This award supports theoretical research and education into understanding the physical principles underlying processes in silica-water systems at macro- and nano-scale. The strength of silica-water interactions ranges from van der Waals to covalent depending on local mechanical and chemical conditions. External or internal stress also plays a vital role in modifying the interaction. The mechanical and optical properties of silica in aqueous environments and electronic structures at the water-silica interface are entangled. Large-scale computer simulations based on first principles are powerful tools for predicting materials behavior. This research develops theoretical descriptions of (1) stability of dry quartz surfaces and solid-to-liquid transition of water on silica surfaces, (2) surface defects and optical properties modified by adsorbed molecules or nano-clusters and water, and (3) silica nano-pores and channels filled with water and phenomena due to nanoconfinement and stress. The second phase of the research consists of (1) improving models for interfaces between quantum and classical regions, (2) constructing a computing architecture to integrate modern computer codes for parallel computing, and (3) implementing the quasi-particle self-consistent so-called GW method.
The effort undertaken has broader impacts with both scientific and educational consequences. The research includes implementing the GW method in a general public license (GPL) electronic structure code so that it is freely available to the larger scientific community. Educationally, students will receive training in the application of both analytical and computational skills and the research helps develop broad interest and skills in the frontiers of electronic structure calculations and multiscale modeling. Aspects of the research, particularly the underlying computational techniques, form part of the subject matter of the advanced physics course ''Physical modeling and simulation'' developed by the PI at her university.
NONTECHNICAL SUMMARY: This award supports theoretical research and education into understanding the physical principles underlying processes in silica-water systems. Silica is a simple molecule with one atom of silicon and two atoms of oxygen bound together. The material is common in nature being recognized as sand or quartz. It is a principal component of glass and substances such as concrete. Silica is the most abundant mineral in the earth's crust. Similarly, water is one of most abundant molecules on the Eath's surface. The proposal research helps understand the complex interactions between these two common materials. Researchers will use theoretical and computational tools to study the electronic structure of water on silica surfaces and will be able to investigate the chemical processes that then take place. Large-scale computer simulations based on first principles are powerful tools used for predicting the materials behavior.
The effort undertaken has broader impacts with both scientific and educational consequences. The research includes implementing the computational methods in a computer code that it is freely available to the larger scientific community. Educationally, students will receive training in the application of both analytical and computational skills and the research helps develop broad interest and skills in the frontiers of electronic structure calculations and multiscale modeling. Aspects of the research, particularly the underlying computational techniques, form part of the subject matter of the advanced physics course ''Physical modeling and simulation'' developed by the PI at her university.
Part I: Research The interaction between oxide materials and water under various physical conditions is an extremely important component in many physical processes. Both crystal growth and materials corrosion are affected greatly by the presence of water. Theoretical investigations based on largescale computational methods and high-performance computing facilities were carried out to investigate the effects of water on silicates, calcites and silica core-shell particles. Our research project has two major components: investigation of silica-water interface and method and software development. We studied stability of water-silica interface, isomorphs of quartz surface, mechanical and electronic properties of Ta2O5 and magnetism in graphene. We also developed a software OPAL-1.0, which is a computing architecture that can run multiple major programs simultaneously. Specifically, seven silica surfaces were [1-4] using quantum mechanical methods [5, 6]. We discovered a highly stable water (ice) bilayer structure on all the surfaces. It effectively shields the long-range interaction between the surface and outer-layer water. Unlike water-cluster interfaces, the collective nature of the hydrogen bond network leads to no change in the water film-surface energy in the presence of defects. For pure silica, we identified five isomer structures of the quartz (0001) surface [7]. In studies of tantala [8], an important dielectric material in optical coatings that is currently used together with silica, we calculated electronic states and the elastic tensor from the stress-strain relationship. [9, 10] We developed and applied the software OPAL to study NaCl dissociation in water. As an important component of our NSF/DMR project, we dedicated much effort to develop methods and codes for multi-scale and high-accuracy modeling and simulations. The concept of OPAL as a multi-task computing architecture based on MPI-2 was developed in the PI’s group [11] (Chao Cao, Ph.D. Thesis, December, 2008, University of Florida). OPAL allows simultaneous execution of multiple computer codes and data passing among codes. We integrated SIESTA [12] for quantum calculations and DL_POLY [13] for the classical region into the framework and studied NaCl dissociation in water. The uniqueness of our approach is there being more than one quantum regions, which is the case in all other multi-scale simulations involving high-level quantum calculations. OPAL1.0 will soon be released via GPL. Without going into details, we also studied reactions at interface of water and carbon nanotube (CNT) thin films and at interface of water and perovikites. These reactions are important for energy research. Part II Education and Broad Impact Our NSF project involved several Ph.D students (with some overlap in time) and two postdoctoral researchers over the last two years. During the funding period, four students from the PI’s group have graduated with Ph.D degrees. One of them, C. Cao (supported by this grant), is the recipient of the APS 2009 Nicholas Metropolis Award for Outstanding Doctoral Thesis Work in Computational Physics. Cao is a professor at Hangzhou University. The second (A.F. Kemper) is a name postdoc at NERSC after being at Standford University. The third (Y.-W. Chen) is a postdoc of Academia Sinica, Taiwan and the fourth one Yu-Ning Wu is during the transition period from my group to a postdoc position. NSF support had been critical for building my research group. 1. Chen, Y.-W. and H.-P. Cheng, Structure and stability of thin water films on quartz surfaces. Applied Physics Letters, 2010. 97(16): p. 161909 2. Chen, Y.-W. and H.-P. Cheng, Interaction between water and defective silica surfaces. Journal of Chemical Physics, 2011. 134(11). 3. Chen, Y.-W., I.-H. Chu, Y. Wang, and H.-P. Cheng, Water thin film-silica interaction on alpha -quartz (0001) surfaces. Physical Review B, 2011. 84(15): p. 155444. 4. Chen, Y.-W., C. Cao, and H.-P. Cheng, Finding stable alpha-quartz (0001) surface structures via simulations. Applied Physics Letters, 2008. 93(18): p. 181911. 5. Kohn, W. and L.J. Sham, Self-Consistent Equations Including Exchange and Correlation Effects. Physical Review, 1965. 140(4A): p. 1133-&. 6. Perdew, J.P., K. Burke, and M. Ernzerhof, Generalized gradient approximation made simple. Physical Review Letters, 1996. 77(18): p. 3865-3868. 7. Chen, Y.W., C. Cao, and H.P. Cheng, Finding stable alpha-quartz (0001) surface structures via simulations. Applied Physics Letters, 2008. 93(18). 8. Wu, Y.-N., L. Li, and H.-P. Cheng, First-principles studies of Ta(2)O(5) polymorphs. Physical Review B, 2011. 83(14): p. 144105. 9. Paier, J., M. Marsman, K. Hummer, G. Kresse, I.C. Gerber, and J.G. Angyan, Screened hybrid density functionals applied to solids. Journal of Chemical Physics, 2006. 124(24): p. 154709. 10. Heyd, J., G.E. Scuseria, and M. Ernzerhof, Hybrid functionals based on a screened Coulomb potential. Journal of Chemical Physics, 2003. 124(18): p. 8207. 11. Cao, C., Y.-W. Chen, Y.-N. Wu, E. Deumens, and H.-P. Cheng, OPAL: A multi-scale multi-center simulation package based on MPI-2 protocol. International Journal of Quantum Chemistry, 2011. 111(15): p. 4020. 12. Universidad-Autónoma-de-Madrid, www.icmab.es/siesta/, current. 13. Smith, W., www.cse.clrc.ac.uk/msi/software/DL_POLY/.
