Final Report Summary - SEPON (Search for emergent phenomena in oxide nanostructures)
The SEPON project has addressed two Grand Challenges of materials design: small size and low dimensionality, both vital aspects for applications in nanotechnology. The focus of the project was on the fabrication of two-, quasi-one and quasi-zero-dimensional oxide nanostructure systems suitable for the elucidation of their emergent properties in terms of structure, electronics, magnetism and catalytic chemistry. This has been achieved by controlled self-assembly in ultrahigh vacuum, with atomic scale precision, and in-situ characterization employing the full palette of modern surface science methodology. Established kinetic preparation methods as well as new approaches to steer self-assembly via graded surface reactivity routes have been successfully applied to the growth of a variety of transition metal oxide nanostructures on suitable substrate surface templates. The stabilization mechanism of polar oxide surfaces in nanoscale oxide objects, the catalytic chemistry of nanoscale "inverse catalysts" consisting of oxide nanowires coupled to arrays of one-dimensional metal step atoms, and the electronic and magnetic properties of surface-supported oxide quantum systems have been probed in this project. These fundamental questions have been addressed in a close collaboration between state-of-the-art experimental and theoretical density functional theory (DFT) techniques. The model systems fabricated in this program have created a new dimension in the understanding of oxide nanophase systems and have broadened the scientific knowledge base of oxide nanostructures for advanced nanotechnology applications.
The main issues in the study of 2-D oxide systems were to draw insights into the relationship between surface atomic structure and electronic properties, the role played by defects and finite size effects for strain relief and phase stability, and their magnetic properties. The reduction of the dimensionality from 3-D in bulk oxides to the 2-D limit results in profoundly modified structure concepts in the latter case, which have been characterized and classified in a number of transition metal oxide systems. Concepts of fabrication, electronic structure and catalytic chemistry were the major topics that have been addressed in the field of 1-D oxide nanostructures. The successful growth concept involved the decoration of the ordered arrays of metallic steps of vicinal metal surfaces by oxidic nanowires. Quantum dots systems involving oxide materials have been prepared and studied in the 0-D part of the project. Magnetic effects at the atomic limit have been investigated using the Kondo response of quasi-0-D Co centers on nanostructured Cu-O surfaces. Ordered arrays of size-selected oxide nanodots have great potential for probing unit and ensemble effects on the properties of oxide materials in the nanoscale regime, and the growth of an ordered superlattice of monodisperse and isomorphic NiO nanodots by directed assembly on a AlOx/Ni3Al(111) template surface has been succcessfully achieved. (WO3)3 clusters constitute oxide material at the molecular limit. The cyclic (WO3)3 clusters, generated in the gas phase by sublimation of WO3 powder in a thermal evaporator and adsorbed on nanostructured Cu-O surfaces, have been investigated by STM at low temperature (5K) and by DFT in terms of their geometry and energetics. For the study of electric field induced effects on oxide nanostructures, a revolutionary novel concept within the SEPON program, a new apparatus has been designed and set up, and the proof of concept of the approach has been established by the observation of the electric field induced reduction of NiO on Ag(100). Using an effective model based on DFT, the interfacial redox process is traced down to a dissociative electron attachment mechanism. The formation of a 2-D ternary CuWO4 phase by reaction of a monolayer of (WO3)3 clusters with a Cu-O surface oxide represents a radically new approach towards the fabrication of 2-D ternary oxides, opening up new lines of research into 2-D materials.
The fabrication and study of low-dimensional materials has produced a veritable hype in the scientific community in recent years. In view of this, we believe that the results of the present project are contributing to stir up and enhance the interest in low-dimensional oxide materials and we envision direct applications, e.g. in the field of electronic nanodevice technology or in the class of novel monolayer catalysts, the latter consisting of a 2-D material in combination with a suitable metal substrate, that have been shown to possess great potential for nanocatalysis.
The main issues in the study of 2-D oxide systems were to draw insights into the relationship between surface atomic structure and electronic properties, the role played by defects and finite size effects for strain relief and phase stability, and their magnetic properties. The reduction of the dimensionality from 3-D in bulk oxides to the 2-D limit results in profoundly modified structure concepts in the latter case, which have been characterized and classified in a number of transition metal oxide systems. Concepts of fabrication, electronic structure and catalytic chemistry were the major topics that have been addressed in the field of 1-D oxide nanostructures. The successful growth concept involved the decoration of the ordered arrays of metallic steps of vicinal metal surfaces by oxidic nanowires. Quantum dots systems involving oxide materials have been prepared and studied in the 0-D part of the project. Magnetic effects at the atomic limit have been investigated using the Kondo response of quasi-0-D Co centers on nanostructured Cu-O surfaces. Ordered arrays of size-selected oxide nanodots have great potential for probing unit and ensemble effects on the properties of oxide materials in the nanoscale regime, and the growth of an ordered superlattice of monodisperse and isomorphic NiO nanodots by directed assembly on a AlOx/Ni3Al(111) template surface has been succcessfully achieved. (WO3)3 clusters constitute oxide material at the molecular limit. The cyclic (WO3)3 clusters, generated in the gas phase by sublimation of WO3 powder in a thermal evaporator and adsorbed on nanostructured Cu-O surfaces, have been investigated by STM at low temperature (5K) and by DFT in terms of their geometry and energetics. For the study of electric field induced effects on oxide nanostructures, a revolutionary novel concept within the SEPON program, a new apparatus has been designed and set up, and the proof of concept of the approach has been established by the observation of the electric field induced reduction of NiO on Ag(100). Using an effective model based on DFT, the interfacial redox process is traced down to a dissociative electron attachment mechanism. The formation of a 2-D ternary CuWO4 phase by reaction of a monolayer of (WO3)3 clusters with a Cu-O surface oxide represents a radically new approach towards the fabrication of 2-D ternary oxides, opening up new lines of research into 2-D materials.
The fabrication and study of low-dimensional materials has produced a veritable hype in the scientific community in recent years. In view of this, we believe that the results of the present project are contributing to stir up and enhance the interest in low-dimensional oxide materials and we envision direct applications, e.g. in the field of electronic nanodevice technology or in the class of novel monolayer catalysts, the latter consisting of a 2-D material in combination with a suitable metal substrate, that have been shown to possess great potential for nanocatalysis.