Molecules on Semiconductors: From Chromogenic Interactions to Information Processing Devices

Rapid development of electronic technologies requires new semiconducting and dielectric materials of superior quality. Recently new applications (like LCD and OLED displays) initiated the employment of various organic materials (pentacene, oligothiophenes) as semiconductors for thin layer field effect transistors. Micro- and nanoelectronics also search for new materials of unusual properties. One of the recent examples is a memristor - a memory resistor. This fourth fundamental passive element (like resistor, capacitor and inductor) was predicted theoretically in early seventies  and demonstrated experimentally 37 years later. Memristors have opened new fields of electronics and enabled fabrication of circuits with synaptic properties  and learning abilities. Not surprisingly the prototype of this amazing device was based on very popular wide band gap semiconductor: titanium dioxide. This material is widely used as photocatalyst and in dye-sensitized solar cells, but its applications in electronic elements are relatively rare. However, TiO₂ is the first material for which the photoelectrochemical photocurrent switching effect was described, which further resulted in new materials for unconventional logic devices. Other metal oxides, especially binary ones (e.g. PbMoO₄, BiVO₄) exhibit similar semiconducting properties (the same n-type conductivity, similar band gap values), but their properties as active materials in micro/nanoelectronic devices are almost not recognized. The role of these oxide semiconductors may be two-fold. Firstly, they can provide a solid substrate and a scaffold for self-assembly of molecular layers. Transition metal oxides should provide self-assembly template for organic molecules. Secondly, due to their non-innocent interactions with organic ligands may substantially modify properties of organic materials. These interactions may be strong enough to generate new (emergent) optical and electronic features which are not present in parent compounds. Photoinduced electron transfer (PET), optical electron transfer (OET) and photoelectrochemical photocurrent switching effect (PEPS) are such phenomena.

            This contribution presents the nature of chromogenic interactions from the quantum-chemical point of view on the example of aromatic hydrocarbons anchored to various d⁰ ions in the coordination environment present in semiconducting oxide phases. The consequences of these interactions are related to the functional properties which are shown from experimental perspective in the second part. Optoelectronic switches, logic gates, binary computing circuits and artificial synaptic devices will be discussed in detail.