# Electronic and Transport Properties of Epitaxial Graphene on SiC and 3C-SiC/Si: A Review
## 5 Fine-Tuning of Transport Properties
### Functionalization of EG  
Controlling the transport properties via surface functionalization makes use of charge compensation from surface adsorption or attachments [139]. [PERSON] et al. demonstrated that the intrinsic n-type doping of EG on SiC (0001) can be compensated by functionalizing graphene (non-covalent) with an electron  
Figure 18: Temperature dependent mobility of QFMLG (\(p=5.7\times 10^{12}\) cm\({}^{-2}\)) and MLG (\(10^{13}\) cm\({}^{-2}\)). Reprinted with permission from Reference [129]. Copyright (2011) AIP Publishing.  
acceptor called as tetrafluorotetracyanoquinodimethane (F4-TCNQ) [79]. Surface transfer doping results in charge transfer from graphene into the doping molecule reducing the sheet density. A 0.8-nm-thick layer of F4-TCNQ on monolayer EG resulted in near charge neutrality, i.e., \(\mathbf{{E_{F}}}=\mathbf{{E_{D}}}\), Figure 19. For a bilayer sample, the band gap more than doubled, moving from about 100 meV to 250 meV [79] with the \(\mathbf{{E_{F}}}\) shifted into the band gap. Yet, both the compensation and band gap widening saturated near 0.8 nm, which indicated that there may be limitations with this approach. An increase of mobility with decreasing carrier density was seen in the F4-TCNQ doped samples by [PERSON] et al. [55]. When the carrier density is decreased, the mobility limited by the electron interaction with substrate phonons rises substantially (up to 29,000 cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\) at 25 K), and Shubnikov-de Haas oscillations and the graphene-like quantum Hall effect are observed [55]. The F4-TCNQ experiments demonstrate that the effects of the substrate interaction can be mitigated by reducing the sheet density, and the quantum Hall effect and pseudo-relativistic physics and linear band structure are possible in the epitaxial graphene.  
[PERSON] et al. [132] studied the transport properties of nitrophenylfunctionalized EG (NP-EG formed on the C-face SiC) and suggested that it changes the graphene from near-metallic to semiconducting (see Figure 15) [132].  
Other interesting work by [PERSON] et al. [140] demonstrate charge neutrality (10\({}^{11}\) cm\({}^{-2}\)) and p-doping (\(5\times 10^{12}\) cm\({}^{-2}\)) by reversible tuning of carrier density via high electrostatic potential gating with ions produced by corona discharge in initially strongly n-doped EG/SiC devices. In this case, the ions deposited on the dielectric layer induce a surface charge density on the graphene. The corona effect is reversible and, depending on the ionic charge, both p-doping and n-doping are possible. The method is utilized in graphene applications such as quantum resistance metrology where specific fixed doping is required.  
[PERSON] et al. [141] used DFT calculations to investigate the graphene interactions with the adsorbed alkali metal ions such as Li\({}^{+}\), Na\({}^{+}\), and K\({}^{+}\) for the applications of graphene-based chemical sensors. In the presence of an external electric field, the positively charged ions get closer to the graphene inducing excess charge transfer from the graphene surface to the metal ions, causing negative charges on the ions and removal of the ions from the surface. [PERSON] et al. [142] demonstrated first direct observation of superconductivity (critical temperature of 5.9 K) for monolayer graphene formed on 6H-SiC (0001) decorated with a layer of lithium atoms at a low temperature, which caused an enhanced electron-phonon coupling.