CdCl2 and MSA solution is preapred first then Na2TeO3 is added and then the pH of the solution is changed to 12.3. In the final step when NaBH4 solution maintained in ice cold water is added to this solution the colour of the solution should change to bright yellow. But instead of that the color is changing to very pale green what may be wrong with the reaction mechanism? How to solve the problem in the reaction mechanism?
Dear Sandeep Radhakrishnan,
I do not see anything wrong with the reaction mechanism. I think the color is changing due to change in the particle size. Quantum dots could change their colors, i) due to particle size , ii) there are several references that particle size may depend on the pH during chemical synthesis. There could also be some complex formation otherwise. However, in case of particles confined in a host matrix (as in case of quantum dots) there could be change in the absorption/luminescence due to defects or change in the band gap. As the color is changing from yellow to pale green, there could be change in the size of the QDs and as a result the band gap might have been increased which would result in the blue shift and change in the color. You should firstly determine the QD size before doing anything else to solve the problem.
--- P D Sahare, Department of Physics & Astrophysics, University of Delhi, Delhi - 110007, India.
Since you have noticed this color change in water, I , humbly , hesitate to base my answer on its particle size.
[ pls find a reference attached]
The reason this observation lies in the fact that:
This results in an increase in the photoluminescence quantum efficiency( 60% at room temperature) and a longer exciton lifetime, monoexponential exciton decays and no observable defect-related emission are obtained
I, reproduce a few lines for your kind perusal[ See page 505--- RHS—bottom – start- 6th line from---In contract ---and end on page 506- LHS top first 6 lines]
“Transfer into water results in a longer lifetime and a monoexponential exciton decay accompanied by an increase in the PL quantum efficiencies. This implies that inter-QD energy transfer is no longer efficient, confirming that the CdTe QDs are fully dispersed in water as individual nanocrystals rather than clusters. Furthermore, the results show that the contribution of nonradiative processes to the exciton decay is significantly reduced after transfer to water. This indicates that the concentration of surface defects is reduced and/or that the exciton trapping at surface defects is less efficient as a consequence of the new capping layer”.
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