Chapter 2 Properties on the Nanoscale Introduction to Nanotechnology

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!

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

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 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).

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?

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.

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).

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 http://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

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

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, 1403-1411

Chameleons

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

Optical Properties Plasmon Resonance -> Surface Plasmon Resonance

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

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

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

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

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

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.

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)?