Researchers from RMIT University in Melbourne have developed a process for “printing” large-scale 2D piezoelectric materials, the first time that such surface deposition has been achieved. Their simple and inexpensive approach enhances the range of materials available to industry at this scale and quality, and could lead to a new generation of piezo-sensors and energy harvesting based on piezoelectric components that are directly integrated onto silicon chips.
Piezoelectric materials can convert mechanical force or strain into electrical energy, forming the basis of sound and pressure sensors, and can use voltages from small mechanical displacement, vibration, bending or stretching to power miniaturised devices. It had not previously been possible to make 2D piezoelectric materials in such large sheets, or to integrate them into silicon chips for large-scale surface manufacturing. This forced piezo accelerometer devices to depend on separate and more expensive components to be embedded onto silicon substrates.
Atomic force microscopy imaging of 2D GaPO4 and piezoelectric measurements at varying applied voltages. Photo credit: FLEETIt opens up the field of optics and electronics to perfect, boundary-less, 2D materials”Kourosh Kalantar-Zadeh
However, in this study, as reported in the journal Nature Communications [Syed et al. Nat. Commun. (2018) DOI: 10.1038/s41467-018-06124-1], the team devised a low temperature, industry-compatible, synthesis technique for making large-scale 2D sheets of gallium phosphate – a quartz-like crystal is used in piezoelectric applications such as pressure sensors and microgram-scale mass measurement – onto any substrate.
The approach, which allows for the growth of large-area, wide-bandgap, 2D gallium phosphate nanosheets of unit cell thickness, is based on a two-step process involving the exfoliation of self-limiting gallium oxide from the surface of liquid gallium due to the lack of affinity between oxide and the bulk of the liquid metal, before “printing” the film onto a substrate and changing it into 2D gallium phosphate via exposure to phosphate vapour.
They investigated this area due to the observation that self-limiting oxides on the surface of liquid metals can easily be removed and placed on substrates. The 2D oxide formed on the surface of liquid metal is a perfect crystal, and ideally has no grain boundaries, as opposed to the conventionally deposited thin films, which originate from the nucleation of elements, and bring either imperfections in grain boundaries or uncontrolled growth.
As team leader Kourosh Kalantar-Zadeh told Materials Today, “It opens up the field of optics and electronics to perfect, boundary-less, 2D materials”. He added “This means we can now have access and create materials that have never been made before, access to two dimensional electronics that can lead to low energy electronics, new optical systems and more efficient piezoelectric devices”.