Thulium 3 chloride hexahydrate

The synthesis
of UCNPs was carried out using a slightly modified method of that
used by Wiesholler et al.^{48 (link)} Specifically,
8 mL of methanol and the precursors with a molar ratio corresponding
to a composition of NaYF₄:50%Yb,0.3%Tm [yttrium(III) chloride
hexahydrate (YCl₃·6H₂O, Sigma-Aldrich)
(0.497 mmol), ytterbium(III) chloride hexahydrate (YbCl₃·6H₂O, Sigma-Aldrich) (0.5 mmol), and thulium(III)
chloride hexahydrate (TmCl₃·6H₂O, Sigma-Aldrich)
(0.003 mmol)] were placed in a 100 mL three-necked flask and stirred
under argon bubbling at room temperature. A mixture of oleic acid
(8 mL) and 1-octadecene (15 mL) was then injected into the flask,
and the temperature was increased to 160 °C, at which point the
solution was stirred for 30 min. The solution was then cooled to room
temperature and a stock solution of ammonium fluoride (NH₄F, Sigma-Aldrich) (4.0 mmol) and NaOH (2.5 mmol) in methanol (20
mL) was added to the flask. The temperature was again increased to
120 °C, at which point the mixture was stirred for 30 min. The
temperature of the reaction solution was then heated to 317 °C
and the reaction was carried out under reflux conditions for 10 min
before cooling to room temperature. The products were collected by
washing as shown in ref (48 (link)). UCNPs with different Yb contents (NaYF₄:x%Yb,0.3%Tm in the range x = 20–60)
were also synthesized by varying the amount of Yb precursor.