Effect of electrostatic screening on apparent shifts in photoemission spectra near metal/organic interfaces

Photoemission spectra of very thin organic films differ from that of thicker bulklike films. In particular, the binding-energy shifts in molecular orbitals at metal/organic interfaces vary as a function of the organic overlayer thickness. Using a simple image-charge model it is found that electrostatic screening can have a significant effect on the final-state relaxation energy. Good agreement between experimental results and theoretical calculations for a variety of dielectric substrates indicate that reported thickness dependent energy-level shifts in organic overlayer spectra can be accounted for by electrostatic screening. Models of organic interfaces based on photoemission spectra of organic thin films should therefore be re-examined.

Figure 1

Dielectric continuum model of an organic overlayer on a substrate with point charge $q=e$ in the organic layer. ${\epsilon }_1$, ${\epsilon }_2$, and ${\epsilon }_3$ are the permittivities of each layer and $k_1$ and $k_2$ are the reflection coefficients at the dielectric boundaries. The image charges shown are from the $n=0$ term of Eq. (2).

Figure 2

(Color online) Correction to the binding energy as a function of the position $\left(z\right)$ in the organic film for different organic layer thickness $\left(d\right)$ and substrate permittivity $\left({\epsilon }_1\right)$. The permittivity of the organic was taken as ${\epsilon }_2=3.0$

Figure 3

(Color online) Binding-energy shift as a function of organic film thickness $\left(d\right)$ for different photoelectron escape depths $\left(\lambda \right)$. The inset shows calculated model spectra for two different film thicknesses on a metallic substrate.

Figure 4

(Color online) $\text{He}\text{I}\alpha$ $\left(h\nu =21.22\right)$ valence-band spectra of the Ag/2T-NATA interface showing the evolution of (a) the secondary electron cutoff and (b) the HOMO of 2T-NATA. The 2T-NATA thickness is shown in (b) in nm. The molecular structure of 2T-NATA is shown in the inset.

Figure 5

(Color online) Comparison of the experimental (symbols) and theoretical (lines) binding-energy shift in the HOMO of 2T-NATA as a function of the organic film thickness $\left(d\right)$ for various substrates. The photoelectron escape depth $\lambda =6.4\text{Å}$ was determined from the attenuation of the Fermi edge of the Ag substrate with film thickness and is in good agreement with Ref. 35. The permittivity of Si and HOPG were taken as ${\epsilon }_1=12$ and ${\epsilon }_1=6$, respectively.