organic semiconductor nanostructures

tuning the color by changing the pitch

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color tuning of nanofibers by periodic organic–organic hetero-epitaxy

Nowadays display technology is dominated by LEDs which have replaced cathode ray tubes enirely. But the next — organic — revolution is already underway which will, or already has, replace(d) inorganic LED displays. If you are an Android user the chances are very high that you already have an O(rganic)LED display in your mobile phone link. But the progress will not stop there - OLED TV-set are already available - and first lamps with OLEDs to replace conventional light bulbs already hit the market.

In this paper we report on well defined nanostructures of two organic molecules para-hexaphenyl (p-6P) and α-sexi-thiophene (6T). By increasing the content of 6T, the emission color of these nanostructures can be continuously tuned. We grow both molecules epitaxially in periodic multilayer heterostructures on top of p-6P nanotemplates. By the chosen approach, 6T molecules are forced to align parallel to the p-6P template molecules, which yields highly polarized photoluminescence (PL)-emission of both species. The PL spectra show that the fabricated multilayer structures provide optical emission from two different 6T phases, interfacial 6T molecules, and 3-dimensional crystallites. By a periodical deposition of 6T monolayers and p-6P spacers it is demonstrated that the strongly polarized spectral contribution of interfacial 6T can be precisely controlled and amplified. By analyzing the PL emission of both 6T phases as a function of p-6P spacer thickness (Δdp–6P) we have determined a critical value of Δdp–6P ≈ 2.73 nm where interfacial 6T runs into saturation and the surplus of 6T starts to cluster in 3-dimensional crystallites. These results are further substantiated by UPS and XRD measurements. Moreover, it is demonstrated by morphological investigations, provided by scanning force microscopy and fluorescence microscopy, that periodical deposition of 6T and p-6P leads to a significant improvement of homogeneity in PL-emission and morphology of nanofibers. Photoluminescence excitation experiments in combination with time-resolved photoluminescence demonstrate that the spectral emission of the organic multilayer nanofibers is dominated by a resonant energy transfer from p-6P host- to 6T guest-molecules. The sensitization time of the 6T emission in the 6T/p-6P multilayer structures depends on the p-6P spacer thickness, and can be explained by well separated layers of host–guest molecules obtained by organic–organic heteroepitaxy. The spectral emission and consequently the fluorescent color of the nanofibers can be efficiently tuned from the blue via white to the yellow-green spectral range.

This work was published under the Auth choice icon license in:
C. Simbrunner, G. Hernandez-Sosa, F. Quochi, G. Schwabegger, C. Botta, M. Oehzelt, I. Salzmann, T. Djuric, A. Neuhold, R. Resel, M. Saba, A. Mura, G. Bongiovanni, A. Vollmer, N. Koch, H. Sitter
ACS Nano 6, 4629 (2012). full-text

  further reading

C. Simbrunner, D. Nabok, G. Hernandez-Sosa, M. Oehzelt, T. Djuric, R. Resel, L. Romaner, P. Puschnig, C. Ambrosch-Draxl, I. Salzmann, G. Schwabegger, I. Watzinger, H. Sitter
Epitaxy of Rodlike Organic Molecules on Sheet Silicates — A Growth Model Based on Experiments and Simulations
Journal of the American Chemical Society 133, 3056 (2011). link

C. Simbrunner, F. Quochi, G. Hernandez-Sosa, M. Oehzelt, R. Resel, G. Hesser, M. Arndt, M. Saba, A. Mura, G. Bongiovanni, H. Sitter
Organic−Organic Heteroepitaxy of Red-, Green-, and Blue-Emitting Nanofibers
ACS Nano 4, 6244 (2010). link

G. Koller, S. Berkebile, J. R. Krenn, F. P. Netzer, M. Oehzelt, T. Haber, R. Resel, M. G. Ramsey
Heteroepitaxy of organic-organic nanostructures
Nano letters 6, 1207 (2006). link

M. Oehzelt, G. Koller, J. Ivanco, S. Berkebile, T. Haber, R. Resel, F. P. Netzer, M. G. Ramsey
Organic Heteroepitaxy: p‐Sexiphenyl on Uniaxially Oriented α‐Sexithiophene
Advanced Materials 18, 2466 (2006). link

M. Oehzelt, L. Grill, S. Berkebile, G. Koller, F. P. Netzer, M. G. Ramsey
The Molecular Orientation of para‐Sexiphenyl on Cu(110) and Cu(110)p(2×1)O
ChemPhysChem 8, 1707 (2007). link

M. Oehzelt, S. Berkebile, G. Koller, J. Ivanco, S. Surnev, M. G. Ramsey
α-Sexithiophene on Cu(110) and Cu(110)–(2×1)O: An STM and NEXAFS study
Surface Science 603, 412 (2009). link

C. Simbrunner
Epitaxial growth of sexi-thiophene and para-hexaphenyl and its implications for the fabrication of self-assembled lasing nano-fibres
Semiconductor Science and Technology 28, 053001 (2013). link

F. Quochi, G. Schwabegger, C. Simbrunner, F. Floris, M. Saba, A. Mura, H. Sitter, G. Bongiovanni
Extending the Lasing Wavelength Coverage of Organic Semiconductor Nanofibers by Periodic Organic–Organic Heteroepitaxy
Advanced Optical Materials 1, 117 (2013). link

L. Tavares, M. Cadelano, F. Quochi, C. Simbrunner, G. Schwabegger, M. Saba, A. Mura, G. Bongiovanni, D. Antônio da Silva Filho, W. Ferreira da Cunha, H.-G. Rubahn, J. Kjelstrup-Hansen
Efficient Exciton Diffusion and Resonance-Energy Transfer in Multilayered Organic Epitaxial Nanofibers
The Journal of Physical Chemistry C 119 15689 (2015). link

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