I tried to achieve a thin film coating of W-doped VO2 nanoparticles over a glass substrate without any surface functionalization of the nanoparticles. I used commercial nanoparticles to prepare the solution. 25mg of nanoparticles was ball milled and grounded thoroughly for 10 mins each, dispersed in 1.5 ml of ethanol solvent, and sonicated for 3 hours. The solution is left to settle overnight, and the supernatant solution is spin-coated over the glass substrate at 1500 rpm for 30s. The substrate was annealed at 150c for 20 mins. When viewed under the microscope, the nanoparticles were not closely packed as I expected. I also tried to repeat the spin coating and annealing process up to three times over the same glass substrate to achieve close-packed nanoparticle film.

Please help me understand why I cannot form a closely packed surface of W-doped VO2 nanoparticles. Also, please suggest alternative routes to achieve it.

I am also attaching the images of W-VO2 nanoparticles taken in the microscope.

Image shown is after the third round of spin coating and annealing.