Creating Nanostructures

Sputtering (a bottom-up approach)

One method used to make thin layers of material that are only a few atoms thick is called "sputtering." Sputtering involves transferring atoms from a block of source metal over to a surface waiting to be coated. The atoms are knocked loose from the source metal by bombarding them with other high-energy particles. The common aproach taken when explaining sputtering is to imagine billiard balls being struck by the cue ball. The cue ball is rather like the hihg-energy incident particle. As it strikes a bunch of billiard balls (atoms in a block of source metal) they scatter from one another. This is where the analogy breaks down, though, as there is no second surface that the billiard balls attach to besides the pool table. In sputtering, however, the loose atoms are free to deposit on some material that needs to be coated. The picture below comes from the wikipedia page on sputtering.

Lithography (4 main types, all of which are top-down approaches)

1) Basic Nanolithography

Lithography was developed about 200 years ago as a method to print on paper. Print lithography involved the patterning of stone with wax. When water based ink is applied to a wax patterned stone, the ink will only stick to the exposed stone, not the wax. The stone effectively becomes a stamp to print on paper.

A different form of lithography is used to make tiny channels and pathways on a computer chip. A silicon wafer is first coated with a thin layer of silicon dioxide or other material. Another chemical, called photoresist, is then layered on top of the silicon dioxide in the desired pattern of the channels and pathways. The photoresist is very durable and chemically resistant. A powerful chemical etch is then used to eat away the silicon dioxide that is exposed, leaving the channels and pathways behind. The channels and pathways are still covered with photoresist, which is removed as the final step. This process can then be repeated over and over to create a complex three dimensional structure containing many layers. In addition, the individual layers can be made of semiconductor materials with whatever properties are desired, enabling the creation of miniatureized transistors, resistors, etc. The picture below was created by our very own Pat Levy.

2) E-beam Nanolithography

Basically, is you understand Basic Nanolithography (#1), then this one isn't such a stretch. The problem with Basic Nanolithopgraphy is that scientists are limited to creating patterns and features no smaller than the wavelength of the light used. One could certainly use smaller and smaller EM radiation (light), but if you recall from your physics class, when EM wavelengths get smaller, frequencies increase and, more importantly, so does the energy that the EM radiation carries. As Ratner and Ratner put it, it's rather like watering your garden plants with a fire hose, you would simply obliterate the structures. Hence, electrons can be used instead of light. Because of their mass, electrons carry with them much less energy, despite exhibiting wavelengths smaller than that of light.

3) Dip Pen Nanolithography

This type of lithography, sometimes referred to as DPN, was developed by Chad Mirkin at Northwestern and is the type of nanotechnology that Richard Feynman used to write his now-famous 1959 paragraph on the possibilities of miniaturization. This type of lithography is named such because the atoms/molecules act as a sort of 'ink' within a scanning probe tip, which is analogous to a pen, as shown in the picture below (click image for source link).

The substrate is either polar or non-polar. As the tip scans across the surface, the atoms/molecules inside align themselves with the substrate in a manner similar to north and south poles of a magnet.

4) Nanosphere Liftoff Lithography

If you imagine a bunch of spheres packed as closely together as possible, you should include in your imagination the fact that each of those spheres will be surrounded by excatly six other spheres, be they tennis balls, marbles, or nanoparticles. Of course, we'll deal with the last of these. If one were to spray a bunch of close-packed nanospheres with some sort of "nano-paint" and then remove those spheres, one would have a repeating pattern of paint that has been left behind, like the one below (click image for source link).

The nice thing about this technique is that lots of different surfaces (those which the nanospheres were initially bound to) and metals/molecules (the paints) can be used. Further, it creates a regularly-occurring pattern and multiple layers can be deposited before the removal of the spheres.

Self-Assmbly (a bottom-up approach)

Though it sounds far-fetched, this approach is simply one of letting molecules find their own lowest states of energy. One way to understand this is to imagine a rock lifted or kicked or thrown or hoisted to a certain point above the ground. Regardless of how that rock gets there and of how high above the ground the rock is, gravity is going to work to bring the rock back to its lowest state of energy, which is on the ground. Likewise, molecules are subject to forces that orient them and/or move them in such a way that their final positions exhibit a lower state of energy than the original position. Forces that are taken advantage of by nanoscientists in this way include hydrogen bonding, magnetic attractions, and hydrophobic and hydrophilic interactions, like those shown below.

Nanoscale Crystal Growth (a bottom-up approach)

Just like it sounds, this method involves rather tricky selection of seed crystals and growing conditions with the hopes of creating crystals that have unusual shapes (think: making rock-candy in a very complex way). Nanowires, which happen to exhibit tremendous conductivity, are typically created in this way (click image for source link).

Other Approaches

There are certainly other methods of nanoscale fabrication with more being developed every year. Some of those other methods include controlled polymerization and molecular assembly, in which atoms and molecules are literally lined up or stacked atom-by-atom.