Charge neutrality and band-gap tuning of epitaxial graphene on SiC by molecular doping

Epitaxial graphene on SiC(0001) suffers from strong intrinsic $n$-type doping. We demonstrate that the excess negative charge can be fully compensated by noncovalently functionalizing graphene with the strong electron-acceptor tetrafluorotetracyanoquinodimethane (F4-TCNQ). Charge neutrality can be reached in monolayer graphene as shown in electron-dispersion spectra from angular-resolved photoemission spectroscopy. In bilayer graphene the band-gap that originates from the SiC/graphene interface dipole increases with increasing F4-TCNQ deposition and, as a consequence of the molecular doping, the Fermi level is shifted into the band-gap. The reduction in the charge-carrier density upon molecular deposition is quantified using electronic Fermi surfaces and Raman spectroscopy. The structural and electronic characteristics of the graphene/F4-TCNQ charge-transfer complex are investigated by x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The doping effect on graphene is preserved in air and is temperature resistant up to $200°\text{C}$. Furthermore, graphene noncovalent functionalization with F4-TCNQ can be implemented not only via evaporation in ultrahigh vacuum but also by wet chemistry.

Figure 1

Dispersion of the $\pi$ bands measured with UV-excited ARPES around the $\overline{K}$ point of the graphene Brillouin zone for (a) an as-grown graphene monolayer on SiC(0001) and [(b)–(e)] for the same sample covered with an increasing amount of F4-TCNQ molecules. The momentum scans are taken perpendicular to the $\overline{\Gamma }\overline{K}$ direction in reciprocal space. The Fermi level $E_{\text{F}}$ shifts progressively toward the Dirac point ($E_{\text{D}}$, dotted black line) with increasing nominal thickness of the deposited F4-TCNQ film. (d) Charge neutrality $\left(E_{\text{F}}=E_{\text{D}}\right)$ is reached for a molecular coverage of 0.8 nm. (e) When depositing additional molecules the Fermi level does not shift any further.

Figure 2

Dispersion of the $\pi$ band around the $\overline{K}$ point of the graphene Brillouin zone measured by ARPES with synchrotron light in scans oriented parallel to the $\overline{\Gamma }\overline{K}$ direction for (a) a pristine epitaxial graphene monolayer, (b) an intermediate F4-TCNQ coverage, and (c) the F4-TCNQ coverage leading to charge neutrality. Panels (d)–(f) show the corresponding constant energy maps at $E_{\text{F}}$. From these Fermi-surface maps we extract a charge carrier concentration of $7.3\pm 0.2\times {10}^{12}{\text{cm}}^{-2}$ for the pristine graphene, $9\pm 2\times {10}^{11}{\text{cm}}^{-2}$ for the intermediate coverage, and $1.5\pm 2\times {10}^{11}{\text{cm}}^{-2}$ for full coverage. All the spectra shown were acquired with circular polarized light with a photon energy of 30 eV and at a sample temperature of 80 K.

Figure 3

(Color online) [(a) and (b)] XPS spectra of the (a) $\text{N}1s$ and (b) $\text{F}1s$ core-level emission regions from submonolayer (bottom spectrum) to multilayer (top spectrum) amounts of F4-TCNQ deposited on a monolayer of graphene which has been grown epitaxially on SiC(0001). Three different components are fitted into the $\text{N}1s$ region and are assigned to ${\text{N}}^{-1}$ and ${\text{N}}^0$ species and to a shake-up process. The blue dashed line indicates the exact energy position of the ${\text{N}}^{-1}$ component as it shifts with molecular layer thickness. (c) Schematic of a F4-TCNQ layer deposited on top of a graphene layer grown on SiC. The charges induced in the graphene layer due to the interface dipole and the molecular charge transfers are indicated.

Figure 4

Dispersion of the $\pi$ bands measured with ARPES through the $\overline{K}$ point of the graphene Brillouin zone for (a) a pristine graphene monolayer grown on SiC(0001) and [(b)–(d)] for increasing amounts of TCNQ deposited on graphene. The Fermi level $\left(E_{\text{F}}\right)$ shifts progressively toward the Dirac point ($E_{\text{D}}$, black dotted line) for increasing molecular coverage up to a value of $E_{\text{D}}-E_{\text{F}}=-0.25\text{eV}$.

Figure 5

(Color online) (a) Secondary electron cut-off region measured by normal emission UPS $\left(h\nu =21.21\text{eV}\right)$ for increasing nominal thickness of a F4-TCNQ film deposited on epitaxial monolayer graphene on SiC(0001) in order to estimate the work-function change $\left(\Delta \Phi \right)$. (b) Near $E_{\text{F}}$ UPS spectra for clean graphene (bottom) and graphene with 0.8 nm of deposited F4-TCNQ (top). The shaded areas highlight the emerging features following F4-TCNQ deposition and are attributed to the HOMO (at 1.4 eV) and to the LUMO which has partially shifted below $E_{\text{F}}$ (at 0.35 eV).

Figure 6

(Color online) (a) Raman spectrum of pristine (bottom black trace) and F4-TCNQ-modified (top red trace) monolayer graphene epitaxially grown on SiC(0001). Molecular peaks are marked with stars and the peaks related to SiC with arrows. The $G$- and 2D-peak regions of graphene are shaded. (b) Differential Raman spectra for different coverages of the F4-TCNQ molecular film ranging from 1.5 to 0.025 nm (see main text). The gray dashed lines are Lorentzians to fit the molecular peaks. The blue solid line is the extracted graphene contribution to the Raman spectrum ($G$ peak).

Figure 7

(Color online) ARPES band-structure plots measured perpendicular to the $\overline{\Gamma }\overline{K}$ direction for an epitaxially grown graphene bilayer on SiC(0001) (a) without F4-TCNQ coverage and [(b)–(e)] with increasing amounts of F4-TCNQ. Bands calculated within a tight-binding model are superimposed to the experimental data. (f) ARPES data showing the band structure of an epitaxial graphene bilayer prepared at a lower annealing temperature. Contributions from monolayer domains are evident. (g) Schematic band structure of monolayer, bilayer, and trilayer epitaxial graphene. (h) Evolution of the energy gap $E_{\text{g}}$, the gap midpoint or Dirac point $E_{\text{D}}$, the minimum of the lowest conduction band $E_{\text{cond}}$, and the maximum of the uppermost valence band $E_{\text{val}}$ as a function of molecular coverage. The evolution of the energies for higher molecular coverages (up to 5 nm, not shown) confirms charge-transfer saturation. The definition of the energies is included in panel (c).

Figure 8

(Color online) (a) Shift of the Fermi level $E_{\text{F}}$ with respect to the Dirac energy $E_{\text{D}}$ as a function of temperature during annealing of F4-TCNQ covered epitaxial monolayer graphene on SiC(0001). The shift was determined from ARPES data recorded after each annealing step in UHV of a graphene sample with an initial molecular coverage of 1.5 nm. (b) Incomplete shift of the $\pi$-band dispersion after F4-TCNQ wet chemical application in chloroform. (c) $\pi$-band dispersion after F4-TCNQ wet chemical application in DMSO. Charge neutrality is achieved, as indicated by $E_{\text{F}}=E_{\text{D}}$.