A new two-dimensional material, the C2N holey 2D (C2N-h2D) crystal, has recently been synthesized. Here we investigate the strain effects on the properties of this new material by first-principles calculations. We show that the material is quite soft with a small stiffness constant and can sustain large strains ≥ 12%. It remains a direct gap semiconductor under strain and the bandgap size can be tuned in a wide range as large as 1 eV. Interestingly, for biaxial strain, a band crossing effect occurs at the valence band maximum close to a 8% strain, leading to a dramatic increase of the hole effective mass. Strong optical absorption can be achieved by strain tuning with absorption coefficient ∼ 10 6 cm −1 covering a wide spectrum. Our findings suggest the great potential of strain-engineered C2N-h2D in electronic and optoelectronic device applications.Since the discovery of graphene, two-dimensional (2D) materials have attracted tremendous interest due to their many fascinating properties.1-4 One current focus is to explore new 2D materials with suitable semiconducting bandgaps for device applications. Recently, such a new 2D crystal, C 2 N holey 2D (C 2 N-h2D) crystal, has been successfully synthesized.5 The material has a direct bandgap (with reported optical gap size around 2 eV), and a field-effect-transistor fabricated based on it shows a high on/off ratio of 10 7 , suggesting its great potential for applications in electronics and optoelectronics. 5-8For application purposes, it is crucial to have the ability to tailor electronic properties of the material. Strain has long been known as an effective mechanism for tuning properties of semiconductors. It is especially useful for low-dimensional systems since they can usually sustain much larger strains than their bulk crystals. In particular, it has been shown that 2D materials, such as graphene, MoS 2 , and phosphorene, have excellent mechanical flexibility (with critical strains ≥ 25%), [9][10][11][12][13] which makes strain an extremely powerful approach for engineering the properties of 2D materials. 14-19Motivated by the urgent need in understanding the physical properties of the newly discovered C 2 N-h2D material and by the great interest in engineering it for applications, in this work, we investigate the effects of biaxial and uniaxial strains on the electronic and optic properties of monolayer C 2 N-h2D crystals using first-principles calculations. We find that the material is quite flexible with a small stiffness constant and can withstand strains ≥ 12%. Under different types of strain, while still maintaining a direct bandgap, the gap size can be tuned in a wide range as large as 1 eV. More interestingly, for biaxial strain, due to different bonding characters of the bands, there is a switch of band ordering near the valence band maximum (VBM) at a critical strain < 8%, leading to a strain-induced dramatic increase of the hole effective mass. Despite its atomic thickness, this material shows fairly large optical absorption over most visible light s...
We report the magnetization, electrical resistivity, specific heat measurements and band structure calculations of layered superconductor SnTaS2. The experiments are performed on single crystals grown by chemical vapor transport method. The resistivity and magnetic susceptibility indicate that SnTaS2 is a type-II superconductor with transition temperature Tc = 3 K. The upper critical field (Hc2) shows large anisotropy for magnetic field parallel to ab plane (H//ab) and c axis (H//c). The temperature dependence of Hc2 for H//ab shows obvious upward feature at low temperature, which may originate from the multiband effect. Band structure of SnTaS2 shows several band crossings near the Fermi level, which form three nodal lines in the kz = 0 plane when spin-orbit coupling is not considered. Drumhead-like surface state from the nodal lines are clearly identified. These results indicate that SnTaS2 is a superconducting topological nodal line semimetal.
Flexible Cu2ZnSnS4 (CZTS) solar cell with 3.82% conversion efficiency is prepared by co-electrodeposited method.
In this work, we have developed a modified way of mechanical exfoliation for making two-dimensional materials by introducing a home-designed exfoliation machine. Optical microscopy was employed to identify the thin-layer (mono- and few-layer) flakes primarily. To testify the high efficiency of our modified exfoliation method, we did a simple statistical work on the exfoliation of graphene and WSe2. Further, we used the Raman spectroscopy and the Atomic Force Microscopy (AFM) to characterize the samples. The results indicated the high quality of the as-fabricated samples. Finally, we developed an exfoliation technique for working with easily oxidizing samples. Our modified exfoliation method would be intriguing and innovative for fabricating two dimensional materials, providing a facile way for making electronic and optoelectronic devices.
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