20 BESSY – Highlights 2003 – Scientific Highligths Bottom-up nano-technology: molecular engineering at surfaces S. Stepanow 1 , M. Lingenfelder 1 , A. Dmitriev 1 , N. Lin 1 , Th. Strunskus 2 , Ch. Wöll 2 , J.V. Barth 3,4 , K. Kern 1,3 1 Max-Planck-Institut für Festkörperforschung, Stuttgart, 2 Lehrstuhl für Physikalische Chemie I, Ruhr-Universität Bochum, 3 Institut de Physique des Nanostructures, Ecole Polytechnique Fédérale de Lausanne, Schweiz 4 Advanced Materials and Process Engineering Laboratory, University of British Columbia, Vancouver, Canada acid (terephthalic acid - tpa) and 1,2,4-benzenetri- carboxylic acid (trimellitic acid - tmla) on the Cu(100) surface. Mol- ecules of this type are frequently employed in 3-D crystal engineering [3] and have proven to be useful for the fabrication of nanoporous supramolecular layers [4-7]. The molecules tpa and tmla with their respective twofold and threefold exodentate functionality are shown in Fig. 1. While the formation of organic layers and nano-structures can be nicely monitored by STM, X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) resolve conclusively the deprotonation of carboxylic acid group and molecular orientation. Distinct hydrogen-bonded assemblies were obtained at low temperature (up to 275 K) with tpa. The STM image in Fig. 2 reveals that molecular ribbons evolve on the square substrate. The corresponding C 1s and O 1s XPS data confirm that the tpa carboxyl groups remain largely complete, i.e., the well-known splitting of the C 1s peak is observed reflecting the contribution from the atoms in carboxyl and phenyl moieties at an References: [1] Lehn, J.-M. Supramolecular Chemistry, Concepts and Perspectives (VCH, Weinheim, 1995). [2] J. V. Barth et al., Appl. Phys. A 76, 645 (2003). [3] O.M. Yaghi et al., Nature 423, 705 (2003). [4] A. Dmitriev et al., J. Phys. Chem. B 106, 6907 (2002). [5] A. Dmitriev et al., Angew. Chem. Int. Ed. 41, 2670 (2003). [6] M. A. Lingenfelder et al., Chem. Eur. J., in print (2004). [7] S. Stepanow et al., Nature Mat., in print (2004). [8] M. Wühn et al., Langmuir 17, 7605 (2001). [9] C. Mainka et al., Surf. Sci. 341, L1055 (1995). [10] D. S. Martin et al., Phys. Rev. B 66, 155427 (2002). A key issue in nanotech- nology is the development of conceptually simple construction techniques for the mass fabrication of identical nanoscale structures. Conventional top down fabrication techniques are both top down fabrication techniques are both energy intensive and wasteful, because many production steps involve depositing unstruc- tured layers and then patterning them by removing most of the deposited films. Furthermore, increasingly expensive fabrica- tion facilities are required as the feature size decreases. The natural alternative to the top- down construction is the bottom-up ap- proach, in which nanoscale structures are obtained from their atomic and molecular constituents by self-organised growth. Self- organised growth can be driven by thermody- namic forces or be the result of kinetic processes. The archetype of a thermody- namically driven structure formation at the mesoscopic scale is molecular self-assem- bly. This refers to the spontaneous associa- tion of molecules under conditions close to equilibrium into stable, well defined nano- structures joined by noncovalent bonds; it is one of the key building principles of all living matter [1]. To make full use of this approach in nanotechnology we have to understand the noncovalent intermolecular interactions and to develop methods to manipulate them in a controlled way [2]. We have performed combined scanning tunnelling microscopy (STM) and synchrotron investigations addressing the bonding, ordering and surface chemistry of the related molecules 1,4-benzenedicarboxylic Fig. 1: Terephthalic acid - tpa (top) and Trimellitic acid - tmla (bottom) comprise two respectively three functional carboxyl groups. Fig. 2: Low-temperature supramolecular ordering of tpa on Cu(100): H-bonded molecular ribbons clearly resolved by STM are stable up to 275 K. In the corresponding O 1s XPS data a broadened peak appears due to overlaping hydroxyl and carbonyl intensities, i.e., tpa carboxylic groups remain complete. In conjunction with the NEXAFS analysis demonstrating flat adsorption, the modeling indicates a 2-D H-bond coupling scheme.