Phonon Intensity Research Articles

Effect of Film Thickness on Energy Transport Characteristics in Aluminum Thin Film

Energy transfer in an aluminum thin film is considered due to a temperature disturbance at the film edges. Temperature oscillation is introduced at the high-temperature edge of the film, and the thermal response of the film is assessed through use of an equivalent equilibrium temperature. Transient frequency-dependent and frequency-independent solutions of the Boltzmann equation are obtained for phonon intensity distributions in the lattice subsystem. An electron–phonon coupling parameter is incorporated in order to account for the thermal communication between electrons and lattice subsystems in the film. The dispersion relations are used in the solution of the frequency-dependent Boltzmann equation. The study is extended to include the effect of film thickness on energy transport characteristics. It is found that oscillation of the equivalent equilibrium temperature takes place within the film because of temperature oscillation at the high-temperature film edges and, as the film thickness increases, the amplitude of the temperature oscillation reduces considerably. The electron temperature closely follows the phonon temperature, provided that the characteristics of the electron temperature oscillation are different than the temperature oscillation in the lattice subsystem.

Nonequilibrium cross-plane energy transport in aluminum–silicon–aluminum wafer

Transient phonon transport across cross-planes of aluminum–silicon–aluminum combined films is investigated and the Boltzmann transport equation is incorporated to formulate the energy transport in the combined films. Since electrons and phonons thermally separate in the thin aluminum film during heating, the Boltzmann equation is used separately in the electron and lattice subsystems to account for the energy transport in the aluminum film. Electron–phonon coupling is incorporated for the energy exchange between electron and lattice subsystems in the film. Thermal boundary resistance (TBR) is introduced at the interfaces of the silicon–aluminum films. In order to examine the ballistic contribution of phonons on the phonon intensity distribution in the silicon film, frequency-dependent solution of the Boltzmann equation is used in the silicon film and the film thickness is varied to investigate the size effect on the thermal conductivity in the film. It is found that equivalent equilibrium temperature of phonons remains high at silicon–aluminum interface because of the ballistic contribution of the phonons. Equivalent equilibrium temperature for the electron subsystem becomes higher than that corresponding to phonon temperature at the aluminum–silicon interface.

Strong Asymmetric Charge Carrier Dependence in Inelastic Electron Tunneling Spectroscopy of Graphene Phonons.

The observation of phonons in graphene by inelastic electron tunneling spectroscopy has been met with limited success in previous measurements arising from weak signals and other spectral features which inhibit a clear distinction between phonons and miscellaneous excitations. Utilizing a back-gated graphene device that allows adjusting the global charge carrier density, we introduce an averaging method where individual tunneling spectra at varying charge carrier density are combined into one representative spectrum. This method improves the signal for inelastic transitions while it suppresses dispersive spectral features. We thereby map the total graphene phonon density of states, in good agreement with density functional calculations. Unexpectedly, an abrupt change in the phonon intensity is observed when the graphene charge carrier type is switched through a variation of the back-gate electrode potential. This sudden variation in phonon intensity is asymmetric in the carrier type, depending on the sign of the tunneling bias.

Structural characterisation of slightly Fe-doped SrTiO3 grown via a sol–gel hydrothermal synthesis

A detailed structural study of the incorporation of Fe into SrTiO3 nanoparticles is reported. Slightly iron-doped strontium titanate nanoparticles with 0, 1, 3 and 5 mol% concentration of iron were grown using a sol–gel hydrothermal process and characterised using Raman scattering, X-ray photoelectron and X-ray diffraction spectroscopy. The amorphisation of the nanostructures was observed as the iron content increased, which was confirmed by the TEM images. The XPS results indicated that the oxidation states of the Sr atoms were maintained in 2+. However, a mixture of Fe3+ and Fe4+ atoms was observed as the Fe content increased, resulting in a significant number of oxygen vacancies in the perovskite structure. The analysis of Raman spectra indicated that the intensity, linewidth and frequency shift of the TO4 phonon can be used as an indicator of the Fe content as well as a local temperature probe for future thermal analysis. Temperature evolution of the Raman spectra of STO:Fe 1 mol%. The peaks with star correspond to the second-order processes. (b) Temperature dependence of the TO4 phonon mode. Blue dots denote measured Raman spectra, and the red solid lines are the Lorentzian fits to respective spectra.

Correlation between reflectance and photoluminescent properties of al-rich ZnO nano-structures

Al rich zinc oxide nano-structured films were synthesized using spin coating sol-gel technique. The films were annealed in oxygen ambient in the temperature range of 200-700 °C. The structural, optical, and photoluminescence (PL) properties of the films were studied at various annealing temperatures using X-ray diffraction spectroscopy, field emission scanning electron microscopy, photoluminescence emission spectra measurement, and Raman and UV-Vis spectroscopy. The optical band gap was found to decrease with the increase of the annealing temperature following the Gauss Amp function due to the confinement of the exciton. The PL peak intensity in the near band region (INBE) was found to increase with the increase of the annealing temperature up to 600 °C, then to decrease fast to a lower value for the annealing temperature of 700 °C due to crystalline quality. The Raman peak of E2 (low) was red shifted from 118 cm-1 to 126 cm-1 with the increase of the annealing temperature. The intensity of the second order phonon (TA+LO) at 674 cm-1 was found to decrease with the increase of the annealing temperature. The normalized values of the reflectance and the PL intensity in the NBE region were highest for the annealing temperature of 600 °C. A special correlation was found between the reflectance at λ = 1000 nm and the normalized PL intensity in the green region due to scattering due to presence of grains.

Electron-Transport Properties of a ZnMgO/ZnO Hetero Structure and the Effect of Interface Roughness and ZnMgO Thickness

The electron-transport properties of a Zn\(_{1-x}\)Mg\(_x\)O/ZnO hetero structure were studied by use of an ensemble Monte Carlo technique. There was no intentional doping in the profile and the entire charge is because of polarization of charges at the interface. Interface roughness, and the intensity of acoustic and optic phonon scattering were used in the ensemble Monte Carlo method to find the transport properties of the two-dimensional electron gas (2DEG). The low-field mobility characteristics of the structure were determined as a function of temperature, the thickness of the ZnMgO layer, and the interface roughness. The interface roughness was found to have a very large effect on the drift mobility of carriers, especially at low temperatures. It was observed that as the thickness of the ZnMgO increases the concentration of electrons increases whereas the mobility of the electrons decreases. The low-field mobility values obtained from ensemble Monte Carlo simulations were in agreement with published experimental data.

Energy transport across thin silicon-diamond films pair with minute vacuum gap at the interface

Energy transfer across the silicon-diamond thin films pairs is investigated due to temperature disturbance at the films edges. A minute vacuum gap satisfying the Casimir limit is considered at the interface films pair. The thermal boundary resistance across the film pair is formulated incorporating the cut-off mismatch model and the heat transfer due to thermal radiation is introduced across the vacuum gap because of temperature difference at the gap interface. The Boltzmann transport equation is used to account for the phonon intensity distribution in the films. The transient and frequency dependent solution of the Boltzmann equation is obtained numerically using the discrete ordinate method. A computer code developed is validated with the thermal conductivity data. It is found that the predictions of thermal conductivity agree well with the data presented in the previous study. The findings revealed that increasing gap size at the interface of the films pair increases interfacial temperature difference across the vacuum gap. Thermal radiation has less influence on the energy transport across the gap as compared to that takes place due to phonon jump across the gap interface.

Phonon Transport Characteristics in a Thin Silicon Film

Phonon transport characteristics in thin silicon film are investigated due to temperature disturbance at the film edges. Phonon intensity distribution ratio , where Iomax is the maximum equilibrium intensity, I is the phonon intensity corresponding to acoustics branch at any location and time in the film) and phonon wavelength ratio , where k is the wavenumber, , and kcutoff is the cutoff wavenumber based on the film length, are introduced to assess the effects of quasi-ballistic and ballistic phonons on the film thermal conductivity and specific heat. It is found that phonon intensity ratio due to longitudinal acoustic branch is considerably higher than that of transverse acoustics branch. The influence of phonon wavelength ratio is not significantly high on the intensity ratio due to longitudinal acoustics branch. Reducing the wavelength ratio creates the size effect in the film while lowering thermal conductivity and specific heat of the film, which is more pronounced when the reduction becomes more than 20%.

Low-energy phonon dispersion inLaFe4Sb12

We studied the vibrational dynamics of a single crystal of ${\mathrm{LaFe}}_{4}{\mathrm{Sb}}_{12}$ by three-axis inelastic neutron spectroscopy. The dispersion of phonons with wave vectors $\mathbf{q}$ along $[xx0]$ and $[xxx]$ directions in the energy range of eigenmodes with high amplitudes of lanthanum vibrations, i.e., at $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\lesssim}12$ meV is identified. Symmetry-avoided anticrossing dispersion of phonons is established in both monitored directions and distinct eigenstates at high-symmetry points and at the Brillouin-zone center are discriminated. The experimentally derived phonon dispersion and intensities are compared with and backed up by ab initio lattice dynamics calculations. Results of the computer model match well with the experimental data.

Origin of coherentG-band phonon spectra in single-wall carbon nanotubes

Coherent phonons in single wall carbon nanotubes (SWNTs) are observed as oscillations of the differential absorption coefficient as a function of time by means of pump-probe spectroscopy. For the radial breathing mode (RBM) of a SWNT, the coherent phonon signal is understood to be a result of the modulated diameter-dependent energy gaps due to the coherent RBM phonon oscillations. However, this mechanism might not be the dominant contribution to other phonon modes in the SWNT. In particular, for the G band phonons, which correspond to bond-stretching motions, we find that the modulation of the interatomic dipole matrix element gives rise to a strong coherent G band phonon intensity comparable to the coherent RBM phonon intensity. We also further discuss the dependence of coherent G band and RBM phonon amplitudes on the laser excitation pulse width.

In silico infrared and Raman spectroscopy under pressure: the case of CaSnO3 perovskite.

The CaSnO3 perovskite is investigated under geochemical pressure, up to 25 GPa, by means of periodic ab initio calculations performed at B3LYP level with local Gaussian-type orbital basis sets. Structural, elastic, and spectroscopic (phonon wave-numbers, infrared and Raman intensities) properties are fully characterized and discussed. The evolution of the Raman spectrum of CaSnO3 under pressure is reported to remarkably agree with a recent experimental determination [J. Kung, Y. J. Lin, and C. M. Lin, J. Chem. Phys. 135, 224507 (2011)] as regards both wave-number shifts and intensity changes. All phonon modes are symmetry-labeled and bands assigned. The single-crystal total spectrum is symmetry-decomposed into the six directional spectra related to the components of the polarizability tensor. The infrared spectrum at increasing pressure is reported for the first time and its main features discussed. All calculations are performed using the Crystal14 program, taking advantage of the new implementation of analytical infrared and Raman intensities for crystalline materials.

Thermal Characteristics of an Aluminum Thin Film due to Temperature Disturbance at Film Edges

Phonon transport in an aluminum thin film is simulated due to a temperature disturbance across the film. The Boltzmann equation is introduced to formulate the radiative transport in the electron and lattice sub-systems. The transient and frequency dependence of the phonon transport is considered, and dispersion relations are accommodated to account for the group velocities in the analysis. Electron-phonon coupling is employed to couple the energy transport across the electron and lattice sub-systems. An equivalent equilibrium temperature is presented to assess the characteristics of the phonon intensity in the film. Temperature predictions are validated with data presented in a previous study. It is found that the equivalent equilibrium temperature differs significantly from that obtained from the two-equation model. The film thickness influences the transport characteristics of the film, in which case the time to reach an almost quasi-steady temperature is shorter for the thin film (\(L_{x}= 0.25 \,\upmu \hbox {m}\), where \(L_{x}\) is the film thickness) than that corresponding to the thick film (\(L_{x}= 2 \,\upmu \hbox {m}\)). In the diffusion limit (when the Knudsen number \(Kn=\varLambda /{L_x }\rightarrow 0\), where \(\varLambda \) is the mean free path), it is demonstrated that the radiative transport equation reduces to the formulation of the two-equation model.

Phonon counting and intensity interferometry of a nanomechanical resonator

In optics, the ability to measure individual quanta of light (photons) enables a great many applications, ranging from dynamic imaging within living organisms to secure quantum communication. Pioneering photon counting experiments, such as the intensity interferometry performed by Hanbury Brown and Twiss to measure the angular width of visible stars, have played a critical role in our understanding of the full quantum nature of light. As with matter at the atomic scale, the laws of quantum mechanics also govern the properties of macroscopic mechanical objects, providing fundamental quantum limits to the sensitivity of mechanical sensors and transducers. Current research in cavity optomechanics seeks to use light to explore the quantum properties of mechanical systems ranging in size from kilogram-mass mirrors to nanoscale membranes, as well as to develop technologies for precision sensing and quantum information processing. Here we use an optical probe and single-photon detection to study the acoustic emission and absorption processes in a silicon nanomechanical resonator, and perform a measurement similar to that used by Hanbury Brown and Twiss to measure correlations in the emitted phonons as the resonator undergoes a parametric instability formally equivalent to that of a laser. Owing to the cavity-enhanced coupling of light with mechanical motion, this effective phonon counting technique has a noise equivalent phonon sensitivity of 0.89±0.05. With straightforward improvements to this method, a variety of quantum state engineering tasks using mesoscopic mechanical resonators would be enabled, including the generation and heralding of single-phonon Fock states and the quantum entanglement of remote mechanical elements.

Temperature dependent Raman scattering and far-infrared reflectance spectra of MgO modified Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 ceramics: A composition effect

Lattice dynamics and phase transition of MgO modified Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 (PZTN-x wt. % MgO, x = 0, 0.1, 0.2, 0.5) ceramics have been investigated by far-infrared (FIR) reflectance in the temperature range of 5.5–300 K and Raman spectra between 77 and 300 K, respectively. With the aid of above complementary methods, the structure of all ceramics was defined as low-temperature ferroelectric rhombohedral phase [FR(LT)] at room temperature. The FIR dielectric functions were extracted from the multi-Lorentz oscillator dispersion model. The lowest frequency phonon mode, which is related to Pb-BO3 (B = Zr, Ti, Nb) vibration, mainly dominates the FIR dielectric response. With increasing MgO composition, the dielectric constants ε(0) at room temperature are estimated to 85.4, 73.4, 73.9, and 41.9, respectively. The decreasing trend can be due to the doubly ionized oxygen vacancies induced by Mg substitution for B-site. The order-disorder phase transition located around 120 K can be clearly clarified from temperature evolution of phonon frequency, damping, and intensity. It decreases slightly with increasing MgO composition, which influence the distortion due to the broken correlation chains and local permanent dipoles creation. Moreover, the transformation from antiferroelectric orthorhombic AO to [FR(LT)] phase has been observed around 250 K, which is associated with the antiferroelectric displacement of Pb atoms along 〈110〉 and coupled rotations of the corner-connected oxygen octahedral. Furthermore, the transition from [FR(LT)] to [FR(HT)] (high-temperature ferroelectric rhombohedral phase) was identified around 290 K for MgO-doped PZTN ceramics. It arises from the shift of cation (Pb and Zr/Ti/Nb/Mg ions) along the 〈111〉 direction and the transition temperature slightly decreases compared to the pure ceramic.

Effect of temperature oscillation on thermal characteristics of an aluminum thin film

Energy transport in aluminum thin film is examined due to temperature disturbance at the film edge. Thermal separation of electron and lattice systems is considered in the analysis, and temperature variation in each sub-system is formulated. The transient analysis of frequency-dependent and frequency-independent phonon radiative transport incorporating electron–phonon coupling is carried out in the thin film. The dispersion relations of aluminum are used in the frequency-dependent analysis. Temperature at one edge of the film is oscillated at various frequencies, and temporal response of phonon intensity distribution in the film is predicted numerically using the discrete ordinate method. To assess the phonon transport characteristics, equivalent equilibrium temperature is introduced. It is found that equivalent equilibrium temperature in the electron and lattice sub-systems oscillates due to temperature oscillation at the film edge. The amplitude of temperature oscillation reduces as the distance along the film thickness increases toward the low-temperature edge of the film. Equivalent equilibrium temperature attains lower values for the frequency-dependent solution of the phonon transport equation than that corresponding to frequency-independent solution.

Optical properties of BiTeBr and BiTeCl

We present a comparative study of the optical properties - reflectance, transmission and optical conductivity - and Raman spectra of two layered bismuth-tellurohalides BiTeBr and BiTeCl at 300 K and 5 K, for light polarized in the a-b planes. Despite different space groups, the optical properties of the two compounds are very similar. Both materials are doped semiconductors, with the absorption edge above the optical gap which is lower in BiTeBr (0.62 eV) than in BiTeCl (0.77 eV). The same Rashba splitting is observed in the two materials. A non-Drude free carrier contribution in the optical conductivity, as well as three Raman and two infrared phonon modes, are observed in each compound. There is a dramatic difference in the highest infrared phonon intensity for the two compounds, and a difference in the doping levels. Aspects of the strong electron-phonon interaction are identified. Several interband transitions are assigned, among them the low-lying absorption $\beta$ which has the same value 0.25 eV in both compounds, and is caused by the Rashba spin splitting of the conduction band. An additional weak transition is found in BiTeCl, caused by the lower crystal symmetry.

Probing high‐energy ion‐implanted silicon by micro‐Raman spectroscopy

The effect of ion implantation (4 MeV12C2+, 5 MeV16O2+, and 8 MeV28Si2+) on [110] silicon wafers in channeling and random orientation is investigated by micro‐Raman spectroscopy. The profiles were measured using Scanning Electron Microscope (SEM) showing that the ions were penetrating deeper inside the wafer in the channeling case creating a 1–2 µm wide strongly modified region and agreeing with the d‐nuclear reaction analysis measurements. Micro‐Raman spectroscopy was employed for the assessment of the lattice damage, probing the side surface of the cleaved wafers at submicron step. The phonon modifications show strong lattice distortions in zones parallel to the front surface of the wafers and at depths, which agree with the results of the characterization techniques. In these strongly damaged zones, there is a substantial reduction in the phonon intensity, a small shift in wavenumber position, and a large increase in the phonon width. On the basis of a modification of the phonon confinement model that takes under consideration the laser beam profile, the reduction in intensity of scattered light, and the nanocrystallite size distribution from the simulation of the lattice displacements, the main characteristics of the Raman spectra could be reproduced for the random C and O implantations. The results indicate that at a critical doping level, the induced defects and lattice distortions relax by breaking the silicon single crystal into nanocrystallites, thus creating the observed zones of strongly distorted lattice. Copyright © 2014 John Wiley & Sons, Ltd.

Effect of temperature on thermal expansion and anharmonicity in Cu2ZnSnS4 thin films grown by co-sputtering and sulfurization

Copper zinc tin sulfide (CZTS, made from the earth abundant and non-toxic materials) is a quaternary semiconducting compound which has received increasing interest for solar cells applications. In this study, CZTS thin film has been grown by co-sputtering Cu, Zn and Sn metal targets and sulfurizing it in H2S. XRD, SEM, EDS, XRF and optical studies show that these films are suitable for solar cell applications. The temperature-dependent Raman spectroscopic study on CZTS thin film was carried out in the temperature range of 80–450 K. A decrease in the intensity of “A” mode Raman peaks, shift of Raman peaks towards lower frequency and increase in the line width have been observed with increase in temperature. Raman “A” mode shifts from 337 cm−1 at 80 K to 329 cm−1 at 450 K. The peak at 288 cm−1 disappears when the measurement was taken at 450 K. Based on experimental results and fittings of anharmonic equation; it has been shown that these effects are due to thermal expansion and interaction of the phonon with other phonons which arise at higher temperature. These phonons interact resulting in damping of the “A” mode phonon intensity. It was concluded that in order to truly analyze multicomponent compounds, Raman studies should be carried out at low temperature.

Excitation energy transfer and electron-vibrational coupling in phycobiliproteins of the cyanobacterium Acaryochloris marina investigated by site-selective spectroscopy

In adaption to its specific environmental conditions, the cyanobacterium Acaryochloris marina developed two different types of light-harvesting complexes: chlorophyll-d-containing membrane-intrinsic complexes and phycocyanobilin (PCB) — containing phycobiliprotein (PBP) complexes. The latter complexes are believed to form a rod-shaped structure comprising three homo-hexamers of phycocyanin (PC), one hetero-hexamer of phycocyanin and allophycocyanin (APC) and probably a linker protein connecting the PBPs to the reaction centre. Excitation energy transfer and electron-vibrational coupling in PBPs have been investigated by selectively excited fluorescence spectra. The data reveal a rich spectral substructure with a total of five low-energy electronic states with fluorescence bands at 635nm, 645nm, 654nm, 659nm and a terminal emitter at about 673nm. The electronic states at ~635 and 645nm are tentatively attributed to PC and APC, respectively, while an apparent heterogeneity among PC subunits may also play a role. The other fluorescence bands may be associated with three different isoforms of the linker protein. Furthermore, a large number of vibrational features can be identified for each electronic state with intense phonon sidebands peaking at about 31 to 37cm−1, which are among the highest phonon frequencies observed for photosynthetic antenna complexes. The corresponding Huang-Rhys factors S fall in the range between 0.98 (terminal emitter), 1.15 (APC), and 1.42 (PC). Two characteristic vibronic lines at about 1580 and 1634cm−1 appear to reflect CNH+ and CC stretching modes of the PCB chromophore, respectively. The exact phonon and vibrational frequencies vary with electronic state implying that the respective PCB chromophores are bound to different protein environments. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.

Magnetic, structural, and electronic properties of the multiferroic compound FeTe 2O 5Br with geometrical frustration

We report electron spin resonance (ESR), Raman scattering, and interband absorption measurements of the multiferroic FeTe2O5Br with two successive magnetic transitions at TN1 = 11.0 K and TN2 = 10.5 K. ESR measurements show all characteristics of a low-dimensional frustrated magnet: (i) the appearance of an antiferromagnetic resonance (AFMR) mode at 40 K, a much higher temperature than TN1, and (ii) a weaker temperature dependence of the AFMR linewidth than in classical magnets, ΔHpp(T) ∝ Tn with n = 2.2–2.3. Raman spectra at ambient pressure show a large variation of phonon intensities with temperature while there are no appreciable changes in phonon numbers and frequencies. This demonstrates the significant role of the polarizable Te4+ lone pairs in inducing multiferroicity. Under pressure at P = 2.12–3.04 GPa Raman spectra undergo drastic changes and absorption spectra exhibit an abrupt drop of a band gap. This evidences a pressure-induced structural transition related to changes of the electronic states at high pressures.