A team of German and Polish scientists has for the first time mapped the conductivity of metal oxide surfaces at atomic resolution at Forschungszentrum Jülich. With the help of the new technology, innovative information processing materials can be examined more accurately and identified more easily. The oxide materials will make computers more energy efficient and efficient in the future. The local-conductivity atomic force microscopy (LC-AFM) technique used by the researchers, unlike other methods, can also be applied to poorly or inhomogeneously conducting surfaces typical of the class of materials.
Oxides from the chemical class of transition metals are considered as promising materials for the computers of tomorrow. Among them are so-called memristive materials. Their electrical resistance is not constant, but can be changed and reset by applying an external voltage. Logic and memory devices made from such materials can operate extremely fast and save energy and preserve stored information even in the event of a power failure. They can also be downsized very well down to the nanometer scale and interconnected to neuromorphic systems that simulate the functioning of the brain.
In order to realize such applications, researchers are particularly interested in so-called filaments along local pathways on the surface of otherwise insulating oxides. The nanometer-scale regions can be switched between conducting and non-conducting. “For the first time, LC–AFM makes it possible to map the local conductivity of the surfaces of such materials at atomic resolution and thus determine where filaments prefer to form”, explains Dr. med. Christian Rodenbücher, physicist at the Jülich Peter Grünberg Institute. LC–AFM, in contrast to scanning tunneling microscopy, a well-established method for imaging the atomic arrangement and electronic structure, is well applicable even on poorly or nano-scale non-homogeneously conducting surfaces typical of the materials studied.
The heart of the method is a conductive tip, which is guided over the surface of the sample with constant force. Between sample and tip, the researchers generate a voltage of a few millivolts. Only where the sample is conducting does electrical current flow, which the researchers detect with a highly sensitive measuring device. “Until now, it was unclear whether LC-AFM could capture true atomic resolution because the tip has a relatively large tip radius of approximately 10-30 nanometers, but nearby atoms in our samples are only one fifth of a nanometer apart,” explains Rodenbücher.
However, test measurements on graphite showed the researchers that it is possible to resolve not only the periodic arrangement of the carbon atoms in the hexagonal lattice, but also single point defects. Graphite is easy to split; This creates “clean” surfaces. The latter is crucial for good measurement results, in addition to a sharp and stable tip and a sensitive power amplifier. The researchers chose commercially available techniques in their further investigations and focused on optimizing the preparation of the surfaces of their actual samples using a proprietary methodology.
They were able to successfully use the method to measure the influence of chemical modifications on the surface conductivities of titanium oxide (TiO 2 ) at atomic resolution. As part of their basic research within the Collaborative Research Center 917 “Nanoswitches”, the researchers had already found that the conductivity of slightly reduced oxide surfaces is inhomogeneous, indicating local reduction processes. Now they wanted to find out how strongly the conductive areas are located.
The result: After slight reduction initially strongly localized conduction paths with a diameter of 1-30 nanometers arise and the border between good and poorly conductive areas is extremely sharp. The researchers were able to confirm the measurement results with simulations on the Jülich supercomputer JURECA. Only after strong reduction they observed increasingly homogeneous conductivity distribution.
The researchers were surprised that not only the arrangement of the atoms in the top monolayer determines the electrical conductivity of the reduced TiO 2 surface. Oxygen vacancies in the further layers of the surface layer also contribute, as simulations have also confirmed. They form together with free electrons during the thermal reduction of TiO 2 under ultrahigh vacuum conditions and help to increase the conductivity in the oxide.
All measurements and simulations took place in Jülich, with the support of guest scientists from the Silesian University of Katowice, the Jagiellonian University of Krakow and the University of Lodz in Poland.
The graph shows examples of the conductivity of slightly reduced TiO 2 surfaces measured at atomic resolution.
Source : Forschungszentrum Jülich