EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Polymorphism represents the ability of materials to crystallize into different forms. These crystal forms are called polymorphs and show different physical properties. It has been reported that some new polymorphic forms could only form near a solid substrate and are called substrate-induced polymorphs (SIPs). Recently, SIPs of organic molecules have drawn increasing attention since they can possess distinct physical properties compared with the bulk phases. SIPs are conceptually different from physisorbed self-assembled molecular networks (SAMNs) of organic molecules as SIPs extend at least over several molecular layers. The SAMNs of organic molecules have been studied extensively in the last couple of decades. However, the connection between the SAMNs and SIPs for organic molecules is still unclear.

The compound of interest for this study is a prototypical molecular semiconductor: lead phthalocyanine (PbPc). We investigated the possibility of SIP formation for PbPc on highly oriented pyrolytic graphite (HOPG) by a combined experimental-modelling approach. The physisorbed self-assembled monolayers of PbPc formed at the solution/HOPG interface were studied using scanning tunneling microscopy (STM). STM images reveal long-range ordered self-assembled networks. A multiscale computational chemistry approach, combining quantum chemical calculations and forcefield simulations, was then employed to gain energetic and structural insights into the assembly. The excellent agreement found between the simulated STM images and the experimental data allowed determining unambiguously the molecular orientation at the surface.

The growth of a SIP templated by the physisorbed monolayer was then modelled by successive adsorption of up to four molecular layers, focusing on the comparison with the two bulk polymorphs known for PbPc. Complementarily, the atomically-resolved structure of few nm-thick PbPc films was determined by transmission electron microscopy (TEM) and compared with the modelled SIP, to gain an integrated view over the 2D to 3D assembly of this molecular semiconductor.