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Self-powered photodetectors have attracted widespread attention due to their low power consumption which can be driven by the built-in electric field instead of external power, but it is very difficult to achieve high responsivity and fast response speed concurrently. Here, a self-powered photodetector with light-induced electric field enhancement based on a 2D InSe/WSe2 /SnS2 van der Waals heterojunction is designed. The light-induced electric field derived from the photo-generated electrons of SnS2 accumulated at the SnS2 /WSe2 interface produces an additional negative gate voltage applied to the WSe2 layer, which enhances the built-in electric field in the InSe/WSe2 /SnS2 heterojunction. Accordingly, the photocurrent and photoresponse speed of the heterostructure device are largely improved. The self-powered photodetector based on the InSe/WSe2 /SnS2 heterostructure exhibits a high responsivity of 550 mA W-1 , which is a 50 times increase compared to the InSe/WSe2 photodetector, and the response speed (110/120 µs) is one order of magnitude faster than that of the InSe/WSe2 photodetector. The high responsivity and fast speed are caused by the stronger built-in electric field modulated by a light-induced electric field, which can separate carriers effectively and reduce drift times. This device architecture can provide a new avenue to fabricate high-responsivity, fast self-power photodetectors by utilizing the van der Waals heterojunction.

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Two-dimensional (2D) materials have attracted great attention in the field of photodetection due to their excellent electronic and optoelectronic properties. However, the weak optical absorption caused by atomically thin layers and the short lifetime of photocarriers limit their optoelectronic performance, especially for weak light detection. In this work, we design a high-gain photodetector induced by carrier recirculation based on a vertical InSe/GaSe heterojunction. In this architecture, the photogenerated holes are trapped in GaSe due to the built-in electric field, suppressing the recombination rate of photocarriers, so the electrons can recirculate for multiple times in the InSe channel following the generation of a single electron-hole pair, resulting a high photoconductive gain (107). The responsivity and detectivity of the InSe/GaSe heterojunction can reach 1037 A/W and 8.6 × 1013 Jones, which are 1 order of magnitude higher than those of individual InSe. More importantly, the InSe/GaSe heterojunction can respond to weaker light (1 µW/cm2) compared to individual InSe (10 µW/cm2). Utilizing GaSe as the channel and InSe as the electrons trapped layer, the same experimental phenomenon is achieved. This work can provide an approach for designing a highly sensitive device utilizing a 2D van der Waals heterojunction, and it also possesses wide applicability for other materials.

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Near-infrared (NIR) synaptic devices integrate NIR optical sensitivity and synaptic plasticity, emulating the basic biomimetic function of the human visual system and showing great potential in NIR artificial vision systems. However, the lack of semiconductor materials with appropriate band gaps for NIR photodetection and effective strategies for fabricating devices with synaptic behaviors limit the further development of NIR synaptic devices. Here, a two-terminal NIR synaptic device consisting of the In2Se3/MoS2 heterojunction has been constructed, and it exhibits fundamental synaptic functions. The reduced band gap and potential barrier of In2Se3/MoS2 heterojunctions are essential for NIR synaptic plasticity. In addition, the NIR synaptic properties of In2Se3/MoS2 heterojunctions under strain have been studied systematically. The ΔEPSC of the In2Se3/MoS2 synaptic device can be improved from 38.4% under no strain to 49.0% under a 0.54% strain with a 1060 nm illumination for 1 s at 100 mV. Furthermore, the artificial NIR vision system consisting of a 10 × 10 In2Se3/MoS2 device array has been fabricated, exhibiting image sensing, learning, and storage functions under NIR illumination. This research provides new ideas for the design of flexible NIR synaptic devices based on 2D materials and presents many opportunities in artificial intelligence and NIR vision systems.

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Tuning the optical and electrical properties of two-dimensional (2D) hexagonal boron nitride (hBN) is critical for its successful application in optoelectronics. Herein, we report a new methodology to significantly enhance the optoelectronic properties of hBN monolayers by substitutionally doping with sulfur (S) on a molten Au substrate using chemical vapor deposition. The S atoms are more geometrically and energetically favorable to be doped in the N sites than in the B sites of hBN, and the S 3p orbitals hybridize with the B 2p orbitals, forming a new conduction band edge that narrows its band gap. The band edge positions change with the doping concentration of S atoms. The conductivity increases up to 1.5 times and enhances the optoelectronic properties, compared to pristine hBN. A photodetector made of a 2D S-doped hBN film shows an extended wavelength response from 260 to 280 nm and a 50 times increase in its photocurrent and responsivity with light illumination at 280 nm. These enhancements are mainly due to the improved light absorption and increased electrical conductivity through doping with sulfur. This S-doped hBN monolayer film can be used in the next-generation electronics, optoelectronics, and spintronics.

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Self-powered photodetectors with great potential for implanted medical diagnosis and smart communications have been severely hindered by the difficulty of simultaneously achieving high sensitivity and fast response speed. Here, we report an ultrafast and highly sensitive self-powered photodetector based on two-dimensional (2D) InSe, which is achieved by applying a device architecture design and generating ideal Schottky or ohmic contacts on 2D layered semiconductors, which are difficult to realize in the conventional semiconductors owing to their surface Fermi-level pinning. The as-fabricated InSe photodiode features a maximal lateral self-limited depletion region and a vertical fully depleted channel. It exhibits a high detectivity of 1.26 × 1013 Jones and an ultrafast response speed of ∼200 ns, which breaks the response speed limit of reported self-powered photodetectors based on 2D semiconductors. The high sensitivity is achieved by an ultralow dark current noise generated from the robust van der Waals (vdW) Schottky junction and a high photoresponsivity due to the formation of a maximal lateral self-limited depletion region. The ultrafast response time is dominated by the fast carrier drift driven by a strong built-in electric field in the vertical fully depleted channel. This device architecture can help us to design high-performance photodetectors utilizing vdW layered semiconductors.

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Solar-blind photodetectors have widespread applications due to the unique merit of a "black background" on the earth. However, most solar-blind photodetectors reported previously exhibited quite low rejection ratios (R200nm/R280nm < 103) and were interfered with by light longer than 280 nm. Herein, by an ambient pressure chemical vapor deposition (CVD) method, large-area, clean, and uniform two-dimensional (2D) multilayer h-BN films with different thicknesses have been successfully synthesized on Au foils. The synthesized multilayer h-BN film is transparent to light longer than 280 nm, showing excellent optical and optoelectronic properties to weak solar-blind light (µW/cm2). This sensitive solar-blind h-BN photodetector exhibits ultrahigh rejection ratios (R220nm/R280nm > 103 and R220nm/R290nm > 104), a low dark current (102 fA), and a large detectivity (3.9 × 1010 Jones). It is noteworthy that the rejection ratio (R220nm/R290nm) here is superior to most of those previously reported based on traditional semiconductors. This large-scale, clean, and uniform multilayer h-BN film will contribute to the progress of next-generation optoelectronic devices.

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Mixed-dimension van der Waals (vdW) p-n heterojunction photodiodes have inspired worldwide efforts to combine the excellent properties of 2D materials and traditional semiconductors without consideration of lattice mismatch. However, owing to the scarcity of intrinsic p-type semiconductors and insufficient optical absorption of the few layer 2D materials, a high performance photovoltaic device based on a vdW heterojunction is still lacking. Here, a novel mixed-dimension vdW heterojunction consisting of 1D p-type Se nanotubes and a 2D flexible n-type InSe nanosheet is proposed by a facile method, and the device shows excellent photovoltaic characteristics. Due to the superior properties of the hybrid p-n junction, the mix-dimensional van der Waals heterojunction exhibited high on/off ratios (103) at a relatively weak light intensity of 3 mW cm-2. And a broadband self-powered photodetector ranging from the UV to visible region is achieved. The highest responsivity of the device could reach up to 110 mA W-1 without an external energy supply. This value is comparable to that of the pristine Se device at 5 V and InSe device at 0.1 V, respectively. Furthermore, the response speed is enhanced by one order of magnitude over the single Se or InSe device even at a bias voltage. This work paves a new way for the further development of high performance, low cost, and energy-efficient photodetectors by using mixed-dimensional vdW heterostructures.

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Thiophenol is a highly toxic compound which is essential in the field of organic synthesis and drug design. However, the accumulation of thiophenols in the environment may cause serious health problems for human bodies ultimately. Therefore, it is critical to develop efficient methods for visualization of thiophenol species in biological samples. In this work, an innovative two-photon fluorescent turn-on probe FR-TP with far-red emission for thiophenols based on FR-NH2 fluorophore and 2,4-dinitrophenylsulfonyl recognition site was reported. The new probe can be used for thiophenol detection with large far-red fluorescence enhancement (about 155-fold), rapid response (completed within 100s), excellent sensitivity (DL 0.363µM), high selectivity, and lower cellular auto-fluorescence interference. Importantly, the probe FR-TP can be successfully employed to visualize thiophenols not only in the living HeLa cells but also in living liver tissues. In addition, through two-photon tissue imaging, the probe was used to monitor and investigate biological thiophenol poisoning in the animal model of thiophenol inhalation for the first time.

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Biothiols such as cysteine (Cys) and glutathione (GSH) are important reductive species that are widely spread in the liver, heart, brain and so on, and are essential for maintaining the fundamental roles of these organs. Fluorescence detection of biothiols in living organisms is of great importance for exploring the metabolic pathways and physiological function in living organisms. Most of the near-infrared probes for biothiols have small Stokes shifts, usually less than 50 nm, which greatly hamper their applications in biological imaging; herein, a novel fluorescent CS-thiol probe based on a Changsha (CS) near-infrared dye analogue with a large Stokes shift was developed and applied for live animal imaging. In the presence of biothiols, such as Cys, CS-thiols show a broad fluorescence emission at 660 nm and exhibit high selectivity toward biothiols among other amino acids and reactive species. CS-Thiols exhibited little toxicity to HeLa cells and were successfully applied to image biothiols in living cells and living animals.

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Full-color fluorescence imaging, which utilizes multiple fluorescent imaging agents with emission covering the whole visible spectrum in the same specimen, is a powerful approach to study molecular processes in living systems. To achieve two-photon full-color bioimaging, two-photon full-color-tunable fluorescent materials (with emission from the blue to near-infrared region) are highly desirable. Herein, in this work, we outline the rational design, synthesis, optical property studies, and biological imaging studies of a unique family of two-photon full-color-tunable functional fluorescent materials. Based on the proposed "hybridization" strategy, we judiciously engineered the first family of two-photon full-color-tunable functional fluorescent materials (TPFC) and their lysosome-targetable derivatives (TPFC-Lyso). The novel TPFC dyes display two-photon full-color-tunable fluorescence emission from the blue to near-infrared region in solutions. By exploiting their unprecedented two-photon full-color-tunable properties, the new TPFC dyes were successfully applied for two-photon full-color imaging in living cells, and in particular, in living tissues at different penetration depths for the first time. In addition, as representative cases, we have described the first report of two-photon full-color fluorescence imaging in subcellular organelles, lysosomes, using the novel lysosome-targetable TPFC-Lyso dyes.

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H2O2 as one of the most important ROS (Reactive Oxygen Species) has more attack activity to biomolecules such as DNA, RNA, protein and enzyme in alkaline environment and leads to a series of disease. However, no attention has been paid to the fluorescent detection of H2O2 in alkaline environment in the past. Herein, we reported the first ratiometric near-infrared fluorescent probe based on a boric acid derivative of Changsha near-infrared dye (CSBOH) for H2O2 detection in alkaline condition and the application for H2O2 imaging in vivo. ICT (intra-molecular charge transfer) mechanism was used in CSBOH to modulate the fluorescence change. The photophysical change of CSBOH was investigated by comparison with a phenol derivative of Changsha near-infrared dye (CSOH), a structural analogue bearing phenol group. In the presence of H2O2, CSBOH exhibited remarkably different fluorescence change at 650 nm and 720 nm when excited by 560 nm and 670 nm light respectively in alkaline buffer and showed high selectivity toward H2O2. Cellular experiments demonstrate that CSBOH can image endogenously generated H2O2 in macrophages and A431 cells. In vivo experiment demonstrates that both CSOH and CSBOH can be used for bio-imaging, and CSBOH can image H2O2 in living animal successfully.

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Most of the near-infrared (NIR) fluorescent copper ion probes are coordination-based, and the fluorescence enhancement is between 10 and 20 folds. Herein, a novel NIR fluorescent dye named CSCN with excellent photostability and a reaction-based Cu(2+) NIR probe named CSCN-Cu were reported. CSCN exhibited good photostability toward photo irradiation. CSCN-Cu showed lower background fluorescent interference and over 40-fold fluorescence enhancement in NIR region, it also exhibited good selectivity toward Cu(2+) in Hepes solution. Biotic experiments demonstrated that CSCN-Cu possessed low toxicity and successfully imaged Cu(2+) in living cells under the conditions performed.

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