Department of Chemistry, National Taiwan University, Taipei, Taiwan 10617.
J Am Chem Soc. 2010 Jan 20;132(2):756-64. doi: 10.1021/ja9084012.
One of the critical issues for the realization of molecular electronics is the development of ideal molecule-electrode contacts that render efficient charge transportation and thus attenuate the unwanted voltage drop and power loss. The conductance at the single-molecule level has long been expected to be correlated strongly with the electrode materials. However, other than gold, systematic studies of a homologous series of molecules to extract the headgroup-metal contact conductance (G(n=0)) have not been reported. Carefully examined herein are the conductances of alkanedithiols anchored onto electrode materials of Au and Pt as well as the conductances of alkanediisothiocyanates on Au, Pd, and Pt by utilizing the method of STM-BJ (scanning tunneling microscopy break junction). In comparison with Au substrate, Pd and Pt are group 10 elements with stronger d-orbital characteristics, and larger local density of states near the Fermi level. The model compounds, SCN(CH(2))(n)NCS (n = 4, 6, and 8), are studied because the isothiocyanate (-NCS) headgroup is a versatile ligand for organometallics, an emerging class of molecular wires, and can bind to substrates of noble metals to complete a metal-molecule-metal configuration for external I-V measurements. Also studied include alkanedithiols, one of the most scrutinized systems in the field of single-molecule conductance. The results show that the conductance for single molecules bridged between a pair of Pt electrodes is about 3.5-fold superior to those between Au electrodes. On all electrode materials, observed are two sets of conductance values, with the smaller set being 1 order of magnitude less conductive. These findings are ascribed to the degree of electronic coupling between the headgroup and the electrode.
实现分子电子学的一个关键问题是开发理想的分子-电极接触，以实现有效的电荷输运，从而减弱不必要的电压降和功率损耗。长期以来，人们一直期望单分子水平的电导与电极材料密切相关。然而，除了金之外，对于同系物分子的系统研究，以提取头基-金属接触电导（G(n=0)）尚未有报道。本文通过利用 STM-BJ（扫描隧道显微镜断键）方法，仔细研究了烷二硫醇在 Au 和 Pt 电极材料上的电导以及烷二异硫氰酸酯在 Au、Pd 和 Pt 上的电导。与 Au 衬底相比，Pd 和 Pt 是第 10 族元素，具有更强的 d 轨道特性和费米能级附近更大的局域态密度。选择模型化合物 SCN(CH(2))(n)NCS（n = 4、6 和 8）进行研究，因为异硫氰酸酯（-NCS）头基是有机金属、新兴的分子导线类别的多功能配体，并且可以与贵金属衬底结合，完成用于外部 I-V 测量的金属-分子-金属构型。同时还研究了烷二硫醇，这是单分子电导领域中最受关注的体系之一。结果表明，在一对 Pt 电极之间桥接的单分子的电导大约比在 Au 电极之间桥接的单分子的电导高 3.5 倍。在所有电极材料上，观察到两组电导值，较小的一组的电导低 1 个数量级。这些发现归因于头基和电极之间的电子耦合程度。
