Essentials in Nanoscience and Nanotechnology

Preface

The turn of twenty-first century has witnessed the emergence of three cutting-edge technologies, namely, Information and Communication Technology (ICT), Biotechnology, and Nanotechnology. In 2005, the United Nations Task Force on the Millennium Development Goals touted Nanotechnology as one of three platform technologies that can reduce hunger, promote health, improve water sanitation, develop renewable resources, and improve the environment, and recommended that developing countries should initiate nanotechnology programs at a national level. Inspired by such forecasts, by 2014, over 60 countries followed the United States and established the National Nanotechnology Initiative. These countries range from advanced industrial countries in Europe and Japan to the emerging markets of Russia, China, Brazil, and India.

Physicist Richard Feynman, in his famous speech of 1959, forecasted the development of nanoscience and the punch line plenty of room at the bottom became reality by the 1980s when scientists developed techniques and tools to explore and manipulate matter at the atomic scale. The term nanoscale defines a size range from 1 to 100 nm, although a scientifically based range goes from the atomic scale (0.2 nm) to 100 nm. The focus on the nanoregime relates to the convenience of some standard definition that can be used to both categorize nanotechnology, nanoscience, and nanoproducts and act as a bridge between quantum mechanical effects and surface area effects.

Nanoscience is not merely about size; it is about the unique physical, chemical, biological, and optical properties that emerge naturally at the nanoscale, whereas nanotechnology is related to the ability to manipulate and engineer such effects. It is a broad new area of science that demolishes boundaries among physics, chemistry, biology, cognitive science, materials science, and engineering at the nanoscale. New technologies, however, are likely to revolutionize the economy and the society only if there is a broader strong National base consisting of trained manpower and infrastructure that allows a new technology to spread and transform to its exciting niche applications, whether civilian or military. To do so, the most important thing is to educate and train the budding manpower at the school and university levels. For that, availability of standard books is necessary, and this book is designed keeping in mind these essential requirements.

Furthermore, keeping in view the unprecedented research and development in the area of nanoscience and nanotechnology and to make the students aware about the latest developments in the field, we have attempted to write this book in a manner as simple as possible while including the latest development in the field. The subject matter of the book, ranging from fundamentals to the latest developments and technological applications, is presented in 10 chapters. The first chapter on introduction gives a historical prospective, provides living examples of nanoscience in nature and artificial nanomaterials, and brings out the likely impact of nanotechnology on human civilization. Chapter 2 describes general synthetic approaches and strategies, while Chapter 3 deals with the characterization of nanomaterials using modern tools and techniques to provide the basic understanding to students who are interested in learning this emerging area. Chapters 4–7 deal with different kinds of nanomaterials such as inorganic, carbon-based nanocomposites, and self-assembled/supramolecular nanostructures, respectively, in terms of their varieties, synthesis, and properties. Following this, Chapters 8 and 9 are devoted to the unique properties and applications of nanotechnology in various disciplines such as information technology, pollution, environment, energy, healthcare, consumer goods, and so on. Finally, the last chapter deals with the toxicological and ethical issues associated with nanotechnology.

We believe this book will generate and promote the basic understanding on the complex and revolutionary disciplines of nanoscience and nanotechnology, which is offered now as a core subject in most of the academic institutions across the globe.

Acknowledgments

Writing a book is always a dedicated effort requiring patience and hidden support of family members, which both the authors have been fortunate to enjoy during the course of writing this book. In this context, Dr. Kumar would like to thank his wife, Mrs. Soma Jain, who supported him at every step and without whom it would have been impossible to accomplish writing this book. She showed immense strength in bearing with little time he could spend with her. Dr. Kumar would like to thank his children and their spouses, Anjali and Vikas; Atul and Parul; Akshaya and Pragati for their support, and his grandchildren Kratyagya, Kritika, and Eshna for always cheering him up. Dr. Kumbhat would like to acknowledge her husband, Air Cmde Jinendra Kumbhat, for being her pillar of strength and standing beside her throughout the course of writing this book. He has been her inspiration and motivation for continuing to improve her knowledge and move her career forward. Dr. Kumbhat would like to thank her children and their spouses, Shruti and Mehul; Kunal and Sonal, who have all been such wonderful and encouraging children.

The authors would like to express their gratitude to a number of distinguished academicians, researchers, and reviewers for their support, discussions, and critical reviews made during the course of writing this book. Dr. Kumar would like to acknowledge the support of students and members of his research group, including Dr. S. R. Vadera, Director, of Defence Laboratory of Jodhpur, Prof. K. Manzoor, Drs Rashi Mathur, A. K. Shukla, M. K. Patra, S. C. Negi, Manoth Mathew, D. R. Sharma, Vivek Kumar Singh, S. Gowda, Vatsana Gupta, Jitendra Singh, and Manoj Dhaka for their ever willing support while Prof. Kumbhat would like to acknowledge the support of Prof. R. P. Singh, Vice Chancellor, of Jai Narain Vyas University, Jodhpur, and her colleagues from the Department of Chemistry for their support in planning and execution of the courses related to nanoscience and nanotechnology. She also acknowledges the ever-willing support of her research students, Drs Urmila Nain, Sushma Dave, Omprakash Khatri, Jaya Jain, Richa Vyas, Kamesh Gangawat, Manjulata Parihar, Rakhee, Kavita, Uravsini Singh, Hema Somani, Ved Prakash, and Menka Khicher.

The authors would like to thank John Wiley & Sons, Inc., for taking up this project and Ms. Anita Lekhwani, Senior Acquisitions Editor, and her team members including Ms. Cecilia Tsai and Ms. Purvi Patel at John Wiley, Hoboken, New Jersey for their consistent interest, timely action in execution of this project, and making the publication of this book a reality.

About the Authors

Dr. N. Kumar, Former Director, Defence Laboratory, Jodhpur

Dr. Narendra Kumar, DRDO fellow, graduated and did his PhD degree in Organometallic Chemistry in 1976 from Delhi University. From 1976 to 1981, he served at the National Physical Laboratory, New Delhi, and worked toward the development and application of materials including organometallics, liquid crystals, electrochromics, and electrode materials. He served as a postdoctoral research fellow at Windsor University, Canada, during 1981–1983 and worked in the field of electrochemical synthesis of metallic and organometallic complexes of transition and actinide elements. In 1984, he joined as Scientist at the Defence Laboratory, Jodhpur, and retired from there as its Director in 2012, where he carried out pioneering research work in the development of conducting polymers, liquid loam, nanomaterials, and products based on them for various defence applications. Dr. Kumar has published more than 100 research and review articles in international journals in the areas of Nanoscience, Organometallics, Conducting Polymers, and Electrochemical Synthesis, and also a chapter entitled Nanotechnology for Sensor and Display Applications in the Encyclopedia of Nanoscience and Technology published by American Scientific Publication, USA. He has 12 patents to his credit and authored one book entitled Nanotechnology and Nanomaterials in the Treatment of Life Threatening Diseases published by Elsevier (USA) in 2013. He also served as a visiting Research Associate of CSIR, New Delhi, during 1992–1995 and is a recognized supervisor of J. N. V. University, Jodhpur, for PhD and has guided 6 students for their PhD degrees in the area of nanoscience. He has delivered several invited talks on Conducting Polymers and Nanomaterials in several International and national conferences/seminars, as well as at universities in India, Japan, and the United States.

He received the DRDO Technology Cash Award in 1996 for his pioneering research work on conducting polymers, and DRDO Scientist of the Year award in 2005 from the Prime Minister of India for products based on conducting polymers and nanomaterials for defense applications. He is a recipient of the National MRSI-ICSC Super Conductivity and Materials Science Annual Award for the year 2010 by Materials Research Society of India. Dr. Kumar is a member of various scientific societies including the prestigious American Chemical Society.

Dr. S. Kumbhat, Professor, Department of Chemistry, Jai Narain Vyas University, Jodhpur

Dr. Sunita Kumbhat, Professor and Chemistry Department at J.N.V. University, Jodhpur, has graduated in Chemistry and obtained her PhD degree in the field of Electrochemistry in 1985 from J.N.V. University, Jodhpur. She did postdoctoral research in the fields of Photoelectrochemistry and Sonovoltammetry with Prof. R.G. Compton at Oxford University, UK. Dr. Kumbhat joined the Department of Chemistry, J.N.V. University, as permanent faculty member in 1986, and has been serving as a Professor since 2001 where she has been involved in teaching graduate and postgraduate courses on Analytical Chemistry, Electrochemistry, Sensors, and Nanoscience and supervising PhD students. Her areas of interest are Electrochemistry and Biosensors for Biomedical and Environmental Analysis. She has more than 50 research papers in international journals, three educational films, and one patent to her credit. She has supervised 14 research students for their PhD degrees. Her awards and recognition include Commonwealth Academic Staff Fellowship (1994–1995) at Oxford, UK, National Associate (1997) at BARC, Mumbai, and the INSA–JSPS Visiting Fellowship (2005) at Kyushu University, Fukuoka, Japan. Dr. Kumbhat is associated with various Indian and International scientific societies and is also an assessing member of the National Assessment and Accreditation Council, Bangalore, India.

Chapter 1

Introduction

1.1 Definitions of Nanoscience and Nanotechnologies

Nanoscienceis a new discipline concerned with the unique properties associated with nanomaterials, which are assemblies of atoms or molecules on a nanoscale. Nanoscience is actually the study of objects/particles and its phenomena at a very small scale, ranging roughly from 1 to 100 nm. Nano refers to a scale of size in the metric system. It is used in scientific units to denote one-billionth of the base unit, approximately 100,000 times smaller than the diameter of a human hair. A nanometer is 10−9 m (1 nm = 10−9 m), a dimension in the world of atoms and molecules (the size of H atom is 0.24 nm and, for instance, 10 hydrogen atoms lined up measure about 1 nm). Nanoparticles are those particles that contain from 100 to 10,000 atoms. Thus, the particles in size roughly ranging from 1 to 100 nm are the building block of nanomaterials.

Nanomaterials: These materials are created from blocks of nanoparticles, and thus they can be defined as a set of substances where at least one dimension is approximately less than 100 nm. However, organizations in some areas such as environment, health, and consumer protection favor a larger size range from 0.3 to 300 nm to define nanomaterials. This larger size range allows more research and a better understanding of all nanomaterials and also allows to know whether any particular nanomaterial shows concerns for human health or not and in what size range. Nanocarbons such as fullerenes, carbon nanotubes, and graphene are excellent examples of nanomaterials. A comparison of the size of nanomaterials with some natural and biological species is illustrated in Figure 1.1.

Nano-object: Material confined in one, two, or three dimensions at the nanoscale. This includes nanoparticles (all three dimensions in the nanoscale), nanofibers (two dimensions in the nanoscale), and nanoplates (one dimension in the nanoscale). Nanofibers are further divided into nanotubes (hollow nanofiber), nanorods (solid nanofiber), and nanowire (electrically conducting or semiconducting nanofiber). However, the term nano-object is not very popular.

Particle: It is a minute piece of matter with defined physical boundaries. A particle can move as a unit. This general particle definition applies to nano-objects.

Nanoparticle: It is a nano-object with all three external dimensions in the nanoscale. Nanoparticles can have amorphous or crystalline form and their surfaces can act as carriers for liquid droplets or gases.

Nanoparticulate matter: It refers to a collection of nanoparticles, emphasizing their collective behavior.

Agglomerate: It is a group of particles held together by weak forces such as van der Waals forces, some electrostatic forces, and surface tension. It should be noted that agglomerate will usually retain a high surface-to-volume ratio.

Aggregate: It is a group of particles held together by strong forces such as those associated with covalent or metallic bonds. It should be noted that an aggregate may retain a high surface-to-volume ratio.

Nanotechnology is the construction and use of functional structures designed from atomic or molecular scale with at least one characteristic dimension measured in nanometers. Their size allows them to exhibit novel and significantly improved physical, chemical, and biological properties, phenomena, and processes because of their size. Thus, nanotechnology can be defined as research and development that involves measuring and manipulating matter at the atomic, molecular, and supramolecular levels at scales measured in approximately 1–100 nm in at least one dimension.

Figure 1.1 Size comparisons of objects, nanomaterials, and biomolecules.

When characteristic structural features are intermediate between isolated atoms and bulk materials in the range of approximately 1–100 nm, the objects often display physical attributes substantially different from those displayed by either atoms or bulk materials. The term nanotechnology is by and large used as a reference for both nanoscience and nanotechnology especially in the public domain. We should distinguish between nanoscience and nanotechnology. Nanoscience is a convergence of physics, chemistry, materials science, and biology, which deals with the manipulation and characterization of matter on length scales between the molecular and the micron size. Nanotechnology is an emerging engineering discipline that applies methods from nanoscience to create products.

1.2 Uniqueness of the Nanoscale

At nanoscale, the laws of physics operate in an unfamiliar way because of two important reasons: high surface-to-volume ratio and quantum effect. The key reason for nano-sized regime being special is the dramatic increase in the surface-to-volume ratio. When the size of building blocks gets smaller, the surface area of the material increases by six orders of magnitude, as illustrated in Figure 1.2, while the volume remaining the same. For example, dissecting a 1 m³ of any material into 1 nm particles increases the total combined surface area from 6 to 60,000,000 m², approximately 10 million times larger [1]. Nanomaterials have a wider range of applications such as catalysts, cleanup, and capture of pollution and any other application where chemical reactivity is important such as medicine. This effect occurs at all length scales, but what makes it unique at the nanoscale is that the properties of the material become strongly dependent on the surface of the material since the amount of surface is now at the same level as the amount of bulk. In fact, in some cases such as fullerenes or single-walled nanotubes, the material is entirely the surface.

Figure 1.2 Exponential increases in surface area for cubes ranging from meter to nanosize.

Another important attribute of nanoscale materials is the fact that it is possible for the quantum mechanical properties of matter to dominate over bulk properties. One example of this is in the change in the optical properties, for example, in the photoemission, of many semiconductor materials as they go nano. Figure 1.3 illustrates how, a material whose optical properties may be considered uninteresting, simply by changing its size to the nanoscale one can control the color of the material [2]. This effect is due to quantum confinement.

Figure 1.3 Change in optical properties of a semiconductor ranging from bulk to nanosize. Courtesy of Grossman, MIT, USA.

Important consequence of each of these properties is that they offer completely new methods of tuning the properties of materials and devices. Nanotechnology can provide unprecedented understanding about materials and devices and is likely to impact many fields. By using structure at nanoscale as a tunable physical variable, we can greatly expand the range of performance of existing chemicals and materials. Nanoscience and nanotechnology are broad and interdisciplinary areas of research and development activity that have been growing explosively worldwide in the past two decades. Nanoscience has the potential for revolutionizing the methods in which materials and products are created and the range and nature of functionalities that can be accessed; nanotechnology already has a significant commercial impact that will increase exponentially in future.

1.3 Nanoscience in Nature

Nanostructures are plentiful in nature. In the universe, nanoparticles are distributed widely and are considered to be the building blocks in planet formation processes. Indeed, several natural structures including proteins and the DNA diameter of around 2.5 nm, viruses (10–60 nm), and bacteria (30 nm to 10 µm) fit the above definition of nonmaterial, while others are of mineral or environmental origin. For example, these include the fine fraction of desert sand, oil fumes, smog, fumes originating from volcanic activity or from forest fires, and certain atmospheric dusts. Biological systems have built up inorganic–organic nanocomposite structures to improve the mechanical properties or to improve the optical, magnetic, and chemical sensing in living species. As an example, nacre (mother-of-pearl) from the mollusk shell is a biologically formed lamellar ceramic, which exhibits structural robustness despite the brittle nature of its constituents. These systems have evolved and been optimized by evolution over millions of years into sophisticated and complex structures. In natural systems, the bottom-up approach starting from molecules and involving self-organization concepts has been highly successful in building larger structural and functional components. Functional systems are characterized by complex sensing, self-repair, information transmission and storage, and other functions all based on molecular building blocks. Examples of these complex structures for structural purposes are teeth, such as shark teeth, which consist of a composite of biomineralized fluorapatite and organic compounds. These structures result in the unique combination of hardness, fracture toughness, and sharpness. The evolution has worked on much smaller scales too, producing finely honed nanostructures, parts less than a millionth of a meter across, or smaller than 1/20th of the width of a human hair help animals climb, slither, camouflage, flirt, and thrive. Figure 1.4a shows an electron microscopic image of a sensory patch in amphibian ears, which consists of a single bundle of stereo cilia projecting from the epithelium of the papilla, and acts as a nanomechanical cantilevers that measure deflections as small as 3 nm because of sound waves. Many of the shimmering colors in butterfly's wings are produced not with pigments but with nanostructures. The scales on their wings are patterned with nanoscale channels, ridges, and cavities made of chitin, a protein. Unlike pigments, which create color by absorbing some wavelengths of light and reflecting the rest, the nanostructures are shaped so that they physically bend and scatter light in different directions, sending particular colors back to our eyes. This scattering can also make them iridescent (i.e., the color changes with the angle one sees it from. When infrared radiation hits the chitin nanostructures, their shape changes because of expansion, thus changing the colors they display. Figure 1.4b shows glittering colors of peacock feather where barbs project directly from the main feather stem, and barbules (∼0.5 mm long) attached to each side of the barb generate the typical shimmer of iridescence. Electron microscopy (Figure 1.4c and d) of barbules reveals a highly ordered structure of melanin rods of high refractive index embedded in keratin of lower refractive index with air tube between each square of melanin rods. The whole array of melanin rods, keratin matrix, and air holes comprises a 2D photonic crystal. There is much interest on mimicking these natural wonders with potential applications in optical engineering and communications. Less seriously, photonic crystal pigment-free paints would not fade, fabrics might be more vibrant.

Figure 1.4 Nanotechnology in nature: (a) electron microscopic image of a sensory patch in amphibian ears. http://scinerds.tumblr.com/post/35542105310/stereocilia-stairsteps; (b) peacock feather showing barbules, representing a photonic lattice; (c and d) electron microscopy image of transverse and longitudinal sections of barbules. Zi et al. [2a] © 2003. With permission of National Academy of Sciences, USA.

The compound eye of arthropods uses nanoscale features to enhance their visual sensitivity. An insect's compound eye has about 50–10,000 individual facets, which are studded with an array of nanoscale protuberances called corneal nipples (Figure 1.5a and b), each with its own set of optical machinery. These tiny structures of size ranging from 50 to 300 nm cut down the glare that reflects off the insect eye. The nanoscale nipple pattern on moth eyes has inspired new antireflective coatings (Figure 1.5c) for solar cells. The male silk moth can detect, with single-molecule precision, the pheromones of a female moth emitted up to 2 miles away. Spider silks are some of the toughest materials known to man, stronger than steel, and their webs can withstand gusts of wind. The spider's silks get their strength from just nanometers of thin crystal proteins, which are stacked with hydrogen bonds, allowing the silk to stretch and flex under pressure.

Figure 1.5 Natural and fabricated antireflective surfaces: (a) schematic of a moth; (b) scanning electron micrograph of antireflective surface of a moth's eye (scale bar = 1 µm); (c) biomimetic replica of a moth eye fabricated with ion- beam etching.

Parker & Townley [2c] © 2007. With permission of Nature Publishing Group.

These are only a few of the countless examples of how nature employs nanotechnology in different methods, of course, with the most important technology to us being the human body itself, which contains billions of nanoscale machines! It is both fascinating and humbling to observe that despite all of the phenomenal technological advances in nanoscale synthesis and characterization, in most cases we are still unable to build nanotechnology-based devices that even come close to nature.

1.3.1 Naturally Occurring Nanomaterials

Naturally occurring nanomaterials may originate from one of the following sources:

i.Natural erosion and volcanic activity

Nanoparticles are part of mineral world since they are naturally produced from erosion and volcanic explosions.

ii.Clays

Minerals such as clays are a type of layered nanostructured silicate materials that are characterized by a fine 2D crystal structure. Mica, one among them, is the most studied [3]. In mica, a large number of silicate sheets are held together by relatively strong bonds. On the other hand, montmorillonite, a smectic type of clay, has relatively weak bonds between layers. Each layer consists of two sheets of silica held together by cations such as Li+, Na+, K+, and Ca²+. The presence of the cations is necessary for compensating the overall negative charge of the single layers. The layers are 20–200 nm in diameter laterally and come into aggregates called tactoids, which can be about 1 nm or more thick. The fine nanostructure of clays determines their properties. As an example, the nanostructured clay swells to several times of the original volume, when water is added to it, due the opening of the layered structure by the water molecules that replaces the cations. Clay swelling is a significant factor in soil stability and is taken into account in constructing roads.

iii.Natural colloids

Naturally occurring liquid colloids, such as milk, blood, aerosols (e.g., fog), are some of the examples of natural colloids. In these materials, nanoparticles are dispersed in the medium (liquid or gas) but do not form a solution, rather they form a colloid. All these materials have the characteristic of scattering light and often their color (such as in the case of milk and blood) are due to the scattering of light by the nanoparticles that makes them up.

iv.Mineralized natural materials

Many of the natural materials such as shells, corals, and bones are formed by the self-assembly of calcium carbonate crystals with other natural materials, such as polymers, to form fascinating three-dimensional (3D) architectures. For instance, a shell is grown layer-by-layer coating of protein supported by chitin, a polysaccharide polymer. The proteins act as a nanoassembly mechanism to control the growth of calcium carbonate crystals. Around each crystal remains a honeycomb-like matrix of protein and chitin. This relatively flexible envelop is fundamental for the mechanical properties of the shell and mitigate cracking. The size of each crystal is around 100 nm. As a result, the mollusk shell has extraordinary physical properties, namely, strength and resistance to compression.

1.3.2 Nanoscience in Action in Biological World

Two most significant examples of active nanoscience in biological world include the following:

i.Lotus effect

Although the water repellency of lotus had long been recognized, its scientific basis was understood only in 1997 when two botanists Wilhelm Berthelot and Christophe Neinhuis, at the University of Bonn in Germany, examined leaf surfaces of lotus using a scanning electron microscope that resolves structures as small as 1–20 nm [4]. Figure 1.6a shows a nonwettable lotus plant leaf. The self-cleaning property is due to the "Super hydrophobicity" of the convex papillae on the surface of leaves, which is coated with wax crystals of nanoscopic dimension of approximately 10–100 nm (Figure 1.6b). Water drop picks up the dirt particles as it rolls off the leaf's surface, showing self-cleaning process (Figure 1.6c). Several other plants such as Nasturtium and cabbages also show lotus effect.

Figure 1.6 (a) Lotus (Nelumbo nucifera) plant; (b) spherical water droplet on a nonwettable lotus plant leaf. Blossey [6] © 2003. With permission of Nature Publishing Group. ; (c) self-cleaning: a drop picks up the dirt particles as it rolls off the leaf's surface.

The papilla greatly reduces the contact area of water droplets with it. Every epidermal cell forms a micrometer-scale papilla and has a dense layer of epicuticular waxes superimposed on it. Each of the papillae consists of branch-like nanostructures on the surface, for example, of the lotus leaves, the almost spherical water droplets will not come to rest and simply roll off if the surface is tilted even slightly, which is now usually referred to as the "Lotus effect." The self-cleaning effects of the surfaces of the lotus flower have been attributed to the combined micro- and nanostructure, which in combination with hydrophobic groups give the surface a water and dirt-repellent behavior. In the past few years, numerous companies have realized products resembling the surface morphology and chemistry of the lotus flower such as paint, glass surface, and ceramic tiles with dirt-repellent properties.

ii.Geckos Technology

Geckos are one of the few species in the animal kingdom that are known for sticky toes that allow them to climb up walls, even hang upside down on ceiling and at the same time can walk on a leaf; they owe this ability to nanoscale attachment elements. As illustrated in Figure 1.7a–e, on the sole of a gecko's toes there are about a billion tiny adhesive hairs, ∼200 nm in both width and length. These hairs put the gecko in direct physical contact with the surface. Spatula-shaped ends on the hairs provide strong adhesion. Industry is researching the evolution of these properties in order to develop artificial dry adhesive systems. Potential applications include reusable adhesive fixtures with the strength of duct tape, which can be removed as easily as a sticky note.

Figure 1.7 Gecko's adhesive system structure: (a) ventral view of a tokay gecko (Gekko gecko); (b) sole of the foot showing adhesive lamellae; (c) microstructure: part of a single lamella showing arrays of setae; (d and e) nanostructure: single seta with branched structure at the upper right area, terminating in hundreds of spatular tips. Hansen and Autumn [6], © 2005. With permission of National Academy of Sciences, USA.

Some more examples of naturally occurring materials such as cotton, spider's silk, and opals are also worth mentioning for their nano features and unique properties. Cotton has nanoscale arrangement of cellulose fibers showing high strength, durability, and absorbance. Spider silk showing five time higher strength than that of steel has natural supramolecular organization of fibroin at nanoscale. Precious stone opal consists of spheres of silicon dioxide (150–300 nm diameter) in a hexagonal or cubic close-packed lattice. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal. The realization that nature can provide the model for improved engineering has created a research field called biomimicking or bio-inspired material science. It has been possible to process several types of nanostructures inspired from biological nanomaterials, represented in Table 1.1, which provide new technological opportunities and potential for applications.

Table 1.1 Bio-Inspired Unique Properties

1.4 Historical Perspective

Thousands of years bc, people used natural fabrics such as flex, cotton, wool, and silk and processed them into products. What makes these fabrics so special that they developed a network of pores of size 1–20 nm for typical nanoporous materials? Owing to their nanoporous structure, natural fabric possesses high utilitarian properties of absorbing sweat, quickly swelling, and getting dried soon. Since ancient times, people mastered the art of making bread, wine, beer, cheese, and other foodstuffs where fermentation processes at the nanolevel are critical. Romans in the pre-Christian era introduced metals with nanometric dimensions in glass-making: a cup describing the death of King Lycurgus (ca 800 bc) contains nanoparticles of silver and gold [7]; when a light source is placed inside the cup, its color changes from green to red (Figure 1.8a). It was found that it was due to the presence of nano-sized particles of silver (66.2%), gold (31.2%), and copper (2.6%) embedded in the glass. Light absorption and scattering by these nanoparticles determine the different colors. The stained-glass windows of the great medieval cathedrals also contain metallic nanoparticles.

Figure 1.8 (a) Lycurgus cups. Courtesy of Trustees of the British Museum. © The Trustees of the British Museum; (b) ancient Maya fresco painting. Reproduced from Sanchez et al. [8] © 2005. With permission of The Royal Society of Chemistry.

The colors of certain Mayan paintings (Figure 1.8b) also stem from the presence of metallic nanoparticles [8]. Mayan artisans concocting in the eighth century the unique pigment we now know as Maya Blue have endured their lively blue tones for more than 12 centuries of harsh jungle environment. Maya Blue is not an ordinary organic dye nor is it any simple mineral; it is a hybrid organic–inorganic nanocomposite in which the organic dye molecules are protected within palygorskite, a complex natural clay. Art history of India and China is also filled with examples of nanotechnology. Photography, which was developed in the 18th and 19th centuries, provides a more recent example of the use of silver nanoparticles. Nanostructured catalysts have also been investigated for over 70 years. In the early 1940s, precipitated and fumed silica nanoparticles were being manufactured and sold in the United States and Germany as substitutes for ultrafine carbon black for rubber reinforcements. Nano-sized amorphous silica particles have found large-scale applications in many everyday consumer products, ranging from nondiary coffee creamer to automobile tires, optical fibers, and catalyst supports. In addition, the definition of nanoparticles based on size allows us to include colloids and soils that have been used for over a hundred years.

In 1857, Faraday had described the use of colloidal gold in his experiments. In his lecture at Royal Society, Faraday presented a purple color slide, stating that it contained gold reduced in exceedingly fine particles, which becoming diffused, produced a ruby red fluid. The various preparations of gold, whether ruby, green, violet or blue etc. consist of that substance in a metallic divided state. Faraday postulated correctly about the physical state of colloids; he also described how a gold colloid would change color (turning blue) on adding salt. Since then, colloidal science has evolved a lot. In the early twentieth century Gustav Mie presented the Mie theory, which is a mathematical treatment of light scattering that describes the relationship between metal colloid size and optical properties of solutions containing them. The Nobel Prize winner for Quantum Electrodynamics, Richard Feynman (Figure 1.9), said, "Nature has been working at the level of atoms and molecules for millions of years, so why do we not? Since his call in a lecture in 1959, nanotechnology has made tremendous progress not only in technical disciplines but also in medicine and pharmaceutics [9]. World began speculating on the possibilities and potential of nanometric materials and on the fact that the manipulation of individual atoms could allow us to create very small structures whose properties would be very different from larger structures with the same composition. Moreover, in an even more radical proposition, he thought that, in principle, it was possible to create nanoscale machines through a cascade of billions of factories. According to him, these factories would be progressively smaller scaled versions of machine, hands, and tools. In these speculations, he also suggested that there are various factors that uniquely affect the nanoscale level. Specifically, he suggested that as the scale got smaller and smaller, gravity would become more negligible, while van der Waals attraction and surface tension would become very important. Feynman's talk has been viewed as the first academic talk that dealt with a main tenet of nanotechnology, the direct manipulation of individual atoms (molecular manufacturing). Richard Feynman is considered as the Father of Nanotechnology, although he never explicitly mentioned the term Nanotechnology."

Figure 1.9 Richard Feynman.

https://commons.wikimedia.org/wiki/Category:Richard_Feynman.

The evolution of integrated chips may also be considered as the part of history of nanotechnology. The first transistor invented in 1947 was a bulk macro-object. To keep with the demand for miniaturization, the dimensions of the transistor have been reduced considerably in the last 30 years. In the year 2002, the nanosize was reached with the achieved size of a single transistor as 90 nm [10]. As on today, a single transistor in an Intel Core 2 Quad Processor is 45 nm. In order to keep pace with Moore's law, transistor would be as small as 9 nm by 2016. However, this dimension is below the fabrication capabilities of last-generation tools used in the microelectronic industry. Numerous novel approaches such as quantum computing and molecular engineering are under investigation to achieve the workable transistor of this size. Material science/engineering is also full with examples of nanomaterials! Often these were produced inadvertently and were not characterized at the nanoscale since the analytic tools were not available. For instance, the process of anodizing was first patented in early 1930s. This represents one of the most important processes used in industry to protect aluminum from corrosion. It consists of depositing a thin protective oxide layer on the aluminum surface. The inventors were not, however, aware that the protective layer is actually a nanomaterial; the anodic layer is composed of hexagonally close-packed channels with diameter ranging from 10 to 250 nm or greater. The first use of the term nanotechnology was by Norio Taniguchi in 1974 at the International Conference on Precision Engineering (ICPE). His definition referred to production technology to get extra high accuracy and ultra-fine dimensions, that is, the preciseness and fineness on the order of 1 nm (nanometer), 10−9 m, in length. The development of nanotechnology has been enabled by the invention of two analytical tools that have revolutionized the imaging (and manipulation) of surfaces at the nanoscale. These are the scanning tunneling microscope (STM) and the atomic force microscope (AFM). The AFM and STM are capable of imaging surfaces at an atomic resolution. Both the instruments were invented by Binning and his coworkers at IBM Zurich. Invention of these versatile tools practically opened the doors of nanoworld to the scientists. With the advent of the STM, scientists were given the tool not only to image surfaces with atomic resolution but also to move individual atoms. The STM is the first step in realizing Feynman's vision of atom-by-atom fabrication. In the 1980s, the basic idea of this definition was explored in much more depth by Eric Drexler, who promoted the technological significance of nanoscale phenomena and devices through speeches and the books, Engines of Creation: The Coming Era of Nanotechnology and Nanosystem: Molecular Machinery, Manufacturing, and Computation [11] and so the term acquired its current sense. Birth of Cluster Science in the late 1980s gave further momentum to the development of nanoscience and nanotechnology. In another development, the studies on the synthesis and properties of metallic and semiconductor nanocrystals led to a fast increasing number of metal and metal oxide nanoparticles and quantum dots. In 2000, the United States National Nanotechnology Initiative (NNI) was founded to coordinate Federal Nanotechnology research and development. In short, the milestones related to the evolution of nanoscience and nanotechnology, from prehistoric to modern era, are given in Table 1.2.

Table 1.2 Milestones Associated with the Evolution of Nanoscience and Nanotechnology

1.5 Nanomaterials

Materials are what the world is made of. They are hugely important and hugely interesting. They are also intrinsically complicated. Materials, in general, are comprised of a large number of atoms and molecules and have properties determined by complex, heterogeneous structures. Historically, the heterogeneity of materials plays crucial roles in determining properties that have been determined largely empirically and manipulated through choice of the compositions of starting materials and the conditions of processing.

Nanomaterial means a material that meets at least one of the following criteria:

Consists of particles with one or more external dimensions in the size range of 1–100 nm for more than 1% of their number.

Internal/surface structures in one or more dimensions in the size range of 1–100 nm.

Specific surface-to-volume ratio >60 m²/cm³, excluding materials consisting of particles with a size less than 1 nm.

Nanomaterials can be nanoscale in one dimension (e.g., surface films), two dimensions (e.g., strands or fibers), or three dimensions (e.g., precipitates, colloids). They can exist in single, fused, aggregated, or agglomerated forms with spherical, tubular, and irregular shapes.

Nanostructures are the ordered system of one, two, or three dimensions of nanomaterials, assembled with nanometer scale in certain pattern that includes nanosphere, nanotubes, nanorod, nanowire, and nanobelt [12]. Nanostructured materials are classified as zero-, one-, two-, and three-dimensional nanostructures, showing typical examples with varied dimensionality in nanomaterials as in Figure 1.10a–i.

Figure 1.10 Typical examples showing varied dimensionality in nanomaterials: (a) fullerene; (b) quantum dot; (c) metal cluster; (d) carbon nanotube; (e) metal oxide nanotube; (f) graphene; (g) metal oxide nanobelts; (h) nanodiamond; (i) metal organic frameworks (MOFs).

A nanocrystallite is generally understood to possess crystalline order in addition to nanoscale size. If one dimension of the 3D nanostructure is at nanoscale, then it is called a Quantum Well. If two dimensions of the 3D nanostructure are at nanoscale, then it is called a Quantum Wire. If all the three dimensions of the nanostructure are at nanoscale, then it is called a Quantum Dot. Nanocrystallites are also called quantum dot. Nanomaterials are of interest because at this scale unique optical, magnetic, electrical, and other properties emerge. These emergent properties have the potential for great impacts in electronics, medicine, and other fields. Nanocarbons such as fullerenes, carbon nanotubes, and graphene are excellent examples of nanomaterials. Nanomaterials are cornerstones of nanoscience and nanotechnology. The creation of functional materials, devices, and systems through control of matter on the nanometer length scale (1–100 nm) is the exploitation of novel phenomena and properties (physical, chemical, biological) at that length scale. The phrase nanostructured materials implies two important ideas: (1) at least some of the property determining heterogeneity in materials occurs in the size range of nanostructures (1–100 nm) and (2) these nanostructures might be synthesized and distributed (or organized), at least in part, by design. The idea of nanostructured materials thus focuses on four key questions:

i. Why nanostructures are interesting?

ii. How can they be synthesized?

iii. How can they be introduced into materials?

iv. How can the relationships between compositions and structures, and matrices and interfaces control the properties of the resultant materials?

Materials by design, pertaining to the last question, have been a goal of material science since its inception. It remains, however, a difficult task because majority of research is still focused to answer the first three questions. All conventional materials such as metals, semiconductors, glass, ceramics, or polymers can in principle be obtained with a nanoscale dimension. The spectrum of nanomaterials ranges from inorganic to organic and from crystalline to amorphous particles, which can be found as single particles, aggregates, powders, or dispersed in a matrix, over colloids, suspensions and emulsion, nanolayers, and nanofilms. Supramolecular structures, such as dendrimers, micelles, or liposomes also belong to the class of nanomaterials.

1.5.1 Nanoparticles

Nanoparticles are solid particles at the intermediate state, that is, between atoms/molecules and macroscopic objects. Nanoparticles constitute of several tens or hundreds of atoms or molecules and can have a variety of sizes and morphologies (amorphous, crystalline, spherical, needles, etc.). The study of nanoparticles relates various scientific fields, for example, chemistry, physics, optics, electronics, magnetism, and mechanism of materials. Some nanoparticles have already reached practical stage. Owning to their small size effect, large surface effect, and quantum tunnel effect, the nanoparticles demonstrate special physical properties and can be widely used in a variety of applications. Some kinds of nanoparticles are available commercially in the form of dry powders or liquid dispersions. The latter is obtained by combining nanoparticles with an aqueous or organic liquid to form a suspension or paste. It may be necessary to use chemical additives (surfactants, dispersants) to obtain a uniform and stable dispersion of particles. With further processing steps, nanostructured powders and dispersions can be used to fabricate coatings, components, or devices that may or may not retain the nanostructure of the particulate raw materials. Industrial scale production of nanoparticulate materials such as carbon black, polymer dispersions, or micronized drugs has been established for a long time. Another important class of nanoparticulate materials is metal oxide nanopowder that includes silica (SiO2), titania (TiO2), alumina (Al2O3), or iron oxide (Fe3O4, Fe2O3); compound semiconductors (e.g., cadmium telluride, CdTe or gallium arsenide, GaAs), metals (especially precious metals such as Ag, Au), and alloys are also included in this category that are being commercialized.

1.5.2 Nanowires and Nanotubes

Linear nanostructures such as nanowires, nanotubes, or nanorods can be generated from different material classes, for example, metals, semiconductors, or carbon, by means of several production techniques. Carbon nanotubes are one of the most promising linear nanostructures that can occur in a variety of modifications (e.g., single-, or multiwalled, filled, or surface modified). At present, carbon nanotubes can be produced by CVD methods on a several tons per year scale.

1.5.3 Nanolayers/Nanocoatings

Nanolayers are one of the most important topics within the range of nanotechnology. Through nanoscale engineering of surfaces and layers, a vast range of functionalities and new physical effects (e.g., magnetoelectronic or optical) can be achieved. Furthermore, a nanoscale design of surfaces and layers is often necessary to optimize the interfaces between different material classes (e.g., semiconductor compound on silicon wafers) and to obtain the desired special properties.

1.5.4 Nanoporous Materials

Materials with defined pore sizes in the nanometer range are of special interest for a broad range of commercial applications because of their outstanding properties with regard to thermal insulation, controllable material separation and release, and their applicability as templates or fillers for chemistry and catalysis. One example of nanoporous material is aerogel, which is produced by sol–gel chemistry. A broad range of potential applications of these materials include catalysis, thermal insulation, electrode materials, environmental filters, and membranes as well as controlled release of drug carriers.

1.6 Strategies for Synthesis of Nanomaterials

Preparation of nanoparticles is an important branch of the materials science and engineering. Nanoparticles can be produced by a whole series of chemical, physical, or biological processes, some of which are totally new and innovative, while others have existed for a very long time. Four major processes are employed in synthesizing the new nanoparticles: gaseous phase, vapor deposition, wet chemistry, and grinding. Several of the processes for producing nanoparticles are similar to the existing chemical production processes.

i. The gas phase method includes gas-phase evaporation method (resistance heating, high-frequency induction heating, plasma heating, electron beam heating, laser heating, electric heating evaporation method, vacuum deposition on the surface of flowing oil, and exploding wire method), chemical vapor reaction (heating heat pipe gas reaction, laser-induced chemical vapor reaction, plasma-enhanced chemical vapor reaction), chemical vapor condensation, and sputtering method.

ii. Liquid phase method for synthesizing nanoparticles mainly includes precipitation, hydrolysis, spray, solvent thermal method (high temperature and high pressure), solvent evaporation pyrolysis, oxidation–reduction (room temperature and atmospheric pressure), emulsion, radiation-assisted chemical synthesis, and sol–gel processes.

iii. Solid phase method includes thermal decomposition, solid-state reaction, spark discharge, stripping, and milling method. Most of these methods result in very fine particles that are more or less agglomerated. The powders are amorphous or crystalline and show a metastable or an unexpected phase, the reasons for which are far from being clear.

Owing to the small sizes, any surface coating of the nanoparticles strongly influences the properties of the particles as a whole. Studies have shown that the crystallization behavior of nanoscale silicon particles is quite different from micron-sized powders or thin films. It was observed that tiny polycrystallites are formed in every nanoparticle, even at moderately high temperatures.

1.7 Properties of Nanomaterials

The interest in nanostructured materials arises from the fact that because of the small size of the building blocks and the high density of interfaces (surfaces, grain, and phase boundaries) and other defects such as pores, new physical and chemical effects are expected or known properties can be improved substantially. The physical and chemical properties of nanostructured materials (such as optical absorption and fluorescence, melting point, catalytic activity, magnetism, electric and thermal conductivity, etc.) differ significantly from the corresponding coarser bulk material. Roughly two types of nanostructure-induced effects can be distinguished:

The size effect, in particular the quantum size effects, where the normal bulk electronic structure is replaced by a series of discrete electronic levels.

The surface- or interface-induced effect, which is important because of the enormously increased specific surface in particle systems.

Although the size effect is mainly considered to describe physical properties, the surface- or interface-induced effect plays an eminent role for chemical processing, in particular in connection with heterogeneous catalysis. Experimental evidence of the quantum size effect in small particles has been provided by different methods, while the surface-induced effect could be evidenced by the measurement of thermodynamic properties such as vapor pressure, specific heat, thermal conductivity, and melting point of small metallic particles. Both types of size effects have also been clearly separated in the optical properties of metal cluster composites. Very small semiconductor (<10 nm), or metal particles in glass composites, and semiconductor/polymer composites show interesting quantum effects and nonlinear electrical and optical properties.

These special properties of nanomaterials are mainly due to quantum size confinement in nanoclusters and an extremely large surface-to-volume ratio relative to bulk materials, thus leading to the presence of a high percentage of atoms/molecules lying at reactive boundary surfaces. For example, in a particle with 10 nm diameter only around 20% of all atoms are forming the surface, whereas in a particle of 1 nm diameter this figure can reach more than 90%. The increase in the surface-to-volume ratio results in an increase in the surface energy of the particle, which leads to, for example, a decrease in melting point or an increase in sintering activity. Furthermore, large surface area of particles may significantly raise the level of otherwise kinetically and thermodynamically unfavorable reactions. For instance, even gold, which is a very stable material, becomes reactive when the particle size is small enough. Fundamentally, there are seven key characteristics that contribute to the uniqueness of nanomaterials [13], and these are summarized in Table 1.3. In general, the unique properties of nanomaterials are an outcome of three effects: reduced size, high surface-to-volume ratio, and supramolecular structure arising because of the self-assembly of molecules that are summarized in Table 1.4.

Table 1.3 Characteristics of Nanomaterial and Their Importance

Source: Courtesy of PerkinElmer, Inc.

Table 1.4 Size and Shape-Related Attributes and Properties of nanomaterials

Source: Patra et al. [15] © 2011. With permission of American Scientific Publisher.

1.8 Significance of Nanoscience

According to much of the information one reads in printed news articles, on websites, and even in many science journals, nanotechnology is projected to hold the key to meet the global energy needs with clean solutions, providing abundant clean water globally, increasing the health and longevity of human life, maximizing the productivity of agriculture, making powerful information technology available everywhere, and even enabling the development of space. Nanotechnology represents an entire scientific and engineering field, and not just a single product or even group of products. As a consequence, there are several areas of nanotechnology with many associated applications. With the increasing understanding on relationship between shape, size, and their physiochemical and biological properties, nanomaterials are considered to be futuristic material for diverse technologies. Current and potential areas of application include transport, manufacturing, biomedicine, sensors, environmental management, information and communications technology, materials, textiles, equipment, cosmetics, skin care, and defense. Although the list is by no means exhaustive, however, emerging application areas are discussed in the following section.

1.9 Commercial Applications

Many nanotech-based products have already been developed and are commercially available. The nanotech industry is poised for rapid growth with many additional nanotech-based products presently in their developmental stage and expected to be commercialized in the near future. The most common commercially exploited nanoparticles in various areas are those of silver, gold, iron metals, oxides of silicon, aluminum, titanium, iron, zinc, and carbon nanomaterials such as carbon nanotubes (CNT) and graphene. These materials are used for their specific and unique chemical, physical, and biological properties together with established technology for their scaled production now. It is also important to note that products are rarely 100% nanotechnology based; nanotechnology will be added to a product and form a part of it. Some of important sectors where nanotechnology-based products are available commercially [15] are given in following sections.

1.9.1 Food Industry

Silver is currently the most common