Masse effective | EPFL Graph Search

redresse=1.5|vignette|Structure de bande générée pour Si, Ge, GaAs et InAs massifs par la méthode . La masse effective est une notion utilisée en physique du solide pour l'étude du transport des électrons. Plutôt que de décrire des électrons de masse fixée évoluant dans un potentiel donné, on les décrit comme des électrons libres dont la masse effective varie. Cette masse effective peut-être positive ou négative, supérieure ou inférieure à la masse réelle de l'électron. La notion de masse effective est notamment utile dans l'étude des semi-conducteurs ou des liquides de Fermi. La masse effective est définie par le tenseur d'ordre 2 des dérivées secondes de l'énergie E par rapport au vecteur d'onde \mathbf{k} : : \left(\frac{1}{m}\right)_{i,j} = \frac{1}{\hbar^2}\frac{\partial^2 E}{\partial k_i\partial k_j} où \hbar est la constante de Planck réduite. Pour un électron libre, la masse effective est bien entendu constante et égale à

Direct measurement of ambipolar diffusion in bulk silicon by ultrafast infrared imaging of laser-induced microplasmas

Alexandros Mouskeftaras

Carrier kinetics in the density range of N = 10(17) - 10(20) cm(-3) is investigated inside the bulk of crystalline silicon. Most conventional experimental techniques used to study carrier mobility are indirect and lack sensitivity because of charging effects and recombination on the surface. An all optical technique is used to overcome these obstacles. By focusing 1.3-mu m femtosecond laser pulses in the volume, we inject an initial free-carrier population by two-photon absorption. Then, we use pump-and-probe infrared microscopy as a tool to obtain simultaneous measurements of the carrier diffusion and recombination dynamics in a microscale region deep inside the material. The rate equation model is used to simulate our experimental results. We report a constant ambipolar diffusion coefficient D-a of 2.5 cm(2) s(-1) and an effective carrier lifetime tau(eff) of 2.5 ns at room temperature. A discussion on our findings at these high-injection levels is presented. (C) 2016 AIP Publishing LLC.

Amer Inst Physics2016

Computational studies for the description of electronic and optical properties for photocatalysis in Metal Organic Frameworks

Andres Adolfo Ortega Guerrero

Metal-organic frameworks(MOFs) are promising materials for photocatalytic applications. MOFs modular and porosity structure provides a platform to assemble photo-active and catalytic-sites building block units that can offer unique photo-physics. This feature is of great interest for H2 evolution photocatalytic applications. However, MOFs current photocatalysts do not meet the industrial need for their implementation as a technology due to performance limitations. Thus, elucidating and predicting the optical properties of MOFs will have wide implications in the development of MOF-based photocatalysts. Computational studies can be of great aid for the discovery of MOFs photocatalyst. Yet, this is a challenging task for computational chemistry given the complexity of the photophysical properties, the inorganic-organic nature, the number of atoms, and the unit cell size of MOFs. The principal goal of this thesis is the study MOFs photophysical properties for predicting their feasibility for photocatalytic applications. Density functional theory (DFT) and linear-response time-dependent DFT (LR-TDDFT) formalisms are used for the description of the light absorption, charge separation, and charge transfer properties of MOFs. We first address the dependence of the ligand in the UV/Vis optical absorption of porphyrin-based MOFs. The characterization of the excited states of the porphyrin ligands in MOFs requires an appropriate approximation for the description in periodic crystal MOFs. The selection of proper functionals and approximations within DFT/TDDFT calculations is necessary to overcome the erroneous description of properties like excitonic effects, optical transitions, and gap-renormalization. Later, we continue with the study of the charge separation in a porphyrin ruthenium-based MOF. We describe the importance of missing coordinated solvents anions in the metal node for the stability and charge transfer mechanism in the material. Within this study, we computed electron-hole interacting energy indicating a low electron-hole recombination rate. We next introduce a computational strategy to describing the long-lived electron-hole pairs and the long-rage charge transfer in MOFs. This description can be depicted by computing charge transfer numbers and the effective mass of carriers, respectively. This methodology can correctly predict the charge separation and the effective mass of 15 representative MOF structures promising for photocatalysis in agreement with the literature. Finally, we present an experimental and theoretical combined study investigation towards optimal photocatalytic hydrogen evolution reaction (HER) in MOFs. Isostructural pyrene-based MOFs with different transition metals allowed us a proper comparison. This study highlights the interplay between tunning electronic properties and crystal morphology on the photocatalytic HER performance of the MOFs. These results presented in this thesis can be exploited and transfer to the study of other MOFs for HER photocatalysis.

EPFL2021

Computational studies for the description of electronic and optical properties for photocatalysis in Metal Organic Frameworks

Andres Adolfo Ortega Guerrero

EPFL2021