On the utility of complementary analytics for on-surface synthesis

Abstract

On-surface synthesis (OSS) facilitates the coupling of larger molecules on solid surfaces into extended covalent nanostructures that are difficult or impossible to achieve by wet chemistry. Its primary analytical tool is scanning probe microscopy (SPM), which provides submolecular views of reactants, products and sometimes intermediates. However, relevant aspects such as subtle chemical changes and structural details remain inaccessible. In addition, direct monitoring of reaction progress in real time by SPM is challenging. This analytical gap is increasingly being filled by complementary analytics: mass spectrometry can be used not only to detect volatile by-products that are released during the reaction, but also to monitor intermediates and higher oligomers. Surface sensitive vibrational spectroscopy, either with electrons or photons, is advantageous for the identification of perceived reaction products, even in cases where the routine approach based on X-ray photoelectron spectroscopy (XPS) is not very promising. X-ray standing wave (XSW) analysis is a less common technique in OSS but well established in surface science, providing experimental access to adsorption heights with picometre accuracy. Its value for detailed comparison and validation of prevailing density functional theory (DFT) based structure calculations cannot be overstated. Recent examples also show the benefits of XSW for less regular structures, such as those often obtained in OSS. Finally, the assessment of reaction kinetics has considerable potential to provide fundamental insights into elementary processes and hidden reaction partners for the unique coupling of larger molecules on surfaces into extended structures. Real-time XPS has sufficient chemical and temporal resolution to monitor reaction kinetics for coupling on surfaces. Ideally, mechanistic insights can be gained by modelling. However, the typically applied linear temperature profiles have limitations that can be overcome by exploring new temperature profiles. Again, the accurate determination of kinetic reaction parameters, such as activation energies, is of paramount importance for benchmarking DFT calculations. Although spectroscopy is already applied for OSS its broader and more systematic implementation appears highly promising for the advancement of the fundamental understanding of OSS, hence eventually also for optimizing the reaction protocols and outcomes.