Souza Amaury de Melo, Rungger Ivan, Pontes Renato Borges, Rocha Alexandre Reily, da Silva Antônio José Roque, Schwingenschlöegl Udo, Sanvito Stefano
School of Physics, AMBER and CRANN, Trinity College Dublin, College Green D2, Ireland.
Nanoscale. 2014 Nov 6;6(23):14495-507. doi: 10.1039/c4nr04081c.
It is often assumed that the hydrogen atoms in the thiol groups of a benzene-1,4-dithiol dissociate when Au-benzene-1,4-dithiol-Au junctions are formed. We demonstrate, by stability and transport property calculations, that this assumption cannot be made. We show that the dissociative adsorption of methanethiol and benzene-1,4-dithiol molecules on a flat Au(111) surface is energetically unfavorable and that the activation barrier for this reaction is as high as 1 eV. For the molecule in the junction, our results show, for all electrode geometries studied, that the thiol junctions are energetically more stable than their thiolate counterparts. Due to the fact that density functional theory (DFT) within the local density approximation (LDA) underestimates the energy difference between the lowest unoccupied molecular orbital and the highest occupied molecular orbital by several electron-volts, and that it does not capture the renormalization of the energy levels due to the image charge effect, the conductance of the Au-benzene-1,4-dithiol-Au junctions is overestimated. After taking into account corrections due to image charge effects by means of constrained-DFT calculations and electrostatic classical models, we apply a scissor operator to correct the DFT energy level positions, and calculate the transport properties of the thiol and thiolate molecular junctions as a function of the electrode separation. For the thiol junctions, we show that the conductance decreases as the electrode separation increases, whereas the opposite trend is found for the thiolate junctions. Both behaviors have been observed in experiments, therefore pointing to the possible coexistence of both thiol and thiolate junctions. Moreover, the corrected conductance values, for both thiol and thiolate, are up to two orders of magnitude smaller than those calculated with DFT-LDA. This brings the theoretical results in quantitatively good agreement with experimental data.
人们通常认为，在形成金-苯-1,4-二硫醇-金结时，苯-1,4-二硫醇硫醇基团中的氢原子会解离。我们通过稳定性和输运性质计算表明，这种假设是不成立的。我们发现，甲硫醇和苯-1,4-二硫醇分子在平坦的金(111)表面上的解离吸附在能量上是不利的，并且该反应的活化能垒高达1电子伏特。对于结中的分子，我们的结果表明，对于所研究的所有电极几何结构，硫醇结在能量上比相应的硫醇盐结更稳定。由于局域密度近似(LDA)中的密度泛函理论(DFT)将最低未占据分子轨道和最高占据分子轨道之间的能量差低估了几个电子伏特，并且没有捕捉到由于镜像电荷效应导致的能级重整化，因此金-苯-1,4-二硫醇-金结的电导率被高估了。在通过约束DFT计算和静电经典模型考虑了镜像电荷效应的校正后，我们应用剪刀算符校正DFT能级位置，并计算硫醇和硫醇盐分子结的输运性质作为电极间距的函数。对于硫醇结，我们表明电导率随着电极间距的增加而降低，而硫醇盐结则呈现相反的趋势。这两种行为在实验中都已被观察到，因此表明硫醇结和硫醇盐结可能共存。此外，校正后的硫醇和硫醇盐的电导率值比用DFT-LDA计算的值小多达两个数量级。这使得理论结果在定量上与实验数据吻合良好。
