1. Chapter 2 Properties on the Nanoscale
Introduction to Nanotechnology

2. Why Miniaturize? Properties start to change as size decreases!
Talking about inorganic bulk materials like metals or semiconductors
Properties start to change as size decreases!

3. Why “Nano” is Interesting
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
(mostly surface!) 3

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!

5. Surface Area and Energy
Surface atoms (%)
Surface energy = Extra energy at surface compared to bulk
Surface energy increases
with surface area
Large surface energy = instability
Driven to grow to reduce surface energy
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
Spatial confinement -> of an electron orbital
Reduced defects due to self purification
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
Particle diameter (nm)
Melting point (K)
Semiconductors, metals, inert gases, molecular crystals
Calculated melting point for gold nanoparticles
Ichimose, N. et al. Superfine Particle Technology Springer-Verlag London, 1992.

8. Mechanical Properties
Mechanical properties improve as size decreases
High atomic perfection
d (µm)
Strength (kg/mm)
Copper:
6 nm grains have hardness 5x higher than 50 µm grains
Palladium:
5-10 nm grains have hardness 5x higher than 100 µm grains
However, one model: Hardness increases as particle size decreases up to a certain point (5 nm), then structure “softens”
Mechanical Strength of NaCl whiskers
Gyulai, Z. Z. Phys. 138, 317 (1954).

9. Mechanical Properties
Mechanical properties improve as size decreases
High atomic perfection
Copper:
6 nm grains have hardness 5x higher than 50 µm grains
Palladium:
5-10 nm grains have hardness 5x higher than 100 µm grains
However, one model: Hardness increases as particle size decreases up to a certain point (5 nm), then structure “softens”
Can fail due to high internal stress

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
220X 1X
20,000X
5000X
Photonic Crystals
Surface Plasmon Resonance
Quantum Dot Fluorescence
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,

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
Bulk plasmons – oscillations of electrons
“Surface” – occur at interface of metal and dielectric

18. Optical Properties Localized Surface Plasmon Resonance
“sea of electrons” excited
Bulk plasmons – oscillations of electrons
“Surface” – occur at interface of metal and dielectric

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

20. Summary Properties that change on the nanoscale: Why? 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 How do we measure size? Characteristic dimension: D
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.

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)?