​​Seminar on Phonon-Mediated Synthesis, Processing and Manipulation of Two-Dimensional and Bulk Crystals

In this talk, we unravel a suite of peculiar observations that appear to arise as a consequence of the highly nonlinear electromechanical fields associated with the coupling of high frequency (>10 MHz) sound energy in the form of surface, bulk and hybrid acoustic waves into fluids at micron and nanometer scales. More specifically, the evanescent eminar lectric field from the piezoelectric substrate, in concert with its unprecedented surface acceleration—on the order of 10 million g’s—are observed to give rise unique charging and polarisation effects that manifest as a range of unique phenomena which can be exploited for the synthesis and assembly of novel materials. In addition to static charging, which can, for example, be utilised for debundling carbon nanotube agglomerates or ionising aerosols for microfluidic mass spectrometry interfacing, we observe molecular polarisation to accompany the acoustic forcing to lead to novel inorganic and organic crystalisation morphologies that have yet to be reported to date, or, in the case of metal–organic frameworks (MOFs), the rapid and facile production of highly oriented and simultaneously-activated freestanding crystals, albeit without the need for any deliberate monolayer self-assembly. Charge transfer effects, on the other hand, from the piezoelectric substrate into the overlying material, on the other hand, are shown to be useful for rapidly exfoliating bulk three-dimensional crystalline transitional metal dichalcogenides such as molybdenum disulphide (MoS₂) and tungsten disulphide (WS₂) into monolayer and few-layer nanosheets or quantum dots with high yield. Additionally, dynamic charging effects have also been observed wherein we show the possibility for manipulating quasiparticles in these two-dimensional materials, such as the possibility for reversibly modulating trion to exciton transition, and their subsequent transport and hence spatial separation within the material. In addition to uncovering the fundamental physicochemical mechanisms that are responsible for this myriad of new phenomena, we also attempt to demonstrate that these phonon sources constitute a powerful, versatile and potentially scalable platform for the synthesis, processing and manipulation of a variety of crystalline materials.

Leslie Yeo is a Distinguished Professor of Chemical Engineering at RMIT University, Australia. Following his PhD from Imperial College London in 2002, for which he was awarded the Dudley Newitt prize, he undertook a postdoctoral stint at the University of Notre Dame USA, after which he held a faculty position at Monash University. He also held the Australian Research Council’s Australian Research Fellowship and Future Fellowship from 2009 to 2017. Dr Yeo was the recipient of the Young Tall Poppy Science Award ‘in recognition of the achievements of outstanding young researchers in the sciences including physical, biomedical, applied sciences, engineering and technology’, and several awards for excellence in research and innovation both at Monash and RMIT. He is co-author of the book Electrokinetically Driven Microfluidics & Nanofluidics, author of over 220 publications and 25 patent applications, Editor-in-Chief of Biomicrofluidics and an editorial board member of Interfacial Phenomena & Heat Transfer and Scientific Reports.