Researchers at the ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET) have developed a revolutionary method to ‘print’ large-scale sheets of two-dimensional piezoelectric material, opening new opportunities for piezo-sensors and energy harvesting. Importantly, the inexpensive process allows the integration of piezoelectric components directly onto silicon chips.
Piezoelectric materials are materials that can convert applied mechanical force or strain into electrical energy. Such materials form the basis of sound and pressure sensors, embedded devices that are powered by vibration or bending, and even the simple ‘piezo’ lighter used for gas BBQs and stovetops. Piezoelectric materials can also take advantage of the small voltages generated by tiny mechanical displacement, vibration, bending or stretching to power miniaturised devices.
Until now, no 2D piezoelectric material has been manufactured in large sheets, making it impossible to integrate into silicon chips or use in large-scale surface manufacturing. This limitation meant that piezo accelerometer devices – such as vehicle air bag triggers or the devices that recognise orientation changes in mobile phones – have required separate, expensive components to be embedded onto silicon substrates, adding significant manufacturing costs.
However, FLEET researchers at RMIT University in Melbourne have now demonstrated a method to produce large-scale 2D gallium phosphate sheets, allowing this material to be formed at large scales in low-cost, low-temperature manufacturing processes onto silicon substrates, or any other surface.
“As so often in science, this work builds on past successes,” explains lead researcher Professor Kourosh Kalantar-zadeh. “We adopted the liquid-metal material deposition technique we developed recently to create 2D films of gallium phosphate through an easy, two-step process.”
Gallium phosphate (GaPO4) is a quartz-like crystal that has been an important piezoelectric material since the late 1980s. It is commonly used in pressure sensors and microgram-scale mass measurement, particularly in high-temperature applications or other harsh environments. Because it does not naturally crystallise in a stratified structure, and hence cannot be exfoliated using conventional methods, its use to date has been limited to applications that rely on carving the crystal from its bulk.
Professor Kalantar-zadeh, now Professor of Chemical Engineering at UNSW, led the team that developed the new method while he was working as Professor of Electronic Engineering at RMIT University. The work was enhanced by a significant contribution from RMIT’s Dr Torben Daeneke, and the extreme persistence and focus shown by PhD researcher Nitu Syed, the first author of the work.
The revolutionary new method allows easy, inexpensive growth of large-area (several centimetres), wide-bandgap, 2D gallium phosphate nanosheets of unit cell thickness. It is the first demonstration of strong, out-of-plane piezoelectricity of the popular piezoelectric material.
The method developed by the FLEET team comprises two steps:
- Exfoliate self-limiting gallium oxide from the surface of liquid gallium – a process made possible by the lack of affinity between oxide and the bulk of the liquid metal.
- ‘Print’ that film onto a substrate and transform it into 2D gallium phosphate via exposure to phosphate vapour.
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Applications
The new process is simple, scalable, low-temperature and cost effective, significantly expanding the range of materials available to industry at such scales and quality. In addition the process is suitable for the synthesis of free-standing gallium phosphate nanosheets. The low-temperature synthesis method is compatible with a variety of electronic device fabrication procedures, providing a route for the development of future 2D piezoelectric materials. This simple, industry-compatible procedure to print large-surface-area 2D piezoelectric films onto any substrate offers tremendous opportunities for the development of piezo-sensors and energy harvesters.
The study ‘Printing two-dimensional gallium phosphate out of liquid metal’ was published in Nature Communications in September. Test materials were synthesised in RMIT’s Micro Nano Research Facility (MNRF) using van der Waals exfoliation followed by a chemical vapour phosphatisation. Measurements included Piezo-force microscopy (PFM), which confirmed a high out-of-plane piezoelectric coefficient of 8-10 pm/V, confirmed by density functional theory (DFT) calculations. This is sufficiently high to provide promise for use in 2D-material based piezotronic sensing and energy harvesting.
FLEET studies novel and atomically-thin (2D) materials for their potential use in new ‘beyond CMOS’ electronic devices. FLEET brings together over 100 Australian and international experts, with the shared mission to develop a new generation of ultra-low energy electronics. The impetus behind such work is the increasing challenge of energy used in computation, which uses 5%-8% of global electricity and is doubling every decade.