Tunable Lattice Constant and Band Gap of Single- and Few-Layer ZnO

Single and few-layer ZnO(0001) (ZnO­(nL), n = 1–4) grown on Au(111) have been characterized via scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. We find that the in-plane lattice constants of the ZnO­(nL, n ≤ 3) are expanded compared to that of the bulk wurtzite ZnO(0001). The lattice constant reaches a maximum expansion of 3% in the ZnO­(2L) and decreases to the bulk wurtzite ZnO value in the ZnO­(4L). The band gap decreases monotonically with increasing number of ZnO layers from 4.48 eV (ZnO­(1L)) to 3.42 eV (ZnO­(4L)). These results suggest that a transition from a planar to the bulk-like ZnO structure occurs around the thickness of ZnO­(4L). The work also demonstrates that the lattice constant and the band gap in ultrathin ZnO can be tuned by controlling the number of layers, providing a basis for further investigation of this material.