Antimicrobial property Crystallite size Escherichia coli Mechanochemical synthesis Silver doped zinc oxide Staphylococcus aureus X ray diffraction antibacterial activity antibiotic agent article controlled study disk diffusion field emission scanning electron microscopy minimum inhibitory concentration nanocrystal nanoparticle nonhuman particle size silver silver doped zinc oxide nanoparticle synthesis transmission electron microscopy unclassified drug zinc oxide