Understanding the electronic contact between molybdenum disulfide (MoS2) and metal electrodes is vital for the realization of future MoS2-based electronic devices. Natural MoS2 has the drawback of a high density of both metal and sulfur defects and impurities. We present evidence that subsurface metal-like defects with a density of ∼1011 cm–2 induce negative ionization of the outermost S atom complex. We investigate with high-spatial-resolution surface characterization techniques the effect of these defects on the local conductance of MoS2. Using metal nanocontacts (contact area < 6 nm2), we find that subsurface metal-like defects (and not S-vacancies) drastically decrease the metal/MoS2 Schottky barrier height as compared to that in the pristine regions. The magnitude of this decrease depends on the contact metal. The decrease of the Schottky barrier height is attributed to strong Fermi level pinning at the defects. Indeed, this is demonstrated in the measured pinning factor, which is equal to ∼0.1 at defect locations and ∼0.3 at pristine regions. Our findings are in good agreement with the theoretically predicted values. These defects provide low-resistance conduction paths in MoS2-based nanodevices and will play a prominent role as the device junction contact area decreases in size.

Understanding the electron transport through transition-metal dichalcogenide (TMDC)-based semiconductor/metal junctions is vital for the realization of future TMDC-based (opto-)electronic devices. Despite the bonding in TMDCs being largely constrained within the layers, strong Fermi-level pinning (FLP) was observed in TMDC-based devices, reducing the tunability of the Schottky barrier height. We present evidence that metal-induced gap states (MIGS) are the origin for the large FLP similar to conventional semiconductors. A variety of TMDCs (MoSe2, WSe2, WS2, and MoTe2) were investigated using high-spatial-resolution surface characterization techniques, permitting us to distinguish between defected and pristine regions. The Schottky barrier heights on the pristine regions can be explained by MIGS, inducing partial FLP. The FLP strength is further enhanced by disorder-induced gap states induced by transition-metal vacancies or substitutionals at the defected regions. Our findings emphasize the importance of defects on the electron transport properties in TMDC-based devices and confirm the origin of FLP in TMDC-based metal/semiconductor junctions.

A novel photoresponsive and fully conjugated N‐heterocyclic carbene (NHC) has been synthesized that combines the excellent photophysical properties of arylazopyrazoles (AAPs) with an NHC that acts as a robust surface anchor (AAP‐BIMe). The formation of self‐assembled monolayers (SAMs) on gold was proven by ToF‐SIMS and XPS, and the organic film displayed a very high stability at elevated temperatures. This stability was also reflected in a high desorption energy, which was determined by temperature‐programmed SIMS measurements. E‐/Z‐AAP‐BIMe@Au photoisomerization resulted in reversible alterations of the surface energy (i.e. wettability), the surface potential (i.e. work function), and the conductance (i.e. resistance). The effects could be explained by the difference in the dipole moment of the isomers. Furthermore, sequential application of a dummy ligand by microcontact printing and subsequent backfilling with AAP‐BIMe allowed its patterning on gold. To the best of our knowledge, this is the first example of a photoswitchable NHC on a gold surface. These properties of AAP‐BIMe@Au illustrate its suitability as a molecular switch for electronic devices.

The distribution of potassium (K+) ions on air-cleaved mica is important in many interfacial phenomena such as crystal growth, self-assembly and charge transfer on mica. However, due to experimental limitations to nondestructively probe single ions and ionic domains, their exact lateral organization is yet unknown. We show, by the use of graphene as an ultra-thin protective coating and scanning probe microscopies, that single potassium ions form ordered structures that are covered by an ice layer. The K+ ions prefer to minimize the number of nearest neighbour K+ ions by forming row-like structures as well as small domains. This trend is a result of repulsive ionic forces between adjacent ions, weakened due to screening by the surrounding water molecules. Using high resolution conductive atomic force microscopy maps, the local conductance of the graphene is measured, revealing a direct correlation between the K+ distribution and the structure of the ice layer. Our results shed light on the local distribution of ions on the air-cleaved mica, solving a long-standing enigma. They also provide a detailed understanding of charge transfer from the ionic domains towards graphene.