1

Surfaces of materialsSurface 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:

€

AV

=4πR2

4

3πR3

=3R

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

€

ΔGformation = ΔGlattice + ΔGsurface ≈ VρΔGlat∧

+ Aγ int

But the volume and area of the particle are related as shown.

Thus as the particle gets smaller the interfacial effect becomes stronger

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Formation ThermodynamicsEnergy involved in nucleation:

(1)The volume(or bulk) free energy released by phase transition

Free energy change ΔG vs. radius of embryo or nucleus

Radius of particle, r

Total free energy change, ΔG T

ΔGs =interfacial energy change

ΔGt= total free energy change

ΔGd =Free energy change due to phase transition

rc1rc2

Homogeneous nucleation

Nucleation on template

ΔG = nRT ln (S) + A

(2)The surface energy required to form the new solid surfaces

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TECHNOLOGICAL APPLICATIONS

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Nano-Technological Application of Interface Engineering

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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

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>>200Å

PROBLEMS INVOLVED IN SURFACE MODIFICATION

• Surface Roughness• Chemical Non-Specificity of the Surface• Non-Efficient Functional Delivery

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Ideal Approach To Surface Modification

Solution: Use Organized Two Dimensional Monolayers

>200Å<50Å

• Utilize coating techniques that provide control at the molecular level

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Lessons From Nature

Hydrophilic Head

Hydrophobic Tail

Water

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Langmuir Films

Water

Langmuir Film

π π

LateralPressure

LateralPressure

Air

Movable Barrier MovableBarrier

0

5

10

15

20

25

30

35

30 40 50 60 70 80 90 100

Surface Pressure

Molecular Area (Å 2)

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

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Langmuir-Blodgett Transfer Monolayers

Water

π π

LateralPressure

LateralPressure

Air

Waterπ π

LateralPressure

LateralPressure

Air

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Langmuir Monolayers of Polymerizable Surfactants

0

10

20

30

40

50

10 20 30 40 50 60 70 80

UnpolymerizedPolymerized

Area per molecule, Å 2/molecule

Surface Pressure,

Π

, /mN m

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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 .

Chemisorptionat the surface

Alkyl, or derivitizedalkyl group

Surface group

Interchain van der Waals andelectrostatic interactions

Surface active headgroup

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Mercaptanes

Alkanethiol Disulfide

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Thiol SAMs

S

H

Long Alkyl ChainThiol

Long Alkyl ChainDialkyldisulfides

S

S

RS −SR+Aun0 ⇒ 2RS−−Au+ +Aun

0

RS−H + Aun0 ⇒ RS−−Au+ +

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H2 + Aun0

or

RS−H +Aun0 + oxidant⇒ RS−−Au+ + 1

2H20 + Aun

0

Chemisorption is Epitaxial.

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Gold Structure Au[111]

Gold atoms are 2.884Å apartx

y

z

Constant current STM

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Thiol Structure on Au[111]

3× 3( )R30o

Constant current STM

C12SH

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Thiol Structure on Au[111]

C(4x2) Superlattice

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Tilt Structure of Thiols

Gold Surface

Even CsOdd Cs

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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 C12SH

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Domain Boundaries

Pure Orientational Domain Boundary

Pure Translational Domain Boundary

C12SHC12SH

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Molecular VacanciesCommercial 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.

t=0

t= 2 min

t= 4 min

t= 16 hours

Vacancy Diffusion Coefficient for C10SH is ~ 1x10-19 cm2/s

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Silane SAMs Surfactant : OTS, C18H37SiCl3

Solvent : CCl4, or other non-competingSilicon Substrate

Angst & Simmons , 1991Maoz & Sagiv , 1985

CH2

CH3

Si

ClO

C18

H37

Si|

|Cl H

2O k C

18H

37Si

|

|OH HCl1− − + − − +⏐ →⏐

C H Si OH HO Si C H C H Si O Si C H H O18 37 |

|

|

|

18 37

k3

18 37 |

|

|

|

18 37 2− − + − − ⏐ →⏐ − − − − +

C H Si OH C Si C H C H Si O Si C H HCl18 37 |

|

|

|

18 37k

18 37 |

|

|

|

18 372− − + − − ⏐ →⏐ − − − − +l

Hydrolysis

Asymmetric Esterification

Symmetric Esterification

Water layer

Native SiO2 Layer

Silicon Substrate

Native SiO2 Layer

Silicon Substrate

W a t e r l a y e r

N a t i v e S i O 2 L a y e r

S i l i c o n S u b s t r a t e

Stevens , 1999

CH3 CH 2( )17SiCl3+H2O⏐ → ⏐ ⏐ CH3 CH2( )17 Si OH( )3 + 3HCl

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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

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DEPOSITION TIME :1 sec

DEPOSITION TIME : 15 secDEPOSITION TIME : 45 sec

DEPOSITION TIME : 5 sec

DEPOSITION TIME : 2 MIN

HEIGHT FRICTION HEIGHT

HEIGHT

HEIGHT

HEIGHT

FRICTION

FRICTIONFRICTION

OTS Adsorption on Hydrated Substrate

IMAGE SIZE : 10 x 10

HEIGHT SCALE

OTS CONC. IN SOLUTION2.06mM

10

0 nm

5 nm

10nm

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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)

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56 5857

Effect of Surface Dehydration on OTS Deposition

• Substrate Treated Under Different Conditions• Same Solvent and Deposition time (30sec)

Hydrated Substrate Substrate Dehydrated partially(100oC)

Dehydrated Substrate(150oC)

10

0 nm

5 nm

10nm

Water Contact angle

92o 90o 72o

(Reduces to ~ 400 after water flow)

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In-situ Study of OTS Adsorption

10 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

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Methods

Micro-contactprinting

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

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Nanoisland

matrix

Chemical Functionalities Differing in size and type

Examples: CH3-, NH2-, CF3-, COOH-, halide, ethylene oxide

Island Surfaces are Formed By Using SAMs with Two Different Functional Groups

Substrate

Terminal FunctionalityAnchoring Functional-

ity

Surfactant type B

Surfactant typeA

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Temp : 220C

OTS

Dehydrated Substrate

In 1.1 mM APhMS 60 sec

rinse in toluene

In 1.1 mM OTS soln60 min

CHCl3 rinse15 min 8 Å

Recessed Islands of APhMSIn OTS Background by Backfilling

0 5 P-Aminophenyltrimethoxy silane

m

Silicon waferOHOH OHOH OHOH OHOH

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OTS

Ht difference :15 Å

Temp : 220C

Dehydrated Substrate

In 1.1 mM APhMS 60 sec

rinse in toluene

In 1.1 mM OTS soln60 min

CHCl3 rinse15 min 0 5

Recessed Islands of APhMSIn OTS Background by Backfilling

octadecyltrichlorosilanem

Silicon wafer

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Method B: Co-Adsorption

Mixed monolayer of OTS and APS(NH2C3H6SiCl3)

Silicon wafer

CH3

23Å

6.5Å

30nmamine

APhMSOTS

octadecyltrichlorosilanes (OTS) P-aminophenyltrimethoxysilanes (APhMS)

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Island Formation of Co-Adsorbed Self-assembling

SurfactantsAPhMS islands in OTS Matrix

0

5

10

15

20

25

30

10 13 15 18 22 25 28 31 34 37 40 43 50

Diameter (nm)

35 islands/µm2, average diameter: 28 nm,distribution width: 10 nm

3:1 OTS:APhMS; Chloroform

2mM total concentration of silane

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Effect of Composition2 mM CHCl3 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 Effect2 mM solution (OTS/APhMS=1:1), deposition time: 2 hrs,

CHCl3Toluene

CCl4 THF

Contact Angles: 103 Contact Angles: 98

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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

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Sequential Adsorption for Mixed Monolayers

Partial OTS monolayers with desired islands Low density surrounding OTS islands at100C.

SOLVENT

SUBSTRATE

OTS SOLUTION

SUBSTRATE

SECOND SILANE SOLUTION

Rinse

Fill surrounding with second silane

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Temperature Effect

100C

~220CReduced Secondary growth at low temperatures

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Control of Morphology and Chemical Functionality at

Nanometer Scale

Mixed monolayer of OTS and BrUTS(BrC11H22SiCl3)

Silicon wafer

CH3

23Å

15Å

30nm to 10 µm Br

10 2

Height HeightFriction Friction

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Control of Morphology at Angstrom Scale

Mixed monolayer of OTS and DTS(C10H21SiCl3)

Silicon wafer

CH3

23Å

14Å

30nm to 10 µm

10

Height Friction

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Nano-dots

5

Low OTS concentration & low deposition time