AVS2011 Session SS+EM-TuA: Organic Electronic Interfaces

In this talk I will highlight recent result on solid-liquid interfaces where the solid has a nanostructure with a characteristic length scale that is only a few liquid molecule wide. The talk will start with the presentation of a new scanning probe technique able to measure interfacial energy (work of adhesion to be precise) with atomic/molecular resolution. It will then show how surfaces with alternating stripe-like domains a few nanometer thick have a structural component to their work of adhesion that can account for as much as 20% of the total energy. Finally novel self assembly approaches to achieve such surfaces will be discussed.

We present an investigation of the evolution of the electronic structure at the interface of the dipolar organic semiconductor vanadyl naphthalocyanine with both highly oriented pyrolytic graphite and Au (111). Using angle-resolved two-photon photoemission and other photoelectron spectroscopies we observe both excitonic as well as strictly interfacial states in both ground and excited state manifolds, with large differences between the two surfaces. Simple electrostatic considerations provide a chemisorption model that is capable of quantitatively describing long- and short-range interface-mediated intermolecular coupling, significantly altering the molecular electronic structure. Additional insights are available from full-scale first-principles calculations at these interfaces. As a consequence, we show that electrostatic multipoles can significantly influence molecular and interfacial electronic structure, with direct and observable impact on interfacial charge-transfer dynamics. Interfacial electrostatic fields may therefore be used to manipulate in a concrete fashion processes of critical importance to solar energy conversion such as photoinduced interfacial electron transfer.

There is considerable interest in the functionalization of metal surfaces by molecules with large dipoles. For this purpose, p-benzoquinonemonimine-type zwitterions represent ideal candidates. These zwitterions can be anchored on gold surfaces where they form homogeneous thin films with the dipole preferentially oriented along the surface normal.^[1] Some zwitterions will selectively adsorb on patterned gold substrates from solution while selective deposition of the zwitterions onto specific ferroelectric domains has been demonstrated. This represents an attractive approach to pattern molecular deposition on optically transparent planar substrates through electrostatic dipolar interactions or orientation dipole controlled surface chemistry. Taking advantage of the high solubility of two zwitterions in both organic solvents and water, we studied the influence of the solvent on the functionnalization of surface.^[2] The goal is to control both packing and selective deposition on a variety of substrates through zwitterion solvent combination.

Acknowledgement. This research was supported by the CNRS and the Ministère de la Recherche et des Nouvelles Technologies, the ANR (07-BLAN-0274-04), the National Science Foundation (grants CHE-0909580 and DMR-0851703), and the Nebraska Center for Materials and Nanoscience at the University of Nebraska-Lincoln.

Abrupt molecular semiconductor interfaces between titanyl phthalocyanine (TiOPc) and C₇₀ were prepared by physical vapor deposition and characterizad by UHV-STM. Molecular TiOPc is a highly anisotropic molecule with a 3.5 dipole moment. Ordered TiOPc monolayer films of the honeycomb phase thus represent a regular 2-d dipolar lattice, which was investigated as an electrostatic template for the growth of the highly polarizable C₇₀. Films of C₇₀ grown layer-by-layer revealed the directed formation of a Kagome lattice. Atomically detailed structural models were obtained for the 0-6 nm C₇₀ thickness (up to 5 ML) range over which the ordering influence of the TiOPc dipolar substrate persists. Unusually low-density C₇₀ molecular packing arrangements result from the ellipsoidal shape, curved surfaces and high polarizability of C₇₀. While Kagome lattices have been frequently observed in colloidal and magnetic systems, this appears to be the first electrostatically-induced Kagome lattice involving a molecular film.

This work has been supported by the National Science Foundation under Surface Analytical Chemistry grant CHE0750203 and under the University of Maryland MRSEC DMR-05-20471

A series of self-assembled monolayers were prepared from para-substituted benzoic acids (X-C₆H₅CO₂H where X = H, F, Cl, Br, I, and CN) onto oxidized Al films and characterized by x-ray photoelectron spectroscopy and contact angle measurements. The acids adsorb to the oxide as a carboxylate group with the plane of the aromatic ring largely perpendicular to the surface, which places the para-substitutent away from the surface. Tunnel junctions were made by vapor deposition of Ag and Pb films as the top electrodes onto the monolayers. Four point probe electrical measurements were made from 4 to 300K. At 4 K the superconducting gap of Pb was observed and unequivocally demonstrates tunneling through a barrier without metallic shorts. When Ag was the top electrode, differential conductance [G(V) = dI /dV] measurements at 4 K showed a quadratic dependence on the applied bias voltage and no zero-bias anomalies. These low temperature measurements suggest the monolayers form pin-hole free tunnel barriers. Two trends emerge when comparing G(V) versus bias voltage for junctions with Ag and Pb top electrodes. When Pb was the top electrode the minimum in G(V) versus bias voltage is offset from zero bias for each monolayer, which scales in a systematic manner with the polarity of the para C-X bond. However, when Ag was the top electrode there was no offset the tunneling conductance. The origin of the different tunneling behavior observed for Pb and Ag top electrodes will be discussed in detail.

In this paper, the mechanisms leading to the enhancement of organic light emitting diodes (OLEDs) with molybdenum oxide MoO₃ incorporated in as hole injecting layers (HILs) will be discussed. The first one is the lowering of hole injection barrier between anodes and organic layers when a thin film of MoO₃ is inserted. The high work function of MoO₃ serves as a carrier ladder which decreases misalignment between the Fermi level of electrodes and highest occupied molecular orbital (HOMO) level of hole transport layers, such as N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidene (NPB). The second model is the formation of gap states above the valence band edge of MoO₃ to the Fermi level of electrodes when NPB molecules are deposited on MoO₃ layers. These gap states enhance the conductivity of MoO₃ and provide transition paths of carrier to assist the injection of hole from indium tin oxide (ITO) anodes to NPB layers. The third mechanism is the p-type doping effect of MoO₃ doped in NPB layers. This p-type doping increases hole concentration in NPB layers and reduces the energy difference between the Fermi level of electrodes and the HOMO of NPB.

We will also demonstrate an effective method to improve the current injection efficiency of OLEDs by modifying the oxidation states of as-deposited MoO₃ as HILs with in-situ argon ion (Ar⁺) sputtering. The injection current of devices incorporating this method is enhanced by one order of magnitude, as compared to that of devices without sputter treatment. The luminance of the devices is also improved. Beside device characterization, X-ray photoemission spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS) were carried out to obtain the chemical and electronic information of MoO₃ thin films treated with Ar⁺ sputter and to unveil the origins of improvement in device performance. It is found that, with slight sputter treatments, MoO₃ layers represent lower oxidation states and show metallic characteristics in energy band structure, which remarkably elevates the carrier injection efficiency from ITO to NPB.