Highly Tunable Colloidal Perovskite Nanoplatelets through Variable Cation, Metal, and Halide Composition

Abstract

Colloidal perovskite nanoplatelets are a promising class of semiconductor nanomaterials-exhibiting bright luminescence, tunable and spectrally narrow absorption and emission features, strongly confined excitonic states, and facile colloidal synthesis. Here, we demonstrate the high degree of spectral tunability achievable through variation of the cation, metal, and halide composition as well as nanoplatelet thickness. We synthesize nanoplatelets of the form L2[ABX3]n-1BX4, where L is an organic ligand (octylammonium, butylammonium), A is a monovalent metal or organic molecular cation (cesium, methylammonium, formamidinium), B is a divalent metal cation (lead, tin), X is a halide anion (chloride, bromide, iodide), and n-1 is the number of unit cells in thickness. We show that variation of n, B, and X leads to large changes in the absorption and emission energy, while variation of the A cation leads to only subtle changes but can significantly impact the nanoplatelet stability and photoluminescence quantum yield (with values over 20%). Furthermore, mixed halide nanoplatelets exhibit continuous spectral tunability over a 1.5 eV spectral range, from 2.2 to 3.7 eV. The nanoplatelets have relatively large lateral dimensions (100 nm to 1 μm), which promote self-assembly into stacked superlattice structures-the periodicity of which can be adjusted based on the nanoplatelet surface ligand length. These results demonstrate the versatility of colloidal perovskite nanoplatelets as a material platform, with tunability extending from the deep-UV, across the visible, into the near-IR. In particular, the tin-containing nanoplatelets represent a significant addition to the small but increasingly important family of lead- and cadmium-free colloidal semiconductors.

Advances in Quantum‐Confined Perovskite Nanocrystals for Optoelectronics

“…We and others have shown that the thickness of individual nanoplatelets can be determined by AFM (Figure 3b), however this becomes impossible in the case that the nanoplatelets form stacks. [9,11,22,27] In x-ray scattering experiments, the stacking of nanoplatelets gives rise to a new series of Bragg peaks, enabling the [15] Copyright 2016, American Chemical Society. b) AFM image of colloidal CsPbBr 3 nanoplatelets; scale bar: 1 µm.…”

“…[9,15] However, the type of monovalent cation can greatly influence the stability and PLQY of perovskite nanocrystals. [15] It has been found that all-inorganic perovskite nanocrystals, which generally contain Cs + as the A-site cation, replacing the MA + , and are fabricated using the hot-injection method, exhibit higher stability as well as higher PLQYs than organic/inorganic hybrid perovskites. [3,10,32] The groups of Manna and Alivisatos extended this approach to 2D nanocrystals, simultaneously reporting CsPbX 3 nanoplatelets.…”

“…[13,14] These components not only dictate the crystal structure, but also control their optical and electronic properties. [2,15] Exemplary is the optical bandgap, which can be tuned throughout the visible range by controlling the halide content. Halide perovskites can form two-dimensional (2D) or quasi-2D layered structures with thickness of one or a few octahedral layers.…”

“…[17] Recent studies have shown that nanocrystalline perovskites exhibit enhanced PLQYs and offer tunable optical properties not only through their constituent ions but also through their size. [3,[8][9][10][11]15,18,19] Quantum size effects, as illustrated in Figure 1a, have been extensively investigated in conventional semiconductor nanocrystals such as metal chalcogenides and utilized for a wide range of applications during the last two decades. [20] As the size of the nanocrystals approaches the exciton Bohr radius of the material, quantum confinement effects start to influence the excitonic wave function and the energetic states of the exciton, leading to blueshifted photoluminescence (Figure 1a).…”

Review—Synthesis, Luminescent Properties, and Stabilities of Cesium Lead Halide Perovskite Nanocrystals

“…12,16,17 The size and shape of the particles are important factors determining the resulting optical properties because of their influence on the bandgap, known as the quantum size effect. 18,19 CsPbX 3 nanocrystals are frequently synthesized in the form of nanocubes, while two-dimensional morphologies such as nanosheets and nanoplatelets are also obtained. Pan et al reported the dependence of the particle morphology on the synthesis temperature and the length of the n-alkyl carboxylic acids and n-alkylamines used in the hot-injection method, as summarized in Figure 5.…”