Nearly-freestanding supramolecular assembly with tunable structural properties (original) (raw)

The synthesis and design of two-dimensional supramolecular assemblies with specific functionalities is one of the principal goals of the emerging field of molecule-based electronics, which is relevant for many technological applications. Although a large number of molecular assemblies have been already investigated, engineering uniform and highly ordered two-dimensional molecular assemblies is still a challenge. Here we report on a novel approach to prepare wide highly crystalline molecular assemblies with tunable structural properties. We make use of the high-reactivity of the carboxylic acid functional moiety and of the predictable structural features of non-polar alkane chains to synthesize 2D supramolecular assemblies of 4-(decyloxy)benzoic acid (4DBA;C 17 H 26 O 3) on a Au(111) surface. By means of scanning tunneling microscopy, density functional theory calculations and photoemission spectroscopy, we demonstrate that these molecules form a self-limited highly ordered and defect-free two-dimensional single-layer film of micrometer-size, which exhibits a nearly-freestanding character. We prove that by changing the length of the alkoxy chain it is possible to modify in a controlled way the molecular density of the "floating" overlayer without affecting the molecular assembly. This system is especially suitable for engineering molecular assemblies because it represents one of the few 2D molecular arrays with specific functionality where the structural properties can be tuned in a controlled way, while preserving the molecular pattern. Complex molecular architectures are usually constructed by connecting molecular building blocks through non-covalent bonding, allowing for a variety of different supramolecular structures and phases to emerge 1-6. When solid surfaces are used as supports, the molecular self-assembly can be regarded as a two-dimensional (2D) process, and the structural and electronic properties, such as molecular packing, the number and typologies of domains, and the binding energy of the molecular levels, strongly depend on the interplay between the intermolecular forces and the interaction between the adsorbate and the underlying support 7-9. A suitable choice of molecular building blocks and supporting substrates would enable, in principle, synthesizing and designing 2D supramolecular assemblies with specific functionalities, disclosing exciting new opportunities in many areas of science and technology 10-13. Key steps to these accomplishments are (a) to understand how molecules self-arrange, and (b) to master the growth of homogeneous, highly ordered and defect-free films with tunable properties 14,15 .