Efficient Blue Electroluminescence Using Quantum-Confined Two-Dimensional Perovskites

Solution-processed hybrid organic–inorganic lead halide perovskites are emerging as one of the most promising candidates for low-cost light-emitting diodes (LEDs). However, due to a small exciton binding energy, it is not yet possible to achieve an efficient electroluminescence within the blue wavelength region at room temperature, as is necessary for full-spectrum light sources. Here, we demonstrate efficient blue LEDs based on the colloidal, quantum-confined 2D perovskites, with precisely controlled stacking down to one-unit-cell thickness (n = 1). A variety of low-k organic host compounds are used to disperse the 2D perovskites, effectively creating a matrix of the dielectric quantum wells, which significantly boosts the exciton binding energy by the dielectric confinement effect. Through the Förster resonance energy transfer, the excitons down-convert and recombine radiatively in the 2D perovskites. We report room-temperature pure green (n = 7–10), sky blue (n = 5), pure blue (n = 3), and deep blue (n = 1) electroluminescence, with record-high external quantum efficiencies in the green-to-blue wavelength region.

Multiscale Analysis for Field-Effect Penetration through Two-Dimensional Materials

Gate-tunable two-dimensional (2D) materials-based quantum capacitors (QCs) and van der Waals heterostructures involve tuning transport or optoelectronic characteristics by the field effect. Recent studies have attributed the observed gate-tunable characteristics to the change of the Fermi level in the first 2D layer adjacent to the dielectrics, whereas the penetration of the field effect through the one-molecule-thick material is often ignored or oversimplified. Here, we present a multiscale theoretical approach that combines first-principles electronic structure calculations and the Poisson–Boltzmann equation methods to model penetration of the field effect through graphene in a metal–oxide–graphene–semiconductor (MOGS) QC, including quantifying the degree of “transparency” for graphene two-dimensional electron gas (2DEG) to an electric displacement field. We find that the space charge density in the semiconductor layer can be modulated by gating in a nonlinear manner, forming an accumulation or inversion layer at the semiconductor/graphene interface. The degree of transparency is determined by the combined effect of graphene quantum capacitance and the semiconductor capacitance, which allows us to predict the ranking for a variety of monolayer 2D materials according to their transparency to an electric displacement field as follows: graphene > silicene > germanene > WS2 > WTe2 > WSe2 > MoS2 > phosphorene > MoSe2 > MoTe2, when the majority carrier is electron. Our findings reveal a general picture of operation modes and design rules for the 2D-materials-based QCs.

Self-Assembled InAs/GaAs Quantum Dot Solar Cells

Our work focuses on experimental and theoretical studies aimed at establishing a fundamental understanding of the principal electrical and optical processes governing the operation of quantum dot solar cells (QDSC) and their feasibility for the realization of intermediate band solar cell (IBSC). Uniform performance QD solar cells with high conversion efficiency have been fabricated using carefully calibrated process recipes as the basis of all reliable experimental characterization. The origin for the enhancement of the short circuit current density (Jsc) in QD solar cells was carefully investigated. External quantum efficiency (EQE) measurements were performed as a measure of the below bandgap distribution of transition states. In this work, we found that the incorporation of self-assembled quantum dots (QDs) interrupts the lattice periodicity and introduce a greatly broadened tailing density of states extending from the bandedge towards mid-gap. A below-bandgap density of states (DOS) model with an extended Urbach tail has been developed. In particular, the below-bandgap photocurrent generation has been attributed to transitions via confined energy states and background continuum tailing states. Photoluminescence measurement is used to measure the energy level of the lowest available state and the coupling effect between QD states and background tailing states because it results from a non-equilibrium process. A basic I-V measurement reveals a degradation of the open circuit voltage (Voc) of QD solar cells, which is related to a one sub-bandgap photon absorption process followed by a direct collection of the generated carriers by the external circuit. We have proposed a modified Shockley-Queisser (SQ) model that predicts the degradation of Voc compared with a reference bulk device. Whenever an energy state within the forbidden gap can facilitate additional absorption, it can facilitate recombination as well. If the recombination is non-radiative, it is detrimental to solar cell performance. We have also investigated the QD trapping effects as deep level energy states. Without an efficient carrier extraction pathway, the QDs can indeed function as mobile carriers traps. Since hole energy levels are mostly connected with hole collection under room temperature, the trapping effect is more severe for electrons. We have tried to electron-dope the QDs to exert a repulsive Coulomb force to help improve the carrier collection efficiency. We have experimentally observed a 30% improvement of Jsc for 4e/dot devices compared with 0e/dot devices. Electron-doping helps with better carrier collection efficiency, however, we have also measured a smaller transition probability from valance band to QD states as a direct manifestation of the Pauli Exclusion Principle. The non-linear performance is of particular interest. With the availability of laser with on-resonance and off-resonance excitation energy, we have explored the photocurrent enhancement by a sequential two-photon absorption (2PA) process via the intermediate states. For the first time, we are able to distinguish the nonlinearity effect by 1PA and 2PA process. The observed 2PA current under off-resonant and on-resonant excitation comes from a two-step transition via the tailing states instead of the QD states. However, given the existence of an extended Urbach tail and the small number of photons available for the intermediate states to conduction band transition, the experimental results suggest that with the current material system, the intensity requirement for an observable enhancement of photocurrent via a 2PA process is much higher than what is available from concentrated sun light. In order to realize the IBSC model, a matching transition strength needs to be achieved between valance band to QD states and QD states to conduction band. However, we have experimentally shown that only a negligible amount of signal can be observed at cryogenic temperature via the transition from QD states to conduction band under a broadband IR source excitation. Based on the understanding we have achieved, we found that the existence of the extended tailing density of states together with the large mismatch of the transition strength from VB to QD and from QD to CB, has systematically put into question the feasibility of the IBSC model with QDs.

Effect of ambient levels of ozone on photosynthetic components and radical scavenging system in leaves of African cowpea varieties

Tropospheric ozone (O3), a main component of photochemical oxidants, adversely affects not only human health but also vegetation. To clarify the long-term effects of ambient levels of tropospheric ozone (O3) on photosynthetic components and radical scavenging system in the leaves of cowpea ( Vigna unguiculata L.), two African varieties, Blackeye and Asontem, were grown in open-top chambers and exposed to filtered air (FA), non-filtered air (NF) or non-filtered air with additional O3 of approximately 50 nl l-1. Ambient levels of O3 significantly reduced chlorophyll concentration, quantum yield and activity of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), thus contributing to the reduction in net photosynthetic rate at the reproductive growth stage of both varieties; with no significant variety difference in the sensitivity to O3. The O3-induced significant reduction in catalase activity was observed in Blackeye at vegetative and reproductive growth stages; and in Asontem at reproductive growth stage. On the other hand, exposure to O3 significantly increased ascorbate peroxidase activity in Blackeye at reproductive stage and did not significantly affect that in Blackeye at vegetative growth stage and that in Asontem at both growth stages. At reproductive growth stage, activities of monodehydroascorbate reductase and glutathione reductase were significantly increased by the exposure to O3 in both varieties. The results obtained in this study suggest that, although ascorbate peroxidase, monodehydroascorbate reductase and glutathione reductase played important roles in scavenging O3-induced reactive oxygen species in the leaves, radical scavenging ability of these enzymes is not sufficient to avoid detrimental effects of ambient levels of O3 on photosynthesis in both African cowpea varieties.

Excitonic behaviour in polymeric semiconductors : the effect of morphology and composition in heterostructures

La compréhension des interrelations entre la microstructure et les processus électroniques dans les polymères semi-conducteurs est d’une importance primordiale pour leur utilisation dans des hétérostructures volumiques. Dans cette thèse de doctorat, deux systémes diffèrents sont étudiés ; chacun de ces systèmes représente une approche diffèrente pour optimiser les matériaux en termes de leur microstructure et de leur capacité à se mettre en ordre au niveau moléculaire. Dans le premier système, j’ai effectué une analyse complète des principes de fonctionnement d’une cellule photovoltaïque hybride à base des nanocristaux d’oxyde de zinc (ZnO) et du poly (3-hexylthiophène) (P3HT) par absorption photoinduite en régime quasi-stationnaire (PIA) et la spectroscopie PIA en pompage modulé dépendant de la fréquence. L’interface entre le donneur (le polymère P3HT) et l’accepteur (les nanoparticules de ZnO), où la génération de charges se produit, joue un rôle important dans la performance des cellules photovoltaïques hybrides. Pour améliorer le mécanisme de génération de charges du P3H: ZnO, il est indispensable de modifier l’interface entre ses constituants. Nous avons démontré que la modification d’interface moléculaire avec cis-bis (4, 40 - dicarboxy-2, 20bipyridine) ruthénium (II) (N3-dye) et a-Sexithiophen-2 yl-phosphonique (6TP) a améliorée le photocourant et la performance dans les cellules P3HT: ZnO. Le 6TP et le N3 s’attachent à l’interface du ZnO, en augmentant ainsi l’aire effective de la surface donneur :accepteur, ce qui contribue à une séparation de charge accrue. De plus, le 6TP et le N3 réduisent la densité de pièges dans le ZnO, ce qui réduit le taux de recombinaison des paires de charges. Dans la deuxième partie, jai introduit une matrice hôte polymérique de polystyréne à masse molaire ulra-élevée, qui se comporte comme un solide pour piéger et protéger une solution de poly [2-méthoxy, 5- (2´-éthyl-hexoxy) -1,4-phénylènevinylène- PPV] (MEHPPV) pour utilisation dans des dispositifs optoèlectroniques quantiques. Des travaux antérieurs ont montré que MEH-PPV en solution subit une transition de conformation, d’une conformation enroulé à haute température (phase bleue) à une conformation de chaîne étendue à basse température (phase rouge). La conformation de la chaîne étendue de la solution MEH-PPV favorise les caractéristiques nécessaires à l’amélioration des dispositifs optoélectroniques quantiques, mais la solution ne peut pas être incorporées dans le dispositif. J’ai démontré que la caractéristique de la phase rouge du MEH-PPV en solution se maintient dans une matrice hôte polymérique de polystyrène transformé de masse molaire très élevée, qui se comporte comme un solide (gel de MEH-PPV/UHMW PS), par le biais de la spectroscopie de photoluminescence (PL) dépendant de la température (de 290K à 80 K). La phase rouge du gel MEH-PPV/UHMW PS se manifeste par des largeurs de raie étroites et une intensité augmentée de la transition 0-0 de la progression vibronique dans le spectre de PL ainsi qu’un petit décalage de Stokes entre la PL et le spectre d’absorption à basse température. Ces approches démontrent que la manipulation de la microstructure et des propriétés électroniques des polymères semi-conducteurs ont un impact direct sur la performance de dispositifs pour leurs développements technologiques continus.

Probing the interactions between ionic liquids and water: experimental and quantum chemical approach

For an adequate choice or design of ionic liquids, the knowledge of their interaction with other solutes and solvents is an essential feature for predicting the reactivity and selectivity of systems involving these compounds. In this work, the activity coefficient of water in several imidazolium-based ionic liquids with the common cation 1-butyl-3-methylimidazolium was measured at 298.2 K. To contribute to a deeper insight into the interaction between ionic liquids and water, COSMO-RS was used to predict the activity coefficient of water in the studied ionic liquids along with the excess enthalpies. The results showed good agreement between experimental and predicted activity coefficient of water in ionic liquids and that the interaction of water and ionic liquids was strongly influenced by the hydrogen bonding of the anion with water. Accordingly, the intensity of interaction of the anions with water can be ranked as the following: [CF3SO3](-) < [SCN](-) < [TFA](-) < Br(-) < [TOS](-) < Cl(-) < [CH3SO3](-) [DMP](-) < [Ac](-). In addition, fluorination and aromatization of anions are shown to reduce their interaction with water. The effect of temperature on the activity coefficient of water at infinite dilution was measured by inverse gas chromatography and predicted by COSMO-RS. Further analysis based on COSMO-RS provided information on the nature of hydrogen bonding between water and anion as well as the possibility of anion-water complex formation.

Effect of the cation on the interactions between alkyl methyl imidazolium chloride ionic liquids and water

A systematic study of the interactions between water and alkyl methyl imidazolium chloride ionic liquids at 298.2 K, based on activity coefficients estimated from water activity measurements in the entire solubility range, is presented. The results show that the activity coefficients of water in the studied ILs are controlled by the hydrophilicity of the cation and the cation-anion interaction. To achieve a deeper understanding on the interactions between water and the ILs, COSMO-RS and FTIR spectroscopy were also applied. COSMO-RS was used to predict the activity coefficient of water in the studied ionic liquids along with the excess enthalpies, suggesting the formation of complexes between three molecules of water and one IL molecule. On the basis of quantum-chemical calculations, it is found that cation-anion interaction plays an important role upon the ability of the IL anion to interact with water. The changes in the peak positions/band areas of OH vibrational modes of water as a function of IL concentration were investigated, and the impact of the cation on the hydrogen-bonding network of water is identified and discussed.

Transição de fase topológica e transporte eletrônica em grafenos e nanofitas zigzag grafeno tensionados

Tese (doutorado)—Universidade de Brasília, Instituto de Física, 2015.

Disorder driven transitions in non-equilibrium quantum systems

This thesis presents studies of the role of disorder in non-equilibrium quantum systems. The quantum states relevant to dynamics in these systems are very different from the ground state of the Hamiltonian. Two distinct systems are studied, (i) periodically driven Hamiltonians in two dimensions, and (ii) electrons in a one-dimensional lattice with power-law decaying hopping amplitudes. In the first system, the novel phases that are induced from the interplay of periodic driving, topology and disorder are studied. In the second system, the Anderson transition in all the eigenstates of the Hamiltonian are studied, as a function of the power-law exponent of the hopping amplitude.

In periodically driven systems the study focuses on the effect of disorder in the nature of the topology of the steady states. First, we investigate the robustness to disorder of Floquet topological insulators (FTIs) occurring in semiconductor quantum wells. Such FTIs are generated by resonantly driving a transition between the valence and conduction band. We show that when disorder is added, the topological nature of such FTIs persists as long as there is a gap at the resonant quasienergy. For strong enough disorder, this gap closes and all the states become localized as the system undergoes a transition to a trivial insulator.

Interestingly, the effects of disorder are not necessarily adverse, disorder can also induce a transition from a trivial to a topological system, thereby establishing a Floquet Topological Anderson Insulator (FTAI). Such a state would be a dynamical realization of the topological Anderson insulator. We identify the conditions on the driving field necessary for observing such a transition. We realize such a disorder induced topological Floquet spectrum in the driven honeycomb lattice and quantum well models.

Finally, we show that two-dimensional periodically driven quantum systems with spatial disorder admit a unique topological phase, which we call the anomalous Floquet-Anderson insulator (AFAI). The AFAI is characterized by a quasienergy spectrum featuring chiral edge modes coexisting with a fully localized bulk. Such a spectrum is impossible for a time-independent, local Hamiltonian. These unique characteristics of the AFAI give rise to a new topologically protected nonequilibrium transport phenomenon: quantized, yet nonadiabatic, charge pumping. We identify the topological invariants that distinguish the AFAI from a trivial, fully localized phase, and show that the two phases are separated by a phase transition.

The thesis also present the study of disordered systems using Wegner's Flow equations. The Flow Equation Method was proposed as a technique for studying excited states in an interacting system in one dimension. We apply this method to a one-dimensional tight binding problem with power-law decaying hoppings. This model presents a transition as a function of the exponent of the decay. It is shown that the the entire phase diagram, i.e. the delocalized, critical and localized phases in these systems can be studied using this technique. Based on this technique, we develop a strong-bond renormalization group that procedure where we solve the Flow Equations iteratively. This renormalization group approach provides a new framework to study the transition in this system.

The effect of notch filtering on the waveform of the newborn auditory brainstem response

OBJECTIVE: To identify whether the use of a notch filter significantly affects the morphology or characteristics of the newborn auditory brainstem response (ABR) waveform and so inform future guidance for clinical practice. DESIGN: Waveforms with and without the application of a notch filter were recorded from babies undergoing routine ABR tests at 4000, 1000 and 500 Hz. Any change in response morphology was judged subjectively. Response latency, amplitude, and measurements of response quality and residual noise were noted. An ABR simulator was also used to assess the effect of notch filtering in conditions of low and high mains interference. RESULTS: The use of a notch filter changed waveform morphology for 500 Hz stimuli only in 15% of tests in newborns. Residual noise was lower when 4000 Hz stimuli were used. Response latency, amplitude, and quality were unaffected regardless of stimulus frequency. Tests with the ABR stimulator suggest that these findings can be extended to conditions of high level mains interference. CONCLUSIONS: A notch filter should be avoided when testing at 500 Hz, but at higher frequencies appears to carry no penalty.

Insulator-to-metal transition in vanadium supersaturated silicon: variable-range hopping and Kondo effect signatures

We report the observation of the insulator-to-metal transition in crystalline silicon samples supersaturated with vanadium. Ion implantation followed by pulsed laser melting and rapid resolidification produce high quality single-crystalline silicon samples with vanadium concentrations that exceed equilibrium values in more than 5 orders of magnitude. Temperature-dependent analysis of the conductivity and Hall mobility values for temperatures from 10K to 300K indicate that a transition from an insulating to a metallic phase is obtained at a vanadium concentration between 1.1 × 10^(20) and 1.3 × 10^(21) cm^(−3) . Samples in the insulating phase present a variable-range hopping transport mechanism with a Coulomb gap at the Fermi energy level. Electron wave function localization length increases from 61 to 82 nm as the vanadium concentration increases in the films, supporting the theory of impurity band merging from delocalization of levels states. On the metallic phase, electronic transport present a dispersion mechanism related with the Kondo effect, suggesting the presence of local magnetic moments in the vanadium supersaturated silicon material.

Topological insulating phases from two-dimensional nodal loop semimetals

Starting from a minimal model for a two-dimensional nodal loop semimetal, we study the effect of chiral mass gap terms. The resulting Dirac loop anomalous Hall insulator’s Chern number is the phase-winding number of the mass gap terms on the loop.We provide simple lattice models, analyze the topological phases, and generalize a previous index characterizing topological transitions. The responses of the Dirac loop anomalous Hall and quantum spin Hall insulators to a magnetic field’s vector potential are also studied both in weak- and strong-field regimes, as well as the edge states in a ribbon geometry.

CAROLYN HALL: EL CAFÉ Y EL DESARROLLO HISTÓRICO-GEOGRÁFICO DE COSTA RICA

El café y el Desarrollo histórico-geográfico de Costa Rica