Definition: The nanowires can be defined as a one-dimensional structure with a limit of 100 nanometers in the transverse direction (longitudinally unrestricted). Suspended nanowires indicate that the nanowires are fixed under vacuum conditions. Typical aspect ratios of nanowires are above 1000, so they are often referred to as one-dimensional materials.
Physical properties of Nanowires:
1. Mechanical properties
Normally, as the size decreases, the nanowires will exhibit better mechanical properties than large pieces of material. Strength becomes stronger, toughness becomes better.
2. Conductive properties
With the significant changes in mechanical properties, the electrical properties of nanowires are also significantly different from those of bulk materials. The conductivity of nanowires is expected to be much smaller than that of bulk materials. The reason is that when the cross-sectional dimension of the nanowires is smaller than the average free path of the bulk material, the scattering effect of the carriers on the boundary will be highlighted. The resistivity will receive a serious effect of the boundary effect. The surface atoms of the nanowires are not sufficiently bonded to the atoms in the bulk material, and these surface atoms that are not sufficiently bonded are often the source of defects in the nanowires, so that electrons can not pass smoothly The nanowires have lower conductivity than body material.
The conductivity of nanowires is expected to be much smaller than that of bulk materials. This is mainly caused by the following reasons. First, when the line width is less than the free electrons of free radicals, the scattering of carriers on the boundary will appear. For example, the average free path of copper is 40 nm. For copper nanowires with a width less than 40 nm, the mean free path will be shortened to line width.
At the same time, because of the scale of the reasons, the nanowires will also reflect other special properties. In carbon nanotubes, the movement of electrons follows the principle of ballistic transport (which means that electrons are free to travel from one electrode to another). In the nanowires, the resistivity is severely affected by the boundary effect. These boundary effects come from the atoms on the surface of the nanowire, which are not fully bonded as those of those atoms in the bulk material. These atoms that are not bonded are usually the source of defects in the nanowires, making the nanowires’ conductive capacity lower than the bulk material. With the decrease of nanowire size, the number of surface atoms increases relative to the number of atoms, so the boundary effect is more obvious.
Further, the conductivity will undergo energy quantization. The nanowires are connected between the electrodes, and scientists can study the conductivity of nanowires. By measuring the conductance of the nanowires at the time of stretching, the scientists found that when the length of the nanowires was shortened, its conductivity was also reduced in the form of ladder, with a Langjian constant G between each order.
Application of Nanowires:
In the electronics, optoelectronics and nanoelectromechanical devices, nanowires may play a very important role. It can also be used as an additive in composites, in a quantum instrument, a field emitter, and a biomolecule nanosensor.
1. Manufacture of electronic equipment
Some early experiments have shown that nanowires can be used in next generation computing devices. In order to make effective electronic elements, the first important step is to chemically method the nanowire doping. This has been implemented on nanowires to produce P-type and N-type semiconductors. The next step is to find the way to make the most simple electronic device for PN junctions. This can be done in two ways. The first is a physical method: put a P-line into an N-line. The second method is chemical: mix different impurities along a line. The next step is to build a logic gate. By simply connecting several PN sections together, the researchers have created all the basic logic circuits: the AND, or the NAND gate can already be crossed by the nanowires. The nanowire crossover may be important for the future of digital computing.
2. Solar energy conversion
The nanowires are able to naturally gather sunlight into a very small area of ??the crystal, which is 15 times the intensity of ordinary light. Since the diameter of the nanowire crystal is smaller than the wavelength of the incident sunlight, the resonance of the interior of the nanowire crystal and the surrounding light intensity can be caused. The study found that the photon emitted by resonance is more concentrated (solar energy conversion is in the process of disseminating the photon), which helps to improve the conversion efficiency of solar energy, making the nanowire-based solar cell technology has been really improved.
3. Promote chemical reactions
Researchers built the nanometer “tree” electrode into the water, and then use the simulated sunlight to illuminate and measure the output of the electricity. The results show that this vertical branch structure can not only capture a lot of solar energy, but also to maximize the increase in hydrogen production. Because in the plane structure, the bubbles must be large enough to float the surface, and the vertical structure can quickly extract very small hydrogen bubbles. The researchers said that this vertical branch structure can provide a chemical surface reaction area of ??400,000 times higher than the surface area. Researchers also have more ambitious goals, their eyes staring at the artificial photosynthesis. In natural photosynthesis, plants not only absorb sunlight, but also absorb carbon dioxide and water, resulting in carbohydrates for their own growth. Researchers hope to one day be able to imitate this process, the use of nano “forest” to absorb the atmosphere of carbon dioxide.
4. Microcell manufacturing
Scientists have made an important step in the manufacture of microcells, and they have developed a microcell with a vertically aligned nickel-tin nanowire, which is evenly wrapped around a multi-cell called PMMA Body material, which is commonly known as plexiglass. The main role of PMMA is insulation, when the current through, it can protect the inside of the nanowires from the reverse electrode. This battery is shorter than the average lithium battery charging time, other performance is also more excellent.
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