Experimental phase diagrams for each transition metal (TM) doped ZnO nanoparticles have been established that explore the relation between growth parameters and single/multiple phases of Zn1-xTMxO. Study of magnetic properties of Zn1-xTMxO nanoparticles have been depicted for TM = Co, Mn, Fe. Technologically relevant "Room temperature ferromagnetism" was achieved in Fe doped ZnO nanoparticles and has been probed by various experimental techniques...