Thursday, September 7, 2017, 4:00 pm

Hosted by: ''Deyu Lu''

First-principles studies often rely on the assumption of equilibrium, which can be a poor approximation, e.g., for epitaxial growth. Here, we propose a general effective chemical potential (μ ¯) approach for non-equilibrium systems. It incorporates growth kinetics into the chemical potential, while maintaining its correct equilibrium limits. In studying molecular beam epitaxy (MBE), we divide the process into three stages: pre-nucleation, nucleation, island growth, and focus our efforts on the first two. For the pre-nucleation stage, we solve the rate equations for small clusters on the surface, which serve as the feedstock for the growth, and find that μ ¯ is determined by the most probable, rather than by the lowest-energy, clusters. While this finding contradicts the equilibrium theory (which is in favor of the lowest-energy state), it reinforces the fundamental principle of statistic mechanics. In the case of Bi2Se3, μ ¯ is found to be highly supersaturated. As μ ¯ determines the nucleation barrier for the nucleation stage, this supersaturation leads to a high nucleus concentration and small-sized islands, in qualitative agreement with experiment.

Thursday, October 5, 2017, 2:45 pm

Hosted by: 'Mircea Cotlet'

Thursday, November 2, 2017, 4:00 pm

Hosted by: ''Chang-Yong Nam''

We have been developing atomic layer deposition (ALD) nanocomposite coatings comprised of conducting, metallic nanoparticles embedded in an amorphous dielectric matrix. These nanocomposite films have proved to be exceptional as resistive coatings in solid-state electron multipliers, as charge drain coatings, and as solar absorbing films in concentrated solar power. All of these applications demand tunable properties so that particular attributes of the film, such as electronic resistivity, can be precisely tailored for maximum efficiency. In our films, the properties are tuned by adjusting the ratio of metallic and dielectric components. For example, nanocomposite films comprised of W:Al2O3 are prepared using alternating exposures to trimethyl aluminum (TMA) and H2O for the Al2O3 ALD and alternating WF6/Si2H6 exposures for the W ALD. By varying the ratio of ALD cycles for the W and the Al2O3 components in the film, we can tune precisely the resistance of these coatings over a very broad range from 1012-105 Ohm-cm. We have used this strategy to synthesize a broad range of ALD nanocomposites combining different metals and dielectrics. These nanocomposite coatings have been utilized to functionalize capillary glass array plates and fabricate large-area microchannel plates suitable for application in large-area photodetectors. In addition, we have applied these films to serve as charge drain coatings in micro electro-mechanical systems (MEMS) devices for a prototype electron beam lithography tool, and obtained high-resolution electron beam patterns without charging artifacts. We have also used these nanocomposite coatings to infiltrate porous scaffolds resulting in selective solar absorbing coatings with high visible absorption and low IR emittance suitable for power tower receivers in concentrated solar power.

Thursday, December 7, 2017, 10:30 am

Hosted by: 'Oleg Gang'