Shifting the Photoresponse of ZnO Nanowires into the Visible Spectral Range by Surface Functionalization with Tailor-Made Carbon Nanodots

We report on the surface functionalization of ZnO nanowires (NWs) with specifically synthesized carbon nanodots (C-dots, CDs) that allows us to shift the photoresponse of the NWs far into the visible spectral range. We modified a well-established citric acid-based synthesis protocol for C-dots by substituting the commonly used aliphatic amine precursors with 2,3-diaminopyridine (CDs-1) and 2,3-diaminonaphthalene (CDs-2). After surface functionalization, we achieve more than a 100-fold increase in the photoresponse of ZnO NW photodetectors at 2.92 eV (425 nm) with CDs-1 and more than 20-fold increase at 2.75 eV (450 nm) with CDs-2. The enhanced absorption of the C-dots in the visible spectral range is attributed to the formation of additional chemical bonds between the carboxyl moieties of citric acid and the amine groups of 2,3-diaminopyridine and 2,3-diaminonaphthalene, which is underlined by results from Fourier transform infrared spectroscopy. We present a model for the microscopic origin of the enhanced photoconductivity that is based on shifts in the lowest unoccupied molecular orbital energy levels in the C-dots synthesized with different amine precursors relative to the conduction band minimum of the ZnO nanowires.