Theoretical Study on Electrical Properties of Molecular Junctions of Viologen Derivatives
- 2021年第27卷 
本文基于密度泛函(DFT)结合非平衡格林函数(NEGF)的方法,以具有氧化还原中心的紫罗碱衍生物(N,N′-bis(4-thioalkyl)-4,4′-bipyridinium, HS-4V4-SH)功能分子构造Au(111)/S-4V4-S/Au(111)分子结,详细分析了分子在三种价态V、V+和V2+下的电学性质与分子的几何结构和电子结构的关系。基于对三种价态透射系数分析结果表明,在零偏压下,V与V+的电导值比V2+高了两个数量级,4V4分子结的电导随两个吡啶环之间夹角的增大呈线性减小。同时,理论计算结果也表明,增加烷基链(HS-nVn-SH, n = 2 ~ 7)的数目,发现分子结电导值呈指数形式衰减,其每个亚甲基的衰减因子约为1,与烷基二硫醇分子的接近。In this paper, the electrical properties of molecular junctions formed N,N′-bis(4-thioalkyl)-4,4′-bipyridinium (viologen) moiety between two gold (Au) electrodes have been investigated by combining density functional theory and non-equilibrium Green’s functional approach. To modulate the viologen molecule to be a cation with one and two positive charges (V+ and V2+), we introduce one and two trifluoroacetic acid ions (TFA-) around the molecule, respectively. The valence states of V+ and V2+ are confirmed by checking Mulliken and NBO charges. Then the relationship between molecular conductance and electronic structures of the neutral state V, the radical state V+ and dication V2+ are analyzed in detail. The results in analyzing transmission spectra of the three states reveal that the conductance values of V and V+ are two orders of magnitude larger than that of V2+. This suggests that the redox states of viologen molecules can be used to realize the function of molecular switches. Our calculated results also show that increasing the torsion angle between two pyridine rings of the S-4V4-S molecule will decrease the conductance. By comparing different ions of TFA、PF6 and BF4, the calculated results show that the molecular junction conductance decreases about 3 times when the torsion angle increases by about 6°. It indicates that increasing the torsion angle of the dication V2+ can improve significantly switching ratio of viologen derivatives molecules. At the same time, the calculated results show that increasing the number of methylene groups in alky chains (HS-nVn-SH, n = 2 ~ 7), the conductance values of molecular junctions decrease exponentially, and the attenuation factor of each methylene is about 1 close to alkanedithiol molecules in literatures experimentally and theoretically. This also shows that as the alkyl chain length increases, the DFT-NEGF theoretical method can better predict the zero-bias conductance of the viologen derivative molecule.