When each atom counts: the world of nanoclusters

Lycurgus Cup- 4th century.

The ruby color is due to gold nanoparticles of different sizes embedded in a silica glass matrix.

The ancient properties of nanoclusters.

What is a nanocluster?

Atom BulkNanocluster

0.5÷5 nm

Ns/N (%)100

50

10

20

Number of atoms

Diameter (nm)

100

101

102

103

104

105

0.1

1

10

Molecule

Cluster

Nanocrystal

Bulk

Nanoscale Microscale

0.1 1 10 100 1000 10000nm

atoms

quantum dots

molecules

GMR layers transistors

field emitters

SETs

cellsVirus

enzymes bacteria

physics@UniTS

The structure at the nanoscale: magic clusters

C60 BuckminsterfullerenePlanar ring

Au6

Pentagonal bipyramid

Si7

Fivefold icosahedron

Al13Cage-like structure

CdSe34

NANOCLUSTERS on solid surfaces

Bimetallic Core-shell nanoclusters

Self-assembly of highly ordered nanoclusters

monolayer

6 indium atoms on Silicon

Self-organized 3D superlattice of

clusters100 nm

C60 molecular crystals

physics@UniTS

The electronic structure at the nanoscale: magic clusters

SuperATOMS

1 2 3 4 Very large

Number of interacting metal atoms

Ener

gy

Small metal

clusters

Large metal

clusters

Bulk metal

P. Cheshnovsky, PRL 64, 1234 (1990) H. Handschuh, JCP 102, 6406 (1995)

Super - rare gas atom

Super- alkali atom

alkali-like electron

MOLECULE

LUMO

HOMO

CB

VB

Eg

Energy

NANOPARTICLE

Eg Eg

BULKSEMICONDUCTOR

Energy Level Diagram: Quantum Size Effects

Among the experimental techniques …

20 40 60 80

Au4f7/2

High-Resolution Core Level Spectroscopy

X-ray Photoelectron Diffraction

• Quantum size effects:

Noble metals, Semiconductors, Oxides.

• Engineer Eg over a wide spectral range:

IR to UV.

• Semiconductor Q Dots:

II-VI: CdS, CdTe, CdSe, PbS, ZnSe

PbS: Eg:0.41 eV 2.34 eV.

(300K, 15 nm) (300K, 1.3 nm)

Eg of PbS nanoparticle vs particle size

Nanoparticles: Quantum Size Effects

Wang et al. J. Chem. Phys. 87, 12 (1987).

CdSe quantum dots

• Semiconducting CdSe nanodots:

Illumination with a single light source

Emission shifts to higher energy

with decreasing particle size.

• Metallic Au nanodots:

Fluorescence shifts to longer

(lower energy) with increasing

nanocluster size.

J. Zheng et al, Phys. Rev. Lett. 93, 077402 (2004).

J.L. West and N. Halas, Ann. Rev. BioMed. Eng. 5, 285 (2003).

Nanoparticles: Quantum Size Effects

Au Nanoclusters

0.5÷5 nmMethods of Synthesis

• RF Plasma• Chemical Methods• Thermolysis• Pulsed Laser

The experimental set-up for size-selected clusters production

Cluster source

Lens system

Quadrupoledeflector

Quadrupole mass spectrometer

Lens system for focussing and soft-landing

Analysis chamber

Nd:Yag Laser

Rotary MotorHe pulse valve

Thermalization chamber

Laser beam

Expansion nozzle

Target

0.5÷5 nmRF Plasma

Illustration of apparatus for the synthesis of nanoparticles using an RF-produced plasma

0.5÷5 nmThermolysis

Apparatus used to make metal nanoparticles by thermally decomposing solids consisting of metal cations and molecular anions, or metal organic solids.

0.5÷5 nmPulsed Laser Method

Apparatus to make silver nanoparticles using a pulsed laser beam that creates hot spots on the surface of a rotating disk.

Magnetic Nanoclusters

Nanocluster Quick Introduction

• From a few atoms to several thousand atoms• High fraction of atoms on the surface• Different elements form different bonds and

different nanocluster structures

A Few Types of NanoclustersVan der Waals Nanoclusters

Figure above from: Alonso, J. A., Structure and Properties of Atomic Nanoclusters, 2005

• Binding energy: < 0.3 eV / atom

• Balance between induced dipole force and quantum closed shell interaction

• Noble gases form icosahedral Van der Waals clusters

A Few Types of NanoclustersVan der Waals Nanoclusters

Figure above from: Echt, O., et al., J. Chem. Soc. Faraday Trans., 86 (1990) 2411

• The drops at 148 and 309 atoms correspond to completed icosahedra.

A Few Types of NanoclustersIonic Nanoclusters

NaCl Cluster

Graph above from: Martin, T. P., Physics Reports, 273 (1996) 199

• Bond Strength: 2-4 eV / atom• Tend to form boxes

A Few Types of Nanoclusters

• Metal clusters have complicated bonding that varies from metal to metal

• Due to this variation the bond strength varies from around 0.5 eV to 3 eV per atom

Metal Nanoclusters

Laser Vaporization

Figure above from: Billas et al., J. Magn. Magn. Mater. 168 (1997) 64

Metal Nanoclusters Produced By Laser Vaporization

Stern-Gerlach Apparatus

Figure above from: Billas et al., J. Magn. Magn. Mater. 168 (1997) 64

Description of magnetic particles

0.5÷5 nm

Stern-Gerlach experiment is used to measure the magnetic moments of nanoparticles. A beam of metal clusters from a source is sent between the poles of permanent magnets shaped to produce a uniform gradient DC magnetic field, which produces a net force on the magnetic dipole moments of the clusters. There by deflecting the beam. The magnetic moment can be determined by the extent of the deflection, which is measured on a photographic plate or fluorescent screen.

A series of electron microscope pictures of gold Stern-Gerlach Experiment les containing approximately 460 atoms taken at various times showing fluctuation-induced changes in the structure.

Band Structure Evolution

Figure above from: Billas et al., J. Magn. Magn. Mater. 168 (1997) 64

Increasing Coordination Number

Magnetic Moment vs. Cluster Size

Figure above from: Billas et al., J. Magn. Magn. Mater. 168 (1997) 64

Magnetic Moment vs. Temperature

Fe

Ni

Co

Graphs from: Billas, M. L., A. Chatelain, and W. A. de Heer, Science 265 (1994) 1682

Superparamagnetism

Magnetization Loops of Fe Nanoclusters

Graph from: Jackson, T. J., et al., J. Phys.: Condens. Matter, 12 (2000) 1399

Nanoclusters applications: properties and applications

OPTICALAnti-reflection coatings.Tailored refractive index of surfaces.

THERMALEnhance heat transfer from solar collectors to storage tanks.Improve efficiency of coolants in transformers .

ELECTRONICHigh performance and smaller components, e,g, capacitors for small consumer devices such as mobile phones.Displays that are cheaper, larger, brighter and more efficient.High conductivity materials.

MAGNETICIncreased density storage media.Nanomagnetic particles to create improved detail and contrast in MRI images.

BIOMEDICALAntibacterial silver coatings on wound dressings.Sensors for disease detection (quantum dots).Programmed release drug delivery systems.

MECHANICALImproved wear resistance.New anti-corrosion properties.New structural materials, composites, stronger and lighter.

ENERGYHigh energy density and more durable batteries.Hydrogen storage applications using metal nanoclusters.Electrocatalysts for high efficiency fuel cells.Renewable energy, ultra high performance solar cells.Catalysts for combustion engines to improve efficiency.

Nanoclusters: inspiration and opportunity for science,not just for artists !

and …

Summary• Metal nanoclusters of an element behave differently than

bulk matter of the same element.• d-orbital overlap reduces magnetic moment per atom.• Metal nanoclusters exhibit magnetic shell phenomenon• Metal nanoclusters do not lose their magnetization as quickly

above the Curie temp.• Metal nanoclusters exhibit superparamagnetic behavior.• Superparamagnetism provides a theoretical minimum size per

bit in magnetic moment based memory systems.