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Eddie McCumiskeyMaster’s Thesis Presentation23 January 2008Virginia Commonwealth University

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1. Motivation2. Problem Statement3. Challenges4. Approach5. Experimental Details6. Results7. Conclusion8. Acknowledgements9. Questions

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Low-cost, light-weight, rollable solar cells

Low-power, high-resolution displays Flexible circuitry

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Sony’s 3-mm thick TVwww.sonystyle.com/oled

http://www.solardirect.com/pv/consumer-ready/power-film.htm#fea

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Nanostructure-polymers hybrids exhibit: Increased Efficiency Tunable Band Gap Tunable Electrical Properties

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http://www.nn-labs.com/CdSe-orderform.htm

Commercial CdSe Quantum Dots

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In order for QD-polymer hybrids to be commercially viable, their reliability

and durability must be known.

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Mechanical Characterization

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Polymer-Clay Nanocomposites invented by Toyota (1985)Greatly enhanced strength, elastic modulus

Large Interaction between NPs and filler material due to high interfacial area

CNT-polymer nanocomposites: Increased hardness, elasticmodulus

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Nanocomposite Mechanical Properties

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Theoretically, QDs should be stiffer than bulk Lattice contraction due to high surface energy

Nanoscale structures shown to impede dislocation formation

Nanoindentation of QD films reveals:* Polymeric behavior of QDs with ligands attached Granular behavior of QD films without ligands

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*D. Lee et al., Phys. Rev. Lett. 98.2 (2007)

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Large gap in research on mechanical properties of QD-polymer nanocomposite films

Challenges Mechanical characterization of thin films (<100 nm) Interference from the underlying substrate Obtaining uniform QD Dispersion

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glass substrate

Applied Load

QDs in a polymer matrix

Conceptual Design of ExperimentExample from the literature: organic solar cell made with blended CdSe nanoparticles and OC1C10-PPV*

*Figure from: B. Sun, E. Marx, and N. C. Greenham, “Photovoltaic Devices Using Blends of Branched CdSe Nanoparticles and Conjugated Polymers.” Nano Lett. 3.7 (2003), pp. 691-963.

blended nanoparticle-

polymer thin film

Nanoindenter tip

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Characterizing the Mechanical Properties of Nanocomposite Films

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I. Prepare QD-Polymer Solutions

II. Deposit Films onto Glass

III. Characterize Film Uniformity

IV. Mechanical Characterization

Nanoindentation

TEMAFM

Stirring Sonicating

Spin-coating

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CdSe QDs NN-Labs # CSE 620-100 Capped with ODA Ligand Dispersed in Toluene (5 mg/mL) Abs 620 nm; PL ~630 nm

CdSe

ODA Ligands

5.6 nm

CdSe QDs

Source: http://www.nn-labs.com/CdSe-orderform.htm

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MEH-PPV* Polymer American Dye Source # ADS200RE Abs 490 nm; PL ~585 nm

MEH-PPV Structure

Dry MEH-PPV

*poly[2-methoxy-5-2(2΄-ethylhexyloxy-phenylenevinylene)]

Source: http://www.adsdyes.com/products/pdf/homopolymers/ADS200RE_DATA.pdf

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Fabrication and Characterization

of QD-Polymer Films

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1. Dissolve MEH-PPV in TolueneA. Weigh MEH-PPVB. Add toluene (5 mg MEH-PPV / mL toluene)C. Stir ~12 hrs at 300 RPM at room temp.

2. Mix MEH-PPV & QD SolutionsA. Add equal volume of QD solution to make 50

wt% QDs in MEH-PPV

B. Stir ~12 hrs at 300 RPM at room temp.

+

MEH-PPV in toluene

QDs in toluene

MEH-PPV + QDs in toluene

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A. Ultrasonicate Combined Solutions for 30 minsB. Filter through a 1.0-µm PTFE filter (optional) C. Spin-Coat @ 1,000-2,000 RPM, 30 secondsD. Anneal on hot plate @ 120 °C, 10 mins

A. SonicateC. Deposit via Spin-

Coating

D. Anneal

Filtering

B.

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Unfiltered Filtered

Aggregation

Aggregation observed at the micrometer scale

Filtering removes manylarge aggregates

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20 μm

200 μm200 μm

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Dispersion is a common problem through the literature

Ligands may lead to aggregationLigands also unfavorable for

devices*

* N. C. Greenham, X. Peng, and A. P. Alivisatos, “Charge Transport in Conjugated-Polymer/ Semiconductor-Nanocrystal Composites Studied by Photoluminescence Quenching and Photoconductivity.” Phys Rev. B 54.24 (1996), pp. 17628-17637.

ligand

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Ligand is an insulator

Removing ligands: enhances charge

separation & transport;

quenches PL more realistic for

commercial applications

QDs w/ Ligands

QDs w/o Ligands

LIGAND

Need to remove the ligands

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QDs precipitated in pyridine 1 mL QD solution 1 mL pyridine ~10 mL hexanes

Retrieved via centrifugation 9,000+ RPM 30 mins

Change to binary solvent: 8% pyridine, 92% choloroform**** Modified method of Sun et al. ** suggested by Dr. David Goorskey

** B. Sun, E. Marx, and N. C. Greenham, “Photovoltaic Devices Using Blends of Branched CdSe Nanoparticles and Conjugated Polymers.” Nano Lett. 3.7 (2003), pp. 691-963.*** W. U. Huynh, J. J. Dittmer, W. C. Libby, G. L. Whitting, and P. Alivisatos, “Controlling the Morphology of Nanocrystal-Polymer Composites for Solar Cells.” Adv. Funct. Mater. 13.1 (2003), pp. 73-79.

2X

Centrifuge

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Lig

and

re

mo

ved

Lig

and

atta

che

d

Unfiltered Filtered

Improved dispersion after ligand-removal process Preliminary film preparation is complete All solutions & films henceforth made w/ ligand-removal procedures

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6 Films to characterize:

wt% QDs

vol% QDs*

0% 0%

50% 15.0%

75% 34.6%

90% 61.4%

95% 77.0%

100%

100%

* Vol% estimated using densities of 1 g/cm3 for MEH-PPV* (Mirzov 2004) and 5.664 g/cm3 for CdSe.** O.Mirzov et al. “Direct Exciton Quenching in Single Molecules of MEH-PPV at 77 K.” Chem. Phys. Lett. 386.4-6 (2004), pp. 286-290.*** S. Adachi, Handbook on Physical Properties of Semiconductors. Volume 3: “II-VI Compounds.” Springer-Verlag (2004).

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Tapping-mode AFM Resonant Freq ~260 kHz

Veeco Multimode AFM

LASER alignment

Piezo tubescanner

sample

10µm

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wt% QDs:

100%

0%

50%

75%

90%

95%

5 μm 5 μm

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Ra = 3.4

Ra = 20.3

Ra = 21.0

Ra = 5.7

Ra = 11.7

Ra = 2.4Ra Average Roughness (nm)

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Drop-cast dilute (1 mg/mL) solutionsonto Cu mesh, thin-carbon film TEM grids 50 wt% QDs in MEH-PPV 90 wt% QDs in MEH-PPV

Drop-cast Original QD solution as well Dry in N2 dry box ~ 24 hrs

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Jeol 2010F TEMTEM grid

Drop-casting

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QDs in toluene (as-received)

90 wt% QDs in MEH-PPV

50 wt% QDs in MEH-PPV

20 nm 20 nm 20 nm

5 nm 5 nm 5 nm

3-D Architecture

No QDs

Noise from amorphous

polymer

~5-6 nm25

QD

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MECHANICALCHARACTERIZATION

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Load

, P

Displacement, h

dPS

dh

ch

hold segment

loading segmen

tunloadin

g segmen

t

fh maxh

maxPhf hmaxhc

aspecimen

Play

h

indenter tip

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hc

cP

HA h

2r c

dPS E A h

dh

hardnessstiffness

elastic modulus (reduced)

cross-sectional area

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Isolating Film Properties

Indenter Tip Bluntness

Viscoelasticity

Surface Roughness

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For thin films, substrate properties may interfere w/ measurements

Analogous to two springs in series Minimize with shallow Indentations

(<10 % film thickness)

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*A. C. Fischer-Cripps, Nanoindentaion. Springer Mechanical Engineering Series. New York (2002).

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Time-Dependence on Deformation Leads to erroneous values of E, hc

Minimizing Creep Error: Long Hold* Rapid Unload**

Correcting throughComputation L

oad

, P

Displacement, h

“Nose”

dh

dP

ch

long hold

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*Briscoe 1998**Yang 2004

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hc

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Indenter Tip Geometry

-Images from: A. C. Fischer-Cripps, Nanoindentaion. Springer Mech. Engineering Series (2002).*Oliver, W. C. and G. M. Pharr. J. Mater. Res. 7.6 (1992), pp. 1564-1583.

θ = 65.3º2( ) 24.5c cA h h

Berkovich Indenter Tip

<Rounded tip>

Ideally:

In actuality: 2 1/ 2 1/ 4 1/8 1/160 1 2 3 4 5( ) ...c c c c c c cA h C h C h C h C h C h C h

Rounded at end

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100 Indentations on Fused Quartz: ~5-170 nm Calibrate using known values of

(Er,H) = (69.6 GPa, 9.25 GPa)

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2 1/2 1/424.5 1204.8 6858.3 7914.4c cc cA h h h h h

Overlapping Load-Displacement Curves

Determine Area Function from Measured Er’s

scattered under 20 nm

Check area function

Er = 69.72 ± 3.26 GPA

H = 8.52 ± 0.75 GPA

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Nanoindenter Setup: Hysitron Triboindenter

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Determining the Film Thickness Compare large (hundreds of nm-deep) indentations in film vs.

clean substrate

92 nm

224 nm

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Film Thicknesses

wt% QDs 0% 50% 75% 90% 95% 100%

Film Thickness (nm) 201.62 85.61 162.62 85.15 156.42 131.36

Standard Deviation (nm) 19.33 11.67 17.11 4.48 6.98 11.35

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Nanoindentation Parameters

Load-Control Test Cycle

5 indentations each at 8 maximum loads, 3 rates Load-control feedback Drift correction enabled Diamond Berkovich indenter with 50-nm tip radius

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Add film thickness, roughness

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75 wt% QDs

00.5

11.5

22.5

33.5

4

0 50 100 150Contact Depth (nm)

Hard

ness

(GPa

)

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Indentation Size Effect

0 wt% QDs

00.5

11.5

22.5

33.5

4

0 50 100 150Contact Depth (nm)

Hard

ness

(GPa

)

50 wt% QDs

00.5

11.5

22.5

33.5

4

0 50 100 150Contact Depth (nm)

Hard

ness

(GPa

)

90 wt% QDs

00.5

11.5

22.5

33.5

4

0 50 100 150Contact Depth (nm)

Har

dnes

s (G

Pa)

95 wt% QDs

00.5

11.5

22.5

33.5

4

0 50 100 150Contact Depth (nm)

Hard

ness

(GPa

)

100 wt% QDs

00.5

11.5

22.5

33.5

4

0 50 100 150

Contact Depth (nm)H

ardn

ess

(GPa

)

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Choose an appropriate range: > 10nm < ~10% thickness

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Red

uced

Mod

ulus

(G

Pa)

Contact Depth (nm)

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Modulus vs. QD Loading

Modulus increases by a factor of 3Linear when plotted vs. vol%

Red

uced

Mod

ulus

(G

Pa)

Wt% QDs in MEH-PPV Vol% QDs in MEH-PPV

4141

75 wt% QDs

00.5

11.5

22.5

33.5

4

0 50 100 150Contact Depth (nm)

Hard

ness

(GPa

)

Choose an appropriate range: > 50nm

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Hardness vs. QD Loading

Hardness increases by a factor of 6Linear when plotted vs. vol% QDs

Har

dnes

s (G

Pa)

Wt% QDs in MEH-PPV Vol% QDs in MEH-PPV

4343

Sample0 wt% QDs(pure MEH-PPV)

100 μN/s

10 μN/s

1 μN/s

1 μN/s

4444

100 µN/s

1 µN/s

Sample0 wt% QDs(pure MEH-PPV)

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Creep During the Hold Segment for Different Loading Rates0 wt% QDs

Time (s)

Cre

ep (

nm)

4646

0 wt%

50 wt%

75 wt%

95 wt%

100 wt%

90 wt%

Creep During the Hold Segment for Different QD Loading

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Incorporation of QDs: Increases elastic modulus (up to 3x) Increases hardness (up to 6x) Reduces viscoelasticity

Implications: Important for assessing the durability and

reliability of QD-polymer hybrid devices Less creep more stable structure over

time

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Special thanks to: Dr. Curtis Taylor (Committee Chair) Dr. Jim McLeskey (Committee Member) Dr. Samy El-Shall (Committee Member) Dr. N. Chandrasekhar VCU Nanomanufacturing Lab

▪ Dr. Tarek Trad, Yezuo Wang, Dongshan Yu, Jon Kodadek, Nikolai Eroshenko

IMRE Staff▪ Dr. Saravanan Shanmugavel, Shen Lu, Dr.

Dominik Janczewski, Luong Trung Dung, Kajen Rasanayagam

Dr. David Goorskey48

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Agilent PicoPlus System User’s Manual v1.2, “Aligning the Photodiode Detector.” pp. 1-18

Used with permission from http://barrett-group.mcgill.ca/yager/art.html

http://www.nanoscience.com/products/AFM_tips.html

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