Raman Peak Intensity Research Articles

Spectroscopic Visualization of Grain Boundaries of Monolayer Molybdenum Disulfide by Stacking Bilayers.

Polycrystalline growth of molybdenum disulfide (MoS2) using chemical vapor deposition (CVD) methods is subject to the formation of grain boundaries (GBs), which have a large effect on the electrical and optical properties of MoS2-based optoelectronic devices. The identification of grains and GBs of CVD-grown monolayer MoS2 has traditionally required atomic resolution microscopy or nonlinear optical imaging techniques. Here, we present a simple spectroscopic method for visualizing GBs of polycrystalline monolayer MoS2 using stacked bilayers and mapping their indirect photoluminescence (PL) peak positions and Raman peak intensities. We were able to distinguish a GB between two MoS2 grains with tilt angles as small as 6° in their grain orientations and, based on the inspection of several GBs, found a simple empirical rule to predict the location of the GBs. In addition, the large number of twist angle domains traced through our facile spectroscopic mapping technique allowed us to identify a continuous evolution of the coupled structural and optical properties of bilayer MoS2 in the vicinity of the 0° and 60° commensuration angles which were explained by elastic deformation model of the MoS2 membranes.

Swift Heavy Ion Induced Optical and Electronic Modifications of Graphene–TiO2 Nanocomposites

The effect of swift heavy ions irradiation on optical and electronic properties of chemically synthesized graphene–TiO2 nanocomposites is presented. Modification of surface properties of these nanocomposites by irradiation with three different ions and with varying fluence was analyzed by Raman spectroscopy, transmission electron microscopy, and scanning Kelvin probe microscopy techniques. Raman spectra of irradiated samples exhibit systematic changes in the characteristic peaks of both graphene and TiO2. The nanocrystallite dimension calculated from Raman peak intensity decreases with fluence, indicating the occurrence of peripheral fragmentation. Furthermore, measurement of the surface contact potential difference using scanning Kelvin probe reveals that the work function of graphene–titanium dioxide nanocomposites can be effectively increased by more than 1 eV.

Reversible Piezofluorochromic Property and Intrinsic Structure Changes of Tetra(4-methoxyphenyl)ethylene under High Pressure.

During the past decade, luminescent mechanochromism has received much attention. Despite the garnered attention, only a few studies have reported the effect of internal molecular structure change on the performance of mechanochromic fluorescence. Here, we chose tetra(4-methoxyphenyl)ethylene (TMOE) as a model molecule to study the correlation between structure and fluorescence property under a hydrostatic pressure produced by a diamond anvil cell (DAC). TMOE is a methoxy-substituted tetraphenylethylene (TPE) derivative and has a nearly centrosymmetric structure and a natural propeller shape. Ultraviolet-visible absorption and fluorescence spectra of TMOE and TPE in solution proved that the presence of methoxy groups in TMOE is responsible for the difference in fluorescence emissions of TMOE and TPE. Under a hydrostatic pressure, the in situ fluorescence spectra of TMOE at different concentrations show that the fluorescence intensity gradually weakens, accompanied by an obvious redshift. The Raman peak intensities decrease gradually, and the peaks disappear eventually with the pressure increasing. These spectral changes are attributed to the changes in the intramolecular conformation, that is, the strengthening of the weak C-H···O hydrogen bonds in TMOE molecules, which is caused by the twisted dihedral angle between the benzene ring and the carbon rigid plane of ethylene. Density functional theory simulation further confirms that the decreased dihedral angle could weaken Raman peak intensity, which is consistent with our experimental results.

Relation between Raman frequency and triaxial stress in Si for surface and cross-sectional experiments in microelectronics components

This paper provides a detailed description explaining how to calculate the relation between the silicon Raman frequency and local stress or strain in the silicon, applied to stress measurements in microelectronics. This relation is well known for measurements from the (100) surface of silicon. However, it is often used in the wrong way, neglecting non-zero stress tensor elements. Especially, in current 3D microelectronics technology, where the stress caused by through Si vias or micro-bumps is of large importance, the vertical stress component, which highly affects the measured Raman frequency shift, is often erroneously neglected. In addition, the equations for the (100) surface are also often used incorrectly for cross-sectional measurements from a (110) surface. In this paper, different ways to calculate the relation between Raman frequency and triaxial stress, and the related Raman peak intensities, are discussed in detail.

Study of the spectro‐chemical signatures of cobalt–manganese layered oxides (asbolane–lithiophorite and their intermediates) by Raman spectroscopy

In this study, Raman microspectroscopy is applied to investigate two manganese oxide phases: lithiophorite [(Al,Li)Mn4+O2(OH)2] and asbolane [(Ni,Co)xMn4+(O,OH)4·nH2O], along with their intermediates (‘asbolane–lithiophorite intermediates’: ALI). These oxides typically incorporate variable concentrations of Co, Ni, Cu and Li. They represent a group of economically interesting phases that are difficult to identify and characterize with classical X‐ray diffraction techniques. We show that Raman micro spectroscopy is useful to the investigation of those phases, but they require to be tested in very low laser power conditions to avoid sample degradation (e.g. 0.2 mW with λ = 532 nm). We propose reference Raman spectroscopic signatures for lithiophorite, asbolane and ALI phases. These spectra are mainly composed of two spectral domains, the first one is located between 370 and 630 cm−1 and the second one between 900 and 1300 cm−1. We then assess the impact of their highly variable chemistry on their Raman peak positions, intensities and FWHM using a semi‐systematic curve‐fitting method profiled for these phases. Copyright © 2015 John Wiley & Sons, Ltd.

Thermal behavior and the evolution of char structure during co-pyrolysis of platanus wood blends with different rank coals from northern China

Abstract Co-thermochemical conversion of biomass and coal has been extensively investigated as an effective means to reduce the emission of greenhouse gas. Successful evaluation on the thermal behavior and product characteristics of the co-pyrolysis process is of crucial importance for predicting performance and improving efficiency of this technology. In this study, thermal behavior of platanus wood, two different rank coals (Pingzhuang lignite and Shenmu bituminous) and their blends during pyrolysis were studied via thermogravimetric analyzer. Raman spectroscopy technique was further employed to study the evolution of char structure. Thermogravimetric experiments indicated that the pyrolysis characteristics of their blends cannot be accurately forecasted based on individual component and blending ratio. The non-additive behavior of thermogravimetric curves of the blends suggested the existence of synergistic effects, which can promote the volatiles yields during the co-pyrolysis process. The most obvious synergistic effects were observed when the coal blending ratios were 30% for Pingzhuang lignite and 50% for Shenmu bituminous, respectively. In addition, iso-conversional method was used to solve activation energy. Non-additivity performance in the mean activation energy values of the blends was observed. The Raman results revealed that as the platanus wood blending ratio increased the Raman peak intensities of the char samples from pyrolysis of the blends also increased, which was probably attributed to the competition of oxygen-containing groups increment and disproportional impact of alkali and alkaline-earth metal species. The intensity ratios of some major Raman bands can provide useful information on char structure evolution. The generation of smaller aromatic rings structures (3–5 fused rings) was favored in these char samples with the higher platanus wood blending ratio.

Transition from a nanocrystalline phase to an amorphous phase in In-Si-O thin films: The correlation between the microstructure and the optical properties

We investigated the structural and optical properties of In-Si-O thin films as the phase abruptly changes from nanocrystalline (nc) to amorphous (a) with increasing Si content. In-Si-O thin films were deposited on Si substrate using a co-sputtering deposition method. The RF power of the In2O3 target was fixed at 100 W, while the power applied to the SiO2 target was varied between 0 W and 60 W. At the Si = 2.8 at. %, i.e., at the onset of amorphous phase, the optical properties, including the dielectric functions, optical gap energies, and phonon modes, changed abruptly which were triggered by changes in the crystallinity and surface morphology. X-ray diffraction (XRD) spectra showed crystalline (c-) In2O3-like peaks below Si = 2.2%. Additionally, a broad peak associated with an amorphous (a-) In2O3 phase appeared above 2.8%. However, the Raman spectra of In-Si-O showed very weak peaks associated with c-In2O3 below 2.2%, and then showed a strong Raman peak associated with a-In-Si-O above 2.8%. X-ray photoelectron spectroscopy measurements showed that oxygen vacancy-related peak intensities increased abruptly above Si = 2.8%. The contrasting results of XRD and Raman measurements can be explained as follows: first, the large enhancement in Drude tails in the a-In-Si-O phase was caused by Si-induced amorphization and a large increase in the density of oxygen vacancies in the In-Si-O thin films. Second, the apparently drastic increase of the Raman peak intensity near 364 cm−1 (for amorphous phase, i.e., above Si = 2.8%) is attributed to a disorder-activated infrared mode caused by both the amorphization and the increase in the oxygen vacancy density in In-Si-O thin films.

Development of time-domain differential Raman for transient thermal probing of materials.

A novel transient thermal characterization technology is developed based on the principles of transient optical heating and Raman probing: time-domain differential Raman. It employs a square-wave modulated laser of varying duty cycle to realize controlled heating and transient thermal probing. Very well defined extension of the heating time in each measurement changes the temperature evolution profile and the probed temperature field at μs resolution. Using this new technique, the transient thermal response of a tipless Si cantilever is investigated along the length direction. A physical model is developed to reconstruct the Raman spectrum considering the temperature evolution, while taking into account the temperature dependence of the Raman emission. By fitting the variation of the normalized Raman peak intensity, wavenumber, and peak area against the heating time, the thermal diffusivity is determined as 9.17 × 10(-5), 8.14 × 10(-5), and 9.51 × 10(-5) m(2)/s. These results agree well with the reference value of 8.66 × 10(-5) m(2)/s considering the 10% fitting uncertainty. The time-domain differential Raman provides a novel way to introduce transient thermal excitation of materials, probe the thermal response, and measure the thermal diffusivity, all with high accuracy.

Raman spectroscopic characterization of the core-rim structure in reaction bonded boron carbide ceramics

Raman spectroscopy was used to characterize the microstructure of reaction bonded boron carbide ceramics. Compositional and structural gradation in the silicon-doped boron carbide phase (rim), which develops around the parent boron carbide region (core) due to the reaction between silicon and boron carbide, was evaluated using changes in Raman peak position and intensity. Peak shifting and intensity variation from the core to the rim region was attributed to changes in the boron carbide crystal structure based on experimental Raman observations and ab initio calculations reported in literature. The results were consistent with compositional analysis determined by energy dispersive spectroscopy. The Raman analysis revealed the substitution of silicon atoms first into the linear 3-atom chain, and then into icosahedral units of the boron carbide structure. Thus, micro-Raman spectroscopy provided a non-destructive means of identifying the preferential positions of Si atoms in the boron carbide lattice.

Characteristics of porous GaN prepared by KOH photoelectrochemical etching

This paper presents the structural and optical studies of porous GaN prepared by photoelectrochemical etching in diluted KOH under different etching durations. Such etching technique is a cheaper and simpler way of fabricating porous structure. The as-grown and porous GaN samples were investigated by field emission scanning electron microscopy, high resolution X-ray diffraction and Raman spectroscopy. Field emission scanning electron microscopy measurement allowed us to observe the morphology of the porous samples, and we found that the pore density increased with etching duration. From high resolution X-ray diffraction characterisation, the width at half maximum value of the rocking curve at (0002) and (102) plane decrease for all porous samples because of the reduction in dislocation density. Furthermore, as compared to the as-grown sample, the porous samples exhibit a significant enhancement of Raman peak intensity because of multiple scattering of light from the GaN crystallite sidewalls. Overall, this study shows that the surface nanostructure could improve the structural and optical properties of GaN. This is an initial demonstration to show the prospect of the porous GaN structure in optical and sensor applications.

Surface Enhanced Raman Scattering Effect of Au-Ag Alloy Films Based on Kretschmann Configuration†

The surface enhanced Raman scattering(SERS) effect of Au-Ag alloy films was investigated,using a 532 nm laser source and the Kretschmann configuration. The experimental results indicate that when the incidence angle of the p-polarized beam meets the surface plasmon resonance(SPR) condition,the intensities of Raman peaks for Nile Blue molecules immobilized on the Au-Ag alloy film reaches maxima,being almost 2times as high as those achieved with the pure gold layer. Gold nanoparticles(GNP) modification of the Au-Ag alloy films resulted in more than 3 times increase in Raman peak intensity. With the GNP-modified Au-Ag alloy film the Raman peaks are 2 times higher than those with the GNP-modified pure gold layer. The Raman signals detected with the probe placed below the prism substrate are completely non-polarized,but those detected along the reflected light beam on the side of the prism are p-polarized due to the SPR-coupled directional emission of Raman light excited in the evanescent-field region. The work demonstrated that the Au-Ag alloy films are advantageous over the conventional gold films for surface plasmon enhanced Raman spectroscopy.

Recyclable three-dimensional Ag nanoparticle-decorated TiO2 nanorod arrays for surface-enhanced Raman scattering

Multifunctional Ag nanoparticle-decorated TiO2 nanorod arrays were prepared by two simple processes. TiO2 nanorod arrays were first fabricated by the hydrothermal route and then Ag nanoparticles were decorated on the nanorods by the chemical reduction impregnation method. Three-dimensional Ag/TiO2 arrays were used as an active substrate for surface-enhanced Raman scattering (SERS). The results show that the detection limit for rhodamine 6G (R6G) was as low as 10−7M and the Raman enhancement factor was as large as 105. After calibrating the Raman peak intensities of R6G, it could be quantitatively detected. More importantly, the photocatalytic activity of TiO2 provides a self-cleaning capability to the SERS substrate, which can be recycled and used to degrade many Ag surface adsorbates such as R6G, methyl orange, Congo red, and methylene blue after exposure to visible light. The absorbed small molecules can all be rapidly and completely removed from the SERS substrate, which has been successfully reused four times without a decrease in accuracy or sensitivity. Our results reveal that the unique recyclable property not only paves a new way to solve the single-use problem of traditional SERS substrates but also provides more SERS platforms for multiple detections of other organic molecular species.

Quantitative Raman spectroscopic investigation of geo-fluids high-pressure phase equilibria: Part I. Accurate calibration and determination of CO2 solubility in water from 273.15 to 573.15 K and from 10 to 120 MPa

We developed an accurate method to calibrate Raman system for measuring the concentrations of dissolved gases in aqueous geo-fluids at high pressure, and determined CO2 solubility in water at temperatures from 273.15 to 573.15K and pressures from 10 to 120MPa. The Raman peak area ratio (PAR) and peak intensity ratio (peak height ratio, HR) of the upper band (at ∼1390cm−1) of CO2 Fermi dyad to the OH stretching band of water in homogeneous CO2 solutions were obtained over wide temperature, pressure, and concentration ranges. Pressure and concentration (mCO2) show little effects on PAR/mCO2 and HR/mCO2, while PAR/mCO2 increases linearly with temperature from 273.15 to 573.15K, and HR/mCO2 decreases linearly with temperature at a different rate at temperatures below and above 404.45K. The solubility data obtained from 298 to 523.15K are consistent with previous experimental and thermodynamic studies. However, predictions from commonly-used thermodynamic models for temperatures below 298.15K and above 523.15K substantially deviate from the new measurements, suggesting that new or refined parameters are needed to improve these models.

Optoelectronic properties of polythiophene thin films and organic TFTs fabricated by oxidative chemical vapor deposition

Fabrication of devices utilizing unsubstituted polythiophene (PT) has received limited attention because thin films of this insoluble material have been difficult to prepare using traditional solution-based methods. However, since the thiophene monomer is volatile, PT films can be readily fabricated by oxidative chemical vapor deposition (oCVD). The pressure of the oCVD process is here shown to significantly affect the optoelectronic properties of the PT films. Shifts in the Raman bands near 1500 cm−1 suggest that the lowest pressure deposition conditions (1 mTorr) result in shorter conjugation. Additionally, relative Raman peak intensities suggest that the polymer chains in these films contain more distortions than films deposited at higher pressures. UV-Vis absorption spectra of the oCVD PT films show that films deposited at the lowest pressure considered (1 mTorr) are slightly blue-shifted and demonstrate lower absorption than films deposited at higher pressures. PT films deposited at 75, 150, and 300 mTorr demonstrated similar UV-Vis absorption spectra, with absorption maxima near 515 nm. Organic thin film transistors (TFTs) were fabricated using oCVD PT as the active channel layer to analyze the effect of deposition pressure on charge transport and electrical properties. Films deposited at 150 mTorr demonstrate the greatest field effect carrier mobility of 4 × 10−3 cm2 V−1 s−1.

Si/ZnO nanocomb arrays decorated with Ag nanoparticles for highly efficient surface-enhanced Raman scattering.

High-density ZnO nanocombs were first grown on a nanoporous silicon pillar array, and pre-prepared 3D Si/ZnO/Ag nanocomb arrays were employed as substrates for surface-enhanced Raman scattering (SERS). The finite-difference time-domain simulation result shows that two kinds of inter-Ag-NP nanogaps in the geometry create a large number of SERS "hot spots," which contributes to the detection limits for rhodamine-6G as low as 10⁻¹² M and the Raman enhancement factor as large as 10⁹. The linear dependence between the Raman peak intensities and the concentrations of thiram provides a new calibration method for rapid and quantitative detection of trace organic molecules.

Recyclable surface-enhanced Raman scattering template based on nanoporous gold film/Si nanowire arrays

Nanoporous gold film (NPGF) composed of gold nanoparticles was used to cover a large Si nanowire array (SiNWA) by simple metal-assisted chemical etching and metal reduction processes. Three-dimensional SiNWA/NPGF was employed as an active substrate for surface-enhanced Raman scattering (SERS). The results show that the detection limit for crystal violet was as low as 10−12M, and the Raman enhancement factor was as large as 107 with a relative standard deviation of less than 20%. After calibrating the Raman peak intensities of crystal violet and thiram could be quantitatively detected. More importantly, the SERS substrates are recyclable and can be used for many gold surface adsorbates such as p-aminothiophenol, crystal violet, Rhodamine 6G, and methyl orange, which can all be rapidly and completely removed from the NPGF surface. Our findings are an important advance in SERS substrates and will allow the quantitative and recyclable detection of trace organic contaminants.

Aluminum doped ZnO sol–gel derived nanocrystals: Raman spectroscopy and solid solubility characterization

In this study, a sol–gel method was used to synthesize Al‐doped ZnO nanocrystals (AZO). For preparation of this highly concentrated sol, zinc acetate concentration was fixed at 2M. UV‐visible absorption and photoluminescence spectroscopy were used to evaluate optical properties of the prepared samples. High temperature annealing was employed to evaluate the solid solubility limit of Al ions. XRD and Raman spectroscopy are then used to evaluate the doping process of aluminum in ZnO nanopowders. Disappearing of the second order Raman peak of ZnO at 1160 cm−1 was used to confirm the solid solubility limit. It can be found that the disappearing of second order Raman peak intensity at 1160 cm−1 is observed for sample with 2 at% Al3+ ion. Experimental results have shown that the solid solubility limit of aluminium ions in sol–gel derived zinc oxide nanocrystals was achieved.

SERS substrates of doped germanium nanowires decorated with silver nanoparticles

Abstract The microstructure and SERS activity of Ag-decorated Ge nanowires (NWs) doped with boron (B) or phosphorous (P) were investigated. The silver nanoparticles (AgNPs) distribution observed for the both types of doped nanowires differed significantly, as well as before and after the annealing procedures. The deposited strong-packed nonspherical Ag nanoparticles favor the Raman surface enhancement compared to spherical nanoparticles distributed along the nanowires with a larger interparticle distances after annealing. The SERS activity of the 3D nanocomposites is also influenced by the initial shape and the diameter of the nanowires and by the surface effects. The strongest SERS signal of Rhodamine 6G (R6G) was registered for the sample doped with P before annealing. Unlike the 2D AgNPs array the thermal annealing at 300 °C for 1 h destroyed the Raman activity of the B-doped composites and reduced the Raman peak intensities of the P-doped one.

Cicada wing decorated by silver nanoparticles as low-cost and active/sensitive substrates for surface-enhanced Raman scattering

A green, low-cost and highly efficient surface-enhanced Raman scattering (SERS) substrate was achieved by a chemical deposition of silver nanoparticles on a cicada wing, which has the large-scale nanosized protrusions on its surface. Employing the already-formed Ag/cicada wing as substrate for SERS detection, the detection limit for rhodamine 6G could reach 10−7M, the Raman enhancement factor of the substrate was as large as 106 and the relative standard deviation remains lower than 7%. The three-dimensional finite-difference time-domain simulation results showed that two types of inter-Ag-nanoparticle nanogaps in the formed geometry created a huge number of SERS “hot spots” where the electromagnetic field is substantially amplified and contributes to the higher SERS sensitivity. Meanwhile, the water contact angle of the SERS substrate is roughly 150°, which indicates the super-hydrophobic surface of the substrate. This feature may be conducive to the gathering of target molecules during the SERS detection, which in turn further improves the detection limit of target molecules. In order to improve the application of the substrate, thiram was used as the probe molecule, and the detection limit also reached 10−7 M. Meanwhile, the calibration of the Raman peak intensities of Rhodamine 6G and thiram allowed their quantitative detection. Therefore, the green and low-cost SERS substrates could be used for fast and quantitative detection of trace organic molecules. Our findings may contribute to the development of the green and low-cost SERS substrates and will allow the fast and quantitative detection of trace organic molecules.

Pressure and temperature dependence of the Raman peak intensity ratio of asymmetric stretching vibration (ν₃) and asymmetric bending overtone (2ν₂) of methane.

Raman peaks of the asymmetric stretching vibration (ν3) and the asymmetric bending overtone (2ν2) of methane were studied at elevated pressures and temperatures, from 3 to 51 MPa and from 298.15 to 473.15 K. The peak intensity ratios of ν3 and 2ν2 were calculated, and the relationship among peak intensity ratio, temperature, and pressure/density were derived using equations. Such relationships allow geologists to determine the pressure and density of methane fluid inclusions using Raman spectroscopic measurements of the peak intensity ratios of ν3 and 2ν2.