Graphene on Ir(111) characterized by angle-resolved photoemission

Angle-resolved photoelectron spectroscopy (ARPES) is extensively used to characterize the dependence of the electronic structure of graphene on Ir(111) on the preparation process. ARPES findings reveal that temperature-programmed growth alone or in combination with chemical vapor deposition leads to graphene displaying sharp electronic bands. The photoemission intensity of the Dirac cone is monitored as a function of the increasing graphene area. Electronic features of the moiré superstructure present in the system, namely, minigaps and replica bands are examined and used as robust features to evaluate graphene uniformity. The overall dispersion of the π band is analyzed. Finally, by the variation of photon energy, relative changes of the π and σ band intensities are demonstrated.

Figure 1

(a)–(d) Set of ($1\times 1$) μm${}^2$ large-scale STM images showing the growth of graphene flakes for (a) one, (b) two, (c) three, and (d) seven TPG cycles. In each figure, in the upper left quadrant graphene was colored white whereas bare Ir(111) was colored black. White arrows in (c) and (d) point to some of the wrinkles discussed in the text. (e) Squares: surface coverage θ [line is a fit to $\theta \left(N\right)=1-e^{-\lambda N}$ ] as a function of the number of TPG cycles. Circles: graphene step-edge density (line to guide the eye) as a function of the number of TPG cycles.

Figure 2

(a) ARPES spectra (excitation energy 21.2 eV, mixed polarization) showing the Dirac cone intensity obtained after the indicated number of TPG cycles. Spectra are focused to the region of the K point of graphene's BZ and were recorded at ∼60 K. (b) Energy-distribution-curve cuts obtained from the series of seventeen spectra, part of which are shown in (a), merged into a two-dimensional map. Cuts were taken at $k_{\left|\right|}=1.784$ Å${}^{-1}$ [as indicated by a yellow line in an eight cycle spectrum from (a)]. (c) Photoemission peak areas extracted from spectra in (b). Lines indicate fits of peak areas (see discussion).

Figure 3

(a) ARPES spectrum (excitation energy 55 eV, $p$ polarization, sample temperature 80 K) showing the Dirac cone and characteristic features of high-quality graphene on Ir(111) discussed in the text. $R$'s indicate replica bands, arrows point to the minigaps, and K indicates the position of the K point of graphene. (b) Zoom into the minigap region from (a). (c) Tight-binding band of graphene (black) and its superperiodictiy replica bands (blue) for the azimuth of 0.4° that best fit spectrum (a). Numbers 1–6 indicate six replica bands.[20] Circled regions denote regions of minigaps indicated by arrows in (a).

Figure 4

Examples of (a) momentum-distribution-curve and (b) and (c) energy-distribution-curve cuts from ARPES spectrum of Fig. 3a. Positions of cuts as well as measured full widths at half maxima of spectral peaks obtained by Lorentzian fits are indicated. (d) MDC- and EDC-derived quasiparticle dispersions are shown as full grey circles and open squares, respectively. Lines above (red) and below (blue) the minigap are linear fits of dispersions in the corresponding energy regions and the values of their slopes are indicated.

Figure 5

ARPES map showing the dispersion of the π band for the ΓK and ΓM directions. Inset indicates how the ARPES scanning was performed. All spectra shown in this map were acquired at a photon energy of 55 eV ($p$ polarization) and the sample temperature of 80 K. Dashed line shows the tight-binding calculation fit, discussed in the text.

Figure 6

(a) ARPES spectra showing dispersion of graphene bands for the ΓK direction. Spectra shown in this map were acquired at photon energy of 36 eV ($p$ polarization) and sample temperature of 80 K. (b) EDC cuts at $k_{\left|\right|}=0.5$ Å${}^{-1}$ for two different photon energies.