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Semiconducting MXenes are an intriguing two-dimensional (2D) material class with promising electronic and optoelectronic properties. Here, we focused on recently prepared Hf-based MXenes, namely, Hf3C2O2 and Hf2CO2. Using the first-principles calculation and excited state corrections, we proved their dynamical stability, reconciled their semiconducting behavior, and obtained fundamental gaps by using the many-body GW method (indirect 1.1 and 2.2 eV; direct 1.4 and 3.5 eV). Using the Bethe–Salpeter equation, we subsequently provided optical gaps (0.9 and 2.7 eV, respectively), exciton binding energies, absorption spectra, and other properties of excitons in both Hf-based MXenes. The indirect character of both 2D materials further allowed for a significant decrease of excitation energies by considering indirect excitons with exciton momentum along the Γ-M path in the Brillouin zone. The first bright excitons are strongly delocalized in real space while contributed by only a limited number of electron–hole pairs around the M point in the k-space from the valence and conduction band. A diverse range of excitonic states in Hf3C2O2 MXene lead to a 4% and 13% absorptance for the first and second peaks in the infrared region of absorption spectra, respectively. In contrast, a prominent 28% absorptance peak in the visible region appears in Hf2CO2 MXene. Results from radiative lifetime calculations indicate the promising potential of these materials in optoelectric devices requiring sustained and efficient exciton behavior. [ABSTRACT FROM AUTHOR]

We present a detailed theoretical analysis of polarized absorption spectra and linear dichroism of cyanine dye aggregates whose unit cells contain two molecules. The studied threadlike ordered system with a molecular exciton delocalized along its axis can be treated as two chains of conventional molecular aggregates, rotated relative to each other at a certain angle around the aggregate axis. Our approach is based on the general formulas for the effective cross section of light absorption by a molecular aggregate and key points of the molecular exciton theory. We have developed a self-consistent theory for describing the orientational effects in the absorption and dichroic spectra of such supramolecular structures with nonplanar unit cell. It is shown that the spectral behavior of such systems exhibits considerable distinctions from that of conventional cyanine dye aggregates. They consist in the strong dependence of the relative intensities of the J- and H-type spectral bands of the aggregate with a nonplanar unit cell on the angles determining the mutual orientations of the transition dipole moments of constituting molecules and the aggregate axis as well as on the polarization direction of incident light. The derived formulas are reduced to the well-known analytical expressions in the particular case of aggregates with one molecule in the unit cell. The calculations performed within the framework of our excitonic theory combined with available vibronic theory allow us to quite reasonably explain the experimental data for the pseudoisocyanine bromide dye aggregate. [ABSTRACT FROM AUTHOR]

Transient absorption (TA) spectroscopy of semiconductor nanocrystals (NCs) is often used for excited state population analysis, but recent results suggest that TA bleach signals associated with multiexcitons in NCs do not scale linearly with exciton multiplicity. In this manuscript, we probe the factors that determine the intensities and spectral positions of exciton and biexciton components in the TA spectra of CdSe quantum dots (QDs) of five diameters. We find that, in all cases, the peak intensity of the biexciton TA spectrum is less than 1.5 times that of the single exciton TA spectrum, in stark contrast to a commonly made assumption that this ratio is 2. The relative intensities of the biexciton and exciton TA signals at each wavelength are determined by at least two factors: the TA spectral intensity and the spectral offset between the two signals. We do not observe correlations between either of these factors and the particle diameter, but we find that both are strongly impacted by replacing the native organic surface-capping ligands with a hole-trapping ligand. These results suggest that surface trapping plays an important role in determining the absolute intensities of TA features for CdSe QDs and not just their decay kinetics. Our work highlights the role of spectral offsets and the importance of surface trapping in governing absolute TA intensities. It also conclusively demonstrates that the biexciton TA spectra of CdSe QDs at the band gap energy are less than twice as intense as those of the exciton. [ABSTRACT FROM AUTHOR]

The development of lasing materials with weak pile‐up effect from triplet excitons, low lasing threshold, and long operational lifetime is much significant for truly continuous wave operation and electrically pumping organic lasers. However, most of organic gain media has been verified to have extreme quenching effect from triplet excitons due to its wide‐band distribution of triplet absorption spectrum. Here, it is found that the phenanthroimidazole derivatives as gain media have congenital advantages on handling triplet's issues. A very low amplified spontaneous emission (ASE) threshold of 0.56 µJ cm−2 and a long operational lifetime of 5.77 × 105 pump pulses in air atmosphere are well obtained. When increasing the pump repetition frequency from 20 Hz to 20 kHz, the operational lifetimes of the thin films have small reduction. Under a high pump repetition frequency of 200 kHz, which is quasi‐continuous wave operational regime, the PI2PhDPO thin film exhibits very low ASE threshold of 0.78 µJ cm−2 due to the tiny overlap between triplet absorption and ASE spectra. [ABSTRACT FROM AUTHOR]

In the era of intelligence, the output colors of foundational phosphors are expected to be controlled by programs, while current activators‐determined afterglow candidates with fixed spectral channel have limitations in creating customized colors. Here, a long‐lived warm‐white emission is successfully demonstrated originating from self‐trapped excitons (STEs) in non‐toxic Cs2NaInCl6:Ag,Bi, ranging from 400 to 850 nm, of which the afterglow color can be easily customized using filters. This investigation indicates that 3% of Ag alloying breaks the dark STEs and introduces traps for efficient afterglow, while 3% Bi doping further improves the quantum yield to ≈100% and greatly enhances afterglow by ≈100‐fold compared with the initial intensity, allowing for an impressive afterglow persistence of over 20,000 s. Intriguingly, the self‐trapped defect bands from Jahn‐Teller distortions are prolonged from hundreds of femtoseconds to several hours, and they are first detected in the steady‐state absorption spectra after the cessation of excitation sources, contributing to the concluded dynamic afterglow model for STEs. And its intelligent application is corroborated by designed multi‐channel information system. These findings offer a novel scheme for understanding dynamic luminescence of STEs and supply an exemplification of designing white afterglow phosphors. [ABSTRACT FROM AUTHOR]

[Display omitted] • The lattice modification of Cs 2 NaInCl 6 was conducted by Ag and Zr dopants. • The emissive color temperature was tailored from warm- to cool-white with Zr content. • Double self-trapped excitons (STEs)-based pure-white afterglow was realized, which could last over 5 h. • 40 % of Zr dopants enhanced the initial afterglow intensity for ∼ 100-fold, compared with Cs 2 Na 0.9 Ag 0.1 InCl 6. • The bands of STEs were observed in steady-state absorption spectra thanks to energy storage. The incommensurable differences between the feeding and actual doping ratios of luminescent dopants and excessive reliance on rare-earth ions become two key problems in halide perovskite-based afterglow materials, restricting the production cost and further application. Here, we prepare a series of novel lead- and rare-earth-free halide afterglow phosphors Cs 2 (Na 0.9 Ag 0.1 In) 1- x Zr x Cl 6 under an ambient condition, in which the Zr dopants modify the lattice thus tailoring the emission spectrum and trap distribution, simultaneously. The focused Cs 2 (Na 0.9 Ag 0.1 In) 0.6 Zr 0.4 Cl 6 with mainly size distribution of ∼ 0.6–1.8 μm exhibits ∼ 350–850 nm ultrabroad white luminescence under ultra-violet excitation, and the introduced traps endow the white emission with a long-persistence of over 5 h after pre-irradiation. The absorption bands of self-trapped excitons (STEs) are observed in the steady-state absorption spectra during the afterglow process, suggesting the general accepted picosecond-scaled lifetimes of STEs have been successfully prolonged to hour-scale, which provide a potential opportunity for investigating the specific behavior of STEs. [ABSTRACT FROM AUTHOR]

Ce+3 doped CsLaSiS 4 single crystals were synthesized using high-temperature flux synthesis. XRD analysis showed that the products are isostructural to CsLaSiS 4 and crystallize in orthorhombic space group Pnma. Absorption spectra were studied at room temperature, and photoluminescence properties were examined in the temperature range of 5–310 K. The energy of interband transitions E g = 3.75 eV was determined at room temperature in the Tauc model. At room temperature, only a non-elementary d → f emission band of Ce+3 ions was observed in the 520 nm region. The luminescence kinetics upon excitation by a pulsed cathode beam or X-ray synchrotron radiation exhibited a dominant nanosecond component. Decay time, as well as build-up time, decreased with increasing concentration of Ce+3 ions. Concentration quenching of Ce+3 emission was not observed up to 11.9 mol% of Ce+3 ions. At a low temperature of 5 K, new wide emission bands at 422 and 688 nm appeared in the photoluminescence spectrum. It is shown that the 422 nm band in the photoluminescence spectrum corresponds to host emission, specifically the luminescence of self-trapped excitons (STE). The 688 nm emission band corresponds to defect-related luminescence. STE emission is quenched according to the Mott law at temperatures above 26 K with an activation energy of 20 meV. An efficient energy transfer channel from the STE to the Ce+3 ion via the radiative resonance mechanism is observed. The emission of Ce+3 ions and defect-related luminescence can be excited by the intracenter way, due to electron-hole recombination, or by the creation of defect bound excitons. Based on the obtained spectroscopic data, a band scheme illustrating the processes of relaxation of electronic excitations at T = 5 K in Ce+3 doped CsLaSiS 4 crystals is proposed. [Display omitted] • Ce+3 doped CsLaSiS 4 single crystals were obtained through high-temperature flux synthesis. • Inter configurational d-f transitions in Ce3+ ions were studied upon both UV- and X-ray excitation. • Luminescence kinetic studies were conducted using a pulsed cathode beam and X-ray synchrotron radiation. • Self-trapped excitons, defect-bound excitons, and impurity luminescence were observed at T = 5 K. [ABSTRACT FROM AUTHOR]