Filling empty states in a CuPc single layer on the Au(110) surface via electron injection

An ordered copper-phthalocyanine (CuPc) single layer deposited on the Au(110) surface has been electron-doped by exposition to potassium. Progressive occupation of the former lowest unoccupied molecular orbitals (LUMOs) up to the filling of the twofold degenerate LUMO and $\text{LUMO}+1$ and $\text{LUMO}+2$ levels, has been detected by near-edge x-ray absorption spectroscopy spectra. The electronic states filled by electron transfer from the alkali atoms are localized at the interface, and the process induces a redistribution of the density of states with a conspicuous decrease in spectral density at the Fermi level. Localization of the final states, sudden decrease in density of states at the Fermi level, minor screening of the electron-hole excitations, and enhanced excitonic effects of doped CuPc with respect to the pristine CuPc single-layer concur to suggest that the electron injection produces a Mott insulator where correlation effects dominate the electronic properties.

Figure 1

Work function change at the CuPc-SL on Au(110) as a function of K deposition time. Inset: sketch of CuPc molecule.

Figure 2

NEXAFS absorption spectra from the CuPc-SL/Au(110), (a) from the carbon $K$ edge, and (b) from the nitrogen $K$ edge. The corresponding $1s$ core-level binding-energy position is marked in the top lines. In the insets, the $\text{C}1s$ and $\text{N}1s$ core-level photoemission data, along with a least-square fitting analysis, are shown.

Figure 3

Carbon $K$-edge NEXAFS absorption spectra at the K-CuPc-Au(110) interface as a function of potassium deposition time.

Figure 4

Nitrogen $K$-edge NEXAFS absorption spectra at the K-CuPc-Au(110) interface as a function of potassium deposition time.

Figure 5

XPS data from at the K-CuPc-Au(110) interface as a function of potassium deposition time: $\text{N}1s$ core-level evolution (dotted lines), least-square fitting analysis with single components, and their sum (continuous lines).

Figure 6

$\text{K}2p$ and $\text{C}1s$ core-level evolution at the K-CuPc-Au(110) interface as a function of potassium deposition time.

Figure 7

HR-UPS normal-emission data from the valence band region of the K-CuPc-Au(110) as a function of potassium deposition time. The spectrum of clean Au(110) is shown for comparison.

Figure 8

HR-UPS data taken at $20°$ out-of-normal emission of the K-CuPc-Au(110) as a function of potassium deposition time.