Upconversion nanoparticles based on rare-earth elements Dmitrii Zharkov, Andrey Leontyev, Artemii Smelev, Victor Nikiforov, Vladimir Lobkov, Masfer Alkahtani and Philip Hemmer EPJ Web Conf., 220 (2019) 03033 Published online: 25 October 2019 DOI: 10.1051/epjconf/201922003033