The local electronic properties of 2D devices are typically controlled by electric fields that come from nearby electrodes. Conventional electrodes, however, are difficult to make with ultra-small feature size (e.g., down to a single nanometer) and also create difficulties in maintaining atomically clean surfaces for scanned probe microscopy experiments.
An alternative for engineering the potential energy landscape of 2D materials is to use charge-tunable molecules and atomic-scale defects. These can provide sub-nanometer structural precision as well as flexible, gate-tunable behavior. With this in mind we have used F4TCNQ molecules as tunable charge centers to engineer the nanoscale energy landscape of graphene. By positioning F4TCNQ molecules into atomically-precise linear arrays we have observed Coulomb-driven charging patterns at the single-molecule level via scanning tunneling microscopy (STM), as well as new multi-impurity super-critical-like extended states.
In addition to molecular charge centers we also find that charge-tunable defects in insulating substrates can be used to manipulate the energy landscape of graphene. This is accomplished by controlling local defect ionization via the electric field of an STM tip. This new technique allows the patterning of gate-tunable graphene quantum dots whose electronic wavefunctions can be directly imaged.
Mike Crommie did his PhD here at UC Berkeley (Go Bears!) and a postdoc at IBM Almaden. He then joined the faculty at Boston University, and moved back to Berkeley Physics in 1999.