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Stokes Shift Engineered Mn:CdZnS/ZnS Nanocrystals as Reabsorption‐Free Nanoscintillators in High Loading Polymer Composites

Plastic scintillators are gaining attention as alternatives to inorganic scintillator crystals owing to their low fabrication cost, customable shape/size, and substantially lighter weight that make them suitable for various radiation detection technologies. These include scintillation panels for national security and industrial monitoring, radiation screens for medical diagnostics, and calorimeters for high energy physics. Because of their low density, plastic scintillators are typically doped with high atomic number (Z) sensitizers that enhance the interaction probability with ionizing radiation and excite molecular emitters. Although effective, such a two‐component design suffers from incomplete sensitization, intrinsically limited efficiency due to multiple radiative steps with non‐unity quantum yield, detrimental phase segregation effects and the fragility of organic emitters to ionizing radiation. In this work, an alternative single‐component scheme is proposed based on high‐Z reabsorption‐free CdZnS/ZnS semiconductor nanocrystals (NCs) doped with manganese embedded in a polyvinyltoluene (PVT) waveguide. Optical‐grade nanocomposites free from optical reabsorption of the scintillation light and with performance comparable to commercial products are obtained through a post‐synthesis resurfacing procedure that maximizes the compatibility between the NCs and PVT and preserves their optical properties upon curing.

Plastic scintillators are gaining attention as alternatives to inorganic scintillator crystals owing to their low fabrication cost, customable shape/size, and substantially lighter weight that make them suitable for various radiation detection technologies. These include scintillation panels for national security and industrial monitoring, radiation screens for medical diagnostics, and calorimeters for high energy physics. Because of their low density, plastic scintillators are typically doped with high atomic number (Z) sensitizers that enhance the interaction probability with ionizing radiation and excite molecular emitters. Although effective, such a two-component design suffers from incomplete sensitization, intrinsically limited efficiency due to multiple radiative steps with non-unity quantum yield, detrimental phase segregation effects and the fragility of organic emitters to ionizing radiation. In this work, an alternative single-component scheme is proposed based on high-Z reabsorption-free CdZnS/ZnS semiconductor nanocrystals (NCs) doped with manganese embedded in a polyvinyltoluene (PVT) waveguide. Optical-grade nanocomposites free from optical reabsorption of the scintillation light and with performance comparable to commercial products are obtained through a post-synthesis resurfacing procedure that maximizes the compatibility between the NCs and PVT and preserves their optical properties upon curing.
high optical quality nanocomposites with relatively large NCs weight fraction. [6b,9] However, because of the characteristic small Stokes shift of direct bandgap materials leading to a large spectral overlap between their absorption and luminescence spectra, one common issue with using NCs as nanoscintillators in highly dense or large volume detectors is represented by the strong reabsorption of the scintillation light along its path to the perimetral photodetectors. The typical strategy adopted to circumvent such a reabsorption bottleneck is a hybrid design where NCs act as high-Z sensitizers for secondary organic emitters featuring a large Stokes shift typically prompted by molecular planarization in the excited state. [2b,9b] However, this approach suffers some intrinsic limitations represented by the high concentration of molecular emitters required for efficiently transferring excitation from the NCs via energy transfer that may result in phase segregation and molecular aggregation, and by the incomplete spectral separation between the optical absorption luminescence profiles of organic dyes, ultimately resulting in substantial residual reabsorption losses in very large detectors and in the intrinsic fragility of conjugated organic emitters exposed to prolonged ionizing radiation.
For these reasons, realizing nanocomposite scintillators in which NCs with specifically engineered large Stokes shift act as both efficient radiation harvesters and reabsorption-free emitters would strongly benefit the field and offer a valuable alternative to existing hybrid technologies. Recently, various examples of this design have been provided by exploiting self-trapped excitons in metal halides. [10] In this work, we aim at contributing to this endeavor by realizing new plastic scintillators consisting in a polyvinyltoluene (PVT) matrix incorporating reabsorption-free CdZnS/ZnS core/shell NCs doped with manganese ions (hereafter referred to as Mn:CdZnS/ZnS, Figure 1a). Manganese doping of large bandgap NCs is an established approach to activate efficient luminescence at ≈580-600 nm arising from the 4 T 1 → 6 A 1 optical transition on Mn 2+ color centers [11] (Figure 1b). Representative optical absorption and photoluminescence (PL) spectra of our NCs are shown in Figure 1c. Crucially, since such a transition is spin-forbidden, the corresponding optical absorption features negligible oscillation strength (see inset of Figure 1c), resulting in an apparent Stokes shift between the band-edge absorption of the host NC and the Mn-PL. As a result, Mn-doped NCs have been successfully implemented as reabsorption-free emitters in luminescent solar concentrators (LSC) [11d,12] which, similar to plastic scintillators, convert impinging radiation (sunlight in the case of LSC) into visible luminescence trapped into total internal reflection propagation modes of a doped optical waveguide and collect it using efficient photodetectors placed along the slab edges. [13] In our approach to reabsorption-free nanoscintillators, we further adopted a synergic strategy in which both the PVT waveguide and the NCs interact with incoming ionizing radiation, while the propagating luminescence is generated by the sole NCs, whose optical properties are specifically engineered to absorb the PVT emission (reported in blue in Figure 1c). The emission efficiency and compatibility of the NCs with the polymer host were optimized by combining inorganic shelling in type-I configuration using wide bandgap ZnS overgrown on top of the Mn:CdZnS cores and surface functionalization with crosslinking ligands that participate in the radical polymerization of the PVT waveguide. Overall, this design results in high optical quality nanocomposites nearly perfectly transparent in the spectral region of their own emission, with scintillation efficiency comparable to commercial plastic scintillators.

Results and Discussion
Mn:CdZnS/ZnS NCs optimized for integration into high optical quality polymeric waveguides were produced following the multi-step strategy schematized in Figure 2a. As a first step, core/shell Mn:CdZnS/ZnS NCs (5% Mn doping in respect Cd and Zn content) were synthetized through a non-injection synthesis [14] where CdZnS cores are grown by heating the precursors in the presence of a manganese source. In our case, we adopted hexadecane as reaction solvent instead of 1-octadecene used in ref, [14], as it ensures a better optical quality of the NCs for further incorporation in the polymeric matrix. ZnS shelling was performed through a layer-by-layer deposition by continuous injection of shell precursor at 230 °C (see Methods section in Supporting Information for details). [14,15] Consistent with previous results, [14] the obtained NCs are in a zincblende crystal structure (as shown by the X-ray diffraction pattern in Figure 2b) and feature a mean diameter <d> of 4.2 nm (Figure 2c). It is important to underline that in this synthetic way the excess of dodecane thiol (DDT) is used both as a source of sulfur (together with the elemental S) and as a capping ligand, which allows to obtain a PL with a high quantum efficiency (PLQY). However, DDT-capped NCs exhibit poor solubility in the vinyl toluene monomer solution used for fabrication of the nanocomposite scintillator resulting in noticeable light scattering even at very low NCs concentrations ( Figure S1, Supporting Information). In order to overcome this limitation, we thus performed a two-step resurfacing procedure to replace DDT ligands, which are strongly bound to the NC's surface, with bis[2-(methacryloyloxy)ethyl] phosphate (BMEP) whose terminal acrylic functionalities participate in the polymerization process and thus lead to homogeneous transparent nanocomposites even at high NCs loadings. With this aim we first grew an additional ZnS layer on top of DDT-capped NCs using an excess of trioctylphosphine (TOP) capping ligands. Such an intermediate step is pivotal since it greatly facilitates the subsequent ligand exchange process, as TOP can be easily substituted by BMEP due to its electronically neutral configuration in both attached and desorbed form. On the contrary, direct replacement of DDT ligand is challenging due to the very low electrostatic stabilization of electronically charged DDT moieties detached from the particle surface, resulting in energetically unfavorable dissociation. [8] Consistently, our efforts to directly exchange DDT to BMEP have yielded no results. Specifically, TOP resurfacing was performed as a one-pot procedure by adding Zn stearate and tri-octyl-phosphine sulfide (TOP-S) to the DDT-capped NCs. The deposition of the TOPcapped ZnS layer was verified by comparing the average size distribution of the NCs from TEM images collected before and after resurfacing. As shown in Figure 2c, TOP-capped NCs feature <d> = 4.6 ± 0.6 nm, which is 0.4 nm larger than DDTcapped NCs corresponding to approximately one monolayer of ZnS. [14,16] As a second step, TOP ligands were exchanged with BMEP, as confirmed by Fourier transform infrared (FTIR) spectroscopy measurements reported in Figure 2d, showing the characteristic peaks of the PO, POR bonds of BMEP in the 1000-1400 cm −1 range. [6b,9a] Finally, nanocomposite samples were produced via thermally activated polymerization of vinyl toluene (VT) loaded with BMEP-capped NCs. To do so, BMEP-capped NCs were washed three times from unreacted BMEP ligands by addition of excess acetone and precipitated and dried with N 2 . Subsequently, NCs were dissolved in VT (containing 5% V/V of divinylbenzene as crosslinker) and sonicated until a clear solution was obtained. The polymerization was then performed by heating the VT/NCs solution in a mold following an optimized thermal ramp for avoiding contraction effects and the formation of defects in the nanocomposite surfaces. After 24 h, the mold was let to cool down to room temperature and the nanocomposite was extracted, cut, and polished for further spectroscopic studies. The right panel of Figure 2a shows the photograph of a PVT/ NCs nanocomposite under ambient light. As it will be quantified below, the nanocomposites exhibit high optical quality and negligible haze due to NCs agglomeration or structural defects, which corroborates the effectiveness of the compatibilization strategy.
Crucially, the optical properties of the NCs are perfectly preserved at all steps of the surface functionalization process, as well as after embedding into the PVT matrix. The optical absorption and PL spectra of DDT-, TOP-, and BMEP-capped NCs and of the nanocomposite sample are reported in Figure 3a showing identical profiles with the absorption edge at 420 nm and a broad Stokes-shifted PL at 595 nm. All samples also exhibit comparable slightly multiexponential PL dynamics with effective lifetime (measured as the time after which the PL intensity has dropped by a factor e) of 0.9 ms (Figure 3b). Possibly more importantly, the PL efficiency is identical for all NCs samples within the experimental uncertainty (PLQY ≈ 65 ± 6%) and only slightly lower in the nanocomposite (PLQY = 55 ± 7%), as shown in the inset of Figure 3b. Crucially, the radioluminescence (RL) spectrum of the nanocomposite, shown as a shaded curve in Figure 3a, is identical to the corresponding PL, indicating that the scintillation process leads to the same recombination mechanism as UV illumination and that no defects related radiative emissions emerge under ionizing radiation. Based on the promising properties of the Mn:CdZnS/ZnS NCs, we proceeded with investigating the scintillation process in PVT/NCs nanocomposites via RL experiments as a function of the NCs loading in PVT matrices (from 0 to 8 wt%, as measured via thermo gravimetric analysis shown in Figure S2, Supporting Information). Consistent with the typical scintillation behavior of PVT, the bare matrix shows the characteristic RL spectrum centered at 320 nm (Figure 3c). Upon increasing the NCs loading leads to the progressive decrease of the PVT-RL, which is accompanied by the raise of the NCs emission, as quantified in Figure 3e, where we report the integrated intensity of the total RL together with the individual contribution by the PVT and the NCs. This is consistent with the complete match between the PVT emission and the NCs absorption. Time-resolved measurements of the PVT luminescence on the same samples (Figure 3d) reveal that the drop of the PVT-RL with increasing NCs content is accompanied by a very minor acceleration of the PVT emission kinetics (from effective τ = 5.8 ns of bare PVT to τ = 5.2 ns for [NC] = 1 wt%), which indicates that the dominant optical process is the absorption of the PVT-RL by the NCs followed by re-emission, whereas non-radiative energy transfer plays a negligible role (see Supporting Information for detailed analysis). A closer look at the integrated RL intensities in Figure 3e further reveals that for NCs contents above 0.2 wt% the total RL emitted by the nanocomposite is dominated by the NCs-RL, whereas the PVT contribution is quenched by over 95%. For lower NCs loadings, on the other hand, the interaction with X-rays occurs mostly with PVT, resulting in PVT-dominated RL. Because of the nonunitary PLQY of the NCs, at intermediate concentrations when the RL is also largely due to indirect NCs excitation by PVT, the total scintillation signal drops with respect to the bare PVT matrix. In order to provide a figure of merit of the scintillation performance of our proof-of-concept nanocomposites, we compared their RL with that of a commercial plastic scintillator (EJ-276 from Eljen Technology) measured in the same experimental conditions. The nanocomposite containing 8 wt% of NCs showed a RL signal comparable to EJ-276 (with reported light yield of 8600 Ph MeV −1 ), corresponding to a relative light yield of 5900 ± 600 Ph MeV −1 (see Figure S4, Supporting Information). Finally, we tested the stability of the RL performance under continuous X-ray irradiation (Figure 3g) up to total absorbed dose of 20 Gy. The RL intensity preserved over 95% of its initial value, independently from the concentration of embedded NCs. The RL spectrum shows no modification upon prolonged X-ray exposure, suggesting that our NCs are suitable also for continuous radiation detection.
Finally, we experimentally validated our approach to reabsorption-free plastic scintillators using doped NCs by fabricating a 20 cm long PVT scintillator waveguide containing 0.2 wt% of NCs and characterizing its light guiding properties under X-ray irradiation. To do so, we collected the RL spectra as a function of the distance, d, between the X-ray excitation beam and the device edge as schematized in Figure 4a. The RL spectra collected at increasing d are reported in Figure 4b and show identical spectral profile and intensity, which confirms that the scintillator is completely unaffected by reabsorption effects. Consistently, as quantified in Figure 4c, the integrated RL intensity is constant for all d-values within the experimental uncertainty, which also confirms that our nanocomposite scintillator waveguide is unaffected by light scattering losses, thus further corroborating the high optical quality of the nanocomposite and the effectiveness of the NCs surface functionalization. The same results are obtained using optical excitation with 3.05 eV light. Also in this case, increasing d does not influence the PL spectrum profile (reported in Figure S5, Supporting Information) or the PL output intensity.

Conclusion
In summary, we have realized a reabsorption-free nanocomposite plastic scintillator based on a PVT matrix loaded with Mn-doped core/shell CdZnS/ZnS NCs. Post-synthesis engineering of the NCs surfaces enabled us to preserve the optical features of the NCs after the thermally activated radical polymerization of the polymeric matrix. Optical and scintillation investigations as a function of the NCs loading highlighted that our PVT-NCs composite scintillators exploit the scintillating properties of both the NCs and the plastic waveguide and exhibit performance comparable to optimized commercial plastic scintillators. Finally, we experimentally validated the effective suppression of reabsorption losses on the radioluminescence by monitoring the waveguiding performance of a large-size NC-based device under X-ray excitation, confirming that our strategy can be successfully implemented also in largescale plastic scintillators.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author. Figure 4. a) Sketch of the experimental configuration used for the RL propagation measurements. b) RL spectra collected at increasing distance, d, from the nanocomposite edge. Spectra collected from the outer portion of nanocomposite (from 0 to 2 and from 18 to 20 cm) have been omitted to avoid erroneous evaluation of the RL intensity due to the different geometrical collection factor. The sample was excited by X-ray beam of 10 mm diameter. c) Integrated RL (orange dots) and PL (black dots) intensities as a function of d extracted from the RL spectra in (b) and the PL spectra in Figure S5, Supporting Information.