Controlled switching of a single CuPc molecule on Cu(111) at low temperature

Low temperature measurements of the tunneling current as a function of the applied bias voltage have been performed on a dense constant-height grid above individual copper phthalocyanine molecules adsorbed on a Cu(111) surface. By appropriate tuning of the applied bias, the molecule can be reversibly switched between two configurations in which pairs of opposite maxima appear rotated by ${90}^{\circ }$ in the tunneling current map. The underlying conformations are revealed by density functional calculations including van der Waals interactions: a $C_{2v}$ symmetric ground state and two energetically equivalent states, in which the molecule is twisted and rotated around its center by $\pm 7^{\circ }$. For tip biases above 200 mV position-dependent current switching is observed, as in previous measurements of telegraph noise [Schaffert et al., Nat. Mater. 12, 223 (2013)]. In a small voltage interval around zero the measured current becomes bistable. Switching to a particular state can be initiated by sweeping the voltage past well-defined positive and negative thresholds at certain positions above the molecule or by scanning at constant current and a reduced reverse bias.

Figure 1

(a) Chemical structure of copper phthalocyanine (CuPc). (b) Lowest energy adsorption configuration according to DFT calculations: the central Cu atom and two benzol rings are located above substrate bridge sites and define the B axis of the molecule, while the other two benzol rings are stabilized above hollow sites to define the molecular H axis. (c) STM topographic image of CuPc on Cu(111) at 5 K. Measurement parameters: $U_{\text{tip}}=-200\mathrm{mV}$; $I_{\text{t}}=-30\mathrm{pA}$. The gas-phase $D_{4h}$ symmetry of the molecules is reduced to $C_{2v}$. The flat-lying molecules adopt three different orientations along the $\left[1\overline{1}0\right]$, $\left[01\overline{1}\right]$, and $\left[\overline{1}01\right]$ substrate directions marked by circles and act as charged scatterers for electrons injected into unoccupied surface states.

Figure 2

(a) Constant height maps of the tunneling current of CuPc on Cu(111) at different tip voltages revealing a bias-dependent contrast transition, size $2.5\times 2.5{\mathrm{nm}}^2$. (b)–(d) Single $I_t\left(U_{\text{tip}}\right)$ curves extracted above the molecular center, above a B and above a H lobe (see black crosses in the inset maps recorded at $-500\mathrm{mV}$). Two position selective characteristic phenomena related to the contrast transition are apparent: telegraph noise (blue area) and a hitherto unobserved current hysteresis around zero bias (green area).

Figure 3

Interpretation of the contrast change as a frustrated rotation of the CuPc molecule around its center. (a), (b) Constant height maps from Figs. 2 and 2 at $-500\mathrm{mV}$, and $300\mathrm{mV}$, but with identical color coding. (c), (d) Side views of the DFT calculated adsorption geometries in the symmetric configuration and that rotated by $7^{\circ }$; out of plane displacements are magnified by a factor of four for better visualization. The direction of local height changes of the H lobes (c) and B lobes (d) are indicated by arrows. (e), (f) Constant current STM images demonstrating frustrated rotation induced by switching the bias between repeated scans over a molecule. Parameters: (e) $U_{\text{tip}}=-150\mathrm{mV}$, $I_t=-20\mathrm{pA}$; (f) $U_{\text{tip}}=50\mathrm{mV}$, $I_t=20\mathrm{pA}$.

Figure 4

Schematic representation of the voltage dependent rotation of the CuPc molecule. Applying a positive tip bias voltage above a certain threshold the molecule randomly jumps between the two $\pm 7^{\circ }$ rotated conformations passing in every jump the $0^{\circ }$ position in the telegraph noise region. By reducing the voltage either the $+7^{\circ }$ or $-7^{\circ }$ rotated conformation is adopted (for a better overview only one of the two orientations is shown) before the molecule switches to the $0^{\circ }$ position at a certain negative tip voltage. Contrarily, while increasing the voltage from negative to positive tip biases the molecule stays in the $0^{\circ }$ position until it switches again to one of the rotated states at positive tip bias values.