Artificial atoms

My mission this winter has been to understand electronics a little better, and in aid of that I listened to an excellent Great Course on nanotechnology.  What a stunning field!  The course fully delivered in terms of explaining what might be accomplished by nanotechnology in the near future in terms of things like cameras, medical treatment, and solar power.  It fell down considerably in the more difficult task of explaining how these things can be accomplished, frequently reverting to jargon and ill-defined terms.  In one breath, the lecturer was unsure whether his audience had heard of the periodic table of the elements.  In the next, he was throwing around terms like "quantum dot" and "band gap" as if they meant something to the average layman.

So with my free time today, I tried to run down popularized sources that would help me understand semiconductors, microelectronics, and quantum dots in a way accessible to people who have no command of the mathematics of wave mechanics.  Here are some of the things I found out.

Any popular discussion of nanotechnology is going to throw out all kinds of exciting possibilities of quantum dots, but what are the little thingummies?  The definition is often given as something like electrons confined in a space measured in a fairly small number of nanometers. But what do we mean by "confining" the electron, exactly?  We've been told we can neither stop them nor ever know exactly where they are.

The answer apparently lies in the same sort of thing that goes on when we say an electron is in orbit around an atomic nucleus.  One of the quandaries that led to the development of quantum mechanics was our inability to visualize how electrons behave in an atom.  In classical mechanics, an oscillating or rotating electron should be generating an electromagnetic wave.  In other words, it should emit light and, in so doing, lose energy.  It should therefore spiral gradually into its nucleus after a while, but it doesn't.  What gives?  Apparently the thinking is that electrons in certain kinds of permitted energy states, which we call "orbits" (though they're not like planetary orbits), are stable and neither emit light nor lose energy and fall into the center of the atom.  If they gain or lose just the right amount of energy, they can jump up or down into the next permitted energy state, but until that happens they're on autopilot.  I gather we don't know why this is so, only that it's the way things happen.  So it must be that electrons in certain states can hang out more or less indefinitely under the influence of a particular atomic nucleus, and that's what we mean--loosely--by their "confined location."  I gather further that a quantum dot pulls off something like the same trick, but without benefit of a nucleus.  In that sense, a quantum dot has been called an "artificial atom."

How does a nanotechnologist confine electrons in a dot, without using an atomic nucleus as an ordering device?  He relies on the special qualities of the materials we call semiconductors.  These are materials that neither freely conduct electricity, as metal does, nor totally insulate it, like glass.  Instead, they allow a very tiny amount of electricity to pass, but only if we excite the system very slightly, such as by introducing a tiny charge or some light.  As I understand it, natural semiconductors tend to be crystalline structures formed from elements with four electrons in their outermost 8-type shell, which form covalent bonds with other similar atoms.  The addition of trace amounts of elements with either 3 or 5 electrons in their outer shells creates a situation in which the crystal is either slightly short of electrons or slightly overstocked with them.  (Something about the regular array of the crystal is conducive to the electrons moving about in a useful way.)  The movement of the spare electrons, or the movement of the missing spaces where an electron should be (as in the case of those little puzzles that are solved by moving around the "hole"), or both, function as the carriers of electric current.  We have developed considerable ingenuity in adding just the right sort of this or that to produce the excess-electron materials ("N"-type) and electron-depleted materials ("P"-type) that we need.  When we sandwich N and P materials together, we find that we can precisely control when current will and will not pass across the boundary between them.

One very handy semiconductor device is a diode, or rectifier, which basically is some N-material stacked against some P-material that functions as a one-way valve:  electric current will go through on one voltage but not on its opposite.  This is what we use, for instance, to convert AC current to DC.  An even handier device is a transistor, which is in essence two diodes back to back.  It is a sandwiching of electron-oversupply and electron-undersupply materials (N-P-N or P-N-P) in such a configuration that applying a tiny current to the middle part of the sandwich permits a current to pass through the whole shebang.  That is, the transistor is either an off-switch or an amplifier, depending on whether it's put in the "on" position by a small current.  (Electrical switches weren't new when transistors were developed, of course, but the old style required a big "gate" of conducting material that could be opened or shut by brute force.  The "switch" in a transistor can be tiny and energy efficient.)

Even cooler, when the current passes through the transistor, it has been amplified.  How does the amplification part work?  George Smalley, one of the winners of the Nobel Prize for nanotechnology in the 1990s, used this analogy:  Tie a bale of hay to the tail of a mule, then put a bit of effort into striking a match to light the hay, and observe the level of energy expended by the mule.  A transistor is a device in which a lot of current is ready to flow once an initial stimulus has triggered it.  Until the trigger happens, nothing flows, but after the trigger happens, the information contained in the trigger is transmitted in vastly louder form.

It turns out that judicious manipulation of semiconductor materials permits us to shave away the crystalline structure in which electrons are permitted to flow until, at last, they are confined to a plane (a "quantum well"), or to a long, narrow tunnel (a "quantum wire"), or a little cube or sphere (a "quantum dot").  Why bother?  Well, to take the case of quantum dots, it turns out that we can fine-tune the size of the dot in order to exert a precise control over the size of the little packet of energy that's required to enter the system and bump the electron to another level.  When it falls back down, it will emit a tiny bit of light at a precise and controllable frequency.  This is proving handy in the development of TV screens in the form of extremely thin layers that emit bright, clear light in any color we like upon the application of an extremely small current.  It's also possible to construct medical nanoparticles that combine antigen-like recognition particles and light-emitting quantum dots, so we can let them roam in the bloodstream until they encounter a microbe or a cancer cell, then emit light that's color-coded to let us know which problem they found.  They can even be programmed to release a toxin to destroy the microbe or cancer cell with minimal effects on surrounding tissue, which should not only decrease side-effects but also lessen the ability of the problem cells to develop resistance.  In the context of solar power, quantum dots hold the promise of something very much like artificial photosynthesis that produces electricity rather than sugar.

It's perfectly amazing how far this field has advance in the last couple of decades.