Probing crystallographic orientation of a single GaN nanotube using polarized Raman imaging

We report the study of crystallographic orientation of a highly anisotropic square‐shaped single wurtzite GaN nanotube (NT) probed by polarized Raman imaging. The polarization‐dependent Raman intensities are collected in two mutually orthogonal directions with respect to the NT long axis. Variation in intensities of symmetry allowed A₁(LO) mode of GaN with two different polarization directions reveals the possible crystalline orientations in the NT. The polarized Raman spectral imaging illustrates the fact of inhomogeneous crystalline orientations along the edges of the NT facets owing to its finite wall thickness. The deviation of polarization selection rule for GaN is attributed to the geometrical anisotropy of the NT. Electron microscopic structural data confirms the spectral imaging analyses. Copyright © 2013 John Wiley & Sons, Ltd.     

Raman scattering enhancement by dielectric spheres

Raman scattering experiments were performed on Si(60 nm)/metal/substrate structures with and without silica microspheres (with a diameter between 0.5 and 5 µm) on top. Raman scattering from the thin Si layer exhibits enhancements (~20) due to the dielectric spheres, where the enhancement factors depend on the diameter of the spheres. The interaction between light and dielectric spheres has been simulated by finite difference time domain calculations (FDTD), wherein particularly the electric energy density (ED) distribution in the thin Si layer was of concern. For microspheres with a diameter less than ~3 µm, the transverse ED distribution (perpendicular to the incident light direction) within the Si layer is characterised by a single peak centered on the optical axis. For larger diameters, a multimodal transverse ED distribution develops where the maximum is not centered on the optical axis. Using an ad‐hoc approach for surface enhanced Raman scattering in combination with the FDTD calculations, the experimental Raman observations are well accounted for. Copyright © 2013 John Wiley & Sons, Ltd.     

Si/SiGe异质结构的硅盖层中应变对Raman谱特征的影响

应变Si/SiGe异质结构通过大剂量Ge离子注入并结合高温快速热退火制备而成。325 nm波长的紫外激光被用于调查应变Si盖层的Raman谱特征。实验发现,硅盖层中的张应变导致硅的520 cm-1的一级拉曼散射峰向低频方向偏移,峰的偏移程度反映硅盖层中横向张应力的大小约为12.5×108 N·m-2。硅盖层中的张应变并未导致1 555和2 330 cm-1的次级拉曼散射峰的变化。     

Polarization resolved stimulated raman scattering: probing depolarization ratios of liquids

Polarization resolved stimulated Raman scattering (SRS) signal is described in the case of isotropic media and linearly polarized incident fields. The model gives simple expressions for the two perpendicularly polarized SRS signals I_X and I_Y, detected along the X and Y directions, respectively, as a function of the incident pump and Stokes polarization angles. We find that Raman depolarization ratio can be simply obtained from the ratio of the SRS intensities detected along the X and Y axis. These theoretical findings are supported by polarization resolved SRS measurements performed on polarized and depolarized bands of cyclohexane. Copyright © 2011 John Wiley & Sons, Ltd.     

The relationship of angle resolved light scattering from surface optical resonances to the surface enhanced Raman effect

For a given mildly rough Ag surface, the angle resolved pattern bears the same shape for elastic light scattering, for Raman scattering from adsorbed pyridine or cyanide, or for the luminescence background. This result suggests that surface enhanced Raman scattering is closely related to topographically related surface optical resonances.     

Thermal conductivity of VLS-grown rough Si nanowires with various surface roughnesses and diameters

In this paper, we synthesize VLS-grown rough Si nanowires using Mn as a catalyst with various surface roughnesses and diameters and measured their thermal conductivities. We grew the nanowires by a combination vapor-liquid-solid and vapor-solid mechanism for longitudinal and radial growth, respectively. The surface roughness was controlled from smooth up to about 37 nm by the radial growth. Our measurements showed that the thermal conductivity of rough surface Si nanowires is significantly lower than that of smooth surface nanowires and decreased with increasing surface roughness even though the diameter of the smooth nanowire was lower than that of the rough nanowires. Considering both nanowires were grown via the same growth mechanism, these outcomes clearly demonstrate that the rough surface induces phonon scattering and reduces thermal conductivity with this nanoscale-hole-free nanowires. Control of roughness induced phonon scattering in Si nanowires holds promise for novel thermoelectric devices with high figures of merit.     

Size effects on phonon localization and Raman enhancement in silicon nanotips

Silicon nanotip arrays exhibit a wide variety of interesting optical and electronic properties associated with their dimensionality. We here investigate the effect of size‐induced changes on phonon localization and explain the enhanced Raman response. The occurrence of normally forbidden transitions in the photoluminescence spectra provides evidence for the predicted localization effect. Spatially resolved Raman spectroscopy reveals a continuous change of the silicon Raman peak position and peak width along the nanotip that is attributed to a smooth change between bulk properties at the base to size‐induced phonon confinement in the apex of the nanotip. This approach allows to exclude heating effects that normally overwhelm the phonon confinement signature. The Raman spectra are in excellent agreement with the spatial correlation model and the extracted correlation length is comparable to the tip dimensions. The observed phonon confinement coincides with an enhancement of the Raman scattering efficiency at the tip apex and results in a 40‐fold increase of the sample's Raman intensity compared with bulk silicon. These results provide a step toward the integration of Si based optoelectronic devices. Copyright © 2012 John Wiley & Sons, Ltd.     

在Si(111)上用有机溶胶凝胶甩膜热解法制备(0001)定向的6H-SiC薄膜

在Si(111)衬底上用聚苯乙烯溶胶凝胶甩膜并经950℃真空(10^-3Pa)热解处理法,制备出晶态SiC薄膜.用FTIR,XRD,TEM,RamanXPS等方法研究了SiC薄膜的晶体结构、微结构、组成以及各元素的化学态等性质.结果表明制得的是沿(0001)高度择优取向的晶态6H-SiC薄膜.膜中SiC晶粒沿c轴柱状生长,其最大尺寸约150nm,膜厚约为0.3μm,SiC中的Si/C比约为1.表层有少许污染C(CH和CO)和少量O(Si₂O₃,CO态氧和吸附氧).从对比实验可知,在热解时将甩膜的Si片与另一空白Si片面面相贴可明显增加SiC的生成量. 关键词： 碳化硅 薄膜 溶胶凝胶     

Plasmon‐enhanced polarized Raman spectroscopy for sensitive surface characterization

Local‐mode and localized surface plasmons generated on the silver thin film can selectively enhance the Raman signal from the surface. Further improvement of surface signal can be obtained by using the polarized Raman technique that results in a dramatic enhancement of the surface sensitivity by up to 25.4 times as compared to that without a silver coating. This technique will be very useful for Raman study on samples that suffer overlapping background signal. In this article, we show that it can be used to significantly improve the signal of thin strained‐Si layer on top of SiGe buffer layer. Copyright © 2008 John Wiley & Sons, Ltd.     

Growth and evolution of epitaxial erbium disilicide nanowires on Si (001)

Sub-monolayer amounts of Er deposited onto Si (001)react with the substrate to form epitaxial nanowires of crystalline ErSi₂. The growth of uniaxial structures occurs because the different crystal structures of ErSi₂ and Si have a good lattice match along one Si<110> crystallographic axis but a significant mismatch along the perpendicular Si<110> axis. The nucleation, growth, and subsequent evolution of ErSi₂ nanowires were investigated as functions of erbium coverage on the Si (001) surface, annealing time, and annealing temperature. Low annealing temperatures (620 °C) and times (5 min) produced ErSi₂ nanowires with widths of a few nanometers, heights less than one nanometer, and lengths of several hundred nanometers. For longer annealing times at low temperature, the surface roughened without significant ripening of the wires. Annealing at intermediate temperatures (∼700 °C) caused stacking faults to form along the long axis of the nanowires and their lengths to ripen. At high temperature (800 °C), the wires broke apart into short segments with stacking faults. Received: 30 January 2002 / Accepted: 31 January 2002 / Published online: 3 May 2002     

Plasmon‐enhanced Raman scattering by suspended carbon nanotubes

We report plasmon‐enhanced Raman scattering of the order of 10³ by a metallic carbon nanotube partially suspended inside a near‐field cavity. The tube is part of a small bundle, and is interfaced with an Au nanodisc dimer using a recently developed assembly scheme based on dielectrophoretic deposition. Spatially resolved Raman measurements with two excitation wavelengths and two orthogonal polarizations confirm that the enhancement arises from a 65 nm long suspended tube segment. We show that the orientation of the tube inside the cavity can be as effective for generating enhancement as placing the nanotube precisely in a plasmonic hotspot. Position and shape of the G‐peak show that the suspended part of the tube is free of strain and doped with a Fermi energy shift ≤40 meV. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Inelastic light scattering and X-ray diffraction from thick free-standing porous silicon films

The results of Raman scattering and X-ray diffraction studies of thick, free-standing, porous Si layers with thickness up to 500 μm are presented. The Raman scattering spectra have a distinctive difference from previous data for porous Si films on Si substrate and for thin, free-standing, porous Si layers. The experimental data can be explained by a modified phonon confinement model that accounts for a comprehensive strained Si nanocrystal. The comprehensive strain is a tensile one, and the value of stress can be up to 3 GPa. This interpretation is supported by data of X-ray diffraction measurements.     

Scattering of Radiation of the Sun and the Sky from Two Dimensional Rough Sea Surfaces

The scattering of optical wave from two dimensional rough sea surfaces is studied first with method of facets. The sea surface is divided into many facets, and each facet is treated as a surface with small roughness instead of a smooth plane, therefore more practical and effective. In addition the shadowing function of sea surfaces for arbitrary incident and scattering angles is numerically calculated with the Z-BUFFER method, which applies to any kinks of rough surfaces compared with the methods available. Finally the spectral irradiance of the sun and the spectral radiance of the sky for different time at sea level with fine weather are obtained with the software of Lowtran7, and the scattering of the radiation of the sun and the sky from two dimensional rough sea surfaces for different time, waveband and wind speed is studied, which is of great reference value for reducing the interference to the infrared detector due to the scattering of the radiation of the sun and the sky from sea surfaces.     

Electronic structure of twinned ZnS nanowires

The electronic properties of twinned ZnS nanowires (NWs) with different diameters were investigated based on first-principles calculations. The energy band structures, projected density of states and the spatial distributions of the bottom of conduction band and the top of the valence band were presented. The results show that the twinned nanowires exhibit a semiconducting character and the band gap decreases with increasing nanowire diameter due to quantum confinement effects. The valence band maximum and conduction band minimum originate mainly from the S-p and Zn-s orbitals at the core of the nanowires, respectively, which was confirmed by their spatial charge density distribution. We also found that no heterostructure is formed in the twinned ZnS NWs since the valence band maximum and conduction band minimum states are distributed along the NW axis uniformly. We suggest that the hexagonal (2H) stacking inside the cubic (3C) stacking has no effect on the electronic properties of thin ZnS NWs.     

Ca_(1-x)Al_2Si_2O_8∶Eu_x表面结构与荧光强度相互关系的研究

采用固相法分别在1 150,1 250,1 350,1 450℃下制备了Ca_(1-x)Al_2Si_2O_8∶Eu_x(x=0,0.01,0.05,0.15)系列微晶材料。通过X射线衍射仪(XRD)、拉曼光谱仪(Raman)、光致发光光谱仪(PL)和X射线荧光光谱仪(XRF)研究了CaAl_2Si_2O_8表面结构与荧光强度之间的相互关系。XRD和Raman结果表明:在制备CaAl_2Si_2O_8材料的过程中,随着温度不断升高,原材料逐渐结晶形成结构较为完整的CaAl_2Si_2O_8相;并且从拉曼光谱可以清晰看出,当Eu掺杂量相同时,随着烧结温度的升高,Si—O非晶相逐渐减少,硅氧四面体逐步形成,其振动峰位置逐渐向低波数移动,但当温度过高时硅氧四面体破坏形成宽化的的非晶峰;Eu的掺杂阻碍了Al取代Si位置的过程,因此在1 620波数处振动峰先增强后减弱。这种材料表面结构的变化与Eu的掺杂密切相关,影响着材料表面Eu原子数量分布。PL和XRF结果表明:相同Eu掺杂量时,温度越高越有利于Eu原子扩散到样品表面,从而使样品的荧光强度更强。因此样品的荧光强度和样品单位表面积Eu原子数量存在正比关系。     

Determination of the Surface Facets of Gold Nanorods in Wet‐Coated Thin Films with Grazing‐Incidence Wide Angle X‐Ray Scattering

This work studies the surface facets of gold nanorods (AuNRs) in wet‐coated nanoparticle thin films with synchrotron‐light‐based grazing‐incidence wide angle X‐ray scattering (GIWAXS), which provides statistically relevant results on many nanoparticles. Air‐brush spraying deposits the monodisperse AuNRs into sparse monolayers where the long axis of rods is parallel to the substrate surface. It is found that the crystalline facets of individual AuNRs in the sparse monolayer are all in the same orientation, as indicated by narrow azimuthal widths of (200) reflections, over a macroscopic scale comparable to the substrate. This alignment is probably due to the rods' sitting on high‐index surface facets such as (520) and (250). A quantitative analysis of the angles between bulk facets and the surface facets leads to a “nested‐octagon” model for the cross sections of AuNRs: shell octagon with high‐index crystalline facets (520), (5‐20), (2‐50), (‐2‐50), (‐5‐20), (‐520), (‐250), and (250), and core octagon consisting of low‐index crystalline facets (100), (1‐10), (0‐10), (‐1‐10), (‐100), (‐110), (010), and (110).     

Polarized SERS study of an individual Ag nanowire with bulb humps

Polarization-dependent surface enhanced Raman scattering (SERS) of an individual Ag nanowire (NW) with bulb humps (including nm-scaled bulb humps on the body part and a bulb NW-tip) was investigated. Strong SERS effect was observed from the bulb NW-tip, which exhibited cos²θ dependence relative to the polarization angle θ of the incident laser. Such dependence is similar to that of the NW-body with bulb humps, but different from that of the other NW-tip with crown shape. Their different polarized SERS behaviors along with the bulb hump effect on SERS were discussed.     

A high‐throughput method for controlled hot‐spot fabrication in SERS‐active gold nanoparticle dimer arrays

We present a high‐throughput method for fabricating large arrays of surface‐enhanced Raman scattering (SERS) active gold dimers. Using a large‐area/low‐cost nanopatterning method in conjunction with a meniscus force deposition technique, we were able to create large arrays of uniformly spaced nanoclusters comprising two 60‐nm gold nanospheres. Raman measurements of a thiophenol monolayer deposited on smaller scale arrays of aligned dimers yielded enhancement factors as high as 10⁹. Polarization‐controlled measurements show spectral peak heights to be 10–100 times smaller when the incident beam is polarized perpendicularly to the dimer axis, confirming that the measured enhancements arise from the ‘hot spots’ between the two nanospheres. Copyright © 2009 John Wiley & Sons, Ltd.     

Terahertz studies of carrier localization in spontaneously forming polar lateral heterostructures

We employ near‐bandgap terahertz emission spectroscopy to study lateral heterostructures resulting from basal plane stacking faults in m ‐plane GaN. The predominant stacking faults have I₁ character and behave as an array of spontaneously forming layers comprised of a single cubic stacking sequence within the wurtzite matrix that terminate the spontaneous polarization along the in‐plane c‐axis, leading to strong lateral electric fields. Spectral tuning of femtosecond excitation pulses enables observation of the transition from carrier transport in the continuum to formation of instantaneous dipoles and nonlinear susceptibility associated with both the quantum‐well‐like regions of the cubic layers and polarization‐induced triangular‐like potentials. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental study of the Raman strain rosette based on the carbon nanotube strain sensor

This work presents a new technique named Raman strain rosette for the micro‐strain measurement of both Raman active and Raman inactive materials. The technique is based on the theoretical model of the carbon nanotube (CNT) strain sensor that applies the resonance and polarization Raman properties of CNTs and calculates the synthetic contributions of uniformly dispersed CNTs to the entire Raman spectrum. In our work, the proposed technique is applied in different experiments on the Raman inactive materials, such as step‐by‐step uniaxial tensile and Raman mapping around a circular hole. The experimental results reached by the Raman strain rosette are consistent with the actual values as a whole. This study verifies that the Raman strain rosette is applicable to quantitative measurement of all the in‐plane components of the strain tensor (including both normal and shear strains) by three polarized Raman detections for each sampling spot on a microscale. The technique is further applicable to achieving the strain fields through Raman mapping. Copyright © 2010 John Wiley & Sons, Ltd.