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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.
Source: Argonne National Laboratory
