Sequential and Site-Specific On-Surface Synthesis on a Bulk Insulator

Kittelmann M, Nimmrich M, Lindner R, Gourdon A, Kühnle A. Sequential and Site-Specific On-Surface Synthesis on a Bulk Insulator. ACS Nano. 2013;7(6):5614-5620.The bottom-up construction of functional devices from molecular building blocks offers great potential in tailoring materials properties and functionality with utmost control. An important step toward exploiting bottom-up construction for real-life applications is the creation of covalently bonded structures that provide sufficient stability as well as superior charge transport properties over reversibly linked self-assembled structures. On-surface synthesis has emerged as a promising strategy for fabricating stable, covalently bound molecular structure on surfaces. So far, a majority of the structures created by this method have been obtained from a rather simple one-step processing approach. But the on-surface preparation of complex structures will require the possibility to carry out various reaction steps in a sequential manner as done In solution chemistry. Only one example exists in literature in which a hierarchical strategy is followed to enhance structural complexity and reliability on a metallic surface. Future molecular electronic application will, however, require transferring these strategies to nonconducting surfaces. Bulk insulating substrates are known to pose significant challenges to on-surface synthesis due to the absence of a metal catalyst and their low surface energy, frequently resulting In molecule desorption rather than reaction activation. By carefully selecting a suitable precursor molecule, we succeeded in performing a two-step linking reaction on a bulk Insulating surface. Besides a firm anchoring toward the substrate surface, the reaction sites and sequential order are encoded In the molecular structure, providing so far unmatched reaction control in on-surface synthesis on a bulk insulating substrate

From dewetting to wetting molecular layers:C60 on CaCO3(1014) as a case study

Rahe P, Lindner R, Kittelmann M, Nimmrich M, Kühnle A. From dewetting to wetting molecular layers:C60 on CaCO3(1014) as a case study. Physical Chemistry Chemical Physics. 2012;14(18):6544-6548.We report the formation of extended molecular layers of C-60 molecules on a dielectric surface at room temperature. In sharp contrast to previous C-60 adsorption studies on prototypical ionic crystal surfaces, a wetting layer is obtained when choosing the calcite (CaCO3)(10 (1) over bar4) surface as a substrate. Non-contact atomic force microscopy data reveal an excellent match of the hexagonal lattice of the molecular layer with the unit cell dimension of CaCO3(10 (1) over bar4) in the [01 (1) over bar0] direction, while a lattice mismatch along the [(4) over bar(2) over bar 61] direction results in a large-scale moire modulation. Overall, a (2 x 15) wetting layer is obtained. The distinct difference observed microscopically upon C-60 adsorption on CaCO3(10 (1) over bar4) compared to other dielectric surfaces is explained by a macroscopic picture based on surface energies. Our example demonstrates that this simple surface-energy based approach can provide a valuable estimate for choosing molecule-insulator systems suitable for molecular self-assembly at room temperature

On-Surface Covalent Linking of Organic Building Blocks on a Bulk Insulator

Kittelmann M, Rahe P, Nimmrich M, Hauke CM, Gourdon A, Kühnle A. On-Surface Covalent Linking of Organic Building Blocks on a Bulk Insulator. ACS Nano. 2011;5(10):8420-8425.On-surface synthesis in ultrahigh vacuum provides a promising strategy for creating thermally and chemically stable molecular structures at surfaces. The two-dimensional confinement of the educts, the possibility of working at higher (or lower) temperatures in the absence of solvent, and the templating effect of the surface bear the potential of preparing compounds that cannot be obtained in solution. Moreover, covalently linked conjugated molecules allow for efficient electron transport and are, thus, particularly interesting for future molecular electronics applications. When having these applications in mind, electrically insulating substrates are mandatory to provide sufficient decoupling of the molecular structure from the substrate surface. So far, however, on-surface synthesis has been achieved only on metallic substrates. Here we demonstrate the covalent linking of organic molecules on a bulk insulator, namely, calcite. We deliberately employ the strong electrostatic interaction between the carboxylate groups of halide-substituted benzoic adds and the surface calcium cations to prevent molecular desorption and to reach homolytic cleavage temperatures. This allows for the formation of aryl radicals and intermolecular coupling. By varying the number and position of the halide substitution, we rationally design the resulting structures, revealing straight lines, zigzag structures, and dimers, thus providing clear evidence for the covalent linking. Our results constitute an important step toward exploiting on-surface synthesis for molecular electronics and optics applications, which require electrically insulating rather than metallic supporting substrates

Influence of charge transfer doping on the morphologies of C-60 islands on hydrogenated diamond C(100)-(2 x 1)

Nimmrich M, Kittelmann M, Rahe P, et al. Influence of charge transfer doping on the morphologies of C-60 islands on hydrogenated diamond C(100)-(2 x 1). Physical Review B. 2012;85(3): 35420.The adsorption and island formation of C-60 fullerenes on the hydrogenated C(100)-(2 x 1):H diamond surface is studied using high-resolution noncontact atomic force microscopy in ultrahigh vacuum. At room temperature, C-60 fullerene molecules assemble into monolayer islands, exhibiting a hexagonally close-packed internal structure. Dewetting is observed when raising the substrate temperature above approximately 505 K, resulting in two-layer high islands. In contrast to the monolayer islands, these double-layer islands form extended wetting layers. This peculiar behavior is explained by an increased molecule-substrate binding energy in the case of double-layer islands, which originates from charge transfer doping. Only upon further increasing the substrate temperature to approximately 615 K, the wetting layer desorbs, corresponding to a binding energy of the charge transfer-stabilized film of 1.7 eV

Tuning Molecular Self-Assembly on Bulk Insulator Surfaces by Anchoring of the Organic Building Blocks

Rahe P, Kittelmann M, Neff JL, et al. Tuning Molecular Self-Assembly on Bulk Insulator Surfaces by Anchoring of the Organic Building Blocks. Advances Materials. 2013;25(29):3948-3956.Molecular self-assembly constitutes a versatile strategy for creating functional structures on surfaces. Tuning the subtle balance between intermolecular and molecule-surface interactions allows structure formation to be tailored at the single-molecule level. While metal surfaces usually exhibit interaction strengths in an energy range that favors molecular self-assembly, dielectric surfaces having low surface energies often lack sufficient interactions with adsorbed molecules. As a consequence, application-relevant, bulk insulating materials pose significant challenges when considering them as supporting substrates for molecular self-assembly. Here, the current status of molecular self-assembly on surfaces of wide-bandgap dielectric crystals, investigated under ultrahigh vacuum conditions at room temperature, is reviewed. To address the major issues currently limiting the applicability of molecular self-assembly principles in the case of dielectric surfaces, a systematic discussion of general strategies is provided for anchoring organic molecules to bulk insulating materials

Transition of Molecule Orientation during Adsorption of Terephthalic Acid on Rutile TiO2(110)

Rahe P, Nimmrich M, Nefedov A, Naboka M, Wöll C, Kühnle A. Transition of Molecule Orientation during Adsorption of Terephthalic Acid on Rutile TiO2(110). Journal of Physical Chemistry C. 2009;113(40):17471-17478.The coverage-dependent mode of adsorption of terephthalic acid [C6H4(COOH)(2), TPA] on rutile TiO2(110) was investigated by means of noncontact atomic force microscopy (NC-AFM) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy under ultrahigh vacuum conditions at room temperature. Individual molecules are observed to adsorb in an disordered, flat-lying geometry at low coverages up to similar to 0.3 monolayer (ML). The molecules are immobile at room temperature. implying a diffusion barrier larger than 0.8 eV. This rather high value might be explained by anchoring to surface defect sites. A transition from flat-lying to upright-oriented molecules is revealed by NEXAFS when saturation coverage is achieved. High-resolution NC-AFM images reveal two different Structures at coverages between similar to 0.8 and 1 ML: (i) a well-ordered (2 x 1) structure and (ii) a structure of single and paired rows oriented along the [001] crystallographic direction. The latter structure might originate from a pairwise interaction of two neighboring molecules through the top carboxyl groups. Further increase in the exposure results in it saturation of the corresponding signal in the NEXAFS spectra, revealing that the growth of TPA oil TiO2(110) at room temperature is self-limiting

Controlled Activation of Substrate Templating in Molecular Self-Assembly by Deprotonation

Kittelmann M, Nimmrich M, Neff JL, et al. Controlled Activation of Substrate Templating in Molecular Self-Assembly by Deprotonation. Journal of Physical Chemistry C. 2013;117(45):23868-23874.Templated assembly of organic molecules constitutes a promising approach for fabricating functional nanostructures at surfaces with molecular-scale control. Using the substrate template for steering the adsorbate growth enables creating a rich variety of molecular structures by tuning the subtle balance of intermolecular and molecule-surface interactions. On insulating surfaces, however, surface templating is largely absent due to the comparatively weak molecule-surface interactions compared to metallic substrates. Here, we demonstrate the activation of substrate templating in molecular self-assembly on a bulk insulator by controlled deprotonation of the adsorbed molecules upon annealing. Upon deposition of 4-iodobenzoic acid onto the natural cleavage plane of calcite held at room temperature, high molecular mobility is observed, indicating a small diffusion barrier. Molecular islands only nucleate at step edges. These islands show no commensurability with the underlying substrate, clearly indicating the absence of surface templating. Upon annealing the substrate, the molecules undergo a transition from the protonated to the deprotonated state. In the deprotonated state, the molecules adopt a well-defined adsorption position, resulting in a distinctly different, substrate-templated molecular structure that is stable at room temperature. Our work, thus, demonstrates the controlled activation of substrate templating by changing the molecule-surface interaction upon annealing

Atomic-resolution imaging of clean and hydrogen-terminated C(100)-(2x1) diamond surfaces using noncontact AFM

Nimmrich M, Kittelmann M, Rahe P, et al. Atomic-resolution imaging of clean and hydrogen-terminated C(100)-(2x1) diamond surfaces using noncontact AFM. Physical Review B. 2010;81(20): 201403.High-purity, type IIa diamond is investigated by noncontact atomic force microscopy (NC-AFM). We present atomic-resolution images of both the electrically conducting hydrogen-terminated C(100)-(2 x 1) : H surface and the insulating C(100)-(2 x 1) surface. For the hydrogen-terminated surface, a nearly square unit cell is imaged. In contrast to previous scanning tunneling microscopy experiments, NC-AFM imaging allows both hydrogen atoms within the unit cell to be resolved individually, indicating a symmetric dimer alignment. Upon removing the surface hydrogen, the diamond sample becomes insulating. We present atomic-resolution images, revealing individual C-C dimers. Our results provide real-space experimental evidence for a (2 x 1) dimer reconstruction of the truly insulating C(100) surface

Molecular Self-Assembly of Enantiopure Heptahelicene-2-Carboxylic Acid on Calcite (1014)

Hauke CM, Rahe P, Nimmrich M, et al. Molecular Self-Assembly of Enantiopure Heptahelicene-2-Carboxylic Acid on Calcite (1014). Journal of Physical Chemistry C. 2012;116(7):4637-4641.Chirality can have a decisive influence on the molecular structure formation upon self-assembly on surfaces. In this paper, we study the structures formed by enantiopure (M)-heptahelicene-2-carboxylic acid ((M)-[7]HCA) on the calcite (10 (1) over bar4) cleavage plane under ultrahigh vacuum conditions. Previous noncontact atomic force microscopy studies have revealed that the racemic mixture of (M)-[7]HCA and (P)-[7]FICA (1:1) self-assembles into well-defined molecular double rows that are oriented along the calcite [01 (1) over bar0] direction. Here, we investigate the enantiopure (M)[7]HCA compound, resulting in distinctly different molecular structures upon deposition onto calcite (10 (1) over bar4). In sharp contrast to the racemate, the (M)-[7]HCA enantiomer forms molecular islands with a (2 x 3) superstructure. Comparison of the results presented here for the enantiopure compound with the results previously obtained from the racemate indicates that heterochiral recognition is responsible for the formation of the unidirectional double row structures formed by the racemate

Growth kinetics of racemic heptahelicene-2-carboxylic acid nanowires on calcite (104)

Einax M, Richter T, Nimmrich M, et al. Growth kinetics of racemic heptahelicene-2-carboxylic acid nanowires on calcite (104). Journal of Chemical Physics. 2016;145(13):134702.Molecular self-assembly of racemic heptahelicene-2-carboxylic acid on a dielectric substrate at room temperature can be used to generate wire-like organic nanostructures consisting of single and double molecular rows. By means of non-contact atomic force microscopy, we investigate the growth of the wire-like pattern after deposition by experimental and theoretical means. From analyzing the time dependence of the mean row length, two distinct regimes were found. At the early post-deposition stage, the mean length grows in time. Subsequently, a crossover to a second regime is observed, where the mean row length remains nearly constant. We explain these findings by a mean-field rate equation approach providing a comprehensive picture of the growth kinetics. As a result, we demonstrate that the crossover between the two distinct regimes is accomplished by vanishing of the homochiral single rows. At later stages only heterochiral double row structures remain. Published by AIP Publishing