Dan Ewing

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 D* 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.

Printable Transfer-Free and Wafer-Size MoS2/Graphene van der Waals Heterostructures for High-Performance Photodetection

Two-dimensional (2D) MoS2/graphene van der Waals heterostructures integrate the superior light-solid interaction in MoS2 and charge mobility in graphene for high-performance optoelectronic devices. Key to the device performance lies in a clean MoS2/graphene interface to facilitate efficient transfer of photogenerated charges. Here, we report a printable and transfer-free process for fabrication of wafer-size MoS2/graphene van der Waals heterostructures obtained using a metal-free-grown graphene, followed by low-temperature growth of MoS2 from the printed thin film of ammonium thiomolybdate on graphene. The photodetectors based on the transfer-free MoS2/graphene heterostructures exhibit extraordinary short photoresponse rise/decay times of 20/30 ms, which are significantly faster than those of the previously reported MoS2/transferred-graphene photodetectors (0.28-1.5 s). In addition, a high photoresponsivity of up to 835 mA/W was observed in the visible spectrum on such transfer-free MoS2/graphene heterostructures, which is much higher than that of the reported photodetectors based on the exfoliated layered MoS2 (0.42 mA/W), the graphene (6.1 mA/W), and transfer-free MoS2/graphene/SiC heterostructures (∼40 mA/W). The enhanced performance is attributed to the clean interface on the transfer-free MoS2/graphene heterostructures. This printable and transfer-free process paves the way for large-scale commercial applications of the emerging 2D heterostructures in optoelectronics and sensors.

Transfer-free and printable graphene/ZnO-nanoparticle nanohybrid photodetectors with high performance

Combining the high mobility of graphene and surface electron depletion effect of zinc oxide nanoparticles (ZnO-NPs), graphene/ZnO-NP nanohybrids can be anticipated for significantly enhanced photoresponsivity and photoconductive gain in optoelectronics. Herein, a transfer-free and printable method was developed for the fabrication of wafer-size graphene/ZnO-NP nanohybrids for high-performance UV photodetectors. These photodetectors achieved the extraordinary photoresponsivity of up to 1000 A W−1 V−1 and high gain of 1.8 × 104, representing more than an order of magnitude improvement compared to that of previously reported UV photodetectors based on various ZnO nanostructures and transferred-graphene/ZnO nanohybrids. Our method provides a low-cost pathway for the run-to-run wafer-size fabrication of high-performance graphene/ZnO optoelectronics.

Printable Nanocomposite FeS2–PbS Nanocrystals/Graphene Heterojunction Photodetectors for Broadband Photodetection

Colloidal nanocrystals are attractive materials for optoelectronics applications because they offer a compelling combination of low-cost solution processing, printability, and spectral tunability through the quantum dot size effect. Here we explore a novel nanocomposite photosensitizer consisting of colloidal nanocrystals of FeS2 and PbS with complementary optical and microstructural properties for broadband photodetection. Using a newly developed ligand exchange to achieve high-efficiency charge transfer across the nanocomposite FeS2-PbS sensitizer and graphene on the FeS2-PbS/graphene photoconductors, an extraordinary photoresponsivity in exceeding ∼106 A/W was obtained in an ultrabroad spectrum of ultraviolet (UV)-visible-near-infrared (NIR). This is in contrast to the nearly 3 orders of magnitude reduction of the photoresponsivity from ∼106 A/W at UV to 103 A/W at NIR on their counterpart of FeS2/graphene detectors. This illustrates the combined advantages of the nanocomposite sensitizers and the high charge mobility in FeS2-PbS/graphene van der Waals heterostructures for nanohybrid optoelectronics with high performance, low cost, and scalability for commercialization.

Facile zinc oxide nanowire growth on graphene via a hydrothermal floating method: towards Debye length radius nanowires for ultraviolet photodetection

Vertically aligned zinc oxide nanowires on graphene (ZnO-NW/graphene) heterojunction nanohybrids combine the superior sensitivity of crystalline ZnO-NWs with high charge mobility of graphene to provide an ideal platform for high-performance detectors and sensors. Controlling the ZnO-NW microstructure and ZnO-NW/graphene interface is of primary importance for the device performance. This work explores floating hydrothermal growth of ZnO-NWs on seedless and ZnO seeded graphene, and investigates the effects of the microstructure and interface on the performance of ZnO-NW/graphene ultraviolet (UV) detectors. It has been found that the ZnO seed layer facilitates the growth of a dense ZnO-NW array with a NW radius approaching the Debye length. In contrast, the seedless process results in a lower NW areal density and a larger NW diameter on the order of sub-to-few micrometers. Consequently, higher UV responsivity up to 728 A W−1 was obtained in the former. However, a strong charge trapping effect was also observed, which is attributed to the poorer crystallinity of the ZnO-NWs originating from the ZnO seed layer. These results shed light on the importance of controlling the microstructure and interface towards high-performance ZnO-NW/graphene nanohybrid optoelectronics.

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 WOX photodetectors, which indicates that the printed WO3 films are promising for various applications of optoelectronics and sensors.