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Argon ion kinetic energy spectra at different discharge voltages (between 480 and 600 V) of a commercial cold cathode ion source IQP10/63 are reported. The high kinetic energy cut-off depends on the discharge voltage and the corresponding plasma potential due to excess positive charges which is found to be about 136 V. Exposure of single layer hexagonal boron nitride on rhodium to the beam of the ion source leads to the formation of nanotents, i.e., stable atomic protrusions. A positive bias voltage is applied to the target sample to block the positive ions produced by the ion source. However, application of a positive bias potential (800 eV), which is higher than the kinetic energy cut-off, still allows the formation of nanotents and its observation with scanning tunneling microscopy. This indicates that the ion source also produces neutral atoms with kinetic energies higher than the penetration threshold across a single layer of hexagonal boron nitride.

Ice nuclei have been studied on the hexagonal boron nitride nanomesh (h-BN/Rh(111)), a template with 2 nm wide molecule traps. Scanning tunneling microscopy shows confined clusters, where oligomers with three protrusions are particularly abundant. Together with local barrier height dI/dz maps, it is found that the dipoles of the water molecules arrange in a homodrome, which is consistent with density functional theory calculations. Hydrogen bonds toward the substrate identify h-BN/Rh(111) to be hydrophilic. The substrate distorts the hexamers (n = 6) and possibly pentamers (n = 5), where the experimentally observed footprints of the three protrusions appear more chiral than expected.

The magnetism of DySc(2)N@C(80) endofullerene was studied with X-ray magnetic circular dichroism (XMCD) and a magnetometer with a superconducting quantum interference device (SQUID) down to temperatures of 2 K and in fields up to 7 T. XMCD shows hysteresis of the 4f spin and orbital moment in Dy(III) ions. SQUID magnetometry indicates hysteresis below 6 K, while thermal and nonthermal relaxation is observed. Dilution of DySc(2)N@C(80) samples with C(60) increases the zero-field 4f electron relaxation time at 2 K to several hours.

The transport properties of graphene change strongly in the presence of electric fields due to graphene's band structure. This makes graphene sensitive to charges in an insulator substrate. Graphene on SiO2/Si is studied under x-ray irradiation in ambient conditions. Using the metal oxide semiconductor structure of their samples, the authors observe remote doping due to the creation of positive charges in the oxide by the irradiation and relate them to resistance and Hall effect measurements performed on the graphene gate. The observed changes in conductivity, Hall charge carrier density, and the corresponding charge carrier mobility are consistent with expectations as well as recent experiments using graphene field effect transistors under ultrahigh vacuum conditions [P. Prochazka et al. Sci. Rep. 7, 563 (2017)]. Furthermore, the stability of the effect under ambient conditions and its recovery using thermal annealing is demonstrated.

The structure of epitaxially grown hexagonal boron nitride (h-BN) on the surface of a Ru(0 0 0 1) single crystal was investigated using surface X-ray diffraction, which showed the system to form a commensurate 14-on-13 superstructure. This result disagrees with previous reports on superstructures of the same system and arguments based on simple thermal expansion coefficient calculations. We argue that the larger observed superstructure forms because of the strong bonding of h-BN to Ru. In comparison to h-BN/Rh(1 1 1) it can accommodate more induced lateral in-plane strain- or lock-in energy over larger regions (referred to as the holes) within the superstructure, which itself can consequently become larger.

The unusual adsorption geometry of the large graphene-like polycyclic aromatic hydrocarbon hexa-peri-hexabenzocoronene (HBC) across kinks on the stepped vicinal Au(11 12 12) surface has been studied by synchrotron radiation X-ray photoelectron diffraction (XPD) and scanning tunneling microscopy (STM). By a combination of the two techniques a complete characterisation of the adsorption behaviour is achieved, yielding information on preferential adsorption sites (STM) and molecular orientation (XPD). At low coverage (

Single sheet boron nitride layers on thin rhodium (111) films were formed upon thermal decomposition of borazine under ultra-high vacuum (UHV) conditions. They were transferred and investigated in an electrolytic environment. In 0.1 M HClO4, the presence and stability of the so-called nanomesh super structure with a lattice constant of 3.2 nm is established with electrochemical impedance spectroscopy, cyclic voltammetry and subsequent imaging with in situ scanning tunneling microscopy (STM) under potential control. In the electrolyte, the BN nanomesh acts as a dielectric layer with an unusual behaviour of the impedance, where the capacitive component is larger than on a Rh(111) reference sample. It exhibits reversible hydrogen adsorption and desorption at a potential of −600 mV vs. a saturated mercury sulphate reference electrode (MSE). The unit cell of the nanomesh is imaged by STM and shows hexagonally arranged two-dimensional pores with a diameter of 2 nm. At a fixed potential, the nanomesh was stable for long time, but after repeated potential sweeps between +260 and −540 mV vs. MSE, STM indicates roughening, though a 12 × 12 superstructure was recovered after annealing in UHV. Keywords: Electrolytic STM, Impedance spectroscopy, Boron nitride, Nanomesh, Template

The hexagonal boron-nitride ‘nanomesh’ surface reconstruction on Rh(1 1 1) [Corso et al., Science 303 (2004) 217–220] has been investigated using surface X-ray diffraction utilizing synchrotron radiation. This unique structure has been found to be stable under ambient atmosphere which provides an important basis for technological applications like templating and coating. The previously suggested (12 × 12) periodicity of this reconstruction has been unambiguously confirmed and structural features are discussed in the light of the X-ray diffraction results.

High-temperature exposure of a Mo(110) surface to borazine (HBNH)3leads to the formation of two distinctly different self-assembling nanostructures. Depending on the substrate temperature during preparation, either well-aligned, ultra-thin boron nanowires or a single-layer stripe structure of hexagonal boron nitride forms. Both structures show one-dimensional (1D) characteristics, but in directions perpendicular to each other. It is also possible to grow the two phases in coexistence. The relative weights are controlled by the sample temperature during preparation.

When a gecko moves on a ceiling it makes use of adhesion and stiction. Stiction--static friction--is experienced on microscopic and macroscopic scales and is related to adhesion and sliding friction. Although important for most locomotive processes, the concepts of adhesion, stiction and sliding friction are often only empirically correlated. A more detailed understanding of these concepts will, for example, help to improve the design of increasingly smaller devices such as micro- and nanoelectromechanical switches. Here we show how stiction and adhesion are related for a liquid drop on a hexagonal boron nitride monolayer on rhodium, by measuring dynamic contact angles in two distinct states of the solid-liquid interface: a corrugated state in the absence of hydrogen intercalation and an intercalation-induced flat state. Stiction and adhesion can be reversibly switched by applying different electrochemical potentials to the sample, causing atomic hydrogen to be intercalated or not. We ascribe the change in adhesion to a change in lateral electric field of in-plane two-nanometre dipole rings, because it cannot be explained by the change in surface roughness known from the Wenzel model. Although the change in adhesion can be calculated for the system we study, it is not yet possible to determine the stiction at such a solid-liquid interface using ab initio methods. The inorganic hybrid of hexagonal boron nitride and rhodium is very stable and represents a new class of switchable surfaces with the potential for application in the study of adhesion, friction and lubrication.

Exposing a monolayer of graphene on ruthenium ( g /Ru(0001)) to low energy Ar + ions leads to nanotent formation and “can-opener” effect, similar phenomena as observed for h -BN/Rh(111) targets (Cun, Iannuzzi, Hemmi, Roth, Osterwalder and Greber, 2013) [1] . Nanotents are extra protrusions in the sp 2 monolayers beneath which atoms are immobilized at room temperature. Annealing the Ar + implanted structures results in the “can-opener” effect, i.e., the formation of voids with a diameter of about 2 nm within the graphene layer. The voids preferentially settle in the “hill” regions of the g /Ru(0001) superstructure and thus display spacial selectivity. This provides a convenient method to control defect positions within graphene membranes with nanometer precision. The results are obtained by scanning tunneling microscopy, low energy electron diffraction and photoemission, and are backed with density functional theory calculations.

The structure of hexagonal boron nitride (h-BN) on Pd(1 1 1) was studied with low energy electron diffraction (LEED), photoelectron spectroscopy and scanning tunnelling microscopy (STM). h-BN forms flat monolayers on the Pd(1 1 1) surface in contrast to Rh(1 1 1) where a complex self-assembled double layer structure, the nanomesh (M. Corso, W. Auwarter, M. Muntwiler, A. Tamai, T. Greber, J. Osterwalder, Science 303 (2004) 217), appears. The LEED patterns reveal a dominating 10 · 10 h-BN superstructure, with a second, distinct structure rotated by 30� and further azimuthally randomly oriented h-BN overlayers. This is consistent with STM images which show several different Moirepatterns associated with different rotation angles of the overlayer. Additionally the use of thin Pd(1 1 1) films instead of single crystal substrates was studied. No significant differences in the h-BN film quality were found. � 2006 Elsevier B.V. All rights reserved.

A new procedure is demonstrated for cross-correlating light and electron pulses in a time-resolved surface diffraction experiment using low-energy electrons: Ultrashort laser pulses produce a space charge that modulates the transmission of the electron pulses across a pinhole. The proposed method is easily implemented into present pump-probe setups and represents a simple means for establishing spatial and temporal overlap of light and electron pulses on the sample. Moreover, the ultrafast space-charge dynamics allow the temporal resolution of the electron pulses to be characterized down to a time scale of less than 20 ps.

Hexagonal boron nitride was grown on nickel (110) by chemical vapor deposition of borazine. The h-BN layers on Ni(110) were investigated by low energy electron diffraction (LEED) and angular resolved photoemission (ARUPS). LEED shows a (1 × 6) and a (7 × 5) super structure. ARUPS from the σ bands of h-BN indicates single layer h-BN, but two different orientations of the h-BN Brillouin zones with a relative weight of 6:1. This is in line with lock in energy arguments, which consider coincidence lattices between nickel top atoms and nitrogen atoms. [DOI:10.1380/ejssnt.2006.410]

A one-monolayer thick covalent network of hexagonal boron nitride (h-BN) forms on Pd(1 1 0) upon thermal decomposition of borazine (HBNH)3. Due to the weak interfacial bonding, the different symmetries of overlayer and substrate, and the lattice mismatch, a variety of rotated domains are created. Several Moire patterns appear in STM images, which can be well explained by simple atomic models. Rings in LEED manifest domain averaging. Non-uniform intensity distributions along these rings measure the abundance of certain domains, depending on particular preparation procedures. The individual Moire domains represent interesting periodic nanostructures that can be imprinted in an adsorbed layer of C60 molecules.

We used angle-resolved photoemission spectroscopy to measure the electronic dispersion and single-particle spectral function in a liquid metal. A lead monolayer supported on a copper (111) surface was investigated as the temperature was raised through the melting transition of the film. Electron spectra and momentum distribution maps of the liquid film revealed three key features of the electronic structure of liquids: the persistence of a Fermi surface, the filling of band gaps, and the localization of the wave functions upon melting. Distinct coherence lengths for different sheets of the Fermi surface were found, indicating a strong dependence of the localization lengths on the character of the constituent atomic wave functions.

On Cu(2 2 1) it is possible to grow long ordered one-dimensional chains of C 60 molecules aligned along the steps of this vicinal template. By scanning tunneling microscopy (STM) a unit cell with two distinctly imaged molecules is found. From distribution of the rare defects a bond energy difference of the two types of molecules is inferred. At least in one of the two species, the molecules have a well-defined orientation as determined by X-ray photoelectron diffraction (XPD).

The spin character of the splitting of the Shockley surface state on Au(111) is directly verified by measurements of the in-plane and out-of-plane spin polarizations in angle-resolved photoemission spectra. The two parabolic sub-bands that are momentum-shifted with respect to each other, reveal a distinct, opposite spin polarization that within the errors lies in the surface plane. The measured in-plane orientation of the spin vectors is consistent with the simple spin structure expected from a nearly-free-electron model, where the polarization axis is tangential to the Fermi surface of the surface state.

Total energy calculations were performed for nitrous oxide and its anion N2O- by Multi Configuration Self-Consistent Field−Configuration Interaction (MCSCF−CI) methods. The shape of the lowest unoccupied molecular orbital (LUMO) and the energy gap between the highest occupied molecular orbital (HOMO) are evaluated for different N2−O distances and bending angles. The vertical electron affinities for the ground state and the first excited bending mode were found to be −2.1 eV, separated by about 50 meV. On the 2A‘ potential energy surface (PES) of negatively charged N2O the dynamics for the reaction after charge transfer on the ground state and thermally excited N2O has been studied by wave packet calculations. The molecules dissociate within 30 fs and strong rotational excitation of the N2 fragment is found. To explain the strong vibrational state selectivity we propose diabatic transitions between the 2A‘ and the 2A‘‘ PES to interrupt the dissociation of N2O- selectively. Implications for the dynamics of ...

The growth of hexagonal boron nitride (h-BN) on Ni(1 1 1) at submonolayer coverages results in triangular islands with two different orientations. In combining X-ray photoelectron diffraction and scanning tunneling microscopy it is demonstrated that these two island types correspond to fcc and hcp domains of h-BN/Ni(1 1 1). The domain boundaries appear as defect lines that are also present in complete h-BN monolayers. Based on the experimental data, structural models for the two domains and the defect lines are discussed. � 2003 Elsevier B.V. All rights reserved.

Cell attachment and spreading on solid surfaces was investigated with a home-made quartz crystal microbalance (QCM), which measures the frequency, the transient decay time constant and the maximal oscillation amplitude. Initial interactions of the adsorbing cells with the QCM mainly induced a decrease of the frequency, coincident with mass adsorption. After about 80 min, the frequency increased continuously and after several hours exceeded the initial frequency measured before cell adsorption. Phase contrast and fluorescence microscopy indicated that the cells were firmly attached to the quartz surface during the frequency increase. The measurements of the maximal oscillation amplitude and the transient decay time constant revealed changes of viscoelastic properties at the QCM surface. An important fraction of these changes was likely due to alterations of cytosolic viscosity, as suggested by treatments of the attached cells with agents affecting the actin and microtubule cytoskeleton. Our results show that viscosity variations of cells can affect the resonance frequency of QCM in the absence of apparent cell desorption. The simultaneous measurements of the maximal oscillation amplitude, the transient decay time constant and the resonance frequency allow an analysis of cell adsorption to solid substratum in real time and complement cell biological methods.

The electronic structure of an incommensurate Pb layer on Cu(1 1 1) is investigated by means of angle scanned ultraviolet photoemission. Despite of the rather complex atomic structure with three competing periodicities, we observe a well defined band structure, reflecting essentially the fundamental (1 1)-periodicity of the adlayer. Comparison with band structure calculations for a free standing Pb-layer shows that all observed bands can be fully attributed to the overlayer. 2003 Elsevier Science B.V. All rights reserved.

Well lattice-matched monolayer films of hexagonal boron nitride (h-BN) can be grown on Ni(111) surfaces, representing a nominally ideal interface for preparing ultimately thin metal-insulator-metal (MIM) structures. In a detailed study, combining local and non-local probes, the presence of characteristic defect lines is uncovered, and a model for their atomic structure is proposed. They have a strong influence on the growth morphologies of metal deposits. For room temperature deposition, they act as anchors for cluster nucleation, thus effectively short circuiting the MIM structure. For high-temperature deposition, the defects collect Co adatoms very efficiently and lead to Co intercalation underneath the h-BN film. [DOI: 10.1380/ejssnt.2003.124]

The magnetic and electronic properties of itinerant ferromagnets and their interplay have been studied in the last few years by spin resolved electron spectroscopy on one hand and by high-resolution angle-resolved photoemission experiments on the other. We discuss how the two approaches can be combined in a high resolution electron spectrometer with spin resolution for angle-scanned Fermi surface mapping experiments. We have built this new instrument, which allows an advance into a deeper understanding of magnetic thin film or multilayer systems, where band structures become intricately dense in momentum space and where the magnetization direction can change from layer to layer. Spin-resolution is thus required to arrive at a correct assignment of spectral features. A fully three-dimensional polarimeter makes the instrument ‘complete’ in the sense that all properties of the photoelectron are measured. First experiments on Ni(111) conclusively confirm previous band and spin assignments at the Fermi level and demonstrate the correct functioning of the apparatus.

The growth of cobalt on a monolayer of hexagonal boron nitride on Ni(1 1 1) has been studied in a combination of scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS). A detailed picture of the growth kinetics is obtained. The Co sticking coefficient strongly decreases at high substrate temperatures. Co deposition results in two different morphologies: (i) three-dimensional clusters that grow on top of the hexagonal boron nitride ( h -BN) film and often align themselves to form chains and (ii) islands with a more two-dimensional character, which are intercalated below the h -BN film dominate at higher substrate temperatures. From XPS measurements at different deposition temperatures an activation energy for intercalation of ∼0.24 eV is inferred. The influence of observed defect lines in the h -BN overlayer on the growth kinetics is discussed.

Nickel (111) and the interface of hexagonal boron nitride on Ni(111) serve as model systems for an itinerant ferromagnet that is truncated with vacuum or a single layer insulator. These systems are investigated with angle-scanned He I and He II photoemission. Upon formation of the h-BN/Ni(111) interface the work function drops by 1.8 eV and the minority Ni Λ3d-band shifts by 0.13 eV to higher binding energy. This indicates that the Ni magnetic moment decreases in the interface. Spin-polarized band structure calculations from bulk nickel identify the observed minority and majority bands. The experimental Fermi surface maps show that h-BN distorts a minority d-band in a way which is consistent with the decreasing magnetic moment. It can be seen that h-BN affects the scattering of the electrons. Effects in the photoemission final and initial state are recognized.

Large scale tetraoctylammonium-assisted electrochemical transfer of graphene grown on single-crystalline Ir(1 1 1) films by chemical vapour deposition is reported. The transferred samples are characterized in air with optical microscopy, Raman spectroscopy and four point transport measurements, providing the sheet resistance and the Hall carrier concentration. In vacuum we apply low energy electron diffraction and photoelectron spectroscopy that indicate transferred large-scale single orientation graphene. Angular resolved photoemission reveals a Fermi surface and a Dirac point energy which are consistent with charge neutral graphene., Journal of Physics: Condensed Matter, 29 (47), ISSN:0953-8984, ISSN:1361-648X

It took a while to discover the first endohedral single molecule magnet. In 2012 it was shown that DySc2N@C80 has, at a temperature of 2 K, magnetic relaxation times longer than 104 s [1]. The Figure depicts an image of this molecule, where the Jz =15/2 groundsate of the Dy 4f9 shell is colored in red. This anisotropic electronic ground state is oriented and stabilized by the strong ligand field, mainly of the central nitrogen ion, which causes the single ion magnetism. After DySc2N@C80 a series of new endofullerene single molecule magnets with a variety of different magnetic ground states were reported. For example, one, two or three encaged Dy ions display tunneling of the magnetisation, long remanence, or a frustrated ground state, which indicates a strong magnetic interaction within the cage [2]. I will discuss x-ray magnetic circular dichroism (XMCD) as a particularly sensitive tool for the detection of the magnetism inside the carbon cages and how these experiments may be complemented with superconducting quantum interference device (SQUID) measurements down to 400 mK temperature. It will furthermore be shown that endohedral units may get preferentially oriented on surfaces [3], and that this proves the carbon cages to be "leaky Faraday cages", with tunable interaction across the carbon shell. References: [1] An Endohedral Single-Molecule Magnet with Long Relaxation Times: DySc2N@C80, Westerström et al. J. Am. Chem. Soc. 134 (2012) 9840. [2] Tunneling, remanence, and frustration in dysprosium-based endohedral single-molecule magnets, Westerström et al. Phys. Rev. B 9 (2014) 060406. [3] Surface Aligned Magnetic Moments and Hysteresis of an Endohedral Single-Molecule Magnet on a Metal, Westerström et al. Phys. Rev. Lett. 114 (2015) 087201. Figure: Model of the first endohedral single molecule magnet: DySc2N@C80 (see: www. spinshuttle.ch, courthesy Ari Seitsonen). Figure 1

The low-temperature magnetic properties of the endohedral metallofullerene HoSc2N@C-80 have been studied by superconducting quantum interference device (SQUID) magnetometry. Alternating current (ac) susceptibility measurements reveal that this molecule exhibits slow relaxation of magnetization in a small applied field with timescales in the order of milliseconds. The equilibrium magnetic properties of HoSc2N@C-80 indicate strong magnetic anisotropy. The large differences in magnetization relaxation times between the present compound and the previously investigated DySc2N@C-80 are discussed.

The paramagnetic NMR study of HoM2N@C-80-I-h and Ho2MN@C-80-I-h nitride cluster fullerenes (M = Sc, Lu, Y) reveals strong dependence of Ho-induced paramagnetic shifts (delta(para)) in C-13 NMR spectra on the size of the diamagnetic metal in the cluster. In particular, the delta(para) value in HoY2N@C-80 is almost doubled in comparison to that in HoSc2N@C-80. X-ray magnetic circular dichroism studies show that all Ho-nitride cluster fullerenes have the same magnetic ground state of Ho3+. Point-charge ligand-field splitting calculations show that the increase of the M3+ radius in going from Sc to Y results in a considerable increase of the energy splitting between different J(z) states. This leads to a 19% higher magnetic anisotropy of Ho3+ in HoY2N@C-80 than in HoSc2N@C-80 at 300 K. Variations of the molecular geometry and cluster dynamics with the size of the cluster are found to have even greater influence on delta(para) values. This work shows that the magnetic properties of the species confined inside the fullerene cages can be tuned using the geometrical factors such as the cluster and the cage size.

We present the design and performance tests of a miniaturized pulsed low-energy electron gun. Electrons photoemitted from a gold cathode are accelerated over a distance of 75 μm and then collimated by a microchannel plate. According to calculations, this novel concept will allow the time spread of the electron pulses to be kept below 5 ps for kinetic energies as low as 100 eV. The achievement of a minimum angular beam divergence (≈1°) along with an energy resolution of 1.1 eV has to be paid for by low signal intensities. We demonstrate the performance of the gun and the high electron-beam coherence by presenting low-energy-electron diffraction images taken from a submonolayer of lead adsorbed on the germanium (111) surface. We anticipate that this electron gun will open up new possibilities for following structural changes on solid surfaces in real time.

We present a comprehensive study on the binding and ordering of C60 on a Pd(110) surface employing scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD). Three well-ordered structures evolving at elevated temperatures were identified, with (∓2 ±5 7 −1), (4×5), (4×8) unit cells and a coverage of 1, 0.82 and 0.77 physical monolayers, respectively. The rearrangement of Pd substrate atoms turns out to play a crucial role in the evolution of thin C60 films. In STM images alternating bright and dark molecular rows are found for the regular structures. This height difference is attributed to a restructuring of the Pd substrate. Whereas the C60 molecules of the bright molecular rows are embedded in one layer deep microscopic pits, C60 accommodated in two layer deep pits account for the dark molecular rows. XPD results complete the understanding of the film structure. They reveal that the orientation of the C60...

Photoelectron diffraction patterns of a large portion of the 2π solid angle above a surface may be used for direct structure determination schemes such as holography. It is shown that the particular experimental set-up of nearnode photoelectron holography greatly improves the quality of holographically obtained three-dimensional images of atomic arrangements. In the near-node set-up the diffraction patterns are recorded at an emission angle near the node of a photoelectron source wave and forward scattering is suppressed. 24 distinct atomic sites were observed for the test case of Al 2s (Ekin = 952 eV) diffraction from Al(111). The physics of photoelectron source waves is recapitulated and the holographic inversion procedure that is based on the multipole expansion is outlined. (Some figures in this article are in colour only in the electronic version)

The orientation and the intramolecular relaxation due to adsorption of the chiral phenanthrene-derivative heptahelicene, C30H18, on Cu(111) and Cu(332) surfaces have been investigated by means of angle-scanned full-hemispherical x-ray photoelectron diffraction. Although the C 1s diffraction patterns of the adsorbed submonolayer coverage helicene films exhibit scattering anisotropies of less than two percent, a detailed analysis involving simple molecular mechanics calculations of the atomic coordinates, photoelectron diffraction single-scattering cluster calculations and an R-factor analysis permits the determination of the helicene molecular orientation. On Cu(111), the molecules are found to bind to the substrate surface via their terminal phenanthrene group oriented parallel to the surface plane, while on Cu(332) the three terminal C-6 rings are oriented parallel to the (111) terrace plane. Six azimuthal molecular orientations are found to coexist on Cu(111), on Cu(332), however, the step–molecule inte...

In angle-resolved photoemission from single-crystal samples, the shape of the Fermi edge can be severely distorted by electronic bands crossing the Fermi energy. Analysing temperature-dependent angle-resolved photoemission data from the (111) surface of nickel metal, we explored how far the spectral background below such bands can be described by a Fermi–Dirac distribution function and, intimately connected with this discussion, how far we can deduce the temperature of the sample from this background. For this purpose, we present a fitting procedure that is applicable to angle- and energy-resolved photoemission data sets which permits one to selectively remove band transitions from such data and to extract this background in a reliable fashion. The sample temperature can be reasonably, but not quantitatively, determined by this procedure.

A new substrate emission X-ray photoelectron diffraction method that subtracts a clean substrate measurement from a measurement of the adsorbate-covered sample is able to reveal important adsorbate–substrate registry information while it suppresses disturbing substrate effects in the diffractograms. Short measurement times and an unambiguous data quality allow for a quick and convincing structural characterization of homogeneous adsorbate layers. The data processing and analysis methods are illustrated with experimental h-BN/Ni(1 1 1) data and multiple-scattering calculations.

We present a high-resolution surface X-ray diffraction study of hexagonal boron nitride (h-BN) on the surface of Rh(1 1 1). The previously observed commensurate 13-on-12 superstructure for this system is stable in the temperature range between room temperature and 830 °C. Surface X-ray diffraction measurements up to 830 °C on the superstructure show no sign of a shift towards a different superstructure, demonstrating the high thermal stability and strong bonding between film and substrate. At lower temperatures, an anomalous thermal expansion behaviour of the topmost surface region of rhodium is observed, where the rhodium in-plane lattice constant remains invariant. This can be explained by the (h-BN) single-layer being compressively strained, whereby the strong bonding to the substrate causes the latter to be tensile strained.

Full kk-maps of the electronic structure near the Fermi level of dierently doped cuprates measured with angle-scanned photoelectron spectroscopy are presented. The valence band maximum of the antiferromagnetic insulator Sr2CuO2Cl2, which is taken as a representative of an undoped cuprate, and the Fermi surfaces of overdoped, optimally doped and underdoped Bi2Sr2CaCu2O8+ high-temperature superconductors are mapped in the normal state. The results conrm the existence of large Luttinger Fermi surfaces at high doping with a Fermi surface volume proportional to (1 +x), where x is the hole concentration. At very low doping, however, we nd that this assumption based on Luttinger's theorem is not fullled. This implies a change in the topology of the Fermi surface. Furthermore the intensity of the shadow bands observed on the Fermi surface of Bi2Sr2CaCu2O8+ as a function of the doping is discussed. PACS. 79.60.-i Photoemission and photoelectron spectra { 74.25.Jb Electronic structure { 71.18.+y Fermi surface: calculations and measurements; eective mass, g factor

We present the Fermi surface map of p(2◊2)O/Mo(110) at room temperature. Changes with respect to the clean surface are discussed. Three surface-state contours (‘hole pockets’) that exist on the clean surface are found to persist on the oxygen-covered surface with slightly changed volumes. The large ‘electron pocket’ shows no significant shift. In combination with a previous electron energy-loss spectroscopy experiment measuring the surface phonon dispersion curves, these data put into question the recently postulated driving force for stabilizing the p(2◊2) superstructure in the low-coverage regime. © 2000 Elsevier Science B.V. All rights reserved.

The routine measurement of full hemispherical photoemission intensity maps gives us the possibility for the combined investigation of structural and electronic phenomena at surfaces. As an example the growth of ultrathin films of Co on Cu(111) is studied as a function of film thickness. While X-ray photoelectron diffraction (XPD) shows the early appearance of stacking faults as a precursor of the hcp structure, Fermi surface maps reveal the very fast evolution of the Co Fermi surface that can be compared to measurements on a clean Co(0001) crystal. For the system O/Rh(111), XPD brings up important structural clues, relating changes in surface reactivity to small amounts of subsurface oxygen, which forces adjacent oxygen atoms to occupy new and more reactive adsorption sites. In the course of this last study we observed for the first time the weak backscattering signals in the angular pattern of adsorbate emission. These cone-like features are extremely sensitive to the adsorbate–substrate bond length.

We present the development of Fermi-level crossings from the clean to the hydrogen-saturated (110) surface of molybdenum at room temperature. The well-known adsorbate-induced phonon anomaly is discussed in terms of a quasi-one-dimensional Fermi-surface nesting and giant Kohn anomaly. Our experiment resolved an existing discrepancy between earlier photoemission data and Fermi-surface contours calculated within density-functional theory. Moreover, it is found that the Fermi-surface nesting is fully established at hydrogen coverages where the phonon anomaly just barely forms.

The doping of a C60/Ag(100) interface with K is studied by high resolution valence band photoemission spectroscopy. With increasing potassium exposure the progressive filling of the C60 lowest unoccupied molecular orbital (LUMO) derived band is observed. The charge transfer onto the C60 molecules is quantified from the observed binding energies of the LUMO derived band. Up to a half filled LUMO no rigid band shift is found. Furthermore the intensity of the highest occupied molecular orbital is not constant but shows in normal emission a pronounced dip between half and complete LUMO filling.

The interaction of state-selected and oriented N2O molecules with one monolayer of Li on Rh(100) was studied by means of exoelectron emission analysis, time-of-flight mass spectroscopy and sticking measurements in a pulsed molecular beam experiment. State-selected and oriented N2O molecules are prepared using electrostatic hexapole and dipole fields applying the linear Stark effect. The sticking probability of N2O on Li/Rh(100) is found to be independent of the molecular state and orientation. The exoemission response on the pulsed beam reveals two different reaction channels: A direct channel which coincides with the arrival of the gas pulse and an indirect channel after the arrival of the gas pulses. The exoemission probability in the direct channel is strongly enhanced for vibrationally excited N2O molecules and further increases when N2O hits the surface with the O-end first. Furthermore, it is associated with the backscattering of N-2 molecules which depends on the molecular orientation, as well. From these observations we identify the direct reaction channel with an Eley-Rideal mechanism, i.e. an abstraction reaction that proceeds during the impact of the N2O where 'harpooning' is the key step that triggers the reaction. The indirect reaction channel, on the other hand, follows the Langmuir-Hinshelwood scheme where the reacting molecules are adsorbed on the surface with complete energy accommodation and thus lose the information on their molecular state and orientation in the gas phase. (C) 1999 Elsevier Science B.V. All rights reserved.

Monolayers of hexagonal boron nitride have been grown by the reaction of benzene-like borazine (BN)3H6 with Ni(111) at 1050 K. The resulting layers are analyzed by means of Si Kα excited N 1s and B 1s X-ray photoelectron diffraction (XPD) and scanning tunneling microscopy (STM). STM shows large terraces without defects and resolves two different atomic species that are commensurate with Ni(111). From XPD it is found that the system discriminates fcc from hcp adsorption sites, that the h-BN layer is corrugated and that nitrogen terminates the surface. The B–N corrugation is determined quantitatively by R-factor analysis between single scattering cluster calculations and experiment. The results are discussed in connection with an existing LEED analysis on the same system [Y. Gamou, M. Terai, A. Nagashima, C. Oshima, Sci. Rep. RITU A 44 (1997) 211].