Atomic structure and defect dynamics of monolayer lead iodide nanodisks with epitaxial alignment on graphene

ARTICLE https://doi.org/10.1038/s41467-020-14481-z OPEN Atomic structure and defect dynamics of monolayer lead iodide nanodisks with epitaxial alignment on graphene 1234567890():,; Sapna Sinha 1, Taishan Zhu2, Arthur France-Lanord Kyriakos Porfyrakis 3 & Jamie H. Warner4* 2, Yuewen Sheng 1, Jeffrey C. Grossman 2, Lead Iodide (PbI2) is a large bandgap 2D layered material that has potential for semiconductor applications. However, atomic level study of PbI2 monolayer has been limited due to challenges in obtaining thin crystals. Here, we use liquid exfoliation to produce monolayer PbI2 nanodisks (30-40 nm in diameter and > 99% monolayer purity) and deposit them onto suspended graphene supports to enable atomic structure study of PbI2. Strong epitaxial alignment of PbI2 monolayers with the underlying graphene lattice occurs, leading to a phase shift from the 1 T to 1 H structure to increase the level of commensuration in the two lattice spacings. The fundamental point vacancy and nanopore structures in PbI2 monolayers are directly imaged, showing rapid vacancy migration and self-healing. These results provide a detailed insight into the atomic structure of monolayer PbI2, and the impact of the strong van der Waals interaction with graphene, which has importance for future applications in optoelectronics. 1 Department of Materials, University of Oxford, 16 Parks Road, Oxford OX1 3PH, UK. 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA. 3 Faculty of Engineering and Science, University of Greenwich, Central Avenue, Chatham Maritime, Kent, ME4 4TB, UK. 4 Department of Mechanical Engineering, University of Texas at Austin, 204 Dean Keeton Street, Austin 78712, USA. *email: NATURE COMMUNICATIONS | (2020)11:823 | https://doi.org/10.1038/s41467-020-14481-z | www.nature.com/naturecommunications 1 ARTICLE T NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-14481-z wo-dimensional (2D) materials attract interest because of their unique chemical and physical properties, facilitating the study of novel physics, e.g., trions, valley polarization, etc1–3. Although graphene exhibits an exceptionally high carrier mobility (>106 cm2 V−1s−1 at 2 K), its zero bandgap poses difficulties for many semiconductor applications4. Monolayer transition metal dichalcogenides (TMDs), such as MoS2, can have direct band gaps of ~1.8 eV, but do not exhibit very high charge carrier mobility5–7. For Mo- and W-based TMDs, such as MoSe2, WS2, WSe2, etc., the band gaps fall within the range of 1.0–2.0 eV, which is the red to near infrared regions8,9. For optoelectronics, blue, green, and ultraviolet light-emitting diodes are also needed for full color displays and cameras. Currently, there are not many experimental studies of 2D materials that can satisfy the demands for the green to UV spectral regions and more research is needed to expand this area. PbI2 is a layered direct bandgap semiconductor with bandgap of 2.4 eV in its bulk form, whereas its 2D monolayer has an indirect bandgap of ~2.5 eV, with possibilities to tune the bandgap between 1–3 eV10–13. PbI2 is frequently used to fabricate organic–inorganic halide perovskite solar cells14,15, and as a highenergy photon detector material for gamma-rays and X-rays16–18. PbI2 has a wide variety of I-Pb-I stacking and this gives rise to more than a dozen polymorphs19. However, the thickness of each single layer (0.7 nm), and the distance between each lead and iodide atoms (0.32 nm) is independent of the polytypes19. The 2H structure is the most commonly found polytype of threedimensional PbI2, where each plane of Pb and I atoms are shifted with respect to each other and form overlapping hexagons. Layered PbI2 has been shown to be excellent candidate for use in optoelectronic applications, photodetectors, and photon detection20–23. Ultrathin PbI2 is an interesting system for studying quantum-confinement effects because the exciton–phonon couplings are dependent on the degree of localization of electronic charge24,25. Cabana et al.26 fabricated PbI2 interfaces with carbon nanotubes and studied the change in the density of states of the system. Zhou et al.10 studied the graphene/PbI2 van der Waals interface and predicted 1.5 eV increase in the visible light absorption capability of the heterostructure as compared to pure 2D PbI2. Recent work on few layer shows PbI2 as a promising candidate for application in the field of ultrafast saturable absorbers27. However, research into the atomic structure of monolayer PbI2 has been limited to date because of the challenges in obtaining high-quality monolayer crystals and preparing suitable samples for transmission electron microscopy studies. Further work is needed to reveal the structure and dynamics of edges, point defects, vacancy clusters, and nanopores in PbI2 monolayer and its interaction with other 2D crystals, such as graphene. Here, we used liquid phase exfoliation (LPE) to isolate PbI2 monolayer flakes from a starting bulk PbI2 powder. Liquid-phase exfoliation is one of the simplest methodologies for producing 2D materials on a large scale28–30. In the past decades, various research has been carried out to find suitable solvents based on their interactions with the 2D material to produce suspended 2D monolayer crystals. Surface tension, the Hilderbrand solubility parameter, the Hansen solubility parameter, surface tension components, etc. have been the widely used parameters to screen appropriate solvents31–35. Using these parameters, we screened the commonly used solvents and found chloroform (CHCl3) to produce monolayer 2D PbI2 crystals. We used annular dark-field scanning transmission electron microscopy (ADF-STEM) to studying the PbI2 atomic structure, by depositing it from solution onto a suspended graphene support and allowing it to dry36–38. The high electron transparency of graphene, enables provides excellent contrast from the Pb and I atoms in ADF-STEM. We 2 report on the fundamental atomic structure, point vacancies, vacancy clusters, vacancy dynamics, edge terminations and edge etching, and the epitaxial interactions with the underlying graphene support. Results Synthesis. PbI2 is a unique exception to all the metal halide compounds that show CdI2 structure, in that it has the largest metal halide bond length39. As a result, the bonds are not as ionic as that of the other compounds that also show CdI2 crystal structure, such as MgI2, FeBr2, etc. However, it is sufficiently ionic to dissolve into polar solvents, such as dimethylformamide (DMF), n-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), and to some extent, water40,41. Recent research on Lewis basicity of solvents, also quantified by Gutmann’s donor number—Dn, has shown that solvents that solubilize the precursor (PbI2) at a total concentration of 1 M, have higher Dn values of >25 (ref. 42). Previous results on the liquid-phase exf (...truncated)