Cesium lead halide pervoskite semiconductors are being extensively studied due to unprecedentedly high luminescence efficiency and concomitant narrow emission line width. Here, we report photophysical properties of CsPbBr 3 quantum dots having different sizes. Notably 5.5 nm sized CsPbBr 3 quantum dots reveal 95% photoluminescence (PL) quantum yield at room temperature. Moreover, signature of the stimulated emission is observed at low temperature for excitation fluence as low as ∼4.16 μW (Xe lamp excitation). Even though CsPbBr 3 quantum dots reveal the red shift in band gap at low temperature, similar to the single crystal, the exciton−phonon interaction is profoundly affected by the quantum size effects. Temperature-dependent optical studies reveal an anomalous decrease in exciton-LO phonon coupling in small-sized quantum dots aside from expected higher exciton binding energy. Observed stimulated emission in low-sized CsPbBr 3 quantum dots has implications in realizing a quantum dot based laser.

Optoelectronic devices of 2D layered Pb-halide perovskites depend on light absorption/emission, exciton dissociation/transfer, and charge transfer processes, which in turn depend on bandgap and band edges. Here, we report that the surface/subsurface region of 2D (C 4 H 9 NH 3 ) 2 PbI 4 perovskite single crystals have a wider bandgap compared to the interior of the crystal. Consequently, single crystals exhibit dual excitonic emission peaks at 2.38 and 2.20 eV arising from the surface and interior, respectively. In contrast, exfoliated layers of (C 4 H 9 NH 3 ) 2 PbI 4 exhibit single photoluminescence peak at 2.38 eV, similar to the surface/subsurface of single crystals. Temperature-dependent (300−10 K) photoluminescence and single-crystal diffraction suggest that the overall structure−bandgap relationships are similar for both single-crystal and few-layer samples, but with some difference in phase transition hysteresis. Similar minor structural differences between the bulk (interior) and surface/subsurface of (C 4 H 9 NH 3 ) 2 PbI 4 single crystals are the probable cause of the dual bandgap. Single crystals of other layered perovskite systems, namely, (C 6 H 13 NH 3 ) 2 PbI 4 and (C 4 H 9 NH 3 ) 2 PbBr 4 , also exhibit a similar dual bandgap.

Lead (II) bromide (99%), Lead (II) chloride (99%), Hydrochloric acid (37%) were purchased from Sigma Aldrich. Dimethyl sulfoxide (DMSO), Dichloromethane (DCM, anhydrous), 1-(2-Aminoethyl) piperazine and Hydrobromic acid (47%) were purchased from TCI Chemicals. All chemicals were used as purchased without further purification. Synthesis of powdered PzPbBr 1D Perovskites: For the preparation of powdered (Pz) 2 PbBr 10 perovskite, 1 mmol (365 mg) of PbBr 2 was dissolved in 3 mL of hydrobromic acid. To this, 1 mmol (140 µL) 1-(2-Aminoethyl) piperazine was added drop by drop. The solution turns turbid white immediately after addition and

CsPbCl3 nanocrystals (NCs) are known to show low photoluminescence (PL) quantum efficiency compared to CsPbBr3 NCs. In the present work, at 20 K, stimulated emission is observed from CsPbCl3 NCs at low excitation intensity. Efforts are made to understand the optical behavior and structural properties of chemically grown, 5.3 ± 1.1 nm sized CsPbCl3 NCs. Bulk CsPbCl3 is known to become cubic at 320 K, while, NCs are cubic or tetragonal at room temperature. Temperature dependent PL studies on CsPbCl3 NCs are carried out in a wide temperature range. The primary experimental finding is, about 90 times rise in PL emission intensity and existence of multiple sharp features at low temperature attributable to free exciton, bound excitons, phonon replica along with the defect related emission and stimulated emission. The optical phonon energy is about 26 meV with coupling constant of 54 meV in ∼5.3 nm sized CsPbCl3 NCs. Structural phase transition in CsPbCl3 NCs at about 200 K is also observed.

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