Simplified structure of an ultrathin PbTiO3 (PTO) ferroelectric film grown on SrTiO3 (STO) substrate covered by conductive SrRuO3 (SRO) interlayer. (a) Striped domain pattern in usual growth conditions. (b) Outward uniform polarization with surface reconstruction in highly oxidizing atmosphere. (c) Growth of antiparallel domains under reducing atmosphere. (d) Inward uniform polarization stabilized by an oxygen deficit surface reconstruction at ultralow oxygen partial pressure.