The scintillator is a luminescence material that absorbs the energy of invisible high-energy radiation and emits UV-visible photons, and has been widely used for medical imaging, nondestructive testing, home security, and scientific research [1–5]. For high-energy photon detection, the absorption energy is strongly dependent on the density, atomic number, and atomic mass, and therefore, the scintillator is expected to have a high Z component.

During the past decades, a series of inorganic materials (such as NaI, CsI, BGO, BaF₂) has been used for gamma-ray detection [6,7]. Among them, alkali-halide scintillators [NaI(Tl), CsI(Tl or Na), SrI2] have achieved great success for commercial applications. NaI(Tl) crystals have a high light output, and emission photons match well with the sensitivity curve of photomultiplier tubes (PMTs) with bialkali photocathodes [8]. CsI crystals have a high density for high gamma-ray stopping power [9]. The physical properties of CsI crystals are dependent on the activator: the emission peak of CsI(Na) crystal at 420 nm matches well with a bialkali photomultiplier; the emission peak of CsI(Tl) crystal at 550 nm is suitable for photodiode readout. However, several issues limit the applications: (1) high cost: growing these crystals is usually done with the Bridgman method, which requires a highly expensive furnace; (2) small Stokes shift, which causes overlapping between light emission and absorption; and (3) stability: these crystals are highly hygroscopic, which is less user friendly [10].

Recently, due to the tunable emission wavelength, high photoluminescence quantum yield (PLQY), and easy synthesis, a series of halide perovskite nanocrystals has been used for X-ray detection [11–15]. These low-dimensional nanoscintillators are ultrasensitive to low-energy X-ray photons with a low detection limit, which can be beneficial for X-ray imaging [16–18]. However, they are not thick enough for high-energy gamma-ray spectroscopy applications. Therefore, a bulk scintillator with low-cost, high-stability, no reabsorption, fine energy resolution, and user friendliness, is urgently needed [19,20]. Recently, a reabsorption-free 0D Cs3Cu2I5 perovskite single crystal with large stopping power was used for gamma-ray detection; however, researchers now are focusing on the high-cost melt growth method [21,22].

Here, we address the issues (high cost, being hygroscopic, user friendliness, self-absorption) for both bulk and alkali-halide nanoscintillators by developing a bulk Cs3Cu2I5 single crystal scintillator using the solution growth method with excellent features such as low cost, large stopping power, high stability, and being reabsorption free. We synthesize the bulk Cs3Cu2I5 single crystal using a simple solution-processed method with the highest temperature lower than 85°C. Its unique Cs+ surrounded isolated [Cu2I5]3− clusters 0D structure corresponds to remarkable stability under various atmospheres. The self-trapped exciton (STE) emission mechanism ascribes to the high PLQY (93.5%, excited with 326 nm laser) at room temperature and large Stokes shift from the absorption spectra. Our results demonstrate that the unique properties of 0D Cs3Cu2I5 single crystal provide a wide scintillation application, especially for high-energy gamma-ray spectroscopy, which requires a scintillator with features such as low cost and being reabsorption free and user friendly.

Dimethyl sulfoxide (DMSO), dimethylformamide (DMF), CsI (99.9%), and CuI (99.5%) were purchased from Aladdin. All chemicals were used without further purification. CsI (0.06 mol) and CuI (0.04 mol) were dissolved in DMSO (12 mL) and DMF (8 mL) solvent at 55°C with continuous stirring for 8 h, yielding a black solution. This solution was filtered with a 5 μm polytetrafluoroethylene (PTFE) filter, and gradually heated to 80°C with 1°C/h. Then, the crystal was harvested from the bottom of the vial. Finally, a clear and transparent crystal was obtained after it was washed with DMSO and DMF.

Powder X-ray diffraction (XRD) patterns were recorded by Panalytical X’pert PRO with a Cu-Kα radiation source. X-ray photoelectron spectroscopy (XPS) was performed on a Kratos Axis ULTRA DLD XPS system with a monochromatic Al Kα source (hν=1486.6eV). Optical absorption spectra were measured by a UV-Vis spectrophotometer (Shimadzu UV-2550 spectrometer) at the wavelength of 300–700 nm. Photoluminescence (PL) emission, PL decay time, and PLQY were investigated on a fluorescence spectrometer (FLS 980, Edinburg Instrument) using a xenon lamp. X-ray excited luminescence (XEL) spectra were excited by an X-ray tube with an Ag target and collected using a portable spectrometer.

Pulse height spectra were measured with an analog system, which consists of a PMT (Hamamatsu R11265U_H11934_TPMH1336E), a preamplifier (Cake 611), and a multichannel analyzer. The crystal was coupled on the window of PMT with optical silicone oil and then wrapped with Teflon. Scintillation decay time consisted of a pulsed X-ray source (XRS-3, pulse width: 25 ns), fast photodetector (GD40, without internal gain), and digital oscilloscope (Tektronix model MSO71254C). The radiation stability of the crystal was measured utilizing a ⁶⁰Co source at Nanjing University of Aeronautics and Astronautics. The dose rate was about 0.52 kGy/h, which was calibrated by dosimetry. All measurements were carried out in a dark atmosphere at room temperature.

We measured the PL intensity as a function of excitation intensity [Fig. 2(c)]. The integrated PL intensity exhibits a linear relationship with the excitation intensity [Fig. 2(d)]. Because the concentration of defects is finite, this linear relationship phenomenon confirms the light emission originates from STEs rather than defects.

We also investigated normalized temperature-dependent PL spectra [Fig. 2(e)]. The FWHM value decreases with the temperature from 511.5 meV at 295 K to 266.9 meV at 85 K [Fig. 2(f)], which indicates that the electron–phonon coupling capability has significantly decreased at a low temperature [30]. This electron–phonon coupling behavior is characterized by the Huang–Rhys factor (S) and phonon frequency (ℏωphoton), which can be calculated with the following equation: FWHM=2.36Sℏωphotoncothℏωphoton2kBT,where kB is the Boltzmann constant, and T is temperature. The fitted S and ℏωphoton are 54.2 and 32.3 meV, respectively. These values are slightly higher than for perovskite nanocrystals [27,31,32]. It is worth noting that the value of S is also higher than that of the reported alkali-halide scintillator (NaI of 42; CsI of 12) [33,34], and it is comparable to the recently reported defect halide perovskites (Cs3Sb2I9, Rb3Sb2I9, and Cs3Bi2I9) [35].

In conclusion, a high-density bulk Cs3Cu2I5 crystal was grown by a simple low-cost, solution-processed method. The crystal with unique Cs+ surrounded isolated [Cu2I5]3− clusters 0D structure results in characteristics such as remarkable air stability with no degradation and in being non-hygroscopic. The broad emission and zero self-absorption are attributed to the recombination of STEs, which has advantages for thick scintillator applications used in high-energy gamma-ray detection. Distinguishable pulse height spectra of C137s were successfully collected using a 5mm×5mm×2mm crystal. We found this 0D bulk crystal has strong radiation hardness under a high-dose gamma ray of C60o. The work demonstrates that this solution-processed, low-cost, strong-radiation-hard, and user-friendly 0D bulk crystal with STE emission could be a promising scintillator for gamma-ray spectroscopy application.