Browsing by Author "Huang, Zhi Feng"

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Honeycomb and triangular domain wall networks in heteroepitaxial systems
(2016-05-07) Elder, Ken R.; Chen, Z.; Elder, K. L M; Hirvonen, Petri; Mkhonta, S. K.; Ying, S. C.; Granato, E.; Huang, Zhi Feng; Ala-Nissilä, Tapio
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A comprehensive study is presented for the influence of misfit strain, adhesion strength, and lattice symmetry on the complex Moiré patterns that form in ultrathin films of honeycomb symmetry adsorbed on compact triangular or honeycomb substrates. The method used is based on a complex Ginzburg-Landau model of the film that incorporates elastic strain energy and dislocations. The results indicate that different symmetries of the heteroepitaxial systems lead to distinct types of domain wall networks and phase transitions among various surface Moiré patterns and superstructures. More specifically, the results show a dramatic difference between the phase diagrams that emerge when a honeycomb film is adsorbed on substrates of honeycomb versus triangular symmetry. It is also shown that in the small deformation limit, the complex Ginzburg-Landau model reduces to a two-dimensional sine-Gordon free energy form. This free energy can be solved exactly for one dimensional patterns and reveals the role of domains walls and their crossings in determining the nature of the phase diagrams.
Influence of dislocations in multilayer graphene stacks : A phase field crystal study
(2024-10) Elder, K. R.; Huang, Zhi Feng; Ala-Nissila, T.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
In this study, the influence of 5|7 dislocations in multilayer graphene stacks (up to six layers) is examined. The study is conducted using a recently developed phase-field crystal (PFC) model for multilayer systems incorporating out-of-plane deformations and parameterized to match to density functional theory calculations for graphene bilayers and other systems. The specific configuration considered consists of one monolayer containing four 5|7 dislocations (i.e., two dislocation dipoles) sandwiched between perfect graphene layers. This study reveals how the strain field from the dislocations in the defected layer leads to out-of-plane deformations, which in turn cause deformations of neighboring layers. Quantitative predictions are made for the defect-free energy of the multilayer stacks as compared to a defect-free system, which is shown to increase with the number of layers and system size. Furthermore, it is predicted that system defect energy saturates by roughly ten sheets in the stack, indicating the range of defect influence across the multilayer. Variations in stress field distribution and layer height profiles in different layers of the stack are also quantitatively identified.
Moiré patterns and inversion boundaries in graphene/hexagonal boron nitride bilayers
(2023-02) Elder, K. R.; Huang, Zhi Feng; Ala-Nissila, Tapio
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
In this paper a systematic examination of graphene/hexagonal boron nitride (g/hBN) bilayers is presented, through a recently developed two-dimensional phase field crystal model that incorporates out-of-plane deformations. The system parameters are determined by closely matching the stacking energies and heights of g/hBN bilayers to those obtained from existing quantum-mechanical density functional theory calculations. Out-of-plane deformations are shown to reduce the energies of inversion domain boundaries in hBN, and the coupling between graphene and hBN layers leads to a bilayer defect configuration consisting of an inversion boundary in hBN and a domain wall in graphene. Simulations of twisted bilayers reveal the structure, energy, and elastic properties of the corresponding moiré patterns and show a crossover as the misorientation angle between the layers increases from a well-defined hexagonal network of domain boundaries and junctions to smeared-out patterns. The transition occurs when the thickness of domain walls approaches the size of the moiré patterns and coincides with the peaks in the average von Mises and volumetric stresses of the bilayer.