lecture 6: individual nanoparticles, nanocrystals and ...€¦ · nanoscience ii spring 2009 3...

Nanoscience II spring 20091

Lecture 6: Individual nanoparticles, nanocrystals and quantum dots

Definition of nanoparticle:

Size definition arbitrary

More interesting: definition based on changein physical properties.

Size smaller than some critical length thatcharacterize physical phenomena

Examples: Electron mean free path, thermaldiffusion length


Metal nanoparticles

Structure:

Common metals have close-packed crystal structure: fcc or hcp (somebcc)

Smallest fcc ”crystal”: one unit cell, 13 atoms


Another atom layer outside: 13 + 42 = 55 atoms

More layers: 13, 55, 147, 309, 561 atoms

Structural magic numbers

N =1310n3 15n2 +11n 3( )

n = number of layers

Surface atoms:

Nsurf =10n2 20n+12


Geometric structure, general comments:

Nanoparticle structure generally similar to bulk

Small particles (< 5 nm) may have different structures

Example Al13: theory predicts icosahedral structure

Also size-dependent deviations from the ideal structure

Indium nanoparticles, tetragonal c/aratio

TheoreticalAl13 clusters


Electronic magic numbers

Simple model: Clusters as”superatoms”, with electronicshell structure

Jellium model: ”free” valenceelectrons in a homogeneouspositive background

Potential:

U r( ) =U0

exp r r0( ) +1[ ]

r0 : cluster effective radiusU0 : potential constant : steepness parameter

Electronic magic numbers: 2, 18, 20, 40 …. corresponding to filled shells

Compare to single atoms: 2, 10, 18, 36 (He, Ne, Ar, Kr)


Electronic structure, general

• Solve the Schrödinger equation for an exact atomicmodel, using realistic potentials

• Density-functional theory and molecular dynamicsmost commonly used

• Find minimum-energy structure (depends onelectronic energy levels)

• Typical solution has discrete energy levels,depending on cluster size (compare ”particle in a box”)- big difference to bulk metal.

• Quantum size effects when cluster size is comparableto Fermi level wavelength. Much larger size forsemiconductors (μm compared to nm)


Experimental studies of nanoparticle energy levels

• Optical spectroscopy (absorption, fluorescence)

• UV photoemission

• STM/STS

Example: combined STM/STS and photoemission studyof gold clusters


Photoemission:

Gold nanoparticles gold surface

Gold nanoparticles on HOPG

STMSTS, current images


Reactivity - catalysis

Depends on size and structure

• most (all) atoms on the surface

• closed shell most stable (compare noble gasatoms)

• strong variations with size

Production of nanoparticles - largecommercial activities

Example: Nanostellar Inc., a Silicon Valleystartup company(www.nanostellar.com)


Fluctuations

Small nanoparticles, many surface atoms with lessmovement restrictions structure changes (fluctuations)


Semiconducting nanoparticles

Strong variation in optical properties compared to bulk - blue shift

Nanoparticles:

• weak confinement: particle radius larger thanexciton radius blue shift

• strong confinement: particle radius smaller thanexciton radius no exciton, electron and holeindependent. blue shift + new set of energylevels

En = 13.6m* m0

r( )2n2

eV

Bulk: excitons are important for the opticalproperties

4.19 Optical absorption in Cu2O


Light absorption in colloidal solution

Figure 7.2. Solutions of quantum dots of varying

size. Note the variation in color of each solution

illustrating the particle size dependence of the

optical absorption for each sample. Note that the

smaller particles are in the red solution (absorbs

blue), and that the larger ones are in the blue

(absorbs red).


4.20 Optical absorption spectrum of CdSe nanoparticles withsizes 20 and 40 Å


Karlstad: ZnO nanocrystals on surfaces

SEM images: STM images:

a) 250 x 250 nmb) 50 x 50 nmc) 28 x 28 nm


Other nanoparticles

• Inert-gas clusters - weak van der Waals forces

• Magnetic nanoparticles… very hot research area (chapter 7)

• Superfluid clusters - Bose-Einstein condensates

• Molecular clusters - example: water, hydrogen-bondedclusters


Synthesis of nanoparticles

Laser evaporation methods:


Aerosol techniques, Lund Univ.Metal nanoparticles

Semiconductor nanoparticles


Differential mobility analyzer - analysis and size selection


Chemical methods - colloidal growth- nanoparticles grown in solution with surfactant layer

- most promising for volume production, good scalability (ExampleNanostellar)

- monodispersive growth possible (similar size of particles)

- control over composition, size, shape, structure, surface properties

Figure 7.3. La Mer model for the growth

stages of nanocrystals.

Kinetic size control


Colloidal growth

Kinetic size control:

Monodisperse colloidalnanocrystals synthesized underkinetic size control. a,Transmission electron microscopy(TEM) image of CdSenanocrystals. b, TEM image ofcobalt nanocrystals. c, TEMmicrograph of an AB13superlattice of _-Fe2O3 and PbSenanocrystals. The precise controlon the size distributions of bothnanocrystals allows theirselfassembly into ordered three-dimensional superlattices. Scalebars, 50 nm.


Colloidal growth - shape control

Shape control of colloidal nanocrystals. a, Kinetic shape control at high growth rate. The high-energy facets grow morequickly than low energy facets in a kinetic regime. b, Kinetic shape control through selective adhesion. The introduction ofan organic molecule that selectively adheres to a particular crystal facet can be used to slow the growth of that side relativeto others, leading to the formation of rod- or disk-shaped nanocrystals. c, More intricate shapes result from sequentialelimination of a high-energy facet. The persistent growth of an intermediate-energy facet eventually eliminates the initialhigh-energy facet, forming complex structures such as an arrow- or zigzag-shaped nanocrystals. d, Controlled branchingof nanocrystals. The existence of two or more crystal structures in different domains of the same crystal, coupled with themanipulation of surface energy at the nanoscale, can be exploited to produce branched inorganic nanostructures such astetrapods. Inorganic dendrimers can be further prepared by creating subsequent branch points at the defined locations onthe existing nanostructures. The red and green dots in a and b represent metal coordinating groups with different affinitiesto nanocrystal facets.


Other synthesis techniques

• Radiofrequency plasma methods

• Epitaxial growth - self-assembled quantum dots (lecture 3)

• Ion implantation

• Thermolysis - high-temperature decomposition

• Mechanical milling

• Cavitation, sonochemistry, detonation

After treatments

• Passivation of cluster surfaces

• Powder consolidation

• Nanoparticle coatings

lecture 6: individual nanoparticles, nanocrystals and ...€¦ · nanoscience ii spring 2009 3...

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