Stability of Single- and Few-Molecule Junctions of Conjugated Diamines

We study the stability of molecular junctions based on an oligo­(phenylenethynylene) (OPE) diamine using a scanning tunneling microscope at room temperature. In our analysis, we were able to differentiate between junctions most probably formed by either one or several molecules. Varying the stretching rate of the junctions between 0.1 and 100 nm/s, we observe practically no variation of the length over which both kinds of junction can be stretched before rupture. This is in contrast with previously reported results for similar compounds. Our results suggest that, over the studied speed range, the junction breakage is caused purely by the growth of the gap between the gold electrodes and the elastic limit of the amine–gold bond. On the other hand, without stretching, junctions would survive for periods of time longer than our maximum measurement time (at least 10 s for multiple-molecule junctions) and can be considered, hence, very stable

Does a Cyclopropane Ring Enhance the Electronic Communication in Dumbbell-Type C₆₀ Dimers?

Two C₆₀ dumbbell molecules have been synthesized containing either cyclopropane or pyrrolidine rings connecting two fullerenes to a central fluorene core. A combination of spectroscopic techniques reveals that the cyclopropane dumbbell possesses better electronic communication between the fullerenes and the fluorene. This observation is underpinned by DFT transport calculations, which show that the cyclopropane dumbbell gives a higher calculated single-molecule conductance, a result of an energetically lower-lying LUMO level that extends deeper into the backbone. This strengthens the idea that cyclopropane behaves as a quasi-double bond

Structural versus Electrical Functionalization of Oligo(phenylene ethynylene) Diamine Molecular Junctions

We explore both experimentally and theoretically the conductance and packing of molecular junctions based on oligo­(phenyleneethynylene) (OPE) diamine wires, when a series of functional groups are incorporated into the wires. Using the scanning tunnelling microscopy break-junction (STM BJ) technique, we study these compounds in two environments (air and 1,2,4-trichlorobenzene) and explore different starting molecular concentrations. We show that the electrical conductance of the molecular junctions exhibits variations among different compounds, which are significant at standard concentrations but become unimportant when working at a low enough concentration. This shows that the main effect of the functional groups is to affect the packing of the molecular wires, rather than to modify their electrical properties. Our theoretical calculations consistently predict no significant changes in the conductance of the wires due to the electronic structure of the functional groups, although their ability to hinder ring rotations within the OPE backbone can lead to higher conductances at higher packing densities

Centimeter-Scale Synthesis of Ultrathin Layered MoO₃ by van der Waals Epitaxy

We report on the large-scale synthesis of highly oriented ultrathin MoO₃ layers using a simple and low-cost atmospheric pressure, van der Waals epitaxy growth on muscovite mica substrates. By this method, we are able to synthesize high quality centimeter-scale MoO₃ crystals with thicknesses ranging from 1.4 nm (two layers) up to a few nanometers. The crystals can be easily transferred to an arbitrary substrate (such as SiO₂) by a deterministic transfer method and be extensively characterized to demonstrate the high quality of the resulting crystal. We also study the electronic band structure of the material by density functional calculations. Interestingly, the calculations demonstrate that bulk MoO₃ has a rather weak electronic interlayer interaction, and thus, it presents a monolayer-like band structure. Finally, we demonstrate the potential of this synthesis method for optoelectronic applications by fabricating large-area field-effect devices (10 μm × 110 μm in lateral dimensions) and find responsivities of 30 mA W^–1 for a laser power density of 13 mW cm^–2 in the UV region of the spectrum and also as an electron acceptor in a MoS₂-based field-effect transistor