Understanding Energy-Level Alignment in Donor–Acceptor/Metal Interfaces from Core-Level Shifts

The molecule/metal interface is the key element in charge injection devices. It can be generally defined by a monolayer-thick blend of donor and/or acceptor molecules in contact with a metal surface. Energy barriers for electron and hole injection are determined by the offset from HOMO (highest occupied) and LUMO (lowest unoccupied) molecular levels of this contact layer with respect to the Fermi level of the metal electrode. However, the HOMO and LUMO alignment is not easy to elucidate in complex multicomponent, molecule/metal systems. We demonstrate that core-level photoemission from donor–acceptor/metal interfaces can be used to straightforwardly and transparently assess molecular-level alignment. Systematic experiments in a variety of systems show characteristic binding energy shifts in core levels as a function of molecular donor/acceptor ratio, irrespective of the molecule or the metal. Such shifts reveal how the level alignment at the molecule/metal interface varies as a function of the donor–acceptor stoichiometry in the contact blend

Photoinduced C–C Reactions on Insulators toward Photolithography of Graphene Nanoarchitectures

On-surface chemistry for atomically precise sp² macromolecules requires top-down lithographic methods on insulating surfaces in order to pattern the long-range complex architectures needed by the semiconductor industry. Here, we fabricate sp²-carbon nanometer-thin films on insulators and under ultrahigh vacuum (UHV) conditions from photocoupled brominated precursors. We reveal that covalent coupling is initiated by C–Br bond cleavage through photon energies exceeding 4.4 eV, as monitored by laser desorption ionization (LDI) mass spectrometry (MS) and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) gives insight into the mechanisms of C–Br scission and C–C coupling processes. Further, unreacted material can be sublimed and the coupled sp²-carbon precursors can be graphitized by e-beam treatment at 500 °C, demonstrating promising applications in photolithography of graphene nanoarchitectures. Our results present UV-induced reactions on insulators for the formation of all sp²-carbon architectures, thereby converging top-down lithography and bottom-up on-surface chemistry into technology