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

Control of binding of C60 molecules to the substrate by Coulomb blockade

We report on a transition in a monolayer of C₆₀ molecules deposited on a WO₂/W(110) substrate. The transition from a static state, where the molecules are rigidly bound to the surface by a coordination bond, to a state where the molecules are loosely bound to the surface by van der Waals force and rotate continuously, has been studied using scanning tunnelling microscopy (STM). The separation between the molecules and the surface increases by 1.2 Å across the transition. The transition from the static state into the rotating state takes place at 259 K. The energy of the spinning state with respect to the lowest energy state, having a single coordinated bond, can be obtained from the statistics of the molecules switching. The binding energy of the molecule in the spinning state can be easily altered by changing the polarity of the bias voltage applied between the STM tip and the surface. The binding energy decreases by 80 meV when the bias polarity of the sample changes from positive to negative with respect to the tip. The results are consistent with the Coulomb blockade model: when electrons travel from the surface to the C₆₀ molecule, and then to the tip; charge accumulates on the molecule due to the Coulomb blockade. This increases the electrostatic interaction between the molecule's charge and a corresponding image charge generated on the metallic surface.

Surface modification on MoO2+x/Mo(110) induced by a local electric potential

Oxygen adatoms on the MoO_2+x/Mo(110) surface are observed to be removed when a sufficiently large bias is applied between the scanning tunneling microscope tip and the surface. Experimental observations, such as the bias polarity dependence of adatom removal and the observation of an intermediate state, indicate that the adatom penetrates the surface oxide layer. Through the comparison of finite element method simulations with various experimental relationships, the electric field is concluded to be the sole contributor to adatom penetration into the surface oxide layer. The energetic barrier to this process is estimated to be approximately 0.45 eV in magnitude. Furthermore, the resolution of this phenomenon is on the atomic scale: individual adatoms can undergo surface penetration whilst their nearest neighbour adatoms, separated by 5 Å, are unaffected. The mechanism reported here has the advantages of not strongly influencing the substrate and is exceptionally localised, which can be beneficial for the synthesis of single atom devices.

Electronic Structure and Band Gap of Fullerenes on Tungsten Surfaces: Transition from a Semiconductor to a Metal Triggered by Annealing

The understanding and control of molecule-metal interfaces is critical to the performance of molecular electronics and photovoltaics devices. We present a study of the interface between C₆₀ and W, which is a carbide-forming transition metal. The complex solid-state reaction at the interface can be exploited to adjust the electronic properties of the molecule layer. Scanning tunneling microscopy/spectroscopy measurements demonstrate the progression of this reaction from wide band gap (>2.5 eV) to metallic molecular surface during annealing from 300 to 800 K. Differential conduction maps with 10⁴ scanning tunneling spectra are used to quantify the transition in the density of states and the reduction of the band gap during annealing with nanometer spatial resolution. The electronic transition is spatially homogeneous, and the surface band gap can therefore be adjusted by a targeted annealing step. The modified molecules, which we call nanospheres, are quite resistant to ripening and coalescence, unlike any other metallic nanoparticle of the same size. Densely packed C₆₀ and isolated C₆₀ molecules show the same transition in electronic structure, which confirms that the transformation is controlled by the reaction at the C₆₀-W interface. Density functional theory calculations are used to develop possible reaction pathways in agreement with experimentally observed electronic structure modulation. Control of the band gap by the choice of annealing temperature is a unique route to tailoring molecular-layer electronic properties.

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.