Upconversion nanoparticles based on rare-earth elementsDmitrii Zharkov, Andrey Leontyev, Artemii Smelev, Victor Nikiforov, Vladimir Lobkov, Masfer Alkahtani and Philip HemmerEPJ Web Conf., 220 (2019) 03033DOI: https://doi.org/10.1051/epjconf/201922003033