Module A-2: SYNTHESIS & ASSEMBLY

Size – Dependent Properties

Electronic Energy Band

Size-Dependent Properties

Nanoscale: High Ratio of Surface Area to VolumeNanoscale: High Ratio of Surface Area to Volume

Dimensions of Materials

• 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

Size – Dependent Properties

The melting point of gold particles decreases dramatically as the particle size gets below 5 nm

Source: Nanoscale Materials in Chemistry, Wiley, 2001

Melting Point

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 nano- particles 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 nano- crystals 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 poly-mer 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 attract-ed to the solvent. However, there are no sur- factants in our gold nanoparticle and polystyrene latex preparations.

R. P. Andres, Science (1996)

OSO- Na+

O

OSDS