Bottom-Up Nano-Technology: Molecular Engineering at Surfaces
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BESSY – Highlights 2003 – Scientific Highligths 1 Max-Planck-Institut für Bottom-up nano-technology: Festkörperforschung, Stuttgart, 2 molecular engineering at surfaces Lehrstuhl für 1 1 1 1 2 Physikalische Chemie I, S. Stepanow , M. Lingenfelder , A. Dmitriev , N. Lin , Th. Strunskus , Ruhr-Universität Bochum, Ch. Wöll2, J.V. Barth3,4, K. Kern1,3 3 Institut de Physique des Nanostructures, A key issue in nanotech- acid (terephthalic acid - Ecole Polytechnique Fédérale de nology is the development tpa) and 1,2,4-benzenetri- Lausanne, Schweiz of conceptually simple carboxylic acid (trimellitic 4 construction techniques acid - tmla) on the Advanced Materials and Process for the mass fabrication Cu(100) surface. Mol- Engineering Laboratory, of identical nanoscale ecules of this type are University of British Columbia, structures. Conventional frequently employed in Vancouver, Canada top down fabrication 3-D crystal engineering techniques are both top [3] and have proven to be down fabrication techniques are both energy useful for the fabrication of nanoporous intensive and wasteful, because many supramolecular layers [4-7]. The molecules production steps involve depositing unstruc- tpa and tmla with their respective twofold tured layers and then patterning them by and threefold exodentate functionality are removing most of the deposited films. shown in Fig. 1. While the formation of Furthermore, increasingly expensive fabrica- organic layers and nano-structures can be tion facilities are required as the feature size nicely monitored by STM, X-ray photoelectron decreases. The natural alternative to the top- spectroscopy (XPS) and near-edge X-ray down construction is the bottom-up ap- absorption fine structure (NEXAFS) resolve proach, in which nanoscale structures are conclusively the deprotonation of carboxylic obtained from their atomic and molecular acid group and molecular orientation. constituents by self-organised growth. Self- Distinct hydrogen-bonded assemblies were organised growth can be driven by thermody- obtained at low temperature (up to 275 K) namic forces or be the result of kinetic with tpa. The STM image in Fig. 2 reveals processes. The archetype of a thermody- that molecular ribbons evolve on the square namically driven structure formation at the substrate. The corresponding C 1s and O 1s mesoscopic scale is molecular self-assem- XPS data confirm that the tpa carboxyl bly. This refers to the spontaneous associa- Fig. 1: groups remain largely complete, i.e., the tion of molecules under conditions close to Terephthalic acid - tpa (top) and well-known splitting of the C 1s peak is Trimellitic acid - tmla (bottom) equilibrium into stable, well defined nano- observed reflecting the contribution from the comprise two respectively three structures joined by noncovalent bonds; it is atoms in carboxyl and phenyl moieties at an functional carboxyl groups. one of the key building principles of all living matter [1]. To make full use of this approach in nanotechnology we have to understand Fig. 2: References: the noncovalent intermolecular interactions Low-temperature supramolecular ordering of tpa [1] Lehn, J.-M. Supramolecular and to develop methods to manipulate them on Cu(100): H-bonded molecular ribbons clearly resolved by STM are stable up to 275 K. In the Chemistry, Concepts and in a controlled way [2]. Perspectives (VCH, Weinheim, corresponding O 1s XPS data a broadened peak 1995). We have performed combined scanning appears due to overlaping hydroxyl and carbonyl [2] J. V. Barth et al., Appl. Phys. tunnelling microscopy (STM) and synchrotron intensities, i.e., tpa carboxylic groups remain complete. In conjunction with the NEXAFS A 76, 645 (2003). investigations addressing the bonding, analysis demonstrating flat adsorption, the [3] O.M. Yaghi et al., Nature 423, ordering and surface chemistry of the 705 (2003). modeling indicates a 2-D H-bond coupling [4] A. Dmitriev et al., J. Phys. related molecules 1,4-benzenedicarboxylic scheme. 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). 20 HL_1403.p65 20 16.03.2004, 10:49 Uhr energy of 285.2 and 289.8 eV, respectively molecule, which is associated with a lateral Fig. 3: [8]. Accordingly, the O 1s peak appears coupling scheme where the anionic tpa A molecular monolayer with a broadened due to overlapping carbonyl and carboxylate groups are engaged in O...HC biterephthalate species is obtained upon room temperature hydroxyl intensities (531.9 and 533.4 eV, hydrogen bridges with phenyl rings of deposition. STM data reveal an Fig. 2). The NEXAFS analysis of this phase adjacent molecules. For comparison, on increased packing density of the indicates that adsorption with the aromatic Cu(110) under similar conditions an upright overlayer lattice oriented along ring parallel to the substrate prevails. standing monoterephthalate layer is formed [110]. While the O 1s XPS data Interestingly, the modelling reveals that the [10]. This demonstrates that the geometry indicate a deprotonation of both H-bonds formed between the adsorbed and symmetry of the substrate can be used tpa carboxyl groups, the NEXAFS molecules do not obey the R-COOH pairing to tune the orientation of functional organic analysis shows that the phenyl motif typically observed for pure tpa or other layers. A somewhat different scenario is ring is oriented parallel to the substrate lattice. carboxylic acids, rather the interplay between encountered with tmla layers evolving with substrate corrugation and functional group the substrate held at elevated temperature interactions makes a lateral 2-D coupling (450 K), whose topography is shown in scheme preferable. For the related case of Fig. 4. Now the XPS data indicate merely a low-temperature tmla adsorption (T < 200 K) partial deprotonation of the carboxyl groups, synchrotron data similarly reveal flat adsorp- which signals the formation of a monocarbo- tion of integral molecules. However, no xylate species. Moreover, the NEXAFS supramolecular ordering could be observed analysis of this phase clearly reveals a by STM, which is associated with the striking change in the bonding geometry reduced symmetry and mobility of the (Fig. 4). The phenyl ring plane of the mono- molecule. trimellate is, on average, oriented at an angle of approximately 65°±5° relative to the At increased temperatures (T=300 - ~500 K) Cu(100) surface. This observation is regular organic layers can be obtained with consistent with the only partial depro- both molecules, already imaged by STM tonation of the tmla molecule, where the (Figs. 3,4). The photoemission data show carboxylate anchoring is dominated by just that a (partial) deprotonation of the carboxy- one of the three functional groups. lic acid groups occurs because of the catalytic activity of the Cu substrate. With tpa the formation of a biterephthalate is Fig. 4: Regular organic layer upon encountered, as deduced from the reduced deposition of tmla at 450 K, with separation of the splitted C 1s contributions a characteristic three-lobe inner (now at 285.1 and 288.2 eV, respectively) structure of individual molecules and the existence of a narrowed O1s peak in STM imaging. Corresponding centred at 531.4 eV (Fig. 3). Again the NEXAFS data indicate a NEXAFS analysis reveals that the tpa phenyl reorientation of the phenyl ring ring is oriented parallel to the substrate oriented approximately 65°±5° lattice - the pronounced C 1s -> * reso- relative to the Cu(100) surface. nance exhibits a marked intensity variation with the angle of incidence. The fact that the intensity of the resonance does not vanish completely at an incidence angle of 90° as expected for a perfectly flat lying tpa mol- ecule is attributed to adsorption induced intramolecular distortions, involving a bent of the CH-bonds out of the ring plane. Such distortions of aromatic compounds interact- Contact ing with metal surfaces are observed frequently [9]. Surprisingly, the STM data Johannes Barth [email protected] show that the packing density with the Thomas Strunskus negatively charged biterephthalate species is [email protected] increased as compared to the integral 21 HL_1403.p65 21 16.03.2004, 10:49 Uhr.