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Control over the exciton binding energy (E b) in two-dimensional (2D) perovskites can enable their application in a broad range of optoelectronic technologies. To investigate the effects of organic cations on the excitonic properties of 2D perovskites, films of lead iodide hybrids basing on four structurally similar cations of phenylethylammonium (PEA), 4-florophenylethylammonium (FPEA), 4-chlorophenylethylammonium (ClPEA) and 4-bromophenylethylammonium (BrPEA) were prepared and their optical properties were characterized. Results show that both the perovskite layer distortion and exciton absorption/emission peak do not vary too much by the substitution of halogen atom in the para H of phenylethyl cation. Further examination of the temperature-dependent exciton emission properties reveals that the thermal activation energy E a , which can also be regarded as exciton binding energy in our work, can be tuned over ca. 170 meV within the set of hybrids. It is found that exciton binding energy is sensitive to the identity of organic cations and to the resulting PbI 6 octahedron distortion. Among the compounds considered, FPEA based perovskite has the biggest PbI 6 octahedron distortion and the largest calculated E a of 330 meV, while the Br-PEA based perovskite shows a smaller octahedron distortion and the lowest calculated E a. We believe that an enhanced polarizability of organic cations may be responsible for the observed decrease in octahedra distortion and E a for the p -halophenylethylammonium lead iodide perovskite derivatives. • Effects of organic cations on the excitonic property of (XPEA) 2 PbI 4 (X = H, F, Cl, Br) were studied. • The exciton binding energy (E a) is correlated to the identity of organic cations and to the octahedra distortion. • E a can be varied over ca. 170 meV within the set of hybrid compounds. • Larger polarizability of organic cations is responsible for the smaller octahedra distortion and reduced E a. [ABSTRACT FROM AUTHOR]

Halide perovskites have attracted enormous attention as a novel optoelectronic material showing bright emission recently. In this paper, we chose CsPbX 3 nanocrystals (NCs) as an example to study the structural, morphological and optical properties. We have synthesized monodispersed colloidal inorganic cesium lead halide perovskites (CsPbX 3 , X = Cl, Br and I, or mixed lead halide CsPb(Cl/Br) 3 and CsPb(Br/I) 3 ) NCs with various morphologies and brightly luminescent properties. The emission wavelength of resulting CsPbX 3 NCs can be tuned from 440 nm to 696 nm by varying the amount of different PbX 2 (X = Cl, Br and I), while maintaining high quantum yields up to 95% and narrow emission spectra. The average lifetime of the NCs is several ns to hundred with changing the composition. The blinking of the NCs in terms of a binary (on/off) process is different from II-VI quantum dots. Interestingly, we find that CsPbBr 3 NCs to Cs 4 PbBr 6 NCs or a mixture of CsPbBr 3 NCs and Cs 4 PbBr 6 NCs were obtained by adjusting the amount of oleic acid (OA)-Cs precursor. The optical properties of these NCs have been controlled by varying their composition. Moreover, the injection speed of Cs-OA precursor enables the control of CsPbI 3 NCs with different morphology, which can be tuned from litter nanoparticles to uniform nanorods. This finding can serve as an ideal platform for the investigation of fundamental properties. [ABSTRACT FROM AUTHOR]

In this article we focus on the optical properties of a luminescent two-dimensional organic–inorganic material (thiopheneethyl ammonium lead iodide) with the formula (C 4 H 3 SC 2 H 4 NH 3 ) 2 [PbI 4 ]. This compound exhibits a self-assembled nano-layered structure, forming a multi-quantum wells (QWs) structure. X-ray diffraction patterns revealed that thin films deposited by a spin coating method were highly oriented with the c -axis perpendicular to the substrate surface. The optical properties were investigated by, absorption, ellipsometry and photoluminescence spectroscopy. At room temperature a strong photoluminescence, due to exciton confined in the PbI quantum well, is found around 2.32 eV. The Stokes shift was determined and compared with those found for homologous materials. The exciton-phonon interaction was also investigated by studying the temperature dependence of photoluminescence. The exciton-phonon coupling constants for both acoustic and optical phonons were extracted using a phenomenological model are more than one order of magnitude larger than in conventional QWs. [ABSTRACT FROM AUTHOR]

Abstract: We report on the scintillation properties of undoped PbI2, and PbI2 doped with La, Cu, Ag, and Te. X-ray luminescence spectra were recorded at 7K and the time responses for 80ps pulses of X-rays were recorded from 14K to 150K. Samples were irregularly shaped crystal pieces that ranged from 300 to 1000μm in size. We found that at 14K the undoped samples had a fast emission at 520nm (0.18eV below the band edge) and a total light output of about 40,000photons/MeV. 10% of this light appears in the first ns, which is 5 times greater than LSO and 1.5 times greater than LaBr3:Ce. 50% of the light appears in the first 100ns. The luminosity decreased dramatically as the temperature was increased above 40K, consistent with a non-radiative path that has an activation energy of 3.8meV. Doping with the donor La3+ only slightly changed these scintillation properties. Doping with the donor La3+ and the deep acceptors Cu+, Ag+, and Te2– produced primarily slow donor–acceptor emissions at peak wavelengths of 605, 653, and 800nm, respectively. The luminosity of the samples doped with La3+ and Cu+ depended strongly on the La3+ concentration, demonstrating its role as an electron donor. First-principles band structure calculations showed (1) that the donor level depth of an isolated iodine vacancy is about 0.8eV, too deep to explain the 520nm emission wavelength, and (2) that a donor–acceptor pair consisting of a spatially associated iodine vacancy donor and a lead vacancy acceptor has shallower donor and acceptor depths that are consistent with the 520nm emission wavelength. [Copyright &y& Elsevier]

Abstract: Relaxation and diffusion processes of high-density excitons in layered-type semiconductor PbI2 have been investigated through femtosecond transient grating and time-resolved luminescence measurements under two-photon absorption. We observed two luminescence bands at low temperature; one is due to an exciton–exciton collision process, while the other originates from localized excitonic states. By utilizing the transient grating spectroscopy, the lifetime and diffusion coefficient of the high-density excitons in PbI2 are estimated to be 72ps and 4.6cm2/s, respectively. The lifetime observed is almost equal to the values of the decay time of the luminescence due to the exciton–exciton collision process and the rise time of the luminescence due to the localized excitons. This strongly shows that the decay of the exciton–exciton collision process is dominated by the exciton trapping to the localized states arising from potential fluctuations due to imperfection of crystal. [Copyright &y& Elsevier]

Abstract: Photoluminescence properties of highly excited lead iodide have been investigated through two-photon absorption for band-to-band transitions at low temperature and a new luminescence band is observed. The intensity of the new luminescence band increases as 4th power of the excitation intensity and the peak energy is about 23meV lower than the lowest exciton energy, whose value is almost equal to the energy difference between the n=1 and 2 excitonic states of lead iodide. These results indicate that the new luminescence band observed originates from an exciton–exciton collision process, in which one of the two excitons scatters into the n=2 excitonic state and the other radiatively recombines. [Copyright &y& Elsevier]

Metal halide perovskite semiconductors often exhibit photoluminescence blinking and flickering when luminescence of individual small nano- or even microcrystals is monitored. The nature of these fluctuations is not well understood but must be related to the presence of metastable non-radiative recombination channels and efficient charge migration in these materials. Here we report on the excitation wavelength dependence of photoluminescence flickering effect in degraded methylammonium lead iodide (MAPbI 3) thin films. While the luminescence intensity is temporary stable when excited in the blue region with wavelength shorter than 530 nm, excitation with red light (wavelength longer than 530 nm) results in luminescence flickering. It is hypothesised that the wavelength dependence reflects the excitation energy dependence of the photochemical mechanism that switches non-radiative recombination channels on and off. The effect can also be related to hindered charge carrier diffusion due to their localization in the interfacial layer between MAPbI 3 and PbI 2 which is formed in the course of degradation. • Perovskite MAPbI3 exhibits enhanced photoluminescence flickering upon degradation. • Photoluminescence flickering is more pronounced under low energy photon excitation. • Lead iodide possibly plays an important role in the photoluminescence blinking phenomenon. [ABSTRACT FROM AUTHOR]

Hybrid organic-inorganic perovskites have emerged among the most promising materials for optoelectronic applications thanks to the great synthetic versatility that allows tailoring their structural and photophysical properties. In the two-dimensional (2D) crystalline form, these semiconductors have multiple-quantum-well structure with stable excitonic resonances at room temperature. In this work we present an easy and effective anti-solvent crystallization method to synthesize n = 1 and n = 2 dodecylammonium (DA) lead iodide perovskite single crystals. Different synthetic conditions have been explored and general guidelines for single crystals preparation are provided revealing that γ-butyrolactone is the best solvent for its ability to weakly coordinate Pb2+ centers and that diethyl ether is the best anti-solvent because fast diffuses into the precursors solution. We exploit the high optical quality of the synthesized perovskites to observe strong exciton−photon coupling in single-crystal waveguides, avoiding the need to embed them within mirror microcavities. • Quasi-2D dodecylammonium lead iodide perovskite single crystals have been synthesized. • Anti-solvent assisted crystallization method provides n = 1 and n = 2 perovskite crystals. • Strong light-matter coupling has been observed in perovskite single crystals waveguide. [ABSTRACT FROM AUTHOR]

Whether optoelectronic devices based on metal-halide perovskite semiconductors will become a commercially viable technology will be determined by their intrinsic and operational stability. Recent results indicate there is some reversibility of perovskite degradation in thin films and devices, although mechanistic insight into the processes driving degradation and recovery are still scarce. We here present a comparative spectroscopic study of methylammonium lead iodide (MAPbI 3) films having undergone either photo- or thermal degradation under controlled conditions. We confirm that the degradation mechanism pertaining to each type of stress is inherently different. Our results from photoluminescence microscopy measurements paint a spatially, spectrally and temporarily resolved picture showing that, unlike thermally degraded samples, photodegraded samples are in a state that intermittently recovers to luminescent MAPbI 3 upon laser excitation. This indicates that rather than irreversibly decomposing, photoinduced degradation leaves MAPbI 3 structurally or compositionally intact but induces defects causing non-radiative recombination losses. • Visual representation of differences between thermal and photoinduced degradation. • Real-time demonstration of the reversibility of photodegraded thin films. • Spatially-resolved PL spectra and PL lifetime provides insight into mechanisms. [ABSTRACT FROM AUTHOR]