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Scanning probe study of organic semiconducting molecules Tom, Gary Ka Wai


When compared to conventional inorganic semiconductors, organic semiconductors are lightweight, flexible, and compatible with less expensive high-throughput manufacturing techniques. Applications of organic semiconductors in power generation and light emitting applications have been realized through the development of organic photovoltaic (OPV) and organic light emitting diode (OLED) devices. However, to optimize the performance and efficiency of these applications, the molecular orbital energy and the role of the exciton in charge generation and luminescence in organic materials need to be further explored. In this work, scanning probe microscopy (SPM) techniques including scanning tunnelling microscopy (STM), scanning tunnelling spectroscopy (STS), and scanning tunnelling microscopy luminescence (STML) were used to probe the electronic and optical properties of individual organic molecules deposited on insulating NaCl layers on a metallic substrate. Pixel-by-pixel STS energetically and spatially resolves molecular orbitals. Concurrent STML induces molecular luminescence through electron tunnelling, giving spectral information of the excitons and vibrational modes of the organic molecule on sub-nanometre length scales. The results presented here are the first signals of molecular emission obtained from our microscope, demonstrating the capability of our system in detecting single molecule luminescence. Conventional organic molecules 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and zinc (II) phthalocyanine (ZnPc) were studied and the results compared to those presented in the literature. SPM was also performed on F₈ZnPc to explore the effects of fluorination. Our results revealed that electronic and structural changes due to the additional fluorine atoms and interactions with the substrate can affect the energy levels and luminescence of the molecule. Organic donor-acceptor molecules based on the hexamethylazatriangulene (HMAT) complex were also studied. The effects of various acceptor groups on the energetic gap and spatial distribution of molecular orbitals were explored for different HMAT derivatives using STS. We demonstrate the effects of gap engineering at the sub-molecular level for this promising class of optoelectronic organic materials.

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