Objective:
CdSe quantum dots will be synthesized anaerobically
from elemental selenium and cadmium oxide.
The resulting quantum dots will be characterized by their emission
properties.
Experimental:
The synthesis of quantum dots
requires a nucleation step, separate from dot growth. To achieve a complete nucleation, solutions
of Se and CdO are prepared separately and then combined. Preparation
of CdO solution: A 25mL 3-neck round
bottom flask is equipped with a stir bar and a septum in all three ports. The top port is used to purge and vent the
system with Ar, the left port is equipped with a temperature probe attached to
a digital thermometer, and the right port is used for addition of
reagents. 0.100mmol (0.0127g) of CdO and
0.400mmol (0.1140g) of stearic acid are added to the round bottom flask and
heated, with a heating mantle, to 150°C until the CdO has fully dissolved. Reaction is cooled to about room temperature
and 5.017mmol (1.9400g) of trioctylphosphine oxide (TOPO) and 8.035mmol
(1.9400g) of hexadecylamine (HDA) are added to the flask. Reaction is heated to 320°C and becomes
optically clear. Preparation of Se Solution: To
a 15mL side arm pear shaped flask equipped with a stir bar, 1.000mmol (0.0790g)
of selenium powder, and 2.10mL of dioctylamine are added. The side arm is equipped with a screw top cap
and Teflon seal facing the inside of the flask, so as to keep material from
clumping in the side arm. The top of the
flask is equipped with a septum and an Argon inlet-needle, and a vent needle. The reaction flask is purged for 10min with
Ar. A 1mL syringe with a 4” stainless
steel needle is purged with Ar and then used to remove 0.290mL of tributylphosphine
(TBP) from a sure seal bottle. The TBP
is then injected into the flask via the Teflon coated disk on the side arm. Combination of Solutions: Once the CdO solution has reached 320°C
the entire Se solution is added using a 5mL glass syringe with a 4” stainless
steel needle. The reaction temperature
is reduced to 290°C and ≈5µL aliquots are removed at 2sec, 40sec, 90sec, 130
sec, 310sec, and 600sec; each aliquot is diluted to 3mL with chloroform in a
microfuge tube. After ten minutes the
reaction is cooled to room temperature and 15mL of chloroform is added to the
reaction. Reaction purification and data collection: The final reaction mixture is transferred
equally into two 15mL centrifuge tubes and centrifuged at max speed for
20min. The supernatant is collected and
centrifugation is repeated until the resulting supernatant is clear. Once optically clear the QD’s were
precipitated by adding methanol into the chloroform solution and isolated by
centrifugation and decantation. The
aliquots collected and diluted in microfuge tubes are used without further
purification and UV-Vis data is obtained from 350-700nm. Samples are
diluted so that they have an optical density at the first excition absorption
peak below .1 in order to avoid any signicant reabsorption; fluorescence
emission data is obtained.
References:
Qu, L.; Peng, X. J. Am.
Chem. Soc. 2002, 124, 2049.
