22 research outputs found
Optimal design and quantum limit for second harmonic generation in semiconductor heterostructures
The optimal design for infrared second harmonic generation (SHG) is
determined for a GaAs-based quantum device using a recently developed genetic
approach. Both compositional parameters and electric field are simultaneously
optimized, and the quantum limit for SHG, set by the trade-off between large
dipole moments (favouring electron delocalization) and large overlaps
(favouring electron localization), is determined. Optimal devices are generally
obtained with an asymmetric double quantum well shape with narrow barriers and
a graded region sideways to the largest well. An electric field is not found to
lead to improved SHG if compositional parameters are optimized.Comment: 5 pages, 2 figures embedded. To apper in J. App. Phys. (Jan 2nd,
2001
Multiscale in modelling and validation for solar photovoltaics
Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared to other sources. The optimization of these aspects involves the exploitation of new materials and development of novel solar cell concepts and designs. Both theoretical modeling and characterization of such devices require a comprehensive view including all scales from the atomic to the macroscopic and industrial scale. The different length scales of the electronic and optical degrees of freedoms specifically lead to an intrinsic need for multiscale simulation, which is accentuated in many advanced photovoltaics concepts including nanostructured regions. Therefore, multiscale modeling has found particular interest in the photovoltaics community, as a tool to advance the field beyond its current limits. In this article, we review the field of multiscale techniques applied to photovoltaics, and we discuss opportunities and remaining challenges
The Large Scale Conformational Change of the Human DPP III–Substrate Prefers the “Closed” Form
Human dipeptidyl peptidase III (DPP III) is a two domain
metallo-peptidase
from the M49 family. The wide interdomain cleft and broad substrate
specificity suggest that this enzyme could experience significant
conformational change. Long (>100 ns) molecular dynamics (MD) simulations
of DPP III revealed large range conformational changes of the protein,
suggesting the <i>pre-existing equilibrium</i> model for
a substrate binding. The binding free energy calculations revealed
tighter binding of the preferred synthetic substrate Arg–Arg–2–naphtylamide
to the “closed” than to the “open” DPP
III conformation. Our assumption that Asp372 plays a crucial role
in the large scale interdomain closure was proved by the MD simulations
of the Asp372Ala variant. During the same simulation time, the variant
remained more “open” than the wild type protein. Apparently,
Ala was not as efficient as Asp in establishing the interdomain interactions.
According to the MM–PBSA calculations, the electrostatic component
of the free energy of solvation turned out to be higher for the “closed”
protein than for its less compact form. However, the gain in entropy
due to water released from the interdomain cleft nicely balanced this
negative effect
A Three- Dimensional Quantitative Structure Activity Relationship (3D-QSAR) Model for Predicting the Enantioselectivity for Candida Antarctica Lipase B
Computational techniques involving molecular modeling coupled with multivariate statistical analysis were used to evaluate and predict quantitatively the enantioselectivity of lipase B from Candida antarctica (CALB). In order to allow the mathematical and statistical processing of the experimental data largely available in the literature (namely enantiomeric ratio E), a novel class of GRID-based molecular descriptors was developed (differential molecular interaction fields or DMIFs). These descriptors proved to be efficient in providing the structural information needed for computing the regression model. Multivariate statistical methods based on PLS (partial least square \u2013 projection to latent structures), were used for the analysis of data available from the literature and for the construction of the first three-dimensional quanititative structure-activity relationship (3D-QSAR) model able to predict the enantioselectivity of CALB. Our results indicate that the model is statisticall
Visible Spectrum Quantum Light Sources Based on In<sub><i>x</i></sub>Ga<sub>1–<i>x</i></sub>N/GaN Quantum Dots
We present a method for designing
quantum light sources, emitting
in the visible band, using wurtzite In<sub><i>x</i></sub>Ga<sub>1–<i>x</i></sub>N quantum dots (QDs) in a
GaN matrix. This system is significantly more versatile than previously
proposed arsenide- and phosphide-based QDs, having a tuning range
exceeding 1 eV. The quantum mechanical configuration interaction method,
capturing the fermionic nature of electrons and associated quantum
effects explicitly, is used to find shapes and compositions of dots
to maximize the excitonic dipole matrix element and optimize the biexciton
binding energy. These results provide QD morphologies tailored for
either bright single-photon emission or entangled-photon-pair emission
at any given wavelength in the visible spectrum