No abstract

This study shows evidence of nanoscale dehydrogenation occurring during tip-enhanced Raman spectroscopy (TERS) measurements. The near-field TERS spectra obtained locally on a self-assembled monolayer of 2,13-bis(aldehyde)-[7]thiaheterohelicene molecules showed vibrational frequencies in good agreement with that predicted by density functional theory calculations, except for the L-mode at ∼2000 cm −1 , which is ascribed to a CC triple bond, implying that the benzene rings had been dehydrogenated during the experiments. Using peak analysis, incorporated with the molecular adsorption orientation deduced from high-resolution STM imaging, we conclude that one side benzene ring protruding upward was dehydrogenated as a result of pyrolysis, with the Ag tip serving as both local heat source and catalyst.

No abstract

Adsorption of the M-type enantiomer of 2,13bis(hydroxymethyl) [7]-thiaheterohelicene molecules deposited on Au(111) was investigated by scanning tunneling microscopy (STM) and current imaging tunneling spectroscopy (CITS). We observed preferential adsorption at step edges, fcc regions, and fcc elbow regions of the herringbone reconstruction at a low coverage after molecular deposition onto a substrate maintained at 150 ± 20 K. Surprisingly, after molecular deposition onto a substrate maintained at 355 ± 20 K, highly ordered islands exhibiting molecular selfassembly surrounded by disordered areas were observed. The islands consist of self-assembled zigzagged twin rows with the preferential direction tilted clockwise and anticlockwise from the ⟨112̅ ⟩ direction of the Au(111) substrate by an angle of 12 ± 2°. A detailed statistical analysis of the twin rows indicated that the unit cell lattice vectors a and b have lengths of 2.4 and 4.4 nm, respectively, with an angle between them of 77°. We postulate an adsorption model without hydrogen bond formation between the hydroxyl groups of neighboring molecules or a strong π−π stacking interaction. We emphasize the advantages of this molecular system investigated for STM-induced luminescence experiments in the near future.

We report on van der Waals epitaxy of two-dimensional (2D) molybdenum trioxide (MoO3−x ) with monolayer thickness directly grown on highly oriented pyrolytic graphite by thermal evaporation under ultrahigh vacuum. The chemical composition, electronic and crystalline lattice structures of the mono-and few-layer MoO3−x sheets are analysed. Using scanning tunnelling microscopy and spectroscopy, we investigate the electronic properties of MoO3−x as a function of the number of layers and measure the apparent energy gap to be 0.4 eV for the first three layers of MoO3−x on graphite. We carried out density functional theory calculations to shed light on the mechanism underlying the observed narrow bandgap with oxygen deficiency. Moreover, the air exposure effect on monolayer MoO3−x is investigated confirming that the apparent bandgap closes, and additionally we show the reduction of the work function from 5.7 to 4.7 eV. We prove that it is possible to synthesize the 2D, non-stoichiometric, and electrically conductive MoO3−x .