To elucidate the mechanism behind this UCL enhancement, we measured the enhancement factors for UCNPs with different physical and chemical parameters. We found that for UCNPs passivated with a 5-nm inert shell, functionalization by Sn2S6 4-ligands does not result in any UCL enhancement (Figure ). We conclude that UCL enhancement of Sn2S6 4-ligands relies upon their proximity to photo-active dopant ions, consistent with energy transfer of lanthanide excited states to ligand vibrational modes. Similarly, we found much smaller UCL enhancements for larger 52-nm UCNPs compared to smaller 15-nm UCNPs (Figure ). The reduced enhancement is likely due to the smaller surface-to-volume ratios of larger UCNPs, which are well known to be less susceptible to surface quenching. We also found that UCNPs with higher doping concentrations (40% Yb 3+ , 60% Er 3+ ) had higher enhancement factors (9-10), while UCNPs with lower doping concentrations (20% Yb 3+ , 2% Er 3+ , with similar sizes) have smaller enhancement factors (~2) (Figure ). Finally, we found comparable enhancements for both cubic-and hexagonal-phase UCNPs (Figure ), revealing a negligible effect of the crystal structure on UCL enhancement. Finally, we investigated whether Sn2S6 4-ligands can be used to enhance the properties of 8-nm-diameter NaYF4: 8% Tm 3+ photon avalanching nanoparticles (ANPs). Preliminary studies show that ANP cores with Sn2S6 4-ligands have a slight increase in the avalanching slopes but with no significant changes in the avalanching thresholds (Figure ). However, more careful studies with different types of ANPs will be necessary to ascertain and understand the effect of these ligands on the avalanching properties.