Technology

# Hybridisation of perovskite nanocrystals with organic molecules for highly efficient liquid scintillators

Sep 7, 2020

### Hybrid CsPbA3 liquid scintillators and radiography

The hybrid liquid scintillators were manufactured by dispersing CsPbA3 NCs and PPO in octane without precipitation (Fig. 1a). The perovskite NCs were synthesised via a hot injection method26,27,28 (see “Methods” for details). Transmission electron microscopy (TEM) measurements revealed that the as-synthesized NCs have a cubic shape with an average size of 12 nm (Fig. 1b). The optical and structural properties of the perovskite NCs were investigated using photoluminescence (PL), ultraviolet-visible (UV-Vis) spectroscopy, X-ray diffraction (XRD) measurements, and TEM images (Supplementary Figs. S1 and S2)29,30,31. To quickly evaluate the suitability of the CsPbBr3 NCs+PPO material as a scintillator for X-ray imaging, we imaged a wide range of biological and inorganic specimens with X-rays using a liquid scintillator panel (Fig. 1c) combined with a charge-coupled device (CCD) camera (Fig. 1d). For radiographic measurements, the specially designed display panel was used. The colloidal hybrid CsPbBr3 NCs+PPO solution was sandwiched by two quartz windows with 4-inch diameters. The X-ray images were taken at an accelerating voltage of 70 kVp. To demonstrate X-ray imaging, the concentrations of the CsPbBr3 NCs and PPO in octane were set at 25 mg/ml and 10 mg/ml, respectively. An object was placed on the panel detector, and an X-ray-excited optical image was projected through a mirror onto the CCD. As will be discussed in further detail, the CsPbBr3 NCs+PPO scintillator was selected to demonstrate the X-ray imaging because the CsPbBr3 NCs have excellent durability and the strongest RL intensity. As shown in Fig. 1e–g and Supplementary Fig. S3, the metal structures within the biological and plastic specimens were clearly imaged on the liquid scintillator panel.

### Enhanced radioluminescence in hybrid CsPbA3 scintillators

Figure 2a shows photographs of the X-ray imaging system and colloidal CsPbA3 NCs+PPO scintillators in the presence of white light and under X-ray irradiation (accelerating voltage: 6 MVp). During X-ray exposure, the CsPbBr3 NCs+PPO scintillator exhibited the brightest RL and emitted a green colour. As anticipated, the hybrid CsPbBr3 NCs+PPO scintillator exhibited the highest RL intensity in both the soft and hard X-ray regimes (Supplementary Fig. S4).

Figure 2b shows a comparison of the RL spectra emitted from the CsPbBr3 NCs (25 mg/ml), PPO (10 mg/ml), and hybrid CsPbBr3 NCs + PPO scintillators. The hybrid NCs+PPO scintillator exhibited strong RL that was several times stronger than those emitted by other scintillators. The hybrid NCs + PPO and pure NCs scintillators had the same RL peak positions, indicating that adding PPO does not significantly affect the emission energy of the CsPbBr3 NCs while enhancing their RL intensity. Another interesting observation is that the RL signal of PPO completely disappears in the spectrum of the hybrid NCs+PPO scintillator, suggesting the likelihood of X-ray-induced charge transfer from PPO to the CsPbBr3 NCs. We have experimentally demonstrated that the RL spectrum of a powder mixture containing the same amounts of PPO and CsPbBr3 NCs without octane exhibited two resolved RL emissions, each corresponding to PPO and CsPbBr3 NCs (Supplementary Fig. S5) and that the PPO peak is not suppressed in the PL spectrum of the hybrid NCs+PPO scintillator under UV irradiation (see Supplementary Fig. S6); collectively, these findings support the proposed mechanism that PPO plays a key role in enhancing the RL of the CsPbA3 NCs in octane. The surface hybridisation of halide perovskite NCs with PPO is highly feasible in a nonpolar liquid solvent medium such as octane. The same dramatic RL enhancement was also observed with the hybrid CsPbCl3 NCs + PPO and CsPbI3 NCs + PPO scintillators (Fig. 2c).

### Scintillation mechanism

In lead halide perovskite NCs, the photoelectric interaction between incident high-energy X-ray photons and heavy lattice atoms produces high-energy electrons, and these energetic electrons subsequently generate secondary high-energy carriers32,33. The hot carriers then undergo a thermalisation process, producing numerous low-energy excitons, and, consequently, high-energy X-ray photons are converted to visible low-energy photons via direct-bandgap luminescence23. For our hybrid CsPbA3 NCs+PPO scintillators, X-ray-induced energetic electrons generated from PPO can transfer to the CsPbA3 NCs via surface hybridisation and amplify the number of energetic electrons in the NCs, thereby enhancing the RL from the CsPbA3 NCs with a significantly improved quantum yield (Fig. 2d).

Density functional theory (DFT) calculations were performed to simulate the surface hybridisation of CsPbBr3 NCs with PPO and elucidate the origin of the improved quantum yield in the hybrid CsPbBr3 NCs+PPO scintillator in terms of X-ray-induced charge transfer from PPO to the NCs. For hybridisation of the CsPbBr3 NCs with PPO, the PPO must compete with the oleic acid (OA) ligand bound to the CsPbBr3 NC surfaces via surface reactions. Thus, we first compared the binding energies of PPO and OA on the CsPbBr3 NC surfaces and assessed how well the desorbed OA could be dissolved in octane.

Neutral PPO and anionic OA showed binding energies of −1.03 eV and −0.30 eV on the Pb site, respectively, and anionic OA had a larger solvation free energy of −36.05 kcal/mol compared with the value of −9.82 kcal/mol for PPO in octane solvent. The calculated results revealed that PPO, with its relatively large binding energy, can replace the OA on the CsPbBr3 NC surface and that the desorbed OA can be stabilised in octane with a large negative solvation free energy (Supplementary Table S1). Therefore, the formation of the hybrid CsPbBr3 NCs+PPO in octane was facilitated by the strong interaction between PPO and Pb ion sites through N-Pb bonding (Supplementary Figs. S7 and S8). XPS measurements of CsPbBr3 NCs, PPO, and CsPbBr3 NCs+PPO provide strong evidence for N-Pb bonding (Supplementary Fig. S9).

We analysed the energy level alignment between PPO and the CsPbBr3 NCs and the frontier orbital distributions. X-ray-induced charge transfer was allowed when the excited state of PPO was much higher than the conduction band state of the CsPbBr3 NCs. In particular, the large contribution of N and Pb in forming N-Pb bonds that led to the aligned states of PPO and the CsPbBr3 NCs effectively led to charge transfer from PPO to the CsPbBr3 NCs (Fig. 2e and Supplementary Fig. S10). Here, we used the p-band centre of the Pb atom as the representative conduction band state of the CsPbBr3 NCs because the valence 6p-orbital of Pb is involved in the N–Pb bond and is distributed over a wide range of conduction bands with various contributions. The energy level alignment in Fig. 2f revealed that the lowest unoccupied molecular orbital (LUMO) state of PPO and the p-band centre of the Pb atom were located at 3.31 eV and 3.64 eV, respectively. Energy levels are denoted based on the aligned Fermi energy (Ef) of the hybrid CsPbBr3 NCs+PPO at 0 eV. In addition, the N atom in PPO significantly contributed to LUMO, LUMO+1, and LUMO+4; therefore, the LUMO+1 and LUMO+4 states, which were located above the p-band centre of the Pb atom and had large contributions from the N atom, could effectively induce charge transfer from PPO to the CsPbBr3 NCs. This implies that a sufficiently high-energy source, such as X-ray irradiation, is required to induce charge transfer from the excited states above the LUMO of PPO to the Pb p-orbital of the CsPbBr3 NCs.

Consequently, the characteristic structural and electronic features of the hybrid CsPbBr3 NCs+PPO scintillator resulted in selective charge transfer under X-ray irradiation, eventually enhancing the scintillation quantum yield. On the other hand, an excited electron in PPO cannot move to an NC upon UV illumination because the energy levels of the allowed states in the NC are too high. This is consistent with the experimental observation that low-energy UV light cannot enhance the quantum yield in the hybrid NCs+PPO scintillator (Supplementary Fig. S11).

We then measured the RL spectra of the CsPbA3 NC (25 mg/ml)+PPO (10 mg/ml) hybrid scintillators as a function of dose rate (Supplementary Fig. S12). The measured RL emission exhibited a linear response to the X-ray dose rate, which is a desirable feature of a good scintillator for X-ray imaging and dosimetry. We also measured the X-ray response characteristics of the hybrid scintillator upon excitation with a single X-ray photon from a portable coin-type 60Co source (Supplementary Fig. S13). The extracted fast scintillation decay time τ was 60–100 ns for the CsPbA3 NCs+PPO hybrid scintillators, which is much shorter than that for the bulk CsI:Tl (on the order of μs). The fast RL decay time of the hybrid scintillators is also expected to act as a favourable trait for use in medical radiography.

We further quantitatively investigated how PPO contributes to the RL of the CsPbBr3 NCs+PPO hybrid scintillator by varying the concentration ratio of the NCs and PPO. As shown in the photographs (Fig. 3a) under X-ray irradiation (accelerating voltage: 6 MVp), the RL emission from the hybrid NCs+PPO scintillator became brighter as the PPO density was increased for a fixed NC density of 5 mg/ml. We measured the RL spectra of the hybrid NCs+PPO scintillators in the soft (dose rate of 37.4 mGy s−1 at an accelerating voltage of 50 kVp) and hard X-ray regimes (Fig. 3b, Supplementary Fig. S14a), plotted the measured RL peak intensity as a function of the PPO density (Fig. 3c), and observed a linear relationship between the RL intensity and PPO density. The scintillation efficiency of a hybrid CsPbBr3+PPO scintillator was enhanced with increasing PPO density (Supplementary Fig. S15). Without CsPbBr3 NCs, the RL intensity of the pure PPO liquid scintillator decreased at high PPO densities (>10 mg/ml), which was likely due to scintillation quenching (namely, self-absorption)34,35 (Supplementary Fig. S16). When the PPO density was greater than 50 mg/ml in the colloidal hybrid scintillator, a yellowish-green dense precipitate was formed. As the PPO density was further increased to above the critical value of ~500 mg/ml, the hybrid NCs+PPO material in octane completely transformed into an opaque dense precipitate that emitted a very strong RL.

We also carried out similar measurements while increasing the CsPbBr3 NC density for a fixed PPO density of 30 mg/ml and observed that precipitates were not formed. In contrast, as the CsPbBr3 NC density increased, the RL emission quickly saturated in both the soft and hard X-ray regions, as shown in the photographs (Fig. 3d) and RL spectra (Fig. 3e and Supplementary Fig. S14b). The observed features are summarised in Fig. 3f, in which the measured RL peak intensity is plotted as a function of NC density.

### X-ray imaging performance

To directly confirm any enhancement in the X-ray image quality when using the hybrid CsPbBr3 NCs+PPO liquid scintillator, we recorded the X-ray images of a portable data storage device using PPO, CsPbBr3 NC, and hybrid CsPbBr3 NCs+PPO scintillators (Fig. 4a). The hybrid NCs+PPO scintillator produced notably clearer X-ray images than those of PPO the CsPbBr3 NCs. The spatial resolution and image quality of the scintillation materials were quantitatively evaluated using a radiography test phantom36,37 (Leeds test objects, model: TOR 18FG, Supplementary Fig. S17). Figure 4b shows X-ray images of the test objects. The imaging performances of the scintillators were comparatively evaluated by counting the maximal number of resolvable line pairs per millimetre (lp/mm) and checking the abruptness of the contrast changes at the boundary. Figure 4c–e shows the intensity variation along the yellow lines in the X-ray images of the line patterns. The largest detectable lp/mm of the hybrid scintillator was at least 3.5 lp/mm, which was several times greater than those of the pure halide perovskite NCs and PPO scintillators.

Figure 4f–h show the edge spread functions (ESFs) at the boundary of the test phantom, indicated by the red dashed boxes in the insets, which characterise the sharpness of the images. The abrupt change in intensity is reflected by the slope across the boundary. The measured slope was 17.6 grey value/pixel for the hybrid NCs+PPO scintillator, which was much larger than those for the other materials (range, 1.36–1.66 grey value/pixel) (pixel size: 9 μm). The line spread functions (LSFs) extracted from the ESFs (red curves above the insets) also indicated how sharp the image was near the boundary in terms of the full width at half maximum (FWHM)38. The estimated FWHM of the NCs+PPO hybrid material was 7.6 pixels, which is much smaller than those of the other scintillators (range: 16–32 pixels). The image contrast is a measure of how clearly an object is distinguishable and can be assessed using the following expression:

$${mathrm{Contrast}}left( % right) = 100 times left( {I_{{mathrm{Object}}} – I_{{mathrm{Background}}}} right)/left( {I_{{mathrm{Object}}} + I_{{mathrm{Background}}}} right)$$

(1)

where IObject and IBackground represent the RL intensities of the object and adjacent material near the boundary, respectively. The contrast near the boundary was 33% for the hybrid NCs+PPO scintillator and 12−13% for the other scintillators.

Ageing and deterioration of the hybrid NCs+PPO scintillator were examined by repeating the X-ray imaging measurements in the same environment after a year (Supplementary Fig. S18) and after continuous irradiation with a very high-energy X-ray for a prolonged period (Supplementary Fig. S19). The colloidal hybrid scintillator exhibited almost no degradation in performance, thereby confirming its stability.

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