The electronic structure of the interface between tris͑8-hydroxyquinolino͒-aluminum ͑Alq 3 ͒ and La 0.7 Sr 0.3 MnO 3 ͑LSMO͒ manganite was investigated by means of photoelectron spectroscopy. As demonstrated recently, this interface is characterized by efficient spin injection in organic spintronic devices. We detected a strong interface dipole of about 0.9 eV that shifts down the whole energy diagram of the Alq 3 with respect to the vacuum level. This modifies the height of the barrier for the injection into highest occupied molecular orbital level to 1.7 eV, indicating more difficult hole injection at this interface than expected for the undistorted energy level diagram. We believe that the interface dipole is due to the intrinsic dipole moment of the Alq 3 layer. The presented data lead to significant progress in understanding the electronic structure of LSMO/ Alq 3 interface and represent a step toward the description of spin transport in organic spin valves.
The self-assembly of organic molecules on surfaces is a promising approach for the development of nanoelectronic devices. Although a variety of strategies have been used to establish stable links between molecules, little is known about the electrical conductance of these links. Extended electronic states, a prerequisite for good conductance, have been observed for molecules adsorbed on metal surfaces. However, direct conductance measurements through a single layer of molecules are only possible if the molecules are adsorbed on a poorly conducting substrate. Here we use a nanoscale four-point probe to measure the conductivity of a self-assembled layer of cobalt phthalocyanine on a silver-terminated silicon surface as a function of thickness. For low thicknesses, the cobalt phthalocyanine molecules lie flat on the substrate, and their main effect is to reduce the conductivity of the substrate. At higher thicknesses, the cobalt phthalocyanine molecules stand up to form stacks and begin to conduct. These results connect the electronic structure and orientation of molecular monolayer and few-layer systems to their transport properties, and should aid in the rational design of future devices.
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