Types of Reflection:

1. If the surface is smooth and shiny, like glass, water or polished metal, the light will reflect at the same angle (i.e., reflected light rays travel in the same direction) as it hits the surface. This is called specular reflection (Figure 7.1).
2. For a rough surface, reflected light rays scatter in all directions. This is called diffuse reflection. With a flat mirror, it is easy to show that the angle of reflection is the same as the angle of incidence. It is possible to make mirrors that behave like humps or troughs, i.e., “concave” or “convex,” and because of the different way they reflect light, they can be very useful. A rough surface has concave and convex surfaces.

Concave mirrors: When parallel light rays hit a concave mirror, they reflect inwards towards a focal point (F). Each individual ray is still reflecting at the same angle as it hits that small part of the surface (Figure 7.2). Concave mirrors are used in certain types of astronomical telescopes called reflecting telescopes. The mirrors condense lots of light from faint sources in space onto a much smaller viewing area and allow the viewer to see far away objects and events in space that would otherwise be invisible to the naked eye. Light rays travel towards the mirror in a straight line and are reflected inwards to meet at a point called the focal point. Concave mirrors are useful for make-up mirrors because they can make things seem larger. This concave shape is also useful for car headlights and satellite dishes.

Convex mirrors: Convex mirrors curve outwards. When parallel light rays hit a convex mirror they reflect outwards and travel directly away from an imaginary focal point (F). Each individual ray is still reflecting at the same angle as it hits that small part of the surface (Figure 7.3). If imaginary lines are traced back, they appear to come from a focal point behind the mirror. Convex mirrors are useful for shop security and rear-view mirrors on vehicles because they give a wider field of vision.

Scattering of Light: Some light is scattered in all directions when it hits either very small particles, such as gas molecules, or much larger particles such as dust or droplets of water. The amount of scattering depends on how big the particle is compared to the wavelength of light that is hitting it. Smaller wavelengths are scattered more. This scattering answers questions such as:

• “Why is the sky blue?” Light from the sun is made up of all the colors of the rainbow. As this light hits the particles of nitrogen and oxygen in our atmosphere, it is scattered in all directions. Blue light has a smaller wavelength than red light, so it is scattered much more than red light. When we look at the sky, we see all the places that the blue light has been scattered from.
• This is similar to the question, “Why are sunsets red?” When the Sun appears lower in the sky, the light that reaches us has already travelled through a lot more of the atmosphere. This means that a lot of the blue light has been scattered in all directions well before the light reaches us, so the sky appears redder.
• “Why do clouds appear white?” It is because the water droplets are much larger than the wavelengths of light. In this situation, all wavelengths of light are equally scattered in all directions.

7.7.1.1 Mechanical

Mechanical milling (MM) of a suitable powder in a high-energy mill, along with a suitable milling medium to reduce the particle size and blending of particles in new phases is the basic concept of MM. Different types of ball milling are used for synthesis of nanomaterials in which fine surface balls impact the powder charge [41]. The balls either roll down the surface of the chamber in a series of parallel layers or they are allowed to fall freely and impact the powder and balls beneath them. For large-scale production of nano-size grain, mechanical milling is a more economical process [42]. The kinetics of mechanical milling or alloying depends on the energy transferred to the powder from the balls during milling [43]. The energy transfer is governed by many parameters such as the type of mill, the powder supplied to drive the milling chamber, milling speed, size and size distribution of the balls, dry or wet milling, temperature of milling and the duration of milling [44].

Mechanical attrition produces its nanostructures by the structural decomposition of coarser grained structures as a result of plastic deformation. By this process elemental powders of Al and β-SiC ceramic/ceramic nanocomposites are fabricated. The ball-milling and rod-milling techniques belong to the mechanical alloying process, which has received much attention as a powerful tool for the fabrication of several advanced materials. Mechanical alloying is a unique process, which can be carried out at room temperature. The process can be performed on high-energy mills, centrifugal-type mill and vibratory-type mill, and low-energy tumbling mill [45–47]. However, the principles of these operations are the same for all the techniques. High-energy mills include:

Attrition Ball Milling: The milling procedure takes place by the stirring action of an agitator, which has a vertical rotator central shaft with horizontal arms (impellers). The rotation speed is later increased to 500 rpm. Also, the milling temperature has greater control.

Planetary Ball Milling: Centrifugal forces are caused by rotation of the supporting disc and autonomous turning of the vial (Figure 7.7). The milling media and charge powder alternatively roll on the inner wall of the vial and are thrown off across the bowl at high speed (360 rpm).

Vibrating Ball Milling: It is used mainly for production of amorphous alloys. The changes of powder and milling tools are agitated in the perpendicular direction at very high speed (1200 rpm).

Low-Energy Tumbling Milling: The tumbler ball mill, shown in Figure 7.8, is a cylindrical container rotated about its axis in which balls impact upon the powder charge. The balls may roll down the surface of the chamber in a series of parallel layers or they may fall freely and impact the powder and balls beneath them. A tumbler ball mill is operated close to the critical speed beyond which the balls are pinned to the inner walls of the mill because of the centrifugal force dominating over centripetal force. For large-scale production, tumbler mills are more economical when compared to the other high-energy ball mills [48]. While a number of ingenious milling devices were developed early in the century, the one high-energy ball mill that has been adopted by industry was invented by Andrew Szegvari in 1922 in order to quickly attain fine sulfur dispersion for use in vulcanization of rubber. This mill is called an attritor or attrition mill and is illustrated in Figure 7.9b.

Low-energy tumbling mills have been used for successful preparation of mechanically alloyed powder. They are simple to operate with low operation costs. A laboratory-scale rod mill was used to prepare homogeneous amorphous Al30Ta70 powder by using stainless steel cylinder rods. Single-phase amorphous powder of AlxTm100-x with low iron concentration can be formed by this technique.

High-Energy Ball Milling: In the ball milling process, a powder mixture placed in the ball mill is subjected to high-energy collision from the balls. The synthesis of materials by high-energy ball milling of powders was first developed by John Benjamin [49] and his coworkers at the International Nickel Company in the late 1960s. The goal of this work was the production of complex oxide dispersion-strengthened (ODS) alloys for high temperature structural applications. It was found that this method, termed mechanical alloying, could successfully produce fine, uniform dispersions of oxide particles (Al₂O₃, Y₂O₃, ThO₂) in nickel-base superalloys, which could not be made by more conventional powder metallurgy methods. Benjamin and his coworkers at Paul D. Merica Research Laboratory in partnership with INCA (Inventors Network of the Capital Area) also explored the synthesis of other kinds of materials, e.g., solid solution alloys and immiscible systems, and pointed out [50] that mechanical alloying (MA), in addition to synthesis of dispersion-strengthened alloys, could make metal composites, compounds, and/or new materials with unique properties [50]. However, one of the major applications of MA so far has been the production of oxide-dispersion-strengthened (ODS) materials [51].

Their innovation has changed the traditional method in which production of materials is carried out by high temperature synthesis. Besides synthesis of materials, high-energy ball milling is a way of modifying the conditions in which chemical reactions usually take place either by changing the reactivity of as-milled solids (mechanical activation increasing reaction rates, lowering reaction temperature of the ground powders) or by inducing chemical reactions during milling (mechanochemistry). It is, furthermore, a way of inducing phase transformations in starting powders whose particles have all the same chemical composition: amorphization or polymorphic transformations of compounds, disordering of ordered alloys, etc.

High-energy ball milling is an already established technology but has been considered dirty because of contamination problems with iron. However, the use of tungsten carbide component and inert atmosphere and/or high vacuum processes has reduced impurity levels to within acceptable limits. Common drawbacks include low surface, highly polydisperse size distribution, and partially amorphous state of the powder. These powders are highly reactive with oxygen, hydrogen and nitrogen. Mechanical alloying leads to the fabrication of alloys, which cannot be produced by conventional techniques. It would not be possible to produce an alloy of Al-Ta because of the difference in melting points of Al (933 K) and Ta (3293 K) by any conventional process. However, it can be fabricated by mechanical alloying using the ball milling process.

7.7.1.2 Melt Mixing

Polymer melting and mixing involves various types of mixing elements or components used for the homogeneous mixing of the polymer melt inside the extruder. The most common dry mixing methods are volumetric mixing and gravimetric mixing. It is possible to form or arrest nanoparticles in glass, which is structurally an amorphous solid, lacking the symmetric arrangement of atoms/molecules. Metals, when cooled at very high cooling rates (10⁵–10⁶ K/s), can form amorphous solids-metallic glasses. Mixing molten streams of metals at high velocity with turbulence forms nanoparticles, e.g., a molten stream of Cu-B and molten stream of Ti form nanoparticles of TiB₂.

7.7.1.3 Hydrothermal and Solvothermal Synthesis

In hydrothermal synthesis the reaction is typically carried out in a pressurized vessel called an autoclave in aqueous solution. The temperature in the autoclave can be raised above the boiling point of water, reaching the pressure of vapor saturation. Hydrothermal synthesis is widely used for the preparation of TiO₂ nanoparticles, which can easily be obtained through hydrothermal treatment of peptized precipitates of a titanium precursor with water [52]. The hydrothermal method can be useful to control grain size, particle morphology, crystalline phase and surface chemistry through regulation of the solution composition, reaction temperature, pressure, solvent properties, additives and aging time [53].

The solvothermal method is identical to the hydrothermal method except that a variety of solvents other than water can be used for this process. This method has been found to be a versatile route for the synthesis of a wide variety of nanoparticles with narrow size distributions, particularly when organic solvents with high boiling points are chosen. The solvothermal method normally has better control of the size and shape distributions and the crystallinity than the hydrothermal method, and has been employed to synthesize TiO₂ nanoparticles and nanorods with/without the aid of surfactants.

7.7.1.4 Templating

The synthesis of nanostructure materials using the template method has become extremely popular during the last decade. In order to construct materials with a similar morphology of known characterized materials (templates), this method utilizes the morphological properties with reactive deposition or dissolution. Therefore, it is possible to prepare numerous new materials with a regular and controlled morphology on the nano- and microscale by simply adjusting the morphology of the template material. A variety of templates have been studied for synthesizing titania nanomaterials [54, 55]. This method has some disadvantages, including the complicated synthetic procedures and, in most cases, templates need to be removed, normally by calcinations, leading to an increase in the cost of the materials and the possibility of contamination [56].

7.7.1.4.1 Inverse Micelles as Nanotemplate

Reverse micelles as nano-sized aqueous droplets existing at certain compositions of water-in-oil microemulsions (the term microemulsion was coined by Schulman et al., in 1959 [57]) are widely used today in the synthesis of many types of nanoparticles. Reverse micelles as nanoscale hydrophilic cavities of microemulsions have been known since the 1960s, but these diverse multimolecular structures were used for the first time as nanotemplates for materials synthesis (of monodispersed Pt, Pd, Rh and Ir particles) in 1982 [58]. After these pioneering studies, many different materials comprising de-agglomerated and monodispersed particles have been prepared [59–62] by using reverse micelles. Microemulsions are transparent, thermodynamically stable dispersions of two immiscible liquids containing appropriate amounts of surfactant. The success of the reverse micellar procedure for materials synthesis is closely related to the fact that particle nucleation can be initiated simultaneously at a large number of locations within reverse micelles, with the nucleation sites well isolated from each other due to the presence of surfactant films that may act as stabilizers of the formed particles. Normally, monodisperse particles are formed only when the nucleation and growth stages are strictly separated, which is a property of reverse micelle material synthesis due to the uniform nanodroplet structure and specific intermicellar interactions. Reverse micelle phases are tiny droplets of water encapsulated by surfactant molecules and thus physically separated from the oil phase (Figure 7.10). Simplified representation of the reverse micellar preparation of particles takes that aqueous “pools” of the reverse micelles act as nanoreactors for performing simple reactions of synthesis, and that the sizes of the microcrystals of the product are directly determined by the sizes of these pools [65–67]. It is possible to control the sizes of reverse micelles by controlling the parameter w, defined as molar ratio of water-to-surfactant. The higher the w, the larger the water pools of the micelles and the nanoparticles formed within, and vice versa.

7.7.1.5 Electron Beam Lithography

Lithography is an old technique used for ink imprinting that dates back to 17th century. Nowadays, the techniques and applications of lithography have been diversified, but lithography is the process of transferring a pattern from one media to another. Base on the same concept, electron beam lithography was developed in the late 60s and consists of electron irradiation of a surface covered with a resist sensitive to electrons by means of a focused electron beam. The energetic absorption in specific places causes the intramolecular phenomena that define the features in the polymeric layer. This lithographic process, capable of creating submicronic structures, consists of three steps: exposure of the sensitive material, development of the resist and pattern transfer. It is important to consider that these should not be realized independently, and that the final resolution is conditioned for the accumulative effect of each individual step of the process. A great number of parameters, conditions and factors within the different subsystems are involved in the process and contribute to the electron beam lithography (EBL) operation and result. In a direct write EBL system, the designs are directly defined by scanning the energetic electron beam, and then the sensitive material is physically or chemically modified due to the energy deposited from the electron beam. This material is called the resist, since, later on, it resists the process of transference to the substrate. The energy deposited during the exposure creates a latent image that is materialized during chemical development. For positive resists, the development eliminates the patterned area, whereas for negative resists, the inverse occurs. As a consequence, the shape and characteristics of the electron beam, the energy and intensity of electrons, the molecular structure and thickness of the resist, the electron–solid interactions, the chemistry of the developer in the resist, the conditions for development and the irradiation process, from the structure design to the beam deflection and control, are determinant for the results, in terms of dimensions, resist profile, edge roughness, feature definition, etc.

7.7.1.6 Vapor Phase Synthesis

Physical vapor deposition (PVD) is a collective set of processes used to deposit thin layers of material (Figure 7.11), typically in the range of a few nanometers to several micrometers [68]. PVD involves vacuum deposition techniques consisting of three fundamental steps:

• Vaporization of the material from a solid source assisted by high temperature vacuum or gaseous plasma.
• Transportation of the vapor in vacuum or partial vacuum to the substrate surface.
• Condensation onto the substrate to generate thin films.

Different PVD technologies utilize the same three fundamental steps but differ in the methods used to generate and deposit material. The two most common PVD processes are thermal evaporation and sputtering. Thermal evaporation is a deposition technique that relies on vaporization of source material by heating the material using appropriate methods in a vacuum. Sputtering is a plasma-assisted technique that creates a vapor from the source target through bombardment with accelerated gaseous ions (typically argon). In both evaporation and sputtering, the resulting vapor phase is subsequently deposited onto the desired substrate through a condensation mechanism [69].

Deposited films can span a range of chemical compositions based on the source material(s). Further compositions are accessible through reactive deposition processes. Relevant examples include co-deposition from multiple sources, reaction during the transportation stage by introducing a reactive gas (nitrogen, oxygen or simple hydrocarbon containing the desired reactant), and post-deposition modification through thermal or mechanical processing [70]. PVD is used in a variety of applications, including fabrication of microelectronic devices, interconnects, battery and fuel cell electrodes, diffusion barriers, optical and conductive coatings, and surface modifications [71–73].

7.7.1.7 Gas Phase Methods

Gas phase methods are ideal for the production of thin films. Gas phase can be carried out chemically or physically. Chemical vapor deposition (CVD) is a widely used industrial technique that can coat large areas in a short space of time. During the procedure, titanium dioxide is formed from a chemical reaction or decomposition of a precursor in the gas phase [39, 74]. Physical vapor deposition (PVD) is another thin film deposition technique. Films are formed from the gas phase but without a chemical transition from precursor to product. For TiO₂ thin films, a focused beam of electrons heats the titanium dioxide material. The electrons are produced from a tungsten wire heated by a current. This is known as electron beam (E-beam) evaporation. Titanium dioxide films deposited with E-beam evaporation have superior characteristics over CVD grown films such as smoothness, conductivity, presence of contaminations and crystallinity. Reduced TiO₂ powder (heated at 900 °C in a hydrogen atmosphere) is necessary for the required conductance needed to focus an electron beam on the TiO₂ [75].

In 1993, the CVD technique was first reported to produce multi-walled carbon nanotubes (MWNTs) by Endo et al., [76]. Three years later, Dai in Smalley’s group successfully adapted CO-based CVD to produce single-walled carbon nanotubes (SWNTs) [77]. CVD technique can be achieved by taking a carbon source in the gas phase and using an energy source, such as plasma or a resistively heated coil, to transfer energy to a gaseous carbon molecule. The CVD process uses hydrocarbons as the carbon sources, including methane, carbon monoxide and acetylene. The hydrocarbons flow through the quartz tube in an oven at a high temperature (~720 °C) (Figure 7.12). At high temperature, the hydrocarbons are broken down into the hydrogen–carbon bond, producing pure carbon molecules. Then, the carbon will diffuse toward the substrate, which is heated and coated with a catalyst (usually a first row transition metal such as Ni, Fe or Co) where it will bind. Carbon nanotubes will be formed if the proper parameters are maintained. The advantages of the CVD process are low power input, lower temperature range, relatively high purity and, most importantly, the possibility of scaling up the process. This method can produce both MWNTs and SWNTs depending on the temperature, in which production of SWNTs will occur at a higher temperature than MWNTs.

This process is often used in the semiconductor industry to produce high-purity, high-performance thin films. In a typical CVD process, the substrate is exposed to volatile precursors, which react and/or decompose on the substrate surface to produce the desired film. Frequently, volatile by-products that are produced are removed by gas flow through the reaction chamber. The quality of the deposited materials strongly depends on the reaction temperature, the reaction rate, and the concentration of the precursors [78]. Cao et al., prepared Sn⁴⁺-doped TiO₂ nanoparticle films by the CVD method and found that more surface defects were present on the surface due to doping with Sn [79]. Gracia et al., synthesized M (Cr, V, Fe, Co)-doped TiO₂ by CVD and found that TiO₂ crystallized into the anatase or rutile structures depending on the type and amount of cation present in the synthesis process. Moreover, upon annealing, partial segregation of the cations in the form of M₂O_n was observed [80]. The advantages of this method include uniform coating of the nanoparticles or nanofilm. However, this process has limitations, including the higher temperatures required, and it is difficult to scale up.

7.7.1.8 Thermal Decomposition and Combustion

Pure and doped metal nanomaterials can be synthesized via thermally decomposing metal alkoxides and salts by applying high energy using heat or electricity. However, the properties of the produced nanomaterials strongly depend on the precursor concentrations, the flow rate of the precursors and the environment. Kim et al., synthesized TiO₂ nanoparticles with a diameter less than 30 nm via the thermal decomposition of titanium alkoxide or TiCl₄ at 1200 °C [81]. Liang et al., produced TiO₂ nanoparticles with a diameter ranging from 3 to 8 nm by pulsed laser ablation of a titanium target immersed in an aqueous solution of surfactant or deionized water [82]. Nagaveni et al., prepared W, V, Ce, Zr, Fe, and Cu ion-doped anatase TiO₂ nanoparticles by a solution combustion method and found that the solid solution formation was limited to a narrow range of concentrations of the dopant ions [38]. However, the drawbacks of these methods are high cost and low yield, and difficulty in controlling the morphology of the synthesized nanomaterials.

Combustion synthesis leads to highly crystalline particles with large surface areas [38]. The process involves a rapid heating of a solution containing redox groups. During combustion, the temperature reaches approximately 650 °C for one or two minutes, making the material crystalline. Since the time is so short, the transition from anatase to rutile is inhibited.

7.7.1.9 Sputtering

The ejection of atoms from the surface of a material (the target) by bombardment with energetic particles is called sputtering [83]. Sputtering is a momentum transfer process in which atoms from a cathode/target are driven off by bombarding ions. Sputtered atoms travel until they strike a substrate, where they deposit to form the desired layer (Figure 7.13). Sputter deposition is a widely used technique to deposit thin films on substrates. The technique is based on ion bombardment of a source material, the target. Ion bombardment results in a vapor due to a purely physical process, i.e., the sputtering of the target material. Hence, this technique is part of the class of physical vapor deposition techniques, which includes, for example, thermal evaporation and pulsed laser deposition. The most common approach for growing thin films by sputter deposition is the use of a magnetron source in which positive ions present in the plasma of a magnetically enhanced glow discharge bombard the target.

The sputtering process is classified as DC (direct current) or RF (radio frequency) depending on the type of power supply used. DC sputtering is mainly used to deposit metals. In the case of insulators after the ions strike the surface, their charge will remain localized and with passage of time positive charge will build up on the target, making it unfeasible to further bombard the surface. This can be prevented by bombarding the insulator by both positive ions and electrons simultaneously [84]. That is done by applying a RF potential to the target. The RF potential provides sufficient energy to the electrons oscillating in the alternating field to cause ionizing collisions, and a self-sustained discharge is maintained. As electrons have higher mobility compared to ions, more electrons will reach the insulating target surface during the positive half cycle than the positive ions during the negative half cycle. Hence, the target will be self-biased negatively. This repels the electrons from the vicinity of the target and forms a sheath enriched in positive ions in front of the target surface. These ions bombard the target and sputtering is achieved.

The simplest source of ions for sputtering is provided by the well-known phenomenon of glow discharge due to an applied electric field between two electrodes in a gas at low pressure. The gas breaks down to conduct electricity when a certain minimum voltage is reached. Such an ionized gas is called plasma. Ions of the plasma are accelerated at the target by a large electric field. When the ions impact the target, atoms (or molecules) are ejected from the surface of the target into the plasma, where they are carried away and then deposited on the substrate. This type of sputtering is called “DC sputtering” (Figure 7.14).

In order to avoid any chemical reaction between the sputtered atoms and the sputtering gas, the sputtering gas is usually an inert gas such as argon. However, in some applications, such as the deposition of oxides and nitrides, a reactive gas is purposely added to argon so that the deposited film is a chemical compound. This type of sputtering is called “reactive sputtering.” When the plasma ions strike the target, their electrical charge is neutralized and they return to the process as atoms. If the target is an insulator, the neutralization process results in a positive charge on the target surface. This charge may reach a level where bombarding ions are repelled and the sputtering process stops. To continue the process, the polarity must be reversed to attract enough electrons from the plasma to eliminate surface charge. This periodic reversal of polarity is done automatically by applying a radio-frequency (RF) voltage on the target assembly. Thus, this type of sputtering is known as “RF sputtering,” (Figure 7.15).

In order to increase the efficiency of the sputtering process, it is common for the sputtering source to have some magnetic confinement through a magnetron source. The effect of the magnetic field is to spiral the electrons so that they have a better chance of undergoing an ionizing collision, thus enabling the plasma to be operated at a higher density. This type of sputtering is called “magnetron sputtering” and it can be used with DC or RF sputtering.

Materials that can be sputtered include elements (such as pure metals and elemental semiconductors), alloys, and compounds (such as oxides, nitrides, sulfides, and carbides). Sputtering allows for the deposition of films having the same composition as the target source. This is the primary reason for the widespread use of sputtering as a thin-film deposition technique. Compared to other deposition techniques, sputtering gives more uniform and reproducible results.

7.7.1.10 Arc Discharge

Arc discharge was the first recognized technique for producing MWNTs and SWNTs. The arc discharge technique generally involves the use of two high-purity graphite electrodes as the anode and the cathode. The electrodes are vaporized by the passage of a DC current (~100 A) through two high-purity graphite electrodes separated (~1–2 mm) in 400 mbar of helium atmosphere. The experimental set up of the arc discharge apparatus is shown in Figure 7.16. After arc discharging for a period of time, a carbon rod is built up at the cathode. This method can mostly produce MWNTs but can also produce SWNTs with the addition of metal catalyst such as Fe, Co, Ni, Y or Mo, on either the anode or the cathode. The quantity and quality, such as lengths, diameters, purity, etc., of the nanotubes obtained depend on various parameters such as the metal concentration, inert gas pressure, type of gas, plasma arc, temperature, the current and system geometry.

Electric arc deposition: Cathodic arc deposition or arc-PVD is a physical vapor deposition technique in which an electric arc is used to vaporize material from a cathode target. The vaporized material then condenses on a substrate, forming a thin film (Figure 7.17). The technique can be used to deposit metallic, ceramic, and composite films. Industrial use of modern cathodic arc deposition technology originated in the Soviet Union around 1960–1970. By the late 70 s the Soviet government released the use of this technology to the West. Among the many designs in the USSR at that time, the design by L. P. Sablev et al., was allowed to be used outside the USSR. The arc evaporation process begins with the striking of a high current, low voltage arc on the surface of a cathode (known as the target) that gives rise to a small (usually a few micrometers wide), highly energetic emitting area known as a cathode spot. The localized temperature at the cathode spot is extremely high (around 15000 °C), which results in a high velocity (10 km/s) jet of vaporized cathode material, leaving a crater behind on the cathode surface. The cathode spot is only active for a short period of time, then it self-extinguishes and reignites in a new area close to the previous crater. This behavior causes the apparent motion of the arc.

7.7.1.11 Laser Ablation and Pulsed Laser Ablation

In 1995, Smalley and coworkers produced carbon nanotubes using laser ablation technique [85]. In the laser ablation technique, a high-power laser was used to vaporize carbon from a graphite target at high temperature. Both MWNTs and SWNTs can be produced with this technique. In order to generate SWNTs, metal particles must be added as catalysts to the graphite targets similar to the arc discharge technique. The quantity and quality of the carbon nanotubes produced depend on several factors such as the amount and type of catalysts, laser power and wavelength, temperature, pressure, type of inert gas, and the fluid dynamics near the carbon target (Figure 7.18). The laser is focused on carbon targets containing 1.2% of cobalt/nickel with 98.8% of graphite composite that is placed in a 1200 °C quartz tube furnace under argon atmosphere (~500 Torr). These conditions were achieved for production of SWNTs in 1996 by Smalley’s group [86]. In such a technique, argon gas carries the vapors from the high temperature chamber into a cooled collector positioned downstream. The nanotubes will self-assemble from carbon vapors and condense on the walls of the flow tube. The diameter distribution of SWNTs from this method varies about 1.0–1.6 nm. Carbon nanotubes produced by laser ablation were purer (up to 90% purity) than those produced in the arc discharge process and have a very narrow distribution of diameters.

7.7.1.12 Ion Implantation

Ion implantation is an alternative to deposition diffusion and is used to produce a shallow surface region of dopant atoms deposited into a silicon wafer. This technology has made significant inroads into diffusion technology in several areas. In this process a beam of impurity ions is accelerated to kinetic energies in the range of several tens of kV and is directed to the surface of the silicon. As the impurity atoms enter the crystal, they give up their energy to the lattice in collisions and finally come to rest at some average penetration depth, called the projected range, expressed in micrometers. Depending on the impurity and its implantation energy, the range in a given semiconductor may vary from a few hundred angstroms to about one micrometer. Typical distribution of impurity along the projected range is approximately Gaussian. By performing several implantations at different energies, it is possible to synthesize a desired impurity distribution, e.g., a uniformly doped region.

A gas containing the desired impurity is ionized within the ion source. The ions are generated and repelled from their source in a diverging beam that is focused before it passes through a mass separator that directs only the ions of the desired species through a narrow aperture. A second lens focuses this resolved beam, which then passes through an accelerator that brings the ions to their required energy before they strike the target and become implanted in the exposed areas of the silicon wafers. The accelerating voltages may be from 20 kV to as much as 250 kV. In some ion implanters, the mass separation occurs after the ions are accelerated to high energy. Because the ion beam is small, means are provided for scanning it uniformly across the wafers. For this purpose, the focused ion beam is scanned electrostatically over the surface of the wafer in the target chamber (Figure 7.19). Repetitive scanning in a raster pattern provides exceptionally uniform doping of the wafer surface. The target chamber commonly includes automatic wafer handling facilities to speed up the process of implanting many wafers per hour.

7.7.1.13 Synthesis of Nanoporous Polymers Using Membranes

Most of the work in template synthesis has entailed the use of two types of nanoporous membranes: track-etch polymeric membranes and porous alumina membranes.

7.7.2.1 Colloidal Methods

Colloidal methods are simple wet chemistry precipitation processes in which solutions of the different ions are mixed under controlled temperature and pressure to form insoluble precipitates. For metal nanoparticles the basic principles of colloidal preparation have been known since antiquity; e.g., gold colloids used for high quality red and purple stained glass from medieval times to date. However, proper scientific investigations of colloidal preparation methods started only in 1857, when Faraday published the results of his experiments with gold. He prepared gold colloids by reduction of HAuCl₄ with phosphorus. Today, colloidal processes are widely used to produce such nanomaterials like metals, metal oxides, organics, and pharmaceuticals.

7.7.2.2 Conventional Sol-Gel Method

The sol-gel method is a versatile process used for synthesizing various oxide materials [94]. This synthetic method generally allows control of the textural, chemical, and morphological properties of the solid. This method also has several advantages over other methods, such as allowing impregnation or co-precipitation, which can be used to introduce dopants. The major advantages of the sol-gel technique include molecular scale mixing, high purity of the precursors, and homogeneity of the sol-gel products with a high purity of physical, morphological, and chemical properties [95]. In a typical sol-gel process, a colloidal suspension, or a sol, is formed from the hydrolysis and polymerization reactions of the precursors, which are usually inorganic metal salts or metal organic compounds such as metal alkoxides [96]. A general flowchart for a complete sol-gel process is shown in Figure 7.20. Any factor that affects either or both of these reactions is likely to impact the properties of the gel. These factors, generally referred to as sol-gel parameters, include type of precursor, type of solvent, water content, acid or base content, precursor concentration, and temperature. These parameters affect the structure of the initial gel and, in turn, the properties of the material at all subsequent processing steps. After gelation, the wet gel can be optionally aged in its mother liquor, or in another solvent, and washed. The time between the formation of a gel and its drying, known as aging, is also an important parameter.

7.7.2.3 LB Technique

The Langmuir-Blodgett (LB) technique is a room temperature deposition process that may be used to deposit monolayer and multilayer films of organic materials. Furthermore, this method permits the manipulation of organic molecules on the nanometer scale, thereby allowing intriguing superlattice architectures to be assembled [97]. Advances to the discovery of Langmuir-Blodgett films began with Benjamin Franklin in 1773, when he dropped about a teaspoon of oil onto a pond. Franklin noticed that the waves were calmed almost instantly and that the calming of the waves spread for about half an acre. What Franklin did not realize was that the oil had formed a monolayer on top of the pond surface. Over a century later, Lord Rayleigh quantified what Benjamin Franklin had seen. Knowing that the oil, oleic acid, had spread evenly over the water, Rayleigh calculated that the thickness of the film was 1.6 nm by knowing the volume of oil dropped and the area of coverage.

7.7.2.4 Microemulsion-Based Methods

Ultrafine metal nanoparticles of diameter between 5 nm and 50 nm can be prepared by water-in-oil microemulsions. The nanodroplets of water are dispersed in the oil phase (Figure 7.21). The size of the droplets can be varied in the range of 5–50 nm by changing the water/surfactant ratio. The surfactant molecules provide the sites for particle nucleation and stabilize the growing particles. Therefore, the microemulsion acts as a microreactor.

7.7.3.1 Nanometal Synthesis Using Microorganisms

By now it is an open secret that microbes take up metal in ionic form and accumulate it within the cell without affecting its regular metabolic activities. This phenomenon is termed “resistance for metal” by microorganisms. But at some stage of the life cycle, if the metal accumulation is uncontrollable by the microorganisms, then the cell produces toxicity, finally leading to the death of the microbe and to the advantage of those who are interested in nanometal synthesis. These microbes die or burst open, releasing nanometals.

Microorganisms have been shown to be important nanofactories that hold immense potential as eco-friendly and cost-effective tools, avoiding toxic, harsh chemicals and the high energy demand required for physiochemical synthesis. Microorganisms have the ability to accumulate and detoxify heavy metals due to various reductase enzymes, which are able to reduce metal salts to metal nanoparticles with a narrow size distribution and, therefore, less polydispersity.

7.7.3.2 Nanometal Synthesis Using Fungi and Actinomycetes

Fungi and actinomycetes have also been stated to be advantageous in the synthesis of nanoparticles [144], which can be scaled up easily, is economically viable, and has the possibility of easily covering large surface areas by suitable growth of the mycelia. This transition from bacteria to fungi as a means of developing natural nanofactories has the added advantage that downstream processing and handling of the biomass would be simpler. To get filtrates from colloidal bacterial broth, sophisticated equipment is a must, whereas filtrates from fungal broths can be obtained by using simple equipment. Fungi have been found to be efficient secretors of soluble proteins and their mutant strains can secrete up to 30 g/l of extracellular protein. It is this character of high level protein secretion that has made fungi a favorite host of heterologous expression of high-value mammalian protein for manufacturing by fermentation.

The ability of fungi and actinomycetes as nanofactories was explored later than bacteria. However, they were found to be more efficient than bacteria. Almost all the nanometals that could be synthesized by bacteria have been successfully biosynthesized also using fungi. The first attempt at biosynthesizing nanometal using fungi was made by Torres-Martínez [145].

7.7.3.3 Nanometals Synthesis Using Plants

Plants respond to heavy metal toxicity in a variety of different ways. Such responses include immobilization, exclusion, chelation and compartmentalization of the metal ions; and the expression of more general stress response mechanisms such as ethylene and stress proteins. These mechanisms have been reviewed comprehensively [146] for plants exposed to cadmium (Cd), the heavy metal for which there has arguably been the greatest number and most wide-ranging studies over many decades. Understanding the molecular and genetic basis for these mechanisms will be an important aspect of developing plants as agents for the phytoremediation of contaminated sites [147]. One recurrent general mechanism for heavy metal detoxification in plants and other organisms is the chelation of the metal by a ligand and, in some cases, the subsequent compartmentalization of the ligand-metal complex. A number of metal binding ligands have now been recognized in plants. The roles of several ligands have been reviewed [148]. Extracellular chelation by organic acids, such as citrate and malate, is important in mechanisms of aluminum tolerance. For example, malate efflux from root apices is stimulated by exposure to aluminum and is correlated with aluminum tolerance in wheat [149]. Some aluminum-resistant mutants of Arabidopsis have also increased organic acid efflux from roots [150]. Organic acids and some amino acids, particularly histidine, also have roles in the chelation of metal ions both within cells and in xylem sap [151]. Peptide ligands include the metallothioneins (MTs), small gene-encoded, cysteine-rich polypeptides. Our current understanding of the functions and expression of MTs in plants, particularly Arabidopsis, have been reviewed elsewhere [148, 152]. In contrast, the phytochelatins (PCs) are enzymatically synthesized cysteine-rich peptides. The PC structure, biosynthesis, and function have been extensively reviewed [148, 153]. Recent advances in our understanding of aspects of PC biosynthesis and function are derived predominantly from molecular genetic approaches using model organisms for the gold and silver nanoparticles synthesis.

7.7.3.4 Nanometals Biosynthesis Using Algae

The capacity of unicellular algae, called diatoms, to take up silica from the surrounding medium and convert it into very fascinating intricate patterns and symmetries on the cell wall has become a field of interest to many materials scientists and has drawn nanotechnologists towards it. Diatoms contain an enzyme called silaffin that can catalyze the polymerization of silica spheres—tiny structures reminiscent of the nanoparticles known to constitute diatom biosilica [154]. Kröger et al., have now further defined the structure of the silaffins and discussed their pivotal role in the nanofabrication of diatom biosilica. A peptide, derived from the silaffin-1A protein of the diatom Cylindrotheca fusiformis, has been found to convert magnesium oxide microfilaments into nanocrystalline magnesium oxide [107]. Fluid (gas or liquid)/silica displacement reactions leading to a variety of other oxides have also been identified. This hybrid (biogenic/synthetic) approach has opened the door to biosculpt ceramic microcomponents with multifarious tailored shapes and compositions for a wide range of environmental, aerospace, biomedical, chemical, telecommunications, automotive, manufacturing, and defense applications. A marine diatom, Nitzschia frustulum, has already been harnessed to fabricate Si-Ge oxide nanocomposite materials [108].

Moreover, apart from diatoms other fresh water as well as marine algae has been successfully used for biosynthesis of nanometals. Singaravelu et al., [109] exploited Sargassum wightii for extracellular synthesis of gold nanoparticles. Oza et al., [155, 156] have also extracellularly synthesized gold nanoparticles using the fresh water algae Chlorella pyrenoidosa and the marine algae Sargassum wightii. Nitrate reductase has been proved to be the most influential reducing enzyme involved in the synthesis of gold nanoparticle by these two algae (Figure 7.22).

7.7.3.5 Nanometals Biosynthesis Using DNA

The search for unique biogenic nanoparticle synthesis methods led scientists to take advantage of a self-replicating biogenic polymer having high specific interactions between complementary bases in DNA, which evolved into a new branch of technology now known as DNA origami [157, 158]. At present, DNA nanostructures in many cases are formed with DNA origami technology. An alternative method employs nonspecific interactions in DNA solution for the construction of nanosystems. The manipulations with solvent quality via the change in temperature or in solvent composition, pH and ionic strength variation, utilization of ligand binding, and organization of supramolecular structures in a solution can be extended to the manufacturing of DNA nanostructures. DNA is a unique polymer with extremely high charge density and huge bending stiffness. The relatively stable and tough double helix structure, however, can easily be transformed into a flexible single-stranded conformation. This allows us to use the conformational transitions for the formation of multicomponent systems and nanostructures. The specific binding of different ligands with a macromolecule provides the targeted modification of the polymer chain. One of the amazing challenges is to create nanowires and waveguides by the metallization of DNA molecules [159–161]. Two alternative approaches can be used for DNA metallization in a solution and on a substrate: the DNA linking with metallic nanoparticles and the reduction of silver ions after their binding to DNA.

Among biological molecules, DNA has been used extensively as a biotemplate to grow inorganic quantum confined structure and to organize non-biological building blocks into extended hybrid materials because of its physicochemical stability, well-defined sequence of DNA base and a variety of superhelix structures [162]. DNA template has also been described as smart glue for assembling nanoparticles. In general, synthesis of nanoparticles based on DNA template has been done by incubating metal ion-coated DNA in a reduction agent solution. The ion DNA complexes controlled the release of metal ions, slowed down their reduction and effectively inhibited metal ions from growing into big clusters. Nanostructures of metals, such as silver, gold, palladium, platinum, copper, and nickel, have been successfully synthesized by using DNA network templates [162, 163].

7.7.3.6 Nanometals Biosynthesis Using Enzymes

Employment of microbial enzymes for synthesis of gold nanoparticles is a new field with growing importance due to the fact that the enzymatically produced nanoparticles have the potential for use in homogenized catalysts and are suitable for nonlinear optics. The biological agents, including fungi, bacteria and plants, secrete a large amount of enzymes, which are capable of hydrolyzing metals and thus bring about enzymatic reduction of metals ions [164]. There are only a few examples of successful synthesis of nanoparticles using purified microbial enzymes [164–166]. Likewise, the majority of such studies have not examined antimicrobial activity and cytotoxicity of synthesized nanoparticles for biological systems.