Two-Dimensional Crystals as a Buffer Layer for High Work Function Applications: The Case of Monolayer MoO3.

We propose that the crystallinity of two-dimensional (2D) materials is a crucial factor for achieving highly effective work function (WF) modification. A crystalline 2D MoO3 monolayer enhances substrate WF up to 6.4 eV for thicknesses as low as 0.7 nm. Such a high WF makes 2D MoO3 a great candidate for tuning properties of anode materials and for the future design of organic electronic devices, where accurate evaluation of the WF is crucial. We provide a detailed investigation of WF of 2D α-MoO3 directly grown on highly ordered pyrolytic graphite, by means of Kelvin probe force microscopy (KPFM) and ultraviolet photoemission spectroscopy (UPS). This study underlines the importance of a controlled environment and the resulting crystallinity to achieve high WF in MoO3. UPS is proved to be suitable for determining higher WF attributed to 2D islands on a substrate with lower WF, yet only in particular cases of sufficient coverage. KPFM remains a method of choice for nanoscale investigations, especially when conducted under ultrahigh vacuum conditions. Our experimental results are supported by density functional theory calculations of electrostatic potential, which indicate that oxygen vacancies result in anisotropy of WF at the sides of the MoO3 monolayer. These novel insights into the electronic properties of 2D-MoO3 are promising for the design of electronic devices with high WF monolayer films, preserving the transparency and flexibility of the systems.

Antimony oxide nanostructures in the monolayer limit: self-assembly of van der Waals-bonded molecular building blocks.

Antimony oxide nanostructures have been identified as candidates for a range of electronic and optoelectronic applications. Here we demonstrate the growth of 2-dimensional antimony oxide nanostructures on various substrates, including highly oriented pyrolytic graphite (HOPG), MoS2 and α-Bi(110) nanoislands. Using scanning tunneling microscopy (STM) we show that the nanostructures formed are exclusively highly crystalline α-Sb2O3(111) monolayers with a lattice constant of 796 pm ± 7 pm. The nanostructures are triangular with lateral dimensions of up to ∼30 nm. Even though elemental antimony nanostructures are grown simultaneously mixed phases are not observed and both materials exhibit their own distinct growth modes. Moiré patterns are also observed and simulated, allowing confirmation of the atomic unit cell and an understanding of the orientation of the Sb2O3 structures with respect to the supporting materials. As in the bulk, the Sb2O3 nanostructures are formed from Sb4O6 molecules that are weakly interacting through van der Waals forces. This allows physical modification of the nanostructures with the STM tip. Scanning tunnelling spectroscopy reveals a wide band gap of at least 3.5 eV. Finally, we show that possible alternative structures that have unit cells comparable to those observed can be excluded based on our DFT calculations. The considered structures are a 2 × 2 reconstruction of ß-Sb with one vacancy per unit cell and a van der Waals solid composed of Sb4 clusters. Previous reports have predominantly demonstrated Sb2O3 structures with much larger thicknesses.