Curious Case of CsPb<sub>2</sub>Br<sub>5</sub>: Extremely Soft Structure-Induced Broadband Emission
Jayita Pradhan, Anustoop Das, Arpita Panda, Kanishka Biswas
Abstract
Ever since the initial report on green emissive two-dimensional (2D) all-inorganic indirect band gap semiconductor, CsPb 2 Br 5, 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 CsPbBr 3 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 CsPb 2 Br 5 phase over time, and interestingly, pristine CsPb 2 Br 5 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 CsPb 2 Br 5 .