A Raman study of CdSe and ZnSe nanostructures

Raman studies have been carried out on CdSe nanotubes and ZnSe nanorods produced by surfactant-assisted synthesis. The Raman spectrum of CdSe nanotubes shows modes at 207.5 and 198 cm-1; the former arises from the longitudinal optic phonon mode red-shifted with respect to the bulk mode because of phonon confinement, and the latter is the l = 1 surface phonon. Analysis based on the phonon confinement model demonstrates that the size of the nanoparticle responsible for the red-shift is about 4 nm, close to the estimate from the blue-shift of the photoluminescence. The Raman spectrum of ZnSe nanorods shows modes at 257 and 213 cm-1, assigned to longitudinal and transverse optic phonons, blue-shifted with respect to the bulk ZnSe modes because of compressive strain. The mode at 237 cm-1 is the surface phonon.     

Raman spectroscopy of single-wall boron nitride nanotubes

Single-wall boron nitride nanotubes samples synthesized by laser vaporization of a hexagonal BN target under a nitrogen atmosphere are studied by UV and visible Raman spectroscopy. We show that resonant conditions are necessary for investigating phonon modes of BNNTs. Raman excitation in the UV (229 nm) provides preresonant conditions, allowing the identification of the A1 tangential mode at 1370 cm(-1). This is 5 cm(-1) higher than the E(2g) mode in bulk h-BN. Ab initio calculations show that the lower frequency of bulk h-BN with respect to large diameter nanotubes and the single sheet of h-BN is related to a softening of the sp2 bonds in the bulk due to interlayer interaction.     

Study of optical phonon modes of CdS nanoparticles using Raman spectroscopy

The reduction in the grain size to nanometer range can bring about radical changes in almost all of the properties of semiconductors. CdS nanoparticles have attracted considerable scientific interest because they exhibit strongly size-dependent optical and electrical properties. In the case of nanostructured materials, confinement of optical phonons can produce noticeable changes in their vibrational spectra compared to those of bulk crystals. In this paper we report the study of optical phonon modes of nanoparticles of CdS using Raman spectroscopy. Nanoparticle sample for the present study was synthesized through chemical precipitation technique. The CdS nanoparticles were then subjected to heat treatment at low temperature (150°C) for extended time intervals. The crystal structure and grain size of the samples were determined using X-ray diffraction and HRTEM. The Raman spectra of the as-prepared and heat treated samples were recorded using conventional Raman and micro-Raman techniques. The spectrum of as prepared sample exhibited an intense, broad peak at 301 cm⁻¹ corresponding to the LO phonon mode. Higher order phonon modes were also observed in the spectra. A noticeable asymmetry in the Raman line shape indicated the effect of phonon confinement. Other features in the spectra are discussed in detail.     

Thermomechanical analysis of residual stresses in brazed diamond metal joints using Raman spectroscopy and finite element simulation

Thermal residual stresses are one of the crucial parameters in engineered grinding tool (EGT) life and its consistency. Predicting failure of brazed diamond metal joints in EGTs is related to analyzing the thermal residual stresses during the cooling process. Thus thermal residual stresses have been simulated in a model with realistic materials properties, for instance isotropic hardening and a hyperbolic-sine creep law for SS316L and the silver–copper–titanium active filler alloy, named Cusil ABA™. Also, special modeling techniques such as tie constraint and sub-modeling have been used to model an intermetallic layer titanium-carbide (TiC) with dimensions in nanometers, where the rest of the model’s dimensions are in millimeters. To verify the simulated stress state of the diamond, Raman-active optical phonon modes at three different paths in the diamond were measured. As the experiments with Raman spectroscopy (RS) do not deliver stress components, the solution is to directly compute the peak shift of Raman spectrum. The splitting in phonon frequencies and the mixing of phonon modes contain information about the thermal residual stresses in the diamond. Finally the shift in the phonon frequencies was calculated from the different numerical residual elastic strain components and compared to the experimental results.     

The Electron–Phonon Interaction of Low-Dimensional and Multi-Dimensional Materials from He Atom Scattering

Atom scattering is becoming recognized as a sensitive probe of the electron–phonon interaction parameter λ at metal and metal-overlayer surfaces. Here, the theory is developed, linking λ to the thermal attenuation of atom scattering spectra (in particular, the Debye–Waller factor), to conducting materials of different dimensions, from quasi-1D systems such as W(110):H(1 × 1) and Bi(114), to quasi-2D layered chalcogenides, and high-dimensional surfaces such as quasicrystalline 2ML-Ba(0001)/Cu(001) and d-AlNiCo(00001). Values of λ obtained using He atoms compare favorably with known values for the bulk materials. The corresponding analysis indicates in addition, the number of layers contributing to the electron–phonon interaction, which is measured in an atom surface collision.     

Surface optical phonon-assisted electron Raman scattering in a free-standing quantum wire with ring geometry

We present a theoretical calculation of the differential cross section for the electron Raman scattering process associated with surface optical phonon modes in semiconductor quantum wires (QWs) with ring geometry. Electron states are considered to be confined within the QWs. We consider the Fröhlich electron-phonon interaction in the framework of the dielectric continuum approach. Some singularities for the ring with various sizes in the Raman spectra are found and interpreted. A discussion of the phonon behavior for the QWs with large and small sizes is presented. The numerical results are also compared with those of experiments.     

Growth and lattice dynamics of single-crystalline SnO2 nanowires prepared by annealing a gel precursor

A large amount of ultra-long single-crystalline SnO₂ nanowires were successfully synthesized through a polymeric sol–gel approach followed by a post-annealing in a crucible covered with a lid. The experimental results indicate that the product is composed of tetragonal SnO₂ nanowires, and moreover, their growth mechanism should arise from the self-catalyzed Sn-terminated polar surfaces due to the enrichment of Sn at the growth front of nanowires. In Raman scattering spectrum, in addition to three fundamental vibrational modes, some infrared (IR) active and surface phonon modes (SPM) are also observed due to size and shape confinement effects and surface disorder. Similarly, low frequency non-active IR and surface phonon mode absorptions are also observed in IR spectra.     

Ab initio calculation of the structural and dynamical properties of layered semiconductors

We present a theoretical first-principles investigation of the structure and lattice dynamics of several layered semiconductors. The equilibrium structure as obtained by minimization of the total energy of the bulk materials is in good agreement with experiment. Furthermore, we have investigated the surface of these materials in order to obtain information on the van der Waals epitaxial growth. We found that the relaxed atomic positions at the surface deviate from the ideal ones in the bulk by less than 1%, which is obviously a consequence of the weak interlayer forces. Additionally, bulk phonon-dispersion curves have been calculated along several high symmetry directions within the density-functional perturbation theory (DFPT). The weak interlayer interaction makes the vibrational properties of the bulk very similar to those of the surface. In fact, our ab initio calculations for the bulk reproduce well both the experimental bulk phonon frequencies obtained by inelastic neutron scattering and the experimental surface phonon dispersion measured with inelastic He-atom scattering (HAS).     

Raman spectroscopy of few-quintuple layer topological insulator Bi2Se3 nanoplatelets

We report on Raman spectroscopy of few quintuple layer topological insulator bismuth selenide (Bi2Se3) nanoplatelets (NPs), synthesized by a polyol method. The as-grown NPs exhibit excellent crystalline quality, hexagonal or truncated trigonal morphology, and uniformly flat surfaces down to a few quintuple layers. Both Stokes and anti-Stokes Raman spectroscopy for the first time resolve all four optical phonon modes from individual NPs down to 4 nm, where the out-of-plane vibrational A(1g)(1) mode shows a few wavenumbers red shift as the thickness decreases below ~15 nm. This thickness-dependent red shift is tentatively explained by a phonon softening due to the decreasing of the effective restoring force arising from a decrease of the van der Waals forces between adjacent layers. Quantitatively, we found that the 2D phonon confinement model proposed by Faucet and Campbell cannot explain the red shift values and the line shape of the A(1g)(1) mode, which can be described better by a Breit–Wigner–Fano resonance line shape. Considerable broadening (~17 cm(–1) for six quintuple layers) especially for the in-plane vibrational mode E(g)(2) is identified, suggesting that the layer-to-layer stacking affects the intralayer bonding. Therefore, a significant reduction in the phonon lifetime of the in-plane vibrational modes is probably due to an enhanced electron–phonon coupling in the few quintuple layer regime.     

Lattice dynamics and Raman spectrum of rutile TiO2: The role of soft phonon modes in pressure induced phase transition

Using the plane-wave pseudopotential technique based on the first-principles density functional perturbation theory (DFPT), we have studied the vibrational properties and Raman susceptibility tensor at ambient and high pressure of rutile phase of TiO₂. Full phonon dispersion curves and phonon densities of states with projected phonon density of states and Raman tensors at high pressures are calculated and given. It is found that rutile TiO₂ shows a pressure induced phase transition, especially when lattice dynamical instabilities are involved, like the soft phonon modes, at a hydrostatic pressure lower than 10 GPa. An analyses of the vibrational displacements is given. The possibility to use Raman line intensities as an additional tool in the study of phase transitions is also discussed.     

h-BN各向异性热膨胀和热输运特性的第一性原理研究

热填料的热膨胀系数和热导率是设计热管理和热防护复合材料的关键参考因素.六方氮化硼(h-BN)由于其独特的优点是最常用的热填料之一.但由于不同测试方法和测试样品的不一致性,其热膨胀系数和热导率的精确数值尚不清楚.本文分别用基于密度泛函理论的准谐近似方法和声子玻耳兹曼输运方程理论精确计算了h-BN沿层间和层内方向的热膨胀系...     

Accurate measurements of the acoustical physical constants of LiNbO (3) and LiTaO(3) single crystals

The acoustical physical constants (elastic constant, piezoelectric constant, dielectric constant, and density) of commercial surface acoustic wave (SAW)-grade LiNbO(3) and LiTaO(3) single crystals were determined by measuring the bulk acoustic wave velocities, dielectric constants, and densities of many plate specimens prepared from the ingots. The maximum probable error in each constant was examined by considering the dependence of each constant on the measured acoustic velocities. By comparing the measured values of longitudinal velocities that were not used to determine the constants with the calculated values using the previously mentioned constants, we found that the differences between the measured and calculated values were 1 m/s or less for both LiNbO(3) and LiTaO(3) crystals. These results suggest that the acoustical physical constants determined in this paper can give the values of bulk acoustic wave velocities with four significant digits.     

Confocal micro-Raman observation of nanometer thick oxide layers on metal nanoparticles

It is shown for the first time that confocal micro-Raman spectroscopy can be used to detect metal oxide nanometer thick layers on metal nanoparticles. The measured frequencies of the observed oxide layers are shifted to lower frequencies and the line widths were asymmetrically broadened compared to Raman spectrum in bulk oxide of the metals. The effects are due to phonon confinement which occurs in materials of nanometer dimensions. Models of phonon confinement are used to estimate the thickness of the oxide layers.     

Lattice Transformation from 2D to Quasi 1D and Phonon Properties of Exfoliated ZrS2 and ZrSe2

Recent reports on thermal and thermoelectric properties of emerging 2D materials have shown promising results. Among these materials are Zirconium-based chalcogenides such as zirconium disulfide (ZrS₂), zirconium diselenide (ZrSe₂), zirconium trisulfide (ZrS₃), and zirconium triselenide (ZrSe₃). Here, the thermal properties of these materials are investigated using confocal Raman spectroscopy. Two different and distinctive Raman signatures of exfoliated ZrX₂ (where X = S or Se) are observed. For 2D-ZrX₂, Raman modes are in alignment with those reported in literature. However, for quasi 1D-ZrX₂, Raman modes are identical to exfoliated ZrX₃ nanosheets, indicating a major lattice transformation from 2D to quasi-1D. Raman temperature dependence for ZrX₂ are also measured. Most Raman modes exhibit a linear downshift dependence with increasing temperature. However, for 2D-ZrS₂, a blueshift for A1g mode is detected with increasing temperature. Finally, phonon dynamics under optical heating for ZrX₂ are measured. Based on these measurements, the calculated thermal conductivity and the interfacial thermal conductance indicate lower interfacial thermal conductance for quasi 1D-ZrX₂ compared to 2D-ZrX₂, which can be attributed to the phonon confinement in 1D. The results demonstrate exceptional thermal properties for Zirconium-based materials, making them ideal for thermoelectric device applications and future thermal management strategies.     

Ultrafast dynamics of surface-enhanced Raman scattering due to Au nanostructures

Ultrafast dynamics of surface-enhanced Raman scattering (SERS) was investigated at cleaved graphite surfaces bearing deposited gold (Au) nanostructures (～10 nm in diameter) by using sensitive pump-probe reflectivity spectroscopy with ultrashort (7.5 fs) laser pulses. We observed enhancement of phonon amplitudes (C═C stretching modes) in the femtosecond time domain, considered to be due to the enhanced electromagnetic (EM) field around the Au nanostructures. Finite-difference time-domain (FDTD) calculations confirmed the EM enhancement. The enhancement causes drastic increase of coherent D-mode (40 THz) phonon amplitude and nanostructure-dependent changes in the amplitude and dephasing time of coherent G-mode (47 THz) phonons. This methodology should be suitable to study the basic mechanism of SERS and may also find application in nanofabrication.     

Direct observation of mode selective electron-phonon coupling in suspended carbon nanotubes

Raman spectra of individual pristine suspended single-walled carbon nanotubes are observed under high electrical bias. The LO and TO modes of the G band behave differently with respect to voltage bias, indicating preferential electron-phonon coupling and nonequilibrium phonon populations, which cause negative differential conductance in suspended devices. By correlating the electron resistivity to the optically measured phonon population, the data are fit using a Landauer model to determine the key scattering parameters.     

Simultaneous spectroscopic and topographic near-field imaging of TiO2 single surface states and interfacial electronic coupling

We have probed single surface states and the involved interfacial charge transfer coupling on the TiO(2) surface using confocal as well as tip-enhanced near-field topographic-spectroscopic imaging analysis on a niobium-doped rutile TiO(2)(110) surface. The confocal images excited with a radially polarized donut mode render ring-shaped excitation patterns typical for quantum systems with two perpendicular transition dipole moments. The tip-enhanced near-field optical images of single surface states are visualized by the strong exciton plasmon-polariton coupling localized at the subdomain boundaries with a spatial resolution of ～15 nm (far beyond the optical diffraction limit). We suggest that the abundant surface states in the doped TiO(2) generate excitons under laser excitation which are strongly coupled to the surface plasmon-polaritons of the Au tip. Moreover, the interfacial electronic molecule-substrate coupling has been characterized by probing the molecule-perturbed surface states distribution and the associated specific Raman vibrational modes. The imaging and characterization of the surface states and their distributions on TiO(2) surfaces at nanoscale are critically relevant to a deep understanding of interfacial electron transfer dynamics and energetics involving in solar energy conversion, photocatalysis, and mechanistic understanding of surface-enhanced Raman scattering spectroscopy.     

Raman spectroscopy of indium phosphide nanowire networks coated with gold clusters

Enhanced Raman signal of the longitudinal optical phonon mode in indium phosphide nanowire networks with gold coating of up to 5 nm thickness was observed experimentally to further study the phonon spectrum of nanowire networks. Indium phosphide nanowire networks coated with different nominal thicknesses of gold were prepared and optically studied. Scanning electron microscopy, photoluminescence spectroscopy and Raman spectroscopy were used to study the dependence of surface morphology and phonon modes of the nanowire networks on the nominal thickness of the gold coating. The Raman peak of longitudinal optical phonon mode for as grown sample was negligible, while the peak intensity for 1 and 5 nm gold coated sample reached to 1,379 and 792 a.u. respectively. Electromagnetic enhancement and extinction coefficient are discussed to qualitatively assess the role of the gold coating on indium phosphide nanowire networks.     

Stable structure of platinum carbides: A first principles investigation on the structure,elastic, electronic and phonon properties

A comprehensive first principles study of structural, elastic, electronic, phonon and thermodynamical properties of novel metal carbide, platinum carbide (PtC) is reported within the density functional theory scheme. The ground state properties such as lattice constant, elastic constants, bulk modulus, shear modulus and finally the enthalpy of PtC in zinc blende (ZB) and rock-salt (RS) structures are determined. The energy band structure and electron density of states for the two phases of PtC are also presented. Of these phases zinc blende phase of PtC is found stable and phase transition from ZB to RS structure occurs at the pressure of about 37.58 GPa. The phonon dispersion curves and phonon DOS are also presented. All positive phonon modes in phonon dispersion curves of ZB-PtC phase indicate a stable phase for this structure. Within the GGA and harmonic approximation, thermodynamical properties are also investigated. All results reveal that the synthesized PtC would favor ZB phase. The compound is stiffer and ductile in nature.     

Surface second-harmonic generation from vertical GaP nanopillars

We report on the experimental observation and analysis of second-harmonic generation (SHG) from vertical GaP nanopillars. Periodic arrays of GaP nanopillars with varying diameters ranging from 100 to 250 nm were fabricated on (100) undoped GaP substrate by nanosphere lithography and dry etching. We observed a strong dependence of the SHG intensity on pillar diameter. Analysis of surface and bulk contributions to SHG from the pillars including the calculations of the electric field profiles and coupling efficiencies is in very good agreement with the experimental data. Complementary measurements of surface optical phonons by Raman spectroscopy are also in agreement with the calculated field intensities at the surface. Finally, polarization of the measured light is used to distinguish between the bulk and surface SHG from GaP nanopillars.