Second-order Raman Spectra Research Articles

Determination of acoustic phonon anharmonicities via second-order Raman scattering in CuI

We demonstrate the determination of anharmonic acoustic phonon properties via second-order Raman scattering exemplarily on copper iodide single crystals. The origin of multi-phonon features from the second-order Raman spectra was assigned by the support of the calculated 2-phonon density of states. In this way, the temperature dependence of acoustic phonons was determined down to 10 K. To determine independently the harmonic contributions of respective acoustic phonons, density functional theory in quasi-harmonic approximation was used. Finally, the anharmonic contributions were determined. The results are in agreement with earlier publications and extend CuI’s determined acoustic phonon properties to lower temperatures with higher accuracy. This approach demonstrates that it is possible to characterize the acoustic anharmonicities via Raman scattering down to zero-temperature renormalization constants of at least 0.1 cm−1.

Some Optical Properties of LiNbO3:Gd3+(0.003):Mg2+(0.65 wt %) Single Crystal: A Promising Material for Laser Radiation Conversion

A nonlinear optical double-doped single-crystal LiNbO3:Gd:Mg (Gd concentration is 0.003, Mg—0.65 wt % in the crystal) has been researched by several optical methods: laser conoscopy, photoinduced light scattering (PILS), optical spectroscopy, and Raman scattering. The crystal has been shown to have no photorefraction effect and a high optical uniformity. Fine features of the crystal structure have been studied via Raman spectra. Spectra have been registered in the first and second orders, they have been excited by visible (532 nm) and near-IR (785 nm) lasers. Registered Raman spectra have the fundamental vibrations of the crystal lattice of the A1(TO,LO)- and E(TO,LO)-type symmetry located in the range of 150–900 cm−1. A number of low-intensity Raman bands in the 900–2000 cm−1 region have been determined to correspond to the second-order Raman spectrum. These bands are polarized and appear only in certain polarization-scattering geometries. They appear in the spectrum excited by visible radiation, but their number and intensity are much lower than those excited by near-IR lasers. Oxygen-octahedral MeO6 clusters in our case can contain Li, Nb, Gd, or Mg in the Me site. The clusters in the LiNbO3:Gd:Mg crystal structure are slightly distorted compared with similar clusters in the nominally pure LiNbO3 crystal. It has been established that the value R = [Li]/[Nb] in the studied crystal is ≈ 1. Such a ratio usually characterizes a nominally pure stoichiometric crystal.

Thermoelectric properties and scattering mechanisms in natural PbS

X-ray diffraction and energy dispersive x-ray spectroscopic analyses showed a natural galena (PbS) crystal from Freiberg in Saxony (Germany) to be a single phase specimen [rock salt (NaCl) structure type, space group $Fm\overline{3}m, a=5.932(1)$ \AA{}] with stoichiometric composition and an enhanced dislocation density ($\ensuremath{\delta}\ensuremath{\approx}{10}^{11}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$). The latter parameter leads to an increase of the electrical resistivity in the high-temperature regime, as well as to the appearance of phonon resonance with a characteristic frequency ${\ensuremath{\omega}}_{\mathrm{PR}}=3.8(1)$ THz. Being in the same range (i.e., 3--5.5 THz) with the sulfur optical modes of highest group velocities, it results in a drastic reduction (by $\ensuremath{\sim}$ 75%) of thermal conductivity ($\ensuremath{\kappa}$) at lower temperatures (i.e., $<100$ K), as well as in the appearance of a characteristic minimum in $\ensuremath{\kappa}$ at $T\ensuremath{\approx}$ 30 K. Furthermore, the studied galena is characterized by phonon-drag behavior and by temperature dependent switch of the charge carrier scattering mechanism regime (i.e., scattering on dislocations for $T<100$ K, on acoustic phonons for 100 K $<T<170$ K and on both acoustic and optical phonons for 170 K $<T<300$ K). The combined theoretical calculation and optical spectroscopic study confirm this mineral to be a direct gap degenerate semiconductor. The possible origins of the second-order Raman spectrum are discussed.

Graphite thermometry by interactive fitting of Raman spectra

The growth conditions of natural graphite crystals are conventionally estimated by the application of petrological methods on the host rocks. As Raman Spectroscopy records detailed information on the microstructure of a carbon-material, this method is applied empirically to graphite from the southern margin of the Bohemian Massif to investigate the correlation between microstructure and metamorphic peak conditions directly, characterizing an economically important geomaterial. Automated fitting of the second-order Raman-spectrum of graphite assesses the strongly asymmetrical S2-band at ca. 2700 cm−1 by a shape parameter, characterizing the microstructural state of graphite. Applying this concept, a confident parameter (S2-shape) estimates the peak metamorphic temperature of natural graphite crystals between 600 °C and 800 °C. The application in the southern Bohemian Massif maps metamorphic peak temperatures related to Variscan tectonics. Comparing the microstructure of large crystals from graphite seams with tiny crystals dispersed in the rock matrix, there is evidence that small graphite particles are not influenced by a late-stage overprint.

Temperature dependence of Raman scattering in CdO: Insights into phonon anharmonicity and plasmon excitations

We present a temperature-dependent Raman scattering study of CdO in the temperature range between 80 and 500 K. Distinctive features present in the second-order Raman spectra of CdO are identified as overtones and combination modes with the help of density functional perturbation theory calculations. A wide, broad band in the region between 250 and $470 {\mathrm{cm}}^{\ensuremath{-}1}$ is attributed to Raman scattering by charge-density fluctuations in the high-density overdamped electron gas present in the sample. Very good agreement is found between the Raman spectra and the expected scattering arising from the overtone density of states and plasmon excitations. The temperature dependence of the modes yields information about the anharmonic phonon interactions. Four-phonon processes arising from quartic anharmonicity play a significant role in the observed temperature dependence.

Investigation of the Structural Perfection of a LiNbO3:Gd3+(0.003):Mg2+(0.65 wt.%) Double-Doped Single Crystal Using the Raman Spectra Excited by Laser Lines in the Visible (532 nm) and Near-IR (785 nm) Regions

A compositionally homogeneous nonlinear optical single crystal of double-doped LiNbO3:Gd3+(0.003):Mg2+(0.65 wt.%) was obtained. Fine features of the LiNbO3:Gd3+(0.003):Mg2+(0.65 wt.%) crystal structure were studied from the Raman spectra of the first and second orders upon excitation by laser lines in the visible (532 nm) and near-IR (785 nm) regions. When the Raman spectrum was excited by a 785 nm laser line in the frequency range of 1000–2000 cm−1 for the first time, a number of low-intensity lines in the range of 900–2000 cm−1, corresponding to the second-order Raman spectrum, were discovered. The same lines also appear in the spectrum upon excitation by a laser line with a wavelength of 532 nm, but their intensities are significantly (by an order of magnitude or more) lower. It is shown that in the structure of the double-doped LiNbO3:Gd3+(0.003):Mg2+(0.65 wt.%), the crystal oxygen-octahedral clusters MeO6 (Me–Li, Nb, Gd, Mg) are slightly distorted, and in addition, the value R = [Li]/[Nb] ≈ 1 is close to that for a nominally pure stoichiometric crystal.

Accurate assignment of double resonant Raman bands in Janus MoSSe monolayer from first-principles calculations

• The double resonant Raman (DRR) spectra of Janus MoSSe monolayer is calculated within the first-principles framework. • Eleven DRR bands are assigned with combined modes compared quantitatively if one DRR band comprises more than one combined mode. • The rationality of the assignments has been verified by group theoretical analysis as well as the calculated DRR spectra under biaxial strains. Janus transition metal dichalcogenides (TMDs) structures, as a new type of two-dimensional layered materials, have drawn increasing research efforts mostly by the Raman characterization technique since their successful synthesis. First- and second-order resonant Raman spectra (RRS) have been reported by experiments. But, unlike much interest paid to the first-order RRS, there has been so far no much discussion dedicated to the second-order double resonant Raman (DRR) bands and band assignments of Janus TMDs, which nevertheless is indispensable but hampered by the difficulty of calculations. In this work, we calculate the DRR spectra of Janus MoSSe monolayer within the first-principles framework and succeed in achieving accurate assignments of the DRR bands. The assignments are in agreement with our group theoretical analysis. Moreover, taking advantage of its strain-sensitive feature, we calculate the DRR spectra under biaxial strain, and further verify the rationality of our assignments by analyzing strain-induced shift of the DRR bands. Our present study supplies an efficient strategy for quantitative understanding on the electron-phonon coupling in the Janus structures.

Polaronic optical transitions in hematite (α-Fe2O3) revealed by first-principles electron-phonon coupling.

Polaron formation following optical absorption is a key process that defines the photophysical properties of many semiconducting transition metal oxides, which comprise an important class of materials with potential optoelectronic and photocatalytic applications. In this work, we use hematite (α-Fe2O3) as a model transition metal oxide semiconductor to demonstrate the feasibility of direct optical population of band edge polaronic states. We employ first-principles electron-phonon computations within the framework of the density functional theory+U+J method to reveal the presence of these states within a thermal distribution of phonon displacements and model their evolution with temperature. Our computations reproduce the temperature dependence of the optical dielectric function of hematite with remarkable accuracy and indicate that the band edge optical absorption and second-order resonance Raman spectra arise from polaronic optical transitions involving coupling to longitudinal optical phonons with energies greater than 50meV. Additionally, we find that the resulting polaron comprises an electron localized to two adjacent Fe atoms with distortions that lie primarily along the coordinates of phonons with energies of 31 and 81meV.

Structure particularities and first, second order Raman spectra of LiNbO3:Tb near the concentration threshold

Defective structure particularities have been studied in pure non-stoichiometric (Li/Nb≠1) lithium niobate (NSLN) crystals and LiNbO3:Tb (2.24 wt%) crystal. NSLN Raman spectra contain low-intensity extra bands in the area 100-1000 cm−1 at different scattering geometries. The bands are not provided by the selection rules for LN. The bands are caused by static disorder in cation sublattice of crystals. Besides, we have established additional bands in the area 1300-2000 cm−1 of a LiNbO3:Tb (2.24 wt%) crystal Raman spectrum. The bands correspond to overtone states of crystal lattice polar fundamental modes. Second-order Raman spectrum of LiNbO3:Tb contains bands with frequencies higher than that of overtones of the highest-frequency fundamental polar modes with А1(Z) and Е(X,Y) symmetry type. The particularities are manifestation of bonded states of polar modes – bipolaritons; they convert in two entangled photons or in a bonded state – biphoton at an exit from the crystal.

Optical absorption and resonance Raman scattering spectra of lead phthalocyanine and chloroaluminum chlorophthalocyanine thin films

The optical absorption spectra (1.2–2.5 eV) and resonance Raman spectra (100–3000 cm−1) of lead phthalocyanine (PbPc) and chloroaluminum chlorophthalocyanine (ClAlClPc) thin films with thickness of 190 and 200 nm, respectively, were studied. The films were deposited using thermal evaporation in vacuum (6.5 mPa) onto silicon substrates held at room temperature. During thermal annealing of PbPc and ClAlClPc films, the transition from monoclinic to triclinic crystal phase was observed. It is shown that the second-order Raman spectra of PbPc and ClAlClPc films are mainly formed by overtones and combinations of the first-order bands with presumed B1 symmetry.

Second-Order Raman Scattering in Ferroelectric Ceramic Solid Solutions LiNbxTa1−xO3

In the second-order Raman spectra of ceramic solid solutions, LiNbxTa1−xO3 weak overtone bands of fully symmetric fundamental polar excitations were observed for the first time. The frequencies of the two bands exceeded the value of the overtone frequency corresponding to the fully symmetrical vibration 4A1(z). The possibility of the existence of phonon bound states of the antipolar type in the vibrational spectrum of LiNbxTa1−xO3 ceramics is predicted.

Optimizing the Properties of Hybrids Based on Graphene Oxide for Carbon Dioxide Capture.

The reduction of CO2 emissions and its elimination from the atmosphere has become one of the major problems worldwide, since CO2 is the main cause of the greenhouse effect and climate change. In recent years, a great number of carbonaceous materials that can be used as CO2 adsorbents have been synthesized. The strategy is usually to synthesize the materials and determine their adsorption capacity without studying previously the factors that influence this capacity. In this work, different properties of the adsorbents are analyzed to study their influence on the CO2 adsorption capacity. For this purpose, 10 adsorbents have been synthesized using different strategies and characterized with X-ray photoelectron spectroscopy, X-ray diffraction, and micro-Raman spectroscopy. The percentage of sp2 carbons, the position of the D + D′ peak of the second-order Raman spectrum, the micropore volume, and the grain size of the C sp2 domains have been related to the amount of CO2 adsorbed by the adsorbents. The results confirm a linear relationship between the volume of the micropores and the CO2 uptake and it proves that CO2 retention is favored in those materials that, in addition to having a high volume of micropores, also have low grain size of C.

Second-order Raman spectra of a LiNbO3:Tb crystal

Additional bands have been discovered for the first time in the Raman spectra of terbium doped lithium niobate crystals. The bands have been determined in the high-frequency region and have been attributed to the overtone states of the second order of polar fundamental modes of the crystal lattice. It has been established that Raman spectrum of terbium doped lithium niobate contains bands with frequencies exceeding the overtones of the highest-frequency fundamental polar modes of A1(Z) and E(X, Y) symmetry types. The observed features of the second-order Raman spectra have been interpreted as a manifestation of bound states of phonons – biphonons.

Effects of Parent Coal Properties on the Pyrolytic Char Chemical Structure: Insights from Micro-Raman Spectroscopy Based on 32 Kinds of Chinese Coals

The chemical structures of pyrolytic chars prepared from 32 kinds of Chinese coals were investigated with micro-Raman spectroscopy in this study. Both first-order and second-order Raman spectra of the chars were curve-fitted and analyzed. The effects of the parent coal properties, including coal rank, volatile, fixed carbon, and ash content, on the pyrolytic char structures were detailed discussed and the correlations between these coal properties and pyrolytic char chemical structures were set up. Multiple-factor analysis was done to propose a comprehensive coal property index that relates well to the pyrolytic char chemical structure. The results indicate that the aromatization degree is the key distinguishable structure of pyrolytic chars prepared from coals with various rank, and the alkyl C−H and aryl C−H structures have no significant difference. The aromatization degree of pyrolytic char decreases with the increase of coal rank, while it increases with the increase of the fixed carbon content in parent coals. The high content of moisture in parent coal can induce condensation of the pyrolytic char, but the inorganic composition probably prevents the condensation of the char. Limited correlations between the coal rank, fixed carbon, moisture and ash content, and the aromatization degree of pyrolytic chars were found. A comprehensive coal property index: (fixed carbon content + moisture content)/(volatile content + ash content) (in air dry basis) combining the coal properties together relates well to the aromatization degree of pyrolytic char and can act as a good indicator for the pyrolytic char chemical structure. This study reveals the effects of the parent coal properties, including coal rank, fixed carbon, moisture, and ash content, on the pyrolytic char chemical structure, and provides a new comprehensive coal property index to predict the pyrolytic char chemical structure.

Characterization of second-order bands in Raman scattering spectra of lead phthalocyanine thin films

The structure, optical absorption (500…950 nm) and resonance Raman spectra (within the range 100…3000 cm–1) of lead phthalocyanine (PbPc) thin solid films with the thickness 190 nm were studied. The films were deposited using thermal evaporation in vacuum 6.5 mPa onto silica substrates held at room temperature. It was found that in the process of depositing the PbPc thin solid films monoclinic and triclinic PbPc crystallites were grown, and the amount of crystallites in the triclinic phase in the as-deposited PbPc films was approximately two times less than those in the monoclinic one. The resonance Raman spectroscopy, with application of the He-Ne laser line 632.8 nm as an excitation source, was used for studying the 190-nm thick PbPc films. Due to resonance enhancement, the second-order Raman spectrum of PbPc films within the region 1700…2950 cm–1 was successfully registered and analyzed for the first time. It has been shown that the second-order PbPc Raman spectrum is mainly formed by the overtones and combination modes of B1 symmetry fundamental vibrations. The second-order Raman region of 2550…2900 cm–1 appeared to be highly specific for PbPc and could be used for its identification along with the finger-print region of fundamental vibrational modes.

First- and second-order Raman spectra of carbonaceous material through successive contact and regional metamorphic events (Ryoke belt, SW Japan)

The evolution of both the first- and second-order Raman spectra of carbonaceous material (CM) through successive contact and regional metamorphic events is explored in the western part of the Ryoke belt (Iwakuni-Yanai area, SW Japan). Thirty-two metasedimentary rock samples were collected along a N-S gradient locally affected by contact metamorphism before and after the main regional tectono-metamorphic event (DP1). First-order spectra document a decreasing peak area ratio R2 and an increasing temperature TCM towards the south and the surrounding granitoids. Domains with intermediate (535–600 °C) TCM values match the extent of the pre-DP1 contact aureole but also image a so far unclear post-DP1 aureole. The axial part of the belt, likely unaffected by granite intrusions, preserves southward increasing TCM from 425 to 660 °C. Second-order spectra show a single S1 band that splits into two peaks (S1- and S1+) whose frequency difference ΔS1 increases stepwise towards the south. The spatial distribution of ΔS1 follows that of the E–W trending regional metamorphic zones. The splitting of S1 indicates a transition from two-dimensional to three-dimensional CM and occurs at ~500 °C, which seems to be common to all metamorphic belts worldwide. Despite regional metamorphism CM was able to record the post-DP1 contact overprint and there is no clear observation of delayed CM recrystallization, which likely depends on the crystallinity of the CM precursor. A discrepancy between first- and second-order Raman parameters suggests that they partly record the influence of different factors; R2 gives an account of thermal events, particularly those related to localized contact metamorphism, whereas ΔS1 potentially yields information on regional variations in heating duration and pressure. This demonstrates the potential of the full Raman spectrum of CM for deciphering the complex thermal history of orogenic systems.

Developing micro-Raman spectroscopy for char structure characterization in the scale of micro- and bulk: A case study of Zhundong coal pyrolysis

Heterogeneity of coal in micro-scale is significant and can affect coal thermal behavior. In this study, micro-Raman spectroscopy was developed for characterizing coal/char in the scale of micro- and bulk with a case study of Zhundong coal pyrolysis. More than 150 particles in micro-scale were analyzed for each sample by micro-Raman mapping technology controlled by a computer program. Individual Raman spectrum of each particles in micro-scale and average Raman spectrum for each sample were analyzed, respectively. The results indicate that the evolution characteristics of aromatic rings, C–H in methyl or methylene, and C = O groups can be revealed by combined deconvolution of first- and second-order Raman spectrum. The fluorescence interference in Raman spectrum defined as drift coefficient α can quantitate volatiles content and H/C atom ratio of individual coal/char particle in micro-scale. The results reveal that the chemical structures of the Zhundong coal in micro-scale are significantly heterogeneous. The structural evolution of char particle in micro-scale differs from each other, especially in the early stage of the Zhundong coal pyrolysis. The chemical structure of char particles in micro-scale tends to converge as the pyrolysis progresses. A more severe pyrolysis condition can generally lead to a char with higher heterogeneity. For the bulk structural evolution, the Zhundong coal pyrolysis can be mainly divided into two stages. Firstly, the C–H in methyl or methylene releases rapidly, and the relative amount of small aromatic rings and C = O groups decrease mildly. And then, the aromatization degree of char increases rapidly along with the significant release of C = O groups.

Anisotropic optical phonons in MOCVD grown Si-doped GaN/Sapphire epilayers

Micro-Raman spectroscopy is employed to study the anisotropic optical phonons of Si-doped GaN/Sapphire epifilms grown by metal organic chemical vapor deposition method. In an undoped 3.6 μm thick sample – our polarized Raman measurements in the backscattering geometry revealed major first order modes of GaN and sapphire. Careful analyses of the second-order Raman spectra using critical-point-phonons from a rigid-ion-model fitted inelastic X-ray spectroscopy data with appropriate selection rules helped us attain expedient data for the lattice dynamics of GaN. In Si-doped films, a modified phonon confinement model is used for simulating Raman line shapes of E2high phonons to monitor crystalline quality. While the optical phonons in lightly doped samples are coupled to electron plasma– at higher carrier concentration the over-damped A1(LO) mode vanished in the background. For each sample we assessed the transport parameters by simulating Raman profiles of A1(LO) line shape with contributions from plasmon-LO-phonon and Lorentzian shaped Eg sapphire mode. A realistic Green’s function theory is adopted to study the vibrational modes of Si donors and Mg acceptors in GaN by including force constant changes estimated from lattice relaxations using first-principles bond-orbital model. Theoretical results of impurity-activated modes compared favorably well with the existing Raman scattering data.

Probing nanoscale variations in strain and band structure of MoS2 on Au nanopyramids using tip-enhanced Raman spectroscopy

We report tip-enhanced Raman spectroscopy and tip-enhanced photoluminescence studies of monolayer and bilayer $\mathrm{Mo}{\mathrm{S}}_{2}$ in which we characterize photoluminescence and first- and second-order Raman spectra in monolayer, bilayer, and inhomogeneously strained $\mathrm{Mo}{\mathrm{S}}_{2}$. From the transition of unstrained $\mathrm{Mo}{\mathrm{S}}_{2}$ from monolayer to bilayer, we determine a spatial resolution of approximately 100 nm through the peak positions of the first-order Raman modes. The strain dependence of the second-order Raman modes reveals changes in the electronic band structure in strained $\mathrm{Mo}{\mathrm{S}}_{2}$ that are directly observed through changes in the Raman peak positions and peak area ratios, which are corroborated through density functional theory calculations.

Evolution of the Raman Spectrum with the Chemical Composition of Graphene Oxide

Raman spectra of graphene oxides (GOs) with different chemical compositions and synthesized by oxidation of distinct starting materials were analyzed to relate the spectral features to structural properties. The chemical compositions of different graphene oxides were determined by X-ray photoelectron spectroscopy (XPS), and nanoplatelets were characterized by zeta potential (ζ) and dynamic light scattering (DLS) measurements. The results indicated that the chemical composition, size, and superficial charge of the nanoplatelets depend on the starting material. We found five reported bands (D, D′, G, D″, and D*) in the first-order Raman spectrum and three bands (2D, D + D′, and 2D′) in the second-order Raman spectrum that successfully interpret the Raman spectra between 1000 and 3500 cm–1. Analysis of the bands allowed linear correlations to be found between the maximum positions of the 2D and D + D′ bands and between the relative intensities of the D and G bands (ID/IG) and the Csp2 percentage. Moreover, o...