Isolant électrique

Résumé

thumb|Câble électrique 2 fils + terre avec isolant plastique thumb|Isolateur céramique utilisé pour supporter les câbles haute tension En électricité comme en électronique, un isolant électrique est une partie d'un composant ou un organe ayant pour fonction d'empêcher le passage de tout courant électrique entre deux parties conductrices soumises à une différence de potentiel électrique. Un isolant est constitué d'un matériau diélectrique qui possède peu de charges libres. Description La faculté isolante d'un matériau s'explique par la notion de bandes d'énergie. Dans un solide constitué de N atomes, les niveaux d'énergies se répartissent en intervalles dits bandes d'énergies qui seront donc occupées par les électrons. Parmi ces bandes , deux seulement sont intéressantes car ils déterminent les propriétés électriques du solide, ces deux bandes sont la bande de valence (BDV) et la bande de conduction, séparées par une bande dite interdite car ne pouvant être occupée par les

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Electric Field Eect on Skyrmion phase in Chiral lattice Ferrimagnet Cu2OSeO3

Naman Agarwal

Insulator materials are the recent research interest in Spintronics because of the absence of joule heating. There has been continued interest in insulator material Cu2OSeO3 because it possesses the novel Skyrmion phase [1], specific type of spin vortices in the lattice, characterized by quantized topological number, near the paramagnetic phase boundary [2]. This Skyrmion phase can be harnessed for the switching applications in the data storage devices [3–6]. We study the effect of positive or negative DC electric field on the Skyrmion phase boundary of this material by magnetoelectric susceptibility(dM/dE)measurements, which is a sensitive technique for scanning phase boundary [7,8]. We scan out the phase diagram for positive, negative and zero DC electric fields and find that electric field effect is maximum at the phase boundaries and decreases inside the phase There is no cut-off for the electric field effect. Also, the electric field effect is maximum at upper skyrmion boundary compared to other phase boundaries. We also show the small expansion(contraction) of Skyrmion phase along temperature axis with the application of positive(negative) DC electric field.

2016

Chargement

Graphene has some drawbacks, like the absence of an electronic band gap which leads to only small gate switching ratios, and its fast carrier recombination that limits its use in optoelectronics. In order to overcome these issues, the aim of this thesis was to tune the properties of graphene by two different modification approaches. The first approach involved chemical functionalization to create lateral graphene heterostructures, whereas the second approach was directed toward vertical heterostructures that combine graphene with another 2D material. For the former case, two novel chemical functionalization methods were investigated. The first method relied upon hyperthermal molecular ion collisions with 4,4¿-azobis(pyridine). This enabled the covalent functionalization of graphene with a functionalization degree of up to 3%, as demonstrated by Raman in combination with XPS. AFM studies revealed that the functionalized sheets retain their topographic integrity. Thus obtained stripe-like patterns of covalently graphene within extended graphene sheets enabled enhanced on/off ratios upon gate switching. In the second functionalization, OsO4 was used to selectively introduce hydroxyl groups to graphene. Best results were obtained using UV light activation, which yielded graphene of high covalent functionalization degree, as concluded from its optical transparency and gate-induced on/off switching ratios of up to 500. Temperature dependent electrical measurements revealed 2D hopping as the dominant transport mechanism. The first type of vertical heterostructure, gr-TiOx-Ti diodes were fabricated and their electrical properties in the dark and under visible light illumination studied. In contrast to conventional MIM diodes, the performance of the graphene-based diodes was found to increase with decreasing thickness of the oxide insulator. Bias-induced modulation of the work function of graphene was identified as the key to the operation mechanism of the graphene diodes, enabling them to reach a very high asymmetry and nonlinearity (9000 and 8). The diodes compete well with state-of-the-art MIM diodes. Furthermore, the graphene-TiOx-Ti diodes could be operated as photovoltaic cell, with a maximum open-circuit voltage of 0.3 V and a short-circuit photocurrent of 14 nA under global illuminationt. This finding constitutes the first proof-of-principle of hot carrier extraction from graphene, based on the photovoltaic effect. The second type of vertical heterostructures was based on black phosphorus (BP). As a first step, thin BP sheets were combined with n-GaAs into novel pn devices. They displayed pronounced rectification behavior that, in the low bias regime, approaches that of ideal diodes. Moreover, when operated as photodiodes they reached external quantum efficiencies (EQE) above 30% under higher reverse bias. On this basis, the bP was then combined with gr into a p+/p heterojunction. Thus obtained devices achieved a maximum internal quantum efficiency (IQE) and responsivity of 13% and 10.7 mA/W, respectively. This IQE is the highest thus far reported for diodes comprising BP and another 2D material. Moreover, with the aid of a top gate it was possible to modulate both, the photo-conversion efficiency and photocurrent generation mechanism of the black phosphorous-graphene diodes. Together with the possibility to tune the photocurrent response by the thickness of the black phosphorous, these diodes emerge as promising photodetectors.

EPFL2016

Chargement

Titanium dioxide is a cheap, chemically stable and non-toxic material. However its electrical properties are unstable and it is a modest semiconductor and a mediocre insulator. For several applications it would be interesting to make it either more insulating or more conducting. The goal of this work was to modify the electrical properties of nano-crystalline TiO2 thin films deposited by reactive sputtering and to understand the mechanism leading to these modifications. The principal factors that influence the electrical conductivity are on the one hand the concentration and nature of the chemical impurities incorporated in TiO2, and on the other hand the morphology of the thin films. The study was split into two parts. The fist part describes the modifications of the sputtered material obtained by chemical doping of the TiO2 thin films. Niobium, cerium, iron, and fluorine were incorporated successfully in TiO2. The second part describes the modifications obtained by modifying the reactive gas used during the sputtering process: thus oxygen was substituted with water vapor. Several analysis techniques have been used to characterize the TiO2 thin films. They are essentially divided in four categories. The chemical analyses included electron probe microanalyses, x-ray photoemission spectroscopy, and secondary ion mass spectrometry. The structure and morphology analyses of thin films were carried out with x-ray diffraction and atomic force microscopy. The electrical properties in dc or ac mode were measured between room temperature and 350°C. Finally optical transmission provided information on the electronic states and morphology of the thin films. With an appropriate choice of impurities, titanium dioxide can be made more insulating (TiO2:Ce), more conducting with an n-type electrical conductivity (TiO2:Nb) or p-type electrical conductivity (TiO2:Fe). Chemical doping is also the cause of important structure and morphology changes, for example it can force the transformation from the anatase to the rutile structure. It is shown in particular that dopant atoms generate defects such as oxygen vacancies. These defects impede the variation of the electrical conductivity produced by the impurities, thus, in spite of dopant concentration in the percent range, the variation of the electrical conductivity is an increase or a decrease of three order of magnitude at most when compared with undoped TiO2. Thin films deposited with water vapor as the reactive gas present an increase in the electrical conductivity by height order of magnitude compared to samples prepared with oxygen in similar conditions. No atom other than titanium or oxygen could be detected in the thin films in significant concentration. The dramatic conductivity increase is due to the injection inside TiO2 grains of electrons donated by unsaturated titanium atoms lying on the grain surface. A nanometric grain size is a requirement to make such high doping levels possible. Depending on the impurities or reactive gas selected, titanium dioxide can be made either into a good insulator with a high breakdown field and a high permittivity or into a reasonably conducting, transparent film. The electrical conductivity of the samples prepared for this study cover the range from 10-10 to 103 S m-1.

EPFL1999

Chargement

EPFL2016

EPFL1999