Using first-principles calculations, we propose a new two-dimensional Ge2S (space group P21212) with unique mechanical and electronic properties. Monolayer Ge2S has excellent thermal, mechanical, and dynamic stabilities, exhibiting a semiconducting behavior with an indirect bandgap and anisotropic carrier mobility. The uniaxial strain along the zigzag direction can induce an indirect-to-direct bandgap transition. Remarkably, Ge2S possesses large in-plane negative Poisson's ratios, comparable with that of well-known penta-graphene. Moreover, we identify Ge2S as a high-performance anode material for metal-ion batteries. It shows metallic features after adsorbing Na, K, and Mg, providing good electrical conductivity during the charge/discharge process. The diffusion of metal ions on Ge2S is anisotropic with modest energy barriers in the armchair direction of 0.12, 0.39, and 0.76 eV for Na, K, and Mg, respectively. Ge2S can adsorb metal atoms up to a stoichiometric ratio of 1:1, which yields storage capacities of 151.17, 151.17, and 302.35 mA h g−1 for Na, K, and Mg, respectively. The volume of Ge2S shrinks slightly upon the adsorption of metal ions even at high concentrations, ensuring a good cyclic stability. Besides, the average open circuit voltage (0.30–0.70 V) falls within the acceptable range (0.1–1.0 V) of the anode materials. These results make Ge2S a promising anode material for the design of future metal-ion batteries.
Two-dimensional (2D) materials provide tremendous opportunities for next-generation energy storage technologies. We theoretically propose 2D group-IV oxides (α-, β-, and γ-CXO, X = Si/Ge). Among them, α-CXO monolayers, composed of the C-O-X skeleton of silyl (germyl) methyl ether molecules, are the most stable phase. α-CXO possess robust dynamical, mechanical, and thermal stabilities. Remarkably, α-CGeO has an unusual negative Poisson’s ratio (NPR). However, α-CSiO displays a bidirectional half-auxeticity, different from all the already known NPR behaviors. The intrinsic moderate direct-band-gap, high carrier mobility, and superior optical absorption of α-CXO make them attractive for optoelectronics applications. A series of α-CXO-based excitonic solar cells can achieve high power conversion efficiencies. Besides, α-CXO monolayers are promising anode materials for sodium- and potassium-ion batteries, exhibiting not only the high specific capacity (532−1433 mA h g−1) but also low diffusion barrier and open-circuit voltage. In particular, the specific capacity of K on α-CSiO exhibits one of the highest values ever recorded in 2D materials. The multifunctionality renders α-CXO promising candidates for nanomechanics, nanoelectronics, and nano-optics.
Two-dimensional (2D) materials have aroused tremendous interest due to their great potential in the applications in electronic, optical, and mechanical devices. We theoretically design a new 2D material SiGeS by regularly arranging the Si-S-Ge skeleton of SiH<sub>3</sub>SGeH<sub>3</sub>. Based on first-principles calculation, the structure, stability, electronic properties, mechanical properties, and optical properties of SiGeS are systematically investigated. Monolayer SiGeS is found to be energetically, dynamically, and thermally stable. Remarkably, SiGeS displays a unique negative Poisson's ratio. Besides, SiGeS is an indirect-semiconductor with a band gap of 1.95 eV. The band gap could be modulated effectively by applying external strains. An indirect-to-direct band gap transition could be observed when the tensile strain along x axial or biaxial is greater than +3%, which is highly desirable for applications in optical and semiconductor technology. Moreover, pristine SiGeS has a high absorption coefficient (~10<sup>5</sup> cm<sup>-1</sup>) from visible to the ultraviolet region. Under tensile strain along x, the absorption edge of SiGeS has a red shift, which makes it cover the whole region of solar spectrum. These intriguing properties render SiGeS a competitive multifunctional material for nanomechanic and optoelectronic applications.
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