Han, X; (2015) Bandgap Engineering and Molecular Selectivity of 2D Nanostructures by First-Principles Simulations. Doctoral thesis , UCL (University College London).

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Abstract

Two-Dimensional (2D) materials often exhibit far more distinguished properties than their 3D counterparts and offer great potential to advance technologies. However, even graphene, the star of 2D materials, still face several challenges, despite its high mobility and high thermal conductivity. One of such challenges is the lack of a bandgap for use in electronics, photonics or photocatalysis. Here, we propose two approaches to tuning the bandgap: One is by vacancy and nitrogen substitution, and another by substrate interaction. Several vacancy/nitrogen configurations were considered in the study. One of the defective complexes, with 2 vacancies and 4 N atoms, can open up the bandgap to 0.27 eV. Diamond substrate of different orientations with or without hydrogen termination was employed to create a heterostructure with graphene. Our calculations indicate that the hydrogen treatment of the diamond surface plays an important role, and so is the surface orientation, in determining the size of the bandgap. A hydrogen-terminated diamond with (100) surface can tune the bandgap of graphene to 61 meV. This agrees well with collaborative experimental measurement of a similar system. Another newly discovered 2D material, phosphorene, was also investigated with a particular focus on the effects of nanostructuring, straining, and hybridization with graphene. Phosphorene nanoribbons (PNR) with different widths and orientations were considered. Significant quantum confinement causes the bandgap to vary substantially with ribbon width and orientation. Furthermore, the straining effect is also shown to alter the electronic properties dramatically. For a diagonally-cut nanoribbon (d-PNR), the direct to indirect bandgap switch-over occurred below -7% compression or beyond +3% extension. As phosphorene may degrade in the air, a graphene/phosphorene/graphene (G/P/G) sandwich heterostructure is designed and studied. The bandgap of this sandwich heterostructure is 19 meV, due to charge redistribution within the interlayers. The calculated dielectric constant show large " 4" directional variation, due to phosphorene’s puckered structure. This suggests G/P/G may be considered for birefringence optical applications. The defective graphene is also a strong contender for gas separation and H2 purification, which is also studied here. The results show that graphene with a vacancy cluster (pore-10) show exceptional selectivity for H2 at room temperature, while inhibiting many other gaseous molecules. Nitrogen-doping can attract more gas molecules to the pore area, increasing the propensity of “trapping” molecular impurities. In particular, a strong energy trap is generated when a CO2 molecule approaches the proposed nanopore, due to its relative strong quadruple moment interacting with N-doped carbon edge sites. A CH4 molecule needs to overcome a sequence of energy barriers in order to pass through the pore. Such interactions inhibit the impurity molecules and enhance the selectivity for H2 purification.

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