Ambient hydrocarbon contamination is an unavoidable issue in van der Waals materials, but until recently its structure was not well understood. In our earlier work we identified that after a few days in air, vdW surfaces become covered by a self-assembled monolayer of normal alkanes, revealed using STM together with infrared spectroscopy. In this new paper, we show what this contamination actually does to STM measurements on graphite.
The tricky part is that the alkane layer can easily go unnoticed. It introduces no obvious molecular features in the dI/dV spectra, since alkanes are wide-gap insulators, and the spectra still look essentially “graphite-like.” In practice, the only clear sign in standard imaging is that atomic-resolution STM topography becomes noisier and less stable, which many users might simply attribute to tip issues.
On graphene and graphite the contamination leaves two very strong fingerprints. First, it suppresses the well-known phonon-induced tunneling gap: on clean UHV-cleaved graphite we clearly resolve the phonon gap edges at 56.8 meV and 80.9 meV, while on contaminated surfaces this feature disappears. Second, the alkane layer strongly modifies current-distance spectroscopy: instead of the expected exponential decay with κ around ~1 Å⁻¹, contaminated surfaces show an anomalously small decay constant, with values around 0.5 Å⁻¹ or even less, meaning the tunneling current decay is flattened by roughly a factor of 1.5-5 compared to the clean case.
These two effects provide a simple practical diagnostic for contamination, and the good news is that the surface can be restored by UHV annealing (≥ 200 °C). Finally, we also present DFT calculations, showing that the alkane overlayer also modifies the decay of the graphite surface wavefunctions into the vacuum, making the imaging of the molecular overlayer possible at low setpoint currents of the order of 10 pA.
See our paper in PRB.
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