First of all, few information are needed before giving any suggestion. (i) your Ag Nps dispersed in which kind of solution? Is it in aqueous or in organic solvent.? (ii) do you need bare Ag NPs films on glass substrate? If so, it is difficult to deposit the film on the substrate by spin coating method. Better you can try with "Dr . Blade Method". (iii) If your Ag NPs dispersed in organic solution, you can make Ag NPs embedded polymer (PMMA, PVA etc) blend in order to get homogeneous film by spin coating method.