Electron hopping transport in two-dimensional semiconductor - zinc oxide nanoflakes

The sequential hydrothermal process is used to synthesize ZnO nanostructures on Si substrates. The synthesized ZnO nanostructures are inspected by scanning electron microscope and transmission electron microscope. They present a morphology of two-dimensional structures, named nanoflakes. The ZnO nanoflakes have a thickness of tens of nanometers. The energy dispersive x-ray spectrum reveals their compositions of only Zn and O elements. In addition, its crystalline structures are investigated by high-resolution transmission electron microscope. The nanoflakes are then dispersed for another morphology measurement using atomic force microscope and their average thickness is determined. The dispersed nanoflakes are contacted with metal electrodes for electron transport measurements. Through the analysis of electrical and temperature dependences of resistivity, it is confirmed that the electron transport in ZnO nanoflakes agree well with the theory of Mott’s two-dimensional variable range hopping. The nature of two-dimensional electron system in ZnO nanoflakes points to the application of this two-dimensional semiconductor as new channel materials for electronic devices