Electronic states and related orientational assembly in potassium-doped C₆₀ layer on SrTiO₃(001)

Abstract

Two-dimensional (2D) fullerene (C₆₀) films grown on various metal and semiconductor substrates have been extensively studied but remain less explored on oxide substrates. By using cryogenic scanning tunneling microscopy and spectroscopy (STM/STS), we investigated the molecular orientation and electronic structure of C₆₀ films grown on a SrTiO₃(001) surface. Our STM/STS results display the variation of electronic energy levels of C₆₀ molecules as a function of both coordination and potassium doping. The orientation of each C₆₀ molecule can be identified through density-functional theory (DFT) calculations and STM simulations. The orientational assembly of the C₆₀ layer near a K-doped C₆₀ shows a chiral 2×2 superstructure of the hexagon-faced up (H) configuration. Intriguingly, the spatial distribution of the density of states (DOS) peak around the K-doped C₆₀ shows an electronic state in the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap that can be attributed to the charge-trapped state. The spatial dependence of this state indicates an upward-bending of the conduction band, induced by the screened electric field of a negatively charged center. Together, the band-bending behavior and the related chiral 2×2 superstructure suggest a possible scenario of a charge-trapped, orientational C₆₀ assembly.

Keywords

Scanning tunneling microscopy / fullerene / K-dopant / charge transfer / conformation

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