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Fundamental Concepts

1.1 Introduction to Nanoscience and Nanotechnology

The word "nano" originated from the ancient Greek ????? (nânos), meaning dwarf. In modern science and technology, nano is specifically used as an SI prefix, referring to the multiplying factor of 10-9. For example, an anti-HIV/AIDs drug can be said to give an inhibitory concentration of a few nanomoles per liter (10-9 mol L-1), an organic fluorescent molecule shows a fluorescence lifetime of tens of nanoseconds (10-9 second), and the third carbon allotrope, Buckminsterfullerene C60 (bucky ball), has a diameter of 1 nanometer (10-9 meter) as illustrated in Figure 1.1. Today the use of "nano" has gone far beyond its numerical meaning. In the academic world, nano has become a buzzword spanning the fields of chemistry, physics, biology, medicine, and engineering, where it is used to describe studies of advanced molecular, macromolecular, and supramolecular materials with the dimension of nanometers. In the industrial sector, many nanotechnology companies have been established to deal with the development and commercialization of nanometer-sized materials and devices for special purposes and tasks. The National Nanotechnology Initiative (NNI) in the US has defined nanotechnology as a science, technology, and engineering conducted at the scale of 1-100 nm. Unique properties and functions would arise from materials on such a small scale, which are often unprecedented and considerably more advantageous in comparison to conventional bulk materials.

Figure 1.1 Illustration of various objects at the meter, micrometer, and nanometer scales. Credit: Alissa Eckert, MS; Dan Higgins, MAM / Wikimedia Commons / Public Domain.

So, how did nanotechnology start? The production and manipulation of nanoscale materials in fact have a surprisingly long history. Take the famous ancient artifact, the Lycurgus cup, as an example. This cup is an impressive Roman treasure made in about AD 400, which was named for its depiction of King Lycurgus of Thrace entangled in grape vines. The Lycurgus cup is well known because of its color-changing glass. When light is shone upon it, the cup displays a jade green color. When light is shone through it, however, the cup turns into a brilliant red color. The reason for this color-changing property is due to the gold-silver alloyed nanoparticles that are distributed in the glass. These nanoparticles scatter reflected light and back-illuminated light in different ways. Another example of ancient nanotechnology is the Damascus steel swords from the Middle East, which were made between AD 300 and AD 1700. They are known for their superior strength, shatter resistance, and exceptionally sharp cutting edge, owing to the use of the so-called wootz steel in their blade making. A recent scientific discovery disclosed that the wootz steel is full of carbon nanotubes, which are a class of appealing nanomaterials discovered by scientists in 1991. Nowadays, those artifacts would be included in a sub-branch of nanotechnology, known as nanocomposites. Beyond a doubt, those ancient artisans possessed masterful skills and empirical knowledge which enabled them to fabricate such stunning artifacts, but the presence and roles of the nanomaterials were not consciously known by them, neither were their related chemical and physical principles.

Many consider that the year of 1959 marked the inception of the concept of modern nanotechnology. In the December of that year, a visionary American physicist, Richard Feynman, who was the Nobel Prize laureate in Physics in 1965, gave a lecture at an annual meeting of the American Physical Society at Caltech, entitled There's Plenty of Room at the Bottom [1]. In this famous lecture, Feynman envisioned a day when devices and machines could be miniaturized in such a way that huge amounts of information could be stored in extremely small spaces, while machines could be fabricated and compacted together at a much smaller scale. Feynman's futuristic ideas sparked the beginning of the modern nanotechnology, although Feynman himself never used the term "nanotechnology" in his lectures. It was Prof. Norio Taniguchi of the Tokyo University of Science who coined the term "nano-technology" fifteen years later. In 1974, Taniguchi published a paper entitled On the Basic Concept of Nanotechnology, in which he wrote "Nanotechnology mainly consists of the processes of separation, consolidation, and deformation of materials by one atom or one molecule" [2]. This term was then adopted and greatly promoted by an American engineer, Eric Drexler, in his popular book Engines of Creation: The Coming Era of Nanotechnology published in 1986 [3]. In this book, Drexler imagined numerous future technologies, including an unprecedented class of tiny machines, which he termed molecular assemblers. He predicted that these assemblers would have the ability to precisely build objects in an atom-by-atom manner.

The era from 1980s to 1990s was full of exciting experimental discoveries and achievements in nanotechnology. In 1981, Gerd Binnig and Heinrich Rohrer at IBM Zurich Research Laboratories in Switzerland developed the first working scanning tunneling microscope (STM), for which they won the Nobel Prize in Physics in 1986. In 1985, these IBM researchers developed the technique called atomic force microscopy (AFM), which added another powerful tool for imagining and manipulating nanoscale objects on various substrates. A tour de force in precise positioning of atoms was achieved by IBM researchers Don Eigler and Erhard K. Schweizer in 1989, who used STM to create a tiny IMB logo with only 35 xenon atoms (see Figure 1.2). A range of nanoscale materials were discovered and developed during this period, which opened many sub-branches in modern nanotechnology.

Figure 1.2 An IBM logo formed by positioning 35 xenon atoms on a nickel (110) surface. Credit IBM.

Nanotechnology is not an individual discipline in science and engineering. Unlike the traditional disciplines (e.g., mathematics, chemistry, physics, and biology), nanotechnology is more like a technological hub that gathers a vast array of research under its umbrella. Research involving nanoscale materials has permeated almost every classical division of science and technology, making it highly multidisciplinary and without a clear boundary. Therefore, it is very hard to define the skills and precise type of backgrounds required to be a "nanotechnologist." Nanotechnology broadly covers engineering, chemistry, biology, medicine, computer science, theoretical simulations, devices and structures fabrication, just to name a few. In most nano-related studies, the preparation of nanoscale materials and precise control over their dimensions and shapes take center stage. To achieve these goals, special methods for nanofabrication are needed. In general, nanofabrication can be carried out using two different approaches, namely top-down and bottom-up. The top-down approach starts with the manipulation of bulk materials and structures. For example, the fabrication of a computer chip is done by "engraving" the surface of a piece of single-crystal silicon using a technique known as photolithography as the key step. Through the top-down approach, miniaturized devices are "chiseled out" of the bulk material in a precisely controlled manner, with the original integrity of the bulk material (e.g., crystallinity and long-range order) still retained. The bottom-up approach produces nanomaterials and devices through the self-assembly or chemical synthesis of certain nanoscale building blocks. The self-assembling process is dictated by specific chemical and/or physical forces (e.g., metal ligand coordination, hydrogen bonding, and p-p stacking) to form defined nanostructures. The building blocks can be obtained from naturally existing materials (e.g., DNA, lipids, carbohydrates) or prepared by chemical synthesis (e.g., synthetic molecules and polymers). The biological world is full of events utilizing the bottom-up approach, such as protein synthesis and cellular growth. Inspired by these natural materials and events, enormous efforts have been dedicated to applying various molecular functions and supramolecular forces to create nanostructures and materials.

1.2 Nanochemistry in Action

As prophesized by Richard Feynman, there's plenty of room at the bottom. Atoms and molecules are at the bottom of our physical world and can be manipulated and controlled. The knowledge and techniques for dealing with the assembly of atoms and molecules have long existed in the field of chemistry. According to the definition provided by Britannica, chemistry is "the science that deals with the properties, composition, and structure of substances (defined as elements and compounds), the transformations they undergo, and the energy that is released or absorbed during these processes". Jean-Marie Lehn, a 1987 Nobel Prize laureate in Chemistry, stated that "the science of chemistry is not just about discovery. It is also, and especially, about creation. It is an art of the complexification of matter. To understand the logic of the latest discoveries in nanochemistry, we have to take a 4-billion year leap back in time". So, it is obvious that chemists have been equipped with the required tools (knowledge of the properties of molecules...