CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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by reducing Se with NaBH 4 with Se: NaBH 4 =1:2 in water maintained at 273K for 1h. NaHSe
was added to cadmium precursor solution with vigorous stirring. The ratio between Cd, MPA
and Se was maintained at 1:2.5:0.5. The above solution was refluxed for six hours. Reaction
mixture was allowed to cool naturally and was concentrated to 1/3rd of its original volume
by using rotary evaporator. QDs were precipitated by adding Isopropyl alcohol as the
nonsolvent. Supernatant was discarded and precipitate was re-dissolved in nanopure water
and finally re-precipitated. The cleaning was complete by a 3 time dissolution and reprecipitation
procedure to remove un-reacted precursors. These samples were dried in
ambient for further use.
5.2.3. Shell Growth:
For shell formation the concentration of the precursor for formation of 2.5nm shell
was calculated according to reported methods. 10 -6 M MPA capped CdSe was dissolved in N 2
purged water. To this solution zinc nitrate hexahydrate and MPA solution in 1:2.5 molar ratio
was added and pH was adjusted to 9-10. NaHTe (tellurium precursor) solution was prepared
fresh by reducing Te with NaBH 4 with Te: NaBH 4 =1:2 in N 2 purged water maintained at
273K for 8h. This was added to the above prepared QD solution with Zn precursor with
vigorous stirring. The Zn: MPA: Te molar ratio was maintained at 1:1.5:0.5. The solution
was refluxed for 6h for the growth of shell. To obtain shells of different shell thickness the
samples were eluted at different time (1/2h, 1h, 2h, and 6h). The cleaning procedure was the
same as described above for the CdSe QDs. The four samples are named as CdSe/ZnTe1,
CdSe/ZnTe2, CdSe/ZnTe3, and CdSe/ZnTe4
with increasing shell thickness order which were eluted at 1/h, 1h, 2h and 6h respectively.