I have a colloidal solution of NaYF4 nanoprticles in hexane solvent. Instead of forming a continuous monolayer some islands with preferred orientation are observed. However increase in particles concentration prefers discontinuous bilayer instead continuous monolayer. I use drop cast assisted spin coating technique for monolayer formation where a drop of NPs solution is spun with definite spinning parameters. Is it because of the fast evaporation of hexane as I see some bursts in SEM images of the film ? any other solvents suggestions in case this is the reason or other possible explanation ?

Thanks a lot :)