Tunneling across SAMs Containing Oligophenyl Groups

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• 作者：Carleen M. Bowers ; Dmitrij Rappoport ; Mostafa Baghbanzadeh ; Felice C. Simeone ; Kung-Ching Liao ; Sergey N. Semenov ; Tomasz Żaba ; Piotr Cyganik ; Alan Aspuru-Guzik ; George M. Whitesides
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：June 2, 2016
• 年：2016
• 卷：120
• 期：21
• 页码：11331-11337
• 全文大小：454K
• 年卷期：0
• ISSN：1932-7455

文摘

This paper reports rates of charge tunneling across self-assembled monolayers (SAMs) of compounds containing oligophenyl groups, supported on gold and silver, using Ga₂O₃/EGaIn as the top electrode. It compares the attenuation constant, β, and the pre-exponential parameter, J₀, of the simplified Simmons equation across oligophenyl groups (R = Ph_n; n = 1, 2, 3) with three different anchoring groups (thiol, HSR; methanethiol, HSCH₂R; and acetylene, HC≡CR) that attach R to the template-stripped gold or silver substrate. The results demonstrate that the structure of the molecular linker between the anchoring group (−S– or −C≡C−) and the oligophenyl moiety significantly influences the rate of charge transport. SAMs of SPh_n and C≡CPh_n on gold show similar values of β and log |J₀| (β = 0.28 ± 0.03 Å^–1 and log |J₀| = 2.7 ± 0.1 for Au/SPh_n; β = 0.30 ± 0.02 Å^–1 and log |J₀| = 3.0 ± 0.1 for Au/C≡CPh_n). The introduction of a single intervening methylene (CH₂) group between the anchoring sulfur atom and the aromatic units generates SAMs of SCH₂Ph_n and increases β to ca. 0.66 ± 0.06 Å^–1 on both gold and silver substrates. (For n-alkanethiolates on gold, the corresponding values are β = 0.76 ± 0.03 Å^–1 and log |J₀| = 4.2 ± 0.2). Density functional theory calculations indicate that the highest occupied molecular orbitals (HOMOs) of both SPh_n and C≡CPh_n extend beyond the anchoring group and onto the phenyl rings; SAMs composed of these two groups of molecules result in indistinguishable rates of charge transport. The introduction of the CH₂ group, to generate SCH₂Ph_n, disrupts the delocalization of the orbitals, localizes the HOMO on the anchoring sulfur atom, and results in the experimentally observed increase in β to a value closer to that of a SAM of n-alkylthiolate molecules.