Speaker
Description
ZnO:Ga scintillation powder is a promising candidate for detectors with ultrafast timing, thanks to its extremely short decay and practically no rising time [1]. For practical use, the powder needs to be embedded in a suitable optically transparent matrix. However, heat treatment of ZnO:Ga particles is needed to increase the luminescence at the cost of particle agglomeration. This in turn results in low transparency of scintillating nanopowder embedded in a matrix and constitutes one of the biggest disadvantages, besides low light yield and low stopping power of ZnO:Ga powder. Limiting ZnO:Ga particle size is therefore a key step in order to prepare highly luminescent and transparent composites with high potential to be appropriate for PET TOF applications.
Therefore, the research presented has two goals – preventing the particles agglomeration during heat treatment to improve homogeneous embedding and the related transparency, and surface passivation of ZnO:Ga particles to preclude non-radiation loses on the grains boundary.
For surface passivation, the covering by either organic polymers or inorganic layers is acceptable, depending on target application [2, 3]. In this work, two methods for limiting growth of particles were studied.
The first method was tested for surface passivation and limiting the particle size;ZnO:Ga particles were annealed at 200 °C and coated by SiO2 amorphous layer via sol-gel method. Annealed powder was dispersed in the solution of absolute ethanol and TEOS (tetraethylorthosilicate) which was hydrolysed by dropwise adding ammonia. The amount of TEOS was calculated from the specific surface area of ZnO:Ga to cover the particles by two layers of SiO2. Product was dried and heat treated at 300, 600 and 1000 °C. XRD measurements show that linear crystallite size is constant up to 600 °C. At higher temperatures, increase of the crystallite size and formation of zinc silicate phase was observed. Particle size was also calculated from the evaluation of specific surface area and it does not exceed 14 nm, regardless of the calculation method. Parasitic Zn2SiO4 phase deteriorates luminescence properties and therefore this surface modification is suitable only for powders not requiring heat treatment.
In the second method for preventing particle agglomeration, ZnO:Ga was dispersed in water and the sufficient amount of PVA (polyvynilalcohol) was dissolved to form gel. This gelatinous suspension was dried at 50 °C and subsequently heat treated at 600, 800 and 1000 °C. TEM images show well separated particles with ~50 nm in diameter after heat treatment at 600 °C. Above 600 °C, the particle size increases with increasing temperature.
The combination of those two methods of post-treatment may allow for improvement of the optical and luminescence properties of ZnO:Ga-based composites.
References:
[1] R. M. Turtos, S. Gundacker, M. T. Lucchini, L. Prochazkova, V. Cuba, H. Buresova, J. Mrazek, M. Nikl, P. Lecoq, E. Auffray, Phys. Stat. Sol. RRL 10, 843-847 (2016)
[2] Xiong, H.-M., Wang, Z.-D., Xia, Y.-Y. Advanced Materials, 18 (6), 748-751 (2006)
[3] Cong Chen, Haiping He, Yangfan Lu, Kewei Wu, and Zhizhen Ye. ACS Applied Materials & Interfaces 5 (13), 6354-6359 (2013)
