Hawa was invited as a seminar speaker to the XVIII IMRC (International Materials Research Congress) 2009 held at Cancun, Mexico August 16 - 21. He gave a seminar talk in Nanostructured Materials and Nanotechnology Symposium, and His talk was highlighted by MRS eNews on August 19.

Following a citation from MRS eNews:

TALKS
Symposium 1. Nanostructured Materials and Nanotechnology

Nanoparticle Synthesis and Assembly from Atomistic Simulation Studies

Nanoparticles are involved in 75% of today's chemical manufacturing processes (tires, toners, food, pharmaceuticals, waste, cosmetics, etc.), and controlled synthesis of these nanoparticles is critical for commercial success. Nanoparticles can be synthesized in a liquid phase, allowing for good control of particle size control but low production rate, or in a vapor phase, allowing for high productivity rates but has significant difficulties in controlling particle size. T. Hawa (University of Oklahoma) investigates synthesis variables on the effects of final nanoparticle physical properties by molecular dynamics simulations.

Vapor synthesis begins with a chemical precursor vapor that eventually breaks down and nucleates nanoparticles on a substrate. Final particle size is a balance particle-particle interactions: coalescence, where two particles combine to form one larger particle; and collision, where two particles collide to form one agglomerate. Hydrogen passivation is a common technique to tie up dangling bonds on the surface of nanoparticles and thus reduce their reactivity, and Hawa seeks to understand the effects of passivation on the balance of coalescence and collision during particle synthesis. Particle interaction is proportional to the contact surface area, and this data can be extrapolated to approximate the critical approach energy for the reaction. If the particle energy is lower than the critical reaction energy, then the particles will not react; if the energy is higher, then they will. Hawa also found that increasing particle temperature and decreasing particle size increases the probability of particle reaction. An equation for the coalescence time was obtained, and Hawa finds that hydrogen stays on the surface of the particle throughout coalescence. Effects of hydrogen passivation on the dynamics of sintering of individual particles and particle aggregates, as well as the stability of nanoparticles facets (from which the final particle shape can be predicted) were also investigated and compared to experimental results in the literature.

Nanoparticle assembly is one of the largest challenges in nanotechnology-based device development, and Hawa investigated electrostatic-directed assembly, both experimentally and by modeling. Electrostatic-directed assembly, where substrates have charged patterns that attract oppositely-charged particles, is a common and robust assembly technique. Hawa employed a GaAs substrate with regions of Si- and Zn-doping which formed p-n junctions. When a voltage was applied across the junction, charged particles were observed to collect on oppositely-charged regions of the substrate. Particle trajectory over like-charged substrate regions was modeled to determine the effect of the electric field to control particle deposition location.

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