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November 11, 2002
Volume 80, Number 45
CENEAR 80 45 p. 15
ISSN 0009-2347


COMPUTATIONAL CHEMISTRY

CALCULATING AT THE INTERFACE
Challenging interfacial properties yield to new theoretical methods

MITCH JACOBY

Using recently developed computational methods, researchers have demonstrated that properties of interfaces can be calculated accurately--giving excellent agreement with experimental results. This means that more complex chemical systems can now be investigated with theoretical tools, hopefully leading to greater understanding of interfacial bonding mechanisms.

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PROMISING Sandia theoreticians Jennison (left) and Mattsson apply computational methods to investigate properties of metal-metal oxide interfaces.
Thin films of one material on another--for example, metals on metal oxides--are at the heart of numerous applications in microelectronics, sensing, heterogeneous catalysis, and other areas. Yet despite the technological importance of such unions of materials, scientists do not have a firm handle on bond strengths and adhesion energies at interfaces. For example, quantum mechanical calculations of adhesion energies based on density functional theory (DFT) give widely varying results, depending on the particular form of computational method employed.

But now, Ann E. Mattsson and Dwight R. Jennison, surface scientists at Sandia National Laboratories, have shown that by using corrected mathematical forms of a metal and metal oxide's surface energy--a property that governs wetting and adhesion between dissimilar materials--calculated adhesion energies match experimentally measured values regardless of the particular DFT method used for computation.

Specifically, the team found that the energy of adhesion of palladium on -alumina determined from DFT-based "local density" and "generalized gradient" approximation methods--if corrected--agree with each other and with experimental data [Surf. Sci., 520, L611 (2002)]. The work builds on recent advances in computing surface energies made by Mattsson and Chemistry Nobel Laureate and University of California, Santa Barbara, chemistry professor Walter Kohn.

In a commentary published in the same issue of Surface Science, Gianfranco Pacchioni, a professor of materials science at the University of Milan, Italy, describes the work as a "new and important contribution." Explaining that bonding at surfaces can change from site to site because of defects, dopants, and variations in electronic and geometric structure, Pacchioni remarks that "it's no wonder that the description of the metal/oxide interface is a challenge for theory."

Charles T. Campbell, a professor of chemistry at the University of Washington, Seattle, says, "The theoretical success promises, at last, some understanding of the elusive bonding mechanism at such interfaces."



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