Functionalized MXene Nanosheets and Al-Doped ZnO Nanoparticles for Flexible Transparent Electrodes

Abstract

Transparent, flexible, and chemically stable conducting electrodes are the most important key components for fabricating optoelectronic devices. However, in conventional bulk metal–semiconductor (MS) junctions, Fermi-level pinning is a major concern that limits the charge transport properties of the devices. In this work, we have designed MS junctions using two-dimensional (2D) MXene nanosheets and Al-doped ZnO nanoparticles as a metal and an n-type semiconductor, respectively. The heterojunction was formed by layer-by-layer self-assembly on an indium tin oxide (ITO) substrate and probed by the Pt/Ir scanning tunneling microscopy (STM) tip at room temperature. By recording the tunneling current of the components that in turn yielded the density of states of the materials, we could identify their energy positions to determine the band alignments. We then proceeded to form heterojunctions and characterized their current–voltage characteristics through scanning tunneling spectroscopy. The junctions showed rectification, and the rectification ratio varied with the semiconductor doping concentrations. However, the shift of the Fermi level toward the conduction band edge of the semiconductor reduces the Schottky barrier width and consequently lowers the rectification ratio. Additionally, the MS junction with the poly-allylamine (PAH)-functionalized MXene nanosheets and the ZnO layer show a nonrectifying nature with low contact resistance at the interface. This work shows how a functionalized 2D-metal MXene and doped ZnO nanoparticle can tune the MS junction properties, used for implementing various flexible optoelectronic devices and transistor applications.