Eddie McCumiskey Master’s Thesis Presentation 23 January 2008 Virginia Commonwealth University
<ul><li>Motivation </li></ul><ul><li>Problem Statement </li></ul><ul><li>Challenges </li></ul><ul><li>Approach </li></ul><...
<ul><li>Low-cost, light-weight, rollable solar cells </li></ul><ul><li>Low-power, high-resolution displays </li></ul><ul><...
<ul><li>Nanostructure-polymers hybrids exhibit: </li></ul><ul><ul><li>Increased Efficiency </li></ul></ul><ul><ul><li>Tuna...
<ul><li>In order for QD-polymer hybrids to be commercially viable, their reliability and durability must be known.  </li><...
<ul><li>Polymer-Clay Nanocomposites invented by Toyota ( 1985 ) </li></ul><ul><ul><li>Greatly enhanced strength, elastic m...
<ul><li>Theoretically, QDs should be stiffer than bulk </li></ul><ul><ul><li>Lattice contraction due to high surface energ...
<ul><li>Large gap in research on mechanical properties of QD-polymer nanocomposite films </li></ul><ul><li>Challenges </li...
QDs in a polymer matrix Conceptual Design of Experiment Example from the literature: organic solar cell made with blended ...
I.  Prepare QD-Polymer Solutions II.  Deposit Films onto Glass III.  Characterize Film Uniformity IV.  Mechanical Characte...
<ul><li>CdSe QDs </li></ul><ul><ul><li>NN-Labs #  CSE  620-100 </li></ul></ul><ul><ul><li>Capped with ODA Ligand </li></ul...
<ul><li>MEH-PPV* Polymer </li></ul><ul><ul><li>American Dye Source  # ADS 200 RE </li></ul></ul><ul><ul><li>Abs  490  nm; ...
<ul><li>Fabrication and Characterization </li></ul><ul><li>of QD-Polymer Films </li></ul>
<ul><li>Dissolve MEH-PPV in Toluene </li></ul><ul><ul><li>Weigh MEH-PPV </li></ul></ul><ul><ul><li>Add toluene ( 5  mg MEH...
<ul><li>Ultrasonicate Combined Solutions for  30  mins </li></ul><ul><li>Filter through a  1.0 -µm PTFE filter (optional) ...
<ul><li>Aggregation observed at the  micrometer scale </li></ul><ul><li>Filtering removes many large aggregates </li></ul>...
<ul><li>Dispersion is a common problem through the literature </li></ul><ul><li>Ligands may lead to aggregation </li></ul>...
<ul><li>Ligand is an insulator </li></ul><ul><li>Removing ligands: </li></ul><ul><ul><li>enhances charge separation  & tra...
<ul><li>QDs precipitated in pyridine </li></ul><ul><ul><li>1  mL QD solution </li></ul></ul><ul><ul><li>1  mL pyridine </l...
Ligand removed Ligand attached Unfiltered Filtered <ul><li>Improved dispersion after ligand-removal process </li></ul><ul>...
<ul><li>6 Films to characterize: </li></ul>* Vol% estimated using densities of 1 g/cm 3  for MEH-PPV* (Mirzov 2004) and 5....
<ul><li>Tapping-mode AFM </li></ul><ul><li>Resonant Freq ~ 260  kHz </li></ul>Veeco Multimode AFM LASER alignment Piezo tu...
wt% QDs: 100% 0% 50% 75% 90% 95% R a  = 3.4 R a  = 20.3 R a  = 21.0 R a  = 5.7 R a  = 11.7 R a  = 2.4 R a     Average Rou...
<ul><li>Drop-cast dilute ( 1  mg/mL) solutions onto Cu mesh, thin-carbon film TEM grids </li></ul><ul><ul><li>50  wt% QDs ...
QDs in toluene (as-received) 90 wt% QDs in MEH-PPV 50 wt% QDs in MEH-PPV 3-D Architecture No QDs Noise from amorphous poly...
<ul><li>MECHANICAL CHARACTERIZATION </li></ul>
Load,  P Displacement,  h specimen Play h indenter tip hold segment loading segment unloading segment h f h max h c a h c ...
<ul><li>Isolating Film Properties </li></ul><ul><li>Indenter Tip Bluntness  </li></ul><ul><li>Viscoelasticity </li></ul><u...
<ul><li>For thin films, substrate properties may interfere w/ measurements </li></ul><ul><li>Analogous to two springs in s...
<ul><li>Time-Dependence on Deformation </li></ul><ul><li>Leads to erroneous values of  E, h c </li></ul><ul><li>Minimizing...
Indenter Tip Geometry  -Images from: A. C. Fischer-Cripps,  Nanoindentaion . Springer Mech. Engineering Series  (2002). *O...
<ul><li>100  Indentations on Fused Quartz: ~ 5-170  nm </li></ul><ul><li>Calibrate using known values of  ( E r , H ) = ( ...
Nanoindenter Setup:  Hysitron Triboindenter
Determining the Film Thickness <ul><li>Compare large (hundreds of nm-deep) indentations in film vs. clean substrate </li><...
Film Thicknesses 11.35 6.98 4.48 17.11 11.67 19.33 Standard Deviation (nm) 131.36 156.42 85.15 162.62 85.61 201.62 Film Th...
Nanoindentation Parameters Load-Control Test Cycle <ul><li>5 indentations each at 8 maximum loads, 3 rates </li></ul><ul><...
Add film thickness, roughness
Indentation Size Effect
<ul><li>Choose an appropriate range: </li></ul><ul><ul><li>>  10 nm </li></ul></ul><ul><ul><li><  ~10%  thickness </li></u...
Modulus   vs. QD Loading <ul><li>Modulus increases by a factor of  3 </li></ul><ul><li>Linear when plotted vs. vol% </li><...
<ul><li>Choose an appropriate range: </li></ul><ul><ul><li>>  50 nm </li></ul></ul>
Hardness vs. QD Loading <ul><li>Hardness increases by a factor of 6 </li></ul><ul><li>Linear when plotted vs. vol% QDs </l...
Sample 0 wt% QDs (pure MEH-PPV) 100  μ N/s 10  μ N/s 1  μ N/s 1  μ N/s
Sample 0 wt% QDs (pure MEH-PPV) 100 µN/s 1 µN/s
Creep During the Hold Segment for Different Loading Rates 0 wt% QDs Time (s) Creep (nm)
0 wt% 50 wt% 75 wt% 95 wt% 100 wt% 90 wt% Creep During the Hold Segment for Different QD Loading
<ul><li>Incorporation of QDs: </li></ul><ul><ul><li>Increases elastic modulus (up to  3 x) </li></ul></ul><ul><ul><li>Incr...
<ul><li>Special thanks to: </li></ul><ul><ul><li>Dr. Curtis Taylor (Committee  Chair) </li></ul></ul><ul><ul><li>Dr. Jim M...
Agilent  PicoPlus System User’s Manual v1.2, “Aligning the Photodiode Detector.”  pp . 1-18 Used with permission from http...
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Nanomechanical Characterization of CdSe QD-Polymer Nanocomposites

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Nanomechanical Characterization of CdSe QD-Polymer Nanocomposites

  1. 1. Eddie McCumiskey Master’s Thesis Presentation 23 January 2008 Virginia Commonwealth University
  2. 2. <ul><li>Motivation </li></ul><ul><li>Problem Statement </li></ul><ul><li>Challenges </li></ul><ul><li>Approach </li></ul><ul><li>Experimental Details </li></ul><ul><li>Results </li></ul><ul><li>Conclusion </li></ul><ul><li>Acknowledgements </li></ul><ul><li>Questions </li></ul>
  3. 3. <ul><li>Low-cost, light-weight, rollable solar cells </li></ul><ul><li>Low-power, high-resolution displays </li></ul><ul><li>Flexible circuitry </li></ul>http://www.solardirect.com/pv/consumer-ready/power-film.htm#fea Sony’s 3-mm thick TV www.sonystyle.com/oled
  4. 4. <ul><li>Nanostructure-polymers hybrids exhibit: </li></ul><ul><ul><li>Increased Efficiency </li></ul></ul><ul><ul><li>Tunable Band Gap </li></ul></ul><ul><ul><li>Tunable Electrical Properties </li></ul></ul>http://www.nn-labs.com/CdSe-orderform.htm Commercial CdSe Quantum Dots
  5. 5. <ul><li>In order for QD-polymer hybrids to be commercially viable, their reliability and durability must be known. </li></ul>Mechanical Characterization
  6. 6. <ul><li>Polymer-Clay Nanocomposites invented by Toyota ( 1985 ) </li></ul><ul><ul><li>Greatly enhanced strength, elastic modulus </li></ul></ul><ul><li>Large Interaction between NPs and filler material due to high interfacial area </li></ul><ul><li>CNT-polymer nanocomposites: </li></ul><ul><ul><li>Increased hardness, elasticmodulus </li></ul></ul>Nanocomposite Mechanical Properties
  7. 7. <ul><li>Theoretically, QDs should be stiffer than bulk </li></ul><ul><ul><li>Lattice contraction due to high surface energy </li></ul></ul><ul><li>Nanoscale structures shown to impede dislocation formation </li></ul><ul><li>Nanoindentation of QD films reveals:* </li></ul><ul><ul><li>Polymeric behavior of QDs with ligands attached </li></ul></ul><ul><ul><li>Granular behavior of QD films without ligands </li></ul></ul>*D. Lee et al., Phys. Rev. Lett. 98 .2 (2007)
  8. 8. <ul><li>Large gap in research on mechanical properties of QD-polymer nanocomposite films </li></ul><ul><li>Challenges </li></ul><ul><ul><li>Mechanical characterization of thin films (<100 nm ) </li></ul></ul><ul><ul><li>Interference from the underlying substrate </li></ul></ul><ul><ul><li>Obtaining uniform QD Dispersion </li></ul></ul>
  9. 9. QDs in a polymer matrix Conceptual Design of Experiment Example from the literature: organic solar cell made with blended CdSe nanoparticles and OC 1 C 10 -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 Characterizing the Mechanical Properties of Nanocomposite Films glass substrate Applied Load
  10. 10. I. Prepare QD-Polymer Solutions II. Deposit Films onto Glass III. Characterize Film Uniformity IV. Mechanical Characterization Nanoindentation TEM AFM Stirring Sonicating Spin-coating
  11. 11. <ul><li>CdSe QDs </li></ul><ul><ul><li>NN-Labs # CSE 620-100 </li></ul></ul><ul><ul><li>Capped with ODA Ligand </li></ul></ul><ul><ul><li>Dispersed in Toluene ( 5 mg/mL) </li></ul></ul><ul><ul><li>Abs 620 nm; PL ~ 630 nm </li></ul></ul>CdSe ODA Ligands 5.6 nm CdSe QDs Source: http://www.nn-labs.com/CdSe-orderform.htm
  12. 12. <ul><li>MEH-PPV* Polymer </li></ul><ul><ul><li>American Dye Source # ADS 200 RE </li></ul></ul><ul><ul><li>Abs 490 nm; PL ~ 585 nm </li></ul></ul>MEH-PPV Structure Dry MEH-PPV *p oly[2- m ethoxy-5-2(2΄- e thyl h exyloxy- p henylene v inylene)] Source: http://www.adsdyes.com/products/pdf/homopolymers/ADS200RE_DATA.pdf
  13. 13. <ul><li>Fabrication and Characterization </li></ul><ul><li>of QD-Polymer Films </li></ul>
  14. 14. <ul><li>Dissolve MEH-PPV in Toluene </li></ul><ul><ul><li>Weigh MEH-PPV </li></ul></ul><ul><ul><li>Add toluene ( 5 mg MEH-PPV / mL toluene) </li></ul></ul><ul><ul><li>Stir ~12 hrs at 300 RPM at room temp. </li></ul></ul><ul><li>Mix MEH-PPV & QD Solutions </li></ul><ul><ul><li>Add equal volume of QD solution to make 50 wt% QDs in MEH-PPV </li></ul></ul><ul><ul><li>Stir ~12 hrs at 300 RPM at room temp. </li></ul></ul>+  MEH-PPV in toluene QDs in toluene MEH-PPV + QDs in toluene
  15. 15. <ul><li>Ultrasonicate Combined Solutions for 30 mins </li></ul><ul><li>Filter through a 1.0 -µm PTFE filter (optional) </li></ul><ul><li>Spin-Coat @ 1,000-2,000 RPM, 30 seconds </li></ul><ul><li>Anneal on hot plate @ 120 °C, 10 mins </li></ul>A. Sonicate C. Deposit via Spin-Coating D. Anneal Filtering B.
  16. 16. <ul><li>Aggregation observed at the micrometer scale </li></ul><ul><li>Filtering removes many large aggregates </li></ul>Unfiltered Filtered Aggregation 20 μ m 200 μ m 200 μ m
  17. 17. <ul><li>Dispersion is a common problem through the literature </li></ul><ul><li>Ligands may lead to aggregation </li></ul><ul><li>Ligands also unfavorable for devices* </li></ul>* 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
  18. 18. <ul><li>Ligand is an insulator </li></ul><ul><li>Removing ligands: </li></ul><ul><ul><li>enhances charge separation & transport; </li></ul></ul><ul><ul><li>quenches PL </li></ul></ul><ul><ul><li>more realistic for commercial applications </li></ul></ul>QDs w/ Ligands QDs w/o Ligands LIGAND  Need to remove the ligands
  19. 19. <ul><li>QDs precipitated in pyridine </li></ul><ul><ul><li>1 mL QD solution </li></ul></ul><ul><ul><li>1 mL pyridine </li></ul></ul><ul><ul><li>~ 10 mL hexanes </li></ul></ul><ul><li>Retrieved via centrifugation </li></ul><ul><ul><li>9,000 + RPM </li></ul></ul><ul><ul><li>30 mins </li></ul></ul><ul><li>Change to binary solvent: 8 % pyridine, 92 % choloroform*** </li></ul>* 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
  20. 20. Ligand removed Ligand attached Unfiltered Filtered <ul><li>Improved dispersion after ligand-removal process </li></ul><ul><li>Preliminary film preparation is complete </li></ul><ul><li>All solutions & films henceforth made w/ ligand-removal procedures </li></ul>
  21. 21. <ul><li>6 Films to characterize: </li></ul>* Vol% estimated using densities of 1 g/cm 3 for MEH-PPV* (Mirzov 2004) and 5.664 g/cm 3 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). wt% QDs vol% QDs* 0% 0% 50% 15.0% 75% 34.6% 90% 61.4% 95% 77.0% 100% 100%
  22. 22. <ul><li>Tapping-mode AFM </li></ul><ul><li>Resonant Freq ~ 260 kHz </li></ul>Veeco Multimode AFM LASER alignment Piezo tube scanner sample 10µm
  23. 23. wt% QDs: 100% 0% 50% 75% 90% 95% R a = 3.4 R a = 20.3 R a = 21.0 R a = 5.7 R a = 11.7 R a = 2.4 R a  Average Roughness (nm) 5 μm 5 μm
  24. 24. <ul><li>Drop-cast dilute ( 1 mg/mL) solutions onto Cu mesh, thin-carbon film TEM grids </li></ul><ul><ul><li>50 wt% QDs in MEH-PPV </li></ul></ul><ul><ul><li>90 wt% QDs in MEH-PPV </li></ul></ul><ul><li>Drop-cast Original QD solution as well </li></ul><ul><li>Dry in N 2 dry box ~ 24 hrs </li></ul>Jeol 2010F TEM TEM grid Drop-casting
  25. 25. QDs in toluene (as-received) 90 wt% QDs in MEH-PPV 50 wt% QDs in MEH-PPV 3-D Architecture No QDs Noise from amorphous polymer ~5-6 nm QD 20 nm 20 nm 20 nm 5 nm 5 nm 5 nm
  26. 26. <ul><li>MECHANICAL CHARACTERIZATION </li></ul>
  27. 27. Load, P Displacement, h specimen Play h indenter tip hold segment loading segment unloading segment h f h max h c a h c hardness stiffness elastic modulus (reduced) cross-sectional area
  28. 28. <ul><li>Isolating Film Properties </li></ul><ul><li>Indenter Tip Bluntness </li></ul><ul><li>Viscoelasticity </li></ul><ul><li>Surface Roughness </li></ul>
  29. 29. <ul><li>For thin films, substrate properties may interfere w/ measurements </li></ul><ul><li>Analogous to two springs in series </li></ul><ul><li>Minimize with shallow Indentations ( <10 % film thickness) </li></ul>*A. C. Fischer-Cripps, Nanoindentaion . Springer Mechanical Engineering Series. New York (2002).
  30. 30. <ul><li>Time-Dependence on Deformation </li></ul><ul><li>Leads to erroneous values of E, h c </li></ul><ul><li>Minimizing Creep Error: </li></ul><ul><ul><li>Long Hold* </li></ul></ul><ul><ul><li>Rapid Unload** </li></ul></ul><ul><li>Correcting through Computation </li></ul>*Briscoe 1998 **Yang 2004 Load, P Displacement, h “ Nose” long hold
  31. 31. 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 = 65.3º Berkovich Indenter Tip <Rounded tip> Ideally: In actuality: Rounded at end h c
  32. 32. <ul><li>100 Indentations on Fused Quartz: ~ 5-170 nm </li></ul><ul><li>Calibrate using known values of ( E r , H ) = ( 69.6 GPa, 9.25 GPa) </li></ul>Overlapping Load-Displacement Curves Determine Area Function from Measured E r ’s scattered under 20 nm Check area function E r = 69.72 ± 3.26 GPA H = 8.52 ± 0.75 GPA
  33. 33. Nanoindenter Setup: Hysitron Triboindenter
  34. 34. Determining the Film Thickness <ul><li>Compare large (hundreds of nm-deep) indentations in film vs. clean substrate </li></ul>92 nm 224 nm
  35. 35. Film Thicknesses 11.35 6.98 4.48 17.11 11.67 19.33 Standard Deviation (nm) 131.36 156.42 85.15 162.62 85.61 201.62 Film Thickness (nm) 100% 95% 90% 75% 50% 0% wt% QDs
  36. 36. Nanoindentation Parameters Load-Control Test Cycle <ul><li>5 indentations each at 8 maximum loads, 3 rates </li></ul><ul><li>Load-control feedback </li></ul><ul><li>Drift correction enabled </li></ul><ul><li>Diamond Berkovich indenter with 50-nm tip radius </li></ul>
  37. 37. Add film thickness, roughness
  38. 38. Indentation Size Effect
  39. 39. <ul><li>Choose an appropriate range: </li></ul><ul><ul><li>> 10 nm </li></ul></ul><ul><ul><li>< ~10% thickness </li></ul></ul>Reduced Modulus (GPa) Contact Depth (nm)
  40. 40. Modulus vs. QD Loading <ul><li>Modulus increases by a factor of 3 </li></ul><ul><li>Linear when plotted vs. vol% </li></ul>Reduced Modulus (GPa) Wt% QDs in MEH-PPV Vol% QDs in MEH-PPV
  41. 41. <ul><li>Choose an appropriate range: </li></ul><ul><ul><li>> 50 nm </li></ul></ul>
  42. 42. Hardness vs. QD Loading <ul><li>Hardness increases by a factor of 6 </li></ul><ul><li>Linear when plotted vs. vol% QDs </li></ul>Hardness (GPa) Wt% QDs in MEH-PPV Vol% QDs in MEH-PPV
  43. 43. Sample 0 wt% QDs (pure MEH-PPV) 100 μ N/s 10 μ N/s 1 μ N/s 1 μ N/s
  44. 44. Sample 0 wt% QDs (pure MEH-PPV) 100 µN/s 1 µN/s
  45. 45. Creep During the Hold Segment for Different Loading Rates 0 wt% QDs Time (s) Creep (nm)
  46. 46. 0 wt% 50 wt% 75 wt% 95 wt% 100 wt% 90 wt% Creep During the Hold Segment for Different QD Loading
  47. 47. <ul><li>Incorporation of QDs: </li></ul><ul><ul><li>Increases elastic modulus (up to 3 x) </li></ul></ul><ul><ul><li>Increases hardness (up to 6 x) </li></ul></ul><ul><ul><li>Reduces viscoelasticity </li></ul></ul><ul><li>Implications: </li></ul><ul><ul><li>Important for assessing the durability and reliability of QD-polymer hybrid devices </li></ul></ul><ul><ul><li>Less creep  more stable structure over time </li></ul></ul>
  48. 48. <ul><li>Special thanks to: </li></ul><ul><ul><li>Dr. Curtis Taylor (Committee Chair) </li></ul></ul><ul><ul><li>Dr. Jim McLeskey (Committee Member) </li></ul></ul><ul><ul><li>Dr. Samy El-Shall (Committee Member) </li></ul></ul><ul><ul><li>Dr. N. Chandrasekhar </li></ul></ul><ul><ul><li>VCU Nanomanufacturing Lab </li></ul></ul><ul><ul><ul><li>Dr. Tarek Trad, Yezuo Wang, Dongshan Yu, Jon Kodadek, Nikolai Eroshenko </li></ul></ul></ul><ul><ul><li>IMRE Staff </li></ul></ul><ul><ul><ul><li>Dr. Saravanan Shanmugavel, Shen Lu, Dr. Dominik Janczewski, Luong Trung Dung, Kajen Rasanayagam </li></ul></ul></ul><ul><ul><li>Dr. David Goorskey </li></ul></ul>
  49. 49.
  50. 50. 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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