  Inorganic lead halide perovskites are promising candidates for optoelectronic
applications, due to their bandgap tunability, high photoluminescence quantum
yield, and narrow emission line widths. In particular, they offer the
possibility to vary the bandgap as a function of the halide composition and
dimension/shape of the crystals at the nanoscale. Here we present an
aberration-corrected scanning transmission microscopy (STEM) study of extended
nanosheets of CsPbBr3 directly demonstrating their orthorhombic crystal
structure and their lateral termination with Cs-Br planes. The bandgaps from
individual nanosheets are measured by monochromated electron energy loss
spectroscopy (EELS). We find an increase of the bandgap starting at thicknesses
below 10 nm, confirming the less dramatic 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.
