RESUMEN
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.
RESUMEN
Halide perovskites (HPs) are fascinating materials whose optoelectronic properties are arguably excitonic. In the HP family, biexcitons are known to exist only in low dimensions where exciton-exciton binding is strongly enhanced by quantum and dielectric confinements. In this paper, however, it is shown that they indeed do exist in 3D bulk CH3 NH3 PbBr3 (MAPbBr3 ) single crystals if the pristine crystal quality is ensured for subtle binding of two excitons. The existence of biexcitons is clearly evidenced below 30 K with a binding energy of ≈3.9 ± 0.3 meV according to i) exciton-biexciton population dynamics, ii) giant resonant two-photon excitation of biexcitons, iii) inverted Boltzmann-type spectral feature, and iv) zero degree of circular polarization in the biexciton photoluminescence. Because of the polariton effect, the two-photon resonance occurs at the excited biexciton state from which longitudinal-transverse splitting is calculated to be 3.7 meV. The discovery of the 3D biexcitons underscores the very quality of HP crystals for generating various many-body excitonic phases in MAPbBr3 and its analogues toward the improved understanding of their fundamental properties and highly efficient optoelectronic applications.
RESUMEN
Two-dimensional (2D) transition metal dichalcogenide (TMD) hetero PN junctions with a van der Waals (vdW) interface have received much attention, because PN diodes are basically important to control the vertical current across the junction. Interestingly, the same vdW PN junction structure can be utilized for junction field-effect transistors (JFETs) where in-plane current is controlled along the junction. However, 2D vdW JFETs seem rarely reported, despite their own advantages to achieve when good vdW junction is secured. Here, we present high-performance p-MoTe2 JFETs using almost perfect vdW organic Alq3/p-MoTe2 junctions and demonstrate organic NPB/n-MoS2 JFETs. The p- and n-channel JFETs stably show high mobilities of 60-80 and â¼800 cm2/V s, respectively, along with a high ON/OFF current ratio (>1 × 105) and minimal gate leakage at 5 V even after a few months. Such performances are attributed to a quality vdW junction at organic layer/TMD interfaces.
