Mapping nanosecond time scale nanoelectromechanical phenomena in 2D Materials on nanometre length scale via ultrasonic and heterodyne force microscopies

Atomically thin layers of graphene (GR) and other two-dimensional materials (2DM) such as hexagonal Boron Nitride (hBN) possess unique mechanical, electronic and thermal properties making them ideal materials for variety of nanoelectromechanical sensors (NEMS) [1].Exploration of such 2DM NEMS requires both ultimate resolution approaching atomic scale as well as ability to explore short time scale phenomena in such devices. Here we explore 2DM nanostructures using combination of scanning probe microscopy (SPM), ultrasonic vibrations and electrostatic interactions that reveal key nanomechanical and nanoelectromechanical properties of 2DM essential for the systems where the atomically thin layers are subjected to the flexural and normal stresses and electrical fields [2].We use quantitative analysis of the stress field generated by the SPM probe vibrated at the ultrasonic frequencies in the 2DM’s and show that in these elastically transversely isotropic materials, a stress propagation of the 2DM and it heterostructures is directly governed by the ratio of the out-of-plane Young modulus and the in-plane shear modulus. This shows direct proof of “ultrasonic transparency” of few layer graphene and MoS2 observed in ultrasonic force microscope (UFM) and allows to observe defects and structures under immediate surface of such materials [3]. We demonstrate that anisotropic properties of 2DMs allow exploration of local electrostatic interactions between the material and the substrate via nanomechanical actuation, revealing and mapping with nanoscale resolution the charges hidden under the layers of such materials. We then use nonlinear detection of nanoelectromechanical actuation via heterodyne force microscopy (HFM) [4] allowing to port the phase and amplitude of the high frequency vibration to kHz frequencies easily detected by atomic force microscopy. This allowed us to detect vibrations in 2DM NEMS with pm vertical resolution, and ps time-scale sensitivity. Significantly, the charges hidden between the 2D materials and substrate interface can be revealed due to resulting electromechanical actuation of such material.[1] J. S. Bunch et al, Science, 315, 490-493, (2007)[2] Kay N.D et al, Nano Letters. 14(6) :3400, 2014.[3] Dinelli, F., Pingue, P., Kay, N., D. & Kolosov, O., V., Nanotechnology 28, 085706 (2017).[4] M. T. Cuberes et al, Journal of Phys D: App Phys, 33, 2347, (2000)[5] Authors acknowledge the support of EU QUANTIHEAT grant and EPSRC Graphene NowNano CDT.