The Scanning Tunneling Microscope:
Eyes and Hands of Nanoscience
by Sarah Laubacher
The computer screens at a surface science lab could be mistaken for those in a graphic design studio – images of colorful concentric circles fill Power Point presentations, and layers of color rippled by pointy protrusions decorate pages in printer trays. Even recognizable letters and shapes can be discerned in various designs.
These images of atoms are products of Dr. Saw-Wai Hla’s perspective on science. The Ohio University physics and astronomy professor said he sees science as not only the study of nature, but as an art form as well.
At the surface science lab, nature is studied at the nanoscale – a relatively new sphere of science where atoms can be closely examined and manipulated. The properties of particles become more interesting at this level, because quantum mechanics comes in to complicate things.
The images on these computer monitors could be thought of as tangible topographies of tiny realities. But whether we approach nanoscience as nature or art, we must learn about the paintbrush capable of controlling a palette of atoms – the scanning tunneling microscope.
What is it?
So what is a scanning tunneling microscope? The “STM,” as it is called, is a microscope that senses and records atomic-scale images. It’s definitely dissimilar to the microscope you might know from high school science – the STM has no eyepiece to peer through, and the sample is not prepared as a wet mount slide. The actual microscope operates in a chamber, and the scientist (or student) controls the tool with a computer mouse and observes results on the screen.
Instead of the onion skin squished between glass slides you might have studied in biology class, the “sample” under the STM is a substrate composed of layers of atoms. (“Sample” and “substrate” are used interchangeably in this context.) The most important part of the STM is its tip, which looks like a little needle. The tip has many capabilities, but most fall into the categories of imaging or manipulation.
Operating Conditions
The graduate students at Dr. Hla’s lab recently constructed their own STM. To manipulate molecules effectively, the STM must operate under low temperatures and in an ultra-high vacuum. Considering these conditions, “LT-UHV” is often added to the acronym.
The low temperatures are maintained by liquid helium and liquid nitrogen, which cool the STM chamber down to around -451 degrees Fahrenheit. This temperature is so low that the STM tip can be stabilized over a single molecule for long periods of time.
The ultra-high vacuum rids the chamber’s atmosphere of oxygen and carbon dioxide, among other molecules, that are usually bumping around. They would certainly interfere with precise molecular manipulation, so the air is pumped out of the chamber to create an atmosphere similar to outer space.
To comprehend the scale at which an STM operates, compare it to a scanning electron microscope. This microscope perceives structures measured in micrometers, such as viruses or bacteria. A nanometer, the unit used for measuring matter under the STM’s scope, is much smaller than that. One nanometer is a billionth of a meter – approximately the size of three atoms.
The Quantum Concept of Tunneling
When we observe nature at the nanoscale, certain phenomena become apparent. Electrons simultaneously act as particles and as waves – a key concept of quantum mechanics. This behavior helps explain the “tunneling phenomenon,” a concept which might be unfamiliar to you.
Classical physics tells us that for electrons in solids to go from point A to point B, they must have a continuous conductor along which to travel. A classic textbook example is electricity traveling through wire. If the electron current is to move from one wire to another, the wires must touch in order for the current to complete its path.
Quantum mechanics challenges this concept. Physical contact of the two wires is not necessary, as long as the gap between them is approximately one nanometer. When the gap is this small, the electrons can “extend” over to the other side.
So, why do scientists call this “tunneling?” The electrons are essentially tunneling through a barrier. Though a barrier made of space may seem counterintuitive to us, the little electrons much prefer metal, so a gap of space would normally be a roadblock for them.
Imaging
What does “tunneling” have to do with taking images? It is important to remember that the images produced by the STM are not like photographs. The STM is not like a camera recording an atom’s light emission on film – it is a tool that “senses” rather than sees. Because of tunneling currents, the STM does not have to touch the sample in order to make a mapping.
The tip is positioned at a fixed, nanoscale distance away from the sample surface. A small voltage is applied between the tip and the substrate to “encourage” the tunneling current. When the tip is close to the substrate, the tunneling current increases and more electrons “leap” over. When the tip is farther away from the substrate, the tunneling current decreases and fewer electrons “make the leap.” Since the researcher fixes the current, the topography of the substrate is responsible for variations in the tip-to-surface relationship, and those variations are recorded and translated into colorful, computerized profiles.
With these profiles, researchers can examine the quality of atomically layered surfaces, or they can study the properties of specific molecules. Dr. Hla sometimes uses the STM’s manipulation capabilities to design patterns with individual atoms, and then takes images of the consequential designs. It is an ephemeral art, however. Atomic activity grows more frenzied with heat, and if it weren’t for the computer’s records, the atomic-scale artwork would be merely a memory after reaching room temperature.
Manipulation
Along with topographies of the atomic landscape, the STM also completes molecular-manipulation tasks. Nanoscience provides a new approach to manufacturing electronics, and could lend major advancements to the world of medicine. Nanotechnology already aids in the design of Pentium processors, and soon, tiny nanomachines may be used to repair damaged cells or tissues in our bodies.
Molecules will be the materials for these machines, and the most mechanically-stable molecules must be used. There are three main ways to manipulate molecules and test their stability. The STM can push them, pull them, or pick them up and put them down.
Pushing or pulling the molecules with the STM falls under the category of “lateral manipulation.” During these techniques, the STM tip does not have direct contact with the sample; it simply applies an electric field to the molecules. The electrons of the molecules “feel” the charge from the tip, and it’s this electrostatic interaction – attractive or repulsive – that produces the “motion” (pushing or pulling) of the molecule.
The STM can also act as a crane, selecting molecules and repositioning them. This is referred to as “vertical manipulation.” In this case, the STM tip does have direct contact with the atoms or molecules. The tip “dips” into the substrate, the atoms cling to the tip, and then are moved to a new location on the surface.
The STM also facilitates methods for breaking and forming molecular bonds. In order to break bonds, the STM “excites” the molecule with a tunneling current until the links between atoms are fractured. Dr. Hla has developed a technique for doing the reverse operation. By exciting two broken pieces of a molecule with an electric field or tunneling current, bonds can be rejoined. Hla used the analogy of a divorced couple. The two liked each other at one time, but after splitting they became caught up in other groups of friends, (other electrons on the substrate.) With the right spark, (a tunneling current) the two rekindle their attraction and are pushed together to form a molecule once again.
The STM in Action
One Ohio University student, Jessica Benson, was recently awarded a Marshall Scholarship for her research using the STM. She decided to use lateral manipulation to put a molecule involved with photosynthesis, Chlorophyll-a, to the test. She took the molecules from spinach, and used a copper surface as a substrate. The molecule remained intact after being pulled across the substrate, proving its mechanical stability.
“The spinach molecule is a promising candidate for environmentally friendly nano-electronic device applications,” Benson explained when presenting her findings at this spring’s American Physical Society conference.
This is just one example of the many possibilities the STM has to offer. Constructing electronics in a more environmentally-sound manner, using nanomachines to navigate our bloodstream, or arranging atoms to achieve fleeting synthesis of nature and art – as the eyes and hands of nanoscience, the STM is the key to understanding the beauty and brawn that matter possesses at the atomic scale.
