1.1. Introduction to Nanoscience and Nanotechnology

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1 1.1. Introduction to Nanoscience and Nanotechnology A Brief Historical Overview Before trying to understand and discuss about synthesis, characterization and application of nanomaterials, it is perhaps best to start with the broad area that is known as nanoscience and nanotechnology. Nanoscience and nanotechnology is an emerging area of science that concerns itself with the study of materials that have very small dimensions as well as the development of reliable processes and techniques^ for the synthesis and characterization of nanosized materials over a wide range of dimensions. It also refers to the fundamental understanding and resulting technological advances arising from the exploitation of new physical, chemical, and biological properties of systems that are intermediate in size, between isolated atoms and molecules and bulk materials,*«where the transitional properties between the two limits can be controlled. It cannot really be called chemistry, physics, or biology; researchers from all domains are studying very small things in order to better understand our world.

2 2 Sunny and Thomas The word "nano" is derived from the Greeic word "nanos," which means dwarfs. Richard Feynman ( ), the Nobel laureate physicist, first mentioned the concept of "nanosized materials" (not yet using that name) in his speech (APS meeting Dec 29, 1959) tided There's Plenty of Room at the Bottom [1]. Later in 1974, Professor Norio Taniguchi from Tokyo Science University, Japan, coined the term nanotechnology to describe the arts and science of manipulating atoms and molecules to create new systems, materials, and devices. The term nanotechnology was then reintroduced and popularized by California scientist and author Eric Drexler. In 1981, the advent of the scanning tunneling microscope enabled atom clusters to be seen, while in 1991 IBM demonstrated the ability to arrange individual xenon atoms using an atomic force instrument [2, 3] Nanomaterials "Nanomaterials" are the materials (crystalline or amorphous) that have one or more dimensions in the range of 1 to 100 nm. The prefix "nano" means one billionth. In general, these particles (nanopowders, nanofibers, tubes, and thin films) could exist in powder form, dispersed in some medium, or as solid films [4]. Figure 1.1 shows the size of the nanoscale relative to some things we are more familiar with. We can see that the difference between a nanometer and a person is roughly the same as the difference between a person and celestial orbits. From the point of view of a chemist, the basic building blocks of matter are atomic nuclei and electrons in which the electrons orbit around the single nucleus and the number of electrons depends on the element. Anything smaller than a nanometer, in size is just a loose atom or small molecule. To get a sense of nanoscale, DNA strands are around 10 nm long, a bacterial cell measures a few hundreds of nanometers across, while a man is billions of nanometer tall (Figure 1.1). t DNA 10 nm Cell 1000 nm Man --^ billions of nm Figure 1.1 with. Scheme showing the size of nanoscale relative to some things we are more familiar

3 Nanoparticles 3 O Quantum dots Nano wires /nanorods Layered structures Bulk Particles Figure 1.2 Classification of nanomaterials. On the basis of the dimensionality, the materials can be generally classified [4] as: Zero-dimensional structures: One-dimensional structures: Two-dimensional structures: Special nanomaterials: Quantum dots or simply "nanoparticles." Nanowires and nanorods. Thin films, layered structures. Carbon fullerenes, carbon nanotubes. Micro- and mesoporous materials, core shell structures, and nanocomposites. Figure 1.2 shows a schematic representation of some examples of nanomaterials, which use the nomenclature adopting the number of growing dimensions [4]. Nanoparticles or zero-dimensional structures, such as metal oxides and their clusters, are typically defined as being less than 100 nm in all three dimensions. One-dimensional materials like nanotubes, nanofiber, etc., will grow preferentially in one dimension. Two-dimensional materials (grown preferentially in two dimensions), such as thin films, layered materials including layered silicate, so-called nanoclay, form part of a class of material with interesting properties, especially by its "sui-generis" structure, which is built by the stacking of "twodimensional" units known as layers, along the basal direction. Already the early theoretical predictions have shown that nanomaterials when compared to their bulk counterpart's exhibit dramatically improved mechanical properties in addition to their interesting functional properdes [4, 5]. Nanomaterials have been important in the materials field for quite a long time. An early example was the incorporation of gold nanoparticles in stained glass in 10 AD at sizes in the nanorange as gold can exhibit a range of colors [3, 6]. The famous "Purple of Cassius" of 17th century consisted of colloidal gold particles and tin dioxide. In 1818, Jermias Benjamine Richter suggested that purple color of drinkable gold results from gold parucles of finest degree of subdivision, whereas yellow color originates from the aggregation of fine particles. In 1857, Faraday had reported the formation of deep red solutions of colloidal gold by the phosphorus reduction of HAuCU in carbon disulfide. He has also observed a reversible color change for a thin film of so prepared nanoparticles from bluish purple to green upon mechanical compression [3]. Also nanosized carbon black particles have been used to reinforce tires for 100 years [7]. At length-scales comparable to atoms and molecules, quantum effects strongly modify properties of matter, like color, reactivity, magnetic, or dipolar moment, etc.

4 4 Sunny and Thomas Bulk matter Micron sized particles Nanoparticles Figure 1.3 A typical illustradon of surface area of bulk matter and nanoparticles. In the last decade, low-dimensional materials have exhibited a wide range of electronic and optical properties that depend sensitively on both size and shape and are of both fundamental and technological interest. The reasons for the change in behavior when solids are in the nano regime may be an increased relative surface area (producing increased chemical reactivity) and the increasing dominance of quantum effects (with effects on materials optical, magnetic, or electrical properties). It has been widely pointed out that while a cube measuring 1 cm on a side possesses 6 cm^ of surface area, the same 1 cm cube, itubdivided into many 1 nm cubes, possesses a surface area equivalent to that of a football field! (Figure 1.3). A tiny piece of any solid would have many of its atoms at its edges, which are unstable. The nanosized particles when compared to their bulk counterparts possess more edges, which makes large fraction of its atoms to locate in the grain or interphase boundaries. Two divergent views exist on the nature of grain boundaries in nanomaterials. The classic interpretation suggests that the nanosized materials have more fractions of its atoms in the grain boundaries and the nature of these boundaries in nanomaterials is identical to that in the coarse-grained materials. But new sophisticated computer simulation techniques reveal the absence of a long-range periodicity in the nanomaterials. It has also been suggested that the grain boundary energy and the width distributions ^e narrower while grain boundaries are wider when compared to that of coarse-grained materials. This results in increased solubility, diffusion, and also considerable changes in the thermodynamic behavior (e.g., lower melting temperature of nanophase) [4, 8, 9]. Fast electronic systems, extremely sensitive sensor device^ to probe confined environments, and multiplexed techniques for high-throughput analysis represent some of today's most prominent nanotechnological needs. The ultimate realization of these technological advancements will be based on our ability to synthesize and organize matter into controlled geometries on the nanoscale. During the past few years, the exploration of

5 Nanoparticles 5 synthetic techniques for the fabrication of nanostructured materials having controllable morphologies has emerged as a fast-growing subfield of nanotechnology research. Advanced functional materials incorporating well-defined nanoarchitectures have shown great potential for nanotechnological applications, such as miniaturized nanoelectronics, ultrafast quantum compuung, high-density memory/data storage media, ultrasensitive chemical sensing/biosensing, generation of high-efficiency catalytic substrates, and high-throughput templating for the growth/attachment of other types of bio- or inorganic nanomaterials. Of particular interest are zero, one- and two-dimensional arrays of patterned nanostructures (nanoparucles, nanowires, nanotubes, layered nanostructures, etc.), which have been shown to display unique optoelectronic, magnetic, or catalytic properties that can be tuned by varying their size and/or interparticle separation distance [10]. For example, patterned gold nanoparticles display plasmon optical properties that can be applied in surface-enhanced Raman scattering detection systems with high sensitivities [8], whereas nanowires with high surface ratios and diameters in the nm range display interesting opfical and electrical properties that are highly desirable in electrochemical sensing technologies, field-emission systems, and lasers [10]. Nanoporous materials displaying molecular sieve properties can act as chemical sensor elements, wherein the plasmon properties of the pores can be used in optical detection systems (Raman, optical waveguides). One of the most important technological challenges that remain to be addressed, however, is the development of effective patterning methods to control materials assembly on a nanometer scale. At present, there is a wide variety of top-down and bottom-up fabrication techniques that are capable of creating nanostructured arrays with varying degrees of speed, cost, and structural quality. We shall provide a brief overview of some of these techniques below, but for more detailed descriptions, the reader is referred to several excellent recent reviews [11-14] and books [15] Synthesis of Nanosized IMaterials In recent years, a wide range of nanostructured materials in various forms such as nanosized powders, nanosized fibers, nanotubes, thin films, etc., of metals, semiconductors, dielectrics, magnetic materials, polymers, and other organic compounds are introduced by means of a number of processing routes. It is seen that the properties of these particles are quite sensitive to their sizes. The intensive research and the contemporary technological advances make it difficult to keep up to date with all the achievements in the field of nanotechnology. In this section, we are trying our best to briefly summarize the various synthetic routes used for the production of low-dimensional nanoparticles with strong application potenuals [10-16]. The drive for finding novel routes for the synthesis of nanomaterials has gained considerable momentum in recent years, owing to the ever-increasing demand for smaller particle sizes. Since nanomaterials have little mass and are dominated by surface area and size effects, the processes and equipment for their manufacturing are expected to differ significandy from conventional approaches. ConvenUonal methods, called top-down techniques (Figure 1.4), include chopping down the bulk material by mechanical means and the resulting collohdal particles are subsequently stabilized by colloidal protecting agents. The simplest example is ball milling. Generally top-down strategies involve either (1) using macroscopic tools to first transfer a computer-generated pattern onto a larger piece of bulk material, and then "sculpting"