Dear Jhony Orbulescu!

I totally agree with your comments "Capping is the KEY in obtaining small size nano-crystals or nanoparticles.", Do you have any references regard to capping agent control the size of quantum dots? Can we synthesized quantum dots without capping agent? What is the Size of crystal OR particle that should be consider as Quantum dot?

I guess capping agent is not responsible for obtaining different size of NPs. It serves different role, as a stabilizer of NPs in formulations, so that they do not agglomerate.

Size of the NPs depends upon your synthesis methods.

Capping agents are used are to arrest the coalescence of synthesized nano-particles.

So, the first step is synthesis of small dia NP, subsequently stabilize them with capping agents.

Synthesis of NPs in micro-emulsions: water + linear alkanes like n-Heptane, octane, nonane with emulsifiers like AOT can give NPs of 8-10 nm

Capping agent plays a significant role in the growth of nanoparticles. So it will affect the size of the quantum dot obtained. Playing with the ratio of Cd or Pb to capping agent can give some idea of how it affect the size. generally increasing the capping agent concentration wrt to metal ion is known to reduce the final particle size obtained. What I mean to say is that capping agent plays a role of stabiliser which avoids agglomeration. secondly the capping agent affects addition of precursor to nanoparticle surface therefore affects growth too. Additionally capping agents also have a role in dispersing particles in a solution medium.

Reverse micelles are some of the agents used for dispersibility for restricting particle size. But after the reaction processing of the nanoparticle become difficult are the interaction between the micelle head or tail group and nanoparticle surface is weak generally. By processing I mean cleaning the nanoparticle. as significant amount of unreacted precursor s generally present in the solution medium which needs to be removed by cleaning. One can use a thiolate capping. like mercaptopropionic acid or thioglycolic acid as capping agents. In thiol capped CdS however PL may be weak due to formation of thiolate levels within band gap.

Synthesize the NP's in organic media with capping as octadecyl thiol.

Prepare a solution of thioglycolic acid in water.

Add water solution to the organic media (usually CHCl3).

Sonicate.

Separate the water phase from organic phase.

Measure absorption and luminescence in the water phase.

This is due to the thioglycolic acid modified NP's.

You could also use thioctic acid as a capping agent.

Thanks to you all.

Dear Amit Mandal,

Capping is the KEY in obtaining small size nanocrystals or nanoparticles.

Without capping, the small NP's will agglomerate forming larger particles that precipitate from the solution.

Thiolates can be used to prepare small CdS nanoclusters in organic solvent (DMF) as well as in water (pH important). These nanoclusters can be isolated as soluble powders. The problem is the photo stability. One also use TOP/TOPO as well.

In your starting solution you have a Cd source, a Sulfur source and the capping (thiols). The size will depend on the three, then the temperature and heating time.

Hi,

You may use 2-Mercaptoethanol(β-mercaptoethanol) for extreme nanosize particles or quantum dot. It is the best capping agent and by controlling its concentration you can control size of your particles.

In addition to the suggestions above around keeping the initial particles small by using microemmulsion techniques, we have used some small polar molecules like cystein, thiourea, thioglycolic esters, and carbohydrate derivatives as direct capping agents without the need to displace TOPO. The trick seems to have either fairly large tails on the capping agent such as octyl, or to have an additional polar group such as a carboxylate or hydroxyl (Suresh's idea if 2-mercaptoethanol). The only drawback of the latter has been that sometimes there is competitive binding of the ligand via either side.

PVP, Polyvinylpyrrolidone, as water-soluble polymer, is a choice of capping agent. Then you could prepare PbS NPs directly in aqueous solution.

You can use water soluble 1-thioglycerol as capping agent. You may consult this article.

Journal of Nanoparticle Research, 2012, 14: 916

for the very small PbS nanoparticles, the ligands should be the strong ligand of oleic acid and thio. For the small CdS nanoparticles, weak ligands such as oleylamine are also ok

For CdS the best is thiglycolic acid which is water soluble, I would stay away from 2-mercaptoethanol due to the fumes. Thiolactic acid along with MPA work well.

Look up Hui Li's Thesis on CdS.

Overview of Stabilizing Ligands for Biocompatible Quantum Dots is also a good open source paper which gives reaction pH's for ligands and some ratios.

Dear Jhony Orbulescu!

I totally agree with your comments "Capping is the KEY in obtaining small size nano-crystals or nanoparticles.", Do you have any references regard to capping agent control the size of quantum dots? Can we synthesized quantum dots without capping agent? What is the Size of crystal OR particle that should be consider as Quantum dot?

Hello, please take a look on,

https://books.google.co.uk/books?id=2k1YCwAAQBAJ&pg=PA61&lpg=PA61&dq=synthises+CdS+nanoparticles+from+oleylamine&source=bl&ots=FuBSwpmfuD&sig=YPwBua7eQYXXT2UOo78ZPsip-5k&hl=en&sa=X&ved=0ahUKEwiB4fC2uerQAhUqIcAKHfO7BzEQ6AEILjAC#v=onepage&q=synthises%20CdS%20nanoparticles%20from%20oleylamine&f=false