Dear Yichen,

Apparently and theoretically, YES! It is possible indeed. Since the synthesis is done inside the micelle (with diameter of around 50 nm), the synthesis of 20 nm particles of CdS is highly possible. You can go through one of my papers (attached here) for more details of this particular method. 

Good luck!

Hi Yichen

We synthesized CdSe nanoparticles below 50 nm inside DPPC vesicles. Changing the phospholipid would change the vesicle size thus enabling to vary the NP Size. We have a paper about it but is in Spanish...