Muon spin relaxation as a probe of electron spin relaxation in organic semiconductors
Abstract
The purpose of this thesis is to study the electron spin relaxation (eSR) in small organic
molecular semiconductors using the muon spin relaxation (MuSR) technique.
One of the inherent problems in utilising the spin degree of freedom is the lack of
understanding of the fundamental mechanisms behind spin relaxation. Two interactions
have been proposed as the dominant mechanisms behind the spin relaxation,
the Hyper ne interaction (HFI) and the Spin Orbit (SO) interaction. There remains
much debate over the models for these interactions and their exact role, a contention
that drives the work carried out in this thesis.
The MuSR technique is utilised providing a novel molecular scale probe sensitive to
relaxation rates in the range of 0.01-10 MHz. The Avoided Level crossing (ALC)
MuSR application is useful in accessing the spin relaxation information. Temperature
dependent ALC-MuSR measurements are performed for a selection of functionalised
acenes and Quinolate molecules. Transverse eld MuSR measurements
are also taken to determine the Hyper ne coupling constants present. DFT and
semi-empirical computational methods are employed to determine theoretical values
for the isotropic and anisotropic terms and the suitability of these methods was
discussed.
It is concluded that an intra-molecular eSR is present in all small organic molecular
semiconductors. The mechanism behind this eSR was found not to be the HFI
but in fact the SO interaction. It is also determined that the eSR is coupling to
the vibrations in the molecule and a possible theory based on the curvature of the
molecule from the vibrational modes inducing an enhanced SO coupling is proposed,
which results in the eSR. The nal part of this thesis looks at the future experiments
that have been initiated or can be conducted to further the understanding of spin
relaxation and determine the role of a vibrationally enhanced SO coupling.
Authors
Willis, MaureenCollections
- Theses [3831]