Chemical and physical properties of nanoparticles - why they are different from conventional materials

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Chemical and physical properties of nanoparticles
- why they are different from conventional
materials

• Tim Senden
• The Browitt Nanoparticle Laboratory
• Dept Applied Mathematics
• Research School of Physics and Engineering

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It isnt size alone that makes a material nano
its how nanoscopic phenomena play on that
material that does matter.

• Summary (some questions to be explored)
• How does matter interact with light?
• How does matter interact with matter?
• Which bulk properties dont scale with size?
• Why does surface chemistry matter?
• What keeps nano-materials dispersed?

The nanoscale characterises a strong cross over
between physics and chemistry (both matter and
energy levels are discrete.)
Ganges River Delta
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Getting a sense of scale
metres
fog / mist
ions molecules
oil / smoke
pollen
macromolecules
bacteria
viruses
micelles
Surface tension beats gravity
Thermal fluctuations
Electronic effects
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Nanoscale measurements
Nanoscale leads to pico-, femto-, atto- effects
Scale of forces 1 N force required to hold an
apple against gravity 1 mN force required to
hold a postage stamp against gravity 1 µN force
required to hold an eye lash against gravity 1 nN
covalent bonds force between clay particles in
water 10 pN a single H-bond
Scale of energy 100 J the energy release by a
sleeping person per second 1 J work required to
pick an apple of the ground (1 metre) 1 fJ
energy required to bend lipid membrane 1 aJ
energy required to do cis - trans rotation
(thermal energy)
thermal energy (kT) is maxm work available to a
molecule
10-18 atto- 10-15 femto- 10-12 pico- 10-9
nano- 10-6 micro-
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What are these forces?Where do they come from?

• (its about how electrons interact with electrons
  interact with photons - think liq. Helium)
• DEMO
• Attraction generally increases will refractive
  index
• - Need to return to the bulk -

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Bulk properties

• Some bulk properties scale with size but the
  explanation might not

Elasticity
stretch
Cooling molecule down
Consider a rubber band
Viscosity
Thermal fluctuations
Ordered layer
etc..
Now consider boiling/melting point, reflectivity,
solubility
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For liquids
Two unequal water drops in zero gravity

• Systems interact to minimise total surface
  energy.
• Pressure difference due to surface energy, so
  material dependent.
• Contaminants always go to the interface.

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Curved Interfaces

• Consider an air-bubble in water
• The bubble is stable when no net air flows
• Surface tension (g) will act to decrease radius
• Can be prevented by raising the pressure such
  that PIgtPO.

Water
PO
PI
r
Air
Laplace pressure
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Points to note

• The pressure inside a curved meniscus is always
  greater than that outside
• This is true also for liquid droplets in air
• Thus, liquid in droplet form is under a greater
  pressure than liquid a flat surface
• This effect is only significant when the
  curvature is high, ie. for small particles

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Plot of vapour pressure vs radius
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o
Droplet
o
2.5
Bubble
o
2
Relative Vapour Pressure
1.5
o
o
o
o
o
o
o
o
o
o
1
o
o
0.5
o
0
1
10
100
1000
Radius (nm)
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Goretex
Goretex is essentially just porous teflon
(polytetrafluoroethylene). The pores allow water
vapour to pass through the fabric and are
essential if one wants a raincoat which
breathes. But why then dont water droplets
penetrate the fabric?
Micron scale
DEMO
It can be the nanoscopic pores that make the
nanomaterial NOT the material itself.
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For solids

• The surface atoms squeeze the internal atoms.
  In nanoscopic systems this could be 1000s of
  atmospheres.
• Physical properties such as opto-electronic,
  phase state, solubility, reactivity and
  conductivity may change

Each atom on the surface has different properties
(colour indicated) thus the surface is defective.
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Reactivity
tipping point
energy
Population of atoms with a given energy
Mg
MgO
Thermal energy
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For gases

• depends on vapour pressure and a balance of
  surface energies
• hydrophobic is qgt90
• roughness makes a huge difference
• If the vapour doesnt adsorb then surface is not
  wet

Its curvature that matters

q
Contact angle is due to balance of surface
energies
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Why are nanomaterials stable?

• Chemical stability - surface passivation
• Physical stability - against aggregation
• - A balance of forces
• Sulfur is hydrophobic, gold has huge attraction
• Dissociation - (Oxides, acidic or amphoteric)
• Crystal lattice effects (Clays)
• Ion adsorption (specific)

DEMO
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The origin of surface charge

• Surface SiOH are acidic

• Some metal oxides are amphoteric
• eg alumina, goethite (a-FeO(OH))

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Most native surfaces are nagative
Two charge colloids repell - Due to osmotic
pressure
tipping point
AFM measurement A tool for seeing (feeling) the
world from a nanoparticles perspective
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van der Waals depends on material
charging depends on solution/surface
attraction
repulsion
combination
tipping point
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Summary of forces
Force approx. range min/max force for
colloidal sized objects Attractive
(negative force) van der Waals lt15 nm lt -1
nN Hydrophobic lt500 nm lt -10 nN Repulsive
(positive force) Double layer repulsion lt100
nm lt 5 nN Hydration lt5 nm lt 10
nN Steric lt20 nm lt 10 nN
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Scattering

• Finely divided insulators become whiter
• Finely divided metals become black and then
  coloured

Aussie sky blue
European sky blue
Colour in metals comes from plasmon resonance,
just ask Paul Blue Karason
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Mary Kathleen uranium mine, near Cloncurry, Qld.
Tyndall effect
It is named after the Irish scientist John
Tyndall. Light with shorter wavelengths scatters
better, thus the color of scattered light has a
bluish tint. This is the reason why the sky looks
blue the blue component of sun light is more
highly scattered.
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Energy Band Representation of Insulators,
Semiconductors and Metals
Empty Conduction band
Conduction band
400 kT
40 kT
Partially filled Conduction band
Filled valence band
valence band
valence band
Insulator
Semiconductor
Metal
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Density of States in semiconductors
Reduced Dimensionality leads to higher
efficiency, lower threshold current, reduced
power consumption and higher operating speed
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Photoluminescence
1.6 nm
4 GaAs QW with AlGaAs barriers
2.2 nm
2
3.4 nm
6.8 nm
3
1
4
S
Colloidal CdSe quantum dots
Courtesy of Prof. Jagadish, ANU
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Summary

• Its not so much the size that matters, its the
  dominance of microscopic phenomena at that length
  scale.
• Bulk, macroscopic properties give way to the fact
  matter is corpuscular, electronic and fluctuating
  with thermal energy.