Stapleton Joshua J, Daniel Thomas A, Uppili Sundarajan, Cabarcos Orlando M, Naciri Jawad, Shashidhar Ranganathan, Allara David L
Department of Chemistry and The Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA.
Langmuir. 2005 Nov 22;21(24):11061-70. doi: 10.1021/la051094z.
Self-assembled monolayers (SAMs) of the isocyano derivative of 4,4'-di(phenylene-ethynylene)benzene (1), a member of the "OPE" family of "molecular wires" of current interest in molecular electronics, have been prepared on smooth, {111} textured films of Au and Pd. For assembly in oxygen-free environments with freshly deposited metal surfaces, infrared reflection spectroscopy (IRS) indicates the molecules assume a tilted structure with average tilt angles of 18-24 degrees from the surface normal. The combination of IRS, X-ray photoelectron spectroscopy, and density functional theory calculations all support a single sigma-type bond of the -NC group to the Au surface and a sigma/pi-type of bond to the Pd surface. Both SAMs show significant chemical instability when exposed to typical ambient conditions. In the case of the Au SAM, even a few hours storage in air results in significant oxidation of the -NC moieties to -NCO (isocyanate) with an accompanying decrease in surface chemical bonding, as evidenced by a significant increase in instability toward dissolution in solvent. In the case of the Pd SAM, similar air exposure does not result in incorporation of oxygen or loss of solvent resistance but rather results in a chemically altered interface which is attributed to polymerization of the -NC moieties to quasi-2D poly(imine) structures. Conductance probe atomic force microscope measurements show the conductance of the degraded Pd SAMs can diminish by approximately 2 orders of magnitude, an indication that the SAM-Pd electrical contact has severely degraded. These results underscore the importance of careful control of the assembly procedures for aromatic isocyanide SAMs, particularly for applications in molecular electronics where the molecule-electrode junction is critical to the operational characteristics of the device.
4,4'-二（亚苯基乙炔基）苯（1）的异氰酸酯衍生物属于当前分子电子学领域备受关注的“分子导线”的“OPE”家族成员，已在光滑的{111}织构的金和钯薄膜上制备了自组装单分子层（SAMs）。对于在无氧环境中与新沉积的金属表面进行组装，红外反射光谱（IRS）表明分子呈现倾斜结构，平均倾斜角相对于表面法线为18 - 24度。IRS、X射线光电子能谱和密度泛函理论计算的结合均支持 -NC基团与金表面形成单一的σ型键，与钯表面形成σ/π型键。当暴露于典型的环境条件时，两种SAMs都表现出显著的化学不稳定性。对于金SAMs，即使在空气中储存几个小时，-NC部分也会显著氧化为 -NCO（异氰酸酯），同时表面化学键合减少，这可通过在溶剂中溶解的不稳定性显著增加来证明。对于钯SAMs，类似的空气暴露不会导致氧的掺入或溶剂抗性的丧失，而是导致化学性质改变的界面，这归因于 -NC部分聚合成准二维聚（亚胺）结构。电导探针原子力显微镜测量表明，降解后的钯SAMs的电导可降低约2个数量级，这表明SAM - 钯电接触已严重退化。这些结果强调了仔细控制芳香族异氰化物SAMs组装程序的重要性，特别是对于分子电子学中的应用，其中分子 - 电极结对于器件的操作特性至关重要。
