Frequency-modulated atomic force spectroscopy on NiAl(110) partially covered with a thin alumina film

Force spectroscopy has been performed using a low-temperature scanning tunneling microscope (STM) and atomic force microscope (AFM) with small amplitude frequency modulation (FM). Frequency shift versus distance curves acquired on NiAl(110) are compared to measurements performed on a thin alumina film. Interaction force and energy are determined from the frequency shift. Due to the high stability of small amplitude frequency modulation in combination with a stiff force sensor, it is possible to observe clear differences in the interaction potential between the metal and oxide surface. The setup also allows us to gain specific information in the repulsive regime of the contact formation, where elastic and plastic stages have been identified.

Figure 1

Schematic setup of the FM-AFM and STM operated in UHV. Definition of parameters: the tip oscillates with amplitude $A$ and the nearest distance between tip apex and sample surface is $D$.

Figure 2

STM image of NiAl(110) substrate partially covered by alumina. The unit cells of the two domains of the thin oxide film are marked by A and B in reference to the orientation of NiAl(110). A dashed line marks the decorated border, between the oxide film and the metal substrate. Scan size $20\mathrm{nm}\times 20\mathrm{nm}$, $I_t=0.025\mathrm{nA}$, $V_{\text{sample}}=-500\mathrm{mV}$.

Figure 3

The measured data [(a), (d)] have been obtained in UHV at $5\mathrm{K}$. $V_{\text{sample}}=0\mathrm{mV}$. (a) $\Delta f\left(z\right)$ curve recorded on NiAl(110), (b) recovered force of [(a), (c)] recovered energy of [(a), (d)] $\Delta f\left(z\right)$ curve recorded on thin alumina film, and (e) recovered force of [(d), (f)] recovered energy of (d).

Figure 4

Frequency shift versus $z$-displacement curves obtained in UHV at $5\mathrm{K}$ on NiAl(110) [(a), (b)] and thin alumina film [(c), (d)]. The black circles mark the approach and the white rectangles the retraction between tip and sample. In (b) an enlargement of (a) is given, where elastic (A) and plastic (B) stages accompanied by atomic structural rearrangement are marked.

Figure 5

STM images of NiAl(110) substrate partially covered with alumina before (a) and after (b) the acquisition of frequency shift versus $z$-displacement curves. A cross surrounded by a square marks the tip position on the oxide and a cross surrounded by a circle marks the tip position on the metal, where spectroscopy curves have been obtained consecutively. Scan size $27\mathrm{nm}\times 27\mathrm{nm}$, $I_t=0.01\mathrm{nA}$, $V_{\text{sample}}=50\mathrm{mV}$, and $T=5\mathrm{K}$.