Diamond (111) surface reconstruction and epitaxial graphene interface

The evolution of the diamond (111) surface as it undergoes reconstruction and subsequent graphene formation is investigated with angle-resolved photoemission spectroscopy, low energy electron diffraction, and complementary density functional theory calculations. The process is examined starting at the $\mathrm{C}\left(111\right)\text{−}(2\times 1)$ surface reconstruction that occurs following detachment of the surface adatoms at 920 ${}^{\circ }\mathrm{C}$, and continues through to the liberation of the reconstructed surface atoms into a freestanding monolayer of epitaxial graphene at temperatures above 1000 ${}^{\circ }\mathrm{C}$. Our results show that the $\mathrm{C}\left(111\right)\text{−}(2\times 1)$ surface is metallic as it has electronic states that intersect the Fermi level. This is in strong agreement with a symmetrically $\pi \text{-bonded}$ chain model and should contribute to resolving the controversies that exist in the literature surrounding the electronic nature of this surface. The graphene formed at higher temperatures exists above a newly formed $\mathrm{C}\left(111\right)\text{−}(2\times 1)$ surface and appears to have little substrate interaction as the Dirac point is observed at the Fermi level. Finally, we demonstrate that it is possible to hydrogen-terminate the underlying diamond surface by means of plasma processing without removing the graphene layer, forming a graphene-semiconductor interface. This could have particular relevance for doping the graphene formed on the diamond (111) surface via tuneable substrate interactions as a result of changing the terminating species at the diamond-graphene interface by plasma processing.

Figure 1

Structure models, surface quality, and schematics detailing the nomenclature of the BZs used throughout the article. (a)–(c) The optimized lowest energy configuration positions calculated by DFT. Namely, the $\mathrm{C}(1\times 1)\text{−}\left(111\right)$:H surface, the $\mathrm{C}\left(111\right)\text{−}(2\times 1)$ symmetric $\pi \text{-bonded}$ chain reconstructed surface, and the graphitized $\mathrm{C}\left(111\right)\text{−}(2\times 1)$ surface, respectively. The surface primitive unit cells are indicated by the black solid lines on each plan view. (d) DFT-calculated electronic structure of the symmetrically $\pi \text{-bonded}\mathrm{C}\left(111\right)\text{−}(2\times 1)$ surface, with the surface state shown in black. (e) and (f) Schematics of the reciprocal space patterns for the $(1\times 1)$ and $(2\times 1)$ lattice, respectively, along with the conventional notation used for the high-symmetry points of the BZs. The black circles represent the reciprocal lattice originating from the diamond $(1\times 1)$ surface and the gray dashed hexagon is the surface BZ with its irreducible part shaded in gray. In (f), the red colored circles represent the new $(2\times 1)$ surface reciprocal lattice that arises following reconstruction and the red dashed rectangle is the new surface BZ with its irreducible part shaded in red. (g) Surface BZ for the three-domain $\mathrm{C}\left(111\right)\text{−}(2\times 1)$ surface along with the notation used when high-symmetry points overlap as a result of the spatially averaging nature of the photoemission techniques used. Each color (red, blue, and green) represents one orientation of the $(2\times 1)$ BZ. (h) Multiple adjacent BZs with solid-colored lines that represent the almost flat band that traverses the short edge of the rectangular $(2\times 1)$ zone in red, green, and blue for their respective rotations. Averaging over space would lead to a repeating hexagram represented by all colors, at binding energies close to $E_{\text{F}}$. The location of the high-symmetry points at the corner of graphene's hexagonal BZ, ${\overline{\text{K}}}_{\text{G}}$, is also shown relative to the $(2\times 1)$ BZs.

Figure 2

$\mu \mathrm{LEED}$, ARPES, and DFT results from the $\mathrm{C}\left(111\right)\text{−}(2\times 1)$ surface reconstruction following in vacuo annealing at 920 ${}^{\circ }\mathrm{C}$. (a) $\mu \mathrm{LEED}$ pattern acquired at a kinetic energy of 64 eV; spots from all three rotational domains are observed with comparable intensity. (b) Schematic diagram of the $\overline{\mathrm{\Gamma }}\text{−}{\overline{\text{K}}}_{\text{I,II}}$ direction through the surface BZs used for the DFT and ARPES data sets presented in panels (d) and (e), respectively. (c) Constant energy surface from the ARPES data set at $E_{\text{B}}=0.1$ eV. The white arrow indicates the direction of the slice used to produce the $E$ vs $k_{\parallel }$ data shown in (e). (d) DFT results of the occupied valence band structure; the $\pi \text{band}$ originating from the reconstruction is shown in magenta and the bulk $\sigma \text{bands}$ in blue. (e) Photoemission intensity on the left-hand side and the simulated intensity on the right. Overlaid on the image is the $\pi \text{band}$ that results from the 64-atom DFT supercell calculation in magenta, along with the bulk $\sigma \text{bands}$ from the simple 2-atom unit cell DFT calculation in blue. The white vertical lines are the boundaries of the integrated area used for the EDCs at ${\overline{\text{K}}}_{\text{I,II}}$ and $\overline{\mathrm{\Gamma }}$ shown in panels (f) and (g), respectively.

Figure 3

ARPES measurements and DFT calculations of graphene formed above the $\text{C:}2\times 1$(111) surface. (a) Constant energy surface at $E_{\text{B}}=0.1$ eV using $h\nu =125$ eV. The graphene ${\overline{\text{K}}}_{\text{G}}$ point is circled in a magenta dotted line, and the white dotted lines are to guide the eye toward the hexagram features already discussed for the $\mathrm{C}\left(111\right)\text{−}(2\times 1)$ surface. (b) Schematic diagram showing the ${\overline{\text{K}}}_{\text{G}}\text{−}{\overline{\text{M}}}_{\text{G}}\text{−}\overline{\text{K}}{G}^{\prime }\text{−}\overline{\mathrm{\Gamma }}{}^{\prime }$ direction along the edge of graphene's hexagonal BZ. (c) Photoemission intensity, DFT-calculated bare bands, and simulated intensity. The ARPES data set on the left-hand side was acquired with $h\nu =40.8$ eV. Overlaid on the image are the DFT-calculated $\pi \text{band}$ as the dashed magenta line, and the same band after performing a rigid shift and stretch as a solid magenta line. The solid line is used for creating the simulated ARPES intensity on the right. (d) EDC taken at the ${\overline{\text{K}}}_{\text{G}}$ point with a width of 0.1 Å${}^{-1}$ integrated between the vertical dashed white lines in (c). (e) Constant energy surface at $E_{\text{F}}$, using $h\nu =40.8$ eV; the white arrow indicates the slice taken to extract the $E$ vs $k_{\parallel }$ data sets shown in (c). (f) The graphene atoms' cohesion energy as a function of distance from the $\mathrm{C}\left(111\right)\text{−}(2\times 1)$ surface both with and without vdW interactions.

Figure 4

XPS and UPS measurements of the diamond surface following high-temperature annealing and atomic hydrogen treatment. (a) and (b) Core level intensity for C $1s$ spectra collected at a pass energy of 20 eV and $h\nu =1253.6$ eV after 950 ${}^{\circ }\mathrm{C}$ and 1000 ${}^{\circ }\mathrm{C}$, respectively, on the type Ia diamond substrate. (c) and (d) UPS spectra at various stages of preparation of the type IIb diamond substrate collected at a pass energy of 5 eV and $h\nu =21.2$ eV. (c) SE cutoff and shape of the SE tail and (d) intensity of the integrated valence band. The regions labeled I and II are related to bulk $\sigma \text{states}$ where region III relates to $\pi \text{states}$. The inset panel in (d) shows a $\mu \mathrm{LEED}$ pattern of the surface following hydrogen termination of the graphene on the $\mathrm{C}\left(111\right)\text{−}(2\times 1)$ diamond system.