Luminescent Lead Halide Ionic Liquids for High-Spatial-Resolution Fast Neutron Imaging

Abstract

The fast neutron imaging technique with recoil proton detection harbors significant potential for imaging of thick, large-scale objects containing high-Z elements. However, the challenge to find efficient fast neutron scintillators with high spatial resolution is ongoing. The list of requirements for such scintillators is long and demanding: a proton-rich, scattering-free material combining high light yield with the absence of light reabsorption. To meet these challenges, we look for a suitable material among a rising class of 0D organic–inorganic Pb(II) halide hybrids. The use of large organic cations, e . g., trihexyltetradecylphosphonium, results in room-temperature ionic liquids that combine highly Stokes-shifted (up to 1.7 eV), reabsorption-free, and efficient emission (photoluminescence quantum yield up to 60%) from molecularly small and dense (PbX 2 molar fraction up to 0.33) emitting centers. We investigate the optical properties of the resulting ionic liquids and showcase their utility as fast neutron imaging scintillators. Concomitantly with good light yield, such fast-neutron scintillators exhibit both higher spatial resolution and lower γ-ray sensitivity compared with commercial ZnS:Cu-based screens.

Highly Concentrated, Zwitterionic Ligand-Capped Mn2+:CsPb(BrxCl1–x)3 Nanocrystals as Bright Scintillators for Fast Neutron Imaging

“…The concentration-dependent light output shows a continuous increase with increasing concentration (Figure S5a), indicating that the charge collection efficiency increases as we obtain more NCs per unit volume. Compared to prior efforts, this increase is significantly closer to linearity, indicating that the self-absorption of these Stokes-shifted NCs is well below that of FAPbBr 3 NCs and closer to self-trapped exciton systems such as the ionic liquid THTDPCl-PbCl 2 that recently demonstrated spatial resolution superior to that of the ZnS:Cu reference screens …”

“…Compared to prior efforts, 1 this increase is significantly closer to linearity, indicating that the self-absorption of these Stokes-shifted NCs is well below that of FAPbBr 3 NCs and closer to self-trapped exciton systems such as the ionic liquid THTDPCl-PbCl 2 that recently demonstrated spatial resolution superior to that of the ZnS:Cu reference screens. 72 Thickness-dependent measurements further confirm that the effect of self-absorption is substantially reduced in these doped NCs, as the light output is essentially linear and reaches 91.9% of the expected value (estimated using the stopping power of the solvent to 1 MeV neutrons) for a 10-fold increase in thickness (Figure 3d). Note that this expected value at 5 and 10 mm is overestimated as the beam includes both fast neutrons and intermediate neutrons, where the latter have smaller penetration depths and therefore would contribute to more light output at lower thicknesses; hence, the 91.9% value is a lower limit of the actual efficiency versus the expected values.…”

“…Compared to prior efforts, 1 this increase is significantly closer to linearity, indicating that the self-absorption of these Stokes-shifted NCs is well below that of FAPbBr 3 NCs and closer to self-trapped exciton systems such as the ionic liquid THTDPCl-PbCl 2 that recently demonstrated spatial resolution superior to that of the ZnS:Cu reference screens. 72 …”

Noncrystalline Hybrid Lead Halides with Liquid-Polymer Characteristics

“…Characterization of the frozen sections shows an undulating stacking of the lamellas, validating the disordered arrangement within the product and hence the noncrystalline nature (Figures c,d and S3). In addition, the lamellar morphology implies that the product differs from an actual liquid, despite exhibiting liquid-polymer properties macroscopically, as a liquid should not present a defined nanostructure under TEM …”

“…As a further point to note, this material, although in the liquid form, has a significantly different PL nature from luminescent molecular liquids. For luminescent molecular liquids, the PL is derived from the luminophore of the molecules. , As the molecules are isolated and disordered, their energy levels are discrete without forming energy bands, thus providing sharp and intense peaks in their absorption and excitation spectra. , This material, in contrast, consists of interconnected lead halide octahedron frameworks, so that the energy levels may evolve into energy bands, leading to the flat and continuous absorption and excitation spectra characteristic of semiconductors. Moreover, its conductivity is estimated to be on the order of 10 –3 S/m, which also falls within the semiconductor conductivity range (Figure S22).…”