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Diamond Materials

Diamond Materials for Quantum Application

23. September 2014: The DFG research group FOR 1493 “Diamond Materials and Quantum Applications” goes into its second funding period. FOR1493 is a national research consortium funded by the Deutsche Forsch-ungsgemeinschaft.

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Rubrene single crystals: Pyramids shed light on excitons

In organic single crystals – in contrast to inorganic systems – the crystal lattice is not stabilized by valence bonds but rather by van-der-Waals forces between the molecules. This yields to optical properties which are fundamentally different from those of inorganic crystalline semiconductors.

Essential questions arising from the properties of weakly bond molecular crystals are the nature of their optical excitations and their spatial transport. In cooperation with scientists from Würzburg, Garching and Cambridge we tackle those questions by studying the photoluminescence (PL) spectra of self-organized surface structures on rubrene single crystals. As a result it appears that the excitonic excitation has to be divided into free i.e. non-localized and so called molecular, “self-trapped” excitons. Spatially resolved intensity measurements along the sides and on the apex of the pyramidal surface structures show an increase of the PL intensity by about one order of magnitude. Beside this strong field enhancement, it is noteworthy that the shape of the PL spectra, and therefore the ratio between free and molecular excitons, changes significantly. This can be explained by spatial confinement of the diffusive transport of free excitons due to scattering at the structure boundaries. By means of temperature-dependent and time-resolved measurements we were able to extract diffusion parameters for free excitons as well as their activation energies for the transition to molecular excitons.
Future studies are to explore the growth mechanism which is behind the forming of this self-organized surface modification.


Original publication:
“Enhanced photoluminescence from self-organized rubrene single crystal surface structures”
by R. J. Stöhr, G. J. Beirne, P. Michler, R. Scholz, J. Wrachtrup, and J. Pflaum
Appl. Phys. Lett. 96, 231902 (2010); doi:10.1063/1.3449124