Investigation into the Photoluminescence Red Shift in Cesium Lead Bromide Nanocrystal Superlattices

Abstract

The formation of cesium lead bromide (CsPbBr₃) nanocrystal superlattices (NC SLs) is accompanied by a red shift in the NC photoluminescence (PL). The values of the PL red shift reported in the literature range from none to ∼100 meV without unifying explanation of the differences. Using a combination of confocal PL microcopy and steady-state optical spectroscopies we found that an overall PL red shift of ∼96 meV measured from a macroscopic sample of CsPbBr₃ NC SLs has several contributions: ∼ 10-15 meV from a red shift in isolated and clean SLs, ∼ 30 meV from SLs with impurities of bulklike CsPbBr₃ crystals on their surface, and up to 50 meV or more of the red shift coming from a photon propagation effect, specifically self-absorption. In addition, a self-assembly technique for growing micron-sized NC SLs on the surface of perfluorodecalin, an inert perfluorinated liquid and an antisolvent for NCs, is described.

Figure 1

(a) Steady-state absorption (solid black curve) and PL (solid green curve) spectra and a low-magnification TEM image (inset) of a batch of ∼6.8 ± 1 nm CsPbBr₃ NCs; (b) schematic of the CsPbBr₃ NC self-assembly on the surface of perfluorodecalin (PFD); (c) photographs of a vial with a luminescent solid of CsPbBr₃ NC SLs after PFD was decanted off (“dried”) and after addition of oleic acid (“+oleic acid”); (d) low-magnification TEM image of a small (<1 μm) CsPbBr₃ NC SL showing a simple cubic packing of individual NCs. This type of packing is typical for nanocubes with sharp edges; (e) optical microscopy image of the CsPbBr₃ NC SL suspension in oleic acid with ∼10 μm SLs; (f) topology of a large SL in three dimensions and an accompanying optical image (insert, scale bar 20 μm).

Figure 2

Confocal PL microscopy images and spectra of (a) three different CsPbBr₃ NC samples: NCs dispersed in ODE (red curve), an individual SL (green curve, λ_max = 511 nm), and a NC film drop-casted from a hexane solution (black curve). Confocal PL microscopy images and spectra of (b) Three CsPbBr₃ NC SLs with varying degrees of impurities and PL red shifts, SL1 (black curve, λ_max = 512 nm), SL2 (red curve, λ_max = 515 nm), SL3 (blue curve, λ_max = 517 nm). The samples were excited with an Ar laser, λ_exc = 488 nm.

Figure 3

(a) Scheme of the CsPbBr₃ NC SLs that were grown on a tilted Si wafer by a slow solvent evaporation and optical microscopy images of three areas on the wafer with different densities of NC SLs. (b) PL spectra from the areas with a different SL coverage (a small peak near 555 nm is possibly an artifact). (c) Effect of the self-absorption on the measured PL. Short-dashed lines are transmission spectra at four different extinctions, and solid lines are the resulting PL spectra.