Experimental Determination of the Excited-State Polarizability and Dipole Moment in a Thin Organic Semiconductor Film

We report the evolution of the electronic structure of excited (sub)monolayer films of vanadyl naphthalocyanine (VONc) at the interface with highly oriented pyrolytic graphite (HOPG). Using two-photon photoemission spectroscopy, an unoccupied state corresponding to the lowest unoccupied molecular orbital of VONc is observed. The energy of this state shows a significant dependence on coverage, interpreted in the context of the electrostatic environment at the interface. On the basis of a simple electrostatic model, we were able to determine the excited-state polarizability and dipole moment of VONc at the interface with HOPG. The results suggest that local electric fields may have a major influence on interfacial energy level alignment in the excited-state manifold, with direct consequences for interfacial charge-transfer dynamics

Near- and Far-Field Effects on Molecular Energy Level Alignment at an Organic/Electrode Interface

We investigate the evolution of the interfacial electronic structure at the interface of (sub)monolayer vanadyl naphthalocyanine (VONc) on highly oriented pyrolytic graphite (HOPG). Both the vacuum level and molecular energy levels show a significant but fundamentally different dependence on coverage, resulting in an overall change of the ionization potential with coverage. We use a simple model to show how this effect arises from the differential sensitivity of these levels to the near- and far-field properties of the dipole-layer-generated interfacial electric potential. These results help to unravel the electronic structure at organic/electrode interfaces, with direct implications for organic electronic devices

Interfacial Electronic Structure of the Dipolar Vanadyl Naphthalocyanine on Au(111): “Push-Back” vs Dipolar Effects

We investigate the interfacial electronic structure of the dipolar organic semiconductor vanadyl naphthalocyanine on Au(111) in a combined computational and experimental approach to understand the role of the permanent molecular dipole moment on energy-level alignment at this interface. First-principles Density Functional Theory (DFT) calculations on such large systems are challenging, due to the large computational cost and the need to accurately consider dispersion interactions. Our DFT results with dispersion correction show a molecular deformation upon adsorption but no strong chemical bond formation. Ultraviolet photoelectron spectroscopy measurements show a considerable workfunction change of −0.73(2) eV upon growth of the first monolayer, which is well reproduced by the DFT calculations. This shift originates from a large electron density “push-back” effect at the gold surface, whereas the large out-of-plane vanadyl dipole moment plays only a minor role

Tin DisulfideAn Emerging Layered Metal Dichalcogenide Semiconductor: Materials Properties and Device Characteristics

Layered metal dichalcogenides have attracted significant interest as a family of single- and few-layer materials that show new physics and are of interest for device applications. Here, we report a comprehensive characterization of the properties of tin disulfide (SnS₂), an emerging semiconducting metal dichalcogenide, down to the monolayer limit. Using flakes exfoliated from layered bulk crystals, we establish the characteristics of single- and few-layer SnS₂ in optical and atomic force microscopy, Raman spectroscopy and transmission electron microscopy. Band structure measurements in conjunction with <i>ab initio</i> calculations and photoluminescence spectroscopy show that SnS₂ is an indirect bandgap semiconductor over the entire thickness range from bulk to single-layer. Field effect transport in SnS₂ supported by SiO₂/Si suggests predominant scattering by centers at the support interface. Ultrathin transistors show on–off current ratios >10⁶, as well as carrier mobilities up to 230 cm²/(V s), minimal hysteresis, and near-ideal subthreshold swing for devices screened by a high-<i>k</i> (deionized water) top gate. SnS₂ transistors are efficient photodetectors but, similar to other metal dichalcogenides, show a relatively slow response to pulsed irradiation, likely due to adsorbate-induced long-lived extrinsic trap states