Lead Sulfide Quantum Dot Synthesis

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Overview

Cadmium selenium Quantum Dots (QDs) are metal nanoparticles that fluoresce in a variety of colors determined by their size. QDs are solid state structures made of semiconductors or metals that confine a countable, small number of electrons into a small space.

Quantum dots are semiconductor nanocrystals on the scale of a few nanometers, or several hundred atoms. At this size quantum effects dominate and the electrons are quantum confined to narrow energy levels determined by the size of the particle. This means that the absorption wavelength the light can be fine-tuned and that quantum dots can be used to adjust the bandgap of the materials they are associated with. Quantum dots being explored for use in photovoltaics and in signal processing.

Significance

Quantum dots can be used increase the efficiency of solar cells by tapping different portions of the spectrum and by increasing the number of excitons that be formed per photon, resulting in higher quantum efficiency.

Procedure

After the synthesis is quenched it is necessary to remove excess oleic acid and other impurities. Two methods are shown.

Method 1: Separatory funnel:

1. pour PbS QDs (dissolved in minimal amount of hexanes) into sep. funnel
2. precipitate with methanol (MeOH)
3. Add back small amounts of hexanes and shake funnel vigorously between additions. If there is still preciptated product clinging to the walls of the funnel between hexanes additions, continue to add more hexanes. Once the product stops sticking to the walls, let the funnel sit for a minute or so to see if any phase separation occurs. If not, continue adding small amount of hexanes until it does. Note that only a small amount of separation will be seen at the bottom of the funnel at first, and the mixture will have to sit for at least 30 mins to fully separate. If no phase separation is observed after a few additions, the funnel can be placed in the freezer to speed the process.
4. Drain off the clear bottom MeOH layer into the waste
5. Drain small amounts (~1 or 2 mL portions) of PbS into a centrifuge tube.
6. Precipitate by filling the tube with MeOH
7. Centrifuge the tube (5 min, 3000 RPM), and pour off clear MeOH layer
8. Pour another addition of PbS into the same tube and repeat steps 6) and 7). Note: for the first two cycles, multiple tubes can be used rather than just one, but on the third (last) cleaning step, it is generally preferred to use a single tube to consolidate product.
9. Dissolve the product in minimal amount of hexanes, votex, sonicate (5 mins, or until dissolved) and add PbS back to the separatory funnel. Repeat the above steps for the first two cleaning cycles.

Method 2: Simply use multiple centrifuge tubes.

1. Dissolve the product in small amount of hexanes, then vortex and sonicate until QDs dissolve.
2. Precipitate by filling each centrifuge tube with MeOH
3. Centrifuge each tube (5 min, 3000 RPM)
4. Discard clear supernatant
5. For the first two cleaning cycles, repeat the above steps

To consolidate into a single tube, just choose one tube to keep and repeat step 1) above for the other tubes. Then successively add the dissolved QDs of ONE of the other tubes to to the dry product in the tube you chose to keep and repeat steps 2-4 above until all tubes have been combined into one (only add the contents of one tube for each step precipitation-centrifuge cycle-do not combine the contents of all remaining tubes at once)

In both cases, once the cleaning cycles are complete, the QDs can be dissolved in hexanes or toluene and left to sit. Alternatively, a septum cap can be placed on the centrifuge tube which can then be placed on the schlenk line to dry.

External Links

wikipedia:Quantum_dot

• Quantum dot blog
• Absence of Photoinduced Charge Transfer in Blends of PbSe Quantum Dots and Conjugated Polymers Kevin M. Noone, Nicholas C. Anderson, Noah E. Horwitz, Andrea M. Munro, Abhishek P. Kulkarni, David S. Ginger ACS Nano 2009 3 (6), 1345-1352 doi:10.1021/nn800871j