Facet Engineering in Halide Perovskite Nanocrystals for Modulating Energy-Level Alignment and Emission Behavior

Halide perovskite nanocrystals (HPNs) have emerged as a promising avenue for optoelectronics due to their near-unity photoluminescence quantum yield (PLQY), high defect tolerance, and tunable emission properties. Facet engineering of such nanocrystals (NCs) offers significant opportunities for optimizing their energetic and electronic behaviors. However, the uneven distribution of surface electrostatic potentials at undercoordinated sites makes it challenging to model the faceted NC and hence to establish an atomistic understanding on the structure-property correlation between faceted NCs and their optoelectronic properties. In this work, using state-of-the-art level of electronic structure calculations, we have systematically investigated the role of facet engineering in the energy-level alignment and emission behavior of CsPbBr₃ NCs with a vis-a-vis comparison with the surface slabs in order to reveal the importance of the consideration of realistic model in studying HPNs and establishing a structure-property correlation upon facet truncation. We have found that, for the almost similar sized NCs, the work function can be tuned up to 2.20 eV with a proper faceting compared to that of slabs, where it can be tuned up to 1.42 eV. We observe a notable variation in the excited-state energetics and emission behavior upon facet tuning, showing a localized self-trapped exciton (STE)-like to non-STE-like electron-hole (e-h) pair formation upon introducing 110-facets into the 100-facet-based hexahedron NC, thus showing more nonradiative recombination in highly faceted cuboctahedra or decahedra NC compared to that of less faceted hexahedron shape. Such modulation of energy-level position and emission behavior originates from the different extents of unsaturation of the facets of the NCs. This study offers a comprehensive understanding of facet engineering in HPNs for an effective design strategy in tuning the electronic and optical behavior of HPNs, thereby tailoring them into a wide range of optoelectronic applications.