Scanning Tunneling Microscope: What the Heck Is It?
Scanning tunneling microscope (STM) is an instrument that reveals individual atoms on the surfaces of objects or matter.
It functions by scanning a surface with a beam of electrons, causing a narrow channel of tunneling electrons to flow between the sample and the beam. This produces very focused three-dimensional images of atomic topography and structure.
The first STM was created in 1981 by German Gerd Binnig and Swiss Heinrich Rohrer at IBM Zurich, who won the 1986 Nobel prize in physics for their groundbreaking invention.
The scanning tunneling microscope is able to scan with such a high resolution that individual atoms within objects can be routinely photographed and manipulated.
The STM is very versatile in that it can be used in a variety of conditions, including ultra-high vacuum, water, air and other gas or liquid surroundings, and at temperatures varying from zero Kelvin (that’s minus 459.67 degrees Fahrenheit or minus 273.15 Celsius!) to a few hundred degrees Celsius (250 degrees Celsius equals 482 degrees Fahrenheit, more than double the temperature at which water boils).
This amazing instrument is based on a concept called quantum tunneling, which refers to a mechanical occurrence in which a particle passes through a barrier that it ordinarily could not penetrate. Quantum tunneling plays a vital role in a number of important physical phenomenon, including the type of nuclear fusion that happens in main sequence stars (the sun, for example) and has essential uses in modern devices such as the semiconductor tunnel diode.
How the STM Works
In a nutshell, the STM works by scanning a very sharp metal wire tip over a surface. By bringing the tip very close to the surface, and by applying an electrical voltage to the tip or sample, we can image the surface at an extremely small scale – down to resolving individual atoms.
When the conducting tip of the scanning tunneling microscope is moved extremely close to a surface being examined, a voltage difference – called a bias – is applied between them. This action can allow electrons to tunnel through the vacuum between them.
The resulting tunneling current is a function of tip position, applied voltage, and the local density of states (LDOS) of the surface being scanned. Information is acquired by monitoring the current as the tip’s position scans across the surface, and is usually displayed in image form.
STM can be a difficult procedure, as it requires very clean and stable surfaces, sharp tips, exceptional vibration control, and complicated electronics.
The components of the space-age microscope include the scanning tip, a piezoelectric controlled height and x,y scanner, a coarse sample-to-tip control, a vibration isolation system and a computer.
The clarity of an image is restricted by the radius of curvature of the STM scanning tip. Furthermore, image artifacts can happen if there are two tips at the end rather than a single atom. This results in an event called “double-tip imaging,” where both tips contribute to the tunneling.
Because of this, it is critical to develop methods for consistently acquiring sharp, usable tips. The tip is often constructed from tungsten or platinum-iridium, although gold can also be utilized. Tungsten tips are usually made by electrochemical etching, and platinum-iridium tips by mechanical shearing.
Due to the extreme sensitivity of tunnel current to height, proper vibration isolation, a very rigid STM body is important for usable results.
Binnig and Rohrer employed magnetic levitation in their first scanning tunneling microscope to prevent vibration. Currently, mechanical spring or gas spring units are common. Additionally, mechanisms for reducing eddy currents are sometimes implemented.
Maintaining the tip position with respect to the sample, scanning the sample and acquiring the data are all controlled by the computer, which can also be used for improving the image with the aid of image processingand also for carrying out quantitative measurements.
No Shovels Involved In THIS Tunneling!
Tunneling is a concept that arises from quantum mechanics: an object colliding with an impenetrable barrier will not pass through. But objects with a very small mass, such as the electron, have wavelike characteristics which allow such an event, which is referred to as tunneling.
A tunneling current happens when electrons go through a barrier that they ordinarily shouldn’t be able to penetrate. If you have insufficient energy to go “over” a barrier, you won’t.
However, electrons have wavelike characteristics. These waves don’t finish abruptly at a wall or barrier, but rapidly taper off. If the barrier is thin enough, the probability function may reach into the next region, right though the barrier!
Because of the small likelihood of an electron popping up on the other side of the barrier, given enough electrons, some will indeed move through and appear on the other side. This is tunneling.
Because of the sharp decline of the probability function through the barrier, the number of electrons that will make the jump is dependent upon the barrier’s thickness. The actual current that penetrates the barrier drops off rapidly with increased barrier thickness.
To apply this to the STM: The starting point of the electron is either the tip or sample, depending on how the microscope is set. The barrier is the gap (air, vacuum, liquid), and the second region is the “other side” – tip or sample, again, depending on the arrangement of the experiment.
By monitoring the current through the gap, we have very good control of the tip-sample distance.
For What Purpose Is a Scanning Tunneling Microscope Used?
The STM is widely used in both industrial and fundamental research to obtain atomic-scale images of metal surfaces.
Because it provides a three-dimensional profile of the surface, it is quite useful in characterizing surface roughness, observing surface defects and determining the size and conformation of molecules and aggregates on the surface. Examples of advanced research using the STM are provided by current nanotechnology studies in the Electron Physics Group at NIST and at the IBM Laboratories.
The scanning tunneling microscope is a key instrument for studies in surface physics and chemistry. With the capability to reveal the maekup of the outer layer of atoms or molecules, the STM can reveal surface defects, show the morphology of various depositions or measure surface roughness. In addition, STM is used to study conduction or charge transport mechanisms.
STM can also be deployed to precisely manipulate single atoms by pushing or dragging them with the tip at low temperatures. Electrons emitted by the tip can also be used to alter the sample. This makes it an important tool in nanosciences.
If You Have Actually Read Down This Far On the Page
… Wow! You REALLY Want to Know About STMs
So, then, let’s have some fun. Stuffy science nerds can quit reading here, unless you want to see what the lesser brainiacs (ordinary folk) think about this foreign language called SCIENCE.
In Layman’s Terms: the Scanning Tunneling Microscope allows engineers, rocket scientists and other super-smart people to see and manipulate individual atoms in order to do things we would never dream of, let alone understand, for purposes we can’t imagine – (hopefully) for the common good.
Should the topic of scanning tunneling microscopy ever come up in conversation, just gaze at someone who has that deer-caught-in-the-headlights look (don’t worry, there will be LOTS of them, unless of course the topic comes up at a Mensa meeting) and explain, “An STM allows us to image and manipulate individual atoms on surfaces” and you will be deemed the Einstein of the soiree.
If mention of the Scanning Tunneling Microscope seldom seems to pop up in your circles, feel free to introduce it offhandedly into the conversation yourself to reap the above benefits!
How, you might ask? Simple!
Should an especially attractive member of the opposite sex walk past, casually mutter, “I’d like to examine THAT under a scanning tunneling microscope.”
Or pay a compliment to a spouse, friend, etc., by congratulating, “You know, you couldn’t have done a better job on that if you had used a scanning tunneling microscope!”
Then there is always the ubiquitous lament when something goes wrong, “Dang it, there’s never a scanning tunneling microscope around when you need one!” Feel free to use a stronger curse than “dang” should the situation and company warrant it.
See, this is loads of fun! Surely you’ll be able to think up clever ways to insert the term “scanning tunneling microscope” into practically ANY conversation!
Oh, and if some other smarty pants tries to impress everyone by explaining exactly what an STM is before you get the chance, simply shake your head sympathetically and mention in talking-down tone of voice, “Yeah, a scanning tunneling microscope was always my favorite … then I heard about the Titan 80-300 Cubed Electron Microscope – makes an STM look like a cheap toy.” You’re welcome!
Now get out there and impress everyone with your knowledge of the scanning tunneling microscope!