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In this study, a molecular dynamics (MD) study has been performed on composition and temperature dependences of mechanical properties of CdTe 1 − x Se x (x = 0. 2 5 , 0.50 and 0.75) nanowires with a diameter of 6.93 nm. The simulation results show that CdTe 0. 7 5 Se 0. 2 5 nanowire seems to be more ductile, whereas CdTe 0. 2 5 Se 0. 7 5 nanowire seems to be more brittle at 1 K. Moreover, the temperature and composition exert significant effects on the mechanical properties of CdTeSe nanowires under stretching. We conclude that the dominancy of Se atoms yields a higher stability and strength at the lower temperature of 1 K, whilst it is the same for the nanowires with both higher Te and Se contents at the higher temperature of 300 K. The radial distribution functions (RDFs) have also been calculated for the CdTeSe nanowires based on the pair separation distance at 1 and 300 K. [ABSTRACT FROM AUTHOR]

Defects in multilayered films have long been a performance-limiting problem for the semiconductor industry. For instance, CdTe/CdS solar cell efficiencies have had significant improvement in the past 15 years or more without addressing the problem of high misfit dislocation densities. Overcoming this stagnation requires a fundamental understanding of interfacial defect formation. Herein, we demonstrate a new first principles-based CdTe bond-order approach that enables efficient molecular dynamics to approach the fidelity of density functional theory. Stringent quantum-mechanical verification and experimental validation tests reveal that our new approach provides an accurate prediction of defects that earlier methods cannot. Using this new capability, we show that misfit dislocations in CdTe/CdS multilayers can be significantly reduced via nano-patterning and composition grading and more importantly, dislocation-free multilayers naturally arise when the pattern dimension is reduced below 90 nm. Our predictive methods are generally applicable to other materials, highlighting a rational approach towards low-defect semiconductor films. [ABSTRACT FROM AUTHOR]

Helically coiled carbon nanotubes are modeled using topological coordinate method which is based on the toroidal triply connected graphs, containing pentagons, hexagons, and heptagons. Their regular incorporation into the hexagonal carbon net induces transition from the straight to the helical geometry. Relaxation of the structural model is performed in two steps: Firstly, molecular dynamics based on the Brenner potential is applied and then the coil parameters are, once again, optimized within symmetry preserving density functional tight binding (DFTB) method. Model of smooth regularly helically coiled single-walled nanotube structure is obtained. Correlations between the helical angle, tubular and helical diameter are found. Cohesive energy of the coiled structure is calculated by DFTB method within symmetry based POLSym code. Its dependence on the diameter of the coil is shown. The calculated energies range from 7.5 to 8.0 eV/atom. [ABSTRACT FROM AUTHOR]

Modeling atomic and molecular systems requires computation-intensive quantum mechanical methods such as, but not limited to, density functional theory [R. A. Friesner, Proc. Natl. Acad. Sci. USA, 102 (2005), pp. 6648-6653]. These methods have been successful in predicting various properties of chemical systems at atomistic scales. Due to the inherent nonlocality of quantum mechanics, the scalability of these methods ranges from O(N³)to O (N7) depending on the method used and approximations involved. This significantly limits the size of simulated systems to a few thousand atoms, even on large scale parallel platforms. On the other hand, classical approximations of quantum systems, although computationally (relatively) easy to implement, yield simpler models that lack essential chemical properties such as reactivity and charge transfer. The recent work of van Duin et al. [J. Phys. Chem. A, 105 (2001), pp. 9396-9409] overcomes the limitations of nonreactive classical molecular dynamics (MD) approximations by carefully incorporating limited nonlocality (to mimic quantum behavior) through an empirical bond order potential. This reactive classical MD method, called ReaxFF, achieves essential quantum properties, while retaining the computational simplicity of classical MD, to a large extent. Implementation of reactive force fields presents significant algorithmic challenges. Since these methods model bond breaking and formation, efficient implementations must rely on complex dynamic data structures. Charge transfer in these methods is accomplished by minimizing electrostatic energy through charge equilibration. This requires the solution of large linear systems (108 degrees of freedom and beyond) with shielded electrostatic kernels at each time-step. Individual time-steps are themselves typically in the range of tenths of femtoseconds, requiring optimizations within and across time-steps to scale simulations to nanoseconds and beyond, where interesting phenomena may be observed. In this paper, we present implementation details of sPuReMD (serial Purdue reactive molecular dynamics program), a unique reactive classical MD code. We describe various data structures, and the charge equilibration solver at the core of the simulation engine. This Krylov subspace solver relies on a preconditioner based on incomplete LU factorization with thresholds (ILUT), specially targeted to our application. We comprehensively validate the performance and accuracy of sPuReMD on a variety of hydrocarbon systems. In particular, we show excellent per-time-step time, linear time scaling in system size, and a low memory footprint. sPuReMD is a freely distributed software with GPL and is currently being used to model diverse systems ranging from oxidative stress in biomembranes to strain relaxation in Si-Ge nanorods. [ABSTRACT FROM AUTHOR]

A new semi-empirical potential for Fe based on the quantum chemistry concept of bond order has been developed. The potential consists of two parts: the repulsive short-range exponential potential, and the attractive potential, also of the exponential form, with a bond-order prefactor. The latter depends on angles between the Fe-Fe bonds, and uses the environmental parameter similar to that of the Tersoff bond-order potential for tetrahedrally bonded semiconductors. The bond order function (depending on the above environmental parameter), however, is of a more general form than that of the Tersoff potential. The new potential was calibrated using the traditional fitting to the Universal Scaling and the equilibrium volume and cohesive energy of BCC Fe. The introduced 'punishment functions' also directed the multi-variate minimization process towards minimizing the deviations between the calculated and experimental values of the elastic moduli C′ and C44, the energies of FCC and HCP Fe modifications, and the (111) free surface energy. With the total of 15 fitted parameters, the potential reproduces with only minor deviations the elastic moduli, the volume–pressure equation of states in BCC phase, the energies in FCC and HCP modifications, the BCC-HCP phase transformation under pressure, and the energy of the (111) free surface. Other tests of the new potential are being currently performed. The potential will be used in atomistic simulations of lattice stability, and deformation and chemisorption processes in Fe. [ABSTRACT FROM AUTHOR]