Discrete nature of inhomogeneity: The initial stages and local configurations of TiOPc during bilayer growth on Ag(111)

TL;DR

This study investigates the initial growth stages and local configurations of TiOPc bilayers on Ag(111), revealing defect structures, vibrational signatures, and high thermal stability crucial for organic optoelectronic device interfaces.

Contribution

It provides a microscopic model of TiOPc bilayer growth on Ag(111) and links defect structures to vibrational signatures and stability, advancing understanding of organic-inorganic interfaces.

Findings

Defect structures influence vibrational signatures.

TiOPc bilayer stable up to 500 K due to hydrogen bonds.

Growth characterized by IRAS and STM analysis.

Abstract

The operation of organic optoelectronic devices relies notably on the bulk properties of compound molecular species, but even more so on the influence of interfaces thereof. The identification and characterization of elemental processes in these critical sections of a device thereby requires well-defined interfaces with low defect density. In this context titanyl phthalocyanine (TiOPc) arises as an excellent candidate that reveals the formation of a stable bilayer structure with a characteristic "up-down" molecular arrangement that optimizes the dipole-dipole interaction within the bilayer. In our experimental study, long-range ordered TiOPc bilayers have been grown on Ag(111) surfaces and analyzed using infrared absorption spectroscopy and scanning tunneling microscopy. By monitoring the prominent Ti=O stretching mode in IRAS and identifying local configurations in STM, a microscopic model for the growth of TiOPc bilayers on Ag(111) is suggested. We demonstrate that defect structures within these bilayers lead to characteristic vibrational signatures which react sensitively to the local environment of the molecules. Thermal desorption spectroscopy reveals a high thermal stability of the TiOPc bilayer up to 500 K, which is attributed to hydrogen bonds between oxygen of the titanyl unit and the hydrogen rim of phthalocyanines in the second layer, in addition to contributions arising from the oppositely oriented axial dipole moments and the ubiquitous van der Waals interactions.