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Shaping and Edge Engineering of Few-Layered Freestanding Graphene Sheets in a Transmission Electron Microscope This may be the author’s version of a work that was submitted/accepted for publication in the following source: Zhao, Longze, Luo, Guangfu, Cheng, Yong, Li, Xin, Zhou, Shiyuan, Luo, Chenxu, Wang, Jinming, Liao, Hong Gang, Golberg, Dmitri, & Wang, Ming Sheng (2020) Shaping and Edge Engineering of Few-Layered Freestanding Graphene Sheets in a Transmission Electron Microscope. Nano Letters, 20(4), pp. 2279-2287. This file was downloaded from: https://eprints.qut.edu.au/135883/ c Consult author(s) regarding copyright matters This work is covered by copyright. Unless the document is being made available under a Creative Commons Licence, you must assume that re-use is limited to personal use and that permission from the copyright owner must be obtained for all other uses. If the docu- ment is available under a Creative Commons License (or other specified license) then refer to the Licence for details of permitted re-use. It is a condition of access that users recog- nise and abide by the legal requirements associated with these rights. If you believe that this work infringes copyright please provide details by email to [email protected] Notice: Please note that this document may not be the Version of Record (i.e. published version) of the work. Author manuscript versions (as Sub- mitted for peer review or as Accepted for publication after peer review) can be identified by an absence of publisher branding and/or typeset appear- ance. If there is any doubt, please refer to the published source. https://doi.org/10.1021/acs.nanolett.9b04524 Shaping and edge engineering of few-layered freestanding graphene sheets in transmission electron microscope Longze Zhao#1, Guangfu Luo#2, Yong Cheng1 , Xin Li1, Shiyuan Zhou3, Chenxu Luo3, Jinming Wang1, Hong-Gang Liao3, Dmitri Golberg4,5, Ming-Sheng Wang1* 1 Department of Materials Science and Engineering, College of Materials, and Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, Fujian 361005, China. 2 Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China. 3 State Key Lab of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China 4 School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology (QUT), 2nd George str., Brisbane, QLD 4000, Australia 5 International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 3050044, Japan *Address correspondence to: [email protected] 1 # These authors contributed equally to this work Abstract Full exploitation of graphene’s superior properties requires the ability to precisely control its morphology and edge structures. We present such a structure-tailoring approach via controlled atom removal from graphene edges. With the use of a graphitic carbon-capped tungsten nanoelectrode as a noncontact “milling” tool in a transmission electron microscope, graphene edge atoms approached by the tool tip are locally evaporated, thus allowing a freestanding graphene sheet to be tailored with high precision and flexibility. A threshold for the tip voltage of 3.6±0.4 V, independent of polarity, is found to be the determining factor that triggers the controlled etching process. The dominant mechanisms involve weakening of carbon-carbon bonds through the interband excitation induced by tunneling electrons, assisted with a resistive-heating effect enhanced by high electric field, as elaborated by first-principles calculations. In addition to the precise shape and size control, this tip-based method enables fabrication of graphene edges with specific chiralities, such as “armchair” or “zigzag” types. The as-obtained edges can be further “polished” to become entirely atomically smooth via edge evaporation/reconstruction induced by in-situ TEM Joule annealing. We finally demonstrate the potential of this technique for practical uses through creating a graphene-based point electron source, whose field emission characteristics can effectively be tuned via modifying its geometry. Keywords: graphene, nanomilling, edge engineering, subtractive manufacturing, In-situ TEM 2 Introduction Atomic structure determines properties; this is particularly true for graphene, an exciting two- dimensional material with extraordinary physical properties.[1-4] Many fascinating applications of graphene in electronics require its zero band gap to be opened, for instance, by reducing its size to form a nanoribbon, whose band gap is expected to be inversely proportional to the ribbon width.[3- 6] The band structure of a graphene nanoribbon (GNR) also strongly depends on its edge structures, including the crystallographic orientation (i.e. edge chirality) and roughness.[7,8] Graphene edges with uncontrolled chirality or poor smoothness may lead to unpredictable properties of graphene. In addition, graphene nanostructures are envisaged to have a significant shape effect on their magnetic and spintronic properties.[9-11] Therefore, the ability to precisely control the geometry and edges of graphene has been highly desired for practical and research purposes. To date, numerous approaches for fabrication of graphene have been reported, although each of them has its own limitations.[12-25] Chemical synthesis methods, such as CVD growth and organic synthesis in solution, have shown a potential for large-scale fabrication of GNRs with well-defined structures, but they can hardly be used to produce graphene pieces with specifically designed shapes or edge structures.[12-14] Likewise, GNRs fabrication by longitudinal unzipping of carbon nanotubes has similar drawbacks.[15-16] Electron-beam lithography provides a sophisticated tool for graphene patterning.[17-19] However, being limited by lithography resolution, it has a poor control over structural details and usually results in rough graphene edges, creating difficulties in determining exact crystallographic orientation of the patterns. Importantly, lithography methods, including scanning probe microscopy (SPM) lithography,[20,21] are only suitable for graphene samples lying on a substrate. This brings considerable inconvenience for a subsequent sample transfer (to other substrates) or in the applications where freestanding graphene components are 3 required. Thus, new methods need to be explored so as to achieve full and precise structural control over freestanding graphene morphologies and atomic structures, including shape, size, crystallographic orientation and the edge type. In this work, we present a tip-based nanomanufacturing method with resemblance to conventional milling processes that allows the material removal from the sample surface or edges by using a machine tool.[26-28] In order to engineer graphene, one of the strongest known materials, the nanoscale subtractive manufacturing process has to be adapted from the conventional forms that involve mechanical shearing.[27-29] Our method employs a biased graphitic carbon-capped metal tip as the “nanomilling” tool working in a non-contact mode (Figure 1a,b). The edge atoms of a freestanding graphene sheet (both graphene and reduced graphene oxide (RGO)) selected by the nanotool tip are locally evaporated and the graphene sheet can thus be shaped to desired geometries and edge orientations, as observed via in situ by transmission electron microscopy (TEM). The as-obtained edges are typically nanometer or subnanometer in smoothness, which can be further improved to become entirely atomically perfect through atom evaporation and edge reconstruction under in situ TEM Joule annealing (Figure 1c,d). Furthermore, we demonstrate a possible application of this technique by creating graphene-based electron emitters with tunable geometry and field emission properties. Results and discussion The graphene-shaping experiments were performed on a 200 keV TEM by using a scanning tunneling microscope (STM)-TEM holder that allows for delicate piezo-driven manipulation.[30, 31] Graphene used in this work was either reduced graphene oxide or nanosheets scraped from 4 highly oriented pyrolytic graphite (HOPG). Thus-obtained graphene samples were mostly few- layered (3-10) sheets. The graphene samples were loaded onto a gold wire or copper grid that served as a stationary electrode on the sample holder. To fabricate the machine tool tip used for graphene tailoring, a small piece of graphitic carbon was deliberately welded onto the end of a tungsten tip, as shown schematically in Figure 1a. This graphitic nanostructure can be viewed as the cutting edge of the machine tool, i.e. the toughest part that would experience high temperature and local field during the atom evaporation process. Such cutting edges can be made of a carbon “onion” (Figure 2a), a small graphene fragment (Figure 2d) or even a graphitized amorphous carbon deposit (Figure 4a) (see Figures S1-3 in the Supporting Information for more details). Without the protection of graphitic carbon, the tungsten tip would be easily melted and deformed upon its interaction with graphene under biasing (see Figure S4 for detailed information on structural damage of the tip end). Figure 2a-c shows a typical case of shape and size control of a pure graphene nanosheet obtained from HOPG. In this case, the tool tip was built with a carbon “onion” as the cutting edge (see the Experimental
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