Self Assembled Monolayers

Transcription

1 Nanotechnology for engineers Winter semester Plastic Electronics - Self Assembled Monolayers - Organic Materials Based Devices - Organic photovoltaic devices Nanotechnology for engineers Winter semester Self Assembled Monolayers - Lotus effect - Langmuir-Blodgett, Self-assembly and Silanisation and their thermal stability

2 Introduction - Importance of SAMs Self Assembly Bottom-up key-word SAM for lithography (ultrathin resists) Functional SAM for (bio-)chemical detection/analysis Formerly : Coatings, e.g. oil spraying on metals since approx. 1900; Stationary phase production in chromatography (analytical chemistry) since approx. 1920; The lotus effect: correlation between ultrastructure, wettability and contamination Water droplet on lotus leaf, with adhering particles Contaminating stain powder (Sudan III) removed by rinsing with water The Lotus Effect is based on surface roughness caused by different microstructures together with the hydrophobic properties of the epicuticular wax.

3 The lotus effect: correlation between ultrastructure, wettability and contamination Monolayer Oleo and hydrophobic surfaces Chen W., Fadeev A.Y.,Hsieh M.C., Oener D., Youngblood J., McCarthy T.J., Langmuir, 15, 1999,

4 The Lotus Effect: Auto-clean Effect - Only based on physical-chemical principles. - A model for the development of artificial surfaces. - Applications: roofs, facades, paints - beneficial to the environment as a result of the reduction of cleansing agent. Molecular SAMs of large molecules Surface monolayers of large oligomers, polyelectrolytes, bio-molecules, proteins, Substrate Thickness depends on molecule size and 3-D conformation (several nm to hundrets of nm) Langmuir-Blodgett, Self-assembled

5 Molecular SAMs of Small Molecules Surface monolayers of alkyl chain X-(CH 2 ) n molecules n = 1,, 22 Non-polar Substrat polar Langmuir-Blodgett, Self-assembly, Silanisation Molecular SAMs: Which SAM type on which substrate? Type Molecules Substrates Langmuir Blodget Self Assembly Silanisation Alkyl-acids (R-COOH) others Thiols (R-SH) Phosphonates (R- PO 3 H) Silanes (R-SiX 3 ) Silanes (R-SiX 3 ; R-R 2 -SiX) metal-oxides, Al 2 O 3, AgO Any polar or ionic surface Au, Ag, Cu (sans oxyde) Ta 2 O 5 ; TiO 2 ; Al 2 O 3 ;..?.. Any substrate silica hydrated, other oxides

6 Langmuir-Blodgett Assembly Langmuir-Blodgett Assembly

7 Langmuir-Blodgett Assembly LB-films Resuming Overview Advantages: relatively simple, large variability many parameters bath: (conc, ph, solvent, (T), ) molecules: (head-, end-groups, chain length, functional) transfer process: (speed, surface pressure, up-down, ) Multi-layers Disadvantages: layer stability, contamination

8 Self-assembly sample covered by gold Ethanol solution with a thiol(r-s-h) From some minutes to 24 hours at RT SAM layer resuming overview Advantages : thousands of publications with recipes very simple (beaker, solvent, molecules) highest density of SAM s large variability of molecules many functional thiols commercially available Disadvantages: thousands of bad publications limited to few substrates (Au, Ag, Cu) substrate quality (islands) layer stability (unstable to oxidation)

9 Silanisation the Ideal Reaction RnSiX 4 n + SiOH SiOSiRn + 4 nhx n = 1 3 X Cl, OR, NH2, NR2 = R=Alkyl, functional chain R Si R R X + H O -HX Si O R STRONG COVALENT BOND Silanisation the Result Ideal and risk O Si O Si O O O O H Si HO O OH

10 SAM s LB, SA, Sil Thermal Stability SAM type Bonding type Energy Langmuir-Blodgett ionic, electrostatic 0.52 ev = 50 kj/mol Self Assembly R- SH on gold Covalent, d-d 1.87 ev = 177 kj/mol Silanization Covalent (Si-O) Covalent (Si-C) 4.59 ev = 443 kj/mol 3.17 ev = 306 kj/mol SAM s LB, SA, Sil Oxidation Stability SAM type Bonding type Langmuir-Blodgett Self Assembly R- SH on gold ionic, electrostatic Covalent, d-d R-COO-Substr. Oxidation of C-chains (identical to polymers) R-S-Substr. Oxidation of R-S to R-SO 2 -> water soluble Silanization Covalent (Si-O) Covalent (Si-C) R-Si-Substr. Oxidation of C-chains (identical to polymers)

11 SAM s LB, SA, Sil Dissolution Stability SAM type Bonding type Solvent - specific Langmuir-Blodgett ionic, electrostatic Easily soluble in strong polar or ionic solutions Self Assembly R- SH on gold Covalent, d-d Moderately soluble in polar and nonpolar solvents Silanization Covalent (Si-O) Covalent (Si-C) Hydrolysis in strong acids and basis retro-formation reaction SAM layer Heat Effect a) Long Molecules C 22 Stearic acid monolayer : a) K: all trans b) ~ 200K: gauche chain ends c) > 400K: irreversible disorder b) c) Shorter Molecules < C 12

12 SAM Layers Resume For all small molecular SAM s : the structures are comparable in functionality LB layers, before transfert densest (high p), Au-SAM s densest silanised SAMs most stable (3 types of stability!!!!) Attention: details are crucial, very often completely underestimated For all other molecular SAM s : the structures and coverages are determined by the same rules as in colloid chemistry zeta-potential Nanotechnology for engineers Winter semester Organic Materials Based Devices - Organic Crystals - Organic Light Emitting Devices (OLEDs) - Organic Field Effect Transistors (OFETs)

13 Evolution of Device Efficiency 1 lumen =1/ nm ( Organic OLED An electronic device made by placing a series of organic thin films between two conductors. When an electric current is applied between the conductors, a bright light is emitted - electrophosphorescence. Discovered in ~1990 by Friend and Holmes group at University of Cambridge.

14 E-diagrams of different metals with organic material sandwiched Organic OLED

15 Small Molecules and Polymer Device Structures Comparison of high-cost vacuum fabrication and lost-cost printing/coatings of polymers Common problem: gas-tight packaging Small Molecules Structures Comparison of high-cost vacuum fabrication

16 OLED architecture cathode + - electron transmitting layer luminescent layer 300 nm substrate and anode hole transmitting layer Electroluminescent Small Molecules

17 Polymer Device Structures Electroluminescent Conjugated Polymers

18 Multiple Emission Colors Polymer OLED Display Fabrication Steps

19 Inkjet Printing to Pattern Polymers (Full Color Applications) The Holy Grails: Flexible OLEDs

20 Comparison between Devices based on Small Molecules and Polymer Small molecule active matrix display products Polymer passive matrix display products Organic OLED Company screen diagonal Method pixels public. date S = shadow mask evaporation Ti = ink jetting P = polymers M = evaporated molecules 16 segment display single layer OLED. PPV, glass ITO Al-cathode, 4.5 V; Univ. Bayreuth / Germany

21 Organic field effect transistors (OFETs) L organic layer source W drain insulator gate Substrate S Regio-regular poly thiophene n S S S S S S hexathiophene pentacene Performance Figures: Charge carrier mobility µ Organic FETs (room temperature): holes: Pentacene µ = cm 2 /Vs Polythiophene µ = cm 2 /Vs Regio-regular Polythiophene µ = cm 2 /Vs Organic single crystals µ 1-8 cm 2 /Vs Y. Y. Lin et al. IEEE, 44 (1997) A. Tsumura et al. Synth. Met. 25 (1988), 11. H. Sirringhaus et al., Science 280 (1998), N. Karl, Mol. Cryst. Liqu. Cryst. 171, (1989) 157. V. Podzorov et al., Appl. Phys. Lett., 83, (2003) Inorganic FETs (room temperature) electrons/holes: Ge: µ = 3900 /1900cm 2 /Vs Si: µ = 1500/450 cm 2 /Vs GaAs: µ = 8500/400 cm 2 /Vs InAs: µ = 80000/1250 cm 2 /Vs Sze, «Physics of Semiconductors»1981, John Wiley OFETs could be used for active matrix displays

22 Organic Crystals and Disordered Semiconductor Polymers Temperature dependence of defect electron mobility in a 8.7 µm thick disordered layer of MPMP Temperature dependence of electron mobility in a 370 nm thick Perylen crystal, inclined orientation relative to the electric field F. Organic thin film photovoltaic devices Interfacial charge transfer mechanism hυ e LUMO Maximum overall yield of 1% (Merocyanine) E f p-doped HOMO h Ghosh, Feng, J. Appl. Phys. 49 (1978) Electrode Electrode 1 hυ LUMO h HOMO Donor LUMO e HOMO Acceptor Electrode 2 Reductive charge transfer mechanism Maximum overall yield of 3% (PPV derivative-c60 blend) Sariciftci et al. Adv. Func. Mater. 11 (2001), 15. Yu, Heeger, Science 270 (1995), 1789.

23 Charge Separation O CH 3 O * n hν e MeO O Electrode 1 Electrode 2 Sariciftci et al. Adv. Func. Mater. 11 (2001), 15. Yu, Heeger, Science 270 (1995), Open circuit conditions built in field Electrode 1 hν + Electrode 2 Short circuit conditions C. J. Brabec et al. Chem. Phys. Lett. 340 (2001), Resume organic electronics Huge effort done in the last 10 years : - electrical problems solved!!! - small molecules UHV evaporation good quality - polymers still problems solvent evaporation - contamination Challenge long term stability! - perfect gas tight sealing needed, glass solder! - internal failure due to photolytic and/or electronic degradation of organics For displays : competition LCD!!