Self Assembled Monolayers

Transcription

1 Nanotechnology for engineers Winter semester Nanotechnology for Engineers : J. Brugger (LMIS-1) & P. Hoffmann (IOA) Outlook Introduction (gas phase solution) Large molecules SAMs Small molecules SAMs Preparation methods (simple low cost) Langmuir Blodgett Self Assembly Silanization SAM Stability 1

2 Introduction - Importance of SAMs Scientific politics 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; 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 2

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

4 LB-films - preparation Langmuir Blodgett balance LB-films deposition control 4

5 LB-films multi-layer preparation 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 5

6 Self Assembly-films - preparation Au covered sample Ethanolic solution of a thiol (R-S-H) After a couple of minutes up to 24 hours 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) 6

7 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 7

8 Silanisation - packing density Teflon Helical solid structure subunits Eby RK, Clark ES, Farmer BL, Piermarini GJ, Block S. Polymer, 1990;31:2227. Clark E.S. The Molecular Conformations of Polytetrafluoroethylene: Forms II and IV. Polymer 40, , 1999 SAM s LB, SA, Sil thermal stability SAM type Langmuir- Blodgett Self Assembly R-SH on gold Silanization Bonding type ionic, electrostatic Covalent, d-d Covalent (Si-O) Covalent (Si-C) Energy 0.52 ev = 50 kj/mol 1.87 ev = 177 kj/mol 4.59 ev = 443 kj/mol 3.17 ev = 306 kj/mol 8

9 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 SAM layers resume For all small molecular SAM s : the structures are comparable functional and multilayers LB before transfert densest (high p), Au-SAM s densest silanised SAMs most stable 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 9