ZnS Quantum Dots

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C. M. Expt. 6

Determination of Number of ZnS Shells on CdSe/ZnS Quantum Dots

Developed by Karen S. Quaal, Department of Chemistry and Biochemistry, Siena

College, Loudonville, NY. Jennifer Z. Gillies, Evident Technologies, Troy, NY.

[email protected]

The Crash and Suspend procedure was used to remove the organic ligands from the

surface of the quantum dot (QD).

Crash and Suspend Procedure: A known volume of approximately 1 micromole* of a

CdSe/ZnS in toluene sample was added to a glass centrifuge tube. The centrifuge tube

was filled ~5/6 full with methanol. The tube was shaken and put in ice for twenty

minutes. After icing, the tube was placed in a centrifuge for twenty minutes. When

removed from the centrifuge, the supernatant was clear and CdSe/ZnS crystals were in

the bottom of the tube. If the supernatant is not clear, repeat the icing and centrifugeprocedure. The supernatant was removed and a minimum amount of toluene was added

in order to resuspend the crystals. A sonicator was used to aid in resuspension. Methanol

was added and the procedure was repeated two additional times.

Note: The quantum dots become progressively more difficult to resuspend as they are

stripped of their ligands.

*Moles based on concentration determined by using Beers Law, UV/VIS spectrum and

molar absorptivity of CdSe, which is available in Yu1et al or at the following website:

www.evidenttech.com.

A sample calculation of molar mass for CdSe quantum dot:

From Yu1et 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
http://www.evidenttech.com/http://www.evidenttech.com/


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mass of CdSe Q.D. that has a diameter of 2.4 nm.

Constant Weight Procedure: After the Crash and Suspend procedure was repeated

three times, the supernatant was removed a final time. A piece of filter paper was

secured over the centrifuge tube, and a small hole was punched in the filter paper to allow

airflow. The centrifuge tube was then placed under vacuum, at 70C, for 1 hour. Thetube was cooled in a desiccator, then weighed. The filter paper was replaced and the tube

was dried in a vacuum at 70C for an additional hour. This process was repeated until a

constant weight for the dried CdSe sample was obtained and recorded.

Digestion Procedure: High Purity concentrated Nitric Acid (2 mL) was added to the

centrifuge tube containing the dry nanocrystals. This solution was allowed to sit

overnight. High purity concentrated Hydrochloric Acid (5-6 drops) was added to the

centrifuge tube. The tube was then placed in a hot water bath and left until the solution

was clear and contained no solids (2-3 hours). The solution was then diluted for AA

spectroscopic analysis to determine the cadmium and zinc concentrations. After thesolution was removed from the centrifuge tube, the tube was cleaned, dried, and weighed

in order to determine the constant mass of the dried crystals.

Analysis of Atomic Absorption Spectroscopic Data:

A. Constant Mass: The constant mass of the dry crystals was divided by the original

sample volume used for the Crash and Suspend procedure to give the constant mass of

crystals per mL (g/mL).

B. Mass of CdSe: Using the concentration of Cd as determined by AA and the

volume of the dilution, the mass of Cd in the constant weight sample was determined.The mass of Cd was divided by the molar mass of Cd (112.411 g/mol) to give the number

of moles of Cd in constant weight sample. The moles of Cd were then multiplied by the

molar mass of Se (78.96 g/mol) to give the mass of Se in the constant weight sample.

The mass of Cd was then added to the mass of Se to give the total mass of CdSe in the

constant weight sample.

C. To determine the concentration of CdSe in mg/mL in the original sample, divide

the total mass of CdSe (as determined by AA spectroscopy) in the constant weight

sample by the original sample volume.

D. To determine the mass of ZnS: Using the concentration of Zn as determined by

AA and the volume of the dilution, the mass of Zn in the constant weight sample was

determined. The mass of Zn was divided by the molar mass of Zn (65.39 g/mol) to give

the number of moles of Zn in constant weight sample. The moles of Zn were then

multiplied by the molar mass of S (32.066 g/mol) to give the mass of S in the constant

weight sample. The mass of Zn was then added to the mass of S to give the total mass of

ZnS (as determined by AA spectroscopy) in the constant weight sample.


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E. The total mass of CdSe/ZnS quantum dots in the original sample, as determined

by AA spectroscopy, was obtained by adding the mass of CdSe to the mass of ZnS.

To verify that the digestion process was valid, the total mass (as determined by AA

spectroscopy) and the constant weight were compared.

Number of ZnS Shells on CdSe/ZnS QD procedure:

Information needed:

Diameter of CdSe (nm) based on spectral data1

# units of CdSe across diameter

# units of CdSe/dot

Density of ZnS (4.1x10-21 g/nm3)

Single ZnS shell thickness (0.31 nm)

Equations:

1) VTOTAL = (4/3) (dTOTAL/2)3

2) VTOTAL = VCORE + VSHELL

3) dTOTAL = d1 +d2 + dCORE

Where d1 = d2 and represent the thickness of the ZnS shells encapsulating the core along a

diameter through the center of the sphere.

4) (mg Cd divided by mg Zn) = (mg CdSe/dot divided by mg ZnS/dot)

The milligrams of CdSe/dot were determined by multiplying the number of units of

CdSe/dot by the molar mass of CdSe (191 g/mol) and then dividing by Avogadros

number to get grams of CdSe/dot, which is then converted to milligrams to give mg

CdSe/dot.

Using equation 4, the mg of ZnS/dot was determined utilizing both the mg CdSe/dot

calculated in step 1, and using the AA data for the left side of the equation.

VSHELL was determined by dividing the mg ZnS/dot by the density of ZnS (4.1x10 -21

g/nm3) to obtain the volume of the shell in nm3.

VCORE was determined using equation 1 and substituting the diameter of the core (CdSe)

for dTOTAL.

Using Equation 2 VTOTAL was determined.

Using equation 1, dTOTAL was determined.


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Using Equation 3, d1 + d2 was determined. Dividing by 2 gives the thickness of the shell.

To determine the number of shells, the thickness of the shell was divided by the Single

ZnS shell thickness (0.31 nm).

References:

1. 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) 2854-

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