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PhD Thesis Presentation
On-surface Synthesis and Characterization of Conjugated Oligomers Using Low Temperature Scanning Tunneling Microscopy and Spectroscopy
Speaker Mr Guowen Kuang
Department of Physics, The Hong Kong University of Science and Technology
Date 17 August 2017 (Thursday)
Time 9:30 - 10:30
Venue Room 5506 (Lifts 25-26), HKUST
Conjugated oligomers are envisionedas basic functional units in molecular electronics, due to their versatility, flexibility and multi-functionality. There are challenges in understanding basic phenomena of charge transport and spin manipulationin conjugated oligomers. This thesis focuseson on-surface synthesis and functionality characterization of porphyrin-based conjugated oligomersutilizing low-temperature scanning tunneling microscopy and spectroscopy. This thesis consists of three parts: on-surface synthesis, charge transport and spin manipulation of porphyrin-based oligomers. In the part of on-surface synthesis, the biphenyl-linked porphyrin oligomers (bp-ppo) is synthesized by homo-coupling of Br2-TPP, the phenyl-linked porphyrin oligomers (p-ppo) is synthesized by cross-couplingof Br2-TPP and Br2-DPP and porphyrin-dimers (ppo) can not be synthesized on surface.
In the part of charge transport, we studied the charge transport behavior of bp-ppo. Firstly, the long-range nearly lengthindependent resonant charge transport is observed in straight bp-ppo using STM vertical manipulation technique. We found that the electrons transfer through delocalized LUMO, and the lowest attenuation factor ( < 0.001Å-1) of differential conductance is achieved. Secondly, the negative differential conductance (NDC) was observed in oligomers of bp-ppo with a 90o-kink, and the mechanism is proposed to be localization of delocalized molecular orbital at higher bias. In the part of spin manipulation, we studied the mediation of Fe spin by molecular backbone. Firstly, in the system of double-Fe metalated and three C-C bonds linked ppo, we found that the spin-excitation on individual spin is quenched due to the formation of spin-spin singlet by indirect coupling mediated by molecular backbone. Secondly, we demonstrated that the spin of Fe is closely related to the conformation of porphyrin hosting the Fe: the transformation from saddle to planar changes the Fe spin state from S=2 to S=1.
In summary, we studied on-surface synthesis, charge transport and spin manipulation of porphyrin-based conjugated oligomers. Our results may contribute to the development of molecular electronics