Module A-2: SYNTHESIS & ASSEMBLY
Size – Dependent Properties
Electronic Energy Band
Size-Dependent Properties
Nanoscale: High Ratio of Surface Area to Volume
Dimensions of Materials
Size – Dependent Properties
• Nanoscale sizes can lead to different physical and
chemical properties
- Optical properties
- Bandgap
- Melting point
- Surface reactivity
• Even when such nanoparticles are consolidated into
macroscale solids, new properties of bulk materials
are possible.
- Example: enhanced plasticity
Melting Point
The melting point of gold particles decreases dramatically as
the particle size gets below 5 nm
Source: Nanoscale Materials in Chemistry, Wiley, 2001
Top-Down and Bottom-Up Processes
Nanoparticle Synthesis
Nanoparticle Properties
Monodisperse Nanoparticles
Monodisperse Silver Nanoparticles
1D Monodisperse Nanorods
3D Nanotetrapods
Nanotetrapods
Solution Phase Gold Nanoparticle Synthesis
Building Blocks (BBs)
Building blocks of nanostructured materials
• Synthesis of nanoscale materials can be divided
into wet and dry methods.
– By dry methods the material is made in solid form from
vapor phase precursors and used directly in the form it
was made.
– By wet methods materials are made by chemical
reactions in solution or on a solid support, and
separation of the desired material from unwanted solid
or liquid materials is necessary
The Building Blocks (BBs)
• Metal nanoparticles and nanowires
• Nanotubes
• Semiconductor nanospheres, rods,
wires, etc.
• Carbon nanotubes
• Organic BBs - DNA, proteins, etc.
• Cells, viruses, etc.
Nanoparticle Synthesis
• Colloidal metal and colloidal semiconductor particles are made from
solutions of precursor chemical compounds by chemical reactions
that produce the insoluble metal or semiconductor particles.
– For gold nanoparticles the reaction is reduction of gold ions by
citrate ions in aqueous solution.
Au3+ + citrate ---> Au0 + oxidized citrate
Synthesis of Nanoparticles in Laboratory
Solution Phase Quantum-Dot Synthesis
Monodisperse QD Synthesis
Hot Solvent-Injection Synthesis
Low-Resolution Monodisperse QDs
High-Resolution Monodisperse QDs
High-Resolution Monodisperse QDs
Optical Properties
Optical Properties
Building Blocks (BBs) and Self Assembly
Many factors must be considered when we approach
the bottom-up nanomanufacturing by self assembly –
including BBs, forces on BBs, and functional
nanotechnological applications.
Forces on BBs
Strategies for Nanostructure Fabrication
• Bottom-up approach for nanostructures using nanoparticles as building blocks
– Example: Opals: The fascinating interference colors
stems from Bragg diffraction of light by the regular
lattice of silica particles 100-500 nm in diameter.
Attractive Features of Self-Assembly
• Self-assembly proceeds spontaneously
• The self-assembled structure is close to
thermodynamic equilibrium
• Self-assembly tends to have less defects,
with self-healing capability
Why Should We Deal With Self Assembly?
• Like atoms or molecules, nanocrystals can be treated
as artificial atoms and used as the building blocks of
condensed matter.
• Assembling nanocrystals into solids opens up the
possibilities of fabricating new solid-state materials and
devices with novel or enhanced physical and chemical
properties, as interactions between proximal nanocrystals give rise to new collective phenomena.
Stabilization Of Colloids
•
Fundamental problem: The thermodynamically stable state of metals,
semiconductors, and polymers is bulk material, not colloidal particles.
Stable colloidal dispersions require an interfacial stabilizer, which is a
chemical that reduces the interfacial free energy between the particle
and the solvent and makes short range forces between the particles
repulsive.
R. P. Andres
Science (1996)
Gold Colloidal Nanoparticles
•
•
In the case of our gold nanoparticles, the
stabilizer is citrate ion, whose negative
charge is opposite to that of positive gold
ions on the particle surface. The excess
negative charge due to adsorption of citrate
on the surface of the particles makes the
particles repel one another. Our polystyrene
latex also is charge stabilized. Dissociation
of a fraction of the sodium ions of the
sodium 4-styrenesulfonate units of the polymer leaves the particles with a negative
charge.
The stabilizer often is a surfactant, which is
a chemical compound such as sodium
dodecyl sulfate (SDS) whose structure has
one end that is chemically attracted to the
particle and the other end chemically attracted to the solvent. However, there are no surfactants in our gold nanoparticle and
polystyrene latex preparations.
O
OSO- Na+
SDS
O
R. P. Andres, Science (1996)