Large exciton binding energy, high photoluminescence quantum yield and improved photostability of organo-metal halide hybrid perovskite quantum dots grown on a mesoporous titanium dioxide template.

Herein, we demonstrated a novel synthetic route to grow size-tunable hybrid perovskite (CH ₃ NH ₃ PbI ₃ and CH ₃ NH ₃ PbBr ₃ ) quantum dots (QDs) using a Fluorine-doped TiO ₂ (F-TiO ₂ ) mesoporous template and these QDs exhibit large exciton binding energy, high photoluminescence quantum yield and improved photostability. The pore size in F-TiO ₂ template is tuned by varying the HF molar concentration during its solvothermal growth and size of the perovskite QDs embedded in F-TiO ₂ pores is tuned in the range 1.7-5.1 nm, as revealed from the TEM analysis. A systematic blue-shift in UV-visible absorption edge, as well as photoluminescence (PL) spectrum, is observed with the reduced size of the perovskite QDs due to strong quantum confinement. The CH ₃ NH ₃ PbI ₃ QD with average size ∼1.7 nm exhibits ∼47 nm blue shift in the PL spectra, ∼43 fold enhancement in PL intensity and ∼25% PL quantum yield (QY). On the other hand, CH ₃ NH ₃ PbBr ₃ QD of similar size exhibits dramatically enhanced (∼124 times) PL emission with narrow line width and a PLQY of ∼57%, which is significant for the template-assisted growth of perovskite QDs film. The quantitative analysis of the PL emission energy vs QD size shows an excellent fit with the Brus equation confirming the strong quantum confinement effect in the perovskite QDs. Analysis of low-temperature PL spectra reveals very high exciton binding energy (162-272 meV) for the QDs as compared to the bulk film (32 meV) due to the high effective dielectric constant, and high electron-hole recombination probability in the QDs, which is consistent with the extremely high PLQY and stable emission from the QDs. The blue shift of the PL peak with increasing temperature is explained on the basis of localization effect. Time-resolved PL analysis for both the perovskite QDs reveals faster life time compared to their bulk counterparts, confirming the significant radiative recombination of carriers in the QDs at the room temperature. The CH ₃ NH ₃ PbBr ₃ QDs embedded in porous F-TiO ₂ template maintain its initial PL intensity up to several hours (≥10 h) under the UV laser exposure (18mW), while that of the bulk film decreases to <67%. Thus, template grown hybrid perovskite QDs exhibiting high photostability and very high PLQY demonstrated here are promising for the next generation optoelectronic applications.
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