共查询到20条相似文献,搜索用时 31 毫秒
1.
Tarek Ragab 《Carbon》2010,48(1):47-53
In this paper the forces induced on the atoms of the lattice due an electric current (electron transport induced wind forces) are calculated based on quantum mechanics. These forces are calculated in metallic single-walled armchair carbon nanotubes (SWCNTs) from the momentum transfer between the charge carriers and the lattice in a quantum mechanical framework. Energy and phonon dispersion relations are the main input for the formulation proposed. Scattering of electrons with longitudinal acoustic and longitudinal optical phonons are considered to be the only scattering mechanisms that are responsible for the momentum exchange. The current-voltage characteristics are also predicted using the same framework and show good agreement with experimental data. The study shows that using the constant effective charge number for SWCNT is inaccurate at higher electric field forces due to saturation of the lowest energy subband. The thermal effect on the effective charge number appears to be very important, due to the increasing scattering probabilities at higher temperatures. 相似文献
2.
P. Djemia F. Ttard K. Bouamama Eric Charron D. Ttard Y. Rabinovitch 《Journal of the European Ceramic Society》2007,27(16):4719-4725
Brillouin light scattering, Raman light scattering and visible–infrared reflectometry techniques have been used to investigate, respectively, the elastic properties, the phonons and the optical properties of bulk textured polycrystalline yttrium–aluminum garnet doped with 2 at% neodymium obtained by the sintering of commercial oxides. From the analysis of the observed bulk longitudinal and transverse acoustic modes with the knowledge of the refractive index 1.81 inferred from the visible reflectometry, the two independent effective elastic constants of the isotropic polycrystal C11 = 362 GPa and (C11 − C12)/2 = 121 GPa are determined leading to the value of the bulk modulus B = (C11 + 2C12)/3 = 200 GPa. The ratio 0/∞ = 3.1 and the optic permittivity ∞ = 3.46 are derived from the infrared reflectivity data. Pair potential calculations of the three single crystal elastic constants c11 = 340, c12 = 127 and c44 = 112 GPa, of the bulk modulus B = (c11 + 2c12)/3 = 198 GPa, of the zone-center (Γ) phonons and of the permittivity function provide good comparison with our experimental results. 相似文献
3.
Photo-excitation and size-dependent Raman scattering studies on the silicon (Si) nanostructures (NSs) prepared by laser-induced
etching are presented here. Asymmetric and red-shifted Raman line-shapes are observed due to photo-excited Fano interaction
in the quantum confined nanoparticles. The Fano interaction is observed between photo-excited electronic transitions and discrete
phonons in Si NSs. Photo-excited Fano studies on different Si NSs show that the Fano interaction is high for smaller size
of Si NSs. Higher Fano interaction for smaller Si NSs is attributed to the enhanced interference between photo-excited electronic
Raman scattering and phonon Raman scattering. 相似文献
4.
With structural miniaturization down to the nanoscale, the detectable quantities of solid materials no longer remain constant but become tunable. For the II-VI semiconductors example, the band gap expands, the elastic modulus increases, the melting point drops, and the Raman optical phonons experience red shift associated with creation of low frequency Raman acoustic modes that undergo blue shift with decreasing the dimensional scale. In order to understand the common origin of the size dependency of these seemingly irrelevant properties, we formulated these quantities for CdS, ZnS, and CdSe semiconductors from the perspectives of bond order-length-strength correlation and the local bond averaging approach. Consistency between the theory predictions and the measured size dependence of these quantities clarified that the undercoordination-induced local strain and quantum entrapment and the varied fraction of undercoordinated atoms of the entire solid correlate these quantities and dominate their size effect. 相似文献
5.
Deepika Chaturvedi Soni Mishra Poonam Tandon José A. Portilla-Arias Sebastián Muñoz-Guerra 《Polymer》2012,53(13):2681-2690
Poly (β-L-malic acid) (PMLA) is a biodegradable polymer and it has various important applications in the biomedical field. In the present work the structural and spectral characteristics of PMLA have been studied by methods of infrared, Raman spectroscopy and quantum chemistry. Electrostatic potential surface, optimized geometry, harmonic vibrational frequencies, infrared intensities and activities of Raman scattering were calculated by density functional theory (DFT) using oligomeric approach employing B3LYP with complete relaxation in the potential energy surface using 6-311++G (d, p) basis set. Based on results, we have discussed the correlation between the vibrational modes and the structure of the PMLA. A complete analysis of the experimental infrared and Raman spectra has been reported on the basis of wavenumber of the vibrational bands and potential energy distribution. The calculated HOMO and LUMO energies shows that charge transfer occur within the molecule. The calculated infrared and the Raman spectra of the polymer based on DFT calculations show reasonable agreement with the experimental results. 相似文献
6.
Shin-ichi Nakashima Makoto Higashihira Kouji Maeda Hidehiko Tanaka 《Journal of the American Ceramic Society》2003,86(5):823-829
Raman scattering measurements have been made on SiC ceramics prepared from two powdered by sintering at different temperatures. The Raman spectra of starting powders have also been measured. The volume contents of the 4H and 15R polytype phases relative to that of the 6H phase in the ceramics are inferred from the Raman intensity of folded modes of the acoustic branches and compared with those determined from X-ray diffraction (XRD) analysis. A strong correlation is found between the results obtained from the two analyses. The 4H polytype contents estimated by Raman measurement for specimens prepared from one powder show a good agreement with those obtained by the XRD analysis. For the 15R polytype component there is a correlation between the contents inferred by the two techniques when the content is not very small. The results obtained by the two techniques demonstrate that the Raman spectroscopy as well as the XRD analysis is useful to study the natures and preparation conditions of SiC ceramics. 相似文献
7.
VA Yuryev LV Arapkina MS Storozhevykh VA Chapnin KV Chizh OV Uvarov VP Kalinushkin ES Zhukova AS Prokhorov IE Spektor BP Gorshunov 《Nanoscale research letters》2012,7(1):414-18
ABSTRACT: Morphology and defects: Issues of Ge hut cluster array formation and growth at low temperatures on the Ge/Si(001) wetting layer are discussed on the basis of explorations performed by high resolution STM and in-situ RHEED. Dynamics of the RHEED patterns in the process of Ge hut array formation is investigated at low and high temperatures of Ge deposition. Dfferent dynamics of RHEED patterns during the deposition of Ge atoms in different growth modes is observed, which reflects the difference in adatom mobility and their 'condensation' fluxes from Ge 2D gas on the surface for different modes, which in turn control the nucleation rates and densities of Ge clusters. Data of HRTEM studies of multilayer Ge/Si heterostructures are presented with the focus on low-temperature formation of perfect films. Photo-emf spectroscopy: Heteroepitaxial Si p-i-n-diodes with multilayer stacks of Ge/Si(001) quantum dot dense arrays built in intrinsic domains have been investigated and found to exhibit the photo-emf in a wide spectral range from 0.8 to 5 mcm. An effect of wide-band irradiation by infrared light on the photo-emf spectra has been observed. Photo-emf in different spectral ranges has been found to be differently affected by the wide-band irradiation. A significant increase in photo-emf is observed in the fundamental absorption range under the wide-band irradiation. The observed phenomena are explained in terms of positive and neutral charge states of the quantum dot layers and the Coulomb potential of the quantum dot ensemble. A new design of quantum dot infrared photodetectors is proposed. Terahertz spectroscopy: By using a coherent source spectrometer, first measurements of terahertz dynamical conductivity (absorptivity) spectra of Ge/Si(001) heterostructures were performed at frequencies ranged from 0.3 to 1.2 THz in the temperature interval from 300 to 5 K. The effective dynamical conductivity of the heterostructures with Ge quantum dots has been discovered to be significantly higher than that of the structure with the same amount of bulk germanium (not organized in an array of quantum dots). The excess conductivity is not observed in the structures with the Ge coverage less than 8 A. When a Ge/Si(001) sample is cooled down the conductivity of the heterostructure decreases. 相似文献
8.
Cheng-Wen Huang Bing-Jie Lin Hsing-Ying Lin Chen-Han Huang Fu-Yu Shih Wei-Hua Wang Chih-Yi Liu Hsiang-Chen Chui 《Nanoscale research letters》2013,8(1):480
Abstract
The interactions between phonons and electrons induced by the dopants or the substrate of graphene in spectroscopic investigation reveal a rich source of interesting physics. Raman spectra and surface-enhanced Raman spectra of supported and suspended monolayer graphenes were measured and analyzed systemically with different approaches. The weak Raman signals are greatly enhanced by the ability of surface-enhanced Raman spectroscopy which has attracted considerable interests. The technique is regarded as wonderful and useful tool, but the dopants that are produced by depositing metallic nanoparticles may affect the electron scattering processes of graphene. Therefore, the doping and substrate influences on graphene are also important issues to be investigated. In this work, the peak positions of G peak and 2D peak, the I2D/IG ratios, and enhancements of G and 2D bands with suspended and supported graphene flakes were measured and analyzed. The peak shifts of G and 2D bands between the Raman and SERS signals demonstrate the doping effect induced by silver nanoparticles by n-doping. The I2D/IG ratio can provide a more sensitive method to carry out the doping effect on the graphene surface than the peak shifts of G and 2D bands. The enhancements of 2D band of suspended and supported graphenes reached 138, and those of G band reached at least 169. Their good enhancements are helpful to measure the optical properties of graphene. The different substrates that covered the graphene surface with doping effect are more sensitive to the enhancements of G band with respect to 2D band. It provides us a new method to distinguish the substrate and doping effect on graphene.PACS
78.67.Wj (optical properties of graphene); 74.25.nd (Raman and optical spectroscopy); 63.22.Rc (phonons in graphene) 相似文献9.
The acoustic modes propagated in the bc plane of triclinic β-BiNbO4 single crystal are investigated by the Brillouin scattering method at room temperature. The Brillouin spectra corresponding to the longitudinal acoustic mode are analyzed to evaluate the elastic constants. Obtained elastic constants are c33 = 16 ± 2, c22=27 ± 3 and c23+2c44=25 ± 3 in the unit of 1011 dyne· mid;cm-2. 相似文献
10.
The Raman spectra of graphene with three different types of point defects, namely, a mono-vacancy, a di-vacancy, and a Stone-Wales defect, was calculated within a non-orthogonal tight-binding model using supercells of graphene with a single defect. The defects were found to modify the electronic structure and the phonons of graphene giving rise to new optical transitions and defect-related phonons. Based on the calculated Raman spectra, we determined the Raman lines that can serve as signatures of the specific defects. The comparison of the calculated Raman intensity of the graphitic (G-) band of perfect graphene and graphene with defects shows that the intensity can be enhanced up to one order of magnitude by the presence of defects. 相似文献
11.
Raman spectra have been observed from two adjacent thin polymer films and the interfacial region. This has been made possible by the application of integrated optical techniques to Raman spectroscopy. The observed Raman spectra have their strongest intensity contributions from regions near the optical field intensity maxima, which can occur in either film or at the interface. Both the optical intensity and the scattering volume are increased significantly and Raman spectra with good signal-to-noise ratios can be obtained. The method will be briefly illustrated and spectra for a thin film of poly(styrene) on a film of polyvinyl alcohol) will be shown for various (modes) optical profiles. 相似文献
12.
Four-bilayer Ge quantum dots (QDs) with Si spacers were grown on Si(001) substrates by ultrahigh vacuum chemical vapor deposition. In three samples, all Ge QDs were grown at 520 °C, while Si spacers were grown at various temperatures (520 °C, 550 °C, and 580 °C). Enhancement and redshift of room temperature photoluminescence (PL) were observed from the samples in which Si spacers were grown at a higher temperature. The enhancement of PL is explained by higher effective electrons capturing in the larger size Ge QDs. Quantum confinement of the Ge QDs is responsible for the redshift of PL spectra. The Ge QDs’ size and content were investigated by atomic force microscopy and Raman scattering measurements. 相似文献
13.
Dzhagan V Lokteva I Himcinschi C Jin X Kolny-Olesiak J Zahn DR 《Nanoscale research letters》2011,6(1):79
Resonant Raman study reveals the noticeable effect of the ligand exchange on the nanocrystal (NC) surface onto the phonon
spectra of colloidal CdTe NC of different size and composition. The oleic acid ligand exchange for pyridine ones was found
to change noticeably the position and width of the longitudinal optical (LO) phonon mode, as well as its intensity ratio to
overtones. The broad shoulder above the LO peak frequency was enhanced and sharpened after pyridine treatment, as well as
with decreasing NC size. The low-frequency mode around 100 cm-1 which is commonly related with the disorder-activated acoustical phonons appears in smaller NCs but is not enhanced after
pyridine treatment. Surprisingly, the feature at low-frequency shoulder of the LO peak, commonly assigned to the surface optical
phonon mode, was not sensitive to ligand exchange and concomitant close packing of the NCs. An increased structural disorder
on the NC surface, strain and modified electron-phonon coupling is discussed as the possible reason of the observed changes
in the phonon spectrum of ligand-exchanged CdTe NCs. 相似文献
14.
Dong Hoon Kang Soo Han Oh Jae-Hyeon Ko Kwang-Sei Lee Seiji Kojima 《International journal of molecular sciences》2021,22(13)
The inelastic interaction between the incident photons and acoustic phonons in the taurine single crystal was investigated by using Brillouin spectroscopy. Three acoustic phonons propagating along the crystallographic b-axis were investigated over a temperature range of −185 to 175 °C. The temperature dependences of the sound velocity, the acoustic absorption coefficient, and the elastic constants were determined for the first time. The elastic behaviors could be explained based on normal lattice anharmonicity. No evidence for the structural phase transition was observed, consistent with previous structural studies. The birefringence in the ac-plane indirectly estimated from the split longitudinal acoustic modes was consistent with one theoretical calculation by using the extrapolation of the measured dielectric functions in the infrared range. 相似文献
15.
Shih CI Lin CH Lin SC Lin TC Sun KW Voskoboynikov OA Lee CP Suen YW 《Nanoscale research letters》2011,6(1):409
In this report, the influence of the intrinsic transitions between bound-to-delocalized states (crossed states or quasicontinuous density of electron-hole states) on photoluminescence excitation (PLE) spectra of InAs quantum dots (QDs) was investigated. The InAs QDs were different in size, shape, and number of bound states. Results from the PLE spectroscopy at low temperature and under a high magnetic field (up to 14 T) were compared. Our findings show that the profile of the PLE resonances associated with the bound transitions disintegrated and broadened. This was attributed to the coupling of the localized QD excited states to the crossed states and scattering of longitudinal acoustical (LA) phonons. The degree of spectral linewidth broadening was larger for the excited state in smaller QDs because of the higher crossed joint density of states and scattering rate. 相似文献
16.
The theory of large radius polaron in the quantum wire is developed. The interaction of charge particles with interface optical
phonons as well as with optical phonons localized in the quantum wire is taken into account. The interface phonon contribution
is shown to be dominant for narrow quantum wires. The wave functions and polaron binding energy are found. It is determined
that polaron binding energy depends on the electron mass inside the wire and on the polarization properties of the barrier
material. 相似文献
17.
Raul D Rodriguez Evgeniya Sheremet Dominic J Thurmer Daniel Lehmann Ovidiu D Gordan Falko Seidel Alexander Milekhin Oliver G Schmidt Michael Hietschold Dietrich RT Zahn 《Nanoscale research letters》2012,7(1):594
Large arrays of multifunctional rolled-up semiconductors can be mass-produced with precisely controlled size and composition, making them of great technological interest for micro- and nano-scale device fabrication. The microtube behavior at different temperatures is a key factor towards further engineering their functionality, as well as for characterizing strain, defects, and temperature-dependent properties of the structures. For this purpose, we probe optical phonons of GaAs/InGaAs rolled-up microtubes using Raman spectroscopy on defect-rich (faulty) and defect-free microtubes. The microtubes are fabricated by selectively etching an AlAs sacrificial layer in order to release the strained InGaAs/GaAs bilayer, all grown by molecular beam epitaxy. Pristine microtubes show homogeneity of the GaAs and InGaAs peak positions and intensities along the tube, which indicates a defect-free rolling up process, while for a cone-like microtube, a downward shift of the GaAs LO phonon peak along the cone is observed. Formation of other type of defects, including partially unfolded microtubes, can also be related to a high Raman intensity of the TO phonon in GaAs. We argue that the appearance of the TO phonon mode is a consequence of further relaxation of the selection rules due to the defects on the tubes, which makes this phonon useful for failure detection/prediction in such rolled up systems. In order to systematically characterize the temperature stability of the rolled up microtubes, Raman spectra were acquired as a function of sample temperature up to 300°C. The reversibility of the changes in the Raman spectra of the tubes within this temperature range is demonstrated. 相似文献
18.
Nikolai Gaponik Yury P. Rakovich Matthias Gerlach John F. Donegan Diana Savateeva Andrey L. Rogach 《Nanoscale research letters》2006,1(1):68-73
We have studied the photoluminescence and Raman spectra of a system consisting of a polystyrene latex microsphere coated by CdTe colloidal quantum dots. The cavity-induced enhancement of the Raman scattering allows the observation of Raman spectra from only a monolayer of CdTe quantum dots. Periodic structure with very narrow peaks in the photoluminescence spectra of a single microsphere was detected both in the Stokes and anti-Stokes spectral regions, arising from the coupling between the emission of quantum dots and spherical cavity modes. 相似文献
19.
Composite films of poly(vinyl alcohol) containing polyynes adsorbed on aggregates of Ag nanoparticles were prepared and the stability of the Ag aggregates and polyynes was investigated by measuring the time evolution of absorption and surface-enhanced Raman spectra. The results indicate that the embedded aggregates of Ag nanoparticles and adsorbed polyynes are stable for a month. The high stability of the composite films made it possible to obtain an excitation profile of the surface-enhanced Raman spectrum. The excitation profile could be well reproduced by a theoretical calculation based on the electromagnetic mechanism of surface-enhanced Raman scattering. 相似文献
20.
In Raman spectroscopy investigations of defective suspended graphene, splitting in the D band is observed. Four double resonance Raman scattering processes: the outer and inner scattering processes, as well as the scattering processes with electrons first scattered by phonons (“phonon-first”) or by defects (“defect-first”), are found to be responsible for these features of the D band. The D sub-bands associated with the outer and inner processes merge with increasing defect concentration. However a Stokes/anti-Stokes Raman study indicates that the splitting of the D band due to the separate “phonon-first” and “defect-first” processes is valid for suspended graphene. For graphene samples on a SiO2/Si substrate, the sub-bands of D band merge due to the increased Raman broadening parameter resulting from the substrate doping. Moreover, the merging of the sub-bands shows excitation energy dependence, which can be understood by considering the energy dependent lifetime and/or scattering rate of photo-excited carriers in the Raman scattering process. 相似文献