, conducting films with extraordinary low material loss during SWCNT processing. The starting material was characterized by means of thermogravimetric analysis, high-resolution transmission electron microscopy and Raman spectroscopy. The quality of the starting material and produced dispersions was evaluated by ultraviolet and visible light absorption spectroscopy and Raman spectroscopy. A transparent conductive film was fabricated by drop-casting, whereas films were obtained with electrical to optical conductivity ratios (σDC/σOp) as high as 2.2, combined with a loss of nanotube material during processing well below 20 wt%. High pressure carbon monoxide conversion (HiPCO) SWCNTs, which are very well described in the literature, were used for comparison.

Formation mechanism of ZnS impurities and their effect on photoelectrochemical properties on a Cu2ZnSnS4 photocathode

Cu2ZnSnS4 (CZTS) nanocrystals were synthesized by a hot injection method. Ultraviolet (UV) Raman spectroscopy measurements showed that ZnS impurities existed in CZTS with a Zn/Sn precursor ratio of 1.2. Combining Raman spectra with energy-dispersive X-ray spectroscopy (EDS) characterization, the formation mechanism of CZTS and ZnS impurities were also investigated. We found that crystalline CZTS and Cu2SnS3, as well as amorphous ZnS, formed after sulfur–oleylamine was injected into the mixture solution. Amorphous ZnS reacted with Cu2SnS3 to form CZTS during sulfur annealing at high temperature. However, if the Zn/Sn ratio was too high, the excess ZnS could not be eliminated even after a longer reaction time. A relatively pure phase of CZTS was only obtained by decreasing the Zn/Sn precursor ratio to 0.6. Moreover, CZTS nanocrystals were assembled into thin films by electrophoretic deposition onto molybdenum-coated soda lime glass and the photoelectrochemical properties of CZTS photocathodes with and without ZnS impurities were investigated. A CZTS photocathode without ZnS demonstrated higher photocurrent than those with ZnS. The results will deepen the understanding of the CZTS formation process and be helpful to explore efficient CZTS solar conversion devices.

Direct fabrication of graphene/zinc oxide composite film and its characterizations

Graphene-based composites represent a new class of materials with potential for many applications. Graphene can be attached to a metal, a semiconductor, or any polymer for enhancing properties. In this work, a new mixed dispersion approach for graphene-based composite has taken on. Graphene flakes (<4 layers) and a well-known semiconductor zinc oxide (ZnO) (<50 nm particle size) have dispersed in N-methyl-pyrrolidone. We deposited graphene/ZnO composite thin film by a simple, low-cost, environmentally friendly and non-vacuum electrohydrodynamic atomization process on silicone substrate. Experiments have been carried out by changing flow rate and applied potential while keeping stand-off distance and substrate velocity constant, to discover the optimum conditions for obtaining a high-quality thin film. It has been explored that high-quality thin composite film is obtained at optimum flow rate of 300 μl/h at 6.3 kV applied potential after curing for 2 h at 300 °C. Graphene/ZnO thin composite film has been characterized using Field emission scanning electron microscopy, Ultra-violet Visible near Infra Red spectroscopy, X-ray diffraction, Raman Spectroscopy and 3D-Nanomap. For electrical behavior analysis, a simple diode Indium tin oxide/(poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/polydioctylfluorene-benzothiadiazole(F8BT)/(Graphene/ZnO) has fabricated. It is observed that at voltage of 0.3 V, the current in organic structure is at low value of 1.20 × 10−3 Amp/cm2 and after that as further voltage was applied, the device current increased by the order of 110 and reaches up to 1.32 × 10−1 Amp/cm2 at voltage 2 V.

Vanadium species supported on inorganic oxides as catalysts for propene epoxidation in the presence of N2O as an oxidant

The surface structure of vanadium species supported on SiO2, Al2O3, TiO2, and MgO and the effect of these supports on the activity of vanadium catalysts for propylene to propylene oxide oxidation with N2O as an oxidant were studied using ultraviolet visible diffuse reflectance spectroscopy and Raman spectroscopy, infrared spectra (IR) of adsorbed reagents, and the measurements of catalytic activity. The acidity of the samples was estimated from temperature-programmed desorption of ammonia and from IR spectra of adsorbed pyridine. On the basis of spectroscopic measurements, it was concluded that positively charged V5+ ions formed on silica support are responsible for N2O adsorption through nitrogen with the formation of weakly electronegative oxygen capable of interacting with a double CC bond with the formation of epoxide. Analysis of different vanadium bonds present on the V/(oxide support) system indicates that mainly VOSi groups are responsible for activity in propylene epoxidation.

Characterization of the thin layer photocatalysts TiO2 and V2O5- and Fe2O3- doped TiO2 prepared by the sol–gel method

The catalysts TiO2 and TiO2 doped with Fe and V were prepared using the sol–gel method. TiO2-modified samples were obtained in the form of a thick film on pyrex glass sticks and tubes and were used as catalysts in the gas phase photo-oxidation of p-xylene. The physico-chemical characteristics of the catalysts were determined using the methods of Brunauer–Emmett–Teller adsorption, x-ray diffraction, and infrared, ultraviolet and visible and Raman spectroscopies. The experimental results show that the introduction of V did not expand the region of light absorption, but slightly reduced the size of the TiO2 particles, and reduced the number of OH-groups, which should decrease the photocatalytic activity and efficiency of the obtained catalysts compared to those of pure TiO2. The Fe-doped TiO2 samples, in contrast, are characterized by an extension of the spectrum of photon absorption to the visible region with wavenumbers λ up to 464 nm and the values of their band gap energy decreased to lower quantities (up to 2.67 eV), therefore they should have higher catalytic activity and conversion efficiency of p-xylene in the visible region than the original sample. For these catalysts, a combined utilization of radiation by ultraviolet (λ = 365 nm) and visible (λ = 470 nm) light increased the activity and the yield in p-xylene conversion by a factor of around 2–3, as well as making these quantities more stable in comparison with those of TiO2–P25 Degussa.

Reduction of graphene oxide through a green and metal-free approach using formic acid

The present work reports on the production of reduced graphene oxide (GO) by the chemical reduction of GO using formic acid. The process involved is simple, environmentally friendly, low cost and metal free. The structural and electrical characterization ascertains that the quality of the material improves with the time of reduction. To compare the effect of reduction time, three samples are prepared for 18, 24 and 30h respectively. The samples produced are characterized to confirm the reduction of GO and formation of reduced GO (FRGO) by high-resolution transmission electron microscopy, field emission scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction pattern, ultraviolet visible spectroscopy and Raman spectroscopy. Among the three samples, FRGO-3 prepared for reduction time of 30h shows a good crystalline behavior and the highest electrical conductivity (11.859S/cm) at room temperature. This value is comparable with other reported values. Further, from thermo-gravimetric analysis reasonable thermal stability for FRGO-3 is observed in the temperature range 400–800°C. Based on the above observations a mechanism of reduction from GO to reduced GO by formic acid (FRGO) is proposed.

Ultraviolet Raman spectroscopy of GaN/AlN core-shell nanowires: Core, shell, and interface modes

The vibrational properties of GaN/AlN core-shell nanowire heterostructures with shell thicknesses ranging from 0.5 to 12 nm have been investigated by means of Raman spectroscopy with ultraviolet and visible excitation. The selection of an excitation line close to the GaN band gap allows the study of the strain state of the GaN core, while excitation at higher energies activates modes localized at the core-shell interface and the AlN shell, which are as well strain dependent. The results are compared with calculations of the vibrational modes based on the dielectric continuum model, including strain through the continuum elasticity theory.

Characterization of thin polycrystalline silicon films deposited on glass by CVD

We deposited polycrystalline silicon (poly-Si) thin films on commercial float glass by chemical vapour deposition from trichlorosilane at temperatures between 735 and 870 °C. The structural properties of the films were evaluated by means of scanning electron microscopy, x-ray diffraction, atomic force microscopy, reflectance in the ultraviolet region and Raman spectroscopy. The electrical characterization involved measurements of dark conductivity and photoconductivity as a function of temperature, Hall effect, ambipolar diffusion length from the steady-state photocarrier grating technique and density of defects by means of modulated photoconductivity. By using boron tribromide as a doping agent, degrees of doping ranging from intrinsic to clearly p-doped were obtained. The process, the reactants and the substrate used are of low cost, and proved to be adequate for direct poly-Si deposition, giving films of good structural and electrical properties.

Ultraviolet (UV) Raman Spectroscopy Study of the Soret Effect in High-Pressure CO2–Water Solutions

Spatially resolved deep-ultraviolet (UV) Raman spectroscopy was applied to solutions of CO(2) and H(2)O or D(2)O subject to a temperature gradient in a thermally regulated high-pressure concentric-tube Raman cell in an attempt to measure a Soret effect in the vicinity of the critical point of CO(2). Although Raman spectra of solutions of CO(2) dissolved in D(2)O, at 10 MPa and temperatures near the critical point of CO(2), had adequate signal-to-noise and spatial resolution to observe a Soret effect with a Soret coefficient with magnitude |S(T)| > 0.03, no evidence for an effect of this size was obtained for applied temperature gradients up to 19 °C. In contrast, the concentration of CO(2) dissolved in H(2)O was shown to vary significantly across the temperature gradient when excess CO(2) was present, but the results could be explained simply by the variation in CO(2) solubility over the temperature range and not by kinetic factors. For mixtures of D(2)O dissolved in scCO(2) at 10 MPa and temperatures close to the critical point of CO(2), the Raman peaks for D(2)O were too weak to measure with confidence even at the limit of D(2)O solubility.