Manipulation of the Charge State of Single Au Atoms on Insulating Multilayer Films

We show charge-state manipulation of single Au adatoms on 2–11 monolayer (ML) thick NaCl films on Cu surfaces by attaching or detaching single electrons via the tip of an atomic force microscope (AFM). Tristate charge control (neutral, negatively charged, and positively charged) is achieved. On Cu(100) and Cu(111) supports, charge tristability is achieved independently of the NaCl layer thickness. In contrast, on Cu(311), only Au anions are stable on the thinnest NaCl films, but neutral and positive charge states become sufficiently long lived on films thicker than 4 ML to allow AFM-based charge-state-manipulation experiments.

Figure 1

Manipulation of the charge state of a Au adatom on $\mathrm{NaCl}(10\mathrm{ML})/\mathrm{Cu}(111)$ by attaching or detaching single electrons via a probe tip. (a) Schematic $\mathrm{\Delta }f(V)$ spectrum of a closed charge-switching cycle (${\mathrm{Au}}^{-}\to {\mathrm{Au}}^0\to {\mathrm{Au}}^{+}\to {\mathrm{Au}}^{-}$). The dotted parabolas visualize the change of the LCPD (the triangles) by a change of the charge state. (b)–(d) Experimental manipulation spectra for stepwise manipulation from ${\mathrm{Au}}^{-}\to {\mathrm{Au}}^0$, ${\mathrm{Au}}^0\to {\mathrm{Au}}^{+}$, and ${\mathrm{Au}}^{+}\to {\mathrm{Au}}^{-}$, respectively. In each case, the tip was moved above the Au adatom in constant-$\mathrm{\Delta }f$ mode and the feedback loop was then interrupted. (e)–(h) Determination of the adsorption position at each stage by constant-height $\mathrm{\Delta }f$ images with a different set point on the NaCl substrate (top and bottom scan lines) and on the Au adatom (middle part). A blue grid is drawn on top of the Na sublattice. Black and white crosses indicate the position of the Au adatom. Imaging conditions are $V=0\mathrm{V}$, $z=13.3$ (8.8) Å in top and bottom (middle) scan lines, $z=0$ corresponds to a STM set point ($V=4.2\mathrm{V}$, $I=40\mathrm{fA}$) above NaCl. All images are of the size $50\times 40{\AA }^2$.

Figure 2

Three-dimensional topographical AFM image of three Au adatoms on a $\mathrm{NaCl}(10\mathrm{ML})/\mathrm{Cu}(111)$ film with charge states neutral, positively charged, and negatively charged. Height profiles are shown in the inset figure (solid line: ${\mathrm{Au}}^{-}$, dashed line: ${\mathrm{Au}}^0$, dotted line: ${\mathrm{Au}}^{+}$). Imaging details: Const. $\mathrm{\Delta }f$ feedback, $\mathrm{\Delta }f=-2\mathrm{Hz}$, $V=0\mathrm{V}$, image size $60\times 35{\AA }^2$.

Figure 3

Neutral, positively charged, and negatively charged Au adatoms on a $\mathrm{NaCl}(2\mathrm{ML})/\mathrm{Cu}(100)$ film. (a) Constant-current image ($V=0.1\mathrm{V}$, $I=0.45\mathrm{pA}$, image size $65\times 65{\AA }^2$). (b) Charge-state readout by recording $\mathrm{\Delta }f(V)$ spectra (dots) and determining the LCPD (triangles) from fitted parabolas (dashed lines). The tip was moved above the Au adatoms in constant-current mode ($V=0.1\mathrm{V}$, $I=0.45\mathrm{pA}$); the tip position is indicated by circles in (a).

Figure 4

Lifetime measurement of the ${\mathrm{Au}}^0$ state on $\mathrm{NaCl}(4\mathrm{ML})/\mathrm{Cu}(311)$. (a) STM topography image indicating the position of the tip during data acquisition; the tip was positioned 8 Å above the Au adatom. Imaging conditions: $V=1.5\mathrm{V}$, $I=20\mathrm{fA}$, image size $200\times 200{\AA }^2$. (b) Time lapse of the $\mathrm{\Delta }f$ signal, reflecting the charge state of the Au adatom. A bias of $V=-1.7\mathrm{V}$ was applied to transfer the electron from the Au anion to the tip.