To evaluate the effectiveness of our ligand exchange procedures, we characterized the Sn2S6 4--capped UCNPs with Raman spectroscopy and electron microscopy. The Raman spectra of Sn2S6 4--capped NaYF4 UCNPs obtained through direct and indirect ligand exchange approaches were similar, showing a distinct peak at 182 cm -1 as well as a cluster of peaks in the region of 250-350 cm -1 (Figure , red). We attribute the 182 cm -1 peak to a vibrational mode of UCNPbound Sn2S6 4-ligands, which is close to the 190 cm -1 peak that we observe in the Raman spectrum of Na4Sn2S6 (Figure , grey). On the other hand, the cluster of peaks at 250-350 cm -1 can be primarily attributed to the phonon modes of hexagonal-phase NaYF4, which were also observed in oleate-capped UCNPs (Figure , blue). We note that Na4Sn2S6 also has a strong peak around 350 cm -1 , but it overlaps with the NaYF4 peaks and cannot be unambiguously determined in Sn2S6 4- -capped NaYF4 UCNPs. To better understand the molecular origin of our Raman spectra, we carried out density functional theory (DFT) calculations. Since Sn2S6 4-and NaYF4 have overlapping peaks in the 200-450 cm -1 region, we focused our analysis on the 180-190 cm -1 peak. Our DFT calculations show that this vibrational mode can be attributed to a bending mode that involves the vibration of all bonds within Sn2S6 4-as well as its bonds to the metal ion. (Figure ). The calculation also revealed that Sn2S6 4-bound to a Na + ion has a higher frequency Raman mode (186 cm -1 ) compared to Sn2S6 4-bound to a Y + ion (163 cm -1 ). Thus, the DFT results support that our experimental redshift in the Raman peak is consistent with the binding of Sn2S6 4-to the UCNP surface. Successful Sn2S6 4-ligand exchange was also supported by zeta potential measurements that show a charge inversion from +30 mV after HBF4 stripping to -40 mV upon Sn2S6 4- functionalization (Figure ). Dynamic light scattering measurements showed an increase in the mean hydrodynamic radius from 16 nm to 68 nm, indicating some minor aggregation upon ligand exchange (Figure ).