Tunneling across SAMs Containing Oligophenyl Groups

Journal Article · Mon Apr 11 00:00:00 EDT 2016 · Journal of Physical Chemistry. C

DOI:https://doi.org/10.1021/acs.jpcc.6b01253· OSTI ID:1387947

Bowers, Carleen M. ^[1]; Rappoport, Dmitrij ^[1]; Baghbanzadeh, Mostafa ^[1]; Simeone, Felice C. ^[1]; Liao, Kung-Ching ^[1]; Semenov, Sergey N. ^[1]; Żaba, Tomasz ^[2]; Cyganik, Piotr ^[2]; Aspuru-Guzik, Alan ^[1]; Whitesides, George M. ^[1]

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1. Harvard Univ., Cambridge, MA (United States). Dept. of Chemistry and Chemical Biology
2. Jagiellonian Univ., Krakow (Poland)

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, $$J_0$$, 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 $$|J_0|$$ ($$β$$ = 0.28 ± 0.03 Å$$^{–1}$$ and log $$|J_0|$$ = 2.7 ± 0.1 for Au/SPhn; $$β$$ = 0.30 ± 0.02 $$Å^{–1}$$ and log $$|J_0|$$ = 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 SCH2Phn 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 $|J)0|$ = 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.

• Research Organization: Energy Frontier Research Centers (EFRC) (United States). Center for Bio-Inspired Energy Science (CBES)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: SC0000989
• OSTI ID: 1387947
• Journal Information: Journal of Physical Chemistry. C, Vol. 120, Issue 21; Related Information: CBES partners with Northwestern University (lead); Harvard University; New York University; Pennsylvania State University; University of Michigan; University of Pittsburgh; ISSN 1932-7447
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
catalysis (homogeneous)
solar (photovoltaic)
bio-inspired
charge transport
mesostructured materials
materials and chemistry by design
synthesis (novel materials)
synthesis (self-assembly)