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Color image steganography based on deep learning is the art of hiding information in the color image. Among them, image hiding steganography(hiding image with image) has attracted much attention in recent years because of its great steganographic capacity. However, images generated by image hiding steganography may show some obvious color distortion or artificial texture traces. We propose a color image steganographic model based on frequency sub-band selection to solve the above problems. Firstly, we discuss the relationship between the characteristics of different color spaces/frequency sub-bands and the generated image quality. Then, we select the B channel of the RGB image as the embedding channel and the high-frequency sub-band as the embedding domain. DWT(discrete wavelet transformation) transforms B channel information and secret gray image into frequency domain information, and then the secret image is embedded and extracted in the frequency domain. Comprehensive experiments demonstrate that images generated by our model have better image quality, and the imperceptibility is significantly increased., Comment: 19 pages,17 figures

Monolayer transition metal dichalcogenides (TMDCs) are two-dimensional (2D) materials with many potential applications. Chemical vapour deposition (CVD) is a promising method to synthesize these materials. However, CVD-grown materials generally have poorer quality than mechanically exfoliated ones and contain more defects due to the difficulties in controlling precursors' distribution and concentration during growth where solid precursors are used. Here, we propose to use thiol as a liquid precursor for CVD growth of high quality and uniform 2D MoS2. Atomic-resolved structure characterizations indicate that the concentration of sulfur vacancies in the MoS2 grown from thiol is the lowest among all reported CVD samples. Low temperature spectroscopic characterization further reveals the ultrahigh optical quality of the grown MoS2. Density functional theory simulations indicate that thiol molecules could interact with sulfur vacancies in MoS2 and repair these defects during the growth of MoS2, resulting in high quality MoS2. This work provides a facile and controllable method for the growth of high-quality 2D materials with ultralow sulfur vacancies and high optical quality, which will benefit their optoelectronic applications., Comment: 23 pages, 6 figures, Small, 2020, Accepted

Large scale implementation of electrochemical water splitting for hydrogen evolution requires cheap and efficient catalysts to replace expensive platinum. Molybdenum disulfide is one of the most promising alternative catalysts but its intrinsic activity is still inferior to platinum. There is therefore a need to explore new active site origins in molybdenum disulfide with ultrafast reaction kinetics and to understand their mechanisms. Here, we report a universal cold hydrogen plasma reduction method for synthesizing different single atoms sitting on two-dimensional monolayers. In case of molybdenum disulfide, we design and identify a new type of active site, i.e., unsaturated Mo single atoms on cogenetic monolayer molybdenum disulfide. The catalyst shows exceptional intrinsic activity with a Tafel slope of 35.1 mV dec-1 and a turnover frequency of ~10^3 s-1 at 100 mV, based on single flake microcell measurements. Theoretical studies indicate that coordinately unsaturated Mo single atoms sitting on molybdenum disulfide increase the bond strength between adsorbed hydrogen atoms and the substrates through hybridization, leading to fast hydrogen adsorption/desorption kinetics and superior hydrogen evolution activity. This work shines fresh light on preparing highly-efficient electrocatalysts for water splitting and other electrochemical processes, as well as provides a general method to synthesize single atoms on two-dimensional monolayers., Comment: 25 pages, 4 figures

A new type of two-dimensional layered semiconductor with weak electrostatic but not van der Waals interlayer interactions, Bi2O2Se, has been recently synthesized, which shown excellent air stability and ultrahigh carrier mobility. Herein, we combined theoretical and experimental approaches to study the Raman spectra of Bi2O2Se and related bismuth oxychalcogenides (Bi2O2Te and Bi2O2S). The experimental peaks were fully consistent with the calculated results, and were successfully assigned. Bi2O2S was predicted to have more Raman-active modes due to its lower symmetry. The shift of the predicted frequencies of Raman active modes was also found to get softened as the interlayer interaction decreases from bulk to monolayer Bi2O2Se and Bi2O2Te. To reveal the strain effects on the Raman shifts, a universal theoretical equation was established based on the symmetry of Bi2O2Se and Bi2O2Te. It was predicted that the doubly degenerate modes split under in-plane uniaxial/shear strains. Under a rotated uniaxial strain, the changes of Raman shifts are anisotropic for degenerate modes although Bi2O2Se and Bi2O2Te were usually regarded as isotropic systems similar to graphene. This implies a novel method to identify the crystallographic orientation from Raman spectra under strain. These results have important consequences for the incorporation of 2D Bismuth oxychalcogenides into nanoelectronic devices., Comment: 40 Pages, 18 figures

The identification of the crystalline axis of anisotropic black phosphorus (BP) is key for the study of its physical properties and for its optical and electronic applications. Herein, we show that by applying an in-plane uniaxial strain and measuring the changes of Raman shifts, the crystalline axis of BP can be reliably determined. The strain effects on Raman shifts are angle-dependent, which can be expressed as a combination of Raman responses under zigzag and armchair strains. Different from previous polarized optical spectroscopic means, the proposed method does not rely on the laser polarization, the excitation wavelength, the sample thickness and the material of the stretchable substrate. Besides, the effective strain applied on BP from the stretched substrate is estimated, showing that only 20% to 40% of the strain can be effectively transferred to BP flakes from polyethylene terephthalate substrates. Our method provides not only an effective and universal approach to identify the crystalline orientation of layered BP, but also a model to extract additional information in strain-related studies, and can be extended to other 2D anisotropic materials., Comment: 28 pages, 10 figures. There are some incorrect description would cause misunderstanding