Sensing Dipole Fields at Atomic Steps with Combined Scanning Tunneling and Force Microscopy

The electric field of dipoles localized at the atomic steps of metal surfaces due to the Smoluchowski effect were measured from the electrostatic force exerted on the biased tip of a scanning tunneling microscope. By varying the tip-sample bias the contribution of the step dipole was separated from changes in the force due to van der Waals and polarization forces. Combined with electrostatic calculations, the method was used to determine the local dipole moment in steps of different heights on Au(111) and on the twofold surface of an Al-Ni-Co decagonal quasicrystal.

Figure 1

(a) STM-AFM configuration using a conductive cantilever bending in response to forces. (b) The $70\mathrm{nm}\times 70\mathrm{nm}$ STM image of a Pt(111) surface ($V_t=-0.2\mathrm{V}$, $I=0.16\mathrm{nA}$). (c) Height and force profile across the steps. The force on the tip is more attractive at the bottom of the steps and less attractive at the top. (d) Force image simultaneously acquired with (b). Yellow and blue colors represent low and high attractive forces, respectively.

Figure 2

(a) Height and force profiles across steps for positive and negatively biased tip ($I=0.1\mathrm{nA}$) on the Al-Ni-Co quasicrystal surface showing steps of multiple heights (0.2, 0.5, 0.8, and 1.3 nm). (b) Height and force profile across steps on a Au(111) surface. Small relative peak shifts in the force profiles are caused by noise and thermal drift. $V_t$ is the tip voltage with respect to the sample.

Figure 3

Force and current-distance curves measured on Au(111) at a tip bias of $-3\mathrm{V}$. Before contact the electrostatic force bends the tip towards the surface. This attraction is used to calculate the tip radius (inset), from the slope of $F$ vs $1/D$, yielding $R=30\mathrm{nm}$.

Figure 4

(a) Difference in the force experienced by the tip at the steps for positive and negative bias, per unit applied volt $[\mathrm{nN}/\mathrm{V}]$. Open symbols correspond to steps on Al-Ni-Co quasicrystal surface. Filled symbols to steps on Au(111). The lines are calculations for 1 and 0.45 D per step atom, respectively; by definition they pass through the origin. The error bar is associated with the noise level of force measurement. (b) Electric field distribution calculated using the GICM in the tip-sample region with a permanent dipole close to the tip apex. ($R=30\mathrm{nm}$, $D=0.5\mathrm{nm}$.)