In this paper, we address the phase stability and the relationship between optical gap and structural perturbations in cesium lead halide nanocrystals. We report photoluminescence (PL) spectra for cesium lead iodide (CsPbI3) perovskite nanocrystals under hydrostatic pressures up to 2.5 GPa. The peak position of the CsPbI3 PL shifts as a function of pressure. Initially, the PL shifts to lower energies, until a reversal occurs near 0.33 GPa. At higher pressures, the PL peak position shifts to the blue until PL vanishes above 2.5 GPa. We explain the pressure response with different modes of deformation of the perovskite crystal structure.

We report the size-dependent pressure response for CsPbBr 3 perovskite nanocrystals in the size range 5.7−10.9 nm using photoluminescence spectroscopy in a diamond anvil cell. As the nanocrystal size decreases below ca. 7.5 nm, we observe a decrease in the transition pressure at which there is a change in the mode of deformation concomitant with an isostructural phase transition. We hypothesize that surface fluctuations regarding the tilt and distortion of surface PbBr 6 octahedra facilitate the change in the mode of deformation and phase transition at lower pressures for smaller nanocrystals.

The size-dependent pressure response of oleatestabilized CdSe quantum dots (QDs) in paraffin is investigated using diamond anvil cell experiments and density functional theory (DFT). For QDs above 3.0 nm, the photoluminescence shows a blue-shift of around 43 meV/GPa, close to the value for bulk CdSe, but the shift increases strongly for nanocrystals less than 3 nm in size. Conversely, the absorption shift is 45 meV/GPa above 3.0 nm but weakens to 35 meV/GPa for particles 1.5 nm in size. No crystallographic phase transitions occur below 2 GPa, and the optical effects are reversible. DFT calculations confirm that shifts in the bulk modulus begin for sizes estimated to be 1/2 of the Bohr radius, which we term the extreme confinement regime.