Multiscale Analysis of Phase Transformations in Self-Assembled Layers of 4,4′-Biphenyl Dicarboxylic Acid on the Ag(001) Surface

Abstract

Understanding the nucleation and growth kinetics of thin films is a prerequisite for their large-scale utilization in devices. For self-assembled molecular phases near thermodynamic equilibrium the nucleation–growth kinetic models are still not developed. Here, we employ real-time low-energy electron microscopy (LEEM) to visualize a phase transformation induced by the carboxylation of 4,4′-biphenyl dicarboxylic acid on Ag(001) under ultra-high-vacuum conditions. The initial (α) and transformed (β) molecular phases are characterized in detail by X-ray photoemission spectroscopy, single-domain low-energy electron diffraction, room-temperature scanning tunneling microscopy, noncontact atomic force microscopy, and density functional theory calculations. The phase transformation is shown to exhibit a rich variety of phenomena, including Ostwald ripening of the α domains, burst nucleation of the β domains outside the α phase, remote dissolution of the α domains by nearby β domains, and a structural change from disorder to order. We show that all phenomena are well described by a general growth–conversion–growth (GCG) model. Here, the two-dimensional gas of admolecules has a dual role: it mediates mass transport between the molecular islands and hosts a slow deprotonation reaction. Further, we conclude that burst nucleation is consistent with a combination of rather weak intermolecular bonding and the onset of an additional weak many-body attractive interaction when a molecule is surrounded by its nearest neighbors. In addition, we conclude that Ostwald ripening and remote dissolution are essentially the same phenomenon, where a more stable structure grows at the expense of a kinetically formed, less stable entity via transport through the 2D gas. The proposed GCG model is validated through kinetic Monte Carlo (kMC) simulations.

Controlled Kinetics of On-Surface Dehydrogenation using a Low-Energy Electron Beam

“…Depending on the electron energy, we distinguish two situations for the Ag(001) substrate. The sequence of LEED patterns presented in Figure 3b reveals that an e-beam with an energy lower than 10 eV first induces the formation of the intermediate  phase, 11 the structure of which is shown in Figure 3f. This is followed by further transformation to a molecular phase not observed in earlier works 11,43,44 (denoted as the  phase in the following).…”

“…The as-deposited  phase (Figure 2b,c; and the corresponding diffraction pattern shown in Figure 3a) comprises an ordered network of hydrogen-bonded BDA molecules on both Ag(001) 11,43,44 and Ag(111) substrates. The e-beam irradiation leads to the formation of new molecular phases.…”

“…These transformations are a response to the chemical transformation of BDA, i.e., dehydrogenation (or deprotonation) of the carboxyl groups. 11 The phase comprises 50 % of deprotonated groups, 11 and the  phase is fully deprotonated (see Supporting Information, Section 1 for a detailed description of the phase). Whereas the formation of the  phase can be induced both by irradiation and thermal annealing above 340 K, the phase is only reached by e-beam irradiation.…”

“…The samples were prepared by the method described in refs. 11,43,44 . In brief, the experiments were performed in the UHV system at the CEITEC Nano Research Infrastructure.…”

“…[8][9][10] The desired functionality is implemented by carefully designing molecular precursors and selecting a proper substrate. Besides thermodynamics, which defines the equilibrium structure, the kinetics of the involved on-surface processes, 11 i.e., reaction rate, onsurface diffusion, nucleation, and growth, plays a crucial role. The self-assembled structure is often defined by a self-assembly pathway, [12][13][14] rather than thermodynamic stability; metastable, thermodynamically inaccessible, self-assembled nanostructures can be obtained under kinetic control.…”