Controlling the electronic and physical coupling on dielectric thin films.

Philipp Hurdax,Georg Koller,Serguei Soubatch,Larissa Egger,Michael Hollerer,Anja Haags,Alexander Gottwald,Martin Sterrer,Xiaosheng Yang,Mathias Richter,Michael G Ramsey,Frank Stefan Tautz,Peter Puschnig

doi:10.3762/bjnano.11.132

Abstract

Ultrathin dielectric/insulating films on metals are often used as decoupling layers to allow for the study of the electronic properties of adsorbed molecules without electronic interference from the underlying metal substrate. However, the presence of such decoupling layers may effectively change the electron donating properties of the substrate, for example, by lowering its work function and thus enhancing the charging of the molecular adsorbate layer through electron tunneling. Here, an experimental study of the charging of para-sexiphenyl (6P) on ultrathin MgO(100) films supported on Ag(100) is reported. By deliberately changing the work function of the MgO(100)/Ag(100) system, it is shown that the charge transfer (electronic coupling) into the 6P molecules can be controlled, and 6P monolayers with uncharged molecules (Schottky–Mott regime) and charged and uncharged molecules (Fermi level pinning regime) can be obtained. Furthermore, it was found that charge transfer and temperature strongly influence the orientation, conformation, and wetting behavior (physical coupling) of the 6P layers on the MgO(100) thin films.

Highlights

Since the first scanning tunneling microscope (STM) imaging of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of pentacene (5A) on NaCl/Cu(111) was performed [1], the concept of decoupling molecules from metal substrates with large bandgap dielectric films has become widely accepted
STM shows 6P molecules arranged in ordered monolayer islands (Figure 2b, Figure 2c), with their long axes aligned parallel to each other and parallel to the substrate surface
This is typical for 6P on atomically clean and ordered substrates obtained from bulk oxides to metal substrates [21,22,23,24,25]

Summary

Introduction

Since the first scanning tunneling microscope (STM) imaging of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of pentacene (5A) on NaCl/Cu(111) was performed [1], the concept of decoupling molecules from metal substrates with large bandgap dielectric films has become widely accepted. The critical substrate work function (Φcrit) for the charge transfer is given by the condition ΦMgO = Φpin.

Results

Conclusion