Scaling Dopant States in a Semiconducting Nanostructure by Chemically Resolved Electron Energy-Loss Spectroscopy: A Case Study on Co-Doped ZnO

Dilute dopant introduces foreign states to the electronic structures of host semiconductors and imparts intriguing properties to the materials. Identifying and positioning the dopant states are of crucial importance for seeking the underlying mechanism in the doped systems. However, such determination has still been challenging, particularly for individual nanostructured materials, due to the lack of the spectroscopic probe that possesses both nanometer spatial resolution and chemical resolution. Here, we shall demonstrate the successful scaling of dopant states of individual semiconducting nanostructures by chemically resolved electron energy-loss spectroscopy (EELS), taking the individual Co-doped ZnO nanorods as an example. Guided by the Co dopant spatial distribution mapped by the core-loss EELS technique, chemical resolution is achieved in the accumulated valence electron energy-loss spectra. Three Co dopant states are successfully identified and positioned in the host ZnO bands. Furthermore, the electron extension degrees of the Co dopant states are assessed on the basis of the multiple-atom effect. The above experimental inputs optimize the density functional theoretical calculations, which generates the corrected full electronic structures of (Zn,Co)O dilute magnetic semiconductors. These results give a carrier-mediated interpretation for the room-temperature ferromagnetism of Co-doped ZnO nanostructures based on a recent theory.