Size-dependent and strain rate effects in quantum dot nanostructures with molecular dynamic simulations

Size and strain rate effects are among several factors which play an important role in determining the response of nanostructures, such as their deformations, to the mechanical loadings. The mechanical deformations in nanostructure systems at finite temperatures are intrinsically dynamic processes. Most of the recent works in this context have been focused on nanowires [1, 2], but very little attention has been paid to such low dimensional nanostructures as quantum dots (QDs). In this contribution, molecular dynamics (MD) simulations with an embedded atom potential method(EAM) are carried out to analyse the size and strain rate effects in the silicon (Si) QDs, as an example. We consider various geometries of QDs such as spherical, cylindrical and cubic. We choose Si QDs as an example due to their major applications in solar cells and biosensing. The analysis has also been focused on the variation in the deformation mechanisms with the size and strain rate for Si QD embedded in a matrix of SiO2 [3] (other cases include SiN and SiC matrices).It is observed that the mechanical properties are the functions of the QD size, shape and strain rate as it is in the case for nanowires [2]. We also present the comparative study resulted from the application of different EAM potentials in particular, the Stillinger-Weber (SW) potential, the Tersoff potentials and the environment-dependent interatomic potential (EDIP) [1]. Finally, based on the stabilized structural properties we compute electronic bandstructures of our nanostructures using an envelope function approach and its finite element implementation.

Compiler-Directed Frequency and Voltage Scaling for a Multiple Clock Domain

Multiple Clock Domain processors provide an attractive solution to the increasingly challenging problems of clock distribution and power dissipation. They allow their chips to be partitioned into different clock domains, and each domain’s frequency (voltage) to be independently configured. This flexibility adds new dimensions to the Dynamic Voltage and Frequency Scaling problem, while providing better scope for saving energy and meeting performance demands. In this paper, we propose a compiler directed approach for MCD-DVFS. We build a formal petri net based program performance model, parameterized by settings of microarchitectural components and resource configurations, and integrate it with our compiler passes for frequency selection.Our model estimates the performance impact of a frequency setting, unlike the existing best techniques which rely on weaker indicators of domain performance such as queue occupancies(used by online methods) and slack manifestation for a particular frequency setting (software based methods).We evaluate our method with subsets of SPECFP2000,Mediabench and Mibench benchmarks. Our mean energy savings is 60.39% (versus 33.91% of the best software technique)in a memory constrained system for cache miss dominated benchmarks, and we meet the performance demands.Our ED2 improves by 22.11% (versus 18.34%) for other benchmarks. For a CPU with restricted frequency settings, our energy consumption is within 4.69% of the optimal.

Size dependent microstructure for Ag-Ni nanoparticles

The Ag-Ni system is characterized by large differences in atomic sizes (14%) and a positive heat of mixing (+23 kJ mol(-1)). The binary equilibrium diagram for this system therefore exhibits a large miscibility gap in both solid and liquid state. This paper explores the size-dependent changes in microstructure and the suppression of the miscibility gap which occurs when free alloy particles of nanometer size are synthesized by co-reduction of Ag and Ni metal precursors. The paper reports that complete mixing between Ag and Ni atoms could be achieved for smaller nanoparticles (<7 nm). These particles exhibit a single-phase solid solution with face-centered cubic (fcc) structure. With increase in size, the nanoparticles revealed two distinct regions. One of the regions is composed of pure Ag. This region partially surrounds a region of fcc solid solution at an early stage of decomposition. Experimental observations were compared with the results obtained from the thermodynamic calculations, which compared the free energies corresponding to a physical mixture of pure Ag and Ni phases and a fcc Ag-Ni solid solution for different particle sizes. Results from the theoretical calculations revealed that, for the Ag-Ni system, solid solution was energetically preferred over the physical mixture configuration for particle sizes of 7 nm and below. The experimentally observed two-phase microstructure for larger particles was thus primarily due to the growth of Ag-rich regions epitaxially on initially formed small fcc Ag-Ni nanoparticles. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Direct mdct domain psychoacoustic modeling

We extend the recently proposed spectral integration based psychoacoustic model for sinusoidal distortions to the MDCT domain. The estimated masking threshold additionally depends on the sub-band spectral flatness measure of the signal which accounts for the non- sinusoidal distortion introduced by masking. The expressions for masking threshold are derived and the validity of the proposed model is established through perceptual transparency test of audio clips. Test results indicate that we do achieve transparent quality reconstruction with the new model. Performance of the model is compared with MPEG psychoacoustic models with respect to the estimated perceptual entropy (PE). The results show that the proposed model predicts a lower PE than other models.

Effect of oxygen pressure on the grain and domain structure of polycrystalline 0.85PbMg(1/3)Nb(2/3)O(3)-0.15PbTiO(3) thin films studied by scanning probe microscopy

0.85PbMg(1/3)Nb(2/3)O(3)-0.15PbTiO(3) ferroelectric-relaxor thin films have been deposited on La(0.5)nSr(0.5)CoO(3)/(1 1 1) Pt/TiO(2)/SiO(2)/Si by pulsed laser ablation at various oxygen partial pressures in the range 0.05 to 0.4 Torr. All the films have a rhombohedral perovskite structure. The grain morphology and orientation are drastically affected by the oxygen pressure, studied by x-ray diffraction and scanning electron microscopy. The domain structure investigations by dynamic contact electrostatic force microscopy have revealed that the distribution of polar nanoregions and their dynamics is influenced by the grain morphology, orientation and more importantly, oxygen vacancies. The correlation length extracted from autocorrelation function images has shown that the polarization disorder decreases with oxygen pressure up to 0.3 Torr. The presence of polarized domains and their electric field induced switching is discussed in terms of internal bias field and domain wall pinning. Film deposited at 0.4 Torr presents a curious case with unique triangular grain morphology and large polarization disorder.

Modulation of Catalytic Activity in Multi-Domain Protein Tyrosine Phosphatases

Signaling mechanisms involving protein tyrosine phosphatases govern several cellular and developmental processes. These enzymes are regulated by several mechanisms which include variation in the catalytic turnover rate based on redox stimuli, subcellular localization or protein-protein interactions. In the case of Receptor Protein Tyrosine Phosphatases (RPTPs) containing two PTP domains, phosphatase activity is localized in their membrane-proximal (D1) domains, while the membrane-distal (D2) domain is believed to play a modulatory role. Here we report our analysis of the influence of the D2 domain on the catalytic activity and substrate specificity of the D1 domain using two Drosophila melanogaster RPTPs as a model system. Biochemical studies reveal contrasting roles for the D2 domain of Drosophila Leukocyte antigen Related (DLAR) and Protein Tyrosine Phosphatase on Drosophila chromosome band 99A (PTP99A). While D2 lowers the catalytic activity of the D1 domain in DLAR, the D2 domain of PTP99A leads to an increase in the catalytic activity of its D1 domain. Substrate specificity, on the other hand, is cumulative, whereby the individual specificities of the D1 and D2 domains contribute to the substrate specificity of these two-domain enzymes. Molecular dynamics simulations on structural models of DLAR and PTP99A reveal a conformational rationale for the experimental observations. These studies reveal that concerted structural changes mediate inter-domain communication resulting in either inhibitory or activating effects of the membrane distal PTP domain on the catalytic activity of the membrane proximal PTP domain.