By eye, we cannot see anything much smaller than the width of a human hair, which is tens of microns. The nanoscale is thousands of times smaller than this. To see, detect and measure objects at the nanoscale, we need to use microscopes to help us. There are many types of microscope which allow you to look at things you cannot see normally with varying levels of detail, or resolution. The level of resolution depends the physical mechanisms behind the microscope, the sample you are using in addition to the quality of the instrument.
Each different type of microscope is designed to observe different types of objects at different magnifications in different environments. Are you trying to look at a bacteria in liquid, a single atom in vacuum, or something inbetween? Often you may want to use a number of different microscopes to look at the same sample to obtain a full understanding of it, as each microscope will give you different pieces of information.
One type of microscope is an Atomic Force Microscope (AFM). We are particularly interested in this type of microscope as we are trying to build one! Also because they are really useful in understanding a wide range of samples.
How does an AFM work?
An AFM ‘sees’ objects in the same way a blind person does, by feeling the surface using a long flexible stick; lumps and bumps on the surface cause the stick to bend up and down, allowing you to build a picture of it. An AFM is basically a scaled down record player which can read any surface, not just records, generating pictures instead of music. This idea may sound a bit confusing, but it simply sums up the basic function of an AFM.
In an AFM there are a few key parts; a cantilever, a laser with its optics system, a stage, a box of electronics and some vibration control. By taking these components and putting them together we can make a state-of-the-art microscope.
The reason we say that an AFM is like a record player is that the cantilever tip acts just like the needle in a record player. In a record player there are grooves and the needle moves up and down as it passes over them. The signal that we hear is basically a measure of the height of the record underneath the needle. In an AFM the record disc is replaced by the sample of interest, which can almost be anything with interesting and very small features, and as the cantilever goes over it, it moves up and down like the needle. The signal that we record is the displacement of the cantilever from its rest position, from which we generate images as opposed to sound.
We generate the electrical signal using the laser and its optics. If we focus a laser on the back of the cantilever then we can monitor the cantilever whilst it moves up and down. We do this through special types of optics; the cantilever moving causes a change in the reflected light, which is measured as a change in the shape of the reflected laser beam that focusses on the photodetector. We can measure this signal electrically. This signal is quite small so we feed it into the box of electronics. Some of these electronics are used to amplify the signal. This amplified signal can be read out and fed into the computer. This allows us to electrically measure the distance between the laser and the substrate at one point.
With this technology we can really sensitively measure the height of the tiny point on the sample that the cantilever is floating over. But we are interested in much more than just this point, so we need to be able to move the sample around. This brings us to the next key component of an AFM, the scanning stage. By using a carefully designed stage we can use an electrical signal to control the motion of our sample. This means that the sample scans underneath the cantilever. The relative motion of cantilever and sample means that we can collect height data at a number of points. When we get all of this data we can stitch it together on the computer and build up an image. This is our microscope image which can be at pretty high magnifications!
We haven’t mentioned vibration control. This is important in pretty much all high magnification microscopes. Without vibration control you find that the sample moves distances larger than the sample size. This means that in the final image the sample is blurred out and you can’t see it. We need some vibration control to stop the sample moving around.
When would we want to use an AFM?
Well AFMs can be used to generate some really high magnification images. In optical microscopes (normal microscopes which just magnify light) you are limited in the feature size which you can see. This is because there is a limit to how much you can focus a light beam. The ultimate limit to how small you can focus a beam of light is around 1um (micrometre). One micrometre is pretty small and so we can see some pretty small things by amplifying light but often the things that we want to see are smaller than this, for example DNA, nanoparticles, viruses, atoms, surface features on bacteria, and all sorts of other things.
One situation where we might consider using an AFM is when we want to see features smaller than 1um. In a really good AFM you can see surface atoms – yeah individual atoms –which are under a nanometre (over 1000 times smaller than 1um!). Not all AFMs can do this, but they can still get some big resolution improvements over an optical microscope.
When else would you want to use AFM? The answer is quite long – there are lots of things that you can do with AFM so this will not be a comprehensive answer! An example of one other thing that you might want to use AFM for is nanofabrication (instead of imaging). Nanofabrication is where you make things at the nanoscale. In an AFM you can do this in the same way that you image but instead of looking at a square and imaging it you scan a selected area in a predefined shape. When you image with the cantilever in contact with the surface of whatever it is you are looking at then the cantilever scrapes away a bit of the surface. Wherever you haven’t imaged there will be a residual layer standing proud of the surface. The shape and depth can be determined by setting your scan parameters which provides one simple tool for nanofabrication. This kind of nanofabrication can give features 10s of nm size.