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A long-range electric field solver for molecular dynamics of fluid-solid interfaces based on atomistic-to-continuum modeling

Templeton, Jeremy A.; Jones, Reese E.; Zimmerman, Jonathan A.; Wong, Bryan M.

Understanding charge transport processes at a molecular level using computational techniques is currently hindered by a lack of appropriate models for incorporating anisotropic electric fields, as occur at charged fluid/solid interfaces, in molecular dynamics (MD) simulations. In this work, we develop a model for including electric fields in MD using an atomistic-to-continuum framework. Our model represents the electric potential on a finite element mesh satisfying a Poisson equation with source terms determined by the distribution of the atomic charges. The method is verified using simulations where analytical solutions are known or comparisons can be made to existing techniques. A Calculation of a salt water solution in a silicon nanochannel is performed to demonstrate the method in a target scientific application.

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TYPE Conference YEAR 2010
OSTI

Towards improved CZT crystals

Ward, Donald K.; Doty, Fred P.; Wong, Bryan M.; Zimmerman, Jonathan A.

Past experimental efforts to improve CZT crystals for gamma spectrometer applications have been focused on reducing micron-scale defects such as tellurium inclusions and precipitates. While these micron-scale defects are important, experiments have shown that the micron-scale variations in transport can be caused by the formation and aggregation of atomic-scale defects such as dislocations and point defect clusters. Moreover, dislocation cells have been found to act as nucleation sites that cause the formation of large precipitates. To better solve the uniformity problem of CZT, atomic-scale defects must be understood and controlled. To this end, we have begun to develop an atomistic model that can be used to reveal the effects of small-scale defects and to guide experiments for reducing both atomic- and micron-scale (tellurium inclusions and precipitates) defects. Our model will be based upon a bond order potential (BOP) to enable large-scale molecular dynamics simulations of material structures at a high-fidelity level that was not possible with alternative methods. To establish how BOP improves over existing approaches, we report here our recent work on the assessment of two representative literature CdTe interatomic potentials that are currently widely used: the Stillinger-Weber (SW) potential and the Tersoff-Rockett (TR) potential. Careful examinations of phases, defects, and surfaces of the CdTe system were performed. We began our study by using both potentials to evaluate the lattice constants and cohesive energies of various Cd, Te, and CdTe phases including dimer, trimer, chain, square, rhomboid, tetrahedron, diamond-cubic (dc), simple-cubic (sc), body-centered-cubic (bcc), face-centered cubic (fcc), hexagonal-close-packed (hcp), graphite-sheet, A8, zinc-blende (zb), wurtzite (wz), NaCl, CsCl, etc. We then compared the results with our calculations using the density functional theory (DFT) quantum mechanical method. We also evaluated the suitability of the two potentials to predict the surface reconstructions and surface energies, various defect configurations and defect energies (interstitials and voids), elastic constants, and melting temperatures of different phases. We found that both potentials predicted incorrect energy trends as compared with those predicted by the DFT method. Most seriously, both potentials predicted incorrect lowest energy phases. These studies clearly showed that the existing potentials are not sufficient for correctly predicting the charge transport properties of CdTe demonstrating the need for a new potential. We anticipate that our BOP method will overcome this problem and will accelerate the discovery of a synthesis approach to produce improved CZT crystals.

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TYPE Conference YEAR 2010
OSTI
Results 126–150 of 192
Results 126–150 of 192