We have observed the energy structure in the density of occupied states of graphene grown on n-type 6H-SiC (0001). The structure revealed with photoelectron spectroscopy is described by creation of the quantum well states whose number and the energy position (E-1 = 0.3 eV, E-2 = 1.2 eV, E-3 = 2.6 eV) coincide with the calculated ones for deep (V = 2.9 eV) and narrow (d = 2.15 angstrom) quantum well formed by potential relief of the valence bands in the structure graphene/n-SiC. We believe that the quantum well states should be formed also in graphene on dielectric and in suspended graphene. (C) 2015 Elsevier Ltd. All rights reserved.

Funding Agencies|Government of the Russian Federation [074-U01]; Graphene Flagship [CNECT-ICT-604391]

The electrical transport in graphene interfaced with different ions in solution gated graphene field effect transistors (GFETs) is the subject of active studies due to its importance in sensor fabrication. Most of the developed GFET biological sensors use graphene that has been modified. The difficulty in the modification procedure and the reduction in quality of graphene that it causes are important drawbacks for applications. Therefore, we focus on GFETs based on unmodified graphene gated by aqueous solutions containing lysine amino acids. We observed that an increase in the ionic concentration of lysine in these solutions leads to a suppression of unipolar electron conductance of graphene in GFETs. This dependence is opposite to the dependence typically observed in gating solutions containing smaller atomic ions. We attribute the observed suppression to electric field screening of the graphene surface from water molecules by lysine ions which are larger and have lower charge density compared to atomic ions. This novel phenomenon leads to an overall decrease of surface charge density in molecular layers formed at the graphene interface and can be applied in GFET sensors with unmodified graphene that detect the presence and concentration of large molecules in the gating solutions.

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