Jesper N Andersen

Oxygen Intercalation under Graphene on Ir(111): Energetics, Kinetics, and the Role of Graphene Edges.

Using X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM) we resolve the temperature-, time-, and flake size-dependent intercalation phases of oxygen underneath graphene on Ir(111) formed upon exposure to molecular oxygen. Through the applied pressure of molecular oxygen the atomic oxygen created on the bare Ir terraces is driven underneath graphene flakes. The importance of substrate steps and of the unbinding of graphene flake edges from the substrate for the intercalation is identified. With the use of CO titration to selectively remove oxygen from the bare Ir terraces the energetics of intercalation is uncovered. Cluster decoration techniques are used as an efficient tool to visualize intercalation processes in real space.

The Pd(100)-(root 5 x root 5)R27 degrees-O surface oxide revisited

Combining high-resolution core-level spectroscopy, scanning tunneling microscopy and density-functional theory calculations we reanalyze the Pd(100)-(root5 x root5)R27degrees-O surface oxide phase. We find that the prevalent structural model, a rumpled PdO(001) film suggested by previous low energy electron diffraction (LEED) work [Surf. Sci. 494 (2001) L799], is incompatible with all three employed methods. Instead, we suggest the two-dimensional film to consist of a strained PdO(101) layer on top of Pd(100). LEED intensity calculations show that this model is compatible with the experimental data of Saidy et al

The new ambient-pressure X-ray photoelectron spectroscopy instrument at MAX-lab

The new instrument for ambient-pressure X-ray photoelectron spectroscopy at the Swedish synchrotron radiation facility MAX IV Laboratory is presented. The instrument is based on the use of a retractable and exchangeable high-pressure cell, which implies that ultrahigh-vacuum conditions are retained in the analysis chamber and that dual ambient pressure and ultrahigh-vacuum use is possible.

The Active Phase of Palladium during Methane Oxidation

The active phase of Pd during methane oxidation is a long-standing puzzle, which, if solved, could provide routes for design of improved catalysts. Here, density functional theory and in situ surface X-ray diffraction are used to identify and characterize atomic sites yielding high methane conversion. Calculations are performed for methane dissociation over a range of Pd and PdOx surfaces and reveal facile dissociation on either under-coordinated Pd sites in PdO(101) or metallic surfaces. The experiments show unambiguously that high methane conversion requires sufficiently thick PdO(101) films or metallic Pd, in full agreement with the calculations. The established link between high activity and atomic structure enables rational design of improved catalysts.

Surface oxides on close-packed surfaces of late transition metals

In recent years, the formation of thin, well-ordered but complex surface oxides on late transition metals has been discovered. The driving force for this line of research has been the strong incentive to increase the partial pressure of oxygen from ultra-high vacuum to conditions more relevant for heterogeneous catalysis. Here we review the present status of the research field. Compared to oxygen adatom superstructures, the structure of the surface oxides has proven to be extremely complex, and the investigations have therefore relied on a combination of several experimental and theoretical techniques. The approach to solving the structures formed on close-packed surfaces of Pd and Rh is presented in some detail. Focusing on the structures found, we show that the surface oxides share some general properties with the corresponding bulk oxides. Nevertheless, of all surface oxide structures known today, only the two-dimensional surface oxides on Pd(100) and Pt(110) have the same lattice as the bulk oxides (PdO and PtO, respectively). In addition to two-dimensional oxides, including the O-Rh-O trilayers found on Rh, one-dimensional oxides were observed at ridges or steps of open surfaces such as (110) or vicinal surfaces. Finally, we briefly report on a few studies of the reactivity of surface oxides with well-known structure.

Low-Temperature CO Oxidation on Ni(111) and on a Au/Ni(111) Surface Alloy

From an interplay between scanning tunneling microscopy, temperature programmed desorption, X-ray photoelectron spectroscopy, and density functional theory calculations we have studied low-temperature CO oxidation on Au/Ni(111) surface alloys and on Ni(111). We show that an oxide is formed on both the Ni(111) and the Au/Ni(111) surfaces when oxygen is dosed at 100 K, and that CO can be oxidized at 100 K on both of these surfaces in the presence of weakly bound oxygen. We suggest that low-temperature CO oxidation can be rationalized by CO oxidation on O(2)-saturated NiO(111) surfaces, and show that the main effect of Au in the Au/Ni(111) surface alloy is to block the formation of carbonate and thereby increase the low-temperature CO(2) production.

Photoemission electron microscopy using extreme ultraviolet attosecond pulse trains

We report the first experiments carried out on a new imaging setup, which combines the high spatial resolution of a photoemission electron microscope (PEEM) with the temporal resolution of extreme ultraviolet (XUV) attosecond pulse trains. The very short pulses were provided by high-harmonic generation and used to illuminate lithographic structures and Au nanoparticles, which, in turn, were imaged with a PEEM resolving features below 300 nm. We argue that the spatial resolution is limited by the lack of electron energy filtering in this particular demonstration experiment. Problems with extensive space charge effects, which can occur due to the low probe pulse repetition rate and extremely short duration, are solved by reducing peak intensity while maintaining a sufficient average intensity to allow imaging. Finally, a powerful femtosecond infrared (IR) beam was combined with the XUV beam in a pump-probe setup where delays could be varied from subfemtoseconds to picoseconds. The IR pump beam could induce multiphoton electron emission in resonant features on the surface. The interaction between the electrons emitted by the pump and probe pulses could be observed.

Mechanism of CO oxidation reaction on O-covered Pd(111) surfaces studied with fast x-ray photoelectron spectroscopy: change of reaction path accompanying phase transition of O domains.

We studied the mechanism of CO oxidation on O-precovered Pd(111) surfaces by means of fast x-ray photoelectron spectroscopy (XPS). The oxygen overlayer is compressed upon CO coadsorption from a p(2 x 2) structure into a (square root(3) x square root(3))R30 degrees structure and then into a p(2 x 1) structure with increasing CO coverage. These three O phases exhibit distinctly different reactivities. (1) The p(2 x 2) phase does not react with CO unless the surface temperature is sufficiently high (<290 K). (2) In the square root(3) x square root(3))R30 degrees phase, the reaction occurs exclusively at island peripheries. CO molecules in a high-density phase formed under CO exposure react with oxygen atoms, leading to quite a small apparent activation energy. (3) The reaction proceeds uniformly over the islands in the p(2 x 1) phase.

Tuning the spin state of iron phthalocyanine by ligand adsorption

The future use of single-molecule magnets in applications will require the ability to control and manipulate the spin state and magnetization of the magnets by external means. There are different approaches to this control, one being the modification of the magnets by adsorption of small ligand molecules. In this paper we use iron phthalocyanine supported by an Au(111) surface as a model compound and demonstrate, using x-ray photoelectron spectroscopy and density functional theory, that the spin state of the molecule can be tuned to different values (S ∼ 0, [Formula: see text], 1) by adsorption of ammonia, pyridine, carbon monoxide or nitric oxide on the iron ion. The interaction also leads to electronic decoupling of the iron phthalocyanine from the Au(111) support.

On the origin of the Ru-3d(5/2) satellite feature from RuO2(110)

High resolution core level spectroscopy in combination with density functional theory calculations are used to study the satellite feature of the Ru-3d(5/2), core level spectrum whose interpretation is still a matter of debate. We present evidence that the satellite peak is not related to any structural properties of the RuO2(1 1 0) surface. The binding energy shift between the Ru-3d(5/2) component and the satellite peak is close to the electron energy loss due to plasmon excitation. We propose therefore that the satellite peak is due to excitation of the RuO2 plasmon