C60 fullerides are uniquely flexible molecular materials that exhibit a rich variety of behaviour1, including superconductivity and magnetism in bulk compounds2,3, novel electronic and orientational phases in thin films4,5,6,7,8,9,10 and quantum transport in a single-C60 transistor11. 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 owing 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 ultrathin films through well-controlled atomic layer indexing and accurate doping concentrations. Using scanning tunnelling microscope techniques, we observe a series of electronic and structural phase transitions as the fullerides evolve from two-dimensional monolayers to quasi-three-dimensional 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 provide essential information for the development of new molecular structures and devices.
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This work was supported in part by NSF Grant EIA-0205641 and by the Director, Office of Energy Research, Office of Basic Energy Science, Division of Material Sciences and Engineering, US Department of Energy under contract No. DE-AC03-76SF0098. Y.W. acknowledges a research fellowship from the Miller Institute for Basic Research in Science.
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Wang, Y., Yamachika, R., Wachowiak, A. et al. Tuning fulleride electronic structure and molecular ordering via variable layer index. Nature Mater 7, 194–197 (2008). https://doi.org/10.1038/nmat2100
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