Controlled sequential dehydrogenation of single molecules by scanning tunneling microscopy

Scanning tunneling microscopy (STM) is today the most powerful and versatile tool available for imaging and manipulating single molecules on surfaces. Here, we explore its ultimate limit by demonstrating the possibility of controlling sequential di-dehydrogenation of single Co-Salen molecules sublimated on Cu. In particular, we are able to explore the final products of the ${\text{H}}_2$ dissociation as well as the intermediate state, in which only one H atom is separated from the molecule. This is achieved by low-temperature STM with the dissociation induced by either point spectroscopy or in the standard constant-current mode. Crucial for the interpretation of the data is our ability to perform state-of-the-art density-functional theory simulations of both topographic and spectroscopical STM images. This work demonstrates that STM combined with theory can give access to the atomic details of a chemical reaction even when the reaction products are completely unknown.

Figure 1

(Color online) The three Co-Salen molecules investigated in this work: (1) intact (${\text{C}}_2{\text{H}}_4{\text{N}}_2$ apex), (2) single dehydrogenated (${\text{C}}_2{\text{H}}_3{\text{N}}_2$ apex), and (3) double dehydrogenated (${\text{C}}_2{\text{H}}_2{\text{N}}_2$ apex). Top and side views show the structures after numerical relaxation on the Cu(111) surface.

Figure 2

(Color online) Manipulation of intact Co-Salen (1) by constant-current imaging into its single-(2) and double-(3) dehydrogenated form at a bias voltage above the threshold for manipulation $\left(m\text{Mode}\right)$. (a) An area with six (1) is selected. Two molecules serve as unaltered reference and are only imaged at low bias voltage $\left(i\text{Mode}\right)$. Molecules in Sec. 1 are imaged once, the molecule in Sec. 2 twice in the $m\text{Mode}$. The top panel (b) shows the same molecule obtained during imaging in the $i\text{Mode}$ and the $m\text{Mode}$. Whereas $i\text{Mode}$ imaging does not affect the molecule, in each of the $m\text{Mode}$ images a characteristic jump in the line scans is revealed $(\cdots )$. The first dehydrogenation is detected by a sudden reduction in the molecule apparent height (second panel). During the second dehydrogenation the molecule adopts a mirror symmetric shape (third panel). (c) shows the surface again in the $i\text{Mode}$. The reference molecules remained unmodified, whereas the molecules in Secs. 1, 2 transformed into (2) and (3). The top panel (b) shows the same molecule obtained during imaging in the $i\text{Mode}$ and the $m\text{Mode}$ (with the scan directions as indicated).

Figure 3

(Color online) The transformation from (1) into (2) and (3) is efficiently achieved by voltage pulses above the ${\text{C}}_2{\text{H}}_4$ bridge at constant height. With the current monitored, characteristic jumps reveal the moment of dehydrogenation which is only observed above the threshold voltage of 1.5 V. In (a) the recorded current during such a manipulation is presented which shows two abrupt changes. This is indicative for the transition from (1) into (2) and finally (3). Respectively, the images before and after (see insets) show complex (1) and (3). (b) With the voltage limited to 1.5 V, STS data on the differential conductivity manifest a quantitative change in the spectra. Whereas no significant change in the occupied states is found $\left(U<0\right)$, unoccupied states $\left(U>0\right)$ are shifted toward higher energies after dehydrogenation. The copper spectrum is recorded for the characterization of the tip status at a location nearby with the clear signature of the present surface state.

Figure 4

(Color online) STM/STS maps for the three molecules investigated. In (a) we present the experimental and the simulated topographs of Co-Salen adsorbed on Cu(111). The images were taken at a voltage of $U=1.4\text{V}$. Note that the mirror symmetry is broken for (1) and (2) but not for (3). In panel (b) the experimentally measured STS maps are compared with the calculated ones at three representative voltages of 800, 1000, and 1400 mV. The color scale is identical within each row and the very same tip is used for all the images. In the simulated images the periodic replica of the unit cell appear at the edge of the figure. In addition to a general increase in the molecule’s lateral extension with bias, we remark three main features: (1) in (1) the largest intensity is on the left-hand side of the ${\text{C}}_2{\text{H}}_4{\text{N}}_2$ apex and shifts to give a more symmetric image at higher $U$; (2) is similar to (1) except that the asymmetry persists at higher $U$; and (3) remains symmetric at all voltages with the maximum intensity at ${\text{C}}_2{\text{H}}_2{\text{N}}_2$ apex for small $U$ and a shallow minimum at the same position for large $U$.

Figure 5

(Color online) Electronic wave-function isosurfaces of the frontier molecular orbitals of Co-Salen in the (1) (left) and (3) (right) configurations. The different colors represent the sign of the wave function and the levels are align to have the HOMO at $E=0$. Note that, while the filled states are all symmetric with respect to a median plane passing through the Co, the empty ones of molecule (1) are asymmetric. Note also that the asymmetry develops for level at energies larger than 1 eV above the HOMO while the STM images are asymmetric already for voltages of around 0.5 V.