From a practical standpoint, I suspect that a smooth, dense coating of a few monolayers without any micro-cracks will be a difficult if not impossible outcome to achieve with spin coating on a glass substrate. What other methods have you considered?

For QDLED applications spin coating is the most widely used method and so I'm following that now. Dip coating is another possibility, but I don't have the facility right now.

Im trying to fabricate the QD LED by layer by layer deposition . When I deposit Ag or Al contact at the top using ebeam evaporation, it penetrates through the QD layer cracks and short circuits with the other contact which is a transparent conductive oxide.

So I wanted to have a crack free surface to avoid short circuit between the top and bottom contact of the LED.

Does the literature tell you anything about the recipes that must be followed to make high-quality films of QDs for LED applications? Are you following one that you could reference here for further information?

When you say … layer by layer … what do you mean? Is your deposition done in one batch spin coat with enough QDs to make multi-layers? Alternatively, do you try to coat the equivalent of one monolayer at a time?

Why would you think that dip coating would be any better? What "facility" is needed to do this? Isn't the procedure as simple as dipping a sample in the solution?

Please find the paper in the link below. Here they just spin coat the QDs and bake at a particular temperature. But the problem that I'm facing with this method is the cracks formed in the film that short circuits the anode and cathode.

http://www.nature.com/nphoton/journal/v7/n5/full/nphoton.2013.70.html

I'm trying both.

i) A single layer (few monolayers) of QDs .Spin coat once and bake to evaporate solvents.

ii) Multilayer of QDs. Spin coat- film bake-spin coat -film bake and so on.

My understanding of dip coating is that the rate of pulling the sample is very critical to form a good film of particular thickness. So it requires at least a stepper motor assembly.

Thank you Dr. Weimer for you help.

Unfortunately, I can only access the first page of the article. My reading suggests, they deposited the QDs with ZnO particles. Do you think this may be an important factor? Are you following their recipe precisely, and do they get films that have no cracks? Perhaps you are forgetting to control the humidity or quality of the solvent of spin rate or … something … to the same value as the reference you quote during your process.

Have you tried dip coating by hand at a rate that would seem reasonable? Sometimes, at recommended rates, you can do a reasonable job even by hand. Some clever physics or engineering student somewhere might even be able to create a simple system for you to pull out samples at a uniform rate using a weight on an incline and attached to a pulley. You could at least test whether you get more uniform films.

One thing I can assure you is in any case is that you will NOT get a single, uniform monolayer of pure QDs with spin-coating. The dots agglomerate too much, and you will always get dispersed, patchy agglomerates or islands.

I realize that I am asking more questions than I am giving you answers. I do have ideas other than spin-coating or dip-coating that might be useful to try, but I'd prefer to talk about them off-line.

I think to get uniform mono layer, you have to make a suspension of the quantum dots in ethanol or other volatile solvent and disperse it using sonicator. Then put your substrate in the solution vertically. As the solution dries, there is possibility that a monolayer will be deposited on the substrate.

@S Khisro: Do you have a reference that confirms this method works for QDs? I would venture that far more is demanded in the recipe, especially to make a monolayer.

A friend has been doing this method for making monolayer of nanoparticles to be used as template for tructured thin film. due to surface tension, monolayer of nanoparticles deposited on the substrate while the solution lowers due to drying. TEM confirmed that.

Sounds as though you are proposing the equivalent of dip coating, only that in this case the evaporation of the solvent substitutes for raising the sample from the solution. That is an interesting way to do this.

What type of nanoparticles? Not all nano particles will behave in the same way. What substrate? The particle-substrate interactions also are important, as is the surface roughness of the substrate. Are measurements done with something that confirms the coverage is a uniform monolayer across a larger range of the sample surface than what is seen in TEM? The spatial range of TEM analysis may be too small for example to see whether a film is contiguous across more than micron size distances, if even that.

From my experience with CdSe/ZnS, InP/ZnS , this was not possible as the long chain organic ligands used while growth of these nanocrystals do not evaporate unless heated to temperatures above 300 C. Even after several purification cycles some ligand molecules are not removed and they pose a problem while forming the film. When I looked such sample under SEM or AFM, it was like nanocrystals suspended in a sea of junk.

Traditional dip coating works with Au or Ag nanoparticles on glass or GaAs substrates treated with alkane dithiols. Metal nanoparticels do not require the ligand molecules while growth. The coverage was fairly uniform though not dense.

The ligands are put on the QDs primarily to stabilize them chemically against degradation. They are not a requirement to make QDs. The ligands also help keep the particles suspended in solution, but they subsequently cause the QDs to agglomerate as the solvent dries during the deposition.

I think that you may realize by now from all of this that one answer to your starting question is ... Spin or dip coating will be difficult if not impossible to use to make the quality of films of pure QDs that you seem to need.

what is the minimum concentration of QD usually required and the maximum spin-coat for coating at least a monolayer of QDs?

The solution concentration can be determined through first principles mass-balance-type calculations taking in to account the size and packing density of the QDs.

If the quantum dot is capped by a negatively charged species such as citrate then why the use of some positively charged polymers could be tried ? I think one can do a LBL assembly of PDDA and QD alternating each other.

what's about Spray coating on a hot plate of 250 deg Celsius. is it possible?

Yes possible but film will be not good. It depends on the material and solvent.