Work function and temperature dependence of electron tunneling through an N-type perylene diimide molecular junction with isocyanide surface linkers
Work function and temperature dependence of electron tunneling through an N-type perylene diimide molecular junction with isocyanide surface linkers
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DOI：
10.1039/c7nr06461f
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发表时间：
2018-01-21
期刊：
影响因子：
6.7
通讯作者：
Frisbie, C. Daniel
中科院分区：
文献类型：
--
作者：
Smith, Christopher E.;Xie, Zuoti;Frisbie, C. Daniel
采用传导探针原子力显微镜 (CP-AFM) 来检查自组装单层 (SAM) 结中的电子隧道效应。合成了带有异氰化物连接基团的2.3 nm长苝四甲酸二酰亚胺（PDI）受体分子，将其吸附到Ag、Au和Pt基底上，并通过CP-AFM测量电流-电压（I-V）特性。低偏置电阻 (R) 对接触功函数的依赖性表明传输是 LUMO 辅助的（“n 型行为”）。采用单能级隧道模型结合跃迁电压谱（TVS）来分析实验I-V曲线，并提取有效LUMO位置epsilon（l）= E-LUMO - EF以及PDI氧化还原核与接触之间的有效电子耦合（Gamma）。该分析揭示了所有结点中都存在很强的费米能级 (E-F) 钉扎效应，这可能是由于界面偶极子随着接触功函数的增加而显着增加，正如扫描开尔文探针显微镜 (SKPM) 所揭示的那样。此外，发现 R 的温度 (T) 依赖性很大。对于 Pt/Pt 结，R 在 248 K < T < 338 K 范围内变化超过两个数量级。重要的是，R(T) 数据与单步电子隧道机制一致，并允许独立确定 el，给出的值与基于完整 I-V 数据分析的 el 估计值兼容。理论分析揭示了明确排除两步传输机制的一般标准：即，如果测量的电阻数据表现出明显的阿累尼乌斯型温度依赖性，则在活化能取决于接触冶金的情况下，应排除两步电子转移情况。总的来说，我们的结果表明（1）分子结中费米能级钉扎现象的普遍性，（2）采用单能级隧道模型来确定基本电子结构参数（el和G）的效用，以及（3）改变接触性质以验证传输机制的重要性。
Conducting probe atomic force microscopy (CP-AFM) was employed to examine electron tunneling in self-assembled monolayer (SAM) junctions. A 2.3 nm long perylene tetracarboxylic acid diimide (PDI) acceptor molecule equipped with isocyanide linker groups was synthesized, adsorbed onto Ag, Au and Pt substrates, and the current-voltage (I-V) properties were measured by CP-AFM. The dependence of the low-bias resistance (R) on contact work function indicates that transport is LUMO-assisted ('n-type behavior'). A single-level tunneling model combined with transition voltage spectroscopy (TVS) was employed to analyze the experimental I-V curves and to extract the effective LUMO position epsilon(l) = E-LUMO - EF and the effective electronic coupling (Gamma) between the PDI redox core and the contacts. This analysis revealed a strong Fermi level (E-F) pinning effect in all the junctions, likely due to interface dipoles that significantly increased with increasing contact work function, as revealed by scanning Kelvin probe microscopy (SKPM). Furthermore, the temperature (T) dependence of R was found to be substantial. For Pt/Pt junctions, R varied more than two orders of magnitude in the range 248 K < T < 338 K. Importantly, the R(T) data are consistent with a single step electron tunneling mechanism and allow independent determination of el, giving values compatible with estimates of el based on analysis of the full I-V data. Theoretical analysis revealed a general criterion to unambiguously rule out a two-step transport mechanism: namely, if measured resistance data exhibit a pronounced Arrhenius-type temperature dependence, a two-step electron transfer scenario should be excluded in cases where the activation energy depends on contact metallurgy. Overall, our results indicate (1) the generality of the Fermi level pinning phenomenon in molecular junctions, (2) the utility of employing the single level tunneling model for determining essential electronic structure parameters (el and G), and (3) the importance of changing the nature of the contacts to verify transport mechanisms.