Dry powders are a problem for AFM. If your particles are loose on flat surface, they will most likely move when the AFM tip bumps into them. Usually, resuspending in water, followed by dilute dispersion onto freshly cleaved mica, then thorough drying will result in particles fixed to the surface.
Other possible fixing techniques are fixing on a membrane, dispersion onto a glue that you later cure, even flaming (for bacterial cells). See Atomic Force Microscopy for a detailed section on sample preparation.