Spectroscopic Fingerprints of Work-Function-Controlled Phthalocyanine Charging on Metal Surfaces

Abstract

The electronic character of a π-conjugated molecular overlayer on a metal surface can change from semiconducting to metallic, depending on how molecular orbitals arrange with respect to the electrode's Fermi level. Molecular level alignment is thus a key property that strongly influences the performance of organic-based devices. In this work, we report how the electronic level alignment of copper phthalocyanines on metal surfaces can be tailored by controlling the substrate work function. We even show the way to finely tune it for one fixed phthalocyanine-metal combination without the need to intercalate substrate-functionalizing buffer layers. Instead, the work function is trimmed by appropriate design of the phthalocyanine's supramolecular environment, such that charge transfer into empty molecular levels can be triggered across the metal-organic interface. These intriguing observations are the outcome of a powerful combination of surface-sensitive electron spectroscopies, which further reveal a number of characteristic spectroscopic fingerprints of a lifted LUMO degeneracy associated with the partial phthalocyanine charging.

Iron phthalocyanine on Cu(111): Coverage-dependent assembly and symmetry breaking, temperature-induced homocoupling, and modification of the adsorbate-surface interaction by annealing

“…5 These interesting properties of MPcs have spurred significant interest in the adsorption of metal phthalocyanines on single crystal surfaces of materials such as copper, silver, and aluminium. [6][7][8][9] Surface-driven modification of the molecular properties plays an important role, and recent research into phthalocyanine compounds includes the control of the Kondo effect, 10 switching of magnetic anisotropy, 11 and uneven charge transfer leading to distortion and symmetry reduction. 12 The preparation of MPc monolayers (ML) on metal surfaces by thermal evaporation onto the room temperature surface is often tedious since it is difficult to determine the exact amount of deposited material.…”

Molecular floating-gate single-electron transistor

“…Borghetti et al . reported that a CuPc layer on Ag(111) is charged negatively, while that on Au(111) remains neutral 38 . Although the charge state of the molecules and their stability strongly depend on the energy alignment and atomic scale details at the junction, three charge states of CuPc, i .…”

A Solvent‐Free Solution: Vacuum‐Deposited Organic Monolayers Modify Work Functions of Noble Metal Electrodes

“…Borghetti et al have argued that the strong electron-accepting nature of this molecule leads to a notable charge transfer from the metal substrates into the F 16 CuPc LUMO, thus counteracting the omnipresent pushback effect. [49] Corresponding STM data show that the adlayer structures for F 16 CuPc on Au(111) (c.f. Figure 1f and Ag(111) (see Supporting Information) are consistent with structures reported in the literature.…”

“…For higher coverage, ΔΦ for Ag(111) (cf. [36,44,48,49] Theoretical investigations show that partial charge transfer depends sensitively on the position of the former LUMO relative to the Fermi level of the substrate, but precise calculation of the coverage dependence remains challenging. For Au(111), on the other hand, a further reduction of 150 meV is observed after completion of the first ML.…”

“…[47] Several explanations for these varying behaviors have been considered, ranging from a reduced pushback effect for CuPc/Ag(111) to charge transfer processes between CuPc and the different metal surfaces. [36,44,48,49] Theoretical investigations show that partial charge transfer depends sensitively on the position of the former LUMO relative to the Fermi level of the substrate, but precise calculation of the coverage dependence remains challenging.…”