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

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) Cuboctahedron Truncated decahedron Icosahedron Medium (~3nm)

Structure of small gold clusters 2D versus 3D structures Johansson et al, Phys. Rev. A 77, (2008)

Gold Gold nanoparticles look red!

Catalysis by Gold Nanoparticles Oxidation of CO: CO + O → CO 2 Goodman et al, Top. Catal. 14, 71 (2001).

Catalysis by Gold Nanoparticles Atomic arrangement on Au surface is critical CO + O → CO 2 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, (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, (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 Convers, Natali et al (to be published) Increase in conductance

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

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 SiO x

Molecular Dynamics Simulations – Nanoparticle Bouncing Awasthi et al, PRL 97, (2006)

Nanoparticle Bouncing Awasthi et al, PRL 97, (2006); PRB 76, (2007)

Nanoparticle Bouncing Awasthi et al, PRL 97, (2006); PRB 76, (2007) Elastic Sticking

Nanoparticle Bouncing Awasthi et al, PRL 97, (2006); PRB 76, (2007) Elastic Bouncing

Nanoparticle Bouncing Awasthi et al, PRL 97, (2006); PRB 76, (2007) Plastic Sticking

Nanoparticle Bouncing Awasthi et al, PRL 97, (2006); PRB 76, (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, (2006). 30nm Bi clusters

Metal Oxide Sensors: SnO 2 Metal Oxides are usually semiconductors Metal oxides can be used for many types of gas sensors Lassesson et al, Nanotechnology 19, (2008).

SnO 2 Sensors: H 2 6nm Sn clusters oxidised: 200ºC, 18hrs doped with 1nm Pd T=80ºC Lassesson et al, Nanotechnology 19, (2008).

Response Mechanism Metal Oxides are commonly n-type semiconductors Electrons carry the current A HHHHHHHH

Response Mechanism A reducing gas reacts with surface HHHHHHHH

Response Mechanism Surface defects (donors) are created

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

SnO 2 Sensors: H 2 Lassesson et al, Nanotechnology 19, (2008)

New nanoparticle products >800 products in market place already Source: Woodrow Wilson Centre, Project on Emerging Nanotechnologies 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