Ultra-High Vacuum Deposition of Pyrene Molecules on Metal Surfaces
Sebastian Schleicher, Bogdana Borca, Jeff Rawson, Frank Matthes, Daniel E. Bürgler, Paul Kögerler, C.M. Schneider
Interfacial properties of organic adsorbates featuring aromatic π-orbitals on metal surfaces play an important role for organic electronics and spintronics. Pyrene is a flat aromatic molecule with a size between ultimately small benzene and extended graphene segments. The deposition of pyrene molecules onto clean and reactive surfaces with a sub-monolayer coverage under ultra-high vacuum (UHV) conditions is challenging, since pyrene is a solid with a high vapor pressure.
In this paper, we present a sublimation procedure under UHV and image pyrene adlayers on in-situ prepared Au(111) and Fe/W(110) substrates by means of low-temperature scanning tunneling microscopy.
Figure: STM image of one ML of pyrene molecules after 1 min deposition time on an atomically flat Au(111) surface. Each roundish protrusion represents a pyrene molecule. Inset: Gauss-filtered close-up of the hexagonal superstructure lattice with a lattice constant a = 1050pm (30x30nm2, Vbias=1.0 V, It = 1.0 nA).
For Au(111), the molecule–surface interaction is weak as indicated by the specific herringbone reconstruction of the Au(111) surface that is visible through the self-assembled pyrene adlayer. Pyrene desorption due to weak intermolecular interaction self-limits the growth to one monolayer (ML).
On the more reactive 2–4 ML thick Fe films on W(110), the molecular order of the pyrene adlayer sensitively depends on the Fe thickness-dependent dislocation pattern at the substrate surface. Irregular arrangements occur for 1 ML Fe and near substrate dislocations for 2–4 ML Fe.
Figure:STM image of irregularly adsorbed pyrene molecules after 10 s deposition on 1 ML Fe on W(110). Arrows indicate some of the few molecular fragments or impurities and ellipses highlight the short-range ordering along the  direction (28x28nm2, Vbias= -300 mV, It = 1.0 nA).
Self-assembled ordered arrays form predominantly for 2 ML Fe, where the dislocation pattern leaves sufficiently large unperturbed areas between the dislocation lines.
Figure: Overview STM image of a sub-ML pyrene molecules after 10 s deposition on 2 ML Fe on W(110) showing irregular adsorption near the dislocation lines and self-assembled patches of molecules in between (40x40nm2, Vbias = 100mV, It = 1.0 nA).