Entropy production in photovoltaic-thermoelectric nanodevices from the non-equilibrium Green's function formalism

We derive the expressions of photon energy and particle currents inside an open nanosystem interacting with light using non-equilibrium Green's functions. The model allows different temperatures for the electron reservoirs, which basically defines a photovoltaic-thermoelectric hybrid. Thanks to these expressions, we formulate the steady-state entropy production rate to assess the efficiency of reversible photovoltaic-thermoelectric nanodevices. Next, quantum dot based nanojunctions are closely examined. We show that entropy production is always positive when one considers spontaneous emission of photons with a specific energy, while in general the emission spectrum is broadened, notably for strong coupling to reservoirs. In this latter case, when the emission is integrated over all the energies of the spectrum, we find that entropy production can reach negative values. This result provides matter to question the second law of thermodynamics for interacting nanosystems beyond the assumption of weak coupling.Comment: 12 pages, 4 figure

Single-molecule conductance studies on quasi- and metallaaromatic dibenzoylmethane coordination compounds and their aromatic analogs

The ability to predict the conductive behaviour of molecules, connected to macroscopic electrodes, represents a crucial prerequisite for the design of nanoscale electronic devices. In this work, we investigate whether the notion of a negative relation between conductance and aromaticity (the so-called NRCA rule) also pertains to quasi-aromatic and metallaaromatic chelates derived from dibenzoylmethane (DBM) and Lewis acids (LAs) that either do or do not contribute two extra dπ electrons to the central resonance-stabilised β-ketoenolate binding pocket. We therefore synthesised a family of methylthio-functionalised DBM coordination compounds and subjected them, along with their truly aromatic terphenyl and 4,6-diphenylpyrimidine congeners, to scanning tunneling microscope break-junction (STM-BJ) experiments on gold nanoelectrodes. All molecules share the common motif of three π-conjugated, six-membered, planar rings with a meta-configuration at the central ring. According to our results, their molecular conductances fall within a factor of ca. 9 in an ordering aromatic < metallaaromatic < quasi-aromatic. The experimental trends are rationalised by quantum transport calculations based on density functional theory (DFT)

Mechanical conductance tunability of a porphyrin–cyclophane single-molecule junction

The possibility to study quantum interference phenomena at ambient conditions is an appealing feature of molecular electronics. By connecting two porphyrins in a cofacial cyclophane, we create an attractive platform for mechanically controlling electric transport through the intramolecular extent of π-orbital overlap of the porphyrins facing each other and through the angle of xanthene bridges with regard to the porphyrin planes. We analyze theoretically the evolution of molecular configurations in the pulling process and the corresponding changes in electric conduction by combining density functional theory (DFT) with Landauer scattering theory of phase-coherent elastic transport. Predicted conductances during the stretching process show order of magnitude variations caused by two robust destructive quantum interference features that span through the whole electronic gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Mechanically-controlled break junction (MCBJ) experiments at room temperature verify the mechanosensitive response of the molecular junctions. During the continuous stretching of the molecule, they show conductance variations of up to 1.5 orders of magnitude over single breaking events. Uncommon triple- and quadruple-frequency responses are observed in periodic electrode modulation experiments with amplitudes of up to 10 Å. This further confirms the theoretically predicted double transmission dips caused by the spatial and energetic rearrangement of molecular orbitals, with contributions from both through-space and through-bond transport

Mechanical conductance tunability of a porphyrin–cyclophane single-molecule junction

The possibility to study quantum interference phenomena at ambient conditions is an appealing feature of molecular electronics. By connecting two porphyrins in a cofacial cyclophane, we create an attractive platform for mechanically controlling electric transport through the intramolecular extent of π-orbital overlap of the porphyrins facing each other and through the angle of xanthene bridges with regard to the porphyrin planes. We analyze theoretically the evolution of molecular configurations in the pulling process and the corresponding changes in electric conduction by combining density functional theory (DFT) with Landauer scattering theory of phase-coherent elastic transport. Predicted conductances during the stretching process show order of magnitude variations caused by two robust destructive quantum interference features that span through the whole electronic gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Mechanically-controlled break junction (MCBJ) experiments at room temperature verify the mechanosensitive response of the molecular junctions. During the continuous stretching of the molecule, they show conductance variations of up to 1.5 orders of magnitude over single breaking events. Uncommon triple-and quadruple-frequency responses are observed in periodic electrode modulation experiments with amplitudes of up to 10 Å. This further confirms the theoretically predicted double transmission dips caused by the spatial and energetic rearrangement of molecular orbitals, with contributions from both through-space and through-bond transport. This journal is QN/van der Zant La

Transport résolu en temps dans les nanodispositifs optoélectroniques quantiques

Les récents progrès en matière de fréquences d’excitation au-delà du gigahertz offrent aujourd’hui la possibilité de sonder la réponse interne d’un système quantique. Résoudre le fonctionnement en temps des futurs composants de la nanoélectronique apparaît aujourd’hui comme le défi majeur de la prochaine avancée en matière de modélisation/simulation. C’est le contexte de cette thèse, qui se concentre sur trois axes. Une première partie sur la méthodologie. Nous avons proposé une technique adaptée à la simulation du transport dépendant du temps dans les nanosystèmes interagissant avec un rayonnement lumineux, en nous appuyant sur l’état de l’art des méthodologies de statistiques quantiques avec une attention particulière au formalisme des fonctions de Green hors-équilibre. La deuxième partie de la thèse est consacrée au développement et à la mise en œuvre d’algorithmes efficaces pour simuler des fonctionnement résolus en temps de nanodispositifs optoélectroniques quantiques. Enfin, cette nouvelle méthode et les algorithmes développés nous ont permis d’étudier les processus de transfert de porteurs dans des nanojonctions moléculaires. Cette étude nous a conduit à l’élucidation d’effets physiques insoupçonnés et à des propositions expérimentales captivantes pour la détermination de caractéristiques quantiques internes de ces nanodispositifs. Ce travail nous fournit un outil précieux pour la simulation du transport quantique ultrarapide. Il donne également un aperçu de la pertinence de la dynamique transitoire dans la compréhension du fonctionnement des nanodispositifs optoélectroniques résolu en temps, et ouvre la voie vers la conception de l’optoélectronique ultrarapide.Recent advances in excitation frequencies beyond gigahertz now offer the ability to probe the internal response of a quantum system. Time dependence in future nanoelectronics has arisen as the major challenge of next advances in device modeling and simulations. Oscillating gate voltages, time-dependent bias but also applied illumination pulses, all are examples of key issues in quantum transport simulations which require novel approaches as well as efficient numerical methods. This is the context of this thesis, which focuses on three areas. A first part concerns the methodology. We proposed a suitable technique for the simulation of time-dependent transport in nano-systems interacting with lightradiation, relying on the state of the art in quantum statistical methodologies, with a special attention to the formalism of non-equilibrium Green’s functions. The second part of the thesis is devoted to the development and implementationof efficient algorithms to simulate time-resolved quantities for quantum optoelectronic nanodevices. Finally, this new method and the developed algorithms have enabled us to investigate carrier transfer processes in molecular nanojunctions. This study led us to the elucidation of unsuspected physical effects and captivating experimental proposals for the determination of internal quantum characteristics of these nanodevices. This work provides us with a valuable toolfor ultrafast quantum transport simulation. It also gives indeed an insight on the relevance of transient dynamics in the understanding of time-resolved optoelectronic nanodevice operations and open avenues towards the design of futureultrafast optoelectronics

Asymmetry induces long-lasting energy current transients inside molecular loop circuits

International audienceEnergy transport and conversion at nanoscale have become an important topic of fundamental and applied research, in particular for conceiving groundbreaking solutions in energy-aware digital electronics and energy production. In this work, we propose a formal framework to address time-dependent energy transport inside quantum networks. The approach permits us to investigate how energy transferred to electrons by a femtosecond laser pulse is stored and released in a molecular circuit consisting of two donor-acceptor branches connected to an acceptor chain. Additionally, the two donors may be coupled, creating a loop inside the circuit. Time-resolved analysis reveals that when a difference exists between the two donor-acceptor branches, a loop current occurs and persists during relaxation, while only a small amount of current flows through the acceptor chain. A long-lasting energy flow thus emerges from the asymmetry of the molecular structure

State hybridization shapes the photocurrent in triple quantum dot nanojunctions

International audienceWe investigated a prototype of a quantum dot based photodetector made of a dot absorber interconnected with two lateral dot filters in contact with semiconducting leads. Using the nonequilibrium Green's function technique, we found that there are two opposite evolutions of the photocurrent in the vicinity of the tunnel resonance for such a kind of nanodevice. This evolution depends on where the strongest hybridization of states happens, and hence still reveals a quantum effect. If the filter states hybridize more with the absorber states than the ones of the leads, the photocurrent shows a maximum at the tunnel resonance, while it is minimized in the opposite case. Published by AIP Publishing

Energy and entropy currents for nanoscaled optoelectronics

Conference on Physics, Simulation, and Photonic Engineering of Photovoltaic Devices V, San Francisco, CA, FEB 15-17, 2016International audienceEnergetic and entropic issues are theoretically addressed in quantum optoelectronic nanodevices. We rely on the nonequilibrium Green's function methodology to provide a framework which combines optoelectronics and thermodynamics in a unified picture of energy conversion for nanoscaled optoelectronics. Indeed, we follow the self-consistent Born approximation to derive the formal expressions of energy and entropy currents flowing inside a nanodevice only interacting with light. These expressions are numerically evaluated in a quantum-dot based nanodevice, where verification of the second law of thermodynamics raises questioning about the system model. We here put the focus on the spontaneous emission energy current to discuss the question

Competitive hybridization in quantum dot-based nanodevices

Conference on Physics, Simulation, and Photonic Engineering of Photovoltaic Devices V, San Francisco, CA, FEB 15-17, 2016International audienceBy means of nonequilibrium Green's functions using the Born approximation to treat the light-matter coupling, we numerically investigate impacts of competitive hybridization on the photocurrent of a quantum dot based optoelectronic device. The model of device is an absorbing quantum dot connected to two semiconducting electrodes through energy filtering quantum dots. Hybridization occurs between the absorber and the filter, via the inter-dot coupling beta, and between the filter and the electrode, via the dot-lead coupling Gamma. At the tunnel resonance between the absorber and the filter, the investigation reveals the existence of two operating regimes in the nanodevice characterized by opposite variations of the photocurrent depending on ratio beta/Gamma

Time-resolved quantum transport for optoelectronics

International audienceWe investigate time-resolved energy currents in a molecular optoelectronic junction made of two donors and an acceptor sandwiched between two electrodes and excited by a Gaussian femtosecond laser pulse. Features of direct energy currents are thus correlated to the intra-molecular structure