Phase transformations of 4,4′-biphenyldicarboxylic acid on Cu(001)

Abstract

The growth and structure of $4,{4}^{\ensuremath{'}}$-biphenyldicarboxylic-acid (BDA) on Cu(001) at temperatures between 300 and 400 K was studied by low energy electron microscopy and $\ensuremath{\mu}$-LEED. First, the adsorbed BDA molecules form a disordered dilute phase. Once this phase reaches a sufficiently high density, a crystalline phase nucleates, in which the molecules form a hydrogen-bonded two-dimensional (2D) supramolecular $c(8\ifmmode\times\else\texttimes\fi{}8)$ network. By a careful analysis of the bright-field image intensity, we can measure the density in the dilute phase, which is up to 30$%$ of that in the crystalline phase. From the respective equilibrium densities at different temperatures, we determine the 2D phase diagram and extract a cohesive energy of 0.35 eV. We also analyze the island decay behavior and estimate the BDA molecule diffusion constants. Steps are found to be highly transparent for diffusing BDA molecules. In the temperature range of 362--400 K, we find chemical diffusion constants between 850--1700$\phantom{\rule{0.28em}{0ex}}{\mathrm{nm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$.