Structure-Property Relation in Self-Assembled Monolayers: Electrostatic Engineering, Charge Transport Properties, and Thermal Stability

The major goals of the thesis are design and characterization of functional self-assembled monolayers (SAMs) in context of electrostatic interfacial engineering and molecular electronics as well as a study of their thermal stability. The issue of electrostatic engineering can be addressed using custom-designed SAMs with either terminal dipolar groups or dipolar groups embedded into the molecular backbone. As for the first task, the novel concept of embedded dipole was successfully applied to the oxide substrates, which are highly important for photovoltaic applications. A variation of the work function of indium tin oxide (ITO) by 0.5 eV as compared to the reference non-polar functionalization was achieved at the invariable character of the SAM-ambient interface, allowing, thus, to decouple electrostatic engineering from the interface chemistry. The extremely low work function value for one of the tested monolayers expands a rather limited selection of SAMs capable of significantly lowering the work function of ITO. As a further task, electrostatic effects in charge transport across monomolecular films were studied, which is currently one of the most intensely discussed topics in molecular electronics. The tuning of the electrostatic properties was achieved by the fabrication of binary SAMs of biphenylthiolates (BPT) on Au(111), namely by mixing of BPT with fluorine-substituted-BPT (F-BPT) and 4-methyl-4′-BPT (CH3-BPT) with 4-trifluoromethyl-4′-BPT (CF3-BPT). The charge tunneling rate across the binary SAMs was found to vary progressively with their composition between the values for the single-component monolayers, and could, consequently, be fine-tuned and correlated with the work function. The observed behavior was tentatively explained by the appearance of an internal electrostatic field in the SAMs, leading to a change of the energy-level alignments within the junction upon contact of the SAMs to the top eutectic GaIn electrode. The height of the respective injection barrier is, however, unaffected by such a field, corresponding to the values of the transition voltage, which do not change notably with the SAM composition. Analysis of the presented and literature data suggests that the position of a dipolar group in SAM-forming molecules has significant impact on the charge transport behavior of the respective SAMs in the context of molecular electronics. As the next sub-project in the latter context, custom-designed SAMs of ferrocene/ruthenocene-substituted biphenylthiolates and fluorenethiolates on Au(111) were studied. The novel element of these SAMs was the fully conjugated molecular backbone, in contrast to the previous studies utilizing alkyl linkers as elements of molecular diodes. The designed SAMs exhibited a highly exceptional charge transport behavior showing conductance switching triggered by the applied bias. The extent of this switching, described by a maximum rectification ratio (RR) higher than 1000, was comparable to the best performing molecular diodes but in contrast to these “devices” was maintained at very low bias, close to zero volts. The observed behavior could be tentatively explained by a non-reversible redox process affecting the electronic structure of the molecules and their coupling to the top electrode. The above results are particularly promising to create novel molecular devices for potential applications in electronic circuits, molecular memory, or as an electrochemical sensor. Finally, the issue of thermal stability of functional SAMs on coinage metal and oxide substrates was addressed. This issue is of a crucial importance for applications, defining the temperature range of SAM-based devices and framing the preparation routes involving high temperature steps. Several representative SAMs with thiol anchoring group on Au(111) substrates and phosphonic acid (PA) anchoring group on Al2O3 substrates were studied by high resolution X-ray photoelectron spectroscopy chosen as the most suitable experimental tool. The range of the thermal stability and the degradation pathways were found to depend on the chemical composition of the SAM-forming molecules and the character of the substrates, with such crucial parameters as the strength of substrate-anchoring group bond and the presence of a backbone-specific “weak links”. In general, PA monolayers on oxide substrates were found to have higher robustness and better thermal stability compared to thiolates SAMs on coinage metal substrates. My results show, however, that is always advisable to test thermal stability of a specifically designed functional SAM in context of possible “weak links” as far as this stability is important for a particular application