Direct Evidence of Exciton-Exciton Annihilation in Single-Crystalline Organic Metal Halide Nanotube Assemblies

Unravelling photoisomerization dynamics in a metastable-state photoacid

Direct access to trans-cis photoisomerization in a metastable state photoacid (mPAH) remains challenging owing to the presence of competing excited-state relaxation pathways and multiple transient isomers with overlapping spectra. Here, we reveal the photoisomerization dynamics in an indazole mPAH using time-resolved fluorescence (TRF) spectroscopy by exploiting a unique property of this mPAH having fluorescence only from the trans isomer. The combination of these experimental results with time-dependent density function theory (TDDFT) calculations enables us to gain mechanistic insight into this key dynamical process.

Temperature dependence of radiative and non-radiative decay in the luminescence of one-dimensional pyridinium lead halide hybrids

The photoluminescence properties of organic-inorganic pyridinium lead bromide [(pyH)PbBr3] and iodide [(pyH)PbI3] compounds were investigated as a function of temperature. The inorganic substructure consists of face-sharing chains of PbX6 octahedra. Diffuse reflectance spectra of the compounds show low energy absorption features consistent with charge transfer transitions from the PbX3 chains to the pyridinium cations. Both compounds display extremely weak luminescence at room temperature that becomes strongly enhanced upon cooling to 77 K. Broad, featureless low energy emission (λem > 600 nm) in both compounds have large Stokes shifts [1.1 eV for (pyH)PbBr3 and 0.46 eV for (pyH)PbI3] and are assigned to transitions from self-trapped excitons on the inorganic chains whereas emission at higher energy in (pyH)PbBr3 (λem = 450 nm) is assigned to luminescence from a free exciton state. Analysis of data from temperature-dependent luminescence intensity measurements gives activation energies (Ea) for non-radiative decay of the self-trapped excitons in (pyH)PbBr3 and (pyH)PbI3, (Ea = 0.077 eV and 0.103 eV, respectively) and for the free exciton in (pyH)PbBr3 (Ea = 0.010 eV). Analysis of temperature dependent luminescence lifetime data indicates another non-radiative decay process in (pyH)PbI3 at higher temperatures (Ea = 0.17 eV). A large increase in the luminescence lifetime of (pyH)PbI3 below 80 K is consistent with thermalization between triplet sublevels. Analysis of the luminescence power dependence for (pyH)PbI3 shows superlinear response suggestive of quenching by static traps.

Luminescent Organic-Inorganic Hybrid Metal Halides: An Emerging Class of Stimuli-Responsive Materials

Luminescent organic-inorganic metal halides (OIMHs) are well known as a new materials family in recent years. Novel materials and applications of luminescent OIMHs have been explored by changing either the organic component or the metal halide species. Thereinto, the stimuli-responsive (SR) phenomena in OIMHs have drawn much attention recently, for not only their attractive application potential but also the helpfulness in understanding the stability of OIMHs to the external environment. Herein, the luminescent OIMHs that are sensitive to external stimuli including contact, pressure, mechanical grinding, light, heat, and gas molecules, are reviewed, with an emphasis on analyses of the structural change during the SR process. The applications of SR luminescent OIMHs in widespread fields, including gas sensing, information encryption, and rewritable luminescent paper are summarized. Finally, the challenges that deserve to be further explored in this research field are discussed, which provides certain guidance for the future study of SR luminescent OIMHs.

Twist-angle-controlled neutral exciton annihilation in WS2 homostructures

Exciton-exciton annihilation (EEA), as typical nonradiative recombination, plays an unpopular role in semiconductors. The nonradiative process significantly reduces the quantum yield of photoluminescence, which substantially inhibits the maximum efficiency of optoelectronic devices. Recently, laser irradiation, introducing defects and applying strain have become effective means to restrain EEA in two-dimensional (2D) transition metal dichalcogenides (TMDCs). However, these methods destroy the atomic structure of 2D materials and limit their practical applications. Fortunately, twisted structures are expected to validly suppress EEA through excellent interface quality. Here, we develop a non-destructive way to control EEA in WS₂ homostructures by changing the interlayer twist angle, and systematically study the effect of interlayer twist angle on EEA, using fluorescence lifetime imaging measurement (FLIM) technology. Due to the large moiré potential at a small interlayer twist angle, the diffusion of excitons is hindered, and the EEA rate decreases from 1.01 × 10^-1 cm² s^-1 in a 9° twisted WS₂ homostructure to 4.26 × 10^-2 cm² s^-1 in a 1° twisted WS₂ homostructure. The results reveal the important role of the interlayer twist angle and EEA interaction in high photoluminescence quantum yield optoelectronic devices based on TMDC homostructures.

Organic cations directed 1D [Pb3Br10]4− chains: syntheses, crystal structures, and photoluminescence properties

To diversify the luminescence properties of 1D perovskites, different organic amine cations were combined with 1D [Pb₃Br₁₀]⁴⁻ chains leading to a series of A₂Pb₃Br₁₀ homologues, displaying broadband near white-light emissions with highest CRI of 96.

Semiconducting crystalline inorganic-organic hybrid metal halide nanochains

One-dimensional (1D) inorganic-organic metal halide hybrids at the molecular level, which can be considered as arrays of nanochains isolated by organic components, have shown remarkable optical and electric properties. This review summarizes their reported structural types and shows how to modify their band gaps and optical and electric properties.

Room Temperature Synthesis of All Inorganic Lead-Free Zero-Dimensional Cs4SnBr6 and Cs3KSnBr6 Perovskites

Lead halide perovskites are excellent candidates for photoelectronic and photovoltaic applications, but the toxicity from lead is extremely concerning. Recently, Sn-based zero-dimensional lead-free perovskites synthesized using solid-state reaction techniques have become a new focus in the field. Here, we report a simple room temperature antisolvent method for the synthesis of all inorganic lead-free green emissive Cs₄SnBr₆ (emission at 524 nm) and cyan emissive Cs₃KSnBr₆ (emission at 500 nm) zero-dimensional perovskites. Their photoluminescence quantum yields reach 20% and 35%, respectively. In addition, they maintain their emission for 46 and 55 h in the air, respectively, compared to only 5 min of CsSnBr₃. This method provides a convenient way to do the research and apply these highly emissive perovskites.

[DMEDA]PbCl4: a one-dimensional organic lead halide perovskite with efficient yellow emission

By using a simple room temperature solution reaction, we prepared a new type of one-dimensional (1D) hybrid lead halide [DMEDA]PbCl₄. The compound gives a bright yellow emission with efficient photoluminescence quantum efficiency (PLQE) and optical stability.

Neutral and defect-induced exciton annihilation in defective monolayer WS2

As defects and exciton-exciton annihilation (EEA) frequently govern the properties of nanoscale optoelectronic devices based on monolayer transition metal dichalcogenides (TMDCs), understanding the interaction between defects and EEA is of fundamental importance. Here we perform a systematic investigation of the effect of defects on EEA of neutral excitons and defect-bound excitons in monolayer WS2, using fluorescence lifetime imaging technology. Scanning transmission electron microscopy confirms the creation of atomic-scale defects introduced by argon plasma treatment in defective WS2. Defects can bind neutral excitons or trions to form defect-bound excitons. And defects have a slight effect on the lifetime of neutral excitons. However, owing to the impeded exciton diffusion caused by defects, the EEA rate of neutral excitons reduces from 0.26 cm2 s-1 in the pristine monolayer to 0.16 cm2 s-1 in the defective monolayer. For defect-bound excitons, the EEA rate of 0.068 cm2 s-1 is obtained, which results from the localized nature of defect-bound excitons and suppressed exciton diffusion. Our results reveal the important role of defect-EEA interactions in tailoring the properties of monolayer TMDCs.