Chapter 2 Properties on the Nanoscale

1. Chapter 2Properties on the Nanoscale NANO 101 Introduction to Nanotechnology

2. Why Miniaturize? Properties start to change as size decreases!

3. Particles are small High surface-to-volume ratio React differently Act differently (new properties) Interact with light differently Are on the scale of small biological structures Quantum Mechanics meet Classical Mechanics Interesting “new” structures Why “Nano” is Interesting

4. How Much Surface Area? • 1,000 1 mm cubes, 2/3 of an index card • 1 x 10^21 1nm cubes, larger than a football field! http://www.nano.gov/nanotech-101/special

5. Surface Area and Energy Surface energy increases with surface area • Large surface energy = instability • Driven to grow to reduce surface energy Surface atoms (%) diameter (nm) C. Nutzenadel et al., Eur. Phys. J. D. 8, 245 (2000).

6. Physical Structure  Physical Property What are the structural differences on the nanoscale? • High percentage surface atoms • Large surface energy • Spatial confinement • Reduced imperfections What properties are affected? What properties can we tune?

7. Melting Points • Lower melting point for nanostructures <100 nm • Surface energy increases as size decreases Melting point (K) Particle diameter (nm) Ichimose, N. et al. Superfine Particle Technology Springer-Verlag London, 1992.

8. Mechanical Properties Mechanical properties improve as size decreases • High atomic perfection Strength (kg/mm) d (µm) Mechanical Strength of NaCl whiskers Gyulai, Z. Z. Phys.138, 317 (1954).

9. Mechanical Properties Mechanical properties improve as size decreases • High atomic perfection Can fail due to high internal stress http://hysitron.com/Portals/0/Updated%20Address/PICO02AN%20r1.f.pdf

10. Electrical Properties • Electron Scattering • Electrons move through a metal to conduct electricity • Electrons are scattered to cause resistivity • Decreasing size  fewer defects  less scattering

11. Band Structure of Nanostructures • Electronic states in between bulk solid and molecule Bahnemann, Kormann, Hoffmann, J Phys. Chem. 91, 3789 (1987)

12. Electronic Structure Band gap decreases as particle size increases E Metal Insulator Semi-conductor NP Semi-conductor

13. Particles & Light Particles interact differently with light Structures are smaller than wavelength of visible light • Photonic Crystals • Surface Plasmon Resonance • Quantum Dot Fluorescence 1X 220X 5000X 20,000X Militaries Study Animals for Cutting-Edge Camouflage. James Owen in England for National Geographic News March 12, 2003, Proc. R. Soc. Lond. B (1999) 266, 1403-1411

14. Chameleons

15. Polymers that can act like chameleons • Polymer spheres self assemble to mimic color change of chameleons

16. Optical Properties • Plasmon Resonance -> Surface Plasmon Resonance

17. Optical Properties Localized Surface Plasmon Resonance • “sea of electrons” excited

18. Optical Properties Localized Surface Plasmon Resonance • “sea of electrons” excited https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr200908sf5.html

19. Optical Properties: Quantum Dots Band gap decreases as particle size increases E Metal Insulator Semiconductor 50 nm QD 10 nm QD

20. Summary • Properties that change on the nanoscale: • Melting point • Mechanical properites • Electrical properties • Optical properties • Why? • Nanostructures are mostly surface • Nanostructres may be smaller than electron orbitals and light wavelengths

21. Scaling Laws Model how properties change as things get miniaturized (minimize theoretical calculations) How do we measure size? Characteristic dimension: D Volume Surface area

22. Using Scale Laws An elephant has D ~ 1 m. What is its S/V ratio? A flea has D ~ 1 mm. What is its S/V ratio?

23. Deriving Scale Laws You are told strength is proportional to D2 and that weight is proportional to D3 Write the expression for strength to weight ratio.

24. Practice with Scaling Laws How many times greater is the strength to weight ratio of a nanotube (D = 10 nm) than… • the leg of a flea (D = 100 µm)? • the leg of an elephant (D = 2 m)?