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of pliny, baseball and quantum dots
A Layman's Guide to Quantum Dots

Pliny the Elder got it right when he wrote in his “Natural History” almost two thousand years ago, “Nature is greatest in the smallest of things.”  Although the classical naturalist was referring to insects at the time, his insight has been borne out with increasing veracity through the centuries; surely across a wider range of nature’s spectrum than the Roman sage himself ever imagined.  Today, a new set of “smallest of things,” called quantum dots, has taken the truth of Pliny’s observation to new levels of significance.

Quantum dots are non-nucleic, nanoscale assemblages of electrons compacted into spaces smaller than they would occupy as part of a bulk scale molecule having the same electron configuration.  Despite the absence of a nucleus around which they would orbit in a conventional bulk state, the electrons in quantum dots organize themselves into substantially the same orbital clouds or shells they would otherwise inhabit.  Surprisingly, to a large degree they behave like atoms without nuclei.  Lack of the nuclear attraction force, in combination with their isolation and confinement within nanoscale crystals, makes quantum dots very susceptible to the manipulation of their electrostatic properties, which play a vital role in virtually every performance characteristic in all materials.  The promise of quantum dots is enormous because their very size suggests the possibility of the reconfiguration of the excitation and response traits of virtually any material at nanoscale.

At this early stage of development, quantum dots exploit three sets of physical principles: first, the precise electrical tunability of semiconductors; second, the extreme structural precision of crystals; and third, the unique effects of confinement on nanoscale crystalline structures. 

Today, most valuable among quantum dots’ performance characteristics is their ability to fluoresce.  Put simply, when stimulated appropriately, quantum dots can “glow in the dark.”  All in all, that’s not a big deal.  Lots of things glow in the dark, all the time: fireflies, television sets and fluorescent lights, for instance.  As Mother Nature has it, many natural forces serve to excite light emitting materials; heat, light, pressure, biological substances, electricity, chemicals, and electrons, to name a few.  Generally speaking, things that glow in the dark share a common operative phenomenon called “electron hole recombination.”  If this explication of “glowing” is casting more darkness than light on the topic, hang in there; there’s a baseball analogy coming to the plate.

When a batter hits a behind-the-plate-pop-foul, obviously the ball the ball goes up, up, up, to its apex, whereupon, thanks to gravity, it comes to a theoretical stop before starting on its return trip to earth.   Were it not for gravity, the ball would go up forever, but since gravity is part of our reality, during one infinitesimally miniscule moment the ball is neither rising nor falling.  During its refreshing pause, all the ball’s energy of motion (kinetic energy) is transformed into potential energy, whereupon the fall begins, along with the energy’s conversion back into kinetic form.  We can see that energy in the visible speed of the ball and hear it from the thwapping sound it makes when it hits the catcher’s mitt.

Same thing with electrons.  Although nobody bothers to sit in the bleachers eating peanuts and cracker jacks while watching a game of electron hole recombination, electrons are getting knocked out of their comfy orbits all the time.  What’s knocking them out of orbit is not a baseball bat, but other electrons.  Let’s take an everyday example:

A fluorescent bulb is a vapor-filled glass tube with a positive electrode on one end and negative electrode on the other.  The inside of the glass is coated with a fine crystalline powder that emits light when its electrons are batted out of their preferred orbits by other electrons.  Here’s how it works:  The electrodes on opposite ends of the tube throw high-energy electrons across the field to each other.  Some of those electrons strike electrons in the vapor atoms, which in turn strike others, creating a cascade of electrons that strike electrons in the atoms of the light emitting crystals.  The last electron struck becomes a proverbial behind-the-plate-pop-foul.  It leaves its orbital shell, otherwise known as a band and ascends to a higher energy band which, it stands to reason, would have more energy, just as a high pop-foul would have more energy than a strike or a foul tip.  In light emitting materials, the band from which the electrons are hit is called the valence band.  The band into which it is hit is the conduction band.  We can think of the valence band as the strike zone and the conduction band as the airspace above the stadium lights.

Just as the pop foul eventually comes back down to the earth behind home plate, drawn by the inescapable force of gravity, pop foul electrons are also drawn back to their home band because when an electron is knocked out of its home band, it leaves an empty spot or hole behind, sort of the way a catcher might feel an empty spot in his mitt when a pop foul prevents the ball from crossing home plate.   The hole is considered to be positively charged; the electron, negative.  Together, the electron and the hole are called an exciton.  When the catcher catches the ball, the energy from its descent results in the thwapping sound of the ball striking the mitt.  Similarly, when an electron falls back to its domicile hole, its accumulated energy results, not in a sound, but in the formation of a photon, otherwise known as a light quanta, more confusingly known as a wave particle.

Electromagnetic wave particles are visible or invisible, depending on their length, which is the distance between peaks.  Their length is dependent upon the optical properties of the subject crystal. Those properties hinge on the band gap, which is the space between the valence band and the conduction band.  The size of that space is known as the exciton Bohr radius and it defines the size limitations of quantum dots, which are, by definition, smaller than the subject material’s bulk state exciton Bohr radius; usually in the 2 to 10 nanometer range.

Exciton Bohr radii are the keys to whether a material conducts electricity and heat all the time, sometimes or never.  A small or overlapping radius gives us a conductor; medium sized radii give us semiconductors; large radii make insulators.  The exploitation of exciton Bohr radii centers on the idea that the electrical charge of one material, will influence the electronic disposition of its molecular partner, when they are chemically bound.  The distance between the valence band and the conduction band is relatively easy to adjust by adding contaminants, called dopants, to a semiconductor material.  Dopants change the electrostatic charge of the material by adding or subtracting electrons to the semiconductor materials.  When the electrostatic charge of the semiconductor is altered, so is its exciton Bohr radius.  The alteration of the exciton Bohr radius results in a change in the excitation and emission traits of the semiconductor crystal, along with its associated photon’s particle wavelength.  The icing on the quantum dot cake is that the exciton Bohr radius is manipulable as a function of the size of the quantum dot.

When these phenomena are forced to work in a space smaller than that of the subject molecule’s bulk scale band gap, the properties expressed are intensified, because any excitation force imposed upon the quantum dot’s nanocrystalline structure has no alternative path of expression, other than to affect the dot’s emissive behavior, i.e. in the case of optoelectronics, to make it glow.

Initially, the value of quantum dots appears to be in their promise as signaling devices, particularly in the fields of medical diagnostics and mobile detection for security.  For the future, the unique properties of quantum dots imply the possibility of the alteration of material characteristics by means of the manipulation of the material’s electrical charge. When viewed dynamically in light of the full array of available excitation forces and responsive behaviors, the future of quantum dots appears to be laden with almost limitless possibilities for the trait-based design and development of novel, performance-optimized materials.

Pliny must be smiling.

This article first appeared in Tom’s column, Views from the Bottom.

© Copyright 2006, Thomas P. Imerito / dba Science Communications

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