Nanomechanics and interfacial properties of supported and suspended graphene layers

We report the nanoscale exploration of the structural, interfacial and nanomechanical properties of supported and suspended graphene films using dynamic scanning probe techniques operating in the regime of a few KHz to several MHz. Furthermore, we extend this approach to study the interaction of the graphene with local liquid environments. In order to study graphene nanostructures of 10 to several 100 nm dimensions which are both suspended and supported, the ability to probe both low and extreme stiffness range is required. For lower range stiffness we used force modulation microscopy (FMM), operating at 1-10 kHz and exploring stiffness k in the range of 0.1 to 10 N/m; whereas for k in range up to 10,000 N/m we used ultrasonic force microscopy (UFM). In UFM a sample is vibrated at low amplitude (5-10 Å) and very high frequency (typically 2 – 100 MHz) which makes the cantilever dynamically extremely rigid. The ultrasonic oscillation is then amplitude modulated at low (few kHz) frequency and “rectified” owing to the extreme nonlinear force-vs-distance dependence of the tip-surface contact, resulting in a net “UFM” force at the modulation frequency that produces excellent material contrast to stiff nanostructures. We observe, with nanoscale resolution, surface and sub-surface features arising from the graphene-substrate and also the graphene-environment interactions. We can, for example, map local bending stiffness of suspended few layer graphene (FLG), elucidate growth processes of graphene films via carbon diffusion through metals and study the nanomechanics of normal (adhesive and elastic) and tangential (friction) forces between AFM tip and exfoliated FLG as a function of surrounding medi