Supramolecular ordering of PTCDA molecules: the key role of dispersion forces in an unusual transition from physisorbed into chemisorbed state

On surface chemical reactions of free-base and titanyl porphyrins with r-TiO2(110): a unified picture

In this Perspective we present a comprehensive study of the multiple reaction products of metal-free porphyrins (2H-Ps) in contact with the rutile TiO₂(110) surface. In the absence of peripheral functionalization with specific linkers, the porphyrin adsorption is driven by the coordination of the two pyrrolic nitrogen atoms of the macrocycle to two consecutive oxygen atoms of the protruding O_br rows via hydrogen bonding. This chemical interaction favours the iminic nitrogen uptake of hydrogen from near surface layers at room temperature, thus yielding a stable acidic porphyrin (4H-P). In addition, a mild annealing (∼100 °C) triggers the incorporation of a Ti atom in the porphyrin macrocycle (self-metalation). We recently demonstrated that such a low temperature reaction is driven by a Lewis base iminic attack, which lowers the energy barriers for the outdiffusion of Ti interstitial atoms (Ti_int) [Kremer et al., Appl. Surf. Sci., 2021, 564, 150403]. In the monolayer (ML) range, the porphyrin adsorption site, corresponding to a TiO-TPP configuration, is extremely stable and tetraphenyl-porphyrins (TPPs) may even undergo conformational distortion (flattening) by partial cyclo-dehydrogenation, while remaining anchored to the O rows up to 450 °C [Lovat et al., Nanoscale, 2017, 9, 11694]. Here we show that, upon self-metalation, isolated molecules at low coverage may jump atop the rows of five-fold coordinated Ti atoms (Ti_5f). This configuration is associated with the formation of a new coordination complex, Ti-O-Ti_5f, as determined by comparison with the deposition of pristine titanyl-porphyrin (TiO-TPP) molecules. The newly established Ti-O-Ti_5f anchoring configuration is found to be stable also beyond the TPP flattening reaction. The anchoring of TiO-TPP to the Ti_5f rows is, however, susceptible to the cross-talk between phenyls of adjacent molecules, which ultimately drives the TiO-TPP temperature evolution in the ML range along the same pathway followed by 2H-TPP.

Self-ordering of chemisorbed PTCDA molecules on Ge(001) driven by repulsive forces

Realization of future hybrid electronic devices combining organic and inorganic semiconductors requires a well-defined interface between both components. Such an interface can be formed generally by self-ordering of organic molecules on inorganic substrates, which is usually hindered by strong covalent bonds to the semiconductor surface. In this paper, the 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) molecules were unexpectedly found to form a locally self-ordered monolayer on a strongly interacting semiconductor surface of the Ge(001). Molecular arrangements with preferential separations between the molecules were observed by the scanning tunneling microscopy at various coverages of the molecules and substrate temperatures, suggesting strong inter-molecular interaction. Atomic structures of two paired molecules and their inter-molecular interaction energies in five different configurations were calculated by density functional theory. Simple Monte Carlo simulations show that mobility of molecules activated only by the inter-molecular interactions is sufficient to reproduce the local self-ordering. A dominant inter-molecular interaction between neighboring chemisorbed molecules has mostly positive energy (destabilizing) except for a single configuration, which leads to the formation of one-dimensional chains of the molecules and finally a periodic two-dimensional array by increasing the coverage.

Nanotribological Study of Supramolecular Template Networks Induced by Hydrogen Bonds and van der Waals Forces

Nanotribology has been given increasing attention by researchers in pursuing the nature of friction. In the present work, an approach that combines the supramolecular assembly and nanotribology is introduced. Herein, the nanotribological study was carried out on seven supramolecular template networks [namely, hydrogen bond induced tricarboxylic acids and van der Waals force induced hexaphenylbenzene (HPB) derivatives]. The template networks, as well as the host-guest assemblies of template molecules induced by different forces, were constructed on the highly oriented pyrolytic graphite (HOPG) surface and explicitly characterized using scanning tunneling microscopy (STM). Meanwhile, the nanotribological properties of the template networks were measured using atomic force microscopy (AFM). Together with the theoretical calculation using the density functional theory (DFT) method, it was revealed that the friction coefficients were positively correlated with the interaction strength. The frictional energy dissipation mainly derives from both the intermolecular interaction energy and the interaction energy between molecules and the substrate. The efforts not only help us gain insight into the competitive mechanisms of hydrogen bond and van der Waals force in supramolecular assembly but also shed light on the origin of friction and the relationship between the assembly structures and the nanotribological properties at the molecular level.

Amidoxime-modified chitosan for pigment red 224 enrichment through reversible assembly

An amidoxime-modified chitosan, featuring favorable porosity and super-lipophilic properties, was successfully prepared for pigment red 224 enrichment.

Imaging prototypical aromatic molecules on insulating surfaces: a review

Insulating substrates allow for in-plane contacted molecular electronics devices where the molecule is in contact with the insulator. For the development of such devices it is important to understand the interaction of molecules with insulating surfaces. As substrates, ionic crystals such as KBr, KCl, NaCl and CaF₂ are discussed. The surface energies of these substrates are small and as a consequence intrinsic properties of the molecules, such as molecule-molecule interaction, become more important relative to interactions with the substrates. As prototypical molecules, three variants of graphene-related molecules are used, pentacene, [Formula: see text] and PTCDA. Pentacene is a good candidate for molecular electronics applications due to its high charge carrier mobility. It shows mainly an upright standing growth mode and the morphology of the islands is strongly influenced by dewetting. A new second flat-lying phase of the molecule has been observed. Studying the local work function using the Kelvin method reveals details such as line defects in the center of islands. The local work function differences between the upright-standing and flat-lying phase can only be explained by charge transfer that is unusual on ionic crystalline surfaces. [Formula: see text] nucleation and growth is explained by loosely bound molecules at kink sites as nucleation sites. The stability of [Formula: see text] islands as a function of magic numbers is investigated. Peculiar island shapes are obtained from unusual dewetting processes already at work during growth, where molecules 'climb' to the second molecular layer. PTCDA is a prototypical semiconducting molecule with strong quadrupole moment. It grows in the form of elongated islands where the top and the facets can be molecularly resolved. In this way the precise molecular arrangement in the islands is revealed.

Selective Adsorption of Coronene atop the Polycyclic Aromatic Diimide Monolayer Investigated by STM and DFT

The self-assemblies of polycyclic aromatic diimide (PAI) compounds on solid surfaces have attracted great interest because of the versatile and attractive properties for application in organic electronics. Here, a planar guest species (coronene) selectively adsorbs on the helicene-typed PAI1 monolayer strongly, depending on the conjugated cores of these PAIs. PAI1 molecule displays evidently a bowl structure lying on the highly oriented pyrolytic graphite surface due to the torsion of the "C"-shaped fused benzene rings. In combination with density functional theory calculation, the selective inclusion of coronene atop the backbone of the PAI1 array might be attributed to the bowl structure, which provides a groove for immobilizing coronene molecules. On the other planar densely packed arrays, it is difficult to observe the unstable adsorption of coronene. The selective addition of coronene molecules would be a strategic step toward the controllable multicomponent supramolecular architectures.

Very high temperature tiling of tetraphenylporphyrin on rutile TiO2(110)

We demonstrate the thermal stability up to 450 °C of a titanium(iv)-porphyrin monolayer grown on the rutile TiO₂(110) surface. Starting from a film of metal-free tetra-phenyl-porphyrin, 2HTPP, deposited at room temperature, we show that, beyond the self-metalation reaction at 150°-200 °C, a second phase transition takes place at ∼350 °C. Using surface diffraction and microscopy, we observe a change of the phase symmetry from (2 × 4)-obliq to (2 × 6)-rect. Core level photoemission indicates that the chemical states of both the molecular tetrapyrrolic macrocycle and the substrate are unchanged. X-ray absorption spectroscopy reveals that the driving mechanism is a rotation of the phenyl terminations towards the substrate (flattening) that triggers a conformational change of the molecule through partial cyclo-dehydrogenation. From comparison with first principles calculations, we show that the common feature of these multiple phase transitions is the chemical nature of the porphyrin bonding atop the substrate oxygen rows: the coordination of the macrocycle central pocket to the oxygen atoms beneath is preserved throughout both the self-metalation and flattening reactions. The molecular orientation and arrangement are determined by steric constraints and intermolecular interactions, whereas the specific adsorption site is further stabilized by the interaction of the peripheral C-H network with the adjacent oxygen rows. Porphyrins are thus trapped at the TiO₂(110) surface, where they demonstrate an exceptionally high thermal stability (up to ∼450 °C), which makes this interface potentially useful for sensors and photocatalysis applications in harsh environments.

Ordering of Zn-centered porphyrin and phthalocyanine on TiO2(011): STM studies

Zn(II)phthalocyanine molecules (ZnPc) were thermally deposited on a rutile TiO₂(011) surface and on Zn(II)meso-tetraphenylporphyrin (ZnTPP) wetting layers at room temperature and after elevated temperature thermal processing. The molecular homo- and heterostructures were characterized by high-resolution scanning tunneling microscopy (STM) at room temperature and their geometrical arrangement and degree of ordering are compared with the previously studied copper phthalocyanine (CuPc) and ZnTPP heterostructures. It was found that the central metal atom may play some role in ordering and growth of phthalocyanine/ZnTPP heterostructures, causing differences in stability of upright standing ZnPc versus CuPc molecular chains at given thermal annealing conditions.

Unveiling universal trends for the energy level alignment in organic/oxide interfaces

Organic/oxide interfaces exhibit an energy-level-alignment universal behaviour when a bias is applied. Coulomb-blockade regime is ruled by the organic electronegativity.

Isotropic thin PTCDA films on GaN(0 0 0 1)

The growth of 3, 4, 9, 10-perylene tetracarboxylic dianhydride (PTCDA) on the Ga-polar GaN(0 0 0 1) surface has been studied by x-ray photoelectron spectroscopy (XPS), spot profile analysis low-energy electron diffraction (SPA-LEED), near edge x-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The stoichiometric ratios derived from XPS indicate that the molecules remain intact upon adsorption on the surface. Furthermore, no chemical shifts can be observed in the C 1s and O 1s core levels with progressing deposition of PTCDA, suggesting none or only weak interactions between the molecules and the substrate. NEXAFS data indicate the PTCDA molecules being oriented with their molecular plane parallel to the surface. High-resolution STM shows PTCDA islands of irregular shape on the sub-micron scale, and together with corresponding SPA-LEED data reveals a lateral ordering of the molecules that is compatible with the presence of (1 0 2) oriented PTCDA nano-crystals. SPA-LEED moreover clearly shows the presence of homogeneously distributed rotational domains of two-dimensionally isotropic PTCDA.

Scanning probe microscopy studies on the adsorption of selected molecular dyes on titania

Titanium dioxide, or titania, sensitized with organic dyes is a very attractive platform for photovoltaic applications. In this context, the knowledge of properties of the titania-sensitizer junction is essential for designing efficient devices. Consequently, studies on the adsorption of organic dyes on titania surfaces and on the influence of the adsorption geometry on the energy level alignment between the substrate and an organic adsorbate are necessary. The method of choice for investigating the local environment of a single dye molecule is high-resolution scanning probe microscopy. Microscopic results combined with the outcome of common spectroscopic methods provide a better understanding of the mechanism taking place at the titania-sensitizer interface. In the following paper, we review the recent scanning probe microscopic research of a certain group of molecular assemblies on rutile titania surfaces as it pertains to dye-sensitized solar cell applications. We focus on experiments on adsorption of three types of prototypical dye molecules, i.e., perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), phtalocyanines and porphyrins. Two interesting heteromolecular systems comprising molecules that are aligned with the given review are discussed as well.

Ordered heteromolecular overlayers formed by metal phthalocyanines and porphyrins on rutile titanium dioxide surface studied at room temperature

Molecular heterostructures are formed from meso-tetraphenyl porphyrins-Zn(II) (ZnTPP) and Cu(II)-phthalocyanines (CuPc) on the rutile TiO2(011) surface. We demonstrate that ZnTPP molecules form a quasi-ordered wetting layer with flat-lying molecules, which provides the support for growth of islands comprised of upright CuPc molecules. The incorporation of the ZnTPP layer and the growth of heterostructures increase the stability of the system and allow for room temperature scanning tunneling microscopy (STM) measurements, which is contrasted with unstable STM probing of only CuPc species on TiO2. We demonstrate that within the CuPc layer the molecules arrange in two phases and we identify molecular dimers as basic building blocks of the dominant structural phase.

Transformations of PTCDA structures on rutile TiO2 induced by thermal annealing and intermolecular forces

Transformations of molecular structures formed by perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecules on a rutile TiO2(110) surface are studied with low-temperature scanning tunnelling microscopy. We demonstrate that metastable molecular assemblies transform into differently ordered structures either due to additional energy provided by thermal annealing or when the influence of intermolecular forces is increased by the enlarged amount of deposited molecules. Proper adjustment of molecular coverage and substrate temperature during deposition allows for fabrication of desired assemblies. Differences between PTCDA/TiO2(110) and PTCDA/TiO2(011) systems obtained through identical experimental procedures are discussed.

TiO2(110) Charge Donation to an Extended π-Conjugated Molecule

The surface reduction of rutile TiO2(110) generates a state in the band gap whose excess electrons are spread among multiple sites, making the surface conductive and reactive. The charge extraction, hence the surface catalytic properties, depends critically on the spatial extent of the charge redistribution, which has been hitherto probed by small molecules that recombine at oxygen vacancy (Ovac) sites. We demonstrate by valence band resonant photoemission (RESPES) a very general charge extraction mechanism from a reduced TiO2(110) surface to an extended electron-acceptor organic molecule. Perylene-tetra-carboxylic-diimide (PTCDI) is not trapped at Ovac sites and forms a closely packed, planar layer on TiO2(110). In this configuration, the perylene core spills out the substrate excess electrons, filling the lowest unoccupied molecular orbital (LUMO). The charge transfer from the reduced surface to an extended π-conjugated system demonstrates the universality of the injection/extraction mechanism, opening new perspectives for the coupling of reducible oxides to organic semiconductors and supported catalysts.

Adsorption and Self-Assembly of Large Polycyclic Molecules on the Surfaces of TiO2 Single Crystals

Titanium dioxide is one of the most frequently studied metal oxides, and its (110) rutile surface serves as a prototypical model for the surface science of such materials. Recent studies have also shown that the (011) surface is relatively easy for preparation in ultra-high vacuum (UHV) and that both the (110) and (011) surfaces could be precisely characterized using scanning tunneling microscopy (STM). The supramolecular self-assembly of organic molecules on the surfaces of titanium dioxide plays an important role in nanofabrication, and it can control the formation and properties of nanostructures, leading to wide range of applications covering the fields of catalysis, coatings and fabrication of sensors and extends to the optoelectronic industry and medical usage. Although the majority of experiments and theoretical calculations are focused on the adsorption of relatively small organic species, in recent years, there has been increasing interest in the properties of larger molecules that have several aromatic rings in which functional units could also be observed. The purpose of this review is to summarize the achievements in the study of single polycyclic molecules and thin layers adsorbed onto the surfaces of single crystalline titanium dioxide over the past decade.