Tunnelmicroscopie en tunnelspectroscopie van zelfassemblages van atomen en van moleculen

Progress in understanding the fundamental principles of self-assembly has been driven by great interest in the application of this approach in nanotechnology. Self-assembly can be used as a bottom-up method to fabricate functional nanostructures, which may overcome the fundamental limits of other, more traditional top-down methods such as nanolithography. This work is focused on investigations of self-assembly of cyanine dye molecules, which can be used effectively as a tool to tune the optical properties of materials and improve their light absorption efficiency. The low-temperature scanning tunnelling microscopy (STM) and spectroscopy (STS) in ultra-high vacuum (UHV) used in this thesis are unique and powerful tools for investigating structural and electronic properties of in situ prepared self-assemblies. It is common to prepare self-assemblies from solutions containing different types of liquids. Compared to these standard wet methods, vacuum deposition of cyanine dyes used in this work has several advantages for experimental characterisation of self-assemblies at the (sub-)molecular scale, such as an extended temperature range and better control over the surface coverage. In this PhD thesis, the self-assembly of thermally deposited cyanine dyes under UHV conditions is explored. This technique is highly compatible with STM, which allows investigations with atomic resolution. The work focuses on two different substrates, i.e., the Au(111) surface as a representative of the metal substrate and Br layers on Ag(111) as interlayer for decoupling the molecules from the metal surface. In the first part of the thesis, cyanine dye molecules deposited on a Au(111) metal surface were found to form one-dimensional (1D) assemblies, i.e., chains. Within the chains, the molecules appear in two different conformations adsorbed flat on the surface. The reconstruction of the Au(111) surface does not allow the formation of two-dimensional (2D) molecular aggregates but rather directs the formation of observed chains. The results suggest that chains are formed due to a combination of different intermolecular bonds, including halogen-nitrogen, halogen-halogen, and halogen-hydrogen bonds. Moreover, the halogen-nitrogen bond dominates over other types of interactions. In addition, a difference between the optical energy gap and the energy gap measured using the STS method is revealed. STS provides an estimate of the electron-hole bond energy in these molecules of 0.8 - 0.9 eV. To find a suitable substrate for the efficient formation of 2D self-assemblies in UHV, the properties of Br layers adsorbed on Ag(111) were studied. The Br layers form different structures depending on coverage. It is shown that Br/Ag(111) layers exhibit a low density of states (DOS) near the Fermi energy, and a high DOS at 1.5 - 2.0 eV. The band structure of Br/Ag(111) obtained from density functional theory calculations confirms the appearance of additional states above the Fermi energy, which are mainly associated with Br orbitals. An increase in the work function of the Br/Ag(111) surface is found, which indicates an increase in the surface dipole barrier. The latter result, combined with a low DOS near the Fermi energy, may explain the reduction in coupling between Ag electrons and molecules adsorbed on the Br/Ag(111) surface. In addition, the structure and electronic properties of ultrathin AgBr islands formed together with Br layers on Ag(111) were studied. This allows construction of a structural model of these silver tribromide islands (AgBr3) based on the observed atomic trimers that compose the surface. The AgBr tunnelling spectra exhibit a negative differential resistance, which may be linked to states formed at the AgBr-Ag interface. In contrast to AgBr islands, the Br/Ag(111) surface is found to be a more appropriate substrate for investigating molecular self-assembly. In the third part of the thesis, Br layers prepared in UHV on Ag(111) were used as substrates to study individual cyanine dye molecules as well as their self-assemblies. Dye molecules form 1D and 2D aggregates depending on coverage. At low coverage, in addition to individual dimers, braid-like aggregates are formed by dimers linked by monomers. Close-packed dimers and brickwork structures are observed at elevated coverages. The structural parameters of these aggregates are further used to model changes in their optical absorption spectra, based on the Frenkel exciton model combined with the extended dipole moment approximation. Coverage-dependent aggregate structures on Br/Ag(111) involve three types of aggregates: J-aggregate (red-shifted spectra) for the brickwork structure, H-aggregate (blue-shifted spectra) for the dimers, and I-aggregate ('intermediate' aggregate) for the braid-like and close-packed dimers. The blue-shifted spectra for the dimers are also well supported by STS. According to the model, the aggregates obtained in UHV can exhibit smaller shifts in optical absorption compared to aggregates formed in solution. The work described in this PhD thesis opens additional opportunities for studying self-assemblies of different dyes on halogenated metals under UHV conditions. The effectiveness of the UHV low-temperature STM and STS methods for studying cyanine dye aggregates has been demonstrated. This work enhances understanding of the link between the optical properties and the structure of dye aggregates at different coverages.