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[18p-E302-6] Screening effect of metal surface on the metal/organic interface energy level alignment
Keywords:Screening effect, Inverse photoelectron spectroscopy, metal/organic interface
The energy level alignment at the metal/organic interface is affected by the screening of the metal surface. In this work, we experimentally examine the screening effect by measuring the HOMO and LUMO levels using ultraviolet photoemission spectroscopy (UPS) and low-energy inverse photoemission spectroscopy (LEIPS)respectively. Ag (111), Au(111) and PTCDA were used as a sample, since it is known that PTCDA keeps growing in a lying orientation.
Fig. 1 shows the results of UPS and LEIPS. With increasing film thickness, HOMO shifted to a higher energy region and LUMO shifted to a lower energy region. Fig. 2 shows the energy of the peak as a function of film thickness. The closer the sample surface to the metal surface the narrower the energy gap due to the screening effect.
The electrostatic energy due to the screening effect can be approximated by the image charge. If the ionized molecule is approximated by the point charge e and the distance to the metal surface is r, the charge induced on the metal surface can be approximated to the image charge -e at the position of the symmetrical distance r to the metal surface. expressed as where is the permittivity of the organic layer is shown by solid lines in Fig. 2. The good agreement between the experimental and calculated values indicates that the change in energy gap due to the screening effect can be explained by the image charge. And the screening effect was found to be as high as 0.25 eV in the first layer, which greatly affects the metal/organic interface.
Fig. 1 shows the results of UPS and LEIPS. With increasing film thickness, HOMO shifted to a higher energy region and LUMO shifted to a lower energy region. Fig. 2 shows the energy of the peak as a function of film thickness. The closer the sample surface to the metal surface the narrower the energy gap due to the screening effect.
The electrostatic energy due to the screening effect can be approximated by the image charge. If the ionized molecule is approximated by the point charge e and the distance to the metal surface is r, the charge induced on the metal surface can be approximated to the image charge -e at the position of the symmetrical distance r to the metal surface. expressed as where is the permittivity of the organic layer is shown by solid lines in Fig. 2. The good agreement between the experimental and calculated values indicates that the change in energy gap due to the screening effect can be explained by the image charge. And the screening effect was found to be as high as 0.25 eV in the first layer, which greatly affects the metal/organic interface.