15 Nanowires: Properties and Applications

Dr. Anchal Srivastava

Contents of this Unit

1. Introduction

2. Properties of Nanowires

2.1. Thermal Conductivity of Nanowires

2.2. Transport Properties

2.3. Optical Properties

3.  Applications

3.1. Electrical Applications

3.2. Thermoelectric Applications

3.3. Optical Applications

3.4. Chemical and Biochemical Sensing Devices

3.5. Magnetic Applications

4.  Summary

Learning Outcomes

• After studying this module, you shall be able to understand
• Transport and optical properties of nanowires and how it is important in various applications such as electrical, thermoelectric, optical, biochemical etc.
• The interrelationship between the structure and properties of nanowires.

1.   Introduction

The applications of the nanowires, structural properties are especially important so that a reproducible relationship between their desired functionality and their geometrical and structural characteristics can be established. Due to the enhanced surface-to volume ratio in nanowires, their properties may depend sensitively on their surface condition and geometrical configuration. Even nanowires made of the same material may possess dissimilar properties due to differences in their crystal phase, crystalline size, surface conditions, and aspect ratios, which depend on the synthesis methods and conditions used in their preparation. In this module we study the interrelationship between the structure and properties of nanowires and their different applications.

2.      Properties of Nanowires

2.1Thermal Conductivity of Nanowires

Experimental measurements of the temperature dependence of the thermal conductivity k(T ) of individual suspended nanowires have been carried out to study the dependence of k(T ) on the wire diameter. In this context, measurements have been made on nanowires down to only 22 nm in diameter. Such measurements are very challenging and are now possible because of technological developments in fabricating and using nanometer size thermal scanning probes. The experiments show that the thermal conductivity of small homogeneous nanowires may be more than one order of magnitude smaller than in the bulk, arising mainly from strong boundary scattering effects. Phonon confinement effects may eventually become important at still smaller diameter nanowires. Measurements on mats of nanowire do not generally give reliable results because the contact thermal resistance between adjacent nanowires tends to be high, which is in part due to the thin surface oxide coating that most nanowires have. This surface oxide coating may also be important for thermal conductivity measurements on individual suspended nanowires because of the relative importance of phonon scattering at the lateral walls of the nanowire. The most extensive experimental thermal conductivity measurements have been done on Si nanowires where k(T ) measurements have been made on nanowires in the diameter range of 22 ≤ dW ≤ 115 nm. The results show a large decrease in the peak of k(T ) associated with umklapp processes as dW decreases, indicating a growing importance of boundary scattering and a corresponding decreasing importance of phonon–phonon scattering. At the smallest wire diameter of 22 nm, a linear k(T ) dependence is found experimentally, consistent with a linear T dependence of the specific heat for a 1-D system and a temperature independent mean free path and velocity of sound.

Fig. 1 Predicted thermal conductivity of Si nanowires of various diameters.

Model calculations for k(T ) based on a radiative heat transfer model have been carried out for Si nanowires. These results show that the predicted k(T ) behavior for Si nanowires is similar to that observed experimentally in the range of 37 ≤ dW ≤ 115 nm regarding both the functional form of k(T ) and the magnitude of the relative decrease in the maximum thermal conductivity kmax as a function of dW. But the model calculations predict a substantially larger magnitude for k(T ) (by 50% or more) than is observed experimentally. Furthermore, the model calculations (see Fig. 1) do not reproduce the experimentally observed linear T dependence for the 22 nm nanowires, but rather predict a 3-D behavior for both the density of states and the specific heat in 22 nm nanowires. Thermal conductance measurements on GaAs nanowires below 6K show power law dependence, but the T dependence become somewhat less pronounced below ~2.5K. This deviation from the power law temperature dependence led to a more detailed study of the quantum limit for the thermal conductance. To carry out these more detailed experiments, a mesoscopic phonon resonator and waveguide device were constructed that included four ~ 200 nm wide and 85 nm thick silicon nitride nanowire-like nano-constrictions (see Fig. 2a) to establish the quantized thermal conductance limit of 0 = 2 2 /3ℎ (see Fig. 2b) for ballistic phonon transport. For temperatures above 0.8K, the thermal conductance in Fig. 2b follows a T3 law, but as T is further reduced, a transition to a linear T dependence is observed, consistent with a phonon mean free path of ~ 1μm, and a thermal conductance value approaching 16g0, corresponding to four mass-less phonon modes per channel and four channels in their phonon waveguide structure (see Fig. 2a). Ballistic phonon transport occurs when the thermal phonon wavelength (380 nm for the experimental structure) is somewhat greater than the width of the phonon waveguide at its constriction.

Fig. 2 (a) Suspended mesoscopic phonon device used to measure ballistic phonon transport. The device consists of a 4×4μm “phonon cavity” (center) connected to four Si3N4 membranes, 60 nm thick and less than 200 nm wide. The two bright “C” shaped objects on the phonon cavity are thin film heating and sensing Cr/Au resistors, whereas the dark regions are empty space. (b) Log–log plot of the temperature dependence of the thermal conductance G0 of the structure in (a) normalized to 16g0 (see text)

2.2 Transport Properties

The study of nanowire electrical transport properties is important for nanowire characterization, electronic device applications, and the investigation of unusual transport phenomena arising from one-dimensional quantum effects. Important factors that determine the transport properties of nanowires include the wire diameter (important for both classical and quantum size effects), material composition, surface conditions, crystal quality, and the crystallographic orientation along the wire axis, which is important for materials with anisotropic materials parameters, such as the effective mass tensor, the Fermi surface, or the carrier mobility. Electronic transport phenomena in low-dimensional systems can be roughly divided into two categories: ballistic and diffusive transport. Ballistic transport phenomena occur when electrons travel across the nanowire without any scattering. In this case, the conduction is mainly determined by the contacts between the nanowire and the external circuit, and the conductance is quantized into an integral number of universal conductance units G0 = 2e2/h. Ballistic transport phenomena are usually observed in very short quantum wires, such as those produced by using mechanically controlled break junctions (MCBJ) where the electron mean free path is much longer than the wire length, and the conduction is a pure quantum phenomenon. To observe ballistic transport, the thermal energy must also obey the relation ≪ −   −1,where −   −1 is the energy separation between sub band levels j and j−1. On the other hand, for nanowires with lengths much larger than the carrier mean free path, the electrons (or holes) undergo numerous scattering events when they travel along the wire. In this case, the transport is in the diffusive regime, and the conduction is dominated by carrier scattering within the wires due to phonons (lattice vibrations), boundary scattering, lattice and other structural defects, and impurity atoms.

2.3 Optical Properties

Optical methods provide an easy and sensitive tool for measuring the electronic structure of nanowires since optical measurements require minimal sample preparation (for example, contacts are not required) and the measurements are sensitive to quantum effects. Optical spectra of 1-D systems, such as carbon nanotubes, often show intense features at specific energies near singularities in the joint density of states formed under strong quantum confinement conditions. A variety of optical techniques have shown that the properties of nanowires are different from those of their bulk counterparts.

Although optical properties have been shown to provide an extremely important tool for the characterization of nanowires, the interpretation of these measurements is not always straightforward. The wavelength of light used to probe the sample is usually smaller than the wire length but larger than the wire diameter. Hence, the probe light used in an optical measurement cannot be focused solely onto the wire, and the wire and the substrate on which the wire rests (or host material, if the wires are embedded in a template) are simultaneously probed. For measurements, such as photo-luminescence (PL), if the substrate does not luminescence or absorb in the frequency range of the measurements, PL directly measures the luminescence of the nanowires and the substrate can be ignored. In reflection and transmission measurements, however, even a non absorbing substrate can modify the measured spectra of nanowires.

3.  Applications

In the preceding sections we have reviewed many of the central characteristics that make nanowires in some cases similar and in some cases very different from their parent materials. We have also shown that some properties are diameter dependent and are therefore tunable during synthesis. Thus it is of great interest to find applications for nanowires that could benefit in unprecedented ways from both the unique and tunable properties of nanowires and the small size of these nanostructures, for use in the miniaturization of conventional devices. As the synthetic methods for the production of nanowires are maturing, and nanowires can be made in reproducible and cost-effective ways, it is only a matter of time before applications will be explored seriously. This is a timely development, as the semiconductor industry will soon be reaching what seems to be its limit in feature-size reduction and approaching a classical-to quantum size transition. At the same time the field of biotechnology is expanding through the availability of tremendous genome information and innovative screening assays. Since nanowires are the size of the shrinking electronic components and of cellular biomolecules, it is only natural for nanowires to be good candidates for applications in these fields. Commercialization of nanowire devices, however, will require reliable mass-production, effective assembly techniques, and quality-control methods.

3.1 Electrical Applications

The microelectronics industry continues to face technology (e.g., lithography) and economic challenges as the device feature size is decreased, especially below 100 nm. The self-assembly of nanowires might present a way to construct unconventional devices that do not rely on improvements in photo-lithography and, therefore, do not necessarily imply increasing fabrication costs. Devices made from nanowires have several advantages over those made by photolithography. A variety of approaches has been devised to organize nanowires via self-assembly, thus eliminating the need for the expensive lithographic techniques normally required to produce devices the size of typical nanowires, which we discuss earlier. In addition, unlike traditional silicon processing, different semiconductors can be simultaneously used in nanowire devices to produce diverse functionalities. Not only can wires of different materials be combined, but a single wire can be made of different materials. For example, junctions of GaAs and GaP show rectifying behavior, thus demonstrating that good electronic interfaces between two different semiconductors can be achieved in the synthesis of multi component nanowires. Transistors made from nanowires could also have advantages because of their unique morphology. For example, in bulk field effect transistors (FETs), the depletion layer formed below the source and drain region results in a source-drain capacitance that limits the operation speed. In nanowires, however, the conductor is surrounded by an oxide and thus the depletion layer cannot be formed. Depending on the device design, the source-drain capacitance in nanowires could be greatly minimized and possibly eliminated.

Fig. 3 a, b Optoelectrical characterization of a crossed nanowire junction formed between 65-nm n-type and 68-nm p-type InP nanowires. (a) Electroluminescence (EL) image of the light emitted from a forward-biased nanowire p-n junction at 2.5V. Inset, photoluminescence (PL) image of the junction. (b) EL intensity as a function of operation voltage. Inset, the SEM image and the I–V characteristics of the junction [4.52]. The scale bar in the inset is 5μm.

Device functionalities common in conventional semiconductor technologies, such the p-n junction diodes, field-effect transistors, logic gates, and light-emitting diodes, have been recently demonstrated in nanowires, showing their promise as the building blocks for the construction of complex integrated circuits by employing the “bottom-up” paradigm. Several approaches have been investigated to form nanowire diodes. For example, Schottky diodes can be formed by contacting a GaN nanowire with Al electrodes. Furthermore, p-n junction diodes can be formed at the crossing of two nanowires, such as the crossing of n and p-type InP nanowires doped by Te and Zn, respectively, or Si nanowires doped by phosphorus (n-type) and boron (p-type). In addition to the crossing of two distinctive nanowires, heterogeneous junctions have also been constructed inside a single wire, either along the wire axis in the form of a nanowire superlattice or perpendicular to the wire axis by forming a core-shell structure of silicon and germanium. These various nanowire junctions not only possess similar current rectifying properties as expected for bulk semiconductor devices, but they also exhibit electro-luminescence (EL) that may be interesting for optoelectronic applications, as shown in Fig. 3 for the electroluminescence of a crossed junction of n and p-type InP nanowires.

In addition to the two-terminal nanowire devices, such as the p-n junctions described above, it is found that the conductance of a semiconductor nanowire can be significantly modified by applying voltage at a third gate terminal, implying the utilization of nanowires as a field-effect transistor (FET). This gate terminal can either be the substrate, a separate metal contact located close to the nanowire, or another nanowire with a thick oxide coating in the crossed nanowire junction configuration. We discuss the operation principles of these nanowire-based FETs. Various logic devices performing basic logic functions have been demonstrated using nanowire junctions, as shown in Fig.4 for the OR and AND logic gates constructed from 2-by-1 and 1-by-3 nanowire p-n junctions, respectively. By functionalizing nanowires with redox active molecules to store charges, the nanowire FETs can exhibit bi-stable logic on or off states, which may be used for nonvolatile memory or as switches.

Fig. 4 a–d Nanowire logic gates: (a) Schematic of logic OR gate constructed from a 2(p-Si) by 1(n-GaN) crossed nanowire junction. The inset shows the SEM image (bar: 1μm) of an assembled OR gate and the symbolic electronic circuit. (b) The output voltage of the circuit in (a) versus the four possible logic address level inputs: (0, 0); (0, 1); (1, 0); (1, 1), where logic 0 input is 0V and logic 1 is 5V (same for below). (c) Schematic of logic AND gate constructed from a 1(p-Si) by 3(n- GaN) crossed nanowire junction. The inset shows the SEM image (bar: 1μm) of an assembled AND gate and the symbolic electronic circuit. (d) The output voltage of the circuit in (c) versus the four possible logic address level inputs.

Nanowires have also been proposed for applications associated with electron field emission, such as flat panel displays, because of their small diameter and large curvature at the nanowire tip, which may reduce the threshold voltage for the electron emission. In this connection, the demonstration of very high field emission currents from the sharp tip (~ 10 nm radius) of a Si cone and from carbon nanotubes stimulates interest in this potential application opportunity for nanowires.

3.2 Thermoelectric Applications

One proposed application for nanowires is for thermoelectric cooling and for the conversion between thermal and electrical energy. The efficiency of a thermoelectric device is measured in terms of a dimensionless figure of merit ZT, where Z is defined as:

In which σ is the electrical conductivity, S is the See beck coefficient, k is the thermal conductivity, and T is the temperature. In order to achieve a high ZT and therefore efficient thermoelectric performance, a high electrical conductivity, a high See beck coefficient, and a low thermal conductivity are required. In 3-D systems, the electronic contribution to k is proportional to σ in accordance with the Wiedemann–Franz law, and normally materials with high S have a low σ. Hence an increase in the electrical conductivity (e.g., by electron donor doping) results in an adverse variation in both the See beck coefficient (decreasing) and the thermal conductivity (increasing). These two trades-offs set the upper limit for increasing ZT in bulk materials, with the maximum ZT remaining at ~ 1 at room temperature for the 1960–1995 time frames. The high electronic density of states in quantum confined structures is proposed as a promising possibility to bypass the See beck/electrical conductivity trade-off and to control each thermoelectric-related variable independently, thereby allowing for an increased electrical conductivity, a relatively low thermal conductivity, and a large See beck coefficient simultaneously.

In addition to alleviating the undesired connections between σ, S, and the electronic contribution to the thermal conductivity, nanowires also have the advantage that the phonon contribution to the thermal conductivity is greatly reduced because of boundary scattering, thereby achieving a high ZT. Figure 5a shows the theoretical values for ZT vs. sample size for both bismuth thin films (2-D) and nanowires (1-D) in the quantum confined regime, exhibiting a rapidly increasing ZT as the quantum size effect becomes more and more important. In addition, the quantum size effect in nanowires can be combined with other parameters to tailor the band structure and electronic transport behavior (e.g., Sb alloying in Bi) to further optimize ZT. For example, Fig. 5b shows the predicted ZT for p-type Bi1−xSbx alloy nanowires as a function of wire diameter and Sb content x. The occurrence of a local ZT maxima in the vicinity of x ~ 0.13 and dW ~ 45 nm is due to the coalesce of ten valence bands in the nanowire and the resulting unusual high density of states for holes, which is a phenomenon absent in bulk Bi1−xSbx alloys. For nanowires with very small diameters, it is speculated that localization effects will eventually limit the enhancement of ZT. But in bismuth nanowires, localization effects are not significant for wires with diameters larger than 9 nm. In addition to 1-D nanowires, ZT values as high as ~   2 have also been experimentally demonstrated in macroscopic samples containing PbSe quantum dots (0-D) and stacked 2-D films.

Fig. 5 (a) Calculated ZT of 1-D (nanowire) and 2-D (quantum well) bismuth systems at 77K as a function of dW, denoting the wire diameter or film thickness. The thermoelectric performance (i. e., ZT) is expected to improve greatly when the wire diameter is small enough so that the nanowire becomes a one-dimensional system. (b) Contour plot of optimal ZT values for p-type Bi1−xSbx nanowires vs. wire diameter and antimony concentration calculated at 77K.

Although the application of nanowires to thermo-electrics seems very promising, these materials are still in the research phase of the development cycle and quite far from being commercialized. One challenge for thermoelectric devices based on nanowires lies in finding a suitable host material that will not reduce ZT too much due to the unwanted heat conduction through the host material. The host material should, therefore, have a low thermal conductivity and occupy as low a volume percentage in the composite material as possible while still providing the quantum confinement and the support for the nanowires.

3.3   Optical Applications

Nanowires also hold promise for optical applications. One-dimensional systems exhibit a singularity in their joint density of states, allowing quantum effects in nanowires to be optically observable, sometimes, even at room temperature. Since the density of states of a nanowire in the quantum limit (small wire diameter) is highly localized in energy, the available states quickly fill up with electrons as the intensity of the incident light is increased. This filling up of the sub-bands, as well as other effects unique to low-dimensional materials, lead to strong optical nonlinearities in quantum wires. Quantum wires may thus yield optical switches with a lower switching energy and increased switching speed compared to currently available optical switches. Light emission from nanowires can be achieved by photo-luminescence (PL) or electro-luminescence (EL), distinguished by whether the electronic excitation is achieved by optical illumination or by electrical stimulation across a p-n junction, respectively. PL is often used for optical properties characterization, but from the applications point of view, EL is a more convenient excitation method. Light emitting diodes (LEDs) have been achieved in junctions between a p-type and an n-type nanowire (Fig. 3) and in superlattice nanowires with p-type and n-type segments. The light emission was localized to the junction area and was polarized in the superlattice nanowire. Light emission from quantum wire p-n junctions is especially interesting for laser applications, because quantum wires can form lasers with lower excitation thresholds compared to their bulk counterparts, and they also exhibit decreased temperature sensitivity in their performance. Furthermore, the emission wavelength can be tuned for a given material composition by only altering the geometry of the wire. Lasing action has been reported in ZnO nanowires with wire diameters much smaller than the wavelength of the light emitted (λ = 385 nm) (see Fig. 6). Since the edge and lateral surface of ZnO nanowires are faceted, they form optical cavities that sustain desired cavity modes. Compared to conventional semiconductor lasers, the exciton laser action employed in zinc oxide nanowire lasers exhibits a lower lasing threshold (~40kW/cm2) than their 3-D counterparts (~ 300 kW/cm2). In order to utilize exciton confinement effects in the lasing action, the exciton binding energy (~ 60 meV in ZnO) must be greater than the thermal energy (~ 26 meV at 300 K). Decreasing the wire diameter increases the excitation binding energy and lowers the threshold for lasing. PL NSOM imaging confirmed the wave guiding properties of the anisotropic and well-faceted structure of ZnO nanowires, limiting the emission to the tips of the ZnO nanowires. Laser action has been also observed in GaN nanowires. Unlike ZnO, GaN has a small exciton binding energy, of only ~ 25 meV. Furthermore, since the wire radii used in this study (15–75 nm) are larger than the Bohr radius of excitons in GaN (11 nm), the exciton binding energy is not expected to increase in these GaN wires and quantum confinement effects. Some tenability of the center of the spectral intensity, however, was achieved by increasing the intensity of the pump power, causing a red-shift in the laser emission, which is explained as a band gap renormalization as a result of the formation of electron-hole plasma. Heating effects were excluded as the source of the spectral shift.

Fig. 6 A schematic of lasing in ZnO nanowires and the PL spectra of ZnO nanowires at two excitation intensities. One PL spectra is taken below the lasing threshold, and the other above it.

Nanowire photo detectors were also proposed. ZnO nanowires were found to display a strong photocurrent response to UV light irradiation. The co ductivity of the nanowire increased by four orders of magnitude compared to the dark state. The response of the nanowire was reversible and selective to photon energies above the band gap, suggesting that ZnO nanowires could be a good candidate for optoelectronic switches. Nanowires have also been proposed for another type of optical switching. Light with its electric field normal to the wire axis excites a transverse free carrier resonance inside the wire, while light with its electric field parallel to the wire axis excites a longitudinal free carrier resonance inside the wire. Since nanowires are highly anisotropic, these two resonances occur at two different wavelengths and thus result in absorption peaks at two different energies. Gold nanowires dispersed in an aqueous solution align along the electric field when a DC voltage is applied. The energy of the absorption peak can be toggled between the transverse and longitudinal resonance energies by changing the alignment of the nanowires under polarized light illumination using an electric field. Thus, electro-optical modulation is achieved. Nanowires may also be used as barcode tags for optical read out. Nanowires containing gold, silver, nickel, palladium, and platinum were fabricated by electrochemical filling of porous anodic alumina, so that each nanowire consisted of segments of various metal constituents. Thus many types of nanowires can be made from a handful of materials and identified by the order of the metal segments along their main axis and the length of each segment. Barcode read out is possible by reflectance optical microscopy. The segment length is limited by the Rayleigh diffraction limit and not by synthesis limitations; thus it can be as small as 145 nm. Figure 7 shows an optical image of many Au-Ag- Au-Ag bar-coded wires, where the silver segments show higher reflectivity. Figure 7b is a backscattering mode FE-SEM image of a single nanowire, highlighting the composition and segment length variations along the nanowire. Both the large surface area and the high conductivity along the length of nanowires are favorable for their use in inorganic-organic solar cells, which offer promise from a manufacture-ability and cost effective standpoint. In a hybrid nano crystal-organic solar cell, the incident light forms bound electron–hole pairs (excitons) in both the inorganic nano-crystal and in the surrounding organic medium. These excitons diffuse to the inorganic–organic interface and disassociate to form an electron and a hole. Since conjugated polymers usually have poor electron mobilities, the inorganic phase is chosen to have a higher electron affinity than the organic phase so that the organic phase carries the holes and the semiconductor carries the electrons. The separated electrons and holes drift to the external electrodes through the inorganic and organic materials, respectively. But only those excitons formed within an exciton diffusion length from an interface can disassociate before recombining, and therefore the distance between the dissociation sites limits the efficiency of a solar cell. A solar cell prepared from a composite of CdSe nanorods inside poly (3-ethylthiophene) yielded 6.9% monochromatic power efficiencies and 1.7% power conversion efficiencies under A.M. 1.5 illumination (equal to solar irradiance through 1.5 times the air mass of the earth at direct normal incidence). The nanorods provide a large surface area with good chemical bonding to the polymer for efficient charge transfer and exciton dissociation. Furthermore, they provide a good conduction path for the electrons to reach the electrode. Their enhanced absorption coefficient and their tunable band gap are also characteristics that can be used to enhance the energy conversion efficiency of solar cells.

Fig. 7 (a) An optical image of many short Au- Ag-Au-Au bar-coded wires and (b) an FE-SEM image of an Au/Ag bar-coded wire with multiple strips of varying length. The insert in (a) shows a histogram of the particle lengths for 106 particles in this image.

3.4 Chemical and Biochemical Sensing Devices

Sensors for chemical and biochemical substances with nanowires as the sensing probe are a very attractive application area. Nanowire sensors will potentially be smaller, more sensitive, demand less power, and react faster than their macroscopic counterparts. Arrays of nanowire sensors could, in principle, achieve nanometer scale spatial resolution and therefore provide accurate real-time information regarding not only the concentration of a specific analyte but also its spatial distribution. Such arrays, for example, could be very useful for dynamic studies on the effects of chemical gradients on biological cells. The operation of sensors made with nanowires, nanotubes, or nano-contacts is based mostly on the reversible change in the conductance of the nanostructure upon absorption of the agent to be detected, but other detection methods, such as mechanical and optical detection, are conceptually plausible. The increased sensitivity and faster response time of nanowires are a result of the large surface-to-volume ratio and the small cross-section available for conduction channels. In the bulk, on the other hand, the abundance of charges can effectively shield external fields, and the abundance of material can afford many alternative conduction channels. Therefore a stronger chemical stimulus and longer response time are necessary to observe changes in the physical properties of a 3-D sensor in comparison to a nanowire.

Cuietal. placed silicon nanowires made by the VLS method between two metal electrodes and modified the silicon oxide coating of the wire by the addition of molecules sensitive to the analyte to be detected. For example, a pH sensor was made by covalently linking an amine containing silane to the surface of the nanowire. Variations in the pH of the solution into which the nanowire was immersed caused protonation and deprotonation of the −NH₂ and the –SiOH groups on the surface of the nanowire. The variation in surface charge densities regulates the conductance of the nanowire; due to the p-type characteristics of a silicon wire, the conductance increases with the addition of negative surface charge. The combined acid and base behavior of the surface groups results in an approximately linear dependence of the conductance on pH in the pH range 2 to 9, thus leading to a direct readout pH meter. This same type of approach was used for the detection of the binding of biomolecules, such as streptavidin using biotin-modified nanowires (see Fig. 8). This nanowire-based device has high sensitivity and could detect streptavidin binding down to a 10 pM (10−12 mole) concentration. The chemical detection devices were made in field – effect transistor geometry, so that the back-gate potential could be used to regulate the conductance in conjugation with the chemical detection and to provide a real time direct read out. The extension of this device to detect multiple analytes, using multiple nanowires each sensitized to a different analyte, could provide for fast, sensitive and in-situ screening procedures.

Fig. 8 (a) Streptavidin molecules bind to a silicon nanowire functionalized with biotin. The binding of streptavidin to biotin causes the nanowire to change its resistance. (b) The conductance of a biotin-modified silicon nanowire exposed to streptavidin in a buffer solution (regions 1 and 3) and with the introduction of a solution of antibiotin monoclonal antibody (region 2).

Favier et al. used a similar approach, making a nano sensor for the detection of hydrogen out of an array of palladium nanowires between two metal contacts. They demonstrated that nano-gaps were present in their nanowire structure, and upon absorption of H2 and formation of Pd hydride, the nano-gap structure would close and improve the electrical contact, thereby increasing the conductance of the nanowire array. The response time of these sensors was 75 msec, and they could operate in the range of 0.5 to 5% H2 before saturation occurred.

3.5 Magnetic Applications

It has been demonstrated that arrays of single domain magnetic nanowires can be prepared with controlled nanowire diameter and length, aligned along a common direction and arranged in a close-packed ordered array, and that the magnetic properties (coercivity, remanence, and dipolar magnetic interwire interaction) can be controlled to achieve a variety of magnetic applications.

Magnetic information storage is the most interesting of these applications. The large nanowire aspect ratio (length/diameter) prevents the onset of the “superparamagnetic” limit at which the magnetization direction in the magnetic grains can be reversed by the thermal energy kBT, thereby resulting in loss of recorded data in the magnetic recording medium. The magnetic energy in a grain can be increased by increasing either the volume or the anisotropy of the grain. If the volume is increased, the particle size increases, and thus, the resolution are decreased. For spherical magnetized grains, the superparamagnetic limit at room temperature is reached at 70 Gbit/in2. In nanowires, the anisotropy is very large and yet the wire diameters are small, so the magnetostatic switching energy can easily be above the thermal energy while the spatial resolution is large. For magnetic data storage applications, a large aspect ratio is needed for the nanowires to maintain a high coercivity, and a sufficient separation between nanowires is needed to suppress inter-wire magnetic dipolar coupling. Thus nanowires can form stable and highly dense magnetic memory arrays with packing densities in excess of 1011 wires/cm2. The onset of superparamagnetism can be prevented in the single domain magnetic nanowire arrays that have already been fabricated either using porous alumina templates to make Ni nanowires with 35 nm diameters or diblock copolymer templates to make Co nanowires, with mean diameters of 14 nm and 100% filling of the template pores. Ordered magnetic nanowire arrays that have already been demonstrated offer the exciting promise for achieving systems permitting 1012 bits/in2 data storage.

4. Summary

• In this module you have understand the followings
• Physical properties of nanowires, such as thermal conductivity, transport and optical with particular reference to nanowire properties that differ from those of their bulk counterparts.
• Various potential applications such as electrical, thermoelectric, optical, magnetic etc. that might result from the special structure and properties of nanowires.

Suggested Readings

Suggested Reading

• Carbon Nanotubes: advanced Topics in the Synthesis, Structure, Properties and Applications, Ado Jorio, Ado Jorio, Gene Dresselhaus, Mildred S. Dresselhaus.
• Springer Handbook of Nanotechnology, Bhushan, Bharat.