Unrippling and Imaging of Extra-large Free-standing Graphene with Atomic Precision
Nanoscale ripple is believed to be a common feature most manifested in free-standing graphene and is expected to play an important role in altering the coupling of graphene's electronic and geometric structures. Direct characterization of free-standing graphene ripple is challenging from atom-resolved transmission electron microscopy (TEM) due to its limited depth resolution. Recent scanning tunneling microscopy (STM) of free-standing graphene uses small suspended area (1 or 5 microns) samples and can introduce uncontrolled tension that alters the intrinsic graphene structure. Here we report an STM study of suspended extra-large (~4000 micron2) CVD graphene that was prepared with a resist-free transfer and characterize its electromechanical response in details.
In our study, a series of controlled “Z-V” spectroscopy were carefully conducted. In Z-V spectroscopy, the tip displacement vs. sample bias in close-loop condition is recorded. This gives hints on the nature of interaction forces and the mechanical response of graphene. In contrast to a solid surface, the graphene membrane is very compliant and Z-V curves are characterized by a fast-rise regime and a plateau regime that follows. Graphene deformation up to 100 nm with simply a small ~1V bias ramp was observed. We discovered that our graphene is in best analogy with a curved rubber band that maintains quasi-static in shape until it is either pulled or pushed to tensile stressregimes. The graphene can be manipulated by the STM tip through electrostatic and van der Waals forces, with the latter being significant when it is repulsive.
In its transit to tensile-stressed state, the graphene exhibits a series of sudden speed rise; we interpret these events as unrippling of graphene ripples and render support with molecular dynamics (MD) simulation. Atom-resolved graphene images provide direct evidence of nanoscale structure ripples in its intrinsic state and the smoothing out of such ripples in thetensile regimes. Surprisingly, on rippled monolayer graphene, coexistence of triangular and hexagonal graphene lattices without tip condition change wasalso observed. We also observed similar Z-V behaviors on partially suspended graphene (e.g., graphene/Cu, graphene/BN). Finally, the mechanical resonance of suspended graphene can be easily detected by its interaction with a vibrating AFM cantilever. Our study provides a foundation to understand and control the electromechanical response of graphene (or other flexural atomic crystals) in its pristine two-dimensional form when subjected to a local proximal probe,therefore paves way to further investigate its structure-property correlation with atomic precision.