Monolayer transition metal dichalcogenides (TMD) have numerous potential applications in ultrathin electronics and photonics. The exposure of TMD-based devices to light generates photo-carriers resulting in an enhanced conductivity, which can be effectively used, e.g., in photodetectors. If the photo-enhanced conductivity persists after removal of the irradiation, the effect is known as persistent photoconductivity (PPC). Here we show that ultraviolet light (λ = 365 nm) exposure induces an extremely long-living giant PPC (GPPC) in monolayer MoS2 (ML-MoS2) field-effect transistors (FET) with a time constant of ~30 days. Furthermore, this effect leads to a large enhancement of the conductivity up to a factor of 107. In contrast to previous studies in which the origin of the PPC was attributed to extrinsic reasons such as trapped charges in the substrate or adsorbates, we show that the GPPC arises mainly from the intrinsic properties of ML-MoS2 such as lattice defects that induce a large number of localized states in the forbidden gap. This finding is supported by a detailed experimental and theoretical study of the electric transport in TMD based FETs as well as by characterization of ML-MoS2 with scanning tunneling spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. The obtained results provide a basis for the defect-based engineering of the electronic and optical properties of TMDs for device applications.

2D materials such as hexagonal boron nitride (h-BN) are widely used to decouple organic molecules from metal substrates. Nevertheless, there are also indications in the literature for a significant hybridization, which results in a perturbation of the intrinsic molecular properties. In this work we study the electronic and optical properties as well as the lateral structure of tetraphenyldibenzoperiflanthene (DBP) on Ni(111) with and without an atomically thin h-BN interlayer to investigate its possible decoupling effect. To this end, we use in situ differential reflectance spectroscopy as an established method to distinguish between hybridized and decoupled molecules. By inserting an h-BN interlayer we fabricate a buried interface and show that the DBP molecules are well decoupled from the Ni(111) surface. Furthermore, a highly ordered DBP monolayer is obtained on h-BN/Ni(111) by depositing the molecules at a substrate temperature of 170 °C. The structural results are obtained by quantitative low-energy electron diffraction and low-temperature scanning tunneling microscopy. Finally, the investigation of the valence band structure by ultraviolet photoelectron spectroscopy shows that the low work function of h-BN/Ni(111) further decreases after the DBP deposition. For this reason, the h-BN-passivated Ni(111) surface may serve as potential n-type contact for future molecular electronic devices.

Poly(3,:polystyrene sulfonate (PEDOT:PSS) is up to date the most popular and commercially most successful conductive polymer. It is being used not only for antistatic, anticorrosion, and even antifouling coatings on one hand, but also, owing to its plasma frequency residing within the far-infrared region, for semi-transparent electrodes or charge extraction layers in solar cells on the other hand. The work function of an electrode plays an important role in the performance of any electronic device. Therefore, proper tuning of electrode work functions in semiconductor devices is crucial. It is worth mentioning that the work function of electrodes can be impacted by uncontrolled conditions during film processing because even monomolecular adsorbates may cause noticeable changes. Due to its ionic properties governed by the sulfonate group and its counterion, PEDOT:PSS is strongly hygroscopic, which may influence its functionality. Furthermore, the acidity of the same may lead to the release of additional ions from the neighboring layer. In this contribution, the work function of PEDOT:PSS films, cast from various commercial formulations, was monitored in dependence of the impact of (a) post-production thermal annealing and (b) storage under wellcontrolled relative humidity conditions for (c) certain durations. Indeed, work functions could be correlated with the surface concentration of metal ions, which clearly depended on the mentioned processing conditions. Finally, the impact of processing conditions on the performance of organic solar cells was demonstrated.

Herein, we have studied highly ordered two-dimensional layers of 1,2:8,9-dibenzopentacene (trans-DBPen) adsorbed on a single-crystalline silver surface. While its parent molecule pentacene is known for a relatively high hole mobility, reports on trans-DBPen are rather scarce. Furthermore, it belongs to polycyclic aromatic hydrocarbons for which superconducting properties have been observed upon intercalation with potassium. Our scanning tunneling microscopy and low-energy electron diffraction results reveal a highly ordered monolayer structure of trans-DBPen on Ag(111). We further used angle-resolved ultraviolet photoelectron spectroscopy to measure photoelectron momentum maps (PMMs). The experimental PMMs agree very nicely with simulations based on our structural data while assuming free molecules for the density functional theory calculations. A comparison with pentacene yields some insights into the properties of the two related molecules. We conclude that the degree of hybridization between the molecular orbitals and substrate states is comparatively weak. These results are expected to serve as a starting point for future investigations of K-doped monolayers of trans-DBPen.