Nam Le

Interlayer Interactions in van der Waals Heterostructures: Electron and Phonon Properties

Artificial van der Waals heterostructures constitute an emerging field that promises to design systems with properties on demand. Stacking patterns and the utilization of different types of chemically inert layers can deliver novel materials and devices. Despite the relatively weak van der Waals interaction, which does not affect the electronic properties around the Fermi level, our first-principles calculations show significant changes in the higher conduction and deeper valence regions in the considered graphene/silicene, graphene/MoS2, and silicene/MoS2 systems. Such changes are linked to strong out-of-plane hybridization effects and van der Waals interactions. We also find that the interface coupling significantly affects the vibrational properties of the heterostructures when compared to the individual constituents. Specifically, the van der Waals coupling is found to be a major factor for the stability of the system. The emergence of shear and breathing modes, as well as the transformation of flexural modes, are also found.

Folded graphene nanoribbons with single and double closed edges

Graphene nanoribbon folds with single and double closed edges are studied using density functional theory methods. Van der Waals dispersive interactions are included via semi-empirical pairwise optimized potential. The geometrical phases of the single and double folded ribbons are obtained. The electronic structure in terms of energy needed for the folding process, van der Waals contribution, energy band gaps, and bandstructures are also calculated. The results are interpreted in terms of peculiarities of the structures and dispersion interactions. It is shown that significant modifications in the electronic structure can be achieved as a result of folding.

Zigzag graphene nanoribbons with curved edges

Zigzag-edged single and double folded graphene nanoribbons are studied using density functional theory methods. Some asymmetric folds and folds with an octagon/hexagonal extended defect line are also considered. The long-range van der Waals interactions are taken into account via semiempirical pairwise optimized potential. The geometrical and magnetic phases of the studied structures are obtained. It is shown that the magnetic states of the folds depend strongly on their stacking patterns. The electronic structures in terms of energy needed for the folding process, van der Waals contribution, energy band gaps, band structures, and densities of states are also calculated. We find that significant changes in the electronic structure can be achieved as a result of folding and adding line defects.

Graphene nanoribbons anchored to SiC substrates.

Graphene nanoribbons are quasi-one-dimensional planar graphene allotropes with diverse properties dependent on their width and types of edges. Graphene nanoribbons anchored to substrates is a hybrid system, which offers novel opportunities for property modifications as well as experimental control. Here we present electronic structure calculations of zigzag graphene nanoribbons chemically attached via the edges to the Si or C terminated surfaces of a SiC substrate. The results show that the edge characteristics are rather robust and the properties are essentially determined by the individual nanoribbon. While the localized spin polarization of the graphene nanoribbon edge atoms is not significantly affected by the substrate, secondary energy gaps in the highest conduction and lowest valence region may emerge in the anchored structures. The van der Waals interaction together with the electrostatic interactions due to the polarity of the surface bonds are found to be important for the structure parameters and energy stability.