2016 ANNUAL REVIEW Ministry of Science and Technology

may create a huge global market. And because near infrared can effectively penetrate animal tissue, they have important medical applications. For instance, they can be used to activate animal cells and accelerate cellular repair. In photodynamic therapy, near infrared illumination can be used to activate photosensitive drugs under the skin or within the body and thereby kill tumor cells. Because conventional near infrared illumination has poor efficiency, the development of relevant medical technologies has been severely restricted. However, this breakthrough in near infrared OLEDs totally eliminates these restrictions, paving the way for further progress . Th i s s t udy was comp l e t ed wi t h f und i ng support from MOST's Science Vanguard Research Program and the Ministry of Education's Aim for the Top University Projects, and a resulting paper was published in the November 28, 2016 online issue of Nature Photonics (2016, DOI: 10.1038/ NPHOTON.2016.230). (4) Self-assembly of C 60 molecules guided by a moiré pattern into magic molecular clusters C 60 is a round, hollow molecule consisting of 60 carbon atoms and shaped almost exactly like a soccer ball; C 60 molecules interact chiefly via the Van der Waals effect. When a small quantity of C 60 molecules is deposited on a silicon substrate via low- temperature (110°K) via vapor deposition, the random aggregation of C 60 molecules can be observed, and the resulting molecular clusters are irregular in form. When the temperature is increased to room temperature, the molecular clusters adjust themselves and assume a regular form, but this form consists of loose small islands. If 0.2 layer of C 60 is deposited at the beginning, when the sample is warmed to room temperature, many medium-size islands will appear. These islands are often hexagonal in form, and tend to be more compact. When a larger amount is deposited, large islands form, and their images under a scanning tunneling electron microscope appear as periodic points of light, with the image as a whole appearing as a moiré pattern consisting of two overlaid patterns with different periods. Although samples resulting from low-temperature deposition contain hexagonal islands with many different sizes after warming to room temperature, molecular migration and selection of molecular clusters occur after a sample remains at room temperature for several hours. If a sample remains at room temperature for 48 hours, almost all remaining islands will have only 37 molecules, and these especially stable islands are referred to as "magic molecular clusters." This study was the first time that molecules had been observed to aggregate via self- organization on a surface, and this phenomenon results in the selection of molecular clusters with uniform size. Theoretical calculations based on first principles indicate that C 60 has a maximum potential energy when located at the central point between three gold atoms. This location is a potential energy peak, and other locations are potential energy valleys. It was further observed in a computer experiment, which minimized the islands' energy in the surface potential energy field, that the potential energy field felt by clusters of C 60 molecules is actually the transversely magnified projection of the periodic potential energy field sensed by a single C 60 molecule. It can therefore be said that the potential energy field has the form of a moiré interference pattern. In order to avoid potential energy peaks, small molecular clusters typically have a loose or even bent form. But when a hexagonal molecular cluster consisting of 37 molecules is centered on a potential energy peak, the cluster's six sides will be in potential energy valleys, and the cluster will consequently be very stable. In summary, this study uncovered a new molecular self-organization phenomenon, which is caused by a moiré interference pattern formed by the molecular cluster lattice and substrate lattice. The study also discovered that the potential energy field felt by the molecular clusters is actually the transversely magnified projection of the periodic potential energy field sensed by a single C 60 molecule. As the molecular clusters adjust their form and size in response, and self-organize into molecular clusters with the lowest energy. We can take advantage of the fact that molecular clusters in the surface lattice feel a potential energy field in a moiré pattern to select and control the shape and size of molecular clusters through the selection of lattice types and unit cell size. Support for Academic Research Ministry of Science and Technology 31