Semiconductor perovskite films are now being widely investigated as light harvesters in solar cells with ever-increasing power conversion efficiencies, which have motivated the fabrication of other optoelectronic devices, such as light-emitting diodes, lasers, and photodetectors. Their superior material and optical properties are shared by the counterpart colloidal nanocrystals (NCs), with the additional advantage of quantum confinement that can yield size-dependent optical emission ranging from the near-UV to near-infrared wavelengths. So far, intensive research efforts have been devoted to the optical characterization of perovskite NC ensembles, revealing not only fundamental exciton relaxation and recombination dynamics but also low-threshold amplified spontaneous emission and novel superfluorescence effects. Meanwhile, the application of single-particle spectroscopy techniques to perovskite NCs has helped to resolve a variety of optical properties for which there are few equivalents in traditional colloidal NCs, mainly including nonblinking photoluminescence, suppressed spectral diffusion, stable exciton fine structures, and coherent single-photon emission. While the main purpose of ensemble optical studies is to guide the smooth development of perovskite NCs in classical optoelectronic applications, the rich observations from single-particle optical studies mark the emergence of a potential platform that can be exploited for quantum information technologies.
Lead-halide perovskite nanocrystals (NCs) have just emerged as a novel type of semiconductor nanostructure possessing great potentials in the optoelectronic, photovoltaic and quantum-information-processing applications. This renders it extremely necessary to have a comprehensive understanding of their electronic energy-level structures, which mysteriously exhibit either a doublet or a triplet exciton peak at the single-particle level. Here we show that transition from doublet to triplet excitons in single CsPbI3 NCs can be triggered by reinforcing quantum confinement in the same batch of sample upon being stored in the ambient environment. Besides size reduction and blue-shifted emission, this enhanced quantum confinement is also manifested by the suppressed emission of multiple and charged excitons in single CsPbI3 NCs with a triplet-exciton configuration. We propose that the doublet and triplet excitons should correspond respectively to the weak and strong quantum confinement regimes of single CsPbI3 NCs, with the electron-hole exchange interaction and the Rashba effect determining the exact energy-level alignments and the fine-structure splitting values.
When a UV photon is absorbed by a single semiconductor nanocrystal (NC), two or more excitons can be simultaneously generated through the carrier multiplication (CM) process. It is still highly debated whether the CM efficiency is truly enhanced in semiconductor NCs because all the routine CM measurements performed exclusively at the ensemble level are incapable of completely excluding the false CM signals contributed by the charged excitons. Here we place single CdSe NCs above an aluminum film and successfully resolve their UV-excited photoluminescence time trajectories where the true and false CM signals are contained in the blinking “on” and “off” levels, respectively. When the UV photon energy is ~2.46 times of the NC energy gap, an average CM efficiency of ~20.2% can be reliably estimated. The ability to detect UV-excited photoluminescence from a single NC will surely provide a great guidance for the CM applications in various light-to-electric conversion devices.
S. N. Zhang, O. Adriani, S. Albergo, G. Ambrosi, Q. An, T. W. Bao, R. Battiston, X. J. Bi, Z. Cao, J. Y. Chai, J. Chang, G. M. Chen, Y. Chen, X. H. Cui, Z. G. Dai, R. D'Alessandro, Y. W. Dong, Y. Z. Fan, C. Q. Feng, H. Feng, Z. Y. Feng, X. H. Gao, F. Gargano, N. Giglietto, Q. B. Gou, Y. Q. Guo, B. L. Hu, H. B. Hu, H. H. He, G. S. Huang, J. Huang, Y. F. Huang, H. Li, L. Li, Y. G. Li, Z. Li, E. W. Liang, H. Liu, J. B. Liu, J. T. Liu, S. B. Liu, S. M. Liu, X. Liu, J. G. Lu, M. Mazziotta, N. Mori, S. Orsi, M. Pearce, M. Pohl, Z. Quan, F. Ryde, H. L. Shi, P. Spillantini, M. Su, J. C. Sun, X. L. Sun, Z. C. Tang, R. Walter, J. C. Wang, J. M. Wang, L. Wang, R. J. Wang, X. L. Wang, X. Y. Wang, Z. G. Wang, D. M. Wei, B. B. Wu, J. Wu, X. Wu, X. F. Wu, J. Q. Xia, H. L. Xiao, H. H. Xu, M. Xu, Z. Z. Xu, H. R. Yan, P. F. Yin, Y. W. Yu, Q. Yuan, M. Zha, L. Zhang, L. Y. Zhang, Y. Zhang, Y. J. Zhang, Y. L. Zhang, Z. G. Zhao
The High Energy cosmic-Radiation Detection (HERD) facility is one of several space astronomy payloads of the cosmic lighthouse program onboard China's Space Station, which is planned for operation starting around 2020 for about 10 years. The main scientific objectives of HERD are indirect dark matter search, precise cosmic ray spectrum and composition measurements up to the knee energy, and high energy gamma-ray monitoring and survey. HERD is composed of a 3-D cubic calorimeter (CALO) surrounded by microstrip silicon trackers (STKs) from five sides except the bottom. CALO is made of about 104 cubes of LYSO crystals, corresponding to about 55 radiation lengths and 3 nuclear interaction lengths, respectively. The top STK microstrips of seven X-Y layers are sandwiched with tungsten converters to make precise directional measurements of incoming electrons and gamma-rays. In the baseline design, each of the four side SKTs is made of only three layers microstrips. All STKs will also be used for measuring the charge and incoming directions of cosmic rays, as well as identifying back scattered tracks. With this design, HERD can achieve the following performance: energy resolution of 1% for electrons and gamma-rays beyond 100 GeV, 20% for protons from 100 GeV to 1 PeV; electron/proton separation power better than 10-5; effective geometrical factors of >3 m2sr for electron and diffuse gamma-rays, >2 m2sr for cosmic ray nuclei. R and D is under way for reading out the LYSO signals with optical fiber coupled to image intensified CCD and the prototype of one layer of CALO.
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