Chapter 2Properties on the Nanoscale
NANO 101Introduction to Nanotechnology
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Why Miniaturize?
Properties start to change as size decreases!2
• 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
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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
Surface energy increases with surface area
• Large surface energy = instability• Driven to grow to reduce surface energy
Surface Area and Energy
C. Nutzenadel et al., Eur. Phys. J. D. 8, 245 (2000).
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diameter (nm)
S
urfa
ce a
tom
s (%
)
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?
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Melting Points• Lower melting point for nanostructures <100 nm
• Surface energy increases as size decreases
Ichimose, N. et al. Superfine Particle Technology Springer-Verlag London, 1992. 7
Particle diameter (nm)
M
eltin
g po
int (
K)
Mechanical Properties
Mechanical properties improve as size decreases• High atomic perfection
Mechanical Strength of NaCl whiskers
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d (µm)
S
tren
gth
(kg/
mm
)
Gyulai, Z. Z. Phys. 138, 317 (1954).
Mechanical Properties
Mechanical properties improve as size decreases• High atomic perfection
Can fail due to high internal stress
9http://hysitron.com/Portals/0/Updated%20Address/PICO02AN%20r1.f.pdf
Electrical Properties
• Electron Scattering– Electrons move through a metal to conduct
electricity– Electrons are scattered to cause resistivity– Decreasing size fewer defects less scattering
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Band Structure of Nanostructures
• Electronic states in between bulk solid and molecule
Bahnemann, Kormann, Hoffmann, J Phys. Chem. 91, 3789 (1987)
Electronic Structure
Band gap decreases as particle size increases
E
Metal Insulator Semi-conductor NP Semi-conductor
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Particles & Light
Particles interact differently with light Structures are smaller than wavelength of visible light
13Militaries 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
220X1X 20,000X5000X
• Photonic Crystals• Surface Plasmon Resonance• Quantum Dot Fluorescence
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Chameleons
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Polymers that can act like chameleons
• Polymer spheres self assemble to mimic color change of chameleons
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Optical Properties• Plasmon Resonance -> Surface Plasmon
Resonance
Optical Properties
Localized Surface Plasmon Resonance– “sea of electrons” excited
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Optical Properties
Localized Surface Plasmon Resonance– “sea of electrons” excited
18https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr200908sf5.html
Optical Properties: Quantum Dots
Band gap decreases as particle size increases
E
Metal Insulator Semiconductor 50 nm QD 10 nm QD
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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
Scaling Laws
Model how properties change as things get miniaturized(minimize theoretical calculations)
How do we measure size?
Characteristic dimension: D
Volume
Surface area 21
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?
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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.
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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)?
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