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. 2013 May 14;8(5):e63292. doi: 10.1371/journal.pone.0063292

Figure 4. Spectral imaging of UCNPs.

Figure 4

Emission spectra of UCNPs in (A) single, (B) small cluster (designated ‘cluster 2’ in Figure 2) and (C) powder form (data points joined by solid lines) captured using hyperspectral epi-luminescence microscopy, overlaid with the ensemble-averaged spectra of UCNP powder captured by a calibrated spectrometer (dashed lines). The corresponding exposure times and EMCCD camera electron-multiplication (EM) gains were (A) 4 sec and 255; (B) 1.5 sec and 44; and (C) 0.014 sec and 9. Since the samples (A) and (B) contained considerably less emitters than the powder sample (C), the excitation intensities at λex = 978 nm were varied, respectively, from 250 W/cm2 to 8 W/cm2 to accommodate for the large disparity in the emission signals that would otherwise exceed the dynamic range of the EMCCD. The decreased Iex resulted in an increased green-to-red emission ratio in (C) due to the varied upconversion energy redistribution between the green and red multiplets. Top panel, schematic diagram of NaYF4:Yb,Er UCNP.