I believe it absolutely could be used to monitor the growths of other types of nanocrystals. However, there may be some obstacles in using this system for the study of typical down-converting systems (such as quantum dots) that our up-conversion nanocrystals avoid. For example: background fluorescence from material in the reaction mixture or from the reaction container itself may need to be addressed.

I’ll begin with your first question. We obtain information on the nanocrystal (UCNC) growth mechanism in two ways. The first information we obtain was the rapid rise in luminescence intensity during our real time monitoring experiments. Since beta-phase NaYF4 are roughly 10x brighter than the alpha phase, and since larger UCNCs tend to be brighter than smaller UCNCs (when the diameter is <50nm), we believed that the rapid rise was due to either a combination of phase transition from alpha to beta, and an increase in UCNC size, see the 2nd figure under the Real Time Monitoring section.
To confirm this we removed samples at various stages of the reaction, before, during and after the rapid rise. TEM images of these samples show small alpha phase UCNCs before the rapid rise and large beta phase UCNCs afterwards, see the images below the Crystal Analysis section. By comparing the size distributions before, during and after the rapid rise we can see there are two distinct populations of nanocrystals, see the figures below the Crystal Size & Size Distribution section. By doing this comparison we could see that the population of small alpha phase UCNCs decreased as the population of the large beta phase increased. Additionally the lack of any intermediary sized UCNCs indicate that the process of forming the large UCNCs occurs very rapidly. The presence of both large and small UCNCs in a single sample indicate that the small alpha UCNCs do not convert all at once to the large beta UCNCs but rather the process of converting the entire population takes some time (10-15 minutes).
As to your second question, our research only shows that, in forming the large beta phase UCNCs, we first make small alpha UCNCs. Portions of the small alpha phase population are then rapidly converted to large beta phase populations resulting in a bimodal population. The remainder of the small alpha phase UCNCs are then converted to large UCNCs, completing the synthesis.
While our experiments only show the timing of these transitions and not how the transitions are occurring, based on results found in the literature our best explanation for how the transitions occur are after the initial alpha phase UCNCs undergo a dissolution re-precipitation process that forms large beta phase UCNCs.
I hope I answered your question, please let me know if you need any further clarification or if you have additional questions.

The size dependence of the optical properties of nanocrystals depend greatly on the what kind of nanocrystals one is dealing with. For example, the energy bands of semiconducting quantum dots are delocalized across the entire nanocrystal and leading to the quantum dots having band gaps (and color) with very strong size dependence. However, for the upconversion nanocrystals (UCNCs) I work with the optical process of interest are atomic transitions of individual lanthanide ions. Since the transitions are localized to individual atoms, the energy of the transitions are unaffected by nanocrystal size. That said, when these nanocrystals are small enough (<50nm) they’re upconversion efficiency decreases. This is due to the higher fraction of ions near the surface which can be quenched by defects on the surface or molecules in solution. The localization of the optical process also limits the affect that crystal defects can have on the process since only transitions located near the defect would be affected, as opposed to a defect state quenching an entire QD.
The biggest factor determining the optical properties of these UCNCs are the types and ratios of lanthanide ions used, the crystal phase of the NaYF4 lattice, and the excitation power. Since these optical process are multi photon process the kinetics are greatly dependent on the excitation power density.
Please, let me know if you still have questions.

That is a really good question and one that we have spent a lot of time thinking about how to answer. It has been shown that small alpha phase NaYF4 nanocrystals (UCNCs) can form larger beta phase UCNCs by dissolution of the small UCNCs followed by precipitation of larger UCNCs. The reason the alpha phase UCNCs would be smaller than the beta phase UCNCs is because the two crystal structures will have different lattice energies, surface energies, and different binding energies to the capping ligand, oleic acid. These three energies contribute significantly to the size of the UCNC.

There are two main factors which contribute to the increased fluorescence as the nanocrystals (UCNCs) grow. The first is not, specifically, due to the increase in size but rather the change in the crystal structure. Beta phase NaYF4: Yb, Er is known to be a ~10x more efficient upconvertor than alpha phase NaYF4: Yb, Er. Therefore as part of the population changes from the weaker alpha phase to the brighter beta phase the fluorescence intensity grows. The second factor is a decreasing fraction of lanthanide ions, which are responsible for emission, near the UCNC surface. The lanthanide ions near the surface are susceptible to quenching either from surface defects or molecules in solution. As the size of the UCNC increases the ratio of surface atoms to atoms in the “core” of the UCNC decreases which results in less quenching and more efficient UCNCs.