“Surface self-assembly of functional architectures for nanotechnology applications”

Abstract of PhD Thesis “Material Science and Nanotechnologies” Università degli Studi di Catania Cycle: XXXIV Andrea Maria Gerardo Valenti : “Surface self-assembly of functional architectures for nanotechnology applications” Nowadays, the continuous miniaturization and the increasing performances required to produce electronic devices is leading the scientific community to explore innovative solutions on nanometric scales. In this dimensional range, solutions based on organic or hybrid (organic-inorganic) systems show considerable interest. There are already several studies [1] that testify the possibility to design, with geometric and functional control, nanometric supramolecular architectures able to exhibit different typologies of properties and / or to manifest / modify the latters under the influence of specific external perturbations of a physical or chemical nature. Many of these molecular or supramolecular systems require solvents to perform their specific functions: unfortunately, this represents a strong restriction for almost all the current applications based on solid-state technologies. To overcome this issue, current research in this field focuses on developing methodologies able to integrate different functions that can be exercised at the molecular or supramolecular level on solid substrates. In this context, systems based on molecular structures anchored on surfaces - in a more or less ordered way - play an important role. Such systems are often realized via self-assembly methodologies. One of the areas in which the anchoring of functional molecular systems on solid substrates has found greater success is that of photochemical molecular (PMD) devices. This acronym refers to a variety of systems ranging from solar cells to dye [3] (Dye-sensitized Solar Cells, DSSCs), organic light-emitting diode OLEDs [4] and DSPEC (Dye Sensitized PhotoElectrosynthesis Cells): in particular, the latter are hoped to achieve water split in hydrogen and oxygen. Another interesting application of molecular functionalities integration on solid substrates concerns molecular wires [5]. Such systems consist in supra-molecular structures capable of conducting and/or photoconducting electrons and they can be realized by starting from specific "molecular bricks" (molecular bricks or molecular building blocks), with a stepwise procedure that allows a controlled assembly of the "wires" on the surface of different materials. In this case, the conductive and photoconductive properties induced on the surfaces can be used to design diodes and molecular photodiodes. Thanks to the self-assembling strategy, such hybrid structures allow a control at molecular level able to overcome the current limits of the top-down approach in the realization of systems operating at nanometric scales. To this aim, it is essential to tune the chemical properties on the surface of solids, since very often they do not allow an "easy" molecular engineering [6]. One of the methods used to implement this tuning is the preliminary preparation of Self-Assembled Monolayers (SAMs), i.e. two-dimensional (monomolecular) films of molecules that self-assembly on a substrate in an ordered way. These ultra-thin layers allow to obtain surfaces with well-ordered structures, molecular composition and thickness. The further functionalization of these SAMs can introduce specific macroscopic properties that can be exploited in devices design. The entire doctoral project, in its various tasks, has been developed through the following steps. Firstly, an identification step in which molecular systems with selected functional properties, to be implemented in surface engineering of different types of solid substrates, to design innovative objects in the field of nanoscience and nanotechnologies are identified. A development step, where suitable strategies for anchoring/assembling, depending on the specific properties of the selected building blocks and substrates are developed. A characterization step in which from a compositional, morphological, and structural point of view, the assembled systems according to the above strategies are studied. The last logical step was the test of assembled of the assembled prototype. In the first step of the development of the PhD project, two bis-terpyridine organic ligands were identified as potential systems that can be used in surface functionalization. Experimental procedures have therefore been developed to allow the chemisorption of these systems and their subsequent growth through iterative steps of coordination mediated by the iron ion. The two systems were studied in the liquid phase by UV-Vis spectroscopy to understand the absorption ranges of the ligands and related coordination complexes. Similarly, ToF-SIMS measurements were made to understand the characteristic fragmentations of the ligands and related precipitates. For the ligands indicated as DT and wDT, experimental anchoring procedures were therefore developed that made possible the chemical function of transparent solid substrates: such as ITO and quartz. The growth according to logic attributable to the supramolecular structures usually referred to as molecular wire was successfully monitored by UV-Vis spectroscopy. The systems assembled based on DT-Fe and wDT-Fe on silicon substrates were instead used for the investigation using ToF-SIMS spectrometry. Both characterization techniques confirmed the chemisorption and usability of these systems in the development of functionalization / or surface modification processes. Other molecular systems based on bis-terpyridine ligands precoordinated with Ru (II) ions have been identified. Specifically, 3 building blocks were identified respectively consisting of: 1 ruthenium ion corrected by two bis terpyridine units (indicated as building block Ru1), 2 ruthenium ions corrected by 3 bis terpyridine units (indicated as building block Ru2), 3 ions ruthenium with 4 bis terpyridine units (referred to as building block Ru2). The bis terpyridine units (DT) referred to are those object of study of the previous task of the doctoral project. Solution studies were conducted on these systems to understand the influence of the number of metal centers present in the preformed building blocks on the absorption spectra related to MLCT transitions. The development of anchoring methodologies has allowed the construction of molecular wire-like systems on different solid substrates including: FTO, ITO, quartz and silicon. The monitoring of the molecular wire growth was possible with the use of UV-Vis spectroscopy which allowed the study of the specific absorption bands of the systems assembled on transparent surfaces. ToF-SIMS mass spectrometry has allowed the study of both physisorbed and chemisorbed building blocks on silicon surfaces and to study their specific ion fragmentations. The promising results obtained on these supramolecular systems based on ruthenium and bis-terpyridine functionalities has directed scientific efforts in order to complete the assembly properties on surfaces of different nature. In this regard, we wanted to investigate the plasmonic absorption properties of nanosturtured semitransparent gold surfaces functionalized with molecular threads based on Fe-DT complexes and Ru2DT3 buinding blocks. The development of a gold sputtering procedure on quartz surfaces made it possible to produce semi-transparent substrates. The assembly of the molecular threads on gold surfaces required the development of a different experimental procedure than those previously used on oxide surfaces. In fact, self-assembled monolayers consisting of thiols were used as anchoring platforms for the building blocks (Fe-DT and Ru2DT3). The investigation carried out using UV-Vis spectroscopy made it possible to evaluate the extent of the shift of the plasmonic peak for each cycle of itaration (growth) of the molecular wires. ToF-SIMS mass spectrometry used in static mode was used to evaluate the different functionalization steps of the semi-transparent gold substrates, thus allowing the study of the fragmentation patterns of the different molecular wires (Fe-DT based and Ru2DT3 based) assembled. on gold. Aware of the satisfactory results obtained through the various chemisorption procedures of the structures indicated above on various types of surfaces, we wanted to explore the capabilities in terms of construction of patterned micrometric functionalized surfaces. To do this, a procedure based on photo-patterning was developed. Specifically, with the aid of a hard mask it was possible to remove, exposing the system to UV light, a self-assembled monolayer of phosphonic acid assembled on silicon pretreated via ZP priming. The spatially resolved “free” region allowed the subsequent FeDT-based molecular wire anchoring. In this task of the project, the chemisorption capabilities of Fe-DT systems on silicon surfaces exclusively treated with the ZP priming procedure were investigated. The use of UV-Vis spectroscopy confirmed the presence of photo active centers attributable to the assembly steps of the molecular filaments on quartz surfaces treated only by ZP priming procedure. ToF-SIMS mass spectrometry has been used here, not only for the characterization of the fragmentation patterns related to the samples in the different functionalization steps but it was an essential technique to understand the steps related to the laterally resolved patterning. Obviously in the last case the optimization of the instrumental parameters was required to make the best use of the imaging modality of ToF-SIMS spectrometry. The use of ruthenium-based systems had been identified from the beginning as an interesting system for the development of technological devices where the thin films made by array of molecular wire could play as a photoelectrically active layer. To investigate these functionalities, it was necessary to develop an experimental set up based on the realization of a heterojunction. This hetero junction was constructed by developing gold tracks on silicon thermal oxide surfaces, contacted via a PEDOT film. Using a pattern approach it was possible to interpose a thin layer made by assembling Ru2-based molecular wire between two high conductivity materials and therefore it is now possible to study their electrical properties in light or dark conditions. Specifically, the electrical properties of the heterojunction were recorded by means of a femto amperometer when it was exposed to illumination from a continuous spectrum source, UV range or dark conditions. The results seem to indicate that when the heterojunction is exposed to a continuous light source or UV source, the measured current is less than in the dark condition. An attempt was made to study the three-dimensional characteristics of the structures assembled with the use of the bis terpyridine systems indicated above by means of dual beam ToF-SIMS depth profiling. ToF SIMS measurements in dynamic mode carried out using low-energy cesium "ultra shallow depth profiling" made it possible to describe the ionic components associated with the fragmentation of multilayer systems assembled using bisterpyridine based building blocks from components that can be associated instead with substrates on multilayer systems assembled. The most representative signals of the systems investigated through depth profiling were post processed. Some parameters related to the fittings have brought out trends that are indicative of an dimensional increase of molecular wire systems along a perpendicular direction to the substrate plane. The dynamic ToF-SIMS mode has also been widely used for the development of a project task parallel to the one developed strictly within the PhD program. Specifically, the aim of this project is the construction of a device based on the ECL phenomenon for tracking tumor targets in biological fluids. The task developed in our laboratories has dealt with the production, functionalization, and characterization of micrometric films of nanostructured titanium oxide. It was possible to develop a functionalization strategy aimed at preparing samples in which the functionalization partially or totally affected the thickness of the nanostructured films. To develop a prototype device, the substrates partially or totally functionalized along their thickness were tested by ECL assay. Several interesting aspects have been found that correlate the partial functionalization of the thickness of the nano-structured titanium oxide systems with different triggering mechanisms of the electrochemiluminescence radiative phenomenon.