Phonon and thermal properties of achiral single wall carbon nanotubes

A detailed theoretical study of the phonon and thermal properties of achiral single wall carbon nanotubes has been carried out using force constant model considering up to third nearest-neighbor interactions. We have calculated the phonon dispersions, density of states, radial breathing modes (RBM) and the specific heats for various zigzag and armchair nanotubes, with radii ranging from 2.8 Å to 11.0 Å. A comparative study of phonon spectrum with measured Raman data reveals that the number of Raman active modes for a tube does not depend on the number of atoms present in the unit cell but on its chirality. Calculated phonon modes at the zone center more or less accurately predicted the Raman active modes. The radial breathing mode is of particular interest as for a specific radius of a nanotube it is found to be independent of its chirality. We have also calculated the variation of RBM and G-band modes for tubes of different radii. RBM shows an inverse dependence on the radius of the tube. Finally, the values of specific heat are calculated for various nanotubes at room temperature and it was found that the specific heat shows an exponential dependence on the diameter of the tube.     

Phonon Spectrum and Structural Transformations at High Pressures in Vanadyl IV Phthalocyanine Crystals

The Raman scattering spectra and crystalline structure of vanadyl IV phthalocyanine (VOPc) at normal and high pressures has been studied. According to the X-ray diffraction data, the initial microcrystalline powder represented a mixture of the triclinic α phase (79%) and the monoclinic β phase (21%) possessing P$$\bar {1}$$ and P21/c symmetry, respectively. Raman spectra of the two phases are similar, but the phonon modes of the β phase are shifted toward higher frequencies (energies). The pressure dependence of the spectra of the α phase has been determined and it is established that the interval of 2.3–3.4 GPa reveals reversible pressure-dependent variations: above 3 GPa, some phonon modes exhibit splitting and the coefficients of pressure-induced (baric) shift for almost all modes show a decrease. A high-pressure feature observed in the Raman spectra can be related to changes in intermolecular interactions in crystalline structure of the α phase. The pressure dependence of the α phase unit cell volume measured at pressures increasing up to 4 GPa is a smooth monotonic function that can be well described by the Murnaghan equation of state. The obtained data were used to calculate the Grüneisen parameters of VOPc phonon modes.     

n-SiC拉曼散射光谱的温度特性

测量了采用离子注入法得到掺N的n-SiC晶体从100—450 K的拉曼光谱. 研究了SiC一级拉曼谱、电子拉曼散射谱及二级拉曼谱的温度效应. 实验结果表明,大部分SiC一级拉曼峰会随温度升高向低波数方向移动,但声学模红移(峰值位置向低频方向移动)的幅度较光学模小. 重掺杂4H-SiC的纵光学声子等离子体激元耦合(LOPC)模频率随温度升高表现出先蓝移(峰值位置向高频方向移动)后红移的变化趋势,表明LOPC模的温度特性不仅会受到非简谐效应的影响,还与实际已离化杂质浓度有关. 电子拉曼散射峰线宽随温度升高而增 关键词： 碳化硅 温度 纵光学声子等离子体激元耦合模 电子拉曼散射     

一种基于荧光材料的聚合物白光发光二极管

制备了一种基于荧光聚合物共混的单发光层聚合物白光发光二极管.器件结构为铟锡氧化物/苯磺酸掺杂聚乙烯基二氧噻吩/发光层/ 1,3,5-三(N-苯基-2-苯并咪唑-2)苯41/Ba/Al,蓝光材料芴-氟化喹喔啉共聚物(PF-BPFQ5)、绿光材料苯基取代的聚对苯乙炔(P-PPV)和红光材料聚(2-甲氧基-5-(2′-乙基己氧基)-1,4-对苯乙炔)(MEH-PPV)共混为发光层.当PF-BPFQ5,P-PPV,MEH-PPV的质量比例为100∶06∶06时,获得标准的白光,色坐标为（033 关键词： 聚合物发光二极管 白光 共混     

Surface optical phonon—assisted electron Raman scattering in a semiconductor quantum disc

We have carried out a theoretical calculation of the differential cross section for the electron Raman scattering process associated with the surface optical phonon modes in a semiconductor quantum disc.electron states are considered to be confined within a quantum disc with infinite potential barriers.The optical phonon modes we have adopted are the slab phonon modes by taking into consideration the Frohlich interaction between an electron and a phonon.The selection rules for the Raman process are given.Numerical results and a discussion are also presented for various radii and thicknesses of the disc,and different incident radiation energies.     

Phonon freedom and confinement in GaAsAlxGa1−xAs superlattices

Phonon modes in GaAsAl_xGa_1?xAs superlattices simplify when the phonon wavevector q is perpendicular to the plane of the layers. We have studied such modes using a Raman back-scattering technique on SL's grown by MBE. The results are consistent with simple ideas of LA phonon freedom and LO phonon confinement suggested by one-dimensional lattice dynamical calculations. The longitudinal acoustic (LA) modes show zone folding due to mini-zone formation. Their frequencies occur in doublets linearly dependent on q and show little mini-gap formation. This is consistent with a picture of approximately free plane wave propagating through the interfaces with Raman coupling due to SL layering of the photoelastic coefficient. By contrast, Raman data on LO modes in small period GaAsAlAs SL's suggest that these modes are standing waves strongly confined in either GaAs or AlAs.     

气相外延GaN的拉曼散射

测量了在蓝宝石衬底上气相外延生长GaN的拉曼散射谱.除观察到已被确认的两个E2,一个A1(TO)和一个E1(TO)声于振动以外,在734±3cm-1处观察到一个散射峰且从实验上确认其为GaN的纵向光学声子模E1(LO).而且发现其强度与外延层晶体质量密切相关.A1(TO)和高频E2散射峰相对强度变化显示不同生长条件引起的外延层质量的变化.     

Microscopic theory on the Raman spectra of transition metal dichalcogenides in CDW state

Microscopic mechanisms are clarified for Raman scattering of collective modes, i.e., amplitude and phase modes in the charge density wave (CDW) state of transition metal dichalcogenides. We study three phonon process in triple CDW state and effects of $\text{partial}$ destruction of the Fermi surface due to the phase transition. It has then been understood how both phase and amplitude modes are given Raman activity and how A-E splittings of both modes and the commensurability pinning of the phase mode are related to the three phonon process. We can also explain change of Raman intensity of the originally Raman active A_1g phonon mode at phase transition as interference between paramagnetic and diamagnetic contributions for 2HTaSe₂ as well as other materials.     

Raman scattering using vortex light

We re-examine the theory of Raman scattering in cubic crystals. The unconventional vector potential of vortex light leads to new selection rules. We show that in this novel optical process, (a) silent phonon modes become active and (b) scattering tensors change for ordinary Raman active phonon modes. Calculation based on a simplified model shows that the vortex Raman scattering intensity can be comparable with that of ordinary Raman process.     

Surface optical Raman modes in GaN nanoribbons

Raman scattering studies were performed in GaN nanoribbons grown along [1 0 0]. These samples were prepared inside Na‐4 mica nanochannels by the ion‐exchange technique and subsequent annealing in NH₃ ambient. Detailed morphological and structural studies including the crystalline orientation were performed by analyzing the vibrational properties in these GaN nanoribbons. Pressure in the embedded structure was calculated from the blue shift of the E₂(high) phonon mode of GaN. Possible red shift of optical phonon modes due to the quantum confinement is also discussed. In addition to the optical phonons allowed by symmetry, two additional Raman peaks were also observed at ∼633 and 678 cm⁻¹ for these nanoribbons. Calculations for the wavenumbers of the surface optical (SO) phonon modes in GaN in Na‐4 mica yielded values close to those of the new Raman modes. The SO phonon modes were calculated in the slab (applicable to belt‐like nanoribbon) mode, as the wavenumber and intensity of these modes depend on the size and the shape of the nanostructures. The effect of surface‐modulation‐assisted electron–SO phonon scattering is suggested to be responsible for the pronounced appearance of SO phonon modes. A scaling factor is also estimated for the interacting surface potential influencing the observed SO Raman scattering intensities. Copyright © 2010 John Wiley & Sons, Ltd.     

Temperature‐dependent Raman scattering studies of the geometrically frustrated pyrochlores Dy2Ti2O7, Gd2Ti2O7 and Er2Ti2O7

The temperature‐dependent Raman studies of A₂Ti₂O₇(A = Dy, Er, Gd) were performed on single crystals and polycrystalline samples in the 4.2–295 K temperature range. The Raman spectra showed softening of the majority of phonon modes upon cooling in the whole temperature range studied and large decrease of linewidths. These changes have been analyzed in terms of strong third‐order phonon–phonon anharmonic interactions. Moreover, the 312 and 330 cm⁻¹ modes of Er₂Ti₂O₇(Gd₂Ti₂O₇) showed hardening upon cooling down to about 130 K (100 K) and then anomalous softening below this temperature. The observed anomalous behavior of the Raman modes indicates that some important changes occur in these materials at low temperatures. However, the origin of this behavior is still not clear. Copyright © 2008 John Wiley & Sons, Ltd.     

Lattice dynamics of FeSb2

The lattice dynamics of FeSb(2) is investigated by first-principles density functional theory calculations and Raman spectroscopy. All Raman- and infrared-active phonon modes are properly assigned. The calculated and measured phonon energies are in good agreement. We have observed strong mixing of the A(g) symmetry modes, with the intensity exchange in the temperature range 210 and 260 K. The A(g) mode repulsion increases by doping FeSb(2) with Co, with no signatures of the electron-phonon interaction for these modes.     

Cascaded and cross-cascaded χ(3) nonlinear optical effects in a new SRS-active YAlO3 crystal

Stimulated Raman scattering (SRS) is discovered under picosecond laser excitation in a YAlO₃ orthorhombic crystal, and its cascaded and cross-cascaded χ⁽³⁾ nonlinear optical interactions are studied. Investigations revealed three SRS-active phonon modes of the crystal and identified all its recorded Stokes and anti-Stokes generation lines.     

Raman scattering from microcrystals

This review discusses the size effects on Raman scattering from microcrystals. For ionic microcrystals, the existence of surface phonon modes is predicted from electromagnetic theories. It is shown that Raman spectroscopy is very effective to detect the surface phonon modes. The size effects on nonpolar phonons in covalent microcrystals can also be studied by Raman spectroscopy. However, the relaxation of the wave-vector selection rule or the phonon confinement explains only some of the experimental data. Development of lattice dynamical theories of Raman scattering from microcrystals including surface effects is highly required. Enhancement of Raman intensities arising from the excitation of electromagnetic normal modes of microcrystals is also discussed.     

准一维电荷密度波材料Ta2NiSe7 的转角及变温拉曼光谱研究

近年来, 低维过渡金属硫族化合物Ta2NiSe7 因其独特的电荷密度波和特殊的拓扑能带结构而引起广泛关注. 本文利用拉曼光谱技术对准一维电荷密度波材料Ta2NiSe7 的声子模式进行了系统研究. 角度依赖的偏振拉曼光谱实验表明室温下可以观测到19 个拉曼峰, 且所有观察到的拉曼模式强度随样品旋转呈周期性变化, 这意味着Ta2NiSe7 具有较高的面内各向异性. 通过群论分析, 我们确定了拉曼峰的具体模式为Ag 和Bg 两种原子振动模式. 在温度依赖的偏振拉曼光谱测量中, 我们在低温下还观察到了新的声子峰, 推测与电荷密度波结构调制引起的晶格畸变有关. 本研究提供了对Ta2NiSe7 声子振动的全面理解, 这可能为进一步研究电荷密度波与声子振动之间的关联提供参考意义.     

MBE生长的In_(1-x-y)Ga_xAl_yAs/InP四元混晶的拉曼散射

ＭＢＥ生长的Ｉｎ１－ｘ－ｙＧａｘＡｌｙＡｓ／ＩｎＰ四元混晶的拉曼散射韩和相汪兆平李国华徐士杰刘南竹朱作明（中国科学院半导体研究所半导体超晶格国家重点实验室北京１０００８３）ＯｐｔｉｃａｌＰｈｏｎｏｎＲａｍａｎＳｃａｔｅｒｉｎｇｆｒｏｍＩｎ１－ｘ－ｙＧ...     

仿岩浆岩玻璃相介观团粒结构类声子限制效应的拉曼光谱研究

应用微晶声子色散理论、量子化学从头计算与拉曼光谱(LWRS、RS)分析技术相结合的方法, 系统地探测了仿超基性岩(MU)、仿基性岩(MB)、仿中性岩(MI)和仿酸性岩(MA)等4个系列玻璃相的介观团粒结构参数及其类声子限制效应, 建立了各自的类声子色散曲线,进而获得了随着体系介观团粒尺度的增加,其低频端声学声子(横模与纵模)振动波数红移、中频及高频端的光学声子(横模与纵模)振动波数紫移、光学声子(横模)之拉曼谱峰的半高宽减小和其介观团粒尺度的变化范围增大等结论。这些结论不但丰富了低维非晶硅酸盐玻璃相及其熔体的类声子色散理论,而且开拓了拉曼光谱研究岩浆岩介观结构的新领域,为探索岩浆的结晶、演化和成矿规律提供重要理论依据及信息。     

High-energy phonon branches of an individual metallic carbon nanotube

We present excitation-energy dependent Raman measurements between 2.05 and 2.41 eV on the same individual carbon nanotube. We find a change in the Raman frequencies of both the D mode (63 cm(-1)/eV) and the high-energy modes. The observed frequencies of the modes at approximately 1600 cm(-1) as a function of laser-energy map the phonon dispersion relation of a metallic tube near the Gamma point of the Brillouin zone. Our results prove the entire first-order Raman spectrum in single-wall carbon nanotubes to originate from double-resonant scattering. Moreover, we confirm experimentally the phonon softening in metallic tubes by a Peierls-like mechanism.     

Surface acoustic phonon modes of Ge nanocrystals embedded in SiO2

Ge nanocrystals (NCs) embedded in SiO₂ are synthesized by ion implantation, and the surface vibrational modes of the Ge NCs are investigated using the low-frequency Raman scattering (LFRS) technique. LFRS studies show distinct low-frequency Raman modes in the range 6.5-21.2 cm⁻¹ for the Ge NCs depending on the implant dose and annealing temperature. These low-frequency Raman modes are attributed to the confined surface acoustic phonon modes of Ge NCs with (0,0) spheroidal mode and (0,3) torsional modes. Our results are in excellent agreement with the recent theoretical predictions of surface vibrational modes in Ge NCs.     

One phonon resonant Raman scattering in a quantized spherical film

We have presented a theoretical calculation of the differential cross section (DCS) for the electron Raman scattering (ERS) process associated with surface optical (SO) phonon modes in a semiconductor quantized spherical film. We consider the Fröhlich electron–phonon interaction in the framework of the dielectric continuum approach. We study the selection rules for the processes. Singularities are found to be size-dependent and by varying the size of the QDs, it is possible to control the frequency shift in the Raman spectrum. A discussion of the phonon behavior for the films with large and small size is presented. The numerical results are also compared with that of experiments.