jueves, 24 de noviembre de 2011

Preparation and Properties of Colloidal CdSe Quantum Dots


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.



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