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# Surfaces of materials Surface Modification Techniques

Surfaces of materials Surface Modification Techniques. U785 Introduction to Nanotechnology Spring 2003 Lecture 3. Surface Oxygen. Surface Oxygen. Bulk Oxygen. Bulk Oxygen. Is a Materials Surface Structure and its Bulk Structure Different ?. Example Quartz.

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## Surfaces of materials Surface Modification Techniques

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1. Surfaces of materialsSurface Modification Techniques U785 Introduction to Nanotechnology Spring 2003 Lecture 3

2. Surface Oxygen Surface Oxygen Bulk Oxygen Bulk Oxygen Is a Materials Surface Structure and its Bulk Structure Different ? Example Quartz

3. Is a Materials Surface Structure and its Bulk Structure Different ? Example Polyethylene-Vinyl Alcohol Copolymer

4. Hard Material Interface Between two Polymers Material and Their Interface

5. R Importance of Surfaces in Nano-Phenomena Assume a 1 nanometer a particle. Its area to volume ratio is: Obviously as the particle diameter becomes smaller the ratio increases.

6. R Formation Energy Again assume the 1 nanometer a particle. When this particle was formed, the free energy of formation, includes the energy to phase separate the particle from its ingredients and the work required to make the surface. But the volume and area of the particle are related as shown. Thus as the particle gets smaller the interfacial effect becomes stronger

7. Gs =interfacial energy change Nucleation on template Homogeneous nucleation Gt= total free energy change Total free energy change, GT rc2 rc1 Radius of particle, r Gd =Free energy change due to phase transition Free energy change G vs. radius of embryo or nucleus Formation Thermodynamics Energy involved in nucleation: (1)The volume(or bulk) free energy released by phase transition (2)The surface energy required to form the new solid surfaces G =nRT ln (S) + A

8. TECHNOLOGICAL APPLICATIONS

9. Nano-Technological Application of Interface Engineering

10. Surface Reactions • Flame Treatment • Plasma Treatment • Corona Treatment • Coating Techniques • Paints on Metal surfaces • Sizing agents on Paper • Bulk Techniques • Alloys • Blending of Surface Active Compounds Surface Modification Techniques

11. PROBLEMS INVOLVED IN SURFACE MODIFICATION • Surface Roughness • Chemical Non-Specificity of the Surface • Non-Efficient Functional Delivery >>200Å

12. Ideal Approach To Surface Modification • Utilize coating techniques that provide control at the molecular level >200Å <50Å Solution: Use Organized Two Dimensional Monolayers

13. Lessons From Nature Hydrophobic Tail Water Hydrophilic Head

14. Langmuir Films Gas: No interaction between molecules Liquid State: Beginning of interaction, no position ordering. Liquid Condensed: Positional ordering of the hydrophobe Liquid Solid: Positional ordering of both head group and hydrophobe LC State LS State

15. Langmuir-Blodgett Transfer Monolayers

16. Langmuir Monolayers of Polymerizable Surfactants

17. Why are SAMs formed? The free energy of a self-assembled monolayer is minized because of three main processes: 1. Chemisorption of the surfactant onto the surface, ~40-45 kcal /mole 2. Interchain van der Waals interaction, <10kcal/mole 3. Terminal Functionality, ~0.7-1.0 kcal/mole for CH3 termination Defects larger than a few molecular diameters cannot be sustained .

18. Mercaptanes Alkanethiol Disulfide

19. Thiol SAMs Chemisorption is Epitaxial. Long Alkyl Chain Dialkyldisulfides Long Alkyl Chain Thiol

20. Gold Structure Au[111] Constant current STM Gold atoms are 2.884Å apart

21. Thiol Structure on Au[111] Constant current STM C12SH

22. Thiol Structure on Au[111] C(4x2) Superlattice

23. Tilt Structure of Thiols Odd Cs Even Cs Gold Surface

24. Domain Boundaries C12SH Constant current STM Pit Defects: depth of defect ~2.5 Å Au[111] single-atom step height is also 2.5 Å Gold vacancies are generated ejection of Au atoms during the surface reconstruction during SAM formation

25. Domain Boundaries C12SH C12SH Pure Orientational Domain Boundary Pure Translational Domain Boundary

26. Commercial applications of alkanethiol monolayers may rely on spatially patterned mixed CnX on Au. A critical parameter for mixed CnX on Au is the rate of exchange diffusion or the rate of vacancy diffusion, because this determines the time scale over which the pattern will retain its integrity. The rate of vacancy diffusion can be addressed by deliberately creating isolated molecular vacancies and following their migration. Molecular Vacancies t= 4 min t=0 t= 2 min t= 16 hours Vacancy Diffusion Coefficient for C10SH is ~ 1x10-19 cm2/s

27. Hydrolysis CH2 CH3 Asymmetric Esterification Cl O Si Symmetric Esterification Surfactant : OTS,C18H37SiCl3 Solvent : CCl4, or other non-competing Silicon Substrate Silane SAMs Stevens , 1999 Maoz & Sagiv , 1985 Angst & Simmons , 1991

28. 10 m Mechanism of Silane Monolayer Formation from a Non-Competitive Solvent • Surface Diffusion and Aggregation into “fractal-like” islands (primary growth). • Continued Adsorption Onto Bare Substrate Areas Leading to Full Coverage (secondary growth). • Further Adsorption onto the surface leading to monolayer completion (dense packing). Such Growth also observed by Bierbaum et al (1995); Davidovitis et al(1996) 10nm 5 nm 0 nm AFM topographic images (10 m x 10 m)

29. OTS Adsorption on Hydrated Substrate 10m 10nm 5 nm 0 nm HEIGHT SCALE HEIGHT FRICTION HEIGHT FRICTION IMAGE SIZE : 10m x 10m DEPOSITION TIME :1 sec DEPOSITION TIME : 5 sec OTS CONC. IN SOLUTION 2.06mM HEIGHT FRICTION HEIGHT FRICTION DEPOSITION TIME : 15 sec DEPOSITION TIME : 45 sec HEIGHT DEPOSITION TIME : 2 MIN

30. Mechanism of Silane Monolayer Formation from a Non-Competitive Solvent Continued Adsorption and surface diffusion on the Substrate Areas Leading to Full Coverage. Molecule immobilize on reaching an existing island Further Adsorption onto the surface leading to monolayer completion (dense packing)

31. 10 m Effect of Surface Dehydration on OTS Deposition • Substrate Treated Under Different Conditions • Same Solvent and Deposition time (30sec) 10nm 5 nm 0 nm 57 56 58 Water Contact angle 92o 90o 72o (Reduces to ~ 400 after water flow) Hydrated Substrate Substrate Dehydrated partially(100oC) Dehydrated Substrate (150oC)

32. In-situ Study of OTS Adsorption 10 m 10 m 10nm 5 nm 0 nm Blank solvents were passed over the substrate before OTS solution No solvents were passed over the substrate before OTS solution

33. Methods ~ 10nm Nano-writing Micro-contact printing Monolayer Phase separated Langmuir-Blodgett Films UV mask Oriented block co-polymers Micro-lithography Intermolecular interaction X~ nm Limited by wavelength X ~ m

34. Island Surfaces are Formed By Using SAMs with Two Different Functional Groups Chemical Functionalities Differing in size and type Examples: CH3-, NH2-, CF3-, COOH-, halide, ethylene oxide matrix Nanoisland

35. Dehydrated Substrate In 1.1 mM APhMS 60 sec rinse in toluene In 1.1 mM OTS soln 60 min OH OH OH OH OH OH OH OH CHCl3 rinse 15 min Recessed Islands of APhMS In OTS Background by Backfilling OTS m P-Aminophenyltrimethoxy silane 8 Å Temp : 220C Silicon wafer

36. Dehydrated Substrate In 1.1 mM APhMS 60 sec rinse in toluene In 1.1 mM OTS soln 60 min CHCl3 rinse 15 min Silicon wafer Recessed Islands of APhMS In OTS Background by Backfilling OTS m octadecyltrichlorosilane Temp : 220C Ht difference :15 Å

37. Method B:Co-Adsorption OTS APhMS CH3 amine 30nm 23Å 6.5Å Silicon wafer Mixed monolayer of OTS and APS(NH2C3H6SiCl3) P-aminophenyltrimethoxysilanes (APhMS) octadecyltrichlorosilanes (OTS)

38. Island Formation of Co-Adsorbed Self-assembling Surfactants APhMS islands in OTS Matrix 35 islands/µm2, average diameter: 28 nm, distribution width: 10 nm 3:1 OTS:APhMS; Chloroform 2mM total concentration of silane

39. Effect of Composition 2 mM CHCl3 solution, deposition time; 2 hrs OTS/APMS=1:1 OTS Pillars OTS/APMS=1:3 OTS Pillars OTS/APMS=3:1 APhMS islands

40. SolventEffect 2 mM solution (OTS/APhMS=1:1), deposition time: 2 hrs, Toluene CHCl3 Contact Angles: 103 Contact Angles: 98 CCl4 THF Contact Angles: 80 Contact Angles: 41

41. Effect of Solvent on Composition • Monolayer Composition in Mixed Adsorption is a balance between • relative affinity of surfactants to the depositing solvent • interfacial energy between the film formed and the depositing solution

42. SUBSTRATE SUBSTRATE SOLVENT OTS SOLUTION SECOND SILANE SOLUTION Sequential Adsorption for Mixed Monolayers Rinse Partial OTS monolayers with desired islands Low density surrounding OTS islands at100C. Fill surrounding with second silane

43. Temperature Effect 100C Reduced Secondary growth at low temperatures ~220C

44. Control of Morphology and Chemical Functionality at Nanometer Scale 2 10 Height Friction Friction Height 30nm to 10 µm Br CH3 23Å 15Å Silicon wafer Mixed monolayer of OTS and BrUTS(BrC11H22SiCl3)

45. CH3 30nm to 10 µm 23Å 14Å Silicon wafer Control of Morphology at Angstrom Scale 10 Height Friction Mixed monolayer of OTS and DTS(C10H21SiCl3)

46. Nano-dots 5 Low OTS concentration & low deposition time

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