From a bistable adsorbate to a switchable interface: tetrachloropyrazine on Pt(111)

Abstract

Virtually all organic (opto)electronic devices rely on organic/inorganic interfaces with specific properties. These properties are, in turn, inextricably linked to the interface structure. Therefore, a change in structure can introduce a shift in function. If this change is reversible, it would allow constructing a switchable interface. We accomplish this with tetrachloropyrazine on Pt(111), which exhibits a double-well potential with a chemisorbed and a physisorbed minimum. These minima have significantly different adsorption geometries allowing the formation of switchable interface structures. Importantly, these structures facilitate different work function changes and coherent fractions (as would be obtained from X-ray standing wave measurements), which are ideal properties to read out the interface state. We perform surface structure search using a modified version of the SAMPLE approach and account for thermodynamic conditions using ab initio thermodynamics. This allows investigating millions of commensurate as well as higher-order commensurate interface structures. We identify three different classes of structures exhibiting different work function changes and coherent fractions. Using temperature and pressure as handles, we demonstrate the possibility of reversible switching between those different classes, creating a dynamic interface for potential applications in organic electronics.

Fig. 1. Cut through the GPR PES of TCP on Pt(111) in [110] direction; (red) PES of the on-state geometry; (blue) PES of the off-state geometry; energies are plotted relative to the respective minimum energy E_min.

Fig. 2. Adsorption energies per area for the 1000 energetically most favourable, as well as the optimised on-, off- and mixed-state motifs at all considered coverages; the lines show the convex hull of the minimum energy; the shaded areas around the convex hulls show the prediction uncertainty.

Fig. 3. (a and b) Thermodynamically populated phase diagram showing (a) the expectation value of ΔΦ and (b) the expectation value of the coherent fraction of C atoms, hatched areas indicate the thermodynamic range where adsorption is not energetically favourable; (c and d) cut through the phase diagram at a constant pressure of 10⁻⁶ Pa showing expectation values for (c) ΔΦ and (d) the coherent fraction separately for on-, off- and mixed-state motifs; the line width shows how much each type of motif contributes to the thermal occupation of all states.

Fig. 4. Surface dipoles and (Mulliken) charge transfer for on- and off-state geometries.