Acoustic synthesis and modulation of two-dimensional materials

Two-dimensional materials have attracted significant research attention due to their unique properties, sometimes distinctive from their bulk counterparts, that have made them promising candidates in electronics, energy storage, and optoelectronics applications among others. Beyond graphene, transition metal carbide and/or carbonitrides (MXenes) offer excellent electrical and energy storage properties, while metal chalcogenides semiconductors have been widely used in optoelectronic devices such as photodetectors. Recent work has previously shown that high-frequency acoustic waves modulate the physical, chemical, and optical properties of 2D materials. As MXenes are prone to oxidation in unprotected environments, the conventional hydrothermal and solvothermal synthesis methods result in high levels of oxidation in quantum dots. Oxidation also destroys the electrical and energy storage performance of MXenes. In the first chapter of this thesis, surface reflected bulk waves (SRBWs) were employed to synthesise pristine Ti3C2Tz MXene nanosheets and quantum dots in an acoustomicrofluidic platform. Contrary to hydrothermal methods, the acoustomicrofluidic method in this thesis gives minimal oxidation to MXene nanosheets and quantum dots. The MXene quantum dots synthesised with this method exhibited higher sensitivity in H2O2 sensing down to nM-concentrations. In the second chapter of this thesis, it was successfully demonstrated that SRBWs are an effective method to remove the formed oxide layer on Ti3C2Tz and Mo2CTz MXene thin films. For case of Ti3C2Tz, the acoustic treatment also restored the film’s electrical conductivity and electrochemical performance. Compared with common restoration methods in the literature, SRBWs operate at room temperature, are very fast, and can restore oxidised MXene films up to three cycles. Another important application field of 2D materials is photodetectors. Typically, the detection range of a photodetector is limited by the band gap of the material(s) used; in addition, conventional photoconductive and photovoltaic mechanisms have disadvantages of large dark currents due to applying a voltage bias and low responsivities, respectively. Therefore, a photodetector with broadband response, especially into infrared region, low dark current, and high responsivity is of specific need. In the third chapter of this thesis, a new phenomenon on phonon–photon–electron interactions in 2D materials, termed as the acoustophotoelectric effect was unravelled, where high-frequency surface acoustic waves (SAWs) were shown to not only enhance the photodetection performance of 2D SnS2, but also lower its band gap to extend its detection range into infrared region. The findings in this thesis suggest the practicality of high-frequency acoustic waves in modulating physical, chemical, and optoelectronic properties of 2D materials to address current challenges in some of their common applications.