Things You Need
0.32g Cadmium Myristate
0.06g Selenium Dioxide
10 mL Octadecene
Round Bottom Flask
Heating Mantle and a Variac
400oC Thermometer
8 Small Test Tubes
Pasteur Pipette
Goggles and Gloves
Step 1: The
Reactants
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•
•
•
gloves and goggles
on
0.32 g Cadmium
Myristate
0.06 g Selenium
Dioxide
10 mL Octadecene
In here!
In fume hood
Step 2: Fume Cupboard
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•
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•
place flask in mantle but do not turn the mantle
on
Connect the mantle to the variac and plug the
variac in and make sure the variac is turned off
place flask into the mantle in the fume hood
set up test tubes and pasteur pipette next to the
mantle
Step 3: Ready Set Go!
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Turn on the variac and turn the dial to HI or MAX and PAY ATTENTION
Once the temperature gets to about 140oC reduce the power to the mantle by turning
down the variac so that the temperature rises slowly (say 2oC/min)
Try to let the highest temperature be 200oC
NEVER LET THE TEMPERATURE EXCEED 210oC - if it starts getting to that
temperature turn off the variac and remove the flask from the mantle to prevent charring
of the reaction mixture
When the reaction mixture starts changing color withdraw some of the mixture with the
pipette (1/2 mL is enough) into one of the test tubes
Collect 6-8 samples of varying color from white to red (see below)
When color goes as dark as the last test tube on the right turn off the mantle, remove
the thermometer and raise the flask out of the mantle
Things You Need for
Getting Spectra
• Hot Plate
• 400 mL Beaker
• 8 small test tubes
• Plastic Pipettes
• 50 mL Hexane or
Toluene
• PC / UV-Vis
Spectrometer
Step 4: Solubilize QDs
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Take 1/2 mL of your QD solution
Add 3-4 mL hexane or toluene
Place test tube in hot water bath at
75oC
QD should dissolve and make a clear
solution
If it doesn’t take 1mL of the solution
and place it in a new test tube and
add more of toluene and reheat
Step 5: Record Spectra
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Connect the USB Vernier UV/VIS spectrometer
Start LoggerPro
Calibrate the Spectrometer with the solvent
(toluene or hexane) in a small test tube NOT
THE PLASTIC CUVETTES THAT COME WITH
THE SPECTROMETER
Record the Absorbance vs. wavelength spectra
for the solubilized samples, by placing these
samples in their test tubes into the spectrometer
Step 5: Interpret Spectra
0.035
0.03
absorbance
0.025
For absorption the energy of the
photon has to equal the difference in
energy between the ground state and
the conduction state
0.02
0.015
0.01
DE fi = E f - Ei = hn =
0.005
hc
l
0
400
450
500
550
600
650
700
wavelength (nm)
In a quantum dot an electron is excited into one of the confinement states
in the conduction band leaving a hole in the valence band where it started.
The difference in energy between the hole and the excited electron is
equal to the photon that is absorbed
Many transitions are possible ….
Step 5: Interpret Spectra
Many transitions are possible and so we end up with a complicated
spectrum
ends up here
electron starts here
Transition seen here
Step 5: Interpret Spectra
While it is hard to identify every transition we can identify the lowest
energy transition (highest wavelength) and this depends on the radius
R of the QD
2
2
2
é
ù
h
h
1.8e
DE fi = ê Egap +
+
- Ecorrelation
2ú
2
8m0 me R û 8m0 mh R 4pe 0e (R)R
ë
Onset wavelength for absorption vs. dot radius
800
700
Onset Wavelength (nm)
600
y = 156.38ln(x) + 432.97
500
Equation 3
400
Experiment
( l -432.97 )
300
R=e
200
100
0
0.00
1.00
2.00
3.00
4.00
Dot Radius (nm)
5.00
6.00
Log. (Equation 3)
156.38
7.00
8.00
Step 5: Interpret Spectra
By identifying the highest wavelength transition we can use the
equation to estimate the radius R of the dot
0.035
R1 = e
= 2.05nm
156.38
( 556.0-432.97 )
R2 = e
= 2.20nm
( l -432.97 )
156.38
R=e
0.025
absorbance
( 545.0-432.97 )
0.03
156.38
0.02
0.015
0.01
556 nm
0.005
545 nm
0
400
450
500
550
wavelength (nm)
600
650
700
Step 6: Model the
Spectra
0.248e m0 æ me mh ö
h
h
1.8e
DE fi = Egap +
+
2
2
8m0 me R 8m0 mh R 4pe 0e (R)R 8( he (R)e )2 çè me + mh ÷ø
0
2
2
4
2
= band gap + (e - + h + ) confinement energy + exciton energy + e - / h+ correlation
Onset wavelength for absorption vs. dot radius
800
lthreshold
where
e (R) = e iErf (R / 7nm)
700
600
Onset Wavelength (nm)
=
hc
y = 156.38ln(x) + 432.97
500
Equation 3
400
Experiment
Log. (Equation 3)
300
200
100
0
0.00
1.00
2.00
3.00
4.00
5.00
Dot Radius (nm)
6.00
7.00
8.00