Self-trapped Exciton Emission Research Articles

Factors influencing self-trapped exciton emission of low-dimensional metal halides

In this review, we mainly summarized the structure distortion, molecular engineering, electron–phonon coupling effect, external temperature and pressure, and metal ion doping that influence the self-trapped exciton emission of low-dimensional metal halides (LDMHs).

Cubic Halide Double Perovskite Nanocrystals with Anisotropic Free Excitons and Self-Trapped Exciton Photoluminescence

In this work, we first developed Cs2KBiCl6 cubic double perovskite nanocrystals and a series of morphologically isotropic double perovskite nanocrystals. Different contributions of different elements to self-trapped states were revealed by density functional theory. Meanwhile, these double perovskite nanocrystals exhibit the coexistence of free and self-trapped exciton dual-color photoluminescence. Femtosecond transient absorption spectroscopy can confirm that the double perovskite nanocrystals produce a relatively obvious structural deformation in the excited state. We infer that this can lead to a large deviation of the excitation and emission transition dipoles, thus causing large photoluminescence anisotropy. Most importantly, we observe for the first time that both free exciton emission and self-trapped exciton emission are highly anisotropic, which are comparable to or even better than that of lead halide perovskites. This research paves the way for exploring more possibilities and practical applications.

Self-Trapped Excitons Mediated Energy Transfer to Sm3+ in Cs2AgIn(1–x)SmxCl6:Bi Double Perovskite Nanocrystals

Lanthanide ions (Ln3+) are well-known dopants for controlling the optoelectronic properties of double perovskites (DPs). However, the excitation energy of Ln3+-doped Cs2AgInCl6 being too high (∼250–290 nm) limits its direct excitation by commercial UV light-emitting diodes (≥365 nm). To overcome this challenge, we employed Bi3+ as a sensitizer to induce the emission of Sm3+ at much lower excitation energy in Sm3+–Bi3+ codoped Cs2AgInCl6 DP nanocrystals (NCs). Spectral analysis shows that a trace amount of Bi3+ (∼1%) doping provides dual emission of self-trapped excitons (STEs) and characteristic emissions of Sm3+ assigned to 4G5/2 to 6HJ (J = 5/2, 7/2, 9/2, and 11/2) transitions with 368 nm excitation energy. Transient absorption spectroscopic results revealed the existence of nonradiative energy transfer from STE states. Subsequently, we propose a mechanism to explain the formation of energy-transfer channels from STEs to excited states of Sm3+. Our study demonstrates that Bi3+ can efficiently sensitize Sm3+ to modify the optical properties of lead-free DP NCs to expand their luminescence application.

Cu+@Sb3+-Codoped All-Inorganic Metal Halide of Cs2ZnCl4 with Tunable Dual Emission for Fluorescence Anticounterfeiting and Information Encryption

In recent years, low-dimensional lead-free metal halides have captured wide interest in the application of fluorescence anticounterfeiting due to their unique optical properties, low toxicity, and excellent environmental stability. Herein, we report an effective multimode photoluminescent material of Cu+@Sb3+-codoped Cs2ZnCl4 microcrystals via a facile solution synthesis method. Upon a 365 nm ultraviolet (UV) excitation, Cu+@Sb3+-codoped Cs2ZnCl4 shines a highly efficient broad red emission band at 714 nm. Under a 254 nm UV irradiation, this codoped compound exhibits a dual-band emission with an additional high-intensity emission band at 492 nm, enabling a bright sky-blue emission to be observed. The study of the photophysical mechanism reveals that the observed dual-emission bands at 492 and 714 nm in this compound stem from the self-trapped exciton emission of [CuCl4]3– and [SbCl4]− clusters, respectively. In addition, inspired by the obvious excitation-wavelength-dependent emission characteristics and excellent stabilities of Cu+@Sb3+-codoped Cs2ZnCl4, we successfully applied this compound to fluorescent anticounterfeiting and information encryption–decryption applications.

Phonon-Assisted Upconversion Photoluminescence of a Self-Trapped Exciton in the Rb2CuCl3 Single Crystal.

Phonon-assisted upconversion photoluminescence (UCPL) plays an important role in a wide range of fields such as optical refrigeration, sensitive optical thermometry, quantum state control, and upconversion optoelectronics. High photoluminescence quantum yield (PLQY) and strong electron-phonon coupling are two basic prerequisites of efficient UCPL materials. The self-trapped exciton (STE) system with the above-mentioned advantages hints that it may be a good candidate for phonon-assisted UCPL. Here, we synthesized Rb2CuCl3 single crystals (SCs) which yield a high PLQY of the STE emission at 400 nm, and an efficient phonon-assisted UCPL was demonstrated at room temperature. By exponentially fitting the intensity of temperature-dependent UCPL spectra, we obtained an optical thermometry sensitivity of SCs up to 6 mK at 295 K. We also propose that net cooling would be possible if the PLQY is improved up to 91.5% with 345 nm excitation. Our results open a new door to explore laser cooling in STE systems.

Exploring a Stable and Dense 3D Lead Chloride Hybrid with Emission of Self‐Trapped Excitons toward X‐Ray Scintillation

AbstractThe exceptional photophysical properties of 3D organic–inorganic lead halide hybrids (OILHs) endow their significant potential for usage in optoelectronics, which has sparked intense research on novel 3D OILHs and associated applications. However, constructing new 3D OILHs based on large organic cations suffers from tough challenges due to the limitation of the Goldschmidt tolerance factor rule, let alone further explorations of their practical applications. Herein, a brand‐new 3D lead chloride hybrid, (1MPZ)Pb4Cl10·H2O (1, 1MPZ = 1‐methylpiperazine) is reported, featuring a dense 3D lead chloride framework made of the corner‐, edge‐, and face‐shared lead chloride polyhedra. 1 presents a broadband white light emission with a large Stokes shift and a nanosecond photoluminescence lifetime, which originates from radiative recombination of self‐trapped excitons (STEs) induced by the highly distorted structure. Such a reabsorption‐free and fast‐decayed STEs emission coupling with the dense 3D architecture further enables 1 with effective X‐ray scintillation with good sensitivity. Impressively, 1 also shows superior environmental and radiation stability. This study provides a new 3D OILH with appealing luminescence, not only expanding the 3D OILH family but also inspiring the exploitation of their optoelectronic applications.

Visualization of X-rays with an Ultralow Detection Limit via Zero-Dimensional Perovskite Scintillators

X-rays play an extremely significant role in medical diagnosis, safety testing, scientific research, and other practical applications. However, as the main sources of radioactive pollution, the hazard of X-rays to human health and the environment has been a major concern. Herein, the explored perovskite scintillator of Cs2Zr1-xPbxCl6 in this work exhibits an ultrahigh radioluminescence intensity owing to the enhanced X-ray absorption for the introduction of Pb2+ ions. The Cs2Zr1-xPbxCl6 crystals are demonstrated as efficient scintillators with a self-trapped exciton emission and extremely high steady-state light yield (∼101,944 photons meV-1). This fascinating scintillator provides a convenient visual tool for X-ray detection even for an indoor lighting environment, reaching a low detection limit of ∼14.2 nGy·s-1, which is about 1/387 of the typical medical imaging dose (5.5 μGy·s-1). Moreover, X-ray imaging with a high resolution of 16.6 lp·mm-1 is achieved with the as-explored Cs2Zr1-xPbxCl6 scintillator film. Herein, the Cs2Zr1-xPbxCl6 scintillator provides a feasible strategy for X-ray monitoring in the field of biomedicine, high-energy physics, national security, and other applications.

Boosting Energy Transfer from Self‐Trapped Exciton to Er3+ Through Sb3+ Doping in Cs2Na(Lu/Er)Cl6 Double Perovskites

AbstractMetal halide double perovskites are highly attractive for optoelectronic applications because of their superior optical properties. Here, a novel halide double perovskite, Cs2NaLuCl6, exhibiting bright blue emission originating from the host self‐trapped exciton (STE) is reported. Through doping with Sb3+, the dopant‐induced STE is formed in the crystals with a photoluminescence quantum yield up to 94%. The energy transfer from the dopant STE to Er3+ is superior to the direct excitation of Er3+, which is more efficient than the energy transfer from the host STE to Er3+. Furthermore, the energy transfer efficiency from STE to Er3+ is boosted from 8% to 33% through Sb3+ doping in Cs2NaLu0.95Er0.05Cl6. Due to the strong electron‐phonon coupling and efficient energy transfer, the as‐prepared crystals exhibit intense dual‐band emissions and remarkable thermal stability. Importantly, the STE emission variation after the Er3+‐substitution is explored based on the electron‐phonon coupling effect. These findings provide new insights into the excited state dynamics of halide double perovskites and extend their luminescence applications such as white light‐emitting diodes and night vision.

Octahedral Distortion Co-Regulation via Dual Strategies toward Luminescence Enhancement for the MA4InBr7 Perovskite Single Crystal.

A series of novel perovskite single crystals are innovatively grown. Aiming to enhance the luminescence performance, octahedral distortion co-regulation via dual strategies for the as-prepared perovskite single crystals is performed. The distortion of the octahedral structure strengthens the electron-phonon coupling and electron localization, resulting in a more stable self-trapped state, which thereby increases the potential for radiative recombination, accompanied by the self-trapped exciton emission. Accordingly, the luminescence spectra of the as-prepared MA4In0.975Sb0.025Br7 single crystal can cover the 450-800 nm range, and the photoluminescence quantum yield is up to 81.25%.

Theory and experiments of pressure-tunable broadband light emission from self-trapped excitons in metal halide crystals

Hydrostatic pressure has been commonly applied to tune broadband light emissions from self-trapped excitons (STE) in perovskites for producing white light and study of basic electron-phonon interactions. However, a general theory is still lacking to understand pressure-driven evolution of STE emissions. In this work we first identify a theoretical model that predicts the effect of hydrostatic pressure on STE emission spectrum, we then report the observation of extremely broadband photoluminescence emission and its wide pressure spectral tuning in 2D indirect bandgap CsPb2Br5 crystals. An excellent agreement is found between the theory and experiment on the peculiar experimental observation of STE emission with a nearly constant spectral bandwidth but linearly increasing energy with pressure below 2 GPa. Further analysis by the theory and experiment under higher pressure reveals that two types of STE are involved and respond differently to external pressure. We subsequently survey published STE emissions and discovered that most of them show a spectral blue-shift under pressure, as predicted by the theory. The identification of an appropriate theoretical model and its application to STE emission through the coordinate configuration diagram paves the way for engineering the STE emission and basic understanding of electron-phonon interaction.

Circularly Polarized Luminescence from Zero-Dimensional Hybrid Lead-Tin Bromide with Near-Unity Photoluminescence Quantum Yield.

Zero-dimensional (0D) hybrid metal halides with perfect host-guest structures are promising candidates to construct circularly polarized luminescence (CPL)-active materials. However, it still remains challenging to obtain 0D chiral metal halides with simultaneously strong CPL and high photoluminescence quantum yield. Here, a new enantiomeric pair of 0D hybrid lead-tin bromides, (RR/SS-C6 N2 H16 )2 Pb0.968 Sn0.032 Br6 ⋅ 2H2 O (R/S-PbSnBr ⋅ H2 O), is reported. The R/S-PbSnBr ⋅ H2 O compounds not only show intriguing self-trapped exciton emissions with near-unity quantum yield, but also present intense CPL with a dissymmetry factor glum of ±3.0×10-3 . Such CPL activities originate from the asymmetric [SnBr6 ]4- luminophores in R/S-PbSnBr ⋅ H2 O, due to the induced structural chirality by the organic ligands via N-H⋅⋅⋅Br hydrogen bonds. Furthermore, CPL emissions with tunable colors from R/S-PbSnBr ⋅ H2 O and dehydrated compounds are reversibly observed, which extends their chiroptical applications.

Blue Light-Excitable Broadband Yellow Emission in a Zero-Dimensional Hybrid Bismuth Halide with Type-II Band Alignment

Zero-dimensional (0D) organic-inorganic hybrid metal halides have captured broad interest in the lighting and display fields because of their unique electronic structures and splendid broadband emission properties. However, the blue light-excitable broadband yellow emissions have been rarely reported in 0D hybrid metal halides. Here, we design a new 0D bismuth hybrid, (4cmpyH)2BiCl5 (1, 4cmpy = 4-(chloromethyl)pyridine), featuring isolated edge-sharing bioctahedral [Bi2Cl10]4- dimers surrounded by rigid, conjugated, and luminescent organic [4cmpyH]+ cations. This material is able to show intrinsic broadband yellow emissions under blue light (468 nm) excitation with a long lifetime of 22.33 μs and a photoluminescence (PL) quantum yield of 5.56%. Solid-state UV-vis spectroscopy studies prove that introducing organic π-conjugated groups into hybrid systems leads to absorption in the visible light region, in favor of photoexcitation by visible light. By comparing the PL data of 1 and the organic template at room temperature and measuring variable-temperature PL spectra of 1, the blue light-excited broadband emission of 1 can be attributed to the synergistic emissions of intramolecular π → π* and n → π* transitions in the organic cations and triple self-trapped exciton (STE) states centralized at the highly distorted Bi-Cl lattices. Moreover, density functional theory calculations reveal a type-II band alignment in 1 with an indirect band gap of 2.64 eV, which is together determined by organic cations and inorganic bioctahedral units. To the best of our knowledge, our work represents the first report on the blue light-excitable STE emission in 0D Bi-based metal halides, which will largely promote the rapid development of novel high-performance yellow light-emitting materials.

Large-area flexible scintillator screen based on copper-based halides for sensitive and stable X-ray imaging

X-ray scintillators, which convert high-energy radiation into visible light signals, have been rapidly developed for their widespread use in medical, security, and commercial diagnostic technologies. However, the development of large-area flexible scintillator screens with excellent stability and scintillation performance for non-planar objects is relatively lagging behind. Here, we fabricated large-area flexible scintillator screens with excellent stability and scintillation properties by loading nanoscale Cs3Cu2I5 particles into an elastic polydimethylsiloxane (PDMS) matrix. Copper halide Cs3Cu2I5, with self-trapped exciton emission, exhibits intense blue emission with a large Stokes shift, which can effectively reduce the light self-absorption effect. The experimental results reveal that the PDMS matrix can effectively passivate the surface defects of Cs3Cu2I5 particles, improve the exciton binding energy and ensure effective exciton recombination, thereby achieving more efficient optical coupling efficiency. In addition, the Cs3Cu2I5@PDMS flexible film exhibits excellent thermal stability, photostability, and water stability, and the radiative luminescence intensity does not decay after a continuous irradiation for 100 min under high-dose X-rays. Large-area flexible scintillator screen with uniform luminescence achieves a high spatial resolution of 8.6 lp/mm and a high light yield of 46000 photons/MeV, and exhibits excellent linear response to X-ray dose rate and low detection limit of 96.54 nGyair/s. Moreover, the large-area flexible scintillator screen exhibits high-quality X-ray imaging for both planar and non-planar objects. Our results demonstrate that the stable and sensitive Cs3Cu2I5@PDMS scintillator screen may have promising application prospects in flexible X-ray imaging fields.

Eu3+, Tb3+ doping induced tunable luminescence of Cs2AgInCl6 double perovskite nanocrystals and its mechanism

Incorporation of lanthanide ions (Ln3+) can remarkably enhance the structural stability and optical properties of lead-free perovskite NCs. Herein, we have established the relationship between the intrinsic emission of Cs2AgInCl6 host and the Ln3+ by Eu3+, Tb3+ doping in Cs2AgInCl6 NCs, realizing tunable luminescence of NCs in the visible region. Experimental studies and density functional theory (DFT) calculations analysis show that Eu3+ and Tb3+ occupy the sites of In3+ in Cs2AgInCl6, the defect formation energy for Cs2AgInCl6:Ln3+ is larger than the pristine Cs2AgInCl6, resulting in a more stable cubic double perovskite structure. The optical band gap increases with the increase in Ln3+ doping percentage. Turning excitation from 250 nm to 300 nm, the broad self-trapped excitons (STEs) emission is more remarkable, and the narrow Ln3+ emission is less notable. The emitted color can be adjusted from white to red and to green light by altering the doping concentration of Eu3+ or Tb3+ and excitation wavelength. This work promotes the design and optoelectronic application of desired lead-free perovskites.

Growth of SnX2 (X = Br, I) Single Crystals with Self-Trapped Exciton Emission.

Broadband emission with a large Stokes shift is important to obtain an excellent color rendering index of the solid-state lighting device. Among low-dimensional material and perovskite-like phosphors with broadband self-trapped emission, Sn-based phosphors have attracted much attention due to their high photoluminescence quantum yield (PLQY). However, the disadvantage is that the synthesis of Sn-based phosphors needs to be performed in a glovebox. Upon photoexcitation, the broadband emission of self-trapped excitons results from exciton-phonon coupling induced by the transient distortion of the lattice. Low-dimensional material structures often promote self-trapped emission because of more vibrational degrees of freedom and easier polarization under photoexcitation. Here, zero-dimensional (0D) SnX2 (X = Br, I) single crystals are synthesized by the solvent evaporation method in the air. SnX2 emits blue light, broadband yellow light, and deep red light, among which SnBr2 has the best luminescence performance. The photoluminescence quantum yield (PLQY) of SnBr2 reaches 85% and the Stokes shift reaches 265 nm. The PL intensity of SnX2 is linearly related to excitation power, which preliminarily indicates that the origin of SnX2 luminescence is attributed to self-trapped emission (STE). The white light-emitting diodes (WLEDs) were fabricated using yellow-emitting SnBr2 and blue-emitting BaMgAl10O17:Eu2+, which has a low correlated color temperature (3160 K) and a relatively common color rendering index (79).

Large-area vapor-deposited Cs3Cu2I5 perovskite thin films for highly effective third-order nonlinear optics

Zero-dimensional (0D) lead-free perovskites hold promise for efficient optoelectronic and photonic devices, but there are still critical challenges in fabricating large-area, high-quality thin films for practical applications. Herein, we report a simple vapor route to prepare 0D lead-free Cs3Cu2I5 perovskite thin films of high optical quality over large areas. Importantly, the deposition approach can be applied to diverse substrates, including mica, fused silica, MgO, and indium tin-oxide (ITO) coated glass. The deposited Cs3Cu2I5 thin films are phase pure and highly uniform. These Cs3Cu2I5 thin films exhibit strong blue emission upon UV light illumination, a broad band centered at 445 nm assigned to the self-trapped exciton emission. Furthermore, third-order nonlinear optical properties of the as-prepared thin films were determined by means of the Z-scan method. These thin films showed the third-order nonlinear absorption coefficient β as ∼ 1.07 × 10−7 m/W, which is five orders of magnitude larger than three-dimensional counterpart, CsPbI3. Thus, we demonstrate that large-area, high-quality Cs3Cu2I5 thin films can be vapor-deposited and have great potentials in optical switching and optical limiting applications.

Electron-Phonon Coupling Mediated Self-Trapped-Exciton Emission and Internal Quantum Confinement in Highly Luminescent Zero-Dimensional (Guanidinium)6Mn3X12 (X = Cl and Br).

We report a large Stokes shift and broad emission band in a Mn-based organic-inorganic hybrid halide, (Guanidinium)6Mn3Br12 [GuMBr], consisting of trimeric units of distorted MnBr6 octahedra representing a zero-dimensional compound with a liquid like crystalline lattice. Analysis of the photoluminescence (PL) line width and Raman spectra reveals the effects of electron-phonon coupling, suggestive of the formation of Frenkel-like bound excitons. These bound excitons, regarded as the self-trapped excitons (STEs), account for the large Stokes shift and broad emission band. The excited-state dynamics was studied using femtosecond transient absorption spectroscopy, which confirms the STE emission. Further, this compound is highly emissive with a PL quantum yield of ∼50%. With chloride ion incorporation, we observe enhancement of the emissive properties and attribute it to the effects of intrinsic quantum confinement. Localized electronic states in flat bands lining the gap and their strong coupling with phonons are confirmed with first-principles calculations.

Direct Electron Transfer Enables Highly Efficient Dual Emission Modes of Mn2+-Doped Cs2Na1-xAgxBiCl6 Double Perovskites.

Double perovskites with bright emission, low toxicity, and excellent stability have drawn considerable attention. Herein, we report the hydrothermal synthesis of Mn2+-doped Cs2Na1-xAgxBiCl6 double perovskites that exhibit dual emission modes. Introducing Ag+ ions to Cs2NaBiCl6 samples enables a bright self-trapped exciton (STE) emission in orange-red color, whereas Mn2+ dopants induce a yellow-orange emission. Importantly, Mn2+ doping into Cs2Na1-xAgxBiCl6 double perovskites with an indirect bandgap enables a high photoluminescence quantum yield of 49.52 ± 2%. Density functional theory calculations reveal that bringing Ag+ ions into Cs2NaBiCl6 can localize wave function to the [AgCl6]5- octahedron and convert dark transitions to bright STE transitions. Moreover, the 3d orbitals of Mn2+ dopants hybridize with Bi-6p and Cl-3p orbitals at the conduction band minimum, resulting in direct electron transfer from the host to Mn2+ and a significant increase in photoluminescence efficiency. These results shed light on the optical physical process of Mn2+-doped systems, providing useful information for further improvement of the photoluminescence efficiency of double perovskites.

Luminescence from Self‐Trapped Excitons and Energy Transfers in Vacancy‐Ordered Hexagonal Halide Perovskite Cs2HfF6 Doped with Rare Earths for Radiation Detection (Advanced Optical Materials 19/2022)

Cs2HfX6 halides show a vacancy-ordered cubic double perovskite structure when X = Cl, Br, and I. As to the first member of the halide family, Zhiping Luo and co-workers (article number 2201374) identify Cs2HfF6 as a new type of vacancy-ordered hexagonal perovskite. It is composed of single layers of building blocks. Tunable emission is obtained based on self-trapped exciton emissions from the host and energy transfers in the rare-earth-doped material. From energy-dependent cathodoluminescence and time-resolved X-ray radioluminescence, the co-doped material is found to be more sensitive to radiation energy than the singly-doped or pristine host.

Zero-dimensional rubidium zinc halide blue-emitting quantum dots for X-ray imaging

Low-dimensional-networked metal halide (MH) materials are emerged as a new class of semiconductors owing to their unique structure and outstanding photoelectric properties. Compared with the extensively studied three-dimensional (3D) MHs, the zero-dimensional (0D) MHs are of great interest to be widely studied. Here we report the successful colloidal synthesis of all-inorganic lead-free 0D MHs, including undoped and Cu-doped Rb2ZnCl4 quantum dots (QDs) with bright broad blue and cyan emissions, respectively. Combining the theoretical and experimental study, the band structures are well characterized and the originations of the blue and cyan emissions are attributed to the singlet and triplet self-trapped exciton (STE) emissions, respectively. Finally, owing to the bright radioluminescence (RL), the Cu-doped Rb2ZnCl4 QDs is prepared into a thin film to demonstrate the feasibility for X-ray imaging application. This work not only provides a cyan-emitting all-inorganic 0D MH QDs, but also exploits it as a scintillator for X-ray imaging.