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Recently developed two-dimensional colloidal semiconductor nanocrystals, or nanoplatelets (NPLs), extend the palette of solution-processable free-standing 2D nanomaterials of high performance. Growing CdSe and CdS parts subsequently in either side-by-side or stacked manner results in core–crown or core/shell structures, respectively. Both kinds of heterogeneous NPLs find efficient applications and represent interesting materials to study the electronic and lattice excitations and interaction between them under strong one-directional confinement. Here, we investigated by Raman and infrared spectroscopy the phonon spectra and electron–phonon coupling in CdSe/CdS core/shell and core–crown NPLs. A number of distinct spectral features of the two NPL morphologies are observed, which are further modified by tuning the laser excitation energy Eexc between in- and off-resonant conditions. The general difference is the larger number of phonon modes in core/shell NPLs and their spectral shifts with increasing shell thickness, as well as with Eexc. This behaviour is explained by strong mutual influence of the core and shell and formation of combined phonon modes. In the core–crown structure, the CdSe and CdS modes preserve more independent behaviour with only interface modes forming the phonon overtones with phonons of the core. / Dieser Beitrag ist aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

A series of experimental results about unusual heating of carbon nanotubes by microwaves is analyzed in this dissertation. Two of vibration types, cantilever type (one end is fixed and the other one end is free), the second type is both ends are fixed, have been studied by other people. A third type of forced vibration of carbon nanotubes under an alternating electromagnetic field is examined in this paper. Heating of carbon nanotubes (CNTs) by microwaves is described in terms of nonlinear dynamics of a vibrating nanotube. Results from the model provide a way to understand several observations that have been made. It is shown that transverse vibrations of CNTs during microwave irradiation can be attributed to transverse parametric resonance, as occurs in the analysis of Melde's experiment on forced longitudinal vibrations of a stretched elastic string. For many kinds of carbon nanotubes (SWNT, DWNT, MWNT, ropes and strands) the resonant parameters are found to be located in an unstable region of the parameter space of Mathieu's equation. Third order wave equations are used to qualitatively describe the effects of phonon-phonon interactions and energy transfer from microwaves to CNTs. This result provides another way to input energy from microwaves to carbon nanotubes besides the usual Joule heating via electron-phonon interaction. This model appears to be the first to point out the role of nonlinear dynamics in the heating of CNTs by microwaves.

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. 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.

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Thermal transport across three-dimensional Lennard-Jones superlattices, two-dimensional heterostructures of graphene and hexagonal boron nitride (hBN), and in C60 molecular crystals is studied atomistically. The first two systems are studied as finite junctions placed between bulk leads, while the molecular crystal is studied as a bulk. Two computational methods are used: molecular dynamics (MD) simulations and harmonic lattice dynamics calculations in conjunction with the scattering boundary method (SBM). In Lennard-Jones superlattice junctions with a superlattice period of four atomic monolayers at low temperatures, those with mass-mismatched leads have a greater thermal conductance than those with mass-matched leads. We attribute this lead effect to interference between and the ballistic transport of emergent junction vibrational modes. The lead effect diminishes when the temperature is increased, when the superlattice period is increased, and when interfacial disorder is introduced, and is reversed in the harmonic limit. In graphene-hBN heterostructure junctions, the thermal conductance is dominated by acoustic phonon modes near the Brillouin zone center that have high group velocity, population, and transmission coefficient. Out-of-plane modes make their most significant contributions at low frequencies, whereas in-plane modes contribute across the frequency spectrum. Finite-length superlattice junctions between graphene and hBN leads have a lower thermal conductance than comparable junctions between two graphene leads due to lack of transmission in the hBN phonon band gap. The thermal conductances of bilayer systems differ by less that 10% from their single-layer counterparts on a per area basis, in contrast to the strong thermal conductivity reduction when moving to from single- to multi-layer graphene. We model C60 molecules using the polymer consistent force-field and compute the single molecule vibrational spectrum and heat capacity. In the face-center cubic C60 molecular crystal at a temperature of 300 K, we find three frequency peaks in the center-of-mass translations at 20, 30 and 38 cm􀀀1, agreeing with the average frequencies of the three acoustic branches of the frozen phonon model of the same system and suggesting that a phonon description of center-of-mass translations. We use both direct method and Green- Kubo MD simulations to predict the thermal conductivity of the molecular crystals at a temperature of 300 K. We find that the thermal conductivity of the molecular crystal is 20 to 50% lower than that of a reduced order model where only molecular center-ofmass translations are present, suggesting that molecular vibrations and rotations act as significant scattering sources for the center-of-mass phonons.

Dans cette thèse, on étudie l’interaction optomécanique dans les cavités des cristaux phoxoniques définis comme des cristaux à la fois photonique et phononique. Ces structures périodiques peuvent présenter simultanément des bandes interdites absolues pour les ondes électromagnétiques et pour les ondes acoustiques. L’introduction de défauts tels qu’une cavité dans le cristal permet d’obtenir à la fois des photons et des phonons localisés. Ce confinement simultané des deux excitations à l’intérieur d’une même cavité permet d’exalter leur interaction et d’envisager de nouveaux dispositifs acousto-optiques à l’échelle submicronique. Nous avons étudié théoriquement cette interaction optomécanique dans différentes structures de cristaux phoxoniques (2D, plaques et nanoguide structuré). Nous avons mis en évidence l’effet du changement du matériau et de la longueur d’onde incidente sur le couplage optomécanique. Les résultats pour le nanoguide structuré ont été comparés à des résultats expérimentaux réalisés par nos partenaires. Enfin, le couplage phonon-plasmon est abordé à la fois en terme de premiers résultats et de perspective. / In this thesis, we study optomechanic interactions in phoxonic crystals which are defined as dual phononic/photonic crystals that can exhibit simultaneously phononic and photonic band gaps. The existence of absolute band gaps allows the simultaneous confinement of both waves that, in turn, can produce the enhancement of their interaction for the purpose of novel and high-performance optomechanical and acousto-optic devices and applications. A main objective is the modulation of light by acoustic waves when both excitations are confined inside the same cavity or propagate with a slow group velocity inside a waveguide. We have studied theoretically the optomechanic interactions in different (2D, slabs and strip) phoxonic crystals cavities. We have demonstrated the dependence of these optomechanic interactions as a function of both the nature of the material and the incoming optical wavelength. The results for strip waveguides have been compared with experimental results performed by our partners. Finally, as a perspective, we began to study the phonon-plasmon coupling.

Les applications émergentes des nanocristaux de CdSe nécessitent une compréhension approfondie des propriétés d’émission et de relaxation des sous-niveaux de structure fine de l’exciton de bord de bande. Cette thèse porte sur l’étude spectroscopique de nanocristaux individuels de CdSe présentant une photostabilité remarquable aux températures cryogéniques. La distribution spectrale de leur photoluminescence en fonction de la température et d’un champ magnétique appliqué fournit une signature précise des niveaux de plus basse énergie, révélatrice de leur morphologie et leur structure cristalline. Une méthode d’excitation de la luminescence de haute résolution spectrale a été développée pour sonder la totalité des niveaux de structure fine. Les raies de recombinaison des huit états ont ainsi été résolues pour la première fois dans une situation de levée totale de dégénérescence produite par l’anisotropie des nanocristaux et l’application d’un champ magnétique. L’excitation sélective des nanocristaux dans les niveaux supérieurs de la structure fine permet aussi d’étudier les mécanismes de relaxation de spin entre les branches excitoniques à trou lourd et à trou léger. Des canaux de relaxation sélectifs peuvent notamment être mis à profit pour préparer un nanocristal dans un niveau quantique unique.Des nanocristaux à double coque ont été conçus pour être efficacement photo-chargés, produisant une émission stable à partir de l’exciton chargé (trion) à la température de l’hélium liquide. La recombinaison du trion est purement radiative, avec une signature spectrale caractérisée par une raie d’émission sans phonon unique et intense. Sous champ magnétique, son éclatement en quatre composantes Zeeman livre les facteurs de Landé de l’électron et du trou. L’analyse des poids de ces composantes permet aussi de trouver le taux de relaxation de spin du trion et le signe de sa charge. Une inhibition remarquable de la relaxation de spin se produit lorsque l’éclatement Zeeman est inférieur à l’énergie du premier mode de phonons acoustiques du nanocristal. / The development of emerging applications of CdSe nanocrystals requires a detailed understanding of the band-edge exciton fine structure and relaxation pathways. This thesis is focused on cryogenic spectroscopy of single nanocrystal with a remarkable photostability. Photoluminescence spectra as a function of temperature and under external magnetic fields provide a spectral fingerprint of the low energy sub-levels, revealing the morphology and the crystal structure of individual nanocrystals. In order to probe the entire band-edge exciton fine structure, a high resolution luminescence excitation technique has been developed. Zeeman and anisotropy-induced splittings are used to reveal the entire 8-state band-edge fine structure, enabling complete comparison with band-edge exciton models. State selective excitation allows the preparation of single quantum states. It is also used to map the hole spin relaxation pathways between the fine structure sub-levels.Charged quantum dots provide an important platform for a range of emerging quantum technologies. Double shell CdSe nanocrystals are engineered to efficiently ionize at cryogenic temperatures, resulting in trion emission with a single sharp zero-phonon line and a near-unity quantum yield. Zeeman splitting of this line enables direct determination of electron and hole g-factors. Spin relaxation is observed in high fields, enabling identification of the trion charge. Importantly, we show that spin flips are completely inhibited for Zeeman splittings below the low-energy bound for confined acoustic phonons. This charac- teristic unique to colloidal quantum dots has potential applications in single spin coherent manipulation.