Nanoscale mechanical and electrical properties of low-dimensional structures

Mucientes, Marta and Kolosov, Oleg (2021) Nanoscale mechanical and electrical properties of low-dimensional structures. PhD thesis, Lancaster University.

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Abstract

In this thesis, we mainly study the mechanical, electrical and electromechanical properties of low-dimensional structures of advanced materials, in particular two-dimensional (2D) materials and compound semiconductor (CS) structures and devices. Given the scarcity of methods for direct nano-mapping of physical properties of complex three-dimensional (3D) multilayer CS and 2D materials heterostructures, we adapted and developed suitable optical methods and functional scanning probe microscopies (SPM) approaches based in atomic force microscopy (AFM). These allowed us to successfully investigate the behaviour of one- and two dimensional (1D and 2D) free oscillating structures, such as AFM cantilevers, tuning forks (TF), Si3N4 membranes and graphene drums using the optical laser Doppler vibrometry (LDV) and dynamic AFM modes, finding governing relations of the dynamic behaviour in real-life systems and comparing these with modelling. In addition to the existing ultrasonic SPM, such as force modulation and ultrasonic force microscopy (FMM and UFM), we developed a new method called modulation ultrasonic force microscopy (M-UFM), which allows for nonlinear local excitation and the probing of membrane vibrations. Furthermore, we probe mechanical, electrical and thermal properties of supported layers and heterostructures of diverse transition metal dichalcogenides (TMDCs) and franckeite, understanding their intrinsic surface and subsurface nanostructure. In the final part of this thesis, we explored the feasibility of combining nano-sectioning via Beam Exit Cross-sectional Polishing (BEXP) and the material sensitive SPM analysis for the investigation of defects in CS structures, such as multiple quantum wells (MQW) and nanowires (NWs), and 2D material heterostructures. We applied this methodology to investigate the propagation of material defects, such as antiphase domains in CS, and their effects on the morphology, nanomechanics and electric properties in MQW structures, and to directly observe reverse piezoelectric domains inside individual GaN NWs.