Quantum Size Effect in Organometal Halide Perovskite Nanoplatelets

Abstract

Organometal halide perovskites have recently emerged displaying a huge potential for not only photovoltaic, but also light emitting applications. Exploiting the optical properties of specifically tailored perovskite nanocrystals could greatly enhance the efficiency and functionality of applications based on this material. In this study, we investigate the quantum size effect in colloidal organometal halide perovskite nanoplatelets. By tuning the ratio of the organic cations used, we can control the thickness and consequently the photoluminescence emission of the platelets. Quantum mechanical calculations match well with the experimental values. We find that not only do the properties of the perovskite, but also those of the organic ligands play an important role. Stacking of nanoplatelets leads to the formation of minibands, further shifting the bandgap energies. In addition, we find a large exciton binding energy of up to several hundreds of meV for nanoplatelets thinner than three unit cells, partially counteracting the blueshift induced by quantum confinement. Understanding of the quantum size effects in perovskite nanoplatelets and the ability to tune them provide an additional method with which to manipulate the optical properties of organometal halide perovskites.

Colloidal lead halide perovskite nanocrystals: synthesis, optical properties and applications

“…By systematically varying the ratio of the organic cations used (octylammonium and MA). Sichert et al 25 showed how the perovskite NCs gradually decrease in thickness from large cubic crystals to ultrathin nanoplatelets down to a thickness of only a single perovskite unit cell. The octylammonium molecule is too large to fit into the perovskite crystal structure and so leads to termination of the crystal growth on binding to the perovskite, however predominantly in one direction.…”

“…The authors reasoned that the increased PLQY of the NCs with respect to micrometer-sized bulk perovskite particles resulted from higher excitonic binding energies of up to nearly 400 meV, when the size of the NCs approached the Bohr radius of the respective perovskite, on the order of 13-30 Å, depending on the material. 25,47,48 By replacing the bromide anion with chloride or iodide, or by using mixtures of the three anions, the authors were able to obtain perovskite NC dispersions emitting throughout the full range of the visible spectrum (Figure 3c). The average PL lifetimes of these NCs were considerably shorter than for the respective bulk films.…”

“…22,23 In addition, the size and dimensionality of perovskites can also be used to tune their optical properties, similar to conventional metal chalcogenide semiconductors. 24,25 Although perovskites can be obtained with dimensionality ranging from the 3D to 0D by varying the synthetic conditions used (Figure 1b), they have a strong tendency to form layered two-dimensional (2D) and quasi-2D structures ( Figure 1c). [25][26][27] Recent studies have shown that colloidal perovskite nanocrystals (NCs) can exhibit very high photoluminescence quantum yields (PL QY).…”

“…24,25 Although perovskites can be obtained with dimensionality ranging from the 3D to 0D by varying the synthetic conditions used (Figure 1b), they have a strong tendency to form layered two-dimensional (2D) and quasi-2D structures ( Figure 1c). [25][26][27] Recent studies have shown that colloidal perovskite nanocrystals (NCs) can exhibit very high photoluminescence quantum yields (PL QY). [23][24][25][28][29][30] Moreover, they show strong quantum confinement effects when their dimensionality changes from 3D to 2D, offering additional tunability of optical properties.…”

“…[25][26][27] Recent studies have shown that colloidal perovskite nanocrystals (NCs) can exhibit very high photoluminescence quantum yields (PL QY). [23][24][25][28][29][30] Moreover, they show strong quantum confinement effects when their dimensionality changes from 3D to 2D, offering additional tunability of optical properties. Perovskite NCs have already shown great promise in many applications such as lasers, LEDs and photovoltaic devices.…”