Kondo effect of cobalt adatoms on nanostructured Cu-O surfaces: Scanning tunneling spectroscopy experiments and first-principles calculations (original) (raw)
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Kondo-effect of substitutional cobalt impurities at copper surfaces
New Journal of Physics, 2009
The influence of the coordination on the Kondo temperature of a magnetic impurity at a noble metal surface and the line shape observed in low temperature scanning tunneling spectroscopy (STS) is investigated for single cobalt atoms adsorbed on and embedded in copper surfaces. Surprisingly, the Kondo temperature for substitutional cobalt atoms is almost the same as that of adatoms on the Cu(100) surface. This is in stark contrast to the behaviour observed at the Cu(111) surface. DFT calculations reveal that in the case of Cu(100) the coupling of the spin of the cobalt atom to the conduction band is not substantially increased by the incorporation of the cobalt atom. At the same time the observed line shape differs strongly from what is observed on adatom systems.
Pinning of Adsorbed Cobalt Atoms by Defects Embedded in the Copper (111) Surface
Physical Review Letters, 2010
Using a low-temperature scanning tunneling microscope (STM), we observe that Co adatoms are unusually strongly bound to a particular type of pinning centers on the Cu(111) surface. Using densityfunctional-theory calculations, the pinning centers are identified as Ag substitutional atoms embedded in the topmost atomic layer of the surface. These impurities are hardly detectable in the STM images as they have low topographic height and produce no standing-wave patterns. They do not affect the exchange coupling of the Co adsorbate with the substrate electrons; thus, the Kondo resonances measured on pinned and free Co adatoms show no detectable differences. Whereas free Co adatoms undergo significant surface diffusion already above 8 K, Ag-stabilized Co adatoms remain pinned up to 12.7 K.
CuO Surfaces and CO 2 Activation: A Dispersion-Corrected DFT+U Study
We have used computational methodology based on the density functional theory to describe both copper(I) and copper(II) oxides, followed by the investigation of a number of different low index CuO surfaces. Different magnetic orderings of all the surfaces were studied, and reconstructions of the polar surfaces are proposed. A detailed discussion on stabilities, electronic structure, and magnetic properties is presented. CuO(111) and CuO(111) were found to have the lowest surface energies, and their planes dominate in the calculated Wulff morphology of the CuO crystal. We next investigated the adsorption of CO 2 on the three most exposed CuO surfaces, viz., (111), (111), and (011), by exploring various adsorption sites and configurations. We show that the CO 2 molecule is activated on the CuO surfaces, with an adsorption energy of −93 kJ/mol on the (011) surface, showing exothermic adsorption, while (111) and (111) surfaces show comparatively weak adsorption. The activation of the CO 2 molecule is characterized by large structural transformations and significant charge transfer, i.e., forming a negatively charged bent CO 2 −δ species with elongated C−O bonds, which is further confirmed by vibrational analyses showing considerable red shift in the frequencies as a result of the activation.
Physical Review Letters, 2004
A nearly-free-electron (NFE) model to describe STM spectroscopy of (111) metal surfaces with Kondo impurities is presented. Surface states are found to play an important role giving a larger contribution to the conductance in the case of Cu(111) and Au(111) than Ag(111) surfaces. This difference arises from the farther extension of the Ag(111) surface state into the substrate. The different line shapes observed when Co is adsorbed on different substrates can be explained from the position of the surface band onset relative to the Fermi energy. The lateral dependence of the line shape amplitude is found to be bulk-like for R || < ∼ 4Å and surface-like at larger distances, in agreement with experimental data.
Kondo versus magnetic coupling of cobalt dimers in a Cu–O (2 × 1) reconstruction
Journal of Physics: Condensed Matter, 2010
Co atoms and dimers embedded in a Cu(110)(2 × 1)-O surface oxide are investigated via low-temperature STM/STS experiments and first-principles simulations. It is found that Co dimers incorporated into adjacent rows of the Cu(110)(2 × 1)-O reconstruction show Kondo resonances decoupled from each other, whereas they are antiferromagnetically coupled (and do not exhibit a Kondo effect) when they are aligned on the same row. This shows that it is possible to decouple single carriers of the Kondo effect via a proper choice of the adsorption and host geometry. It is also shown that an oxidic environment presents features remarkably different from those of pure metal substrates.
Selective molecular adsorption in sub-nanometer cages of a Cu2O surface oxide
Physical Chemistry Chemical Physics, 2013
In this study the identity of diverse adsorption sites on a 5-7 Cu 2 O/Cu(111) surface oxide structure has been identified. The 5-7 membered rings formed by a topological defect on stoichiometric Cu 2 O present different electronic structures from the originating hexagonal rings, as shown by combined bias dependent scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The adsorption of CO as a probe molecule on the 5-7 structure, studied using infrared reflection-absorption spectroscopy (IRRAS), shows the existence of special adsorption sites. By combining experimental and theoretical results, it is determined that CO molecules can be selectively confined inside the 7-membered oxide rings with internal dimensions of B0.85 nm, leading to a marked different adsorbate-substrate interaction than in either clean Cu(111) or Cu 2 O. The implication of these newly discovered sites on the chemistry of copper for catalytic reactions is discussed. a 1.134 Å for gas-phase CO bond length. b Cu (CUS) denotes the coordinateunsaturated-site of Cu on the Cu 2 O(111) surface.
STM and DFT studies of CO2 adsorption on O-Cu(100) surface
Surface Science, 2019
We characterized CO2 adsorption and diffusion on the missing row reconstructed (2√2 x √2) R45° Cu(100)-O surface using a combination of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations with dispersion. We deposited CO2 molecules in situ at 5K, which allowed us to unambiguously identify individual CO2 molecules and their adsorption sites. Based on a comparison of experimental and DFT-generated STM images, we find that the CO2 molecules sit in between the O atoms in the missing row reconstructed Cu(100)-O surface. The CO2 molecules are easily perturbed by the STM tip under typical imaging conditions, suggesting that the molecules are weakly bound to the surface. The calculated adsorption energy, vibrational modes, and diffusion barriers of the CO2 molecules also indicate weak adsorption, in qualitative agreement with the experiments. A comparison of tunneling spectroscopy and DFT-calculated density of states shows that the primary change near the Fermi level is associated with changes to the surface states with negligible contribution from the CO2 molecular states. † Steven J. Tjung and Qiang Zhang contributed equally to this work.
Oxygen adsorption and stability of surface oxides on Cu(111): A first-principles investigation
Physical Review B, 2006
As a first step towards gaining microscopic understanding of copper-based catalysts, e.g., for the lowtemperature water-gas shift reaction and methanol oxidation reactions, we present density-functional theory calculations investigating the chemisorption of oxygen, and the stability of surface oxides on Cu͑111͒. We report atomic geometries, binding energies, and electronic properties for a wide range of oxygen coverages, in addition to the properties of bulk copper oxide. Through calculation of the Gibbs free energy, taking into account the temperature and pressure via the oxygen chemical potential, we obtain the ͑p , T͒ phase diagram of O/Cu͑111͒. Our results show that for the conditions typical of technical catalysis the bulk oxide is thermodynamically most stable. If, however, formation of this fully oxidized surface is prevented due to a kinetic hindering, a thin surface-oxide structure is found to be energetically preferred compared to chemisorbed oxygen on the surface, even at very low coverage. Similarly to the late 4d transition metals ͑Ru, Rh, Pd, Ag͒, sub-surface oxygen is found to be energetically unfavorable.