STM 101: Microscope Basics

How do we image atoms and molecules?

Have you ever used a magnifying glass to make a small object, like a bug,appear bigger?  Or a microscope in biology class to look at plant cellwalls or tiny little organisms like plankton?  A magnifying glass and amicroscope make small objects look bigger by using lenses to manipulate light,and you can change the amount that an object is magnified by using differentsized lenses.  This technique works great for bugs, tiny organisms, and evencells because these objects are the perfect size to reflect visible light, andour eyes are really great at seeing visible light.

But what about atoms?  Atoms are everywhere, but have you ever seenan atom?  Chances are your answer to this question is no.  Atoms aretoo small to reflect light like bugs, plankton, or cells, so we can't see atomswith our eyes.  This is where a scanning tunneling microscope, or STM forshort, comes in.  An STM uses electrons to help us visualize atoms and moleculesand works because electrons are similar in size to atoms and molecules. What exactly is an STM, and how does it work?

Let's start with a picture of an STM. 

 

(For scale, the yellow tube in the center of the picture is about a 1/4" in diameter and the whole microscope can fit in the palm of your hand.)

Not what you expected?  That's ok, let us explain.  An STM consistsof three main parts.

First, a tip, which is really just a piece of electrically conducting wire cutto be extremely sharp at the end.  In the image above, the tip was made bycutting a platinum/iridium alloy wire with wire cutters.  (Seriously!) 

Second, we need some atoms to look at, and that's our sample. Samples are also (generally) good conductors of electricity.  Here you'relooking at a very nicely polished and shiny copper sample, it's the orange colored round disk that's really close to the tip.

And you can't actually see the third component, which is a voltage source, muchlike a battery in the remote control for your tv or an electrical outlet in the wall of your house.  Tounderstand why we need a voltage source, let's zoom in, a lot, and look at theatoms that make up the tip and the sample.

 

Here, each grey or orange circle represents a single atom in the platinum/iridium tip or copper sample, respectively.  

After zooming WAAAAY in like this, you can now see that the tip and the sample aren't actually touching.  They're separated by a very small gap, a 1 nanometer or 0.0000000001 meter gap to be exact, and *this* gap is the reason that we need a voltage source.  When we turn the voltage source on, an electric current can flow between the tip and the sample even though they're separated by that gap.  It's kind of like your tv working even if, for example, your cat chews straight through the cord.  

In physics, this flow of electrons through a gap is called tunneling, which partially explains the name scanning tunneling microscope.  And this electric current, composed of tiny electrons, is what makes it possible to see tiny objects like atoms and molecules.