Molecular tribology, or nanotribology, gives us an atomic-scale understanding of the fundamental processes that take place when surfaces in relative motion interact. Researchers at LLNL and elsewhere need to know more about these processes to design and build many ultra- precise components, including optical devices, very smooth surfaces, and computer chips. We are applying a type of realistic computer modeling developed at LLNL, called molecular dynamics modeling, to study what happens when different materials, such as metals and glass, undergo cutting, grinding, cracking, and other processes associated with fabrication. We have found, for example, that both metals and ceramics behave in a ductile manner when we simulate machining on the nanometer length scale. However, the mechanisms underlying deformation are quite different in the two types of materials. Metals, such as copper, remain crystalline and deform through dislocation mechanisms. In contrast, covalent materials, such as silicon, transform into an amorphous state, which flows. We are applying such information to develop more practical engineering guidelines for researchers at LLNL and in the industrial community.
Our capabilities in theoretical and computational condensed-matter physics allow us to gain an atomic- level understanding of the structure, formation, and electronic properties of the surfaces and interfaces of a wide variety of materials. So far, we have simulated such experimentally observed phenomena as the relaxation of metal and semiconductor surfaces and the growth of metal- semiconductor interfaces. Recent applications of our work include the etching of tantalum with chlorine for thermal ink-jet printers, the deposition of molybdenum and silicon atoms for x-ray mirrors, and the metallization of thin films on semiconductor surfaces. As the computing environment grows and matures, we will be able to custom design materials at the atomic level solely from the identities of the atoms and the laws of quantum mechanics.
Scintillators, materials that emit short flashes of light in response to ionizing radiation, are used to detect high- energy radiation (charged particles, x rays, or gamma rays) from various sources. We are modeling the properties of these materials for the purpose of guiding the synthesis of new scintillators with improved detection capabilities. Our calculational tools include methods based on the local density approximation (LDA), such as pseudopotential and all-electron methods, and quasiparticle approaches. We have used our all-electron LDA method to calculate the atomic structure of barium fluoride and lead fluoride, both of which can exist in one of two phases (either cubic or orthorhombic) at low pressures. Our calculations have accurately reproduced the experimentally observed structural properties of these materials. We have also provided insight into their electronic properties. Ultimately, we want to calculate the energies of defect excitations. This is of great practical interest because the optical properties of a material can be tailored by introducing defect levels inside the fundamental band gap.
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