Imaging Pauli Repulsion in Scanning Tunneling Microscopy

A scanning tunneling microscope (STM) has been equipped with a nanoscale force sensor and signal transducer composed of a single ${\mathrm{D}}_2$ molecule that is confined in the STM junction. The uncalibrated sensor is used to obtain ultrahigh geometric image resolution of a complex organic molecule adsorbed on a noble metal surface. By means of conductance-distance spectroscopy and corresponding density functional calculations the mechanism of the sensor and transducer is identified. It probes the short-range Pauli repulsion and converts this signal into variations of the junction conductance.

Figure 1

(a) STM (top) and STHM image (bottom) of PTCDA (3,4,9,10-perylenetetracarboxylic-dianhydride) on Au(111): $1.3\times 0.7{\mathrm{nm}}^2$, constant height, ${\mathrm{D}}_2$, $V=316\mathrm{mV}$ (STM) and $V=-5\mathrm{mV}$ (STHM). The chemical structure formula of PTCDA is shown for comparison. (b) $dI/dV$ spectrum measured in the center of $\mathrm{PTCDA}/\mathrm{Au}(111)$, recorded with lock-in detection (10 mV modulation, frequency 2.3 kHz). $G_0=\frac{2e^2}{h}=(12.9\mathrm{k}\Omega {)}^{-1}$ is the quantum of conductance.

Figure 2

(a) $\frac{dI}{dV}(z)$ spectra recorded at fixed bias, measured with ${\mathrm{D}}_2$ on $\mathrm{PTCDA}/\mathrm{Au}(111)$ ($z$-axis scale given by scale bar). $G_0=\frac{2e^2}{h}$ is the quantum of conductance. $G_{{\mathrm{D}}_2}$ (magenta): $V=-5\mathrm{mV}$. $G_{\mathrm{vac}}$ (black): $V=-130\mathrm{mV}$. All spectra recorded from the same stabilization point. (b) $R_G(z)$ curves (cf. text) measured at positions ${\overrightarrow{r}}_1$ (blue) and ${\overrightarrow{r}}_2$ (red) marked in the lower inset. The ratio $G_{\mathrm{vac}}({\overrightarrow{r}}_1)/G_{\mathrm{vac}}({\overrightarrow{r}}_2)$ is shown in green. (c) STHM images ($1.3\times 1.3{\mathrm{nm}}^2$, constant height, $V=-5\mathrm{mV}$) of $\mathrm{PTCDA}/\mathrm{Au}(111)$ measured with ${\mathrm{D}}_2$ at different $z$ indicated by black triangles in panel b.

Figure 3

DFT-LDA simulated $n_{t,{\mathrm{D}}_2}(E_F,z^{\prime })/n_{t,\mathrm{vac}}(E_F)$ vs ${\mathrm{D}}_2$-tip distance $z^{\prime }$ for $s$- (black) and $p$-type (red) orbitals at the Au atom below the deuterium molecule. $n_{t,{\mathrm{D}}_2}(E_F,z^{\prime })$ is the LDOS of the model tip at the Fermi level at given ${\mathrm{D}}_2$-tip distance $z^{\prime }$ and $n_{t,\mathrm{vac}}(E_F)$ is LDOS at the Fermi level of the bare tip electrode. Equilibrium distance $z_{\mathrm{eq}}^{\prime }$ is indicated by the vertical line. For details of the simulation cf. Ref. 17.

Figure 4

STHM image of a Au dimer on Au(111) (constant height, $V=2\mathrm{mV}$, ${\mathrm{D}}_2$) (bottom panel) and schematic sketch of contrast generation. See text for details.