Long, N. V., D. T. Xuan Thao, N. D. Chien, and P. T. Binh. “Structure and Physico-Chemical Properties of Silica Gels Doped With Optically Activated Er3+ Ions by Sol-Gel Process”. Vietnam Journal of Chemistry, vol. 45, no. 1, Sept. 2014, p. 114, doi:10.15625/4721.