OrganicâInorganic Composite Films Based on Gd3Ga3Al2O12:Ce

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Organic-Inorganic Composite Films Based on GGAG:Ce Scintillator Nano Particles For X-ray Imaging Applications Shashwati Sen, Mohit Tyagi, Kusha Sharma, Partha Sarathi Sarkar, Sudip Sarkar, Chandra Bhanu Basak, Shreyas Pitale, Manoranjan Ghosh, and Sanjay C. Gadkari ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11289 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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ACS Applied Materials & Interfaces

Organic-Inorganic Composite Films Based on GGAG:Ce Scintillator Nano Particles For X-ray Imaging Applications Shashwati Sen1,*, Mohit Tyagi1, Kusha Sharma2, P S Sarkar1, Sudip Sarkar3,C B Basak3, S. Pitale1, M. Ghosh1, S. C. Gadkari1 1Technical Physics Division,3 Glass and Advanced Materials Division, Bhabha Atomic Research Centre, Mumbai, India 2

Department of Converging Technology, University of Rajasthan, Jaipur

KEYWORDS Scintillators, organic-inorganic, nano-composite, luminescence, X ray radiography . ABSTRACT Organic-Inorganic nanocomposite self-standing films of Gd3Ga3Al2O12 (GGAG) uniformly dispersed in PMMA and PS polymer are prepared for radiography application. GGAG:Ce nano-scintillator has been chosen because of its high light output and fast decay time. The nano powder of GGAG is synthesized by co-precipitation method and dispersed in the polymer matrix by a simple blending technique. The nanocomposite films of thickness in the range of 150-450 µm with a very high inorganic content is achieved by this technique. These films are characterized for their uniformity, optical absorption, photo luminescence and radio luminescence. These films are further tested for their application in radiography by recording Xray image using commercially available CCD camera. A resolution of 10 lp/mm is obtained

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using GGAG: PMMA composite film with 50% loading confirming their application in imaging devices. 1.

INTRODUCTION A new class of smart materials has emerged recently consisting of a blend of organic and

inorganic materials. These hybrid composite materials capitalize on the diverse characteristics of available materials to generate smart materials having the properties of both the constituents. The organic-inorganic composite films consist of nano-sized particles of the inorganic sensitizer dispersed in a matrix of an organic polymer. The motivation being the optical properties of the inorganic constituent like high light output, band gap engineering and activator tailored emission, amalgamates with the transparency, environmental stability, mechanical ruggedness, resistance to radiation and laser damage, low cost, flexibility and large size along with the ease of preparation of organic materials.1 These materials have found applications in display,2 lighting devices, solar cells,3,4 dentistry,5 scintillation,6,7,8 radiography, tissue engineering, shielding of waves, thermal stabilization, flame retardancy, antibacterial and hydrophilicity/hydrophobicity. 916

The most common method employed to fabricate organic-inorganic devices is to incorporate nano particles in a polymer matrix and molding it into a desired shape.17,18 This method retains the functionality of both the constituents of the composite and in few cases has also been found to generate new properties which do not exist in either of the material.19 The other method which has been reported to prepare an organic-inorganic nano composite is to load inorganic nano particles into a monomer matrix followed by in-situ bulk polymerization.20-21


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The main challenge in the processing of these composite materials is to stabilize the inorganic nano particles homogeneously by preventing agglomeration in the polymer matrix while maintaining the transparency and of the polymer.22-23 The factors affecting the transparency are size of the inorganic particle which acts as scattering centers and the refractive index matching of the constituent organic and inorganic components. Agglomeration increases the light scattering which limits the higher loading concentration of the inorganic component in the organic matrix. Many capping agents, surfactants, ionic liquids, and complicated synthesis techniques are generally reported to prevent the aggregation.24 Reports also suggest that increased loading of the inorganic hampers the in-situ polymerization process and leads to phase separation including formation of nanocrystals within the monomer or polymer media.25,19 The bulk insitu polymerization process also leads to loss of transparency and luminescence quenching. This has restricted the percentage of inorganic loading in most of the cases.26 The use of organic-inorganic composite films has also been reported in X-ray imaging applications.27-30 The field of X-ray imaging is continuously evolving with new applications in the up-coming field of medical imaging, industrial digital radiography as well as tomography and homeland security.31,32 The biggest challenge at this moment is the advent of new materials with superior properties like high light output, better uniformity, higher spatial resolution and minimum after-glow effects and the most difficult one is large active area.33,34 Most of the currently available radiography devices use CsI:Tl scintillator thin films or gadolinium oxysulfide scintillator as the imaging screen.35 Since it is difficult and uneconomical to prepare large area screens using single crystals, organic-inorganic scintillators films are alternate class of material which can replace the existing ones.29 The other advantage of the composite films for radiography are their amiable optical properties, flexibility, radiation hardness and economical


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for industrial applications. This is compounded with the fact that in case of X-ray imaging the thickness of the film should be just to stop the incoming radiation and at the same time should be thin enough not to affect the spatial resolution of the formed image. The most commonly used polymer for composites reported in literature is poly-methyl methacrylate (PMMA) and polysterene (PS) both having refractive index in the range of 1.5. 6,28,36

The PMMA polymer is most extensively studied matrix for nanocomposites because of it

excellent properties such as exceptional optical clarity (92% light transmission), weather; water and chemical resistant, light weight, high mechanical strength, excellent dimensional stability, and resistance to laser damage and stable in UV environment (UV radiation and high energy radiation doses up to 100 kGy). It is soluble in most non-polar solvents such as benzene and toloune and polar solvents like acetone, dimethyl formamide and tetrahydrofuran which makes it easy to blend into composite matrix. The PS is chosen as a host matrix for organic-inorganic composites because it is colorless and

flexible

as

thin

films,

which

are

ideal

for

investigating optical properties and has low-cost and shape flexibility. PS can with stand high energy gamma radiation up to 10000 kGy making it a radiation hard material suitable for X-ray imaging.37 Polystyrene is soluble in most non-polar solvents such as toluene and benzene. In this study we have used Gd3Ga3Al2O12:Ce (GGAG:Ce) as the inorganic scintillator.38 GGAG is a newly discovered garnet scintillator with promising properties like high light output, high density and stability, fast decay time and radiation hardness that make it an ideal material for imaging applications (also referred as GAGG).39 The GGAG single crystals have been grown by the Czochralski technique and have shown tremendous potential as radiation detector. The single crystal growth of this material has its drawback because of high melting temperature of 1850°C and multiple components. These problems have been addressed by preparing transparent


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ceramics of GGAG which has shown equivalent properties to that of single crystals.40-42 Preparation of highly transparent ceramics requires nano-particles of GGAG but very few studies are available on the synthesis of nano particles of GGAG in the literature.43 There is further scope to investigate better, facile and economical method for GGAG nano powder synthesis. No study has been reported on use of GGAG for imaging application and its use for the synthesis of organic-inorganic composite. We have synthesized GGAG:Ce scintillator nano powder by solid state sintering as well as chemical methods. The co-precipitation technique was found to be a low temperature synthesis method for large-scale synthesis of nano-sized particles. Further GGAG and polymer based organic-inorganic composite films have been fabricated using a simple and facile technique for X-ray imaging applications. A loading percent upto 80% has been achieved with uniform dispersion in the flexible films. Our results have shown that the nano-composite of GGAG and polymer can be successfully utilized for X-ray imaging. 2.

EXPERIMENTAL SECTION The polymer-GGAG nanocomposites were prepared in two steps. In the first step the

nano powder of inorganic scintillator GGAG:Ce was synthesized. Subsequently it was physically mixed with the organic polymer to prepare the composite screen. The GGAG powder was synthesized by chemical as well as solid state route. In the solid state sintering technique high purity constituent oxides namely Gd2O3, Ga2O3, Al2O3 and Ce2O3 were taken in stiochiometeric ratio and physically mixed together in a mortar and pestle. The powder was sintered at 1400°C for 24 hours in a box furnace under air to get a pure phase of GGAG:Ce material.


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For the synthesis via co-precipitation method the nitrate

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salts of gadolinium, gallium,

aluminium and cerium (gadolinium nitrate hexahydrate (purity 99.9%), gallium nitrate (purity 99.99%) aluminum nitrate (Al(NO3)3 .9H2O, 98% purity) and cerium nitrate hexahydrate (99.9% purity) were taken as the starting charge and dissolved in DM water separately to obtain a 0.02M solution. Ammonia solution was used as the precipitating agent. The individual solutions of the metal salts were mixed together under constant stirring and kept for 8 hours for proper mixing. Ammonia solution was added drop-wise into this solution under vigorous stirring and then aged for 24 hrs to assist full precipitation. The precipitate was washed repeatedly and separated by centrifugation. The dehydrated powder was finally annealed in box furnace at temperatures between 500-1000°C for proper phase formation. The Ce doping amount was kept at 1 mol% in all the samples for all experiments. To make the organic inorganic nano-composites polymer powder of PMMA and PS were used. Commercially available PMMA or PS (Aldrich make) was dissolved in organic liquids like hexane, toloune, acetone etc by ultra-sonication to achieve a transparent solution of the polymer. The final solvent was decided based on the clarity and the viscosity of the solution. The GGAG:Ce powder (both solid state sintered and chemically synthesized) was first sieved through a mesh of 25 micron and dispersed in acetone and ultra-sonicated for 2-3 h to achieve a homogeneously dispersed solution. The amount of GGAG powder was varied from 5% to 80% by weight to that of polymer powder. This solution was mixed with the polymer solution under constant stirring followed by ultra-sonication to ensure proper dispersion of GGAG in the polymer solution. The most homogeneous GGAG nano particle dispersion was achieved for PMMA-Toloune and PS-hexane solutions. All further data in this report is for composite films made with these combinations. The organic-inorganic composite mixture solution was spread in


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a flat glass dish and kept at room temperature overnight for solidification by drying. After the evaporation of the organic solvents a film of GGAG-Polymer composite was obtained. This film on complete drying leaves the wall of the container and a self-standing film of diameter 50 mm is achieved as shown in Fig. 1. Films of GGAG-PMMA and GGAG-PS were prepared by this method for 5%, 10%, 20%, 50% and 80% loading fractions of GGAG scintillator powder. The films are named as GGAG:Polymer-loading% such as GGAG:PMMA-80 means composite film of GGAG:Ce with PMMA polymer having 80% loading percentage of GGAG. The thickness of the films was around 150 µm. A few experiments were also carried out to fabricate films of various thicknesses from 150 µm to 700 µm.

[a]

[b]

Fig 1. GGAG:PS-80 free standing nano composite films (a) under normal light showing the flexibility of the films (b) under UV-illumination. The GGAG power was synthesized by the coprecipitation method.

The phase purity of the synthesized powder was evaluated using X-Ray diffractometer (Rigaku Model: RINT 2000 Dmax, CuKα line). A Carl Zesis-Auriga field emission scanning electron microscope (FESEM) system was employed to study the microstructure of the GGAG nano


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powder. The nano-powder was dispersed in ethanol by ultrasonication, deposited drop wise onto a holey carbon coated copper grid, and transmission electron microscopy was carried out using a JEOL, 2000FX microscope operated at 160 kV. To investigate the morphology of the nanocomposite film, synthesized using both the methods, X-ray micro-imaging technique was employed. The system comprises of a 160 kV-1 mA (5 µm focal spot) microfocus X-ray tube and a fibre-optic coupled X-ray CCD camera (1k × 1k with 16 bit digitization).44 The composite film was imaged with tube parameters set at 30 kV and 325 µA, acquisition time of 10 seconds and at various geometrical magnifications. The films were optically characterized by measuring absorption spectra using Shimadzu UV3600 spectrophotometer in diffuse reflectance geometry and

photo-luminescence

spectroscopy using a fluorescence spectrometer (Edinburg Model-FLP920) in the range 200-800 nm. Fluorescence curve were recorded in a reflection geometry using an Edinburgh makeFLP920 spectrometer with a continuous Xe lamp as the excitation sources. The samples were positioned at 45° with respect to the excitation beam. The recorded luminescence spectra were corrected for the spectral sensitivity function of the instrument. The X-ray excited luminescence, also known as radio-luminescence (RL) measurements were carried out using a customized Seiferet X-ray generator with a W tube operating at 40 kV and 30 mA. The RL emission was recorded by the help of an optical fiber based Avamatsu spectrometer. The same X-ray setup was used for X-ray imaging applications. The free standing film was mounted on a stand and images were recorded using commercially available Nikon make CCD camera. The distance between the X-ray source and the object and that of the image and the composite film was around 50 mm. 3.

RESULTS AND DISCUSSION


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The X-ray diffraction spectra of the GGAG: Ce nano powder synthesized by co-precipitation method, solid state method and GGAG-PS-80 composite film are given in Fig 2 (a). The nanopowder synthesized by the co-precipitation method has been sintered at 1000°C for 24 h and the solid state sintered powder is sintered at 1400°C for 24 h. The peaks are indexed and matched with JCPDS-44-0448 database. All the samples show formation of the pure phase of GGAG:Ce.38,39 No peaks corresponding to constituent oxides were observed. The peak width of powder prepared by co-precipitation is broader than powder synthesized by solid state which indicated that the grain size is smaller in case of GGAG:Ce powder synthesized by chemical technique. The XRD pattern of the GGAG:PS-80 sample shows the presence of all the peaks of GGAG along with a hump at lower θ values which is arising because of the polymer matrix. The GGAG powder in the composite sample has been synthesized by the co-precipitation technique.

Figure 2. XRD of (a) GGAG nanopowder synthesized by solid state sintering and Coprecipitation methods. The XRD of GGAG-PS-80 composite film is also given for comparison, (b) XRD of GGAG:Ce powder synthesized by co-precipitation and sintered at various temperatures for 24 h.


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Figure 2(b) shows the XRD of the GGAG sample prepared by the co-precipitation method. The various XRD graphs show the phase of the powder after sintering at different temperatures for 24 h. The as-prepared sample after co-precipitation is amorphous in nature. No phase formation is observed for sintering upto 800° C. A few peaks start appearing at 800° C and powder sintered at 900° C for 15 h starts displaying the major reflections of GGAG: Ce. Sintering at 900° C for 24 h caused considerable crystallization, and nearly all of the characteristic peaks corresponding to GGAG: Ce appeared with a slight trace of un-reacted oxides. Sintering at 1000°C for 24 h results in the formation of phase pure GGAG powders from the precipitates. It is further observed that sintering at a temperature above 900° C leads to continued refinements in peak shapes and intensities indicating growth of crystallite size. It is to be noted that precipitates formed from solution with molarity less than 0.02M does not gives a single phase GGAG power on sintering upto 1400°C. Thus the molarity of the initial solution plays an important role in the phase formation of the garnet powder. Fig 3. shows FESEM micrographs of the GGAG:Ce powder synthesized by the two techniques. The average grain size of the co-precipitated sample before annealing is around 2540 nm (Fig 3a). On annealing at 1000C for 24 h the average grain size increases to ~100 nm (Fig 3b). The increase in the size is because of agglomeration of multiple grains at high temperature. However, a few grains of size even greater than 500 nm are also visible in the chemically synthesized powder followed by sintering. The grain size reported by other group by co precipitation technique is also in the range of 50 nm to 100 nm.43 The FESEM micrograph of the solid state sintered powder shows the formation of grains by agglomeration of smaller crystallites (Fig. 3c). The smaller crystallites are of size is in the range of 300-500 nm (Fig 3d) which agglomerate to form larger grains of size in the range of few microns (Fig 3c). These


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FESEM micrographs further confirm that the grain size of the GGAG powder synthesized by coprecipitation technique is smaller than that synthesized by the solid state technique. The FESEM micrograph of composite films prepared using chemically synthesized GGAG are given in Fig. 3(e) and Fig. 3(f). The films are found to be quite uniform in the scale of 1 micron. At still higher magnification we can see GGAG particles homogeneously dispersed in the polymer matrix.


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Figure 3. Field Emission Scanning Electron Microscopy images of as-prepared GGAG:Ce (1 at%) synthesized by co-precipitation method (a,b) and solid state sintering method (c,d). (a) as prepared precipitates of GGAG and (b) co-precipitate powder sintered at 1000 ºC for 24 h at higher magnifications. (c,d) Solid state sintered powder at 1400°C for 24 h under various magnifications. (e,f) GGAG:PMMA-50 film of thickness 150 micron, GGAG is prepared by coprecipitation. The morphology of the chemically synthesized GGAG powder was further investigated using the transmission electron microscopy technique (TEM) as shown in Fig 4. Fig 4(a) is the TEM micrograph of the as-synthesized precipitates of GGAG. It is quite evident that the powder is amorphous in nature with no grain formation. The TEM image after sintering (Fig 4b) shows that the sintering leads to grain growth as well as crystallization. The grain size is around 100 nm and the presence of the crystalline planes in the HRTEM image of Fig 4(c-d) confirmed the high crystalline nature of the nano-sized grains of GGAG. Figure 4(d) shows a high-resolution TEM (HRTEM) image and selected area electron diffraction (SAED) pattern of the nanoparticles. The lattice spacing as calculated from the HRTEM image is ~6 Å, which corresponds to the (200) lattice plane of GGAG garnet lattice. Fig 4(c) further shows how the grain growth of the nanoparticles takes place during the sintering process by neck formation between two nano-particle and then fuse together to form bigger grains. These images confirm that the nanoparticles are single crystalline in nature and have (200) axis as the major growth direction.


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Fig 4 TEM images of nano-particles of GGAG chemically synthesized by the co-precipitation technique. (a) micrograph of as-synthesized particles, (b) micrograph of precipitate sintered at


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1000°C, (c) HRTEM image showing the high crystalline nature of the nano particles and the atomic planes of GGAG along (200) direction, (d) SAED pattern of the nano particles. The morphology and homogeneity of the nano-composite film were also investigated using the X-ray tomography technique. The uniformity of the screens were evaluated from their X-ray images using standard Beer-Lambert's law and finding out the integrated linear attenuation (linear attenuation coefficient × thickness) at various locations of the images. From this the coefficient of variation (CV) or the relative standard deviation were calculated to find out the degree of uniformity in the screens. The coefficient of variation is defined as the ratio of standard deviation to the mean in the area of interest. The more the CV, the uniformity is less. The CV for composite films prepared using nano powder synthesized by co-precipitation method was 3.27 % and by solid state sintering technique was 3.5 % at highest geometrical magnification (pixel resolution~5 microns). Fig. 5 shows the X-ray tomography images at various magnifications for composite films prepared using nano powder synthesized by co-precipitation method (Fig 5 a,c,e) and by solid state sintering technique (Fig 5 b,d,f). Images at lower magnification shows both the films uniform with no noticeable voids, however the density of the films synthesized by the solid state sintered powder appears to be higher. At higher magnification as evident from Figure 5(f) the composite films prepared from chemically synthesized nano-particles are more homogeneous in a range of 20 × 20 micron with lesser number of voids. Both the above mentioned results are because of the larger grain size of the solid state sintered particles which makes the films of higher density but less homogeneous. All further studies were carried out on films prepared with co-precipitate synthesized GGAG powder.


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\ Fig 5. X-ray micro tomography images of nano-composite films under different magnifications. The left panel (a,c,e) shows images of film with co-precipitate synthesized GGAG and on the right panel (b,d,f) are images of composite films with solid state sintered GGAG powder.


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The optical images of the GGAG:polymer composite films with different loading percentages of chemically synthesized GGAG nano particles under visible as well as UV illumination are given in Fig 6. Fig 6(a) are the photographs of GGAG:PMMA films and Fig 6(b) are of GGAG:PS films with different loading percentage of GGAG. The thicknesses of all the films were around 150 microns. In both the types of composite films as the loading percentage increases the transparency decreases and the films are totally opaque at a loading percent above 20%. We have been able to prepare homogeneous and free standing films with maximum 80% loading. Beyond this loading the films are found to be under stress and starts cracking. The self-standing capability is an additional benefit for imaging applications as the effect of substrate could be totally avoided and images could be directly captured in transmission or reflectance mode as they form on the film which acts like a screen. Secondly these films can be mounted directly on a CCD sensor for better sensitivity and resolution. We were able to prepare films with maximum diameter of 100 mm with 50% loading. All the films are flexible in nature with PS based composite showing higher flexibility than PMMA based films. However increasing the loading percentage makes the film brittle. Under UV illumination all the composite films show a green emission because of the Ce doping. The intensity of the green emission increases with the loading concentration as observed in the last row of Fig 6. The composite films with PS and PMMA do not show any visible difference under visible as well as UV light. Also the effect of different solvents i.e. toloune and benzene used to dissolve the polymer powder do not render any difference in the physical or optical properties of the composite films.


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Fig 6. Optical photograph of (a) GGAG:PMMA films with different loading amount under white light, (b) GGAG:PS composite films with different loading percentages under white light, (c) GGAG:PMMA and GGAG:PS films with different loading amount under UV illumination. (GGAG nano powder used to prepare these films was synthesized by co-precipitation method). For application in radiography it is important to optically characterize the GGAG:Ce nano particles as well as the nano-composite films. The optical characterization was carried by recording the absorption spectra and the photo-luminescence spectra.


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(b)

(a)

Fig 7 (a) Absorption spectra of GGAG:PMMA, GGAG:PS film and GGAG:Ce nano powder synthesized by co-precipitation technique, (b) Absorption spectra of GGAG:PMMA-80 film of thickness 150 µm and 300 µm, (c) Transmission spectra of GGAG:PS film with different loading percentage along with the emission curve.

The absorption curve for GGAG:polymer film was studied through diffused reflectance setup. The absorption spectra of GGAG:Ce nano powder synthesized by co-precipitation method and that of GGAG:PMMA and GGAG:PS film are compared in Fig 7 (a). The absorption spectra


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are similar to that reported for single crystals.39 The absorption edge for the GGAG:Ce nanoparticles is observed to be around 210 nm and therefore indicates that the band gap of GGAG is in the range of ~5.8 eV. The main absorption bands for all the three samples are observed at 440 and 340 nm which can be assigned to Ce3+ 5d to 4f transition.38,39 However in the composite of GGAG:PS we observe another absorption band at 280 nm attributed to the PS matrix.45 GGAG:PMMA composite does not show any absorption band of PMMA. The comparison between the absorption spectra of two GGAG:PMMA polymer composites film of 150 and 300 micron thickness is given in Fig. 7b. No significant change in the absorption property for different thicknesses is observed confirming that the polymer matrix is transparent to the GGAG:Ce emission and thicker films can also be employed as imaging screens. Fig 7c shows the transmission spectra of GGAG-PS film with different loading percentage. The emission spectra is also given along the transmission spectra to show that the films are transparent to its own emission. It is observed that with increase in the loading percentage the transmittance decreases due to the increased scattering from the GGAG particles.

4000

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Fig 8.PL spectra of (a) GGAG:PMMA-20 (b) GGAG:PS-20 film


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Fig 8 (a) and (b) show the PL spectra of the composite film GGAG:PMMA-20 and GAGG:PS-20. The emission spectra for both the composite film consist of a broad peak at 550 nm. This emission band is due to the transition of Ce3+ from the lowest excited 5d state to the 4f ground state. This emission spectrum is exactly the same as that observed for a GGAG;Ce single crystal.30 This confirms that the luminescence property of the composite film is dominated by the inorganic component (GGAG:Ce) and organic polymer is transparent to this emission and it does not give any additional emission or it affects the luminescence property of GGAG. The advantage of the green emission at 550 nm is that it can be directly coupled to a CCD which has its maximum sensitivity in this range for imaging applications. However the excitation spectra of the two composite films are remarkably different. The excitation spectrum of GGAG:PMMA consists of two peaks corresponding to that of GGAG:Ce (350 and 450 nm) with no separate peak corresponding to PMMA.38 In case of the composite film of GGAG:PS an additional peak at 270 nm which is a characteristic of PS polymer is observed.45 This excitation peak is also seen in the absorption spectra of Fig. 7(b). The scintillation properties of the nano-composite films were evaluated by measuring their radio-luminescence (RL) characteristics. The main RL peak observed at 550 nm as given in Fig 9(a) closely resembles that of the PL peak and hence can be attributed to the Ce emission. Fig 9(b) and (c) are the integrated RL intensity of the composite films plotted as a function of Xray tube voltage and tube current. The data show that the integrated RL intensity initially increases with increasing voltage and then starts to saturate. The RL property of the composite films confirms that these composite films can be used for radiation detection as well as X-ray imaging screens. The high density of GGAG power (5.9g/cc) and the high loading concentration


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also renders higher X-ray stopping power. The measured X-ray stopping of the 500 micron GGAG:PMMA composite film found to be 40% .

10000

11000

8000

(a)

20kV 22kV 24kV 26kV 28kV 30kV 32 kV 34kV 36kV 40kV

Varying voltage at current 30 mA

6000 4000 2000 0 400

450

500

550

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650

10000

Integral Intensity (a.u.)

(a)

Intensity (a.u.)

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X-ray Tube Voltage 40kV

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10

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15

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(c)

Integral intensity

X-ray Tube Current 30 mA

9000 8000 7000 6000 5000 4000

20

25

30

35

40

X-ray Tube voltage (kV)

Fig 9. Radioluminescence spectra of (a) PMMA GGAG film under different X-ray tube voltages. (b) Integrated RL intensity for the GGAG:PMMA composite film by varying tube current. (c) Integrated RL intensity for the GGAG:Ce-PMMA composite film as function of varying tube voltage at fixed current.

The efficiency of the composite films for X-ray imaging was tested in a setup shown in Fig.10(a). The composite film was kept at the screen position and direct image of the X-ray beam as well as image shadow of various SS mesh was recorded using a commercially available CCD camera in the transmission mode. Fig 10(b) shows the X-ray images recorded with GGAG:PS as


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well as GGAG:PMMA films with different loading percentages. In the first row of Fig.10(b) images of the X-ray spot (2 mm diameter) generated on the GGAG:PS composite films are shown. We observe that the brightness (light output) and the sharpness of the image increases with the loading from 5 to 50%. The second row shows images captured on GGAG:PS and GGAG:PMMA composite for SS mesh with wires of 500 µm separated by the same distance The best image is recorded for a 50% loaded film. The image recorded with 80% loading film was bright but found to be diffused due to scattering of light by the GGAG grains. As stated earlier increasing the loading percentage of GGAG beyond 50% also rendered brittleness to the films and generation of stress which makes the films to curl at the edges. The PMMA based composites were found to be more brittle at 80% loading as compared to PS based film. 3rd row are the images of SS cross wire and standard line pair (LP) on GGAG:PMMA composite films. Similar trend of increasing brightness with loading concentration is observed in these composites. The best results have been achieved in 50% loading. A spatial resolution of 100 µm was achieved with GGAG:PMMA-50 composite film using standard line pair (last row). The last radiography image is of a 100 µm (10 lp/mm) thin standard line pair sample, imaged successfully by a GGAG:PMMA-50 composite film. We would like to add that this value is comparable to a resolution of 5 lines/mm reported by Kang et al29 and much better than resolution of 2.8 lines/mm reported for commercial Gd2O2S:Tb screen.


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(a)

(b) Fig 10. (a) X-ray radiography setup consisting of X ray source, object, CCD camera and the Composite Screen. (b) X-ray image on GGAG:PS and GGAG:PMMA composite films of loading 10%, 20%, 50% and 80% showing the full X-ray beam, images formed by a SS line mask of spacing 0.5 mm, images of SS cross wire mask and standard lp phantom with 10 lp/mm.


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4. Conclusions

We have successfully synthesized single phase nano powder of Gd3Ga3Al2O12:Ce inorganic scintillator by chemical co-precipitation technique followed by sintering at 1000°C. Organic-inorganic free standing composite film of GGAG nano-particles with PMMA and PS polymer of thickness 150 µm were prepared by a simple technique with high loading concentrations upto 80%. The composite films show similar optical characteristics as that of a single crystal of GGAG:Ce. These GGAG:PMMA and GGAG:PS films were also tested for their X-ray imaging applications. The X-ray radiographs showed that the films with 50% inorganic loading has bright image quality and can spatially resolve 100 µm features. This technique can be further scaled up to prepare larger screens for industrial applications. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. REFERENCES (1)

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Table of Contents Graphic


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Figure 1. GGAG:PS-80 free standing nano composite films (a) under normal light showing the flexibility of the films (b) under UV-illumination. The GGAG power was synthesized by the co-precipitation method. 254x190mm (96 x 96 DPI)


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Figure 2. XRD of (a) GGAG nanopowder synthesized by solid state sintering and Co-precipitation methods. The XRD of GGAG-PS-80 composite film is also given for comparison, (b) XRD of GGAG:Ce powder synthesized by co-precipitation and sintered at various temperatures for 24 h. 254x190mm (96 x 96 DPI)



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Figure 3. Field Emission Scanning Electron Microscopy images of as-prepared GGAG:Ce (1 at%) synthesized by co-precipitation method (a,b) and solid state sintering method (c,d). (a) as prepared precipitates of GGAG and (b) co-precipitate powder sintered at 1000 ºC for 24 h at higher magnifications. (c,d) Solid state sintered powder at 1400°C for 24 h under various magnifications. (e,f) GGAG:PMMA-50 film of thickness 150 micron, GGAG is prepared by co-precipitation. 304x245mm (96 x 96 DPI)


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Figure 4 TEM images of nano-particles of GGAG chemically synthesized by the co-precipitation technique. (a) micrograph of as-synthesized particles, (b) micrograph of precipitate sintered at 1000°C, (c) HRTEM image showing the high crystalline nature of the nano particles and the atomic planes of GGAG along (200) direction, (d) SAED pattern of the nano particles. 254x190mm (96 x 96 DPI)



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Figure 5. X-ray micro tomography images of nono-composite films under different magnifications. The left panel (a,c,e) shows images of film with co-precipitate synthesized GGAG and on the right panel (b,d,f) are images of composite films with solid state sintered GGAG powder. 254x190mm (96 x 96 DPI)


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Figure 6. Optical photograph of (a) GGAG:PMMA films with different loading amount under white light, (b) GGAG:PS composite films with different loading percentages under white light, (c) GGAG:PMMA and GGAG:PS films with different loading amount under UV illumination. (GGAG nano powder used to prepare these films was synthesized by co-precipitation method). 254x190mm (96 x 96 DPI)



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Fig 7 (a) Absorption spectra of GGAG:PMMA, GGAG:PS film and GGAG:Ce nano powder synthesized by coprecipitation technique, (b) Absorption spectra of GGAG:PMMA-80 film of thickness 150 µm and 300 µm, (c) Transmission spectra of GGAG:PS film with different loading percentage along with the emission curve. 324x241mm (96 x 96 DPI)


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Figure 8.PL spectra of (a) GGAG:PMMA-20 (b) GGAG:PS-20 film 254x190mm (96 x 96 DPI)



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Figure 9. Radioluminescence spectra of (a) PMMA GGAG film under different X-ray tube voltages. (b) Integrated RL intensity for the GGAG:PMMA composite film by varying tube current. (c) Integrated RL intensity for the GGAG:Ce-PMMA composite film as function of varying tube voltage at fixed current. 254x190mm (96 x 96 DPI)


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Fig 10. (a) X-ray radiography setup consisting of X ray source, object, CCD camera and the Composite Screen. (b) X-ray image on GGAG:PS and GGAG:PMMA composite films of loading 10%, 20%, 50% and 80% showing the full X-ray beam, images formed by a SS line mask of spacing 0.5 mm, images of SS cross wire mask and standard lp phantom with 10 lp/mm. 254x190mm (96 x 96 DPI)


OrganicâInorganic Composite Films Based on Gd3Ga3Al2O12:Ce

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