Self-trapped Exciton Emission Research Articles

Highly efficient blue emissive copper halide Cs5Cu3Cl6I2 scintillators for X-ray detection and imaging

In recent years, copper-based halide scintillators have attracted tremendous attention due to their excellent luminescent properties and low manufacturing costs. Herein, a high-performance and low-cost Cs5Cu3Cl6I2 scintillator is prepared by simple mechanical grinding. The mixed-halide Cs5Cu3Cl6I2 phosphor exhibits a blue emission centered at 475 nm with a large stokes shift, originating from the self-trapped exciton emission of Cu sites. The photoluminescence quantum yield (PLQY) of Cs5Cu3Cl6I2 can be increased to 88.4% from 20.4% by the efficient suppression of the anion vacancy defects. In addition, the Cs5Cu3Cl6I2 scintillator shows a high steady-state X-ray to light conversion efficiency of about 57000 photons/MeV, good X-ray linear response, and low detection limit of 71.9 nGyair/s. The X-ray imaging results based on the Cs5Cu3Cl6I2 scintillator screen display a high spatial resolution of 9.0 line-pairs per millimeter (lp/mm). Furthermore, Cs5Cu3Cl6I2 exhibits good radiation stability under a total dose of 113.58 Gyair. Overall, the Cs5Cu3Cl6I2 scintillator prepared by mechanical grinding method has the advantages of low cost, high efficiency, and good radiation resistance, suggesting its huge potential in X-ray detection and imaging.

Zero-Dimensional (Piperidinium)2MnBr4: Ring Puckering-Induced Isostructural Transition and Strong Electron-Phonon Coupling-Mediated Self-Trapped Exciton Emission.

We report on the synthesis, structure, and photophysical properties of a lead-free organic-inorganic hybrid halide, (Piperidinium)2MnBr4 (PipMBr). It crystallizes in a monoclinic P21/n structure, with isolated MnBr4 tetrahedra representing a zero-dimensional compound. It undergoes a reversible isostructural transition at 422/417 K in the heating/cooling cycle owing to the hydrogen-bonding rearrangement mediated by ring puckering of piperidinium cations. This compound exhibits green emission with a photoluminescence quantum yield of 51%. Interestingly, strong electron-longitudinal optical phonon coupling with γLO of 237 meV is evidenced from the broadening of the temperature-dependent emission linewidth and the Raman spectrum. Such strong electron-phonon coupling and a relatively low Debye temperature (137 K) suggest the self-trapped exciton emission in this compound.

Self‐Trapped Excitons in 2D SnP2S6 Crystal with Intrinsic Structural Distortion

AbstractExploring and utilizing novel materials with self‐trapped excitons (STEs) is highly desired due to their unique physical properties and multiple optoelectronic applications. Here, a 2D SnP2S6 crystal is reported with obvious STEs emission caused by distorted [SnS6]8‐ and [P2S6]4‐ octahedral units and strong electron‐phonon coupling. The STEs feature wide photoluminescence (PL) spectra range from 600 to 850 nm and a large Stokes redshift of ≈0.6 eV. Carrier dynamics measurements including temperature‐dependent PL and transient absorption reveal a large Huang–Rhys factor of ≈18.3 and a self‐trapping process of ≈101–103 ps in the 2D SnP2S6 crystal. Such self‐trapped states enable 2D SnP2S6 crystal a wide spectral response range and excellent photodetection performance. As a result, high responsivity (22.8 A W‐1) and detectivity (3.98 × 1010 Jones) are achieved under 365 nm light illumination. These results provide a deep insight into the photophysical process of STEs, which lays the foundation for developing novel STEs‐based materials and optoelectronic devices.

Efficient Broadband Near-Infrared Emission from Lead-Free Halide Double Perovskite Single Crystal.

Ultra-broadband near-infrared (NIR) luminescent materials are the most important component of NIR light-emitting devices (LED) and are crucial for their performance in sensing applications. A major challenge is to design novel NIR luminescent materials to replace the traditional Cr3+ -doped systems. We report an all-inorganic bismuth halide perovskite Cs2 AgBiCl6 single crystal that achieves efficient broadband NIR emission by introducing Na ions. Experiments and density functional theory (DFT) calculations show that the NIR emission originates from self-trapped excitons (STE) emission, which can be enhanced by weakening the strong coupling between electrons and phonons. The high photoluminescence quantum efficiency (PLQY) of 51 %, the extensive full width at half maximum (FWHM) of 270 nm and the stability provide advantages as a NIR luminescent material. The single-crystal-based NIR LED demonstrated its potential applications in NIR spectral detection as well as night vision.

Efficient Er3+ doped single-component Cs2Ag(Na)In(Bi)Cl6 phosphors for full-spectrum light-emitting diodes

Single-component phosphors with efficient and stable white-light emission are designed to overcome the trouble of various deteriorations and reabsorptions in the conventional mixing multiple-component phosphors for lighting applications. Here, Er3+ is introduced partially into the double perovskite Cs2Ag(Na)In(Bi)Cl6 lattice to tune white-light emission through adjusting its content. The as-synthesized material itself possesses strong tunable white-light emission under irradiation by 405 nm light due to the combination of self-trapped excitons emission and efficient characteristic emission of Er3+. Steady-state photoluminescence (PL) and time-resolved PL spectra are measured to characterize luminescence properties and clarify the luminescence mechanism. The PL quantum yield of undoped Cs2Ag(Na)In(Bi)Cl6 can achieve 64%. After doping 2 mmol Er3+, energy transfer efficiency from perovskite to Er3+ can reach 30.1%, with an improved PL quantum yield of 71%. The full-spectrum white-light emission can be realized by the Er3+ doped Cs2Ag(Na)In(Bi)Cl6 phosphors-converted 410–420 nm light-emitting diode. Commission International de I'Eclairage (CIE) color coordinates of white light can be improved from (0.47, 0.41) to (0.33, 0.35) with the increase of Er3+ content, and the corresponding color temperature is tuned from 2532 to 5760 K. These characteristics demonstrate that Er3+ doped Cs2Ag(Na)In(Bi)Cl6 phosphors are promising single-component white-light phosphors for next-generation lighting.

Crystal growth and scintillation properties of Tm3+ doped LaCl3 single crystal for radiation detection

Single crystal of Tm-doped LaCl3 (LaCl3:Tm3+) with the dimension of diameter 7 mm and length 55 mm was successfully grown using an in-house-developed Bridgman furnace. The grown crystal has the self-trapped exciton (STE) emission in the range of 300–500 nm along with the characteristic emissions corresponding to the 4f-4f transition of Tm3+ ions under X-ray excitation. The scintillation characterizations of the grown crystal were performed by pulse height spectra, scintillation decay time, as well as pulse shape discrimination studies under γ-ray and α-particle excitation sources. The LaCl3:Tm3+ crystal was found to have a high light yield (42,000 ± 4,200 ph/MeV) with 4.2% energy resolution under γ-rays excitation. Moreover, the pulse shape discrimination was studied using 137Cs and 241Am as the radiation sources with a good FOM value. The results conclude the LaCl3:Tm3+ single crystal to be a promising candidate for X-ray and γ-ray detection purposes.

A heterovalent doping strategy induced efficient cyan emission in Sb3+-doped CsCdCl3 perovskite microcrystal for solid state lighting

Metal halides can produce a broadband emission spectrum of self-trapped excitons (STEs) with a large stokes shift because they have strong electron-phonon coupling and soft lattice. In this study, CsCdCl3:xSb3+ perovskite microcrystals were prepared using a simple co-precipitation strategy at room-temperature. Their composition, morphology and phase structure were characterized by EDS, XPS, SEM and XRD. The result showed that the structure of the CsCdCl3:xSb3+ perovskite microcrystals was hexagonal. In terms of luminescence, the un-doped CsCdCl3 sample showed a quite broadband emission from the host STEs with a large Stokes shift of 340 nm. The PLE and PL of the CsCdCl3:0.007Sb3+ sample show two absorptions at the wavelength of 300 and 355 nm, which is because of the 1S0 → 1P1 and 1S0 → 3P1 transitions of Sb3+ ions and the enhanced cyan emission from STEs emission of [SbCl6] octahedra induced by a heterovalent doping strategy, respectively. In addition, in the PL spectra, there are two luminescence centers in the CsCdCl3:xSb3+ samples. The emission peak at 500 nm is higher than that at 600 nm, which can be attributed to the STEs of [SbCl6] and the two face-sharing octahedra [Sb2Cl9], respectively. The formation of STEs can be demonstrated by the large S value. The CsCdCl3:xSb3+ samples undergo two thermal processes, with an activation energy of 127 meV (Ea2) at a high temperature and 101 meV (Ea1) at a low temperature. Finally, they have good thermal and anti-water stability, and thus can be possibly applied in white LED with CIE color coordinates of 0.4119 and 0.3897, the CCT of 3227 K and CRI of 79.3.

Light Emission of Self-Trapped Excitons in Inorganic Metal Halides for Optoelectronic Applications.

Self-trapped excitons (STEs) have recently attracted tremendous interest due to their broadband emission, high photoluminescence quantum yield, and self-absorption-free properties, which enable a large range of optoelectronic applications such as lighting, displays, radiation detection, and special sensors. Unlike free excitons, the formation of STEs requires strong coupling between excited state excitons and the soft lattice in low electronic dimensional materials. The chemical and structural diversity of metal halides provides an ideal platform for developing efficient STE emission materials. Herein, an overview of recent progress on STE emission materials for optoelectronic applications is presented. The relationships between the fundamental emission mechanisms, chemical compositions, and device performances are systematically reviewed. On this basis, currently existing challenges and possible development opportunities in this field are presented.

Temperature-Dependent Luminescence and Anisotropic Optical Properties of Centimeter-Sized One-Dimensional Perovskite Trimethylammonium Lead Iodide Single Crystals.

Low-dimensional hybrid halide perovskite materials with self-trapped exciton (STE) emissions and anisotropic properties are highly attractive for their great potential in many applications. However, to date, reports on large one-dimensional (1D) perovskite single crystals have been limited. Here, centimeter-sized 1D single crystals of trimethylammonium lead iodide (TMAPbI3) with typical STE emission are synthesized by an antisolvent vapor-assisted crystallization method. Thermal quenching and antiquenching with a high relative sensitivity of photoluminescence (PL) are observed and studied via temperature-dependent photoluminescence spectroscopy. Further analysis indicates that the temperature-dependent PL behaviors are influenced by the self-trapping of the free exciton and the migrations between self-trapped excitons and intermediate nonradiative states. The TMAPbI3 single crystal also exhibits a linearly polarized emission and a large birefringence that is higher than those of commercial birefringent crystals. This 1D perovskite with high structural anisotropy has promise for applications in versatile optical- and luminescence-related fields.

Blue-emitting 0D Cs3ZnX5 (X = Cl, Br) perovskite nanocrystals based on self-trapped excitons

All-inorganic Pb-free halide perovskites have recently received extensive attention due to their unique structures and optoelectronic properties. However, the realization of pure-blue emitting perovskite nanocrystals (NCs) with satisfactory stability and rational interpretation for their corresponding self-trapped excitons (STEs) emissions remains an arduous task. Herein, a modified hot-injection method is proposed to prepare a new family of zero-dimensional (0D) Cs3ZnX5 (X = Cl, Br) perovskite NCs with pure-blue and sky-blue photoluminescence (PL) emissions. Benefitting from the 0D tetragonal-phase structure, the Cs3ZnX5 NCs possess outstanding long-term stability in ambient air conditions. We employ the experimental results and first-principles calculations to confirm that the blue STEs emission of Cs3ZnX5 NCs is dominated by the halide vacancies (VX) in [ZnX4]2- tetrahedra. Our findings not only provide a simple and valid method to control the synthesis of blue-emitting 0D Zn-based perovskite NCs, but also clarify their underlying PL mechanism.

Small Organic Molecular-Based Hybrid Halides with High Photoluminescence Quenching Temperature.

Organic-inorganic metal halides (OIMHs) exhibit excellent photoelectric properties; however, their high-temperature light-emission stability requires further improvement. Here, we report three isostructural OIMHs (C2H8N)4InCl7, (C2H8N)4SbCl7, and (C2H8N)4SbBr7 (C2H8N+ = dimethylammonium). They are all crystallized in the P21212 space group with a zero-dimensional (0D) structure, with orange-red photoluminescence (PL) under 365 nm UV excitation. Among them, (C2H8N)4InCl7 exhibits the strongest PL with a photoluminescence quantum yield (PLQY) of 13.9% at room temperature. Optical property measurements and density functional theory unveil that the luminescence of (C2H8N)4InCl7 at 405 and 620 nm is due to free exciton and self-trapped exciton emission, respectively. It is worth noting that (C2H8N)4InCl7 shows a high PL quenching temperature, maintaining 50% of its room-temperature PL intensity at 425 K, which is rare in OIHMs. This is much higher than the application temperature of phosphors in practical solid-state lighting applications (363-383 K). In this temperature range, the luminous intensity of (C2H8N)4InCl7 exceeds 60% of that at room temperature. The high PL quenching temperature observed in (C2H8N)4InCl7 indicates the potential of OIMHs for applications in phosphor-converted light-emitting diodes.

Reabsorption-free luminescent solar concentrator based on environmentally friendly cesium copper halide nanocrystals

Luminescent solar concentrator (LSC) based on colloidal nanocrystals (NCs) is the key component of building integrated photovoltaics (BIPVs). But strong reabsorption effect, or expensive/toxic components (e.g., In, Cd, or Pb) included in NCs hinder their applications. In this study, we fabricated low-cost, non-toxic LSCs based on cesium copper halide (Cs3Cu2X5, X = I or Cl) NCs. These materials possessed highly efficient self-trapped exciton emission with a large Stokes shift, with the consequent LSCs exhibiting clear reabsorption-free features with edge efficiencies close to the theoretical limits (∼70%) with varying device sizes. Furthermore, waveguiding PV devices consisting of above LSCs (8 × 8 × 0.5 cm3) and commercial polysilicon PV cells exhibit power conversion efficiencies (PCEs) of up to 1.8%. In addition to the waveguiding effect on luminescent photons, these excellent PCEs were confirmed induced by the scattering of visible solar photons. These results clearly illustrated the photon propagation mechanism in LSCs based on Cs3Cu2X5 NCs or similar colloidal NCs. The obtained PCEs in the range 0.8%–2.0% imply considerable potential for fabricating practical PV products with high output per Si PV unit area and low cost.

Strong Electron–Phonon Coupling in β-Ga2O3: A Huge Broadening of Self-Trapped Exciton Emission and a Significant Red Shift of the Direct Bandgap

The temperature dependence of self-trapped exciton (STE) emission and the optical absorption edge of monoclinic gallium oxide (β-Ga2O3) has been carefully studied. According to this research, it is found that as temperature increases (from 10 to 300 K), the STE and the direct bandgap of β-Ga2O3 exhibit a huge broadening (∼120 meV) and a significant red shift (∼250 meV), respectively. Combined with theoretical analysis, these temperature-dependent change trends are found related to the strong electron-phonon coupling (EPC) effect in crystal, which is caused by the high localization (i.e., self-trapping) of carriers in β-Ga2O3. This finding further indicates that in the transition process of carriers' absorbing and releasing photons, the influence of lattice vibration needs to be considered and described by the configuration coordinate model. The strength of EPC can be measured by the Huang-Rhys factor, which is about S ≈ 9 for β-Ga2O3 with the polar longitudinal optical phonon mode of lower energy (∼31 meV) being involved.

Integrated Afterglow and Self-Trapped Exciton Emissions in Hybrid Metal Halides for Anti-Counterfeiting Applications.

0D hybrid metal halides (0D HMHs) are considered to be promising luminescent emitters. 0D HMHs commonly exhibit self-trapped exciton (STE) emissions originating from the inorganic metal halide anion units. Exploring and utilizing the emission features of the organic cation units in 0D HMHs is highly desired to enrich their optical properties as multifunctional luminescent materials. Here, tunable emissions from organic and inorganic units are successfully achieved in triphenylsulfonium (Ph3 S+ )-based 0D HMHs. Notably, integrated afterglow and STE emissions with adjustable intensities are obtained in (Ph3 S)2 Sn1- x Tex Cl6 (x= 0-1) via the delicate combination of [SnCl6 ]2- and [TeCl6 ]2- . Moreover, such a strategy can be readily extended to develop other HMH materials with intriguing optical properties. As a demonstration, 0D (Ph3 S)2 Zn1- x Mnx Cl4 (x= 0-1) are constructed to achieve integrated afterglow and Mn2+ d-d emissions with high efficiency. Consequently, these novel 0D HMHs with colorful afterglow and STE emissions are applied in multiple anti-counterfeiting applications.

Efficient red photoluminescence in holmium-doped Cs2NaInCl6 double perovskite

Metal halide perovskites with efficient and stable self-trapped exciton emission have attracted considerable attention for next-generation solid-state lighting owing to their ultrabroadband emission. However, it remains challenging to widen the color gamut and realize high color-rendering index due to the lack of highly saturated red color-emitting inorganic lead-free perovskite materials with narrow full width at half-maximum. Here, we report a lanthanide-codoped system (Cs2NaInCl6:Sb3+/Ho3+) that enables efficient blue and red emissions with photoluminescence quantum yields of up to 99% and 72%, respectively. The energy transfer efficiency in Ho3+-doped Cs2NaInCl6:Sb is up to 73%, which can overcome the weak absorption cross-section of the f-f transition of lanthanide ion and improve its luminescence performance. Benefitting from the efficient energy-transfer channel from the self-trapped excitons to Ho3+, the emission color is continuously tuned from blue to red by controlling the Ho3+-doping concentration.

Light emission of self-trapped excitons from ion tracks in silica glass: Interplay between Auger recombination, exciton formation, thermal dissociation, and hopping

The initial luminescence yield of amorphous silica under ion irradiation has been studied at temperatures between 30 and 100 K, using swift ions of different masses and energies (3 MeV H, 3.5 MeV He, 19 MeV Si and 19 MeV Cl). The intensity of the 2.1 eV emission band, ascribed to the intrinsic recombination of self-trapped excitons (STEs), has been found to vary systematically with ion mass, energy and irradiation temperature. A detailed model has been developed to quantitatively describe those variations in terms of the competition between non-radiative Auger recombination, STE formation, STE thermal dissociation, and subsequent STE hopping and capture at non-radiative sinks. The model, which uses a thermal spike approach to describe the effect of swift ion bombardment, is found to quantitatively predict the experimental data without adjustable parameters. It provides new insights into the interactions of carriers in an ion track and the behavior of the luminescence emissions during ion irradiation (ionoluminescence). The model is found to predict the correct temperature dependence of the yield if an activation energy for STE thermal migration of 0.12 eV is assumed, which is in good agreement with values previously reported.

Correlating Symmetries of Low-Frequency Vibrations and Self-Trapped Excitons in Layered Perovskites for Light Emission with Different Colors.

The soft hybrid organic-inorganic structure of two-dimensional layered perovskites (2DLPs) enables broadband emission at room temperature from a single material, which makes 2DLPs promising sources for solid-state white lighting, yet with low efficiency. The underlying photophysics involves self-trapping of excitons favored by distortions of the inorganic lattice and coupling to phonons, where the mechanism is still under debate. 2DLPs with different organic moieties and emission ranging from self-trapped exciton (STE)-dominated white light to blue band-edge photoluminescence are investigated. Detailed insights into the directional symmetries of phonon modes are gained using angle-resolved polarized Raman spectroscopy and are correlated to the temperature-dependence of the STE emission. It is demonstrated that weak STE bands at low-temperature are linked to in-plane phonons, and efficient room-temperature STE emission to more complex coupling to several phonon modes with out-of-plane components. Thereby, a unique view is provided into the lattice deformations and recombination dynamics that are key to designing more efficient materials.

Temperature-Responsive Photoluminescence and Elastic Properties of 1D Lead Halide Perovskites R- and S-(Methylbenzylamine)PbBr3.

Low-dimensional metal halide perovskites (MHPs) have received much attention due to their striking semiconducting properties tunable at a molecular level, which hold great potential in the development of next-generation optoelectronic devices. However, the insufficient understanding of their stimulus-responsiveness and elastic properties hinders future practical applications. Here, the thermally responsive emissions and elastic properties of one-dimensional lead halide perovskites R- and S-MBAPbBr3 (MBA+ = methylbenzylamine) were systematically investigated via temperature-dependent photoluminescence (PL) experiments and first-principles calculations. The PL peak positions of both perovskites were redshifted by about 20 nm, and the corresponding full width at half maximum was reduced by about 40 nm, from ambient temperature to about 150 K. This kind of temperature-responsive self-trapped exciton emission could be attributed to the synergistic effect of electron–phonon coupling and thermal expansion due to the alteration of hydrogen bonding. Moreover, the elastic properties of S-MBAPbBr3 were calculated using density functional theory, revealing that its Young’s and shear moduli are in the range of 6.5–33.2 and 2.8–19.5 GPa, respectively, even smaller than those of two-dimensional MHPs. Our work demonstrates that the temperature-responsive emissions and low elastic moduli of these 1D MHPs could find use in flexible devices.

Boosting the Self-Trapped Exciton Emission in Alloyed Cs2 (Ag/Na)InCl6 Double Perovskite via Cu+ Doping.

Fundamental understanding of the effect of doping on the optical properties of 3D double perovskites (DPs) especially the dynamics of self‐trapped excitons (STEs) is of vital importance for their optoelectronic applications. Herein, a unique strategy via Cu+ doping to achieve efficient STE emission in the alloyed lead‐free Cs2(Ag/Na)InCl6 DPs is reported. A small amount (1.0 mol%) of Cu+ doping results in boosted STE emission in the crystals, with photoluminescence (PL) quantum yield increasing from 19.0% to 62.6% and excitation band shifting from 310 to 365 nm. Temperature‐dependent PL and femtosecond transient absorption spectroscopies reveal that the remarkable PL enhancement originates from the increased radiative recombination rate and density of STEs, as a result of symmetry breakdown of the STE wavefunction at the octahedral Ag+ site. These findings provide deep insights into the STE dynamics in Cu+‐doped Cs2(Ag/Na)InCl6, thereby laying a foundation for the future design of new lead‐free DPs with efficient STE emission.

Sublattice Distortion Enabled Strong Interplay between Phonon Vibrations, Electron-Phonon Coupling, and Self-Trapped Excitonic Emissions in Cs2Ag1-xNaxBiCl6 Double Perovskites.

Sublattice distortion resulting from alloying compositionally distinct double perovskites is shown to influence photoluminescence emission in Cs2Ag1-xNaxBiCl6 (0 < x < 1). The end members show negligible photoluminescence, whereas interestingly the alloys exhibit broad photoluminescence. These emissions are attributed to self-trapped excitons (STE) resulting from sublattice distortions arising due to the mismatch in [AgCl6]5- and [BiCl6]3- octahedra. Change in sublattice distortions plays significant role in the formation and recombination of STEs. The STE emission intensity and quantum yield greatly depend on x, with highest intensity observed for x = 0.75, consistent with a large change in sublattice found at this x. Variation in photoluminescence properties with composition follows a similar trend as that of bandgap and phonon vibrational changes observed due to sublattice distortion. Temperature-dependent phonon vibrations and photoluminescence studies reveal a giant electron-phonon coupling. A strong synergy between STE emissions, electron-phonon coupling, bandgap, and phonon vibrations in double perovskites with sublattice distortions is demonstrated.