Kinetically inhibiting lying-standing transitions to control growth of TCNE on Cu(111)

Interface polymorphs strongly govern the functionality of organic semiconductors. Especially when planar molecules adsorb in different orientations relative to the surface, properties significantly vary. To utilize this asset for tailoring interfaces often metastable structures must be stabilized, which might not be put into practice straightforwardly.

Here, we study tetracyanoethylene (TCNE) on Cu(111). Its flat-lying and upright-standing structures feature a notable difference in the work function. In thermodynamic equilibrium, TCNE adsorption starts in the lying position until a coverage-driven reorientation to the standing position takes place within the first monolayer.

To prohibit this work function change, the first monolayer has to be retained in the lying orientation even at higher coverages. This requires a temperature that is (i) sufficiently high to allow unconstrained diffusion in the lying orientation (ensure ordered growth) and (ii) low enough to inhibit the lying-standing transition. For this purpose, we combine density functional theory with harmonic transition state theory and the nudged elastic band method to obtain diffusion and reorientation rates. Based on these rates a suitable range of process temperatures is proposed to stabilize the lying structure.