Perovskite Nanocrystal Self-Assemblies in 3D Hollow Templates

Abstract

Highly ordered nanocrystal (NC) assemblies, namely, superlattices (SLs), have been investigated as materials for optical and optoelectronic devices due to their unique properties based on interactions among neighboring NCs. In particular, lead halide perovskite NC SLs have attracted significant attention owing to their extraordinary optical characteristics of individual NCs and collective emission processes like superfluorescence (SF). So far, the primary method for preparing perovskite NC SLs has been the drying-mediated self-assembly method, in which the colloidal NCs spontaneously assemble into SLs during solvent evaporation. However, this method lacks controllability because NCs form random-sized SLs at random positions on the substrate, rendering NC assemblies in conjunction with device structures, such as photonic waveguides or microcavities, challenging. Here, we demonstrate template-assisted self-assembly to deterministically place perovskite NC SLs and control their geometrical properties. A solution of CsPbBr₃ NCs is drop-casted on a substrate with lithographically defined hollow structures. After solvent evaporation and removal of excess NCs from the substrate surface, NCs remain only in the templates, thereby defining the position and size of these NC assemblies. We performed photoluminescence (PL) measurements on these NC assemblies and observed signatures of SF, similar to those in spontaneously assembled SLs. Our findings are crucial for optical devices that harness embedded perovskite NC assemblies and enable fundamental studies on how these collective effects can be tailored through the SL geometry.

Keywords: colloidal nanocrystals; lead halide perovskites; nanocrystal assembly; nanocrystal superlattice; superfluorescence; templated assembly.

Figure 1

(a) Schematic illustration of the process flow to fabricate transparent, hollow templates that are subsequently filled with NCs. (b) SEM image of a template structure. Scale bar: 5 μm. (c) Optical images of an array of template structures before NCs deposition (Top) and template-assisted NC assemblies after NCs deposition (Bottom). Scale bar: 25 μm. (d) Bright-field STEM image showing a cross section of a template-assisted NC assembly. Scale bar: 200 nm.

Figure 2

Absorption (red solid lines) and PL (black solid lines) spectra of NC solutions (a–c), optical microscopic images (d–f, scale bars: 10 μm), and spatially resolved PL maps (g–i) of NC assemblies prepared by the template-assisted method. The surface capping ligands and solvent of the solution are (a,d,g) ligands: oleic acid (OA) + oleylamine (OLA) + didodecyldimethylammonium bromide (DDAB), solvent: toluene, (b,e,h) ligands: phosphatidylserine (Ptd-l-Ser), solvent: toluene, and (c,f,i) ligands: DDAB, solvent: cycloheptane.

Figure 3

Statistical assembly yield determined from optical microscopic image analysis for different numbers of openings in the hollow template. (a–c) Histograms of the ratio of the area filled with NCs to the total area of the template structure for different numbers of openings: (a) on one side, (b) on two sides, and (c) on four sides. Insets show schematics of each template design.

Figure 4

Spatially resolved PL from a NC assembly. (a) Optical microscopic image of the measured template assembly. Crosses indicate the measured positions (scale bar: 10 μm). (b) PL spectra and (c) time traces obtained at 6 K from the four different positions in the assembly. The time traces were obtained by integrating the wavelength range from 510 to 580 nm.

Figure 5

Excitation fluence dependence of PL from a NC assembly at 6 K. (a) PL spectra and (b) integrated emission intensity as a function of excitation fluence. α indicates the exponent value obtained from a power-law fit (solid line). The integrated wavelength range is from 535 to 570 nm. (c) PL time traces for different excitation fluences. The PL signal within the wavelength range from 532 to 548 nm was selected with a band-pass filter. (d) Emission lifetime (top panel) and emission peak intensity (bottom panel) as functions of excitation fluence. α indicates the exponent value obtained from a power-law fit (solid line).

Figure 6

Ultrafast spectroscopy under strong femtosecond excitation at 6 K. (a,b) Time-integrated spectra of two different template-assisted NC assemblies and a spin-coated NC film (c) as a control sample. (d–f) Spectrally integrated emission time traces for different excitation fluences of both template-assisted NC assemblies (d,e) and for the spin-coated NC film (f). Each time trace is offset by additional 100 counts (d–f) in order to visually separate the curves. For both assemblies, the decay and delay times are extracted from the respective time traces and fit with an SF model function (red lines, see text). For the spin-coated film, the origin of the delay time has been set to the average value of the experimental data since the SF model function fit is not applicable.