University of Technology Graz - Institute of Solid State Physics 1 Harald Etschmaier LB-films and SAMs in OTFTs Langmuir-Blodgett films and chemically reactive Self Assembled Monolayers in Organic Thin film Transistors Harald Etschmaier IF – Seminar, Graz,

University of Technology Graz - Institute of Solid State Physics 2 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Outline  Part I: Langmuir Blodgett films –The Method –Characterization –Modified OTFTs  Part II: Chemically Reactive SAMs –Theory –Setup –Measurements and results

University of Technology Graz - Institute of Solid State Physics 3 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs 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)

University of Technology Graz - Institute of Solid State Physics 4 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Idea of the LB-film Principle copied from nature Cell membrane is stabilized by surrounding water

University of Technology Graz - Institute of Solid State Physics 5 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs The technical approach Binks, B.P. Adv. Colloid Interface Sci., 34 (1991) 343 Apply non-soluble molecules on water surface transfer one by one monolayer to substrate by dipping and removing

University of Technology Graz - Institute of Solid State Physics 6 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs The Langmuir Blodgett Trough Force Sensor is measuring the surface pressure (=surface tension of pure water – surface tension of water with floating molecules)

University of Technology Graz - Institute of Solid State Physics 7 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs 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

University of Technology Graz - Institute of Solid State Physics 8 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Layer formation Diacetylene is dissolved in chloroform and dropped on the surface The barrier compresses the molecules to a solid state liquid gaseous solid Multiple layers

University of Technology Graz - Institute of Solid State Physics 9 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Layer transfer Adam, G.; Läuger, P.; Stark, G.; Physikalische Chemie und Biophysik wettability of sample changes for each dipping in theory any number of monolayers possible

University of Technology Graz - Institute of Solid State Physics 10 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Sample preparation  SiO x substrates with different pretreatments: 1.Oxygen plasma etched  hydrophilic 2.HDMS layer  hydrophobic 3.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

University of Technology Graz - Institute of Solid State Physics 11 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Recorded area transfer out in out Decrease of surface area gives a good feedback for layer deposition

University of Technology Graz - Institute of Solid State Physics 12 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs XRR - Different layer thickness Some kind of wetting layer is always the same The Bragg Peak develops with increasing number of LB layers DI Heinz-Georg Flesch

University of Technology Graz - Institute of Solid State Physics 13 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs 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

University of Technology Graz - Institute of Solid State Physics 14 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs XRR  D-spacing of layers approximately 4.8 nm (one repeated unit consists of two LB layers)

University of Technology Graz - Institute of Solid State Physics 15 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs AFM Center of sample (LB-film) Edge of sample (SIOx substrate) AFM can proof that something was deposited DI Heinz-Georg Flesch

University of Technology Graz - Institute of Solid State Physics 16 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs AFM ~1,25nm ~2nm AFM data gives no clear result for the layer thickness DI Heinz-Georg Flesch

University of Technology Graz - Institute of Solid State Physics 17 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Pentacene Deposition No LB-film1 monolayer diacetylene Dr. Anja Haase Higher nucleation density, three dimensional grains Better homogeneity in depth and a lower density of defects Disordered solid made of dendritic grains High concentration of defects, mainly located at grain boundaries Kalb et al. / Synthetic Metals 146 (2004)

University of Technology Graz - Institute of Solid State Physics 18 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Light Microscope Pentacene on LB-film Pentacene on SiO x SiO x Pentacene on SiO x

University of Technology Graz - Institute of Solid State Physics 19 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs FETs No improvement achieved so far – Further experiments in process!

University of Technology Graz - Institute of Solid State Physics 20 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Part II  Time resolved measurements of chemically reactive SAMs

University of Technology Graz - Institute of Solid State Physics 21 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Setup  Application of CSTS SAM on SiOx substrate  Spin coating of P3HT  Evaporation of Au contacts  Electrical characterization P3HT Pacher et al., submitted

University of Technology Graz - Institute of Solid State Physics 22 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Doping and Dedoping mechanism Pacher et al., submitted Shifts the threshold voltage by up to 60V to negative values

University of Technology Graz - Institute of Solid State Physics 23 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Creating the gas mixture Argon Ammonia Flow meters Valves Measuring cell

University of Technology Graz - Institute of Solid State Physics 24 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs The measuring cell Potassic-bromide windows Three-way valves Spring contacts Source Drain Gate

University of Technology Graz - Institute of Solid State Physics 25 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Results – threshold voltage

University of Technology Graz - Institute of Solid State Physics 26 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Results – threshold voltage

University of Technology Graz - Institute of Solid State Physics 27 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Results - mobility

University of Technology Graz - Institute of Solid State Physics 28 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs 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

University of Technology Graz - Institute of Solid State Physics 29 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Additional observations After reaction with ammonia FETs are very sensitive to light: Turn-on-voltage shifts by up to 70V upon illumination.

University of Technology Graz - Institute of Solid State Physics 30 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs Additional observations Measurements influence device characteristics

University of Technology Graz - Institute of Solid State Physics 31 Harald Etschmaier Seminar - Graz, LB-films and SAMs in OTFTs 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