Almost all the nanoparticles tend to aggregate upon drying, so, you are probably forming mountains of QDs when you do the spin coating. To prevent this, you can either functionalize your QDs with an appropriate molecule/surfactant or, you can functionalize the substrate. In this way. you might obtain a "monolayer-like" film with dispersed QDs. (Hint: for the surface chemistry of your QDs, start by checking the zeta potential). 

For that QD should washed several times by using methanol and water, after that by using propanol. The collected QD should be dried totally and again should dispersed  in volatile solution and should spin coat. But  the film deposited in the process is not so good in quality.

Almost all the nanoparticles tend to aggregate upon drying, so, you are probably forming mountains of QDs when you do the spin coating. To prevent this, you can either functionalize your QDs with an appropriate molecule/surfactant or, you can functionalize the substrate. In this way. you might obtain a "monolayer-like" film with dispersed QDs. (Hint: for the surface chemistry of your QDs, start by checking the zeta potential). 

Dear Saikia,

I agree with Civan and It is difficult indeed. You may also try etching the substrate before deposition of the QDs. I am not sure if you coat on ITO or FTO substrate. You can also deep your  substrate in  shorter ligand solvents such as ethanedithiol or mercapto-propionic acid and in QD solution. More info may be found in these articles:

1o) Kongkanand et al. J.Am.Chem.Soc., 130, (2008), 4007

2o) Robel et al. J.Am.Chem.Soc.,128, (2006), 2385

3o) Klem et al. Appl.Phys.Lett,, 90, (2011)183113

4o) Seo et al. Adv.Mater. 23, (2011)3984

Precipitate the QDs and make a thick paste by continuous grinding of the QDs with drop wise addition of n-butanol. and then the paste can be doctor bladed on the desired substrate.   

Dear Abhishek,thank you for sharing the idea. Doctor Blade method is really helpful in many instances.May you elaborate on the answer to Saikia question: Is this method size-dependent ? Is there a specific concentration to make thick film? Does the film stick on the substrate or it can peel off easily? Any reference about this specific approach? Regards

Hi Pierre, I have tried this doctor bald technique to make film of minimum thickness of 1 micro meter. We can get a good (uniform) quality film if we can make well grinded  paste without any agglomerated particles. In my experience there is no any size dependence. Concentration for paste making can be optimized for different systems. The critical step is the dropwise addition of n-butanol while grinding the paste so that there should be very little agglomeration. If u can sinter the film then you can get a good sticky film on substrate.  Till now I don't have any reference.

Thank you for more info. Optimization is the key, I hope Saikia gets a better film. Good luck.

I think that chemical bath is the simplest way 

Dr. blade technique is good but preparation of proper paste and selection of wright  solvent is very important. you can paste a thin tape on opposite side and make a area between them and go for coating.I thin this will help you to prevent your paste from free flow and will give good coating.

good luck

Dear all,

you may alternatively disperse your QDs in water (through surface chemistry modification or micellization) and create a colloidal crystal through evaporation by the angle method described by Michelletto et al. (Langmuir, 1995). This method allows to create perfectly arranged 2D colloidal crystals of water dispersible particles through a simple method.

Good luck 

You can treat surface of substrate using oxygen/Ar plasma (etching ) Either to make it hydrophilic or phobic. Then depositing  QDs by spin coating; DIP COATING (@ step size in few nanometers) (you can perform DIP coating at controlled temperature or RT). You can also use langmuir blodgett technique (In this technique Liquid used for suspension should not dissolve QDs). 

Tou can follow this attached  paper 

Article Electronic Grade and Flexible Semiconductor Film Employing O...

Spray substrate surface with QD colloid or immerse substrate into QD colloid. Then place in a very low pressure chamber. The timing in both processes will determine the monolayer of the QD thin film.