Insulating state of electron-doped Cu-phthalocyanine layers

Abstract

A copper-phthalocyanine (CuPc) thin film and a highly ordered CuPc single layer prepared on the Au(110) surface are electron doped by exposition to alkali metal. The CuPc thin film remains insulating when the donated electrons flow into the unoccupied molecular orbitals. The CuPc/Au interface is metallic in the absence of doping, and transition to an insulating state is observed by the filling up of the former empty states. In particular, two occupied states are generated by filling the lowest unoccupied molecular orbitals with the doping electrons. Sweeping of the former empty states through the Fermi energy does not induce any enhancing of the electronic spectral density, as detected by high-resolution photoelectron spectroscopy. Electron correlation and Jahn-Teller effects act to determine the insulating response of electron-doped CuPc

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

“…Furthermore, the slight feature close to E F shifts to higher BE and increases its intensity, indicating the occupation of the first former LUMO state, as recently observed for analogous system ͑Cs-doping of a CuPc-SL͒. 11 A second structure emerges below E F and becomes clearly visible at 13Ј of K deposition time. Thus, after the occupation of the first former-LUMO, subsequent former-empty states are filled up to K saturation coverage.…”

“…It lies slightly shifted to lower BE with respect to the HOMO measured in the CuPc thin film. 11 At the first potassium deposition, the HOMO-related structure remains almost unperturbed, and a potassium induced feature appears close to the Fermi level. At subsequent K dose ͑3Ј͒, the HOMO peak decreases its intensity eventually disappearing Binding energy (eV) ͑6Ј͒ in the redistributed spectral density.…”

“…3,4 Experimental investigation of electron doping of -conjugated systems shows a reduction in the hole injection barrier. [5][6][7][8][9][10][11] Metal-phthalocyanines ͑MPcs͒, semiconductor in the condensed phase, can be turned metallic through alkali doping as observed by transport measurements, 12,13 and the electron doping appears to take place in the twofold degenerate lowest unoccupied molecular orbital ͑LUMO͒ as a function of the molecular level occupation. On the other hand, photoemission studies of MPc layer n-doped with alkali metals do not present any spectral density of electronic states at the Fermi level, both when the MPc molecules are arranged in thin films 14,15 and when they are adsorbed as few layers.…”

“…On the other hand, photoemission studies of MPc layer n-doped with alkali metals do not present any spectral density of electronic states at the Fermi level, both when the MPc molecules are arranged in thin films 14,15 and when they are adsorbed as few layers. 11 However, neither the charge donation processes nor the physical reasons of the occurrence of insulator-metalinsulator transition in the molecular film and/or at the hybrid interfaces has been yet fully clarified. The experimental results for a single-layer of CuPc ͑CuC 32 H 16 N 8 ͒, compared with those on CuPc thin-films, present a different response to the electron doping, albeit both systems do not present any insulator-metal transition as detected by the electronic spectral density at the Fermi level.…”

“…The experimental results for a single-layer of CuPc ͑CuC 32 H 16 N 8 ͒, compared with those on CuPc thin-films, present a different response to the electron doping, albeit both systems do not present any insulator-metal transition as detected by the electronic spectral density at the Fermi level. 8,11 Within this context, we electron-doped a CuPc single layer ͑SL͒, controlling the filling of the empty states by means of near-edge x-ray absorption fine-structure spectroscopy ͑NEXAFS͒, and the evolution of the filled-induced density of states by means of high-resolution photoemission. We show that the electron injection of the CuPc single layer concerns not only the LUMO state, as observed for the CuPc thin films, but also the empty states located at higher energy ͑LUMO+ 1 and LUMO+ 2͒, giving rise to a more complex redistribution of the density of the electronic states.…”