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Alkali atoms are known to promote or poison surface catalytic chemistry. To explore alkali promotion of catalysis and to characterize discharge species in alkali-oxygen batteries, we examine coadsorption of K and O 2 on Au(111) surface at the atomic scale by scanning tunneling microscopy (STM) and density functional theory (DFT). On a clean Au(111) surface, O 2 molecules may weakly physisorb, but when Au(111) is decorated with K + ions, they chemisorb into structures that depend on the adsorbate concentrations and substrate templating. At low K coverages, an ordered quantum lattice of K 2 O 2 complexes forms through intramolecular attractive and intermolecule repulsive interactions. For higher K and O 2 coverages, the K 2 O 2 complexes condense first into triangular islands, which further coalesce into rhombohedral islands, and ultimately into incommensurate films. No structures display internal contrast possibly because of high structural mutability. DFT calculations explain the alkali-promoted coadsorption in terms of three center, cation-π interactions where pairs of K + coordinate the π-orbitals on each side of O 2 molecules, and in addition O 2 forms a covalent bond to Au(111) surface. The K promoted adsorption of O 2 is catalyzed by charge transfer from K atoms to Au(111) substrate and ultimately to O 2 molecules, forming O 2 -δ in a redox state between the peroxo and superoxo. Tunneling d I/d V spectra of K 2 O 2 complexes exhibit inordinately intense inelastic progression involving excitation of the O-O stretching vibration, but absence of a Kondo effect suggests that the magnetic moment of O 2 is quenched.

Hot electron processes at metallic heterojunctions are central to optical-to-chemical or electrical energy transduction. Ultrafast nonlinear photoexcitation of graphite (Gr) has been shown to create hot thermalized electrons at temperatures corresponding to the solar photosphere in less than 25 fs. Plasmonic resonances in metallic nanoparticles are also known to efficiently generate hot electrons. Here we deposit Ag nanoclusters (NC) on Gr to study the ultrafast hot electron generation and dynamics in their plasmonic heterojunctions by means of time-resolved two-photon photoemission (2PP) spectroscopy. By tuning the wavelength of p-polarized femtosecond excitation pulses, we find an enhancement of 2PP yields by 2 orders of magnitude, which we attribute to excitation of a surface-normal Mie plasmon mode of Ag/Gr heterojunctions at 3.6 eV. The 2PP spectra include contributions from (i) coherent two-photon absorption of an occupied interface state (IFS) 0.2 eV below the Fermi level, which electronic structure calculations assign to chemisorption-induced charge transfer, and (ii) hot electrons in the π*-band of Gr, which are excited through the coherent screening response of the substrate. Ultrafast pump-probe measurements show that the IFS photoemission occurs via virtual intermediate states, whereas the characteristic lifetimes attribute the hot electrons to population of the π*-band of Gr via the plasmon dephasing. Our study directly probes the mechanisms for enhanced hot electron generation and decay in a model plasmonic heterojunction.

The Kondo effect, a widely studied phenomenon in which the scattering of conduction electrons by magnetic impurities increases as the temperature T is lowered, depends strongly on the density of states at the Fermi energy. It has been predicted by theory that magnetic impurities on free-standing monolayer graphene exhibit the Kondo effect and that control of the density of states at the Fermi level by external means can be used to switch the effect on and off. However, though transport data for Co adatoms on graphene monolayers on several substrates have been reported, there exists no evidence for a Kondo effect. Here we probe the role of the substrate on the Kondo effect of Co on graphene by combining low-temperature scanning tunneling microscopy and spectroscopy measurements with density functional theory calculations. We use a Ru(0001) substrate that is known to cause graphene to ripple, yielding a moiré superlattice. The experimental data show a sharp Kondo resonance peak near the Fermi energy from only Co adatoms at the edge of atop regions of the moiré pattern. The theoretical results show that the variation of the distance from the graphene to the Ru substrate, which controls the spin polarization and local density of states at the Fermi energy, is the key factor for the appearance of the Kondo resonance. The results suggest that rippling of graphene by suitable substrates is an additional lever for tuning and selectively switching the appearance of the Kondo effect.