C.M. Expt. 4
Synthesis of Zinc Sulfide shell on Cadmium Selenide Core
Developed by Karen S. Quaal, Department of Chemistry and Biochemistry, Siena
College, Loudonville, NY. quaal@siena.edu
Synthesis was as reported in the literature with the following modifications1:
Time: Aliquots were removed after 30-minute time intervals
Temperature:
TOPO was degassed at 120oC.
Solution was cooled to 70oC while under vacuum.
Solution was heated to 150oC while under nitrogen.
The working solution containing TMS, diethylzinc and TOP was added drop wise.
Solution was heated to 170oC and allowed to sit for 1 hour.
Solution was heated to 190oC and allowed to sit for 30 minutes.
Temperature Controller: 5°C/minute temperature intervals.
Sample Aliquots: Quenched in 1 mL room temperature toluene.
Synthesis of CdSe/ZnS core/shell:(Student Prep)
1. Concentration determined by using Beer’s Law, UV/VIS spectrum, and molar
absorptivity of CdSe, which is available in Yu2 et al or at the following website:
www.evidenttech.com.
A sample calculation of molar mass for CdSe quantum dot:
From Yu2 et al article or Evident Technologies’ website:
 max= 528 nm
=57,000 cm-1M-1
Diameter= 2.4 nm

Cd-Se bond length 0.36 nm
Calculation of Formula units of CdSe across diameter of QD:
Diameter/CdSe bond length: 2.4 nm/0.36 nm= 6.66 units
Calculation of # of CdSe units in a sphere of d=2.4 nm diameter.
(4/3)(d/2)3=(4/3)(3.14)(6.66/2) 3 =154.6 units of CdSe in the Q.D.
Calculation of Molar Mass of Q.D:
(154.6 units of CdSe)(molar mass Cd+ molar mass Se)=29,580 g/Mol, the molar
mass of CdSe QD that has a diameter of 2.4 nm.
2. Preparation of the working solution:
Equal molar amounts of diethylzinc and bis(trimethylsilyl) sulfide ( 300-1000 fold excess
of TMS and Et2Zn ) dissolved in a 4-fold excess of TOP was prepared in a glovebox.
3. Using standard airless techniques, 10 g Triocylphosphine oxide (TOPO) was dried and
degassed under vacuum at 120ºC for 30 minutes. The solution was cooled to 70ºC under
nitrogen, and 100 nmoles of the CdSe nanoparticles dissolved in toluene were added to
the TOPO using a syringe. The solution containing the TOPO and CdSe was then heated
to 150ºC under nitrogen. A small aliquot was removed and the UV/Visible spectrum
obtained to establish a baseline spectrum for this system.
The working solution containing the diethylzinc and bis(trimethylsilyl) sulfide was added
drop wise using a dropping funnel into the reaction vessel at 150ºC. A small aliquot was
removed and quenched in toluene and the UV/Visible spectrum was obtained to see if
there was any change in the spectrum. Note: Solutions are very hot and the room
temperature syringe clogs easily if too much time passes in the quenching stage. A
second syringe should always be prepared in case a syringe becomes clogged.
The temperature was raised to 170ºC and allowed to stir for 1/2 h. After the growth of
the nanocrystals/shell stabilized, as determined by no additional changes in the spectrum,
the Core/Shell nanoparticles were isolated by precipitation using methanol and
centrifugation. The solid product, after removal of supernatant, was dissolved in toluene
and stored.
Quantum yield determinations were performed and compared to the original CdSe
nanoparticles to determine if there was any change in the intensity of fluorescence.
References:
1. Cumberland, S. L., Hanif, K. M., Javier; A., Khitrov, G. A., Strouse, G. F., Woessner,
S. M., Yun, C. S., Inorganic Clusters as Single-Source Precursors for Preparation of
CdSe, ZnSe, and CdSe/ZnS Nanomaterials, Chem. Mater. 2002, 14, 1576-1584.
2. Yu, W., Qu L., Guo W., and Peng X., Experimental Determination of Extinction
Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chem. Mater. 2003, 15 (14) 28542860.