Department of Chemistry, National University of Singapore , 3 Science Drive 3, Singapore 117543.
Department of Chemical Engineering and Materials Science, University of Minnesota , 421 Washington Avenue SE, Minneapolis, Minnesota 55455, United States.
J Am Chem Soc. 2016 Jun 15;138(23):7305-14. doi: 10.1021/jacs.6b02039. Epub 2016 Jun 3.
We have utilized DC and AC transport measurements to measure the resistance and capacitance of thin films of conjugated oligophenyleneimine (OPI) molecules ranging from 1.5 to 7.5 nm in length. These films were synthesized on Au surfaces utilizing the imine condensation chemistry between terephthalaldehyde and 1,4-benzenediamine. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy yielded molecular tilt angles of 33-43°. To probe DC and AC transport, we employed Au-S-OPI//GaOx/EGaIn junctions having contact areas of 9.6 × 10(2) μm(2) (10(9) nm(2)) and compared to previously reported DC results on the same OPI system obtained using Au-S-OPI//Au conducting probe atomic force microscopy (CP-AFM) junctions with 50 nm(2) areas. We found that intensive observables agreed very well across the two junction platforms. Specifically, the EGaIn-based junctions showed: (i) a crossover from tunneling to hopping transport at molecular lengths near 4 nm; (ii) activated transport for wires >4 nm in length with an activation energy of 0.245 ± 0.008 eV for OPI-7; (iii) exponential dependence of conductance with molecular length with a decay constant β = 2.84 ± 0.18 nm(-1) (DC) and 2.92 ± 0.13 nm(-1) (AC) in the tunneling regime, and an apparent β = 1.01 ± 0.08 nm(-1) (DC) and 0.99 ± 0.11 nm(-1) (AC) in the hopping regime; (iv) previously unreported dielectric constant of 4.3 ± 0.2 along the OPI wires. However, the absolute resistances of Au-S-OPI//GaOx/EGaIn junctions were approximately 100 times higher than the corresponding CP-AFM junctions due to differences in metal-molecule contact resistances between the two platforms.
我们利用直流和交流输运测量技术，测量了长度在 1.5nm 到 7.5nm 之间的共轭聚对苯撑亚胺（OPI）分子薄膜的电阻和电容。这些薄膜是在 Au 表面上通过对苯二甲醛和 1,4-苯二胺之间的亚胺缩合反应合成的。近边 X 射线吸收精细结构（NEXAFS）光谱得出分子倾斜角为 33-43°。为了探测直流和交流输运，我们采用了 Au-S-OPI//GaOx/EGaIn 结，其接触面积为 9.6×10^2μm^2（10^9nm^2），并与之前使用接触面积为 50nm^2 的 Au-S-OPI//Au 传导探针原子力显微镜（CP-AFM）结在相同的 OPI 系统上获得的直流结果进行了比较。我们发现，这两种结平台上的强观测结果非常吻合。具体来说，基于 EGaIn 的结表现出：（i）在分子长度接近 4nm 时，从隧道传输到跳跃传输的转变；（ii）长度大于 4nm 的电线的激活输运，OPI-7 的激活能为 0.245±0.008eV；（iii）电导与分子长度的指数依赖性，在隧道区的衰减常数β=2.84±0.18nm^-1（直流）和 2.92±0.13nm^-1（交流），在跳跃区的β=1.01±0.08nm^-1（直流）和 0.99±0.11nm^-1（交流）；（iv）以前未报道的 OPI 导线上的介电常数为 4.3±0.2。然而，由于两种平台之间金属-分子接触电阻的差异，Au-S-OPI//GaOx/EGaIn 结的绝对电阻大约是相应 CP-AFM 结的 100 倍。
