Behind the buzz and beyond the hype:
Our Nanowerk-exclusive feature articles
Posted: Jun 15, 2007
The phenomena behind nanotechnology's many promises
(Nanowerk Spotlight) "Canadians spy on U.S. with nanotechnology coins!" You might remember this hilarious story that made the rounds a few weeks ago. The U.S. Defense Department had issued an espionage warning after U.S. Army contractors traveling in Canada had filed confidential espionage accounts about Canadian coins as "anomalous" and "filled with something man-made that looked like nanotechnology." It just exemplifies how the term "nanotechnology" gets thrown around and misused for all kinds of purposes. Just because something is really small doesn't mean it has to do with nanotech- nology. Of course, here at Nanowerk we sometimes fall into the same trap and use "nanotechnology" in a story headline just to make it catchier, even if the underlying story is not so much about a "technology" but rather a nanoscale phenomenon. Today's Spotlight therefore takes a step back and looks at some of the various nanoscale phenomena that make new technologies - nanotechnologies - possible and that hold the key to many technological advances that lie ahead of us.
Chemists have dealt with naturally occurring nanoparticles all along. Think molecules or viruses. Toxicologists have dealt with nanoparticles that are the result of modern human life such as carbon particles in combustion engine exhaust. Without being aware of it, tire manufacturers used nanoparticles – carbon black – to improve the performance of tires as early as the 1920s. Medieval artists used gold nanoparticles to achieve the bright red color in church windows (gold particles in nanometer size are red, not golden). You might even say that we are surrounded by, and made of, nanomaterials – atoms and molecules are nanoscale objects after all.
So to start off, and before we go into the details, let's introduce a definition for nanotechnology that we are using here on the Nanowerk site. 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 nm and quite often they make a comparison with a human hair, which is about 80,000 nm wide. Some definitions include a reference to molecular systems and devices and the molecular nanotechnology fanclub argues that any definition of nanotechnology needs to include a reference to "functional systems". The inaugural issue of Nature Nanotechnology asked 13 researchers from different areas what nanotechnology means to them and the responses, from enthusiastic to skeptical, reflect a variety of perspectives.
It seems that a size limitation of nanotechnology to the 1-100 nm range, the area where size-dependant quantum effects come to bear, would exclude numerous materials and devices, especially in the pharmaceutical area, and some experts caution against a rigid definition based on a sub-100 nm size. We found a good definition that is practical and unconstrained by any arbitrary size limitations (source):
Nanotechnology is 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 size-dependant phenomena in nanostructures all represent effects that are not seen in larger, i.e. micron-scale and up, structures. Below a certain size barrier the quantization of energy for the electrons in solids becomes relevant. This so-called "quantum size effect" describes the physics of electron properties in solids with great reductions in particle size. The result is that materials reduced to the nanoscale can suddenly show very different properties compared to what they show on a macroscale. For instance, opaque substances become transparent (copper); inert materials become catalysts (platinum); stable materials turn combustible (aluminum); solids turn into liquids at room temperature (gold); insulators become conductors (silicon). Here are some of the most important effects (the typical length scale denotes the size below which the phenomenon can be observed:
Size quantization (typical length scale: tens of nanometers)
Hodes explains that size quantization is probably the best-known of the different size-dependent properties of nanomaterials. "Size quantization refers to changes in the energy-level structures of materials as the material-unit (most often a crystal) size drops below a certain size. This size, which can be identified with the Bohr diameter of the material, can vary from almost 100 nm to a single nanometer or even less. For most semiconductors, it is typically between a few nanometers and several tens of nanometers. For metals, it is smaller. For gold clusters it is approximately 1 nm (often less than 100 atoms).
"Size quantization is characterized by an increase in bandgap (blue-shift in optical spectra) and increasing separation of energy levels with decrease in crystal size. As the energy-level structure changes continuously with change in crystal size (in the size-quantization regime), a material of a particular fixed chemical composition can be made with varying and tunable physical properties. In other words, basic material properties are determined by the physics rather than by the chemistry of the material."
Single-electron charging and single-electron effects (typical length scale: ca. 50 nm at room temperature)
Single-electron transport is important for a number of reasons. "There is the obvious one of allowing the clear visualization of the number of charges in a device" says Hodes. "Another is the fact that metals (which are also subject to the same phenomena as semiconductors with regard to charging) can exhibit various types of rectifying behavior in the single-electron-transport regime and can be used instead of semiconductors to build devices.
"By cycling biasing gate voltages in single-quantum-dot systems, one electron per cycle would pass through the dot. Thus by controlling the cycle frequency, a very accurate current could be passed through the dot (a single-electron turnstile). This allows a very sensitive current standard. Single-electron transistors represent another important potential application. Following the demonstration of using an imbalance of a single electron to store information, considerable
work has been carried out on what is known as single-electron memories. This list is not comprehensive but should serve to demonstrate the potential of single-electron devices."
Metastable crystal phases (typical length scale: tens of nanometers)
It has been observed in a number of cases that high-pressure crystal phases that are normally unstable at atmospheric pressure can be obtained in nanocrystalline form and are stable under normal atmospheric conditions. Without going into the details here, there are two main factors believed to be responsible for this: Thermodynamic factors and kinetic factors (phase transitions tend to be initiated at defects and small nanocrystals are often defect free).
Charge depletion (typical length scale: ca. 100 nm)
Charge depletion is based on the screening length for electrons in a material. Hodes explains that the best-known application of this principle is the dye-sensitized solar cell (DSSC). "In this cell, an absorber (usually an organometallic dye, sometimes a semiconductor) is adsorbed onto a porous, high-bandgap semiconductor (most often TiO2). Although the porous TiO2 itself is rather insulating, injection of electrons from the photoexcited absorber renders it effectively conductive. An important factor for the operation of this type of cell is the size of the porous oxide particles. They are typically approximately 20 nm. There are at least two reasons for this: One is the main reason for using this porous structure – the high surface area of the oxide allows efficient absorption of light by even a monomolecular layer of dye (resulting in efficient electron transfer from the excited dye to the oxide). The second is that this particle size allows complete depletion (removal of free electrons) of the individual particles by the electrolyte.
Ballistic electron transport (typical length scale: hundreds of nanometers)
Ballistic transport of electrons refers to transport over distances where no scattering of the electrons occurs (distances less than the mean free electron path). "Although structures utilizing ballistic transport are not new, the use of nanotubes and nanowires, where the mean free path can be hundreds of nanometers in size, greater than the dimension of devices made with these nanotubes/wires, is a rapidly evolving field" says Hodes. "Obvious advantages of devices based on ballistic electron transport are less energy loss (as heating) and more speed. On a more fundamental level, certain nanostructures where ballistic
transport occurs over the size of the structure allow a number of different functionalities to be obtained. One of the best known is the Y junction, where Y-shaped nanotubes or nanowires form three terminal devices allowing rectification and logic gating."
Apart from the various phenomena of the quantum size effect, a second important aspect of the nanoscale is that the smaller a nanoparticle gets, the larger its relative surface area becomes. Surface phenomena, together with the dramatic changes in the particle's electronic structure, lead to greatly improved catalytic activity but can also lead to aggressive chemical reactivity.
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 40 mm, 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 these 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 these particles is more than 4 million times greater than the surface area of the silver dollar! (Source)
To sum it up: the fascination with nanotechnology stems from these unique quantum and surface phenomena that matter exhibits at the nanoscale, making possible novel applications and interesting materials.