UVM Theses and Dissertations
Format:
Print
Author:
Pan, Zhenwen
Dept./Program:
Materials Science Program
Year:
2012
Degree:
Ph. D.
Abstract:
Small molecule organic semiconductors such as phthalocyanine (PC), pentacene and their derivatives have generated increasing interest in the scientific and engineering research communities for the last decade because of their low cost for application and processing, tunable chemical properties of molecules and polymorphism in their solid state. This tunability leads to surprisingly different electronic, magnetic and optical properties of films fabricated from similar molecular building blocks.
Small molecular ordered crystalline organic semiconductors should in principle exhibit higher mobilities then their polymeric or amorphous counterparts, because of the large [Pi] orbital overlap causing delocalization of carriers through the organic crystal and the relatively low defect densities. Moreover, the significant long range ordering and molecular interactions will lead to unique optical phenomena and collective magnetism within an organic crystal. Based on all these special properties, applications of crystalline organic semiconductors have been developed in various fields such as electronic media, light emitting devices, photo sensing and light (photon) harvesting. Nevertheless, the fundamental understanding of electronic, optical and magnetic properties of these materials is still lacking.
The objective of this research is to study the electronic and magnetic properties of phthalocyanine-based organic semiconductor crystalline thin film using optical spectroscopy techniques.
Specifically, the electronic states and their correlation with long range ordering in metal-free phthalocyanine crystalline thin film was investigated by using polarization and spectrally resolved microscopy techniques. The measurements simultaneously map the molecular stacking orientation and the polarization-resolved photoluminescence spectra indicating that the selection rules for exciton recombination are preserved in the crystalline phase. The same measurements also probe the electronic states at a single grain boundary and reveal the presence of a monomer-like sharp transition exclusively localized at the grain boundaries indicating the presence of a potential barrier for exciton diffusion across the boundary. Temperature studies of time and polarization resolved photoluminescence (PL) on the same sample shows that long-lived intermolecular charge transfer excitons dominate its PL at low temperature whereas, at room temperature, only Frenkel-exciton PL are mainly observed.
Magnetic properties of metal Phthalocyanines were investigated through highmagnetic field, low temperature magnetic circular dichroism (MCD) spectroscopy. Our system of choice was Cu-PC a quasi 1D spin 1/2 system where direct exchange is negligible and long range magnetic order is achieved via indirect exchange involving ligand [Pi] electrons. The evolution of MCD by magnetic field identifies which electronic states of the valence band manifold mediate the Cu spin exchange. This study reveals for the first time that, in analogy to inorganic diluted magnetic semiconductors, an enhancement of ligand electron g-factors is possible in the presence of long range magnetic ordering. This fundamental observation opens the possibility of engineering organic magnetic semiconductors for all-organic spin devices.
Small molecular ordered crystalline organic semiconductors should in principle exhibit higher mobilities then their polymeric or amorphous counterparts, because of the large [Pi] orbital overlap causing delocalization of carriers through the organic crystal and the relatively low defect densities. Moreover, the significant long range ordering and molecular interactions will lead to unique optical phenomena and collective magnetism within an organic crystal. Based on all these special properties, applications of crystalline organic semiconductors have been developed in various fields such as electronic media, light emitting devices, photo sensing and light (photon) harvesting. Nevertheless, the fundamental understanding of electronic, optical and magnetic properties of these materials is still lacking.
The objective of this research is to study the electronic and magnetic properties of phthalocyanine-based organic semiconductor crystalline thin film using optical spectroscopy techniques.
Specifically, the electronic states and their correlation with long range ordering in metal-free phthalocyanine crystalline thin film was investigated by using polarization and spectrally resolved microscopy techniques. The measurements simultaneously map the molecular stacking orientation and the polarization-resolved photoluminescence spectra indicating that the selection rules for exciton recombination are preserved in the crystalline phase. The same measurements also probe the electronic states at a single grain boundary and reveal the presence of a monomer-like sharp transition exclusively localized at the grain boundaries indicating the presence of a potential barrier for exciton diffusion across the boundary. Temperature studies of time and polarization resolved photoluminescence (PL) on the same sample shows that long-lived intermolecular charge transfer excitons dominate its PL at low temperature whereas, at room temperature, only Frenkel-exciton PL are mainly observed.
Magnetic properties of metal Phthalocyanines were investigated through highmagnetic field, low temperature magnetic circular dichroism (MCD) spectroscopy. Our system of choice was Cu-PC a quasi 1D spin 1/2 system where direct exchange is negligible and long range magnetic order is achieved via indirect exchange involving ligand [Pi] electrons. The evolution of MCD by magnetic field identifies which electronic states of the valence band manifold mediate the Cu spin exchange. This study reveals for the first time that, in analogy to inorganic diluted magnetic semiconductors, an enhancement of ligand electron g-factors is possible in the presence of long range magnetic ordering. This fundamental observation opens the possibility of engineering organic magnetic semiconductors for all-organic spin devices.