In this Letter, we present a new class of near-infrared photodetectors comprising Au nanorods-ZnO nanowire hybrid systems. Fabricated hybrid FET devices showed a large photoresponse under radiation wavelengths between 650 and 850 nm, accompanied by an "ultrafast" transient with a time scale of 250 ms, more than 1 order of magnitude faster than the ZnO response under radiation above band gap. The generated photocurrent is ascribed to plasmonic-mediated generation of hot electrons at the metal-semiconductor Schottky barrier. In the presented architecture, Au-nanorod-localized surface plasmons were used as active elements for generating and injecting hot electrons into the wide band gap ZnO nanowire, functioning as a passive component for charge collection. A detailed investigation of the hot electron generation and injection processes is discussed to explain the improved and extended performance of the hybrid device. The quantum efficiency measured at 650 nm was calculated to be approximately 3%, more than 30 times larger than values reported for equivalent metal/semiconductor planar photodetectors. The presented work is extremely promising for further development of novel miniaturized, tunable photodetectors and for highly efficient plasmonic energy conversion devices.

We investigated the photoconductivity of single ZnO nanowire device as a function of the surrounding atmosphere, considering the comparison between reduced pressure, inert gas environments, and air. We show that after UV excitation the photocurrent persists for hours, in particular in vacuum, nitrogen, and argon. In the presence of oxygen, the photodecay rate is initially fast but then becomes considerably slower, resulting in a long persisting photo-conductivity tail. Our proposed model explains the persistence of the photoconductivity (PPC) in terms of band bending at the surface of the nanowires, which is related to the trapping of electrons from the conduction band.

We report on the surface functionalization of ZnO nanowire (NW) arrays by attachment of carbon nanodots (C-dots) stabilized by polyethylenimine. The photoconductive properties of the ZnO NWs / C-dots devices were investigated under photoexcitation with photon energies below and above the ZnO bandgap. The results indicate an increased photoresponse of the functionalized devices in the visible spectral range, as well as enhanced UV photoconductivity. This is attributed to the fast injection of photoexcited electrons from the C-dots into the conduction band of the ZnO NWs, and the subsequent slower desorption of molecular species from the NW surface, which reduces the surface depletion region in the NWs. The surface functionalization

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.

We investigated the modulation of the photoconductivity under below-band gap excitation in single ZnO nanowire field effect transistors. Light excitation at 550 nm does not induce any change in the drain–source current when the gate voltage is kept at V gs = 0 V, but results in a current increase when it is set to V gs = −50 V. At this negative value of the gate voltage we further investigated the photo-reaction in the below-band-gap range 400–800 nm, observing a qualitative similar profile for all the photo-current curves. These results were attributed to a local effect, suggesting that the change in conductivity is due to the release of electrons from interface states located between the ZnO nanowire active channel and the gate dielectric SiO2.