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Originally published in Science Express on 13 December 2007
Science 18 January 2008:
Vol. 319. no. 5861, pp. 312 - 315
DOI: 10.1126/science.1150532
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Reports

Elementary Structural Motifs in a Random Network of Cytosine Adsorbed on a Gold(111) Surface

Roberto Otero,1* Maya Lukas,1{dagger} Ross E. A. Kelly,2{ddagger} Wei Xu,1 Erik Lægsgaard,1 Ivan Stensgaard,1 Lev N. Kantorovich,2 Flemming Besenbacher1§

Nonsymmetrical organic molecules adsorbed on solid surfaces may assemble into random networks, thereby providing model systems for organic glasses that can be directly observed by scanning tunneling microscopy (STM). We investigated the structure of a disordered cytosine network on a gold(111) surface created by thermal quenching, to temperatures below 150 K, of the two-dimensional fluid present on the surface at room temperature. Comparison of STM images to density functional theory calculations allowed us to identify three elementary structural motifs (zigzag filaments and five- and six-membered rings) that underlie the whole supramolecular random network. The identification of elementary structural motifs may provide a new framework for understanding medium-range order in amorphous and glassy systems.

1 Interdisciplinary Nanoscience Center, Centre for DNA Nanotechnology, and Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark.
2 Department of Physics, School of Physical Sciences and Engineering, King's College London, Strand, London WC2R 2LS, UK.

* Present address: Departamento de Física de la Materia Condensada, Facultad de Ciencias, Universidad Autónoma de Madrid, and Instituto Madrileño de Estudios Avanzados en Nanociencia, 28049 Madrid, Spain.

{dagger} Present address: Institute for Nanotechnology, Forschungzentrum Karlsruhe, Postfach 3640, D-76021 Karlsruhe, Germany.

{ddagger} Present address: Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK.

§ To whom correspondence should be addressed. E-mail: fbe{at}inano.dk

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