UC Berkeley

Understanding of the electronic properties of new materials is of central importance for science and technology. In particular, low-dimensional nano-materials exhibit exotic properties that are determined by quantum mechanics. Rapid development of reticular chemistry has enabled the syntheses of a variety of low-dimensional condensed matter systems that are formed by linking up molecular building blocks. This dissertation focuses on the on-surface synthesis of such materials and consequent characterization of their local electronic structure with scanning tunneling microscopy (STM).

The structure － property relationship of two low-dimensional nano-materials will be described: quasi-one-dimensional (1D) graphene nanoribbons (GNRs) and two-dimensional (2D) covalent organic frameworks (COFs). In the case of GNRs, we first show that two compatible molecular precursors with different sizes lead to GNR heterojunctions with atomically precise interfaces. In order to test predictions for how doped GNRs should behave, we synthesized B-doped GNRs with different doping concentrations on Au(111). Characterization of their electronic properties revealed a symmetry-dependent hybridization between the dopant states and the underlying Au substrate. We also describe our efforts to synthesize N=11 armchair GNRs through sp3-to-sp2 conversion of carbon atoms. In the case of 2D COFs, we first show that formation of imine linkages within COF366-OMe causes a downshift in the frontier orbital energy of porphyrin cores due to the electron-withdrawing characteristics of imine bonds. We then demonstrate the ability to achieve a 2D lattice of type II heterojunctions through judicious choice of molecular precursors that result in an asymmetrical bonding scheme within a 2D COF.

Our studies show that on-surface synthesis can lead to the realization of target nanostructures that are unreachable via solution-based methods, as demonstrated by intra-molecular reactions that lead to new “nano-graphene” species.