Band-gap engineering by Bi intercalation of graphene on Ir(111)

We report on the structural and electronic properties of a single bismuth layer intercalated underneath a graphene layer grown on an Ir(111) single crystal. Scanning tunneling microscopy (STM) reveals a hexagonal surface structure and a dislocation network upon Bi intercalation, which we attribute to a $\sqrt{3}\times \sqrt{3}R{30}^{\circ }$ Bi structure on the underlying Ir(111) surface. Ab initio calculations show that this Bi structure is the most energetically favorable and illustrate that STM measurements are most sensitive to C atoms in close proximity to intercalated Bi atoms. Additionally, Bi intercalation induces a band gap ($E_{\mathrm{g}}=0.42$ eV) at the Dirac point of graphene and an overall $n$ doping $\left(\sim 0.39\mathrm{eV}\right)$ as seen in angular-resolved photoemission spectroscopy. We attribute the emergence of the band gap to the dislocation network which forms favorably along certain parts of the moiré structure induced by the graphene/Ir(111) interface.

Figure 1

Surface structure of the G/Ir and G/Bi/Ir system. STM constant-current topographic images (a)–(d) and differential conductance maps (e) and (f) of graphene on Ir(111), (c)–(f) with intercalated bismuth of different coverage: (a) resolves the moiré superstructure of G/Ir after graphene growth $(I_{\mathrm{t}}=35\mathrm{pA},V_{\mathrm{S}}=50\mathrm{mV})$. In addition (b) resolves the honeycomb lattice of the G/Ir system ($I_{\mathrm{t}}=290\mathrm{pA},V_{\mathrm{S}}=480\mathrm{mV}$). (c) displays a G/Bi/Ir island and a thin G/Bi/Ir terrace (on the low side of the iridium step edge) on a G/Ir background; the G/Bi/Ir coverage on this sample is $5\% (I_{\mathrm{t}}=100\mathrm{pA},V_{\mathrm{S}}=500\mathrm{mV})$. (d) shows G/Bi/Ir terraces (on the low side of iridium step edges) on a G/Ir background; the G/Bi/Ir coverage on this sample is $95\% (I_{\mathrm{t}}=250\mathrm{pA},V_{\mathrm{S}}=100\mathrm{mV})$. (e) and (f) display the contrast between areas of G/Bi/Ir to G/Ir in $dI/dV$-maps (modulation voltage $V_{\mathrm{mod}}=5\mathrm{mV}$ for both), simultaneously acquired for the topographies (c) and (d).

Figure 2

Surface investigation of the G/Bi/Ir system. (a) STM constant current topographic image resolving the $\left(\sqrt{3}\times \sqrt{3}\right)$ structure, the dislocation network and the moiré superstructure $(I_{\mathrm{t}}=684\mathrm{pA},V_{\mathrm{S}}=-10\mathrm{mV})$. (b) FFT of (a) topography featuring clear spots corresponding to the moiré and a halo corresponding to the $\left(\sqrt{3}\times \sqrt{3}\right)$ structure and the dislocation network as marked [47]. (c) and (d) are FFT filtered versions of (a) restricted to (c) the contribution of the moiré and (d) the dislocation network (software wsxm4.0 [47]). Ball-stick model (e) and (f) display the $\left(\sqrt{3}\times \sqrt{3}\right)$ structure for a bismuth layer underneath the graphene for the (e) intercalation and (f) surface alloy case.

Figure 3

(a) Ball-stick models illustrating the most stable structures found for Bi (purple) adsorbed on Ir(111) (white) underneath graphene (gray) at different coverages. (b) Calculated average and differential $E_{\mathrm{int}}$ and $E_{\mathrm{alloy}}$ for increasing Bi coverages, revealing that systems with intercalated Bi are more stable than those where the Bi-Ir surface alloy is formed. The most stable system corresponds to 0.33 ML Bi coverage ordered in the $\left(\sqrt{3}\times \sqrt{3}\right)$ configuration. Average and differential energies correspond to total adsorption energy per Bi atom and to the energy gain for each discrete coverage increase. (c) Tersoff-Hamann STM simulation ($V_{\mathrm{S}}=-0.2\mathrm{V},I_{\mathrm{t}}=0.01\mathrm{nA}$) for the moiré supercell model with a 0.33 ML coverage of intercalated Bi arranged in the $\left(\sqrt{3}\times \sqrt{3}\right)$ structure. The bottom panels illustrate the local structure of selected regions, showing that C atoms in the proximity of intercalated Bi have higher intensities. The color scale indicates corrugation with respect to average Bi height.

Figure 4

Electronic structure around the $K$ point of (a) and (b) G/Ir and of (c), (d), and (f) $95\%$ G/Bi/Ir: (a) and (c) Constant energy surface around the $K$ point of graphene at 0.5 eV below the DP position; black dashed lines indicate the edges of the first Brillouin zone, and the colored dashed lines indicate the high-symmetry directions along which the dispersion is shown. (b) and (d) left panel: dispersion of the graphene Dirac cone along the Γ-K-M direction. Right panel: dispersion of the graphene Dirac cone along the A-K-A′ direction. The dashed line indicates the constant energy cut represented in (a) and (c). (e) Sketch of the graphene Brillouin zones with high-symmetry points and directions. (f) Zoom into the Dirac cone in G/Bi/Ir close to the Fermi level along the A-K-A′ direction. A 0.42 eV energy gap is opened at the level of the Dirac point. The symbols on top of the spectrum show the location of the energy distribution curves (EDCs) maxima. The solid lines show the hyperbolic fitting on the upper and lower cones. (g) The red data points are the EDC intensity profile at the $K$ point integrated over a $0.01{\AA }^{-1}$ range and fitted by the black solid line using two Lorentzians and a Fermi function cutoff. The two Lorentzian components of the fit are shown. The Fermi function is plotted on the left, and an offset is applied for clarity. All the reciprocal space coordinates in the figure are rescaled with respect to the $K$ point.

Figure 5

Unfolded band structure for (a) free-standing graphene, (b) Bi intercalated graphene with a $\sqrt{3}\times \sqrt{3}R{30}^{\circ }$ interface structure (0.33 ML equal to $100\%$), and (c) for the same $\sqrt{3}\times \sqrt{3}R{30}^{\circ }$ structure under an artificial compressive strain applied in the surface normal direction. The size of each data point indicates the intensity (spectral weight) at each corresponding $k$ point and energy interval. In order to visualize the $\pi$ band more clearly, only data points with intensities above a given threshold are shown. Red arrows point to the induced gaps in the $\pi$ band, and blue arrows point to the center of the Dirac point. We note that the reduction from the equilibrium Bi-graphene distance of the (b) stable physisorbed regime to a value more characteristic of (c) chemisorbed graphene leads to strong distortions of the band structure of graphene, which is further $n$ doped and with the presence of gaps in the $\pi$ band.