Control of the Charge State of Metal Atoms on Thin MgO Films

The arrangement of single gold and palladium atoms deposited on the surface of a 3 monolayer thin film of MgO was investigated using low-temperature scanning tunneling microscopy. While Pd atoms are arranged in a random fashion, Au forms an ordered array on the surface. The long-range ordering as well as the scanning tunneling microscopy appearance of single Au atoms on a 3 monolayer thin MgO film can be explained by partial charge transfer from the substrate to Au atoms as predicted recently by density functional theory calculations [Phys. Rev. Lett. 94, 226104 (2005)]. In contrast with that, Au atoms on a thick film were found to be essentially neutral.

Figure 1

STM images ($30\mathrm{nm}\times 30\mathrm{nm}$) of (a) Au atoms adsorbed on 3 ML thin MgO films $\theta =2.9\%\mathrm{ML}$ Au, $V_S=+0.5\mathrm{V}$, $I_T=11\mathrm{pA}$); (b) Pd atoms adsorbed on 3 ML thin MgO films ($\theta =2.3\%\mathrm{ML}$ Pd, $V_S=+0.2\mathrm{V}$, $I_T=13\mathrm{pA}$); (c) Au atoms adsorbed on bare Ag(001) ($\theta =2.9\%\mathrm{ML}$ Au, $V_S=+0.1\mathrm{V}$, $I_T=100\mathrm{pA}$).

Figure 2

Normalized radial pair distribution function $\rho =f_{\exp }(r)/f_{\mathrm{rand}}(r)$ for pair distances up to $r=50\AA$ for (a) Au atoms adsorbed on $\mathrm{MgO}(3\mathrm{L})/\mathrm{Ag}(001)$, (b) Pd atoms adsorbed on $\mathrm{MgO}(3\mathrm{L})/\mathrm{Ag}(001)$, and (c) Au atoms adsorbed on bare Ag(001). $\rho =1$ corresponds to random site occupation.

Figure 3

Experimental STM images and height profiles, and corresponding simulated STM images of single Au (a–c) and Pd (d–f) atoms on the surface of 3 ML $\mathrm{MgO}/\mathrm{Ag}(001)$, respectively. The bias voltage in the experimental and simulated STM images is $V_S=+0.5\mathrm{V}$.