Observation of 3D Biexcitons in Pristine-Quality CH3 NH3 PbBr3 Single Crystals

Visualizing the Low-Energy Electronic Structure of Prototypical Hybrid Halide Perovskite through Clear Band Measurements

Organic-inorganic hybrid perovskites (OIHPs) are a promising class of materials that rival conventional semiconductors in various optoelectronic applications. However, unraveling the precise nature of their low-energy electronic structures continues to pose a significant challenge, primarily due to the absence of clear band measurements. Here, we investigate the low-energy electronic structure of CH3NH3PbI3 (MAPI3) using angle-resolved photoelectron spectroscopy combined with ab initio density functional theory. We successfully visualize the electronic structure of MAPI3 near the bulk valence band maximum by using a laboratory photon source (He Iα, 21.2 eV) at low temperature and explore its fundamental properties. The observed valence band exhibits a highly isotropic and parabolic band characterized by small effective masses of 0.20-0.21 me, without notable spectral signatures associated with a large polaron or the Rashba effect, subjects that are intensely debated in the literature. Concurrently, our spin-resolved measurements directly disprove the giant Rashba scenario previously suggested in a similar perovskite compound by establishing an upper limit for the Rashba parameter (αR) of 0.28 eV Å. Our results unveil the unusually complex nature of the low-energy electronic structure of OIHPs, thereby advancing our fundamental understanding of this important class of materials.

Strong Exciton-Exciton Scattering of Exfoliated van der Waals InSe toward Efficient Continuous-Wave Near-Infrared P-Band Emission

P-band emission is a superlinear low-coherence emission through exciton-exciton (X-X) scattering into photon-like states. It occurs without the prerequisites of population inversion or macroscopical coherence, rendering lower power consumption than the widely explored superlinear low-coherence emissions including superfluorescence, amplified spontaneous emission, and random lasing, and holds great potential for speckle-free imaging and interferometric sensing. However, competition processes including exciton dissociation and annihilation undermine its operation at room temperature and/or low excitation conditions. Here we report room-temperature P-band emission from InSe microflakes with excitation density of 10¹⁰ cm^-2, offering 2-orders-of-magnitude lower operation density compared to the state-of-the-art superlinear low-coherence emissions. The efficient P-band emission is attributed to a large X-X scattering strength of 0.25 μeV μm² due to enhanced spatial confinement along with intrinsic material metrics of 3D/2D exciton complex and asymmetric electron/hole mass. These findings open an avenue toward strong low-coherence near-infrared light sources based on van der Waals semiconductors.