Coverage-Dependent Disorder-to-Order Phase Transformation of a Uracil Derivative on Ag(111)

Enache, L. Maggini, A. Llanes-Pallas, T. A. Jung, D. Bonifazi, M. Stoehr,
J. Phys. Chem. C 2014, 118, 15286-15291
DOI: 10.1021/jp503188m


The self-organization of an angular bis(uracil-ethynyl) benzene derivative is investigated on Ag(111) by means of scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. It is found—starting at low submonolayer coverage—that upon increasing the molecular coverage a disorder-to-order phase transformation occurs. Specifically, at low and intermediate molecular coverage a glassy phase consisting of one-dimensional (1D) chains and 2D aggregates is observed, while close to a first complete molecular layer, a well-ordered 2D close-packed phase is revealed. The main driving forces responsible for the structure formation are (i) the high self-complementarity of the uracil (U) moiety, resulting in UU homopairs through 2-fold C═O···H–N H-bonds and (ii) the steric hindrance induced in the system by the alkyl chains. The delicate balance between the molecule–molecule and the molecule–substrate interactions leads to a complex phase behavior of the uracil derivative at the solid–vacuum interface. On the basis of this detailed study, we present a qualitative understanding of the peculiar phase behavior of the system.

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