Hello, everyone. In this lecture, we are going to talk about the zero-dimensional and one-dimensional nanomaterials. The firstly, zero-dimensional nanomaterials are nanoparticles. The particles can be defined as small objects that behave as a whole unit with respected to its transport and properties. So particles can be classified into three categories according to their diameter. If the diameter of a particle is ranged from 2,500-10,000 nanometer, they are coarse particles. If the diameter of a particle is ranging from 100 nanometer to 2,500 nanometer, they are fine particles. If we reduce the diameter of particles be lower than 100 nanometer, they are nanoparticles. Nanoparticles are important bridging material between bulk material and atomic molecular structure. As you know that we can obtain the constant physical properties regardless of its size in broken material. However, the nanomaterial shows the sided dependent properties, due to the large surface area. Sometimes, we can obtain the unexpectedly high or low the physical and chemical properties in nanomaterials. Nanoparticles are small enough to confine their electrons and produce quantum effect. We already discuss on this phenomena in semiconducting nanocrystals such as cadmium selenide. The color of gold nanoparticles can be changed deep-red to black in solution. Gold nanoparticle can be melt at much lower temperature. So as you know that melting temperature of gold nanomaterial is about 1,064 degrees C. But we can significantly reduce the melting temperature of the gold in level about 300 degree C by reduction of gold nanoparticles size in level about 2.5 nanometer. Nanoparticles have improved the absorption of solar radiation. So by controlling the size and shape of nanoparticles, we can control the solar absorption. Titanium nanoparticles have enhanced the self-cleaning effect, and zinc oxide nanoparticles have improved UV blocking effect. High surface area to volume, which can be found in nanoparticles provide tremendous driving force for diffusion. So normally, the sintering temperature of nanoparticles is very lower than that of microscale particles. Then, one-dimensional nanomaterials are nanowires. The nanowires are nanostructures with the diameter of the order of one nanometer, the ratio of the length to width being greater than 20. The typical nanowires exhibit aspect ratios. Aspect ratio is a length to width ratio of more than 1,000. The structures with the diameter constrained to 10s of nanometers or less. We can find the three different types of nanowires according to the electronic transport properties. The first one is metallic nanowires, for example, nickel, platinum, gold nanowires. Second type is the semiconducting nanowires, including silicon, indium phosphide, gallium nitride nanowires. Last one is the insulating nanowires, for example, silica and titanium nanowires. Now, how about the property of nanowires? We can obtain the unexpected physical and chemical properties in nanowires, which is not seen in three-dimensional bulk materials. Electrons are quantum confined literally due to the quantum confinement effect. This means that the electrons can occupy different energy levels with those in bulk material. So we can obtain the enlarged density of state in things like one-dimensional nanomaterials. So the nanowires can be used as the electronic, opto-electronic, and nanoelectromechanical devices, and can be also used as an additive in advanced composites. So for example, the metallic interconnects in nanoscale quantum devices and leads for the biomolecular nanosensors. The last thing about the electrical conductivity of nanowires, the, normally, the electrical conductivity of nanowire is much less than that of the corresponding bulk material. The [inaudible] intensify the surface electrons scattering. So in order to change and control the electrical conductivity of nanowires, you should consider the value of free electron mean free path. For example, the copper nanowire has about 40 nanometer scale, the electron mean free path. So if we reduce the dimension diameter of nanowires lower than 40 nanometer, the electrical conductivity of copper should be deteriorated. For application, we need welding process. So for example, by applying electric current, we can fuse the wire end. We can find some special case in gold nanowire. The single-crystalline ultrathin gold nanowires with diameter about 3-10 nanometer, they can be cold-welded together within seconds by mechanical contact alone. Then these like nanowires can be applicable to active electronic elements. So let's think about the several steps to fabricates the electronic element. The first step is the to fabricate the chemically doped semiconducting nanowires. Then we should create the p-n junction by physically cross a p-type wire over an n-type wire, or chemically doped single wire with different dopants along length. Finally, we should build a logic gate by connecting several p-n junction together or semiconductor nanowire crossing. One important form of nanowires is carbon nanotube, CNTs. The CNTs are allotropes of carbon with cylindrical nanostructures. The length to diameter ratio in CNTs of up to 132,000,000 to 1. We can obtain the extraordinary high thermal conductivity, more than 3,000 watts per meter Kelvin in CNTs, and also obtain the high mechanical property, for example, the tensile strength of CNTs is about 63 gigapascal. So due to this high mechanical properties, if we use this, like CNTs as an adaptable materials for structural materials, we can obtain the significantly-enhanced mechanical properties in composite material. CNTs also have high electrical property. So for example, electrical conductivity of a CNTs is about 100,000 siemens per centimeter. This is the very higher value compared with the bulk graphite material. Then we can find the some special CNTs. The first one is the single-walled nanotube, the SWNT. The diameter of close to one nanometer and tube length is millions of times longer than the diameter. This is wrapping one atom thick layer of graphite. It is graphene into a cylinder. So band gap can be tuned from the 0-2 electron volt. This means that we can control the electrical conductivity of a singular entity from metallic to semiconductors. Another important form of CNT is the multi-walled CNT, MWNT, the multiple rolled layers of graphene. So one important material is double-walled carbon nanotubes, DWNT. So this material is a special class of nanotubes, and morphology and properties are similar to those of SWNT. The main advantages of these material is good resistance to chemicals compared with single-walled CNTs. Then let's think about how can we fabricate the nanomaterials. So if the material has spontaneous anisotropic crystal structure, we can easily fabricate the nanowires. We can also use the seed-initiated growth, the templating, and kinetic control of organic surfactant, and size reduction, and electrospinning the preparation of one-dimensional nanowires. Sometimes, we can use the self-assembly of nanoparticles. By using the transforming via chemical reaction, we can also fabricate the heterostructured nanowires. In this lecture, we are talking about the zero-dimensional and one-dimensional nanowires. In the next lecture, we are talking about the two-dimensional nanomaterials. Thank you.
