Solar-blind ultraviolet (SBUV) detection has important applications in wireless secure communication, early warning, and so forth. However, the desired key device for SBUV detection and high-sensitivity and low-noise "sandwich" photodetector with large detective area is difficult to be fabricated because it is usually hard for traditional wide band gap semiconductors to boast both high conductivity and high SBUV transparency. Here, we proposed to use graphene as the transparent conductive layer to form graphene/β-GaO heterojunction. With the help of large-area graphene and hot carrier multiplication, a SBUV photodetector with large detective area, low dark current, and high sensitivity was successfully assembled. Its photoresponsivity is 1-3 orders of magnitude higher than that of the conventional SBUV photodetectors, and its response speed can rival the best device ever reported.
A chemical vapor deposition method is developed for thickness‐controlled (one to four layers), uniform, and continuous films of both defective gallium(II) sulfide (GaS): GaS0.87 and stoichiometric GaS. The unique degradation mechanism of GaS0.87 with X‐ray photoelectron spectroscopy and annular dark‐field scanning transmission electron microscopy is studied, and it is found that the poor stability and weak optical signal from GaS are strongly related to photo‐induced oxidation at defects. An enhanced stability of the stoichiometric GaS is demonstrated under laser and strong UV light, and by controlling defects in GaS, the photoresponse range can be changed from vis‐to‐UV to UV‐discriminating. The stoichiometric GaS is suitable for large‐scale, UV‐sensitive, high‐performance photodetector arrays for information encoding under large vis‐light noise, with short response time (<66 ms), excellent UV photoresponsivity (4.7 A W–1 for trilayer GaS), and 26‐times increase of signal‐to‐noise ratio compared with small‐bandgap 2D semiconductors. By comprehensive characterizations from atomic‐scale structures to large‐scale device performances in 2D semiconductors, the study provides insights into the role of defects, the importance of neglected material‐quality control, and how to enhance device performance, and both layer‐controlled defective GaS0.87 and stoichiometric GaS prove to be promising platforms for study of novel phenomena and new applications.
The solid progress in the study of a single two-dimensional (2D) material underpins the development for creating 2D material assemblies with various electronic and optoelectronic properties. We introduce an asymmetric structure by stacking monolayer semiconducting tungsten disulfide, metallic graphene, and insulating boron nitride to fabricate numerous red channel lightemitting devices (LEDs). All the 2D crystals were grown by chemical vapor deposition (CVD), which has great potential for future industrial scale-up. Our LEDs exhibit visibly observable electroluminescence (EL) at both 5.5 V forward and 7.0 V backward biasing, which correlates well with our asymmetric design. The red emission can last for at least several minutes, and the success rate of the working device that can emit detectable EL is up to 80%. In addition, we show that sample degradation is prone to happen when a continuing bias, much higher than the threshold voltage, is applied. Our success of using high-quality CVD-grown 2D materials for red light emitters is expected to provide the basis for flexible and transparent displays.
Heterophase homojunction formation in atomically thin 2D layers is of great importance for next-generation nanoelectronics and optoelectronics applications. Technologically challenging, controllable transformation between the semiconducting and metallic phases of transition metal chalcogenides is of particular importance. Here, we demonstrate that controlled laser irradiation can be used to directly ablate PdSe2 thin films using high power, or trigger the local transformation of PdSe2 into a metallic phase PdSe2-x using lower laser power. Such transformations are possible due to the low decomposition temperature of PdSe2 compared to other 2D transition metal dichalcogenides. Scanning transmission electron microscopy is used to reveal the laser-induced Se-deficient phases of PdSe2 material. The process sensitivity to the laser power allows patterning flexibility for resist free device fabrication. The laser patterned devices demonstrate that a laser-induced metallic phase PdSe2-x is stable with increased conductivity by a factor of about 20 compared to PdSe2. These findings contribute to 2 the development of nanoscale devices with homojunctions and scalable methods to achieve structural transformations in 2D materials.
Monolayer transition metal dichalcogenides (TMDs) have demonstrated great potential in next-generation electronics due to their unique optical and electronic properties. However, it remains challenging to produce uniform highquality TMDs over a large scale. The direct sulfurization method holds great promise in achieving large-scale synthesis, but the obtained materials suffer from small grain size and multilayer regions. Herein, low-cost glass substrate is used to achieve facile growth of large-area uniform and large-size monolayer MoS 2 crystals via a modified direct sulfurization method. We find that the ability of glass to incorporate predeposited precursors into its molten state is the key to the production of high-quality monolayer MoS 2 crystals. The monolayer MoS 2 crystals possess the largest average crystal size (∼100 μm) for MoS 2 grown by the direct sulfurization method, with large-area uniformity, which is only limited by the size of the substrate. A combination of low-cost, uniformity, scalability, simplicity, and high quality is achieved in our method which showed great promise for the development of wafer scale electronic devices based on 2D materials. Our work also provides a new look at the role substrates could play in the synthesis and the possibilities of using other glass or liquid substrates for the growth of high-quality 2D materials.
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