From sintering to particle discrimination: New opportunities in Metal-Organic Frameworks scintillators

: The characterization of a scintillating Metal Organic Framework (MOF) is not straightforward, 18 mainly due to the small size and low density of the material. In this context, we present herein a generic method to give an easy access to the determination of a key parameter in the 20 scintillation field, namely the light output. To reach this, MOF-205 was first synthesized as 21 millimetric-size single crystals then sintered under pressure and temperature conditions to 22 afford a pellet. The density was increased by 300% while maintaining optical properties on par 23 with scintillation application. The as-prepared scintillator was then characterized in terms of photoluminescence (UV-excited emission spectrum, time-correlated single photon counting) and radioluminescence spectroscopy (beta-excited emission spectrum, alpha, beta and gamma pulse height spectra, alpha/beta and alpha/gamma discrimination). Results were compared with 27 commercial BC-404 plastic scintillator performances as well as supported by

afford a pellet. The density was increased by 300% while maintaining optical properties on par 23 with scintillation application. The as-prepared scintillator was then characterized in terms of 24 photoluminescence (UV-excited emission spectrum, time-correlated single photon counting) 25 and radioluminescence spectroscopy (beta-excited emission spectrum, alpha, beta and gamma 26 pulse height spectra, alpha/beta and alpha/gamma discrimination). Results were compared with 27 commercial BC-404 plastic scintillator performances as well as supported by MCNP6.2 28 simulation. 29 30

Introduction 31
Methods to detect, qualify and quantify ionizing radiations were introduced soon after the 32 discovery of radioactivity by Henri Becquerel in 1896. Currently, numerous applications benefit 33 from this field, ranging from nuclear activities, research in high-energy physics, astronomy, 34 homeland security and medicine. Depending on the radionuclide to be detected, various 35 disintegrations can occur, the most common and probable leading to the emission of alpha or 36 beta particles often followed by de-emissions producing X and/or gamma rays. These ionizing 37 radiations can be detected with scintillators, which are materials that are efficient to produce 38 light when exposed to such radiations. This specific class of photoluminescent materials is 39 divided into two main categories, namely inorganic and organic scintillators. [1 The former 40 subclass appeared as early as 1895 (barium tetracyanoplatinate(II) BaPt(CN)4) [2 ). The later was 41 pioneered when the use of naphthalene was first reported in 1947. [3 Since these seminal 42 publications, many efforts have been performed in the two chemistry worlds for the quest of 43 the 'best' scintillator. However, both have pros and cons and currently no photoluminescent 44 material represents the Holy Grail that could fulfil all requirement in terms of radiation 45 detection (among others: detection efficiency against production cost). In this context, scientists 46 have considered using advantages from both worlds, hence leading to a various range of 47 scintillators such as sol-gel, hybrid materials or nanoparticles-loaded plastics. [ 4 Most 48 particularly, composite scintillators stand out as they can bypass a lot of limitations. The core 49 idea is to take a known efficient scintillator, mainly an organic or inorganic single crystal, and 50 embed them into a matrix of suitable polymer. This will give access to what can be described 51 as a polycrystalline scintillator. As single crystals are often hard to produce in large scale or are 52 not very stable towards ambient condition (mechanical weakness, humidity and temperature 53 dependency), this technology affords a way to combine large quantity of efficient scintillator 54 and stability-aimed encapsulation. 55 Metal organic frameworks (MOFs) are a class of hybrid materials. [ However, several limitations are foreseen with the incorporation of a MOF inside a matrix, and 77 in general to the characterization of MOFs as scintillators. Despite efforts by chemists to 78 synthesize MOF nanocrystals, these are subject to strong light scattering already at a low 79 percentage of incorporation in the polymer matrix, which can lead to turbidity observed at 80 loading as low as 0.5 weight%. This is mainly cause by the incorrect matching between the 81 matrix and MOFs refractive index which lead to light scattering. This effect coupled to the 82 numerous interfaces between the matrix and the embedded MOF can thus lead to strong 83 deviation from the optimal light collection. These cumulated factors are altering the global 84 optical properties and leading to a moderate scintillating material (6% the light output of 85 anthracene, which is ca. 1,000 ph·MeV -1 ). Other literature from this field generally describes 86 analytical methods that have to be adapted to small-size and low-density MOF materials, for 87 example with Ion Beam Induced Luminescence [7 (IBIL) or small X-ray tubes, [8 . This 88 experiments require high dose delivery [8,9 or tedious characterization in liquid suspension. [8 89 Such techniques are useful but developing a universal characterization method for scintillating 90 MOFs the closest to their final use, which means confronted to the presence of radionuclides 91 and without form factor (e.g. single crystals dispersed in a liquid) would be of great value for 92 the scientific community, and that was the core idea at the root of this study. 93 To overcome these issues, this work presents two major contributions leading to scintillating 94 materials made from MOFs. The first concerns the densification by sintering until translucent 95 media are reached. [ Detectors [ ). [ 11 , 12 . Furthermore the hybrid nature of our sintered MOF was put in the perspective 103 of classical inorganic and organic crystal scintillation. Those fields are known to demonstrate 104 good particle discrimination by PSD. This approach was applied to our materials and 105 unprecedented particle discrimination with scintillating MOFs has been reached, confirming 106 precedent hint from Allendorf et al. [ 13 mm and thickness 400 µm). Area integration allows to recover the scintillation efficiency 127 values. 128 129 As demonstrated in many contributions, MOF synthesis is tricky and requires attention as an 130 impurity can have a large impact on the final photophysical properties. [13 As a case study, we 131 chose a MOF where the secondary building unit is Zn4O, linked with two different organic 132 linkers: 1,3,5-tris(4-carboxyphenyl)benzene (H3BTB) and 2,6-naphthalene dicarboxylate (2,6-133 NDC), which is also named MOF-205 or DUT-6. [14,15 It was selected as a potent candidate 134 thanks to its photoluminescent properties that comply with standard plastic scintillators: fast 135 decay time and emission wavelength centered around 420 nm. These interesting features are 136

MOF-205 37
A B C D carry by the naphthalene moiety, which is a well-known molecule in the scintillation field. [16 137 The second reason is that this framework presents a cubic lattice structure, which is compliant 138 with sintering application, a key in densification. Theoretically, under uniaxial pressure planes 139 of cubic structures should move isotropically and finally result in a material densification, a 140 result that would be less easy to achieve with non-cubic lattices [10 , or anisotropic collapses. 141 Here we propose a densification of MOF under two external stimuli: pressure and temperature. Thus, MOF-205 was synthesized to obtain large, pure, millimeter-sized crystals (Inset of Figure  147 1.A), was sintered and fully characterized (see Supporting Information, Figure S1). As heat can 148 promote the plastic displacement leading to densification, temperature limits should be defined 149 in order to prevent any parasitic degradation of the (photo)physical properties. Thus, thermal 150 decomposition behavior was investigated in order to characterize its thermal stability. As shown 151 in Figure  thickness of 400 ± 20 µm and a mass of 82 mg. Considering the pellet as a perfect cylinder, a 160 density of 1.56 ± 0.08 was calculated, which represents a remarkable increase of 300% 161 compared to its original density (0.38). [14 Furthermore, the resulting pellet displayed promising 162 photophysical properties. Main spectral characteristics were obtained from either UV 163 photluminescence (PL) or ionizing radiation such as radioluminescence (RL) with an 90 Sr/ 90 Y 164 beta source or cathodoluminescence (CL) with an X-ray excitation. The results are shown in 165 with characteristic vibronic structure of linker in its dilute form (Figure 1.A). This is 168 characteristic of a ligand-centered emission. As already mentioned in many publications, 169 frameworks are likely to be dependent on their environment, guest molecules or impurities 170 trapped inside their porosity. Fluorescence is especially sensitive to external stimuli when it 171 arises from the linker only as is the case for MOF-5 for example. [18 Hence, upon activation the 172 material looses its fine structure and shows a Gaussian-type emission centered at higher 173 wavelengths (394 nm). Then after pressing, the pellet shows a slightly different steady-state 174 photoluminescence as the fluorescence maximum undergoes a shift to 409 nm (Figure 1.B). 175 This wavelength increase could be explained by larger π overlaps between the ligands due to 176 the densification of the material and the reduction of the ligands distance to each other. Thus, 177 the energy gap would be reduced and would result in a bathochromic shift at the image of the 178 ligand in its solid form (λem = 452 nm) ( Figure S3). This assumption is confirmed by a 179 comparison of the time-resolved fluorescence spectra. Under the effect of pressure and 180 temperature the material therefore tends to amorphise and favours a spatial rearrangement of 181 the ligands which leads to emission at a higher wavelength. This is confirmed as the pellet 182 shows no X-ray diffraction. This trend is as also demonstrated by Zacharia et al. for a similar 183 MOF. [17 However, we assume that this structural change remains minor as the average lifetime 184 is only slightly changed compared to pure activated MOF-205 single crystal ( Figure S4). This 185 results collectively show that MOF-205 as a single crystal or sintered as a pellet have the same 186 photophysical behavior. Sintered pellets are hence a good sudo-sample to judge the scintillation 187 response of a MOF. Pellets are also more practical to use andstarting from this point, we are 188 considering the pellets as scintillating material in their own rights. 189 Radioluminescence (RL) and cathodoluminescence (CL), contrary to PL, allow the 190 investigation of excited states by ionization with radionuclides. As known, ionization process 191 is quite different from PL as ionization can lead to several changes in the electronic and 192 molecular structure of matter, thus expectable discrepancies in emission wavelength or/and in 193 lifetime. Figure 1.B compares normalized PL, RL and CL state spectra. Since RL and CL/PL 194 are recorded in transmission and front face, respectively, it is possible to notice several changes 195 in the shape of the Gaussian-type emission. This is mainly due to reabsorption and diffusion 196 occurring within the pellet. However, since traditional scintillation measurements are usually 197 performed in transmission, the RL experiment is closer to the application measurement method. 198 words, this means that MOF-205 has a light output of 37% compared to anthracene, as the BC-212 404 is 68% according to its datasheet. [11 Considering that anthracene is ≈ 15,000 ph·MeV -1 , 213 the light output of the sintered MOF-205 is thus around 5,500 ph·MeV -1 . The above results 214 validate the concept of a sintered MOF-205 as an intrinsic scintillator and places it above other 215 MOF-based scintillators as far as light output is concerned. 216 As the pellets are quite thin, the use of alpha-emitting source is obvious in terms of 217 characterization with radionuclides. Alpha emitters have a short penetration distance in matter, 218 and therefore ionize the pellet by depositing all their energy as shown by simulation (Inset of 219  higher energy emitter 36 Cl led to an increasing response in channels, thus in deposited energy. 260 However, comparing 36 Cl spectrum with a much higher energy emitter such as 90 Sr/ 90 Y, no 261 important change of the spectrum was noticed. This is not surprising considering the high-262 energy 90 Sr beta particles compared to the size of the pellet. Simulated detection efficiency 263 (Inset of Figure 2.D) confirms that the generation of less photons comes therefore from a partial 264 energy deposition within the pellet, as both energy deposition spectra are similar in shape and 265 intensity. 266 Moving on to gamma detection possibility, and knowing that the geometry of our scintillators 267 is not ideal for such detection (which requires large detector volume in general), several 268 scintillation spectra were recorded using low-energy gamma emitter such as 241  a Compton edge (CE) whereas MOF-205 is composed of a unique Gaussian-type spectrum. We 276 expect that it is a convolution of CE and PE with the corresponding maximum attributed to the 277 59 keV gamma ray. As the considered energy is low and therefore near to the background noise, 278 we used a coincidence assembly to go deeper in our interpretation. Comparison between forms 279 of both spectra ( Figure S7) also shows discrepancy. The fact that there are two patterns for 280 MOF-205 is in agreement with our above explanation. So far, the best explanation is that due 281 to the poor resolution of our measurement chain, it is not possible to correctly separate the 282 Compton edge from the PE. Instead, we have a convolution of both corresponding distributions. 283 This trend was also observed for 133 Ba ( Figure S8). Contrary to the 241 Am configuration it is 284 possible to distinguish two contributions. We hypothesized a probable 356 keV full absorption 285 peak but it was difficult to investigate and no formal conclusion was drawn even after 10 million 286 pulses recorded. We estimate that the first visible maximum around channels 1900 corresponds 287 to the 81 keV full absorption peak. By comparing the channels between 241 Am and 133 Ba, these 288 contributions seem to correspond to the two emitted gamma at 81 keV and 356 keV confirming 289 the above hypothesis. To the best of our knowledge, the observation of PE in MOF was never 290 achieved yet and we believe that this was possible in this study due to increased densification. 291 Considering both simulation and experimental data, it was possible to establish a calibration 292 curve by making a parallel with 244 Cm alpha spectrum, 60 Co and 36 Cl beta emitters and gamma 293 emitter such as 241 Am and 133 Ba. To do so, an energy deposition endpoint for beta distributions 294 was read as the mean value between the first value that reaches zero and the last. For the specific 295 alpha emitter 244 Cm, the point was read as the average mean value of the peak. For 241 Am and 296 133 Ba gamma emitters, the point was taken into account only for MOF-205 as it presents a full 297 absorption peak and it was read at the maximum of the curve endorsed by simulation. Figure  298 2.F shows the channels versus the corresponding simulated maximum energy deposition for 299 BC-404 and sintered MOF-205. For BC-404, it is possible to say with confidence that our model 300 fits well as the trend curve passes through the three points with an R² factor of 0.9998. This 301 furthers confirms our beta endpoint determination method, which has sufficient precision for 302 an energy calibration curve. Regarding MOF-205 the model looks consistent with the exception 303 of 244 Cm. This confirms the previous hypothesis that the auto-quenching for alpha ionization is 304 more important in the MOF than within the BC-404, which means that the output energy is 305 lower than the would be perceived 560 keV. It is also important to note that the trend is linear, 306 even at low energy. However, it is well known that both organic and inorganic scintillator are 307 not linear with the incident energy, this effect appearing below 100 keV. [21 This observation 308 remains far beyond the scope of this study as the MOF scintillation is still a new field and 309 requires further exploration to draw consistent conclusions. 310 Having explored the scintillation performances of the sintered MOF-205, we tried to challenge 311 the material up a bit with the study of its potential discrimination properties. In particular, 312 alpha/beta and alpha/gamma discrimination were evaluated. It is noteworthy that such 313 properties are not straightforward for all-purpose scintillators and have never been studied in 314 MOF scintillation to the best of our knowledge, even if it was hinted by previous results. [15 315 This discrimination is related to higher ionization densities within the material when the 316 incoming particle becomes heavier. This lead to a denser population of excited state, causing 317 increase proximity of triplet state. . [ First is the alpha/beta discrimination, second is the alpha/gamma discrimination. Due to the 324 small size of the MOF-205 scintillator compared with our 2.5 cm diameter sources, experiments 325 were performed sequentially, that is to say alpha then beta or gamma spectra. Figure 3, top  326 shows the bidimensional spectra of 244 Cm (left), 36 Cl (center) and their addition (right). Since 327 the tail of alpha-related pulses is slightly longer than beta-or gamma-related pulses, the 328 integration of the delayed charge over the total charge allows sorting the nature of the excitation 329 that led to scintillation. Such pellet configuration is favorable for this discrimination as the 330 scintillator is intrinsically poorly sensitive to gamma rays and alpha emitters see the full 331 absorption of their energy within the material. But still and as expected, alpha/gamma 332 discrimination using a gamma-emitting 133 Ba source was also possible (Figure 3, middle). As 333 a visual comparison, alpha/beta discrimination of BC-404 was less pronounced (Figure 3, 334 bottom), with the two lobes being tilted with a positive slope for an unknown reason. BC-400, 335 a close equivalent to BC-404 was found to display moderate α/β discrimination as well. [25 . In 336 addition, a noticeable Figure of Merit (FOM) of 0.55 was calculated over the full spectrum for 337 both α/β and α/γ discrimination ( Figure S9). Ultimately, fast neutron/gamma discrimination 338 with MOF-205 was also tested but the results were harsh to interpret, mainly due to the small 339

MOF-205
their energy is fully absorbed by the material and the stopping range is not too elevated. Thus, 353 a 400 µm thick MOF-205 pellet displayed interesting scintillation properties, an emission 354 wavelength of 409 nm, a mean decay time of 14.3 ns and a scintillation yield 37% the one of 355 anthracene. Both alpha, beta and gamma experimental spectra were supported by MCNP6.2 356 calculations. Our sintered MOF was not fully transparent but the as-prepared pellet was 357 prepared exclusively from MOF-205, as this was our main goal. Pellets potentially prepared 358 with diluted MOF-205 with cubic powder of the same refractive index would lead to materials 359 with better transparency. It is the first time that alpha/beta and alpha/gamma discrimination is 360 qualitatively acknowledged for a MOF. Finally, this study opens a new and exciting research 361 topic. First, we guess that sintered transparent MOFs, achieved for the first time in this work, 362 will be an ongoing and explored field in the next years for optical application mainly. Secondly, 363 this derivative class of metal organic frameworks constitutes a brand new class of scintillator 364 full of opportunities. For instance by using the unique versatility of MOFs and by playing on 365 the composition with heavy metal as nodes and on sintering parameters, we guess that it should 366 be possible to be more sensitive to some ionization and therefore increase the energy response. 367 The goal is to be positioned between organic and inorganic scintillators as a new class of hybrid 368 materials. We hope that this report will be a tremendous input in the field as it brings two new 369 concept relative to the already rich area of MOF: sintering and scintillation discrimination. 370

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Supporting Information 372 Supporting Information is available from the Wiley Online Library or from the author. 373 374

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The Authors wish to thank Pr. Christophe Dujardin for X-ray TCSPC measurement. This 376 project has received funding from the European Union's Horizon 2020 research and innovation 377 programme under Grant Agreement No 899293. This document reflects only the authors' view 378 and the Commission is not responsible for any use that may be made or the information it 379 contains 380 Received: ((will be filled in by the editorial staff)) 381 Revised: ((will be filled in by the editorial staff)) 382 Published online: ((will be filled in by the editorial staff)) 383 384