Designing molecular nano-architectures on metals and on graphene

Nano-engineering of molecular two-dimensional materials brings exciting opportunities to achieve novel and tunable surface functionalities. Among these nanomaterials, graphene (a single-layer of carbon atoms) and supramolecular architectures on surfaces are the central topic of this thesis. Scanning tunneling and non-contact atomic force microscopy have been used to study the fundamental aspects of molecular self-assembly on surfaces pushing the real space resolution to the current achievable limit by “looking inside the molecules”. In this way, self-organized molecular architectures could be characterized in great detail. In this work, a new way for the creation of graphene on a substrate relevant for industrial microchips production was also developed. This is regarded an important step for the realization of graphene devices. Moreover, the possibility to integrate molecular nano-architectures and graphene is discussed and pursued toward a fine engineering of molecular two-dimensional materials

Photophysics of pentacene-doped picene thin films

Here were report a study of picene nano-cristalline thin films doped with pentacene molecules. The thin films were grown by supersonic molecular beam deposition with a doping concentration that ranges between less than one molecules of pentacene every 104 picene molecules up to about one molecule of pentacene every 102 of picene. Morphology and opto-electronic properties of the films were studied as a function of the concentration of dopants. The optical response of the picene films, characterized by absorption, steady-state and time-resolved photoluminescence measurements, changes dramatically after the doping with pentacene. An efficient energy transfer from the picene host matrix to the pentacene guest molecules was observed giving rise to an intense photoluminescence coming out from pentacene. This efficient mechanism opens the possibility to exploit applications where the excitonic states of the guest component, pentacene, are of major interest such as MASER. The observed mechanism could also serve as prototypical system for the study of the photophysics of host guest systems based on different phenacenes and acenes.Comment: 15 pages, 6 figure

Controlling molecules\u27 momentum with SuMBD: the early stages of Pentacene growth on SiOx/Si

Organic semiconductors are suitable candidates for applications in fields ranging from photonics[1] to devices and sensors realization[2]. Good electrical properties, needed for these applications, can be achieved through molecular assembly in a crystalline structure. On the other hand, the feeble nature of the van der Waals forces between the molecules make this goal not easy to reach in organic thin films and the growth process often gives rise to different polymorphs and molecular orientations that strongly limit the final performances. Such limits need to be overcome by a higher control during the growth process. For this reason, the development of methods that permit to role the molecular assembly through the formation of a crystalline structure is of great interest. Between the vacuum deposition techniques, that up to now give the best results in term of order and purity of the materials, supersonic molecular beam deposition (SuMBD) has shown to improve the control on the growth, giving rise to better morphologies and enhanced electrical properties of the films[3,4]. Using SuMBD we can easily tune the kinetic energy, momentum and internal energy of the impinging molecules[5] influencing the molecules assembly, the island formation and coalescence[6]. The precursor state of the molecule is also important for the activation of new pathways for the adsorption of the molecules on the surface. The possibility to control the different energetic parameters of the impinging molecules and understand how they can influence the molecular assembling is of great importance for the realization of high performances devices. We report a systematic atomic force microscopy study of Pentacene sub-monolayer morphologies on SiOx/Si (60? contact angle), resulting from depositions at room temperature, in different growth conditions. During these early stages of growth, we found that the kinetic energy and the momentum of the impinging molecules play a key role in determining the monolayer morphology. In particular, we investigate the effects induced by changing the parallel and perpendicular momentum of the molecules arriving on the surface. Variations in the energy relaxation mechanisms involving the two component of the momentum activate different adsorption processes leading to modified island fractal dimension, island density and sticking coefficient. The parallel momentum favours a longer mean free path of the molecules adsorbed on the surface giving the possibility for a better assembly and surface energy minimization. Instead, the perpendicular momentum can be fundamental for the activation of deeper adsorption sites on the surface. Our measures show a progressive lowering of the island density when increasing the component of the molecules\u27 momentum parallel to the surface (k//) with the formation of more compact (less fractal) islands, better for the realization of ordered films (Figure1). Large single crystal domains are formed, which is a key aspect for increasing the devices performances

Thermolubricity of gas monolayers on graphene

Nanofriction of Xe, Kr and N2 monolayers deposited on graphene was explored with a quartz crystal microbalance (QCM) at temperatures between 25 and 50 K. Graphene was grown by chemical vapour deposition and transferred to the QCM electrodes with a polymer stamp. Graphene was found to strongly adhere to the gold electrodes at temperatures as low as 5 K and at frequencies up to 5 MHz. At low temperatures, the Xe monolayers are fully pinned to the graphene surface. Above 30 K, the Xe film slides and the depinning onset coverage beyond which the film starts sliding decreases with temperature. Similar measurements repeated on bare gold show an enhanced slippage of the Xe films and a decrease of the depinning temperature below 25 K. Nanofriction measurements of Kr and N2 confirm this scenario. This thermolubric behaviour is explained in terms of a recent theory of the size dependence of static friction between adsorbed islands and crystalline substrates

Unveiling Adatoms in On-Surface Reactions:Combining Scanning Probe Microscopy with van’t Hoff Plots

[Image: see text] Scanning probe microscopy has become an essential tool to not only study pristine surfaces but also on-surface reactions and molecular self-assembly. Nonetheless, due to inherent limitations, some atoms or (parts of) molecules are either not imaged or cannot be unambiguously identified. Herein, we discuss the arrangement of two different nonplanar molecular assemblies of para-hexaphenyl-dicarbonitrile (Ph(6)(CN)(2)) on Au(111) based on a combined theoretical and experimental approach. For deposition of Ph(6)(CN)(2) on Au(111) kept at room temperature, a rhombic nanoporous network stabilized by a combination of hydrogen bonding and antiparallel dipolar coupling is formed. Annealing at 575 K resulted in an irreversible thermal transformation into a hexagonal nanoporous network stabilized by native gold adatoms. However, the Au adatoms could neither be unequivocally identified by scanning tunneling microscopy nor by noncontact atomic force microscopy. By combining van’t Hoff plots derived from our scanning probe images with our density functional theory calculations, we were able to confirm the presence of the elusive Au adatoms in the hexagonal molecular network

Growth and performance of polycrystalline -Sexi-thiophene thin films deposited by Supersonic Molecular Beam Deposition

Conjugated small molecules are very interesting both as a model to study the growth model of crystalline organic films and as a very good performance organic material. Vacuum deposition is the most suitable technique to obtain high purity and order films. Nevertheless, the high anisotropy of organics makes easy the formation of different polymorphs or/and orientations that strongly limit the quality of the films. The innovative supersonic molecular beam deposition (SuMBD) technique, developed at the IFN-Lab, allows a wider control on the growth. The kinetic energy (EK) of the impinging molecules is the key factor that affects the growth modifying the assembling processes of molecules and their surface mobility. We report on the alfa-sexithiophene sub-monolayer growth, investigating the influence of energetic state of the impinging molecules, surface energy and substrate temperature. Each growth parameter affects the morphology of the molecular film in terms of coverage and fractality of the sub-monolayer islands. Optimizing the different parameters, we obtain larger and smoother islands and low density of grain-boundaries. The best conditions, including high kinetic energy of the beam, give rise to organic thin film transistors (OFETs) with a field effect mobility value of 1.5.10-1 V∙cm-1∙s-1, twice higher than the best values in literature. This work was financially supported by Provincia Autonoma di Trento Project Nanosmart and the Fondazione CARITRO Project ODINO

Low-dimensional metal-organic coordination structures on graphene

We report the formation of one- and two-dimensional metal-organic coordination structures from para-hexaphenyl-dicarbonitrile (NC-Ph-6-CN) molecules and Cu atoms on graphene epitaxially grown on Ir(111). By varying the stoichiometry between the NC-Ph-6-CN molecules and Cu atoms, the dimensionality of the metal-organic coordination structures could be tuned: for a 3:2 ratio, a two-dimensional hexagonal porous network based on threefold Cu coordination was observed, while for a 1:1 ratio, one-dimensional chains based on twofold Cu coordination were formed. The formation of metal-ligand bonds was supported by imaging the Cu atoms within the metal-organic coordination structures with scanning tunneling microscopy. Scanning tunneling spectroscopy measurements demonstrated that the electronic properties of NC-Ph-6-CN molecules and Cu atoms were different between the two-dimensional porous network and one-dimensional molecular chains