Dissertations / Theses on the topic 'Phononic Properties'

This dissertation contains research directed at investigating the behavior and properties of a class of composite materials known as phononic crystals. Two categories of phononic crystals are explicitly investigated: (I) elastic phononic crystals and (II) nano-scale phononic crystals. For elastic phononic crystals, attention is directed at two-dimensional structures. Two specific structures are evaluated (1) a two-dimensional configuration consisting of a square array of cylindrical Polyvinylchloride inclusions in air and (2) a two-dimensional configuration consisting of a square array of steel cylindrical inclusions in epoxy. For the first configuration, a theoretical model is developed to ascertain the necessary band structure and equi-frequency contour features for the realization of phase control between propagating acoustic waves. In contrasting this phononic crystal with a reference system, it is shown that phononic crystals with equifrequency contours showing non-collinear wave and group velocity vectors are ideal systems for controlling the phase between propagating acoustic waves. For the second configuration, it is demonstrated that multiple functions can be realized of a solid/solid phononic crystal. The epoxy/steel phononic crystal is shown to behave as (1) an acoustic wave collimator, (2) a defect-less wave guide, (3) a directional source for elastic waves, (4) an acoustic beam splitter, (5) a phase-control device and (6) a k-space multiplexer. To transition between macro-scale systems (elastic phononic crystals) and nano-scale systems (nano-phononic crystals), a toy model of a one-dimensional chain of masses connected with non-linear, anharmonic springs is utilized. The implementation of this model introduces critical ideas unique to nano-scale systems, particularly the concept of phonon mode lifetime. The nano-scale phononic crystal of interest is a graphene sheet with periodically spaced holes in a triangular array. It is found through equilibrium molecular dynamics simulation techniques, that phonon-boundary collision effects and coherent phononic effects (band-folding) are two competing scattering mechanisms responsible for the reduction of acoustic and optical phonon lifetimes. Conclusions drawn about the lifetime of thermal phonons in phononic crystal patterned graphene are linked with the anharmonic, one-dimensional crystal model.

Thesis (Ph. D.)--University of California, San Diego, 2006.<br>Title from first page of PDF file (viewed June 5, 2006). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 252-265).

[EN] Phononic crystals are artificial materials formed by a periodic arrangement of inclusions embedded into a host medium, where each of them can be solid or fluid. By controlling the geometry and the impedance contrast of its constituent materials, one can control the dispersive properties of waves, giving rise to a huge variety of interesting and fundamental phenomena in the context of wave propagation. When a propagating wave encounters a medium with different physical properties it can be transmitted and reflected in lossless media, but also absorbed if dissipation is taken into account. These fundamental phenomena have been classically explained in the context of homogeneous media, but it has been a subject of increasing interest in the context of periodic structures in recent years as well. This thesis is devoted to the study of different effects found in sonic and phononic crystals associated with transmission, reflection and absorption of waves, as well as the development of a technique for the characterization of its dispersive properties, described by the band structure. We start discussing the control of wave propagation in transmission in conservative systems. Specifically, our interest is to show how sonic crystals can modify the spatial dispersion of propagating waves leading to control the diffractive broadening of sound beams. Making use of the spatial dispersion curves extracted from the analysis of the band structure, we first predict zero and negative diffraction of waves at frequencies close to the band-edge, resulting in collimation and focusing of sound beams in and behind a 3D sonic crystal, and later demonstrate it through experimental measurements. The focusing efficiency of a 3D sonic crystal is limited due to the strong scattering inside the crystal, characteristic of the diffraction regime. To overcome this limitation we consider axisymmetric structures working in the long wavelength regime, as a gradient index lens. In this regime, the scattering is strongly reduced and, in an axisymmetric configuration, the symmetry matching with acoustic sources radiating sound beams increase its efficiency dramatically. Moreover, the homogenization theory can be used to model the structure as an effective medium with effective physical properties, allowing the study of the wave front profile in terms of refraction. We will show the model, design and characterization of an efficient focusing device based on these concepts. Consider now a periodic structure in which one of the parameters of the lattice, such as the lattice constant or the filling fraction, gradually changes along the propagation direction. Chirped crystals represent this concept and are used here to demonstrate a novel mechanism of sound wave enhancement based on a phenomenon known as "soft" reflection. The enhancement is related to a progressive slowing down of the wave as it propagates along the material, which is associated with the group velocity of the local dispersion relation at the planes of the crystal. A model based on the coupled mode theory is proposed to predict and interpret this effect. Two different phenomena are observed here when dealing with dissipation in periodic structures. On one hand, when considering the propagation of in-plane sound waves in a periodic array of absorbing layers, an anomalous decrease in the absorption, combined with a simultaneous increase of reflection and transmission at Bragg frequencies is observed, in contrast to the usual decrease of transmission, characteristic in conservative periodic systems at these frequencies. For a similar layered media, backed now by a rigid reflector, out-of-plane waves impinging the structure from a homogeneous medium will increase dramatically the interaction strength. In other words, the time delay of sound waves inside the periodic system will be considerably increased resulting in an enhanced absorption, for a broadband spectral range.<br>[ES] Los cristales fonónicos son materiales artificiales formados por una disposición periódica de inclusiones en un medio, pudiendo ambos ser de carácter sólido o fluido. Controlando la geometría y el contraste de impedancias entre los materiales constituyentes se pueden controlar las propiedades dispersivas de las ondas. Cuando una onda propagante se encuentra un medio con diferentes propiedades físicas puede ser transmitida y reflejada, en medios sin pérdidas, pero también absorbida, si la disipación es tenida en cuenta. La presente tesis está dedicada al estudio de diferentes efectos presentes en cristales sónicos y fonónicos relacionados con la transmisión, reflexión y absorción de ondas, así como el desarrollo de una técnica para la caracterización de sus propiedades dispersivas, descritas por la estructura de bandas. En primer lugar, se estudia el control de la propagación de ondas en transmisión en sistemas conservativos. Específicamente, nuestro interés se centra en mostrar cómo los cristales sónicos son capaces de modificar la dispersión espacial de las ondas propagantes, dando lugar al control del ensanchamiento de haces de sonido. Haciendo uso de las curvas de dispersión espacial extraídas del análisis de la estructura de bandas, se predice primero la difracción nula y negativa de ondas a frecuencias cercanas al borde de la banda, resultando en la colimación y focalización de haces acústicos en el interior y detrás de un cristal sónico 3D, y posteriormente se demuestra mediante medidas experimentales. La eficiencia de focalización de un cristal sónico 3D está limitada debido a las múltiples reflexiones existentes en el interior del cristal. Para superar esta limitación se consideran estructuras axisimétricas trabajando en el régimen de longitud de onda larga, como lentes de gradiente de índice. En este régimen, las reflexiones internas se reducen fuertemente y, en configuración axisimétrica, la adaptación de simetría con fuentes acústicas radiando haces de sonido incrementa la eficiencia drásticamente. Además, la teoría de homogenización puede ser empleada para modelar la estructura como un medio efectivo con propiedades físicas efectivas, permitiendo el estudio del frente de ondas en términos refractivos. Se mostrará el modelado, diseño y caracterización de un dispositivo de focalización eficiente basado en los conceptos anteriores. Considérese ahora una estructura periódica en la que uno de los parámetros de la red, sea el paso de red o el factor de llenado, cambia gradualmente a lo largo de la dirección de propagación. Los cristales chirp representan este concepto y son empleados aquí para demostrar un mecanismo novedoso de incremento de la intensidad de la onda sonora basado en un fenómeno conocido como reflexión "suave". Este incremento está relacionado con una ralentización progresiva de la onda conforme se propaga a través del material, asociado con la velocidad de grupo de la relación de dispersión local en los planos del cristal. Un modelo basado en la teoría de modos acoplados es propuesto para predecir e interpretar este efecto. Se observan dos fenómenos diferentes al considerar pérdidas en estructuras periódicas. Por un lado, si se considera la propagación de ondas sonoras en un array periódico de capas absorbentes, cuyo frente de ondas es paralelo a los planos del cristal, se produce una reducción anómala en la absorción combinada con un incremento simultáneo de la reflexión y transmisión a las frecuencias de Bragg, de forma contraria a la habitual reducción de la transmisión, característica de sistemas periódicos conservativos a estas frecuencias. En el caso de la misma estructura laminada en la que se cubre uno de sus lados mediante un reflector rígido, la incidencia de ondas sonoras desde un medio homogéneo, cuyo frente de ondas es perpendicular a los planos del cristal, produce un gran incremento de la fuerza de<br>[CAT] Els cristalls fonònics són materials artificials formats per una disposició d'inclusions en un medi, ambdós poden ser sòlids o fluids. Controlant la geometría i el contrast d'impedàncies dels seus materials constituents, és poden controlar les propietats dispersives de les ondes, permetent una gran varietatde fenòmens fonamentals interessants en el context de la propagació d'ones. Quan una ona propagant troba un medi amb pèrdues amb propietats físiques diferents es pot transmetre i reflectir, però també absorbida si la dissipació es té en compte. Aquests fenòmens fonamentals s'han explicat clàssicament en el context de medis homogenis, però també ha sigut un tema de creixent interés en el context d'estructures periòdiques en els últims anys. Aquesta tesi doctoral tracta de l'estudi de diferents efectes en cristalls fonònics i sònics lligats a la transmissió, reflexió i absorció d'ones, així com del desenvolupament d'una tècnica de caracterització de les propietats dispersives, descrites mitjançant la estructura de bandes. En primer lloc, s'estudia el control de la propagació ondulatori en transmissió en sistemes conservatius. Més específicament, el nostre interés és mostrar com els cristalls sonors poden modificar la dispersió espacial d'ones propagants donant lloc al control de l'amplària per difracció dels feixos sonors. Mitjançant les corbes dispersió espacial obtingudes de l'anàlisi de l'estructura de bandes, es prediu, en primer lloc, la difracció d'ones zero i negativa a freqüències próximes al final de banda. El resultat és la collimació i focalització de feixos sonors dins i darrere de cristalls de so. Després es mostra amb mesures experimentals. L'eficiència de focalització d'un cristall de so 3D està limitada per la gran dispersió d'ones dins del cristall, que és característic del règim difractiu. Per a superar aquesta limitació, estructures axisimètriques que treballen en el règim de llargues longituds d'ona, i es comporten com a lents de gradient d'índex. En aquest règim, la dispersió es redueix enormement i, en una configuració axisimètrica, a causa de l'acoblament de la simetría amb les fonts acústiques que radien feixos sonors, l'eficiència de radiació s'incrementa significativament. D'altra banda, la teoria d'homogeneïtzació es pot utilitzar per a modelar, dissenyar i caracteritzar un dispositiu eficient de focalització basat en aquests conceptes. Considerem ara una estructura periòdica en la qual un dels seus paràmetres de xarxa, com ara la constant de xarxa o el factor d'ompliment canvia gradualment al llarg de la direcció de propagació. Els cristalls chirped representen aquest concepte i s'utilitzen ací per a demostrar un mecanisme nou d'intensificació d'ones sonores basat en el fenòmen conegut com a reflexió "suau". La intensificació està relacionada amb la alentiment progressiva de l'ona conforme propaga al llarg del material, que està associada amb la velocitat de grup de la relació de dispersió local en els diferents plànols del cristall. Es proposa un model basat en la teoria de modes acoblats per a predir i interpretar este efecte. Dos fenòmens diferents cal destacar quan es tracta d'estructures periòdiques amb dissipació. Per un costat, al considerar la propagació d'ones sonores en el plànol en un array periòdic de capes absorbents, s'observa una disminució anòmala de l'absorció i es combina amb un augment simultani de reflexió i transmissió en les freqüències de Bragg que contrasta amb la usual disminució de transmissió, característica dels sistemes conservatius a eixes freqüències. Per a un medi similar de capes, amb un reflector rígid darrere, les ones fora del pla incidint l'estructura des de un medi homogeni, augmentaran considerablement la interacció. En altres paraules, el retràs temporal de les ones sonores dins del sistema periòdic augmentarà significativament produint un augmen<br>Cebrecos Ruiz, A. (2015). Transmission, reflection and absorption in Sonic and Phononic Crystals [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/56463<br>TESIS<br>Premiado

A detailed understanding of fundamental material properties can be obtained through the study of atomic vibrations, performed experimentally with neutron scattering techniques and coupled with the two powerful new computational methodologies I have developed. The first approach involves phonon-based simulations of the pair distribution function - a histogram of localised atomic positions generated experimentally from total scattering data. This is used to reveal ordering behaviour, to validate interatomic models and localised structure, and to give insights into how far dynamic behaviour can be studied using total scattering techniques. Most importantly, the long-standing controversy over dynamic disorder in β-cristobalite is resolved using this technique. Inelastic neutron spectroscopy (INS) allows \emph{direct} study of vibrational modes through their interaction with the neutron beam, and is the experimental basis for the second strand of the new methodology. I have developed new simulation and refinement tools based on the next generation of spectrometers currently being commissioned at the ISIS pulsed neutron source. This allows a detailed powder spectroscopy study of cristobalite and vitreous silica demonstrating that the Bose peak and so-called 'fast sound' features can be derived from standard lattice dynamics in both the crystal and the amorphous counterpart, and allowing discussion of their origins. Given the controversy in the literature, this is a key result. The new methodology also encompasses refinement of interatomic models against powder INS data, with aluminium providing a successful test-case. A more complex example is seen in calcite, with experimental data collected during the commissioning of the new MERLIN spectrometer. Simulated one-phonon coherent INS spectra for the single crystal and powder (the latter including approximations to multi-phonon and multiple scatter) are fully convolved with experimental resolution functions. These are used in the analysis of the experimental data, yielding previously unpublished dispersion curves and soft mode information, as well as allowing the effectiveness of powder refinement of more complex materials to be assessed. Finally, I present further applications with technologically important materials - relaxor ferroelectrics and high temperature pnictide superconductors. The conclusions draw together the different strands of the work, discussing the importance of these new advances together with future developments and scientific applications.

The focus of this thesis is the study of the electronic and vibrational properties of single layer graphene, silicene and germanene, and bilayer graphene and silicene. Specifically the electronic band structure and Fermi velocity of the carriers as well as the phonon dispersion are calculated; Raman active modes are identified as well. Material stability and electron-phonon effects are also investigated. It is found that in both silicene and germanene linear band dispersion behaviour is found in a planar and a low buckling configuration where the Fermi velocity is 5  105 m/s; about 35% lower than in graphene. From the phonon dispersion curves, the electron-phonon coupling matrix elements are shown to be about a factor of 25 lower when compared with graphene. The applicability of the Born-Oppenheimer approximation to carrier relaxation in silicene and germanene is found to be invalid, as it is in graphene. The phonon dispersion curves show that free-standing bilayer graphene with AB stacking is stable whereas AA stacking is marginally unstable and an optical identification method to distinguish between the two different stacking configurations is proposed. Results for bilayer silicene show there are eight geometries with an energy minimum; it is found that a low buckling AA and a high buckling AA’ stacking configuration have the lowest energies for their groups. Metallic properties are found for all configurations although unusual band structure is found in low buckling forms including linear dispersion behaviour in AA’ form (vf ≈ 5.3  105 m/s). The phonon dispersion curves show that only AB stacked low buckling bilayer silicene was completely stable although it had a higher energy state than AA stacking. The high buckling configuration also shows significant changes to the properties of in-plane vibrational modes suggesting that they can be controlled or engineered by the introduction of additional layers.

The objective of this PhD thesis was the study of friction in carbon nanotubes by analytical methods and molecular dynamics simulations. The goal of this research was to characterize the properties of friction in nanotubes and from a more general point of view the understanding of the microscopic origin of friction. Indeed, the relative simplicity of the system allows us to interpret more easily the physical phenomenon observed than in larger systems. In order to achieve this goal, non-equilibrium statistical mechanics permitted first to develop models based on Langevin equations describing the dynamics of rotation and translation in double walled nanotubes. The molecular dynamics simulations then permitted to validate these analytical models, and thus to study general properties of friction such as the dependence on area of contact, temperature and the geometry of the nanotubes.<p><p>The results obtained shows that the friction increases linearly with the sliding velocity or the angular velocity until very high values beyond that non-linearities appear enhancing dissipation. In the linear regime, it is shown that the proportionality factor between the dynamic friction force and the velocity is given by the time integral of the autocorrelation function of the restoring force for the sliding friction and of the torque for the rotational friction. Furthermore, a novel resonant friction phenomenon increasing significantly dissipation was observed for the sliding motion in certain types of nanotubes. The effect arises at sliding velocities corresponding to certain vibrational modes of the nanotubes. When the dynamics is described by the linear friction in velocity, the empirical law stating that friction is proportional to the area of contact is very well verified thanks to the molecular dynamics simulations. On the other hand, friction increases with temperature. The fact that friction increases as well with the area of contact as the temperature can be easily interpreted. Indeed, if the temperature is large enough so that the electronic effects can be negligible, dissipation is only due to the phonons. Indeed, it is the phonons who give the sliding or rotation energy to the other degrees of freedom until thermodynamic equilibrium is achieved. Thus, if the temperature increases, the coupling between the phonons and the rotational or translational motions increases, as well as friction. In the same manner, when the area of contact increases, the number of available phonons to transport energy increases, explaining thus the increase of the friction force.<p><br>Doctorat en sciences, Spécialisation physique<br>info:eu-repo/semantics/nonPublished

La tendencia de la miniaturización tecnológica ha conducido a la generación de densidades de potencia que exceden los 100 watts/cm2, los cuales están en el orden del calor producido en los reactores termonucleares. La necesidad de nuevas técnicas y procesos de enfriamiento ha posicionado al manejo térmico en el escenario científico en estos últimos años. Por otro lado, la ingeniería de la conducción térmica puede abrir rutas nuevas rutas para recolección energética, por ejemplo, a través de la generación termoeléctrica. Como consecuencia, el control y la ingeniería de fonones a nivel nanométrico es una indispensable herramienta para el ajuste de propiedades físicas deseadas en dispositivos en la búsqueda de un adecuado compromiso entre performance y el consumo de potencia. En el siguiente trabajo se presenta un exhaustivo estudio teórico y experimental de la dependencia con el grosor de las propiedades térmicas en membranas de silicio con espesores que fluctúan entre los 9 a los 2000 nm. La relación de dispersión fononica y la correspondiente velocidad de fase han sido investigadas por medio de la espectroscopia inelástica de la luz Brillouin. La reducción de la velocidad de fase/grupo del modo fundamental de flexión fue observada y teóricamente explicada. Además, la reducción de los tiempos de vida media de fonones acústicos coherentes con frecuencias máximas que bordean los 500 GHz también ha sido estudiada utilizando una de las últimas técnicas ultrarrápidas de resolución temporal (ultrafast pump-probe) conocida como: asynchronous optical sampling (ASOPS). Por medio de esta técnica se observo la dramática disminución de los tiempos de vida del primer modo de dilatación desde los ~ 4.7 ns a los 5 ps en mebranas con grosores entre los ~ 194 a los 8 nm. Finalmente, la conductividad térmica de estas membranas fue investigada utilizando tres diferentes técnicas de caracterización conocidas como: Single laser termometría Raman, doble-laser termometría Raman y gradiente térmico transitorio. Por medio de estas técnicas hemos encontrado que la conductividad térmica de las membranas se reduce sistemáticamente con el grosor de estas, alcanzando valores tan bajos como 9 Wm-1K-1 para la membrana más delgada. Para entender estos drásticos cambios en las propiedades térmicas de las membranas, diferentes aproximaciones teóricas han sido desarrolladas. La simulación de la relación de dispersión acústica fue desarrollada utilizando tres modelos diferentes: el modelo elástico continuo, el modelo de Debye y un modelo ajuste. La dependencia de los tiempos de vida media fueron simulados considerando procesos de dispersiones extrínsecas e intrínsecas. Mientras que conductividad térmica fue modelada utilizando la modificación del modelo bidimensional de Debye (Modelo de Huang), el modelo de Srivastava-Callaway-Debye y el modelo de Fuchs-Sondheimer. Estas observaciones tienes consecuencias significativas para la tecnología basas en el silicio, estableciendo las bases para la investigación de las propiedades térmicas de otros sistemas de baja dimesnionalidad. Ademas, este estudio proporcionaría las directrices de diseño generando nuevo enfoques para el manejo térmico a escalas nanométricas.<br>The miniaturization trend of the technology has led to power level densities in excess 100 watts/cm2, which are in the order of the heat produced in a nuclear reactor. The need for new cooling techniques has positioned the thermal management on the stage the last years. Moreover, the engineering of the thermal conduction opens a route to energy harvesting through, for example, thermoelectric generation. As a consequence, control and engineering of phonons in the nanoscale is essential for tuning desirable physical properties in a device in the quest to find a suitable compromise between performance and power consumption. In the present work we study theoretically and experimentally the thickness-dependence of the thermal properties of silicon membranes with thicknesses ranging from 9 to 2000 nm. We investigate the dispersion relations and the corresponding modification of the phase velocities of the acoustic modes using inelastic Brillouin light scattering spectroscopy. A reduction of the phase/group velocities of the fundamental flexural mode by more than one order of magnitude compared to bulk values was observed and is theoretically explained. In addition, the lifetime of the coherent acoustic phonon modes with frequencies up to 500 GHz was also studied using state-of-the-art ultrafast pump-probe: asynchronous optical sampling (ASOPS). We have observed that the lifetime of the first-order dilatational mode decreases significantly from ∼ 4.7 ns to 5 ps with decreasing membrane thickness from ∼ 194 to 8 nm. Finally, the thermal conductivity of membranes was investigated using three different contactless techniques known as single-laser Raman thermometry, two-laser Raman thermometry and transient thermal gradient. We have found that the thermal conductivity of the membranes gradually reduces with their thickness, reaching values as low as 9 Wm-1K-1 for the thinnest membrane. In order to account for the observed thermal behaviour of the silicon membranes we have developed different theoretical approaches to explain the size dependence of thermal properties. The simulation of acoustic dispersion was carried out by using models based on an elastic continuum approach, Debye and fitting approaches. The size dependence of the lifetimes was modelled considering intrinsic phonon-phonon processes and extrinsic phonon scatterings. The thermal conductivity was modelled using a modified 2D Debye approach (Huang model), Srivastava-Callaway-Debye model and Fuchs-Sondheimer approach. Our observations have significant consequences for Si-based technology, establishing the foundation to investigate the thermal properties in others low-dimensional systems. In addition, this study would provide design guidelines and enable new approaches for thermal management at nanometric scales.

This thesis addresses a theoretical study of the problem of a single impurity embedded in a one-dimensional system of interacting electrons in presence of electron-phonon coupling. First we consider a system with a featureless point-like potential impurity, followed by the case of a resonant level hybridised with a Luttinger Liquid. The stress is made on a more fundamental problem of a featureless scatterer, for which two opposite limits in the impurity strength are considered: a weak scatterer and a weak link. We have found that, regardless of the transmission properties of phonons through the impurity, the scaling dimensions of the conductance in these limits obey the duality condition, \( \triangle_{WS}\) \( \triangle_{WL}\) = 1, known for the Luttinger Liquid in the absence of phonons. However, in the case when the strength of phonon scattering is correlated with electron scattering by the impurity, we find a nontrivial phase diagram with up to three fixed points and a possibility of a metal-insulator transition. We also consider the case of a weakly interacting electron-phonon system in the presence of a single impurity of an arbitrary scattering potential. In the problem of a resonant level attached to the Luttinger Liquid we show that the electron-phonon coupling significantly modifies the effective energy-dependent width of the resonant level in two different geometries, corresponding to the resonant and anti-resonant transmission in the Fermi gas.

Thesis (Ph. D.)--University of Florida, 2001.<br>Title from title page of source document. Document formatted into pages; contains vii, 147 p.; also contains graphics. Includes vita. Includes bibliographical references.

Phonon properties of AB (Bernal) stacked bilayer graphene (BLG) with various types of defects have been investigated theoretically. Forced Vibrational (FV) method has been employed to compute the phonon modes of disordered BLG. A downward linear shift of E2g mode frequencies has been observed with an increasing amount of defect concentration. Moreover, two identical E2g peaks have been observed in PDOS of the bilayer system where the individual layer contains 12C and 13C atoms respectively. From computed typical mode patterns of in-plane K-point optical mode phonons, it has been noticed that phonons become strongly localized around a few nanometers area at the presence of defects and localized modes increase with the increasing amount of defect concentration. The edge effect on the localized phonon modes has also been discussed for bilayer armchair graphene nanoribbons (BiAGNRs). The impact of defects on the phonon conduction properties has also been studied for BiAGNRs. My investigated results can be convenient to study the thermal conductivity and electron-phonon interaction of bilayer graphene-based nanodevices and to interpret the Raman and infrared spectra of disordered system.

At low temperatures the predominant energy loss mechanism for a Joule-heated two dimensional electron gas (2DEG) in a metal oxide semiconductor field effect transistor (MOSFET) is by acoustic phonon emission. By very accurately measuring the temperature gradient developed along the silicon substrate the phonon emission has been investigated as a function of electron concentration, device power, magnetic field and temperature. In zero magnetic field the results show the cut-off predicted theoretically in the maximum phonon momentum that can be emitted in the plane of the 2DEG for low electron concentrations. It is also found that the momentum of the emitted phonons perpendicular to the plane of the 2DEG is restricted by the width of the 2DEG for the high resistivity (1000 [omega]cm) substrates used. For carrier concentrations greater than 4.9 x 1016 m-2 phonon emission from an upper subband is seen. Electrical measurements indicate that the high mobility (1.2 m2 V-1 S-1) of the devices used leads to changes in the screening of scattering potentials by the electrons being important. This is also seen in the phonon emission experiments. Experiments performed in quantising magnetic fields up to 7 T show that for the powers used (0.2 uW mm-2 – 500 uW mm-2) the phonons emitted arise from Lars-Landau level scattering. Oscillations in the temperature of a thermometer situated directly opposite the middle of the 2DEG are attributed to the movement of the phonon emission to the corners of the 2DEG when the Fermi level is between Landau levels (the Quantum Hall regime). Other trends are attributed to the width of the Landau level limiting the maximum phonon energy that can be emitted. Attempts to use a stress tuned phonon filter to probe the frequency dependence of the phonon emission failed due to experimental difficulties.

Les besoins technologiques toujours grandissants dans la société moderne suscitent la nécessité de développer des matériaux multifonctionnels innovants. Ceci est vrai surtout dans des domaines de pointe, tels que la microélectronique et la conversion d'énergie, où on demande aux matériaux de limiter la dissipation de chaleur tout en ayant de bonnes propriétés électroniques.L'optimisation d'un tel type de matériaux est toutefois complexe: une forte réduction de la conductivité thermique se fait en général aux dépenses de la conductivité électrique. Une stratégie qui a été récemment introduite est de développer des matériaux hétérogènes à l'échelle nanométrique, dits « nanocomposites ». Malgré le potentiel dont ils ont déjà fait preuve, à ce jour la compréhension fondamentale de leurs propriétés reste encore limitée. Dans cette thèse nous présentons une étude fondamentale des propriétés de transport dans des composites basés sur une matrice amorphe contenant des inclusions cristallines de tailles nanométriques, afin d'acquérir une compréhension microscopique des mécanismes en jeu. Pour ce faire, nous avons effectué une étude expérimentale dans deux composites intermétalliques, un verre métallique et un verre chalcogénure, obtenus par cristallisation directe du verre. Nous avons pu mettre en évidence un comportement fortement dépendant du contraste de propriétés entre la matrice vitreuse et les inclusions cristallines. Nos conclusions ont trouvé confirmation dans des simulations numériques par dynamique moléculaire que nous avons effectué' sur des systèmes modèles, qui ont en effet permis de mettre en évidence l'effet d'un contraste de rigidité sur les propriétés vibrationnelles de ce type de composite<br>Face to the growing technological needs in the modern society, the need has arisen of developing novel multifunctional materials, able to simultaneously assure different functions.This is especially important in advanced technologies, such as microelectronics and energy harvesting, where heat dissipation reduction is essential, while keeping good electrical properties.Optimizing such materials represents however a challenging task: lowering thermal conductivity generally implies lowering the electrical conductivity as well. A new strategy has recently aroused consisting in exploiting heterogeneous materials at the nanoscale, so-called “nanocomposites”. Despite their great potential, the fundamental understanding of their properties is still lacking.In this thesis, we present a fundamental investigation of the transport properties in composites made of nano-inclusions embedded in an amorphous matrix, aimed to get a microscopic insight into the mechanisms ruling transport in such materials.To this purpose, we have carried on an experimental study in two intermetallic composites, based on a metallic glass and a chalcogenide glass, where crystalline inclusions were directly obtained from the glass temperature-induced recrystallization.We find that transport behavior is strongly dependent on the properties’ contrast between the amorphous matrix and the crystalline inclusions.Our findings are comforted by our theoretical results, obtained by molecular dynamics simulations on a model composite system, which highlight the effect of the rigidity contrast on the vibrational properties of such material, and thus on thermal transport

This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.<br>Thesis: S.M., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2019<br>Cataloged from student-submitted PDF version of thesis.<br>Includes bibliographical references (pages 87-91).<br>The thermal conductivities of electrochemically tuned strontium cobalt oxides (SCO) are significantly different among the perovskite SrCoO3 (P-SCO), the brownmillerite SrCoO2.5 (BM-SCO) and the hydrogenated HSrCoO2.5 (H-SCO)1. The underlying mechanism causing this large difference is still unclear. And phonon properties in SCO have not been investigated thoroughly or have some contradictive predictions. In this work, we have calculated the thermal conductivities in P-SCO and BM-SCO by applying molecular and lattice dynamics, and successfully reconstructed the large difference of the thermal conductivities, consistent with measurements. Furthermore, several phonon properties including heat capacities, group velocities, lifetimes and mean free paths have been calculated, and the key roles of local atomic environment and crystal symmetry in determining the thermal conductivities have been identified. We have also analyzed the impact of interfaces, isotropic strains and defects on thermal conductivities, predicted the neutron scattering intensity in P-SCO, and tested the accuracy and performance of molecular dynamics based on deep learning. Additionally, even though the calculations about the phonon properties in H-SCO are not complete, it still offers some inspirations about its thermal conductivity. The thorough investigations about the phonon properties and the mechanisms determining the thermal conductivities in SCO may benefit future research about tunable thermal conductivities in complex oxides.<br>by Hantao Zhang.<br>S.M.<br>S.M. Massachusetts Institute of Technology, Department of Nuclear Science and Engineering

This thesis is devoted to the study of the interactions of phonons in indium nitride (InN) and materials of the (In,Ga)N system with wurtzite structure. For this purpose, we present Raman spectroscopy on nanowires (NWs), thin films and bulk samples, in order to adress phonon interactions in these materials. We also present Brillouin spectrsocopy measurements of InN thin films, from which a reliable set of elastic constants is proposed. We have studied the phonon anharmonic interactions and phonon decay channels of InN, both in thin films and NWs. The temperature dependence of Raman peak width of all the phonon modes has been studied using a model that considers the contribution of three- and four-phonon processes, taking into account the phonon density of states obtained by ab-initio calculations. In InN thin films, we find that the E2h phonon mode mainly decays through 4-phonon processes, whereas the extremely narrow E2l mode can decay only through up-conversion processes. The LO and the TO modes are found to decay through 3-phonon and 4-phonon interactions. In InN NWs we found the same phonon decay channels but phonon linewidths are significantly reduced, indicating a higher crystalline quality. The lifetimes of the phonon modes are derived from the measured phonon linewidths. The long-lived E2l phonon exhibits the largest lifetime, which is mainly limited by impurity scattering. We also study the anharmonic decay of high-frequency LVMs H complexes in Mg-doped InN, which can be explained by considering dephasing due to quasi-elastic acoustic phonon scattering. We have discussed the relevant electronic resonances that affect Raman scattering in the (In,Ga)N system. We show that the optical excitation of the longitudinal optical modes in InN occurs via the Martin's double resonance both in InN layers and nanostructures, even though the defect density of the latter is significantly lower. By performing wavelength-dependent measurements on InN thin films and NWs, the A1(LO) and the E1(LO) wave-vector dispersion close to zone-center have been obtained. We have also studied the impurity-mediated cascade mechanism of multiphonons in InGaN layers. To ascertain the role of the impurities we have studied as-grown samples in comparison with He+-implanted InGaN layers. UV Raman-scattering measurements allow us to measure up to fifth order multiphonon scattering due to cascade mechanism. Relative multiphonon intensities depend on the indium concentration and implantation dose. Finally, we have studied the LO-Phonon-Plasmon Coupled Modes (LOPCMs) in InN and GaN using the Lindhard-Mermin model. We have determined the electron density in undoped, Si-doped and Mg-doped NWs. We have also studied a bulk, ammonothermally-grown Si-doped GaN sample. No evidence of LOPCMs was detected in the Ga-polar face, probably due to the higher defect density existing in this sample sector. We have detected both branches of the LOPCMs in the N-polar face, and we have made a study of the distribution of the free charge density by means of confocal micro-Raman measurements.<br>Aquesta tesi està dedicada a l’estudi de les interaccions dels fonons en nitrur d'indi (InN) i en semiconductors del sistema (In.Ga)N amb estructura wurtzita. Amb aquest objectiu es presenten estudis d'espectroscòpia Raman en capes primes, nanofils (NWs), i mostres bulk, que han permès abordar de manera global les interaccions dels fonons en aquests materials. Hem estudiat les interaccions anharmòniques i els canals de decaïment dels fonons de InN, tant en capes primes com en NWs. La dependència de l’amplada del pic Raman amb la temperatura de tots els modes fonònics s’ha estudiat utilitzant un model anharmònic que considera la contribució dels processos de tres i quatre fonons, i tenint en compte la densitat d'estats de fonons obtinguda mitjançant càlculs ab-initio. L'anàlisi dels temps de vida fonònics i de la dependència amb temperatura de les freqüències permet afirmar que els NWs tenen una estructura més relaxada que les capes primes. També hem estudiat el decaïment anharmònic de modes locals de vibració corresponents a complexos d'H en InN fortament dopat amb Mg. Hem estudiat les ressonàncies en el sistema (In,Ga)N i la influència de la densitat d’impureses en l’eficiència dels mecanismes ressonants. Hem demostrat que la dispersió Raman de modes òptics longitudinals en el InN es produeix a través de la doble ressonància del Martin tant en capes primer com en nanoestructures, tot i que la densitat de defectes d'aquestes últimes és significativament menor. Hem estudiat també el mecanisme de cascada mediat per impureses, a través del qual es produeix la dispersió de multifonons, en capes primes de InGaN amb diferent composició i diferent grau d’implantació d'ions d'He, i hem comprovat que les intensitats relatives dels multifonons depenen de la concentració d’indi i de la dosi de la implantació. Finalment, hem estudiat l’acoblament de fonons polars amb els plasmons mitjançant el model dielèctric de Lindhard-Mermin, amb la finalitat d’investigar la densitat d’electrons lliures utilitzant espectroscòpia Raman. Hem determinat la concentració d'electrons en NWs de InN sense dopar, dopats amb Si i dopats amb Mg. També hem fet un estudi de la distribució de la densitat de càrrega en una mostra de GaN ammonotermal mitjançant mesures de micro-Raman confocal.

Thesis (Ph.D.)--Mechanical Engineering, Georgia Institute of Technology, 2009.<br>Committee Chair: Graham, Samuel; Committee Co-Chair: Nair, Sankar; Committee Member: Grover - Gallivan, Martha; Committee Member: McDowell, David; Committee Member: Schelling, Patrick; Committee Member: Zhang, Zhuomin

The transformation of the vibrational spectrum of Cu2Zn(Sn1−xGex)S4 single crystals over the entire composition range (0 ≤ x ≤ 1) is studied experimentally by low-temperature Raman scattering and photoluminescence spectroscopies, as well as theoretically in the framework of density functional theory (DFT). It is shown that unlike “classic” mixed binary II–VI and III–V compounds, which are characterized by either one- or two-mode behavior of spectra transformation upon composition variation, the vibrational modes of the quaternary semiconductor Cu2Zn(Sn1−xGex)S4 exhibit both types of behavior within the same alloy system. DFT calculations reveal that the two-mode transformation is in fact observed for the vibrational modes, which possess a very small dispersion across the Brillouin zone, that is typical for a molecular crystal. These modes are due to the “breathing” motion of sulfur within GeS4 and SnS4 tetrahedra. The effects of structural (positional) disorder of mixed crystals are analyzed based on Raman scattering as well as photoluminescence results<br>Dieser Beitrag ist aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich

Negli ultimi anni, c’è stato un rapido sviluppo delle tecniche di crescita dei materiali nanostrutturati, e un forte impulso è stato dato dall’introduzione delle tecniche di crescita colloidale. Tali tecniche consentono di crescere un ampia gamma di materiali nanostrutturati, metalli e semiconduttori, con elevata cristallinità, dimensioni ridotte (< 5 nm) e con una distribuzione delle dimensioni molto stretta. Il solfuro di cadmio (CdS) nanostrutturato ha promettenti future applicazioni tecnologiche, come ad esempio nei dispositivi optoelettronici, celle solari ad alta efficienza e come tracciante fluorescente in biologia. Tuttavia, per poter sfruttare al meglio le proprietà fisiche a favore delle citate applicazioni è di fondamentale importanza una conoscenza approfondita delle proprietà fisiche. In questa tesi, sono state studiate le proprietà optovibrazionali e optoelettroniche dei quantum dots (QDs) di solfuro di cadmio cresciuti tramite un metodo di crescita colloidale. Tramite i metodi di crescita colloidale è possibile produrre QDs con dimensione ridotta e una distribuzione della dimensione molto stretta. La sintesi dei CdS-QDs consiste nella termolisi a circa 260 °C dello stearato di cadmio in presenza di solfuro di idrogeno in un solvente organico altobollente (1-ottadecene). La velocità della crescita e la dimensione finale dei QDs sono regolate dalla presenza di una molecola surfattante, l’ossido di triottilfosfina (TOPO). In particolare, QDs con una determinata dimensione e con una sua distribuzione molto stretta possono essere ottenuti regolando opportunamente la temperatura di crescita, la concentrazione dei precursori e principalmente la concentrazione del surfattante e del tempo di reazione (crescita arrestata). La morfologia, la dimensione (diametro) e la distribuzione dei diametri sono state determinate tramite TEM. Tramite spettroscopia di assorbimento, si ottengono informazioni sugli stati elettronici, inoltre, sfruttando la relazione esistente tra la band gap e il diametro, si può determinare il diametro medio di un campione di QDs. Le proprietà emissive dei QDs sono state studiate tramite spettroscopia di fotoluminescenza (PL) e dall’energia della banda di PL si può ottenere una stima del diametro medio dei QDs. Dalla larghezza di banda degli spettri di assorbimento e di PL si può ottenere anche una stima sulla distribuzione del diametro dei QDs. Un estesa parte del lavoro riguarda lo studio delle proprietà vibrazionali dei CdS-QDs, tramite spettroscopia Raman. Queste indagini sono state effettuate su campioni di CdS-QDs cresciuti appositamente con diversi diametri. Per eseguire misure micro-Raman, i campioni di CdS-QDs coordinati da molecole di TOPO che hanno una consistenza gelatinosa, sono stati trattati con acido tioglicolico (TGA). Questo trattamento è necessario per avere CdS-QDs in forma di polvere, la quale è più adatta per essere studiata tramite spettroscopia Raman. Per evitare effetti termici negli spettri o il danneggiamento del campione, le misure micro-Raman devono essere effettuate con potenze del laser molto basse. Negli spettri Raman di CdS-QDs si osserva uno spostamento del picco del fonone LO verso frequenze più basse , in particolare, tale spostamento è più marcato per i QDs più piccoli, mentre, al crescere del diametro, la frequenza si avvicina progressivamente a quella del bulk. Questa diminuzione di frequenza è causata dall’espansione del cristallo che avviene nei QDs, con il conseguente indebolimento dei legami per i quali diminuisce la frequenza di risonanza. Oltre a questo, il confinamento quantistico dei fononi è visibile come un allargamento asimmetrico della linea dei fononi, e come l’apparizione di un nuovo picco a circa 270 cm-1. Alcune pubblicazioni assegnano questo picco ai modi di superficie, mentre altri lo descivono come la conseguenza delle nuove regole di selezione dovute dalla bassa dimensionalità. Lo studio ha anche lo scopo di comparare le previsioni teoriche basate sia sul modello “dielectric continuum” che sui fononi di superficie con i risultati sperimentali. È stata trovata una relazione tra i valori di frequenza dei fononi predetti teoricamente e i risultati sperimentali, in particolare, le frequenze dei fononi di superficie sono in accordo con i risultati sperimentali. In conclusione, lo scopo di questo studio consiste nello sviluppo di un metodo per crescere CdS-QDs con le caratteristiche fisiche desiderate (diamtero voluto e distribuzione di diametro stretta) per poterne poi effettuare uno studio sistematico delle proprietà vibrazionali ed elettroniche.<br>In recent years, there has been a rapid development of the growth techniques of nanostructured materials, and a particular breakthrough was given by the introduction of colloidal growth techniques. These techniques allow to grow by affordable facilities, a wide range of nanostructured materials, metals and semiconductors, with high crystallinity, reduced size, narrow size distribution. Nanostructured cadmium sulfide (CdS) has promising future applications as in the realization of optoelectronic devices, high efficiency solar cells as well as fluorescent biological probe. However, in order to fully exploit the potential technological applications, the study of the physical properties of such materials is of crucial importance. In this thesis, the optoelectronic and optovibrational properties of cadmium sulfide quantum dots (QDs) grown by colloidal chemical method are studied. By the means of colloidal growth, it is possible to grow QDs with reduced size and narrow size distribution. The synthesis of CdS-QDs consists in the thermolysis (T=260 °C) of cadmium stearate in presence of hydrogen sulfide in a high temperature boiling point solvent (1-octadecene). The growth rate and final QDs size are regulated by the presence of the surfactating molecule trioctylphosphine oxide (TOPO). QDs with a determined size and a narrow size distribution can be obtained properly adjusting the growth parameters such as temperature, precursors concentrations, and principally the surfactant concentration and reaction time (arrested growth). The QDs morphology, their size and their size distribution is determined by TEM imaging. By absorption spectroscopy, information regarding the electronic states in QDs are obtained, and exploiting the relation existing between band gap and QD diameter, the mean diameter of the QDS is determined. The emissive properties of the QDs are probed by photoluminescence spectroscopy (PL). From the energy of PL band, an estimation of the QDs diameter can be obtained. Based on the width of absorbance and PL bands, the width of QDs size distributions can be estimated. A large part of the work is concerned with the study of vibrational properties of CdS-QDs by Raman spectroscopy. These investigations are carried out on the CdS-QDs samples purposely grown with different average sizes. In order to perform micro-Raman measurements, the gel-like TOPO-coated CdS-QDs are treated to replace the TOPO layer by thioglycolic acid (TGA). This treatment is necessary in order to have powder-like CdS-QDs being more suitable to a Raman scattering study. To avoid thermal effects or damage to the sample, the micro-Raman measurements must to be performed using very low laser powers (on the sample). In the Raman spectra of CdS-QDs, a decrease of the phonon frequency (red-shift) with respect to the bulk CdS frequency is observed. In particular, the red-shift is expected to be more pronounced for the smallest QDs, while at the increasing of QDs size, the phonon frequency will approach progressively to the bulk value. This red-shift is caused by the lattice expansion and by a subsequent weakening of the bonds which causes a reduction of the resonance frequency. Beyond the red-shift, the quantum confinement is visible also as an asymmetric broadening of the phonon line and by the apparition of a new peak a circa 270 cm-1. Some reports assign this peak to surface modes, while other reports describe this mode as a consequence of new selection rules arising from the reduced dimensionality. The study has also the aim to cross check the theoretical prediction based on the dielectric continuum model and on the surface modes with the experimental results. A relation between the theory and the experiment has been found, in particular, the predicted surface frequencies are in good agreement with the experiments. In conclusion, the goal of this thesis work is to develop a method to grow CdS-QDs with the desired physical characteristics (narrow size distribution) suitable for a systematic study of optical properties (vibrational and electronic).

The main focus of the thesis is to have better understanding of the atomic and electronic structures, vibrational dynamics and thermodynamics of metallic surfaces and bi-metallic nanoparticles (NPs) via a multi-scale simulational approach. The research presented here involves the study of the physical and chemical properties of metallic surfaces and NPs that are useful to determine their functionality in building novel materials. The study follows the  bottom-up approach for which the knowledge gathered at the scale of atoms and NPs serves as a base to build, at the macroscopic scale, materials with desired physical and chemical properties. We use a variety of theoretical and computational tools with different degrees of accuracy to study problems in different time and length scales. Interactions between the atoms are derived using both Density Functional Theory (DFT) and Embedded Atom Method (EAM), depending on the scale of the problem at hand. For some cases, both methods are used for the purpose of comparison. For revealing the local contributions to the vibrational dynamics and thermodynamics for the systems possessing site-specific environments, a local approach in real-space is used, namely Real Space Green s Function method (RSGF). For simulating diffusion of atoms/clusters and growth on metal surfaces, Molecular Statics (MS) and Molecular Dynamics (MD) methods are employed.<br>Ph.D.<br>Department of Physics<br>Sciences<br>Physics PhD

В даній роботі розглядались електро-фізичні властивості сполук групи А2В6 та їх дослідження за допомогою раманівської спектроскопії. При цитуванні документа, використовуйте посилання http://essuir.sumdu.edu.ua/handle/123456789/29353<br>В данной работе рассматривались электро-физические свойства соединений группы А2В6 и их исследования с помощью рамановского спектроскопии. При цитировании документа, используйте ссылку http://essuir.sumdu.edu.ua/handle/123456789/29353<br>In this work shown electro-physical properties of the compounds of group A2B6 and their research using Raman spectroscopy. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/29353

Thesis advisor: Zhifeng Ren<br>Solid-state cooling and power generation based on thermoelectric effects are attractive for a wide range of applications in power generation, waste heat recovery, air-conditioning, and refrigeration. There have been persistent efforts on improving the figure of merit (ZT) since the 1950's; only incremental gains were achieved in increasing ZT, with the (Bi1-xSbx)2(Se1-yTey)3 alloy family remaining the best commercial material with ZT ~ 1. To improve ZT to a higher value, we have been pursuing an approach based on random nanostructures and the idea that the thermal conductivity reduction that is responsible for ZT enhancement in superlattices structures can be realized in such nanostructures. The synthesis and characterization of various nanopowders prepared by wet chemical as well as high energy ball milling methods will be discussed in this dissertation. The solid dense samples from nanopowders were prepared by direct current induced hot press (DC hot press) technique. The thermoelectric properties of the hot pressed samples have been studied in detail. By ball milling ingots of bulk alloy crystals and hot pressing the nanopowders, we had demonstrated a high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this dissertation, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 can be achieved. Our material also exhibits a ZT of 0.7 at 250 °C, close to the value reached when ingot was used. This process is more economical and environmentally friendly than starting from bulk alloy crystals. The ZT improvement is caused mostly by the low thermal conductivity, similar to the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the high density grain boundaries and defects. The performance of thermoelectric materials is determined by its dimensionless figure-of-merit (ZT) which needs to be optimized within a specific temperature range for a desired device performance. Hence, we show that by varying the Bi/Sb ratio, the peak ZT can be shifted to a higher or lower temperature for power generation applications or a cooling mode operation. A peak ZT of about 1.3 is achieved from a Bi0.4Sb1.6Te3 composition which is highest among the different compositions. These nanostructured bulk samples have a significantly low lattice thermal conductivity compared to the bulk samples due to the increased phonon scattering in the grain boundaries and defects. This study shows that Bi0.5Sb1.5Te3 may potentially perform better for cooling devices, while Bi0.3Sb1.7Te3 should be able to show better power generation efficiency. Several issues related to accurate measurement of thermoelectric properties were identified and many of them were solved during my studies and these are discussed in this thesis. With the data we obtained, it is clear that nanopowder-based thermoelectric materials hold significant promise. Therefore, a review of synthesis of nanostructured materials by solution-based methods, including a hydrothermal process for the Bi2Te3, Bi2Se3, and Bi2Te2.25Se0.75 nanoparticles, a solvothermal route for Sb2Te3 nanostructures, and a polyol process for the preparation of Bi nanostructures is presented in this dissertation. These new nanostructures may find applications in enhancing the thermoelectric performance. Although small sized and well dispersed nanopowders of various thermoelectric materials could be prepared by a solution method in large scale, contamination and partial oxidation are always big challenges in a chemical approach. Hence, a high energy ball milling technique to prepare thermoelectric nanopowders in large scale and without major contamination is still found to be more efficient and preferred<br>Thesis (PhD) — Boston College, 2009<br>Submitted to: Boston College. Graduate School of Arts and Sciences<br>Discipline: Physics

Cet exposé est scindé en deux grandes parties. Dans la première, nous estimons des constantes photo-physiques du saphir dopé au titane à partir d'un modèle analytique simple exploitant une théorie de Huang-Rhys pour la détermination du profil spectral des bandes simples et une hypothèse réaliste de superposition de ces dernières. Nous déterminons une formule pour l'indice de réfraction total du Ti:saphir en fonction de la concentration de dopant. Dans une seconde partie, nous évaluons, selon la vérification d'un concept, la capacité de laser sana inversion de populations pour un cristal dopé possédant une basse symétrie. Nous appuyons notre démonstration en établissant une condition de seuil généralisée d'effet laser. Ce concept pourrait être une rupture technologique dans le domaine des grands cristaux dopés et n'a pas encore été investigué par la communauté<br>This presentation is split into two main parts. In the first, we estimate photo-physical constants of titanium doped sapphire from a simple analytical model using a Huang-Rhys theory for the determination of the spectral profile of simple bands and from a realistic hypothesis of superposition of the latter. We define a formula for the total refractive index of Ti:sapphire as a function of dopant concentration. In a second part, we evaluate, according to the verification of a concept, the laser capability without population inversion for a doped crystal with low symmetry. We support our demonstration by establishing a generalized laser threshold condition. This concept would be a technological breakthrough in the field of large doped crystals and has not yet been investigated by the community

Engineers know well from an early point in their training the trials and tribulations of having to make design tradeoffs in order to optimize one performance parameter for another. Discovering tradeoff conditions that result in the elimination of a loss associated with the enhancement of some other parameter (an improvement over a typical tradeoff), therefore, ushers in a new paradigm of design in which the constraints which are typical of the task at hand are alleviated. We call such a design paradigm “tuning” as opposed to “trading off”, and this is the central theme of this work. We investigate two types of microwave electronic devices, namely GaN-based heterostructure field effect transistors (HFETs) and tunable ferroelectric-ferrite-based microwave phase shifters. The “tuning” associated with these types of devices arises from the notion of an optimal 2DEG density, capable of achieving higher performance in terms of electron velocity and enhanced reliability in the case of the HFET, and the coupling of ferroelectric and ferrite materials in tunable microwave phase shifters, capable of achieving high differential phase shifts while at the same time mitigating the losses associated with impedance mismatching which typically arise when the phase is tuned. Promises and problems associated with HFET devices based on the intriguing InAlN/GaN material system will be described. We focus on the fundamental problem associated with the induction of the large density of carriers at the interface, namely the disintegration of an excess of longitudinal optical phonons (hot phonons) in the channel. We use microwave measurements in conjunction with stress tests to evidence the existence of an optimal 2DEG density wherein the hot phonon effect can be “tuned,” which allows for enhanced high frequency performance as well as device reliability. Next, we focus on the design, fabrication, and measurement of tunable phase shifters consisting of thin films of BaxSr1-xTiO3 (BST), which has the advantage of having high dielectric tunability as well as relatively low microwave loss. We discuss the design, fabrication, and measurement of a simple coplanar waveguide (CPW) type of phase shifter as well as a more complicated “hybrid” phase shifter consisting of a ferrite (YIG) in addition to BST. The use of such a bilayer allows one to “tune” the impedance of the phase shifters independently of the phase velocity through careful selection of the DC biasing magnetic fields, or alternatively through the use of an additional piezoelectric layer, bonded to YIG whose permeability can then be tuned through magnetostriction.

We employ the empirical potential function Airebo to computationally model free-standing Carbon-12 graphene in a classical setting. Our objective is to measure the mean square displacement (MSD) of atoms in the system for different average temperatures and Carbon-13 isotope concentrations. From results of the MSD we aim to develop a technique that employs Transmission Electron Microscopy (TEM), using high-angle annular dark filed (HAADF) detection, to obtain atomic-resolution images. From the thermally diffusive images, produced by the vibrations of atoms, we intent to resolve isotopes types in graphene. For this, we establish a relationship between the full width half maximum (FWHM) of real-space intensity images and MSD for temperature and isotope concentration changes. For the case of changes in the temperature of the system, simulation results show a linear relationship between the MSD as a function of increased temperature in the system, with a slope of 7.858×10-6 Å2/K. We also note a power dependency for the MSD in units of [Å2] with respect to the FWHM in units of [Å] given by FWHM(MSD)=0.20MSD0.53+0.67. For the case of increasing isotope concentration, no statistically significant changes to the MSD of 12C and 13C are noted for graphene systems with 2,000 atoms or more. We note that for the experimental replication of results, noticeable differences in the MSD for systems with approximately 320,000 atoms must be observable. For this, we conclude that isotopes in free-standing graphene cannot be distinguished using TEM.

Nous développons des métamatériaux élastiques à piliers pour manipuler les ondes de Lamb. Dans un premier temps, les propriétés négatives associées aux résonances de flexion, de compression et de torsion dans deux structures constituées de piliers sur un seul côté d’une membrane sont examinées. Nous décrivons deux mécanismes différents des propriétés de double négativité. Le potentiel de ces structures pour la réfraction négative et le cloaking acoustique est démontré. Deuxièmement, nous présentons le transport protégé topologiquement des ondes de Lamb par analogie avec les effets Hall quantiques de spin et de vallée. En réorganisant les structures précédentes en un réseau en nid d'abeille, un cône de Dirac simple et un cône de Dirac double sont introduits. Nous discutons de l’apparition d’états de bord protégés topologiquement par une vallée dans une structure à piliers double face asymétrique. La propagation unidirectionnelle des états de bord est étudiée. De plus, nous considérons un système double face symétrique. Les états de bord protégés topologiquement sur le pseudospin et sur le pseudospin-vallée sont démontrés. Troisièmement, nous proposons une approche pour contrôler activement la transmission de l’onde de Lamb antisymétrique se propageant à travers une ligne infinie de piliers. Deux situations différentes avec les résonances de flexion et de compression respectivement séparées ou superposées sont étudiées. Une force de traction externe et une pression sont appliquées sur les piliers, ce qui permet de les coupler avec les vibrations de flexion et de compression. La transmission est étudiée en fonction de l’amplitude et de la phase relative des sources externes<br>We develop elastic pillared metamaterials to manipulate Lamb waves. Firstly, the negative properties associated with bending, compression and torsion resonances in two structures consisting of pillars on one side of a thin plate are examined. We describe in details two different mechanisms at the origin of doubly negative property. The potential of these structures for negative refraction of Lamb waves and acoustic cloaking is demonstrated numerically. Secondly, we present the topologically protected transport of Lamb waves by analogy with quantum spin and valley quantum Hall effects. By rearranging the previous structures into a honeycomb network, a single Dirac cone and a double Dirac cone are introduced. We discuss the appearance of topologically valley-protected edge states in an asymmetrical double-sided pillar structure. The unidirectional propagation of edge states on different domain walls is studied. In addition, we consider a symmetrical double-sided system allowing the separation of the symmetric and antisymmetric modes. Combined edge states protected topologically by pseudospin and pseudospin-valley degree of freedom are demonstrated. Third, we propose an approach to actively control the transmission of the antisymmetric Lamb wave propagating through an infinite line of pillars. Two different situations with bending and compression resonances respectively separated or superimposed are studied. External tensile force and pressure are applied to the pillars, which allows them to couple with the bending and compressive vibrations. The transmission is studied as a function of the amplitude and the relative phase of the external sources

The first part of my PhD work is about the lattice vibrations in silicon nanowires. First-principles calculations based on the linear response are performed to investigate the quantum confinement effect in lattice vibrations of silicon nanowires (SiNW). The radial breathing modes (RBM) are found in our calculations, which have a different size-dependent frequency shift compared with the optical modes. They are well explained by the elastic model. Finally, the relative activity of the Raman scattering in the smallest SiNW is calculated. The RBM can be clearly identified in the Raman spectrum, which can be used to estimate the size of nanowires in experiment. In the second part of my PhD work, we focus on the electron-hole pair (exciton) in semiconductor nanowires and its influence on the optical absorption spectra. First-principles calculations are performed for a hydrogen-passivated silicon nanowire with a diameter of 1.2 nm. Using plane wave and pseudopotentials, the quasiparticle states are calculated within the so-called GW approximation, and the electron-hole interaction is evaluated with the Bethe-Salpeter Equation (BSE). The enhanced excitonic effect is found in the absorption spectrum. The third part of my work is about the electronic structure in Si/Ge core-shell nanowires. The electronic band structure is studied with first-principles methods. Individual conduction and valence bands are found in the core part and the shell part, respectively. The band offsets are determined, which give rise to the spatial separation of electron and hole charge carriers in different regions of the nanowires. This allows for a novel-doping scheme that supplies the carriers into a separate region in order to avoid the scattering problem. This is the key factor to create high-speed devices. With the confinement effect, our results show important correction in the band offset compared with the bulk heterostructure. Finally, an optimum doping strategy is proposed based on our band-offset data.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico<br>FundaÃÃo Cearense de Apoio ao Desenvolvimento Cientifico e TecnolÃgico<br>FundaÃÃo de Amparo à Pesquisa e ao Desenvolvimento CientÃfico e TecnolÃgico do MaranhÃo<br>CoordenaÃÃo de AperfeÃoamento de Pessoal de NÃvel Superior<br>Perovskitas duplas com fÃrmula na forma A2BBâO6 (A = La, Sr, Ca, B/Bâ = Co/Mn ou Fe/Mo) sÃo materiais multiferrÃicos de grande interesse na ciÃncia devido Ãs notÃveis propriedades, que permitem aplicaÃÃes em capacitores e em memÃrias, por exemplo. AlÃm disso, mudanÃas estruturais sÃo observadas entre a forma policristalina e o filme fino. Essas propriedades podem mudar de acordo com a rota utilizada para sintetizaÃÃo do material. No nosso trabalho estudamos filmes de La2CoMnO6 sintetizados pelo mÃtodo de Pechini Modificado (MPM), com trÃs deposiÃÃes e tratados em diferentes temperaturas: 700, 800 e 900 ÂC. Os filmes foram caracterizados por difraÃÃo de raios X, Microscopia EletrÃnica de Varredura (MEV), Microscopia de ForÃa AtÃmica e espectroscopia Raman à temperatura ambiente e tambÃm variando a temperatura. As tÃcnicas de MEV e AFM foram bastante eficientes na determinaÃÃo do diÃmetro mÃdio dos filmes, fornecendo resultados correspondentes. A tÃcnica de EDS trouxe dados qualitativos da composiÃÃo do material. A difraÃÃo de Raios X confirmou a fase do filme, alÃm de ter identificado a composiÃÃo do substrato: silÃcio e platina. A espectroscopia Raman em diversos pontos possibilitou identificar a regiÃo de melhor formaÃÃo do LCMO. O estudo da mudanÃa do fÃnon mais intenso com a temperatura mostrou diferentes temperaturas para a transiÃÃo de fase paramagnÃtica-ferromagnÃtica, possibilitando confirmar que a temperatura de calcinaÃÃo dos filmes determinou elevada desordem estrutural para os filmes de LCMO tratados a 700 e 900 ÂC. Discutimos nossos resultados com outros estudos na literatura, que mostram as medidas magnÃticas como um fator determinante para comprovar a temperatura do acoplamento spin-fÃnon.

The structural, electronic, and superconducting properties of one dimensional materials are calculated from first principles, using the density functional theory. Nanotubes and nanowires are important building blocks in nanotechnology, in particular for nanoelectronics. In this manuscript, the growth of carbon nanotubes is studied through the interaction between carbon and the transition metal atoms used as growth catalysts. The accepted model for a new phase of nanotube-like molybdenum disulfide is critically examined using comparisons of energetic stability and types of chemical bonding in different candidate structures which have similar compositions. The epitaxial growth of diamond carbon on (100) iridium is exceptionally favorable. The differences between various substrates used experimentally are studied, and the specificity of Ir is shown. Finally, the characteristics of the electron-phonon interaction in aluminium nanowires are determined. The structural instabilities and the differences in the electron-phonon coupling are calculated for straight monoatomic wires, zigzag wires, and thicker straight wires. The constrained geometry of the wires generates a coupling which can be very strong or almost vanish, depending on the structural details, but which is concentrated in the longitudinal high-frequency phonons.

Thesis (Ph.D.)--Chemical Engineering, Georgia Institute of Technology, 2009.<br>Committee Chair: Prof. Sankar Nair; Committee Co-Chair: Prof. Samuel Graham; Committee Member: Prof. Amyn S. Teja; Committee Member: Prof. Mo Li; Committee Member: Prof. Peter Ludovice.

Les générateurs thermoélectriques convertissent directement l'énergie thermique en énergie électrique. Ils pourraient devenir de plus en plus utiles à des fins de récupération d'énergie et font l'objet de recherches actives. Cependant, les meilleurs matériaux thermoélectriques sont rares et polluants.Le Silicium et le Germanium seraient des matériaux intéressants si leur efficacité thermoélectrique était améliorée. Pour ce faire, la nanostructuration est une voie possible, par exemple en introduisant des faces rugueuses ou de nouvelles interfaces semi-transparentes.Récemment, des nanofils polyphasés (composés d'une alternance de phases cubiques et hexagonales de Si et Ge) ont été fabriqués, mais la caractérisation expérimentale de nanostructures aussi complexes comprenant des matériaux exotiques peut être difficile. Dans cette thèse, nous étudions en détail le transport thermique dans des nanostructures avec des simulations numériques. Une méthode Monte Carlo originale a été développée, avec une description "full band" des matériaux. Elle inclut des modèles pour les faces rugueuses et les interfaces entre matériaux. Des simulations de Dynamique Moléculaire sont également effectuées pour caractériser les propriétés des interfaces.Nous confirmons que les phases hexagonales de Si et Ge ont une conductivité thermique inférieure à celle des phases cubiques correspondantes. Le modèle "full band" montre que le flux thermique est fortement anisotrope. Des modèles semi-analytiques habituels n'ont pas pu reproduire la conductivité thermique des nanostructures simulées avec des faces rugueuses.De plus, ces faces ont tendance à concentrer le flux de chaleur dans la direction principale de la nanostructure. Enfin, certaines interfaces polyphasées peuvent avoir une conductance thermique presque aussi faible que les interfaces Si/Ge, et pourrait ainsi améliorer significativement l'efficacité thermoélectrique des nanofils polyphasés. La méthode Monte Carlo présentée peut facilement être utilisée pour étudier une large gamme de matériaux, et elle est capable de modéliser des nanostructures arbitrairement complexes. A l'avenir, les simulations en Dynamique Moléculaire seront utilisées pour paramétrer un modèle plus réaliste d'interfaces<br>Thermoelectric generators are able to directly convert heat into electrical energy. They could have a great potential in terms of energy harvesting, but unfortunately, the best thermoelectric materials are rare and pollutant.Silicon and Germanium would be attractive materials if their thermoelectric efficiency were improved. For this purpose, nanostructuring is a possible route, for instance via the introduction of rough boundaries or interfaces between materials.Recently, polytype nanowires (composed of a sequence of cubic and hexagonal phases of Si and Ge) have been fabricated, but the experimental characterization of such complex nanostructures with exotic materials is challenging.In this thesis, we study the details of thermal transport in nanostructures with numerical simulations. An original Monte Carlo method is developed, with a full band emph{ab initio} description of materials. It includes models for the rough boundaries and the solid-solid interfaces. Molecular Dynamics simulations are also performed to characterize the properties of interfaces.We confirm that the hexagonal phases of Si and Ge have lower thermal conductivity than their cubic counterparts. The full band model shows a strong anisotropy in the heat flux.Usual semi-analytical models failed to reproduce the thermal conductivity of simulated nanostructures with rough boundaries. Besides, those boundaries tend to focus the heat flux in the main direction of the nanostructure. Finally, some polytype interfaces can have an interfacial conductance almost as low as Si/Ge interfaces, and thus could improve significantly the thermoelectric efficiency of polytype nanowires. The presented Monte Carlo method could easily be used with a wide range of materials,and it can model arbitrarily complex nanostructures. In the future, the results from Molecular Dynamics simulation will be used to parametrize a more realistic model of solid-solid interfaces