Effect of Heteroatom Substitution on Transport in Alkanedithiol-Based Molecular Tunnel Junctions: Evidence for Universal Behavior.
Effect of Heteroatom Substitution on Transport in Alkanedithiol-Based Molecular Tunnel Junctions: Evidence for Universal Behavior.
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DOI：
10.1021/acsnano.6b06623
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发表时间：
2017-01
期刊：
影响因子：
17.1
通讯作者：
Zuoti Xie;I. Bâldea;S. Oram;Christopher E. Smith;C. Frisbie
中科院分区：
文献类型：
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作者：
Zuoti Xie;I. Bâldea;S. Oram;Christopher E. Smith;C. Frisbie
将基于具有三种不同亚甲基链长度的烷二硫醇的分子连接的传输特性与基于类似链的连接（其中每三个-CH2-被O或S取代，即遵循通式HS(CH2CH2X)nCH2CH2SH，其中X = CH2、O或S且n = 1、2或3）的分子连接的传输特性进行比较。进行探针原子力显微镜显示，在以CH2<O的顺序取代时，链的低偏压电阻增加< S。电阻的这种变化归因于观察到的接触电阻 Rc 的相同趋势，而所有链的指数前因子 β（长度敏感性）基本上相同。使用已建立的分析单能级模型，我们根据电流-电压 (I-V) 数据计算了有效能量偏移 εh（即相对于有效 HOMO 能级的费米能级）和电子耦合强度 Г。 εh 值仅受杂原子取代的微弱影响，而界面耦合强度 Γ 的变化超过一个数量级。因此，我们将 Rc 的强烈变化归因于 Γ 的系统变化。量子化学计算表明，HOMO 密度从链烷二硫醇的末端 SH 基团转移到取代链中的杂原子，这为具有富电子杂原子的二硫醇的 Γ 显着降低提供了合理的解释。结果表明，电子耦合以及链烷二硫醇的电阻可以通过分子中间的单个原子的取代来调节。重要的是，当适当归一化时，使用不带可调参数的单级模型在整个偏置范围 (±1.5 V) 上准确模拟实验 I-V 曲线。这些数据可以折叠成模型预测的一条通用曲线，提供明确的证据表明这种分析方法捕获了基本物理原理，并支持其在分子电子学中的实用性。
The transport properties of molecular junctions based on alkanedithiols with three different methylene chain lengths were compared with junctions based on similar chains wherein every third -CH2- was replaced with O or S, that is, following the general formula HS(CH2CH2X)nCH2CH2SH, where X = CH2, O, or S and n = 1, 2, or 3. Conducting probe atomic force microscopy revealed that the low bias resistance of the chains increased upon substitution in the order CH2 < O < S. This change in resistance is ascribed to the observed identical trend in contact resistance, Rc, whereas the exponential prefactor β (length sensitivity) was essentially the same for all chains. Using an established, analytical single-level model, we computed the effective energy offset εh (i.e., Fermi level relative to the effective HOMO level) and the electronic coupling strength Γ from the current-voltage (I-V) data. The εh values were only weakly affected by heteroatom substitution, whereas the interface coupling strength Γ varied by over an order of magnitude. Consequently, we ascribe the strong variation in Rc to the systematic change in Γ. Quantum chemical calculations reveal that the HOMO density shifts from the terminal SH groups for the alkanedithiols to the heteroatoms in the substituted chains, which provides a plausible explanation for the marked decrease in Γ for the dithiols with electron-rich heteroatoms. The results indicate that the electronic coupling and thus the resistance of alkanedithiols can be tuned by substitution of even a single atom in the middle of the molecule. Importantly, when appropriately normalized, the experimental I-V curves were accurately simulated over the full bias range (±1.5 V) using the single-level model with no adjustable parameters. The data could be collapsed to a single universal curve predicted by the model, providing clear evidence that the essential physics is captured by this analytical approach and supporting its utility for molecular electronics.