Since the discovery and subsequent production of graphene, two-dimensional materials have been firmly on the radar of research scientists. Similar to graphene, silicene is a two-dimensional allotrope of silicon that has a hexagonal honeycomb lattice structure. Unlike graphene, however, silicene shows surface irregularities that influence its electronic properties.
Now, for the first time, University of Basel physicists have been able to determine its corrugated structure by using a method that is suitable for also analyzing other two-dimensional materials.
Silicene is Not Entirely Flat
Led by Professor Ernst Meyer of the Department of Physics and the Swiss Nanoscience Institute at the University of Basel, the research team succeeded in the quantitative representation of tiny height differences in silicene’s honeycomb lattice and the different arrangement of atoms moving in a range of less than one angstrom, or less than one ten-millionth of one millimeter, using atomic force microscopy.
Quantitative measurement of forces between sample and tip. Image Credit: University of Basel, Department of Physics via EurekAlert!
Dr. Rémy Pawlak, who played a pivotal role in the team’s experiments, said, “We use low-temperature atomic force microscopy with a carbon monoxide tip… Force spectroscopy allows the quantitative determination of forces between the sample and the tip. Thus, the height in relation to the surface can be detected and individual atoms can be chemically identified.”
Silicene’s unevenness, known as ‘buckling’, is a characteristic that influences the material’s properties. Unlike graphene, a material that is known for its excellent conductivity and strength, silicene behaves more like a semiconductor, something that graphene in its raw form does not. "In silicene, the perfect honeycomb structure is disrupted. This is not necessarily a disadvantage, as it could lead to the emergence of interesting quantum phenomena, such as the quantum spin hall effect," says Meyer.
Atomic Force Microscopy
The team used its own strategy, atomic force microscopy, to measure height differences in silicene’s lattice structure. Here, the conductive microscope tip, made from gold, oscillates back and forth just above the two-dimensional surface of silicene. When voltage is applied to the tip, the movement of the pendulum induces a small electrical current on the material’s surface.
This strategy, which has been developed by Meyer’s researchers in Basel, provides new insights into the world of two-dimensional materials and the connection between construction and digital properties.