11 research outputs found

    Photocurrent Response in Multiwalled Carbon Nanotube Core–Molybdenum Disulfide Shell Heterostructures

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    In this report, a few-layer molybdenum disulfide (MoS<sub>2</sub>) shell was coated on core multiwalled carbon nanotube (CNT) by a facile solvothermal method. The morphology and high crystallinity of this structure were demonstrated and verified by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). After being integrated into a planar device, the CNT–MoS<sub>2</sub> core–shell structure exhibits clear photoresponse and a wide response range upon laser illumination. In addition, the device shows a bias-dependent and position-sensitive photocurrent effect. Further experiments show that larger photocurrent was obtained under laser illumination with longer wavelength. Both the photocurrent and response speed are enhanced when the device is placed under vacuum condition. The simple material synthesis and device fabrication method used in this work may provide a practical strategy for low-cost and large-scale optical applications

    NIR Schottky Photodetectors Based on Individual Single-Crystalline GeSe Nanosheet

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    We have synthesized high-quality, micrometer-sized, single-crystal GeSe nanosheets using vapor transport and deposition techniques. Photoresponse is investigated based on mechanically exfoliated GeSe nanosheet combined with Au contacts under a global laser irradiation scheme. The nonlinearship, asymmetric, and unsaturated characteristics of the <i>I</i>–<i>V</i> curves reveal that two uneven back-to-back Schottky contacts are formed. First-principles calculations indicate that the occurrence of defects-induced in-gap defective states, which are responsible for the slow decay of the current in the OFF state and for the weak light intensity dependence of photocurrent. The Schottky photodetector exhibits a marked photoresponse to NIR light illumination (maximum photoconductive gain ∼5.3 × 10<sup>2</sup> % at 4 V) at a wavelength of 808 nm. The significant photoresponse and good responsitivity (∼3.5 A W<sup>–1</sup>) suggests its potential applications as photodetectors

    Plasmon-Enhanced Photocatalytic Properties of Cu<sub>2</sub>O Nanowire–Au Nanoparticle Assemblies

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    Cu<sub>2</sub>O–Au nanocomposites (NCs) with tunable coverage of Au were prepared by a facile method of mixing gold nanoparticles (Au NPs) with copper­(I) oxide nanowires (Cu<sub>2</sub>O NWs) in various ratios. These Cu<sub>2</sub>O–Au NCs display tunable optical properties, and their photocatalytic properties were dependent on the coverage density of Au NPs. The photocatalytic activity of Cu<sub>2</sub>O–Au NCs was examined by photodegradation of methylene blue. The presence of Au NPs enhanced the photodegradation efficiency of Cu<sub>2</sub>O NCs. The photocatalytic efficiency of Cu<sub>2</sub>O–Au NCs initially increased with the increasing coverage density of Au NPs and then decreased as the surface of Cu<sub>2</sub>O became densely covered by Au NPs. The enhanced photocatalytic efficiency was ascribed to enhanced light absorption (by the surface plasmon resonance) and the electron sink effect of the Au NPs

    Desorption of Ambient Gas Molecules and Phase Transformation of α‑Fe<sub>2</sub>O<sub>3</sub> Nanostructures during Ultrahigh Vacuum Annealing

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    Desorption and readsorption of gas molecules from ambient air onto the surface of α-Fe<sub>2</sub>O<sub>3</sub> quasi-1D nanostructures (nanoflakes and nanostrips) were studied in situ by X-ray photoelectron spectroscopy (XPS) and ex situ by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XPS revealed that carbon and oxygen species were physisorbed and chemisorbed as C–C/C–H, C–O, O–CO, and O<sub>2</sub> states on both surfaces of α-Fe<sub>2</sub>O<sub>3</sub> quasi-1D nanostructures upon exposure in air. The physisorbed carbon species (C–C/C–H) and O<sub>2</sub> desorbed from the surfaces when the two nanostructures were heated to 100 °C inside the vacuum chamber of XPS. Significant desorption of chemisorbed O–CO and O–C occurred above 200 °C, which resulted in a reduction of Fe<sub>2</sub>O<sub>3</sub> into Fe<sub>3</sub>O<sub>4</sub> for both samples between 200 and 300 °C. Complete desorption of carbon and O–C/O–CO/O<sub>2</sub> species in O1s occurred at 400 °C, where Fe<sub>3</sub>O<sub>4</sub> in nanoflakes (sample 1) was reduced further into FeO by excess metallic Fe from the bulk, while Fe<sub>3</sub>O<sub>4</sub> in nanostrips (sample 2) was largely oxidized into Fe<sub>2</sub>O<sub>3</sub> by the oxygen from the bulk of Fe<sub>2</sub>O<sub>3</sub>. Although no band bending was observed during the annealing and desorption of ambient gases, the valence band changed as the phase transformation occurred. After the annealed samples were exposed to air for two days, the same chemical states associated with C and O species were again detected on the surfaces of the two nanostructures. In addition, FeO (sample 1) was found to be oxidized into a mixture of Fe<sub>2</sub>O<sub>3</sub> and Fe<sub>3</sub>O<sub>4</sub> on the surface. The adsorption of gas molecules from ambient environment thus has a strong influence on the chemical and physical properties of nanostructures with large surface to volume ratio

    Ultrasensitive Phototransistor Based on K‑Enriched MoO<sub>3</sub> Single Nanowires

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    An ultrasensitive phototransistor was fabricated based on K-intercalated MoO<sub>3</sub> single nanowire. Devices with ultrafast photoresponse rate, high responsivity, and broad spectral response range were demonstrated. Detailed analysis of the charge transport in the device revealed the coexistence of both thermal-activation and photoactivation mechanisms. The promising results are expected to promote the potential of this material in nano/micro-scaled photoelectronic applications

    Microsteganography on WS<sub>2</sub> Monolayers Tailored by Direct Laser Painting

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    We present scanning focused laser beam as a multipurpose tool to engineer the physical and chemical properties of WS<sub>2</sub> microflakes. For monolayers, the laser modification integrates oxygen into the WS<sub>2</sub> microflake, resulting in ∼9 times enhancement in the intensity of the fluorescence emission. This modification does not cause any morphology change, allowing “micro-encryption” of information that is only observable as fluorescence under excitation. The same focused laser also facilitates on demand thinning down of WS<sub>2</sub> multilayers into monolayers, turning them into fluorescence active components. With a scanning focused laser beam, micropatterns are readily created on WS<sub>2</sub> multilayers through selective thinning of specific regions on the flake

    Improved Photoelectrical Properties of MoS<sub>2</sub> Films after Laser Micromachining

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    Direct patterning of ultrathin MoS<sub>2</sub> films with well-defined structures and controllable thickness is appealing since the properties of MoS<sub>2</sub> sheets are sensitive to the number of layer and surface properties. In this work, we employed a facile, effective, and well-controlled technique to achieve micropatterning of MoS<sub>2</sub> films with a focused laser beam. We demonstrated that a direct focused laser beam irradiation was able to achieve localized modification and thinning of as-synthesized MoS<sub>2</sub> films. With a scanning laser beam, microdomains with well-defined structures and controllable thickness were created on the same film. We found that laser modification altered the photoelectrical property of the MoS<sub>2</sub> films, and subsequently, photodetectors with improved performance have been fabricated and demonstrated using laser modified films

    Reduced Graphene Oxide Conjugated Cu<sub>2</sub>O Nanowire Mesocrystals for High-Performance NO<sub>2</sub> Gas Sensor

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    Reduced graphene oxide (rGO)-conjugated Cu<sub>2</sub>O nanowire mesocrystals were formed by nonclassical crystallization in the presence of GO and <i>o</i>-anisidine under hydrothermal conditions. The resultant mesocrystals are comprised of highly anisotropic nanowires as building blocks and possess a distinct octahedral morphology with eight {111} equivalent crystal faces. The mechanisms underlying the sequential formation of the mesocrystals are as follows: first, GO-promoted agglomeration of amorphous spherical Cu<sub>2</sub>O nanoparticles at the initial stage, leading to the transition of growth mechanism from conventional ion-by-ion growth to particle-mediated crystallization; second, the evolution of the amorphous microspheres into hierarchical structure, and finally to nanowire mesocrystals through mesoscale transformation, where Ostwald ripening is responsible for the growth of the nanowire building blocks; third, large-scale self-organization of the mesocrystals and the reduction of GO (at high GO concentration) occur simultaneously, resulting in an integrated hybrid architecture where porous three-dimensional (3D) framework structures interspersed among two-dimensional (2D) rGO sheets. Interestingly, “super-mesocrystals” formed by 3D oriented attachment of mesocrystals are also formed judging from the voided Sierpinski polyhedrons observed. Furthermore, the interior nanowire architecture of these mesocrystals can be kinetically controlled by careful variation of growth conditions. Owing to high specific surface area and improved conductivity, the rGO-Cu<sub>2</sub>O mesocrystals achieved a higher sensitivity toward NO<sub>2</sub> at room temperature, surpassing the performance of standalone systems of Cu<sub>2</sub>O nanowires networks and rGO sheets. The unique characteristics of rGO-Cu<sub>2</sub>O mesocrystal point to its promising applications in ultrasensitive environmental sensors

    Fluorescence Concentric Triangles: A Case of Chemical Heterogeneity in WS<sub>2</sub> Atomic Monolayer

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    We report a novel optical property in WS<sub>2</sub> monolayer. The monolayer naturally exhibits beautiful in-plane periodical and lateral homojunctions by way of alternate dark and bright band in the fluorescence images of these monolayers. The interface between different fluorescence species within the sample is distinct and sharp. This gives rise to intriguing concentric triangular fluorescence patterns in the monolayer. The novel optical property of this special WS<sub>2</sub> monolayer is facilitated by chemical heterogeneity. The photoluminescence of the bright band is dominated by emissions from trion and biexciton while the emission from defect-bound exciton dominates the photoluminescence at the dark band. The discovery of such concentric fluorescence patterns represents a potentially new form of optoelectronic or photonic functionality

    Atomic Healing of Defects in Transition Metal Dichalcogenides

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    As-grown transition metal dichalcogenides are usually chalcogen deficient and therefore contain a high density of chalcogen vacancies, deep electron traps which can act as charged scattering centers, reducing the electron mobility. However, we show that chalcogen vacancies can be effectively passivated by oxygen, healing the electronic structure of the material. We proposed that this can be achieved by means of surface laser modification and demonstrate the efficiency of this processing technique, which can enhance the conductivity of monolayer WSe<sub>2</sub> by ∼400 times and its photoconductivity by ∼150 times
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