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Berlin, Germany

Schmitt S.,SPECS GmbH | Scholl A.,University of Wurzburg | Umbach E.,University of Wurzburg
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. Source


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. Source


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. Source


Schmitt S.,University of Wurzburg | Schmitt S.,SPECS GmbH | Scholl A.,University of Wurzburg | Umbach E.,University of Wurzburg
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. Source


Lubbe J.,University of Osnabruck | Troger L.,University of Osnabruck | Torbrugge S.,University of Osnabruck | 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. Source

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