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Register a new userBright Luminescent Surface States on the Edges of Wide-bandgap Two-dimensional Lead Halide Perovskite
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Three-dimensional lead halide perovskites have surprised people for their defect-tolerant electronic and optical properties, two-dimensional lead halide layered structures exhibit even more puzzling phenomena: luminescent edge states in Ruddlesden-Popper perovskites and conflicting reports of highly luminescent versus non-emissive CsPb$_{text{2}}$Br$_{text{5}}$. In this work, we report the observation of bright luminescent surface states on the edges of CsPb$_{text{2}}$Br$_{text{5}}$ microplatelets. We prove that green surface emission makes wide-bandgap single crystal CsPb$_{text{2}}$Br$_{text{5}}$ highly luminescent. Using polarized Raman spectroscopy and atomic-resolution transmission electron microscopy, we further prove that polycrystalline CsPb$_{text{2}}$Br$_{text{5}}$ is responsible for the bright luminescence. We propose that these bright edge states originate from corner-sharing clusters of PbBr$_{text{6}}$ in the distorted regions between CsPb$_{text{2}}$Br$_{text{5}}$ nanocrystals. Because metal halide octahedrons are building blocks of perovskites, our discoveries settle a long-standing controversy over the basic property of CsPb$_{text{2}}$Br$_{text{5}}$ and open new opportunities to understand, design and engineer perovskite solar cells and other optoelectronic devices.
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Methylammonium lead iodide perovskites are considered direct bandgap semiconductors. Here we show that in fact they present a weakly indirect bandgap 60 meV below the direct bandgap transition. This is a consequence of spin-orbit coupling resulting in Rashba-splitting of the conduction band. The indirect nature of the bandgap explains the apparent contradiction of strong absorption and long charge carrier lifetime. Under hydrostatic pressure from ambient to 325 MPa, Rashba splitting is reduced due to a pressure induced ordering of the crystal structure. The nature of the bandgap becomes increasingly more direct, resulting in five times faster charge carrier recombination, and a doubling of the radiative efficiency. At hydrostatic pressures above 325 MPa, MAPI undergoes a reversible phase transition resulting in a purely direct bandgap semiconductor. The pressure-induced changes suggest epitaxial and synthetic routes to higher efficiency optoelectronic devices.
Inorganic lead halide perovskites are promising candidates for optoelectronic applications, due to their bandgap tunability, high photoluminescence quantum yield, and narrow emission line widths. In particular, they offer the possibility to vary the bandgap as a function of the halide composition and dimension/shape of the crystals at the nanoscale. Here we present an aberration-corrected scanning transmission microscopy (STEM) study of extended nanosheets of CsPbBr3 directly demonstrating their orthorhombic crystal structure and their lateral termination with Cs-Br planes. The bandgaps from individual nanosheets are measured by monochromated electron energy loss spectroscopy (EELS). We find an increase of the bandgap starting at thicknesses below 10 nm, confirming the less dramatic effect of 1D confinement in nanosheets compared to the 3D confinement observed in quantum dots, as predicted by density functional theory calculations and optical spectroscopy data from ensemble measurements.
Bulk quantum materials based on zero-dimensional (0D) lead-free organic tin halide perovskites have been developed for the first time, which give broadband Gaussian-shaped and strongly Stokes shifted emissions with quantum efficiencies of up to near-unity at room temperature due to excited state structural reorganization.
Solution-processed lead halide perovskites have shown very bright future in both solar cells and microlasers. Very recently, the nonlinearity of perovskites started to attract considerable research attention. Second harmonic generation and two-photon absorption have been successfully demonstrated. However, the nonlinearity based perovskite devices such as micro- & nano- lasers are still absent. Here we demonstrate the two-photon pumped nanolasers from perovskite nanowires. The CH3NH3PbBr3 perovskite nanowires were synthesized with one-step solution self-assembly method and dispersed on glass substrate. Under the optical excitation at 800 nm, two-photon pumped lasing actions with periodic peaks have been successfully observed at around 546 nm. The obtained quality (Q) factors of two-photon pumped nanolasers are around 960, and the corresponding thresholds are about 674?J=cm2. Both the Q factors and thresholds are comparable to conventional whispering gallery modes in two-dimensional polygon microplates. Our researches are the first demonstrations of two-photon pumped nanolasers in perovskite nanowires. We believe our finding will significantly expand the application of perovskite in low-cost nonlinear optical devices such as optical limiting, optical switch, and biomedical imaging et al.
Hybrid organic-inorganic semiconductors feature complex lattice dynamics due to the ionic character of the crystal and the softness arising from non-covalent bonds between molecular moieties and the inorganic network. Here we establish that such dynamic structural complexity in a prototypical two-dimensional lead iodide perovskite gives rise to the coexistence of diverse excitonic resonances, each with a distinct degree of polaronic character. By means of high-resolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various low-frequency ($lesssim 50$,cm$^{-1}$) optical phonons involving motion in the lead-iodide layers. We thus conclude that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. This insight on the energetic/configurational landscape involving globally neutral primary photoexcitations may be relevant to a broader class of emerging hybrid semiconductor materials.
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