50 51 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 Investigating the formation and healing of defects in 2D semiconductors Related publication: Hopkinson D. G., ZólyomiV., Rooney A. P., Clark N.,Terry D. J., Hamer M., Lewis D. J., Allen C. S., Kirkland A. I., AndreevY., Kudrynskyi Z., Kovalyuk Z., Patanè A., Fal’koV., Gorbachev R. &Haigh S. J. Formation and healing of defects in atomically thin GaSe and InSe. ACS Nano 13 , 5112 (2019). DOI: 10.1021/acsnano.8b08253 Publication keywords: 2Dmaterials; Point defects; III-VI semiconductors T heamazingelectrical andmechanical properties of graphenehaveattractedhuge interest.Theyhavealsopromptedworldwideefforts to investigate other 2Dmaterials, crystallinematerials consisting of a single layer of atoms. 2D III-VI semiconductors, including gallium selenide (GaSe) and indium selenide (InSe), have excellent optoelectronic properties when reduced to ‘monolayers’ just four atoms thick. Such materials are easily damaged by air and water vapour, with atomic point defects at the seleniumsites in the crystal considered key to their instability. To improve our understanding of the presence and nature of defects in these materials, researchers conducted the first study of defects in monolayer crystals of GaSe and InSe. They used the E02 microscope at ePSIC to image samples at atomic resolution with annular dark-field scanning transmission electronmicroscopy (ADF-STEM). Theirwork identified several different defect types inbothGaSeand InSemonolayers.They observedpoint defects (missingor additional atoms) moving from atomic site to atomic site under the electron beam, with the perfect crystal structure able to recover during imaging. In thicker crystals (two layers ormore), they observed extended defects, such as lines ofmissing atoms and changes in layer stacking order. 2D III-VI semiconductors are a very new class of material, and we are only just beginning to explore their vast potential. This research suggests that heating or laser annealingmay provide a route to improving the crystal quality of atomically thin 2D III-VI semiconductors. III-VI semiconductors are layered compounds with each constituent monolayer comprised of two atomic sublayers of group III post transition metals, such as gallium (Ga) or indium (In) sandwiched between sublayers of group VI chalcogenides, such as sulphur (S), selenium (Se), or tellurium (Te) (Fig. 1a). The layers are held together by weak, van der Waals forces allowing them to be separated to produce atomically thin sheets via ‘scotch-tape’ mechanical exfoliation; this being the same technique that drew enormous attention as a key method for isolating high quality monolayer graphene from graphite. In particular, InSe has attracted considerable interest for having extremely high electronmobility, comparable to graphene, which has the highest known electron mobility of any material. However, unlike graphene, an almost perfect conductor, InSe is a semiconductor with a thickness-dependent bandgap in the optical range 1 . Furthermore, when atomic layers of InSe are stacked together with other III-VI semiconductors, and graphene or boron nitride (a 2D insulator) to form so- called‘van derWaals heterostructures’, fully tuneable optical activity from infrared up to violet is possible 2 .This platformhas the potential to revolutionise low power optoelectronic devices for high speed applications, such as sensing, optical fibre waveguides, photodetectors, single photon emission devices, and ring lasers. A key challenge for the commercial exploitation of atomically thin III-VI semiconductors is maintaining their crystal quality when the materials are made atomically thin. The materials properties are found to degrade in humid and oxygen-richenvironmentsandthishasoftenbeenassignedtopoorenvironmental stability. Despite the suggested importance of crystal defects, no experimental studies of point defects existed prior to the current study. The objective was therefore to understand and propose strategies to mitigate this behaviour. In particular,missingSeatoms(termedSevacancies,V Se ),werepredictedtodrastically reduce the crystal’s stability, as they provide reactive sites for the dissociation of oxygen (O) 3 . In this work, in an attempt to preserve the pristine quality of the bulk crystal, the GaSe and InSe sheets were exfoliated in an argon glove box and sandwiched between graphene for protection (as graphene is completely impermeabletoairandmoisture)(shown inthecartoon inFig.1a). Imagingofthe atomically thin GaSe and InSe, was performed using state-of-the-art, lowvoltage, annular dark field scanning transmission electron microscopy (ADF-STEM) at the ePSIC facility, Diamond Light Source. Low voltage STEM, operating at 60–80 kV acceleration voltage instead of the typical 200–300 kV range, minimises electron beam-induced damage to the sample and graphene encapsulation. In most STEM instruments these imaging conditions would severely limit the achievable spatial resolution, but the advanced aberration correcting optics and highly stable environment allow atomic resolution imaging to be routinely achievable, even at low acceleration voltage. Both pristine and defective regions of monolayer were observed (examples for InSe given in Fig. 1). From the effect of the electron beam, new defects were also observed to form, move, and heal, returning to their original pristine state (Fig. 1f–j). Image simulations were performed matching the experimental parameters of the microscope, using atomic structures containing candidate defects, relaxed using density functional theory (DFT). When compared with the experimental data, these simulations enabled the identification of the point defects in the crystal, such as the substitution of O to a Se site in GaSe (see Related Publication) and single In and Se vacancies in InSe (Fig. 1b–c, and d–e, respectively). Point defects were also observed to have coalesced to form extended defects in few layer materials and these also appeared morphologically different in isostructural GaSe and InSe. GaSe typically formed trigonal faceted defects, believed to consist of regions where both Ga and Se atoms have been equally lost from the crystal (Fig. 2a, c). In contrast, InSe was observed to predominantly form networks of line defects, suggested to comprise of mainly Se vacancies (Fig. 2b). InSe, in addition, showed interesting damage structures at the edges of the crystal, where a sharp transition was seen from pristine crystal to a semi- crystalline and amorphous edge (Fig. 2d). In addition to vacancies, large areas of altered stacking of layers were observed in both GaSe and InSe were seen, where the crystal stacking deviated from the most energetically stable phase (named ε andγforGaSeandInSe,respectively),totheunexpectedAAsequence,whereboth metalandchalcogenatomsareverticallyaligned.Fig.3demonstratesthisfor InSe, where the transition fromγ to AA stacking is seen to extend over 4 nm. Given the excellent properties of devices carefully produced in the same controlled argon environment, the abundant presence of defects found in these materials suggests that preparing 2D III-VI semiconductors in ultra-high vacuum may produce even better properties yet. Furthermore, the observed changes in stackingwere found to have only a small energy penalty but could have significant effectsontheelectronicstructure,motivatingfurtherworktoexplorethepotential of engineered restacking for optoelectronic device applications. References: 1. Bandurin D. A. et al. High electronmobility, quantumHall effect and anomalous optical response in atomically thin InSe. Nat. Nanotechnol. 12 , 223–227 (2017). DOI: 10.1038/nnano.2016.242 2. Terry D. J. et al. Infrared-to-violet tunable optical activity in atomic films of GaSe, InSe, and their heterostructures. 2DMater. 5 , 041009 (2018). DOI: 10.1088/2053-1583/aadfc3 3. Kistanov A. A. et al. Atomic-scalemechanisms of defect- and light-induced oxidation and degradation of InSe. J. Mater. Chem. C 6 , 518–525 (2018). DOI: 10.1039/C7TC04738J Funding acknowledgement: Engineering and Physical Sciences Research Council (EPSRC) U.K Grants EP/ K016946/1, EP/M010619/ 1, EP/R031711/1, EP/P009050/1, EP/S019367/1, EP/ N010345/1, and EP/P026850/1, EPSRC Graphene NoWNano CDT, DefenseThreat Reduction Agency Grant HDTRA1-12-1-0013, European Union Horizon 2020 research and innovation program (European Research Council Grant Agreement ERC-2016-STG-EvoluTEM-715502, the Hetero2D Synergy grant, and the European Graphene Flagship Project), National Academy of Sciences of Ukraine, the LeverhulmeTrust. Corresponding authors: David G. Hopkinson, National Graphene Institute and Department of Materials, The University of Manchester,
[email protected] Prof Sarah J. Haigh, National Graphene Institute and Department of Materials,The University of Manchester,
[email protected] Imaging andMicroscopy Group ePSIC Figure 1: ADF imaging of point defects in monolayer InSe. (a) Cartoon showing the principle of imaging graphene-encapsulated monolayer InSe. In atoms are grey, Se atoms are gold, C atoms are black. (b, d) ADF image of (b) In vacnancy, VIn, and (d) Se vacancy, VSe, with (c, e) their corresponding DFT-relaxed structural models, tilted slightly to aid visualisation of the defect. (f – j) Time series of defect (g) formation, (h – i) migration, and (j) healing. All scale bars: 1 nm. Adapted with permission from the Related Publication. Copyright 2019 American Chemical Society. Figure 2: Comparison of extended defects observed in GaSe and InSe. (a) Time averaged image of stable trigonal defects in thick (>10 layer) GaSe. Scale bar: 5 nm. (b) Dense networks of line defects in bilayer InSe. Scale bar: 2 nm. (c) Detail of a trigonal defect in bilayer GaSe. Scale bar: 1 nm. (d) Atomically sharp crystalline-amorphous transition towards the edge of a few layer InSe. Scale bar: 5 nm. Adapted with permission from the Related Publication. Copyright 2019 American Chemical Society. Figure 3: Strain-induced changes in stacking order in bilayer InSe. (a) Transition from stable γ stacking (bottom) to AA stacking (top) in InSe. Scale bar: 2 nm. Detail of (b) AA, (d) intermediate, and (f) γ stacking, with (c, e, g) corresponding structural models. Scale bars: 1 nm. Adapted with permission from the Related Publication. Copyright 2019 American Chemical Society.