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Interface Properties of Functional Molecules on Au{111}: From Photoinduced Charge Transfer to Exchange Reactions

  • Author(s): Zhao, Yuxi
  • Advisor(s): Weiss, Paul S
  • et al.
Abstract

Understanding electron transfer at the molecular level is crucial for the rational design and performance improvement of organic optoelectronics and photovoltaics. Scanning tunneling microscope (STM) enables measurement of electronic properties with atomic precision. Here, we introduce a custom-built, laser-assisted STM that integrates optical excitation into the tunneling junction. The evanescent field generated by rear-incident laser light excites surface adsorbates on gold films, enabling the detection of optically induced changes in tunneling current under ambient conditions. Thin and atomically flat Au films epitaxially grown on c-cut sapphire prisms provide both efficient optical coupling and long-term stability to monitor optical activities of molecular donor-acceptor heterojunctions isolated in two-dimensional matrices on the surface. Our method offers a versatile platform to elucidate the involvement of individual states that affect charge generation and separation in a C60-tethered 2,5-dithienylpyrrole triad, a molecular donor-acceptor heterojunction, and to correlate the particular molecular orientations and local environment with the measured photocurrent of the anthracene-terminated aromatic thiolates in the tunneling junction.

When alkanethiolate self-assembled monolayers on Au{111} are exchanged with alkaneselenols from solution, replacement of thiolates by selenols is rapid and complete, and is well described by perimeter-dependent island growth kinetics. The monolayer structures change as selenolate coverage increases, from being epitaxial and consistent with the initial thiolate structure to being characteristic of selenolate monolayer structures. At room temperature and at positive sample bias in scanning tunneling microscopy, the selenolate-gold attachment is labile, and molecules exchange positions with neighboring thiolates. The scanning tunneling microscope probe can be used to induce these place-exchange reactions.

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