Experimental and Theoretical Analysis of Nanotransport in Oligophenylene Dithiol Junctions as a Function of Molecular Length and Contact Work Function.
Experimental and Theoretical Analysis of Nanotransport in Oligophenylene Dithiol Junctions as a Function of Molecular Length and Contact Work Function.
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DOI：
10.1021/acsnano.5b01629
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发表时间：
2015-07
期刊：
影响因子：
17.1
通讯作者：
Zuoti Xie;I. Bâldea;Christopher E. Smith;Yanfei Wu;C. Frisbie
中科院分区：
文献类型：
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作者：
Zuoti Xie;I. Bâldea;Christopher E. Smith;Yanfei Wu;C. Frisbie
我们报道了几种类型的电极(M1-S-(C6H4)n-S-M2，1≤n≤4，M1，2=Ag，Au，Pt)上基于低聚苯二硫醇(OPD)的金属-分子-金属隧道结的广泛研究结果，以考察分子长度(N)和金属功函数(Φ)对结性质的影响。我们的研究包括：(1)用扫描开尔文探针显微镜测量ΔΦ=Φ自组装单分子膜(SAM)化学吸附引起的电极功函数变化(OPDSAM-Φ)；(2)用导电探针原子力显微镜测量线性和非线性偏置范围内的结电流-电压(I-V)特性；(3)直接定量分析全I-V曲线。此外，我们还利用跃迁电压谱(TVS)估算了优势分子轨道(HOMO)相对于结费米能EF的能量取向εh=Ef-EHOMO。在有光电子能谱数据的情况下，εh值与TVS测定的值符合得很好。使用单能级模型，我们在后密度泛函理论水平上通过从头算量子化学计算和额外的紫外可见吸收测量，我们能够定量地再现所研究的整个偏置范围(∼1.0V-1.5V)的I-V测量，并理解从实验中提取的εh和Γ(接触耦合强度)的行为。我们发现，由金属-S键的强偶极引起的费米能级钉扎导致吸附分子的HOMO能量发生显著的移动，导致εh与功函数Φ表现出弱的依赖关系。这两个参数在确定隧道衰减系数(β)和结电阻(R)方面起着关键作用。Φ，ΔΦ、R、跃迁电压(Vt)和εh之间的相关性和精确的模拟提供了这些典型分子结中隧道输运的非常完整的图景。
We report the results of an extensive investigation of metal-molecule-metal tunnel junctions based on oligophenylene dithiols (OPDs) bound to several types of electrodes (M1-S-(C6H4)n-S-M2, with 1 ≤ n ≤ 4 and M1,2 = Ag, Au, Pt) to examine the impact of molecular length (n) and metal work function (Φ) on junction properties. Our investigation includes (1) measurements by scanning Kelvin probe microscopy of electrode work function changes (ΔΦ = ΦSAM - Φ) caused by chemisorption of OPD self-assembled monolayers (SAMs), (2) measurements of junction current-voltage (I-V) characteristics by conducting probe atomic force microscopy in the linear and nonlinear bias ranges, and (3) direct quantitative analysis of the full I-V curves. Further, we employ transition voltage spectroscopy (TVS) to estimate the energetic alignment εh = EF - EHOMO of the dominant molecular orbital (HOMO) relative to the Fermi energy EF of the junction. Where photoelectron spectroscopy data are available, the εh values agree very well with those determined by TVS. Using a single-level model, which we justify via ab initio quantum chemical calculations at post-density functional theory level and additional UV-visible absorption measurements, we are able to quantitatively reproduce the I-V measurements in the whole bias range investigated (∼1.0-1.5 V) and to understand the behavior of εh and Γ (contact coupling strength) extracted from experiment. We find that Fermi level pinning induced by the strong dipole of the metal-S bond causes a significant shift of the HOMO energy of an adsorbed molecule, resulting in εh exhibiting a weak dependence with the work function Φ. Both of these parameters play a key role in determining the tunneling attenuation factor (β) and junction resistance (R). Correlation among Φ, ΔΦ, R, transition voltage (Vt), and εh and accurate simulation provide a remarkably complete picture of tunneling transport in these prototypical molecular junctions.