2, National Institute of Advanced Industrial Science and Technology, Tsukuba, , Japan
All-inorganic perovskite nanocrystals (CsPbX3, X=Cl,Br,I; IP-NCs) enjoy currently great research interest due to their stability and outstanding optical properties and low production costs, serving as promising candidates for optoelectronic and photovoltaic applications. The IP-NCs combine the advantages of perovskites (bandgap engineering through composition, low temperature, fast and low-cost synthesis) with the benefits of quantum confinement. As the NC diameter decreases and approaches the Bohr radius, the quantum confinement (QC) sets in, modifying the wave function of the free electron and hole. Hence, the energy band structure is affected and their bandgap energy increases. Here we synthesize CsPbBr3 NCs, and investigate the bandgap energies of individual IP-NCs making use of electron energy loss spectroscopy (EELS) in a state-of-the-art low-voltage monochromatic scanning transmission electron microscope (STEM). This is made possible due to recent developments in the low-energy monochromatic transmission electron microscope with advanced aberration correctors have enabled the combination of electron spectroscopy with ultrahigh spatial (below 1.6 Å) and energy resolutions (FWHM of the zero loss peak ~50 meV).[2,3] In that way, the absorption spectrum is directly correlated with the structural parameters of a single NC. A direct relation between the NC size and its bandgap is obtained on a single object level. We demonstrate that the bandgap is governed by the smallest dimension of the cuboidal perovskite NC. Further, we explicitly show an effective coupling between proximal NCs in an ensemble, leading to their band structure modification. In addition, our methods allowed us to discover the presence of non-perovskite configurations, which we identified as nanocrystals of Cs4PbBr6 and CsPbBr3/Cs4PbBr6 nanohybrids. Cs4PbBr6 introduces additional emission and absorption bands, which affect the emission quantum yield of the ensemble in the UV. These results highlight the incredible developments in the fields of microscopy techniques and materials science, and are of a general interest to the scientific community working on perovskites.
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 de Weerd, J. Lin & L. Gomez et al., J. Phys.Chem. C. 2017, 121.