Atomic processes in molecular beam epitaxy on strained InAs(137): A density-functional theory study

The atomic processes in molecular beam epitaxy of InAs on the InAs(137) surface are investigated by means of first-principles total-energy calculations. We consider layer-by-layer growth on InAs(137) facets as a typical process during the evolution of shallow InAs islands in the Stranski-Krastanov growth mode of InAs on GaAs that is exploited for the self-assembly of heteroepitaxial quantum dots. From the calculated energetics we conclude that a growth scenario where an As2 molecule adsorbs on a single In adatom, followed by capture of another In adatom, is most likely. Moreover, our calculations of the potential-energy surface for In adatoms on the InAs(137) surface show that In adatoms are highly mobile. Surface diffusion on InAs(137) is found to be almost isotropic with energy barriers 2 molecule is destabilized by compressive strain in excess of −5%. This finding leads us to the conclusion that layer growth on InAs(137) facets ceases in highly strained regions of InAs islands on GaAs, in line with the observed shape evolution of such islands

Erratum: Analytic many-body potential for InAsÕGaAs surfaces and nanostructures: Formation energy of InAs quantum dots [Phys. Rev. B 77, 235303 (2008)]

In our paper we have proposed a parametrization of the Abell-Tersoff potential for In, Ga, As, InAs and GaAs. This is to report corrections to the results presented there. Specifically, surface energies of non-stoichiometric surfaces, quoted for a specific value of the arsenic chemical potential, were in error due to inconsistent usage of the As chemical potential. The correct surface energies as calculated with the previously published parametrization T1,1 T2,2 T3,3 T4,4 T5,5 T6,6 T7,7 and our parametrization (denoted as T9) are reported in Tables I and II, replacing the according entries in Tables VII to X of our paper. The surface energies of stoichiometric surface reconstructions and the relaxation differences (||F0|| and ‹Δr›) given in these original Tables are not shown here as they are not affected by the inconsistent usage of the As chemical potential. The correct surface energies of our parametrization (T9) deviate from the DFT values since the inconsistent usage of the As chemical potential obstructed the fitting of parameters. In extension to Sec. III.C and Sec. III.D of our paper, we note that the relaxation of surface slabs was limited to 100 iterations and that T5 referred to a modified version of the parameters from Ref. 5 using Rcij=3.1 Å and Dcij=0.1 Å as cutoff parameters for the As-As interaction. These cutoff parameters effectively define a nearest-neighbour scheme in order to reproduce the results for the GaAs bulk phases presented in the original work.5 For potential parameters T8,8 we again report all surface energies. Previous results were ambiguous, because the cutoff parameters for the potential T8 had not been provided in Ref. 8. The new results reported in Table IV and V are obtained with the cutoff parameters listed in Table III

Analytic many-body potential for InAs/GaAs surfaces and nanostructures: Formation energy of InAs quantum dots

A parametrization of the Abell–Tersoff potential for In, Ga, As, InAs, and GaAs is presented by using both experimental data and results from density-functional calculations as input. This parametrization is optimized for the description of structural and elastic properties of bulk In, Ga, As, InAs, and GaAs, as well as for the structure and energy of several reconstructed low-index GaAs and InAs surfaces. We demonstrate the transferability to GaAs and InAs high-index surfaces and compare the results to those obtained with previously published parametrizations. Furthermore, we demonstrate the applicability to epitaxial InAs/GaAs films by comparing the Poisson ratio and elastic energy for biaxial strain, as obtained numerically with our potential and analytically from continuum-elasticity theory. Limitations for the description of point defects and surface diffusion are pointed out. This parametrization enables us to perform atomically detailed studies of InAs/GaAs heterostructures. The formation energy of InAs quantum dots on GaAs(001) obtained from our atomistic approach is in good agreement with previous results from a hybrid approach

Antireflective nanotextures for monolithic perovskite silicon tandem solar cells

Recently, we studied the effect of hexagonal sinusoidal textures on the reflective properties of perovskite silicon tandem solar cells using the finite element method FEM . We saw that such nanotextures, applied to the perovskite top cell, can strongly increase the current density utilization from 91 for the optimized planar reference to 98 for the best nanotextured device period 500 nm and peak to valley height 500 nm , where 100 refers to the Tiedje Yablonovitch limit. [D. Chen et al., J. Photonics Energy 8, 022601, 2018 , doi 10.1117 1.JPE.8.022601] In this manuscript we elaborate on some numerical details of that work we validate an assumption based on the Tiedje Yablonovitch limit, we present a convergence study for simulations with the finite element method, and we compare different configurations for sinusoidal nanotexture

Tradeoff breaking as a model of evolutionary transitions in individuality and limits of the fitness-decoupling metaphor

Evolutionary transitions in individuality (ETIs) involve the formation of Darwinian collectives from Darwinian particles. The transition from cells to multicellular life is a prime example. During an ETI, collectives become units of selection in their own right. However, the underlying processes are poorly understood. One observation used to identify the completion of an ETI is an increase in collective-level performance accompanied by a decrease in particle-level performance, for example measured by growth rate. This seemingly counterintuitive dynamic has been referred to as fitness decoupling and has been used to interpret both models and experimental data. Extending and unifying results from the literature, we show that fitness of particles and collectives can never decouple because calculations of fitness performed over appropriate and equivalent time intervals are necessarily the same provided the population reaches a stable collective size distribution. By way of solution, we draw attention to the value of mechanistic approaches that emphasise traits, and tradeoffs among traits, as opposed to fitness. This trait-based approach is sufficient to capture dynamics that underpin evolutionary transitions. In addition, drawing upon both experimental and theoretical studies, we show that while early stages of transitions might often involve tradeoffs among particle traits, later-and critical-stages are likely to involve the rupture of such tradeoffs. Thus, when observed in the context of ETIs, tradeoff-breaking events stand as a useful marker of these transitions

Inducing resistance: a summary of papers presented at the First International Symposium on Induced Resistance to Plant Diseases, Corfu, May 2000

The First International Symposium on Induced Resistance to Plant Diseases, organized by Eris Tjamos, brought together over 150 participants to discuss the complexities, questions and future direction of research on the mechanisms by which plants can become better able to defend themselves against pathogen attack. Although the term immunization has been used to denote treatments that enhance the defensive capacity of plants, the correspondence to vaccination in vertebrates is far-fetched: the induced state is by no means specific, but rather constitutes a more general increase in plant resistance to various types of pathogens. Moreover, it seldom prevents disease from occurring but generally reduces its extent or severity. These characteristics make induced resistance a powerful mechanism to exploit for enhancing the overall resistance in crop plants. Indeed, the first commercial chemical triggering induced resistance in plants, acibenzolar-Smethyl (BTH) was recently introduced on the market by Novartis under the tradenames Actigard (USA) and BION (Europe)

In vitro transition of Schistocephalus solidus (Cestoda) from coracidium to procercoid and from procercoid to plerocercoid

With the present study, a culture system for successive life-cycle stages of the tapeworm Schistocephalus solidus was developed and this report documents for the first time, cultivation of the procercoid stage of S. solidus from eggs. Additionally we have transformed procercoids dissected from experimentally infected copepods and cultured procercoids into the early plerocercoid stage in vitro. Observations in the culture suggest that the coracidia can interact with their external environment and need no host specific stimuli, except for the components in the culture medium, for activation and hatching from the embryophore. Increasing the culture medium pH from 7.3 to 8.0 improved escape rates and frequencies of hook contractions, suggesting that the oncosphere may recognize and respond to environmental conditions along the host intestine. Procercoids in the culture did not stop growing indicating that conditions within the copepod may be important to limit growth and to induce transformation to plerocercoids. When procercoids are dissected from copepods and transferred to the culture, the outer tegument layers and cercomer starts to loosen. Comparison of the lectin staining of the loosened outer tegument layers and cercomer in procercoids dissected from copepods confirms that transitions of both, the oncosphere to procercoid and procercoid to plerocercoids, has taken place in the in vitro cultures

Analytic many-body potential for GaAs(001) homoepitaxy: Bulk and surface properties

We employ atomic-scale simulation methods to investigate bulk and surface properties of an analytic Tersoff- Abell type potential for describing interatomic interactions in GaAs. The potential is a modified form of that proposed by Albe and colleagues [Phys. Rev. B 66, 035205 (2002)] in which the cut-off parameters for the As-As interaction have been shortened.With this modification, many bulk properties predicted by the potential for solid GaAs are the same as those in the original potential, but properties of the GaAs(001) surface better match results from first-principles calculations with density-functional theory (DFT). We tested the ability of the potential to reproduce the phonon dispersion and heat capacity of bulk solid GaAs by comparing it to experiment and the overall agreement is good. In the modified potential, the GaAs(001) β2(2 × 4) reconstruction is favored under As-rich growth conditions in agreement with DFT calculations. Additionally, the binding energies and diffusion barriers for a Ga adatom on the β2(2 × 4) reconstruction generally match results from DFT calculations. These studies indicate that the potential is suitable for investigating aspects of GaAs(001) homoepitaxy