We interviewed László Óvári, Head of the Surface and Chemical Dynamics Group, about the experiment.
Nanoobjects are extremely small (a nanometre is one millionth of a millimetre), yet this size range plays a major role in our everyday lives. For example, the basic components of modern computers are less than 10 nm in size. Another nanoscale application is catalysis, where metal particles, such as platinum, with a diameter of about 10 nanometres, are used as catalysts. In this case, the miniature size is useful because it increases the surface area of the substance that enhances the rate of the chemical reaction. Nanotechnology was used, albeit unknowingly, already during the construction of medieval cathedrals, as the colour of stained glass stems from the different nanosized particles used as colouring agents.
“In many areas of life, miniaturization is an essential requirement. Consumers expect devices to become smaller and smarter. However, apart from gradual size reduction, there is another direction: under certain conditions, atoms arrange themselves into nanostructures. Some atoms and molecules can assemble form larger units. The result of our research project is an example for this,” said László Óvári.
The structure of hexagonal boron nitride is similar to that of graphene. The difference is that graphene has carbon atoms at each vertex of its hexagonal rings, whereas boron nitride has alternating boron and nitrogen atoms. Because the electronegativity of the two chemical elements is different, boron nitride is insulating while graphene is conductive. When a layer of boron nitride is deposited on the surface of certain metal single crystals, a periodically undulating monolayer, called nanomesh, is produced with periodic tiny pores on it. If molecules are adsorbed onto this undulating surface, they will occupy these pores. When metals are evaporated onto this mesh, they too will preferentially seek out these pores. In this case, a regularly arranged, self-organizing metal cluster matrix is formed, which is an excellent model system for catalysis. (The advantage of model systems is that a complex, difficult-to-understand phenomenon can be explained by simplifying the process. If you can find out what is happening in the model and why, you can use this knowledge to understand the complex system.) The question is why molecules and metal atoms prefer nanomesh pores. Previous experiments have shown that the phenomenon can be explained by the locally generated electrostatic field, which also varies periodically.
László Óvári, Gyula Halasi and Csaba Vass
“The local work function is different at the bottom of the tiny pores and on the edges. This potential difference generates an inhomogeneous electrostatic field. We have manipulated this field,” said László Óvári, explaining that the undulating boron nitride nanomesh is a kind of template, i.e. a material that arranges the atoms or molecules deposited onto it. Our researchers wanted to further improve this template effect.
In experiments conducted within the framework of a user project of the University of Szeged, a layer of gold was first evaporated onto the surface of a one-millimetre-thick single crystal of rhodium. Then the rhodium, thinly coated with gold, was heated to 1050 Kelvin to form a surface alloy. (This mixing is interesting because in the three-dimensional /bulk/ state these two metals do not form an alloy.) Boron nitride was then synthesised on this alloy in vacuum conditions (also at 1050 Kelvin). Our researchers have found that the resulting monolayer rearranges the underlying gold, resulting in the atoms of this precious metal being located beneath the edges rather than sitting below the boron nitride pores.
According to László Óvári, the periodic electrostatic field is enhanced in the case of the rhodium–gold alloy compared to what was found for the rhodium single crystal. This is explained by the fact that gold can draw electrons away from rhodium, i.e. it becomes slightly negative while leaving the rhodium slightly positive. The experiment used the NanoESCA endstation, which measures the band structure of electrons very precisely. This allowed researchers to determine not only the energy but also the momentum of surface electrons. In one of the electron bands, they detected a splitting, which they explained with the enhancement of the periodic electrostatic field. The experiments were performed by the Surface Dynamics Group at ELI ALPS, and were evaluated in cooperation with scientists from the University of Szeged, the University of Kaiserslautern-Landau, and the HUN-REN Wigner Research Centre for Physics.
“These experiments have demonstrated that by using a rhodium–gold surface alloy instead of a pure rhodium single crystal surface we can enhance the periodic electrostatic field, which can lead to a more efficient template. We have also shown that the boron nitride nanomesh rearranges not only the molecules deposited onto it but also the underlying metal surface,” László Óvári said, summarizing the results available here.
What could come next? The next logical step is to prove that this field enhancement influences the template effect, and we would also like to test this phenomenon with other materials. Our physicists also want to learn how regular the arrangement is and how stable the system is thermally. Another intriguing question is why silver and nickel atoms fail to self-assemble on a boron nitride mesh.
According to László Óvári, further work could even lead to the development of new catalysts.
Photos: Gábor Balázs
Author: Zoltán Ötvös