Exciton many-body interaction is the fundamental light−matter interaction that determines the optical response of the new class of colloidal perovskite nanocrystals of the general formula CsPbX 3 [X = Cl, Br, or I]. However, the understanding of exciton many-body interactions manifested through the transient biexcitonic Stark effect at the early time scales and the Auger recombination process in this new class of materials still remains rather incomplete. In this Article, we studied the many-body exciton interactions under controlled conditions through ultrafast transient absorption spectroscopy. A large biexcitonic redshift ∼30 meV to the effect of hot excitations on the excitonic resonance is observed at the early time scales. From the fluence-dependent studies, it is evident that the samples have only single and biexciton lifetimes, suggesting that the band edges are 2-fold degenerate. This explicit experimental evidence for the exciton many-body interactions in CsPbBr 3 nanocrystals provides a powerful tool to explore the development of their prospective applications in light-emitting devices, lasers, and solar cells.
Two-dimensional (2D) covalent organic frameworks (COFs) are an emerging class of porous materials with potential for wide-ranging applications. Intense research efforts have been directed at tuning the structure and topology of COF, however the bandgap engineering of COF has received less attention, although it is a necessary step for developing the material for photovoltaic or photonic applications. Herein, we have developed an approach to narrow the bandgap of COFs by pairing triphenylamine and salicylideneaniline building units to construct an eclipsed stacked 2D COF. The ordered porous structure of 2D COF facilitates a unique moisture-triggered tautomerism. The combination of donor−acceptor charge transfer and tautomerization in the salicyclidineaniline unit imparts a large bandgap narrowing for the COF and turns it color to black. The synthesized COF with donor−acceptor dyad exhibits excellent nonlinear optical properties according to open aperture Z-scan measurements with 532 nm nanosecond laser pulses.
Defect tolerant perovskite nanocrystals of the general formula Cs-Pb-X3 (where X= Cl, Br and I) have shown exceptional potential in fundamental physics as well as in novel optoelectronic applications as the next generation solar cells. Although exciton many-body interactions such as biexciton Stark shift, state filling, and Auger recombination are studied extensively, other important correlated effects like bandgap renormalization (BGR) and hot phonon bottleneck are not explored in these nanocrystals. Here we experimentally demonstrate the carrier density dependence of the BGR and an effective hot phonon bottleneck in CsPb(Cl0.20Br0.80)3 mixed halide nanocrystals. The results are compared with two other halide compositions, namely, CsPbBr3 and CsPb(Br0.55I0.45)3 nanocrystals with varying bandgaps. The optical response of the nanocrystals changes dramatically across the spectral range of many hundreds of meV at high carrier density due to large BGR. We have calculated the BGR constant ≈ (6.0±0.3)×10-8 eVcm for CsPb(Cl0.20Br0.80)3 nanocrystals that provides the amount of bandgap shift as a function of carrier density. In these nanocrystals, an efficient hot phonon bottleneck is observed at a carrier density of 3.1×10 17 cm-3 that slows down the thermalization by one order of magnitude. Our findings reveal that the complex kinetic profile of the exciton dynamics can be analyzed by the global target analysis using the sequential model with increasing lifetimes.
Herein we report the colloidal synthesis of Cs 3 Sb 2 I 9 and Rb 3 Sb 2 I 9 perovskite nanocrystals,a nd explore their potential for optoelectronic applications.D ifferent morphologies,s uch as nanoplatelets and nanorods of Cs 3 Sb 2 I 9 ,a nd spherical Rb 3 Sb 2 I 9 nanocrystals were prepared. All these samples show band-edge emissions in the yellow-red region. Exciton many-body interactions studied by femtosecond transient absorption spectroscopyofCs 3 Sb 2 I 9 nanorods reveals characteristic second-derivative-type spectral features,suggesting red-shifted excitons by as much as 79 meV.Ah igh absorption cross-section of ca. 10 À15 cm 2 was estimated. The results suggest that colloidal Cs 3 Sb 2 I 9 and Rb 3 Sb 2 I 9 nanocrystals are potential candidates for optical and optoelectronic applications in the visible region, though ab etter control of defect chemistry is required for efficient applications. Figure 1. a) Schematic representation of the relationship among the perovskitec rystal structures of i) 3D CsMI 3 (M = Pb 2+ /Sn 2+ ), ii)2D Cs 3 Sb 2 I 9 ,and iii)0DCs 2 SnI 6 .b)Schematic representation of the synthesis of Cs 3 Sb 2 I 9 NPLs and Cs 3 Sb 2 I 9 NRs. ODE = 1-octadecene, OnA = octanoic acid, OAm = oleylamine.[*] J. Pal, S. Manna, [+] A. Nag
Noble metal nanoclusters protected with carboranes, a 12-vertex, nearly icosahedral boron–carbon framework system, have received immense attention due to their different physicochemical properties. We have synthesized ortho-carborane-1,2-dithiol (CBDT) and triphenylphosphine (TPP) coprotected [Ag42(CBDT)15(TPP)4]2– (shortly Ag42) using a ligand-exchange induced structural transformation reaction starting from [Ag18H16(TPP)10]2+ (shortly Ag18). The formation of Ag42 was confirmed using UV–vis absorption spectroscopy, mass spectrometry, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and multinuclear magnetic resonance spectroscopy. Multiple UV–vis optical absorption features, which exhibit characteristic patterns, confirmed its molecular nature. Ag42 is the highest nuclearity silver nanocluster protected with carboranes reported so far. Although these clusters are thermally stable up to 200 °C in their solid state, light-irradiation of its solutions in dichloromethane results in its structural conversion to [Ag14(CBDT)6(TPP)6] (shortly Ag14). Single crystal X-ray diffraction of Ag14 exhibits Ag8–Ag6 core–shell structure of this nanocluster. Other spectroscopic and microscopic studies also confirm the formation of Ag14. Time-dependent mass spectrometry revealed that this light-activated intercluster conversion went through two sets of intermediate clusters. The first set of intermediates, [Ag37(CBDT)12(TPP)4]3– and [Ag35(CBDT)8(TPP)4]2– were formed after 8 h of light irradiation, and the second set comprised of [Ag30(CBDT)8(TPP)4]2–, [Ag26(CBDT)11(TPP)4]2–, and [Ag26(CBDT)7(TPP)7]2– were formed after 16 h of irradiation. After 24 h, the conversion to Ag14 was complete. Density functional theory calculations reveal that the kernel-centered excited state molecular orbitals of Ag42 are responsible for light-activated transformation. Interestingly, Ag42 showed near-infrared emission at 980 nm (1.26 eV) with a lifetime of >1.5 μs, indicating phosphorescence, while Ag14 shows red luminescence at 626 nm (1.98 eV) with a lifetime of 550 ps, indicating fluorescence. Femtosecond and nanosecond transient absorption showed the transitions between their electronic energy levels and associated carrier dynamics. Formation of the stable excited states of Ag42 is shown to be responsible for the core transformation.
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