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Complex Monolayer Growth Dynamics of a Highly Symmetric Molecule: NTCDA on Ag(111)

Schmidt, Thomas, Marchetto, Helder, Groh, Ullrich, Fink, Rainer H., Umbach, Eberhard
Journal of physical chemistry 2019 v.123 no.13 pp. 8244-8255
adsorption, carboxylic acids, chemical structure, desorption, droplets, geometry, islands, phase transition, silver, temperature
The growth dynamics of the highly symmetric planar organic molecule NTCDA (1,4,5,6-naphthalene tetracarboxylic acid dianhydride) on Ag(111) is rather complex, already in the monolayer regime. This dynamics was investigated in situ with high lateral resolution using the aberration-corrected spectro-microscope SMART. Although the molecular structure of NTCDA is very similar to that of the slightly larger molecule PTCDA, the growth behavior of these two molecules is very different. Several differences of the static geometric, electronic, and vibrational structure have been described previously by several authors, but some dynamic properties hardly accessible by other techniques could be observed in the present real time experiments using the SMART. For instance, it is found by direct observation that after decoration of steps and step bunches and depending on substrate temperature and surface morphology, a two-dimensional lattice gas of diffusing NTCDA molecules is formed. This increases in density upon continuous deposition of molecules before islands nucleate which subsequently grow in size while the density of the lattice gas first remains constant and then is reduced. Furthermore, in a certain temperature and coverage range some macroscopic islands (of micrometer size) abruptly change their shape and position on the time scale of few seconds. These “jumping 2-dim droplets” are observable for adsorption as well as for desorption. Moreover, previously observed 2-dimensional phase transitions can now be followed in real time, yielding further insight into an interesting but complex adsorption system.