Nanowire Semiconductors

A nanowire is a wire with a diameter of the magnitude of a nanometer. They can be defined as structures having a lateral size restricted to ten or less nanometers and a free length. At these scales, the effects of quantum mechanics are important so they are also called “quantum wires”. There are many different types of nanowires, including metal wires, semiconductors and insulators. Molecular nanowires are composed of repetitive molecular units whether organic or inorganic.

Nanotubes for optoelectronics

The relationship between length and width is almost infinite, even several microns, so we can describe them as one-dimensional materials. The nanowires have many interesting properties that have not been seen in three-dimensional materials, unaffected by edge effect. With this filiform morphology, if the material that forms them is semiconductor we will get a confinement of load that circulates mainly in the longitudinal direction that will act as an electric wire.

The peculiar characteristics of this quantum confinement exhibited by certain nanowires such as carbon nanotubes manifest themselves in discrete values ​​of electrical conductance. These discrete values ​​arise from a restriction of quantum mechanics in the number of electrons that can travel through the nanometer-scale wire. These discrete values ​​are often referred to as the quantum of conductance and are integer values. There are many applications where nanowires can become important: in electronics, optoelectronics and nano electromechanical devices, as additives in advanced compounds, for metallic interconnections in quantum nanoscale devices, as field emitters and as contacts or terminals for biomolecular nanosensors.

Prof. Andrew Watt is conducting research on nanotubes and optoelectronics. This project will involve the synthesis of metal alloys for nanotubes by thin film processing in a wide variety of substrates, conductivity and mobility. All these characteristics are measured along with the physical-chemical characterization: XRD, SEM, XPS, TEM. On the other hand, the semiconductor nanowires are the protagonists of diverse lines of investigation on future generations of devices of the electronic world in nanoscale. They are considered as one of the basic elements to develop applications of nanotechnology. It is important that they have a high surface-to-volume ratio where certain properties may change.

One of the most inexpensive ways to obtain nanofiles is found in thermal systems. By placing a metal, zinc oxide for example, on a substrate and heating the latter, we will be acting on the former in such a way as to lengthen and obtain a nanowire with a “drop” of metal at the tip. We can also place a powder inside a furnace and introduce gas inside. This will cause the dust to evaporate and hit the gas to form nanowires.

Current technique allows to select the location where you want to generate the nanowires and has almost absolute control of the deposit of atomic layers of different materials on them. We can create electronic devices or optoelectronic with a high density of transistors, diodes or LEDs and therefore complex circuits or light panels that occupy very little space. The smaller space also implies a lower energy consumption and an increase of the benefits. In this sense, semiconductor nanowires have been used both for the creation of electronic and optoelectronic nanodevices as well as for sensors or for the creation of high performance energy and solar cell generators.

PN and PIN junctions in nanowires are used in high-performance solar cells, driving new renewable energies. One of the “star” applications of zinc oxide semiconductor nanowires is their ability to be used as nanogenerators.

With proper development, nanogenerators will work by converting the mechanical energy of body movement, contraction of muscles or water flow, into electricity. Scientists at the Memorial Sloan-Kettering Cancer Center in New York announced in 2001 the creation of a molecular nanogenerator designed to fight cancer. Its functioning is based on the fact that, once in the human body, it is introduced into the tumor cells. Here it releases a series of atomic particles that would destroy the diseased cells without affecting the healthy ones.

With other types of nanowires we obtain other applications. Using gallium nitride for the growth of nanowires we could obtain light emitting diodes, short wavelength ultraviolet nano-lasers and special biochemical sensors.

Optoelectronics is simply dedicated to everything that is related to light, such as mobile phones, electronic devices, etc. And it is in this field that we are focusing our research to create devices more efficient and respectful with the environment.

Prof. Andrew Watt