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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