2 Introduction: Nanoscience and Nanotechnology

S.S. islam

Introduction:

Nanoscience and nanotechnology are being envisaged as the science and technology of the future. They are being expected to transform our understanding of almost every natural phenomenon. Nanotechnology is predicted to revolutionize every aspect of human life. However, the technology itself is in its nascent stage and needs comprehensive understanding of the various aspects of objects at which it is applicable. This chapter provides the essential insights into the dimensions which are relevant to this technology.

1. Length, energy and time scales

This module presents an introduction about nanoscience, nanotechnologies, and nanomaterials. Historical account of the events leading to the formal introduction of the field is also given.

Nanoscience

Nanoscience is the study of processes and manipulation of materials at atomic or molecular scale, such that the properties vary considerably than at larger scales, i.e., bulk materials.

Bulk and micron-sized materials (such as a sand granule) demonstrate continuous (macroscopic) physical properties. However, as the size of the materials is decreased down to the nanoscale, classical physics fails to explain their behaviors or properties (e.g., energy, momentum, etc.), and quantum mechanics needs to be applied to describe them. For example, depending upon the particle size, gold demonstrates very different properties (optical, electrical, mechanical, etc.) at nanoscale (Figure 1).

Figure 1 Different sized gold particles produce different colors at nanoscale.

Nanotechnologies include “nanoscale designing, characterizing, and producing structures, devices and systems by controlling shape and size for their applications in various fields”.

Analogous to several other disciplines, nanotechnology was in use several centuries before any formal definition of the field. For example, nanotechnology was widely used in steel and painting industries. Early contributions came from James Clark Maxwell and Richard Adolf Zsigmondy. Zsigmondy investigated colloidal solutions including sols of gold as well as other nanomaterials. Irvin Langmuir and Katherine B. Blodgett contributed significantly to the field in first half of twentieth century.

American physicist Richard Feynman is credited with the first modern systematic discussion and formal announcement of nanotechnology as an important field of scientific endeavor. In this regard, his famous lecture “There’s Plenty of Room at the Bottom” given in 1959 has been adapted in various discussions of the field. Though he did not coin the term ‘nanotechnology’, he emphasized the significance of manipulation of matter at very small scale such that these studies will allow the understanding of processes occurring in complex situations. He talked about size dependent behavior of various phenomena and proposed various challenges such as creating a nanomotor, embedding whole Encyclopedia Britannica on a pin’s head.

Norio Taniguchi, Japanese scientist, mentioned the term ‘nanotechnology’ for the first time in 1974 in his paper on synthesis technology to create objects and features of nanometer dimension. He writes:

“In the processing of materials, the smallest bit size of stock removal, accretion or flow of materials is probably of one atom or one molecule, namely 0.1–0.2 nm in length. Therefore, the expected limit size of fineness would be of the order of 1 nm.… Nano-Technology mainly consists of the processing … separation, consolidation and deformation of materials by one atom or one molecule”.

K. Eric Drexler, an American engineer, is often cited with the developer of molecular nanotechnology which led to the development of nanosystem machinery.

The respective discoveries of scanning tunneling and atomic force microscopes in 1980s are considered milestone for the development of nanotechnology as a filed. These microscopes allowed atomic level imaging of materials which is crucial for manipulating matter at atomic/molecular scales. The parallel advancements in computer technology facilitated large scale simulations and analyses of materials by supercomputers, thereby providing significant insights into the structure as well as properties of the materials. The simultaneous modeling, visualizing, as well as manipulation activities, greatly stimulated research investigations in the twentieth century.

One such effort was made by IBM wherein the physicists performed atomic scale manipulation of xenon atoms to produce the pattern “IBM” in 1990 (see Figure 2). The discovery of new carbon structure, buckyball made up of 60 carbon atoms, further increased the interest in the field. This propelled the investigation to discover other nanostructures made up of carbon as well as other materials. The 1991 finding of carbon nanotubes has been instrumental in enthusing research activities in the field of nanoscience and nanotechnology; consequently, numerous nanotechnologies have been unearthed. Carbon nanotubes are one of the most studied materials owing to its amazing properties such as it gives ~100 times strength than steel at 1/6th of the weight. Similarly, novel structures including Quantum dots have been produced which demonstrate properties intermediary to the bulk and single molecular structures.

Figure 2 Demonstration of manipulating atoms with atomic-scale precision with Xenon atoms by IBM.

The astounding physical, chemical as well as mechanical behaviors of nanomaterials, such as large surface area, tiny size, etc.; offer extensive applications in numerous fields including electronics, optoelectronics, etc. Presently, molecular electronics is being proposed as new technological breakthrough. As silicon IC technology is approaching its physical limits, nanotechnology is expected to drive the future generation electronics. If devices can be made from a small cluster of atoms or molecules, computer chips can be fabricated to house 10 times more transistors than possible by the present technology.

1.1 Nanometre Scale

Conventionally, nanoscale has been defined as 1-100 nm. A nanometer is one billionth part of one meter. The minimum size is set to 1 nm, thus a single atom or very small clusters of atoms cannot be considered as nanoparticles. Consequently nanoscience and nanotechnology involve a cluster of atoms at least 1 nm in size. However, the necessary condition for denoting clusters of atoms as nanomaterials is the onset of a quantum phenomenon instead of the actual dimension at which the effect occurs. Furthermore, above nanoscale, the properties of a material result from bulk or volume effects, namely the type of the atoms present, type of bonding existing between them, and the stoichiometry. Below this point, the properties of the materials change and even though the type of atoms and their organizational orientation are still important, surface area effects begin to dominate. The surface area effects include the size as well as the shape of the object.

Therefore, nanoscience is not just the science of the small, but the science where materials with small dimensions demonstrate new physical phenomena, collectively described as quantum effects. Quantum effects are size-dependent and significantly differ from the properties of macroscaled or bulk materials. The changed properties may involve, though not limited to, color, solubility, strength, electronic and thermal conductivities, magnetic behavior, mobility, chemical and biological activities, etc.

Figure 3 Three and a half gold atoms (each with the covalent radius of 0.144 nm) positioned adjacently in a row equal 1 nm.

    1.2   Nanomaterials

The objects with at least one dimension in nanometer range (i.e., ~1-100 nm) are termed as nanomaterials.

Dimension dependent classification of nanomaterials is enumerated in Table 1:

   Broadly, nanomaterials are classified into two types:

• “non-intentionally produced nanomaterials”, these are the nanosized particles or materials which are present in the environment naturally (e.g. proteins, viruses, nanoparticles released from volcanic eruptions, etc.) or a result of human activities (e.g. particles emitted from fossil fuel combustions).
• “intentionally produced nanomaterials” are the nanomaterials produced by following a specific synthesis technique. These are the desired nanomaterials which are produced for a particular application.

    Nanotechnologies, as a matter of fact, do not include the non-intentionally produced nanomaterials.

Figure 4 includes a scale to compare the size of various objects on a nanometer scale.

Some examples of nano-scaled objects encountered in daily life are as follows:

• Our fingernails grow 1nm per second
• The diameter of human hair is ~80,000 nm
• Size of a DNA molecule is ~1-2nm
• A pin head is ~one million nanometer across.
• The transistor of Pentium Core Duo processor is 45nm.

   Nanoscience is “interdisciplinary” in that it includes concepts of multiple disciplines such as chemistry, physics, biology, etc. it is a “horizontal encompassing interdisciplinary sciences cutting across all vertical sciences and engineering disciplines”, as shown in Figure 5(a).

Figure 5 Impact areas of (a) Nanoscience, and (b) Nanotechnologies.

Nanotechnologies involve employing nanoscience to industrial and commercial purposes. Since all industries heavily rely on materials and devices, all of which are made up of atoms and molecules, and therefore, all industries can benefit from the nanotechnologies. Thus, similar to nanoscience, nanotechnologies are also horizontal-enabling convergent technologies. Nanotechnologies are “horizontal” as they intersect several industrial sectors; and “enabling” because they offer the platform and the means to realize specific devices. Additionally, since they draw together scientific disciplines which were hitherto considered separate, they are considered as “convergent” technology, as illustrated in Figure 5(b).

2. Nanotechnology or Nanotechnologies?

Originally, the term was used in its singular form to specify single technology. However, past few decades have witnessed steady rise in the field in terms of science and technology development. It has proposed many new technologies to study various materials and disciplines. Even though, all these technologies are different, they have the common concept of examining the material properties at nanoscale dimensions. Thus a more appropriate use of the term involves using it in plural form.

3. Nanoscience in Nature

Although nanoscience is generally considered as the futuristic science, it forms basis for the majority of the systems in living as well as mineral world. There are various examples of nanoscience in our surroundings including geckos walking upside down on a ceiling, apparently against gravity, to butterflies with iridescent colours, to fireflies glowing in the night (Figure 6). Nature provides excellent solutions to various problems where fine nanostructures have precise associated functions. Owing to the recent advancements in analytical tools, the examination of these nanostructures and their functions has become possible resulting in profuse research in nanoscience and nanotechnologies.

Figure 6 High magnification view of Morpho rhetenor showing the scales, and photonic crystal structures, which in turn show a cross section displaying setae looking like fir trees.

4. Time scale

Time scale relevant in nanoscience and nanotechnologies ranges from 10-15s to several seconds. The temporal scale varies linearly with the number of particles N, the spatial scale goes as N(logN), yet the accuracy scale can go as high as N7 to N! with a significant prefactor.

Figure 7 shows the typical time scales for a variety of events encountered in the study of nanoscaled objects. From this figure, it can be understood that instead of a single time scale, various different time scales exists and all of these are significant. Any specific time scale and its importance vary from process to process. Thus a particular time scale important for one process may not be important for any other process.

Therefore, the studies involve multiscale problems wherein different processes are considered over different time scales. The selection of a particular time scale depends on the process under investigation.

Figure 7 Typical time span of different processes.

5. How do we build nanostructured objects?

Two approaches can be followed to synthesize nanostructured materials. One is top-down strategy which involves taking the bulk materials and then repeatedly breaking it into smaller and small pieces. This can be achieved by chemical, mechanical, or any other method. The second procedure is the bottom-up technique, wherein more complex assemblies are built atom by atom, molecule by molecule or cluster by cluster. Top-down approach can produce structures with long range order, and can also be used to make macroscopic connections. Bottom-up approach is ideal for assembly and establishes short-range orders at nanoscales. Both of these methods are possible in gas, liquid, solid states, or vacuum. The important concerns in nanofabrication are controlling the – (a) size, (b) shape, and (c) clustering – of the particles. The hybrid of both these approaches can offer appropriate means for controlled synthesis of nanoparticles and this is the current approach followed in nanofabrication.

Figure 8 Schematic representation of top-down and bottom-up methods in nano-synthesis.

a) Top-down Approach

In top-down approach, the nanosized feature or pattern is created by cutting away material from the bulk structure – similar to a sculptor, never dealing with atomic level of matter. One of the most popular techniques using this approach is lithographic patterning where short wavelength light source is used to create the desired structure. Its main advantage is that the patterns as well as the objects are created at the desired place and no further assembling is required. Lithography is a highly matured and advanced field owing to the developments in the laser technologies as well as chip manufacturing. Visible as well as UV and X-ray sources are used in this technique to achieve the desired level of resolution. For achieving still higher resolutions and minimizing the pattern size, electrons beams have also been shown to hold great promise. In these cases, the patterns are created by scanning tightly focused electron beam across the surface to be patterned. Ion beams can also be employed for direct processing and pattering of wafers. Scanning probes can be used to create still smaller patterns by depositing or removing very thin layers.

Mechanical printing methods involving nanoscale imprinting, stamping and molding have reached 20-40 nm dimensions. The basic principle behind all these is similar – make a master stamp using high-resolution techniques like electron-beam lithography, and apply the stamp onto a substrate for creating the desired pattern. Additionally, for making thin films in the pattern of the stamp, a very thin layer of the material (ink) is deposited on the surface of the stamp. These nanoscale printing techniques offer several advantages such as – facile and economic equipments.

Even though top-down methods can easily create microscaled patterns, achieving nanoscale resolution is the major concern for all top down methods. Additionally, they are planar technology and producing complex and arbitrary objects is not possible.

b) Bottom-up Approach

Bottom-up approach uses assembling the object atom by atom, by selectively placing an atom at a particular position to create the feature. For assembling basic units to form larger structures, it makes use of chemical or physical forces operable at nanoscale. As size of the object decreases bottom-up approach becomes more and more significant complement of top-down technique. This technique reflects the natural phenomena occurring in biological systems, wherein chemical forces effectively form all structures required for life. Producing clusters of specific atoms which have the capability to self-assemble to produce more complex and elaborate structures.

Over the years, several techniques have been devised which follow bottom-up approach to create nanostructured materials. These techniques vary from condensation of atomic vapors on a substrate to coagulation of atoms within liquids to produce nanoparticles. Processes to fabricate nanostructured materials from atoms involve transformation processes in solutions, e.g. sol–gel technique, plasma synthesis, spraying, chemical and physical vapor depositions, etc. Availability of starting materials (often termed as precursors) is a critical aspect in nanoparticle synthesis processes.

References:

1. Taniguchi, Norio. “On the basic concept of nano-technology.” Proc. Intl. Conf. Prod. London, 1974. British Society of Precision Engineering, 1974.
2. https://www.nano.gov/nanotech-101/what/definition.
3. https://en.wikipedia.org/wiki/Nanotechnology.
4. http://futurehumanevolution.com/history-of-nanotechnology-updated.