**2.2. Energy level tuning**

Experiment and theory studies demonstrated that semiconductor graphene (p-type and ntype) can be obtained by modifying graphene with organic semiconductor molecules (electron-acceptor and electron-donor), which provides a simple and nondestructive way of tuning the Femi level and controlling the charge carriers concentration of graphene.[39] As shown in Figure 3, for the p-type graphene, the Fermi level shifts upward relative to the Dirac point when the electron-acceptor coverage increases. In contrast, for the n-type graphene, the Fermi level shifts downward relative to the Dirac point when the electrondonor coverage increases.[40]

**Figure 3.** Doping graphene with semiconductor donor and acceptor molecules move the Fermi level up or down with respect to the Dirac point.

Wang *et al.* presented an effective route for preparation of both n-type and p-type graphene through adsorbing organic semiconductor molecules: one is the tetracyanoquinodimethane derivative (F4-TCNQ) and the other, the vanadyl-phthalocyanine (VOPc). The Kelvin probe force microscopy characterization results demonstrated that the F4-TCNQ molecules obtained electrons from graphene, but VOPc donated electrons to graphene. This phenomena indicated that chemical functionalization of graphene is a feasible approach for bandgap opening and tuning of graphene, which may have great implications for future large-scale applications of graphene-based nanoelectronics.[41] Pati *et al.* studied the modification in the electronic structure, as well as optical and transport properties of graphene induced by molecular charge transfer using ab initio density functional theory and Raman spectroscopic studies of modified graphene systems. They found that donor and acceptor molecules adsorbed onto the graphene surface exhibited effective molecular charge transfer, giving rise to mid-gap molecular levels with tuning of the band gap region near the Dirac point.[42]
