Electronic Structure of MgO-Supported Au Clusters: Quantum Dots Probed by Scanning Tunneling Microscopy

We investigate via density functional theory (DFT) the appearance of small MgO-supported gold clusters with 8 to 20 atoms in a scanning tunneling microscope (STM) experiment. Comparison of simulations of ultrathin films on a metal support with a bulk MgO leads to similar results for the cluster properties relevant for STM. Simulated STM pictures show the delocalized states of the cluster rather than the atomic structure. This finding is due to the presence of $s$- derived delocalized states of the cluster near the Fermi energy. The properties of theses states can be understood from a jellium model for monovalent gold.

Figure 1

(a) ${\mathrm{Au}}_8$ at $F_s$ on $\mathrm{MgO}(3\mathrm{ML})/\mathrm{Ag}(001)$ (in the order of increasing darkness: Mg white, Au yellow, Ag grey, O red). (b) The corresponding cluster adsorbed at $F_s$ on MgO(001). (c) a LDOS plot and (e) a simulated STM image for structure (a). (d) a LDOS plot and (f) a simulated STM image for structure (b). The Fermi energy is at zero in (c) and (d). The horizontal and vertical arrows in (c) and (d) show the state(s) from which the corresponding STM images have been constructed. The positions of the Au atoms are indicated by the diamonds in (e) and (f).

Figure 2

A side view (a) and a top view (b) of a flat, geometrically closed-shell ${\mathrm{Au}}_{20}/F_s/\mathrm{MgO}$. The Au atom closest to the $F_s$ is marked by dark (blue) shading.

Figure 3

(a) Left: LDOS of ${\mathrm{Au}}_{20}$ from Fig. 2. The energy levels of the Kohn-Sham states with a clear delocalized character are indicated in the middle, and the corresponding wave functions are plotted on the right. The simulated STM pictures (b) and (c) show the two occupied, $2p$-like states below the Fermi energy (d) the sum of (b) and (c). (e) and (f) show the first two empty states and (g) their sum. The atom positions are indicated by diamonds in (b), (c), (e), and (f).

Figure 4

Energy levels and electron shell closings of the 2D harmonic oscillator model for a planar quantum dot.

Figure 5

(a) ${\mathrm{Au}}_{13}/\mathrm{MgO}$ and simulated STM images for (b) HOMO, (c) LUMO. The atom positions are marked by diamonds in (b) and (c). The pinning Au atom is marked by dark (blue) shading.