Atomic force microscopes (AFMs) are powerful tools that allow scientists to see things at the atomic level.
They can measure forces between atoms and molecules, such as friction or adhesion. This video shows how they operate.
An atomic force microscope uses a tiny cantilever tip attached to a microfabricated silicon chip.
When the cantilever moves over a surface, it feels the topography of the surface through its spring constant.
By measuring the deflection of the cantilever, the position of each atom on the surface can be determined.
In this article, we will explore the operation of an atomic force microscope.
We will begin with the basics and then move into some more complicated examples in depth. So let’s begin!
What Is An Atom?
The word “atom” comes from the Greek word meaning indivisible particle. An atom is the smallest unit of matter within an element.
It is composed of protons, neutrons, and electrons. Protons have a positive charge and neutrons have no charge.
Electrons orbit around the nucleus like planets orbiting the sun. Each electron has a specific amount of energy associated with it which is called its orbital energy.
Atoms are arranged in groups based on the number of protons. These groups are known as elements. For example Hydrogen, Helium, Lithium, etc.
How Many Types Of Atoms Are There?
There are 118 naturally occurring elements on Earth. All other elements were created in stars during supernova explosions.
Elements heavier than iron cannot be made in our universe because all the mass needed for them to form would collapse under their gravity.
There are also artificial elements. Scientists create new elements by bombarding targets with high-energy particles.
Some of these elements exist only briefly before decaying back into lighter elements. Others may become stable enough to last longer.
Why Do We Care About Atoms?
We care about atoms because every material thing we encounter is made up of atoms. Our bodies are mostly water, but most of that water is not pure H2O.
Most of the oxygen in your body is bound to carbon, nitrogen, and hydrogen atoms. Your bones are primarily calcium hydroxyapatite.
Iron is found in hemoglobin. Carbon is found in DNA.
We use atoms to make everyday objects. Metals are built out of atoms of gold, silver, copper, lead, tin, zinc, aluminum, and nickel.
Wood is made of cellulose fibers held together by lignin. Glass is made of silica atoms bonded together.
You could say that everything you see around you is made of atoms.
What Is Atomic Force Microscopy?
Atomic force microscopy is a type of scanning probe microscope. Scanning probe microscopy allows us to examine surfaces at very small scales.
In AFM, a sharp tip is scanned across a sample’s surface. As the tip approaches the surface, forces between the tip and the sample cause the tip to deflect.
This deflection is measured using a laser beam reflected off the end of the cantilever.
A computer translates the deflection data into a three-dimensional image of the surface.
AFM tips come in two varieties:
1) Contact mode – The tip touches the surface and measures the force required to maintain contact.
2) Non-contact mode – The tip does not touch the surface. Instead, it is moved close to the surface while maintaining a constant distance from it.
Contact mode is used to measure topography (roughness), adhesion, and friction.
Non-contact mode is used to measure elastic properties such as Young’s modulus and hardness.
Atomic Force Microscope
An atomic force microscope (AFM) uses a tiny needle-like tip attached to a cantilever beam. When the tip touches an atom, it bends slightly.
We then measure how far the tip has bent. We repeat this process thousands of times per second to create a 3D map of the surface topography.
This allows us to see tiny changes in the surface of the material being studied.
For example, we can see the difference between a smooth metal surface and a rough metal surface. Or, we can see the differences between two different materials.
One type of AFM uses a laser beam to heat the tip. Then, we watch how the tip moves when it comes into contact with the surface.
Another type of AFM uses magnetic forces to move the tip. In this case, we use a magnet on the other end of the cantilever beam.
When the tip touches the surface, the magnetic field from the magnet pulls the tip away from the surface.
We can also use an AFM to detect defects on surfaces. For example, we can use an AFM to look at the surface of a silicon wafer before it goes into production.
If there are any problems with the surface, we will know about them early enough to fix them.
The major advantage of an AFM is that it doesn’t require a vacuum chamber. It works well in the air and doesn’t damage the samples being studied.
How Does An AFM Work?
When we use an AFM, we first place the tip near the surface of the object under investigation.
Next, we carefully lower the tip towards the surface. Once the tip reaches the surface, we stop moving it.
Then, we wait for some amount of time. During this waiting period, the tip continues to vibrate due to thermal noise.
As long as the tip remains stationary, we can measure its vibrations. These vibrations tell us information about the surface.
For example, if the tip hits a bump on the surface, it will bounce back. But, if the tip hits the flat part of the surface, it won’t bounce back.
That tells us something important: the tip hit the bump.
If we keep lowering the tip until it finally touches the surface, we can record the height of the bumps.
By comparing these heights to the original surface profile, we can determine exactly where the bumps were located.
This is just one way that an AFM can help us study the structure of our sample. There are many others.
What Are Some Applications Of An AFM?
There are many applications of an AFM. One common application is to examine the surface of semiconductors.
Semiconductors are very useful because they allow electricity to flow through them. However, sometimes they have imperfections.
These imperfections can be caused by impurities in the crystal lattice. They can also be caused by dust particles.
Using an AFM, we can check the surface of a semiconductor to make sure that it’s free of such imperfections.
Another common application is to examine microelectromechanical systems (MEMS). MEMS are miniature devices used in everything from cars to computers.
An AFM can be used to examine the surface of a MEMS device to make sure that it has no flaws or cracks. This would prevent the device from breaking down during operation.
Another common application is to examine biological cells. We can use an AFM microscope to see what kind of shape different types of cells take.
A third common application is to examine materials like plastics. Plastics are widely used in products ranging from food containers to medical equipment.
Using an AFM, we could examine the surface of a plastic product to make sure that it was free of imperfections.
An AFM can also be used to examine the surfaces of metals. Metals are often used in electrical components.
Because of their hardness, they don’t easily wear out. However, they do need a periodic examination to make sure that they’re still working properly.
An AFM can be used in other ways too. For example, we might want to examine the surface of the paper. Paper is made up of tiny fibers.
If we look at the surface of the paper with an optical microscope, we’ll notice that there are lots of little holes and craters. Those holes and craters are made of tiny fibers.
We can use an AFM to examine the surface of those fibers so that we know how much force they exert when you try to tear off a piece of paper.
What Is Electron Microscopy?
Electron microscopes are similar to light microscopes except that they use beams of electrons instead of light.
Like light microscopes, electron microscopes can magnify images up to millions of times.
Unlike light microscopes, however, electron microscopes have a much higher resolution. They can resolve features smaller than 1/1000th the width of an atom.
Electron microscopes are often referred to as transmission electron microscopes or TEMs. They work by passing electrons through a thin slice of metal called a specimen holder.
The electrons interact with the atoms inside the specimen, causing some electrons to scatter away.
Those scattered electrons leave the specimen holder and travel toward a detector where their paths are recorded.
By repeating this process for each pixel on the screen, the image is created.
There are many types of electron microscopes. We will focus on one particular type called STEM.
STEM stands for “Scanning Transmission Electron Microscope”. It works like this:
1) An electron gun fires electrons at a target.
2) When the electrons hit the target, they knock loose electrons from the atoms in the target. These freed electrons move towards the back of the microscope.
3) At the back of the microscope, there is a special lens system that focuses the electrons onto a second target.
4) When the electrons hit this second target, more electrons are knocked loose. These free electrons move towards the front of the microscope.
5) At the front of the microscope, another lens system focuses these electrons onto the final screen.
6) The pattern of electrons hitting the screen is what creates the image.
Why Do We Need An Electron Microscope?
To understand why we need an electron microscope, let’s look at how light microscopes work. Light microscopes work by shining light through a thin section of glass or plastic.
If you shine light through a liquid, the light scatters and makes the liquid appear cloudy.
That’s because the light particles bounce around too much when they pass through the liquid.
But if you make the liquid transparent, then the light travels straight through without bouncing around so much. That’s why glass and plastic are used as lenses in light microscopes.
The same thing happens with electrons. When electrons pass through a solid material, they bounce off the atoms in the material and lose energy. This causes them to scatter.
If you make the material transparent, then the electrons don’t scatter so much. That’s what allows us to see things under a light microscope.
But electrons aren’t just any old particles. They’re charged particles. So when they bounce off the atoms, they also gain energy. And when they gain energy, they move faster.
So even though the electrons aren’t moving very fast, they still move faster than the speed of light. They move close to the speed of light.
When an electron moves closer to the speed of light, it becomes less likely that it’ll be slowed down again by scattering off the atoms.
Instead, it’ll keep going until it hits something else. That’s what causes it to scatter.
So when an electron passes through a piece of transparent material, it doesn’t scatter as much.
Because of this, it can travel right through the material and end up on the other side. That means we can use the material to magnify the object.
How Does An Electron Microscope Work?
So now that we know how a light microscope works, let’s take a look at how an electron microscope works. First, we start with the same materials we used before.
We put them into a vacuum chamber. Then we fire a beam of electrons at the sample.
As soon as the electrons hit the sample, they knock loose some electrons from the atoms in our sample.
These freed electrons move towards the rear of the microscope where there’s a special lens system called a magnetic objective lens.
This lens focuses the electrons onto a small spot on the back wall of the microscope.
This spot is called the secondary emission point. It’s located about 1/10th of a millimeter away from the first emission point.
Next, we turn on the high-voltage power supply. This turns on a bunch of magnets inside the microscope.
These magnets pull the electrons coming from the secondary emission point towards the front of the microscope.
As the electrons get closer to the front of the microscope, they begin to slow down. Eventually, they stop completely.
Now we have a bunch of electrons stuck on the front wall of the microscope. At this point, the electrons are all lined up like soldiers waiting for their orders.
Now we flip the switch on the low voltage power supply. This lets the electrons go free.
As the electrons leave the microscope, they continue to move forward. But instead of hitting another atom in the material, they hit a tiny metal plate.
As the electrons hit the metal plate, they give off some of their energy. The electrons are scattered backward, but not enough to stop them.
So these electrons continue to move towards the rear of our microscope.
The electrons eventually reach the secondary emission point. There, the electrons lose more of their energy. Finally, they come to rest.
The process repeats itself over and over again. Each time, the electrons lose some of their energy. And each time, they move closer to the front of our microscope.
Eventually, the electrons will reach the front of the microscope and escape. When they do, they’ve lost some of their energy.
So they won’t be able to pass through the same piece of glass or plastic that we started with.
Instead, they’ll bounce around inside the microscope. They’ll hit different pieces of metal. Some of those metals will reflect the electrons to us. Others will absorb them.
And that’s why you see the image on your screen. You’re seeing the reflected electrons bouncing off the surface of the sample.
So What Happens If We Don’t Use A Magnet?
If we don’t use any magnets, then the electrons just keep moving forwards until they run out of energy.
Once they’ve lost all their energy, they’ll fall to the bottom of the microscope.
But if we use a magnet, then the electrons will be forced to follow a curved path. That means they’ll travel further than they would without the magnet.
In fact, they’ll travel so far that they can no longer return to the secondary emission point. Instead, they’ll end up falling to the top of the microscope.
That’s why the electrons appear brighter when using a magnet. They’re traveling farther than they would otherwise.
What Does It Mean To “See” An Electron?
When we look at an object under an electron microscope, we actually aren’t looking at the atoms themselves. We’re looking at the electrons that orbit the atoms.
When we say that we’re “seeing” an electron, we really mean that we’re seeing the effects that the electron has on other particles.
For example, if we shine light onto a piece of paper, the photons that make up the light interact with the electrons in the paper. Those interactions cause the electrons to vibrate.
This vibration causes the electrons to emit waves of energy called “photons”. These photons are the things that we call light.
Similarly, when we look at something under an electron microscope, the electrons that make up the object interact with the electrons in our detector.
These interactions cause the electrons to emit waves. Just like the electrons in the paper, these waves of energy are called “electrons”.
So when we look at something with an electron microscope, we’re really seeing the effects that the electrons have on the detector.
How Many Types Of Microscopes Are There?
There are two main types of electron microscopes: Transmission Electron Microscopes (TEMs) and Scanning Electron Microscopes(SEMs).
Transmission electron microscopes work by shining electrons into a piece of material.
As the electrons enter the material, they are slowed down as they pass the material. This slows them down enough for us to see the structure of the material.
The electrons eventually leave the material and strike a piece of film behind the material. This is where the image gets recorded.
Scanning electron microscopes work by scanning over the surface of the material.
As they scan the electron across the surface of the material, they slow down as they pass through it. This slows them down, even more, allowing us to see much smaller details.
Scanning electron microscopes also record images directly onto film. However, unlike transmission electron microscopes, this isn’t done after the electrons have left the material.
Instead, the electrons are scanned across the surface of the sample while still inside the material.
Both TEM and SEM microscopes are used to study materials such as metals, semiconductors, polymers, ceramics, glasses, rocks, etc…
However, both types of microscopes are limited because they only allow us to view one side of the sample at a time.
In addition, they require very high voltages to operate. For example, a typical SEM requires around 5 kV to operate.
That means that you need to be careful about what you touch!
What’s So Special About Using Electrons Instead Of Light?
Electrons can penetrate objects better than light, making it possible to get a clearer picture of the internal structures of those objects.
In addition, electrons don’t scatter or absorb as easily as light does. So, if we use electrons instead of light, we can get a sharper image.
AFM and Electron Microscopy are incredibly useful tools for scientists. They give us a way to examine samples without damaging them.
They also help us learn about the properties of different substances. This article discussed how AFM and EM work and some of their applications.
These tools are widely used in research labs all over the world and help scientists understand the world around us.
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