Nanotechnology: Smaller is Better

"There is plenty of room at the bottom!"

It was 29th December, 1959. Nobel laureate physicist Prof. Richard Feynman was delivering a speech. The occasion was the annual meeting of the prestigious American Physical Society, and the audience was full of eminent scientists.

Though Feynman's speeches were generally considered to be masterpieces of scientific imagination, most of the scientists present there thought that Feynman was trying to be funny.

Nobody in the audience could think at that time when Feynman uttered the words ," ...there is plenty of room at the bottom..." he was directly looking into the future some 30 years ahead! What the scientists are busy working at today and technologists considered as technology of 21st century; Feynman was dreaming it in 1959.

By his key words Feynman strongly pointed out that there is a tremendous scope for research on materials of very small size. When Feynman delivered his visionary talk before the American Physical Society members it was just the beginning of miniaturization.

People were talking of palm sized electric motors, and were still proud of their 'small' computers that filled big halls and could do jobs which are done by today's pocket calculators. The computers also were very 'power hungry' as they used 'valves', which have become almost obsolete. These are seen nowadays only in science museums.

In 1949, Bardeen and others discovered a solid state transistor, which was already revolutionizing the electronics industry. But Feynman was not satisfied with this, and was asking why not have palm size computers? And how to achieve it?

Feynman suggested that by using electrons, ions, or atoms, one could carve the materials and make a whole galaxy of wires, capacitors, transistors, diodes, and other complicated circuitry in a small space.

He dared to suggest that by controlled evaporation of atoms and molecules materials of the required size could be achieved. It sounded like a fantasy in 1959 to the scientists and technologists, but has turned into reality now.

Scientists and technologists of today find themselves practicing what Feynman had imagined. Today we are flooded with miniaturization, i.e., table-top computers, laptops, mobiles, CDs, DVD's, etc. What do they have?

Very Large Scale Integrated (VLSI) circuits with thousands of components compacted together in a single chip of few mm dimension are employed everywhere from kitchen appliances to space vehicles.

But Feynman's envisioned beyond this miniaturization. He was not only pointing towards such developments but still beyond that. By 'small size' Feynman meant something closer to biological systems like a living cells and organisms.

With their interior structure of submicrometer size, they are able to perform a variety of functions. They are able to move, change shapes, respond to optical, acoustic, or other signals. They would also be able to identify other shapes or cells. They are not only able to store information, but would even be able to take decisions.

In other words, they are miniature, highly efficient machines created by Mother Nature. In fact, the beauty of this creation by nature lies in that the atoms or molecules come together, assemble, grow, reproduce, find their own food, develop, and die.

Nanotechnology, also called molecular manufacturing, is 'a branch of engineering that deals with the design and manufacture of extremely small electronic circuits and mechanical devices built at the molecular level of matter'..

The goal of nanotechnology is to be able to manipulate materials at the atomic level to build the smallest possible electromechanical devices, given the physical limitations of matter.

It has been said that a nanometer is 'a magical point on the length scale, for this is the point where the smallest man-made devices meet the atoms and molecules of the natural world'. Much of the mechanical systems we know how to build will be transferred to the molecular level as some atomic analogy.

Materials that can be made at nanoscale, give them the potential to have some very interesting properties which sharply differ from one's characteristic for bulk materials. In the nanoscale regimes neither quantum chemistry nor classical laws of physics hold.

It is this scale where many properties of materials are controlled by phenomena that have their critical dimensions at the nanoscale. By being able to fabricate and control the structure of nanoparticles, the designers can influence the resulting properties and, ultimately, design materials to give desired properties.

The electronic properties of the material in consideration can be controlled at this scale. This phenomenon forms the basis for modern electronics industry.

A host of properties depend on the size of such nanoscale particles, including magnetic properties, melting points, specific heats, and surface reactivity. Furthermore, when such ultra fine particles are consolidated into macroscopic solids, these bulk materials sometimes exhibits new properties (e.g. enhanced elasticity).

Nanoparticle synthesis can be achieved through a wide variety of routes. Basically there are four generic routes to make nanoparticles.

1) Wet Chemical Route
2) Mechanical Route
3) Form-in-place
4) Gas-phase Synthesis

Wet chemical processes include chemistry, hydrothermal methods, sol-gels, and other precipitation processes. Essentially, solutions of different ions are mixed in well-defined quantities and under controlled conditions of heat, temperature, and pressure to promote the formation of insoluble compounds, which precipitate out of the solution.

These precipitates are then collected through filtering and/or spray drying to produce a dry powder. Mechanical processes include grinding, milling, and mechanical alloying techniques. Provided that there is a coarse powder, this coarse powder mechanically is transformed into finer and finer powder.

The most common processes are either planetary or rotating ball mills. The advantages of these techniques are that they are very simple, require low cost equipment, and, provided that a coarse feedstock powder and be made, the powder can be processed.

However, there are difficulties such as agglomeration of particles, broad particle size distribution, contamination from the process equipment, and often difficulty in getting to the very fine particle sizes with viable yield. It is commonly used for metals and inorganics.

Form-in-place processes vacuum deposition processes such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), and spray coating. These processes are focused to the production of nanostructured layers and coatings with enhanced properties for different applications.

Gas phase synthesis includes flame pyrolysis, electro-explosion, laser ablation, and plasma synthesis techniques. The production of fullerenes and carbon nanotubes is a specific subset of gas-phase synthesis techniques. These processes are quite inefficient for the fabrication of powders which can be manufactured by scraping the deposits from the collector.