Electronic structures of TPD/metal interfaces studied by photoemission and Kelvin probe method

Interfacial electronic structures for TPD/metal (Au, Ag, and Ca) systems were investigated by Kelvin probe method (KP), ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and metastable atom electron spectroscopy (MAES). KP results showed that an abrupt vacuum level shift occurred right at the interface, and no further shift was observed by further deposition of TPD up to the thickness of about 100 nm. Similar abrupt shift was also observed from the onset of the lower kinetic electron energy in the UPS spectra. These results suggest that band bending does not occur in the TPD/metal interface. From the MAES and XPS measurements, chemical interaction between TPD and Ca was observed, while the interaction between TPD and Au or Ag was weak. These results show that the origin of the vacuum level shift for TPD/Ca is of more chemical nature, and is different from that for TPD/Au or Ag systems. The highest occupied molecular orbital (HOMO) and C 1s levels of the TPD films gradually shifted by deposition of the TPD and their trends were different from the abrupt shift of the vacuum level. We consider that such gradual shift is due to the final-state screening effect in photoemission measurements, not due to the band bending in the space charge layer.