| Summary: | 碩士 === 國立清華大學 === 物理學系 === 98 === The electronic structures of interface between an organic bistable material 2-amino-4,5-imidazoledicarbonitrile (AIDCN) and electrode metal surfaces (Ag and Al) were investigated by ultraviolet photoemission spectroscopy. The electrical bistability strongly depends on metal electrodes, and the working mechanism is still controversial. The photoemission spectra of the highest occupied molecular orbital (HOMO) of AIDCN on Ag and Al substrate and the corresponding vacuum level (VL) shift are investigated in this thesis. The thickness dependence of VL shift shows a dip in both systems. In AIDCN/Ag system, the HOMO peak shows two-component line shape, which can be divided into the contributions from the monolayer state and mutilayer state. In AIDCN/Al system, a diffuse tailing structure was observed below Fermi edge (~1 eV), and attributed to random molecular orientation. The tailing structure causes lower hole injection barrier than Ag surface (~2 eV). Angle dependence photoemission data also infer different growth mechanism for AIDCN/Ag and AIDCN/Al interfaces.
For AIDCN/Ag, the intensity of HOMO peak increases with increasing incident angle for monolayer coverage, indicating that the pi bonds are normal to Ag surface according to the symmetry selection rule. In multilayer region, the AIDCN dipole moment orients at a certain angle with respect to surface normal on top of monolayer AIDCN. For AIDCN/Al, of the HOMO peak (and also the other peaks) is absent for 2.99 nm-thick AIDCN film on Al. As increasing AIDCN thickness, angular dependence of the HOMO peak gradually generated and it finally became similar to the bulk-like behavior, where the HOMO intensity shows a local maximum representing formation of the bulk phase. We also examined the AIDCN/Al2O3 system by dosing 11000 L oxygen on clean Al surface. The HOMO peak of AIDCN thin film is observed at lower coverage than that for AIDCN/Al. The hole injection barriers from Ag and Al to AIDCN were estimated to be 1.6 and 0.7 eV,
respectively.
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