Hunan Provincial Key Laboratory of Flexible Electronic Materials Genome Engineering, School of Physics and Electronic Science, Changsha University of Science and Technology, Changsha 410114, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11674039, 61701431), the Natural Science Foundation of Hunan Province, China (Grant Nos. 2020JJ4597, 2020JJ4625), the Science Foundationcd of Education Bureau of Hunan Province, China (Grant No. 18B157), and the Postgraduate Scientific Research Innovation Program of Hunan Province, China (Grant No. CX2019703)
Received Date:30 June 2020
Accepted Date:21 August 2020
Available Online:07 December 2020
Published Online:20 December 2020
Abstract:With the miniaturization of molecular devices, high-performance nano devices can be fabricated by controlling the spin states of electrons. Because of their advantages such as low energy consumption, easy integration and long decoherence time, more and more attention has been paid to them. So far, the spin filtration efficiency of molecular device with graphene electrode is not very stable, which will decrease with the increase of voltage, and thus affecting its applications. Therefore, how to enhance the spin filtration efficiency of molecular device with graphene electrode becomes a scientific research problem. Using the first principle calculations based on density functional theory combined with non-equilibrium Green’s function, the physical mechanism of regulating the spin polarization transport properties of single anthracene molecule device with graphene nanoribon as electrode is investigated by molecular oxygen adsorption. In order to explore the effect of the change of the connection mode between single anthracene molecule and zigzag graphene nanoribbon electrode on the spin transport properties of the device, we establish two models. The first model is the model M1, which is the single anthracene molecule longitudinal connection, and the second model is the model M2, which is the single anthracene molecule lateral connection. The adsorption model of single oxygen molecule is denoted by M1O and M2O respectively. The results show that when none of oxygen molecules is adsorbed, the spin filtering effect of single anthracene molecule connecting graphene nanoribbons laterally (M2) is better than that of single anthracene molecule connecting graphene nanoribbons longitudinally (M1). After oxygen molecules are adsorbed on single anthracene molecule, the enhanced localized degree of transport eigenstate will make the spin current of the two kinds of devices decrease by nearly two orders of magnitude. However, molecular oxygen adsorption significantly improves the spin filtering efficiency of the device and enhances the application performance of the device. The maximal spin filtering efficiency of single anthracene molecule connecting graphene nanoribbons longitudinal (M1O) can be increased from 72% to 80%. More importantly, the device with single anthracene molecule connecting graphene nanoribbons laterally (M2) maintains nearly 100% spin filtering efficiency in a bias range from –0.5 V to +0.5 V. These results provide more theoretical guidance for practically fabricating spin molecular devices and regulating their spin transport properties. Keywords:graphene nanoribbons/ molecular adsorption/ spin transport/ spin filtering
${T_\sigma }\left( {E,{V_{\rm{b}}}} \right) = {\rm{Tr}}\left[ {{\varGamma _{\rm{L}}}\left( E \right){G^{\rm{R}}}\left( E \right){\varGamma _{\rm{R}}}\left( E \right){G^{\rm{A}}}\left( E \right)} \right], $
其中${G^{\rm{R}}}\left( E \right)$和${G^{\rm{A}}}\left( E \right)$分别为散射延迟和超前格林函数; ${\varGamma _{{\rm{L, R}}}} = {\rm i}\left( {\sum\nolimits_{{\rm{L, R}}}^{\rm{R}} {\left( E \right) - \sum\nolimits_{{\rm{L, R}}}^{\rm{A}} {\left( E \right)} } } \right)$为展宽函数, $\sum\nolimits_{{\rm{L, R}}}^{\rm{R}} {\left( E \right)}$和$\sum\nolimits_{{\rm{L, R}}}^{\rm{A}} {\left( E \right)}$是左右电极对散射区的自能. 本文中器件的几何结构优化、电子结构和电流-电压特性的计算全部都是由基于非平衡格林函数和密度泛函理论相结合的第一性原理计算方法(ATK软件) 所完成[39,40].
3.结果与讨论为了研究氧气分子吸附对单蒽分子纵向连接器件自旋输运性质的影响, 图2分别给出了器件M1和M1O的自旋输运谱和费米能级处的自旋输运本征态. 众所周知, 器件在低偏压下的电子输运能力主要取决于费米能级附近的输运峰分布以及输运峰的透射系数. 如图2(a)所示, 器件M1在费米能级两侧分布着透射系数很大的输运透射峰. 然而, 在费米能级处的α和β自旋的输运透射系数都很小. α自旋输运透射系数会略大于β自旋输运透射系数. 这个可以由图2(b)费米能级处的自旋输运本征态进行解释. 从图2(b)可以清楚地看到, 两种自旋的输运本征态都主要局域在左半部分的石墨烯电极和单蒽分子的中间区域. 右半部分的石墨烯电极几乎没有输运本征态分布. 因此, 两种自旋态的输运透射系数都很小. 但是通过细致地对比, 可以发现α自旋输运本征态分布在单蒽分子的中间区域并延伸到了右电极, 相比β自旋输运本征态具有更好的空间扩展性. 因此, α自旋输运透射系数会略大于β自旋输运透射系数. 当氧气分子吸附在单蒽分子的中间苯环上, 器件M1O在费米能级两侧仅有两个透射系数很小的输运峰分布, 如图2(c)所示. 更重要的是, 费米能级处的α自旋和β自旋的输运透射系数相比于器件M1大幅度降低. 也就是说, 氧气分子吸附有效地抑制了器件M1两种自旋态的输运能力. 图2(d)明确地显示氧气分子吸附导致费米能级处的两种自旋输运本征态被阻断在氧气分子处, 导致对应的输运透射系数减小. 图 2 (a)和(c)分别表示M1和M1O的零偏压自旋输运谱; (b)和(d)分别表示M1和M1O费米能级处的自旋输运本征态 Figure2. (a), (c) The zero-bias spin-resolved transmission spectra of M1 and M1O; (b), (d) the transmission eigenstate of M1 and M1O on Fermi energy.
为了研究氧气分子吸附对单蒽分子横向连接器件自旋输运性质的影响, 图3分别给出了器件M2和M2O的零偏压自旋输运谱和费米能级处的自旋输运本征态. 如图3(a)所示, 器件M2在费米能级及其两侧分布着透射系数很大且连续的α自旋输运谱. 然而, 在费米能级及其两侧的β自旋输运谱的透射系数几乎为零, 仅在0.3 eV能量位置分布了一个透射系数很小的透射峰. 这个现象可以由图3(b)费米能级处的自旋输运本征态进行解释. 从图3(b)可以清楚地看到, α自旋输运本征态分布在整个器件上且空间扩展性非常优秀, 而β自旋输运本征态仅仅局域在器件的左边石墨烯电极上. 这说明β自旋态的输运能力在器件M2中被明显抑制. 因此, α自旋和β自旋之间巨大的传输能力差异导致M2可以表现明显的自旋过滤行为. 当氧气分子吸附在单蒽分子的中间苯环上, 器件M2O在费米能级及其两侧的β自旋输运谱的透射系数仍然很小, 但是费米能级及其两侧的α自旋输运透射系数相比于器件M2大幅度降低, 如图3(c)所示. 也就是说, 氧气分子吸附仍然起到了抑制器件M2自旋输运能力的效果. 此外, 图3(c)在0.8 eV能量位置出现了一组由氧气分子吸附导致的α和β自旋对称输运峰. 尽管两个输运峰具有很大的透射系数, 但是由于其距离费米能级的位置较远, 因此对器件低偏压下的自旋输运性质没有帮助. 图3(d)给出了器件M2O在费米能级处的自旋输运本征态. 如图3(d)所示, 氧气分子吸附将α自旋输运本征态局域在器件的左侧, 右电极的贡献几乎为零. 因此, 氧气分子吸附强烈地抑制了α自旋输运. 对于β自旋态来说, 氧气分子吸附导致单蒽分子的中间局域地分布了很小区域的输运本征态. 但是与石墨烯电极相连的两个苯环仍然无输运本征态分布. 因此, 器件M2O在费米能级处的β自旋态输运谱系数仍然非常小. 图 3 (a)和(c)分别表示M2和M2O的零偏压自旋输运谱; (b)和(d)分别表示M2和M2O费米能级处的自旋输运本征态 Figure3. (a), (c) The zero-bias spin-resolved transmission spectra of M2 and M2O; (b), (d) the transmission eigenstate of M2 and M2O on Fermi energy.