Physicists and chemists in Switzerland have defied the belief that electrical resistance of a material can't be adjusted by developing thin films with controllable electronic properties. The discovery, they say, could have a big impact on future applications in sensors and computing. Dr. Meike Stöhr and Manfred Matena work at the ultrahigh vacuum system. (Photo: University of Basel)Dr. Meike Stöhr and her collaborators at the universities of Basel and Heidelberg and the Paul Scherrer Institute developed a substance which, after heating on a copper surface, exhibits a two-dimensional network with nanometer-sized pores. The interaction of this network with the existing electron gas on the metal surface leads the electrons underneath the network to be pushed into the pores, forming small bunches of electrons called quantum dots. The researchers found that varying parameters such as the height and diameter of the pores allowed them to selectively tune the properties of the material. By filling the pores with different molecules they get direct access to the material's properties that are dependent on the electronic structure, such as conductivity, reflectivity and surface catalysis. Having access to these properties could lead to the development of new materials with adjustable electronic properties. The underlying physical mechanisms can best be understood by a comparison of the electron-gas with waves in water. Waves on a water surface are reflected by any obstacle they meet. If the obstacle on the surface resembles a honeycomb structure, standing waves are set up in each cell of the honeycomb. This then leads to a wave pattern that mimics the honeycomb structure. A two-dimensional “electronic metamaterial” is generated by supramolecular self-assembly on a metal surface. The periodic influence of the porous molecular network on the otherwise free-electron-like surface state results in the formation of an electronic band.“Applying this analogy to the electron gas, we see that the interaction of the network structure with the electron gas on the metal surface confines the electrons giving rise to a characteristic electron wave structure of the new material,” said Stöhr. These pore networks are good candidates for new metamaterials, which are man-made materials that have specific optical and electronic properties not found in nature. These properties can be tuned by changing the properties of their component materials. In the case of pore networks, it is the electronic surface properties which can be tuned by careful selection of the nanopores. A paper on the research, "Band Formation from Coupled Quantum Dots Formed by a Nanoporous Network on a Copper Surface," appears in the July 16 edition of the journal Science.The University of Basel and the Paul Scherrer Institute (PSI) are long-term partners of the Swiss Nanoscience Institute (SNI), which is financed by the Canton of Aargau. A key partner in this project was the Swiss Light Source of PSI. For more information, visit: http://www.psi.ch/