PREDICTION AND CONTROL OF ATOMIC SELF ASSEMBLY IN Al(110) HOMOEPITAXY
Open Access
- Author:
- Tiwary, Yogesh
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 21, 2009
- Committee Members:
- Kristen Ann Fichthorn, Dissertation Advisor/Co-Advisor
Kristen Ann Fichthorn, Committee Chair/Co-Chair
Costas D Maranas, Committee Member
Michael John Janik, Committee Member
Jorge Osvaldo Sofo, Committee Member - Keywords:
- aluminum
self assembly
directed assembly
homoepitaxy
kinetic monte carlo
first principles
quantum mechanical calculation
ab initio
transition state theory
nudged elastic band
atomic interaction
surface relaxation - Abstract:
- Controlled fabrication of nanostructures is a daunting challenge using conventional techniques. An attractive alternative for achieving this in an economical manner is the use of atomic self assembly. However, to achieve control over atomic self assembly and tailor surface morphologies on demand, we need to understand the inherent interplay between kinetics and thermodynamics. In order to achieve these objectives, we use multi-scale simulations to elucidate the mechanisms of self assembly in Al(110) homoepitaxy, in which, hut-shaped nanostructures or ``nanohuts" were observed in experiments cite{Mongeot03}. At the atomic scale, we use first-principles calculations to study the atomic interactions that govern assembly, and find that many-body effects are important. We propose a ``Connector Model", which is more accurate and efficient in representing high-order, many-body interactions than the traditional lattice-gas approach, and it may be suitable for describing a variety of surface phenomena, such as thin-film and crystal growth, adsorption, phase transitions, and catalysis at surfaces. Further, we elucidate diffusion mechanisms and quantify energy barriers for atomic diffusion on flat (110), (100), and (111) surfaces, (100) and (111) steps, and between the (100), (111) and (110) surfaces of Al. We uncover novel mechanisms for atomic diffusion in this system, including diagonal exchange on flat (110) surface, and three-atom diagonal exchange for inter-layer transport on the (100) and (111) steps. We find that the energy barriers for diffusion are significantly reduced under certain configurations of neighboring adatoms, which makes it easier for a dimer to climb the (100) and (111) facets than an isolated adatom. Finally, we incorporate the results obtained at the atomic scale in a 3-dimensional kinetic Monte Carlo model of homoepitaxial growth on Al(110). We use this to predict shapes and spatial organization of self-assembled nanostructures under different growth conditions, and explain the trends observed in experiments. We also demonstrate that thermal-field-directed self assembly can be employed to achieve uniform spatial distribution of nanostructures during Al(110) homoepitaxy, by using dual-beam or four-beam laser interference irradiation of the substrate. These results are promising and encourage experimental studies to explore the use of thermal-field-directed assembly for such systems.