Recombination Of Self-trapped Excitons Research Articles

Pressure-induced abnormal blue-shift emission enhancement and phase transition of perovskite DMAPbI3

Designing low-dimensional hybrid organic–inorganic perovskites (HOIPs) with enhanced broadband emission under ambient conditions remains a pressing challenge. Here, interesting results of emission blueshift and enhancement were successfully achieved by pressure modulation of the (NH2(CH3)2PbI3 ((NH2(CH3)2 = DMA) structure. The observed rapid blueshifted emission is closely related to the distortion of PbI6 octahedron induced by phase transition from the P63/mc phase to the P1 phase at 0.8 GPa, which has been confirmed by in situ high-pressure PL, UV–vis absorption, synchrotron XRD and Raman spectroscopy. The pressure-induced PL enhancement of DMAPbI3 can be ascribed to the mechanism that the distorted PbI6 octahedrons promote the radiative recombination of self-trapped excitons by increasing the activation barrier energy for detrapping. The PL intensity enhancement was retained at 4 times the initial value after the pressure was released. These findings deepen the understanding of the structure-property relationships of one-dimensional perovskite and reveal the roles of pressure in regulating the optical properties of perovskites.

Small Polaron Dynamics in Transition Metal Oxide Photoelectrodes

The transition metal oxides are widely used in many optoelectronic applications, such as working as charge-transporting layers in solar cells and photoelectrodes in light-driven water-splitting. Considering the ionic bonding nature, the lattice of these oxides can be polarized by the charge carriers due to the strong electron-phonon coupling, giving rise to lattice distortion that fosters a local potential well. This lattice distortion together with the trapped carrier is regarded as a quasi-particle, i.e., a polaron. Here, the photocarrier dynamics in transition metal oxides were investigated using the transient absorption and time-resolved terahertz spectroscopies. We identified the spectral features of small polarons, self-trapped excitons, and free carriers and extracted their dynamics independently. We proposed a molecular model that could successfully explain the non-barrier self-trapping and quantitatively describe the temperature-dependent recombination of self-trapped excitons and the dopant effect on the photocarrier dynamics. Our model also suggested that the recombination of self-trapped excitons at room temperature was dominated by nuclear quantum tunneling. This model also predicted the micro-heterogeneity of the self-trapped species, and thus be able to quantitatively describe the Auger annihilation dynamics of self-trapped excitons.

Fabrication and luminescence properties of (K,Rb)2CuCl3 single crystal scintillators with one-dimensional structures

(K,Rb)2CuCl3 single crystal scintillators with one-dimensional structures were successfully fabricated by the slow cooling method, and their luminescence properties were evaluated. Broad photoluminescence (PL) peaks were observed at 350–500 nm, and the highest PL quantum yields were 96.4 % for (K0.5,Rb0.5)2CuCl3 among the samples. Under X-ray irradiation, scintillation peaks attributed to the recombination of self-trapped excitons were observed within the range of 350–500 nm. Both PL and scintillation decay time constants were obtained to be 10–15 μs. The best afterglow levels at 20 ms after X-ray irradiation for 2 ms were 5 ppm for (K0.75,Rb0.25)2CuCl3. The highest scintillation light yield was 16,000 photons/MeV for (K0.75,Rb0.25)2CuCl3 under 137Cs γ-rays (662 keV) irradiation. The results indicate that (K,Rb)2CuCl3 single crystals are promising as scintillators with high LY, low afterglow, and no hygroscopicity.

Dual-modal optical temperature sensing based on Sb3+/Mn2+ co-doped Cs2NaYCl6 double perovskites

Optical temperature sensing based on luminescence intensity ratio shows significant potential in achieving accurate temperature detection. In this work, Sb3+/Mn2+ co-doped Cs2NaYCl6 double perovskites were synthesised via a hydrothermal method. Introducing Sb3+ ions to the host significantly enhances the radiative recombination of self-trapped excitons (STEs), resulting in a broad blue emission. The co-doping of Mn2+ ions to the composition leads to a tunable dual-emission in the spectra, originating from the energy transfer between the STEs and Mn2+ ions. The dependence of the dual-emission on temperature is employed for optical temperature sensing. The maximum relative sensitivity, optimal thermal resolution, and optimal thermal repeatability are determined as 1.40 % K−1, 0.002 K and 99.3% at 250 K, respectively. The results indicate that the synthesised Cs2NaYCl6: Sb3+/Mn2+ double perovskites are promising candidates for high-sensitivity optical thermometers.

Crystal Growth, Photoluminescence and Radioluminescence Properties of Ce3+-Doped Ba3Y(PO4)3 Crystal

Inorganic scintillation crystals have been widely used in applications of high-energy physics, nuclear medical imaging, industrial nondestructive inspection, etc. In this work, a single crystal Ba3Y(PO4)3 (BYP) with 1.0 at% Ce3+-doping concentration was first grown by the Czochralski method, and the electronic structure was calculated using first principles based on density functional theory. In addition, a series of Ce3+-doped BYP phosphors were synthesized, and the fluorescence emission under UV excitation was measured through low-temperature spectroscopy, containing double-peaked emission from 5d–4f transition and self-trapped exciton recombination. A comparison of the UV and X-ray-excited fluorescence spectra reveals the existence of oxygen vacancies as well as F+ centers in the crystal. The air annealing of the crystal effectively reduces the thermoluminescence defects but reduces the emission intensity under UV or X-ray excitation. The BYP:Ce crystal shows a fast decay lifetime of 15.5 ns, and the fast component is as short as 8 ns. The results show that the Ce3+-doped BYP crystal has potential as a kind of scintillator with fast decay properties.

Pressure-triggered intense natural white emission via controllable distortions of antimony chloride dimers

Zero-dimensional (0D) hybrid metal halides (HMHs), featured with unique self-trapped exciton (STE) emissions, are emerging photoluminescence (PL) materials for photoelectronic applications. Compared with metal-halogen monomers, dimeric units in 0D HMHs possess more structural flexibility and less spatial confinement of excitons, creating new opportunities for PL engineering. Herein, high pressure is employed on 0D (C3H12N2)2Sb2Cl10 to controllably distort inter- and intra-octahedral structures within [Sb2Cl10]4− dimers. Intense natural white emission is achieved with dramatically increased photoluminescence quantum yield from less than ≈1% to 74.2 %. The high-pressure emission of (C3H12N2)2Sb2Cl10 is proven to originate from the STE recombination from triplet states with varied emission equilibrium, as well as the promoted excitonic trapping with restrained nonradiative recombination. This work offers valuable insights into the structure effects on STE emission and the transition mechanisms of metal-halogen dimers, which are crucial for developing novel metal halides for illumination.

Br-substituent effect on photoluminescence and scintillation properties of (C6H5C2H4NH3)2PbCl4 perovskite-type compounds

The photoluminescence and scintillation properties of n-bromophenethylammonium lead chloride ((n-BrPEA)2PbCl4) (n = 4, 3, and 2) and phenethylammonium lead chloride ((PEA)2PbCl4) have been investigated. All the samples showed broad emission peaks at 400–700 nm upon excitation by UV light or X-ray. The decay time constants of 8–28 ns were obtained which originated from the recombination of self-trapped exciton (STE) within inorganic layers. The afterglow levels at 20 ms after 2 ms of X-ray irradiation of (4-BrPEA)2PbCl4 and (3-BrPEA)2PbCl4 (76 and 55 ppm) were better than (PEA)2PbCl4 (379 ppm). The light yield of (4-BrPEA)2PbCl4 (3100 photons/5.5 MeV-α) was higher than (PEA)2PbCl4 (1400 photons/5.5 MeV-α).

Blue light emissive zero-dimensional hybrid cadmium bromide as fluorescence sensor toward benzaldehyde

Organic-inorganic hybrid low-dimensional metal halides have been widely studied as a new class of luminescence materials due to abundant structural diversity and intriguing photophysical property. Herein, we explored two new Cd-based 0D halides of [BHEPZ]CdBr4 (BHEPZ = 1,4-Bis(2-hydroxyethyl) piperazine) and [EPIPZ]CdBr4 (EPIPZ = 1-ethylpiperazine) through facile wet-chemistry method. These 0D halides exhibit blue light emissions derived from radiative recombination of self-trapped excitons (STEs) based on detailed spectroscopy investigations. More interestingly, [BHEPZ]CdBr4 exhibits rapid luminescence off switching toward benzaldehyde but fails in any other organic solvents. In-depth investigations demonstrate that the detection limit can fall to 2.65 ppm indicating perfect real-time fluorescent probe to detect benzaldehyde with high sensitivity and selectivity. This work enriches the cadmium halide materials and provide a new perspective for potential applications of 0D hybrid metal halides.

Synthesis of Metastable Silver-Lanthanide Double Perovskite Nanocrystals with White-Light Emission.

Four silver-lanthanide double perovskite nanocrystals, namely, Cs2AgSmCl6, Cs2AgEuCl6, Cs2AgGdCl6, and Cs2AgErCl6, were synthesized for the first time. These four double perovskites have yet to be reported in any form and are found to be metastable in the bulk phase. By using the colloidal hot-injection synthesis method, the metastable double perovskite phase can be arrested kinetically. These lead-free nanocrystals show a broadband emission owing to the self-trapped exciton recombination, with Cs2AgEuCl6 exhibiting the characteristic f-f emission from Eu3+.

Radiation-induced luminescence and photoluminescence properties of (K,Rb)2CuBr3 crystals synthesized by the slow cooling method

(K,Rb)2CuBr3 crystals were fabricated by the slow cooling method, and their photoluminescence (PL) and scintillation properties were examined. Emission bands were observed at 350–550 nm under UV and X-ray irradiation. PL quantum yields of K2CuBr3, (K0.25,Rb0.75)2CuBr3, (K0.5,Rb0.5)2CuBr3, (K0.75,Rb0.25)2CuBr3, and Rb2CuBr3 were respectively 75.8, 71.0, 82.3, 96.9, and 96.3%. The scintillation decay curves were approximated by a sum of three exponential functions, and the decay time constants due to recombination of self-trapped excitons were 58–63 μs. The afterglow levels of the K and Rb-mixed samples at 20 ms passed after X-ray irradiation decreased to 10–40 ppm, compared with those of K2CuBr3 and Rb2CuBr3. Among these samples, (K0.75, Rb0.25)2CuBr3 showed the highest light yield of 7400 ph/MeV among the samples calculated from the pulse height spectra of 137Cs γ-rays (662 keV).

Composition engineering of lead-free double perovskites towards efficient warm white light emission for health and well-being

Lead-free halide double perovskites (DPs) [A2B(I)B(III)X6] (A: monovalent cation, B(I): monovalent cation, B(II): trivalent cation, X: halide anion) has drawn tremendous attention because of their environmental-friendliness and high stability compared to traditional lead-based perovskites ABX3 (A: monovalent cation; B: Pb2+; X: halide anion). The optoelectronic properties of DPs are, however, compromised. This work provides a methodology for improving the photophysical properties of DPs and achieving high-efficiency warm white light emission. First principles density functional theory (DFT) was used in material design and the ligand-free synthesis of halide DP Cs2Ag1−xKxIn0.875Bi0.125Cl6 (x ≤ 0.60) via a facile recrystallization process was demonstrated. The compositional tuning of K+ to Ag+ resulted in an improvement in photophysical properties. Independent of K+ content, all the samples exhibit strong absorption at a wavelength of λmax ∼ 375 nm with a direct bandgap which is consistent with our DFT calculation results. Broadband photoluminescence (PL) emission profiles covering the spectra from blue to deep red (405–820 nm at λmax ∼ 629 nm) were observed. Besides, with an optimized K+ alloying content of 0.20, the optical performance of these halide DPs was impressively regulated, demonstrating improved photoluminescence quantum yield (PLQY) of host Cs2AgInCl6 crystals from ∼ 2.70 % (x = 0) to ∼15.96 % (x = 0.20), due to increased radiative recombination of self-trapped excitons. the highest report for such structures. It also shows high stability over 180 days. Furthermore, the use of blue-emitting CsPbBr3 and DP Cs2Ag0.80K0.20In0.875Bi0.125Cl6 as color conversion layers in white light-emitting diodes (LEDs) demonstrates that high-quality white light emission with the CIE color coordinate of (0.387, 0.385) and a correlated color temperature (CCT) of 3878 K, color rendering index (CRI) of 85, and luminous efficacy of radiation (LER) of 214 lm/W. Hence, the research work provides insight into realizing environmentally friendly and high-efficiency white light emission with high color quality by using earth-abundant elements and the low-cost synthesis method.

Growth of β-Ga2O3 Single-Crystal Microbelts by the Optical Vapor Supersaturated Precipitation Method

Monoclinic β-Ga2O3 microbelts were successfully fabricated using a one-step optical vapor supersaturated precipitation method, which exhibited advantages including a free-standing substrate, prefect surface, and low cost. The as-grown microbelts possessed a well-defined geometry and perfect crystallinity. The dimensions of individual β-Ga2O3 microbelts were a width of ~50 μm, length of ~5 mm, and thickness of ~3 μm. The SEM, XRD, HRTEM, XPS, and Raman spectra demonstrated the high single-crystalline structure of β-Ga2O3 microbelts. Twelve frequency modes were activated in Raman spectra. The optical band gap of the β-Ga2O3 microbelt was calculated to be ~4.45 eV. Upon 266 nm excitation, 2 strong UV emissions occurred in photoluminescence spectra through the radiative recombination of self-trapped excitons, and the blue emission band was attributed to the presence of donor-acceptor-pair transition. The individual β-Ga2O3 microbelt was employed as metal-semiconductor-metal deep-ultraviolet photodetector, which exhibits the photoresponse under 254 nm. This work provides a simple and economical route to fabricate high-quality β-Ga2O3 single-crystal microbelts, which should be a potential synthetic strategy for ultra-wide bandgap semiconductor materials.

Pressure Effect on Self-Trapped Exciton and Dopant Energy Transfer in Neodymium-Doped CsPbBr3 Perovskite Nanocrystals

The CsPbBr3 nanocrystal is a highly attractive light emitter with outstanding monochromaticity and good thermal stability and has garnered tremendous interest in its application in luminescent materials and devices. Here, we propose a novel strategy to improve the luminescence intensity of CsPbBr3 nanocrystals by doping with Nd3+ ions and introducing pressure. Under high pressure, the CsPbBr3:Nd nanocrystals show an abnormal enhancement of the photoluminescence intensity from 0.19 to 0.34 GPa. The time-resolved photoluminescence measurements reveal that pressure facilitates the recombination of self-trapped excitons and suppresses the recombination of the Nd energy levels. This discovery shows that pressure can modulate the photoluminescence kinetics of CsPbBr3:Nd nanocrystals. This work deepens the understanding of the influence of high pressure on rare-earth-doped luminescent materials and provides insight to improve the capabilities of optical devices.

High-resolution flexible X-ray luminescence imaging enabled by eco-friendly CuI scintillators.

Solution-processed scintillators hold great promise in fabrication of low-cost X-ray detectors. However, state of the art of these scintillators is still challenging in their environmental toxicity and instability. In this study, we develop a class of tetradecagonal CuI microcrystals as highly stable, eco-friendly, and low-cost scintillators that exhibit intense radioluminescence under X-ray irradiation. The red broadband emission is attributed to the recombination of self-trapped excitons in CuI microcrystals. We demonstrate the incorporation of such CuI microscintillator into a flexible polymer to fabricate an X-ray detector for high-resolution imaging with a spatial resolution up to 20 line pairs per millimeter (lp mm-1), which enables sharp image effects by attaching the flexible imaging detectors onto curved object surfaces.

Barrierless Exciton Self‐Trapping and Emission Mechanism in Low‐Dimensional Copper Halides

AbstractLow‐dimensional copper halides having nontoxic elements are attracting increasing attention for their peculiar emission properties. Self‐trapped excitons (STEs) account for their high photoluminescence quantum yields (PLQYs) with emission that can stretch across the entire visible spectrum. However, intrinsic factors that influence the formation or loss of the emissive species in low‐dimensional copper halides remain elusive. Here, a comprehensive study on the STE formation dynamics of one‐dimensional CsCu2I3 and zero‐dimensional Cs3Cu2I5 is presented. It is found from STE kinetic analysis that a slower STE formation demonstrated by the 1D structure is not hindered by a potential barrier, but instead related to the number of phonons released in the self‐trapping process. It is further revealed that in 1D CsCu2I3, the non‐radiative recombination of STEs mainly occurs via the intersection between the STE state and the ground state in the configuration coordinate diagram, placing an intrinsic limit on the PLQY at room temperature. These findings show that the STE formation is affected by both the self‐trapping depth and the phonon energy as opposed to a potential barrier in low‐dimensional copper halides. The better understanding of STE formation and recombination processes provide basis for improving design and performance for broadband light emitting devices.

Yb/Er: Cs2Ag(In/Bi)Cl6 lead-free double perovskite for dual-modal optical temperature sensing

Exploring new optical thermometric materials is important to realize highly accurate temperature detecting. Herein, Yb/Er activators are successfully doped into Cs2Ag(In/Bi)Cl6 lead-free double perovskite (DP) semiconductor, providing dual-modal photoluminescence originated from self-trapped exciton recombination and 4f-4f transitions of Yb/Er dopants as well as Er3+ upconversion emissions. Remarkably different temperature-dependent emissive behaviors from DP host and Yb/Er activators are experimentally demonstrated, which enables to design novel fluorescence intensity ratio (FIR) between DP and Yb/Er based thermometric technology. In addition, upconversion FIR-based temperature sensing via thermal population between the 2H11/2 level and 4S3/2 one of Er3+ dopants in DP host is also realized. It is believed that the present Yb/Er: Cs2Ag(In/Bi)Cl6 DP with excellent long-term stability and dual-functional temperature detecting is a suitable candidate as the self-calibrated optical thermometric material.

Pressure-induced emission enhancement and bandgap narrowing: Experimental investigations and first-principles theoretical simulations on the model halide perovskite Cs3Sb2Br9

We report high-pressure photoluminescence, Raman scattering, and x-ray diffraction measurements on a lead-free halide perovskite ${\mathrm{Cs}}_{3}{\mathrm{Sb}}_{2}{\mathrm{Br}}_{9}$. At about 3 GPa, an electronic transition manifests itself through a broad minimum in linewidth, a maximum in the intensity of ${E}_{g}$, ${A}_{1g}$ Raman modes, and the unusual change in the $c/a$ ratio of the trigonal lattice. The large compressibility and observed Raman anomalies indicate to a soft material with strong electron-phonon coupling. The observed below-bandgap broadband emission in the photoluminescence measurement indicates the recombination of self-trapped excitons. The initial blueshift of the photoluminescence peak reinforces itself to the redshift at around 3 GPa due to the change in the electronic landscape. A first-order trigonal to a monoclinic structural transition is also seen at 8 GPa. The first-principles density functional theory (DFT) calculations reveal that the electronic transition is associated with direct-to-indirect bandgap transition due to changes in the hybridization of $\text{Sb-5}s$ and $\text{Br-4}p$ orbitals near the Fermi level in the valence band. The experimentally observed Raman modes are assigned to their symmetry using the density functional perturbation theory. In addition, the DFT calculations predict a 27.5% reduction of the bandgap in the pressure range 0--8 GPa.

Compact ultrabroadband light-emitting diodes based on lanthanide-doped lead-free double perovskites

Impurity doping is an effective approach to tuning the optoelectronic performance of host materials by imparting extrinsic electronic channels. Herein, a family of lanthanide (Ln3+) ions was successfully incorporated into a Bi:Cs2AgInCl6 lead-free double-perovskite (DP) semiconductor, expanding the spectral range from visible (Vis) to near-infrared (NIR) and improving the photoluminescence quantum yield (PLQY). After multidoping with Nd, Yb, Er and Tm, Bi/Ln:Cs2AgInCl6 yielded an ultrabroadband continuous emission spectrum with a full width at half-maximum of ~365 nm originating from intrinsic self-trapped exciton recombination and abundant 4f–4f transitions of the Ln3+ dopants. Steady-state and transient-state spectra were used to ascertain the energy transfer and emissive processes. To avoid adverse energy interactions between the various Ln3+ ions in a single DP host, a heterogeneous architecture was designed to spatially confine different Ln3+ dopants via a “DP-in-glass composite” (DiG) structure. This bottom-up strategy endowed the prepared Ln3+-doped DIG with a high PLQY of 40% (nearly three times as high as that of the multidoped DP) and superior long-term stability. Finally, a compact Vis–NIR ultrabroadband (400~2000 nm) light source was easily fabricated by coupling the DiG with a commercial UV LED chip, and this light source has promising applications in nondestructive spectroscopic analyses and multifunctional lighting.

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.

Investigating the Influence of Bath Temperature on the Chemical Bath Deposition of Nanosynthesized Lead Selenide Thin Films for Photovoltaic Application

Thin films of CBD are formed on metal surfaces from an aqueous solution containing Pb(NO3)2 and Na2O4Se. The impact of the bath temperature upon lead selenide NPs is analyzed. The evaluation of X-ray diffraction demonstrates that the produced NPs were polycrystalline with (111) orientation. The morphological analysis of the surface shows that the grains are spherical gemstones. As the bath temperature was raised from 20 to 85 degrees Celsius, the energy bandgap decreased from 2.4 to 1.2 eV, indicating a reduction in the band gap. Micron-sized nanoparticles produced at 85 degrees Celsius exhibited the best crystallinity and were uniformly spread across the surface of the substrate with excellent particle sizes. If the solution bath temperature increases from 20°C to 85°C, the average strength of PL decreases. The maximum photoluminescence strength is predominantly because of self-trapped exciton recombination, formed from O2 vacancy and particle size called defect centers, for the deposited thin films at 45°C and 85°C. The photoluminescence intensity rises sequentially with all temperatures. Therefore, the finest solution temperature is 85 degrees Celsius.