Abstract
Graphene-based sensors are among the most promising of graphene’s applications. The ability to signal the presence of molecular species adsorbed on this atomically thin substrate has been explored from electric measurements to light scattering. Here we show that the adsorbed molecules can be used to sense graphene properties. The interaction of porphyrin molecules with nitrogen-doped graphene has been investigated using scanning tunneling microscopy and ab initio calculations. Molecular manipulation was used to reveal the surface below the adsorbed molecules allowing to achieve an atomic-scale measure of the interaction of molecules with doped graphene. The adsorbate’s frontier electronic states are downshifted in energy as the molecule approaches the doping site, with largest effect when the molecule sits over the nitrogen dopant. Theoretical calculations showed that, due to graphene’s high polarizability, the adsorption of porphyrin induces a charge rearrangement on the substrate similar to the image charges on a metal. This charge polarization is enhanced around nitrogen site, leading to an increased interaction of molecules with their image charges on graphene. Consequently, the molecular states are stabilized and shift to lower energies. These findings reveal the local variation of polarizability induced by nitrogen dopant opening new routes towards the electronic tuning of graphene.
Highlights
The properties of graphene, arising from its two dimensional sp[2] hybridized carbon lattice, make it an enticing material for developing novel carbon-based electronics and a formidable playground to explore fundamental solid state physics[1]
We found that the molecular resonances associated with the porphyrin frontier orbitals shifted to lower energy when the molecules were located nearby a nitrogen site leading to the appearance of bright molecules in the molecular islands
Using a statistical analysis of the density of nitrogen dopants and bright molecules we concluded that the effect of a nitrogen dopant extends over 0.8 nm leading to an average number of bright molecule 2.2 times larger than the number of nitrogen atoms
Summary
All the STM and STS measurements reported here were obtained at 4.6 K. Graphene samples containing multilayer (≈5–10) were obtained on SiC(0001) by annealing the substrates in ultrahigh vacuum (UHV) at 1320 °C for 12 min under a silicon flux of ≈1 ML/min. The doping of nitrogen into graphene was obtained by exposing the graphene sample to a flux of nitrogen radicals produced by a remote radio-frequency plasma source[13]. Pristine and nitrogen-doped graphene samples were taken out of the UHV synthesis chamber and transferred in air to another UHV system for low temperature STM measurements. DFT calculations were carried out within Gaussian 09 package[33] and post-processing data were obtained with Multiwfn[34] and Chimera[35] packages
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