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Introduction

Instrumentation and sample preparation

Transmission electron microscopy (TEM) of graphene has developed into a thriving field, especially over the past five years, during which feats such as imaging single carbon atoms have become almost routine. These developments, however, are just the latest in a long, symbiotic history between graphitic materials and electron microscopy. At least as early as 1960,1 electron microscopists prepared few-layer graphene by exfoliation, sometimes producing graphite flakes less than 2 nm thick.2 These graphite flakes were used in some of the earliest TEM studies of crystal defects3–6 and as imaging substrates for biological samples such as deoxyribonucleic acid (DNA).1,2,7 Over the intervening decades, this synergy, in which graphene acts as both a TEM sample and a substrate for other studies, has persisted. In this review, we attempt a broad, materials-oriented review of TEM applications to graphene. We focus on recent developments, first with respect to instrumentation and sample preparation. We then explain how TEM has contributed to our understanding of the physical and electronic properties of graphene sheets and their defects and finish by discussing how graphene is becoming a platform for diverse TEM investigations using in situ techniques, as well as an imaging substrate.

Like all materials investigated by TEM, graphene must be suspended on TEM grids for study. Graphene sample preparation for high-resolution microscopy is particularly challenging because it requires extraordinary cleanliness: When a sample is a single atom thick, even a monolayer of contamination is unacceptable. Whereas cross-sectional TEM samples of graphene on substrates can be prepared using conventional TEM preparation techniques, most TEM studies investigate graphene in plan view, perpendicular to the graphene basal plane. Fabricating these plan-view samples is nontrivial—still more art than science—and is tailored specifically for graphene imaging. In plan-view sample preparation, graphene membranes are transferred onto a TEM grid, typically a metallic or SiN mesh, possibly with additional support provided by perforated amorphous carbon (for example, Quantifoil® holey carbon films). Graphene produced by mechanical or chemical exfoliation is either (1) scooped from solution onto a TEM grid8 or (2) transferred from another surface by placing the graphene and grid in contact and using isopropanol to adhere them.9 Graphene grown by chemical vapor deposition on copper or nickel substrates can be transferred in large sheets to TEM grids by supporting the graphene with

Pinshane Y. Huang, School of Applied and Engineering Physics, Cornell University; [email protected] Jannik C. Meyer, Physics Department, University of Vienna, Austria; [email protected] David A. Muller, School of Applied and Engineering Physics, Cornell University; [email protected] DOI: 10.1557/mrs.2012.183

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MRS BULLETIN • VOLUME 37 • DECEMBER 2012 • www.mrs.org/bulletin

© 2012 Materials Research Society

FROM ATOMS TO GRA