The bias and temperature dependence of both dark and photo-induced currents in carbon based molecular junctions were examined over a wide range of oligomeric layer thickness (d) from 4 to 60 nm. The dark current density vs bias (JV) response of nitroazobenzene molecular junctions exhibits the exponential thickness dependence consistent with coherent tunneling when d<5 nm, but becomes weakly dependent on d and temperature (T) for d=15-60 nm. The photocurrent (PC) response is orders of magnitude higher than the dark current for the same d and bias, with very different curve shape and much earlier onset with bias. Although the dark and PC different greatly in magnitude for d>14 nm, they both exhibit near zero attenuation coefficients (β <0.05 nm-1), and are activationless (Eact < 5 meV) below ~200 K. For d>14 nm both dark and PC become electric field (E) dependent, exhibiting approximate overlap of J vs E response for d=14-60 nm. lnJ vs E1/2 is linear for both PC and dark current, with very different magnitudes and slopes. We propose orbital mediated transport for PC, involving sequential tunneling of photogenerated electrons and holes between frontier orbitals of adjacent, weakly interacting oligomeric subunits. Such transport is "bulk-limited", E dependent, and nearly activationless due to small tunneling barriers and short distances between adjacent molecular orbitals. In contrast, the dark current is activated and injection limited due to a much larger interfacial energy barrier than bulk transport in the junction interior. Rapid, low-barrier transport between orbitals in adjacent molecules should significantly extend the "range" of molecular electronics to >50 nm, and avoid the usually strong temperature dependence observed in thicker organic films.

中文翻译：

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碳基分子结中超过 60 nm 的光激励近共振电荷传输

在从 4 到 60 nm 的广泛低聚物层厚度 (d) 上检查了碳基分子结中暗电流和光感应电流的偏置和温度依赖性。当 d < 5 nm 时，硝基偶氮苯分子结的暗电流密度与偏置 (JV) 响应表现出与相干隧穿一致的指数厚度依赖性，但在 d = 15-60 nm 时变得微弱地依赖于 d 和温度 (T)。对于相同的 d 和偏置，光电流 (PC) 响应比暗电流高几个数量级，具有非常不同的曲线形状和更早的偏置开始。尽管 d>14 nm 时暗和 PC 的量级差异很大，但它们都表现出接近零的衰减系数 (β <0.05 nm-1)，并且在 ~200 K 以下是无激活的 (Eact < 5 meV)。对于 d> 14 nm 暗和 PC 变得依赖于电场 (E)，在 d=14-60 nm 时表现出 J 与 E 响应的近似重叠。lnJ vs E1/2 对于 PC 和暗电流都是线性的，具有非常不同的幅度和斜率。我们提出了用于 PC 的轨道介导传输，包括光生电子和空穴在相邻、弱相互作用的寡聚亚基的前沿轨道之间的顺序隧穿。由于小的隧道势垒和相邻分子轨道之间的短距离，这种传输是“体积受限的”，依赖于 E 并且几乎没有激活。相比之下，由于比结内部的体传输大得多的界面能垒，暗电流被激活并且注入受到限制。相邻分子轨道之间的快速、低势垒传输应该显着延长“