Controlling the self-doping of epitaxial graphene on SiC via Ar ion treatment

We present a simple but efficient way to control substrate-induced doping of graphene via mechanical deformations induced by Ar ions. Graphene on SiC is n-doped because of the substrate-to-graphene charge transfer. Using angle-resolved photoemission spectroscopy (ARPES), core level shift, and scanning tunneling microscopy (STM), the treatment with 0.5-keV Ar ions was found to reduce the charge transfer. First-principles calculations suggest that Stone-Wales defects among various defect structures are likely responsible for the change in the substrate-graphene charge transfer.

Figure 1

Band images taken by ARPES measurement with a photon energy of 22.1 eV (UV lamp, He I). (a) Image obtained from the pristine graphene. (b) and (c) Images were from Ar-ion irradiated graphene at two different irradiation times, respectively. (d) A plot of Dirac point vs Ar-ion bombardment time. The Dirac point moves up to near-Fermi level after irradiation.

Figure 2

(a) C 1s spectra of the pristine (black) and ion-bombarded (red) single-layer graphene taken with a photon energy of 340 eV. Fitting results of the C 1s spectra of the (b) pristine and (c) ion-bombarded graphene. Blue, orange, pink, and dark green lines correspond to the bulk (SiC), graphene (G), and buffer layers (S1 and S2), respectively.

Figure 3

STM images of the pristine (a) and ion-bombarded single-layer graphene at different tunneling conditions (b) and (c). These images were obtained using constant current mode. Image size (a) 500 × 500 Å${}^2$, (inset) 50 × 50 Å${}^2$, (b) 500 × 500 Å${}^2$, and (c) 100 × 100 Å${}^2$. Tunneling parameters (a) $I_{\mathrm{t}}$ $=$ 0.13 nA, $V_{\mathrm{t}}=940$ mV; (inset) 0.17 nA, 196 mV; (b) 0.15 nA, 490 mV; (c) −0.19 nA, −10.0 mV.

Figure 4

The atomic structure of defective graphenes with (a) SW defect (as depicted in red) and (b) 7557 defect (as depicted in green). Graphic depiction of the change of the C–C bond length in graphene with (c) SW defect and (d) 7557 defect. The red (blue) color indicates the elongation (contractration) of the C–C bond from the normal bond. We observe that the C–C bonds around the SW defect are contracted, while the bonds at only the defect site are elongated. (e) Calculated work-function changes of graphene from the value of pristine graphene at varying (isotropic) strains. Expansion of C–C bonds increases the work function, while the contraction does the opposite. The opposite behavior occurs for 7557 defects. (f) A simulated STM image from a graphene with SW defects.