Abstract
Inorganic lead halide perovskites are promising candidates for optoelectronic applications, due to their high photoluminescence quantum yield and narrow emission line widths. Particularly attractive is the possibility to vary the bandgap as a function of the halide composition and the size or shape of the crystals at the nanoscale. Here we present an aberration-corrected scanning transmission electron microscopy (STEM) and monochromated electron energy-loss spectroscopy (EELS) study of extended nanosheets of CsPbBr3. We demonstrate their orthorhombic crystal structure and their lateral termination with Cs-Br planes. The bandgaps are measured from individual nanosheets, avoiding the effect of the size distribution which is present in standard optical spectroscopy techniques. We find an increase of the bandgap starting at thicknesses below 10 nm, confirming the less marked effect of 1D confinement in nanosheets compared to the 3D confinement observed in quantum dots, as predicted by density functional theory calculations and optical spectroscopy data from ensemble measurements.
Highlights
The interest in lead halide perovskites (LHPs) was boosted recently after the demonstration of high efficiency of this class of materials in thin film photovoltaic devices, where the LHPs act as absorbing layers, with the power conversion efficiency of LHP-based solar cells having risen to over 25% in the last years.[1]
Communication single nanocrystals with cubic shape been measured by high resolution electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM).[12]
The provided quantitative values result from the average spectrum obtained from a large area of the [001]-oriented NS used for annular dark field (ADF)-STEM imaging, and a conservative relative error of 5% was considered to take into account possible channeling effects
Summary
The interest in lead halide perovskites (LHPs) was boosted recently after the demonstration of high efficiency of this class of materials in thin film photovoltaic devices, where the LHPs act as absorbing layers, with the power conversion efficiency of LHP-based solar cells having risen to over 25% in the last years.[1].
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