You can try slow addition of Se salt precursor. And try to maintain oxygen free environment.

Are you cleaning your nanocrystals sufficiently before you record emission spectra? It could be the emission of a species other than your quantum dots...

I would interpret the appearance of the second emission maximum as presence of the smaller nanocrystals, which means that you get secondary nucleation event for whatever reason. The simple solution would be to do size selective precipitation during the cleaning step. With a bit of practice you can do it with minimal loss to the majority fraction. To avoid secondary nucleation I would suggest insuring that the precursors are all in the proper chemical form. To give you an example of what I mean: it is important that when your prepare Cd acetate from CdO and acetic acid, you fully convert it and remove the water products. I remember adding slight excess of acid to make sure the conversion is full.

thanks pavlo... we are also assuming that this is due to some smaller particles...we also tried the size selective precipitation, its helping but not completely....and we are using CdO and TOPO/TDPA as solvent. here i just want to ask you whether rapid quenching with ice is necessary or we can gradually reduce the temperature... and thanks all for your suggestions...

Usually we would just remove the heating mantle and air cool the flask with pressurized air. The temperature goes down very quickly. If you need a more rapid quenching you can inject cold solvent to the reaction mixture. This might be difficult with TOPO as a solvent. You can probably substitute it with anhydrous ODE or something sufficiently inert

Both temperature and reaction time control the growth of the nanocrystals. In some cases like CdTe, it is possible to control the particle size by playing with these two parameters. For instance, reaction can be carried out at higher temperatures, but it needs a shorter time for growth. On the other hand, reaction at lower temperatures takes a little more time for the growth, but you will have a better control over the quenching. So, to stop the reaction at a specific particle size with narrow size distribution, I would prefer to work at little lower temperatures and quenching by stopping/removing heating mantle. Check whether it works for your particles.