On-Surface Synthesis.

Abstract

On-surface synthesis represents a chemical approach whereby molecular building blocks holding adequate functional groups are dosed onto surfaces that support or even drive their covalent linkage. The new environment created by the surface confinement and the frequent lack of solvents (most commonly being performed under vacuum conditions) makes this approach fully complementary to conventional chemistry. Among the many differences that this brings, one can find original reaction mechanisms, as well the absence of solubility or chemical instability problems when working under vacuum, each opening new doors to the synthesis of novel materials. Examples thereof include the linear polymerization of alkanes, access to an increasingly large pool of differently structured graphene nanoribbons with atomic precision, or elusive molecules like higher acenes or triangulene. However, it is not only the reaction mechanisms and products which are different in on-surface synthesis, but also the subsequent characterization techniques. With samples consisting of molecular materials adsorbed on well-defined surfaces and under vacuum conditions, a whole battery of surface science techniques appears as ideal tool to characterize these systems. In particular, scanning probe microscopy clearly stands out as the most prominent one within this research field, allowing for single molecular characterization of the chemical structures and often also their respective properties and functionalities. On-surface synthesis has gone through an impressive development since its relatively recent “birth”. Early works still relied on the manual guidance of simple chemical reactions by a scanning tunneling microscopy (STM) tip. It was not until 2007 that the possibility to grow tailored covalently bonded structures relying solely on self-assembly processes of molecules holding an appropriate molecular design was revealed. Only little more than one decade later, the growth of ever more complex structures and heterostructures has been achieved, surpassing the mere proof-of-concept stage and truly aiming at particular functionalities. This great development draws from the advancements in the understanding and control of each of the different parameters that have been identified to date as influencing on-surface synthesis reactions. Such parameters comprise among others the precursor design, which in turn includes key points such as the functional groups that define the coupling chemistry, or the size and symmetry of the reactants. Also the substrate design is extremely important, whereby its catalytic activity can be modulated, but also its potential use as nanostructured template. Other parameters include the activation method, which typically is either tip-induced, thermal or optical; as well as the growth kinetics, which are largely determined by the substrate temperature, reactant stoichiometry and coverage. Within this special issue, a compendium of on-surface synthesis-related works is presented that report the latest advancements in the understanding and implementation of many of these parameters, review particularities of activation methods, as well as the growth and properties of certain materials. The perspectives of this young and very active field are numerous. Indeed, the achievement of functional devices from OSS products is already a reality, as proven with field-effect transistors based on graphene nanoribbons. Now it is time to vary the device types and functionalities, as well as to improve their processing and performance. This will require further advances in, for instance, the extension of defect-free 2D networks or the scaling-up of fabrication processes. Along the way, some of the challenges to face are the development of routes to purify OSS products and/or limit by-products, the improvement of transfer processes from catalytic substrates to other functional interfaces, or the on-surface synthesis on insulators and large-band gap semi-conductors. A growing research community is working on it, partially represented within this special issue that is a great contribution to the field and a good read for newcomers and experts alike. André Gourdon is Director of Research at the Centre National de la Recherche Scientifique (CNRS) in the NanoSciences Group, CEMES, Toulouse, France. After a PhD in chemistry in 1978 and a post-doc in Oxford, he entered CNRS in 1981 and got a PhD in physics in 1986. His work focuses on the design and synthesis of molecular devices for single molecule electronics and optics, in particular of large polyaromatic hydrocarbons for experiments by scanning probe microscopes in ultra-high vacuum. Dimas G. de Oteyza performed his PhD work at the Max-Planck Institute for Metals Research and obtained the degree from the Universidad Autónoma de Madrid in 2007. Thereafter he has worked at the National Institute for Materials Science, Donostia International Physics Center (DIPC), at the Molecular Foundry of the Lawrence Berkeley National Laboratory, at the University of California at Berkeley and at the Centro de Física de Materiales. He is currently an Ikerbasque Research Professor at DIPC. The research throughout his career has been mainly devoted to the investigation of physicochemical phenomena in organic materials and organic-inorganic interfaces, including thin film growth, self-assembly, interface electronics and chemical reactions. To date he has received honors like the Fonda Fasella Award, the Friedrich Wilhelm Bessel Award and an ERC Starting Grant with the objective to study and advance the field of “on-surface synthesis”. Junfa Zhu received his Ph.D. in Physical Chemistry from University of Science and Technology of China (USTC) in 1999. After several years working in the Institute of Experimental Physics, Johannes-Kepler-Universität Linz, Austria, Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany, and Department of Chemistry, University of Washington, USA, he returned to USTC in December, 2006, and became a professor at National Synchrotron Radiation Laboratory, USTC, under the support of “Hundred Talent Program” of Chinese Academy of Sciences. His research interests mainly focus on in situ studies of surface and interface chemistry of organic molecules on metal or oxide surfaces, oxide-supported metal nanoparticles and surface/interface properties of functional materials.