Controlling the electronic structure of graphene using surface-adsorbate interactions

Hybridization of atomic orbitals in graphene on $\mathrm{Ni}\left(111\right)$ opens up a large energy gap of $\approx 2.8\mathrm{eV}$ between nonhybridized states at the $K$ point. Here we use alkali-metal adsorbate to reduce and even eliminate this energy gap, and also identify a new mechanism responsible for decoupling graphene from the Ni substrate without intercalation of atomic species underneath. Using angle-resolved photoemission spectroscopy and density functional theory calculations, we show that the energy gap is reduced to 1.3 eV due to moderate decoupling after adsorption of Na on top of graphene. Calculations confirm that after adsorption of Na, graphene bonding to Ni is much weaker due to a reduced overlap of atomic orbitals, which results from $n$ doping of graphene. Finally, we show that the energy gap is eliminated by strong decoupling resulting in a quasifreestanding graphene, which is achieved by subsequent intercalation of the Na underneath graphene. The ability to partially decouple graphene from a Ni substrate via $n$ doping, with or without intercalation, suggests that the graphene-to-substrate interaction could be controlled dynamically.

Figure 1

(a) Electronic structure of pristine $\mathrm{Gr}/\mathrm{Ni}\left(111\right)$ measured along the $\mathrm{\Gamma }K$ direction in the Brillouin zone; the $\pi , {\sigma }_2$, and ${\sigma }_3$ states of graphene and the $3d$ band of $\mathrm{Ni}\left(111\right)$ are indicated with symbols, while high symmetry points of the Brillouin zone $(\mathrm{\Gamma },K)$ are indicated with vertical white arrows. (b) Calculated majority band structure of $\mathrm{Gr}/\mathrm{Ni}\left(111\right)$. The graphene $2p_z$ contributions are highlighted in red (thick lines).

Figure 2

(a) Experimental band structure of graphene on $\mathrm{Ni}\left(111\right)$, where the $K$ point is indicated by vertical white arrows $(\downarrow )$. The insets above show the energy distribution curves (EDCs) near the $K$ point [the $K$-point EDC is indicated in red (thick line)] as a function of detection angle. The EDCs energy range is from $-12\mathrm{to}1\mathrm{eV}$. (b) Same as (a) but after adsorption of 0.8 monolayer Na on top; the minimum of the ${\pi }^{*}$ state is visible at the Fermi level. The yellow vertical arrows $(\uparrow )$ indicate roughly the maximum of the $\pi$ state, established from the state turning point. (c) Same as (b) after further annealing to intercalate Na to underneath graphene.

Figure 3

Band structures for (a) freestanding graphene, (b) $\mathrm{Gr}/\mathrm{Ni}\left(111\right)$, (c) $\mathrm{Na}/\mathrm{Gr}/\mathrm{Ni}\left(111\right)$, and (d) $\mathrm{Gr}/\mathrm{Na}/\mathrm{Ni}\left(111\right)$ in a $(2\times 2)$ supercell, corresponding to a Na coverage of 0.75 ML. The contributions of graphene $2p_z$ orbitals are highlighted in red (thick line) in panels (b)–(d).

Figure 4

Charge density difference plots, using an isosurface of $\pm 0.02e/{\AA }^3$ for (a) $\mathrm{Gr}/\mathrm{Ni}\left(111\right)$, (b) $\mathrm{Na}/\mathrm{Gr}/\mathrm{Ni}\left(111\right)$, and (c) $\mathrm{Gr}/\mathrm{Na}/\mathrm{Ni}\left(111\right)$; charge density accumulation is shown in red and depletion in blue. The adsorption energy of Na on $\mathrm{Gr}/\mathrm{Ni}$ is $-1.00\mathrm{eV}$ at a Na coverage of 1 ML, referenced to the total energies of atomic Na and $\mathrm{Gr}/\mathrm{Ni}\left(111\right)$.