All-inorganic perovskites nanostructures, such as CsPbCl 3 nanocrystals (NCs), are promising in many applications including light-emitting diodes, photovoltaics, and photodetectors. Despite the impressive performance that was demonstrated, a critical issue remains due to the instability of the perovskites in ambient. Herein, we report a method of passivating crystalline CsPbCl 3 NC surfaces with 3-mercaptopropionic acid (MPA), and superior ambient stability is achieved. The printing of these colloidal NCs on the channel of graphene field-effect transistors (GFETs) on solid Si/SiO 2 and flexible polyethylene terephthalate substrates was carried out to obtain CsPbCl 3 NCs/GFET heterojunction photodetectors for flexible and visible-blind ultraviolet detection at wavelength below 400 nm. Besides ambient stability, the additional benefits of passivating surface charge trapping by the defects on CsPbCl 3 NCs and facilitating highefficiency charge transfer between the CsPbCl 3 NCs and graphene were provided by MPA. Extraordinary optoelectronic performance was obtained on the CsPbCl 3 NCs/graphene devices including a high ultraviolet responsivity exceeding 10 6 A/W, a high detectivity of 2 × 10 13 Jones, a fast photoresponse time of 0.3 s, and ambient stability with less than 10% degradation of photoresponse after 2400 h. This result demonstrates the crucial importance of the perovskite NC surface passivation not only to the performance but also to the stability of the perovskite optoelectronic devices.

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 × 10 A/W and a photoconductive gain of 3.6 × 10 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 × 10 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.

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

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 FeS 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 FeS-PbS sensitizer and graphene on the FeS-PbS/graphene photoconductors, an extraordinary photoresponsivity in exceeding ∼10 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 ∼10 A/W at UV to 10 A/W at NIR on their counterpart of FeS/graphene detectors. This illustrates the combined advantages of the nanocomposite sensitizers and the high charge mobility in FeS-PbS/graphene van der Waals heterostructures for nanohybrid optoelectronics with high performance, low cost, and scalability for commercialization.

Zinc oxide nanoparticles (ZnO-NPs) with radius of the Debye length have the optimal electron depletion effect for high-performance optoelectronic devices. However, a major challenge remains in assembling ZnO-NPs into three-dimensional (3D) interlinked networks for high-efficiency electron transport. Here an ultrafast thermal annealing (UTA) process has been developed by exposing the ZnO-NPs to excessive heat for a short period of 2 s. This enables the formation of NP-NP interface nanojunctions, resulting in nearly two orders of magnitude decrease of the dark current I Dark and more than an order of magnitude increase of the photocurrent I Ph under ultraviolet (UV) illumination. Moreover, the UV photodetectors based on such 3D interlinked ZnO-NP networks exhibit extraordinary performance with high I Ph /I Dark ratio of 3.1×10 5 , responsivity of up to 95.4 A/W at 340 nm UV power of 0.1 mW/cm 2 (and up to 430 A/W at 0.003 mW/cm 2 ), detectivity of 1.4× 10 13 Jones, and rise/decay time of 9.4/13.5 s. These results illustrate the critical importance of the NP-NP interface nanojunctions and provide a low-cost pathway for high-performance ZnO-NP optoelectronics.