36 research outputs found
Action d'un champ magnétique et d'un champ électrique sur un exciton lié à une impureté de type donneur ionisé dans un puits quantique de semi-conducteur
No full textNot availableLe travail présenté dans ce mémoire de thèse est consacré à l'étude des propriétés électroniques d'un exciton lié à une impureté ionisée de type donneur (D+, X), dans les puits quantiques de type GaAs/Ga1-xAlxAs et CdTe/Cd1-xZnxTe, en présence des champs magnétique et électrique. Cette étude s'inscrit dans un contexte de recherche en plein développement, en raison des applications potentielles des nanostructures dans le domaine de l'optoélectronique. dans le cadre de l'approximation de la masse effective et dans un modèle à deux bandes paraboliques simples et non dégénérées, nous avons comparé l'énergie de corrélation du complexe (D+, X), du donneur neutre (D0) et de l'exciton libre (X) dans un puits quantique. Nous avons déduit que le complexe est toujours stable dans le domaine de variation de la largeur L du puits étudié. Nous avons discuté également l'effet de la position du donneur dans un puits quantique sur la stabilité du complexe pour une largeur du puits donnée. Enfin nous avons examiné l'influence d'un champ magnétique et d'un champ électrique sur l'énergie de l'état fondamental et sur la stabilité du complexe dans un puits quantique. Dans le cas d'un puits quantique de type GaAs/Ga1-xAlxAs, l'application d'un champ magnétique conduit à une augmentation de l'énergie du complexe. La stabilité du complexe doit être étudiée soit par rapport au donneur, soit par rapport à l'exciton suivant une valeur critique du champ qui diminue quand la largeur du puits augmente. Dans les puits quantiques de type GaAs/Ga1-xAlxAs et CdTe/Cd1-xZnxTe le complexe est stable par rapport aux produits de dissociation les plus probables, dans tout le domaine de la largeur du puits et de l'intensité de champ magnétique étudié. En présence d'un champ électrique, l'énergie de corrélation du complexe dépend de l'intensité du champ applique : dans le cas d'un puits quantique de type GaAs/Ga1-xAlxAs, cette dépendance, faible pour un puits étroit, devient significative pour un puits large. Le complexe est stable dans le domaine de la largeur du puits et d'intensité du champ électrique étudiés
Unveiling the catalytic potential of two-dimensional boron nitride in lithium–sulfur batteries
No full textLithium–sulfur (Li–S) batteries, renowned for their potential high energy density, have attracted attention due to their use of earth-abundant elements. However, a significant challenge lies in developing suitable materials for both lithium-based anodes, which are less prone to lithium dendrite formation, and sulfur-based cathodes. This obstacle has hindered their widespread commercial viability. In this study, we present a novel sulfur host material in the form of a two-dimensional semiconductor boron nitride framework, specifically the 2D orthorhombic diboron dinitride (o-B2N2). The inherent conductivity of o-B2N2 mitigates the insulating nature often observed in sulfur-based electrodes. Notably, the o-B2N2 surface demonstrates a high binding affinity for long-chain Li-polysulfides, leading to a significant reduction in their dissolution into the DME/DOL electrolytes. Furthermore, the preferential deposition of Li2S on the o-B2N2 surface expedites the kinetics of the lithium polysulfide redox reactions. Additionally, our investigations have revealed a catalytic mechanism on the o-B2N2 surface, significantly reducing the free energy barriers for various sulfur reduction reactions. Consequently, the integration of o-B2N2 as a host cathode material for Li–S batteries holds great promise in suppressing the shuttle effect of lithium polysulfides and ultimately enhancing the overall battery performance. This represents a practical advancement for the application of Li–S batteries.Team Poulumi De
Action d'un champ magnétique et d'un champ électrique sur un exciton lié à une impureté de type donneur ionisé dans un puits quantique de semi-conducteur
No full textMETZ-SCD (574632105) / SudocSudocFranceMoroccoFRM
Photocatalytic and thermoelectric performance of asymmetrical two-dimensional Janus aluminum chalcogenides
No full textThrough a density functional theory-driven survey, a comprehensive investigation of two-dimensional (2D) Janus aluminum-based monochalcogenides (Al2XY with X/Y = S, Se, and Te) has been performed within this study. To begin with, it is established that the examined phase, in which the Al-atoms are located at the two inner planes while the (S, Se, and Te)-atoms occupy the two outer planes in the unit cell, are energetically, mechanically, dynamically, and thermally stable. To address the electronic and optical properties, the hybrid function HSE06 has been employed. It is at first revealed that all three monolayers display a semiconducting nature with an indirect band gap ranging from 1.82 to 2.79 eV with a refractive index greater than 1.5, which implies that they would be transparent materials. Furthermore, the monolayers feature strong absorption spectra of around 105 cm−1 within the visible and ultraviolet regions, suggesting their potential use in optoelectronic devices. Concerning the photocatalytic performance, the conduction band-edge positions straddle the hydrogen evolution reaction redox level. Also, it is observed that the computed Gibbs free energy is around 1.15 eV, which is lower and comparable to some recently reported 2D-based Janus monolayers. Additionally, the thermoelectric properties are further investigated and found to offer a large thermal power as well as a high figure of merit (ZT) around 1.03. The aforementioned results strongly suggest that the 2D Janus Al-based monochalcogenide exhibits suitable characteristics as a potential material for high-performance optoelectronic and thermoelectric applications.Team Poulumi De
Harnessing intrinsic electric fields in 2D Janus MoOX (X=S, Se, and Te) monolayers for enhanced photocatalytic hydrogen evolution
No full textTwo-dimensional (2D) Janus monolayers, distinguished by their intrinsic vertical electric fields, emerge as highly efficient and eco-friendly materials for advancing the field of hydrogen evolution reactions (HER). In this study, we explore, for the first time, the potential viability of the oxygenation phase of two-dimensional Janus transition metal dichalcogenides MoOX (X = S, Se, and Te) monolayers as an exceptionally efficient photocatalyst for hydrogen production. Based on first-principles computations, we demonstrate that all three monolayers exhibit semiconductor behavior, characterized by a band gap ranging from 0.66 to 1.55 eV. This narrow band gap renders the proposed materials highly efficient at absorbing light within the visible region. Excitingly, the introduction of an electrostatic potential difference ΔΦ has granted us the ability to surpass the conventional bandgap limit (Eg≥1.23). Consequently, all monolayers exhibit favorable band alignment with respect to the vacuum level. Moreover, the calculated solar-to-hydrogen efficiency for the envisaged monolayer exceeds the established theoretical limit. Particularly, the MoOTe monolayer emerges as an infrared-light-driven photocatalyst, demonstrating a remarkable solar-to-hydrogen efficiency limit of up to 25,21% when considering the entire solar spectrum. A thorough examination of the Gibbs free energy differences across these monolayers has revealed that the values during the oxygenation phase are significantly smaller and approach the optimum, in contrast to the parental two-dimensional Janus transition metal dichalcogenides. Our results conclusively establish that the proposed materials exhibit exceptional efficiency as photocatalysts for hydrogen evolution reactions. Notably, their efficacy is demonstrated even in the lack of co-catalysts or sacrificial agents.Team Poulumi De
Janus Ga2SeTe and In2SeTe nanosheets: Excellent photocatalysts for hydrogen production under neutral pH
No full textIn the past few years, Janus nanosheets have attracted much interest according to their specific structure and considerable potential to address the energy and environmental issues. Herein, the electronic, optical and photocatalytic properties of two-dimensional Janus Ga2SeTe and In2SeTe have been studied using ab-initio computations based on the density functional theory. The obtained results show that these nanomaterials exhibit a semiconductor behavior with direct and moderate bandgaps using hybrid HSE06 func-tional. Subsequently, the understudied compounds present suitable optical conductivity, absorption, transmission and reflectivity for water splitting under the ultraviolet-visible light irradiation. Interestingly, the band edge positions of Janus Ga2SeTe and In2SeTe excellently straddle the redox potentials of water under neutral pH. Additionally, the free energy values for the formation of H2 from H adsorbed on the Ga2SeTe and In2SeTe com-pounds are respectively 1.304eV and 0.976eV at pH = 7. More excitingly, the present study proposes strain engineering approach to improve the photocatalytic performance of the Janus Ga2SeTe and In2SeTe monolayers. Specifically, the investigated semiconductors show more appropriate band edge alignment and better hydrogen evolution reaction ac-tivity under biaxial tensile strain, which fulfil the water splitting requirements at neutral pH conditions. Our findings conclude that the Janus Ga2SeTe and In2SeTe nanosheets are promising candidates for photocatalytic hydrogen production
Computational prediction of two-dimensional o-Al2N2 under applied strain for boosting the photocatalytic hydrogen evolution reaction performance
No full textPhotocatalytic water splitting for clean hydrogen fuel production provides a promising approach to solve the energy and environmental issues. Recently, two-dimensional (2D) photocatalysts have attracted growing interest owing to their short carrier diffusion path, abundant active sites and large surface area. This study explores the photocatalytic performance of 2D orthorhombic dialuminum dinitride (o-Al2N2) using density functional theory. The computational results show that the o-Al2N2 monolayer has a semiconductor character with indirect and moderate bandgap. Moreover, this system exhibits high light absorption in the visible region, referring to its high capacity for harvesting sunlight. Meanwhile, under neutral pH, the band edge positions are suitable to straddle water redox potentials and the hydrogen evolution reaction is energetically favorable to allow hydrogen production on the surface of 2D o-Al2N2 compound. More importantly, the photocatalytic activity of o-Al2N2 monolayer is significantly improved under slight biaxial compressive strain. Therefore, our findings suggest that the o-Al2N2 nanomaterial is a highly efficient 2D photocatalyst for hydrogen production via water splitting under neutral pH.(c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved
Entanglement assisted via Kerr effect in exciton-optomechanics
No full textEntanglement lies at the core of numerous quantum information processing applications. A main obstacle against implementing powerful quantum information tasks is decoherence that degrades, or even destroys, entanglement. Therefore, developing strategies to mitigate decoherence is essential for practical implementations. Here, we propose a scheme to enhance exciton-exciton entanglement in an exciton-optomechanical cavity incorporating a Kerr medium. Under experimentally feasible conditions, we demonstrate that the Kerr nonlinearity not only strengthens excitonic entanglement, but also allows it to persist over a wide range of operating parameters. While strong exciton-phonon coupling is required for entangling the two excitonic modes in the Kerr medium-free case, we show that such coupling is no longer necessary when the Kerr interaction is introduced. Remarkably, the presence of the Kerr medium enables the excitonic entanglement to survive up to room temperature without requiring a very high mechanical quality factor. Our scheme provides a promising route for generating robust entanglement and may open new possibilities for quantum information applications
Binding energy of an exciton in a GaN/AlN nanodot: Role of size and external electric field
No full textInternational audienceWe report the impact of an external electric field on the energy spectrum of an exciton inside a spherical shaped GaN/AlN core/shell nanodot. The modulation of the confined exciton lowest state energy by the nanodot size is also treated. Our theoretical approach, based on a variational calculation, predicts a remarkable decrease in the exciton's energy when the electric field is switched on. Furthermore, our investigation shows that for a fixed nanodot size, the energy redshift is a unique function of the external electric field strength. On the other hand, it was observed that as the nanodot size increases the lowest exciton energy decreases and vice versa
Revealing the superlative electrochemical properties of o-B2N2 monolayer in Lithium/Sodium-ion batteries
No full textPromising flexible electrochemical energy storage systems (EESSs) are currently drawing considerable attention for their tremendous prospective end-use in portable self-powered electronic devices, including roll-up displays, and "smart "garments outfitted with piezoelectric patches to harvest energy from body movement. However, the lack of suitable battery electrodes that provides a specific electrochemical performance has made further development of these technologies challenging. Two-dimensional (2D) lightweight and flexible materials with outstanding physical and chemical properties, including mechanical strengths, hydrophilic surfaces, high surface metal diffusivity, and good conductivity, have been identified as a potential prospect for battery electrodes. In this study, taking a new 2D boron nitride allotrope, namely 2D orthorhombic diboron dinitride monolayer (o-B2N2) as representatives, we systematically explored several influencing factors, including electronic, mechanical, and their electrochemical properties (e.g., binding strength, ionic mobility, equilibrium voltage, and theoretical capacity). Considering potential charge-transfer polarization, we employed a charged electrode model to simulate ionic mobility and found ionic mobility has a unique dependence on the surface atomic configuration influenced by bond length, valence electron number, electrical conductivity, excellent ionic mobility, low equilibrium voltage with excellent stability, good flexibility, and extremely superior theoretical capacity, up to 8.7 times higher than that of widely commercialized graphite (3239.74 mAh g(-1) Vs 372 mAh g(-1)) in case of Li-ion batteries and 2159.83 mAh g(-1) in case of Na-ion batteries, indicating that the new predicted 2D o-B2N2 monolayer possess the capability to be ideal flexible anode materials for Lithium and Sodium-ion battery. Our finding provides valuable insights for experimental explorations of flexible anode candidates based on 2D o-B2N2 monolayer
