62 63 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 2 1 / 2 2 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 2 1 / 2 2 Understanding lattice reconstruction of 2Dmaterials for tuning electronic behaviour Related publication: Weston, A., Zou, Y., Enaldiev, V., Summerfield, A., Clark, N., Zólyomi, V., Graham, A., Yelgel, C., Magorrian, S., Zhou, M., Zultak, J., Hopkinson, D., Barinov, A., Bointon, T. H., Kretinin, A.,Wilson, N. R., Beton, P. H., Fal’ko, V. I., Haigh, S. J., & Gorbachev, R. Atomic reconstruction in twisted bilayers of transition metal dichalcogenides. Nature Nanotechnology 15 , 592–597 (2020). DOI: 10.1038/s41565-020-0682-9 Publication keywords: 2Dmaterials; Moiré superlattices; Van derWaals heterostructures T wisting two sheets of monolayers (a layer of material one/fewatoms thick) has proven to be a novel way to engineer strikingly different properties compared to their monolayer counterparts. When two Transition-metal dichalcogenide (TMD) monolayers are stacked on top of each other, two common stacking polytypes can be formed - referred to as 3R and 2H. Introducing a slight twist angle to either polytype produces a long-range periodic moiré pattern. This was expected to significantlymodify the optoelectronic properties of the twisted bilayer systembut had not been experimentally investigated. ResearchersusedePSIC’s E02ScanningTransmissionElectronMicroscopy (STEM) instrument toexamine theTMDcrystal lattice. The instrument’s multiple detectors allowed visualisation of the domain structure from the low angle annular dark field detector (LAADF). High angle annular dark field (HAADF) imaging revealed the atomic lattice information. Their results show that at twist angles of below around 2 o , a structural transition occurs from a rigidly twisted lattice to an atomically reconstructed lattice where the energy is reduced by having regions (or domains) of commensurate (perfect) stacking separated by partial dislocations. It further demonstrated that this structural relaxation changes the electronic behaviour of the material, providing potential applications in non-volatilememory storage and quantum technologies. The results obtained from ADF STEM combined with c-AFM suggest that this new regime of lattice reconstruction can further tailor the properties of 2Dmaterials and broaden the scope for optoelectronic applications. Transition-metal dichalcogenides (TMDs) are a sub-group of layered crystalline materials that can be easily exfoliated down to the monolayer thickness owing toweak inter-layer van derWaals forces. Eachmonolayer consists of a groupVI transitionmetal atom sublayer sandwiched between two chalcogen atom sublayers. This study focussed on semiconducting TMDs, namely, MoS 2 and WS 2 with trigonal prismatic metal coordination. Besides graphene, TMDs are one of the most widely studied 2-dimensional (2D) materials owing to their excellent light-matter interaction, electrical properties and wide range of band-gaps that render them ideal for a host of optoelectronic applications including LEDs, transistors and sensors. In this work, adhesion-enhanced TEM grids 2 were used to reliably fabricate clean suspended samples of twisted bilayer TMDs. The twisted bilayers were prepared in a controlled argon environment to ensure a clean interface between the layers. A modified ‘tear-and-stack’ method 3 was employed for twisted homobilayers to selectively determine the stacking polytype and the twist angle with high accuracy (± 0.1 o ). For heterobilayers (e.g. MoS 2 on WS 2 ), straight crystallographic edges with angles that are multiples of 30 o of two dissimilar crystals were aligned with a relative twist to achieve the required small twist angle. With the latter method, there is equal probability of creating the 3R or 2H stacking polytype. ePSIC’s E02 Scanning Transmission Electron Microscopy (STEM) instrument was used at low accelerating voltage (60 or 80 kV) to avoid damaging the TMD crystal lattice. The instrument’s multiple detectors allowed visualisation of the domain structure from the low angle annular dark field detector (LAADF) (Fig. 1c-d and g-h) as well as atomic lattice information to be revealed via high angle annular dark field (HAADF) imaging (Fig. 1b and f). Lattice reconstruction was imaged for both the 3R and 2H hetero/homobilayer crystals at the atomic scale focusing on the reconstruction that occurs when the twist angle is below~2 o Fig. 1a-d presents the 3R-type TMD bilayers which structurally reconfigure into a triangular domain network (Fig. 1b). Periodically repeating stacking configurationswereobservedand identified in the imageaswell as corresponding insets asMX’(metal in top layer above chalcogens in lower layer), XM’(chalcogens in top layer above metals in lower layer), XX’ (chalcogens/metals in top layer above chalcogens/metals in lower layer) and B (boundary- no direct alignment). Note that theMX’and XM’stackings are the identical (3R) structure‘flipped’upside down. Nonetheless, as the 3R-like atomic stacking lacks mirror and inversion symmetry, these two configurations behave differently. The triangular domains alternate between MX’ and XM’ stacking, while the intersections are XX’ and the domain boundaries (partial dislocations) showno direct vertical alignment of the two layers. LAADF STEM revealed boundary widths of ~3 nm and that for these 3R typeTMD bilayers, the tessellated triangular domains growequally in sizewith decreasing twist angle (Fig. 1c-d). In contrast, 2H-type TMD bilayers twisted close to 2 o initially reconfigure into a triangular domain network, similar to that seen for 3R. However, unlike the 3R-polytype, 2H stacking has mirror/inversion symmetry. Adjacent domains of 2H (the most common TMD stacking) and MM’ stacking (where the metal atoms in the top layer are directly above metal atoms in the lower layer) are observed (Fig. 1f.) The MM’ stacking is a metastable-stacking configuration introduced via twisting and has not been previously observed. The domains are again separated by boundaries (3.5 nm in width) with XX’ (chalcogens in the top layer above chalcogens in the bottom layer) stacking configurations at the intersections. At smaller twist angles, the domains generally increase in size, but in this case the MM’domains reach amaximum size of 5 nm for θ~0.9 o (Fig. 1g). At increasingly small twist angles, 2H domains eventually take on a hexagonal domain configuration in which 2H domains are adjacent to each other and the MM’ stacking remains at alternating intersections (Fig. 1h). All our experimental observations agree well with complementary modelling (Fig. 1a and 1e) 4 . Complementary, conductive atomic forcemicroscopy (c-AFM) (this work) and Kelvin probe force microscopy (KPFM) measurements 5 were used to reveal the dramatic effect such local structural relaxation has on the electronic behaviour. For the 3R-type configuration, the triangular MX’stacking domains have a higher tunnelling current than XM’ domains (Fig. 2a). This is attributed to the ‘layer polarised’ electron wavefunction weightings of the top and bottom layers due to inversion asymmetry. In other words, the top layer has a higher probability of finding an electron than the bottom; this scenario reverses as we scan across MX’ to XM’ stacking domains during c-AFM measurements. A follow on study from the same group found the 3R-type domain structure to be a ferroelectric 5 . KPFM revealed an out-of-plane potential difference between oppositely polarised MX’ and XM’ domains and ferroelectric switching was demonstrated in prototype devices; such behaviour is ideal for non-volatile memory storage applications. For the 2H-type configuration, c-AFM revealed regions of high tunnelling current were confined to the MM’ stacking domains (with a maximum width of 5nm) and an intermediate tunnelling current at the domain boundaries (Fig. 2b). The increased tunnelling current was attributed to the piezoelectric charge induced by the localised strain, concentrated in the MM’ and boundary regions. Monolayers of TMDs are known piezoelectrics; although the piezoelectric effect is expected to cancel out in 2H-type bilayers, the induced deformation at the MM’ and boundary sites that is occurring in opposite directions means that piezo- charge of each layer can be added together, effectively doubling the piezoelectric induced charge confined to the MM’ and B regions of the domain structure. This type of charge-confinement has the exciting potential for creating twist controlled networks of quantum dots and nanowires with novel optoelectronic properties. References: 1. Wilson N. P. et al . Excitons and emergent quantumphenomena in stacked 2D semiconductors. Nature 599, 383–392 (2021). DOI: 10.1038/s41586-021- 03979-1 2. Hamer M. J. et al. Atomic Resolution Imaging of CrBr 3 Using Adhesion- Enhanced Grids. Nano Letters 20, 6582–6589 (2020). DOI: 10.1021/acs. nanolett.0c02346 3. KimK. et al. van derWaals Heterostructures with High Accuracy Rotational Alignment. Nano Letters 16, 1989–1995 (2016). DOI: 10.1021/acs. nanolett.5b05263 4. EnaldievV. V. et al. Stacking Domains and Dislocation Networks inMarginally Twisted Bilayers of TransitionMetal Dichalcogenides. Physical ReviewLetters 124, 206101 (2020). DOI: 10.1103/PhysRevLett.124.206101 5. Weston, A., Castanon, E.G., Enaldiev, V. et al. Interfacial ferroelectricity in marginally twisted 2D semiconductors. Nat. Nanotechnol . (2022). https:// doi.org/10.1038/s41565-022-01072-w Funding acknowledgement: We acknowledge support from the Engineering and Physical Sciences Research Council (EPSRC) grants EP/N010345/1, EP/P009050/1, EP/S019367/1, EP/ S030719/1, EP/P01139X/1, EP/R513374/1, the Centre for Doctoral Training (CDT) Graphene-NOWNANO and the Diamond Light Source for access and support in the use of the electron Physical Sciences Imaging Centre (proposal numbers EM19315 andMG21597).We also acknowledge support from the European Graphene Flagship Project, European QuantumTechnology Flagship Project 2D-SIPC (820378), European Research Council (ERC) Synergy Grant Hetero2D, ERC Starter grant EvoluTEM (715502), Royal Society and Lloyd Register Foundation Nanotechnology grant. Corresponding authors: Dr AstridWeston, The University of Manchester,
[email protected] Professor Sarah J. Haigh, The University of Manchester,
[email protected] Imaging andMicroscopy Group ePSIC Figure 1: ADF Imaging of lattice reconstruction in twisted TMD homo- and hetero-bilayers; DFT and multiscale modelling for reconstructed (a) 3R-type and (e) 2H-type stacking domain structures. Corresponding ADF images with the four primary stacking configurations enlarged in the inset as well as atomic schematics for both (b) 3R-type and (f ) 2H-type configurations. LAADF images of 3R-configuration periodically repeating triangular domains that grow equally in size with decreasing twist angle (from (c) 1.5o to (d) 0.67o) unlike the 2H-configuration that transitions from a (g) kagome-like structure to a (h) hexagonal shaped domain structure with decreasing twist angle. Figure 2: Conductive-AFM images of reconstructed homobilayers of MoS2 in both the 3R- and 2H- stacking configuration; (a) Map of the tunnelling current acquired on a 3R-type MoS2 homobilayer with a tip-sample bias Vb = 600 mV; (b) Map of the tunnelling current acquired on a 2H-type MoS2 homobilayer with a tip-sample bias Vb = 400 mV. In the top image, the periodically repeating bright spots correspond to regions of MM’ stacking and in the bottom image the regions of higher tunnelling current correspond to both MM’ as well as extended 2H/2H domain boundaries (extending to the top RHS of the image).