Deterministic nucleation of nanocrystal superlattices on 2D perovskites for light-funneling heterostructures

Abstract

Semiconductor heterostructures that combine components with different dimensionality provide an interesting way to manipulate the physical properties of the resulting material. Two-dimensional lead halide perovskites crystallize as flat microcrystals and have efficient in-plane exciton mobility, while perovskite nanocrystals are efficient emitters with a tunable bandgap that can self-assemble into microscopic superlattices. However, combining such intricate architectures into heterostructures has been challenging due to the mismatch in solubility properties and the challenging transfer procedures. Here we realize heterostructures where CsPbBr3 nanocrystal superlattices are deterministically grown along the faces of PEA2PbBr4 two-dimensional layered perovskite microcrystals. The growth can be limited to the lateral faces of the microcrystals and result in core-crown epitaxial heterostructures, or extended to the vertical direction leading to core-shell-like structures. The growth method is simple yet effective and versatile, and promises to be expanded to a large variety of other materials. We demonstrate that these heterostructures can be employed as efficient light-harvesting systems. In fact, energy can be transferred from the two-dimensional microcrystal domain to the superlattices, enabling switching between linear and non-linear carrier recombination regimes by tuning the excitation fluence. Moreover, by exploiting the lifetime shortening of CsPbBr3 nanocrystal emission upon sample cooling, we ensure that energy transfer occurs after the biexcitonic and single-excitonic decays of the nanocrystals, effectively extending the radiative recombination of superlattices.

Figures (5)

• Figure 1: Cartoon summarizing core-crown heterostructures band alignement and optical interaction. After laser excitation, the 2DLP (donor) transfers (non-radiatively and radiatively) the excitation to the superlattice (acceptor).
• Figure 1: a) Formation of 3D CsPbBr3 NC SL/2D PEA2PbBr4 crystal heterostructures. 2DLP microcrystals are grown on a designated substrate by an anti-solvent-assisted fast crystallization procedure ($\approx$ 4 hours).schleusener2024heterostructures Then a solution of nanocrystals is dropcast onto the same substrate, which is subsequently enclosed in a Petri dish to ensure slow solvent evaporation. After $\approx$ 6 hours, heterostructures are formed. b) Representative microscope images of regions of the substrate before (left) and after (right) core-crown heterostructure formation. c) Scheme of the inverted microscope set-up used to optically monitor the heterostructures formation (scale bar: 15 $\upmu$m). On the right a colored SEM image of a typical core-crown heterostructure formed on the substrate top part is displayed (scale bar: 15 $\upmu$m). d) In situ PL spectra acquired during heterostructure formation. Inset: nanocrystal PL peak position during heterostructure growth.
• Figure 2: Structural characterization of heterostructures. a) Top: Low-magnification SEM images of the heterostructures. 2DLP microcrystals are located at the center of the structures while superlattices grow at the edges. Bottom: high-resolution image of a representative heterostructure interface, where arrays of nanocrystals are observed. b) SEM image of one heterostructure and corresponding EDX maps for Br, Cs and Pb. c) Left: Low-magnification SEM image of heterostructure with fully covered 2DLP microcrystal (nanocrystals assemble also on the top surface). Right: High-resolution image of the heterostructure interface. d) Comparison between ensemble XRD patterns of pure C18-capped CsPbBr3 superlattices, pristine PEA2PbBr4 microcrystals, and PEA-C18 heterostructures (black trace on top). e) Cartoon depicting a possible crystallographic alignment between 2DLP and nanocrystals.
• Figure 3: a) Top: Representative microscope images of one heterostructure excited with (left) visible and (right) UV light. Bottom: Absorption spectrum of several heterostructure ensembles and representative PL spectrum of a single heterostructure. b) Time-resolved emission decays of a pristine 2DLP microcrystal and a heterostructure extracted at 3 eV, and collected in the low- (top) and high-fluence (bottom) regimes. c) Similar comparison for traces extracted at the CsPbBr3 nanocrystal emission peak ($\approx$ 2.4 eV) for a pristine C$_8$ superlattice (orange line) and a C$_8$ heterostructure (spring green line). d, e) Effective lifetimes (weighted average from biexponential fits) as a function of fluence, extracted at 3 eV and 2.4 eV emission energies, respectively. The underlying pink regions indicate the fluence ranges where energy transfer starts to significantly impact biexcitonic recombination relative to single excitonic decays. f) Cartoon depicting the influence of energy transfer on the average exciton population per recombination site $\langle N \rangle$.
• Figure 4: a,b) Representative microscope images of a pristine 2DLP microcrystal at 293 K (left) and 80 K (right) (a) and of a heterostructure (b), both excited with a 343 nm laser. c,d) Temperature-dependent emission spectra of a pristine 2DLP microcrystal and a C8 heterostructure, respectively. In (c) the inset shows the emergence of the STE at 150 K and in (d) the inset highlights the temperature evolution of the 2DLP peak. e) Energy transfer decays extracted at 80 K and at 293 K from the 2DLP decays (see Figure S10). The orange dashed lines correspond to the best single-exponential fits. f) Biexcitonic decays extracted from the superlattice decays at 293 K and 80 K (see Figure S13). g,h) Time-resolved emission of a heterostructure and of a pristine superlattice extracted at the CsPbBr3 emission peak in the low- (g) and high-fluence (h) regimes. i) Scheme of all the excitation and recombination mechanisms occurring in the heterostructure in the superlattice domain at 293 K and 80 K.