Optical and acoustic phonon modes confined in gallium phosphide nanoparticles

The main aim of this paper is to discuss the confinement effects on the optical and acoustic phonon vibrational modes in gallium phosphide(GaP) nanoparticles(cylindric grain).The Raman scattering from the GaP nanoparticles was investigated.It was found that the red-shifts of the longitudinal optical(LO) mode and transverse optical(TO) mode were 15 cm?1 and 13.8 cm?1,respectively.It is generally accepted that the red-shifts of the optical phonon modes are due to the presence of smaller nanosized particles(～1.2 nm) acting as the nanoclustered building blocks of the GaP nanoparticles.In the low frequency Raman spectrum,a set of Stokes lines with almost the same spacing was clearly observed.The scattering feature originates from the discrete phonon density of states of the nanoclustered building blocks.According to Lamb's vibrational theory,the Raman shift wavenumbers of the spheroidal mode and torsional mode of the lowest energy surface modes for the nanoclustered building blocks were calculated.Good agreement can be achieved between the calculated results and the observed scattering peaks.These results indicate that the corresponding Raman peaks are due to scattering from the localized acoustic phonons in the nanoclustered building blocks in the GaP nanoparticles.     

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 annealing 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).     

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).     

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…     

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.     

Properties of nanostructured cubic boron nitride films prepared by the short-pulse laser plasma deposition techniques

Nanostructured cubic boron nitride (BN) films were synthesized on molybdenum substrates by using the short-pulse laser plasma deposition techniques. The surface morphologies, chemical compositions, bond structures, and mechanical properties of the obtained BN thin films have been investigated by scanning electron microscopy, Raman scattering, Fourier transform infrared spectroscopy (FTIR), energy dispersive spectra, and hardness measurements. High power density laser deposition yielded boron-rich BN nanorod arrays where tBN component dominates. Reducing power density down to 8 × 10⁷ W/cm² during laser plasma deposition produced flat cBN thin films. Typical TO and LO bands in the Raman and FTIR spectra of the cBN samples were identified. The cBN sample with hardness up to 40 GPa was obtained.     

Spectroscopic and conductivity studies of doping in chemically synthesized poly(3,4-ethylenedioxythiophene)

Spectroscopic methods (Raman (514.5 nm excitation), infrared (IR), X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR)) and electrical conductivity measurements were used to characterize the electrically conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) prepared by persulfate oxidation under aqueous conditions. Elemental analysis was carried out on the resulting sulfate-doped PEDOT to determine the sulfate doping level. Due to the difficulty in dedoping highly stable p-doped PEDOT, the sulfate-pre-doped PEDOT was directly treated with iodine solution in order to investigate the secondary doping processes between PEDOT and iodine. Two processes were observed: (1) the further oxidation of the polymer by iodine to produce an increase in the doping level, with triiodide as the dopant and (2) the replacement of sulfate by triiodide in an ion-exchange process involving triiodide in equilibrium with iodine in the iodine solution. Conductivity studies, in conjunction with XPS and EPR experiments, were used to analyse the relationship between the doping level and conductivity that was explained by differences in the nature of SO₄²⁻ and I₃⁻ as dopant ions and a shift in the polaron/bipolaron equilibrium. The Raman spectra (785 nm excitation) of these PEDOT samples were also investigated and a significant change has been observed in the wavenumber of the symmetric C_α = C_β stretching bands with varying levels of dopants. A correlation was observed between the ratio of the integrated intensities of the symmetric C_α = C_β stretching bands and the doping level in PEDOT that can be useful for estimating the doping level of a PEDOT sample from its Raman spectrum.     

Effects of Indium Doping on the Properties of Cd1-xMnxTe Crystals

通过近红外和红外透过谱表征,确定出掺铟后碲锰镉晶体透过率迅速下降,这是晶格吸收和自由载流子吸收共同作用的结果;而光致发光谱的分析结果表明,掺杂后施主-受主对峰增加,受主束缚激子峰减弱,且随着In含量的增加,受主束缚激子峰消失,只剩下施主-受主对峰。这一系列变化是因为替代的In原子作为施主补偿了Cd空位的缘故。拉曼光谱的测量显示,In掺杂导致"类CdTe"的纵向光学声子峰减弱;而磁学的测试结果则说明In的引入几乎不引起碲锰镉晶体磁化强度的变化。     

Crystal Perfection in GaP Films Heteroepitaxially Grown on GaAs by Low-pressure Metalorganic Chemical Vapor Deposition

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Resonant Raman spectroscopy of conducting organic polymers. (CH)x,and an oriented analog

The results of low-temperature Raman scattering experiments on cis and trans (CH)_x are reviewed, and the Raman bands corresponding to carbon -carbon stretches of the polymer backbone discussed. The Raman spectra of a variety of commercial polarizing sheet, Polaroid K, are presented. The spectra indicate that the dehydrated polyvinyl alcohol chain in Polaroid K is an oriented analog of trans (CH)_x for purposes of Raman spectroscopy.For films of trans (CH)_x, changes in the Raman profile with excitation wavelength reveal the presence of a distribution of effective chain lengths. The shorter the length of conjugated chain, the higher the carbon-carbon stretch frequencies, and the further to the blue the optical absorption and wavelength necessary for maximum resonant enhancement of the Raman scattering. The shape and positions of the carbon-carbon band corresponding to double bond stretching suggest the presence of lengths of chain (terminated by conjugation or bonding defects) having x (number of monomer units) from x ? 10 to x ? 100. This is based on a limiting C=C frequency in an infinite chain of 1450 cm^?1, determined from the wavelength-independent, low-frequency origin of the band. A similar band-shifting phenomenon in Polaroid K is displayed.Spectra of iodinated (CH)_x and iodinated Polaroid K are presented which suggest a similarity between the dopant species present in the two different materials.     

Raman scattering investigation of Bi2Te3 hexagonal nanoplates prepared by a solvothermal process in the absence of NaOH

Hexagonal Bi₂Te₃ nanoplates were synthesized by a simple solvothermal process in the absence of NaOH. The composition, morphology and size of the as-prepared products were characterized by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). Raman scattering optical properties of the as-prepared Bi₂Te₃ nanoplates were investigated by micro-Raman spectroscopy. The Raman spectrum shows that infrared (IR) active mode (A_1u), which must be odd parity and is Raman forbidden for bulk crystal due to its inversion symmetry, is greatly activated and shown up clearly in Raman scattering spectrum. We attribute the appearance of infrared active (A_1u) in Raman spectrum to crystal symmetry breaking of Bi₂Te₃ hexagonal nanoplates. The as-grown Bi₂Te₃ hexagonal nanoplates, exhibiting novel Raman optical properties compared with bulk crystals, may find potential applications in thermoelectric devices.     

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.     

Raman spectra and structure study of silicate glasses and their liquids

Stress index of tetrahedron （SIT） was defined to describe the topological connectivities among various sili- con-oxygen tetrahedra （SiOT） in anionic clusters of binary silicate crystals, glasses, and melts. It was found that the value of SIT was well correlated with the wavenumber of Raman active symmetric stretching vibration of non-bridging oxygen of SiOT. The spatial fractional dimension of hyperfine structure was introduced while comparative analysis was made with the value of SIT. It can be concluded that the concepts of SIT, vibrational wavenumber, and spatial fractional dimension were inherently and holographically correlated and exhibit isomorphic representations of complex structure of binary silicates. Experimental Raman spectra of binary silicates with different alkali cations were investigated. It was demonstrated that alkali cations have little effect on the vibrational wavenumber of symmetric stretching of non-bridging oxygen （NBO） of SiOT, but remarkably affect its Raman active optical cross section, as was consensus resulted from ab initio calculation. It can also be concluded that the spatial fractional dimension of binary silicate is predominantly determined by the hyperfine structure of the anionic clusters and little affected by alkali cations, although the species of anionic clusters and their distributions were originally assigned by the content of alkali oxides. And Raman optical activity extinct effect of isolated SiOT at high basicity should be considered while being applied to quantitatively analysis.     

Correlation between vibrational spectra and electrical conductivity of polythiophene

Polythiophene films synthesized by electrochemical polymerization under various conditions were undoped and redoped maximally with iodine. The electrical conductivity of the redoped films was in the range 2 × 10² − 5.9 S cm⁻¹. Vibrational spectra of the undoped films were analysed on the basis of those of model compounds including 2,2′-bithiophene, 2,2′:5′,2″-terthiophene and 2,2′:5′,2″:5″,2′-quaterthiophene. First, the intensity ratio of the infrared CH out-of-plane bending bands at 697 (2-substituted thiophene) and 787 cm⁻¹ (2,5-substituted thiophene) is a clue to the degree of polymerization. Secondly, the intensity ratio of the infrared double-bond symmetric and antisymmetric stretching bands reflects the distribution in length of the conjugated segment. Finally, the intensities of three Raman bands at 1155, 682 and 652 cm⁻¹ give information about the amount of distorted structure around the inter-ring C-C bonds. From the comparison of the spectroscopic and conductivity data, it is concluded that a high degree of polymerization alone is not adequate for a doped film to have high electrical conductivity but abundance of long conjugated coplanar segments is the requisite condition.     

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。     

Structural and tribological properties of DC reactive magnetron sputtered titanium/titanium nitride (Ti/TiN) multilayered coatings

Ti/TiN multilayered coatings of 200 layers with the thickness of 1.5 μm were deposited by a reactive DC magnetron sputtering technique using a mixture of Ar and N₂ gas. XRD technique was employed to elucidate the structural parameters. The presence of different phases like TiN, TiO_xN_y and TiO₂ were confirmed by XPS analyses. The observation of longitudinal optic (LO) phonon modes in the Raman spectra confirmed the highly crystalline nature of the deposited films. A microhardness value of 25.5 GPa was observed for Ti/TiN multilayers. The observed lower friction coefficient value for the Ti/TiN multilayers on mild steel (MS) indicated that the stack layers have better wear resistance property. Results from the electrochemical polarization and impedance studies showed the favorable behavior of the Ti/TiN multilayers, which have improved the corrosion resistance property of MS in 3.5% NaCl solution. The results of this study demonstrate that these multilayers can improve the corrosion resistance of mild steel substrates.     

Superconductivity in acenes

The vibronic coupling between the lowest unoccupied molecular orbital (LUMO) and molecular vibrations is calculated and analyzed with respect to a series of acenes. The Raman active CC stretching modes of 1400–1600 cm⁻¹ have large coupling constants in small acenes, while both low Raman active modes of 200–300 cm⁻¹ and CC stretching modes of 1400–1600 cm⁻¹ play an important role in the electron–phonon coupling in large acenes. The possible superconducting transition temperatures of the monoanions of benzene (C₆H₆) and acenes are roughly estimated, on the basis of the hypothesis that vibronic interactions between the LUMO and intramolecular vibrations would play an essential role in the occurrence of superconductivity in these molecules.     

Infrared and Raman studies of the phase transition in the organic conductor (TTM-TTP)I3

We report the polarized IR reflectance as well as Raman scattering investigations of the organic charge transfer salt (TTM-TTP)I₃ as a function of temperature, below and above the metal–insulator phase transition at T = 160 K. The IR reflectance was measured in the frequency region from 600 to 10,000 cm⁻¹, for the electrical vector of the polarized light parallel and perpendicular to the TTM-TTP stacking axis. For the polarization parallel to stacks the IR spectra are typical for semiconducting charge transfer salts. The electronic part of IR spectra was analysed in terms of a Lorentz model and temperature dependence of the optical transport parameters was determined. For the polarization perpendicular to the stacks we observed two electronic bands at about 5000 and 8000 cm⁻¹. The phase transition at 160 K has nearly no influence on the IR spectrum. The Raman scattering for different excitations (λ = 514.5, 632.6 and 785 nm) was mainly studied within the region of CC stretching vibrations. In this frequency range, three Raman lines at 1426, 1453 and 1486 cm⁻¹ attributed to TTM-TTP molecules are observed. Below 160 K a splitting of the band 1486 cm⁻¹ into two peaks at about 1488 and 1498 cm⁻¹ is found. The intensity and temperature behaviour of the split band at 1498 cm⁻¹ is strongly dependent on sample. The observed spectral modifications are related to an asymmetric deformation of TTM-TTP. Taking into account temperature dependence of bands attributed to the CH stretching and SCH₃ bending vibrations, we suggest that the TTM-TTP deformation can exist also above the phase transition temperature. Above 160 K molecules fluctuate between distorted and symmetrical state forming non-stable domains (pre-transitional effects), but below 160 K the molecular distortion and domains are stable. The existence of electronic band at 5000 cm⁻¹ for the polarization perpendicular to TTM-TTP supports this picture.     

Absorption and fluorescence spectra of gallium phosphide（GaP） nanoparticles

The optical absorption spectrum ranging from 200 to 800 nm and fluorescence spectra ranging from 300 to 650 nm of GaP nanoparticles at room temperature were reported. From the optical absorption spectrum it is inferred that the GaP nanoparticles exhibit a direct transition of about 410 nm （3.02 eV） and an indirect transition around 480 nm （2.58 eV）. In addition, an absorption peak at about 308 nm （4.02 eV） corresponding to the direct transition at higher energy was observed. The absorption peak was attributed to the transition from the spin-orbit-split valence band to the lowest conduction band along the A direction. By observing the fluorescence of the GaP nanoparticles, it follows that multiple emission bands corresponding to the violet, blue, and yellow light are shown peaking at about 400.4-414.1 nm （3.097-2.994 eV）, 450.1-466.8 nm （2.755-2.656 eV）, and 582.4 nm （2.129 eV）, respectively. The violet and blue light emissions are ascribed to the direct and indirect transitions from conduction band to valence band of the GaP nanoparticles. As to the weak yellow emission, it may be attributed to the radiative recombination from defect centers. The spin-orbit-splitting of the GaP nanoparticles is determined as about 100 meV.