Heteroepitaxy—atomically aligned growth of a crystalline film atop a different crystalline substrate—is the basis of electrically driven lasers, multijunction solar cells, and blue-light-emitting diodes1,2,3,4,5. Crystalline coherence is preserved even when atomic identity is modulated, a fact that is the critical enabler of quantum wells, wires, and dots6,7,8,9,10. The interfacial quality achieved as a result of heteroepitaxial growth allows new combinations of materials with complementary properties, which enables the design and realization of functionalities that are not available in the single-phase constituents. Here we show that organohalide perovskites and preformed colloidal quantum dots, combined in the solution phase, produce epitaxially aligned ‘dots-in-a-matrix’ crystals. Using transmission electron microscopy and electron diffraction, we reveal heterocrystals as large as about 60 nanometres and containing at least 20 mutually aligned dots that inherit the crystalline orientation of the perovskite matrix. The heterocrystals exhibit remarkable optoelectronic properties that are traceable to their atom-scale crystalline coherence: photoelectrons and holes generated in the larger-bandgap perovskites are transferred with 80% efficiency to become excitons in the quantum dot nanocrystals, which exploit the excellent photocarrier diffusion of perovskites to produce bright-light emission from infrared-bandgap quantum-tuned materials. By combining the electrical transport properties of the perovskite matrix with the high radiative efficiency of the quantum dots, we engineer a new platform to advance solution-processed infrared optoelectronics.
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This publication is based, in part, on work supported by an award (KUS-11-009-21) from the King Abdullah University of Science and Technology (KAUST), by the Ontario Research Fund Research Excellence Program and by the Natural Sciences and Engineering Research Council (NSERC) of Canada. Computations were performed using the BlueGene/Q supercomputer at the SciNet HPC Consortium provided through the Southern Ontario Smart Computing Innovation Platform (SOSCIP). E.Y. acknowledges support from an FAPESP-BEPE (14/18327-9) fellowship. The authors thank L. Levina for assistance in CQD synthesis, E. Beauregard for assistance in PHC synthesis, Z. Yang and M. Adachi for discussions, and E. Palmiano, R. Wolowiec and D. Kopilovic for their help during the course of study.
The authors declare no competing financial interests.
Extended data figures and tables
The total charge difference (Δρ) is measured along the Z axis of the material.
a, b, Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), respectively, of a matched CQD. c, d, HOMO and LUMO, respectively, of mismatched dots. The states are mostly localized within the CQD in matched dots, whereas they are localized on the interface between the CQDs and perovskite in mismatched dots, indicating the formation of defects in the latter. e, f, The projected density of states (PDOS) as a function of energy (E) of matched and mismatched (rotated) CQDs, respectively.
a, TEM image of CQDs after solution-phase iodide–ligands exchange. The average CQD size is approximately 4 nm. b, Scanning electron microscopy (SEM) image of the film surface with CQDs embedded in perovskite matrix (CQD volume percentage of 3.8%). The average perovskite grain size is about 60 nm. c, SEM image of the cross-section of the film with CQDs embedded in perovskite matrix (CQD volume percentage of 3.8%). The film thickness is about 240 nm (top layer). d, e, Simulated TEM images of matched (d) and mismatched (e) CQDs and perovskite. Yellow line, perovskite lattice plane; red line, CQD lattice plane.
a–d, XPS analysis of the CQDs embedded in PHC film (Pb 4f, S 2p, N 1s, I 3d). All the elements (as labelled) are observed in the film, indicating the existence of both PbS and MAPbI3. e, XPS spectra of the N 1s orbital for high-loading PbS films, which follows the ligand exchange to perovskite ligands in butylamine, before and after annealing at 70 °C for 10 minutes, before the methylammonium post-treatment step. We detect only the ammonium signal at 402 eV and no amine signal at 398–399 eV, which suggests that the methylammonium iodide ligand is more stable on the surface and displaces weakly bound buylamine, even before film annealing. f, Plot of the nominal volume ratio of CQDs to perovskite versus the ratio measured using RBS (both with the perovskite value scaled to one), with a linear fit (red line) superimposed. The ideal case, where the measured ratio and the nominal ratio are equal, is indicated by the dashed grey line, showing the agreement between the nominal value and the one evaluated using RBS.
a, b, HAADF-STEM and TEM images of pure perovskite, respectively. c, HAADF-STEM image of dots in perovskite. The region indicated by the dashed yellow circle contains three PbS nanocrystals (white spots) located inside perovskite crystal (grey area). Inset, TEM image of part of this region. d, A close up of part of the inset in c.
a, b, Absorption and photoluminescence (PL) spectra for a pure CQD film (a) and a pure perovskite film (b). c, d, CQD photoluminescence signal from PHC films with 28% (c) and 17% (d) CQD concentration, acquired at λ = 635 nm (blue), 680 nm (green), and 815 nm (red). The photoluminescence signal has been normalized by the excitation intensity at the different wavelengths used. The shaded area corresponds the region of the spectrum used to calculate the photoluminescence integrated area in equation (3). e, Corresponding optical absorption spectra for a CQD concentration of 28% (dark red) and 17% (red) that were used to evaluate the absorption parameters in, for example, equation (3).
a, Cross-section of the relaxed 3-nm PbS CQD in vacuum. b, DFT-optimized geometry of the same 3-nm CQD epitaxially matched with perovskite in a unit cell corresponding to unstrained perovskite. Colours represent the following: grey, lead; yellow and green, sulphur; pink, iodine. The green lines and corresponding numbers are the long-range Pb-to-Pb distance (23.949 Å, 24.069 Å and 44.197 Å); the large green numbers are the average lattice constant (average distance between lead atoms), along those lines.
a, Photoluminescence (PL) excitation profile from a hybrid film of 1,300-nm-emission CQDs dispersed in 950-nm-emission CQDs (1:10 volume ratio, ‘dots-in-dots’), acquired by measuring the photoluminescence emission (at λ ≈ 1,300 nm) from the small-bandgap CQDs (1,300-nm-emission CQDs) as a function of excitation wavelength. The absorption profile from the large-bandgap CQDs (950-nm-emission CQDs) is overlaid as the shaded grey area, and exhibits the characteristic excitonic peak at λ ≈ 950 nm. The wavelength dependence of the photoluminescence intensity from the small-bandgap CQDs closely follows the absorption profile of the large-bandgap CQDs, analogous to the dots-in-perovskite films (see Fig. 4b), and is therefore suggestive of carrier funnelling from the large- to the small-bandgap dots. b, Photoluminescence decay for 1,300-nm-bandgap CQDs in a CQD-based matrix (light red) and in a perovskite (MAPbI3) matrix (dark red), with 632 nm excitation. The longer dynamics for the CQDs embedded in the perovskite matrix (approximately 150 ns) compared to those for the ‘dots-in-dots’ (approximately 30 ns) provides evidence for a better passivation of CQDs in the former.
Plot of the diffusion length LD of carriers in the perovskite matrix (round markers) for various CQD concentrations, calculated using equation (11) and different possible values for the diffusion coefficient D, as labelled. The red line shows the calculated average inter-dot spacing as a function of CQD:MAPbI3 volume ratio.
A stacked plot showing the total photoluminescence yield of the CQDs (shaded area), which is made up of contributions from free carrier recombination and exciton recombination: filled circles represent the integrated (with respect to time) photoluminescence (PL) contribution as a result of free carriers injected from the perovskite (‘free carrier recombination’); filled squares represent the integrated (with respect to time) photoluminescence as a result of the dissociation of excitons generated directly in the CQDs (‘excitation recombination’); see also equation (10). Consistent with the photoluminescence excitation spectra, the largest contribution to the CQDs photoluminescence is ascribed to radiative recombination of free carriers transferred from the perovskite.
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Ning, Z., Gong, X., Comin, R. et al. Quantum-dot-in-perovskite solids. Nature 523, 324–328 (2015). https://doi.org/10.1038/nature14563
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