Role of Phase Stabilization and Surface Orientation in 4,4′-Biphenyl-Dicarboxylic Acid Self-Assembly and Transformation on Silver Substrates

Abstract

Molecular functionalization of nanoparticles and metallic substrates can be used to tune their properties for specific applications. However, polycrystalline substrates and nanoparticles exhibit surface planes with distinct crystallographic orientations. Therefore, the development of reliable strategies for molecular functionalization requires knowledge of the role of the surface plane orientation in the growth kinetics, structure, and properties of the molecular layer. Here, we apply a combination of low-energy electron microscopy and diffraction, scanning tunneling microscopy, and photoelectron spectroscopy to investigate the self-assembly of 4,4′-biphenyl-dicarboxylic acid (BDA) on Ag(111) and critically discuss the difference to Ag(100). The structural motifs for intact and fully deprotonated BDA are similar on both surfaces, however, the intermediate phases comprising partially deprotonated BDA differ in structure and chemical composition. A real-time view of the phase transformations enables us to present a generalized picture of the phase transformations between the self-assembled molecular phases on the surfaces and underline important features such as the phase stabilization of the chemical composition and the mechanism of the related burst transformation. The influence of the substrate orientation on the structure of molecular layers and phase transformations provides the necessary background for developing functionalization strategies of the substrates displaying multiple surface planes and kinetic models for the growth near thermodynamic equilibrium.

Robust Dipolar Layers between Organic Semiconductors and Silver for Energy-Level Alignment

“…Figure b shows the evolution of the degree of deprotonation with annealing temperature for both Ag surfaces. On both surfaces, BTB molecules gradually deprotonate; on Ag(100), the deprotonation occurs at lower temperatures (consistently with BDA), and complete deprotonation is observed at 170 °C, whereas on Ag(111), it is reached at 240 °C. For the Ag(111) substrate, this temperature is already very close to the threshold for the decarboxylation of BTB molecules, i.e., a complete removal of carboxyl groups that occurs around 250 °C for both surfaces.…”

“…The intensity ratio of these pairs is 2:1. The higher binding energy component from each pair (highlighted by a lighter color in Figure a) is associated with hydroxyl oxygen (C–OH) and the darker one with carbonyl oxygen (−CO) of the carboxyl group (−COOH) by comparison with previous works. ,, Two distinct pairs of peaks point to the existence of two different chemical environments of the carboxyl groups; these are probably associated with the ribbon-like structure of the compressed as-deposited phase (see Figure S3 in Supporting Information Section 2).…”

“…In the case of BTB, there are enough molecules in the second layer as their desorption temperature from the second BTB layer is close to the temperature required for their full deprotonation. On the contrary, the BDA molecules desorb from the second layer between 75 and 100 °C, i.e., ∼100 °C below the temperature required for the full deprotonation (180–200 °C) . Hence, in the BDA case, there are no BDA molecules left to fill the gaps formed during the formation of the fully deprotonated phase.…”

“…(e) Position of the C 1s peak associated with phenyl rings within the first BTB molecular layer plotted as a function of the sample WF compared with earlier results for BDA. 34 The line has a slope of −1, whereas the fitted experimental values have a slope of −1.03 ± 0.06. tion occurs at lower temperatures (consistently with BDA 48 ), and complete deprotonation is observed at 170 °C, whereas on Ag(111), it is reached at 240 °C. For the Ag(111) substrate, this temperature is already very close to the threshold for the decarboxylation of BTB molecules, i.e., a complete removal of carboxyl groups that occurs around 250 °C for both surfaces.…”

“…On the contrary, the BDA molecules desorb from the second layer between 75 and 100 °C, 34 i.e., ∼100 °C below the temperature required for the full deprotonation (180−200 °C). 48 Hence, in the BDA case, there are no BDA molecules left to fill the gaps formed during the formation of the fully deprotonated phase.…”