An electron microscope is a highly advanced microscope that, depending on the type of electron microscope, blasts electrons through a specimen, excites electrons that make up the specimen, or maps the tunneling of electrons through a specimen and reconstructs the feedback from these methods to form an image. The ability of these microscopes to help us visualize specimens that are smaller than the wavelength of light has helped to propel us into the nano world.
The 2000s and beyond, phrases like “Nanotechnology revolution” or “Quantum (insert device name here)” pop up everywhere. Many times, people think these are way “out of reach” topics to understand, but some of it isn’t as complicated as it seems!
Discoveries from the nano world changed the way we view life. It gives great insight into things we couldn’t even process in the past. One of these technologies, is literally how we view things we couldn’t in the past!
The typical question is: as technology gets smaller, how can people see what is going on at that level? Or maybe, we want to check out things that are so tiny normal microscopes don’t work. How do we zoom in on the face of something already so small? Like going from this:
To this:
Through high-tech microscopes! That image was completed by an Electron Microscope – a microscope that uses tiny particles called electrons and a bit of quantum mechanics. It might seem like a super new technology, but electron microscopes have been around since 1931.
From checking out bugs, to new materials, to medicine, electron microscopes have opened the nano world up to us. While they seem like very complicated devices, this article will help illuminate everything there is to know about how these revolutionary devices work!
What Are Normal Microscopes?
But first, what are normal microscopes and how do they work? Most people have seen these microscopes before. This is a typical compound light microscope.
A little glass slide gets snapped into place. You turn on a light, you look through a magnifying lens, and you see something tiny… but bigger! You move some knobs about to focus the image you are seeing, and then you can scan around and check out what you are looking at.
So how does this work? You have that white light coming from underneath the object. It bounces around the object, it gets absorbs, reflected, etc. Eventually, some of that light will end up going through that magnifying lens, which than goes from that tiny image all the way up that 50X or 100X image your eyeball is seeing. For more information on how a microscope works see Microscope Magnification: Explained and What is Microscope Resolution?
Basically, this whole process is dependent on two things: (1) What you are looking at and (2) How are you looking at it. What you are looking at is maybe some bacteria or a mosquito. How you are looking at it is… light! It is because that light hits that object, gets magnified, and then hits your eye.
Just a quick review. What is light? I’m sure we have all seen this before (especially if you are a Pink Floyd fan).
Light can be a huge range of wavelengths. We see the colors between red and violet – the visible spectrum. These colors, as the bottom of the graph shows, have a wavelength associated with it. So red is around 700 micrometers, while violet is around 400 micrometers. Light is a wave that has this wavelength.
That is the mechanism of a normal microscope! Say we have an object that is 2 millimeters (2mm is 2000 micrometers). We send in light that is 500 micrometers in. It has plenty of room to “bounce around” and “collect” information about the object we want to see. The light eventually makes it way to our magnifying glass and into our eyes.
This works because the object we want to see if much bigger than our wavelength of light we are using. We can say that the image is resolved. That is – the wavelength of light is small enough so it can accurately capture everything about the much bigger object we want to see. But what happens if the light isn’t small enough?
Why Does Light Fail?
As scientists and engineers got more interested in the micro/nano worlds, the size scales became more and more of an issue. They wanted to see materials at smaller and smaller scales. Eventually, scientists got to the point where the object is smaller than the wavelength of light, so our images were unresolved.
Light cannot bounce around the object enough times to produce a clear image by the time it reached our eyes. This is because for a smaller object, we need a smaller wavelength. And eventually, they got out of the visible range. So, we can’t really use light waves anymore to image things. But as Einstein proclaimed: Things can be both waves and images, and that is where things get Quantum!
Quantum Mechanics: It’s more than just Schrodinger’s Cat!
Light waves aren’t small enough to produce images anymore. Smaller wavelengths are needed, yet the limit is reached with light. At the turn of the 19th century, scientists were introduced to the quantum world. The first real instance of this was Einstein. Everyone was comfortable with the idea light was a wave, but Einstein also proved it was a particle too. This particle was then named a photon.
Meanwhile, for decades everyone knew of the particle called the electron. This electron is how scientists studied electricity. This little bundle of energy is what caused metals to shock or how lightning hits the ground. The electron is super small and super light (we actually still don’t know what it looks like!). However, they knew it was a particle. But according to quantum mechanics, could it act like a wave too?
That is where another famous scientist came in – Dr. Louis DeBroglie. He definitively made the statement that all particles could act like waves, with a defined wavelength. This wavelength was based on the mass of the object. Now because the electron is so light and so small, its wavelength is smaller than that of light! Much smaller! So, it could be used for imaging!
A quick summary:
- Scientists knew light was a wave. This wave could be used to see things
- Things that scientists wanted to see got smaller and smaller. Light didn’t work anymore.
- The discovery was made that light (which everyone treated as a wave) could be treated as a particle
- So, scientists thought “could we take particles and use them as a wave?”
- The electron was the first candidate in this endeavor!
- The electron’s wavelength was much smaller than light so smaller things could be seen!
Great! We have a few general reasons why scientists made the switch from typical light to electrons to image objects. The trick now is, how do we construct these microscopes? Light microscopes involve some mirrors and lens and a light source. Electron microscopes are slightly more complicated
Types of Electron Microscopes
In general, there are three major types of electron microscopes, each of them using these electrons to image but using a different mechanism.
Transmission Electron Microscope (TEM)
The first type of electron microscope is the transmission electron microscope. This is the first type of electron microscope. The key mechanism here is transmission. The electron travels through the object to collect information. It is very similar to the typical light microscope.
These microscopes take these electrons, speed them up very quickly, and then lets the electrons “seep” through the object. After transmitting through the object, they are then again focused on some lens which force the electrons to land on some kind of sensor to give a computer feedback on the “image” they saw. There are five main components to these microscopes:
- Vacuum System – as one can imagine, electrons can interact with a lot of things! So, our sample needs to be in an environment with as little stuff to interfere with the electron. The volume where the sample sits gets pumped down to very low pressures so that there is nothing “floating” around to affect the electrons while they image.
- Specimen Stage – is exactly what it sounds like! It simply holds the object you want to see.
- Electron Gun – once again… it is exactly what it sounds like! There are a lot of components to an electron gun, but it basically shoots electrons. There is a bit of electronics that control how electrons are emitted in a controlled manner towards the electron lens.
- Electron Lens – analogous to typical light lenses. They help focus the electron path towards the object (or part of the object) you’d like to see.
- Apertures – Plates that help control errors, stray electrons, reduce measurement noise, etc.
You can see most of the components of the microscope here. The computer is used to communicate to and from the microscope. The microscope itself is the tall thing to the left of the computer.
The main “chamber” is the metallic tube running down the middle. This is where the sample is placed, and the lens and apertures are adjusted. The top of this tube holds the electron gun. The large white portion in the back is mostly electronics, monitors, and other components to make sure the microscope runs safely.
As one can see, there are a lot of components that make up an electron microscope. Usually, these components are all made by different companies. For example, one company will make the lens, one company will make the electron gun, another the body, etc. But the end product is usually assembled by a company that collects all the individual components and creates the final product.
Companies include Leica/Cambridge, Hitachi, Amray, ASPEX, and many more. They can cost anywhere from $6000 to $100,000 per microscope – depending on if it is new and what kind of features come with it.
As the first kind of electron microscope, the transmission electron microscope was used to see a lot of different things. The first few uses for it was to explore how materials look and act on a small scale. Other uses were in biology to explore what viruses and bacteria looked like on small scales, and how we can better understand how they replicate and move about our bodies:
Scanning Electron Microscope (SEM)
The next type of microscope is the scanning electron microscope. In the transmission electron microscope, electrons are sent through the object to “process” the object for us to view it.
In the scanning electron microscope, electrons are used to scan the object we’d like to view. Electrons are sent to the object and interact with the atoms in the sample. Usually, these atoms that are sent to the surface of the object excites other electrons in the object. It is these emitted electrons that are sensed by the electron microscope to process the object and produce an image.
The trick here is that the incoming electrons need to interact with the surface to get these other electrons out of object to get imaged. Thus, the object itself needs to be electrically conductive to be processed. For example, biological specimens are not electrically conductive. So, to be processed, they are usually coated with a very thin layer of a conductive metal. This is a spider coated in Gold.
This is an extra step when compared to what was needed for the transmission electron microscope. These microscopes are also under very low pressure (high vacuum), so it is important that objects aren’t placed in it that might produce gases.
For example, putting a living tissue under a vacuum might destroy what a person wishes to see from it. However, unlike the TEMs, SEMs do not need to lens to image the object. They have lens to focus the incoming electron beam to a small enough area.
Within SEMS, there is another useful set of electron microscopes known as Environmental Scanning Electron Microscope… or ESEMs. Aside from objects themselves, ESEMS allow processes at the micro/nano scale to be investigated. For example, how does condensation actually work? How does a water droplet start to grow on a surface?
Here, a sample can be cooled down, and the sample can be exposed to water to start condensation. ESEMs are useful to explore how there is energy and mass transfer at very small scales like this.
Scanning Tunneling Microscope (STM)
The final type of microscope we will talk about is the scanning tunneling microscope. Earlier in the article we talked about one big premise of quantum mechanics – how particles can be waves and how waves can be particles. In pop culture, there is usually one other topic in quantum mechanics people usually hear about… Quantum Tunneling.
Imagine there are two hills. One is much bigger than the next. If the ball is rolling from the big one to the small one, it will make it over.
Now if the ball starts on the small hill, it won’t make it past the big hill:
This is a law in classical mechanics. But in Quantum mechanics, it COULD make it through the hill. This is called quantum tunneling.
The trick here is, that there is a CHANCE of it going through. Sometimes it doesn’t, sometimes it does. As our intuition might tell us, the chance if it is going through might depend on the size of the hill it wants to go through. The less wide the hill is, the more likely it will go through. This is the premise of the tunneling electron microscope.
Say we have some crystals that range from 1 nanometer to 3 nanometers in height on a flat sample. If we send some electrons in the 1 nanometer height area, we notice that more of the electrons are tunneled. We also notice that in the 3 nanometer region, less electrons are tunneled. Based upon how many electrons are tunneled, we can map out the topography (fancy word for map) of the object we want to see.
Scanning tunneling electron microscopes move back and forth on a sample to map out this surface. STMs usually are composed of around four components:
- Computer- helps process the image.
- Vibration Isolation System – as one can imagine, sending single electrons can be very tricky. So STMs have some kind of way to dampen and make sure the go on their correct trajectories.
- X-Y-Z stages – in STMs, usually it is the sample that moves. So, there must be controllers to move the sample up and down (z-stage), left and right (x-stage), and forward and backwards (y-stage).
- Scanning Tip – this is worth talking about in detail. So how do we eject a single electron at a time to probe the surface in great detail? Well, we need a really REALLY skinny tip. Picture a pencil tip… sharp right? Well that pencil tip has around 10,000,000,000,000 atoms across the diameter. That is a lot of space of electrons to roam around on, so we can’t really focus on an object using a tip the size of a pencil. We need a smaller tip. So STMs have tips that could be almost the size of one or two atoms across.
These tips can be very very small. But that is what is needed to probe in such detail. As a result, STM electron microscopes have some of the highest resolution imaging we can get to date. This is an image of clean gold. This image is atomically resolved such that individual atoms are starting to be recognized.
STMs are revolutionary in material science and physics for the ability to measure atomic structure and molecule imaging. However, there are limitations such as moving from flat (2D) to fancier structures (3D). However, they are by far the priciest microscope available. They can cost up to millions of dollars. Those tips alone can cost up to a $1,000 and you need to reuse those tips all the time.
Electron Microscopes and Science
Since their invention, electron microscopes have revolutionized how we see the micro and nano world. They immediately had an impact on material science and physics. Being able to see how materials are built up from their atoms changed how we can convert energy, manage heat, absorb sunlight, and many other things.
By looking deep into the structure of materials, it quickly spilled into other fields like food science to see how we can better improve food quality and safety. Electron microscopes are used in mining and industry to better quality of metals and chemicals that we use all the time. And most creatively, electron microscopes changed how we view some biological structures and animals.
By seeing insects on crazy up-close scales, it can help capture the complexity of life. Finally, they have helped with disease and drug research. The incredible viewing scales of electron microscopes helped clear up how viruses could reproduce or how our own cells react to drugs.
The Future
Electron microscopes seem to have a potential to be a tool for the near to distant future. While they may be expensive and costly to build and maintain, they have changed how many professions have embraced the micro and nano scale world to improve the world we live in. There are consistent improvements to the resolution and operation of these microscopes, like: low vibration buildings to reduce how accurate electron beams are, new lens and materials for focusing electron beams, and much more.
All these types of electron microscopes have become common equipment at labs, universities, research companies, and many more places. From the heart of it all, electron microscopes are probably the most obvious example of how quantum mechanics, a seemingly abstract and far-fetched type of physics, improved our understanding of the non-quantum world we all live in.