1. Department of Physics; South University of Science and Technology of China; Shenzhen 518055 China
2. Shenzhen Key Laboratory of Thermoelectric Materials; Shenzhen 518055 China
3. School of Materials Science and Engineering; Beihang University; Beijing 100191 China
4. Department of Chemistry; Northwestern University; Evanston IL 60208 USA
5. Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons (ER-C); Forschungszentrum Jülich GmbH; Jülich 52425 Germany
6. Peter Grünberg Institute; Forschungszentrum Jülich GmbH; Jülich 52425 Germany