In this dissertation, the studies aim to characterize the electronic structure at the internal interface of hybrid materials. The characterization challenge is originating from the spectral superposition of hybrid constituents. A characterization protocol based on photoemission spectroscopy (PES) was developed and applied to investigate the orbital alignment at the internal interface of the oligothiophene-TiO2 and ArS-CdSe hybrid materials by characterizing the individual constituents and the assembly hybrids respectively. Electrospray deposition technique was used to deposit targeting materials which enabled preparation of thin films in vacuum minimizing ambient contaminations while transmission electron microscopy (TEM) was used to investigate the morphology and the particle size of the pure nanoparticles and the hybrids. Ultraviolet-visible (UV-vis) spectroscopy was also used in the estimation of the optical band gap of the pure nanoparticles and the HOMO-LUMO gap of the organic ligands. One of the hybrid materials studied in this dissertation is oligothiophene-TiO2 nanoparticle hybrids in which the oligothiophene ligands are bonded to the surface of TiO2 nanoparticles covalently. This hybrid system was used to develop and demonstrate a measurement protocol to characterize the orbital alignment at the internal interface. Low intensity X-ray photoemission spectroscopy (LIXPS) was used to determine the work function of the oligothiophene ligands and the TiO2 nanoparticles. In combination with the highest occupied molecular orbital (HOMO) cutoff and the valence band maximum (VBM) measured by ultraviolet photoemission spectroscopy (UPS), the ionization energies (IE) of these two constituents were determined. X-ray photoemission spectroscopy (XPS) was used to characterize the core level emissions of the constituents and the hybrid assembly, which were used to determine the charge injection barriers at the internal interface. The results showed that there was an interface dipole at the internal interface between organic and inorganic constituents of the hybrid. The dipole was determined to be 0.61 eV and the hole injection barrier at the internal interface amounted to 0.73 eV. The electron injection barrier was estimated by taking into account the gap between highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO). The procedure followed only suggested the presence of an insignificant barrier in the oligothiophene-TiO2 nanoparticle hybrids. Arylthiol functionalized Cadmium Selenide (ArS-CdSe) is a novel hybrid material which can be used in hetero-junction solar cells. The ArSH ligands are bonded on the surface of the CdSe nanoparticles covalently through sulfur atoms serving as anchors. The internal interface in the ArS-CdSe hybrids between the organic constituent and the inorganic constituent was studied by the same characterization protocol developed in this dissertation. Furthermore, a physisorbed interface between the ArSH ligands and the CdSe nanoparticles was created through multi-step in-vacuum deposition procedure. The electrospray deposition technique enabled the formation of a well-defined physisorbed interface which was characterized by LIXPS, UPS and XPS for each deposition step. Accordingly, the orbital alignment at the physisorbed interface was determined. Based on the results obtained, detailed orbital alignments at the ArSH/CdSe physisorbed interface and the internal interface in the ArS-CdSe hybrid materials were delineated and discussed. The hole injection and electron injection barrier at the physisorbed ArSH/CdSe interface are 0.7 eV and 1.0 eV respectively. An interface dipole of 0.4 eV was observed at the interface. In the ArS-CdSe hybrid materials, the electronic system of the ArSH component shifts down due to the charge transfer induced by the covalent hybridization. The hybridization also shifts the electronic system of the CdSe constituent to a lower energy level due to saturation of the unoccupied bonds of the Cd atoms on the surface. The hole injection barrier and electron injection barrier were determined to be 0.5 eV and 1.2 eV respectively. A small interface dipole (0.2 eV) was observed at the internal interface as a result of the presence of covalent bonds.

The studies presented here address the characterization of the electronic structure of various self-assembled monolayers (SAMs) of peptide nucleic acid (PNA) and tetraphenylporphyrin (TPP) SAMs and arrays, formed on gold substrates. PNA is a promising alternative to DNA for bio-sensing applications, as well as for strategies for self-assembly based on nucleic acid hybridization. In recent years charge transfer through PNA molecules was a focus of research due to possible applications in self-assembled molecular circuits and molecular tools. In light of this research it is interesting to investigate the electronic structure of PNA interfaces to gold, a potential electrode material. TPP is, due to its electronic structure, an organic p-type molecular semiconductor. Such a material can provide an alternative to standard micro- and optoelectronic devices and in recent years more attention was paid to semiconducting polymers and organic compounds offering these low-cost and flexible alternatives. Therefore, it is of high importance to investigate the prospect of using modified TPP molecules for the formation of interconnected molecular networks on metallic surfaces. All investigated monolayers were formed from solution in a nitrogen atmosphere inside a homemade glove box. This process allowed for PNA SAM and TPP SAM and array formation on clean Au substrates without the exposure to the ambient atmosphere. Ultraviolet and X-ray photoemission spectroscopy (UPS and XPS) measurements on the resulting PNA SAMs and TPP SAMs and arrays, which were performed in a to the glove box attached vacuum chamber containing a photoemission spectrometer, revealed the hole injection barriers at the interfaces and the interface dipoles. In addition to the UPS and XPS measurements on PNA, electronic structure calculations based on molecular dynamics sampling of the PNA structure were obtained, yielding the HOMO-LUMO gap and the electronic density of states for PNA. Combined with the UPS data, the theoretical calculations enabled estimation of the charge injection barriers for the PNA SAMs at the interface, as well as the assignment of individual UP-spectral features to specific molecular orbitals. The orbital line-up at the interface between the Au substrate and the PNA indicated a significant interface dipole resulting in the alignment of the Au Fermi level near the center of the PNA HOMO-LUMO gap. This alignment causes large charge injection barriers for both holes and electrons, and thus impedes charge transfer from Au into the PNA SAM. The study of PNA molecules with ferrocene termini showed that this hole injection barrier is shifted to lower energies at the PNA/ferrocene interface. This shift was explained with a molecular orbital reconfiguration through the presence of the ferrocene terminus. The further investigation of the dependence of the electronic structure of PNA SAMs, based on their orientation, showed that incomplete films containing flat lying molecules can have a significant impact on the charge injection barriers. The close proximity of the nucleobases to the Au surface offers new ways for charge transfer between the substrate and the PNA molecule through its nitrogen sites, leading to a lowering of the hole injection barrier at the interface. The TPP arrays were formed by depositing AgNO3 on the Au substrate prior to TPP incubation using the electrospray technique. The interaction of AgNO3 with the TPP promoted the formation of an interconnected thin film forming a network on the Au substrate. The line-up at the Au/TPP interface without AgNO3 exposure showed an interface dipole formation with injection barriers that would potentially obstruct charge injection into the molecule. However, the addition of AgNO3 to the process resulted in the formation of fine structures, and lead to a lower hole injection barrier due to an induced dipole, which would ultimately improve charge transfer between the substrate and the thin film. A separate thiolated TPP derivative was used to form SAMs on a gold substrate. The SAM exhibited an even lower injection barrier than the mentioned TPP thin film with AgNO3 exposure, leading to the conclusion that a mix of both TPP derivatives could potentially lead to a SAM with long range interconnectivity and a low hole injection barrier towards the substrate.