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. 2015 Nov 16;6:8920. doi: 10.1038/ncomms9920

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(a) Up-conversion photoluminescence spectra at 297 (red curve) and 281 K (blue curve) for an excitation energy of 1.13 eV. (b) Up-conversion (red circles, excitation energy=1.13 eV, I_UCPL) and normal photoluminescence intensities (black circles, excitation energy=2.18 eV, I_PL). The temperature-dependent normal photoluminescence intensity is fitted with a linear function (dotted line), which was used to normalize the up-conversion photoluminescence intensity at each temperature in c. (c) Arrhenius plot of the up-conversion intensity (I_UCPL) normalized by the normal photoluminescence intensity (I_PL) at the corresponding temperature (red squares). I_UCPL was divided by I_PL to account for the temperature-dependent variation of I_PL. The black solid line represents the Arrhenius equation for an activation energy of 120 meV.