Nanomechanical and nanoelectromechanical phenomena in 2D-atomic crystals : a scanning probe microscopy approach

Kay, Nicholas (2016) Nanomechanical and nanoelectromechanical phenomena in 2D-atomic crystals : a scanning probe microscopy approach. PhD thesis, Lancaster University.

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Abstract

In this thesis we probe the morphological, nanomechanical and nanoelectromechanical properties of 2D materials: graphene, MoS2 and h-BN. Throughout this study we extensively use scanning probe techniques of ultrasonic force microscopy (UFM), direct-contact electrostatic force microscopy (DC-EFM) and heterodyne force microscopy (HFM). With the use of these techniques we report the observation of the nanoscale Moir`e pattern when graphene is aligned on h-BN and we propose that the imaging with atomic force microscopy of such a sample is partly due to the variance in both sample adhesion and mechanical stiffness. In addition to this we probe the ability for UFM to detect the subsurface mechanical properties in 2D materials and confirm that the anisotropy present effectively enhances its ability to do so. We apply this knowledge of UFM and 2D materials to detect the decoupling of graphene, grown on 4H-SiC, from the substrate through the intercalation with hydrogen. In the final part of this thesis we discuss the electromechanical phenomena observable in 2D materials and related devices. Through the electrostatic actuation of graphene resonator type devices we are able to probe the electrostatic environment beneath the graphene layer, information that is unavailable to non-contact mode techniques. We then develop this method of DC-EFM to incorporate a sensitivity to the time-dependent properties by introducing the heterodyne mixing principle. This new technique developed, called electrostatic heterodyne force microscopy (E-HFM) is sensitive in the nano-second time domain whilst maintaining the nanoscale lateral and vertical resolution typical of an atomic force microscope. We propose that E-HFM will prove to be a valuable tool in characterising the behaviour of high-frequency small-scale nano electromechanical systems (NEMS) currently beyond the reach of conventional characterisation techniques. Finally we pave the way forward to future NEMS and demonstrate some of the steps taken towards progress in the field.