Nanowires are one of the most promising candidates for nanoscale electronics and optoelectronics. They are defined as cylindrical wires with a diameter of less than 100 nm and a length-to-diameter ratio greater than 1000. Due to their very small dimensions, nanowires offer a number of benefits over conventional microscale devices, including increased surface area-to-volume ratios, quantum confinement effects, and ballistic charge transport. In addition, due to their small size and high aspect ratio, nanowires can be easily integrated into existing microelectronic devices.
The first reported synthesis of nanowires was by Wang et al. in 2001, who used the vapor-liquid-solid (VLS) method to grow zinc oxide (ZnO) nanowires on a silicon substrate. The VLS method involves the deposition of a metal onto a substrate, followed by heat treatment to vaporize the metal and form a liquid phase. The metal then diffuses into the substrate where it nucleates and grows crystals. This method has since been used to synthesize a variety of other materials including metals (Au, Ag), semiconductors (SiC), and insulators (Al2O3).
One major challenge in the field of nanowire research is controlling the composition and structure of the wires at the atomic level. This is necessary in order to tailor the properties of thenanowires for specific applications. For example, in order for nanowires to be used in electronic devices they must be made from materials that are electrically conducting or semi-conducting. Additionally, it is often desirable to dope semiconductor nanowires with impurities in order to modify their electrical properties. Doping can be achieved either during synthesis or post synthesis by diffusion from an external source such as an ion implantation process.
Another way researchers have attempted to control composition is by using templates during growth such that only wires with desired compositions will nucleate and grow inside the pores while others are excluded due to size restrictions . An example of this would be growing ZnO wires inside alumina membranes with nanopores that have diameters just slightly larger than that of the wire being grown.. By carefully choosing template material one can also control other aspects about wire growth such as directing lateral overgrowth along certain crystallographic planes which might improve mechanical strength .
Templating can also be combined with VLS methods resulting in what’s called “ seeded growth ” where instead of randomly deposited atoms on a surface you start with molecules or even preformed crystals which act as sites for further crystal growth . This technique allows much more precise placement of individual wires as well as providing some degree of control over initial structure/composition . It should be noted that not all templating techniques require an additional seeding step as sometimes VLS will produce homogeneous surfaces without any need for extra help . A common problem when working with templates though is pore collapse which happens when smaller pores close up due partially to capillary forces but also because many templates aren’t perfectly symmetric so there’s an imbalance in these forces . If pore collapse isn’t taken into account it could lead to disastrous results like shorts between adjacent wires !