Printing two-dimensional gallium phosphate out of liquid metal

Nitu Syed, Ali Zavabeti, Jian Zhen Ou, Md Mohiuddin, Naresh Pillai, Benjamin J. Carey, Bao Yue Zhang, Robi S. Datta, Azmira Jannat, Farjana Haque, Kibret A. Messalea, Chenglong Xu, Salvy P. Russo, Chris F. McConville, Torben Daeneke, Kourosh Kalantar-Zadeh

Two-dimensional piezotronics will benefit from the emergence of new crystals featuring high piezoelectric coefficients. Gallium phosphate (GaPO4) is an archetypal piezoelectric material, which does not naturally crystallise in a stratified structure and hence cannot be exfoliated using conventional methods. Here, we report a low-temperature liquid metal-based two-dimensional printing and synthesis strategy to achieve this goal. We exfoliate and surface print the interfacial oxide layer of liquid gallium, followed by a vapour phase reaction. The method offers access to large-area, wide bandgap two-dimensional (2D) GaPO4 nanosheets of unit cell thickness, while featuring lateral dimensions reaching centimetres. The unit cell thick nanosheets present a large effective out-of-plane piezoelectric coefficient of 7.5 ± 0.8 pm V-1. The developed printing process is also suitable for the synthesis of free standing GaPO4 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.