Concentration-Dependent Bandgap Modulation and PL Enhancement of CsPbBr₃ QDs Using Bisbenzimidazolium Salt

Abstract

All-inorganic lead halide perovskite CsPbBr₃ quantum dots (QDs) are renowned for their size-tunable bandgap and sharp, high-yield green emission. However, their long-term stability and peak photoluminescence quantum yield (QY) are limited by surface defect sites. In this study, we systematically investigate how the precursor concentration (0.1 M, 0.2 M, 0.4 M) influences the optical bandgap and photophysical properties of CsPbBr₃ QDs synthesized by a ligand-assisted reprecipitation method with bisbenzimidazolium salt as ligand (S-1) for surface passivation. UV–Vis absorption spectra exhibit a clear concentration-dependent shift of the band edge: as the precursor molarity increases, the absorption onset red-shifts, yielding optical bandgaps of approximately 2.29 eV (0.1 M), 2.17 eV (0.2 M), and 2.03 eV (0.4 M). Steady-state PL measurements show that S-1 passivation dramatically enhances the emission intensity and narrows the emission linewidth, indicating suppressed trap-mediated nonradiative recombination. S-1-treated QDs exhibit high PL QY, reduced full-width at half-maximum (FWHM), and prolonged carrier lifetimes under all conditions, highlighting the key role of S-1 in stabilizing surface states. These findings demonstrate the strong interplay between precursor stoichiometry and optical performance in CsPbBr₃ QDs, providing a practical framework for tuning their luminescent properties for LEDs, lasers, and other optoelectronic applications.

Keywords: CsPbBr₃ quantum dots, perovskite nanocrystals, bandgap engineering, surface passivation, photoluminescence (PL), precursor concentration effects, optical characterization.