Prenons en exemple une salle de classe composée d’un professeur et de nombreux élèves, lorsque trop d’élèves s’adressent en même temps au professeur ce dernier n’est plus en mesure de comprendre les paroles transmisses par les élèves. Cette illustration s’étend évidemment aux systèmes de communications sans fil (la 4G par exemple). Dans ces systèmes, les terminaux (les élèves par analogie) transmettent sur un canal, nommé canal en accès aléatoire, des messages qui sont potentiellement réceptionnés par la station de base (le professeur par analogie). Ces canaux ne sont habituellement pas surchargés car leur capacité (nombre de messages reçus par seconde) est tellement importante qu’il est très complexe de surcharger le canal. L’émergence de l’Internet des objets où des milliards de petits objets devraient être déployés partout dans le monde a changé la donne. Étant donné leur nombre et leur type de trafic, ces derniers peuvent surcharger les canaux en accès aléatoire. Ainsi, le sujet : « contrôle de charge des canaux en accès aléatoire » a connu un gain d’intérêts ces dernières années. Dans cette thèse nous avons développé des algorithmes de contrôle de charge permettant d’éviter qu’une station de base soit surchargée. Cela est très utile pour les opérateurs Télécoms, ils sont désormais certains qu’il y n’y aura pas de perte de service à cause de ces surcharges. Tous les principes développés dans cette thèse seront intégrés dans un futur proche aux produits IoT d’Airbus.

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Les excitations d'´echange de charge dans les noyaux constituent l'un des sujets importants et actuels en physique nucl´eaire et en astrophysique. En principe, une connaissance syst´ematique de l'´evolution du comportement de ces excitations `a travers la table des ´el´ements fournirait des informations directes sur les propri´et´es en spin et isospin de l'interaction entre nucl´eons dans le milieu nucl´eaire, et sur l'´equation d'´etat de la mati`ere nucl´eaire. Par ailleurs, une quantit´e d'importance essentielle pour la structure des noyaux, l'´epaisseur de la peau de neutrons, peut ˆetre d´etermin´ee par la r`egle de somme de la r´esonance spin-dipolaire (RSD) ou par la s´eparation en ´energie entre l'´etat isobarique analogue (EIA) et la r´esonance de Gamow-Teller (RGT). Plus g´en´eralement, les excitations d'´echange de charge permettent d'aborder des probl`emes d'int´erˆet g´en´eral tels que l'´etude de l'´evolution des ´etoiles `a neutrons et des supernovae, la d´ecroissance β des noyaux le long du processus r dans la nucl´eosynth`ese stellaire, ou les interactions neutrino-noyau. Elles jouent aussi un rˆole essentiel pour extraire la valeur de l'´el´ement Vud de la matrice de Cabibbo-Kobayashi- Maskawa par le biais de la r´eaction de d´ecroissance β super-permise 0+ → 0+ dans les noyaux. Pour toutes ces raisons, il est important de d´evelopper des th´eories microscopiques des excitations d'´echange de charge, et ceci constitue la principale motivation de notre recherche. Dans ce travail, nous ´etablissons le formalisme et les m´ethodes num´eriques pour d´ecrire les excitations d'´echange de charge dans le cadre de la Random Phase Approximation (RPA) selfconsistante construite sur l'approximation de Hartree-Fock relativiste (RHF). Un test important de pr´ecision num´erique est r´ealis´e sur l'´etat isobarique analogue. La m´ethode est ensuite utilis´ee pour mener des applications num´eriques r´ealistes sur un certain nombre de questions physiques: les r´esonances de spin-isospin dans les noyaux proches des noyaux magiques, les corrections dˆues aux m´elanges d'isospin dans les transitions β super-permises, les interactions neutrino-noyau dans les voies de courants charg´es. Pour les deux modes importants de spin-isospin que sont la RGT et la RSD nous trouvons qu'un excellent accord avec l'exp´erience est obtenu sans aucun r´eajustement des param`etres du mod`ele. De plus, les termes d'´echange de l'interaction induite par les m´esons isoscalaires jouent un rˆole essentiel dans les excitations de spin-isospin, `a la diff´erence de la RPA construite sur l'approximation de Hartree relativiste. En ce qui concerne notre ´etude des transitions β 0+ → 0+ super-permises l'une des conclusions est que les corrections δc dˆues aux violations de la sym´etrie d'isospin d´ependent sensiblement du champ moyen d'´echange produit par les interactions coulombiennes, mais ne changent pas sensiblement avec le mod`ele de Lagrangien utilis´e. Nous utilisons ces valeurs de δc pour d´eduire des plus r´ecentes valeurs exp´erimentales de ft dans les noyaux T = 1, et en tenant compte des corrections radiatives, les valeurs de Ft ”ind´ependantes de noyaux”. Nous obtenons ainsi des valeurs de l'´el´ement de matrice |Vud| de Cabbibo-Kobayashi-Maskawa en bon accord avec les valeurs d´eduites des d´ecroissances neutronique et pionique, et les transitions dans les noyaux miroirs, tandis que la somme des carr´es des ´el´ements de la premi`ere ligne d´evie l´eg`erement de la condition d'unitarit´e. Nous avons ´egalement utilis´e nos fonctions d'onde RPA pour ´evaluer les amplitudes de transition correspondant `a l'interaction faible lepton-hadron sous la forme standard courant-courant. Ainsi, les processus faibles semi-leptoniques tels que les r´eactions neutrino-noyau, capture leptonique charg´ee, d´esint´egration β, peuvent ˆetre ´etudi´es. Nos premi`eres applications concernent la r´eaction 16O(νe, e−)16F pour laquelle nous comparons nos pr´edictions avec celles d'autres auteurs. Dans la discussion des r´esultats nous nous effor¸cons en particulier de clarifier l'influence appr´eciable des diff´erentes prescriptions que l'on peut adopter pour le choix de la constante de couplage vecteur axiale et l'inclusion ou non des ´etats excit´es de basse ´energie dans le noyau final.

The parasitic components of MOS switches at high speeds affect the linearity and resolution of CMOS sample and hold circuits. CCD-based circuit design can offer good performance at high speeds. This thesis presents the design of sample and hold circuits using charge-coupled device structures in a standard CMOS process. Three sample and hold circuits have been built and tested for linearity and speed performance. The CCD S/Hs have been characterized using a continuous-time integrator and a Δ∑ ADC. The CCDs, with a switched capacitor amplifier at the output, achieve an SFDR of 54dB for an input signal V[subscript in]=2.6V+0.4Vpp at f[subscript in]=10.1KHz. / Graduation date: 2003

Wet thermal oxidations of polycrystalline Si₀.₅₄Ge₀.₄₆ films at 600°C for 30 and 50 min were carried out. A stable mixed oxide was obtained for films that were oxidized for 50 min. For film oxidized for 30 min, however, a mixed oxide with Ge nanocrystallites embedded in the oxide matrix was obtained. A trilayer gate stack structure that consisted of tunnel oxide/oxidized polycrystalline Si₀.₅₄Ge₀.₄₆/rf sputtered SiO₂ layers was fabricated. We found that with a 30 min oxidized middle layer, annealing the structure in N₂ ambient results in the formation of germanium nanocrystals and the annealed structure exhibits memory effect. For a trilayer structure with middle layer oxidized for 50 min, annealing in N₂ showed no nanocrystal formation and also no memory effect. Annealing the structures with 30 or 50 min oxidized middle layer in forming gas ambient resulted in nanocrystals embedded in the oxide matrix but no memory effect. This suggests that the charge storage mechanism for the trilayer structure is closely related to the interfacial traps of the nanocrystals. / Singapore-MIT Alliance (SMA)

The research field of organic electronics experiences tremendous developments since the discovery of conducting polymers upon chemical doping and the developments of applications where organic materials replace the traditionally used inorganic semiconductors. Devices such as light-emitting diodes (OLEDs), solar cells, and field-effect transistors (OFETs) based on organic ð-conjugated materials as active materials represent the key applications of the domain. In OLEDs, charge carriers (holes and electrons) are injected from the electrodes into the organic semiconductor and emit light when they meet. Solar cells have an opposite working principle compared to OLEDs: light is absorbed and dissociated in charge carriers that migrate to the electrodes to give rise to an electric current. OFET plays the role of current modulator in electronic circuits by tuning the current flowing in its channel. The gain of better device performances (better conversion efficiency for OLEDs and solar cells or high ON/OFF ratios for OFETs) requires a better understanding at a molecular scale of the charge transport properties that are quantified at the experimental level by the charge carrier mobility ì. Since organic conjugated materials are typically disordered, the charge carriers are mostly localized over a single molecule and charge transfers between molecules occur via a hopping mechanism. In our Ph.D. thesis, we have characterized the charge transport properties at the molecular scale by calculating the parameters entering into the Marcus hopping rate by means of semi-empirical Hartree-Fock methods and Density Functional Theory (DFT) calculations. On that basis, we have propagated a single charge carrier in molecular assemblies by means of a Dynamic Monte-Carlo procedure that we have developed in order to estimate mobility values as the ratio of the total distance travelled by the charge divided by the product of the total time needed to travel that distance and the norm of the electric field. The systems under study were model one-dimensional array of pentacene molecules, single molecular crystals and structures simulated by Molecular Dynamics (liquid crystalline phthalocyanine derivatives) and by Molecular Mechanics (grain boundaries in pentacene layers). The principle results shows anisotropic behaviour and electric field dependence for the charge carrier mobility, the impact of energetic as well as the positional disorder on the charge migration were investigated and we emphasize the importance to describe both disorders at a molecular scale in order to get a reliable picture for the charge transport properties calculations.

In large molecular systems such as DNA, supramolecular complexes and dendrimers, functional groups located at different parts of the molecular structure can act as charge donors or acceptors, and photoinduced intramolecular charge transfer can occur. An analogous scenario can be found in colloidal semiconductor nanocrystals, most evident in type-II heterostructures, where the relative band-alignment of the constituent materials are in a stagger configuration. Such a configuration, provides an energetically favourable situation for an photo-generated electron to be transferred from one material to the other, confining the electron and the hole in different domains of the nanostructure. A less obvious scenario in nanocrystals is when the core is thought of as the donor group, and the surface as the acceptor group. In such a scenario, the localization of electron or hole at surface defect sites, a process that occurs in every nanocrystal, can be thought of as an ``intramolecular" charge transfer. The studies presented in this dissertation are an attempt to further understand charge transfer processes in semiconductor nanostructures, in particular, those occurring within the same nanocrystals. This is carried out by a combination of spectroscopic techniques and modelling. First, time-resolved fluorescence measurements are used to investigated surface trapping/de-trapping dynamics in CdSe and CdSe/CdS/ZnS core/shell/shell quantum dots. A kinetic model, in which trapping/de-trapping is described with Marcus' classical electron transfer theory, is used to analyzed our results, yielding excellent agreement between model and experiment. Second, the influence of temperature and solvent environment in the optical spectra of CdSe/CdTe nanorods are examined. Solvatochromic shifts in these heterostructures are found to be larger than those observed in core-only quantum dots. Finally, ultrafast dynamics and biexciton states in CdSe/CdTe quantum dots are probed using two-dimensional optical spectroscopy. The fine structure of the lowest exciton and biexciton states are calculated for a model system with type-II band-alignment and simulations of 2D spectra are performed.

In large molecular systems such as DNA, supramolecular complexes and dendrimers, functional groups located at different parts of the molecular structure can act as charge donors or acceptors, and photoinduced intramolecular charge transfer can occur. An analogous scenario can be found in colloidal semiconductor nanocrystals, most evident in type-II heterostructures, where the relative band-alignment of the constituent materials are in a stagger configuration. Such a configuration, provides an energetically favourable situation for an photo-generated electron to be transferred from one material to the other, confining the electron and the hole in different domains of the nanostructure. A less obvious scenario in nanocrystals is when the core is thought of as the donor group, and the surface as the acceptor group. In such a scenario, the localization of electron or hole at surface defect sites, a process that occurs in every nanocrystal, can be thought of as an ``intramolecular" charge transfer. The studies presented in this dissertation are an attempt to further understand charge transfer processes in semiconductor nanostructures, in particular, those occurring within the same nanocrystals. This is carried out by a combination of spectroscopic techniques and modelling. First, time-resolved fluorescence measurements are used to investigated surface trapping/de-trapping dynamics in CdSe and CdSe/CdS/ZnS core/shell/shell quantum dots. A kinetic model, in which trapping/de-trapping is described with Marcus' classical electron transfer theory, is used to analyzed our results, yielding excellent agreement between model and experiment. Second, the influence of temperature and solvent environment in the optical spectra of CdSe/CdTe nanorods are examined. Solvatochromic shifts in these heterostructures are found to be larger than those observed in core-only quantum dots. Finally, ultrafast dynamics and biexciton states in CdSe/CdTe quantum dots are probed using two-dimensional optical spectroscopy. The fine structure of the lowest exciton and biexciton states are calculated for a model system with type-II band-alignment and simulations of 2D spectra are performed.

Copper is an essential trace element present in all living systems and is important for the function of many cellular enzymes. It ranks third in intracellular abundance behind only zinc and iron and plays a very important role as a catalytic cofactor in various cellular processes such as mitochondrial respiration, iron uptake, and the redox processes of a number of enzymes, including superoxide dismutase, lysyl oxidase, or tyrosinase. Any abnormality in copper trafficking pathways can lead to serious diseases such as Wilsons disease, Menkes syndrome and has been implicated in the neurodegenerative diseases amyotropic lateral sclerosis (ALS) and Alzheimers disease. While free copper in the cytoplasm would prove toxic, there is compelling evidence for the existence of a labile pool of copper that remains kinetically accessible. In order to investigate the existence of such a pool the development of Cu(I) selective probes is necessary. Chapter I provides the background for the role of copper in biology and elucidates the main trafficking pathways discovered to date. This chapter also provides recent developments of fluorescent sensors for selective visualization of biologically relevant metals. Chapter II discusses the exploration of a phenanthroline-based ligand for the selective detection of Cu(I). A series of derivatives incorporating chelating substituents in the 2- and 9-positions to enforce a 1:1 binding stoichiometry were synthesized and the properties of their respective Cu(I) complexes were characterized by x-ray structural analysis, and their photophysical properties were investigated by absorption and emission spectroscopy. Visible light excitation yielded metal-to-ligand charge-transfer (MLCT) excited states with luminescence lifetimes up to 155 ns. Electrochemical measurements further indicate that coordinative rearrangements are involved in nonradiative deactivation of the excited states. According to time-dependent density functional theory calculations (B3LYP/6-31G**), the major MLCT transitions are polarized along the C2 axis of the complex and originate predominantly from the dxz orbital. In chapter III, the development of a ratiometric Cu(I) sensor based on a donor-acceptor functionalized biphenyl fluorophore platform is discussed. The fluorescence emission energy for such fluorophores is highly dependent upon the interannular twist angle and this property was harnessed to provide a ratiometric sensor selective for Cu(I). Coordination of Cu(I) leads to a flattening of the biphenyl backbone and was confirmed by absorbance and emission spectroscopy as well as 2D NOESY experiments. The peak emission energy was shifted by 39 nm towards higher energy upon metal cation binding with a concomitant 7 bathochromic shift in absorption energy. The photophysical data accompanied by 1H NMR analysis confirms a well-defined 1:1 binding stoichiometry between metal and ligand. The findings from this study showed ratiometric behavior for this probe, albeit with a lowered quantum yield. While the quantum yield for the fluorophore discussed in chapter III was low (8.0%), the focus of chapter IV was the elucidation of the fluorescence quenching mechanism. To investigate the possibility of a twisted intramolecular charge transfer (TICT) state a donor-acceptor biphenyl fluorophore was synthesized incorporating a conformationally restricted amine donor group incapable of rotating out of plane in the excited state. Analysis of this derivative, as well as the sensor discussed in chapter III, reveals that fluorescence quenching is most likely due to hydrogen bonding to the acceptor subunit in they excited state. Finally, in chapter V, a pyrazoline fluorophore library with varying numbers of fluorine substituents was synthesized. The photophysical and electrochemical properties of these fluorophores were measured in order to determine if careful tuning of the excited state electron transfer thermodynamics is possible. The compounds cover a broad range of excited state energies and reduction potentials, and the data suggest that selective and differential tuning of both the reduction potential of the acceptor as well as the excited state equilibrium energy. These findings show that the individual parameters involved in excited state electron transfer can be tuned by the modular architecture of the pyrazoline fluorophore.

This study uses an magnetohydrodynamics (MHD) model to simulate unsteady one-dimensional transport variables in argon plasma, under low pressure and weak ionization between two planar electrodes suddenly biased by a negative voltage or electric field. Plasma has been widely used in etching, ion implantation, light source, and nuclear fusion, etc. Studying transport processes of plasmas therefore is important. Ignoring magnetic field, collisions between ions and electrons, the computed results in this work shows density, velocity, voltage, electric field, energy and temperature transport phenomenon in different cases. The results give insight by theory and simulation the surfaces behavior in plasma. Keywords: magnetohydrodynamics (MHD), transport variables in sheath,space charge