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The production of high-quality hexagonal boron nitride (h-BN) is essential for the ultimate performance of two-dimensional (2D) materials-based devices, since it is the key 2D encapsulation material. Here, a decisive guideline is reported for fabricating high-quality h-BN on transition metals: It is crucial to exclude carbon from h-BN related process. Otherwise carbon prevails over boron and nitrogen due to its larger binding energy, thereupon forming graphene on metals after high-temperature annealing. We demonstrate the surface reaction-assisted conversion from h-BN to graphene with high-temperature treatments. The pyrolysis temperature Tp is an important quality indicator for h-BN/metals. When the temperature is lower than Tp, the quality of h-BN layer is improved upon annealing. While the annealing temperature is above Tp, in case of carbon-free conditions, the h-BN disintegrates and nitrogen desorbs from the surface more easily than boron, eventually leading to clean metal surfaces. However, once the h-BN layer is exposed to carbon, graphene forms on Pt(111) in the high-temperature regime. This not only provides an indispensable principle (avoid carbon) for fabricating high-quality h-BN materials on transition metals, but also offers a straightforward method for the surface reaction-assisted conversion from h-BN to graphene on Pt(111)., Accepted by SMALL on October 7, 2022

Realization of stable spin states in surface-supported magnetic molecules is crucial for their applications in molecular spintronics, memory storage or quantum information processing. In this work, we studied the surface magnetism of dimetallo-azafullerene Tb-2@C79N, showing a broad magnetic hysteresis in a bulk form. Surprisingly, monolayers of Tb-2@C79N exhibited a completely different behavior, with the prevalence of a ground state with antiferromagnetic coupling at low magnetic field and a metamagnetic transition in the magnetic field of 2.5-4 T. Monolayers of Tb-2@C79N were deposited onto Cu(111) and Au(111) by evaporation in ultra-high vacuum conditions, and their topography and electronic structure were characterized by scanning tunneling microscopy and spectroscopy (STM/STS). X-ray photoelectron spectroscopy (XPS), in combination with DFT studies, revealed that the nitrogen atom of the azafullerene cage tends to avoid metallic surfaces. Magnetic properties of the (sub)monolayers were then studied by X-ray magnetic circular dichroism (XMCD) at the Tb-M-4,M-5 absorption edge. While in bulk powder samples Tb-2@C79N behaves as a single-molecule magnet with ferromagnetically coupled magnetic moments and blocking of magnetization at 28 K, its monolayers exhibited a different ground state with antiferromagnetic coupling of Tb magnetic moments. To understand if this unexpected behavior is caused by a strong hybridization of fullerenes with metallic substrates, XMCD measurements were also performed for Tb-2@C79N adsorbed on h-BN|Rh(111) and MgO|Ag(100). The co-existence of two forms of Tb-2@C79N was found on these substrates as well, but magnetization curves showed narrow magnetic hysteresis detectable up to 25 K. The non-magnetic state of Tb-2@C79N in monolayers is assigned to anionic Tb-2@C79N- species with doubly-occupied Tb-Tb bonding orbital and antiferromagnetic coupling of the Tb moments. A charge transfer from the substrate or trapping of secondary electrons are discussed as a plausible origin of these species. Deutsche Forschungsgemeinschaft [PO 1602/7-1, PO 1602/8-1, LI 3055/3-1, AV 169/3-1]; Swedish Research Council [201500455]; Sklodowska Curie Actions [INCA 600398] Published version Computational resources were provided by ZIH/TU Dresden. The work was performed with financial support from Deutsche Forschungsgemeinschaft (grants PO 1602/7-1 and PO 1602/8-1 to A. A. P., LI 3055/3-1 to F. L., and AV 169/3-1 to S. M. A.), the Swedish Research Council (Grant No. 201500455), and Sklodowska Curie Actions cofounding project INCA 600398. We acknowledge the Paul Scherrer Institute for providing synchrotron beamtime at the X-treme and PEARL beamlines of the Swiss Light Source.

A key parameter for the low-temperature magnetic coupling of in dinuclear lanthanide single-molecule magnets (SMMs) is the barrier $U_{FA}$ resulting from the exchange and dipole interactions between the two $4f$ moments. Here we extend the pseudospin model previously used to describe the ground state of dinuclear endofullerenes to account for variations in the orientation of the single-ion anisotropy axes and apply it to the two SMMs Dy$_2$ScN@C$_{80}$ and Dy$_2$TiC@C$_{80}$. While x-ray magnetic circular dichroism (XMCD) indicates the same $J_z=15/2$ Dy groundstate in both molecules, the Dy-Dy coupling strength and the stability of magnetization is distinct. We demonstrate that both the magnitude of the barrier $U_{FA}$ and the angle between the two $4f$ moments are determined directly from precise temperature-dependent magnetization data to an accuracy better than $1^{\circ}$. The experimentally found angles between the $4f$ moments are in excellent agreement with calculated angles between the quantisation axes of the two Dy ions. Theory indicates a larger deviation of the orientation of the Dy magnetic moments from the Dy bond axes to the central ion in Dy$_2$TiC@C$_{80}$. This may explain the lower stability of the magnetisation in Dy$_2$TiC@C$_{80}$, although it exhibits a $\sim 49\%$ stronger exchange coupling than in Dy$_2$ScN@C$_{80}$., 4 figures, 6 pages

Engineering intramolecular exchange interactions between magnetic metal atoms is a ubiquitous strategy for designing molecular magnets. For lanthanides, the localized nature of 4f electrons usually results in weak exchange coupling. Mediating magnetic interactions between lanthanide ions via radical bridges is a fruitful strategy towards stronger coupling. In this work we explore the limiting case when the role of a radical bridge is played by a single unpaired electron. We synthesize an array of air-stable Ln2@C80(CH2Ph) dimetallofullerenes (Ln2 = Y2, Gd2, Tb2, Dy2, Ho2, Er2, TbY, TbGd) featuring a covalent lanthanide-lanthanide bond. The lanthanide spins are glued together by very strong exchange interactions between 4f moments and a single electron residing on the metal–metal bonding orbital. Tb2@C80(CH2Ph) shows a gigantic coercivity of 8.2 Tesla at 5 K and a high 100-s blocking temperature of magnetization of 25.2 K. The Ln-Ln bonding orbital in Ln2@C80(CH2Ph) is redox active, enabling electrochemical tuning of the magnetism., Dilanthanide complexes that possess radical bridges exhibit enhanced magnetic exchange coupling, affording molecular magnets with high blocking temperatures. Here, the authors explore a series of dilanthanide-encapsulated fullerenes where the radical bridge is taken to its limit and the role is played by a single unpaired electron.

Single-layer hexagonal boron nitride is produced on 2 inch Pt(111)/sapphire wafers. The growth with borazine vapor deposition at process temperatures between 1000 and 1300 K is in situ investigated by photoelectron yield measurements. The growth kinetics is slower at higher temperatures and follows a tanh2 law which better fits for higher temperatures. The crystal-quality of hexagonal boron nitride (h-BN)/Pt(111) is inferred from scanning low energy electron diffraction (x-y LEED). The data indicate a strong dependence of the epitaxy on the growth temperature. The dominant structure is an aligned coincidence lattice with 10 h-BN on 9 Pt(1 × 1) unit cells and follows the substrate twinning at the millimeter scale.

The production of high-quality two-dimensional (2D) materials is essential for the ultimate performance of single layers and their hybrids. Hexagonal boron nitride (h-BN) is foreseen to become the key 2D hybrid and packaging material since it is insulating, impermeable, flat, transparent, and chemically inert, though it is difficult to attain in ultimate quality. Here, a scheme is reported for producing single layer h-BN that shows higher quality in view of mosaicity and strain variations than material from chemical vapor deposition (CVD). We delaminate CVD h-BN from Rh(111) and transfer it to a clean metal surface. The twisting angle between BN and the second substrate yields metastable moire structures. Annealing above 1000 K leads to 2D distillation, i.e., catalyst-assisted BN sublimation from the edges of the transferred layer and subsequent condensation into superior quality h-BN. This provides a way for 2D material production remote from CVD instrumentation.

We have deposited Sn on corrugated hexagonal boron nitride (h-BN) nanomeshs formed on Rh(111) and found that Sn atoms are intercalated between h-BN and Rh, flattening the h-BN. Our reflection high-energy electron diffraction (RHEED) analysis showed that the average in-plane lattice constant of h-BN increases due to the loss of the corrugation. Furthermore, electronic structure measurements based on angle-resolved photoemission spectroscopy (ARPES) showed that the h-BN π band width increases significantly while the σ band width does not change as much. These behaviors were partly different from previous reports on the intercalation of h-BN/Rh system. Our results offer a novel, simple method to control the electronic structure of h-BN.

Fused‐pentagons results in an increase of local steric strain according to the isolated pentagon rule (IPR), and for all reported non‐IPR clusterfullerenes multiple (two or three) metals are required to stabilize the strained fused‐pentagons, making it difficult to access the single‐atom properties. Herein, we report the syntheses and isolations of novel non‐IPR mononuclear clusterfullerenes MNC@C76 (M=Tb, Y), in which one pair of strained fused‐pentagon is stabilized by a mononuclear cluster. The molecular structures of MNC@C76 (M=Tb, Y) were determined unambiguously by single‐crystal X‐ray diffraction, featuring a non‐IPR C 2v(19138)‐C76 cage entrapping a nearly linear MNC cluster, which is remarkably different from the triangular MNC cluster within the reported analogous clusterfullerenes based on IPR‐obeying C82 cages. The TbNC@C76 molecule is found to be a field‐induced single‐molecule magnet (SMM).

International audience; Ions inside of fullerene molecules are model systems for the study of the electrostatic interaction across a single layer of carbon. For TbSc2N@C80 on h-BN/Ni(111), we observe with high-resolution X-ray photoelectron spectroscopy a splitting of the C 1s core level. The data may be explained quantitatively with density functional theory. The correlation of the C 1s eigenvalues and the Coulomb potential of the inside ions at the corresponding carbon sites indicates incomplete screening of the electric field due to the endohedral ions. The screening comprises anisotropic charge transfer to the carbon atoms and their polarization. This behavior is essential for the ordering of endohedral single-molecule magnets and is expected to occur in any single-layer material.

The Photo-Emission and Atomic Resolution Laboratory is a new soft X-ray beamline at the Swiss Light Source for the study of surface structure using photoelectron diffraction and scanning tunneling microscopy., The Photo-Emission and Atomic Resolution Laboratory (PEARL) is a new soft X-ray beamline and surface science laboratory at the Swiss Light Source. PEARL is dedicated to the structural characterization of local bonding geometry at surfaces and interfaces of novel materials, in particular of molecular adsorbates, nanostructured surfaces, and surfaces of complex materials. The main experimental techniques are soft X-ray photoelectron spectroscopy, photoelectron diffraction, and scanning tunneling microscopy (STM). Photoelectron diffraction in angle-scanned mode measures bonding angles of atoms near the emitter atom, and thus allows the orientation of small molecules on a substrate to be determined. In energy scanned mode it measures the distance between the emitter and neighboring atoms; for example, between adsorbate and substrate. STM provides complementary, real-space information, and is particularly useful for comparing the sample quality with reference measurements. In this article, the key features and measured performance data of the beamline and the experimental station are presented. As scientific examples, the adsorbate–substrate distance in hexagonal boron nitride on Ni(111), surface quantum well states in a metal-organic network of dicyano-anthracene on Cu(111), and circular dichroism in the photoelectron diffraction of Cu(111) are discussed.

This chapter summarizes the investigations of the endohedral metallofullerenes (EMFs) with a C80 carbon shell in view of their magnetic properties, where the recently discovered single-molecule magnetism in dysprosium based species are highlighted.

Increasing the temperature at which molecules behave as single-molecule magnets is a serious challenge in molecular magnetism. One of the ways to address this problem is to create the molecules with strongly coupled lanthanide ions. In this work, endohedral metallofullerenes Y2@C80 and Dy2@C80 are obtained in the form of air-stable benzyl monoadducts. Both feature an unpaired electron trapped between metal ions, thus forming a single-electron metal-metal bond. Giant exchange interactions between lanthanide ions and the unpaired electron result in single-molecule magnetism of Dy2@C80(CH2Ph) with a record-high 100 s blocking temperature of 18 K. All magnetic moments in Dy2@C80(CH2Ph) are parallel and couple ferromagnetically to form a single spin unit of 21 μB with a dysprosium-electron exchange constant of 32 cm−1. The barrier of the magnetization reversal of 613 K is assigned to the state in which the spin of one Dy centre is flipped., Single molecule magnets have demonstrated promise for information storage, molecular spintronics and quantum computing, but are limited by their low operational temperatures. Here, Popov and coworkers prepare a SMM with a high blocking temperature of 18 K by trapping two lanthanide ions with a single-electron bond inside a fullerene.

For the endohedral fullerene molecule HoLu_{2}N@C_{80}, it is shown that the endohedral HoLu_{2}N unit may be oriented in a magnetic field. The Ho magnetic moment is fixed in the strong ligand field and aligns along the holmium-nitrogen axis. The torque of a magnetic field on the Ho magnetic moment leads to a hopping bias of the endohedral unit inclining to an orientation parallel to the externally applied field. This endohedral cluster distribution remains frozen below the onset of thermally induced rotation of the endohedral units. We derive an analytical statistical model for the description of the effect that scales below 7 T with the square of the external field strength, and that allows us to resolve the freezing temperature of the endohedral hopping motion. The freezing temperature is around 55 K and depends on the cooling rate, which in turn determines an activation energy for the hopping motion of 185 meV and a prefactor of 1.8×10^{14} s^{-1}. For TbSc_{2}N@C_{80} we find the same behavior with a 3.5% higher freezing temperature.

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.

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

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.

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.

Two limiting factors for a new technology of graphene-based electronic devices are the difficulty of growing large areas of defect-free material and the integration of graphene with an atomically flat and insulating substrate material. Chemical vapor deposition (CVD) on metal surfaces, in particular on copper, may offer a solution to the first problem, while hexagonal boron nitride (h-BN) has been identified as an ideal insulating substrate material. The bottom-up growth of graphene/h-BN stacks on copper surfaces appears therefore as a promising route for future device fabrication. As an important step, we demonstrate the consecutive growth of well-aligned graphene on h-BN, both as single layers, by low-pressure CVD on Cu(111) in an ultrahigh vacuum environment. The resulting films show a largely predominant orientation, defined by the substrate, where the graphene lattice aligns parallel to the h-BN lattice, while each layer maintains its own lattice constant. The lattice mismatch of 1.6% between h-BN and graphene leads to a moiré pattern with a periodicity of about 9 nm, as observed with scanning tunneling microscopy. Accordingly, angle-resolved photoemission data reveal two slightly different Brillouin zones for electronic states localized in graphene and in h-BN, reflecting the vertical decoupling of the two layers. The graphene appears n-doped and shows no gap opening at the K[overline] point of the two-dimensional Brillouin zone.

Single atoms, and in particular the least reactive noble gases, are difficult to immobilize at room temperature. Ion implantation into a crystal lattice has this capability, but the randomness of the involved processes does not permit much control over their distribution within the solid. Here we demonstrate that the boron nitride nanomesh, a corrugated single layer of hexagonal boron nitride (h-BN) with a 3.2 nm honeycomb superstructure formed on a Rh(111) surface, can trap individual argon atoms at distinct subsurface sites at room temperature. A kinetic energy window for implantation is identified where the argon ions can penetrate the h-BN layer but not enter the Rh lattice. Scanning tunneling microscopy and photoemission data show the presence of argon atoms at two distinct sites within the nanomesh unit cell, confirmed also by density functional theory calculations. The single atom implants are stable in air. Annealing of implanted structures to 900 K induces the formation of highly regular holes of 2 nm diameter in the h-BN layer with adjacent flakes of the same size found on top of the layer. We explain this "can-opener" effect by the presence of a vacancy defect, generated during the penetration of the Ar ion through the h-BN lattice, and propagating along the rim of a nanomesh pore where the h-BN lattice is highly bent. The reported effects are also observed in graphene on ruthenium and for neon atoms.

A detailed understanding of the organic molecule/substrate interface is of crucial importance for the design of organic semiconducting devices, as the interface determines the contact resistance and the charge injection. Generally, two different adsorption situations are considered: physisorption and chemisorption. For small molecular adsorbates like CO or N2, the adsorption energy alone can be used as a criterion to classify the adsorption in chemisorption (adsorption energies larger than 1 eV) and physisorption (few tens of meV). This classification fails for complex π-conjugated organic molecules. Here we discuss on the basis of a pentacene/Cu(110) model system a different set of criteria to distinguish between chemisorption and physisorption beyond the total bond energy argument. We analyze the bonding situation on the basis of density functional theory (DFT) calculations and photoelectron spectroscopy. Theory predicts (i) a significant bending of the molecule after adsorption, (ii) a buckling of the ...

We report on experimental evidence for the formation of a two dimensional Si/Au(110) surface alloy. In this study, we have used a combination of scanning tunneling microscopy, low energy electron diffraction, Auger electron spectroscopy and ab initio calculations based on density functional theory. A highly ordered and stable Si-Au surface alloy is observed subsequent to growth of a sub-monolayer of silicon on an Au(110) substrate kept above the eutectic temperature.

The atomic and magnetic structure of surfaces and interfaces can be determined by angle-scanned resonant x-ray photoelectron diffraction. For the case of magnetized nickel the resonant L2 excitation with circularly polarized light yields electrons with a dichroic signature from which the dipolar part may be retrieved. The dichroic dipoles in the electron emission patterns rotate by distinct angles, which indicates different electron source waves with angular momenta parallel or antiparallel to those of the photons. For precise determination of the magnetization direction, the asymmetry between electron diffraction patterns as excited by left and right circularly polarized light has to be used. ©2011 American Physical Society

a r t i c l e i n f o The synthesis of high quality single layer graphene on rhodium, g/Rh(111), is reported. The graphene layers are grown at 1060 K by low pressure chemical vapor deposition (CVD) using 3-pentanone as a precursor molecule. The presented growth technique shows an easy high quality production method for epitaxial graphene monolayers. The chemical composition and structural properties of such self-assembled monolayers were characterized by X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Scanning Tunneling Microscopy (STM) con! rms the formation of a 3 nm super cell and a unique surface morphology which establishes the potential of g/Rh(111) as a template for molecules.

In a recent letter [Phys. Rev. Lett. 105 (2010) 036804] the unoccupied electronic states of single layers of graphene on ruthenium are investigated. Here we comment on the interpretation, which deviates in four points from [J. Phys.: Condens. Matter 22 (2010) 302001] and outline the corresponding consequences.

Field-emission patterns from a clean tungsten tip apex induced by femtosecond laser pulses have been investigated. Strongly asymmetric field-emission intensity distributions are observed depending on three parameters: (1) the polarization of the light, (2) the azimuthal and (3) the polar orientation of the tip apex relative to the laser incidence direction. In effect, we have realized an ultrafast pulsed field-emission source with site selectivity of a few tens of nanometers. Simulations of local fields on the tip apex and of electron emission patterns based on photo-excited nonequilibrium electron distributions explain our observations quantitatively. Electron emission processes are found to depend on laser power and tip voltage. At relatively low laser power and high tip voltage, field-emission after two-photon photo-excitation is the dominant process. At relatively low laser power and low tip voltage, photoemission processes are dominant. As the laser power increases, photoemission from the tip shank becomes noticeable., 12 pages, 12 figures, submitted to Physical Review B

Single layers of carbon (graphene) and hexagonal boron nitride may be grown on transition metal surfaces. The substrates take the role of the support and allow due to their catalytic activity the growth of perfect layers by chemical vapor deposition. The layers are sp2 hybridized honeycomb networks with strong in plane σ- and weaker out of plane π-bonds to the substrate and to the adsorbates. This hierarchy in bond strength causes anisotropic elastic properties, where the sp2 layers are stiff in plane and soft out of plane. A corrugation of these layers imposes a third hierarchy level in bond energies (α-bonds), with lateral bonding of molecular objects with sizes between 1 and 5 nanometer. The lateral extra bond energies due to the polarizability α of the adsorbates are in the range of thermal energies kTR at room temperature and are particularly interesting for nanotechnology. The concomitant template function will be discussed. The peculiar bond hierarchy also imposes intercalation as another property of sp2 layer systems. Last but not least sp2 layer systems are particularly robust, i.e. survive immersion into liquids, which is a promise for sp2 layers being useful outside ultra high vacuum. [DOI: 10.1380/ejssnt.2010.62]

6 páginas, 4 figuras.-- PACS number(s): 71.20.Tx, 71.38.-k, 79.60.-i, 31.70.-f.-- et al., The low-energy single-particle excitations of highly ordered C60 chains adsorbed on a vicinal copper substrate are investigated by angle-resolved photoemission spectroscopy. The interface state previously identified on C60/Cu(111) shows a one-dimensional dispersion on Cu(553). In contrast, no significant momentum dependence is detected for emission from the lowest unoccupied molecular orbital (LUMO). The LUMO displays similar phonon features as in C60/Cu(111) but it does not peak toward the Fermi level for all considered potassium dopings and its photoemission lineshape is broader than in any other monolayer system investigated so far. This behavior is not easily reconciled with existing theory and indicates that the one-dimensional character of the chains affects the electronic structure of the monolayer in an intricate way., The project was supported by the Swiss National Science Foundation.

We have investigated field emission patterns from a clean tungsten tip apex induced by femtosecond laser pulses. Strongly asymmetric modulations of the field emission intensity distributions are observed depending on the polarization of the light and the laser incidence direction relative to the azimuthal orientation of tip apex. In effect, we have realized an ultrafast pulsed field-emission source with site selectivity on the 10 nm scale. Simulations of local fields on the tip apex and of electron emission patterns based on photo-excited nonequilibrium electron distributions explain our observations quantitatively., Comment: 4 pages, submitted to Physical Review Letters

Single sheets of hexagonal boron nitride on transition metals provide a model system for single layer dielectrics. The progress in the understanding of h-BN layers on transition metals of the last 10 years are shortly reviewed. Particular emphasis lies on the boron nitride nanomesh on Rh(111), which is a corrugated single sheet of h-BN, where the corrugation imposes strong lateral electric fields. Fermi surface maps of h-BN/Rh(111) and Rh(111) are compared. A h-BN layer on Rh(111) introduces no new bands at the Fermi energy, which is expected for an insulator. The lateral electric fields of h-BN nanomesh violate the conservation law for parallel momentum in photoemission and smear out the momentum distribution curves on the Fermi surface., 14 pages, 6 figures, 1 table, 1 equation, Accepted for publication in the Special Surface Science issue in honor of Gerhard Ertl's Nobel Prize

Charge transfer in highly-ordered C60 chains grown on a Cu(5 5 3) vicinal surface is studied by means of resonant photoemission. Tuning the light polarization, autoionization of the highest occupied molecular orbital (HOMO) was expected to detect anisotropy in this one-dimensional system. For one monolayer C60 we found no signature of autoionization. This indicates that for an electron which is excited from the C 1s level of C60 to the lowest unoccupied molecular orbital (LUMO), hybridization leads to delocalization on the femtosecond time-scale and no influence of the light polarization is observed.

An extended set of temperature-dependent ARUPS data from Ni(111) is presented. The ferromagnetic and the paramagnetic state as well as the phase transition are examined in great detail. Rather new and unconventional modes of data acquisition in ARUPS are applied with high angular and energy resolution, exhibiting great power near the Fermi energy EF. Even up to 5kBT above EF energy bands are readily observed. The understanding of these ARUPS data is strongly enhanced by spin-polarized band structure calculations. Exchange-split bands of both, sp- and d-character, are resolved in angular scans and in photoemission Fermi-surface maps. From two-dimensional data sets in energy and angle the dispersion and the exchange splitting are obtained with high precision. All the observed sp- and d-bands clearly exhibit a Stoner-like collapsing-band behavior. The exchange splitting ΔEex vanishes above TC in all cases, and ΔEex closely follows the temperature dependence of the macroscopic magnetization. The apparent deviations from the Stoner-like band behavior stated in P. Aebi et al. , Phys. Rev. Lett. 76, 1150 (1996) are explained. Furthermore we detect anomalously high intensity from a minority d-band close to an sp-band. This strongly suggests that sp-d-fluctuations at the Fermi level are a driving force for the magnetic phase transition of nickel.

Heteroepitaxial nucleation and growth of twin-free diamond on Ir(111) is reported. The bias enhanced nucleation (BEN) technique was applied in a microwave plasma chemical vapor deposition setup to induce diamond nucleation on the new multilayer stack Ir/YSZ/Si(111). We demonstrate that the gathering of the diamond nuclei in so-called “domains,” a pattern formation process unique for diamond nucleation on iridium, is also present on Ir(111). The 1–2 nm thick carbon layer deposited by BEN does not show any crystalline diamond structures in electron diffraction and high resolution lattice imaging microscopy. In contrast, x-ray photoelectron diffraction (XPD) measurements yield C 1s diffractograms with clear threefold symmetry. The main features are comparable to measurements on diamond (111) single crystal surfaces. The weaker fine structure in the XPD patterns of the BEN layers is attributed to some disorder due to the harsh ion bombardment. However, this ion bombardment did not induce any measurable amount...

Accurate momentum mapping of bulk and surface electronic states by angle-resolved two-photon photoemission is demonstrated on Cu(111) and one of its vicinal surfaces, Cu(443), using laser light of $3.08\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ photon energy for excitation. The surface state dispersion found agrees well with that expected from the periodic arrangement of terraces and monatomic steps on Cu(443). Polarization dependent data suggest that the state consists of out-of-plane ${p}_{z}$ orbitals like on the flat (111) copper surface, mixed with in-plane orbitals at the step edges. Maps of the Fermi surface taken from the vicinal surface are found to be in excellent agreement with conventional photoemission data and density-functional calculations. This proves that multiphoton photoemission can be used like direct one-photon photoemission as initial state spectroscopy with high energy and momentum resolution provided that no real intermediate states are involved in the excitation process.

The low-energy electronic excitations in cobalt are studied by a theoretical method that includes many-body effects and a realistic description of the band structure. Angle-resolved photoemission spectra measured on a thick film of hexagonal close-packed Co on Cu(111) agree well with calculated spectral functions. Because of many-body effects no sharp quasiparticle peaks exist for binding energies larger than $2\mathrm{eV}$ and in this energy region the spectrum is essentially incoherent. The many-body corrections are much stronger in the majority-spin channel and drastically affect the spin polarization of the spectra.

The interaction of state-selected and oriented N2O molecules in the first excited n(2) = l bending mode with one monolayer of Cs on Pt(111) is studied by means of time-of-flight mass spectroscopy in a molecular beam experiment. State-selected and oriented N2O molecules were prepared using electrostatic hexapole and dipole fields. The emission of N-2 molecules induced by the abstraction reaction (N2O + Cs --> CsO + N-2) depends strongly on the molecular state and its orientation. Direct abstraction and backscattering of N-2 molecules is observed for N2O ground-state molecules in the n(2) = 1 first vibrationally excited bending mode. A distinct steric effect has been found for N-2 molecules emitted specularly with respect to the incoming N2O molecules. These findings are explained in a picture where direct N-2 emission is triggered by resonant N2O ionization (harpooning). The results are discussed in the context of recent exoelectron emission experiments. (C) 1998 Elsevier Science B.V. All rights reserved.

The reaction of O 2 with Li and Cs films is accompanied by the emission of electrons. The variations of the initial yields with the velocity of the impinging molecules is exploited to access the dynamics of the adsorption process. The data are analyzed in terms of a model where chemisorption proceeds via an intermediate [Formula: see text] species. This [Formula: see text] state readily dissociates while the de-excitation of the resulting O − into O 2− may cause electron emission. From a comparison with experimentally observed electron yields the reaction time and velocity uptake of the products is accessed.

Energy distributions of electrons emitted from alkali-metal surfaces by impact of metastable He atoms reveal that there is a high probability for transformation of singlet atoms (excitation energy ${\mathit{E}}^{\mathrm{*}}$=20.6 eV) into triplet atoms (${\mathit{E}}^{\mathrm{*}}$=19.8 eV) prior to deexcitation into the ground state. The conversion probability (as expressed by the ratio R of the intensities of valence-band emission due to triplet and singlet ${\mathrm{He}}^{\mathrm{*}}$ deexcitation, respectively) increases with increasing alkali-metal coverage on a Ru(0001) substrate, and in turn decreases with increasing oxygen exposure at a fixed alkali coverage. These findings indicate that R is a qualitative measure for the degree of ``metallization'' of the adlayer. R also increases with temperature due to broadening of the nearest-neighbor distribution whereby, on the average, a larger part of the adlayer becomes metalliclike. For Cs overlayers exhibiting work functions 2 eV the mechanism of deexcitation changes and may proceed via ${\mathrm{He}}^{\mathrm{*}\mathrm{\ensuremath{-}}}$ (1${\mathit{s}}^{1}$2${\mathit{s}}^{2}$) formation as reflected by the R data as well as by the widths of the electron spectra.

Two isomers of metallofullerene Dy2S@C82 with sulfur-bridged Dy ions exhibit broad magnetic hysteresis with sharp steps at sub-Kelvin temperature. Analysis of the level crossing events for different orientations of a magnetic field showed that even in powder samples, the hysteresis steps caused by quantum tunneling of magnetization can provide precise information on the strength of intramolecular Dy⋯Dy inter-actions. A comparison of different methods to determine the energy difference between ferromagnetic and antiferromagnetic states showed that sub-Kelvin hysteresis gives the most robust and reliable values. The ground state in Dy2S@C82 has ferromagnetic coupling of Dy magnetic moments, whereas the state with antiferromagnetic coupling in C s and C 3v cage isomers is 10.7 and 5.1 cm−1 higher, respectively. The value for the C s isomer is among the highest found in metallofullerenes and is considerably larger than that reported in non-fullerene dinuclear molecular magnets. Magnetization relaxation times measured in zero magnetic field at sub-Kelvin temperatures tend to level off near 900 and 3200 s in C s and C 3v isomers. These times correspond to the quantum tunneling relaxation mechanism, in which the whole magnetic moment of the Dy2S@C82 molecule flips at once as a single entity.