| Summary: | 碩士 === 輔仁大學 === 物理學系 === 90 === C60 molecules adsorbed on Ag(100) exhibit rather peculiar bright and dim contrast as large as ~2Å in scanning tunneling microscopy(STM). Origin of this contrast is ambiguous and has not been resolved. Here I combined STM and non-contact atomic force microscopy (NC-AFM) to demonstrate that this contrast was of topographic origin. First, I observed significant substrate mass flow occurs with the emergence of dim C60 molecules. This showed a restructuring at the C60-Ag(100) interface had occurred. From local number conservation of silver atoms, I propose a model of substrate reconstruction involving an <111> microfaceted pit underneath each dim C60. Second, I studied the temperature evolution of bright-dim contrast and shows the above interfacial restructuring starts at ~270K, and was both thermally activated and irreversible. Finally, high-resolution NC-AFM micrographs showed evidence of the molecular contrast consistent with that of STM. By measuring local contact potential difference (CPD), I also show electrons were transferred into the C60 layer. The resolution in CPD is still inadequate to assess whether the bright and dim C60s are of different charge states.
(In addition to the study of C60-Ag(100) system, I made a series of testing and fixing of the Oxford LTSTM system as desorbed in Appendix A)
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