Department of Chemistry, ‡Department of Chemical Engineering and Materials Science, and §Chemical Theory Center, and Supercomputing Institute, University of Minnesota , Minneapolis, Minnesota 55455, United States.
ACS Nano. 2016 Apr 26;10(4):4372-83. doi: 10.1021/acsnano.5b08126. Epub 2016 Apr 4.
We report the synthesis, transport measurements, and electronic structure of conjugation-broken oligophenyleneimine (CB-OPI 6) molecular wires with lengths of ∼4 nm. The wires were grown from Au surfaces using stepwise aryl imine condensation reactions between 1,4-diaminobenzene and terephthalaldehyde (1,4-benzenedicarbaldehyde). Saturated spacers (conjugation breakers) were introduced into the molecular backbone by replacing the aromatic diamine with trans-1,4-diaminocyclohexane at specific steps during the growth processes. FT-IR and ellipsometry were used to follow the imination reactions on Au surfaces. Surface coverages (∼4 molecules/nm(2)) and electronic structures of the wires were determined by cyclic voltammetry and UV-vis spectroscopy, respectively. The current-voltage (I-V) characteristics of the wires were acquired using conducting probe atomic force microscopy (CP-AFM) in which an Au-coated AFM probe was brought into contact with the wires to form metal-molecule-metal junctions with contact areas of ∼50 nm(2). The low bias resistance increased with the number of saturated spacers, but was not sensitive to the position of the spacer within the wire. Temperature dependent measurements of resistance were consistent with a localized charge (polaron) hopping mechanism in all of the wires. Activation energies were in the range of 0.18-0.26 eV (4.2-6.0 kcal/mol) with the highest belonging to the fully conjugated OPI 6 wire and the lowest to the CB3,5-OPI 6 wire (the wire with two saturated spacers). For the two other wires with a single conjugation breaker, CB3-OPI 6 and CB5-OPI 6, activation energies of 0.20 eV (4.6 kcal/mol) and 0.21 eV (4.8 kcal/mol) were found, respectively. Computational studies using density functional theory confirmed the polaronic nature of charge carriers but predicted that the semiclassical activation energy of hopping should be higher for CB-OPI molecular wires than for the OPI 6 wire. To reconcile the experimental and computational results, we propose that the transport mechanism is thermally assisted polaron tunneling in the case of CB-OPI wires, which is consistent with their increased resistance.
我们报告了具有约 4nm 长度的共轭断裂聚亚苯基亚胺 (CB-OPI6) 分子线的合成、输运测量和电子结构。这些线是通过在 Au 表面上使用间苯二胺和对苯二甲醛（1,4-苯二甲醛）之间的逐步芳基亚胺缩合反应生长而成的。在生长过程中，通过在特定步骤用反式-1,4-二氨基环己烷取代芳族二胺，在分子主链中引入饱和间隔物（共轭断裂物）。FT-IR 和椭圆光度法用于跟踪 Au 表面上的亚胺化反应。通过循环伏安法和紫外可见光谱法分别确定线的表面覆盖率（约 4 个分子/nm2）和电子结构。使用导电探针原子力显微镜（CP-AFM）获取线的电流-电压（I-V）特性，其中 Au 涂覆的 AFM 探针与线接触，形成接触面积约为 50nm2 的金属-分子-金属结。低偏压电阻随饱和间隔物数量的增加而增加，但与间隔物在线中的位置无关。所有线的电阻随温度的测量均符合局域电荷（极化子）跳跃机制。激活能范围为 0.18-0.26eV（4.2-6.0kcal/mol），其中最高值属于完全共轭的 OPI6 线，最低值属于 CB3,5-OPI6 线（两条饱和间隔物的线）。对于另外两条带有单个共轭断裂物的线，CB3-OPI6 和 CB5-OPI6，分别发现其激活能为 0.20eV（4.6kcal/mol）和 0.21eV（4.8kcal/mol）。使用密度泛函理论的计算研究证实了载流子的极化子性质，但预测 CB-OPI 分子线的半经典跳跃激活能应高于 OPI6 线。为了调和实验和计算结果，我们提出对于 CB-OPI 线，输运机制是热辅助极化子隧道，这与它们增加的电阻一致。
