Materials with switchable electrical properties are in demand for memory applications – like thumb drives. New research from a team led by The Hong Kong Polytechnic University (PolyU) achieves a sought-after type of electrical behaviour in nano-sized materials. Electronics manufacturers are expected to show a keen interest, as the valuable ferroelectric characteristics can be reproduced at large scales with unprecedented ease.
A one-atom-thick layer of any substance – most famously graphene, made of atomically thin carbon – can have dramatically different properties from the bulk material. Stacking such layers on top of each other may give rise to new properties which do not exist in their individual layer. The PolyU research team, led by Prof. Daniel LAU, Chair Professor of Nanomaterials and Head of Department of Applied Physics of PolyU, studied two-layer stacks of molybdenum disulphide and tungsten disulphide (MoS2 and WS2). Excitingly, these materials showed not just piezoelectric but also ferroelectric effects.
Ferroelectric materials have an intrinsic electrical polarisation that can be switched by simply applying a current. The ability to "toggle" between two states gives them wide-ranging applications in sensors, capacitors and data storage. The electronics industry is particularly interested in developing ultra-thin devices, based on ferroelectrics that retain their properties even when produced at the nanometre scale. This has proven a major hurdle until now.
In contrast to single-molecule layers of pure MoS2 or WS2, the nano-scale stacking of both compounds gave rise to a strong ferroelectric response. The team produced different versions of the bilayers by tuning the stacking angle – similar to how one clockface might be overlain on another, with the two 12-o'-clocks either aligned or displaced. Both types of stacked bilayers displayed remarkably strong piezoelectricity as well as ferroelectricity.
To verify the switchable polarisation of MoS2/WS2, the researchers pulled off an impressive feat of "domain writing". Within a triangular slice of the thin material, they established a square-in-a-square pattern that could be seen under an atomic microscope. The smaller inner square, approximately one micron across, was clearly distinct from the larger outer square due to the opposite voltages of the two poled areas.
This is not the first report of exotic electrical behaviour in hetero-structured bilayers, in which the two layers are made of different chemicals. Usually, though, piezo- and ferroelectricity depend on subtle geometric details of such materials. This can make them difficult to manufacture consistently and at industrial scale. In particular, conventional hetero-bilayers tend to have moiré patterns (named after a type of fine fabric), due to the stacking of two layers with similar but not quite identical crystal structures.
The moiré effect is fascinating, but the researchers ruled it out as an explanation of the piezo- and ferroelectricity of MoS2/WS2. Despite the slight difference between the inter-atom distances in the two layers, they accommodated one another to produce perfectly aligned stacking, without the tiny twists or discrepancies required for moiré interference. The PolyU team's process involved simply "baking" MoS2 and WS2 together and letting the layers stack spontaneously.
Indeed, the perfect stacking of two layers with identical crystal structures but different atom types is key to the electrical properties. According to physics, ferroelectricity can only arise in such materials if they have a certain symmetry, or rather lack of it. Compared with two identical layers, a stack of MoS2 on WS2 has no centre of symmetry (formally, inversion centre) and also lacks several other symmetric transformations. This symmetry-breaking allows the material to show ferro- and piezoelectricity when one layer slides slightly relative to the other.
Prof. Daniel Lau said, "High-tech industries, such as the computer memory sector, will benefit from this new class of nano-scale ferroelectrics. When manufacture is scaled up, the low cost, low energy demand and faithful reproducibility of these atomically thin bilayers promises to advance the frontier of modern electronics."
The PolyU research was in collaboration with researchers from the Renmin University of China, University of Cambridge, and Nanjing University. The study was published on Science.
