Nanowire Growth By Pressure Injection

Published Date: 02 Nov 2017

Nanomaterial research has attracted comprehensive attention owing to their potential applications. Among various synthetic processes, porous alumina template synthesis has been one of the most commonly used approaches, because it is easily fabricated and can be used to fabricate many types of nanostructures. Applications have been studied in the areas of plasmonics, nanoelectronics, nanobiotechnology, biology, superconductivity, magnetic sensors based on the giant magneto-resistance effect, ultra high-density magnetic recording media systems, low-voltage field emitter arrays, and thermoelectric devices. Moreover, with the rapid decrease in the size of electronic devices, metallic nanowires can play an important role in the middle scale of 10–200 nm to connect molecular scale devices to the macroscale world, and to connect different elements in nano-electronics. During the past couple of decades, various nanostructure materials have been obtained by different kinds of methods (such as electrochemical template synthesis, electrospinning, and electrochemical deposition etc.)

Nanowires are particularly attractive for nanoscience studies and also for nanotechnology applications. Nanowires have two quantum confined directions as compared to other low dimensional systems, whereas leaving one unconfined direction for electrical conduction. This unconfined direction permits nanowires to be used in applications where electrical conduction is required relatively to tunneling transport. Nanowires are likely to show considerably different optical, electrical and magnetic properties from their bulk 3D crystalline counterparts due to their unique density of electronic states. The augmented surface area, very high density of electronic states and joint density of states near the energies of their van Hove singularities[1], improved exciton binding energy, diameter-dependent band gap, and increased surface scattering for electrons and phonons are just some of the ways in which nanowires differ from their corresponding bulk materials. Yet the sizes of nanowires are typically large enough (> 1nm in the quantum confined direction) to have local crystal structures closely related to their parent materials, thereby allowing theoretical predictions about their properties to be made on the basis of an extensive literature relevant to their bulk properties. Nanowires, not only having numerous similar properties but also the definitely different properties from those of their bulk counterparts, have the benefit from an applications viewpoint. Some of the materials strictures that are critical for certain properties can be freely controlled in nanowires however not in their bulk counterparts, for example, their thermal conductivity. Additionally, many properties can be improved non-linearly in small diameter nanowires by developing the singular features of the 1D electronic density of states. Moreover, nanowires have been exposed to deliver a capable outline for applying the "bottom-up" approach [2] for the intention of nanostructures for nanoscience studies and for potential nanotechnology applications.

1.2 Synthesis

Previous study data present many synthesis methods of simple and multilayered nanowires such as: photochemical synthesis [3], catalytical synthesis [4], vapour-liquid-solid growing [5], electrochemical deposition [6-8]. The preparation of nanowires by electrochemical deposition in nanosized pores is more frequently used because of the low cost and the better energetic efficiency of process. The electro deposition is a preparation method which allows the controlled deposition from solution of metallic materials. Generally, such a solution contains dissolved salts of metals which are going to be deposited. Passing of a current through the electrochemical cell (formed by three electrodes: the reference electrode, the counter electrode and the working electrode) allows the ions migration from the electrochemical bath to working electrode and their deposition in metallic state.

1.3 Template Synthesis

The template-assisted synthesis of nanowires is a conceptually simple and intuitive way to fabricate nanostructures [9]. These templates contain very small cylindrical pores or voids within the host material, and the empty spaces are filled with the chosen material, which adopts the pore morphology, to form nanowires. In template-assisted synthesis of nanostructures, the chemical stability and mechanical properties of the template, as well as the diameter, uniformity and density of the pores are important characteristics to consider. Templates frequently used for nanowire synthesis include anodic alumina (Al2O3), nano-channel glass, ion track-etched polymers and mica films.

1.4 Track-Etched Membrane

The polycarbonate membranes are obtained by the "track-etch" method. This method uses the bombardment with heavy atoms of a nonporous material to create holes. This step is followed by chemical treatment to transform the holes in nanopores. The nanoporous membrane contains cylindrical pores of uniform diameters but which are randomly distributed on its surface. The porous template that is commonly used for nanowire synthesis is the template type fabricated by chemically etching particle tracks originating from ion bombardment [10], such as track-etched polycarbonate membranes [11, 12] and also mica Films[13].

1.5 Anodic Alumina Template

Anodic aluminum Template (AAO) are obtained by anodization of aluminum foils in acids electrolytes containing bivalent or trivalent anions such as: phosphoric acid H3PO4 [14]., sulphuric acid H2SO4 [15], or oxalic acid (COOH)2 [16]. One of the methods proposed in literature, which leads to the preparation of plane and good quality membrane, is the anodization in two steps. This method was proposed for the first time by Masuda and Fukuda [17].

Porous anodic alumina templates are produced by anodizing pure Al films (Templates) in various acids [18, 19]. Under carefully chosen anodization conditions, the resulting oxide film possesses a regular hexagonal array of parallel and nearly cylindrical channels. The self-organization of the pore structure in an anodic alumina template involves two coupled processes: pore formation with uniform diameters and pore ordering. The pores form with uniform diameters because of a delicate balance between electric-field-enhanced diffusion which determines the growth rate of the alumina, and dissolution of the alumina into the acidic electrolyte [20]. The pores are believed to self-order because of mechanical stress at the aluminum-alumina interface due to expansion during the anodization. This stress produces a repulsive force between the pores, causing them to arrange in a hexagonal lattice [15]. Depending on the anodization conditions, the pore diameter can be systematically varied from < 10nm up to 200nm with a pore density in the range of 109 (1011 pores/cm2 [18, 21, 22]. It has been shown by many groups that the pore size distribution and the pore ordering of the anodic alumina templates can be significantly improved by a two-step anodization technique [14, 23], where the aluminum oxide layer is dissolved after the first anodization in an acidic solution followed by a second anodization under the same conditions.

1.6 Other Templates

There are other porous materials that can be used as host templates for nanowire growth, as discussed by [24]. Nano-channel glass (NCG), for example, contains a regular hexagonal array of capillaries similar to the pore structure in anodic alumina with a packing density as high as 3x1010 pores/cm2 [9]. Porous Vycor glass that contains an interconnected network of pores less than 10nm was also employed for the early study of nanostructures [25]. Mesoporous molecular sieves [26], termed MCM-41, possess hexagonally-packed pores with very small channel diameters which can be varied between 2nm and 10nm. Conducting organic filaments have been fabricated in the nanochannels of MCM-41[27]. Recently, the DNA molecule has also been used as a template for growing nanometer-sized wires [28].

Diblock copolymers, which consist of two different polymer chains with different properties, have also been utilized as templates for nanowire growth. When two components are immiscible in each other, phase segregation occurs, and depending on their volume ratio, spheres, cylinders and lamellae may self-assemble. To form self-assembled arrays of nanopores, copolymers composed of polystyrene and polymethylmethacrylate [P(S-b-MMA)] were used [29]. By application of an electric field while the copolymer was heated above the glass transition temperature of the two constituent polymers, the self-assembled cylinders of PMMA could be aligned with their main axis perpendicular to the film. Selective removal of the PMMA component afforded the preparation of 14 nm diameter ordered pore arrays with a packing density of 1.9 x 1011 cm-3.

1.7 Nanowire Growth by Pressure Injection

The pressure injection technique is often employed for fabricating highly crystalline nanowires from a low-melting point material or when using porous templates with robust mechanical strength. In the high-pressure injection method, the nanowires are formed by pressure injecting the desired material in liquid form into the evacuated pores of the template. Due to the heating and the pressurization processes, the templates used for the pressure injection method must be chemically stable and be able to maintain their structural integrity at high temperatures and at high pressures. Anodic aluminum oxide films and nano-channel glass are two typical materials used as templates in conjunction with the pressure injection filling technique. Metal nanowires (Bi, In, Sn, and Al) and semiconductor nanowires (Se, Te, GaSb, and Bi2Te3) have been fabricated in anodic aluminum oxide templates using this method [30, 31].

1.8 Electrochemical Deposition

An attractive synthesis method, electrochemical deposition (ECD) is controllable and inexpensive, and provides great opportunities for the preparation of new materials and nanostructures [32, 33].The electrochemical deposition technique has attracted increasing attention as a promising alternative for fabricating nanowires. Traditionally, electrochemistry has been used to grow thin films on conducting surfaces. Since electrochemical growth is usually controllable in the direction normal to the substrate surface, this method can be readily extended to fabricate 1D or 0D nanostructures, if the deposition is confined within the pores of an appropriate template. In the electrochemical methods, a thin conducting metal film is first coated on one side of the porous membrane to serve as the cathode for electroplating. The length of the deposited nanowires can be controlled by varying the duration of the electroplating process.

In the electrochemical deposition process, the chosen template has to be chemically stable in the electrolyte during the electrolysis process. Cracks and defects in the templates are detrimental to the nanowire growth, since the deposition processes primarily occur in the more accessible cracks, leaving most of the nanopores unfilled. Particle track-etched mica films or polymer membranes are typical templates used in the simple DC electrolysis. To use anodic aluminum oxide films in the DC electrochemical deposition, the insulating barrier layer which separates the pores from the bottom aluminum substrate has to be removed, and a metal film is then evaporated onto the back of the template membrane [11].

It is also possible to employ an AC electrodeposition method in anodic alumina templates without the removal of the barrier layer, by utilizing the rectifying properties of the oxide barrier. In AC electrochemical deposition, although the applied voltage is sinusoidal and symmetric, the current is greater during the cathodic half-cycles, making deposition dominant over the etching, which occurs in the subsequent anodic half-cycles. Since no rectification occurs at defect sites, the deposition and etching rates are equal, and no material is deposited. Hence, the difficulties associated with cracks are avoided.

In the case of materials prepared by the electrochemical method, besides the condition that can be easily used for the process development, the quality of the synthesized material can be better controlled by fine-tuning the electrolyte composition and electrolysis parameters control such as: the applied potential, the current density, electrical charge, temperature and the type of the electrolysis (potentiostatic or galvanostatic).

1.9 Vapor Deposition

Vapor deposition of nanowires includes physical vapor deposition (PVD) [34], chemical vapor deposition (CVD) [35], and metallorganic chemical vapor deposition (MOCVD) [36].

An especially designed experimental setup given by Heremans et al. [34] has been used in the physical vapor deposition technique. The material to be filled is first heated to produce a vapor, which is then introduced through the pores of the template and cooled to solidify.

Compound materials that result from two reacting gases have also been prepared by the chemical vapor deposition (CVD) technique. In this method, the nanochannels are filled with one liquid precursor (e.g., Me3Ga or Et3In) via a capillary effect and the nanowires are formed within the template by reactions between the liquid precursor and the other gas reactant (e.g., AsH3).

1.10 Vapor-Liquid-Solid Method

Semiconductor nanowires those are recently synthesized based on the vapor-liquid-solid (VLS) mechanism of anisotropic crystal growth. The proposed mechanism, for the first time, for the growth of single crystal silicon whiskers 100 nm to 100 microns in diameter contains the absorption of source material from the gas phase into a liquid droplet of catalyst (a molten particle of gold on a silicon substrate in the original work) [37]. A nucleation event generates a solid precipitate of the source material upon super saturation of the liquid alloy. This seed works as a favored site for further deposition of material at the interface of the liquid droplet, supporting the elongation of the seed into a nanowire or a whisker, and suppressing further nucleation events on the same catalyst. Since the liquid droplet catalyzes the incorporation of material from the gas source to the growing crystal, the deposit grows anisotropically as a whisker whose diameter is dictated by the diameter of the liquid alloy droplet.

1.11 Properties and Applications

Nanowires owing to their high density of electronic state, diameter-dependent band gap, enhanced surface scattering of electrons and phonons, increased excitation binding energy, high surface to volume ratio and large aspect ratio, nanowires of metals and semiconductor exhibit unique electrical, magnetic, optical, thermoelectric and chemical properties compared to their bulk parent counterparts. The interesting properties of nanowires hold lot of promises for applications in the fields of electronics, optics, magnetic medium, thermoelectronic, sensor devices etc. [38, 39].

1.11.1 Magnetic Properties

The magnetic nanowires represent a class of nanosized materials in the shape of nanowires intensively studied in the last years is. This family of nanowires is interesting because of their magnetical and transport properties (giant magnetoresistance, reversal magnetization in only one nanowire) being of significant interest due to their potential to work as sensing elements in chemical biological sensors or in optical and electronic devices. The special properties of nanowires can be used in various applications (spintronics, miniaturization of magnetic sensors, ultrahigh-density magnetic storage media, etc.).

The interesting physical properties of magnetic nanowires reside in their geometry and in their dimensionality. The studies presented in the literature on simple magnetic nanowires based on Fe, Co, Cu and Ni show that the magnetic properties of nanowires materials are different from the bulk material. This is especially related to the shape anisotropy [40-42]. The giant magnetoresistance (GMR) studies of magnetic nanowire arrays started in the nineties [43] and is continuing nowadays [44].

Hexagonally arranged Ni-nanowires embedded in anodic alumina templates have been found to exhibit a strong enhancement in their magneto-optical (MO) response [45]. It has been reported [46], that the magnetic nanowires of Fe, Co, and Ni show much enhanced magnetic coercivity than that of their bulk counterpart. It is important to note that the coercivity is strongly influenced by annealing of the wire at different temperatures, the aspect ratio and the wire diameter.

1.12 Thermoelectric Properties

Nanowires are predicted to be promising for thermoelectric applications [47, 48], due to their novel band structure compared to their bulk counterparts and the expected reduction in thermal conductivity associated with enhanced boundary scattering. Metal nanowires exhibit many fold increase in Seebeck coefficient due to their enhanced density of electronic states at the one-dimensional sub-band edges, which is attributed to the quantum confinement effect. It has been reported [49] that in Sb and Si doped Bi nanowires; the thermopower (V/K) can be increased by decreasing the wire diameter. A similar phenomenon has also been observed in case of alumina doped Zn nanowires [49].

1.13 Optical Properties

Optical methods offer a quiet and delicate tool for measuring the electronic structure of nanowires, subsequently optical measurements need nominal sample preparation and the measurements are thoughtful to quantum effects. Optical spectra of 1D system frequently indicate deep features at specific energies near individualities in the joint density of states which are formed under strong quantum confinement circumstances. A wide range of optical techniques and the available characterization of nanowires showed the discrimination of the properties from those of their parent bulk materials. Most of differences in properties emphasis on quantum confinement issues and some narrate to geometric differences such as the aspect ratio.

The optical properties of nanowires have been studied extensively by employing different optical characterization and analytical techniques. By considering the nanowires and the host matrix to act as a single material, the effective medium theories helped in the interpretation of complex dielectric function (ε1 + ε2) of the nanowires which are embedded in the host material [50, 51]. For the composite medium, the refractive index (n) is linked with ε1 and the absorption coefficient (k) is related to ε2. The combination of standard reflection and transmission measurements with Maxwell’s equations directly determine the complex dielectric function of the nanowires[52]. The complex refractive index measurements can be helpful for defining the band gap and temperature variation of band gap of the nanowires, which are considered to be key parameters for the choice of materials for specific photonic applications [53].

1.14 Electrical Properties

For understanding the exclusive one-dimensional carrier transport mechanism, electrical, and electronic applications, the electron transport properties of nanowires are very significant.

The electron transport mechanism of nanowires is affected by wire surface condition, chemical composition, wire diameter, crystal structure and its quality, crystallographic orientation along the wire axis etc.

The various studied showed that quasi one-dimensional nanowires exhibit both ballistic and diffusive type electron transport mechanism, which depends upon the wire length and diameter. Ballistic type transport phenomena is associated with predominant carrier flow without scattering which is due to the fact that the carrier mean free path is longer than that of the wire length. At the contact junction of nanowire and other external circuits ballistic type transport mechanism is usually perceived [54, 55], where the conductance is quantized into an integral multiple of 2e2/h, called the universal conductance unit (e is the electronic charge and h the Plank’s constant) [56, 57].

1.15 Applications

Applications of nanowires could be beneficial in extraordinary ways for use in the miniaturization of conventional devices from both the unique and tunable properties of nanowires and the small size of these nanostructures. 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. Commercialization of nanowire devices, however, will require reliable mass-production, effective assembly techniques and quality-control methods.

In this section, applications of nanowires to electronics, thermoelectrics, Optics, chemo- and bio-sensing, and magnetic media are discussed

1.15.1 Electrical Applications

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 have been devised to organize nanowires via self-assembly. The 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 [58], thus demonstrating that good electronic interfaces between two different semiconductors can be achieved in the synthesis of multicomponent nanowires.

Device functionalities common in conventional semiconductor technologies, such the p-n junction diodes [59], field-effect transistors [60], logic gates [59], and light-emitting diodes [58, 61] , have been recently demonstrated in nanowires, showing their promise as the building blocks toward 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 [62]. 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 [61], or Si nanowires doped by phosphorus (n-type) and boron (p-type) [63]. 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 [58], or perpendicular to the wire axis by forming a core-shell structure of silicon and germanium [64].

Nanowires have also been proposed for applications associated with electron field emission [65], such as flat panel display, because of their small diameter and large curvature at the nanowire tip, which may reduce the threshold voltage for the electron emission [66]. In this connection the demonstration of very high field emission currents from the sharp tip (»10nm radius) of a Si cone [65] and from carbon nanotubes stimulates interest in this potential applications opportunity for nanowires.

1.15.2 Thermoelectric Applications

The enhanced thermopower and manifold increase in the Seebeck coefficient of nanowires make them very attractive for thermoelectric cooling system and energy conversion devices [67]. The application of nanowires to thermoelectrics seems very promising; these materials are still in the research phase of the development cycle and quite far from being commercialized.

1.15.3 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 that is unique to low-dimensional materials, lead to strong optical non-linearities 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.

Uniform morphology and interesting optical properties of nanowires have raised their potential for various optical applications. The n–p junction of nanowires has been found to be capable of light emission, by virtue of their photoluminescence (PL) or electroluminescence (EL) properties. The use of p–n junction nanowires has been contemplated for laser applications. It has been established that ZnO nanowires of wire diameter smaller than the wavelength of emitted light exhibits lasing actions [68] at lower threshold energy compared to their bulk counterpart. This has been attributed to the exciton confinement effect in the laser action, which decreases the threshold lasing energy in nanowires. This effect has been observed in small diameter ZnO (385 nm diameter) and GaN nanowires [68]. The n–p junction nanowires or superlattice nanowires with p–n junctions can also be used as light emitting diodes [69]. The huge surface area and the high conductivity along the length of nanowires are suitable for inorganic–organic solar cell [70]. The solar cell made of CdSe nanowires has high efficiency [71].

Nanowires made of various metal segments like Ag, Au, Ni, Pd etc can be used as barcode tags [72] for different optical read outs.

1.15.4 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, as well as providing the corresponding information on other analytes within the same submicron volume. The development of nanowire based pH sensor [73] and Pb nanowire based hydrogen gas sensor [74] have been reported so far.

1.15.5 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 [29, 75].

The most attractive potential applications of nanowires lie in the magnetic information storage medium. Studies have shown that periodic arrays of magnetic nanowire arrays possess the capability of storing 1012 (bits/in2) of information per square inch of area. The small diameter, single domain nanowires of Ni, Co fabricated into the pores of porous anodic alumina [29, 75] has been found to be most suitable for the above purpose. The high aspect ratio of the nanowires results in enhanced coercivity and suppresses the onset of the "super paramagnetic limit", which is considered to be very important for preventing the loss of magnetically recorded information between the nanowires. Suitable separation between the nanowires is maintained to avoid the inter-wire interaction and magnetic dipolar coupling. It has been found that nanowires can be used to fabricate stable magnetic medium with packing density > 1011 wires/cm2.

Chapter 2 Experimental Procedures

2.1 Preparation of templates

High purity aluminum foils (99.999%) via a two-step anodization procedure [76] was used to create porous anodic alumina oxide (AAO) templates. Prior to anodizing, aluminum templates first degreased in acetone, and then annealed at 500 °C for 5 h in a vacuum of 10-3 Pa to remove the mechanical stress and obtain homogeneous structure over a large area. Anodized templates with η =40 V (DC) in a 0.3 M Oxalic acid solution at 2~5 °C for 6 hours. To remove the alumina layer that formed by first anodization, immerse templates in a mixture of phosphoric acid (6 wt %) and chromic acid (1.8 wt %) for 12 hours. After that, anodize templates again at the same conditions at first anodize but for 12 hours this time. To remove the remaining aluminium on the back side of the templates that experienced above, etch templates by a saturated CuCl2 solution, then, dissolved barrier layer of alumina in a 5 wt % phosphoric acid solution at 40 °C. Finally, in order to deposit silver and copper into alumina membrane, gold was sputtered onto back-side of the alumina to provide conductive contact.

2.2 The electrolyte for preparation of Ag & Cu nanowire

In order to synthesis Ag and Cu nanowires in alumina membrane, electrolyte solutions were prepared from a mixture of AgNO3 (45 g/L) and H3BO3 (45 g/L) in distilled water, and adjusted to a pH 2.2 by adding HNO3 solution to synthesis Ag nanowires, and a mixture of CuSO4 (90 g/L) and H3BO3 (45 g/L) in distilled water, and adjusted to a pH 2.2 by adding H2SO4 to synthesis Cu nanowires at room temperature.

2.3 Experiments of the deposition of metal nanowires

The electroplating was employed in a three-electrode system; the counter electrode was a Pt plate, the reference electrode was Ag/AgCl electrode, and the working electrode was Au-sputtered AAO membrane. Electrochemical synthesis into anodic alumina pore channels was employed to fill porous aluminum oxide (PAO) template.

Ordered nanoporous alumina templates (Fig. 2. SEM image of the Alumina surface) have been fabricated using oxalic acid solutions in order to employ it to growth nanowires. Values of interpore distances obtained by anodizing aluminum in oxalic acid solutions is in the range of 100 to 110 nm and the pore diameters between 45 and 55 nm

2

Fig. 2. SEM image of the Alumina surface

Electroplating of the nanowires was done in a cylindrical. The metallic nanowires (Ag, Cu) were grown inside the pores by electrochemical potentiostatic deposition. The main parameters of the deposition process were the applied potential (E) and the deposition time (which controls the nanowires length).

2.4 Characterization of Nanowires

2.4.1 X-Ray Diffraction

X-Ray Diffraction (XRD) the characterization technique is commonly used to study the crystal structure and chemical composition of nanowires. The peak positions in the x-ray diffraction pattern can be used to determine the crystal phase structure of the nanowires.

X-rays are electromagnetic radiation similar to light, but with a much shorter wavelength. They are produced when electrically charged particles of sufficient energy are decelerated. In an X-ray tube, the high voltage maintained across the electrodes draws electrons toward a metal target (the anode). X-rays are produced at the point of impact, and radiate in all directions. Tubes with copper targets, which produce their strongest characteristic radiation (K 1) at a wavelength of about 1.5 angstroms, are commonly used for geological applications.

Max von Laue, in 1912, discovered that crystalline substances act as three-dimensional diffraction gratings for X-ray wavelengths similar to the spacing of planes in a crystal lattice. X-ray diffraction is now a common technique for the study of crystal structures and atomic spacing.

X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample. These X-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the sample. The interaction of the incident rays with the sample produces constructive interference (and a diffracted ray) when conditions satisfy Bragg's Law

This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays are then detected, processed and counted. By changing the geometry of the incident rays, the orientation of the centered crystal and the detector, all possible diffraction directions of the lattice should be attained.

Figure .2 SCEMATIC CROSS SECTION OF AN X-RAY TUBE

Fig. Schematic Cross section of an x-ray tube [77]

Structural characterization of the copper and silver nanowires embedded in alumina membranes was examined by X-ray diffraction (XRD).

2.4.2 Scanning Electron Microscopy (SEM)

SEM usually produces images down to length scales of >10nm and provides valuable information regarding the structural arrangement, spatial distribution, wire density, and geometrical features of the nanowires.

The concept of a Scanning Electron Microscope was first described by Knoll in 1935. Basically a SEM is built-up of a column on a sample chamber. At the top of the column, electrons are generated. These electrons are focused on the sample by means of condensers and coils. The electron beam is scanned over the sample. Around the sample, detectors sense the different signals generated by the electron beam. The signals detected are used to generate an image on the computer screen. What basically happens is that the electron beam scans an array of pixels. Every pixel is filled with a grey-value from one of the detectors. The most important signals that are detected are Secondary Electrons (SE) (relatively slow electrons that lost part of their energy in the sample), Backscattered Electrons (BSE) (electrons that retained most of their energy) and X-rays. SE gives topographic contrast, which is topographic information about the surface of the sample. BSE gives compositional contrast, which is information about the composition of the sample: heavier elements appear brighter on the screen.

In order to image and characterise the copper and silver nanowires by scanning electron microscopy (SEM), nanowires were produced by dissolving the alumina membrane in 5 wt% NaOH solution at 25 â—¦C for 30 min

2.4.2.1 Scanning Electron Microscopy (SEM) Instrumentation –

Essential components of all SEMs (Fig. 2.) include the following:

• Electron Source ("Gun").

• Electron Lenses.

• Sample Stage.

• Detectors for all signals of interest.

• Display / Data output devices.

• Infrastructure Requirements.

• Power Supply.

• Vacuum System.

• Cooling system.

• Vibration-free floor.

• Room free of ambient magnetic and electric fields.

Fig. 2. Scanning Electron Microscopy (SEM)

SEMs always have at least one detector (usually a secondary electron detector), and most have additional detectors. The specific capabilities of a particular instrument are critically dependent on which detectors it accommodates[77].