Abstract:
In the wake of the discovery of graphene, the search for new and remarkable 2D materials
with astounding electronic and mechanical properties has led to the fabrication of
germanene, a 2D germanium allotrope similar to silicene. Unlike the planar structure of
the graphene lattice, germanene has a buckled honeycomb structure with two vertically
displaced sublattices. Free-standing germanene is a semimetal where the electrons behave
as massless relativistic particles leading to enhanced carrier mobility. Indeed, recent
studies have shown germanene to have an intrinsic carrier mobility on the order of 6x105
cm2V-1s-1, more than double that of graphene's. Another advantage over graphene is
germanene's larger spin-orbit gap (23 meV), which when compared to graphene's (<0.05
meV) makes germanene a superior candidate to exhibit the quantum spin Hall effect at
experimentally viable temperatures. Lastly, the germanene lattice allows for an opening
of the band gap via an applied electric field or adsorption of foreign atoms, enabling the
creation of germanene based field-effect devices. In this thesis we analyze the effect of edgefunctionalizing
species H, C, N, P, As, O, S, Se, Te, F, Cl, Br, and I on the electronic and
geometric properties of germanene armchair and zigzag nanoribbons. The effect of strain
application on the band gap of H-passivated armchair nanoribbons is also examined.
We found that for each species, the armchair nanoribbons transition between semiconductor
and semimetal in a cyclical pattern depending on width. The band gap of the
nanoribbon, as well as the semimetal point, are tunable through width, edge-functionalization,
and mechanical strain. In the case of zigzag germanene nanoribbons, the ribbons are nodal
semimetals at small atomic widths but transition to metallic behavior at higher widths.
While the metallic nature of large zigzag nanoribbons has been studied, the nodal semimetal
behavior of thin (4-8 atoms wide) zigzag germanene ribbons is previously unreported. The
different edge species reveal distinct groups showing similarities in both the geometric structure
and band structure of the zigzag nanoribbons over various widths. The tunability of the
band gap in germanene armchair nanoribbons nearly covers the entire infrared spectrum, a
property only previously realized in HgCdTe.
