Localized Corrosion of Passive Metals
R. C. Alkire,* E. Webb, M. Knight, T. Pricer, L. Zhu, J. Gray, D. Veyret, M. Georgiadou, F. Topin, T. Suter, C. Paik, J. Ganley
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

The materials investigated in this study are those metals and alloys which tend spontaneously to form protective surface layers and become thereby susceptible to localized corrosion when the protective layers are disturbed: Al, Ni, Cu, and their alloys including stainless steels. Special attention is given to integration of the experimental approach with a Web-based modeling approach that accounts for transport, kinetic, and thermodynamic phenomena over a wide range of time- and length scales. Topics under current study include local breakdown in pits, crevices, and cracks.

Use of Very High Pressure to Investigate the Structure of Matter
H. G. Drickamer,* G. Yang, Y. Li
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory and the Departments of Physics and Chemistry)

The basic thesis of our research is that high pressure is an essential tool for understanding electronic phenomena in condensed systems. With increasing compression there is increased overlap among electronic orbitals. Different types of orbitals are perturbed to different degrees. A study of these perturbations permits one to characterize electronic states and excitations, to test theories, and, under some circumstances, to induce electronic transitions to new ground states. Our current work involves: (1) Nonlinear optical phenomena at high pressure including both second harmonic generation and two photon processes. (2) The relation between steady-state emission data and kinetic data for molecules with excited states with two different geometries. (3) Pressure effects on light-emitting polymers potentially useful as light-emitting diodes.

Photochromic Self-assembled Surfaces Formed Using Polypeptides
V. K. Gupta,* A. Finnell
University of Illinois; National Science Foundation, CTS 9732691

This research explores molecular-level principles for photocontrol of the optical and the interfacial properties of self-assembled monolayers. Towards the goal of enhancing the photostimulated response we are exploring the use of photoresponsive polymers such as a-helical polypeptides because these biological macromolecules are structurally anisotropic and possess macrodipoles. By establishing principles based on which structure and organization of surfaces can be engineered for optimal control of physico-chemical properties, the proposed research will permit light-assisted manipulation of adsorption-desorption of biomolecules on surfaces, wetting-dewetting of polar or nonpolar fluids, and optical anisotropy such as birefringence or dichroism in thin films.

Chemical Selectivity of Organized Assemblies of Macrocycles on Solid Substrates
V. K. Gupta,* J. Faull
University of Illinois; National Science Foundation, CTS 9732691

Chemically selective surfaces are essential to chemical and biochemical sensing as well as in new processes for chemical purification/separation. This research centers on surfaces that contain immobilized macrocycles as model receptor molecules with a potential for complexing with organic adsobates in solution via guest-host interactions. The proposed research addresses the need for understanding how characteristics such as surface density of receptor molecules, steric barriers to binding or molecular flexibility of receptor chains can be manipulated to enhance the guest-host binding. Optimization of the guest-host complexation properties will facilitate new analytical/diagnostic procedures and applications where catalytic activity can be confined to an interface through guest-host complexation.

Ion- and Photon-enhanced Surface Diffusion
E. G. Seebauer,* R. Ditchfield
National Science Foundation, CTS 98-06329

Our recent measurements of surface diffusion on Si have demonstrated that both photon illumination and low-energy ion bombardment can significantly alter surface diffusion on Group IV semiconductors. Neither effect has ever been observed directly before. Photon-induced modifications seem to be mediated electronically, as substrate doping affects the results. Ion-induced modifications clearly involve some sort of momentum transfer. Both effects can have direct implications for semiconductor processing.

Laser Measurements of Thermal Surface Diffusion
E. G. Seebauer,* R. Ditchfield, D. Llera-Rodriquez
National Science Foundation, CTS 95-06419; U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Surface diffusion on semiconductors is important in several aspects of microelectronic device fabrication. We are making measurements of surface diffusion under real processing temperatures and pressures using our recently developed laser technique of second harmonic microscopy. Under such conditions, we find that the diffusion mechanism changes from simple site hopping to a previously unknown vacancy-mediated form. We are probing surface diffusion in a variety of adsorption systems to determine the precise nature of this mechanism.

Simulations of Surface Diffusion by Molecular Dynamics
E. G. Seebauer,* R. Ditchfield, Z. Wang
National Science Foundation, CTS 95-06419

We are performing computer simulations of surface diffusion on silicon and germanium by molecular dynamics. This approach uses selected interatomic potentials and integrates the equations of motion for an ensemble of surface atoms. We have shown good correspondence between the simulational results and experiments for Ge on Si. We are now examining the effects of low-energy ion bombardment.

Chemical Vapor Deposition of Titanium Silicide
E. G. Seebauer,* Z. Wang
Sematech

Chemical vapor deposition of titanium silicide is being investigated for metallizing future generations of integrated circuits. Based on ultrahigh-vacuum kinetic studies, we have developed a quantitative predictive model for growth, and have confirmed potential growth conditions in real deposition experiments. This work represents the first such optimization performed based on fundamental kinetic surface studies in any adsorption system of practical interest. Work now focuses on bringing the process into suitable form for large-scale production.

Chemical Vapor Deposition of Cobalt Silicide
E. G. Seebauer,* Z. Wang, Y. Jung
Sematech

The chemical vapor deposition of cobalt silicide represents a potentially attractive way to form metallization contracts to transistors in under future design rules for silicon-based integrated circuits. We are investigating the feasibility of this technology from the perspective of surface chemistry, with a focus on issues of substrate consumption and selectivity with respect to silicon oxides.

Flow of Weakly Flocculated Suspensions
C. F. Zukoski,* S. J. Yoon
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

In this investigation we examine the flow properties of weakly flocculated suspensions. A model system has been chosen in which, by solution pH, the suspension can be reversibly gelled. By mapping out a phase boundary in pH/volume fraction space, we are able to explore the relationship between flocculation in colloidal suspensions and sol-gel transitions observed in molecular systems. The mechanical properties of the gelled samples are of importance in determining porosity and suspension processibility. We are currently seeking general descriptions of yielding and flow in terms of the depth of the interparticle attractive potential.

Flow of Aqueous Polyurethane Dispersions
C. F. Zukoski, G. Flickenger
U.S. Department of Energy, DE-FG02-96ER45439; 3M Corp. (In cooperation with the Materials Research Laboratory)

Aqueous polyurethane dispersions are of growing significance in the castings technologies where there is a need to reduce volatile organic emissions. In this study, we characterize dispersion flow properties as a function of ionic strength and polyurethane weight fraction.

Formation of Silver Sols
C. F. Zukoski,* D. Van Hyning
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

The mechanisms of formation of silver sols produced by the sodium borohydride reduction of silver perchlorate are explored. These studies suggest the silver ions are rapidly reduced resulting in 1-2 nm particles which aggregate over a substantially longer period of time to produce uniform 10-15 nm silver particles. Factors controlling aggregation include ionic strength and borohydride concentration. Detailed kinetic experiments are used to develop a predictive growth model.

Structure and Rheology of Bimodel Suspensions of Uniform Spheres
C. F. Zukoski,* W. Hunt
U.S. Department of Energy, DE-AC02-96ER45439 (In cooperation with the Materials Research Laboratory)

The structure and flow of dense suspensions of uniform particles is investigated with particular attention paid to how materials flow at high-volume fraction. Methods of achieving flowable suspensions at volume fractions above 0.6 are sought through the use of bimodel mixtures of particles. Effects of particle size ratio and number ratio are studied. Small-angle neutron scattering studies are used to characterize microstructures at rest and under shear flow.

Assembly of Nanoparticles
C. F. Zukoski,* S. Ramakrishnan
U.S. Department of Energy, DE-AC02-96ER45439 (In cooperation with the Materials Research Laboratory)

Methods of manipulating the interactions of 1-nm particles are explored. The strength of attraction of heteropolymetal oxyanions controlled through ionic strength and characterized through light scattering and viscosity. Links between the strength of attraction and anion solubility are studied. Methods of building nanoporous structures of these superacid particles are developed.

Platelet Orientation in the Flow of Dense Suspensions
C. F. Zukoski,* S. Jogun
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

The orientation of clay particles has been investigated as a funtion of shear rate and packing fraction using wide-angle x-ray scattering and conductivity. These results are used to confirm predictions developed for dilute suspensions. The influence of particle orientation on the flow of dense suspensions has been subject to theoretical or experimental investigation. Our studies demonstrate a limited volume fraction sensitivity of the fractional degree of orientation at a given shear rate.

Drying and Cracking in Ceramic Fabrication
C. F. Zukoski,* L. Brown
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Understanding the stresses built up in ceramic green bodies is essential to producing low-flaw-density ceramics. In this study, we look at the role of interparticle forces in determining rates of drying and compaction in slurries. Conditions resulting in cracking are explored with the goal of generating dense, uniform, crack-free green bodies.