X-ray radiation excited ultralong (>20,000 seconds) intrinsic phosphorescence in aluminum nitride single-crystal scintillators

Visualizing temperature inhomogeneity using thermo-responsive smart materials

Despite the substantial progress made, the responsiveness of thermo-responsive materials upon various thermal fields is still restricted to monochromatic visualization with single-wavelength light emission. This stems from a poor understanding of the photophysical processes within the materials and the unvarying optical performance of luminescent centers' response to various ambient temperatures. Conventional techniques to assess the inhomogeneities of thermal fields can be time-consuming, require specialized equipment and suffer from inaccuracy due to the inevitable interference from background signals, especially at high temperature. To this end, we overcome these limitations for the first time, to flexibly visualize temperature inhomogeneities by developing a thermochromic smart material, SrGa12-xAlxO19:Dy3+. Two distinct modes of thermochromic properties (steady-state temperature-dependent luminescence and thermally stimulated luminescence) are investigated. It is revealed that the abundant colors (from yellow, green to red) and amazing color-changing features are due to the superior optical integration of the host (SrGa12-xAlxO19) and dopant (Dy3+) emissions under specific thermal stimulations. We suggest that this thermo-responsive smart material can be used to realize highly efficient and simple visualization of invisible thermal distribution in industry and beyond.

Multispectral Large-Panel X-ray Imaging Enabled by Stacked Metal Halide Scintillators

Conventional energy-integration black-white X-ray imaging lacks the spectral information of X-ray photons. Although X-ray spectra (energy) can be distinguished by the photon-counting technique typically with CdZnTe detectors, it is very challenging to be applied to large-area flat-panel X-ray imaging (FPXI). Herein, multilayer stacked scintillators of different X-ray absorption capabilities and scintillation spectra are designed; in this scenario, the X-ray energy can be discriminated by detecting the emission spectra of each scintillator; therefore, multispectral X-ray imaging can be easily obtained by color or multispectral visible-light camera in a single shot of X-rays. To verify this idea, stacked multilayer scintillators based on several emerging metal halides are fabricated in a cost-effective and scalable solution process, and proof-of-concept multispectral (or multi-energy) FPXI are experimentally demonstrated. The dual-energy X-ray image of a "bone-muscle" model clearly shows the details that are invisible in conventional energy-integration FPXI. By stacking four layers of specifically designed multilayer scintillators with appropriate thicknesses, a prototype FPXI with four energy channels is realized, proving its extendibility to multispectral or even hyperspectral X-ray imaging. This study provides a facile and effective strategy to realize multispectral large-area flat-panel X-ray imaging.

Multicolor ultralong phosphorescence from perovskite-like octahedral α-AlF3

Designing organic fluorescent and phosphorescent materials based on various core fluorophore has gained great attention, but it is unclear whether similar luminescent units exist for inorganic materials. Inspired by the BX₆ octahedral structure of luminescent metal halide perovskites (MHP), here we propose that the BX₆ octahedron may be a core structure for luminescent inorganic materials. In this regard, excitation-dependent color-tunable phosphorescence is discovered from α-AlF₃ featuring AlF₆ octahedron. Through further exploration of the BX₆ unit by altering the dimension and changing the center metal (B) and ligand (X), luminescence from KAlF₄, (NH₄)₃AlF₆, AlCl₃, Al(OH)₃, Ga₂O₃, InCl₃, and CdCl₂ are also discovered. The phosphorescence of α-AlF₃ can be ascribed to clusterization-triggered emission, i.e., weak through space interaction of the n electrons of F atoms bring close proximity in the AlF₆ octahedra (inter/intra). These discoveries will deepen the understanding and contribute to further development of BX₆ octahedron-based luminescent materials.

Unravelling the origin of emission in luminescent inorganic materials is challenging. Here, the authors report that AlF₆ octahedrons exhibit excitation-dependent color-tunable phosphorescence; structurally related compounds are also luminescent.

Direct Determination of Band Gap of Defects in a Wide Band Gap Semiconductor

Crystal defects play an important role in the degradation and failure of semiconductor materials and devices. Direct determination of band gap of defects is a critical step for clarifying how the defects affect the physical properties of semiconductors. Here, high-quality aluminum nitride (AlN) thin films were grown epitaxially on single-crystal Al₂O₃ substrates via pulsed laser deposition. The atomic structure and band gap of three types of inversion domain boundaries (IDBs) in AlN were determined using aberration-corrected transmission electron microscopy and atomic-resolution valence electron energy-loss spectroscopy. It was found that the band gap of all of the IDBs reduces evidently compared to that of the bulk AlN. The maximum band gap reduction of the IDBs is 1.0 eV. First-principles calculations revealed that the band gap reduction of the IDBs is mainly due to the rise of p_z orbital at the valence band maximum, which originates from the elongated Al-N bonds along the [0001] direction at the IDBs. The successful band gap determination of defects paves an avenue for quantitatively evaluating the effect of defects on the performance of semiconductor materials and devices.

Single-Crystalline Perovskite Nanowire Arrays for Stable X-ray Scintillators with Micrometer Spatial Resolution

X-ray scintillation detectors based on metal halide perovskites have shown excellent light yield, but they mostly target applications with spatial resolution at the tens of micrometers level. Here, we use a one-step solution method to grow arrays of 15-μm-long single-crystalline CsPbBr₃ nanowires (NWs) in an AAO (anodized aluminum oxide) membrane template, with nanowire diameters ranging from 30 to 360 nm. The CsPbBr₃ nanowires in AAO (CsPbBr₃ NW/AAO) show increasing X-ray scintillation efficiency with decreasing nanowire diameter, with a maximum photon yield of ∼5 300 ph/MeV at 30 nm diameter. The CsPbBr₃ NW/AAO composites also display high radiation resistance, with a scintillation-intensity decrease of only ∼20-30% after 24 h of X-ray exposure (integrated dose 162 Gy_air) and almost no change after ambient storage for 2 months. X-ray images can distinguish line pairs with a spacing of 2 μm for all nanowire diameters, while slanted edge measurements show a spatial resolution of ∼160 lp/mm at modulation transfer function (MTF) = 0.1. The combination of high spatial resolution, radiation stability, and easy fabrication makes these CsPbBr₃ NW/AAO scintillators a promising candidate for high-resolution X-ray imaging applications.

Nanosecond and Highly Sensitive Scintillator Based on All-Inorganic Perovskite Single Crystals

The scintillator is a unique class of luminescent materials, which is of great significance in clinical diagnosis, security inspection, and radiation detection. Herein, an all-inorganic Cs₄PbI₆ single crystals (SCs) as a nanosecond and an efficient X-ray and α particle scintillator is described. The radioluminescence (RL) spectrum of Cs₄PbI₆ SCs under X-ray excitation consists of a band gap emission at 310 nm and a broadband emission at 552 nm at room temperature. Furthermore, Cs₄PbI₆ SCs demonstrate nanosecond decay times of 0.95 and 6.86 ns, a high sensitivity to low-energy X-ray (30 keV) with a low detection limit (187 nGy_air/s), and a favorable linearity detection range, potentially enabling their broad application in X-ray imaging. Under ²³⁷Np α particle irradiation, the light yield of Cs₄PbI₆ SCs is about 49.5% of that of a BGO scintillator with an energy resolution of 35% at 4.78 MeV. Our results demonstrate the potential of Cs₄PbI₆ SCs as a nanosecond and low-cost scintillator in radiation detection applications.

Effect of Substrate and Thickness on the Photoconductivity of Nanoparticle Titanium Dioxide Thin Film Vacuum Ultraviolet Photoconductive Detector

Vacuum ultraviolet radiation (VUV, from 100 nm to 200 nm wavelength) is indispensable in many applications, but its detection is still challenging. We report the development of a VUV photoconductive detector, based on titanium dioxide (TiO₂) nanoparticle thin films. The effect of crystallinity, optical quality, and crystallite size due to film thickness (80 nm, 500 nm, 1000 nm) and type of substrate (silicon Si, quartz SiO₂, soda lime glass SLG) was investigated to explore ways of enhancing the photoconductivity of the detector. The TiO₂ film deposited on SiO₂ substrate with a film thickness of 80 nm exhibited the best photoconductivity, with a photocurrent of 5.35 milli-Amperes and a photosensitivity of 99.99% for a bias voltage of 70 V. The wavelength response of the detector can be adjusted by changing the thickness of the film as the cut-off shifts to a longer wavelength, as the film becomes thicker. The response time of the TiO₂ detector is about 5.8 μs and is comparable to the 5.4 μs response time of a diamond UV sensor. The development of the TiO₂ nanoparticle thin film detector is expected to contribute to the enhancement of the use of VUV radiation in an increasing number of important technological and scientific applications.

Fermi-Surface Modulation of Graphene Synergistically Enhances the Open-Circuit Voltage and Quantum Efficiency of Photovoltaic Solar-Blind Ultraviolet Detectors

Increasing the open-circuit voltage (V_OC) is of a great significance to achieve high photoelectric conversion efficiency in photovoltaic applications. Here, we present a simple NO₂ doping strategy that can significantly modulate the V_OC of graphene-based solar-blind ultraviolet photodetectors from 0.96 to 1.84 V. The intriguing result can be demonstrated by the fact that NO₂ doping lowers the Fermi surface of graphene and thus enhances quasi-Fermi level splitting of the whole device under illumination. The >103% increase of both external quantum efficiency and photoresponsivity compared to before doping is the result of a 0.88 V increase in the V_OC. Our work sheds light on the forming mechanism of V_OC in graphene-based photovoltaic detectors and further suggests alternative pathways to enhance the V_OC of photovoltaic devices with high efficiency.

High-Efficiency Down-Conversion Radiation Fluorescence and Ultrafast Photoluminescence (1.2 ns) at the Interface of Hybrid Cs4PbBr6-CsI Nanocrystals

The research of fast scintillators in positron emission tomography and other applications based on time-of-flight technology promotes the development of radiation detection. However, because of the current lack of efficient and fast carrier radiation recombination pathways, the research on scintillator radioluminescence (RL) still faces severe challenges. Here, we propose an effective interface carrier transport mechanism: CsI:Na crystal and Cs₄PbBr₆ nanocrystals (NCs) interface to form a new phase and a continuous heterostructure, providing an effective channel for X-ray excited carrier transfer to Cs₄PbBr₆. Then, the excited carriers realize efficient recombination luminescence through the self-trapped excitons inside Cs₄PbBr₆. On the basis of this mechanism, the heterostructure composite scintillator composed of CsI:Na/Cs₄PbBr₆ exhibits high-efficiency radiant fluorescence and an ultrafast photoluminescence (PL) decay time of 1.22 ns. The effective interface carrier transport shown in this work provides an optimization idea that can be used for reference in the research of fast scintillators.

High-Pressure O2 Annealing Enhances the Crystallinity of Ultrawide-Band-Gap Sesquioxides Combined with Graphene for Vacuum-Ultraviolet Photovoltaic Detection

(Al_xGa_1-x)₂O₃ is emerging as a promising wide-band-gap sesquioxide for vacuum-ultraviolet (VUV, 10-200 nm) photodetectors and high-power field-effect transistors. However, how the key parameters such as the band gap and crystalline phase of the (Al_xGa_1-x)₂O₃-based device vary with stoichiometry has not been explicitly defined, which is due to the unclear underlying mechanism of the Al local coordination environment. In this work, a high-pressure O₂ (20 atm) annealing (HPOA) strategy that can significantly improve the crystallinity of β-(Al_xGa_1-x)₂O₃ and achieve a tunable optical band gap was proposed, facilitating the revelation of the local structure of Al³⁺ varying with Al content and the kinetic mechanism of Al³⁺ diffusion. By combining the as-HPOA-treated single-crystalline β-(Al_0.69Ga_0.31)₂O₃ films with p-type graphene (p-Gr), which serves as a transparent conductor, a VUV photovoltaic detector is fabricated, showing an improved photovoltage (0.80 V) and fast temporal response (2.1 μs). All of these findings provide a rewarding and important strategy for enhancing the band-gap tunability of sesquioxides, as well as the flexibility of zero-power-consumption photodetectors.

Experimental Evidence on Stability of N Substitution for O in ZnO Lattice

Although the dispute remains, the N substitution for the lattice O (N_O) in zinc oxide (ZnO) demonstrates the promising future in achieving the p-type ZnO-based semiconductor. In this context, a highly crystallized N-doped ZnO (ZnO:N) film is fabricated with ultralow defect density. Based on the synchrotron radiation X-ray absorption near-edge structure (XANES) and low-temperature photoluminescence (PL) spectra combined with first-principles calculations, the results demonstrate that the majority of N ions locate stably at the lattice O site to succeeding the N substitution for lattice O as the N_O defects. A prototype LED device is built based on the homojunction of ZnO:N film and ZnO:Ga wafer with good electroluminescence performance. These important findings provide a rewarding avenue to the p-type ZnO semiconductor design and device fabrication, and demonstrate a prevailing guidance on the materials design and development as well.

Quasiphonon polaritons

Mid-infrared reflection spectra of c- and m-plane bulk AlN show a reststrahlen band related to the formation of phonon polaritons. However, it is worth noting that there are additional hump- and spike-shaped peaks in the spectra, which cannot be explained by the phonon-polaritons model applicable to optically isotropic crystals. Here, considering the existence of quasiphonons in wurtzite crystals, we suppose that the extra peaks result from the generation of quasiphonon polaritons (QPPs) induced by the coupling between photon and quasi-transverse optical phonon. On the basis of this point, a QPPs model applicable to optically anisotropic wurtzite crystals is developed, which successfully explains the reststrahlen band of bulk AlN. Besides, on the ground of our model, a series of reststrahlen band of bulk AlN under various configurations is also predicted and presented.