Transcript Simon Brown size matters v4

Size Matters
Why small is different
Simon Brown
MacDiarmid Institute and Department of Physics
University of Canterbury,
Christchurch, New Zealand
NZIP 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
Icosahedron
Small
(<2nm)
Truncated decahedron
Medium
(~3nm)
Cuboctahedron
Large
(>4 nm)
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
Sn
S. L. Lai et al., PRL 77, 99 (1996)
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
“Frozen” by immediate
exposure to air
30nm Bi clusters
Convers, Natali et al (to be published)
Allowed to evolve in
vacuum for 3 days
Coalescence
0.05
P=210-7 Torr
0.04
P=410-7 Torr
P=410-6 Torr
 G/G0
0.03
0.02
0.01
0
-0.01
0.5
1
1.5
t (s)
2
2.5
3
4
x 10
Increase in
conductance
Convers, Natali et al (to be published)
Rayleigh Instability
0.05
P=210-7 Torr
0.04
P=410-7 Torr
P=410-6 Torr
 G/G0
0.03
0.02
0.01
0
-0.01
0.5
1
1.5
t (s)
2
2.5
Decrease in
conductance
Lord Rayleigh, On the instabilities of jets, Proc. Lond. Math. Soc. 10, 4 (1878)
3
4
x 10
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
Elastic Sticking
Awasthi et al, PRL 97, 186103 (2006); PRB 76, 115437 (2007)
Nanoparticle Bouncing
Elastic Bouncing
Awasthi et al, PRL 97, 186103 (2006); PRB 76, 115437 (2007)
Nanoparticle Bouncing
Plastic Sticking
Awasthi et al, PRL 97, 186103 (2006); PRB 76, 115437 (2007)
Nanoparticle Bouncing
Plastic Bouncing
Awasthi et al, PRL 97, 186103 (2006); PRB 76, 115437 (2007)
Templated devices
• Bouncing of clusters off flat surfaces governs cluster assembly
30nm Sb clusters
Partridge et al, Nanotechnology 15, 1382 (2004)
No Lift-off lithography
30nm Bi clusters
Reichel et al, Appl. Phys. Lett, 89, 213105 (2006).
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
resistance [Ohm]
1.0E+10
1.0E+09
100ppm
1.0E+08
200ppm
1.0E+07
500ppm
1000ppm
1.0E+06
T=80ºC
5000ppm
1.0E+05
0
100
200
300
400
500
600
700
800
time [minutes]
• 6nm Sn clusters
• oxidised: 200ºC, 18hrs
• doped with 1nm Pd
Lassesson et al, Nanotechnology 19, 015502 (2008).
Response Mechanism
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•
Metal Oxides are commonly n-type semiconductors
Electrons carry the current
H
H
H
H
A
H
H
H
H
Response Mechanism
• A reducing gas reacts with surface
H
H
H
H
H
H
H
H
Response Mechanism
• Surface defects (donors) are created
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Response Mechanism
• Surface defects (donors) are created
• Additional electrons are released into the wire
• The current increases
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SnO2 Sensors: H2
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++ +++ + +++ + + + + + + + + +
+
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+ + + + + + + ++ + + +
Response [G/G0]
800
600
400
200
0
0
4
8
cluster coverage [ML]
Lassesson et al, Nanotechnology 19, 015502 (2008).
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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