A Primer on Nanotechnology

Ask 10 people what nanotechnology is and you will get 10 different answers. And then there are all these terms floating around: "bottom-up" and "top-down" fabrication, "atomically precise manufacturing", "molecular assembly", "self-assembly", "nanorobots", "nanofactories" and so forth.

Of course you have also heard about all these "nanotechnology" products already hitting the market - but they seem decidedly low-tech, such as golf balls, 'no-smell' socks, toothpaste, scratch-resistant car paint, and so on. That's what we have been investing billions and billions of dollars for?

Image: Nano-Explosions - Color-enhanced scanning electron micrograph of an overflowed electrodeposited magnetic nanowire array (CoFeB), where the template has been subsequently completely etched. It's a reminder that nanoscale research can have unpredicted consequences at a high level. (Image: Fanny Beron, École Polytechnique de Montréal, Canada)

One of the problems facing nanotechnology is the confusion about its definition. Most definitions revolve around the study and control of phenomena and materials at length scales below 100 nanometers (nm) and the most overused comparison you read about all the time is that with a human hair, which is about 80,000 nm wide.

Some definitions include a reference to molecular systems and devices and nanotechnology 'purists' argue that any definition of nanotechnology needs to include a reference to "functional systems".

Another important criteria for the definition is the requirement that the nano-structure is man-made. Otherwise you would have to include every naturally formed biomolecule and material particle, in effect redefining much of chemistry and molecular biology as 'nanotechnology. The most important requirement for the nanotechnology definition is that the nano-structure has special properties that are exclusively due to its nanoscale proportions.

A good definition for "nanotechnology" that is practical and unconstrained by any arbitrary size limitations is this one (source):

The design, characterization, production, and application of structures, devices, and systems by controlled manipulation of size and shape at the nanometer scale (atomic, molecular, and macromolecular scale) that produces structures, devices, and systems with at least one novel/superior characteristic or property.

The bulk properties of materials often change dramatically when reduced to nanoscale dimensions. This has to do with two main reasons:

Surface area

Nanomaterials have a relatively larger surface area when compared to the same mass of material in bulk form. This can make materials more chemically reactive (in some cases materials that are inert in their larger form are reactive when produced in their nanoscale form), and affect their strength or electrical properties.

Image: Climatic change on carbon nanotubes - Carbon nanotubes have many characteristics that promise to revolutionize the world of structural materials. There are different ways to grow carbon nanotubes, especially the CVD technique, which allows obtaining SWCNT's on a silicon surface. These SWCNT can be carried from the silicon surface to another surface, as HOPG, without suffering changes on their properties. That means nanomanipulation of carbon nanotubes. (Mr Miguel Ângel Fernández Vindel, Universidad Autonoma de Madrid/Spain)

To understand the effect of particle size on surface area, consider a U.S. silver dollar. The silver dollar contains 26.96 grams of coin silver, has a diameter of about 4 centimeters, and has a total surface area of approximately 27.70 square centimeters. If the same amount of coin silver were divided into tiny particles - say 1 nanometer in diameter - the total surface area of those particles would be 11,400 square meters. In other words: when the amount of coin silver contained in a silver dollar is rendered into 1 nm particles, the surface area of those particles is over 4 million times greater than the surface area of the silver dollar! (Source)

Quantum Effects

Quantum effects can begin to dominate the behavior of matter at the nanoscale - particularly at the lower end - affecting the optical, electrical and magnetic behavior of materials. Materials can be produced that are nanoscale in one dimension (for example, very thin surface coatings), in two dimensions (for example, nanowires and nanotubes) or in all three dimensions (for example, nanoparticles and quantum dots).
Composites made from particles of nano-size ceramics or metals smaller than 100 nanometers can suddenly become much stronger than predicted by existing materials-science models. For example, metals with a so-called grain size of around 10 nanometers are as much as seven times harder and tougher than their ordinary counterparts with grain sizes in the hundreds of nanometers.

The causes of these drastic changes stem from the weird world of quantum physics. The bulk properties of any material are merely the average of all the quantum forces affecting all the atoms. As you make things smaller and smaller, you eventually reach a point where the averaging no longer works and you have to deal with the specific behavior of individual atoms or molecules - behavior that can be very different to when these atoms are aggregated into a bulk material.

Systematic control and manufacture at the nanoscale are envisioned to evolve in four overlapping generations of new nanotechnology product types that start with nanoscale building blocks and evolve over time into complex heterogeneous systems. Each anticipated generation of products will provide a nanotechnology base for further innovation, leading to succeeding generations of products of increasing complexity and functionality (source).

Image: Jumper Sometimes the thesis just gets to be too much... This image is of some contamination (probably monodisperse polystyrene spheres from a previous user) in the microscope which just happened to collect at the corner of a Ta2O5 particle. The image was collected using secondary electrons in a Hitachi S-4700 SEM and was colorized using the Gnu Image Manipulation Program (GIMP). (Image: Georff Brennecka, Sandia National Laboratories)

First Generation (beginning ~2000): passive nanostructures, illustrated by nanostructured coatings, nanoparticles, dispersion of nanoparticles, nanocomposites, and bulk nanostructured materials - nanostructures made of metals, polymers, ceramics; bio-building blocks. The nanotech sunscreens and golf balls that you read about are only incremental improvements of previous products where thanks to the use of such passive nanostructures - in the case of sunscreen zinc oxide or titanium dioxide; and carbon nanotubes in the case of golf balls - better performance characteristics could be achieved.

Second Generation (beginning ~2005): active nanostructures, illustrated by transistors, amplifiers, targeted drugs and chemicals, biological and non-biological sensors, actuators, and adaptive structures.

Third Generation (beginning ~2010): three-dimensional nanosystems and systems of nanosystems using various synthesis and assembly techniques such as bio-assembly, networking at the nanoscale, and multiscale architectures.

Fourth Generation (beginning ~2015): materials by design and heterogeneous molecular nanosystems, where each molecule in the nanosystem has a specific structure and plays a different role. Molecules will be used as devices, and from their engineered structures and architectures will emerge fundamentally new functions.

Nanotechnology is not an industry; nor is it a single technology or a single field of research. What we call "nanotechnology" consists of sets of enabling technologies applicable to many traditional industries (that's why it would be more correct to speak of nanotechnologies in the plural).

Image: Nano Rings - The image shows a four-terminal quantum ring structure defined in a two-dimensional electron gas (2DEG) with local anodic oxidation using an atomic force microscope tip. The elevated white lines represent the oxide on the surface of the GaAlAs heterosstructure containing the 2DEG. These oxide lines are on average 15nm high and penetrate just as deep into the sample surface, forming barriers in the electron gas below. The ring has an average diameter of 1 micron and the four outer rectangular areas enclosed by oxide lines are used as in-plane gates to tune the electron density of the four arms of the ring. Measuring Aharonov-Bohm oscillations in the ring conductance this device is used to interferometrically detect the relative phaseshift of Coulomb blockade resonances in two quantum dots induced in the arms of the ring. (Dr Andreas Fuhrer, Nanophysics Group of Prof. Ensslin at ETH Zürich/Switzerland)

You often hear nanotechnology described as a "platform technology" - these are technologies that are so pervasive that they serve as springboards for other technologies and as foundations for many diverse applications; they are also regarded as essential for progress in multiple fields. Computer operating systems are a good example for a platform technology. Rather than having to deal with computer hardware directly, programmers work with an abstraction of the underlying hardware (the operating systems) to build diverse applications from games to control software for nuclear power plants.

In a similar fashion, nanotechnologies will allow the precise control of individual atoms and molecules, leading to an unprecedented ability in many diverse areas to develop new materials, devices or medical treatments.

There are different ways of manipulating matter at the nanoscale. The two notions you hear most are 'top-down' and 'bottom-up' methods.

Top-down and bottom-up

Michelangelo was a "top-down" artist. He took one big, raw block of Carrara marble and after years of chiseling away produced David. In the process he reduced the original block of marble to half its original volume and left the other half as waste. This is the nanotechnology equivalent of lithography and other top-down methods where you start by taking a block of material and remove the bits and pieces you don't want until you get the shape and size you do want. In the process you spend (relatively much) energy, use (sometimes very toxic) chemicals, produce (often quite a bit of) waste, need a lot of patience (these processes are relatively slow) and often the results are quite unique and not easily replicable.

Bottom-up methods are much more elegant and efficient. Take lego blocks. Just pick the shape and sizes you need and - one by one - build more or less anything you want with them. Replace your hands with a (tiny) machine, or some other assembly process, and the legos with atoms or molecules and you have molecular assembly. Unfortunately, this analogy is too simplistic.

The other way of doing bottom-up nanotechnology is man's way: molecular assembly. It sounds like self-assembly but it is a very different concept. If you take a look at the nanofactory animation 'Molecular desktop manufacturing Productive Nanosystems: From Molecules to Superproducts' you'll get the idea.

Image: Scanning near-field optical microscopy (SNOM) uses nanoscale metal tips to scan a surface. Here, a standard tip has been modified and sharpened to increase its precision. The tip in the middle of this structure measures a few tens of nanometers. (Image: G.C. Gazzadi, S3 (INFM-CNR), Modena; P.Gucciardi, CNR-IPCF, Messina. Artwork: Lucia Covi)

This is the vision that proponents or revolutionary nanotechnology put forward: molecular assembly as a factory concept, assembly lines and all, just scaled down to the nano level. The notion of 'self-assembly' becomes relevant in this context with regard to 'self-replicating' nanomachines, i.e. machines that self-assemble themselves; but this is very different from the type of self-assembly found in nature.

There is one - very big - catch though: Today, universal molecular assembly is a vision; in a scientific sense it is not even a theoretical concept yet.

With our technical capabilities today, the most advanced bottom-up nanotechnologies are a combination of chemical synthesis and self-assembly. But they already allow us to perform atomically precise manufacturing on a modest scale and this will lead to vastly improved materials, much more efficient manufacturing processes and entirely new medical procedures. And we haven't seen the end yet of where nanotechnology "doing it nature's way" will lead. Maybe we will never have, and never need, molecular assembly modeled after today's factories.