Mapping nanomechanical phenomena in graphene nanostructures using force modulation and ultrasonic force microscopy
Authors: Kolosov O., Kay N., Robinson B., Rosamond M., Zeze D., Falko V., Dinelli F.
Autors Affiliation: Physics Department, Lancaster University, Lancaster, LA1 4YB, United Kingdom; School of Engineering and Computing Sciences, Durham University, Durham DH1 3LE, United Kingdom; CNR – INO, Pisa, Italy
Abstract: Graphene is a novel two dimensional nanomaterial that possesses outstanding electrical, thermal, and mechanical properties 1,2. Whereas graphene electronic properties were extensively studied 3,4, mechanical properties of graphene nanostructures are much less experimentally explored even for simple graphene structures5-11. At the same time, the nanoscale morphology of atomically thin graphene films, including rippling at various length scale 12-14, nanomechanical graphene-substrate and inter-layer interactions 15 affects electrical and thermal transport in graphene and therefore is of ultimate importance for development on graphene devices. We used a scanning force microscopy to probe dynamic nanomechanical properties of graphene at frequencies from kHz through MHz frequency range and to map stiffness of graphene nanostructures with extremely wide dynamic range from 0.02 to 2000 N/m and lateral resolution of few nanometres. That allowed us for the first time to map shell buckling instability in suspended few layer graphene (FLG) with 5 graphene layers, manifesting itself as “buckling lines” of 10 nm wide where local dynamic stiffness abruptly dropped. The spatial locations of buckling instability were correlated to “nano-ripples” of 20 to 60 nm in size in the adjacent areas of supported graphene, observed using ultrasonic force microscopy (UFM). It revealed extreme nanomechanical non-uniformity of supported FLG in areas of “nano-ripples” confirming break down of mechanical contact between graphene sheet and supporting substrate in such areas, leading to local stresses affect buckling instabilities in FLG layer. UFM maps if such FLG non-uniformities allowed estimating compressive in-plane stress in the layer of 2.2 ± 0.4 GPa. Finally, we explored transition of FLG bending from plate to stretched membrane regime and identified range of forces where buckling phenomena dominate layer nanomechanical properties. We believe that this study offers new concept for nano-mechanical exploration of graphene and other two-dimensional materials based nanostructures.
KeyWords: Atomic force microscopy; Buckling; Graphene; Nanomechanics; Ultrasonic force microscopy