ZnO, a wide bandgap semiconductor, has attracted much attention due to its multifunctionality, such as transparent conducting oxide, light-emitting diode, photocatalyst, and so on. To improve its performances in the versatile applications, numerous hybrid strategies of ZnO with graphene have been attempted, and various synergistic effects have been achieved in the ZnO-graphene hybrid nanostructures. Here we report extraordinary charge transport behavior in Al-doped ZnO (AZO)-reduced graphene oxide (RGO) nanocomposites. Although the most challenging issue in semiconductor nanocomposites is their low mobilities, the AZO-RGO nanocomposites exhibit single crystal-like Hall mobility despite the large quantity of nanograin boundaries, which hinder the electron transport by the scattering with trapped charges. Because of the significantly weakened grain boundary barrier and the proper band alignment between the AZO and RGO, freely conducting electrons across the nanograin boundaries can be realized in the nanocomposites. This discovery of the structurally nanocrystalline-electrically single crystalline composite demonstrates a new route for enhancing the electrical properties in nanocomposites based on the hybrid strategy.
We propose a strategy for enhancing thermoelectric performance through the realization of a 'phonon-glass electron-crystal' (PGEC) by interface control using multiwalled carbon nanotubes (MWCNTs). By the consolidation of undoped ZnO nanoparticles with MWCNTs (0.5, 1, and 2 wt%) using spark plasma sintering, we fabricated the interface-controlled ZnO-MWCNT nanocomposites, in which ZnO grains were surrounded with a MWCNT network. Both single crystal-like charge transport (electron-crystal) and considerably reduced thermal conductivity (phonon-glass) were achieved simultaneously thanks to the beneficial effects of the MWCNT network, and this led to the enhancement of the thermoelectric figure of merit. We discussed these findings on PGECs in the ZnO-MWCNT nanocomposites from the viewpoint of interface control in detail, and our strategy may provide a promising way to the realization of PGEC in other hybrid thermoelectric materials.
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