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August 2017

Graphene Visualizes the Ion Distribution on Air-Cleaved Mica

Figure 2. Scanning tunneling microscopy of the Graphene-Ice-Mica interface. (a) UHV STM topography (190 × 190 nm2) of a few layers graphene deposited on mica recorded at 0.2 V and 100 pA. Ice crystals (darker regions) are observed intercalated between graphene and mica surrounded by two water layers (brighter region). (b) A high resolution image (17 × 17 nm2) at the edges between an ice crystal and two water layers. A ripple-like structure is observed. (c) An atomic resolution image (6 × 6 nm2) clearly showing the ripple-like structure of graphene. (d) FFT of the hills (red circle in panel (c)) and valleys (green circle in panel (c)) of the rippled graphene surface and the higher edges of the fractal (blue circle in panel (b)).

The distribution of potassium (K+) ions on air-cleaved mica is important in many interfacial phenomena such as crystal growth, self-assembly and charge transfer on mica. However, due to experimental limitations to nondestructively probe single ions and ionic domains, their exact lateral organization
is yet unknown. We show, by the use of graphene as an ultra-thin protective coating and scanning probe microscopies, that single potassium ions form ordered structures that are covered by an ice layer. The K+ ions prefer to minimize the number of nearest neighbour K+ ions by forming row-like structures as well as small domains. This trend is a result of repulsive ionic forces between adjacent ions, weakened due to screening by the surrounding water molecules. Using high resolution conductive atomic force microscopy maps, the local conductance of the graphene is measured, revealing a direct correlation between the K+ distribution and the structure of the ice layer. Our results shed light on the local distribution of ions on the air-cleaved mica, solving a long-standing enigma. They also provide a detailed understanding of charge transfer from the ionic domains towards graphene.

Scientific reports 7 (2017): 43451.

Pantelis Bampoulis1,2,*, Kai Sotthewes1,*, Martin H. Siekman1, Harold J. W. Zandvliet1 & Bene Poelsema1

1Physics of Interfaces and Nanomaterials, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands.
2Physics of Fluids and J.M. Burgers Centre for Fluid Mechanics, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands.
*These authors contributed equally to this work. Correspondence and requests for materials should be addressed to P.B. (email: [email protected])

UHV Beetle 3000 STM