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

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 since it is at the interface where elemental electronic processes take place. Their identification and characterization thereby requires that these critical sections of a device are well defined and can be prepared 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[double bond, length as m-dash]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.

A Solvent‐Free Solution: Vacuum‐Deposited Organic Monolayers Modify Work Functions of Noble Metal Electrodes

“…[38] The latter values correspond reasonably well to those reported in the literature for TiOPc monolayers on HOPG and bilayers on Ag(111). The best fit was obtained for a permanent TiO group dipole moment of µ TiO = 0.6 eÅ and a molecular dipole moment of µ TiOPc = 0.4 eÅ for mole cules in the bilayer, with a polarizability of α TiOPc = 100 Å 3 and using a bilayer unit cell area of A uc = 189 Å 2 .…”

“…[38,41] Previous SPA-LEED studies of TiOPc on Ag(111) by Fernández et al have shown that this arrangement results in a POL-phase for the monolayer with a primitive unit cell of A uc = 199 Å 2 , [37] while the bilayer is somewhat compressed and has a unit cell area of 189 Å 2 . [38,41] Previous SPA-LEED studies of TiOPc on Ag(111) by Fernández et al have shown that this arrangement results in a POL-phase for the monolayer with a primitive unit cell of A uc = 199 Å 2 , [37] while the bilayer is somewhat compressed and has a unit cell area of 189 Å 2 .…”

“…[28][29][30][31] A recent work by Petritz et al reports a strong decrease in OFET contact resistance upon shortening SAM chain length, thus emphasizing the necessity for extremely thin monolayers. [38] Although there are several studies correlating the structural information to the electronic properties of individual phthalocyanines on metal substrates, [37][38][39][40][41] a systematic study comparing the coverage-dependent change in work function of different noble metals for different (polar and nonpolar) phthalocyanines has not yet been reported. [9,33] Koch et al have demonstrated the feasibility of this approach for monolayers of small organic molecules on noble metals.…”

Interface Structure and Evolution of Dinaphthothienothiophene (DNTT) Films on Noble Metal Substrates

“…Sample Preparation : The presently used Ag(111) and Au(111) samples were prepared by epitaxial growth of metal onto freshly cleaved and carefully degassed mica substrates under high‐vacuum conditions (typical thickness: 1500 Å). As demonstrated elsewhere, this yielded highly ordered (111) surfaces with atomically flat terraces extending over more than 1000 Å, separated by monatomic steps. For the complementary XRD measurements, also Ag(111) and Au(111) single crystals were used to avoid overlapping diffraction peaks of the mica substrate.…”

Electronic Structure of Titanylphthalocyanine Layers on Ag(111)

“…21,22 These latest studies also suggest that the oxygen atom is pointing to the vacuum in the monolayer. Molecules in the second layer grow on top of the first one with the oxygen atom facing towards the surface in order to minimize the dipole-dipole interaction between layers 23 (see Figure 1 for a sketch). The different layer morphologies reveal characteristic molecular arrangements on the surface of Ag(111) ranging from the submonolayer till the completion of the monolayer (ML) and the bilayer formation.…”