Converting solar energy into electricity or chemical energy with high efficiency and low cost plays an important role in our nation’s long term energy security, economy, and environmental sustainability. A key issue in this endeavor is to find or design new renewable energy materials with stable structure and superb electronic and optoelectronic properties. Two-dimensional (2D) materials have many novel properties suitable for electronic and optoelectronic applications. However, all current 2D materials have shortcomings that limit their performances. For example, graphene, as the first extensively studied 2D material, has excellent carrier mobility, but its semi-metallic behavior and lack of bandgap limit its applications in electronics. Monolayer TMDs such as MoS2, show direct bandgaps and self-healing properties in air, but their less dispersive band edges with relatively localized transition metal d character result in relatively heavy carrier effective masses and thus not so good mobility for high-performance applications. Moreover, bandgaps change from being direct into indirect when TMDs change from mono- to multi-layers, thus limiting the flexibility of tuning its electronic properties through layer controls. Phosphorene and few-layer black phosphorous have shown good mobility and good flexibility of tuning properties by layer control and anisotropic engineering. However, their poor stability leads to rapid degradation when exposed to air. While much effort has been devoted to improve the properties of these current 2D materials, it is still highly desirable to find new 2D materials with exceptional material properties for renewable energy applications.
Prof. Su-Huai Wei at the Beijing Computational Science Research Center, in collaboration with Dr. Ji-Hui Yang at the National Renewable Energy Laboratory and Rice University, USA, and a group led by Prof. X. G. Gong in Fudan University, have predicted, by using atomic transmutation and differential evolution global optimization methods in conjunction with first-principles density functional theory (DFT) calculations, two group IV-VI 2D materials, Pma2-SiS and silicene sulfide with superp material properties. Pma2-SiS is found to be both chemically, energetically, and thermally stable. Most importantly, Pma2-SiS has shown good electronic and optoelectronic properties, including direct bandgaps suitable for solar cells, good mobility for nanoelectronics, low thermal conductivity for thermal electric devices, good flexibility of property tuning by layer control and strain appliance, and good air stability as well. Therefore, Pma2-SiS is expected to be a promising 2D material in the field of 2D electronics and optoelectronics. The designing principles demonstrated in identifying these two tantalizing examples have great potential to accelerate finding of new functional 2D materials.
For more information, please see the paper: “Two-dimensional SiS layers with promising electronic and optoelectronic properties: Theoretical prediction”, J.-H. Yang, Y. Zhang, W.-J. Yin, X. G. Gong, B. I. Yakobson, and S.-H. Wei*, Nano Lett. 16, 1110-1117 (2016), (Pub. 7 Jan. 2016). DOI: 10.1021/acs.nanolett.5b04341.
This research was supported by Development Fund of China Academy of Engineering Physics and NSFC.
Fig. 1. Top and side views of structures for (a) Pmma-SiS, (b) silicene sulfide of Cmmm symmetry, and (c) Pma2-SiS. Blue is for Si and yellow is for S atoms.
Fig. 2. Band structures and (partial) density of states for (a) Pma2-SiS and (b) silicene sulfide.