Nanolayers

Technically speaking, nanolayers has been around longer than other nanotechnology applications.  In fact, since the 1970's, nanolayers have been important parts of lasers and other optical devices that require good 2-D confinement in a thin layer.

A nanolayer is exactly what it sounds like: a layer of material no thicker than a few nanometers.  Nanolayers are more commonly known as quantum wells.  To make a quantum well, all you have to do is sandwich a material with a small bandgap with two materials that have significantly higher bandgaps.  The result is that electrons excited in the middle layer cannot escape into the outer layers since they don't have the requisite energy to make the jump into the higher bandgap.

A laser takes advantage of this fact by creating a very thin layer of semiconductor sandwiched between similar semiconductors with high bandgaps and similar lattice constants.  A good example of this is the III-V compound semiconductor, Gallium Arsenide (GaAs) that is a direct bandgap light emitter with fast switching properties.  You can lattice match Aluminum-Gallium Arsenide (AlGaAs) almost exactly.  AlGaAs is not like GaAs.  It has a similar structure but acts more like an insulator. 

By trapping electrons in the GaAs and then stimulating electrons with a voltage, you get a large number of excited electrons.  While you've got the workings of a modern laser right there, a few other important steps have to be finished.  First of all, let's assume you have a rectangular quantum well.  In this model, we have 4 edges where the GaAs is exposed to the surface and the external environment.  Ideally we should surround the entire quantum well with reflectors.  Why?  When the excited electrons fall back to their ground states, they will emit photons of a specific energy.  We don't' want to lose the photons right away otherwise we'll never achieve coherent emission of monochromatic photons.  The photons will need to bounce around the quantum well (thanks to the reflectors) and aid in the stimulation of more photons of the exact same wavelength.  When a condition called population inversion is reached, a photon can trigger an entire chain of electrons to cascade back to their ground states and emit more light of the same wavelength.  This sequence of events is known as light amplification by stimulated emission of radiation, or LASER for short.  It is the way all modern lasers are made.  Choosing the right semiconductor mix in the quantum well can tune the laser's frequency within a specific range.   

So while nanolayers are not really a new invention, they are still one of the fundamental structures of nanotechnology.  New advances in their fabrication offer hope in taking the idea even further.  One instance of a new approach to quantum well fabrication is the inclusion of highly mismatched layers of semiconductor that can function despite the tremendous strain involved.