In the year 2000, when they look back at this age, they will wonder why it was not until the year 1960 that anybody began seriously to move in this direction, so said Nobel-prize-winning physicist Rechard Feynman in a 1960. He is commonly considered to have launched nanotechnology, but even this was a bit premature.
While miniaturisation continued at a breakneck pace, machines continued to shrink one step at a time in what we now call very prolonged top-down nano-fabrication. No one immediately took up the challenge to start thinking from the bottom up, and it wasnt until the year 2000, as Feynman predicted with uncanny accuracy, that devices started to break into the nanoscale and people started asking why we hadnt thought of this long before.
The reason is simple. We didnt have the tools. None of the manufacturing techniques that have allowed us to make smaller and smaller devices-macrolates, etchers, visible-light lithography equipment-are operable at the nanoscale. And not only couldnt we manipulate individual atoms and molecules, but we couldnt even see them until electron and atomic force microscopies were invented.
The reason why nanotechnology is coming to the surface now is that tools to see, measure, and manipulate matter at the nanoscale exist now. Though they are still crude, and the techniques with which we employ them are unrefined, but that is changing rapidly. It is now possible for a scientist in Washington, DC, using just an Internet connection to a remote controlled laboratory in San Jose, California, to move a single atom across a platform in the lab. Technology continues to improve, and we have taken the, ahem, quantum leap into the nanoscale.
TOOLS FOR MEASURING NANOSTRUCTURES
Scanning Probe Instruments
Some of the first tools to help launch the nano-science revolution were the so called scanning probe instruments. All types of scanning probe instruments are based on an idea first developed at the IBM Laboratory in Zurich in the 1980s. Essentially, the idea is a simple one: if you rub your finger along a surface, it is easy to distinguish velvet from steel or wood from tar. The different materials exert different forces on your finger as you drag it along the different surfaces. In these experiments, your finger acts like a force measurement structure. It is easier to slide it across a satin sheet than across warm tar because the warm tar exerts a stronger force dragging back the finger. This is the idea of scanning force microscope, one of the common types of scanning probe.
In scanning probe measurements, the probe, also called a tip, slides along a surface in the same way your finger does. The probe is of nanoscale dimensions, often only a single atom in size where it scans the target. As the probe slides, it can measure several different properties, each of which corresponds to a different scanning probe measurement. For example, in atomic force microscopy (AFM), electronics are used to measure the force exerted on the probe tip as it moves along the surface. This is exactly the measurement made by your sliding finger, reduced to the nanoscale.
In scanning tunnelling microscopy (STM), the amount of electrical current flowing between a scanning tip and a surface is measured. Depending on the way the measurement is done, STM can be used either to test the local geometry (how much the surface protrudes locally) or to measure the local electrical conducting characteristics. STM was actually the first of the scanning probe methods to be developed, and Gerd Binning and Heinrich Rohrer shared the 1986 Nobel Prize for its development.
In magnetic force microscopy (MFM), the tip that scans across the surface is magnetic. It is used to sense the local magnetic structure on the surface. The MFM tip works in a similar way to the reading head on a hard disk drive or audio cassette player.
Computer enhancement is often used to get a human usable picture from any scanning probe instrument, such as the nano-scale abacus. It takes a great deal of enhancement just to make the raw results look as good as the ghostly x-ray pictures taken of your luggage at the airport. Scanning probe instruments cant imagine anything as large as luggage, however; they are more useful for measuring structures on length scales from the single atom level to the micro-scale. Nanotechnology will offer us other ways catching baggage offenders. Other types of scanning microscopies also exist. They are referred to as scanning probe microscopies because all are based on the general idea of the STM. In all of them, the important idea is that a nanoscale tip that slides or scans over the surface is used to investigate nanoscale structure by measuring the forces, currents, magnetic drag, chemical identity, or other specific properties.
Scanning probe microscopy made it possible to see thing of atomic dimensions for the first time. It has been critical for measuring and understanding nanoscale structures.
Spectroscopy refers to shining light of a specific color on a sample and observing the absorption, scattering, or other properties of the material under those conditions. Spectroscopy is a much older, more general technique than scanning probe microscopy and it offers many complementary insights.
Some types of spectroscopy are familiar from the every day world. X-ray machines, for example, pass very high-energy radiation through an object to be examined and see how the radiation is scattered by the heavy nuclei of things like steel or bone. Collecting the x-ray light that passes through yields an image that many of us have seen in the doctors office after a slip on the ice or in the bathtub. Magnetic resonance imaging, or MRI, is another type of spectroscopy that may be familiar from its medical applications.
Many sorts of spectroscopy using different energies of light are used in the analysis of nanostructure. The usual difficulty is that all light has a characteristics wavelength. Since visible light has a wavelength of between approximately 400 and 900 nanometers, it is clear that it isnt too much help in looking at an object only a few nanometers in size. Spectroscopy is of great importance for characterising nanostructure en masse, but most types of spectroscopy do not tell us about structures on the scale of nanometers.
Electrochemistry deals with how chemical processes can be changed by the application of electric currents, and how electric currents can be generated from chemical reactions. The most common electrochemical devices are batteries that produce energy from chemical reactions. The opposite process is seen in electroplating, wherein metals are made to form on surfaces because positively charged metal ions absorb electrons from the current flowing through the surface to be plated and become neutral metals.
Electrochemistry is broadly used in the manufacturing of nano-structures, but it can be used in their analysis. The nature of the surface atoms in an array can be measured directly using elector-chemistry, and advanced electrochemical techniques(including some scanning probe electrochemical techniques) are often used both to construct and to investigate nano-structures.
Even before the development of scanning probe techniques, methods that could se individual nano-structures were available. These methods are based on the use of electrons rather than light to examine the structure and behaviour of the material. There are different types of electron microscopy, but they are all based on the same general idea. Electrons are accelerated and passed through the sample. As the electrons encounter nuclei and other electrons, they scatter. By collecting electrons that are not scattered, we can construct and image that didnt make it through. Under favourable conditions, TEM images can have a resolution sufficient to see individual atoms, but samples can only measure physical structure, not forces like those from magnetic electric fields. Still, electron microscopy has many uses and is broadly used in nanostructure analysis and interpretation.