click to enlarge STM 3D-rendered image of a C60 self-assembled monolayer at a domain boundary of graphene and bare SiC(0001); each C60 molecule is 1 nm in diameter.

Cryogenic
ultrahigh vacuum scanning tunneling microscopy (STM) was employed by
researchers in the Center for Nanoscale Materials Electronic & Magnetic
Materials & Devices Group at Argonne National Laboratory to uncover
exceptionally weak molecule-surface interactions between fullerene C₆₀
deposited onto epitaxially grown graphene on silicon carbide substrates.

The first
layer of C₆₀ molecules self-assembles into well-ordered,
close-packed islands. In situ
scanning tunneling spectroscopy reveals a highest occupied molecular
orbital–lowest unoccupied molecular orbital gap of 3.5 V, which is close to the
value of solid and gas-phase C₆₀. This finding indicates a
significantly smaller amount of charge transfer from the C₆₀ to the
graphene as compared with C₆₀ adsorbed onto metallic surfaces.

Usually interface effects dominate over the properties of adsorbed
molecules. Here, however, a perfect 2D material (graphene) has completely
decoupled the organic system from the charged interface states of the silicon
carbide surface reconstruction. Improving molecule-based organic photovoltaics
and biosensors relies on minimal substrate-molecule interaction to preserve
intrinsic molecular functionalities, which was achieved in this case via an
inert graphene “barrier” layer.

Study Abstract

Source: Argonne National Laboratory