It’s a whole new world down there 

At the nanoscale, where classical science mingles with quantum mechanics, the rules of the game change.  How do scientists keep up?

By Anita Martin

            Ever seen an atom?  You might refer to the recent image captured by Ohio University’s Saw-Wai Hla, Ph.D. of silver atoms lined up like The Marching 110 to form the letters “OU.”  But don’t be fooled—the human eye does not “see” individual atoms, even through the lens of a microscope, in the same way that it views magnified bacteria from a pond or the texture of a human hair.  Sight requires light, which travels in waves that shift their lengths and frequencies after passing through or bouncing off objects.  These adjusted wavelengths enter your eye, revealing characteristics of those objects: color, texture, size, shape. 

World’s smallest OU, created by atom manipulation and imaged using the same STM tip.

            So why can’t light waves do the same for a single atom?  The answer to this, and just about every question regarding nanoscience, is a matter of scale.  Atoms are just too small.  In order to see a standing particle, the light wavelength must be roughly equal to the size of the particle.  The human eye can see light waves between 300 and 700 nanometer (nm) wavelengths.  This is the range of visible light (a.k.a. Roy G. Biv): red around 700nm wavelengths and violet around 300nm.  The average single atom, however, measures to only about .1nm.  To detect such a particle, a light wavelength of about .1nm is needed.  That is the wavelength of an x-ray, far from the visible light range. 

On the scale of the nanoworld, the world at one billionth of a meter—things get tricky.  Nanoscience may be the wave of the future, but in order to direct this wave toward useful shores, scientists must learn to navigate the waters.  That means adjusting to nanoscale properties, restraints and techniques of research.

            In the case of the atomic OU snapshot, Dr. Hla used something called a scanning tunneling microscope to “feel,” rather than see the atoms.  He arranged the atoms on a substrate, or surface, usually of aluminum or silver, where they remain like nanoscopic bumps.  The scanning tunneling microscope reads the letters like a blind man reads Braille.

Making waves

            These days, with “nanotechnology” as the new buzzword, the nanoscience research team at Ohio University has more to worry about than atomic photo shoots.  When Dr. Hla and his colleagues move atoms around, they are laying the groundwork for machines of the future—in a world where chemistry and classical physics meet quantum mechanics. 

            To better understand what that means, let’s take a closer look at that OU picture.  See the lines in the center of the O that look like ripples in a lake?  In reality, those lines are ripples in the surface of the silver substrate. 

            “Imagine Stonehenge,” explains Dr. Arthur Smith, another brain on the nanoscience case at Ohio University. “You don’t see waves on the ground between those rocks.  Think of legos arranged on a circle on your desk—no waves there either.  Now look at these atoms.  Here we see the quantum mechanical scale.”

            Wait a minute—waves in silver?  We’ve all learned that silver is a metal—that’s a solid, not a liquid.  How can a solid have waves?  The answer is: electrons.  To understand quantum mechanics, first forget everything you’ve learned about electrons and their nuclei as minute solar systems of distinct particles.  According to quantum mechanics, electrons exhibit properties of both particles and waves.  Sometimes they appear distinct—they can be counted, unlike water.  But like a wave, electrons can appear to exist in more than one place at the same time; and like a liquid, they can flow.  Metals, like silver, have a weak hold on their valence (outer) electrons.  These electrons break orbit and flow unattached, creating an electron sea around the atoms.  We call them free electrons and they’re the ones making waves.

Breaking bonds

            The wave-like quality of electrons is really the trick to the scanning tunneling microscope.  Earlier, the scanning feature was addressed—the microscope scans the atomic bumps on the surface of a substrate to create an image like the OU photo.  But the microscope scans with the help of the electrons on the tip of the microscope.  That is, by tunneling them.

            Before jumping into the tunneling process itself, let’s take a look at that silver atom again.  The protons and neutrons are pretty stable couples.  For the most part, they’re in it for the long haul.  Electrons, however, tend to come and go.  The proton-electron affairs continue thanks to the universal law: opposites attract.  Positively charged protons hanker for the negative charge of electrons.  When the proton-electron ratio is equal, the atom is fulfilled, balanced—neutrally charged.  But these relationships are often one-night stands.  Electrons want to be close to the nucleus.  But, as soon as they stray, they are easily distracted and stolen away, leaving a positively charged ion yearning in its wake.  Fidelity aside, the attraction is still strong enough to keep the sea of free electrons within the metal.

            Even if electrons were normal particles, they would make for a pretty lousy solar system—they share orbits, they change orbits and sometimes, they break orbits altogether and fly about like a meteor.  So what’s really going on?  The reality of electrons brings us into the weird science of quantum mechanics.  Their wave properties are best explained by plotting the probability of finding a given electron in a given place at a given time.  The plot reveals that even if an electron were confined to a box, it would have a finite probability of being found outside the box.  The exact whereabouts are unpredictable and undetectable.  Electrons behave like wave impulses that can extend themselves through boundaries and across expanses. 

            To create the OU image, Hla lowered a silver tipped scanning tunneling microscope (STM) to about 1nm away from the bumpy silver substrate.  He then applied a controlled voltage to the microscope, sending a current of free electrons between the substrate.  Think of the space between the tip of the STM and the substrate as a steep ravine between two cliffs.  People, as macroparticles, can ride bicycles up to the clip, but cannot just peddle across the air to the other side of a ravine.  We need either a bridge or a ramp and a fast rolling start to get over the gap.  If a subway were rushing through the center of one of the cliffs, it would need a tunnel to get across the ravine.

            If electrons were ordinary particles, the empty space between the STM tip and the substrate would be as much an obstacle as a deep ravine between two cliffs is to bicycles and subway trains.  But due to their wavelike quality, when the STM tip draws near enough to the substrate, free electrons can extend themselves far enough to establish a current between the substrate and the STM tip, that is—they tunnel.  The closer the STM tip comes, the stronger the current becomes.  So, by detecting the current intensity, Dr. Hla can scan the bumps and valleys of the substrate.

Color me red, color me yellow, color me green

            As it turns out, electrons are responsible for a great deal of unusual behavior at the nanoscale.  Take the case of the multicolored quantum dots.  No, these are not the latest Willy Wonka creation.  According to Ohio University’s Sergio E. Ulloa, Ph.D, quantum dots are artificially constructed clusters of atoms that fall within the range of a few hundred nanometers.  If we take a molecule containing cadmium (Cd) and another containing selenium (Se) atoms, for example, and toss them together in a beaker, the Cd and Se will join into what Dr. Ulloa refers to as “little hairy balls” of about 5nm.  The “little hairs” of the quantum dots are remnants of the original atoms, still dangling from the spherical surface.  Despite their scruffy façade, on the inside CdSe dots are perfect crystals with atoms arranged in perfectly ordered repeating structures—similar to the construction of a beehive.

            Now let’s think about quantum dots in terms of electrons.  Electrons fill orbitals of atoms with varying energy levels.  The highest of these atomic energy levels, as stated earlier, is the valence.  For atoms in a crystal, we refer to this as the valence band.  Above the valence band is a gap and above the gap is the conduction band.  As we know, metals contain swarms of electrons at the conduction band.  But our CdSe dots are not metals; they are crystalline semiconductors. 

Electrons of semiconductors are not the vagabond playboys that metal electrons are.  For the most part, they are homebodies, content to remain within the valence band.  Semiconductor electrons do get out occasionally, but to do so, they need an extra kick—like a dose of ultraviolet light.  Once energized, valence electrons have the energy to leap over the band gap into the conduction band, where they are free to fly around and bounce off of the “walls” of the quantum dot… but not for long.  These electrons in a semiconductor run on borrowed energy.  Their atoms are waiting up for them and they must return home.  This means that they jump back down to the valence band.  When they drop, they emit the excess electromagnetic energy in the form of little light energy packets called photons.  Put enough energized CdSe dots together, and the light emitted from their combined photons will appear to glow.  That is called fluorescence.

What makes quantum dots different from other fluorescent material is this: change the size of the quantum dots, and the color of the fluorescence changes as well.

“Think about it like this,” says Dr. Hla, “if you add a glass of water to the ocean, the water level will not rise—you will not notice anything.  But add a glass of water to another glass of water and you see the change.  On the nanoscale, you can see how little changes make a big difference.”

Accordingly, add a few hundred atoms to a hunk of metal, and the properties of that metal will not change.  But add a few hundred atoms to something only a few hundred atoms in size already, like a CdSe quantum dot, and you will see the change.  To further understand their color-size connection, consider the case of the multi-colored quantum dots in terms of energy bands and band gaps.  Larger dots have smaller gaps between the valence band and the conduction band.  With a smaller band gap, these electrons require less energy to make the jump and emit less energy on their return.  They produce long wavelengths of around 700nm—red light. 

The valence and conduction bands of smaller dot, on the other hand, are farther apart.  Their electrons have further to go.  They absorb more energy on their journey to freedom and emit shorter wavelengths upon homecoming—like green light.

Look out below

The first step to understanding nanoscience is to practice seeing things in a whole new way, and in terms of the scanning tunneling microscope (STM)—quite literally.  Rather than eyes and lenses, the STM uses measurements of electron tunneling to see-feel nanoscale surfaces.  This is one example of how observing the nanoscale requires new techniques and tools that not only accommodate, but utilize and even profit from strange quantum mechanical phenomena. 

It really is a whole new world down there—one of multihued quantum dots, where minute additions change everything and the boundaries blur between waves and particles, solids and liquids.  The nanoscale is, for the most part, unknown territory.  But every time pioneering scientists stake out a new claim of discovery, we find more evidence of the technological resources available down below.  Forget outer space, the final frontier may be right under our nose—all we have to do is find the right way to look.

For additional fact-checking/source-checking purposes:

Written sources used:

Ratner, Mark & Daniel. Nanotechnology, a Gentle Introduction to the Next Big Idea,

Pearson Education Inc. 2003

And these exact links:

http://www.chem1.com/acad/webtut/bonding/TunnelBond.html

http://csep10.phys.utk.edu/astr162/lect/light/bohr.html

http://www.ece.utep.edu/courses/ee3329/ee3329/Studyguide/ToC/Fundamentals/BDiagrams/conduction.html