Nonintuitive Surface Self-Assembly of Functionalized Molecules on Ag(111)

Abstract

The fabrication of nanomaterials involves self-ordering processes of functional molecules on inorganic surfaces. To obtain specific molecular arrangements, a common strategy is to equip molecules with functional groups. However, focusing on the functional groups alone does not provide a comprehensive picture. Especially at interfaces, processes that govern self-ordering are complex and involve various physical and chemical effects, often leading to unexpected structures, as we showcase here on the example of a homologous series of quinones on Ag(111). Naively, one could expect that such quinones, which all bear the same functionalization, form similar motifs. In salient contrast, our joint theoretical and experimental study shows that profoundly different structures are formed. Using a machine-learning-based structure search algorithm, we find that this is due to a shift of the balance of three antagonizing driving forces: adsorbate–substrate interactions governing adsorption sites, adsorbate–adsorbate interactions favoring close packing, and steric hindrance inhibiting certain otherwise energetically beneficial molecular arrangements. The theoretical structures show excellent agreement with our experimental characterizations of the organic/inorganic interfaces, both for the unit cell sizes and the orientations of the molecules within. The nonintuitive interplay of similarly important interaction mechanisms will continue to be a challenging aspect for the design of functional interfaces. With a detailed examination of all driving forces, we are, however, still able to devise a design principle for self-assembly of functionalized molecules.

First-principles calculations of hybrid inorganic–organic interfaces: from state-of-the-art to best practice

“…252 Å 2 large supercell, we detect 10 different stable local minimum structures. The displayed data have been calculated with PBE+vdW surf 161 and were taken from Jeindl et al 162 The different structures and their adsorption energies are depicted in Fig. 8a .…”

“…Even for the relatively small molecule tetracyanoethylene (TCNE), a large variety of different structures has been found on various metal surfaces both theoretically 153,158,163,164 and experimentally 134,153 and there are plenty of additional examples. 162,165–167 The existence of multiple minima of the potential energy surface for even a single adsorbed molecule illustrates a fundamental problem for first-principles studies of interfaces: Unless a systematic structure search is performed, it is easy to miss the global energetic minimum, which is arguably the most relevant structure. A survey of literature indicates that systematic searches are the exception rather than the rule.…”

Interfacial Charge Transfer Influences Thin-Film Polymorphism

“…As we have previously shown, the intermolecular interactions of this molecule are qualitatively similar to those of technologically more relevant, larger analogues like 5,12-pentacenequinone. 29 − 32 …”

“…The repeated slab approach was employed using a dipole correction 76 to electrostatically decouple the periodic replicas in the z direction. Default “tight” basis sets were used for all chemical elements except Ag, for which a mixed quality numerical basis set (see ref ( 29 ) for details) was employed. A unit cell height of >80 Å was selected.…”

“…For Ag(111), the geometry of the first layer of benzoquinone was taken from an earlier work. 29 The SAMPLE approach was applied to predict the structure of the second layer. As a substrate for the adsorption of the second layer, we used a geometry that includes Ag atoms and the first layer of benzoquinone.…”