Electronic structure engineering in chemically modified ultrathin ZnO nanofilms via a built-in heterointerface

Zinc oxide, a typical semiconducting material crystallizing in either hexagonal wurtzite or cubic zinc blende structure in the bulk, exhibits a graphite-like structure in ultrathin nanofilms with a few layers. On the basis of first-principles calculations, we show that the ultrathin ZnO nanofilms regain their bulk structures with either surface hydrogenation or fluorination. In particular, a heterointerface containing only Zn-Zn or O-O bonds can be built spontaneously in ultrathin ZnO nanofilms depending on their surface chemical modification, dividing the nanofilm into two different domains. An extended impurity state, constrained in the heterointerface, is created around the Fermi energy level. The ZnO nanofilms with a Zn-Zn heterointerface are metallic, whereas those with an O-O heterointerface are still semiconducting. The built-in heterointerface presents a novel channel for charge collection and transport in ZnO nanofilms for their potential applications in electronic and optoelectronic devices.