Metal nanowires are promising materials for many novel applications, ranging from chemical and biological sensors to optical and electronic devices. This is not only because of their unique geometry, but also because they possess many unique physical properties, including electrical, magnetic, optical, as well as mechanical properties. While most efforts to date have focused on developing better methods to fabricate the nanowires and on characterizing the various properties, applications are becoming an important area of research and development. Some of the applications are discussed below.
1.3.1. Magnetic Materials and Devices
The electrodeposition methods described above have been used to fabricate magnetic nanowires of a single metal [1.28], multiple metals in segments [1.29], as well as alloys [1.30].
Since the pioneering works nearly a decade ago, much progress has been made in understanding the magnetic properties of the nanowires [1.31]. Arecent review provides a detailed description of the properties [163]. For magnetic nanowires (Fe, Co, and Ni) with relative large aspect ratios (e.g., >50), they exhibit an easy axis along the wires. An important parameter that describes magnetic properties of materials is the remanence ratio, which measures the remanence magnetization after switching off the external magnetic field. The remanence ratios of the Fe, Co, and Ni nanowires can be larger than 0.9 along the wires and much smaller in the perpendicular direction of the wires. This finding clearly shows that the shape anisotropy plays an important role in the magnetism of the nanowires. Another important parameter that describes the magnetic properties is coercivity, which is the coercive field required to demagnetize the magnet after full magnetization. The magnetic nanowires exhibit greatly enhanced magnetic coercivity [1.32, 1.33]. In addition, the coercivity depends on the wire diameter and the aspect ratio, which shows that it is possible to control the magnetic properties of the nanowires by controlling the
fabrication parameters. The diameter dependence of the coercivity reflects a change of the magnetization reversal mechanism from localized quasi-coherent nucleation for small diameters to a localized curling like nucleation as the diameter exceeds a critical value [1.34]. Another technically important novel property observed in the magnetic nanowires is giant magnetoresistance (GMR) [1.29]. For example, Evans et al. have studied Co-Ni-Cu/Cu multilayered nanowires and found a magnetoresistance ratio of 55% at room temperature and 115% at 77K for current perpendicular to the plane (along the direction of the wires) [1.35].
Giant magnetoresistance has also been observed in semimetallic Bi nanowires fabricated by electrodeposition [1.36]. Hong et al. have studied GMR of Bi with diameters between 200 nm and 2µm in magnetic fields up to 55T and found that the magnetoresistance ratio is between 600–800% for magnetic field perpendicular to the wires and 200% for the field parallel to the wires [1.37]. The novel properties and small dimensions have potential applications in the miniaturization of magnetic sensors and the high-density magnetic storage devices. The alignment of magnetic nanowires in an applied magnetic field can be used to assemble the individual nanowires [1.38]. Tanase et al. studied the response of Ni nanowires in response to magnetic field [1.39]. The nanowires are fabricated by electrodeposition using alumina templates and functionalized with luminescent porphyrins so that they can be visualized with a video microscope. In viscous solvents, magnetic fields can be used to orient the nanowires. In mobile solvents, the nanowires form chains in a head-to-tail configuration when a small magnetic field is applied. In addition, they demonstrated that three-segment Pt-Ni-Pt nanowires can be trapped between lithographically patterned magnetic microelectrodes [1.40]. The technique has a potential application in the fabrication and measurement of nanoscale magnetic devices.
1.3.2. Optical Applications
Dickson and Lyon studied surface plasmon (collective excitation of conduction electrons) propagation along 20 nmdiameter Au, Ag, and bimetallic Au-Ag nanowires with a sharp Au/Ag heterojunction over a distance of tens of µm [1.41]. The plasmons are excited by focusing a laser
with a high numerical aperture microscope objective, which propagate along a nanowire and reemerge as light at the other end of the nanowire via plasmon scattering. The propagation depends strongly on the wavelength of the incident laser light and the composition of the nanowire. At the wavelength of 820 nm, the plasmon can propagate in both Au and Ag nanowires, although the efficiency in Ag is much higher than that in Au. In the case of bimetallic nanowire, light emission is clearly observed from the Ag end of the nanowire when the Au end is illuminated at 820 nm. In sharp contrast, if the same bimetallic rod is excited at 820 nm via the Ag end, no light is emitted from the distal Au end. The observation suggests that the plasmon mode excited at 820 nm is able to couple from the Au portion into the Ag portion with high efficiency, but not from the Ag portion into Au. The unidirectional propagation has been explained using a simple two-level potential model. Since surface plasmons propagate much more efficiently in Ag than in Au, the Au→ Ag boundary is largely transmissive, thus enabling efficient plasmon propagation in this direction. In the opposite direction, however, propagation from Ag to Au gains a much steeper potential wall, allowing less optical energy to couple through to the distal end. Their experiments suggest that one can initiate and control the flow of optically encoded information with nanometer-scale accuracy over distances of many microns, which may find applications in future high-density optical computing.
1.3.3. Biological Assays
We have mentioned that by sequentially depositing different metals into the nanopores, multisegment or striped metal nanowires can be fabricated [1.27]. The length of each segment can be controlled by the charge passed in each plating step and the sequence of the multiple segments is determined by the sequence of the plating steps. Due to the different chemical reactivities of the “stripe” metals, these stripes can be modified with appropriate molecules. For example, Au binds strongly to thiols and Pt has high affinity to isocyanides. Interactions between complementary molecules on specific strips of the nanowires allow different nanowires to bind to each other and form patterns on planar surfaces. Using this strategy, nanowires could assemble
deterministically into cross- or T-shaped pairs, or into more complex shapes [1.41]. It is also possible to use specific interactions between selectively functionalized segments of these nanowires to direct the assembly of nanowire dimers and oligomers, to prepare two-dimensional assembly of nanowire-substrate epitaxy, and to prepare threedimensional colloidal crystals from nanowire-shaped objects [1.42]. As an example, single-stranded DNA can be exclusively modified at the tip or any desired location of a nanowire, with the rest of the wire covered by an organic passivation monolayer. This opens the possibility for site specific DNA assembly [1.27].
Nicewarner-Pena et al. showed that the controlled sequence of multisegment nanowires can be used as “barcodes” in biological assays [1.43]. The typical dimension of the nanowire is 200 nm thick and 10 µm long. Because the wavelength dependence of reflectance is different for different metals, the individual segments are easily observed as “stripes” under an optical microscope with unpolarized white-light illumination. Different metal stripes within a single nanowire selectively adsorb different molecules, such as DNA oligomers, which can be used to detect different biological molecules simultaneously. These multisegment nanowires have been used like metallic barcodes in DNAand protein bioassays. The optical scattering efficiency of the multisegment nanowires can be significantly enhanced by reducing the dimensions of the segment, such that the excitation of the surface plasmon occurs. Mock et al. [1.44] have studied the optical scattering of multisegment nanowires of Ag, Au, and Ni that have diameters of 30 nm and length up to 7µm. The optical scattering is dominated by the polarization-dependant plasmon resonance of Ag and Au segments. This is different from the case of the thicker nanowires used by Nicewarner-Pena et al., where the reflectance properties of bulk metals determine the contrast of the optical images [1.43]. Because of the large enhancement by the surface plasmon resonance, very narrow ( 30 nm diameter) nanowires can be readily observed under white light illumination and the optical spectra of the individual segments are easily distinguishable [1.44]. The multisegment nanowires can host a large number of segment sequences over a rather small spatial range, which promises unique applications.
1.3.4. Nanoelectronic and Nanoelectrochemical Applications
In addition to multisegment metal nanowires, one can also fabricate a metal/organic film/metal junction [1.45] and metal/nanoparticle/metal junctions [1.46] in a single nanowire, which have been used to study electron transport properties of the small amount of molecules and nanoparticles. It has been demonstrated that a Au nanowire containing 4-_[2-nitro-4 (phenylethynyl) phenyl ethynyl_benzenethiol molecule junction exhibits negative differential resistance at room temperature [1.47], while a 16-mercaptohexadecanoic acid nanojunction exhibits a coherent nonresonant tunneling [1.48]. If some of the metal segments or “stripes” are being replaced with semiconductor, colloidal, and polymer layers, one can introduce rectifying junctions, electronic switching, and photoconductive elements in the composite nanowires. If selectively modifying the nanowire further, using the distinct surface chemistry of different stripes, the nanowires can be positioned on a patterned surface to fulfill nano-logic and memory circuits by self-assembling [1.46, 1.49]. An array of metal nanowires can be used as a nanoelectrode array for many electrochemical applications [1.50]. For this purpose, a large array of nanowires with long-range hexagonal order fabricated with the anodic alumina templates is particularly attractive.
1.3.5. Chemical Sensors
Penner, Handley, and Dagani et al. exploited hydrogen sensor applications using arrays of Pd nanowires [1.50,1.52]. Unlike the traditional Pd-based hydrogen sensor that detects a drop in the conductivity of Pd upon exposure to hydrogen, the Pd-nanowire sensor measures an increase in the conductivity. The reason is because the Pd wire consists of a string of Pd particles separated with nanometer-scale gaps. These gaps close to form a conductive path in the presence of hydrogen molecules as Pd particles expand in volume. The volume expansion is well known, which is due to the disassociation of hydrogen molecules into hydrogen atoms that penetrate into the Pd lattice and expand the lattice. Although macroscopic Pd-based hydrogen sensors are
readily available, they have the following two major drawbacks. First, their response time is between 0.5 s to several minutes, which is too slow to monitor gas flow in real time. Second, they are prone to the poisoning of a number of gas molecules, such as methane, oxygen, and carbon monoxide, which adsorb onto the sensor surfaces and block the adsorption sites for hydrogen molecules. The Pd nanowires offer remedies to the above problems. They have a large surface-to-volume ratio, so the response time can be as fast as 20 ms. The large surface-to-volume ratio also make the nanowire sensor less prone to the poisoning by common contaminations.