Nanoclusters and nanolines: the effect of molybdenum oxide substrate stoichiometry on iron self-assembly

The growth of Fe nanostructures on the stoichiometric MoO₂/Mo(110) and oxygen-rich MoO₂+x /Mo(110) surfaces has been studied using low-temperature scanning tunnelling microscopy (STM) and density functional theory calculations. STM results indicate that at low coverage Fe nucleates on the MoO₂/Mo(110) surface, forming small, well-ordered nanoclusters of uniform size, each consisting of five Fe atoms. These five-atom clusters can agglomerate into larger nanostructures reflecting the substrate geometry, but they retain their individual character within the structure. Linear Fe nanocluster arrays are formed on the MoO₂/Mo(110) surface at room temperature when the surface coverage is greater than 0.6 monolayers. These nanocluster arrays follow the direction of the oxide rows of the strained MoO₂/Mo(110) surface. Slightly altering the preparation procedure of MoO₂/Mo(110) leads to the presence of oxygen adatoms on this surface. Fe deposition onto the oxygen-rich MoO₂+x /Mo(110) surface results in elongated nanostructures that reach up to 24 nm in length. These nanolines have a zigzag shape and are likely composed of partially oxidised Fe formed upon reaction with the oxygen-rich surface

Homolytic cleavage of molecular oxygen by manganese porphyrins supported on Ag(111).

Oxygen binding and cleavage are important for both molecular recognition and catalysis. Mn-based porphyrins in particular are used as catalysts for the epoxidation of alkenes, and in this study the homolytic cleavage of O2 by a surface-supported monolayer of Mn porphyrins on Ag(111) is demonstrated by scanning tunneling microscopy, X-ray absorption, and X-ray photoemission. As deposited, {5,10,15,20-tetraphenylporphyrinato}Mn(III)Cl (MnClTPP) adopts a saddle conformation with the average plane of its macrocycle parallel to the substrate and the axial Cl ligand pointing upward, away from the substrate. The adsorption of MnClTPP on Ag(111) is accompanied by a reduction of the Mn oxidation state from Mn(III) to Mn(II) due to charge transfer between the substrate and the molecule. Annealing the Mn(II)ClTPP monolayer up to 510 K causes the chlorine ligands to desorb from the porphyrins while leaving the monolayer intact. The Mn(II)TPP is stabilized by the surface acting as an axial ligand for the metal center. Exposure of the Mn(II)TPP/Ag(111) system to molecular oxygen results in the dissociation of O2 and forms pairs of Mn(III)OTPP molecules on the surface. Annealing at 445 K reduces the Mn(III)OTPP complex back to Mn(II)TPP/Ag(111). The activation energies for Cl and O removal were found to be 0.35 ± 0.02 eV and 0.26 ± 0.03 eV, respectively

Correlation between charge-transfer and rotation of C60 on WO2/W(110).

Understanding molecular switching between different charge states is crucial to further progress in molecule-based nano-electronic devices. Herein we have employed scanning tunnelling microscopy to visualize different charge states of a single C60 molecule within a molecular layer grown on the WO2/W(110) surface. The results obtained demonstrate that individual C60 molecules within the layer switch between neutral and negatively charged states in the temperature range of 220-260 K over the time scale of the experiment. The charging of the C60 causes changes in the local density of electron states and consequently a variation in tunnelling current. Using density functional theory calculations, it was found that the charged state corresponds to the negatively charged C60(-), which has accepted an electron. The switching of the molecule into the charged state is triggered continuously by tunnelling electrons when the STM tip is static above an individual C60 molecule with a bias applied. Molecular movement accompanies the molecule's switching between these states