二维硼化物纳米材料的结构和性能预测的第一性原理研究

The boron bulk and their metal borides, as the ultrahigh temperature ceramics and superhard materials, have excellent potentials to be used in the area of aerospace, machinery and electronics. Accompanying with extensively theoretical explorations on 2D boron nanomaterials, the 2D boron is predicted to have extraordinary potentials for the applications to the superconductors, hydrogen-storage materials, Li-ion/ Na-ion battery or supercapacitors, etc. However, the fabrications of 2D boron crystals are extremely difficult due to boron’s active chemical features and the contaminations of carbon sources. Until in the end of 2015, 2D boron icosahedral sheets with just a few atomic thickness have been synthesized on the Cu substrate by using CVD technology, which was reported by Key Laboratory of Intelligent Nano Materials and Devices of Ministry of Education on Nanjing University of Aeronautics and Astronautics (Angew. Chem., 2015, 127: 15693-15697). Later, three types of 2D boron sheets with single atomic layer thickness have been observed on Ag substrate, which were reported by Argonne National Laboratory of USA (Science, 2015, 350: 1513-1516), Institute of Physics in Chinese Academy of Sciences, respectively (Nat. Chem. 2016, 8: 563-569). Our subsequent theoretical studies (Adv. Funct. Mater. 2016 DOI:10.1002/adfm.201603300) found that the bonding features and bandgap of 2D boron sheets can be tuned by the type and numbers of atoms passivated on the surface of 2D boron sheets. These semiconducting boron sheets are stable up to 1000K. In this project, in order to achieve 2D metal boride nanosheets with semiconducting characters and high thermal stability, and to cover their performance dependency on the element components, we will try to modulate the physical properties of the 2D transition-metal borides by adding the transition metals dopants or passivating on the surface via first principles calculations and structural search methods. We wish these explorations could discover new type of 2D nanomaterials and pave the way for their applications on the high-temperature nanodevices in future.

由于硼单质和金属硼化物是重要的高温陶瓷和超硬材料，在航空航天和机械电子等领域有巨大的应用前景。随着对二维硼纳米材料理论研究的深入，人们发现二维硼在超导、储氢和超级电容器等方面有潜在的应用前景。然而二维硼的实验制备和应用研究一直很难突破，直到2015年底，德国应用化学、科学和自然·化学上报道成功制备出少数原子层的半导体性的二维硼薄膜和金属性的二维硼单层，更引发了对二维硼的结构和器件应用的研究热潮。最近申请人在先进功能材料的理论研究发现，氢表面钝化可以实现1000K下稳定的禁带可调的少数原子层二维硼薄膜。本项目拟使用第一性原理和结构搜索方法，通过元素掺杂和表面修饰，研究过渡族金属二维硼化物的物性和高温稳定性，希望实现具有高温稳定性和半导体特性的二维硼化物薄膜，揭示过渡族金属元素与二维硼化物薄膜物性的关系，拓展二维纳米材料在高温电子器件等方向的应用潜力，为二维硼基纳米材料和硼化物纳米材料的器件，应用提供理论基础。

在本项目的资助下，我们研究了二维硼化物薄膜和二维金属硼化物薄膜的物性及结构稳定性。研究结果表明，氢表面钝化和硼氢表面钝化二维金属硼化物薄膜，是热力学比较稳定的二维硼化物薄膜的结构构型。过渡金属元素和稀土元素与硼元素具有较强的成键相互作用，可以导致部分嵌入型和表面修饰型二维金属硼化物薄膜退化为准三维结构，而不能形成热力学稳定的二维金属硼化物薄膜材料。然而，过渡金属表面修饰型二维金属硼化物薄膜具有较好的热力学稳定性和独特的物理性能。Mn和Mo表面修饰型二维金属硼化物薄膜倾向于表现为铁磁特性二维材料，而Cr、Fe表面修饰型二维金属硼化物薄膜倾向于表现为反铁磁特性。而Nb、V以及Ce、Nd、Pr等表面修饰型二维金属硼化物薄膜，是无磁性的半导体材料，禁带宽度随元素掺杂而改变，约为0.18~0.57 eV。同时，我们利用第一性原理方法预测了一种全新结构的铁磁性四方相二维半导体材料(NbSeH2)，不仅可以表现出明显的铁磁性质，更重要的是，它还具有半导体性，带隙为0.94 eV。我们通过电荷掺杂和应变可以对NbSeH2的磁性耦合状态和居里温度进行大幅度的性能调控。这种二维结构可以推广到其他过渡金属元素和硫族元素，进而形成一系列全新结构的四方相“双面神”材料。