Noble-metal intercalation process leading to a protected adatom in a graphene hollow site

In previous studies, we have shown that gold deposited on a monolayer (ML) of graphene on SiC(0001) is intercalated below the ML after an annealing procedure and affects the band structure of graphene. Here we prove experimentally and theoretically that some of the gold forms a dispersed phase composed of single adatoms, being intercalated between the ML and the buffer layer and in a hollow position with respect to C atoms of the ML on top. They are freestanding and negatively charged, due to the partial screening of the electron transfer between SiC and the ML, without changing the intrinsic n-type doping of the ML. As these single atoms decouple the ML from the buffer layer, the quasiparticles of graphene are less perturbed, thus increasing their Fermi velocity. Moreover, the hollow position of the intercalated single Au atoms might lead to spin-orbit coupling in the graphene layer covering IC domains. This effect of spin-orbit coupling has been recently observed experimentally in Au-intercalated graphene on SiC(0001) [D. Marchenko, A. Varykhalov, J. Sánchez-Barriga, Th. Seyller, and O. Rader, Appl. Phys. Lett. 108, 172405 (2016)] and has been theoretically predicted for heavy atoms, like thallium, in a hollow position on graphene [C. Weeks, J. Hu, J. Alicea, M. Franz, and R. Wu, Phys. Rev. X 1, 021001 (2011); A. Cresti, D. V. Tuan, D. Soriano, A. W. Cummings, and S. Roche, Phys. Rev. Lett. 113, 246603 (2014)].

Figure 1

(a) STM picture showing the two phases (i.e., IC and AuF) due to the intercalation of gold below the graphene monolayer (ML) after several cycles of annealing. (b) Zoom-in of an IC domain, showing that the majority of gold dots are due to the aggregation of three clusters, as shown in the STM image (c). Inset in (b) Its corresponding self-correlation image, showing a quasiperiodic hexagonal arrangement of Au dots. (d) Graph of the distribution of the number of clusters per Au dots, from five STM images totaling more than 500 dots. (a) $86\times 86 {\mathrm{nm}}^2, -1.5$ V; (b) $26\times 26$ ${\mathrm{nm}}^2, -2.2$ V, $\partial z/\partial x$-derivative representation of the topography; (c) $2.3\times 2.7 {\mathrm{nm}}^2, -1.5$ V (image processing using Gwyddion and WSxM software [46]).

Figure 2

Direct comparison of (a) the experimental STM image with (b) the simulated STM image of an individual Au atom intercalated between the ML and the BuL. Image (a), $2.1\times 2.1 {\mathrm{nm}}^2$, wa made at $-1.1$ V. Image (b), simulated at $-1.3$ V, is the same size as (a), with the C atoms of the ML on top of the Au atom displayed in green. (c) The hollow position of the intercalated Au atom.

Figure 3

(a) Side view of the DFT optimized structure of a single Au atom intercalated between the BuL and the ML. The Au atom is displayed in yellow, the C in gray, and the Si in blue. Inset: Corresponding top view, showing the position of the intercalated single Au atom: nearly at a hollow site with respect to the ML (distance of 2.40 Å; C atoms displayed in light gray) and nearly at a top site with respect to the BuL (distance of 2.06 Å; C atoms in dark gray). The BuL-to-ML distance varies from 3.20 to 4.20 Å. (b) Charge density difference upon intercalation of a single Au atom between the BuL and the ML. Red (blue) regions mark accumulation (depletion) of electrons. Units are ${10}^{-3}e$.

Figure 4

C $1s$ and Si $2p$ spectra measured on pristine EG (experimental data are displayed in red) and in IC domains (in black) are presented for direct comparison. Insets: Deconvoluted peaks related to the BuL for both spectra. We also show two Au $4f$ spectra, the one in red being measured on a clean Au(111) surface as a reference [50] and the other one, in black, measured on IC domains. Inset: The two deconvoluted peaks from the IC-domain spectrum, one related to the intercalated monolayers of Au from the AuF domains (Au ML) and the other to the intercalated Au 2D dots (Au dots). The deconvolutions of all spectra are shown in Supplemental Material Fig. 3 [45] and the positions of all deconvoluted peaks are listed in Supplemental Material Table 1 [45]. See text for details.

Figure 5

(a) STM picture of an IC domain, with its corresponding self-correlation image in the inset, showing a quasiperiodic hexagonal arrangement of Au dots. (b) Pair distribution function, $f\left(r\right)$, whose envelope decays exponentially $[\propto exp(-0.997r$); fit shown by dashed red line]. (c) Graph of the self-correlation function of the atomic positions, $C\left(r\right)$, which stems from a radially averaged self-correlation image of an STM picture. From $C\left(r\right)$, we measure a mean distance between the centers of mass of two neighboring Au dots of $2.25\pm 0.07$ nm. These measurements, using Fiji [62, 63] for $f\left(r\right)$ and WSxM for $C\left(r\right)$, were made on 14 STM images at different bias voltages, using different tips and on different samples. Image (a): $22.7\times 55.6 {\mathrm{nm}}^2, -1.5$ V.

Figure 6

(a) STM picture showing an IC domain separated from a pristine ML by a rim. (b) Profile curve corresponding to the dashed blue line in the STM image. One can directly see that the graphene layer in the IC domain is almost flat between intercalated Au dots. (c) Root-mean-square roughness of different graphene layers: BuL, pristine ML; QFMLG, ML with intercalated H atoms [75]; BL, pristine bilayer graphene; and TL, pristine trilayer graphene. Some were measured without removing the atomic corrugation (black squares), whereas red triangles represent measurements of the “real” roughness of the layer. This figure is adapted from Refs. [6, 74], and [75]. Image (a): $45.4\times 27.2 {\mathrm{nm}}^2, -1.3$ V.