Nanowires

Nanowires are structures with diameters of the nanometer order and very high length to diameter ratios. They can consist of a variety of materials or combinations of materials and are usually up to several microns in length. Nanowires are characterized by properties that are affected by quantum limits imposed on the travel of electrons, where the limits exist because of the linear constraint along the length of the nanowire. Smaller diameter nanowires exhibit more quantum effects than higher diameter nanowires. Nanowires can be metallic, semiconducting, or insulating, depending on their material, and they can be doped during growth to create contrasting properties along their lengths.  The atomic structures of nanowires can range from crystalline to polycrystalline to nearly amorphous. The chemistry of nanowires can also be modified after growth through processing and functionalization. Nanowires are generally compatible with post-processing techniques, including harsh chemical treatments and plasma treatments.

Thermal chemical vapor deposition (thermal CVD), low pressure CVD (LPCVD), laser CVD (LCVD), plasma enhanced CVD (PECVD) and metalorganic CVD (MOCVD) have been used to synthesize several types of nanowires via the vapor-liquid-solid (VLS) mechanism. Thermal CVD is a simple method for producing several high quality nanowires from materials including gallium nitride (GaN), cadmium oxide (CdO), and germanium (Ge).  In one example of thermal CVD, a solid source is placed upstream of a bulk catalyst or substrate. Through heating, the solid source forms a vapor, which is transported by a carrier gas to feed a catalyst which attracts the source molecules and initiates nanowire growth.  Low pressure CVD allows for environments with selectable pressures below atmospheric pressure and can greatly reduce gas and material consumption compared to processing via thermal CVD.  A large selection of metal organics can be used as precursors to nanowire growth with MOCVD, expanding the selection of synthesized nanowires. Load-locks and precise gas control are used to affect the nanowire purity.  The CVD setup can be modified in many ways to enhance nanowire growth by a variety of methods. In LCVD, a laser is used to vaporize precursor targets in well-controlled doses, allowing for controlled wire lengths. Such precision allows for the formation of superlattices with alternating structures or dopant levels, while also achieving reduced target material consumption.

Nanowires are fascinating materials and have enormous potential for research and experimentation to understand and develop useful applications. Their unique properties set the stage for the replacement of many conventional materials in a variety of fields with large advances in performance. Electrical applications include transistors, chemical sensors, biological sensors, integrated circuits, quantum wires, and logic devices.  Mechanical applications include composites, advanced coatings, and scanning probe microscopy tips.  Nanoelectromechanical systems (NEMS), energy storage technologies, medical devices, and optical devices, such as photonics, LEDs, and lasers, are also possible with nanowires.