Nahid Ilyas

Resonance and localization effects at a dipolar organic semiconductor interface

The image state manifold of the dipolar organic semiconductor vanadyl naphthalocyanine (VONc) on highly oriented pyrolytic graphite is investigated by angle-resolved two-photon photoemission (AR-TPPE) spectroscopy in the 0-1 monolayer regimes. Interfacial charge-transfer from the image potential state of clean graphite populates a near-resonant VONc anion level, identifiable by the graphite image potential state by its distinct momentum dispersion obtained from AR-TPPE. This affinity level is subject to depolarization by the neighboring molecules, resulting in stabilization of this state with coverage. Near a coverage of one monolayer, a hybrid image potential/anion state is also formed, showing progressive localization with coverage. Intensities for all these features develop rather differently with molecular coverage, pointing towards the different types of charge-transfer interactions at play at this interface.

Electronic structure and dynamics of quasi-2D states of vanadyl naphthalocyanine on Au(111)†

We investigate the evolution of the interfacial electronic structure and dynamics of thin films of the organic semiconductor vanadyl naphthalocyanine on Au(111). Using angle-resolved two-photon photoemission, a comprehensive coverage- and excitation-energy-dependent characterisation of the electronic structure and the resulting dynamics of short-lived image potential resonances (IPRs) on Au(111) are presented. The study of these quasi-two-dimensional (quasi-2D) bands is enabled by molecular adsorption and reveals a significant lengthening of their lifetimes. The resonances remain, however, significantly coupled to the continuum of bulk bands of Au(111) even in the presence of the organic adsorbate, giving rise to Fano-like quantum interference and ‘intensity switching’ effects. Coupling to the continuum is also responsible for providing excitation pathways to the image potential manifold above and below optical resonance with the Shockley surface state. The organic semiconductor interface and quasi-2D bands investigated here provide a model for understanding the role of quantum effects in ultrafast dynamics of confined systems and at interfaces such as those that are relevant e.g. for interfacial charge-transfer processes in organic electronics.

Sticking with the Pointy End? Molecular Configuration of Chloro Boron-Subphthalocyanine on Cu(111)

In this combined low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) study, we investigate self-assembly of the dipolar nonplanar organic semiconductor chloro boron-subphthalocyanine (ClB-SubPc) on Cu(111). We observe multiple distinct adsorption configurations and demonstrate that these can only be understood by taking surface-catalyzed dechlorination into account. A detailed investigation of possible adsorption configurations and the comparison of experimental and computational STM images demonstrates that the configurations correspond to “Cl-up” molecules with the B–Cl moiety pointing toward the vacuum side of the interface, and dechlorinated molecules. In contrast to the standard interpretation of adsorption of nonplanar molecules in the phthalocyanine family, we find no evidence for “Cl-down” molecules where the B–Cl moiety would be pointing toward the Cu surface. We show computationally that such a configuration is unstable and thus is highly unlikely to occur for ClB-SubPc on Cu(111). Using these assignments, we discuss the different self-assembly motifs in the submonolayer coverage regime. The combination of DFT and STM is essential to gain a full atomistic understanding of the surface–molecule interactions, and our findings imply that phthalocyanines may undergo surface-catalyzed reactions hitherto not considered. Our results also indicate that care has to be taken when analyzing possible adsorption configurations of polar members of the phthalocyanine family, especially when they are adsorbed on comparably reactive surfaces like Cu(111).

van der Waals Interaction Activated Strong Electronic Coupling at the Interface between Chloro Boron-Subphthalocyanine and Cu(111)

In this article, we investigate the interface between shuttlecock-shaped chloro boron-subphthalocyanine molecules and the Cu(111) surface. We highlight how molecular planarization induced by van der Waals forces can fundamentally alter the interface properties and how it can enable a particularly strong hybridization between molecular and metal states. In our simulations, we start from a situation in which we disregard van der Waals forces and then introduce them gradually by rescaling the interaction parameter, thereby “pulling” the molecule toward the surface. This reveals two adsorption regimes with significantly different adsorption distances, molecular conformations, and adsorbate-induced changes of the work function. Notably, the above-mentioned massive hybridization of electronic states, also observed in photoelectron spectroscopy, is obtained solely for one of the regimes. We show that this regime is accessible only as a consequence of the planarization of the molecular backbone resulting from the van der Waals attraction between the molecule and the surface. The results of this study indicate that for certain metal–molecule combinations unusually strong interfacial electronic interactions can be triggered by van der Waals forces creating a situation that differs from the usually described cases of physisorptive and chemisorptive interactions.