7Size – 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
8Melting PointThe melting point of gold particles decreases dramatically as the particle size gets below 5 nmSource: Nanoscale Materials in Chemistry, Wiley, 2001
19Building 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
20The Building Blocks (BBs) Metal nanoparticles and nanowiresNanotubesSemiconductor nanospheres, rods, wires, etc.Carbon nanotubesOrganic BBs - DNA, proteins, etc.Cells, viruses, etc.
21Nanoparticle 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
31Building 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
32Strategies for Nanostructure Fabrication Bottom-up approach for nanostructures using nano- particles as building blocksExample: Opals: The fascinating interference colors stems from Bragg diffraction of light by the regular lattice of silica particles nm in diameter.
33Attractive Features of Self-Assembly Self-assembly proceeds spontaneouslyThe self-assembled structure is close to thermodynamic equilibriumSelf-assembly tends to have less defects, with self-healing capability
34Why 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 chemicalproperties, as interactions between proximal nano crystals give rise to new collective phenomena.
35Stabilization 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. AndresScience (1996)
36Gold 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)