Unconventional Patterning at the Nanoscale
NANOSCALE SYNTHESIS
Synthesis of CdSe nanocrystals
The objective of this experiment is to synthesize semiconducting CdSe nanocrystals in
solution and observe their size-dependent properties.
BACKGROUND:
Nanocrystals are interesting because they exhibit unique properties depending on their
size. For example, CdSe is a semiconducting material whose energy bandgap is in the visible
region (~650 nm). When these particles are made very small (2-5 nm), quantum confinement
produces a range of different colors depending on the size of the particle. The origin
of this effect is because the size of the particle becomes smaller than the size of the
charge carriers responsible for light emission. In this lab you will synthesize CdSe nanoparticles
and characterize their optical absorption and emission spectra.
MATERIALS:
- CdO
- TDPA (Tetra decyl phosphonic acid)
- Selenium
- TOPO (Tri octyl phosphine oxide, high boiling point solvent)
- TOP (Tri octyl phosphine)
- Three neck round bottom flask, Thermometers, stir bars, glass vials, spatulas, pipettes,
bulbs, hot plate, heating
- mantle, clamps, cuvettes
PROCEDURE: (Click on pictures to view the videos)
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Weigh 0.062 g of CdO (0.5 mmol) and 0.278 g of TDPA (1 mmol) to a 10 ml three
neck round bottom flask. (Warning: CdO is an inhalation
hazard and this operation should be done in a fumehood)
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To the same flask, add 3.678 g (~10 mmol) TOPO.
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Insert a thermometer in one neck and add a magnetic stir bar in the flask.
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Close the other two necks with a rubber septa.
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Clamp the flask in a heating mantle.
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Heat the solution to 320 degrees C to dissolve the CdO.
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If you have access, plug in a nitrogen inlet.
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Use a syringe needle as an outlet. Otherwise you can skip this and the previous
step.
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As you wait for CdO to dissolve, add 0.041 g of Se (0.5 m mol) to 3 ml TOP in
a bottle. Heat the solution to about 150 degrees C to dissolve the Se.
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The CdO, TDPA and TOPO mixture initially becomes a red solution. Continue heating
it.
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It takes anywhere between 1 - 2 hours for the solution to become clear. Once the
solution is clear, drop the temperature to 260 degrees C.
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Using a syringe, take about 2 ml of TOP-Se solution.
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Inject the solution into the round bottom flask and start observing the color
change.
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The solution turns yellow and start taking aliquots (remove a small amount of
solution and place in seperate vials after mixing) every 30 sec - 1 min depending
on the rate of reaction. Obtain 6-7 samples. You can take aliquots at 1 min, 2 min,
5 min, 10 min.
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The solution turns orange in a minute or so.
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The solution color gets darker with time.
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The solution becomes deep red in color in about 10-15 minutes after the TOP-Se
solution is injected.
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You can shine a broadband fluorescent source behind the vial and take a picture.
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Dilute the CdSe nanoparticle solutions (approximately 1:3 with hexane). Line them
in a row and take a digital picture. Also record the absorbance spectrum of the solutions
to find the peak wavelength. Include these in your write-up. |
QUESTIONS:
- How does the absorption spectrum of the aliquots change from samples that were taken
from the solution at earlier times (for example, 10 s) compared to later times (60 s)?
- Why do the samples exhibit different colors since they are all the same material?
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Authors:
Prof. Teri W. Odom,
Dr. M. Viswanathan and Y. Babayan
Institution:
Northwestern University
Evanston, IL USA
Level:
College and above
In the classroom:
This Course is a video lab manual for hands on fabrication and characterization of materials at the nanoscale.
Materials requirements range from simple chemicals, benchtop tools and CDs to necessary access to
advanced characterization equipment such as an Scanning Tunneling Microscope. |
