Nanowire
More generally, nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometers or less and an unconstrained length.
Other important examples are based on semiconductors such as InP, Si, GaN, etc., dielectrics (e.g. SiO2,TiO2), or metals (e.g. Ni, Pt).
Initial synthesis via either method may often be followed by a nanowire thermal treatment step, often involving a form of self-limiting oxidation, to fine tune the size and aspect ratio of the structures.
For nanowires, the best catalysts are liquid metal (such as gold) nanoclusters, which can either be self-assembled from a thin film by dewetting, or purchased in colloidal form and deposited on a substrate.
For example, a method termed ENGRAVE (Encoded Nanowire GRowth and Appearance through VLS and Etching)[11] developed by the Cahoon Lab at UNC-Chapel Hill allows for nanometer-scale morphological control via rapid in situ dopant modulation.
Unlike VLS, the catalytic seed remains in solid state when subjected to high temperature annealing of the substrate.
By using metal nanocrystals as seeds,[15] Si and Ge organometallic precursors are fed into a reactor filled with a supercritical organic solvent, such as toluene.
Thermolysis results in degradation of the precursor, allowing release of Si or Ge, and dissolution into the metal nanocrystals.
As more of the semiconductor solute is added from the supercritical phase (due to a concentration gradient), a solid crystallite precipitates, and a nanowire grows uniaxially from the nanocrystal seed.
Spontaneous nanowire formation by non-catalytic methods were explained by the dislocation present in specific directions[20][21] or the growth anisotropy of various crystal faces.
More recently, after microscopy advancement, the nanowire growth driven by screw dislocations[22][23] or twin boundaries[24] were demonstrated.
[citation needed] The conductance in a nanowire is described as the sum of the transport by separate channels, each having a different electronic wavefunction normal to the wire.
In practical terms, this means that a MOSFET with such nanoscale silicon fins, when used in digital applications, will need a higher gate (control) voltage to switch the transistor on.
[38] For nanowires with diameters less than 10 nm, existing welding techniques, which require precise control of the heating mechanism and which may introduce the possibility of damage, will not be practical.
Recently scientists discovered that single-crystalline ultrathin gold nanowires with diameters ≈ 3–10 nm can be "cold-welded" together within seconds by mechanical contact alone, and under remarkably low applied pressures (unlike macro- and micro-scale cold welding process).
[39] High-resolution transmission electron microscopy and in situ measurements reveal that the welds are nearly perfect, with the same crystal orientation, strength and electrical conductivity as the rest of the nanowire.
Combined with other nano- and microfabrication technologies,[40][41] cold welding is anticipated to have potential applications in the future bottom-up assembly of metallic one-dimensional nanostructures.
[42] Moreover, nanowires continue to be actively studied, with research aiming to translate enhanced mechanical properties to novel devices in the fields of MEMS or NEMS.
One of the key challenges of building future nanoscale MOS transistors is ensuring good gate control over the channel.
By connecting several p-n junctions together, researchers have been able to create the basis of all logic circuits: the AND, OR, and NOT gates have all been built from semiconductor nanowire crossings.
Dispersions of conducting nanowires in different polymers are being investigated for use as transparent electrodes for flexible flat-screen displays.
Because nanowires appear in bundles, they may be used as tribological additives to improve friction characteristics and reliability of electronic transducers and actuators.
[56] The high aspect ratio of nanowires makes this nanostructures suitable for electrochemical sensing with the potential for ultimate sensitivity.
[57][58] Nanowire lasers are Fabry–Perot resonator cavities defined by the end facets of the wire with high-reflectivity, recent developments have demonstrated repetition rates greater than 200 GHz offering possibilities for optical chip level communications.
[59][60] In an analogous way to FET devices in which the modulation of conductance (flow of electrons/holes) in the semiconductor, between the input (source) and the output (drain) terminals, is controlled by electrostatic potential variation (gate-electrode) of the charge carriers in the device conduction channel, the methodology of a Bio/Chem-FET is based on the detection of the local change in charge density, or so-called "field effect", that characterizes the recognition event between a target molecule and the surface receptor.
Moreover, the wire, which serves as a tunable conducting channel, is in close contact with the sensing environment of the target, leading to a short response time, along with orders of magnitude increase in the sensitivity of the device as a result of the huge S/V ratio of the nanowires.
[62] Silicon nanowires could also be used in their twisted form, as electromechanical devices, to measure intermolecular forces with great precision.
One approach of overcoming this limitation employs fragmentation of the antibody-capturing units and control over surface receptor density, allowing more intimate binding to the nanowire of the target protein.
This approach proved useful for dramatically enhancing the sensitivity of cardiac biomarkers (e.g. Troponin) detection directly from serum for the diagnosis of acute myocardial infarction.
[64] For a minimal introduction of stress and bending to transmission electron microscopy (TEM) samples (lamellae, thin films, and other mechanically and beam sensitive samples), when transferring inside a focused ion beam (FIB), flexible metallic nanowires can be attached to a typically rigid micromanipulator.
