Imaging Suzuki Precipitates on $\mathrm{NaCl}:{\mathrm{Mg}}^{2+}(001)$ by Scanning Force Microscopy

We present a dynamic scanning force and Kelvin microscopy study of Suzuki precipitates on clean (001) surfaces of UHV cleaved $\mathrm{NaCl}:{\mathrm{Mg}}^{2+}$. Nanometer sized precipitates with a rectangular shape on flat terraces and also at steps were found. Images with atomic resolution confirm that the precipitates are embedded in the NaCl matrix and permit the identification of each type of ion in the unit cell of the Suzuki structure. Kelvin microscopy images show always a strong bright contrast at the precipitates, which is probably due to the negative cation vacancies in the Suzuki structure.

Figure 1

Topography (a) and corresponding Kelvin image (b) of a (001) surface of a $\mathrm{NaCl}:{\mathrm{Mg}}^{2+}$ crystal, which was cleaved and annealed in UHV. Topography (c) and Kelvin image (d) of the surface region labeled by the dotted rectangular in image (a). $A_{p-p}=23\mathrm{nm}$, $\Delta f=-32\mathrm{Hz}$, $U_{\mathrm{ac}}=0.8\mathrm{V}$, $f_{\mathrm{ac}}=474\mathrm{Hz}$; (a),(b) $770\times 560{\mathrm{nm}}^2$; (c),(d) $140\times 115{\mathrm{nm}}^2$.

Figure 2

The same crystal surface as shown in Fig. 1 at another position. The topography and corresponding Kelvin signal are shown in (a) and (b), respectively. The steps (s1) and (s2) have a height of $\sim 2\mathrm{nm}$, all other steps are single or double monoatomic high steps. $800\times 402{\mathrm{nm}}^2$, $A_{p-p}=23\mathrm{nm}$, $\Delta f=-35\mathrm{Hz}$, $U_{\mathrm{ac}}=1.5\mathrm{V}$, $f_{\mathrm{ac}}=474\mathrm{Hz}$.

Figure 3

(a) Detuning image with atomic resolution of the corner of a Suzuki precipitate that was taken in the surface region marked by the white dotted square in image (c) of Fig. 1. The rectangular shape of the precipitate appears much less distorted than in the images of Fig. 1, which was due to the considerably higher scanning speed of 12 Hz ($12\times 10{\mathrm{nm}}^2$, $A_{p-p}=23\mathrm{nm}$, $\Delta f=-177\mathrm{Hz}$). (b) Drawing of the expected Suzuki phase inside the NaCl matrix.

Figure 4

(a) Detuning image with atomic resolution of a Suzuki precipitate (recorded without Kelvin modulation). $3.3\times 3.3{\mathrm{nm}}^2$, $A_{p-p}=46\mathrm{nm}$, $\Delta f=-24.9\mathrm{Hz}$. (b) Drawing explaining the atomic structure of the precipitate.