Curious Case of CsPb₂Br₅: Extremely Soft Structure-Induced Broadband Emission

Abstract

Ever since the initial report on green emissive two-dimensional (2D) all-inorganic indirect band gap semiconductor, CsPb2Br5, arguments have arisen concerning its origin of photoluminescence, which has sparked continuous debate. Following the identification of the photoluminescence centers being primarily either strongly green emissive CsPbBr3 impurities or different amorphous lead-bromide ammonium complexes present at the surface of the solution-based synthesized product, here in this work, an all-solid-state synthesis approach has been implemented to avoid any involvement of incertitude that may arise from unwanted external particles. For the first time, we record the progressive formation of thermodynamically favored CsPb2Br5 phase over time, and interestingly, pristine CsPb2Br5 exhibits a highly Stokes-shifted broadband pinkish-red emission spanning from the visible-to-near infrared spectrum (∼500 to 850 nm). Emission wavelength-independent excitation and excitation wavelength-independent emission coupled with the similarity in the decay kinetics, all indicate toward the intrinsic nature of the broadband emission. The occurrence of low bulk and shear moduli (8.59 and 7.07 GPa, respectively) estimated from sound velocity measurements, the soft vibrational modes from Raman spectroscopy, and the significantly low Debye temperature (113 K) obtained from heat capacity (C p) measurements in the low-temperature range (2–31 K) reveal the structural softness on both global and local scales. These traits lead to the emergence of short-range elastic lattice deformation resulting from strong electron–phonon coupling upon photoexcitation, generating self-trapped excitonic emission in CsPb2Br5.

Strain-Induced Valence Band Splitting Enabling Above-Bandgap Exciton Luminescence in Epitaxial Mg3N2 Thin Films

“…Calculated experimental BE from the difference of bandgap (E g ) and near band edge emission (E ex ) comes out to be ∼40 meV for A-exciton, which is higher than GaN at room temperature. However, the determination of exciton BEs for both the peaks labeled as 1 and 3 (A- and B-excitons) by the fitting of the quenching of PL emission intensity with the increase in temperature from the Arrhenius model, , considering only one nonradiative recombination channel, comes to be 35.51 and 32.60 meV, respectively (see Supporting Information Section 5 and Figure S11). Such a higher BE stabilizes the exciton in Mg 3 N 2 against thermal dissociation at room temperature.…”