# NaYF4 Microstructure, beyond Their Well-Shaped Morphology
## 3 Results and Discussion
### TEM Observation and Doping Homogeneity  
The doping of the NaYF\({}_{4}\) NPs (nature and concentration of the rare earth ions) plays an unquestionable part in the shape and structure of the NPs [3]. Several batches of NPs were synthetized with different nominal doping concentrations. The effective doping of NPs was measured by EDX analysis. The Table 3 presents the expected and effective doping of particles obtained from different syntheses and shown in Figure 3. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**Sample** & \multicolumn{2}{c}{**TEM Size (nm)**} & \multicolumn{2}{c}{**Aspect**} & \multicolumn{2}{c}{**Expected Doping (mol\%)**} & \multicolumn{2}{c}{**Measured Doping (mol\%)**} & \multicolumn{2}{c}{**Area in**} \\
**Name** & **L** & **D** & **Ratio** & **Y** & **Er** & **Yb** & **Gd** & **Y** & **Er** & **Yb** & **Gd** & **Figure 3** \\ \hline NR00 & 300 & 50 & 6 & 50 & 2 & 18 & 30 & 40.2 & 9.4 & 18.2 & 32 & \\ \hline NR09 & 150 & 60 & 2.5 & 50 & 2 & 18 & 30 & 40.0 & 6.9 & 15.6 & 38.6 & \\ \hline NR11 & 200 & 30 & 6.6 & 50 & 2 & 18 & 30 & 41.5 & 8.4 & 17.5 & 32 & 1 \\  & 640 & 100 & 6.4 & & & & & 56 & 6.7 & 13.4 & 18 & 2 \\ \hline NR13 & 55 & 40 & 1.4 & 20 & 2 & 18 & 60 & 13.3 & 14.5 & 13.5 & 58 & 4 \\  & 130 & 35 & 3.7 & & & & & 23.9 & 14.6 & 13.9 & 47.6 & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Expected and effective doping concentration of different NPs measured by EDX. Two lines in the tabular correspond to two population measured by EDX. The NPs sizes given come from the measured NPs and can differ from the mean values. Figure 2: **(a,b)** Spectrum for two orthogonal polarizations (red and black) and the difference between them (green) for two different nanoparticles (NPs) of the same batch NR07. **(c)** PL intensity versus polarization for different wavelengths. The three first samples were prepared under similar conditions (doping concentration, heating temperature and time). However, the effective dopings are found to be different: NRs are less rich in Y of about 20 mol%, more than 3 times more rich in Er whereas Yb and, in a minor way, Gd doping are close to the expected values. This shows the sensitivity of the synthesis to some parameters that can be associated to the volume of solvent and the type of container, that could affect for example the pressure during the synthesis (see Table 3). TEM observations evidences two situations. On one hand, we sometime observe in the same batch two (or more) populations of NPs, each one having a relatively low size dispersion. However, the size of each population can be very different as it is illustrated in Figure 3 for two syntheses (NR11 and NR13). The EDX measurements of the delimited area give us the chemical composition as reported in the Table 3. For each population, we measure a strong difference in the gadolinium concentration (and in consequence the yttrium concentration) while erbium and ytterbium concentrations present only small variations. In Figure 3a, the NP in the area (1) delimited by the dotted green line area has a concentration of gadolinium of 32 mol% and its size is typical from this synthesis (50 \(\times\) 260 nm), while the NP shown by the continuous red line (area 2) has a concentration of only 18 mol% and its size is twice the average size (110 \(\times\) 650 nm) with a similar aspect ratio. Another example for the batch NR013 is presented in Figure 3b where the same behavior is observed with a complete different aspect ratio. While the experimental conditions are exactly the same, one (NR00) or two populations of particle sizes (NR11) are observed, with identical aspect ratios (about 6). At this stage, no clear parameter can explain these results but this is probably an evidence that the two kind of particles formed at different time during the synthesis, probably because the kinetic of formation of Gd rich or Gd poor particles is different. On the other hand, the expected doping and the effective doping can differ strongly.