Adsorption of tetracene on Ag(111) surface has been studied by means of low temperature scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (HREELS). At sub-monolayer coverage, tetracene forms a disordered phase due to repulsive interactions between flat-laying molecules on the metal surface. When the coverage increases, a tendency for local ordering is registered (disorder-order phase transition as a function of coverage). For compact layers of tetracene on Ag(111), two different phases of long-range order are found, depending on the surface temperature during deposition, in agreement with ref. [1]. The so-called α-phase undergoes a reversible order-disorder phase transition at 230K, while the metastable high density β-phase irreversibly transforms into the α-phase on annealing above 230 K. In this contribution, the nature of the various phases and the nature of the driving forces behind the various phase transitions are discussed in the light of new LT-STM data and vibrational spectroscopy. One of the disorder-order transitions may in fact be entropically driven.
[1] A. Langner, A. Hauschild, S. Fahrenholz, and M. Sokolowski, Surf. Sci. 574 (2005) 153.
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