In situthermal noise measurements under nanoindentation of suspended graphene

We demonstrated thermal noise measurement under the nanoindentation of monolayer and bilayer graphene nanodrums. The resonant oscillation of the cantilever excited only by a thermal energy is detectable even in the case of contact with a suspended graphene. The contact resonance fRequency can be obtained in 1 millisecond intervals during the force curve measurement by optimizing the parameters of a real-time spectrum analyzer. The pretension value of the graphene nanodrum is evaluated by the minimum frequency just when the applied force of the cantilever becomes zero. The simultaneous measurement of the force and the resonant frequency with respect to the deformation of the graphene nanodrum enables us to determine the value of InvOLS (inverse optical lever sensitivity) more accurately in each measurement. From the analysis scheme, force curve measurements of the graphene nanodrums with the same diameters show good reproducibility. We also revealed that the effective spring constant of the graphene nanodrums consists of a weak sample-dependent pretension factor and a deformation-dependent factor proportional to the number of graphene layers.