Correlation between HOMO alignment and contact resistance in molecular junctions: Aromatic thiols versus aromatic isocyanides
Correlation between HOMO alignment and contact resistance in molecular junctions: Aromatic thiols versus aromatic isocyanides
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DOI：
10.1021/ja0607990
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发表时间：
2006-04-19
影响因子：
15
通讯作者：
Frisbie, CD
中科院分区：
文献类型：
--
作者：
Kim, B;Beebe, JM;Frisbie, CD
了解金属-分子-金属（MMM）结中的电子传递对分子电子学的发展具有重要意义。决定MMM结电导率的关键因素包括金属分子接触的性质和分子主链的电子结构。我们利用导电探针原子力显微镜（CP-AFM）和紫外光电子能谱（UPS）研究了巯基（- S）或异氰酸酯（- CN）连接的刚性、共轭长度增加的寡聚烯烃的电子输运性质和价电子结构。我们发现，对于这些共轭体系，Au - CN接触比Au - S接触具有更大的电阻性。接触电阻的差异与UPS测量结果相关，该测量结果显示异氰化物系列的最高占据分子轨道（HOMO）的能量（相对于Au的费米能级）低于硫醇系列的HOMO，这表明异氰化物连接分子在接触处存在更高的隧道势垒。相比之下，两个系列分子的HOMO位置的差异似乎不会影响结电阻的长度依赖性（即β值= 0.5 Å-1）。
Understanding electron transport in metal−molecule−metal (MMM) junctions is of great importance for the advancement of molecular electronics. Critical factors that determine conductivity in a MMM junction include the nature of metal−molecule contacts and the electronic structure of the molecular backbone. We have studied the electronic transport property and the valence electronic structure on rigid, conjugated oligoacenes of increasing length with either thiol (−S) or isocyanide (−CN) linkers using conducting probe atomic force microscopy (CP-AFM) and ultraviolet photoelectron spectroscopy (UPS). We find that for these conjugated systems the Au−CN contact is more resistive than Au−S. The difference in contact resistance correlates with UPS measurements that show the highest-occupied molecular orbital (HOMO) of the isocyanide series is lower in energy (relative to the Fermi level of Au) than the HOMO of the thiol series, indicating the presence of a higher tunneling barrier at the contact for the isocyanide-linked molecules. By contrast, the difference in the HOMO positions for the two series of molecules does not appear to affect the length dependence of the junction resistance (i.e., the β value = 0.5 Å-1).