UC Berkeley

The ability to modify the electronic properties of monolayer graphene via charge-donating or charge-accepting molecules creates new opportunities for fabricating nano-scale hybrid devices. Introducing donor and acceptor molecules to a graphene surface induces charge transfer and creates localized Coulomb potentials. Combined with an electrostatic back-gate on a graphene/BN device, it is possible to engineer a variety of electrostatic potentials for studying fundamental electronic phenomena. In this dissertation, we discuss local probe investigation of such phenomena in graphene/molecule hybrid systems using scanning tunneling microscopy and atomic force microscopy.

We first describe the procedures for fabricating atomically clean graphene/BN field effect transistors (FETs) for molecular self-assembly and local probe studies. Using these graphene/BN FETs, we are able to adjust the energy alignment of aromatic molecular orbital levels with respect to the Fermi level. These molecules are weakly coupled to the graphene and show clear vibronic resonances in their energy spectra. By choosing a more electronegative molecule, fluorinated TCNQ, with orbital levels closer to the Dirac point, we are able to demonstrate charge-state switching in single molecules. The electric field-induced energy shift of the molecular levels is influenced here by gate-dependent screening effects arising from the graphene substrate. Additionally, we find that inert fatty acid islands deposited on the surface can help to stabilize single molecules for local probe characterization.

Such control of single molecules on a device surface allows us to create more complex self-assembled molecular nanostructures using bottom up techniques. Inert fatty acid islands, for example, are used to act as a template for the self-assembly of one-dimensional molecular arrays. These arrays are electrically charged by applying an electrostatic back-gate to the graphene/BN FET, thus generating strong one-dimensional Coulomb potentials. Such Coulomb potentials are found to induce new localized Dirac fermion states. The formation of these states are explained by an atomic collapse picture whereby an electron cannot form stable bound states around an isolated Coulomb potential, but rather spirals inward toward the center of the impurity. Self-assembly processes also lead to the formation of densely-packed, charged two-dimensional molecular islands on the graphene surface. The formation mechanism of these islands is explained by work function heterogeneity on the surface, a mechanism unique to poorly screened substrates like graphene. This work reveals new fundamental behavior for Coulomb potentials anchored to graphene surfaces at different length scales and geometries, enabling the engineering of 2D potential landscapes and electron wave-functions in graphene devices.