Water dispersible SrF2: Tm3+, Yb3+ upconverting nanoparticles (9 nm diameter) have been synthesized to elucidate their potential as ultraviolet emitters. High intensity upconversion ultraviolet emission bands centered at 350 and 360 nm were detected following excitation at 980 nm in the near-infrared region for H2O and D2O colloidal dispersions of the upconverting nanoparticles, corresponding to transitions from the I-1(6) and D-1(2) energy levels of thulium (Tm3+) ions. Both emission intensities were strongly dependent on the temperature and the H2O/D2O molar ratio of the media, two important criteria that define the different environments in which the nanoparticles can be applied. Since the variations observed were different for each emitting level, the relevance of each band is discussed in relation to these two criteria.
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