ChemE 554

Nano Science IRheology and Processes in

NanotechnologyInstructor: René M. Overney

roverney@u.washington.edu

http://depts.washington.edu/nanolab/

Department of Chemical EngineeringUniversity of Washington

Seattle, WA 98195

Introduction Mesoscale Research and Perception Novel Surface Technique: Scanning Force Microscopy (also

called: Atomic Force Microscopy) Motivation for Mesoscale Research:

- Confinement Effects in Thin Spincast Films Examples of Sensible Mesoscale Technologies:

- Thin Film Lubrication Mesoscale Kinetics

- Ultrathin LED Materials

Exciton Annihilation

- PEM Fuel Cell Proton Transport

Mesoscale Science and Technology

Applications:

- Lubrication

- Photonics

- Fuel Cells

- Data Storage

The realm of the Mesoscale fosters new perceptions and approaches.

Classical Sciences

Molecular SciencesAtomistic &

Illustration of a 2D PhenomenonA highly organized plastic deformation: often referred to as Schallamach Waves

a slow moving (~ m/s ) sliding contact (~10-5 m2) affects a scan area on the order of 103 m2 in an apparently coherent fashion.

AFM Scan:

Probe Width: ~ 10 nm

Line Separation: > 100 nm

(a)

(d)(c)

(b)

An entertaining satire by Edwin A. Abbott

A Sphere, an inhabitant from

Spaceland,

E.A. Abbott, Flatland, A Romance of Many Dimensions. Dover Publ. Inc., New York (1992) first published 1884 under the title “A. Square”

Flatland

a Square,an inhabitant from

Flatland

introducesto a higher

Dimensionality

Perception

• Objects are perceived in Flatland as lines or points.

Dimensionality nurtures perceptions and limits possibilities.

Flatland – An entertaining satire by Edwin A. Abbott

• To distinguish objects in Flatland the observer has to travel around the objects. A Circle is perceived as an

angularly length- invariant object.

A Sphere is reduced to its cross-sectional area with Flatland, and thus, perceived as a Circle.

Boundaries in lower dimensionalities are lifted from the perspective of a higher dimensionality

Flatland – Shrinking, Disappearing and Reappearing Act

DoorDoor

??

North

South

Direction of Rain

Flatland: “Physical Laws”Abbott spent a significant part for a his satire on developing the two-dimensional world of Flatland by introducing imaginary laws of nature that apply in one and 2-dimensions. Although these laws that for instance explain how rain is experienced in 2-dimensionsare unrealistic, they impressively illustrate the mystery of lower dimensionalities.

Huygens Body Waves (3D solution)A spatially localized initial disturbance gives rise to alimited and fast decaying disturbance only, at any accessible location away from the source of the disturbance.

Rayleigh Surface WavesWaves, propagating over the surface of a body with a smallpenetration distance into the interior of the body, acquireat a great distance from the source a continually increasingpreponderance.

--> important in the study of seismic phenomena

Illustration of Dimensionality:Acoustic Wave (3D vs. 2D)

SAMPLE

CANTILEVER

PIEZO

Atomic Force Microscopy (AFM)

Photodiode LASER

Topography

NanoScience ToolNanoScience Tool

AFM

Friction

Material Distinction

100 µm

0 µm

50 µm

100 µm0 µm 50 µm

A

B32.33 µm

0 µm

16.17 µm

32.33 µm0 µm 16.17 µm

ElasticityTg = 374K

Glass Transition

In environmental chamber with sample heating /cooling stage

SFM Setup

C

SFM/AFM Heating/Cooling Stage:Explorer (Topometrix) (MMR Techn.)

Confined Boundary Layer of Spincast Films

Mean field theories consider the effect of pinning at interfaces only within a pinning regime (0.6 – 1 nm « Rg)

BU

LK

SIC

Z

~ 1 nm

Lateral Force and Dewetting Studies suggest that the PEP phase is rheological modified within a 100 nm boundary region that exceeds by two orders of magnitude the theoretically predicted pinning regime of annealed elastomers at interfaces with negative spreading coefficient.

BU

LK

SIC

ZS

RZ

~ 100 nm

DisentangledSublayer

Diffusioninto

Classical Actual

Mean field theories consider the effect of pinning at interfaces only within a pinning regime (0.6 – 1 nm « Rg)

R.M. Overney et al., J. Thermal Analysis and Calorimetry, 59, 205-225 (2000).

S.Ge, et al. Phys. Rev. Lett., 85, 2340-2343 (2000)

Glass Transition Properties of Confined Films

Glass transition studies on polystyrene indicate confinement effects similar to those found in PEP shear studies.

85

90

95

100

105

0 50 100 150 200 250 300

FILM THICKNESS, ( nm )

Tg

( o

C )

12.0 kDa PS

FOX-FLORY (BULK)

BULKS ICZ SRZ

xmod

Shear Response

ShearDisplacement

Sample

CantileverTip

xL

Heating/Cooling Stage

xmodxmod

Shear Response

ShearDisplacement

Sample

CantileverTipCantileverTip

xL

Heating/Cooling Stage

Near-Surface Tg Measurements of “Thick” Films (t > 100 nm) are bulk-like.

Liquid Structuring: Entropic Cooling• Interfacial confinement leads to an entropically cooled boundary layer in simple fluids like hexadecane.

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

-6 -5 -4 -3 -2 -1 0 1 2 3 4

DISPLACEMENT (nm)

NO

RM

AL

IZE

D V

AL

UE

(A

.U.)

MODULUS

CONTACT FORCE

I II III

I: BULK LIQUID

II: BOUNDARY LAYER

III: SOLID SUBSTRATE

Shear Modulated SFM approach curves indicate a hexadecane boundary layer thickness of ~2.5 nm on SiO2.

He, et al., Phys. Rev. Lett. 88, 15 (2002)

Molecular Dynamics simulations predict a boundary layer thickness of 1.5-1.8 nm.

Xia, et al., Phys. Rev. Lett. 69, 1967 (1992)

Lubrication

Examples of Sensible Mesoscale Technologies

- Thin Film Lubrication

Mesoscale Kinetics

- Thin Film LED Materials

Exciton Annihilation

- PEM Fuel Cell

Proton Transport

Mesoscale Kinetics -Fractal Bonding Kinetics and Lubrication

10 Å100 Å

400 Å

Si

Cr

CHxZdol

X - (CF2O)y (CF2CF2O)z CF2 – X

X CH2OH (functional group) y perfluoromethylene oxide groups (C1)z perfluoroethylene oxide groups (C2)

polargroup

polargroupbackbone

Monolayer Lubricant:Hydroxyl-terminated perfluoropolyether (PFPE-OH) film (Fomblin Zdol©)

Interaction: Hydroxylated chain ends form hydrogen bonds with carbon surface.

Lubrication Performance: Depends on the molecular mobility

Bonding Kinetics and Shear Mechanical Response

Kinetic Experiments 10.5 ± 0.5Å Zdol, 2500 CHx

0 20 40 60 80 100 120 140 1600.00

0.05

0.10

0.15

0.20

T = 54°C

T = 86°C

t-1. 0

= 0.8

t-

t -0. 5

k b

TEMPERATURE (°C)

LOW TEMPERATURE5.0)( tktk b

• “GLASS LIKE”• SHORTER RANGE INTERACTIONS• DIFFUSION LIMITED

HIGH TEMPERATURE0.1)( tktk b

• “LIQUID LIKE”• DISPERSIVE INTERACTIONS• ACTIVATION BARRIER LIMITED

Shear Modulated SFM

0 20 40 60 80 100 120 140 1600.010

0.015

0.020

0.025

0.030

0.035 T = 56°C

SH

EA

R R

ES

PO

NS

E (

a.u

.)

TEMPERATURE (°C)

T = 85°C

Monolayer Lubrication and Kinetics

i. The monolayer confined system exhibits a glass transition value of 52 oC that is significantly exceeding the bulk material value of -115 oC.

ii. The confined system exhibits a “Fractal Reaction Kinetics”.

iii. The rheological transition at 52 oC separates two fundamental kinetic bounding regimes:

- diffusion limited reaction- activation barrier limited reaction

iv. The shear rheological analysis with SM-SFM was found to provide valuable material information that explains the “exotic” reaction kinetics.

Photonics and Thin Semiconducting Polymer Films

Film preparation parameters have shown to effect significantly the optoelectronic properties (e.g, conversion temperature)

Ultrathin films have shown very exotic optoelectronic properties (bias-voltage dependent color emission)

Motivations for Mesoscopic Rheological Analysis:

Thickness dependent Luminescence

Bias-voltage dependent Reversible Color Emission

Zhang, X., S. A. Jenekhe, et al. (1996) Chem. Mater. 8: 1571-1574

tc 50 nm

67 nm40 nm

Aln-type (hole)

p-type (electron) ITO

PPQ

PPV tPVP

V (bias)tPPQ

PPQt)V(h

polyquinolines (PPQ)

p-phenylenes (PPV)

tPPQ = 67 nm; tPVP = 25 nm

10-30 V: orange-red

tPPQ = 67 nm; tPVP = 25 nm 8-10 V: orange13-20 V: green

PPV is not soluble in conventional solvents used for spin coating. A two-step process is needed, which involves a tetrahydrothiofenium (THT) precursor polymer, which is soluble (e.g., in methanol). After spin casting the film is converted by thermal annealing into PPV

- silicon substrates: cleaned sequentially with acetone and methanol

- THT films: spin coated at 1000 rpm onto Si (sulfonium precursor in a methanol solution, 1 wt%)

- Conversion in vacuum oven at 10°C/min starting from 100°C.

- Samples were cooled to room temperature at a rate of approximately 40°C/min

Film Preparation of PPV

nn

S+

n

Cl-

S+

n

Cl-

h

Conversion Temperature: 175 oC

SHEARDISPLACEMENT,

XMOD

SHEAR RESPONSE

XR

SAMPLE

CANTILEVER

TIP

NO-SLIPCONTACT

HEATING / COOLING STAGE

40 45 50 55 60 65 70 75 80 85 90 95 100

0.038

0.040

0.042

0.044

0.046

0.048

0.050

0.052

Tg = 66 oC

Shea

r R

espo

nse

x RTEMPERATURE (oC)

Rheological Transition Measurements

Qualitative comparison of

(a) the rheological transition temperature ■, (Tg ) (measured with the SM-SFM method) with

(b) the photo-luminescence (PL) efficiency ▲ Morgado, J., F. Cacialli, et al. (1999). J. Appl. Phys. 85(3): 1784-

1791. as function of the PPV conversion temperature.

55

60

65

70

75

80

85

90

140 160 180 200 220 240 260 280

Conversion Temperature (oC)

Tran

sitio

n Te

mpe

ratu

re

(oC

)

0.085

0.105

0.125

0.145

0.165

0.185 PL Q

uantum E

fficiency

Photoluminescence Efficiency vs. Glass Transition

Below 205 oC, the degree of conversion to PPV (which is increasing with temperature) is dominating both, the rheological transition properties and the EL efficiency.

Interpretation

55

60

65

70

75

80

85

90

140 160 180 200 220 240 260 280

Conversion Temperature (oC)

Tran

sitio

n Te

mpe

ratu

re

(oC

)

0.085

0.105

0.125

0.145

0.165

0.185 PL Q

uantum E

fficiency

Above 205 oC, polymer degradation is dominating. For instance, it is known that carbonyl groups are formed excessively (Morgado, J., F. Cacialli, et al. (1999). J. Appl. Phys. 85 (3): 1784-1791)

Effects on EL Intensity:

- (left) Degree of conversion to PPV increase EL intensity.

- (right) Residual byproducts from conversion and oxygen degradation act as exciton quenching sites (radiation-free annihi-lation), i.e., lower the EL intensity.

Effects on Rheological Transition:

- (left) Degree of conversion to PPV increases thermal transition values

- (right) Degradation lowers the transition values

Proton Exchange Membrane (PEM) Fuel Cell

R.M. Overney University of Washington

PEM

50-175m5-50m

ACTIVE LAYER

ANODE CATHODE

4H++4e-+O22H2OH+H22H+ + 2e-

PEM Fuel Cell

Nafion: Commercially available perfluorosulphonate cation exchange membrane SO3H form: Tg 376 K [S.C. Yeo, A. Eisenberg, J. Appl. Polym Sci. 21, 875 (1977)]

A. Sen, et al., Mat. Res. Soc. Symp. Proc. Vol. 393, 157 (1995)Temperature (oC)

Mem

bran

e R

esis

tivi

ty (

Ohm

-cm

)

Tc 80-90 oC

(CF2-CF2)x (CF2-CF)y

(O-CF2-CF)m O-CF2- CF2-SO3H

CF3

Nafion consists of a hydrophobic tetrafluoroethylene (TFE) backbone with pendant side chains of perfluorinated vinylethers terminated by ion-exchange groups. Ionic Cluster Model involving sulphonate groups.

Polymer relaxation properties affect the proton transport properties.

SM-SFM Rheological Response

Temperature (oC)

Mem

bran

e R

esis

tivi

ty (

Ohm

-cm

)

Tc 80-90 oC

A. Sen, et al., Mat. Res. Soc. Symp. Proc. Vol. 393, 157 (1995)

Transition

Regime

Low PEM

Efficiency

High PEM

Efficiency

60 70 80 90 100 110 120

0.009

0.010

0.011

0.012

0.013

12

14

16

18

20

Vis

cous

Res

pons

e

Mod

uli R

espo

nse

Temperature (oC)

~83 oCRheological transition at