Submonolayer and multilayer growth of titaniumoxide-phthalocyanine on Ag(111)

Abstract

For exploiting the full potential of organic materials for future organic electronic devices it is of crucial importance to understand structural and electronic properties of metal-organic interfaces and adsorbate systems, in particular electronic interactions and growth mechanisms. Phthalocyanine molecules represent one class of materials which are very frequently discussed in this context. They feature an appealing tunability in terms of structural, electronic and magnetic properties, simply by exchanging the central (metal) atom or group of atoms. Here we present a comprehensive study of one of the model systems in this field, TiOPc on Ag(111). We discuss structure formation and growth from submonolayer to multilayer films, based on results obtained by electron diffraction, scanning tunneling microscopy, electron energy loss spectroscopy, x-ray standing waves, photoelectron spectroscopy and pair potential calculations. Similar to related metal-phthalocyanine adsorbate systems we find three distinct phases in the submonolayer regime, a disordered gas-like 'g-phase', a commensurate 'c 2 -phase' at low temperature, and a 'p.o.l.-phase' consisting of a series of point-online structures with continuously shrinking unit cells. For the latter a uniform TiO-up configuration (Ti-O group pointing towards vacuum) was found. Hence, the first-layer molecules form a strong dipole layer, the dipole moment of which is compensated by molecules adsorbing in the second layer at hollow-sites in TiO-down geometry (Ti-O group pointing towards the surface). The Coulomb interaction between the dipole moments in the first and second layer stabilizes this bilayer structure and causes a bilayer-by-bilayer growth mode of molecular films above a thickness of 2ML. We report the structural properties (vertical adsorption heights, inter-layer distances, inplane orientations and positions) of the molecules in all phases in detail, and discuss the effect of inelastic dynamical charge transfer. Our results contribute to a comprehensive understanding of this interesting adsorbate system and, in comparison with earlier studies on CuPc, H 2 Pc and SnPc on Ag(111), we shine new light on the interesting interplay of molecule-molecule and molecule-substrate interactions.

Configuration-specific electronic structure of strongly interacting interfaces: TiOPc on Cu(110)

“…An f -LUMO is expected on Cu(110) as well since this surface is both more reactive and has a lower work function (∼200 meV) relative to Ag(111). Each of these factors shifts the LUMO further below E F , thus supporting our assignment [24,[37][38][39]. The accompanying STM image [ Fig.…”

“…5(c) and 6(b)]. Again, a partially filled f -LUMO for TiOPc on Cu(110) is expected given reports of an f -LUMO for TiOPc and CuPc on Ag(111) [37], and other phthalocyanines on Cu(110) [52]. Charge flow out of the surface and into the molecule counteracts pushback effects, and equilibrium is reached at ϕ ads = −400 meV for 1-ML coverage.…”

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

“…[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 .…”

“…[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 . [41] It is noteworthy that especially for the case of polar phthalocyanines, post-annealing at around 480 K after each deposition step is important in order to obtain reproducible Φ values and long-range ordered films. For both substrates, four different regimes can be identified.…”

“…[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.…”