| AVS 59th Annual International Symposium and Exhibition | |
| Late Breaking Session | Wednesday Sessions |
| Session LB+EM+GR+MN+TR-WeA |
| Session: | Select Topics in Surface and Interface Science |
| Presenter: | W.W. Pai, National Taiwan University, Taiwan, Republic of China |
| Authors: | W.W. Pai, National Taiwan University, Taiwan, Republic of China R. Breitweiser, National Taiwan University, Taiwan, Republic of China Y.C. Hu, National Taiwan University, Taiwan, Republic of China Y.C. Chao, National Taiwan University, Taiwan, Republic of China Y.R. Tzeng, Institute of Nuclear Energy Research of Taiwan, Republic of China L.J. Li, Academia Sinica, Taiwan, Republic of China K.C. Lin, Catholic Fu Jen University, Taiwan, Republic of China |
| Correspondent: | Click to Email |
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) Cu 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 ~1 V 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 stress regimes. 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 jump; 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 the tensile regimes. Surprisingly, on rippled monolayer graphene, coexistence of triangular and hexagonal graphene lattices without tip condition change were observed. 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.