Carriers induced ferromagnetism in Co(II)-doped ZnO monolayers and their optical properties: First principles calculations

•The Na co-doping changed the ground state of Co-doped ZnO monolayer from AFM to FM state.•The observed room temperature ferromagnetism in the co-doped monolayer with 5% Na doping is due to exchange coupling via electron trapping oxygen vacancies with the magnetic Co ions.•The d-d transition bands of Co(II) ions and the fundamental band gap of the ZnO monolayer show a red/blue shift for FM/AFM coupled Co(II) ion systems in a near configuration.•The d-d transition peaks in coupled ion systems of FM and AFM occur at the same energy level in a far configuration due to the paramagnetic behavior of Co ions.