| Summary: | This thesis focusses on the elastic properties of the two dimensional (2D) van der Waals (vdW) crystal Indium Selenide (InSe) and heterostructures that combine InSe with hexagonal Boron Nitride (hBN) and graphene. Nanolayers and heterostructures are formed by mechanical exfoliation and dry transfer techniques, which have been developed and documented herein. Picosecond acoustics are used to probe the dynamic elastic properties of vdW nanolayers and heterostructures through the generation and detection of coherent acoustic phonons.
Coherent acoustic phonons are generated in nanolayers of InSe through the absorption of pump laser pulses leading to the generation of nanomechanical resonances whose amplitude and frequency depend upon the materials elastic properties, layer thickness and elastic coupling of InSe to its substrate and other layers of the heterostructure. Detection of these nanomechanical resonances is possible through the modulation of secondary probe pulses. A ‘spring’ model enables one to quantify the strength of interlayer coupling through the comparison of experimental and theoretical Fourier spectra of the coherent acoustic phonon spectra generated in the layers.
The data and analysis show that the vdW heterostructure interface (for InSe/hBN heterostructures and InSe/InSe homojunctions) is elastically ideal and may be described by acoustic mismatch for frequencies of up to 100 GHz. In contrast, vdW nanolayers tend to be weakly coupled to the supporting substrate (SiO2/Si or sapphire) with a varying degree of strength described by the spring model. Picosecond acoustic imaging techniques were also used to spatially map the elastic coupling, revealing extended regions (~10 um2) of elastically ideal and broken interfaces. This information gained on vdW interfaces could prove valuable for future development of high-quality vdW heterostructures and functional devices beyond the scope of the materials used in this study.
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