  C60 fullerides are uniquely flexible molecular materials that exhibit a rich
variety of behavior, including superconductivity and magnetism in bulk
compounds, novel electronic and orientational phases in thin films, and quantum
transport in a single-C60 transistor. The complexity of fulleride properties
stems from the existence of many competing interactions, such as
electron-electron correlations, electron-vibration coupling, and intermolecular
hopping. The exact role of each interaction is controversial due to the
difficulty of experimentally isolating the effects of a single interaction in
the intricate fulleride materials. Here we report a unique level of control of
the material properties of KxC60 ultra-thin films through well-controlled
atomic layer indexing and accurate doping concentrations. Using STM techniques,
we observe a series of electronic and structural phase transitions as the
fullerides evolve from two-dimensional monolayers to quasi-threedimensional
multilayers in the early stages of layer-by-layer growth. These results
demonstrate the systematic evolution of fulleride electronic structure and
molecular ordering with variable KxC60 film layer index, and shed new light on
creating novel molecular structures and devices.
