ESR-STM of Organic Nano-Adsorbates on Silicon

The combination of semiconductor surfaces and nanostructures with single functional organic molecules is one of the most exciting topics of contemporary nanoscale science. It unites the indispensable properties of semiconductors with the versatility of functional molecules offering prospects for future information processing and storage, sensor applications or interfacing with biological/medical systems. Due to the paramount technological relevance of (doped) silicon in contemporary semiconductor devices, compatibility of future molecular electronics devices with conventional MOS technology is desired. Despite the ubiquitous presence of silicon in microelectronics, the knowledge of the interaction between silicon surfaces and organic molecules is still in its infancy. In particular, electronic and magnetic effects from the underlying (doped) bulk or from surface impurities may crucially modify the organic/silicon interface. Since it is the interface, that decisively effects the properties of nano-devices, present research focuses on understanding and controlling the underlying fundamental processes. Our proposal concerns the development of novel nanoscopic hybrid structures between silicon and organic molecules from the metallo-porphyrin and metallo-phthalocyanine classes, which play important roles in fundamental biological processes (photosynthesis, cell respiration, bio-catalysis). The research focuses on the understanding of elementary properties and processes underlying the adsorption, bonding, conformation, and atomic-scale electronic and magnetic properties of organic adsorbates on silicon. Low-temperature scanning tunneling microscopy (LT-STM) will be utilized for controlled positioning and consecutive characterization of individual molecules at specific surface sites, both with atomic precision. Atomically resolved elastic and inelastic tunneling spectroscopy with LT-STM will reveal the electronic properties effected by the local interaction with the substrate. For exploring atomic-scale magnetic properties (molecular g-factor, spin orientation, spin relaxation times, etc.) and achieving nanoscopic access to atomic/molecular single-spin properties, we will expand the LT- STM towards an analytical tool for performing magnetic resonance experiments on single magnetic molecular adsorbates and single-spin systems. Our long term vision is a full control of the interactions between functional organic molecules with structural and chemical modifications of the hosting surface (dopants, surface defects, adsorbates, etc.) suitable for tailored engineering of electrically and/or magnetically switchable organic molecular nanostructures compatible with silicon technology.

A new route for the resonant excitation of low-energy transitions in single molecules and molecular clusters has been successfully demonstrated by scanning tunneling microscopy at radio-frequencies. This is the key result of FWF project P20773-N20 "ESR-STM of Organic Nano-Adsorbates on Silicon". The ongoing route of miniaturizing structures and devices in nano-science increasingly demands for new analytical tools with sensitivity down to individual molecules and atoms. Our vision is to combine the best of two well-established methods, namely the analytical power of magnetic-resonance methods with the ultimate spatial resolution and manipulation power of scanning tunneling microscopy (STM). This would enable spectroscopy of the electronic, magnetic and chemical properties of single molecules and their immediate atomic surrounding with sub-molecular spatial resolution. In this ambitious project we have successfully achieved a number of firsts: We have pioneered the technical development and experimental proof-of-principle of a novel analytic functionality of STM that is based on the detection and modulation of radio-frequency (RF) signals inside the tunnel junction. This is an important step towards STM-based single-molecule and single-spin RF spectroscopy. Our approach is based on the modification of a commercial low-temperature STM instrument with a dedicated RF-generation and -detection system with a bandwidth of 10MHz to 1GHz. This frequency regime is suitable for electron magnetic resonance spectroscopy at a static field of about 20mT. We investigated at the single-molecule level the frontier-orbital related electronic and magnetic properties of different molecular organic/inorganic hybrid systems previously unexplored. We have obtained fundamental contributions to diverse fields: Our fundamental study of a AuIII-porphyrin anticancer agent revealed an ionic adsorption mode suitable for stabilizing the desired AuIII state in a metal-supported agent system with promising impact for an improved drug delivery. Our results on surface-supported free-based corroles represent a promising starting point for realizing exceptionally high oxidation states and stereo-selectivity in transition metal-tetrapyrroles for surface-based catalysis. We pioneered the investigation of surface-supported stable hydrocarbon pi radicals with the discovery of a novel carbon-based Kondo system and a novel type of molecular-chain based quantum resonators.