Negative Differential Resistance by Molecular Resonant Tunneling between Neutral Tribenzosubporphine Anchored to a Au(111) Surface and Tribenzosubporphine Cation Adsorbed on to a Tungsten Tip

Tribenzosubporphyrins are boron(III)-chelated triangular bowl-shaped ring-contracted porphyrins that possess a 14π-aromatic circuit. Their flat molecular shapes and discrete molecular orbital diagrams make them ideal for observation by scanning tunneling microscopy (STM). Expanding their application...

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Published in:Journal of the American Chemical Society 2013-09, Vol.135 (38), p.14159-14166
Main Authors: Majima, Yutaka, Ogawa, Daisuke, Iwamoto, Masachika, Azuma, Yasuo, Tsurumaki, Eiji, Osuka, Atsuhiro
Format: Article
Language:English
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Summary:Tribenzosubporphyrins are boron(III)-chelated triangular bowl-shaped ring-contracted porphyrins that possess a 14π-aromatic circuit. Their flat molecular shapes and discrete molecular orbital diagrams make them ideal for observation by scanning tunneling microscopy (STM). Expanding their applications toward single molecule-based devices requires a fundamental knowledge of single molecular conductance between tribenzosubporphines and the STM metal tip. We utilized a tungsten (W) STM tip to investigate the electronic properties of B-(5-mercaptopentoxy)tribenzosubporphine 1 at the single molecular level. B-(5-mercaptopentoxy)-tribenzosubporphine 1 was anchored to the Au(111) surface via reaction with 1-heptanethiol linkers that were preorganized as a self-assembled monolayer (C7S SAM) on the Au(111) substrate. This arrangement ensured that 1 was electronically decoupled from the metal surface. Differential conductance (dI/dV – V) measurements with the bare W tip exhibited a broad gap region of low conductance and three distinct responses at 2.4,–1.3, and −2.1 V. Bias-voltage-dependent STM imaging of 1 at 65 K displayed a triangle shape at −2.1 < V < −1.3 V and a circle shape at V < −2.1 V, reflecting its HOMO and HOMO–1, respectively. In addition, different conductance behaviors were reproducibly observed, which has been ascribed to the adsorption of a tribenzosubporphine-cation on the W tip. When using a W tip doped with preadsorbed tribenzosubporphine-cation, negative differential resistance (NDR) phenomena were clearly observed in a reproducible manner with a peak-to-valley ratio of 2.6, a value confirmed by spatial mapping conductance measurements. Collectively, the observed NDR phenomena have been attributed to effective molecular resonant tunneling between a neutral tribenzosubporphine anchored to the metal surface and a tribenzosubporphine cation adsorbed on a W tip.
ISSN:0002-7863
1520-5126
DOI:10.1021/ja404512w