Berlin, Germany
Berlin, Germany

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Ensling D.,TU Darmstadt | Ensling D.,VARTA Microbattery GmbH | Thissen A.,TU Darmstadt | Thissen A.,SPECS GmbH | And 3 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The electronic properties of LiCoO2 have been studied by theoretical band-structure calculations (using density functional theory) and experimental methods (photoemission). Synchrotron-induced photoelectron spectroscopy, resonant photoemission spectroscopy (ResPES), and soft x-ray absorption (XAS) have been applied to investigate the electronic structure of both occupied and unoccupied states. High-quality PES spectra were obtained from stoichiometric and highly crystalline LiCoO2 thin films deposited "in situ" by rf magnetron sputtering. An experimental approach of separating oxygen- and cobalt-derived (final) states by ResPES in the valence-band region is presented. The procedure takes advantage of an antiresonant behavior of cobalt-derived states at the 3p-3d excitation threshold. Information about the unoccupied density of states has been obtained by OK XAS. The structure of the CoL absorption edge is compared to semiempirical charge-transfer multiplet calculations. The experimental results are furthermore compared with band-structure calculations considering three different exchange potentials [generalized gradient approximation (GGA), using a nonlocal Hubbard U (GGA+U) and using a hybrid functional (Becke, three-parameter, Lee-Yang-Parr [B3LYP])]. For these different approaches total density of states and partial valence-band density of states have been investigated. The best qualitative agreement with experimental results has been obtained by using a GGA+U functional with U=2.9eV. © 2010 The American Physical Society.


Lubbe J.,University of Osnabrück | Troger L.,University of Osnabrück | Torbrugge S.,University of Osnabrück | Torbrugge S.,SPECS GmbH | And 5 more authors.
Measurement Science and Technology | Year: 2010

The effective Q-factor of the cantilever is one of the most important figures-of-merit for a non-contact atomic force microscope (NC-AFM) operated in ultra-high vacuum (UHV). We provide a comprehensive discussion of all effects influencing the Q-factor and compare measured Q-factors to results from simulations based on the dimensions of the cantilevers. We introduce a methodology to investigate in detail how the effective Q-factor depends on the fixation technique of the cantilever. Fixation loss is identified as a most important contribution in addition to the hitherto discussed effects and we describe a strategy for avoiding fixation loss and obtaining high effective Q-factors in the force microscope. We demonstrate for room temperature operation, that an optimum fixation yields an effective Q-factor for the NC-AFM measurement in UHV that is equal to the intrinsic value of the cantilever. © 2010 IOP Publishing Ltd.


Tromp R.M.,IBM | Hannon J.B.,IBM | Wan W.,Lawrence Berkeley National Laboratory | Berghaus A.,SPECS GmbH | Schaff O.,SPECS GmbH
Ultramicroscopy | Year: 2013

In Part I we described a new design for an aberration-corrected Low Energy Electron Microscope (LEEM) and Photo Electron Emission Microscope (PEEM) equipped with an in-line electron energy filter. The chromatic and spherical aberrations of the objective lens are corrected with an electrostatic electron mirror that provides independent control of the chromatic and spherical aberration coefficients Cc and C3, as well as the mirror focal length. In this Part II we discuss details of microscope operation, how the microscope is set up in a systematic fashion, and we present typical results. © 2012 Elsevier B.V.


Schramm S.M.,Leiden University | Kautz J.,Leiden University | Berghaus A.,SPECS GmbH | Schaff O.,SPECS GmbH | And 3 more authors.
IBM Journal of Research and Development | Year: 2011

We describe the layout and the capabilities of a new aberration-corrected low-energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) facility, which features real- and reciprocal-space spectroscopy. This new setup, named Electronic, Structural, and Chemical Nanoimaging in Real Time (ESCHER), was recently installed at Leiden University. It has three major instrumentation-related goals. First, we aim to reach the ultimate spatial resolution facilitated by aberration correction using an electron mirror, together with advanced electron detection. Second, we want to develop and exploit the spectroscopic possibilities of LEEM and PEEM in a standard laboratory environment. To this end, ESCHER is equipped with an inline energy filter and advanced photon sources. Third, we plan to extend the sample temperature range down to approximately 10 K, which is significantly lower than that achieved to date. Combined, these efforts will broaden the scientific reach of LEEM and PEEM beyond the areas of surface and materials science and into the realms of biosciences and life sciences. Here, we also present images of the first experiments performed with ESCHER focused on the growth of graphene on SiC(0001). © 2011 by International Business Machines Corporation.


Tromp R.M.,IBM | Hannon J.B.,IBM | Ellis A.W.,IBM | Wan W.,Lawrence Berkeley National Laboratory | And 2 more authors.
Ultramicroscopy | Year: 2010

We describe a new design for an aberration-corrected low energy electron microscope (LEEM) and photo electron emission microscope (PEEM), equipped with an in-line electron energy filter. The chromatic and spherical aberrations of the objective lens are corrected with an electrostatic electron mirror that provides independent control over the chromatic and spherical aberration coefficients Cc and C3, as well as the mirror focal length, to match and correct the aberrations of the objective lens. For LEEM (PEEM) the theoretical resolution is calculated to be ~1.5nm (~4nm). Unlike previous designs, this instrument makes use of two magnetic prism arrays to guide the electron beam from the sample to the electron mirror, removing chromatic dispersion in front of the mirror by symmetry. The aberration correction optics was retrofitted to an uncorrected instrument with a base resolution of 4.1nm in LEEM. Initial results in LEEM show an improvement in resolution to ~2nm. © 2010 Elsevier B.V.


Gunther S.,TU Munich | Danhardt S.,Ludwig Maximilians University of Munich | Ehrensperger M.,Ludwig Maximilians University of Munich | Zeller P.,Ludwig Maximilians University of Munich | And 3 more authors.
ACS Nano | Year: 2013

The ordering transition of an amorphous carbon layer into graphene was investigated by high-temperature scanning tunneling microscopy. A disordered C layer was prepared on a Ru(0001) surface by chemical vapor deposition of ethylene molecules at ∼660 K. The carbon layer grows in the form of dendritic islands that have almost the same density as graphene. Upon annealing of the fully covered surface, residual hydrogen desorbs and a coherent but still disordered carbon layer forms, with almost the same carbon coverage as in graphene. The ordering of this layer into graphene at 920 to 950 K was monitored as a function of time. A unique mechanism was observed that involves small topographic holes in the carbon layer. The holes are mobile, and on the trajectories of the holes the disordered carbon layer is transformed into graphene. The transport of C atoms across the holes or along the hole edges provides a low-energy pathway for the ordering transition. This mechanism is prohibited in a dense graphene layer, which offers an explanation for the difficulty of removing defects from graphene synthesized by chemical methods. © 2012 American Chemical Society.


Gunther S.,Ludwig Maximilians University of Munich | Gunther S.,TU Munich | Danhardt S.,Ludwig Maximilians University of Munich | Wang B.,Ecole Normale Superieure de Lyon | And 4 more authors.
Nano Letters | Year: 2011

The epitaxial growth of graphene by chemical vapor deposition of ethylene on a Ru(0001) surface was monitored by high-temperature scanning tunneling microscopy. The in situ data show that at low pressures and high temperatures the metal surface facets into large terraces, leading to much better ordered graphene layers than resulting from the known growth mode. Density functional theory calculations show that the single terrace growth mode can be understood from the energetics of the graphene-metal interaction. © 2011 American Chemical Society.


Torbrugge S.,SPECS GmbH | Schaff O.,SPECS GmbH | Rychen J.,SPECS Zurich GmbH
Journal of Vacuum Science and Technology B:Nanotechnology and Microelectronics | Year: 2010

Combined atomic-resolution scanning tunneling microscopy (STM) and noncontact atomic-force microscopy (NC-AFM) studies are carried out with the piezoelectric KolibriSensor in ultrahigh vacuum at room temperature. The sensor exhibits a very low spectral deflection noise density of only 6.5 fm/ Hz which favors in combination with its high spring constant of 540 000 N/m stable NC-AFM operation at subnanometer oscillation amplitudes. The authors present atomic-resolution imaging on the Si (111) (7×7) surface recorded in STM and NC-AFM feedback mode. They find that the tip surface distance during atomic-resolution imaging on silicon is much smaller for NC-AFM compared to STM. It is shown that atomic-resolution NC-AFM and dynamic STM images of the same area on the Si (111) (7×7) surface enable a discrimination of vacancies and adsorbates. Furthermore, the topography of graphite imaged in dynamic STM and NC-AFM feedback mode is compared. © 2010 American Vacuum Society.


Schmitt S.,SPECS GmbH | Scholl A.,University of Würzburg | Umbach E.,University of Würzburg
Surface Science | Year: 2015

The organic molecule PTCDA preferentially adsorbs on steps of vicinal Ag(111) surfaces and bunches them to well defined facet planes. These depend on coverage and annealing temperature and are independent of the nominal step direction and angle of inclination of the unreconstructed initial surface. We study the development of the facets and present a map of all 16 types of facets in a stereographic triangle of 35° off the [111]-direction. The faceting mechanism is interpreted as orientational phase separation originating from different bonding strengths of PTCDA on various facets. The faceting drives the system to the minimum of its surface free energy. © 2015 Elsevier B.V.


Schmitt S.,University of Würzburg | Schmitt S.,SPECS GmbH | Scholl A.,University of Würzburg | Umbach E.,University of Würzburg
Surface Science | Year: 2016

The organic molecule PTCDA preferentially adsorbs on steps of vicinal Ag(111) surfaces and bunches them to well defined facet planes. These depend on coverage and annealing temperature and are independent of the nominal step direction and angle of inclination of the unreconstructed initial surface. We study the development of the facets and present a map of all 16 types of facets in a stereographic triangle of 35° off the [111]-direction. The faceting mechanism is interpreted as orientational phase separation originating from different bonding strengths of PTCDA on various facets. The faceting drives the system to the minimum of its surface free energy. © 2015 Elsevier B.V.

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