UC Berkeley

This dissertation is divided into two segments, both of which focus on creating and manipulating one-dimensional (1D) conduction channels in novel 1D and two-dimensional (2D) systems, characterized by scanning tunneling microscopy (STM).

The first half describes how the electronic properties of quasi-1D graphene nanoribbons (GNRs) are manipulated by controlling their width and edge geometry at the atomic scale. A bottom-up approach is used for fabricating three different armchair GNR (AGNR) systems, allowing the geometry and hence the electronic properties of resultant AGNRs to be controlled. Successful molecular bandgap engineering in 1D AGNR heterojunctions is described, as well as the electronic and topographic characterization of the concentration dependence of boron-doped AGNRs. The discovery of two new in-gap dopant states with different symmetry is described. The successful fabrication and characterization of S-AGNRs having sulfur atoms substitutionally doped at the AGNR edges is also described. Our results indicate that S-doping induces a rigid shift of the energies for both the valence and conduction bands.

The second half of this thesis describes how the 1T’ phase of monolayer transition metal dichalcogenides (TMDs) can be used as a platform to create 2D topological insulators (TIs). These novel TI systems are characterized in great detail. The successful growth and characterization of single-layer 1T’–WTe2 is described. This material is shown to host a bulk bandgap and helical edge states at the 1T’–vacuum interface. The growth and characterization of mixed phase-WSe2 is described. New techniques for creation and manipulation of edge conduction channels at interfaces between materials of different topologies are described.