l 6 nanoclusters
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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.