François OZANAM Laboratoire de Physique de la Matière Condensée,

1. Functionalization of Si surfaces: grafting and molecular layer modifications François OZANAM Laboratoire de Physique de la Matière Condensée, Ecole polytechnique, 91128 Palaiseau,France francois.ozanam@polytechnique.fr NaS-ERA workshop – Algiers 21-23 May 2012

2. OUTLINE INTRODUCTION: Direct grafting on Si GRAFTING METHODS: A rapid review of the various strategies FUNCTIONALIZATION OF GRAFTED LAYERS: Patterning and introduction to multi-step strategies ADRESSING MORE DIFFICULT CHALLENGES: Well-defined surfaces for pushing boundaries

3. DIRECT GRAFTING OF ORGANIC SPECIES ON SILICON WHY ? - surface passivation or stabilisation - surface functionalization, sensing - primer for deposit adhesion - surface (nano)structuration HOW ? - quick and dirty: silanization on native oxide film - well organized films, controlled interfaces : on oxide-free surfaces WHERE ? - on crystalline Si, but also on polycrystalline, amorphous or porous Si

4. Si  H Si  C THE HYDROGENATED Si SURFACES "atomically rough" "atomically flat" "porous" Grafting via Si  C bond formation

5. THREE NICE RECENT REVIEWS Wet chemical routes to the assembly of organic monolayers on silicon surfaces via the formation of Si-C bonds: surface preparation, passivation and functionalization S. Ciampi, J. B. Harper, J. J. Gooding, Chem. Soc. Rev. 39 (2010) 2158-2183. Electrografting: a powerful method sor surface modification D. Bélanger and J. Pinson, Chem. Soc. Rev. 40 (2011) 3995-4048. Covalent functionalizations of silicon surfaces and their application to biosensors J.-N. Chazalviel, P. Allongue, A. C. Gouget-Laemmel, C. Henry de Villeneuve, A. Moraillon, F. Ozanam, Sci. Adv. Mater. 3 (2011) 332-353.

6. FIRST REPORTS : RADICAL INITIATION [R’C(O)O]2 Si - H + R - CH = CH2 Si - (CH2)2 - R STRUCTURE OF THE SURFACE LAYER - Molecular layers better organized and more stable than on SiO2 - Good versatility (few constraints on R groups) BUT - A few Si-O-C linkages (initiators) and some oxidation (Si-OH groups)

7. OXI- DATION ! TWO-STEP GRAFTING IDEA : starting from a surface more reactive than SiH Good candidates: HALOGENATED SURFACES, but: Preparation of an Si-Cl or Si-Br surface: - Liquid-phase halogenation using PCl5 or NBS (with peroxides or thermal activation) - Vapor-phase halogenation(Cl2, Br2) under UV or thermal activation

8. SECOND STEP: GRAFTING ON THE HALOGENATED SURFACE Reaction with an organo-magnesium (or organo-lithium) compound : - alkyl groups (saturated termination or not) - other functional groups (need further steps) A few applications : Photoanode stabilization against oxidation Grafting of conductive oligomers on Si EN RÉSUMÉ : - Very good chemical stability - No oxidation (if all sites are grafted) - multi-step process

9. THE HYDROSILYLATION REACTION A versatile reaction for binding through Si-C bonds leaves unreacted sites hydrogenated, but needs “activation”  A convenient route to ideal grafted surfaces ?

10. . . . THERMAL ACTIVATION Hydrosilylation reaction Experimental conditions: - Naked or diluted reactant, thoroughly purified - Reaction during a few hours at 100-200°C under Argon ALKENES : ALKYNES : ? BUT trace amounts of impurities readily oxidize Si during grafting

11. MICROWAVE ACTIVATION Local heating activation HYDROSILYLATION REACTION (microwave absorption mostly in silicon) Experimental conditions: - quartz vessel, stirring and Ar bubbling in microwave reactor - reaction with alkenes  30 min - temperature monitor for adjusting microwave power ALSO reaction with diazirine, alkyl halides, … EN RÉSUMÉ : • - very fast grafting • high surface coverage (with proper temp. setting) • surface oxidation with alkyl halides • only on porous Si (so far)

12. EtAlCl2 RCCR’ Si -RC=CR’H Si - H + EtAlCl2 RR’C=R”H Si-R”HC-CRR’H CATALYTIC ACTIVATION USING LEWIS ACID ALKENE OR ALKYNE ADDITION ON Si-H (porous Si) Other usual catalysts (Pt, Rh, Pd): oxidation more efficiently activated Experimental conditions (porous Si): - EtAlCl2 solution in hexane - Reaction with alkynes  1 h, ambient temperature, inert atmosphere - Reaction with alkenes  16 h, ambient temperature, inert atmosphere

13. need to heat (100°C) but NO Si OXIDATION and COMPACT LAYER on crystalline Si: Interferences : - A few (no alkyl halides) - Saturated or oxygen-bearing groups: protection by using a catalyst excess (otherwise: no reaction)  good VERSATILITY for the grafting of functional molecules EN RÉSUMÉ : - Rather simple process (albeit glove box) - Versatility (at least on porous Si) - High surface coverage on flat crystalline Si

14. PHOTOCHEMICAL ACTIVATION Hydrosilylation reaction Sensitivity of surface SiH groups to UV irradiation: Experimental conditions: - Reactant thoroughly purified and deoxygenated - A few hours under sous Argon - UV irradiation l<350 nm Mechanism: analogous to that suggested for thermal activation

15. PHOTOCHEMICAL ACTIVATION Alkynes: Main problem: easy oxidation BUT GOOD VERSATILITY  NUMEROUS APPLICATIONS EN RÉSUMÉ : - Rather high surface coverage - Good versatility - Vinylic coupling to Si granted - Some tendency to Si oxidation

16. ELECTROCHEMICAL ACTIVATION OXIDATIVE GRAFTING OF ALKYL LAYERS ALKYL SUBSTITUTION ON Si - H Si - H + 2 RMgX + 2 h+ Si - R + RH + 2 MgX+ Experimental conditions: - Concentrated solution (1M to 3M) of RMgX in diethyl ether or THF - Anodic oxidation  2 min, 0.5 mA/cm2, under inert atmosphere - Careful rinsing before exposition to atmosphere On porous Si: - Si-C signature - minute oxidation Reaction kinetics on Si (111)  FAST reaction

17. ELECTROCHEMICAL OXIDATION OF RMgX 1 mm IMAGING AFTER GRAFTING AFM image of the grafted layer  Grafting keeps unchanged Surface topography Special case of METHYLATEDSURFACES - Substitution of ALL SiHs - ORDERED SiCH3 layer on the surface EN RÉSUMÉ : - Fast and simple process (but glove box) - Efficient on c-Si and porous Si - Rather compact layers - Usable for polymer electrografting (no self-limited growth)

18. 20 nm NUCLEOPHILIC SUBSTITUTION ON Si-H REACTION OF AN ORGANO-MAGNESIUM (-LITHIUM) COMPOUND Experimental conditions: - Concentrated reagent, in a thoroughly outgased ether - 80°C, a few hours, under argon - n-Si substrate Very compact layers: A few applications: Conducting polymer coupling on Si : Ultrathin capacitors : EN RÉSUMÉ : • - good surface coverage • - poorly versatile (alkyl and a few other one) • sensitive to Si doping type (electrochem. mechanism !) • some sensitivity to oxidation (insaturated R groups) • polymer formation (insaturated R groups)

19. ELECTROCHEMICALACTIVATION ALKYL LAYER FORMATION BY ELECTROCHEMICAL REDUCTION ALKYL HALIDE REDUCTION ON Si - H Si - H + 2 RX + 2 e- Si - R + 2 X- + RH Experimental conditions: - Concentrated solution (~0.3 M) of RI in acetonitrile + supporting salt - 10 mA/cm2,  2 minutes, under inert atmosphere EN RÉSUMÉ : - GOOD VERSATILITY - Fast and simple (but prone to oxidation ?) - Only demonstrated on porous Si

20. ELECTROCHEMICAL ACTIVATION ALKYNE GRAFTING BY ELECTROCHEMICAL REDUCTION ON Si - H Experimental conditions: - Alkyne solution (~3 à 4 % vol) in CH2Cl2 + supporting salt - 10 mA/cm2,  2 minutes, under inert atmosphere Numerous possible grafting: Mechanism: attack of Si – Si backbonds  hardly efficient on crystalline Si (no compact layer) EN RÉSUMÉ : - good versatility - Fast and simple - Only on porous Si

21. ELECTROCHEMICAL ACTIVATION FORMATION OF ARYL LAYERS BY ELECTROCHEMICAL REDUCTION SUBSTITUTION OF ARYL GROUPS ON Si - H Si - H + 2 XArN2+ + 2 e- Si - ArX + 2 N2 + HArX Experimental conditions: - 2 mM solution of 4-X-benzenediazonium in 0.1 M H2SO4 or acetonitrile - Reaction time < 1 minute, potentiostatic control - Thorough rinse of modified surfaces using solvents REACTION IS (nearly) SELF-LIMITED ! NO OXIDATION ! (robust process) A few interferences (when X is easily reducible): rather good VERSATILITY

22. ELECTROCHEMICAL REDUCTION OF DIAZONIUM SALTS XArN2 MAIN ISSUE: MONO vs. MULTILAYER FORMATION • Need to precisely control the COULOMETRIC CHARGE • Problem with open-circuit deposition in some cases GRAFTED (MONO)LAYER STRUCTURE H-Si(111) [11-2] 1 nm [1-10] - DENSE et ORDERED arrangement of phenyl groups on the surface - SiH substitution ratio = 50% - Non reacted sites remain under SiH form EN RÉSUMÉ : • - Simple and attractive process • - Good versatility, but needs to synthesize the • diazo precursor (possibly in situ) • Not easily usable on porous Si (potential control) • Monolayer control difficult

23. MECHANOCHEMICAL ACTIVATION GRAFTING OF ALKENES, ALKYNES AND HALOGENOALKANES Experimental conditions: - oxide covered Si surface, wet by a liquid film of the reagent - Experiment under air, as it is reagent (no outgasing) - Scribing substrate using a tungsten carbide ball Principle and mechanism: Grafting geometries: EN RÉSUMÉ : • - likely rather versatile • very simple and fast • no large area • - no compact layer, no well-controlled surface

24. FUNCTIONALIZATION OF GRAFTED LAYERS SURFACE PATTERNING MASKING BY PHOTO-OXYDATION Principe: two UV irradiations: first one in air, second one in outgased decene 140m Mask (step 25 mm) AFM topo AFM friction Optical picture after wetting Oxygen Carbon SEM image and Auger profile

25. SURFACE PATTERNING OF GRAFTED LAYERS PHOTOCHEMICAL PATTERNING Hydrosilylation of alkenes or alkynes photoactivated through a mask MECHANICAL PATTERNING hydrophilic domains Line sharpness: 20 mm

26. SURFACE PATTERNING OF GRAFTED LAYERS ELECTRON-BEAM PATTERNING Pattern printing via electron induced desorption MEB picture of a decylated surface, after e-beam patterning and Cu wet chemical deposition

27. FUNCTIONNALIZATION OF GRAFTED LAYERS MULTI-STEP PROCESSES: AN ATTRACTIVE APPROACH ? DILUTION OF REACTIVE GROUPS IN THE GRAFTED LAYER ... Grafted functional molecules are present in the same proportion at the surface and in the reacting solution … AND VARIOUS REACTIONS: hydrolysis reduction Re-esterification

28. 0.4 3 total 0.3 2 acid COVERAGES n/n0 AREAL CONCENTRATION OF GRAFTED SPECIES (1014 cm-2) 0.2 chains 1 0.1 decyl chains 0 0 0 0.5 1 ACID FRACTION IN SOLUTION FUNCTIONNALIZATION OF GRAFTED LAYERS MULTI-STEP PROCESSES: DEVIL IS IN DETAILS DILUTION OF REACTIVE GROUPS IN THE GRAFTED LAYER ... Grafted functional molecules are present in the same proportion at the surface and in the reacting solution … but not always … YIELDS AND SIDE REACTIONS: hydrolysis … but prone to oxidation

29. OXI- DATION ! FUNCTIONNALIZATION OF GRAFTED LAYERS REACTIVE GROUP GRAFTING REACTIVE GROUP DEPROTECTION AFTER GRAFTING Experimental conditions: - When R = acetamide, deprotection in HCl 4M (24h) - When R = phtalimide, deprotection in ethanol + 5% hydrazine (48h) Deprotection often requires HARSH conditions (more than in solution)

30. MILDER APPROACHES TO FUNCTIONALIZATION Click coupling very versatile but sometimes need to manage steric limitations

31. CLICK COUPLING 170°C 95°C oxidation vs. crowding  Trade-off between various constraints, needs of a systematic approach

32. H H H WELL-DEFINED, HYDROGENATED Si SURFACES Si (111) Preparation of atomically flat surfaces: NH4F (40%) Si SiO2

33. SiH SiHx WELL-DEFINED, HYDROGENATED Si SURFACES A tool for a surface-science approach NH4F: SiH HF: SiHx SiH

34. COMBINING TOOLS FOR MORE RELIABLE GRAFTING PROCEDURES AFM on WELL-DEFINED SURFACES Grafting of undecylenic acid QUANTITATIVE IR SPECTROSCOPY

35. OTHER TOOLS FOR MOLECULAR LAYER CHARACTERIZATION EIS: XPS: electronic quality of the interface chemical bonding but sometimes layer density but also coverage Grafted alkyl layers

36. SPECIFIC TOOLS FOR CHARACTERIZING MOLECULAR LAYER TARGETED PROPERTIES Si photoluminescence A very sensitive indicator of the electronic passivation of the surface acid Wet environment is more demanding than air methyl decyl alkaline environment is degrades the surface

37. MOLECULAR LAYER PROPERTIES pH-driven change in wetting intrinsic functional limitation organisation defects: weak points intrinsic (?) assembly limitation

38. CCl3 CH3 CCl3 CH3 CH3 CH3 CCl3 CCl3 Cl2 , l = 350 nm FUNCTIONALIZATION OF NON-REACTIVE ALKYL LAYERS alkyl layers: the most dense and best organized ones CHEMICAL DERIVATIZATION aryl diazirine sulfochloration PHOTOCHLORATION Experimental conditions: - 0.2 Torr Cl2 after thorough outgasing - UV l = 350 nm, 15 s

39. FUNCTIONALIZATION OF NON-REACTIVE ALKYL LAYERS PLASMA TREATMENT Experimental conditions: - thermally grafted alkyl layers - O2 plasma ultra low power density (10 mW/cm3), 15 min Oxidation and slight etching of the distal end of the chains by O2* Complete layer etching and Si oxidation by atomic oxygen at higher power density

40. SOFT-PLASMA OXIDATION OF ALKYL LAYERS Deeper in the reaction mechanism in situ IR Identification of two C=O oxidation products Modelling kinetics: an intermediate is formed before C=O species

41. SOFT-PLASMA OXIDATION OF ALKYL LAYERS Still deeper in the reaction mechanism ex situ XPS Mechanism: EN RÉSUMÉ : - Easily controllable - Selective oxidation of chain ends on dense layers - Minute substrate oxidation

42. hydrolyse FUNCTIONNALIZATION OF GRAFTED LAYERS BIO-MOLECULES IMMOBILIZATION DNA GRAFTING: on a non-reactive alkyl surface: or using complex stacks ...

43. USE OF COUPLING AGENTS activation / amidation scheme: Mild coupling in neutral aqueous environment IR: it works ! but: • yields • contamination

44. EDC/NHS COUPLING SCHEME mechanism: systematic exploration on porous Si

45. EDC/NHS COUPLING KINETICS In situ IR: monitoring activation in real time - acid disappearance - ester formation  kinetics curve fit to a bi-exponential law

46. EDC/NHS COUPLING: LIMITING STEPS II: slow III+V: fast no adjustable parameter simulation on mixed acid/alkyl layers

47. Controlling monolayers Understanding • side reactions (oxidation…) • absence of by-products possible painfull Addressing demanding applications BUT • stability (chemical, thermal) • good electronic passivation • specific physico-chemical properties mandatory Functionalization of silicon surfaces easy Grafting a molecule on Si One-step or multi-step process

48. Absorbance per reflection Wavenumber Transfer to non-ideal substrates 7.8 7.2 Smooth transfer of processes Efficient device

49. In Ecole Polytechnique Philippe Allongue Jean-Noël Chazalviel Anne Chantal Gouget-Laemmel Catherine Henry de Villeneuve Anne Moraillon Ionel Solomon Damien Aureau Carine Douarche Anne Faucheux Samira Fellah Juliana Salvador Andresa Larbi Touahir In Alger In Versailles Noureddine Gabouze Sabrina Sam Arnaud Etcheberry Jacky Vigneron In Lille Rabah Boukherroub Sabine Szunerits Xavier Wallart Elisabeth Galopin In Paris Willy Morscheidt In Berlin Jorg Rappich THANKS