Chemical and physical properties of nanoparticles - why they are

different from conventional materials

Tim SendenThe Browitt Nanoparticle Laboratory

Dept Applied Mathematics

Research School of Physics and Engineering

Ganges River Delta

Summary (some questions to be explored)

• How does matter interact with light?

• How does matter interact with matter?

• Which bulk properties don’t scale with size?

• Why does surface chemistry matter?

• What keeps nano-materials dispersed?

It isn’t size alone that makes a material “nano” it’s how nanoscopic phenomena play on that material that does matter.

The nanoscale characterises a strong cross over between physics and chemistry (both matter and energy levels are discrete.)

Getting a sense of scalemetres

colloidsfog / mistions

molecules

macromoleculespollen

bacteriamicelles

oil / smoke

viruses

10-10 10-9 10-8 10-7 10-6 10-5 10-4

micro-pico- milli-nano-

10-310-12 10-11

Electronic effects

Thermal fluctuations

Surface tension beats gravity

Nanoscale measurements

Scale of forces1 N ≈ force required to hold an apple against gravity1 mN ≈ force required to hold a postage stamp against gravity1 µN ≈ force required to hold an eye lash against gravity1 nN ≈ covalent bonds; force between clay particles in water10 pN ≈ a single H-bond

Scale of energy100 J ≈ the energy release by a sleeping person per second1 J ≈ work required to pick an apple of the ground (1 metre)1 fJ ≈ energy required to bend lipid membrane1 aJ ≈ energy required to do cis - trans rotation (thermal energy)

10-18 atto- 10-15 femto- 10-12 pico- 10-9 nano- 10-6 micro-

Nanoscale leads to pico-, femto-, atto- effects

thermal energy (kT) = is maxm work available to a molecule

What are these forces?Where do they come from?

(it’s 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 -

Bulk properties

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

Consider a rubber band

stretch

Now consider boiling/melting point, reflectivity, solubility……

Elasticity

Viscosity

etc…..

Thermal fluctuations

Ordered layer

Cooling molecule down

Connect them with a straw

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

Two unequal water drops in zero gravity

For liquids

Curved Interfaces• Consider an air-bubble

in water• The bubble is stable

when no net air flows• Surface tension () will

act to decrease radius• Can be prevented by

raising the pressure such that PI>PO.

Water

Air

PI

PO

r

€

ΔP=2γr

[Laplace pressure]

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

Plot of vapour pressure vs radius

o

oo

o o o o

o

oo

o o o o

0

0.5

1

1.5

2

2.5

3

Rel

ativ

e V

apou

r P

ress

ure

Radius (nm)

o Dropleto Bubble

1 10 100 1000

GoretexGoretex 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 don’t water droplets penetrate the fabric?

[ DEMO ]

It can be the nanoscopic pores that make the nanomaterial NOT the material itself.

Micron scale

•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

For solids

Each atom on the surface has different properties (colour indicated) thus the surface is defective.

Reactivity“tipping point” 2Mg + O2 2MgO

Mg

MgO

ener

gy

Po

pula

tion

of a

tom

s w

ith a

giv

en

en

erg

y

Thermal energy

Heating or finely dividing

• depends on vapour pressure and a balance of surface energies• hydrophobic is >90°• roughness makes a huge difference•If the vapour doesn’t adsorb then surface is not wet

For gases

It’s curvature that matters

Contact angle is due tobalance of surface energies

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 ]

The origin of surface charge

• Surface SiOH are acidic

Si

O

– O

Si

SiSiO

OO

O O

H+

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

-M+–OH2 -M–OH -M–O– + H2OH+ OH–

AFM measurementA tool for seeing (feeling) the world from a

nanoparticles perspective

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

Solid

Diffuse Layer

Aqueous Solution

Bulk Electrolyte

Two charge colloids repell- Due to osmotic pressure

Most native surfaces are nagative

“tipping point”

attraction

repulsion

combination

van der Waals depends on material charging depends on solution/surface

“tipping point”

Force approx. range min/max forcefor colloidalsized objects

Attractive (negative force)van der Waals <15 nm < -1 nNHydrophobic <500 nm < -10 nN

Repulsive (positive force)Double layer repulsion <100 nm < +5 nNHydration <5 nm < +10 nNSteric <20 nm < +10 nN

Summary of forces

• Finely divided insulators become whiter

• Finely divided metals become black and then coloured

Scattering

Colour in metals comes from plasmon resonance, just ask Paul “Blue” Karason

Aussie sky blue European sky blue

Tyndall effect

Mary Kathleen uranium mine, near Cloncurry, Qld.

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.

Energy Band Representation of Insulators, Semiconductors and Metals

Insulator Semiconductor Metal

EmptyConduction band

Filledvalence band

Conduction band

Partially filledConduction band

valence band valence band

400 kT

40 kT

Bulk (3D)

Quantum Well (2D)

Quantum Dot (0D)

Quantum Wire (1D)

Energy

Energy

(E)

Energy

Energy

Density of States in semiconductors

Reduced Dimensionality leads to higher efficiency, lower threshold current, reduced power consumption and higher operating speed

4 GaAs QW with AlGaAs barriers

600 650 700 750 800 8500

5000

10000

15000

20000

25000

PL Intensity (a.u.)

Wavelength (nm)

1

2

3

4

S

Photoluminescence

1

2

3

4 S

S

Transmission Electron Micrograph

Courtesy of Prof. Jagadish, ANU

1.6 nm

2.2 nm

3.4 nm

6.8 nm

Colloidal CdSe quantum dots

It’s not so much the size that matters, it’s 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.

Summary