Molecular Dynamics Simulation Of Thermal Processes For Selected Nano-Structures
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Date
2018-09
Authors
Min Tjun Kit
Journal Title
Journal ISSN
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Publisher
Universiti Sains Malaysia
Abstract
The core premise of this thesis is the adoption of molecular dynamics (MD) in simulating and measuring three different nanoscale systems. namely (i) epitaxial graphene growth on 6H-SiC (0001) surface induced by simulated annealing, (ii) free-standing silicene subjected to extensive thermal heating, and (iii) wurtzite ZnO slab which is subjected to simulated annealing. Epitaxial growth of graphene on the (0001) surface of 6H-SiC substrate is simulated via molecular dynamics using LAMMPS code. A specially designed protocol to reconstruct the surface via a simulated annealing procedure, is prescribed to simulate the epitaxial graphene formation on the substrate surface. Two empirical potentials, the Tersoff potential and the TEA potential are used in the MD simulations to investigate and compare the growth mechanisms resulted. Results obtained from MD simulated in this thesis show that TEA potential is more accurately in describing the growth process of graphene formation, in which the result is generally more physical and realistic. Graphene is shown in the MD simulation using TEA potential to be accurate at an annealing temperature of ๐โ1200 K, comparable to that observed in a reported experiment in which graphene nucleates at a pit-forming temperature of 1298 K. The numerical evaluation of the average bond-length, binding energy as well as pair correlation function in the MD experiments allows for the measurement and quantification of the graphene formed. Double and triple layer graphene can also be grown from the same substrate after the first layer of graphene is formed. The technique to grow double and triple layer graphene on top of the already-formed single layer graphene follows a similar but slightly modified procedure used in growing the first layer graphene. In addition to epitaxial graphene growth, MD simulations are also performed in this thesis to measure the melting temperature of free-standing silicene by using optimized Stillinger-Weber (SW) potential by Zhang et al.. The data are systematically analysed using a few qualitatively different indicators, including caloric curve, radial distribution function and a numerical indicator known as global similarity index. The optimized SW potential consistently yields a melting temperature of 1500 K for the simulated free-standing, infinite silicene. The third nanoscale system investigated in this thesis via MD is a thick wurtzite ZnO slab terminated in two surfaces, namely, (0001ฬ
) (which is oxygen terminated) and (0001) (which is Zn-terminated). The MD experiment is performed to measure the effect of thermal annealing on the ZnO slab. To this end, reactive force field (ReaxFF) is used. Is it observed that annealing results in the sublimation of surface oxygen atoms from the (0001ฬ
) surface at a threshold temperature range of 700 K < ๐๐กโค 800 K, while no atoms leave the (0001) surface. The ratio of oxygen leaving the surface increases with temperature ๐ (for ๐ โฅ ๐๐ก). The relative luminescence intensity of the secondary peak in the photoluminescence (PL) spectra, interpreted as a measurement of amount of vacancies on the sample surfaces, qualitatively agrees with the threshold behaviour as found in the MD simulations. The formation of oxygen dimers on the surface and evolution of partial charge distribution during the annealing process has also been depicted in the MD simulations. The MD simulations have also revealed the formation of oxygen dimers on the surface and evolution of partial charge distribution during the annealing process. The results from the MD simulations based on the ReaxFF are consistent with experimental observations.
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Keywords
Molecular dynamics , Nanostructures