三角形石墨烯量子点阵列的磁电子学特性和磁输运性质

Magneto-electronic and magnetic transport properties of triangular graphene quantum-dot arrays

基于密度泛函理论的第一性原理计算方法,研究了三角形石墨烯纳米片用不同连接方式拼接而成的四种一维量子点阵列(1D QDAs)的磁电子学性质和磁输运性质.结合能计算表明所有1D QDAs是非常稳定的.特别是研究发现1D QDAs的电子和磁性质不仅依赖于磁性态,也明显依赖于连接方式,如在无磁态时,不同量子点阵列(QDAs)可为金属或窄带隙半导体.在铁磁态时,不同QDAs能为半金属(half-metal)或带隙不同的双极化磁性半导体.而在反铁磁态时,不同QDAs为带隙不等的半导体.这些结果意味着连接方式对有效调控纳米结构电子和磁性质扮演重要的角色.1D QDAs呈现的半金属或双极化磁性半导体性质对于发展磁器件是非常重要的,而这些性质未曾在本征石墨烯纳米带中出现.同时,我们也研究了一种阵列的磁器件特性,发现其拥有完美的(100%)单或双自旋过滤效应,尤其是呈现超过109%的巨磁阻效应.

Graphene （GN）, a monolayer two-dimensional （2D） system closely arranged into a benzene ring structure by C atoms, has so far aroused considerable research interest due to its novel electronic, magnetic, mechanical and thermal properties. But 2D GN is a semimetal with zero band gap, and the lowest conduction band touches the highest valence band at Fermi level, leading to the inability to achieve the ＂off＂ effect in the electronic device. Therefore, many researchers are searching the solutions. A simple and feasible method is to convert 2D GN into quasi-one-dimensional （1D） graphene nanoribbons, quantum-dot arrays （QDAs） and zero-dimensional （0D） quantum-dot by tailoring it along a specific single crystallographic direction. The QDAs, due to their structural diversity, have great potential applications in future nano-integrated circuit. In this work, first-principles method based on density functional theory is used to study the magneto-electronic and magnetic transport properties of four 1D quantum-dot arrays （1D QDAs） consisting of triangular graphene nanoflakes with different linking modes. The calculated binding energy suggests that these structures are very stable, and the arrays that are linked by the ＂bottom-side＂ are more stable than that only by the ＂vertex＂. In particular, it is found that the electronic and magnetic features are not only related to the different magnetic states, but also depend on linking modes. For example, in the non-magnetism state, different QDAs can be a metal or a narrowed band-gap semiconductor. In the ferromagnetic state, different QDAs can be half-metal materials or bipolar magnetic semiconductors with different gaps, and have greatly different magnetic moments from 1.985 to 7.994#B/unit cell, reaching a difference almost as large as four times. While in the antiferromagnetie state, all QDAs are semiconductors but with different gaps. These results imply that the linking modes play a crucial role in effectively tuning the electronic and magnetic features for nanostructures. The calculated atom-projected density of states indicates that the highest valence band and the lowest conduction band are determined by the edge C atoms. The half-metallic and bipolar magnetic semiconducting behaviors presented by 1D QDA are extremely important for developing magnetic devices, which is not found in the intrinsic graphene nanoribbons. And, we also investigate the magnetic device properties based on one kind of QDA, and the single or dual spin-filtering effect with the perfect （100%） spin polarization and a rectification ratio of about 104 can be predicted. Particularly, a giant magnetoresistanee over 109% is found unambiguously, which is two orders of magnitude higher than the value predicted based on the zigzag graphene nanoribbons and five orders of magnitude higher than previously reported experimental values for the MgO tunnel junction. Our results thus provide strong evidence for the effectiveness of QDAs on the magneto-electronic properties.