Investigation of metal/organic interfaces for organic and molecular electronics

The investigation of the interface between metal substrate and adsorbed organic molecules is of primary interest in organic and molecular electronics research. The electronic properties of the devices are affected by the interface interaction, thus determining the efficiency and functionality of the device. This work consists of the study of metal phthalocyanines, respectively CoPc, ZnPc, and MgPc, deposited through thermal evaporation PVD on a polycrystalline gold substrate, following a step-wise method to investigate both monolayer and thin films. The samples are analyzed by photoemission spectroscopy (XPS and UPS). Where available, the experimental results are compared to ab initio simulations in the density functional theory framework of idealized systems, that consider an Au(111) surface. Particular attention is dedicated to the self-organization of the organic monolayer, obtained through a geometry optimization that simulates the relaxation processes, and on the calculation of the electron density, that is connected to the intensity and direction of the interface charge transfer. From the gathered results, it is demonstrated that the combination of both XPS and UPS analysis is mandatory for the complete investigation of the metal/organic interface, providing both qualitative and quantitative outcomes. The reduction of the work function after the adsorption is mainly attributed to the pushback effect, and the ionization potential, which are directly measured with UPS, thus defining the energy band diagram at the interface. From the energy shifts of the characteristic peaks, it is demonstrated that no chemical bonds are built between metal and molecules, nevertheless, a bi-directional charge transfer mechanism occurs for CoPc. Therefore, the central metal atom of the molecule affects the interaction. The evolution of carbon satellite peaks proves the self-organization of the phthalocyanines in the multi-layer, composed of stacks of molecules with aligned benzene rings that generate resonance effects. The non-linear reduction of the Au4f peak intensity at the initial deposition stages can be attributed to the transition from a sub-monolayer film to a full coverage of the gold surface. This aspect is particularly relevant for the fabrication of monolayers and ultrathin structures, that also better reflect the simulated ideal systems. Finally, the calculations of the transmission spectrum and the IV characteristic denote completely different conductive properties between the laying down and the standing configurations. The obtained results highlight the importance of precisely controlling the deposition process to get the desired interface properties. Moreover, a good match with theoretical simulation results is obtained when a full coverage of the metal surface is achieved. Future works might focus on the investigation of the fabrication to improve the interface properties control against the process variations, for reliable electronic device productions.