Harald Etschmaier

1. Harald Etschmaier Langmuir-Blodgett films and chemically reactive Self Assembled Monolayers in Organic Thin film Transistors IF – Seminar, Graz, 9.1.2008

2. Outline • Part I: Langmuir Blodgett films • The Method • Characterization • Modified OTFTs • Part II: Chemically Reactive SAMs • Theory • Setup • Measurements and results

3. Motivation • LB-films form a very regular structure on the substrate and allow a easy control of the surface wettability. • We hope that these properties can yield a better grain formation when growing pentacene on top and hence result in a better mobility in the transistor. • Two-dimensional charge transport in self-organized, high-mobility conjugated polymers, Sirringhaus et al., Nature Vol 401, 1999 • Improved organic thin-film transistor performance using novel self-assembled monolayers, McDowell, Hill, APL 88, 2006 Organic Electronics, Hagen Klauk, Wiley-VCH Verlag (2006)

4. Idea of the LB-film • Principle copied from nature • Cell membrane is stabilized by surrounding water www.molecularexpressions.com

5. The technical approach • Apply non-soluble molecules on water surface • transfer one by one monolayer to substrate by dipping and removing Binks, B.P. Adv. Colloid Interface Sci., 34 (1991) 343

6. The Langmuir Blodgett Trough • Force Sensor is measuring the surface pressure (=surface tension of pure water – surface tension of water with floating molecules)

7. The molecule Pentacosa-10,12-diynoic acid so-called diacetylene Polymerization upon exposure to UV stabilizes layer Single Monolayer transistors have been build from these films Monolayer Transistor Using a Highly Ordered Conjugated Polymer as the Channel, Scott et al., Nano letter Vol.6, 2006

8. Layer formation • Diacetylene is dissolved in chloroform and dropped on the surface • The barrier compresses the molecules to a solid state Multiple layers solid liquid gaseous

9. Layer transfer • wettability of sample changes for each dipping • in theory any number of monolayers possible Adam, G.; Läuger, P.; Stark, G.; Physikalische Chemie und Biophysik

10. Sample preparation • SiOx substrates with different pretreatments: • Oxygen plasma etched  hydrophilic • HDMS layer  hydrophobic • Perflourinated SAM  super hydrophobic • Optional polymerization on water surface or directly on substrate • Cd-Ions in the head group of the diacetylene to stabilize the layer and improve XRR-contrast

11. Recorded area transfer in out in out Decrease of surface area gives a good feedback for layer deposition

12. XRR - Different layer thickness DI Heinz-Georg Flesch Some kind of wetting layer is always the same The Bragg Peak develops with increasing number of LB layers

13. XRR – illuminated and not illuminated • Smaller and broader Bragg Peak for not polymerized sample (worse multilayer stacking) • Minimum shifts upon polymerization, but stays the same independent from number of LB layers DI Heinz-Georg Flesch

14. XRR • D-spacing of layers approximately 4.8 nm (one repeated unit consists of two LB layers)

15. AFM Center of sample (LB-film) DI Heinz-Georg Flesch Edge of sample (SIOx substrate) AFM can proof that something was deposited

16. AFM ~2nm ~1,25nm DI Heinz-Georg Flesch AFM data gives no clear result for the layer thickness

17. Pentacene Deposition No LB-film 1 monolayer diacetylene Dr. Anja Haase • Disordered solid made of dendritic grains • High concentration of defects , mainly located at grain boundaries • Higher nucleation density, three dimensional grains • Better homogeneity in depth and a lower density of defects Kalb et al. / Synthetic Metals 146 (2004) 279-282

18. Light Microscope SiOx Pentacene on SiOx Pentacene on SiOx Pentacene on LB-film

19. FETs No improvement achieved so far – Further experiments in process!

20. Part II • Time resolved measurements of chemically reactive SAMs

21. Setup P3HT • Application of CSTS SAM on SiOx substrate • Spin coating of P3HT • Evaporation of Au contacts • Electrical characterization Pacher et al., submitted

22. Doping and Dedoping mechanism Pacher et al., submitted Shifts the threshold voltage by up to 60V to negative values

23. Creating the gas mixture Flow meters Ammonia Argon Valves Measuring cell

24. The measuring cell Spring contacts Source Drain Gate Three-way valves Potassic-bromide windows

25. Results – threshold voltage

26. Results – threshold voltage

27. Results - mobility

28. Conclusions from preliminary experiments • Ammonia increases hysteresis: Molecules diffuse into bulk and cause additional trap states • Ammonia decreases mobility: Additional scattering centres • The threshold voltage drops irreversibly upon exposure to Ammonia by. Quantity is dependent on time and concentration.  further experiments

29. Additional observations After reaction with ammonia FETs are very sensitive to light: Turn-on-voltage shifts by up to 70V upon illumination.

30. Additional observations Measurements influence device characteristics

31. Co-Workers • FETs • Peter Pacher, Andrej Golubkov, Egbert Zojer • SAMs • Alexandra Lex, Gregor Trimmel, Christian Slugovc • AFM, XRR • Heinz Georg Flesch, Roland Resel, Anja Haase • LB-films • Martin Weis, Julius Cirak