Excitation energy-dependent nature of Raman scattering spectrum in GaInNAs/GaAs quantum well structures

The excitation energy-dependent nature of Raman scattering spectrum, vibration, electronic or both, has been studied using different excitation sources on as-grown and annealed n- and p-type modulation-doped Ga_1 − xIn_xN_yAs_1 − y/GaAs quantum well structures. The samples were grown by molecular beam technique with different N concentrations (y = 0%, 0.9%, 1.2%, 1.7%) at the same In concentration of 32%. Micro-Raman measurements have been carried out using 532 and 758 nm lines of diode lasers, and the 1064 nm line of the Nd-YAG laser has been used for Fourier transform-Raman scattering measurements. Raman scattering measurements with different excitation sources have revealed that the excitation energy is the decisive mechanism on the nature of the Raman scattering spectrum. When the excitation energy is close to the electronic band gap energy of any constituent semiconductor materials in the sample, electronic transition dominates the spectrum, leading to a very broad peak. In the condition that the excitation energy is much higher than the band gap energy, only vibrational modes contribute to the Raman scattering spectrum of the samples. Line shapes of the Raman scattering spectrum with the 785 and 1064 nm lines of lasers have been observed to be very broad peaks, whose absolute peak energy values are in good agreement with the ones obtained from photoluminescence measurements. On the other hand, Raman scattering spectrum with the 532 nm line has exhibited only vibrational modes. As a complementary tool of Raman scattering measurements with the excitation source of 532 nm, which shows weak vibrational transitions, attenuated total reflectance infrared spectroscopy has been also carried out. The results exhibited that the nature of the Raman scattering spectrum is strongly excitation energy-dependent, and with suitable excitation energy, electronic and/or vibrational transitions can be investigated.     

Experimental methods in chemical engineering: Raman spectroscopy

When photons impinge on a substrate, most scatter with the same frequency (elastic scattering or Rayleigh dispersion) and only 10^?7 scatter with a different energy (inelastic scattering). This inelastic interaction (Raman scattering) exchanges energy in the region of molecular vibrational transitions for crystalline and amorphous materials. Raman bands in a spectra represent vibrational transitions, like infrared, however the selection rules are different. Typically, the vibrations that are intense in Raman are weak in infrared and vice versa. A remarkable feature of the Raman effect is that it is highly sensitive to nanocrystals, even below 4 nm, which are too small to generate XRD patterns. Plasmonic enhancement, like surface‐enhanced Raman spectroscopy (SERS) boost the Raman signal by 10⁴, providing single‐molecule detection capability. Glass, quartz, and sapphire are transparent to Raman effect (depending on the energy of the incident excitation radiation), which makes it ideal to examine materials under reaction conditions (in‐situ cells and operando reactors that operate over a broad range of temperature, pressures, and environments). Raman spectroscopy emerged in the 1930s; however, infrared spectrometry displaced it. With the advent of powerful lasers in the 1970s, more researchers began to apply Raman routinely. In 2019, the Web of Science indexed 20 400 articles mentioning Raman against 50 000 articles mentioning infrared. Chemical engineers apply Raman less frequently than in material science, physical chemistry, and applied physics, with 887 articles vs 6250, 3700, and 3510 for the other disciplines. A bibliometric analysis identified four research clusters centred on thin films and optics, graphene and nanocomposites, nanoparticles and SERS, and photocatalyst.     

Doping level change of polyaniline film during its electrochemical growth process

Polyaniline (PANI) films doped with hydrochloric acid have been electrochemically deposited on mirrorlike platinum electrode surfaces by a cyclic voltammogram method. The Raman spectra of doped PANI films were investigated by excitation with a 633‐ or 785‐nm laser beam. It was found that the overall features of the Raman spectra depend strongly on the film thickness, due mainly to that the doping level of PANI film increases during the film‐growth process. X‐ray photoelectron spectroscopic (XPS) analysis and ultraviolet (UV)‐visible absorption spectrum results also confirmed this finding. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 171–177, 2004     

Synthesis and properties of polydiacetylene-containing polyesters

A series of diacetylene-containing polyesters with number-average molar masses (GPC) in the range 900–4200 g mol⁻¹ were prepared from terephthaloyl chloride and hexa-2,4-diyne-1,6-diol using benzoyl chloride as a monofunctional reactant for control of molar mass. Degrees of crystallinity were estimated from WAXD to be up to 29%. Correlations between molar mass, melting behaviour, degree of crystallinity and thermal cross-polymerisation of diacetylene-containing polyesters have been established using hot-stage microscopy, DSC and resonance Raman spectroscopy. The polyester with M̄_n of 1264 g mol⁻¹ gave the best balance between processability and the ability to cross-polymerise efficiently. Its degree of crystallinity before cross-polymerisation was estimated from WAXD measurments to be 24%, a value coincident with the percentage conversion of diacetylene units to polydiacetylene chains measured by ¹³C solid-state NMR. The optimum conditions for compression moulding the polyester to produce a material with a strong Raman spectrum involved heating under vacuum at 120°C for 6h after an initial 3 h heat-up period. The material thus produced gave an intense Raman CC stretching band, which upon tensile deformation shifted linearly with strain to lower wavenumber by 12.0 cm⁻¹ %⁻¹. The potential use of the diacetylene-containing polyesters in the preparation of model blends for use in quantitative micromechanics studies of stress transfer between phases is briefly discussed.     

Raman spectroscopic studies of nomex and kevlar fibres under stress

The vibrational Raman spectra of single fibres of Nomex and Kevlar have been studied and vibrational assignments made. Commercial samples of Kevlar 29 and Kevlar 49 were found to be identical spectroscopically. Distinguishing features in the Raman spectra of Kevlar and of Nomex were identified for the purpose of spectroscopic or microprobe characterisation of these materials as single fibres or as small particles (～ 2 μm). The Raman spectra of Kevlar and Nomex fibres which had been subjected to mechanical stresses of up to 4.5 kg mm^?2 were also analysed and comparisons made with previous results on stressing experiments in the literature for Kevlar.     

Quantitative evaluation of sp/sp hybridization ratio in cluster-assembled carbon films by in situ near edge X-ray absorption fine structure spectroscopy

We present a near edge X-ray absorption fine structure spectroscopy characterization of nanostructured carbon films containing carbynoid species. By a careful data analysis and normalization of the spectra at the carbon K-edge we have quantitatively evaluated the extent of valence sp hybridization of the films. A sp/sp² ratio between 10% and 25% has been obtained. This result allowed the evaluation of the ratio between the sp and sp² Raman cross section at different excitation laser wavelengths.     

Vibrational properties of nitrogen-doped ultrananocrystalline diamond films grown by microwave plasma CVD

Ultrananocrystalline diamond (UNCD) films grown in an argon-rich Ar/CH₄/H₂ microwave plasma with nitrogen gas added in amounts of 0%–20% were studied by Raman spectroscopy with multiple excitation wavelengths in the range of 244–647 nm and by optical absorption in UV–visible. The Raman spectra have demonstrated the presence of diamond, amorphous carbon and polyacetylene in the UNCD films. Analysis of vibrational and optical properties of amorphous carbon phase proves that nitrogen stimulates the transition from amorphous carbon into an ordered graphite-like structure with narrowed optical band gap, which is supposed to be responsible for the high electrical conductivity of the N-doped UNCD.     

Subpicosecond solvent dynamics in charge-transfer transitions: challenges and opportunities in resonance Raman spectroscopy

Resonance Raman spectroscopy is an established tool for the determination of structure and dynamics in electronically excited states. In condensed-phase systems, Raman excitation profiles and electronic absorption spectra depend on changes in molecular geometry and solvation structure induced by electronic excitation. Recent studies of solvent isotope effects on resonance Raman intensities in charge-transfer excitations reveal solvent dynamics taking place on a subpicosecond time scale and vibrational mode-specific solute-solvent interactions. These discoveries present challenges to the current working theories for analysis of resonance Raman and absorption spectra.     

Raman spectroelectrochemical study of self-doped copolymers of aniline and selected aminonaphthalenesulfonates

Novel self-doped polyaniline-like copolymers have been prepared by electrochemical copolymerization of aniline with four aminonaphthalenesulfonates. All copolymer films prepared show their electrochemical redox activity even in pH-neutral solutions at a midpoint potential around 0.0 V versus Ag/AgCl. Raman spectroelectrochemical study of the copolymers prepared has been done with a red laser excitation (632.8 nm) within a broad electrochemical potential window of 0.0-1.0 V, and specific Raman features have been identified. Raman bands within the range of 1300-1400 cm⁻¹ have been discussed regarding localized or delocalized polaronic ν_s(CN⁺) vibrations. The influence of sulfonate group position in aminonaphthalenesulfonates on the parameters of polaronic bands has been demonstrated.     

Control of polymerization processes of 10,12-pentacosadiynoic acid LB films

Studies have been carried out on KrF excimer laser light (EX), X-ray or electron beam (EB) induced polymerization of 10,12-pentacosadiynoic acid (PDA) Langmuir-Blodgett (LB) films in relation to molecular density or molecular arrangement of the films using X-ray diffraction analysis, infrared (IR) spectroscopy and Raman spectroscopy. The molecular arrangement or density of the PDA LB films was controlled by subphase conditions when the films were built up, such as pH, temperature of a subphase or salt concentration in the subphase. Polymerization sensitivity of the PDA LB film was affected by the arrangement or molecular density. On low density (A type) films the polymerization occurred by irradiation with EX, X-ray or EB, but on high density (B type) films the polymerization occurred only when the irradiation was carried out by high energy beams such as X-ray or EB. Decomposition of polymerized films was observed further by excessive irradiation of EX or X-ray, but not on the B type films. It was revealed by X-ray diffraction analysis that in the A type film, the PDA molecules bent to a larger extent than those in the B type film and the polymerization proceeded topochemically, that is, the thickness decreased little after EB irradiation in a helium atmosphere. On the other hand, in the B type film, the thickness decreased by about 10% as a result of EB irradiation. By IR reflection-absorption (RA) and Raman measurements, it was confirmed that conjugated diacetylenic bonds disappeared and conjugated double and new conjugated triple bonds appeared after high energy beam irradiations. These results support the supposition that 1,4-polymerization, i.e. polydiacetylene type polymerization, occurs easily in the A type film and 1,2- or 3,4-polymerization, i.e. polyacetylene type polymerization, occurs in the B type film, and that the polymerized A type film was decomposed at the polydiacetylenic bond when the irradiation continued further. It was also shown that the polyacetylene type polymer was obtained only when the B type film was irradiated with the high energy beam.     

Studying cubic boron nitride by Raman and infrared spectroscopies

In this paper, optical properties of cubic boron nitride (cBN) in forms of films, powders and monocrystals are studied in vibrational spectral regions. For cBN single crystals, the reststrahlen peak reflectivity of about 95% has been obtained. The second-order Raman scattering spectra induced by visible light excitation is also presented. The infrared two-phonon absorption bands are suggested as additional fingerprints for identification of the cBN nature.     

Structures and morphologies of cast and plastically strained polyamide 6 films as evidenced by confocal Raman microspectroscopy and atomic force microscopy

Both confocal Raman microspectroscopy and atomic force microscopy (AFM) have been undertaken to study the crystalline and the morphological aspects of cast PA 6 films at a sub-microscopic scale. The percentages of the different crystalline structures present within PA 6 cast films, i.e. the monoclinic α, the pseudo-hexagonal β, and the monoclinic γ, have been measured by confocal Raman microspectroscopy. In cast films, the prevailing structure is the β one. AFM has been used to characterize the morphology of the PA 6 films. Simultaneously, the deformed state has been considered as well. Our main interest has been to follow the evolution of the percentage of each crystalline structure as a function of the plastic deformation mechanisms which are responsible of the yielding of PA 6 films: shear banding for temperatures T lower than 160 °C and formation of fibrils for      

Low-temperature deposition of optically transparent diamond using a low-pressure flat flame

The radial uniformity and scaleable nature of flat flames make them an attractive technique for diamond deposition. Due to the high temperatures involved in combustion synthesis, typically molybdenum and silicon have been used as substrates. Here we report low-temperature diamond deposition on glass substrates. Diamond deposition was achieved on ordinary sodium silicate glass at substrate temperatures of 500°C; however, film delamination occurred during cooling after deposition. Vycor™ and Pyrex™ are two glasses that have thermal expansion coefficients that are similar to diamond. Continuous, optically transparent films were successfully deposited on both glasses. The diamond films have been characterized by scanning electron microscopy, Raman spectroscopy and secondary ion mass spectroscopy (SIMS). The dependence of hydrogen and sp²-bonded carbon incorporation in the films on reactant composition was quantified. These films were optically transparent and showed good adhesion as measured by a simple tape test.     

Spatially resolved near-infrared excited Raman spectroscopy of nanocrystalline diamond films

Scanning Raman spectroscopic measurements were performed on nanocrystalline diamond thin films with 0.5 μm lateral steps and excitation spot limited to 1 μm in diameter using 488 nm and 785 nm excitation wavelengths. The comparison of the spectra measured with different excitation energies showed that in contrast to the well-known five bands in the 488 nm excited Raman spectra of nanocrystalline diamond a number of narrow peaks appears in the spectra when using near-infrared excitation. The intensity and position of the latter vary when moving the excitation spot along the sample. The detailed analysis of the sequences showed that the 785 nm excited Raman spectroscopy allows the detection and identification of the Raman peaks arising from individual diamond crystallites of the nanocrystalline diamond films.     

Characterization of d.c.jet CVD diamond films on molybdenum

Diamond films grown using a thermal plasma technique are characterized using a variety of techniques. The relationships between the chemistry, morphology, and mechanical properties are explored using microscopy, Raman spectroscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. The characteristics of films grown using two different nucleation enhancement techniques are shown. Films grown using high methane concentrations at the beginning of growth produce large grained columnar films, whereas films grown on substrates which have been treated with a diamond polishing step show nanocrystalline structures. Variations in sp³ and sp² bonding and peak shifts are tracked through the thickness of the film, corresponding to variations in the methane concentration during growth. Stresses are measured using peak shifts and beam bending techniques. Adhesion is tested using indentations, and is shown to increase both as growth temperatures and surface roughness increase.     

Resonant Raman studies of tetrahedral amorphous carbon films

Resonant Raman scattering has been used to study the tetrahedral amorphous carbon films deposited by the filtered cathodic vacuum arc technique. The excitation wavelengths were 244, 488, 514 and 633 nm, corresponding to photon energies of 5.08, 2.54, 2.41 and 1.96 eV, respectively. In the visible Raman spectra only vibrational modes of sp²-bonded carbon (G and D peaks) are observed, while a wide peak (called the T peak) can be observed at approximately 1100 cm⁻¹ by UV-Raman spectra which is associated with the vibrational mode of sp³-bonded carbon. Both the position and the width of the G peak decrease almost linearly with increasing excitation wavelength, which is interpreted in terms of the selective ππ* resonant Raman scattering of sp²-bonded carbon clusters with various sizes. The G peak position in the UV-Raman spectra, the T peak position and the intensity ratios of I_D/I_G and I_T/I_G all exhibit maximum or minimum values at the carbon ion energy of 100 eV. The changes of these spectral parameters are discussed and correlated with the sp³ fraction of carbon atoms in the films.     

Measurement of Concentration Profiles in Thin Film Binary Polymer-Solvent Coatings Using Confocal Raman Spectroscopy: Free Volume Model Validation

Concentrations of solvent and polymer have been measured using confocal Raman spectroscopy in poly(styrene)-tetrahydrofuran, poly(methyl methacrylate)-tetrahydrofuran, and poly(styrene)-p-xylene systems. Free volume theory parameters have been regressed from the measured concentration data. Model-predicted concentration profiles are in very good agreement with the measured profiles in the case of a highly volatile solvent. For instance, free-volume-model-predicted profiles are in very good agreement in poly(styrene)–tetrahydrofuran system and poly(methyl methacrylate)–tetrahydrofuran system. However, the free volume model is not able to predict entire measured profiles in the case of less-volatile solvent in poly(styrene)–p-xylene system.     

Correlation of vibrational intensity with fluorescence lifetimes in π conjugated polymers

A series of novel pi (π) conjugated polymers, originating from the archetypical Polyphenylene vinylene, in which the phenyl units are successively replaced by the larger naphthyl and anthryl acene units, were previously found to have a well-defined relationship between their relative fluorescence yields and their vibrational characteristics, as determined by Raman spectroscopy. In this study the Strickler-Berg equation is used to probe the influence of continual substitution of higher order acene units into the conjugated backbone in terms of the variation of the radiative and non-radiative rates. The deconvolution of the radiative and non-radiative rates enables the correlation of the reduction of the Raman intensity and concomitant increase in the fluorescence yield with the reduction of the non-radiative rate. This confirms that the reduction of the non-radiative rate is the dominant process introduced by the vibrational confinement originating from systematic substitution of higher order acene units into the polymer backbone.     

Formation and stability of β-structure in biodegradable ultra-high-molecular-weight poly(3-hydroxybutyrate) by infrared, Raman, and quantum chemical calculation studies

Infrared (IR) and Raman spectra were measured for four kinds of ultra-high-molecular-weight poly[(R)-3-hydroxybutyrate] (UHMW-PHB) films: a solvent-cast film, a cold-drawn film, a two-step-drawn film, and a hot-drawn film. Quantum chemical calculations were made for octamer models of UHMW-PHB with a helix conformation (α-form) and a planar zigzag conformation (β-form). Comparison of the results between the Raman spectra of four kinds of films and the quantum chemical calculations of octamer models revealed that only two-step-drawn film yields additional bands at 1735, 966, 935, 908, and 858 cm⁻¹ assignable to the β-structure, suggesting that it contains the β-form as well as the α-form. Detailed comparison of the frequencies and intensities of Raman bands between the observed and calculated values for the β-form indicates that the amount of β-form is relatively small and that the β-structure has a less ordered structure. The infrared and Raman spectra of two-step-drawn film also indicate that it has more amorphous parts than other films. When the two-step-drawn film was further heated up to 130 °C and then cooled down to room temperature, the above additional bands due to the β-form disappeared in the Raman spectra, suggesting that the β-form is less stable than α-form.     

Bi2O3-Ga2O3-CdO系统玻璃的Raman光谱和X射线光电子能谱研究

用X射线光电子能谱和Raman光谱研究了Bi2O3-Ga2O3-CdO系统玻璃的结构，Raman光谱曲线被分离成6个谱带，4条谱带分属于不同键长的Bi-O振动，一条谱带属于Ga-O振动，Bi2O3-Ga2O3二元系统玻璃的Raman散射最强峰位于400－420cm^-1,当Ga^3 被Cd^2 离子取代后，Raman散射最强峰移向595－630cm^-1，随着Ga2O3含量的增加，位于高波数属于Bi-O振动的2条谱带强度降低并朝低波数移动，位于低波数属于Bi-O振动的2条谱带强度增加并朝高波数移动，添加CdO则出现相反的效应，X射线光电子能谱显示出非常低的O1s电子结合能，甚至低于碱硅酸盐玻璃中非桥氧的O1s电子结合能，并且不可能分为桥氧和非桥氧，O1s和Bi4f的电子结合能都随Ga2O3和C dO含量增加而增加。