Second-order Raman Spectra Research Articles

Electronic Raman scattering and the second-order Raman spectra of the n-type SiC

The Raman scattering spectra of the nitrogen doped n-SiC is studied. The theoretical line shape analysis indicates that, compared with 4H-SiC, the shift of the LO phonon-plasma coupled mode in 6H-SiC with free carrier concentration is smaller. From the electronic Raman spectra, which were obtained with laser excitation at 5145nm, there are four spectral lines in 6H-SiC and two lines in 4H-SiC, which correspond to the 1s(A1) to 1s(E) valley orbit transitions at the inequivalent k site. The explanation of the high-frequency signals of 6303 and 635 cm-1 is that they are velated with transitions at active deep level of defect. Finally, the second-order Raman features of 6H- and 4H-SiC are identified using the selection rules for second-order scattering in wurtzite structure.

Size dependence of the electronic structures and electron-phonon coupling in ZnO quantum dots

The electronic structures and optical properties of various sizes of ZnO quantum dots (QDs) were studied using x-ray absorption, photoluminescence, and Raman spectroscopy. The increase in the intensity ratio of the second-order Raman spectra of longitudinal optical mode and its fundamental mode, which is related to the strength of the electron-phonon coupling (EPC), is found to increase with the size of QD. The trend of EPC also correlates with the increase of the intensity ratio of the O 2pπ (Iπ) and 2pσ (Iσ) orbital features in the O K-edge x-ray absorption near-edge structure (XANES) as the size of QD increases. The EPC and XANES results suggest that the crystal orientations of ZnO QDs are approximately aligned with the c axis parallel with the polarization of x-ray photons.

On the curve of the density of phonon states for cubic boron nitride (from the results of photoluminescence measurements)

The fine structure of the phonon wing associated with the zero-phonon line (ZPL) of the BN1 center in the cubic boron nitride is analyzed in comparison with the structure of the phonon wing of the luminescence center at 3.188 eV in diamond, the second-order Raman scattering spectrum, and the theoretically calculated densities of phonon states of the c-BN compound. Taking into account the similarity of the structures of the phonon wings in the spectra of the above centers and the previously made assumptions that the structure of the phonon wing of the center at 3.188 eV is due to the specific features in the density of phonon states of diamond, it is assumed that, in the observed density of phonon states of cubic boron nitride, the critical points are represented by the specific features of the structure of the phonon wing associated with the zero-phonon line (at 3.294 eV) of the BN1 luminescence center. In turn, these latter specific features coincide accurate to within 5–10cm−1 with the theoretically calculated lattice vibrations of the c-BN compound and the experimental data obtained from the second-order Raman spectra.

Effect of phosphorus content on structural properties of phosphorus incorporated tetrahedral amorphous carbon films

With phosphorus incorporated tetrahedral amorphous carbon (ta-C:P) films prepared using filtered cathodic vacuum arc technique with PH3 as the dopant source, we investigate the effect of phosphorus content on the structural properties of the films by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. XPS analysis indicates that a function is established between the atomic fraction of phosphorus in the samples and the flow rate of PH3 during deposition, and that phosphorus implantation increases the graphite-like trihedral sp2 bonds deduced from fitted C 1s and P 2p core level spectra. Raman spectra of a broad range show that there are two notable features for all ta-C:P films: the first-order band centered at about 1560 cm-1 and the second-order band between 2400 and 3400 cm-1. The broad first-order band demonstrates that the amorphous structure of all samples does not remarkably change when a lower flow rate of PH3 is implanted, while a higher concentration of phosphorus impurity enhances the clustering of sp2 sites dispersed in sp3 skeleton and the evolution of structural ordering. Furthermore, the second-order Raman spectra confirm the formation of small graphitic crystallites in size due to a finite-crystal-size effect.

Boron mediated synthesis of multiwalled carbon nanotubes by chemical vapor deposition

A CVD procedure has been used for the attempted synthesis of boron-doped multiwalled carbon nanotubes using BF 3/MeOH as the boron-source, Fe/Ca(BO 3) 2/CaCO 3 as catalyst and acetylene as the carbon-source. The boron-doped multiwalled carbon nanotubes were characterized by TEM, Raman spectroscopy and laser ablation mass spectrometry analysis. Variation of the boron concentration was achieved by diluting the boron-source with MeOH and dilution resulted in: (i) a higher yield of carbon material, (ii) a less disordered material and (iii) a decrease in intensity in the first- and second-order Raman spectra of boron-doped multi walled carbon nanotubes.

Further development of Raman Microprobe spectroscopy for characterization of char reactivity

The effect of heat treatment on reactivity of cellulose char was investigated, using two methods: (1) Raman Microprobe spectroscopy analysis (RMA) and (2) thermogravimetric analysis (TGA). The heat-treatment was in the temperature range of 600–2600 °C, temperature prevailing in combustion of coal-chars. In the RMA, first- and second-order Raman spectra in the range of 800–2000 and 2000–3600 cm −1, respectively, were measured for all samples. In the first-order Raman spectra, the following bands have been observed: D band and G (at 1350 and 1590 cm −1 respectively), 1150 and 1450 cm −1. In the second-order Raman spectra, four bands have been observed at 2450, 2700, 2940 and 3250 cm −1. Both first- and second-order Raman spectra were fitted by Lorentzian functions. The Lorentzian parameters (bandwidth and intensity ratio) showed significant changes with heat treatment, which is consistent with structural modification. Also, from TGA experiments we observed the expected significant influence of heat treatment on char reactivity. Attempts were made to correlate the Lorentzian parameters with char reactivity. A good correlation was found between the 2940 cm −1 bandwidth in the second-order Raman spectrum and char reactivity, confirming the strong connection between char structure and its reactivity, and illustrating the usefulness of RMA in such studies.

Comparative study of first- and second-order Raman spectra of MWCNT at visible and infrared laser excitation

Comparative studies of first- and second-order Raman spectra of multi-walled carbon nanotubes (MWCNT) and three other graphitic materials – carbon fiber, powdered graphite and highly ordered pyrolytic graphite – are reported. Three laser excitation wavelengths were used: 514.5, 785 and 1064 nm. In first-order Raman spectra, the positions of the bands D, G and D′ (1100–1700 cm −1) presented very similar behavior, however the intensity ( I) ratio I D/ I G ratio showed differed behaviors for each material which may be correlated to differences in their structural ordering. In the second-order spectra, the G′ band varied strongly according to structure with the infrared laser excitation.

Raman scattering inβ-ZnS

The first- and second-order Raman spectra of cubic ZnS $(\ensuremath{\beta}$-ZnS, zinc-blende) are revisited. We consider spectra measured with two laser lines for samples with different isotopic compositions, aiming at a definitive assignment of the observed Raman features and the mechanisms which determine the linewidth of the first order TO and LO Raman phonons. For this purpose, the dependence of the observed spectra on temperature and pressure is investigated. The linewidth of the TO phonons is found to vary strongly with pressure and isotopic masses. Pressure runs, up to 15 GPa, were performed at 16 K and 300 K. Whereas well-defined TO Raman phonons were observed at low temperature in the whole pressure range, at 300 K the TO phonons appear to hybridize strongly with the two-phonon background and lose their identity, especially in the (3--10)-GPa region. The intensity of the TO phonons, which nearly vanishes when measured with a red laser line, is shown to result from a destructive interference of the amplitudes of the band-edge resonance and that of a background of opposite sign. The analysis of these effects is aided by calculations of the densities of one- and two-phonon states performed with the adiabatic bond charge model of the lattice dynamics.

Photoluminescence quenching effect on porous silicon films for gas sensors application

Porous silicon (PS) films were investigated by Raman, and photoluminescence (PL) spectroscopies using different laser excitations: 488.0, 514.5, 632.8, and 782.0 nm. The analysis of the first-order and second-order Raman spectra have shown that the band gaps of the PS films are indirect as in the bulk c-Si. The Raman phonon and the PL spectra as well as the spectral distribution of the linear polarisation degree (LPD) of PS layers have shown to be dependent on the laser excitation energy. This dependence cannot be explained within the quantum confinement model. A mechanism for the PL emission in PS layers is presented in which the radiative recombination of electron–hole pairs occurs in localised centres (the Si–O–SiR moieties) at the pore/crystallite interface. These quasi-molecular centres are Jahn–Teller active, i.e. the radiative recombination is a phonon-assisted phenomena. The adsorption of gas molecules on the porous silicon surface was studied throughout photoluminescence quenching effect. The adsorption experiments were performed at 10 −6 bar of pressure using gas molecules of organic solvents. In all these cases, the PL intensity was recovered after gas desorption. The PL quenching effect was explained in the sense of electron transfer mechanism (ET).

Graphite Materials Prepared from an Anthracite: A Structural Characterization

The purpose of this research was to study the influence of the temperature, treatment time, and initial coal particle size on the evolution of the structural order of graphite materials that have been prepared from an anthracite at temperatures >2273 K. Crystalline parameters such as the interlayer spacing and crystallite sizes were calculated from X-ray diffractometry measurements. The analysis of the first- and second-order Raman spectra allowed the assessment of the degree of orientation at the outermost layers of these materials. The graphitization of the anthracite happened in two different stages. The temperature of 2673 K seems to be the inflection point for the change in the graphitization rate of the anthracite. Highly crystalline materials were obtained at 2673 K. Temperatures of treatment >2673 K led to minor changes in the degree of structural order of the graphite materials obtained. The initial particle size of the anthracite affected the evolution of the graphitization process with temperature, because of differences in the ratio of particles that contain organic matter and mineral matter associations. The degree of graphitization achieved with this coal was comparable to that of other natural and synthetic graphites.

Analysis of surface modifications on graphite induced by slow highly charged ion impact

Modifications of the highly oriented pyrolytic graphite surfaces induced by the single impacts of slow Ar+ and Ar8+ were investigated by Raman spectroscopy. The difference in the irradiation-induced disorder between Ar+ and Ar8+ was clearly revealed by comparing two kinds of changes in Raman features against ion fluences: one is the peak intensity ratio of the disorder-induced peak with respect to the E2g-mode peak in the first-order Raman spectra, and the other is the full width at half maximum of the E2g-mode peak. Judging from the peculiar dependence of them on the fluence of Ar8+, it was assumed that the defects introduced by Ar8+ impacts is not simple vacancies as is the case of Ar+ impacts but vacancy clusters. The formation mechanism of vacancy clusters under Ar8+ irradiation was also discussed from the change in the second-order Raman spectra.

Translational and rotational mode coupling in disordered ferroelectric KTa 1− xNb xO 3 studied by Raman spectroscopy

The coupling of translational modes to the reorientational motion is an essential property of systems with internal orientational degrees of freedom. Due to their high complexity most of those systems (molecular crystals, glasses, etc.) present a major puzzle for scientists. In this paper we analyze the Raman scattering of a relatively simple ferroelectric system, KTa 1− x Nb x O 3, which may serve as a model for more complicated cases. We show the presence of a strong coupling between translational and reorientational motion in the crystal. Our data suggest that this coupling is the main reason for the depolarized component of the second-order Raman spectra and that it is also responsible for the frequency decrease (softening) of the transverse acoustic mode down to the third of three transitions, below which reorientational motion is no longer allowed.

First- and second-order Raman spectra of galena (PbS)

Raman spectra of cleaved, millimeter-sized single crystals of mineral galena (PbS) have been measured between 100 and 1200 cm−1 over the temperature range 80 to 373 K. Three low wave number bands are observed which can be reconciled with the results of earlier reports. However, changes in the peak wave numbers and in the relative intensities of these bands in response to changes in temperature do not conform with previous assignments for the observed bands. Detailed assignments are proposed which are based on resonance Raman processes. The observed bands are assigned to forbidden longitudinal optical excitations, which are allowed under resonance conditions by a Fröhlich interaction mechanism, and two-phonon excitations.

Raman characterization of boron-doped multiwalled carbon nanotubes

We present first- and second-order Raman spectra of boron-doped multiwalled carbon nanotubes. The Raman intensities are analyzed as a function of the nominal boron concentration. The intensities of both the D mode and the high-energy mode in the first-order spectra increase with increasing boron concentration, if normalized with respect to a second-order mode. We interpret this result as an indication that the high-energy mode in carbon nanotubes is defect-induced in a similar way as the D mode. Based on this result, we provide a preliminary quantitative relation between the boron concentration and the Raman intensity ratios.

Old and new aspects in lattice-dynamical theory

An overview of the basics of theoretical lattice dynamics is presented.Starting from the classical formulation, a tutorial description of theevolution of the lattice-dynamical theories is reported. After a briefsummary of the mostly used phenomenological models, the modern conceptsof first-principles lattice dynamics are introduced, paying specialattention to the methods based on the density-functional theory. Amongthese methods, the perturbative ab initio approach is highlighted.Furthermore, the application of this formalism to some cases of generalinterest is shown. In particular, we report on the controversy aboutthe origin of a peak in the second-order Raman spectrum of diamond, on thephonon spectrum of the wurtzite nitrides and on the temperature-inducedphase transitions in tin.

Raman scattering studies of the graphitization process in anthracene- and saccharose-based carbons

Abstract We report Raman scattering studies of the graphitization process in a series of carbons produced by the pyrolysis of saccharose and anthracene and then annealed at 1000, 1900 and 2300°C. Using 488 nm light, the first- and second-order Raman spectra of six samples have been examined in the range 50-4000cm−1. The low-frequency Raman range 50-lOOOcn−1 has been carefully examined using the laser excitation wavelengths 457.2, 476.5, 488.0 and 514.3 nm. A dependence of the Raman spectra in the first- and second-order ranges on annealing temperature and degree and size of ordered domains has been observed and correlated with neutron diffraction observations, indicating almost complete graphitization of the carbon prepared from anthracene and a disordered structure of the saccharose carbon at 2300°C. The occurrence of a sharp peak at about 464cm−1 and broadened peaks in the range 75-210cm−1, which shift with excitation energy as result of the one-dimensional auantum confinement effect of electrons in suc...

Changes in the porous silicon structure induced by laser radiation

AbstractPorous silicon (PS) films were investigated by Raman, and photoluminescence spectroscopies using different laser excitations at 488.0, 514.5, 632.8, and 782.0 nm. The exposure of PS layers to high laser powers causes an increase in the 480 cm−1 Raman intensity and a shift and enhancement of the PL emission. A laser‐assisted surface reaction is proposed to explain these observations. The analysis of the first‐ and second‐order Raman spectra showed that the band gaps of the PS films are indirect as in bulk c‐Si. The Raman phonon and the PL spectra and also the spectral distribution of the linear polarization degree (LPD) of PS layers were shown to be dependent on the laser excitation energy. This dependence cannot be explained within the quantum confinement model. A mechanism for the PL emission in PS layers is presented in which the radioactive recombination of electron–hole pairs occurs in localized centres (the Si—O—SiR moieties) at the pore/crystallite interface. These quasi‐molecular centres are Jahn–Teller active, i.e. the radioactive recombination is a phonon‐assisted phenomenon. Copyright © 2001 John Wiley & Sons, Ltd.

MBE growth and Raman studies of cubic and hexagonal GaN films on (0 0 1)-oriented GaAs substrates

Using different nucleation layers, we have grown cubic and hexagonal GaN epilayers on (0 0 1)-oriented GaAs substrates by radio-frequency plasma-assisted molecular beam epitaxy. First- and second-order Raman spectra are taken from these epilayers in back-scattering configurations at room temperature. For the first-order Raman spectra, transverse-optical (TO) phonons and longitudinal-optical (LO) phonons of cubic GaN are found to be at 533 and 739 cm −1, as well as the E 2 and A 1(LO) frequency of hexagonal GaN are found to be at 568 and 733 cm −1, and the polarized spectra are in good agreement with Raman selection rules for cubic and hexagonal GaN, respectively. These results confirm the single crystalline nature of the samples. As for the second-order Raman spectra, the bands show a continuum, at about twice the energy of the LO phonon of cubic GaN and A 1(LO) phonon of hexagonal GaN, which are nearly 10 times weaker than the first-order scattering. Two-phonon spectra are dominated by contributions due to longitudinal optical phonons.

Anharmonicity of short-wavelength acoustic phonons in silicon at high temperatures

Second-order Raman spectra corresponding to transverse acoustic phonons are studied in detail for crystalline Si over the temperature range 20–620°C. The largest relative softening and anharmonicity at the boundaries of the Brillouin zone were observed for the TA(X) mode. Extrapolation of the TA(X) frequency to high temperatures suggests that the Si lattice should be dynamically unstable at temperatures on the order of a doubled melting temperature. It is found that the main contribution to the softening of the transverse acoustic phonons in silicon comes from the anharmonicity and not from the volume expansion.

Effects of boron doping for the structural evolution of vapor-grown carbon fibers studied by Raman spectroscopy

The structural deviation of boron-doped vapor-grown carbon fibers (VGCFs) with diameters around 10 μm relative to their undoped counterparts was investigated by polarized microprobe Raman spectroscopy and field-omission scanning electron microscopy as a function of heat-treatment temperature (HTT). Boron doping induces the formation of dislocation loops in the surface, which combine into larger loops with increasing HTT. The depolarization ratio, Dp, of the E2g2 mode for VGCFs increases gradually with increasing HTT, and finally approaches the value of highly oriented pyrolytic graphite, which is consistent with the asymmetric shape of the peak at ∼2725 cm−1 in the second-order Raman spectra. On the other hand, the Dp ratios of the E2g2 mode for boron-doped VGCFs show no deviations up to an HTT of 2100 °C, as compared to that of VGCFs, and decrease with increasing HTT, whereas the Dp ratios of the D peak show a maximum value at 2100 °C, and decrease gradually with increasing HTT. Consistent with these Raman results, boron atoms in the graphite lattice introduce a decreased d002 spacing (accelerating graphitization), but also hinder two-dimensional structural development and increase the amount of disorder. This is done by introducing tilt boundaries and vacancies, which make the Dp ratio of the E2g2 mode lower than the value for polycrystalline graphite, even though the fibers are heat treated at 2800 °C.