Quantum Dots-Facilitated Printing of ZnO Nanostructure Photodetectors with Improved Performance

A nanocomposite ink composed of zinc oxide precursor (ZnOPr) and crystalline ZnO quantum dots (ZnOPrQDs) has been explored for printing high-performance ultraviolet (UV) photodetectors. The performance of the devices has been compared with their counterparts’ printed from ZnOPr ink without ZnO QDs. Remarkably, higher UV photoresponsivity of 383.6 A/W and the on/off ratio of 2470 are observed in the former, which are significantly better than 14.7 A/W and 949 in the latter. The improved performance is attributed to the increased viscosity in the nanocomposite ink to enable a nanoporous structure with improved crystallinity and surface-to-volume ratio. This is key to enhanced surface electron-depletion effect for higher UV responsivity and on/off ratio. In addition, the QD-assisted printing provides a simple and robust method for printing high-performance optoelectronics and sensors

Heat-Assisted Inkjet Printing of Tungsten Oxide for High-Performance Ultraviolet Photodetectors

An ammonium metatungstate precursor (WO₃Pr) ink was printed for tungsten oxide (WO₃) UV detectors on SiO₂/Si wafers with prefabricated Au electrodes. A systematic study was carried out on the printing parameters including substrate temperatures in the range of 22–80 °C, WO₃Pr molar concentrations of 0.01, 0.02, and 0.03 M, and printing scan numbers up to 7 to understand their effects on the resulted WO₃ film morphology and optoelectronic properties. It has been found that the printing parameters can sensitively affect the WO₃ film morphology, which in turn impacts the WO₃ photodetector performance. In particular, the printed films experienced a systematic change from discontinuous droplets at below 40 °C to continuous films at 40–60 °C of the substrate temperature. At higher temperatures, the excessive heat from the substrate not only caused drastic evaporation of the printed ink, resulting in highly nonuniform films, but also detrimental heating of the ink in the printer nozzle in proximity of the substrate, preventing continuous printing operation. An optimal printing window of the substrate temperature of 45–55 °C at a molar concentration of 0.02 M of ammonium metatungstate and three printing scans was obtained for the best UV detector performance. A large on/off ratio of 3538 and a high responsivity up to 2.70 A/W at 5 V bias (0.54 A/W·V) represent a significant improvement over the best report of ∼0.28 μA/W·V on WO_<i>X</i> photodetectors, which indicates that the printed WO₃ films are promising for various applications of optoelectronics and sensors

All-Printable ZnO Quantum Dots/Graphene van der Waals Heterostructures for Ultrasensitive Detection of Ultraviolet Light

In ZnO quantum dot/graphene heterojunction photodetectors, fabricated by printing quantum dots (QDs) directly on the graphene field-effect transistor (GFET) channel, the combination of the strong quantum confinement in ZnO QDs and the high charge mobility in graphene allows extraordinary quantum efficiency (or photoconductive gain) in visible-blind ultraviolet (UV) detection. Key to the high performance is a clean van der Waals interface to facilitate an efficient charge transfer from ZnO QDs to graphene upon UV illumination. Here, we report a robust ZnO QD surface activation process and demonstrate that a transition from zero to extraordinarily high photoresponsivity of 9.9 × 10⁸ A/W and a photoconductive gain of 3.6 × 10⁹ can be obtained in ZnO QDs/GFET heterojunction photodetectors, as the ZnO QDs surface is systematically engineered using this process. The high figure-of-merit UV detectivity <i>D*</i> in exceeding 1 × 10¹⁴ Jones represents more than 1 order of magnitude improvement over the best reported previously on ZnO nanostructure-based UV detectors. This result not only sheds light on the critical role of the van der Waals interface in affecting the optoelectronic process in ZnO QDs/GFET heterojunction photodetectors but also demonstrates the viability of printing quantum devices of high performance and low cost

Interface Nanojunction Engineering of Electron-Depleted Tungsten Oxide Nanoparticles for High-Performance Ultraviolet Photodetection

This work reports a general and facile route, i.e., thermal decomposition of a precursor followed by ultrafast thermal annealing (TDP-UTA), to the in situ fabrication of a nanojunction-interlinked tungsten oxide nanoparticle (WO₃-NP) networks for extraordinary ultraviolet (UV) photodetection. TDP leads a spin-coated ammonium metatungstate thin layer to in situ self-assemble into a highly crystalline WO₃-NP mesoporous film on SiO₂/Si substrates with prepatterned electrodes. The as-synthesized WO₃-NPs have dimensions comparable to the Debye length (≈43 nm), which is critical to the optimal electron-depletion effect for high gain in photodetection. UTA creates the NP–NP interface nanojunctions between neighboring WO₃-NPs, which is the key to high-efficiency electron transport with minimized charge recombination in optoelectronic processes. The photodetectors based on such nanojunction-interlinked WO₃-NP networks exhibit a photocurrent-to-dark-current ratio of 5600, the highest value for any WO_<i>x</i>-based photodetectors ever reported. Moreover, the obtained photoresponsivity is up to 139 A/W (or 27.8 A/W·V) upon 360 nm illumination, which is over 1 order of magnitude higher than that of any previously reported WO_<i>x</i>-nanostructure film photodetectors. These results demonstrate that the TDP-UTA route is a low-cost, robust, and scalable pathway to the in situ fabrication of interlinked semiconducting-nanostructure networks for high-performance optoelectronics and sensors