1. Introduction
Nano is derived from the Greek word, Nanos, meaning extremely small. Nanotechnology, as the name indicates, is the science and technology of preparation, characterization and application of matter in the size range of a billionth of a meter (1 x 10-9). Nanotechnology can be used to manipulate atoms and molecules, creating new materials and machines with greatly improved qualities as compared to those of their precursor materials. This is achieved by constructing layers of atoms or molecules through various fabrication methods such as solution processing, melt processing, in-situ deposition etc. In addition to functioning as nano-scale devices, nanoparticles often display substantially altered properties in the bulk, thus making them valuable candidates in constructing novel materials. These nano materials are used in construction, medicine, textile, engineering and many other industries. In biology, silver nanoparticle embedded polymers are used as drug delivery agents, in tissue engineering, in DNA probing, as antimicrobial and antifungal agents, and in wound management (Sharma 84). Gold, silver, copper, titanium and iron nano particles are used as electronic sensors, catalytic agents, industrial cutters and as embedded agents in other applications. While a wide variety of nano materials and their application areas exist, perhaps one the most widely investigated are Carbon Nanotubes (CNTs), which are allotropes of carbon. Research into the synthesis and characterization of CNT composites is expected to yield not only new insights into the physics of the extremely small, but also novel construction materials that can take advantage of their high tensile strength, thermal conductivity and electrical conductivity (Konsta-Gdoutos, Metaxa, and Shah 1052).
2. History of Nanotechnology
Human civilizations from different eras have employed nano-scale effects while fabricating artifacts, without being aware of the underlying principles. For example, the Lycurgus cup of Roman antiquity, exhibited at the British Museum, is impregnated with alloyed gold and silver nano particles. Because of an effect known as Surface Plasmon Resonance, explained later in this paper, it changes in color from green to a vivid red when it is illuminated, respectively, in reflected light and transmitted light (Maier and Atwater 011101-3). Another example is the development of photography during the eighteenth century, which made use of silver nano particles produced by photo-reduction of silver halide to take photographs. The first scientific exposition of the science of particles in the atomic range was, however, due the celebrated physicist Richard Feynman. Feynman delivered a lecture in 1959 at a meeting of the American Physical Society, in which he speculated on the rearranging of atoms and the creating of circuits and machines at the atomic scale. Experiments involving atomic scale manipulations began during the 1960s, and porous silicon was synthesized by Uhlir (333) in 1956 (however, this material was seriously investigated only after the 1990s, after fluorescence under ambient conditions was observed). The technique of ball milling in the presence of a surfactant and a solvent was used to develop ferro-fluids, consisting of nano dispersed magnetic particles suspended in liquids. Quantum wells were manufactured during the 1970s, and the technique of atomic layer deposition (synthesis of individual layers one atom thick) was developed by Tuomo Suntola in 1974. During this same year, the term “nano technology” was coined by Norio Taniguchi to describe the above techniques as a whole, thus bringing into focus this nascent science of manipulation of materials one atom at a time. One of the most significant developments in this field occurred in 1981, when Gerd Binnig and Heinrich Rohrer developed the Scanning Tunneling Microscope (STM), thus enabling scientists to take photographs of atomic surfaces. This was followed by the invention of the Atomic Force Microscope (AFM), which could image and manipulate atoms at nanometer resolutions, by Calvin Quate and Christoph Gerber in 1986. But before that, in 1985, came the ground-breaking discovery of fullerenes by Harry Kroto, Richard Smalley, and Robert Curl. They used laser irradiation to vaporize graphite and produce icosahedral structures, having 60 vertices and 32 faces, of the C60 allotrope of carbon. Another allotrope, CNT, was discovered by Sumio Ijima in 1991, originally synthesized as seamless cylinders of graphite sheets. It was found that CNTs can have a single wall (SWNT), double wall (DWNT) or a multiple wall (MWNT) structure, with diameters between 0.4-30 nm and lengths in the micrometer range (Ijima 1). These different types of carbon nano-structures are shown in Figure 1 below:
Figure 1: Different forms of Carbon nanostructures (a) Graphene sheet; (b) C60 fullerene; (c) SWCNT; (d) DWCNT; (e) MWCNT. Source: Grady 12.
During the 1980s and 1990s many different fabrication techniques were discovered, such as electron-beam lithography, chemical vapor deposition, organic colloidal dispersion (the famous Brust Schiffrin method) etc. Manufacturing techniques continued to progress and become more simplified. While traditionally Nano materials and devices were constructed by using the top-down approach, which involved breaking down materials through techniques of solid-state physics, now such materials are increasingly being manufactured by the bottom-up approach, which consists of synthesis of nanostructures of supra-molecular and biomimetic materials (Steed, Turner & Wallace, 2007). The facile techniques, as well as increasingly sophisticated investigating tools such as Transmission Electron Microscopes (TEM), X-Ray Diffraction Analysis (XRD) and Fourier Transform Infrared spectroscopy (FT-IR), have led to an explosion of nano materials having tailored properties and novel applications.
3. Synthesis
Depending on their application area – industrial and biological – nano materials can be synthesized using physical or chemical means. Common physical synthesis methods include electric arc discharge in a gaseous medium, Physical Vapor Deposition (PVD), ion sputtering, pyrolysis and laser ablation; common chemical methods include nanocrystal reduction, solvothermal and photochemical synthesis, thermolysis, sonication, preparation from micelles and micro emulsions, etc. Methods that are appropriate for preparing biocompatible nano materials are grouped under “green chemistry” synthesis. Industrial applications usually require manufacture through “top-down” methods, examples being semiconductor processing and precision engineering. Biological applications usually require manufacturing through “bottom-up” methods, including nano-material growth and self-assembly. A third approach is molecular manufacturing. A few of these methods are briefly discussed below.
Arc discharge and Physical Vapor Deposition (PVD): In the arc-discharge method, an electric arc is created in an inert atmosphere between two electrodes to vaporize metals. In a variant, a metallic foil is placed in a ceramic container and heated to a high temperature in the presence of an inert gas. Collisions between the metal vapor and the inert gas produce nanoparticles, and this method can be used to create deposits of metals such as iron, titanium, magnesium etc. (Rao, Thomas and Kulkarni 25). PVD is used to deposit thin films of very uniform thickness and high quality on to a substrate. The material to be deposited is first thermally evaporated and then deposited onto the substrate from a reservoir under high vacuum. This method is applied for preparing ultrathin films of great hardness on industrial cutting tools.
Laser ablation and pyrolysis: A pulsed or Nd:Yag laser can be used to ablate material from actinide metals, and the resulting plume is expanded supersonically to create nano particles. In laser pyrolysis, the metal is heated with a pulsed laser in the presence of an inert gas to produce nano particles. A popular method, albeit chemical, is flame spray pyrolysis, in which a fine mist of the solution containing a precursor material is decomposed through combustion. This thermal decomposition technique is also known as chemical vapor deposition (CVD), which involves the catalytic reduction of hydrocarbons. CVD is popularly used for manufacturing SWNTs and MWNTs. The physical variant of this method involves generating heat within a furnace. Carbon black has been prepared for a long time by the pyrolysis of hydrocarbons (Hunt et al. 140). CVD is also often used for depositing films with multiple materials, through co-deposition, infiltration or alternate layer deposition
Electro-spinning, freeze-thawing and other chemical methods: A variety of polymers can be embedded with nano particles using these methods. Electro-spinning consists of taking a mixture that will function either as a melt or a solution, and transferring it into a nozzle aligned to the deposition surface. The surface and the nozzle are connected to electrodes that are oppositely charged, so that in the presence of a high enough applied voltage droplets are deposited in the form of thin streams, thus forming a mesh like substance. This is followed by rapid evaporation of the solvent (in case of a solution), or cooling (in case of a melt) to form the network structure (Wendorff, Agarwal, and Greiner 11). In the method of freeze-thawing, nano particles are impregnated within the bulk of polymer networks. A filler material is used at first to model the network structure; the solvent is frozen to form the crystalline, interconnected structure around which polymerization occurs; then the solvent is thawed out to leave the nano impregnated superstructure behind (Lanza and Vacanti 291). A variety of methods involving biological reagents at room temperature have also been developed for synthesis of nano impregnated drug delivery systems. These methods collectively come under “green chemistry”, and they are especially useful for medicinal applications.
4. Properties of nano materials
Nano particles exhibit properties that are substantially different from those in the bulk, and this is due to their extremely high surface area compare to their volume. For example, CNTs have remarkable properties, such as higher mechanical strength than iron and lower density than aluminium – while metallic CNTs can theoretically carry charge densities up to 4 x 109 Amp/cm2, which is a thousand times higher than those carried by copper. Their elastic stiffness approaches that of diamond (1000Gpa), while they have a yield strength of 100Gpa, making them almost ten times stronger than steel (Hong and Myung 207). Nanomaterials have also been observed to possess advanced magnetic and spintronic properties. These properties arise in materials constructed out of two or more elements, deposited alternatively as thin films. For example, when chromium films are alternated with iron films, this gives rise to a phenomenon known as giant magneto-resistance (GMR), which involves low electric resistance for a parallel alignment of the iron films and high electric resistance for an anti-parallel alignment (Shinjo 4). These effects are particularly useful in constructing nano machines and micro-electromechanical systems (MEMS).
Another interesting property is surface plasmon resonance, which is the oscillation of a group of valence electrons when they are excited by radiation of correct frequencies. This effect gives rise to a variety of optical properties, such intense coloration of colloidal suspensions, increased light absorption in photovoltaic cells, resonance shifting through changes in refractive index etc. Colloidal gold nano particles change color from red to purple or blue, while colloidal silver nanoparticles exhibit a variety of colors depending upon their state of aggregation in the suspension (Eustis and El-Sayed 212). Figure 2 shows this remarkable change in color:
Figure 2: Optical properties of nanomaterials exhibited in colloidal suspensions. Left: A suspension of gold nano particles changes in color from red to purple. Right: A suspension of gold nano particles changes in color from clear yellow to dark yellow to violet to grayish.
Another property that is being actively investigated is molecular optoelectronics, in which nano scale devices can be to convert electricity into light signals and vice versa through quantum mechanical effects. These effects can be observed both in molecular wires as well as individual nano scale particle collections. An allied phenomenon is transport of electric currents across molecular wire junctions, mediated by single molecules or a group of self-assembled molecules (Mujica and Ratner 12-1).
5. A few applications and benefits
Nano technology has found a wide variety of applications across industries. Materials impregnated with nano particles not only exhibit better physical and material properties, but these properties can also often be tailored for individual applications. In the construction industry, CNT impregnated concrete shows superior strength, resistance to crack formation and resistance to crack formation. Polymers embedded with a variety of nano particles are used as additives in the automobile industry to create lightweight and yet stronger body components. These additives are also used in the textile industry to create fabric that is wrinkle resistant and anti-bacterial. Biological thin film polymers are used for making displays, and also used as coatings on windows. A variety of nano particle impregnated ceramics are being manufactured, and they are used in applications such as solid oxide fuel cells, wear resistant coatings etc. The field of nano chemistry involves nano mediated catalytic reactions that have found applications in the petroleum industry as well as in automobile catalytic converters. In the computing and electronic industries, GMR effects are expected to yield vastly enhanced disk storage capabilities and flash memory devices; nano particles are also being used to develop conductive inks for display on e-readers.
6. Possible Adverse Effects
Although nano particles are being widely used, the impact of CNTs and other nano materials on human health, as well as their long term environmental effects, need to be investigated further. A number of studies have pointed to the risks associated with long term exposure to such novel materials. For example, nano materials, because of their extremely small size, can directly enter the body through the skin, and then diffuse through cell membranes or cross the blood-brain barrier. While this property can be useful for controlled drug delivery, it can also have unknown side effects through uncontrolled exposure on workers at construction sites (Lee, Mahendra and Alvarez 1). In order to address these concerns, the Netherlands has initiated a pilot project to determine Nano Reference Values (NRVs), to be implemented as safety warnings for workers. It is expected, however, that safe handling limits and usage methods of nanomaterial composites will be found out in the coming years.
7. Conclusion
The science and engineering of nano materials is expected to be one of the defining technologies of the 21st century. Nano particle impregnated composites as well as nano scale devices have led to novel possibilities in engineering and medicine. Nano scale devices can be programed to act either individually or in groups to perform a variety of activities, such as sensing and controlling events, that lead to effects at the macro scale. It is expected that worldwide research areas will find many more phenomena at the atomic scale, leading to many more benefits in the areas of guided self-assembly and DNA processing. Nano science and engineering will in the near future restructure almost all industries toward the next industrial revolution, and assure quality of life for everyone.
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