Phantom Force Induced by Tunneling Current: A Characterization on Si(111)

Simultaneous measurements of tunneling current and atomic forces provide complementary atomic-scale data of the electronic and structural properties of surfaces and adsorbates. With these data, we characterize a strong impact of the tunneling current on the measured force on samples with limited conductivity. The effect is a lowering of the effective gap voltage through sample resistance which in turn lowers the electrostatic attraction, resulting in an apparently repulsive force. This effect is expected to occur on other low-conductance samples, such as adsorbed molecules, and to strongly affect Kelvin probe measurements when tunneling occurs.

Figure 1

(a) From a Morse potential, the force and force gradient can be calculated. At $V_{\mathrm{tip}}=-1.5\mathrm{V}$, (b) current and (c) force data are collected without $I$ or $\Delta f$ feedback. Similar data are shown at $V_{\mathrm{tip}}=+1.5\mathrm{V}$; the (d) current data appears inverted because in opposite bias voltages, current flow is reversed, however (e) $\Delta f$ again increases above the adatoms. Data were collected at $A=400\mathrm{pm}$, $f_0=25908\mathrm{Hz}$. In STM data, a unit cell of $7\times 7$ is highlighted; images are $10\mathrm{nm}\times 5.5\mathrm{nm}$.

Figure 2

(a) From Figs. 1b, 1c, a comparison between $I$ and $\Delta f$ can be made. Their relationship is linear; the line shown has a slope of $4.36\mathrm{Hz}/\mathrm{nA}$. (b) Following similar analyses at other biases ($V_{\mathrm{tip}}=-1.5$, $-1.0$, 1.0 and 1.5 V) the slopes of the fits can be plotted as a function of the bias. $I$ has the opposite sign as $V_{\mathrm{tip}}$. A linear fit is shown as a guide to the eye.

Figure 3

Top: Schematic of the experiment. Bottom: Simultaneous $I$ and $\Delta f$ data acquired at 4.5 K. (a) $V_{\mathrm{tip}}=-1.5\mathrm{V}$.(b) $V_{\mathrm{tip}}=0\mathrm{V}$.(c) Tip is approached 340 pm closer to the surface. Data were collected at $A=100\mathrm{pm}$, $f_0=16777\mathrm{Hz}$, and images are $8\mathrm{nm}\times 8\mathrm{nm}$.

Figure 4

Top: Schematic of the experiment. Middle: Simultaneous $I$ and $\Delta f$ data. Bottom: $\Delta f$ versus $I$ data. (a) $R=0$ between sample and ground. (b) $R_1=10\mathrm{M}\Omega$ (c) $R_2=30\mathrm{M}\Omega$. See text for details. Data were acquired with $V_{\mathrm{tip}}=1.5\mathrm{V}$, $A=400\mathrm{pm}$, $f_0=19390\mathrm{Hz}$ and are $10\mathrm{nm}\times 7\mathrm{nm}$.