Nanowire Based Sensors

In this section of the blog we will describe nanowires, their properties such as electrical, optical, thermoelectric and mechanical. Then we will describe types of fabrication of nanowires and their application in sensing applications.

• Metallic materials
• Semiconducting materials,
• Insulating nanowires and
• Molecular nanowires composed of organic or inorganic repeating molecular units and
• Core-shell super lattice nanowires.

In comparison to other low dimensional systems, nanowires have two quantum confined directions, while still leaving one unconfined direction for electrical conduction. This allows nanowires to be used in applications where electrical conduction, rather than tunneling is required. Because of their unique density of electronic states, nanowires exhibit significantly different optical, electrical and magnetic properties. 

 Properties of Nanowires 

The nanowires are of great interest to scientist and researchers since their unique electrical, mechanical, thermal and optical properties. Large nanowire surface-to-volume ratio allows the creation of extremely sensitive sensors in chemical and biological systems for the detection of charged particles and molecules at low concentrations. Nanowires produced from semiconducting materials have interesting optical properties because they absorb and emit light efficiently over a very broad energy range from the UV to visible IR wavelengths. This type of nanowires are used in commercial products such as visible LEDs and blue laser diodes.

1D nanostructures based on III-nitride semiconductors, including nanowires and nano-rods, have attracted attention as potential nano scale building blocks for enhanced performance or functionality for optoelectronics, sensing, photovoltaic, and electronic applications. 

Electrical properties of nanowires

One-dimensional arrangements such as nanowires have outstanding potential in nanoscale electronic devices. They are often configures in field effect transistor structures. Important factors that determine the transport properties of nanowires include the wire diameter, which is important for both classical and quantum size effects, material composition, surface conditions, crystal quality, and the crystallographic orientation along the wire axis.

Electronic transport phenomena in low dimensional systems can be divided into two categories and these categories are:

• Ballistic transport phenomena which occur when the electrons travel across nanowire without scattering. This type of electron transport happens in short nanowires with the length similar to the mean free path of the carrier (electrons)
• Diffusive regime - Nanowires with lengths much larger than the carrier mean free path, the electrons (or holes) undergo numerous scattering events when travelling along the wire. The transport is diffusive and the conduction is dominated by carrier scattering within the wires.

Optical properties of nanowires

Two several advantages and applications that arise from the optical properties on nanowire include:

-          The flow of optically encoded information with nm-scale accuracy over distances of many microns may be controlled for nanowire structures. This can be applied in future high-density computing.

-          Devices that are based on optically sensitive nanowires have a high potential for photovoltaic cells of phototransistors. In this context, it is important to note that photo-transistors are a subtype of transistors in which the incident light intensity can modulate the charge-carrier density in the channel. Organic nanowire phototransistors exhibit interesting photo electronic properties upon different types of light irradiation. These nanowires yield much higher photoconductive gains and external quantum efficiencies than their thin-film counterparts.

Their properties are highly promising alternative to conventional thin film type photodiodes, and can pave the way for optoelectronic device miniaturization.

 Thermo electronic properties

Thermoelectric conversion relies on a difference between hot and cold areas in a device. Heat flowing from the hot side to the cold side creates current, which can be captured and used to power a device or stored for subsequent use. Bulk material has traditionally been considered a poor material for thermoelectric conversion, because the thermal conductivity is too high. Heat travels across it so well that it is difficult to create the necessary temperature differential. However, it has been evidenced that nanowires have improved thermoelectric properties, and examples of applications that take advantage of this property are given in the following figures.

Next figure shows that transverse thermoelectric devices exhibit distinctive characteristics compared to ordinary thermoelectric devices as follows:

• The voltage signal develops perpendicular to the applied temperature gradient.
• Compatibility between n-type and p-type thermoelectric material is not necessary since either is sufficient to construct the device.
• The macroscopic physical properties of the multilayer material can be tuned by changing the combination and periodicity of the constituents. These features provide additional degrees of freedom when designing alternative thermoelectric devices.

In addition to the unique properties of nanowires that can be utilized for thermo-electric applications, the fabrication of the devices is growing increasingly cost effective. Multiple studies have been executed that pursue relatively easy and cost-effective fabrication of such devices. The presented image relates to stoichiometric and single-phase lead telluride (PbTe) nanowire arrays that were prepared using photoresist-bottomed lithographically patterned nanowire electro deposition (PB- LPNE). This fabrication approach has been found to provide a control over wire width and thickness and allows the preparation of suspended nanowires across 25-micrometer air gaps.