Materials in Flatland Twist and Shine

NEWS & VIEWS RESEARCH

species between the Permian and the end of 1. Erwin, D. Sci. Am. 275, 72–78 (1996). 7. Gastaldo, R. A., Neveling, J., Geissman, J. W. & the Middle Triassic (between 251.9 million and 2. Burgess, S. D., Bowring, S. & Shen, S. Z. Proc. Natl Kamo. S. L. Geol. Soc. Am. Bull. 130, 1411–1438 Acad. Sci. USA 111, 3316–3321 (2014). (2018). around 237 million years ago) has focused on 3. Payne, J. L. & Clapham, M. E. Annu. Rev. Earth 8. Collinson, J. W., Hammer, W. R., Askin, R. A. & the effects of a global trend towards aridifica- Planet. Sci. 40, 89–111 (2012). Elliot, D. H. Geol. Soc. Am. Bull. 118, 747–763 (2006). tion. It was proposed that, after a worldwide 4. Fielding, C. R. et al. Nature Commun. 10, 385 9. Benton, M. J. Phil. Trans. R. Soc. A 376, 20170076 collapse of plant communities and a mass (2019). (2018). 5. Nowak, H., Schneebeli-Hermann, E. & 10. Smith, R. H. M. & Botha-Brink, J. Palaeogeogr. extinction of species that cascaded through the 9 Kustatscher, E. Nature Commun. 10, 384 (2019). Palaeoclimatol. Palaeoecol. 396, 99–118 (2014). food chain , there was a change in the floral 6. Behrensmeyer, A. K., Kidwell, S. M. & Gastaldo, R. A. 11. Sues, H. D. & Fraser, N. C. Triassic Life on Land: The species across global landscapes by the Middle Paleobiology 26, 103–147 (2000). Great Transition (Columbia Univ. Press, 2010). Triassic period. For the demise of Glossopteris, Fielding and colleagues find no evidence of an aridification trend in their region that would CONDENSED-MATTER PHYSICS suggest that a hot terrestrial landscape pro- moted a mass extinction of plants during the time of the end-Permian crisis. This conclusion of Fielding and colleagues’ Materials in flatland regional work is supported by a comprehensive analysis of plant fossil records on a global scale conducted by Nowak and colleagues. twist and shine The authors analysed the patterns of previ- ously reported plant fossils from 259.1 mil- Four studies demonstrate the vast opportunities provided by stacking pairs of lion to around 237 million years ago, which monolayer materials and changing the resulting optical properties by twisting spans the end-Permian mass extinction and one material with respect to the other. See Letters p.66, p.71, p.76 & p.81 the Early and Middle Triassic. They generated a database that includes information on more than 7,300 plant macrofossils and nearly BERNHARD URBASZEK & AJIT SRIVASTAVA be stacked on top of each other to form what is 43,000 fossil records of pollen or spores. So known as an artificial heterobilayer (Fig. 1a). far, this is the most comprehensive database tomically thin materials are currently If the layers are periodic crystals that have generated for floral analysis before and after being investigated for fundamental slightly different lattice constants, the elec- the end-Permian crisis. It amasses the evidence research and applications in optics tronic properties of each layer are modified that has been considered by many palaeontol- Aand electronics, because they interact strongly by the presence of the other layer. Specifically, ogists to indicate a trend in mass extinction with light and have fascinating magnetic the electronic states and band structure of of terrestrial plants that mirrors that of the properties. When two different monolayer the heterobilayer depend on the spacing and marine mass extinction9. materials are brought into contact to form a relative alignment of the atoms. The authors present origination, extinction bilayer, electrons can no longer move freely in Because of the slight mismatch in the and turnover patterns at the level of species and the planes of the atomic layers. Instead, they lattice constants of the two layers, arrangements genera on a stage-by-stage basis (stages being are trapped in spatially periodic potential- of atoms in the heterobilayer change periodi- steps in the geological timescale). The diver- energy variations called moiré potentials, as cally: atoms of metals (such as molybdenum sity of genera was relatively constant across the a result of interactions between the layers1. and tungsten) are positioned on top of each time interval, although the species diversity of These nanometre-scale potentials are caused other at certain points, whereas atoms of chal- macrofloral fossils dropped 251.9 million years by the layers having different orientations or cogens (such as sulfur and selenium) are aligned ago. The diversity of genera represented by lattice constants — parameters that describe at other points. These different configurations spores and pollen remained constant across the dimensions of a unit cell in a crystal of atoms, known as registries, result in differ- the time frame studied, although Nowak et al. lattice. Moiré potentials have been predicted ent energies for the valence and conduction note a small decline in species-level diversity to strongly modify the optical properties of bands, as verified by scanning tunnelling micro around 251.9 million years ago. Of the groups such bilayers2. Four papers in this issue3–6 scopy1. Consequently, electrons in the plane of of plants that have either pollen or spores, report observations of optical emission and the heterobilayer are subject to periodically the spore-bearing ferns, as well as the pollen- absorption that confirm this prediction. changing bands, and, if the variations in the producing seed ferns and cycads, declined Monolayers of materials called transition- band energies are sufficiently large, electrons in diversity during this time, whereas the metal dichalcogenides (TMDs) have strong are trapped in moiré potentials. pollen-bearing conifers and ginkgos increased in-plane covalent bonds, and can therefore An attractive feature of such heterobilayers in diversity. be produced by exfoliation (the removal of is the tunability of their periodic potential. In contrast to prevailing wisdom, Nowak sheets from a bulk crystal) in a similar way to Adding a slight twist during the stacking pro- and colleagues demonstrate that land plants graphene — a single layer of carbon atoms. cess, or exchanging one lattice with another did not experience widespread extinction However, unlike graphene, atomically thin that has a different lattice constant, leads to during Earth’s most severe biological crisis. TMD crystals are semiconductors that have moiré potentials that have a different spatial Their conclusion is similar to that drawn for an energy gap between their higher-energy periodicity. This feature enables the electronic terrestrial vertebrates11. This leaves the rela- (conduction) and lower-energy (valence) properties of layered materials to be tailored in tionship between the end-Permian marine electronic bands. Consequently, charge carri- a fundamentally new way. As a result, hetero mass extinction and the effect on land at the ers in these materials that are excited by light or bilayers are becoming a playground for explor- time enigmatic for now, and still up in the air that are injected using a voltage can relax from ing exotic quantum phenomena — rather like for further investigation. ■ the conduction band to the valence band by how the strong interactions between electrons emitting particles of light (photons). in twisted bilayers of graphene have led to the Robert A. Gastaldo is in the Department of Just as the weak attraction between layers spectacular observation of superconductivity Geology, Colby College, Waterville, Maine lends itself to exfoliation, it also allows two in electronic-transport studies7. 04901, USA. monolayers of different TMDs (such as tung- The four current papers investigate the e-mail: [email protected] sten disulfide and molybdenum diselenide) to impact of moiré potentials on light emission

RESEARCH NEWS & VIEWS

a b c

Conduction Hybridized band Electron exciton

Interlayer exciton

Energy

Intralayer exciton Hole Valence band

Figure 1 | Features of moiré heterobilayers. a, Four papers3–6 report on The two groups explored interlayer excitons, in which the electron and hole the optical properties of moiré heterobilayers — stacks of two different exist in different layers of the heterobilayer (indicated by the different colours monolayer materials that display a ‘moiré’ pattern. b, The teams studied of the charge carriers). Jin et al.5 focused on intralayer excitons, in which heterobilayers in which light emission and absorption are governed by the electron and hole reside in the same layer. c, Alexeev et al.6 found that electrons and holes (electron vacancies) forming bound states called intralayer excitons can be mixed (hybridized) with interlayer excitons in excitons. Tran et al.3 and Seyler et al.4 studied molybdenum diselenide– heterobilayers of molybdenum diselenide and tungsten disulfide, the band tungsten diselenide heterobilayers. Shown here are electronic-band diagrams diagram of which is shown in purple. The conduction band is delocalized for molybdenum diselenide (green) and tungsten diselenide (blue) that over both layers, which permits the existence of a hybridized electron state illustrate the higher-energy (conduction) and lower-energy (valence) bands. (indicated by the arrow). and absorption in heterobilayers of TMDs. Whereas Tran et al. and Seyler et al. focused In these four papers, an optical microscope The work has many new aspects compared on light emission, Jin et al.5 (page 76) detected with a spatial resolution of typically 1 micro- with studies on graphene–boron nitride experimental signatures of changes in light metre was used to investigate moiré potentials heterobilayers 1. First, the variation in the band absorption caused by moiré potentials in that, by contrast, have a periodicity of only tens energies for the different atomic registries is tungsten diselenide–tungsten disulfide of nanometres. By using ‘near-field’ techniques larger. Second, light emission3,4 and absorp- heterobilayers. The authors examined intra- that have a spatial resolution of the order of tion5,6 in the TMD structures are governed by layer excitons10, in which the electron and hole 10 nanometres, individual potentials could electrons and holes (electron vacancies) form- exist in the same layer (Fig. 1b). They showed be addressed16. Depending on the size and ing bound states called excitons8. And third, that the strong, intriguing absorption features depth of the moiré potentials, the number of the excitons interact with light of a specific associated with these excitons can be tuned by trapped excitons could really go down to one, polarization that depends on the local sym- an applied voltage. offering a source of single photons. Finally, the metry of the atoms arranged periodically in Finally, Alexeev et al.6 (page 81) found potentials themselves could provide a periodic the heterobilayer. evidence that intralayer excitons can be array of tunable light emitters — one of the Tran et al.3 (page 71) explored interlayer mixed (hybridized) with interlayer excitons many fascinating prospects for optics in two- excitons, in which the electron and hole in molybdenum diselenide–tungsten disulfide dimensional ‘flatland’, although, admittedly, reside in different layers (Fig. 1b). The authors heterobilayers, because the conduction band now the material is not completely flat, but obtained emission spectra from molyb is delocalized over both layers (Fig. 1c). They two monolayers thick. ■ denum diselenide–tungsten diselenide that showed that this feature amplifies the impact had a small twist angle between the layers. of the moiré potentials on optical properties. Bernhard Urbaszek is at the Laboratory They located peaks in the spectra that cor- These hybridized states could combine the of Physics and Chemistry of Nano-Objects, responded to interlayer excitons, and found strong optical absorption of intralayer exci- INSA-CNRS-UPS, Toulouse University, that the energy separation between the peaks tons with the tunability of interlayer excitons in Toulouse 31077, France. Ajit Srivastava is in and the polarization of the emission depended electric fields — this tunability is attributable the Department of Physics, Emory University, on the twist angle. These results indicate that to the permanent electric dipole of interlayer Atlanta, Georgia 30322, USA. it might be possible to achieve nanoscale excitons, which results from the electron and e-mails: [email protected]; patterning of optical and magnetic properties hole being in different layers. [email protected] in heterobilayers2,4. These studies and others10–12 give a first 4 1. Zhang, C. et al. Sci. Adv. 3, e1601459 (2017). Seyler et al. (page 66) studied individual glimpse of the opportunities offered by com- 2. Yu, H., Liu, G.-B., Tang, J., Xu, X. & Yao, W. Sci. Adv. 3, interlayer excitons trapped in moiré poten- bining two materials and changing the result- e1701696 (2017). tials, also in molybdenum diselenide–tungsten ing optical properties simply through the 3. Tran, K. et al. Nature 567, 71–75 (2019). 4. Seyler, K. L. et al. Nature 567, 66–70 (2019). diselenide structures. They detected lines in twist angle. A challenge will be to control the 5. Jin, C. et al. Nature 567, 76–80 (2019). emission spectra that corresponded to highly natural tendency of two stacked monolayers to 6. Alexeev, E. M. et al. Nature 567, 81–86 (2019). polarized light and that split into multiple rotate after deposition towards an energetically 7. Cao, Y. et al. Nature 556, 43–50 (2018). peaks when magnetic fields were applied. favourable configuration, as has been observed 8. Wang, G. et al. Rev. Mod. Phys. 90, 021001 (2018). 13 9. Michler, P. et al. Science 290, 2282–2285 (2000). These emission lines provide a fingerprint of for graphene–boron nitride heterobilayers . 10. Zhang, N. et al. Nano Lett. 18, 7651–7657 (2018). the magnetic moment and valley — minimum This local rearrangement of atoms, termed 11. Ciarrocchi, A. et al. Nature Photon. 13, 131–136 or maximum of the band structure — of the reconstruction, can result in deviations from (2019). electron and hole that formed a particular a completely periodic potential and therefore 12. Paik, E. Y. et al. Preprint at https://arxiv.org/ 14 abs/1901.00598 (2019). exciton. However, to verify that the emission inhomogeneities in the moiré potentials . 13. Woods, C. R. et al. Nature Commun. 7, 10800 really stems from a single exciton trapped in a Further insight into this issue could be gained (2016). specific moiré potential, measurements based by studying the differences that might occur 14. Alden, J. S. et al. Proc. Natl Acad. Sci. USA 110, 11256–11260 (2013). on a phenomenon called photon antibunching between exfoliated heterobilayers and samples 15. Hsu, W.-T. et al. Nature Commun. 9, 1356 (2018). will be necessary, to prove that, as expected, grown by a technique called chemical-vapour 16. Schmidt, P. et al. Nature Nanotechnol. 13, only one photon is emitted at a time9. deposition15. 1035–1041 (2018).