Molecular self-assembly on an insulating surface: interplay between substrate templating and intermolecular interactions

Kittelmann M, Rahe P, Kühnle A. Molecular self-assembly on an insulating surface: interplay between substrate templating and intermolecular interactions. Journal of Physics : Condensed Matter. 2012;24(35): 354007.We report on molecular self-assembly of biphenyl-4,4'-dicarboxylic acid (BPDCA) on CaCO3(10 (1) over bar4) under ultra-high vacuum conditions. Two-dimensional, ordered islands are obtained upon deposition at room temperature, coexisting with a streaky structure that is ascribed to individual, mobile molecules forming a two-dimensional gas-like phase. High-resolution non-contact atomic force microscopy (NC-AFM) images of the molecular islands reveal an ordered inner structure that is dominated by rows of molecules aligned side by side running along the [(42) over bar 61] crystallographic direction. A detailed analysis of these rows exhibits inter-row distances that are multiples of the calcite unit cell dimension along the [01 (1) over bar0] direction, clearly demonstrating the templating effect of the substrate. Our results indicate that an excellent size match of the molecular structure with respect to the underlying substrate results in an increased binding of the BPDCA molecules to the surface. In between the rows, a different molecular structure is coexisting with the molecules aligning head to tail. This structure is explained by intermolecular hydrogen bond formation very similar to the BPDCA bulk structure. The coexistence of the bulk-like structure with the row structure suggests a close balance of intermolecular and molecule-surface interactions to be responsible for the observed structure formation

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

Direct Visualization of Molecule Deprotonation on an Insulating Surface

Kittelmann M, Rahe P, Gourdon A, Kühnle A. Direct Visualization of Molecule Deprotonation on an Insulating Surface. ACS Nano. 2012;6(8):7406-7411.Elucidating molecular-scale details of basic reaction steps on surfaces is decisive for a fundamental understanding of molecular reactivity within many fields, including catalysis and on-surface synthesis. Here, the deprotonation of 2,5-dihydroxybenzoic acid (DHBA) deposited onto calcite (101;4) held at room temperature is followed in situ by noncontact atomic force microscopy. After deposition, the molecules form two coexisting phases, a transient striped phase and a stable dense phase. A detailed analysis of high-resolution noncontact atomic force microscopy images indicates the transient striped phase being a bulk-like phase, which requires hydrogen bonds between the carboxylic acid moieties to be formed. With time, the striped phase transforms into the dense phase, which is explained by the deprotonation of the molecules. In the deprotonated state, the molecules can no longer form hydrogen bonds, but anchor to the surface calcium cations with their negatively charged carboxylate group. The deprotonation step is directly confirmed by Kelvin probe force microscopy images that unravel the change in the molecular charge

Decisive influence of substitution positions in molecular self-assembly

Neff JL, Kittelmann M, Bechstein R, Kühnle A. Decisive influence of substitution positions in molecular self-assembly. Physical Chemistry Chemical Physics. 2014;16(29):15437-15443.Molecular self-assembly provides a versatile tool for creating functional molecular structures at surfaces. A rational design of molecular structure formation requires not only an in-depth understanding of the subtle balance between intermolecular and molecule-surface interactions, but might also involve considering chemical changes of the molecules, such as deprotonation. Here, we present a systematic investigation of a comparatively simple class of molecules, namely dihydroxybenzoic acid, which, nevertheless, enables creating a rich variety of structures when deposited onto calcite (10.4) held at room temperature. Based on non-contact atomic force microscopy measurements in ultra-high vacuum, our study demonstrates the decisive impact of the positions of the hydroxyl groups on the structure formation. Six isomers of dihydroxybenzoic acid exist which form six different molecular structures on the calcite surface. Surprisingly, only two isomers arrange into stable, ordered structures at sub-monolayer coverage: 2,5-dihydroxybenzoic acid forms a commensurate (1 x 5) structure, composed of deprotonated molecules. A double-row structure consisting of protonated molecules is observed for 3,5-dihydroxybenzoic acid. The positions of the functional groups steer the molecular self-assembly of dihydroxybenzoic acids in three distinct ways, namely by (a) affecting the deprotonation tendency of the acid group, (b) influencing the intermolecular interaction as already indicated by greatly different bulk structures and (c) altering the molecule-substrate matching. Our results, thus, shed light on the impact of rather small changes in the molecular structure on the structural variety in molecular self-assembly on surfaces

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

Increasing the templating effect on a bulk insulator surface: from a kinetically trapped to a thermodynamically more stable structure

Molecular self-assembly, governed by the subtle balance between intermolecular and molecule-surface interactions, is generally associated with the thermodynamic ground state, while the competition between kinetics and thermodynamics during its formation is often neglected. Here, we present a simple model system of a benzoic acid derivative on a bulk insulator surface. Combining high-resolution noncontact atomic force microscopy experiments and density functional theory, we characterize the structure and the thermodynamic stability of a set of temperature-dependent molecular phases formed by 2,5-dihydroxybenzoic acid molecules, self-assembled on the insulating calcite (10.4) surface. We demonstrate that a striped phase forms before the thermodynamically favored dense phase, indicating a kinetically trapped state. Our theoretical analysis elucidates that this striped-to-dense phase transition is associated with a distinct change in the chemical interactions involved in the two phases. The striped phase is characterized by a balance between molecule-molecule and molecule-substrate interactions, reminiscent of the molecular bulk. In contrast, the dense phase is formed by upright standing molecules that strongly anchor to the surface with a comparatively little influence of the intermolecular interactions, i.e., in the latter case the substrate acts as a template for the molecular structure. The kinetic trapping stems from a relatively strong intermolecular interaction between molecules in the striped phase that need to be broken before the substrate-templated dense phase can be formed. Thus, our results provide molecular level insights into two qualitatively different bonding motifs of a simple organic molecule on a bulk insulator surface. This understanding is mandatory for obtaining predictive power in the rational design of molecular structures on insulating surfaces. © 2016 American Chemical Society

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

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

Substrate Templating Guides the Photoinduced Reaction of C-60 on Calcite

Lindner R, Rahe P, Kittelmann M, Gourdon A, Bechstein R, Kühnle A. Substrate Templating Guides the Photoinduced Reaction of C-60 on Calcite. Angewandte Chemie. International Edition. 2014;53(30):7952-7955.A substrate-guided photochemical reaction of C-60 fullerenes on calcite, a bulk insulator, investigated by non-contact atomic force microscopy is presented. The success of the covalent linkage is evident from a shortening of the intermolecular distances, which is clearly expressed by the disappearance of the moire pattern. Furthermore, UV/Vis spectroscopy and mass spectrometry measurements carried out on thick films demonstrate the ability of our setup for initiating the photoinduced reaction. The irradiation of C-60 results in well-oriented covalently linked domains. The orientation of these domains is dictated by the lattice dimensions of the underlying calcite substrate. Using the lattice mismatch to deliberately steer the direction of the chemical reaction is expected to constitute a general design principle for on-surface synthesis. This work thus provides a strategy for controlled fabrication of oriented, covalent networks on bulk insulators