Effect of heat-treatment on the surface properties of gallium phosphide nanosolids by Raman spectroscopy

Raman spectra of gallium phosphide （GAP） nanosolids （unheated and heat-treated at 598 and 723 K, respectively） were investigated. It was observed that both the longitudinal optical mode （LO） and the transverse optical mode （TO） displayed an asymmetry on the low-wavenumber side. The scattering bands were fitted to a sum of four Lorentzians which were assigned to the LO mode, surface phonon mode, TO mode, and a combination of Ga-O-P symmetric bending and sum band formed from the X-point TA ＋ LA phonons, respectively. Analysis of the characteristic of surface phonon mode revealed that the surface phonon peak of the GaP nanosolids could be confirmed. In the infrared spectrum of the GaP nanoparticles, we observed the bands on account of symmetric stretching and bending of PO2, as well as stretching of Ga-O The Raman scattering intensity arising from the Ga-O-P linkages increased as increasing the heat-treatment temperature.     

Localized vibrations around a soliton in polyacetylene

Localized vibrations and optical and acoustic phonons are calculated for finite length C_nD_n+2 containing a charged soliton. The molecular orbital method and normal coordinate analysis have been used in this calculation. The localized translational mode couples with the CH bending mode when the chain length is short (n ≈17). The coupling explains well the two broad peaks in the i.r. spectrum. It is also found that dopant pinning has effects similar to chain shortening; i.e. raising the frequency of the T mode and causing coupling of the T and TH modes. Approximate phonon dispersions are calculated for finite trans-polyacetylene with a charged soliton. Branch modes of the previously known localized modes are discussed in connection with the corresponding optical phonons.     

Structure and Raman scattering of Mg-doped ZnO nanoparticles prepared by sol-gel method

Mg-doped ZnO (Mg_xZn_(1-x)O, x=0-0.10)nanoparticles were prepared by sol-gel method. Structural characterization by X-ray diffraction (XRD) indicates that the lattice parameter a increases and c decreases linearly with the increase in Mg content (x) due to the substitution of Mg~(2+) for Zn~(2+) in ZnO lattice. The blueshift of Raman modes is observed, impling the increase in force constant of atom vibration in the Mg_xZn_(1-x)O (MgZnO) nanoparticles. Resonant Raman spectra show longitudinal optical phonon overtones up to fifth order, revealing that the short part of the electron-phonon interaction is enhanced and long-range part is weakened by Mg doping.     

Micro-Raman Spectroscopy for Stress Evaluation of 3C-SiC Epitaxially Grown on Si Substrate by Hot Wall CVD

GROWTH of large area3C-SiC films on Si hasattracted great interest since the early report onhetero-epitaxial growth by means of chemical vapordeposition(CVD)[1],due to its outstanding thermal,chemical and electrical properties for deviceapplications,such as,wide-band gap(2.3eV),highthermal conductivity(4.9W/cmK),high electronmobility(lOOOcmVVs)and high electron saturationdrift velocity(2.7xl07cm/s)[l,2].To our knowledge,the reports on the growth of SiC involved largely on the(100)Si,wi…     

Structure and optical characterization of GaP-SiO2 co-sputtered films

The films of GaP nanocrystals embedded in SiO2 matrix were prepared by radio frequency magnetron co-sputtering and subsequent anneal- ing technology. The structure and morphology of the films were investigated by scanning electron microscope, X-ray diffraction, and energy dispersive spectrum. Raman spectra results showed that the transverse optical phonon model (TO) and the longitudinal optical phonon model (LO) of GaP nanocrystals were both discovered to undergo red shift, broadening, and asymmetry. The red shift degree of the TO model was about 8.8 cm-1. The luminescence spectrum of the GaP/SiO2 film consisted of several emission peaks. 2.84-2.54 eV blue light emission was explained by the quantum confinement-luminescence centers model (QC-LCs).     

不同品种立方氮化硼晶体的Raman光谱

利用Raman光谱仪研究了我公司用高温高压合成的不同品种立方氮化硼(CBN)晶体。结果表明不同品种CBN晶体的Raman光谱有明显差别,可分为以下几种类型:(1)典型的CBN拉曼光谱,只有两个对应CBNTo和Lo模的Raman峰,分别位于1055cm-1和1305cm-1左右;(2)除了CBN的特征峰外,在925cm-1、955cm-1和1245cm-1左右出现额外峰;(3)有CBNTo模的峰,但对应于Lo模的峰消失,在1286cm-1左右出现一个不对称的宽峰。另外在760cm-1、920cm-1和1015cm-1处出现多个额外峰。(4)在1286cm-1左右出现一个峰形不对称的宽峰,对应To模的峰有较大的位移,Raman光谱中有强烈的从低波数端到高波数端呈上升趋势的背底。本文对产生这些Raman峰的原因进行了分析探讨。     

Raman Spectroscopy of Layered Compound YbB2C2

REB2C2 (RE =Y and lanthanides) compounds have gained attention for their unique layered crystal structure.However,there have been few reports about Raman spectroscopy of REB2C2 compounds up to now.Here,the Raman spectrum of YbB2C2 is obtained by a micro-Raman spectroscope and the first-principles calculations.Raman active vibrational modes of YbB2C2 are confirmed as A1g (627 and 1311 cm-1),B1g (944 and 1172 cm-1),B2g (330 and 885 cm-1) and Eg (357 and 530 cm-1).Atomic displacements of these modes are different,they can be divided into two groups:A1g,B1g and B2g correspond to ring breathing (δin,in the plane) of B2C2 layer;Eg is due to ring deformation (δoop,out of the plane) of B2C2 layer.These results are helpful to understand the individual structure of REB2C2.     

Spectroscopic investigations of copolymers incorporating various thiophene and phenylene monomers

Experimental and theoretical studies are combined in order to describe copolymers involving various thiophene and phenylene monomers (TBT). The copolymers structures are confirmed and their optical properties are examined. The incorporation of a block unit, such as biphenylene, bipyridine or anthracene into the full conjugated backbone has a significant effect not only on the structural and vibrational properties of the compound but also on its optical behavior, resulting in different electronic transitions and emission colors from blue to blue-greenish and green. Therefore, a better correlation between photoluminescence and Raman vibrational modes assigned to the phonon emission is established. Theoretical studies, based mainly on density functional theory (DFT) calculations, are performed to explain the effect of the block unit incorporation in the optical property modifications. As a result, the blue shift is related to a less planar structure of the copolymer.     

Growth and structural properties of pulsed laser-ablated CuInSe2 nanoparticles by pulsed-laser ablation and selenization process

Nanoparticles of CuInSe₂ (CIS) were synthesized by pulsed laser ablation. The effect of the preparation conditions on the structural properties of the CIS nanoparticles was investigated. The CIS nanoparticles showed a more developed structural property after treatment in Se-evaporated atmosphere. By X-ray photoelectron spectroscopy, increase of Se amounts in the CIS nanoparticles was confirmed. Enhancement of phonon modes in the CIS nanoparticles was observed in Raman scattering spectroscopy while secondary phases like In₂Se₃ or CuAu structure-related peak were identified in the spectra. High-resolution transmission electron microscopy indicated that the individual nanoparticles were embedded in matrix of some amorphous layers and diffraction patterns representing the chalcopyrite structure were also scrutinized.     

Optical and Raman scattering studies on SnS nanoparticles

Tin sulfide nanoparticles were synthesized through wet chemical route. Structure and phase purity were confirmed by powder XRD. Morphology and size were identified from TEM and AFM. The room temperature photoluminescence spectrum shows the band edge emission at 1.57 eV. The direct and indirect band gaps are estimated from UV-vis-NIR absorption spectrum as 1.78 and 1.2 eV, respectively. Blue shift of 0.48 eV observed for direct transition and 0.2 eV for indirect transition as compared to bulk band gap is due to quantum confinement effect. The Raman spectrum of SnS nanoparticles shows all the predicted Raman modes which show shift towards lower wave number side in comparison with those of the SnS single crystal. This is attributed to phonon confinement.     

Role of vibrational optical phonons in the heat capacity of K3C60

The reported heat capacity C(T) data of alkali doped fulleride K₃C₆₀ is theoretically investigated in the temperature domain 5 ≤ T ≤ 25 K. Calculations of C(T) have been made within the two component scheme: one is the phonon (optic and acoustic) and the other is electronic contribution. We begin with the intercage interactions between the adjacent C₆₀ cages and expansion of lattice due to the intercalation of alkali atoms based on the spring model to estimate vibrational optic and acoustic phonon frequencies from the dynamical matrix for the intermolecular alkali-C₆₀ phonon mode. Lattice specific heat is well estimated from the Debye and Einstein approximation. Fermionic component as the electronic specific heat coefficient is deduced using the band structure calculations for metallic phase. Comparison of the coefficient of the normal state electron contribution to C with band structure calculations gives an estimate of the electron–phonon coupling strength. It is notice that electron correlations are essential to enhanced density of state over simple Fermi liquid approximation in the metallic phase. The present numerical analysis of specific heat shows dominating role of vibrational optical phonons.     

Raman active phonons of RFe3(BO3)4, R=La or Nd, single crystals

Oriented single crystals of RFe₃(BO₃)₄, with R=La or Nd, have been studied by Raman spectroscopy. Spectra with the relevant polarization configurations have been recorded in order to obtain the symmetry of the observed phonons. The factor group analysis and the correlation with the free (BO₃)³⁻ ion are done in order to identify most of the phonons associated with the two different types of (BO₃)³⁻ ion present in the crystal. The number and symmetries of the optical Raman active modes are 7A₁+19E, among which 4A₁+8E can be assigned as mostly due to (BO₃)³⁻ vibrations. 7A₁+18E modes were observed.

The highest energy peaks have been assigned to the regular planar (BO₃)³⁻ and to the three irregular (BO₃)³⁻ groups. The two lowest energy peaks of A₁ symmetry (around 180 and 300 cm⁻¹) are very probably related to the BO₃ rotatory mode and to Fe displacements. R ions do not participate in A₁ symmetry modes. The E mode around 90 cm⁻¹ (the lowest frequency mode) is probably due to the R ions which have the longest bonds and are the heaviest ions.     

CeO2的晶格动力学性质和热输运性质的第一性原理计算

采用基于密度泛函理论的有限位移法和玻尔兹曼方程,计算了CeO2的晶格动力学性质、热力学性质和热输运性质,计算结果和实验结果基本符合。通过分析CeO2所有声子模式的振动频率、Gruneisen系数和散射率,揭示了光学声子对增强晶格振动的非简谐性和声子散射率所起的重要作用。此外,还计算了不同自由程的声子模式对热导率的贡献,发现CeO2的晶格热导率主要由声子自由程在1~10 nm之间的声子所贡献。     

Fully oriented cis-(CH)x: Experimental and theoretical analysis of the polarized Raman spectra

New experimental results of polarized Resonant Raman Scattering (RRS) spectra of fully oriented cis-rich (CH)_x films for the excitation wavelengths 676.4 and 600 nm are presented together with the theoretical interpretation. The three Raman active bands due to the totally symmetric vibrational modes of cis-(CH)_x are observed only in the / / / / polarized spectrum, indicating that the films are fully oriented. The contribution of the Raman active vibrational modes of the trans segments is observed in all polarized spectra and for both incident wavelengths. Also, we report the RRS spectra of the same sample thermally isomerized, without stretching, for λ_L = 676.4 nm and λ_L = 457.9 nm. We find that the depolarization ratios of the Raman bands due to the vibrational modes of trans-segments are much higher after isomerization than for an usual stretched trans (CH)_x sample.     

Influence of phonon focusing on the Knudsen flow of phonon gas in single-crystal nanofilms of spintronic materials

Influence of the anisotropy of elastic energy on the phonon transport has been investigated in single- crystal nanofilms of Fe, Cu, MgO, InSb, and GaAs materials used for spintronic instruments and devices in the Knudsen flow regime of phonon gas. The dependences of the lattice thermal conductivity and lengths of free paths of phonons for all acoustic modes on the geometric parameters of the films have been considered for low temperatures with the dominance of the diffuse scattering of phonons at the boundaries. Physical aspects of the propagation of phonon modes in the films have been analyzed. It has been shown that the anisotropy of phonon transport in single-crystal films is due to the features of the propagation of phonon modes in elastically anisotropic films with a different relationship of the geometric parameters. The directions of heat flow and orientations of the film planes that yield the maximum and minimum thermal conductivity of phonons in film planes have been determined.     

Structural and optical properties of encapsulated ZnO in porous host matrix

ZnO nanoparticles were encapsulated in the porous activated carbon matrix by incipient-wetness impregnation. The use of the small host matrix allowed the size confinement of ZnO by utilizing the porous nature of the host matrix. Partial fixation of ZnO in the porous matrix determines the size and the dispersion of the particles. Experiments at different calcination temperatures were carried out to investigate structural and optical properties of ZnO nanoparticles in the porous activated carbon matrix using X-ray diffraction, scanning electron microscopy, Raman spectroscopy and photoluminescence. The optimal calcination temperature was found to be ～450 °C in order to confine ZnO nanoparticles in the porous ACP matrix. Near-band-edge UV emission and green emission were both associated with the deep-level defect state. A decrease in full width at half maximum of E₂ mode in Raman spectrum confirmed an increase in crystallite size due to higher calcination temperature, causing an increase in phonon lifetime.     

Raman scattering study of electrochemically doped single wall nanotubes

Sheets of single wall carbon nanotubes (SWNT) bundles were studied by Raman scattering spectroscopy when doped electrochemically in an aqueous NaCl solution. At non-resonant conditions, the radial breathing mode (RBM) and tangential displacement modes (TDM) change very little intensity and frequency with electrochemical doping. Resonant Raman scattering for the TDM band, however, show large changes when applying or negative voltages. We conclude that the electronic resonance condition disappears due to larger change of the electronic occupancy with electrochemical doping.     

Determination of ionic and pure electronic contributions to the electro-optic coefficient of cadmium telluride and gallium arsenide single crystals

High electro-optic figure of merit n₀³r₄₁ and the absence of natural birefringence make semi-insulating gallium arsenide (GaAs) and cadmium telluride (CdTe) attractive materials for the fabrication of electro-optical devices. In this context we characterized both acoustic and optical phonon contributions to the electro-optic coefficient of CdTe and GaAs. Accurate measurements of n₀³r₄₁ as a function of modulation frequency and temperature were carried out on CdTe:In and GaAs crystal at 1.5 μm. For CdTe, to the best of our knowledge, this is the first time that a contribution to the electro-optic coefficient due to optical phonons has been determined. A positive value for the ionic contribution, due to optical phonons in GaAs, is obtained, in disagreement with that previously inferred from Raman scattering efficiency measurements. The pure electronic contribution was then isolated, and the second-order non-linear optical coefficient was derived. The latter was compared to previously reported data from non-linear wavelength conversion measurements.     

Temperature-dependent Raman scattering and photoluminescence in YBa_2Cu_3O_7 doped with SiO_2 and Zn_(0.95)Mn_(0.05)O nanoparticles:comparative study

A combined study examining the temperature dependencies of Raman scattering and photoluminescence(PL)of a YBa_2 Cu_3 O_7(YBCO)matrix doped with SiO2(12 nm;0.01 wt%.,0.10 wt%)and Zn_(0.95)Mn_(0.05)O(20 nm;0.02 wt%,0.10 wt%)nanoparticles was presented.X-ray diffraction(XRD)analysis confirms that both YBCO types exhibit aperovskite structure with the orthorhombic Pmmm phase.The microstructure was examined using environmental scanning electron microscopy(ESEM).Raman scattering and photoluminescence measurements as functions of temperature were conducted in the 77-837 K range.The photoluminescence intensity is observed to decrease for the doped YBCO than for the pure YBCO,because of localized defects.The photoluminescence spectrum is primarily composed of three bands at 1.60,1.88,and 2.40 eV.A clearly pronounced correlation is observed between electronic and structural changes in the doped YBCO,which is due to the temperature,illumination,added oxygen or metal ions,and spectral parameters.The PL integrated intensity as a function of the inverse temperature was simulated using the Arrhenius model.This analysis reveals that the energy exchange between the different levels in the pure and doped YBCO was conducted via two vibration modes only,which are strongly linked to the oxygen and copper atoms in the YBCO matrix.The temperature dependencies of the modes at 340 and 500 cm~(-1) exhibit softening with temperature increase,resulting from microstructure control,which may be due to small concentrations of Si,Zn,and Mn substitutions at the chain Cu(1)and plane Cu(2)sites.     

Study of interactions in carbon nanotubes systems by using Raman and SERS spectroscopy

We present results obtained on carbon nanotube systems by means of Raman scattering and surface enhanced Raman scattering (SERS). Such systems consist of single-walled nanotubes (SWNTs), multi-walled nanotubes (MWNTs), or polymer/nanotubes composites. In particular, we have focused our studies on the interactions taking place in these compounds which lead to an upshift of the radial breathing mode (RBM) in the case of SWNTs, either when they are arranged in bundles or embedded in polymers. Similar calculations has allowed us to interpret the low frequency Raman modes observed in purified MWNTs. We show that they come from the RBM of individual tubes. From intensity calculations performed to simulate Raman spectra, we demonstrate the importance of two experimental parameters, namely the diameter of internal tubes and the number of shells which constitute the MWNTs, such parameters being extracted experimentally from high resolution transmission electron microscopy images.