Novel 2D CaCl crystals with metallicity, room-temperature ferromagnetism, heterojunction, piezoelectricity-like property and monovalent calcium ions

Lei Zhang; Guosheng Shi; Bingquan Peng; Pengfei Gao; Liang Chen; Ni Zhong; Liuhua Mu; Lijuan Zhang; Peng Zhang; Lu Gou; Yimin Zhao; Shanshan Liang; Jie Jiang; Zejun Zhang; Hongtao Ren; Xiaoling Lei; Ruobing Yi; Yinwei Qiu; Yufeng Zhang; Xing Liu; Minghong Wu; Long Yan; Chungang Duan; Shengli Zhang; Haiping Fang

doi:10.1093/nsr/nwaa274

Novel 2D CaCl crystals with metallicity, room-temperature ferromagnetism, heterojunction, piezoelectricity-like property and monovalent calcium ions

Natl Sci Rev. 2020 Nov 7;8(7):nwaa274. doi: 10.1093/nsr/nwaa274. eCollection 2021 Jul.

Authors

Lei Zhang¹, Guosheng Shi², Bingquan Peng¹, Pengfei Gao¹, Liang Chen³, Ni Zhong⁴, Liuhua Mu⁵, Lijuan Zhang⁵, Peng Zhang¹, Lu Gou¹, Yimin Zhao¹, Shanshan Liang⁵, Jie Jiang⁵, Zejun Zhang⁵, Hongtao Ren¹, Xiaoling Lei⁶, Ruobing Yi¹, Yinwei Qiu³, Yufeng Zhang⁷, Xing Liu², Minghong Wu², Long Yan⁵, Chungang Duan⁴, Shengli Zhang¹, Haiping Fang⁶

Affiliations

¹ MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, School of Physics, Xi'an Jiaotong University, Xi'an 710049, China.
² Shanghai Applied Radiation Institute, Shanghai University, Shanghai 200444, China.
³ Department of Optical Engineering, Zhejiang A&F University, Hangzhou 311300, China.
⁴ Key Laboratory of Polar Materials and Devices of Ministry of Education, East China Normal University, Shanghai 200241, China.
⁵ Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China.
⁶ Department of Physics, East China University of Science and Technology, Shanghai 200237, China.
⁷ College of Physical Science and Technology, Xiamen University, Xiamen 361005, China.

Abstract

Under ambient conditions, the only known valence state of calcium ions is +2, and the corresponding crystals with calcium ions are insulating and nonferromagnetic. Here, using cryo-electron microscopy, we report direct observation of two-dimensional (2D) CaCl crystals on reduced graphene oxide (rGO) membranes, in which the calcium ions are only monovalent (i.e. +1). Remarkably, metallic rather than insulating properties are displayed by those CaCl crystals. More interestingly, room-temperature ferromagnetism, graphene-CaCl heterojunction, coexistence of piezoelectricity-like property and metallicity, as well as the distinct hydrogen storage and release capability of the CaCl crystals in rGO membranes are experimentally demonstrated. We note that such CaCl crystals are obtained by simply incubating rGO membranes in salt solutions below the saturated concentration, under ambient conditions. Theoretical studies suggest that the formation of those abnormal crystals is attributed to the strong cation-π interactions of the Ca cations with the aromatic rings in the graphene surfaces. The findings highlight the realistic potential applications of such abnormal CaCl material with unusual electronic properties in designing novel transistors and magnetic devices, hydrogen storage, catalyzers, high-performance conducting electrodes and sensors, with a size down to atomic scale.

Keywords: 2D CaCl crystals; coexistence of piezoelectricity-like property and metallicity; heterojunction; monovalent calcium ions; room-temperature ferromagnetism.