27 Nanowires

S.S. islam

epgp books

 

 

 

Introduction

 

A tubular nanostructure having the diameter of the order of a nanometer (10-9 m) is called a nanowire. Another way to define a nanowire is that it is a nanostructure whose length to width ratio is in access of 1000. Alternately, nanowire may be described as structure whose diameter is limited to a few nanometers and has an unconstrained length. Since quantum effects become dominant at this scale, the term ‘quantum wire’ if often used for a nanowire. There are a variety of nanowires prepared over the years, these include superconducting (YBCO), metallic (Ni, Pt, Au, etc.), semiconducting (silicon nanowires, InP nanowires, GaN, etc.), and insulating nanowires (SiO2, TiO2, etc.). Additionally, molecular nanowires also exist which are made of repeating molecular units that may be either organic (DNA) or inorganic (Mo6S9−xIx).

 

Nanowires, are also termed as one-dimesional materials, and have novel properties which are not present in their bulk or three-dimensional counterparts. These properties arise due to the quantum confinement of their electrons to lateral directions. As a result, these electrons have energy levels much different from the continuous energy bands present in bulk materials.

 

This quantum confinement leads to the quantisation of electrical conductance of the nanowires. These discrete nature of electrical conductance can be attributed to the quantum mechanical limitation on the number of electrons travelling through the wires at nanometer scales. These discrete values are called ‘quantum of conductance’.

 

The nanowires find applications in a variety of fields such as electronic, optoelectronic, nano-electromechanical devices, as conductive fillers in functional composites, as metallic interconnects in nanoscaled quantum devices, field emitting devices, electrical connects to nanoscaled biosensors, etc.

 

Applications of Nanowires

 

a. Electronic Devices

 

One of the most important and frequent applications of nanowires is in transistors. Transistors are the building blocks in present days’ electronic devices. As per Moore’s law, the size of transistors is getting smaller and smaller to reach the nanoscale. However, a major challenge for nanoscale transistors is achieving high gate control over the channel. Owing to their large aspect ratios, nanowires are ideal candidates for channels in transistors. If gate dielectric is wrapped over the nanowire based channel, a high gate control over the channel electrostatic potential can be achieved, leading to efficient operation of the transistor.

 

The nanowires with unique one-dimensional structures and excellent optical properties are ideal for producing highly efficient photovoltaic devices. Compared with its bulk counterparts, the nanowire solar cells are relatively immune to impurities caused by bulk recombination. Therefore, low purity silicon wafers with can also be used to obtain acceptable efficiency, leading to the reduction on material consumption.

 

The steps involved in creating active electronic elements from semiconducting nanowires, can be summarised as:

–    Chemically dope the nanowire with appropriate species in order to produce p-type or n-type nanowires, as a first step.

–    Secondly, a p-n junction needs to be created. p-n junctions have been prepared in two ways: (a) physically crossing a p-type nanowire oven n-type nanowire, and (c) chemically dope single nanowire with different dopants along its length. The second method creates a p-n junction within a single nanowire.

–    With the p-n junctions prepared from nanowires, logic gates can be fabricated. This can be achieved by interconnecting several p-n junctions in specific manner. The most basic gates have been produced in this manner, which include AND, OR, and NOT gates. These logic gates have been prepared via nanowire crossing technique.

 

However, accurately doping a semiconductor to obtain p-type or n-type nanowire still remains a challenge. This has been overcome by researchers in 2012, when first NAND gate was prepared from undoped silicon nanowires. A silicide layer was deposited over the metal-semiconductor (silicon) interface to obtain the Schottky barrier having low contact resistance.

 

The semiconductor nanowire crossings are expected to benefit the future generation digital computing. Although, there are a wide variety of applications of nanowires, their use in electronic industry is their most fundamental application.

 

Another use of nanowires involve their use as ballistic waveguides for photons as interconnects in quantum dot (QD) or quantum effect well photon logic arrays. In these structures, photons travel inside the tube (waveguide), and electrons move in the outer shell.

 

Molecular computers can be a reality thanks to the conducting nanowires, wherein nanowires will be used to connect the molecular scale (nanostructured) objects. Dispersions of conducting nanowires in different polymers are being investigated for their use as transparent electrodes in flexible flat-screen displays.

 

Owing to their excellent mechanical properties such as large Young’s modulus, nanowires are being studied for improving the mechanical strength of composites. For this application, their use as fillers inside the composite matrix is being examined. The tendency of nanowires to aggregate can be exploited in tribological additives for improving friction performance and reliability of electronic transducers and actuators.

 

Nanowires are particularly suitable for dielectrophoresis manipulation owing to their large aspect ratios. This provides an economic, bottom up approach for integrating suspended nanowires (made of dielectric metal oxides) in electronic devices including UV, water vapour, and ethanol sensors.

 

b. Chemical Sensing

 

Similar to the FET devices, wherein the modulation of conductance (flow of electrons or holes) between the input (source) and the output (drain) electrodes in the semiconductor is controlled via variations in electrostatic potential (gate-electrode) of the charge carriers in conduction channel of the device, a Bio/Chem-FET operates by detecting local changes in the charge density. These changes are often termed as ‘field-effect’. These changes in the local charge density mark the recognition between the target molecule and surface receptor.

 

The changes in surface potential controls the Chem-FET device in an identical manner the gate voltage does, resulting in a detectable and measurable change in the conduction of the device. When a nanowire is used as the transistor element, the binding of a chemical or biological species to the surface of the sensor can cause depletion or accumulation of charge carriers in the ‘bulk’ of the nanowire, i.e., within the small cross section available for conduction channels. Since the wire serving as tunable conducting channel remains in close contact with the sensing environment of the target, the response time is very short and the sensitivity is very high due to large S/V ratio of the nanowires.

 

Numerous inorganic semiconductor materials have been used for the fabrication of nanowires including Si, Ge, and metal oxides (In2O3, SnO2, ZnO, TiO2, etc.). However, for FET applications, Si based nanowire are the most preferable. Silicon nanowires (SiNWs) are extensively used in sensing devices including ultra sensitive relatime sensing of biomarker proteins for cancer detection, detecting isolated virus particles, and detecting explosive materials of nitro-aromatic type, e.g., 2,4,6 Tri-nitrotoluene (TNT) in much higher sensitives than canines. SiNWs can also be employed in electromechanical devices, where they can be used in twisted shapes to determine the intermolecular forces with high accuracy.

 

c. Nanowire Battery

 

Nanowire batteries are the battery systems wherein nanowires are used to enhance the surface area of the electrodes. The semiconducting nanowires proposed for use in battery applications include Si, Ge, and transition metal oxides based nanowires. The use of these nanowires as electrodes in lithium-ion batteries has been demonstrated to greatly enhance the battery performance. These electrodes are usually used as anodes where they replace the conventional anode (graphite). A short description of these electrodes has been given below:

 

a.    Silicon: Silicon is the most preferable material for lithium ion battery anodes owing to its extremely favourable properties. In fact, it was the only commercially available variant of anodes in lithium ion batteries till 2013. Silicon has a low discharge potential along with a very high theoretical charge capacity, which is about ten times higher than that of the conventionally used graphite anode. Nanowires enhance these properties due to their large surface area in contact with the electrolyte. This increases the power and current densities of the anode, and also results in faster charging. Such batteries could deliver higher current density. Nonetheless, practical application of silicon as anode in lithium-ion batteries is precluded by colossal volume expansions of the anode during lithiation (lithium insertion in anode during charging). Silicon undergoes upto 400% swelling during lithium intercalation while charging, causing material degradation. Additionally, the volume inflation is not isotropic because of crack propagation immediately following a moving lithiation front. The crack cause pulverisation of the electrode resulting in considerable capacity loss within first few cycles. The theoretical capacity of SiNWs is around 4200 mAh/g, which is highest among other structures of silicon. For comparison, the theoretical capacity of traditional anode, graphite, is only 372 mAh/g. SiNW based batteries provide potential opportunities to produce dimensionally flexible energy sources. This can be beneficial for the production of wearable technological devices. Researchers from Rice University have shown this possibility where they deposited porous copper nanoshells around a SiNW in a polymer matrix. This lithium-polymer silicon nanowire battery (LIOPSIL) demonstrated operational full cell voltage of 3.4 V. In addition to this, this battery was mechanically flexible as well as scalable for large scale production.

 

b.    Transition Metal Oxides (or TMO): These include several oxides of transition metals such as Cr2O3, Fe2O3, MnO2, Co3O4 and PbO2. These materials have numerous advantages as anode materials over conventional electrode materials for lithium-ion battery (LIB) as well as other battery systems. These materials are naturally abundant, have large theoretical energy capacities, non-toxic and environmentally benign. As the concept of the nanostructred battery electrode has been introduced, experimentalists start to look into the possibility of TMO-based nanowires as electrode materials. Some recent investigations into this concept are discussed in the subsequent subsections.

  • Lead Oxide Anode: Lead-acid battery is the oldest type among rechargeable battery cells. Although the raw material (PbO2) for producing these cells is easily available and reasonable priced, lead-acid battery cells have relatively small specific energy. Besides, the paste thickening (or volume expansion) effects during the operation cycle blocks the effective flow of the electrolyte. Due to these problems, the potential of the cell is limited and the cell is not useful for energy-intensive operation. In 2014, researchers successfully prepared PbO2 nanowire via a facile template based electrodeposition technique. The performance of these nanowire as anode material for lead-acid cell was investigated. The increased surface area of the nanowire resulted in a relatively consistent capacity of about 190 mAh/g even after 1,000 cycles. This result showed this nanostructured PbO2 as a fairly promising substitute for the normal lead-acid anode. 
  • Manganese Oxide Anode: Because of high energy capacity, non-toxicity and low cost, MnO2 is a potential candidate for electrode materials. However, lithium-ion insertion into the crystal matrix during charging/discharging cycle causes large volume expansions. To avoid this problem, Li-enriched MnO2 nanowires (with a nominal stoichiometry of Li2MnO3) have been proposed as anode materials for lithium-ion batteries. This new proposed anode material resulted in the energy capacity of 1279 mAh/g and the corresponding current density of 500 mA even after 500 cycles. This performance is much higher than that of pure MnO2 anode or MnO2 nanowire anode cells.
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References

  1. Applications of Nanowires: Wipedia
  2. Boston, R.; Schnepp, Z.; Nemoto, Y.; Sakka, Y.; Hall, S. R. (2014). “In Situ TEM Observation of a Microcrucible Mechanism of Nanowire Growth”. Science. 344 (6184): 623–6.
  3. Holmes, J. D.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. (2000). “Control of thickness and orientation of solution-grown silicon nanowires”. Science. 287 (5457): 1471–3.
  4. Tilke, A. T.; Simmel, F. C.; Lorenz, H.; Blick, R. H.; Kotthaus, J. P. (2003). “Quantum interference in a one-dimensional silicon nanowire”. Physical Review B. 68 (7): 075311.
  5. “Nanowires get reconfigured”. nanotechweb.org. October 19, 2006.