The University of Massachusetts Amherst
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Lin Du Ph.D. Defense, “Driven Morphological Evolution of Crystal Surfaces, Epitaxial Thin Films, and Two-Dimensional Materials: Morphological Stability and Pattern Formation”


Monday, November 20, 2017 - 2:00pm


Gunness Student Center inside Marcus Hall



Mass transport in solid materials driven by externally applied fields, such as mechanical stresses, electric fields, and temperature gradients, can cause morphological instabilities, leading to failure of materials used in electronic and optoelectronic devices. However, properly controlled applied fields can also stabilize planar surface morphology, reduce surface roughness, and drive the formation of intriguing nanoscale morphological features, providing a path toward precise nanopatterning for the development of electronic and photonic materials with optimal functionality. 
Toward this end, we have studied the surface morphological evolution of stressed crystalline solids and thin films based on a continuum model of driven surface mass transport that accounts for stresses, electric fields, temperature gradients, surface energy, wetting potential, and surface diffusional anisotropy. Based on linear stability analysis and self-consistent dynamical simulations, we found that long-wavelength plane-wave perturbations from the planar surface of a uniaxially stressed solid can trigger not only the Asaro-Tiller/Grinfeld (ATG) instability but also a nonlinear tip-splitting instability, while sufficiently strong and well controlled electric fields and thermal gradients can alone or synergistically stabilize the planar surface morphology. For conducting thin films with nanoscale surface roughness, we established the electrical stressing of the films as a viable physical processing strategy for surface roughness reduction and optimized this strategy to minimize the electric field strength requirement. For heteroepitaxial thin films, we found that burying quantum dot (QD) arrays in the corresponding substrates can be used to engineer the initial surface morphological perturbation of the strained epitaxial film in order to form quantum dot molecules (QDMs) by design. We also found that thermal annealing of epitaxial QDs can induce additional stress due to thermal mismatch, leading to further morphological evolution of the QDs and their transformation to nanorings, or multiple concentric nanorings, and eventually to multiple QDs. The study provides a promising way to stabilize the planar surface morphology and smoothen the surface of thin films, and also sets the stage for precise engineering of tunable-size nanoscale surface features in strained thin film growth by exploiting film surface nonlinear pattern forming phenomena. 
Moreover, we have conducted a systematic analysis of pore-edge interactions in graphene nanoribbons (GNRs) using first-principles density functional theory (DFT) calculations, as well as molecular-statics (MS) and molecular-dynamics (MD) simulations based on reliable interatomic potentials. We identified and parameterized the strongly attractive pore-edge interactions for nanopores in the vicinity of GNR edges, which can drive nanopores to migrate toward and coalesce with the GNR edges. The post-coalescence morphological evolution of an armchair GNR edge leads to the formation of a V-shaped edge pattern consisting of zigzag linear segments (facets). DFT calculations show that the zigzag segments forming at the armchair edges can be used to tune the electronic band structure of the GNR. The bandgap of the patterned GNRs exhibits a linear dependence on the density of the zigzag edge atoms, which is controlled by the size and concentration of the pores introduced in the defect-engineered GNR. 
The public is cordially invited to attend. Refreshments will be served.

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