Size Matters

Why small is different

Simon Brown

MacDiarmid Institute and Department of PhysicsUniversity of Canterbury, Christchurch, New ZealandNZIP Conference, Christchurch, July 2009

Silicon

Silicon

• Diamond Structure• Lowest energy configuration

The surface of Silicon (111)

• Model• But what happens to the dangling bonds?

The best way of imaging surfaces

• Scanning Tunneling Microscope (STM)• UHV STM / AFM installed at UC, Jan 2009

The surface of Silicon (111)

• Model• But what happens to the dangling bonds?

The surface of Silicon (111)

• Image from Scanning Tunnelling Microscope (STM)• “Reconstruction” minimises energy

The surface of Silicon (001)

• Image from Scanning Tunnelling Microscope (STM)

The surface of Silicon (001)

• Image from Scanning Tunnelling Microscope (STM)

Gold

Gold – a close packed structure

• Face-centred cubic

Surface of Gold

Paweł Kowalczyk (UC)

Surface of Gold

Paweł Kowalczyk (UC)

Surface of Gold (111)

• “Herringbone” reconstruction

Nanoparticles

• Mostly surface!• Here: 42 / 55 atoms are on surface

Size matters

• In small metal particles (e.g. Au)• Five-fold symmetry is forbidden in large crystals

• not space-filling

Small(<2nm)

Large(>4 nm)

CuboctahedronTruncated decahedronIcosahedron

Medium(~3nm)

Structure of small gold clusters

• 2D versus 3D structures

Johansson et al, Phys. Rev. A 77, 053202 (2008)

Gold

• Gold nanoparticles look red!

Catalysis by Gold Nanoparticles

• Oxidation of CO: CO + O → CO2

Goodman et al, Top. Catal. 14, 71 (2001).

Catalysis by Gold Nanoparticles

• Atomic arrangement on Au surface is critical• CO + O → CO2

Goodman et al, Top. Catal. 14, 71 (2001).

Melting point changes

• Dramatic decrease at small sizes

S. L. Lai et al., PRL 77, 99 (1996)

Sn

Surface melting

Shaun Hendy, IRL

Its not all about the surface

• Quantum Effects

Its not all about the surface

• New materials, new properties• Carbon nanotubes are

• Strongest material known• Highest conductivity known

Some “new” phenomena for metal nanoparticles

• Coalescence• Bouncing

• Sometimes nanoparticles act more like liquids than solids

How to make nanoparticles (“clusters”)

Cluster source: highly flexible e.g. Si for transistors, Cu for interconnects, Pd for hydrogen sensors Proof of concept with Sb, Bi – interesting electronic properties Change cluster size through temperature, gas type and pressure Change cluster velocity through gas flow rate

Simple Nanodevices Made from Nanoparticles

Schmelzer et al, Phys. Rev. Lett. 88, 226802 (2002)

Large metal particles do not coalesce

• (Obviously!)

But liquid drops do…

Spreading of droplets of silicone oil on a highly wet-able substrate

Ristenpart et al, PRL 97, 064501 (2006)

Metal nanoparticles coalesce

Convers, Natali et al (to be published)

“Frozen” by immediate exposure to air

Allowed to evolve in vacuum for 3 days

30nm Bi clusters

Coalescence

0.5 1 1.5 2 2.5 3

x 104

-0.01

0

0.01

0.02

0.03

0.04

0.05

t (s)

G

/G0

P=210-7 Torr

P=410-7 Torr

P=410-6 Torr

Convers, Natali et al (to be published)

Increase inconductance

Rayleigh Instability

0.5 1 1.5 2 2.5 3

x 104

-0.01

0

0.01

0.02

0.03

0.04

0.05

t (s)

G

/G0

P=210-7 Torr

P=410-7 Torr

P=410-6 Torr

Lord Rayleigh, On the instabilities of jets, Proc. Lond. Math. Soc. 10, 4 (1878)

Decrease inconductance

Large balls bounce

Liquid droplets also bounce….

Jayaratne and Mason, Proc. Roy. Soc. London. Ser. A, 280, 545 (1964)

…. but they also wet surfaces

Clusters partially wet surfaces

• Bismuth on SiOx

Molecular Dynamics Simulations – Nanoparticle Bouncing

Awasthi et al, PRL 97, 186103 (2006)

Nanoparticle Bouncing

Awasthi et al, PRL 97, 186103 (2006); PRB 76, 115437 (2007)

Nanoparticle Bouncing

Awasthi et al, PRL 97, 186103 (2006); PRB 76, 115437 (2007)

Elastic Sticking

Nanoparticle Bouncing

Awasthi et al, PRL 97, 186103 (2006); PRB 76, 115437 (2007)

Elastic Bouncing

Nanoparticle Bouncing

Awasthi et al, PRL 97, 186103 (2006); PRB 76, 115437 (2007)

Plastic Sticking

Nanoparticle Bouncing

Awasthi et al, PRL 97, 186103 (2006); PRB 76, 115437 (2007)

Plastic Bouncing

Templated devices

30nm Sb clusters

Partridge et al, Nanotechnology 15, 1382 (2004)

• Bouncing of clusters off flat surfaces governs cluster assembly

No Lift-off lithography

Reichel et al, Appl. Phys. Lett, 89, 213105 (2006).

30nm Bi clusters

Metal Oxide Sensors: SnO2

• Metal Oxides are usually semiconductors

• Metal oxides can be used for many types of gas sensors

Lassesson et al, Nanotechnology 19, 015502 (2008).

SnO2 Sensors: H2

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

1.0E+10

0 100 200 300 400 500 600 700 800

time [minutes]

resi

stan

ce [

Oh

m]

5000ppm

1000ppm

500ppm

200ppm

100ppm

• 6nm Sn clusters• oxidised: 200ºC, 18hrs• doped with 1nm Pd

T=80ºC

Lassesson et al, Nanotechnology 19, 015502 (2008).

Response Mechanism

• Metal Oxides are commonly n-type semiconductors • Electrons carry the current

A

H HH H

H H H H

Response Mechanism

• A reducing gas reacts with surface

H H H HH H H H

Response Mechanism

• Surface defects (donors) are created

++ + + + + + + + + + + + + + + + +

Response Mechanism

• Surface defects (donors) are created• Additional electrons are released into the wire • The current increases

++ + + + + + + + + + + + + + + + +

SnO2 Sensors: H2

Lassesson et al, Nanotechnology 19, 015502 (2008).

0

200

400

600

800

0 4 8 12

cluster coverage [ML]

Res

po

nse

[G

/G0]

++ ++++ ++++ ++ ++

++ +++ +++ ++++++++

++ +

New nanoparticle products• >800 products in market place already

• Source: Woodrow Wilson Centre, Project on Emerging Nanotechnologies• http://www.nanotechproject.org/inventories/consumer/

• Mostly “low tech”• Sunscreens, cosmetics, nappies, washing machines, fuel additives• FOE report: 100 nanoproducts in Food and packaging

• We are unaware of most of them

New hazards

• Long carbon nanotubes work like asbestos in the lungs

• Silver nanoparticles are toxic

• Nanoparticles can cause DNA damage

• Sunscreens cause photo-catalytic damage to colour-steel roofing*

• Very many unknowns

* Barker and Branch, Progress in Organic Coatings 62, 313 (2008)

New Uncertainties

• All new technologies have risks• In this case we don’t know what they are• Risk Assessment protocols yet to be developed

• Problem: Incredible number of unknowns• Do nanoparticles penetrate the skin, lungs? • What do they do inside the body?

• Huge number of challenges• Example: Regulation

• Same materials, different sizes• 50,000 types of carbon nanotube

New Uncertainties

• All new technologies have risks• In this case we don’t know what they are• Risk Assessment protocols yet to be developed

• Problem: Incredible number of unknowns• Do nanoparticles penetrate the skin, lungs? • What do they do inside the body?

• Huge number of challenges• Example: Regulation

• Same materials, different sizes• 50,000 types of carbon nanotube

Size really does matter• Nanoparticles and nanowires are mainly surface

• Properties are very different to bulk materials• New Science

• Surface reconstructions• New “crystal” structures• Catalysis

• New Technology• Sensors• Catalysts• Transistors

• New Hazards• Penetration of skin and lungs• Carcinogens• Business risks