Photoelectric Conversion by Organic Ultra-Thin Film with High Efficiency by Nanogap Optical Antennae

2010.11.22

National Institute for Materials Science
Hokkaido University

NIMS and Hokkaido University developed a method which dramatically increases light utilization efficiency in organic ultra-thin film type molecular photoelectric conversion devices, which are expected to be applied in molecular electronics.

Abstract

  1. Dr. Kohei Uosaki, Coordinator of the Nano Green Area, International Center for Materials Nanoarchitectonics (MANA; Director-General: Masakazu Aono) of the National Institute for Materials Science (President: Sukekatsu Ushioda), in joint work with Associate Professor Katsuyoshi Ikeda of the Graduate School of Science, Hokkaido University (President: Hiroshi Saeki), developed a method which dramatically increases light utilization efficiency in organic ultra-thin film type molecular photoelectric conversion devices, which are expected to be applied in molecular electronics. In this research, nanogap-type optical antennae were successfully introduced by sandwiching a monolayer (i.e., single molecule) ultra-thin film comprising molecules coupling parts having the necessary functions of light absorption/electron transmission between gold nanoparticles and a flat gold electrode surface, in which the surface was controlled at the atomic level. As a result, an improvement of approximately 50 times in the efficiency of the photo-induced electron transfer reaction per optical antenna was demonstrated.
  2. In functional design using monolayer ultra-thin films, precise structural control at the molecular level and free coupling of the parts which manifest functions has the potential to realize advanced functions. However, when designing light-sensitive organic ultra-thin films, there are limits to the improvement of light absorptivity with monolayers, as well as problems in achieving high efficiency in the system as a whole. On the other hand, if light absorptivity is increased by use of a multilayer film, precise structural control at the molecular level becomes difficult.
  3. It is known that gold, silver, and other metallic nanostructures show plasmon resonance, in which the free electrons of the substance display collective motion coupled with the electrical field of light. Because localized electrical fields are formed near the metal structure by coupling of light with plasmons, it is considered possible to increase the efficiency of interaction between dye molecules and irradiated light. However, various problems arose when attempting to realize optical antennae using this phenomenon. In particular, in application to ultra-thin films with optical functions, it was difficult to satisfy both control of the metallic surface at the atomic level, which is necessary for molecular functions, and construction of a metallic nanostructure which shows plasmon resonance.
  4. In this research, both control of the interfacial structure and control of antenna properties were satisfied by a process which was the opposite of the conventional approach. In the new process, first, a highly-oriented functional molecular ultra-thin film was formed on an electrode surface. This was followed by construction of a nanogap structure in which the ultra-thin film was sandwiched between the electrode and nanoparticles by adsorbing gold nanoparticles on the molecular layer. This enabled strict design and control of the properties of the optical antennae, while continuing to make the maximum use of the function of the molecular layer. Because this method makes it possible to fabricate the optical antennae after formation of the ultra-thin film, application to a variety of electrode surfaces is expected.
  5. These research results have been accepted by Angewandte Chemie, which is a scientific journal published by the German Chemical Society. This research was carried out a part of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Grants-in-Aid for Scientific Research on Priority Areas, "Creation of Strong Photon-Molecule Coupling Reaction Fields."