ABSTRACT:Phototransistors based on monolayer transition metal dichalcogenides (TMD) have high photosensitivity due to their direct band gap transition. However, there is a lack of understanding of the effect of metal contacts on the performance of atomically thin TMD phototransistors. Here, we fabricate phototransistors based on large-area chemical vapor deposition (CVD) tungsten diselenide (WSe2) monolayers contacted with the metals of different work function values. We found that the low Schottky-contact WSe2 phototransistors exhibit a very high photo gain (10 5 ) and specific detectivity (10 14 Jones), values higher than commercial Si-and InGaAs-based photodetectors; however, the response speed is longer than 5s in ambient. In contrast, the response speed of the high Schottky-contact phototransistors display a fast response time shorter than 23ms, but the photo gain and specific detectivity decrease by several orders of magnitude.Moreover, the fast response speed of the high Schottky-contact devices is maintained for a few months in ambient. This study demonstrates that the contact plays an important role in TMD 2 phototransistors, and barrier height tuning is critical for optimizing the photoresponse and photoresponsivity. KEYWORDS:Photodetector · Contact effect · Schottky barrier · Tungsten diselenide · 2d materialThe monolayer two dimensional (2D) transition metal dichalcogenides (TMDs) are potentially important building blocks for nanoscale optoelectronics due to their unique optical properties. 1-9Recently, there has been much interest in the molybdenum disulfide (MoS2) Jones. However, the photocurrents for the Pd-contacted devices take more than 5s to saturate in ambient air. In contrast, the high Schottky-barrier WSe2 devices using low work function Ti metal contacts present a much faster response time of <23ms, and better photocurrent linearity as a function of incident optical power. We suggest a qualitative mechanism based on metal/semiconductor junction energy band diagrams to explain these distinctly different phenomena. RESULTS AND DISCUSSIONThe large area WSe2 monolayers were grown on sapphire by the vapour-phase reaction of WO3 and Se powders in a hot-wall CVD chamber as described in our previous work. 22 After growth, the films were transferred to 300nm SiO2/Si substrates. Figure 1a shows the optical 4 image of a ~1×1cm transferred film on SiO2/Si substrate, and Figure 1b is the magnified optical image with no obvious optical contrast difference across the field, indicating that the transferred film was uniform. 23Atomic force microscopy (AFM), Raman spectroscopy, and photoluminescence (PL) were used to characterize the number of layers and quality of the transferred films. As shown in Figures 1c and 1d, the representative thickness of the transferred films is ~0.97nm, which is slightly thicker than the as-grown monolayer WSe2 on SiO2/Si, 22,24,25 and this could be attributed to residual chemical contamination of the transferred process or the substrate effect. 26 The Raman spectra for th...

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All investigated reservoirs were eutrophicated based on the comprehensive TSI values; thus, our results provided an early warning of water degradation in Fujian reservoirs. Furthermore, the trophic state plays an important role in shaping community structure and in determining species diversity of algae. Therefore, long-term and regular monitoring of Euglenophyta, Cyanophyta, TN, TP and chlorophyll a in reservoirs is urgently needed to further understand the future trend of eutrophication and to develop a watershed-based strategy to manage the Cyanophyta bloom hazards.

Designing high-activity catalysts and revealing the in-depth structure-property relationship is particularly important for Li-O 2 batteries. Herein, the self-boosting catalysis of LiCoO 2 as an electrocatalyst for Li-O 2 batteries and the investigation of its self-adjustment mechanism using in situ X-ray absorption spectroscopy and other operando characterization techniques is reported. The intercalation/extraction of Li + in LiCoO 2 not only induces the change in Co valence and modulates the electronic/crystal structure but also tunes the surface disorder degree, lattice strain, and local symmetry, which all affect the catalysis activity. In a discharge, highly ordered LiCoO 2 acts as a catalyst to boost oxygen reduction reaction. During charging, the initial extraction of Li + from LiCoO 2 induces Li/oxygen vacancy and Co 4+ , which deforms CoO 6 octahedron as well as lowers the symmetry, and accordingly promotes oxygen evolution reaction. This article offers insights into tuning the activity of catalysts for Li-O 2 batteries with the intercalation/extraction of alkali metal ions in traditional cathodes.