Size, strain, and band offset engineering in GaAs(Sb)(N)-capped InAs quantum dots for 1.3 - 1.55 µm LEDs and LDs

The optical and structural properties of InAs/GaAs quantum dots (QD) are strongly modified through the use of a thin (~ 5 nm) GaAsSb(N) capping layer. In the case of GaAsSb-capped QDs, cross-sectional scanning tunnelling microscopy measurements show that the QD height can be controllably tuned through the Sb content up to ~ 14 % Sb. The increased QD height (together with the reduced strain) gives rise to a strong red shift and a large enhancement of the photoluminescence (PL) characteristics. This is due to improved carrier confinement and reduced sensitivity of the excitonic bandgap to QD size fluctuations within the ensemble. Moreover, the PL degradation with temperature is strongly reduced in the presence of Sb. Despite this, emission in the 1.5 !lm region with these structures is only achieved for high Sb contents and a type-II band alignment that degrades the PL. Adding small amounts of N to the GaAsSb capping layer allows to progressively reduce the QD-barrier conduction band offset. This different strategy to red shift the PL allows reaching 1.5 !lm with moderate Sb contents, keeping therefore a type-I alignment. Nevertheless, the PL emission is progressively degraded when the N content in the capping layer is increased

Self-assembled MgxZn1−xO quantum dots (0 ≤ x ≤ 1) on different substrates using spray pyrolysis methodology

By using the spray pyrolysis methodology in its classical configuration we have grown self-assembled MgxZn1−xO quantum dots (size [similar]4–6 nm) in the overall range of compositions 0 ≤ x ≤ 1 on c-sapphire, Si (100) and quartz substrates. Composition of the quantum dots was determined by means of transmission electron microscopy-energy dispersive X-ray analysis (TEM-EDAX) and X-ray photoelectron spectroscopy. Selected area electron diffraction reveals the growth of single phase hexagonal MgxZn1−xO quantum dots with composition 0 ≤ x ≤ 0.32 by using a nominal concentration of Mg in the range 0 to 45%. Onset of Mg concentration about 50% (nominal) forces the hexagonal lattice to undergo a phase transition from hexagonal to a cubic structure which resulted in the growth of hexagonal and cubic phases of MgxZn1−xO in the intermediate range of Mg concentrations 50 to 85% (0.39 ≤ x ≤ 0.77), whereas higher nominal concentration of Mg ≥ 90% (0.81 ≤ x ≤ 1) leads to the growth of single phase cubic MgxZn1−xO quantum dots. High resolution transmission electron microscopy and fast Fourier transform confirm the results and show clearly distinguishable hexagonal and cubic crystal structures of the respective quantum dots. A difference of 0.24 eV was detected between the core levels (Zn 2p and Mg 1s) measured in quantum dots with hexagonal and cubic structures by X-ray photoemission. The shift of these core levels can be explained in the frame of the different coordination of cations in the hexagonal and cubic configurations. Finally, the optical absorption measurements performed on single phase hexagonal MgxZn1−xO QDs exhibited a clear shift in optical energy gap on increasing the Mg concentration from 0 to 40%, which is explained as an effect of substitution of Zn2+ by Mg2+ in the ZnO lattice.

Effect of concentration on the performance of quantum dot intermediate-band solar cells

Implementation of a high-efficiency quantum dot intermediate-band solar cell (QD-IBSC) must accompany a sufficient photocurrent generation via IB states. The demonstration of a QD-IBSC is presently undergoing two stages. The first is to develop a technology to fabricate high-density QD stacks or a superlattice of low defect density placed within the active region of a p-i-n SC, and the second is to realize half-filled IB states to maximize the photocurrent generation by two-step absorption of sub-bandgap photons. For this, we have investigated the effect of light concentration on the characteristics of QDSCs comprised of multi-layer stacks of self-organized InAs/GaNAs QDs grown with and without impurity doping in molecular beam epitaxy.

A numerical study into the influence of quantum dot size on the sub-bandgap interband photocurrent in intermediate band solar cells

A numerical study is presented of the sub-bandgap interband photon absorption in quantum dot intermediate band solar cells. Absorption coefficients and photocurrent densities are calculated for the valence band to intermediate band transitions using a four-band k · p method. It is found that reducing the quantum dot width in the plane perpendicular to the growth direction increases the photocurrent from the valence band to the intermediate-band ground state if the fractional surface coverage of quantum dots is conserved. This provides a path to increase the sub-bandgap photocurrent in intermediate band solar cells.

Effect of field damping layer on two step absorption of quantum dots solar cells

Multi-stacked InAs/AlGaAs quantum dot solar cells (QDSCs) introduced with field damping layers (FDL) which sustain the junction built-in potential have been studied. Without an external bias condition, the external quantum efficiency (EQE) of QD layers are reduced by introducing the thick FDL, because the carrier escape due to built-in electric field was suppressed. On the other hand, the photocurrent production due to two-step absorption is increased by the formation of flat-band QD structure for QDSC with thick FDL.

Effect of Sb on the quantum efficiency of GaInP solar cells

The energy bandgap of GaInP solar cells can be tuned by modifying the degree of order of the alloy. In this study, we employed Sb to increase the energy bandgap of the GaInP and analyzed its impact on the performance of GaInP solar cells. An effective change in the cutoff wavelength of the external quantum efficiency of GaInP solar cells and an effective increase of 50 mV in the open-circuit voltage of GaInP/Ga(In)As/Ge triple junction solar cells were obtained with the use of Sb. Copyright © 2016 John Wiley & Sons, Ltd.

The effect of flow speed and food size on the capture efficiency and feeding behaviour of the cold-water coral Lophelia pertusa

Acknowledgements The authors acknowledge L. Wicks and B. de Francisco for helping in coral sampling and coral care in the aquaria facilities at SAMS. Thanks to C. Campbell and the CCAP for kind support and help. Scientific party and crew on board the RVs Calanus and Seol Mara, as well as on board the RRS James Cook during the Changing Oceans cruise (JC_073) are greatly acknowledged. Thanks to colleagues at SAMS for their support during our stay at SAMS. We are in debt with A. Olariaga for his help modifying the cylindrical experimental chambers used in the experiments, and C.C. Suckling for assistance with the flume experiment. Many thanks go to G. Kazadinis for preparing the POM used in the feeding experiments. We also thank two anonymous reviewers and the editor for their constructive comments, which contribute to improve the manuscript. This work has been supported by the European Commission through two ASSEMBLE projects (grant agreement no. 227799) conducted in 2010 and 2011 at SAMS, as well as by the UK Ocean Acidification Research Programme's Benthic Consortium project (awards NE/H01747X/1 and NE/H017305/1) funded by NERC. [SS]

The effect of cultivation on the size, shape, and persistence of disease patches in fields

Epidemics of soil-borne plant disease are characterized by patchiness because of restricted dispersal of inoculum. The density of inoculum within disease patches depends on a sequence comprising local amplification during the parasitic phase followed by dispersal of inoculum by cultivation during the intercrop period. The mechanisms that control size, shape, and persistence have received very little rigorous attention in epidemiological theory. Here we derive a model for dispersal of inoculum in soil by cultivation that takes account into the discrete stochastic nature of the system in time and space. Two parameters, probability of movement and mean dispersal distance, characterize lateral dispersal of inoculum by cultivation. The dispersal parameters are used in combination with the characteristic area and dimensions of host plants to identify criteria that control the shape and size of disease patches. We derive a critical value for the probability of movement for the formation of cross-shaped patches and show that this is independent of the amount of inoculum. We examine the interaction between local amplification of inoculum by parasitic activity and subsequent dilution by dispersal and identify criteria whereby asymptomatic patches may persist as inoculum falls below a threshold necessary for symptoms to appear in the subsequent crop. The model is motivated by the spread of rhizomania, an economically important soil-borne disease of sugar beet. However, the results have broad applicability to a very wide range of diseases that survive as discrete units of inoculum. The application of the model to patch dynamics of weed seeds and local introductions of genetically modified seeds is also discussed.

Fano-Rashba effect in quantum dots

We consider the electronic transport through a Rashba quantum dot coupled to ferromagnetic leads. We show that the interference of localized electron states with resonant electron states leads to the appearance of the Fano-Rashba effect. This effect occurs due to the interference of bound levels of spin-polarized electrons with the continuum of electronic states with an opposite spin polarization. We investigate this Fano-Rashba effect as a function of the applied magnetic field and Rashba spin-orbit coupling.

Quantum anomalous Hall effect in graphene coupled to skyrmions

Skyrmions are topologically protected spin textures, characterized by a topological winding number N, that occur spontaneously in some magnetic materials. Recent experiments have demonstrated the capability to grow graphene on top Fe/Ir, a system that exhibits a two-dimensional skyrmion lattice. Here we show that a weak exchange coupling between the Dirac electrons in graphene and a two-dimensional skyrmion lattice withN = ±1 drives graphene into a quantum anomalous Hall phase, with a band gap in bulk, a Chern number C = 2N, and chiral edge states with perfect quantization of conductance G = 2N e2 h . Our findings imply that the topological properties of the skyrmion lattice can be imprinted in the Dirac electrons of graphene.

Electrocatalytic reduction of CO2 to formate using particulate Sn electrodes: Effect of metal loading and particle size

The development of electrochemical processes for the conversion of CO2 into value-added products allows innovative carbon capture & utilization (CCU) instead of carbon capture & storage (CCS). In addition, coupling this conversion with renewable energy sources would make it possible to chemically store electricity from these intermittent renewable sources. The electroreduction of CO2 to formate in aqueous solution has been performed using Sn particles deposited over a carbon support. The effect of the particle size and Sn metal loading has been evaluated using cyclic voltammetry and chronoamperometry. The selected electrode has been tested on an experimental filter-press type cell system for continuous and single pass CO2 electroreduction to obtain formate as main product at ambient pressure and temperature. Experimental results show that using electrodes with 0.75 mg Sn cm−2, 150 nm Sn particles, and working at a current density of 90 mA cm−2, it is possible to achieve rates of formate production over 3.2 mmol m−2 s−1 and faradaic efficiencies around 70% for 90 min of continuous operation. These experimental conditions allow formate concentrations of about 1.5 g L−1 to be obtained on a continuous mode and with a single pass of catholyte through the cell.

Quantum theory of spin waves in finite chiral spin chains

We calculate the effect of spin waves on the properties of finite-size spin chains with a chiral spin ground state observed on biatomic Fe chains deposited on iridium(001). The system is described with a Heisenberg model supplemented with a Dzyaloshinskii-Moriya coupling and a uniaxial single ion anisotropy that presents a chiral spin ground state. Spin waves are studied using the Holstein-Primakoff boson representation of spin operators. Both the renormalized ground state and the elementary excitations are found by means of Bogoliubov transformation, as a function of the two variables that can be controlled experimentally, the applied magnetic field and the chain length. Three main results are found. First, because of the noncollinear nature of the classical ground state, there is a significant zero-point reduction of the ground-state magnetization of the spin spiral. Second, there is a critical external field from which the ground state changes from chiral spin ground state to collinear ferromagnetic order. The character of the two lowest-energy spin waves changes from edge modes to confined bulk modes over this critical field. Third, in the spin-spiral state, the spin-wave spectrum exhibits oscillatory behavior as function of the chain length with the same period of the spin helix.

The effect of truck size and weight on accident experience and traffic operations. Volume 1: executive summary. Final report.

Federal Highway Administration, Washington, D.C.