Scalable photonic sources using two-dimensional lead halide perovskite superlattices

ARTICLE https://doi.org/10.1038/s41467-019-14084-3 OPEN Scalable photonic sources using two-dimensional lead halide perovskite superlattices 1234567890():,; Jakub Jagielski1, Simon F. Solari1, Lucie Jordan1, Declan Scullion 2, Balthasar Blülle3, Yen-Ting Li4,5, Frank Krumeich 6, Yu-Cheng Chiu 4,7, Beat Ruhstaller3,8, Elton J.G. Santos 2 & Chih-Jen Shih 1* Miniaturized photonic sources based on semiconducting two-dimensional (2D) materials offer new technological opportunities beyond the modern III-V platforms. For example, the quantum-confined 2D electronic structure aligns the exciton transition dipole moment parallel to the surface plane, thereby outcoupling more light to air which gives rise to highefficiency quantum optics and electroluminescent devices. It requires scalable materials and processes to create the decoupled multi-quantum-well superlattices, in which individual 2D material layers are isolated by atomically thin quantum barriers. Here, we report decoupled multi-quantum-well superlattices comprised of the colloidal quantum wells of lead halide perovskites, with unprecedentedly ultrathin quantum barriers that screen interlayer interactions within the range of 6.5 Å. Crystallographic and 2D k-space spectroscopic analysis reveals that the transition dipole moment orientation of bright excitons in the superlattices is predominantly in-plane and independent of stacking layer and quantum barrier thickness, confirming interlayer decoupling. 1 Institute for Chemical and Bioengineering, ETH Zürich, 8093 Zürich, Switzerland. 2 School of Mathematics and Physics, Queen’s University Belfast, BT7 1NN Belfast, UK. 3 Fluxim AG, 8400 Winterthur, Switzerland. 4 Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan. 5 National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan. 6 Laboratory of Inorganic Chemistry, ETH Zürich, 8093 Zürich, Switzerland. 7 Advanced Research Center for Green Materials Science and Technology, National Taiwan University, Taipei 10617, Taiwan. 8 Institute of Computational Physics, Zurich University of Applied Sciences (ZHAW), 8400 Winterthur, Switzerland. *email: NATURE COMMUNICATIONS | (2020)11:387 | https://doi.org/10.1038/s41467-019-14084-3 | www.nature.com/naturecommunications 1 ARTICLE B NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-14084-3 right excitons in quantum-confined two-dimensional (2D) materials have their transition dipole moment (TDM) oriented in parallel to the surface plane1–4, which is essential to enable high-efficiency quantum optics5,6 and electroluminescent devices7,8. It is desirable to fabricate the decoupled multi-quantumwell (MQW) superlattices3 by inserting atomically thin quantum barriers (QBs) between individual 2D material layers9. However, despite intense research efforts in layer-by-layer assembly10–12, van der Waals (vdW) epitaxy13–15, intercalation16, and colloidal chemistry2,17,18, controllable high-order MQW superlattices have not yet been realized in a scalable manner. Indeed, following the path of III–V semiconductors, a main motivation for the development of 2D material-based MQW superlattices is to gain strong spontaneous emission without triggering the multiexciton quenching mechanisms such as the Auger process19 that reduce the photoluminescence (PL) quantum yield (ηPL) by orders of magnitude20. To this end, inserting sizable, atomically thin QBs between 2D material layers that screens interlayer coupling becomes increasingly attractive21,22. The interlayer coupling yields the charge-transfer (CT) and the momentum-forbidden dark excitons that often annihilate radiative recombination because of their long lifetime23,24. Nevertheless, little is known of QB’s fundamental prerequisites to fully decouple neighboring 2D material layers in their stacks. In particular, it is desirable to gain fundamental insights into their correlation with the QB and 2D material thicknesses, dQB and d2D, respectively. For example, in the system of stacked CdSe nanoplatelets (NPLs) having d2D of 15 Å, 2D layers remain strongly coupled even with dQB of ~37 Å[2]. On the other hand, in the monolayer WSe2/MoS2 (d2D = 6.2 Å) heterostructures, a QB of trilayer hexagonal boron hydride (hBN), corresponding to dQB ≈ 13 Å, was observed to considerably reduce the CT exciton emission but not completely supressed21. In this report, from a fundamental point of view, we aim to elucidate principles decoupling two stacked 2D layers, as well as the spectroscopic techniques characterizing the extent of interlayer coupling. Since the quantum emission characteristics are mediated by the TDMs that orthogonally interact with the electromagnetic fields of the emitted photons, the dipole orientation of bright excitons in 2D materials is predominantly in-plane (IP) 1–4, analogous to those in the planar molecules7. When interlayer coupling comes into play, the symmetry is broken and the outof-plane (OP) components are induced23,25. Consequently, the probability of IP dipoles, or the IP dipole ratio, RIP, is lowered. This scenario is supported by recent observations that RIP in CdSe NPLs monolayer reaches 0.95 but drops to ~0.67 in the coupled multilayers corresponding to isotropic dipole orientation2,3. To our knowledge, decoupled 2D materials have never been demonstrated in their high-order superstructures. Here, we demonstrate that colloidal quantum wells (CQWs) of lead halide perovskites26–30, can form fully decoupled MQW superlattices with ultrathin organic QBs, equivalent to the insertion of monolayer h-BN. Not only is the TDM orientation of bright excitons in the superlattices predominantly IP, but also independent of stacking layer, which is proven by employing crystallographic and 2D k-space spectroscopic analysis. We attribute the observed localization of Wannier–Mott (WM)-like excitons to the strong ionic dielectric response that screens the interlayer electrostatic interactions. The preferential orientation of TDM is retained in the mixed-halide superlattices, covering the entire blue-to-orange visible spectrum. The findings reported here lay the foundation of ultrathin 2D material-based quantum emitters. 2 Results Fabrication of lead halide perovskite MQWs. We developed the synthetic and processing protocols based on our previous work28,31 to fabricate stacking-controlled MQW superlattices using the CQWs of lead halide perovskites. The CQWs are monodispersed quantum-confined 2D nanocrystals synthesized in solution with the formula (RNH3)2[CH3NH3PbBr3]nPbBr4, where R is an alkyl group with a low dielectric constant, ε ≈ 232, and n is the number of perovskite unit cell along the c axis (Fig. 1a–d). Upon the formation of MQW superlattices, the organic ligands attached to individual nanocrystals uniformly separate the perovskite QW layers, serving as a QB owing to their low dielectric constant and conductivity. Accordingly, depending on the length of R, one c (...truncated)