Raman scattering of InAs/AlAs quantum dot superlattices grown on (001) and (311)B GaAs surfaces

ABSTRACT: We present a comparative analysis of Raman scattering by acoustic and optical phonons in InAs/AlAs quantum dot superlattices grown on (001) and (311)B GaAs surfaces. Doublets of folded longitudinal acoustic phonons up to the fifth order were observed in the Raman spectra of (001)- and (311)B-oriented quantum dot superlattices measured in the polarized scattering geometries. The energy positions of the folded acoustic phonons are well described by the elastic continuum model. Besides the acoustic phonons, the spectra display features related to confined transverse and longitudinal optical as well as interface phonons in quantum dots and spacer layers. Their frequency positions are discussed in terms of phonon confinement, elastic stress, and atomic intermixing.     

Temperature-dependent Raman scattering in round pit of 4H-SiC

The Raman spectra of N-doped 4H-SiC single crystal films is investigated between 100 and 600 K. The temperature dependence of the three optical modes is obtained. These measurements reveal that all Raman peaks shift to lower frequencies with increasing temperature, except A₁(LO). The temperature dependence of A₁(LO) phonon modes in the round pit also manifests different features with temperature increasing, but the demarcation temperature point of the blueshift and the redshift in the round pit is higher than that in the outer area. At high temperature, all active phonon modes clearly become broader, but the linewidth of the E₁(TO) phonon mode from round pit increases with temperature more rapidly than that from the outer area, this indicates that the lifetime of the E₁(TO) phonon in round pit is more sensitive than that in the outer area.     

Anharmonicity and local noncentrosymmetric regions in BaTiO3 pressed powder studied by the Raman line temperature dependence

The temperature dependencies of position, width, and integral amplitude the E(TO) line near 307 cm⁻¹ in Raman spectra in barium titanate powders with different non-hydrostatic pressure and temperature treatment were studied. It was found that parameters of the E(TO) line near 307 cm⁻¹ are different in the crystal, the untreated powder, the powder treated by the non-hydrostatic pressure, and the powder annealed after the pressure treatment. The line width (FWHM) increase with temperature according to Klemens model. It indicates that the origin of the line broadening is the anharmonicity of the E(TO) phonons. The pressure treatment changes the anharmonicity of the phonon potential. It was found that the temperature dependence of the integral intensity of the E(TO) line is similar to that of the second optical harmonic signal and reflects the presence of the local polar regions. Thus, the E(TO) line in the Raman spectrum allows one to characterize the average polarity of local regions and their anharmonicity depending on non-hydrostatic pressures and thermal treatment.     

氮化铝镓纳米固体材料的合成及其拉曼光谱

利用氨热法合成了AlxGa1-xN合金的纳米固体，其中Al含量的摩尔数可在x＝0－1的范围内变化，在x值从0到1整个范围内，对该合金的纳米固体进行了激光Raman光谱分析，证实了A1（LO）声子的模式行为和E2声子的的双模式行为，从实验现象中还可以推断出A1（TO）声子可能具有单模式行为，A1（LO）声子频率在x＝0－0．5范围内迅速单调增加，当x＞5时则缓慢增加，这可能是由于纳米颗粒之间的相互作用造成的，对于A1（TO），T1（LO），和E2（GaN）声子模式，其线宽在x＝0．5时达到最大值。     

Structural and optical characterization of Zn0.95−xMg0.05AlxO nanoparticles

The structural and optical properties of ZnO nanoparticles doped simultaneously with Mg and Al were investigated. XRD results revealed the hexagonal wurtzite crystalline structure of ZnO. The FE-SEM study confirmed the formation of nano-sized homogeneous grains whose sizes decreased monotonously with increasing doping concentrations of Mg and Al. The absorption spectra showed that band gap increased from 3.20 to 3.31 eV with Mg doping. As the Al concentration changed from x=0.01 to x=0.06 mol% at constant Mg concentration the band gap observed to be decreased. Particle sizes estimated from effective mass approximation using absorption data and these values are in good agreement with the crystallite sizes calculated from XRD data. Raman spectra of ZnO showed a characteristic peak at 436 cm⁻¹ correspond to a non-polar optical phonon E₂ (high). With increase of the Al doping concentrations, E₂ (high) phonon frequency shifted to 439 cm⁻¹ from to 436 cm⁻¹. The origin of E₂ (high) peak shift in ZnO nanoparticles is attributed to optical phonon confinement effects or the presence of intrinsic defects on the nanoparticles. PL spectra indicated that with increase of Al co-doping along with Mg into ZnO, intensity of the peak positioned at 395 nm was initially increased at x=0 and then decreased with increase of the Al concentrations from x=0.01 to x=0.06 mol%.     

声子态密度对掺稀土玻璃材料上转换发光强度的影响

用熔融法制备了碲酸盐玻璃(PWT)和锗酸盐玻璃(PWG),测试了它们的上转换发光。试验发现声子能量高的锗酸盐玻璃上转换发光强度远强于声子能量低的碲酸盐玻璃上转换荧光的“异常”现象。从物理模型及其物理意义上,并从Raman光谱定性分析得知,当基质材料的声子能量相差不大时,声子态密度对上转换发光强度起决定性作用,从而能对试验现象进行较为圆满的解释。     

Rolled-up tubes and cantilevers by releasing SrRuO3-Pr0.7Ca0.3MnO3 nanomembranes

ABSTRACT: Three-dimensional micro-objects are fabricated by the controlled release of inherently strained SrRuO3/Pr0.7Ca0.3MnO3/SrRuO3 nanometer-sized trilayers from SrTiO3(001) substrates. Freestanding cantilevers and rolled-up microtubes with a diameter of 6 to 8 μm are demonstrated. The etching behavior of the SrRuO3 film is investigated, and a selectivity of 1:9,100 with respect to the SrTiO3 substrate is found. The initial and final strain states of the rolled-up oxide layers are studied by X-ray diffraction on an ensemble of tubes. Relaxation of the sandwiched Pr0.7Ca0.3MnO3 layer towards its bulk lattice parameter is observed as the major driving force for the roll-up of the trilayers. Finally, μ-diffraction experiments reveal that a single object can represent the ensemble proving a good homogeneity of the rolled-up tubes.PACS: 81.07.-b; 68.60.-p; 68.37.Lp; 81.16.Dn.     

NBT基无铅压电陶瓷的拉曼光谱研究

利用拉曼散射并结合群论系统研究了(Na0.5Bi0.5)(1-x)TiO3-BaxTiO3(简称NBT-BT)的晶体振动及铁电相到顺电相的结构相变。结果表明,在室温下随着Ba2 含量的增加,521.4cm-1光学声子模劈裂为双峰且高频声子模向高频漂移;随着温度的升高,BT含量为6%的样品在460￣620cm-1范围内,高频峰出现了明显的软化现象。因此,通过拉曼散射的分析,可以表征NBT基陶瓷中相结构的变化趋势。     

绝热圆腔内4根冷热微管阵列振动强化传热实验

目前针对微细管管外同时有自然对流和周期横向振动存在情况下的混合对流问题鲜有研究。为了探究这种现象, 进而开发一种新型的换热器, 本文提出了一种由4根冷热微细管组成的微管阵列, 将其平行放置于有中间流体的圆形腔体内, 并对管束施加微小横向振动的混合对流传热机理进行研究。详细描述了实验系统及实验方法, 以及后期对实验数据的处理方法, 得到了传热Nu数随管间自然对流强度Ra数, 强迫振动Re_v数等量纲为1参数的变化曲线;揭示了该结构形式下传热特性随冷热管温差、振动频率、微管振幅等传热规律。结果表明, 横向振动在小温差情况下传热强化效果更为明显。将本文实验数据与Lemlich单管振动实验中的数据进行对比, 表明了振动对4根微管阵列的强化传热效果比单根管更明显。     

UV Raman characteristics of nanocrystalline diamond films with different grain size

Nanocrystalline diamond films with different size were characterized by ultraviolet (UV) (244 nm) Raman spectroscopy. It was found that a diamond peak at 1333 cm⁻¹ was enhanced, while the D and G peak of graphite as well as photoluminescence was suppressed, compared with that measured by visible (514.5 nm) Raman. With decreasing the particle size from 120 to 28 nm, the diamond peak shifts from 1332.8 to 1329.6 cm⁻¹, the line width of the peak becomes broader, the intensity ratio of diamond and G peak decreases. The down shift and broadening of the diamond peak depending on the particle size by UV Raman measurements are consistent with the phonon confinement model.     

Raman spectroscopy on isolated single wall carbon nanotubes

A review is presented on the resonance Raman spectra from one isolated single wall carbon nanotube. The reasons why it is possible to observe the spectrum from only one nanotube are given and the important structural information that is provided by single nanotube spectroscopy is discussed. Emphasis is given to the new physics revealed by the various phonon features found in the single nanotube spectra and their connection to spectra observed for single wall nanotube bundles. The implications of this work on single wall carbon nanotube research generally are also indicated.     

掺铈铌酸锂晶体的生长和表征

采用提拉法生长了掺Ce的同成分LiNbO3（Ce：CLN）晶体。X射线粉末衍射表明,掺杂并不影响晶体结构,其空间群为R3c。采用X射线荧光光谱分析方法,计算得到铈离子的有效分凝系数为0.4。室温下,晶体在700～3500nm波段的透过率达75%。利用X射线光电子谱（X-ray photoelectron spectroscopy,XPS）和Raman光谱分别表征了铈离子对铌酸锂的电子和声子结构的影响。XPS结果表明,铌离子仅存在纯的＋5价,铈离子由于浓度较小,未检测到信号。Raman光谱结果显示,除Ce：CLN晶体的A1（TO4）模波数和A1（TO1）模半峰宽较纯的LiNbO3晶体发生明显变化外,其余模都与其基本一致,表明铈离子主要进入晶格的Nb位置。     

Laser Ablation Synthesis and Optical Characterization of Silicon Carbide Nanowires

Silicon carbide (SiC) nanowires were synthesized at 900°C by the laser ablation technique. The growth morphology, microstructure, and defects in SiC nanowires were characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The Raman scattering study indicated that the Raman peaks corresponding to the TO and LO phonon modes of the SiC nanowires had larger red shifts compared to those of bulk SiC material. The red shift, broadening peak, and the asymmetry of the Raman peak could be explained by the size confinement effect in the radial and growth directions. The growth mechanism of SiC nano-wires was discussed based on the vapor–liquid–solid reaction.     

Photoluminescence and thermal expansion of zirconium vanadate

The optical property and phase transition of ZrV₂O₇ were investigated by temperature-dependent X-ray diffraction, photoluminescence (PL), and Raman spectra. It was found that the material exhibited PL emission in the visible region. The temperature-dependent PL emission showed that the peak energy for emission (at red wavelength) presented a red shift from 10 to 80 K, and then a blue shift with the increase in temperature. The PL emission bands of ZrV₂O₇ shifted with change in temperature relating to its phase transition (80-100 K), which could be confirmed by temperature-dependent Raman spectra.     

Theory of Raman Scattering by Phonons in Germanium Nanostructures

Within the linear response theory, a local bond-polarization model based on the displacement–displacement Green’s function and the Born potential including central and non-central interatomic forces is used to investigate the Raman response and the phonon band structure of Ge nanostructures. In particular, a supercell model is employed, in which along the [001] direction empty-column pores and nanowires are constructed preserving the crystalline Ge atomic structure. An advantage of this model is the interconnection between Ge nanocrystals in porous Ge and then, all the phonon states are delocalized. The results of both porous Ge and nanowires show a shift of the highest-energy Raman peak toward lower frequencies with respect to the Raman response of bulk crystalline Ge. This fact could be related to the confinement of phonons and is in good agreement with the experimental data. Finally, a detailed discussion of the dynamical matrix is given in the appendix section.     

Pressure and Temperature Dependence of the Raman Spectra of Zirconia and Hafnia

The pressure dependence of the phonon frequencies in the isomorphic materials ZrO₂ and HfO₂, and the temperature dependence of the phonon frequencies of HfO₂, were investigated by Raman spectroscopy in the ranges 0 to 16 GPa and 20 to 800 K. The comparison of the pressure dependence of the Raman spectra of the two materials helps to elucidate the correlation of their vibrations in the intermediate spectral region (240 to 410 cm⁻¹). The high-pressure phase of ZrO₂ is stable up to 16 GPa and of tetragonal symmetry. HfO₂ does not show any pressure-induced phase transition up to 12 GPa, while the temperature effect on the phonon frequencies is similar to that of ZrO₂.     

Applications of laser Raman microprobe spectroscopy to the characterization of coals and cokes

Optical microscopy is widely used in the characterization of coals and cokes. This Paper shows that the laser Raman microprobe (MOLE) which combines an optical microscope and a Raman spectrometer can provide useful additional information. Three main areas were investigated: identification of minerals in coal and coke; structural characterization of coals and cokes; and the interaction of inorganic additives and coal. Where possible, the results were compared with conventional optical microscopy measurements whereby it was shown that the optical texture and Raman spectra of cokes are not closely related. The Raman spectra of high temperature cokes were used to estimate the size of microcrystalline regions.     

用于硅酸盐熔体微结构研究的高温Ramam光谱技术

硅酸盐熔体微结构通常在熔态下进行实验研究，而不是采用淬冷物质或玻璃样品的间接研究方法。为此比较了当前若干微结构测试方法用于高温的可能性，方法的特点，适用范围和缺陷。此处介绍一种高温Raman光谱技术，它配备了显微热台，实现了高温达1623K的共焦显微Raman；并采用了光谱信号的时间分辨检测技术，实现了可达2023K温度下的宏观Raman，拓展了Raman光谱研究高温物质结构的应用。通过对硅酸盐等熔体的Raman光谱测定和相关淬冷玻璃光谱的比较研究，表明高温Raman光谱是实时和原位研究熔体微结构的有效实验方法，同时探讨了将电荷耦合探测技术引入高温Raman光谱技术的可行性。     

Isostructural phase transition (IPT) in CaMoO4 with Scheelite structure

The ceramic compound CaMoO₄ is synthesized via a solid-state reaction technique. Rietveld refinement studies were done on the powder X-ray diffraction data of CaMoO₄ and revealed that the compound is crystallized in the tetragonal Scheelite structure with I4₁/a space group. The differential scanning calorimetry (DSC) studies on CaMoO₄ divulged an anomaly around 440 °C. This anomaly is further probed using the temperature-dependent Raman and dielectric spectroscopic measurements and are corroborating with the results obtained from DSC. A detailed investigation on the temperature-dependent Raman spectroscopic data revealed that the A_1g mode of CaMoO₄ showed a soft phonon behavior up to the phase transition temperature. It is observed that the A_1g mode displayed phonon hardening behavior with further increasing the temperature. The anomaly is attributed to an isostructural phase transition (IPT), a rarely observed phenomenon in the compounds with Scheelite structure. The IPT in CaMoO₄ is elucidated with a phonon softening mechanism.     

Raman and dielectric spectroscopic analysis of magnetic phase transition in Y(Fe0.5Cr0.5)O3 multiferroic ceramics

Here, we report the Raman and dielectric spectroscopic studies as a function of temperature of orthorhombically distorted Y(Fe_0.5Cr_0.5)O₃ (YFC) ceramics, measured from 80 to 300 K. The dc-magnetization measurements under field cooled (FC)-zero field cooled (ZFC) protocol indicate a small onset of magnetic ordering at T_N∼270 K. The field dependent magnetization plot recorded at 50 K, 150 K and 200 K show a clear opening in hysteresis loops. The linear dependence of magnetization plot at high field without any saturation of magnetization indicates the coexistence of weak ferromagnetic (WFM) component within the canting antiferromagnetic (CAFM) matrix. Temperature evolution of Raman line-shape parameter of B_2g(4) phonon mode clearly exhibits an anomalous behavior of phonon shift near T_N∼270 K, indicating the spin-phonon coupling in the ceramics. From the temperature dependent dielectric permittivity (ε(T)) study, two dielectric relaxation peaks are detected below 200 K and above 250 K. The appearance of former relaxation peak is responsible for polaronic conduction mechanism, while the later one is associated with magnetic phase transition which might be relevant to the presence of magnetoelectric coupling in YFC ceramics. The observed P-E hysteresis loops at room temperature indicate weak ferroelectric nature of the ceramics.