Friday 30 Nov 2012Colloquium: Local probe investigation of spin and charge dynamics in organic semiconductors

Dr. Alan Drew - Queen Mary University of London

Newman F 12:00-13:00

Organic semiconductors fall into a class of materials that shows significant potential for future applications and as a result, the field is becoming extremely topical. This is due to their ease of processing, low cost, highly tuneable electronic properties, favourable mechanical properties and long spin coherence times. The latter point makes them extremely promising for future spintronic applications. However, there is a lack of suitable techniques that can yield information on intrinsic spin and charge carrier dynamics in organic materials. For example, many of the experimental techniques available that probe the spin polarisation of charge carriers in inorganic spintronic devices/materials are not always applicable to organic materials. Muon spectroscopy is a technique that has rarely been applied to study spintronic problems in inorganic systems, yet is ideally suited to studying them in organic semiconductors.

Low energy muon spin rotation can directly measure the depth resolved spin polarisation of charge carriers in organic spin injection devices [1]. After giving a brief introduction to muon spin rotation in the context of these results, I will go on to demonstrate that it is possible to separate out the various contributions to spin decoherence in organic spin valves, differentiating between interface and bulk spacer layer effects [2].

A more exotic application of the muon technique, known as avoided level crossing (ALC) spectroscopy, can be used to probe the spin dynamics in organic semiconductors on a molecular lengthscale [3,4]. After briefly introducing this application of muons, I will go on to present measurements of temperature dependent electron spin relaxation rates, on a series of organic molecules of different morphology and molecular structure. These measurements, when combined with some of the latest results on the mass-dependence of electron spin relaxation rates, offer clues as to the underlying relaxation mechanism in organic semiconductors [4,5].

Finally, taking advantage of the intrinsic spatial sensitive of ALC spectroscopy, I will show how laser excited ALC spectroscopy (a technique I am currently developing) can offer unique insight into electron transfer in organic molecules. Not only is this an important process in organic electronics/spintronics, it also is fundamental to many biological processes, including photosynthesis, DNA repair and cell respiration.

[1] A. J. Drew et al., Nature Materials 8, 109 (2009)
[2] L. Schulz et al., Nature Materials 10, 39 (2011)
[3] A. J. Drew et al., Phys. Rev. Lett. 100, 116601 (2008)
[4] L. Schulz et al., Phys. Rev. B 84, 085209 (2011)
[5] L. Nuccio et al., submitted.

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