Graduation Year

2014

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Electrical Engineering

Major Professor

Rudy Schlaf

Keywords

charge injection barrier, dipole, electrospray, photoemission spectroscopy, work function

Abstract

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.

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