1 Surfaces of materials Surface Modification Techniques U785 Introduction to Nanotechnology Spring 2003 Lecture 3

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

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

4 Hard Material Material and Their Interface Interface Between two Polymers

5 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. R

6 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. R But the volume and area of the particle are related as shown. Thus as the particle gets smaller the interfacial effect becomes stronger

7 Formation Thermodynamics Energy involved in nucleation: (1)The volume(or bulk) free energy released by phase transition Free energy change  G vs. radius of embryo or nucleus Radius of particle, r Total free energy change,  G T  G s =interfacial energy change  G t = total free energy change  G d =Free energy change due to phase transition r c1 rc2rc2 Homogeneous nucleation Nucleation on template  G = nRT ln (S) +  A (2)The surface energy required to form the new solid surfaces

8 TECHNOLOGICAL APPLICATIONS

9 Nano-Technological Application of Interface Engineering

10 Surface Modification Techniques 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

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

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

13 Lessons From Nature Hydrophilic Head Hydrophobic Tail Water

14 Langmuir Films LC State LS State 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

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, ~ kcal/mole for CH 3 termination Defects larger than a few molecular diameters cannot be sustained.

18 Mercaptanes Alkanethiol Disulfide

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

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

21 Thiol Structure on Au[111] Constant current STM C 12 SH

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

23 Tilt Structure of Thiols Gold Surface Even Cs Odd Cs

24 Domain Boundaries 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 Constant current STM C 12 SH

25 Domain Boundaries Pure Orientational Domain Boundary Pure Translational Domain Boundary C 12 SH

26 Molecular Vacancies Commercial applications of alkanethiol monolayers may rely on spatially patterned mixed C n X on Au. A critical parameter for mixed C n X 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. t=0 t= 2 min t= 4 min t= 16 hours Vacancy Diffusion Coefficient for C 10 SH is ~ 1x cm 2 /s

27 Silane SAMs Surfactant : OTS, C 18 H 37 SiCl 3 Solvent : CCl 4, or other non-competing Silicon Substrate Angst & Simmons, 1991Maoz & Sagiv, 1985 CH 2 CH 3 Si Cl O Hydrolysis Asymmetric Esterification Symmetric Esterification Stevens, 1999

28 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) 0 nm 5 nm 10nm 10  AFM topographic images (10  x 10 

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

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)

Effect of Surface Dehydration on OTS Deposition Substrate Treated Under Different Conditions Same Solvent and Deposition time (30sec) Hydrated SubstrateSubstrate Dehydrated partially(100 o C) Dehydrated Substrate (150 o C) 10  0 nm 5 nm 10nm Water Contact angle 92 o 90 o 72 o (Reduces to ~ 40 0 after water flow)

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

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

34 Nanoisland matrix Chemical Functionalities Differing in size and type Examples: CH 3 -, NH 2 -, CF 3 -, COOH-, halide, ethylene oxide Island Surfaces are Formed By Using SAMs with Two Different Functional Groups

35 Temp : 22 0 C OTS Dehydrated Substrate In 1.1 mM APhMS 60 sec rinse in toluene In 1.1 mM OTS soln 60 min CHCl 3 rinse 15 min 8 Å Recessed Islands of APhMS In OTS Background by Backfilling P-Aminophenyltrimethoxy silane mm Silicon wafer OH

36 OTS Ht difference :15 Å Temp : 22 0 C Dehydrated Substrate In 1.1 mM APhMS 60 sec rinse in toluene In 1.1 mM OTS soln 60 min CHCl 3 rinse 15 min Recessed Islands of APhMS In OTS Background by Backfilling octadecyltrichlorosilane mm Silicon wafer

37 Method B: Co-Adsorption Mixed monolayer of OTS and APS(NH 2 C 3 H 6 SiCl 3 ) Silicon wafer CH 3 23Å 6.5Å 30nm amine APhMS OTS octadecyltrichlorosilanes (OTS) P-aminophenyltrimethoxysilanes (APhMS)

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

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

40 Contact Angles: 80 Contact Angles: 41 Solvent Effect 2 mM solution (OTS/APhMS=1:1), deposition time: 2 hrs, CHCl 3 Toluene CCl 4 THF Contact Angles: 103 Contact Angles: 98

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 Sequential Adsorption for Mixed Monolayers Partial OTS monolayers with desired islands Low density surrounding OTS islands at10 0 C. SOLVENT SUBSTRATE OTS SOLUTION SUBSTRATE SECOND SILANE SOLUTION Rinse Fill surrounding with second silane

43 Temperature Effect 10 0 C ~22 0 C Reduced Secondary growth at low temperatures

44 Control of Morphology and Chemical Functionality at Nanometer Scale Mixed monolayer of OTS and BrUTS(BrC 11 H 22 SiCl 3 ) Silicon wafer CH 3 23Å 15Å 30nm to 10 µmBr 10  22 Height Friction

45 Control of Morphology at Angstrom Scale Mixed monolayer of OTS and DTS(C 10 H 21 SiCl 3 ) Silicon wafer CH 3 23Å 14Å 30nm to 10 µm 10  Height Friction

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