AFMs are complicated and advanced pieces of kit. It can be mind boggling trying to figure out what each part is for. In this month’s blog Jamie Goodchild, resident NuNano Applications Engineer, leads a tour around the parts of the AFM, to help you get the most out of yours…
It takes many years to master AFM. I have been doing AFM for around 8 years and it can still stump me. It can feel like something new goes wrong every week or there’s a new part of the machine/software or new physics associated with it that I need to learn about.
But that’s also what makes this work fascinating. The AFM is an instrument and a technique which continues to evolve. We’re still a long way from having exhausted all the possibilities of characterisation and measurement that AFM offers.
That’s why it’s well worth spending some time getting to know your AFM. And the very good news is that there are some things about the AFM you can learn upfront and which will apply to nearly every type of instrument you might come across.
In this post I help you get to know the different parts of an AFM - and I recommend you take the time to get to know them on the AFM in your lab…trust me, it’s worth it!
The manufacturer
NB Our AFM is a Park Systems NX10 Instrument. We have used photographs and diagrams of our instrument in order to illustrate the information we’re sharing below.
NuNano’s Park Systems NX10 AFM
There are a number of different AFMs, made by different manufacturers – Park Systems, Asylum, Bruker, Nanosurf etc. Whoever the manufacturer, most AFMs work on the same principles and have the same basic parts.
Own Software and Design - What is different about AFMs, depending on the manufacturer, is they all have their own software and have slightly different designs. Check out your manufacturer’s website for documentation on the instrument specs and for webinars and videos on basic use.
Defunct Manufacturers - the company your AFM came from may no longer exist…Company buyouts are a regular feature of the market, and some companies are no longer trading. This is worth keeping in mind if you’re struggling to find information about it online.
A probe by any other name will still fit - Most AFMs work with the same standard probe chip shape (3.4 x 1.6 x 0.3 mm). This means you’re not tied to buying probes from the instrument manufacturer. You can use NuNano probes on almost all AFM instruments (except if you have a certain type of older AFM that need alignment grooves).
HELP! Most AFM companies offer help – online help, applications scientists etc., and so do we. If you’re stuck with any question or issue with your AFM you can always get in touch with me, I’d be happy to help.
The parts you can see – and what they do
The AFM Head – This is where you’ll find the probe or tip holder (more on this in a moment) and a number of knobs and levers for focussing the laser on the cantilever and for focussing the laser in the photodetector.
Probe holder – It’s worth noting that tip holders come in all sorts of designs, and the success of inserting your probe is dependent on your familiarity with the holding mechanism. Some have a clip that you press in, others you need to screw in and out with a screwdriver; some have clips that rotate around and in some cases the probes are held in place by a small taut piece of wire.
It’s important to note that this is one of the first and biggest challenges in getting to know your AFM – learning how to insert the probe correctly and without dropping it. Trust me, this is such a common thing to do in the beginning! By their very nature probes are small, you need to use tweezers and almost everyone drops a probe when they are first getting used to using the AFM. Don’t worry too much about this! But familiarising yourself with the tip holder and how it works before you start trying to insert the probe is a good way to minimise droppage.
AFM head and probe holder
Sample stage – This is where you place your sample. The stage allows you to move the sample laterally/sideways to look at different parts of the surface. Getting to know the parts of the sample stage is really helpful to prevent the second most common error when first working with AFM which is crashing your probe into the sample…
Z Motor – sometimes sits underneath the stage, sometimes sits above the AFM head. Its job is to move the sample and tip up and down, relative to each other.
Simple optical microscope – is like a magnified camera, enabling you to focus on your probe and focus on the level below, so you can see where the sample is. Using the optical microscope you can bring the tip and the sample safely together. Without this it becomes very slow to get your tip the right distance from the surface so it’s a real timesaver.
Handily there is normally also a light/lamp with the optical microscope, as it is hard to focus on anything in the dark!
(I know we labelled this section ‘parts you can see’, but in fact for our system the motor and most of the optical microscope are hidden above the AFM head. It depends on the system, and we thought it was best to organise these parts together with sample stage for this explanation)
PC – software for controlling the AFM. This is where you control the engage on the surface and optimise imaging parameters such as setpoint and gains. The feed from the optical microscope/camera is displayed here and it is where your beautiful AFM images will also appear.
Controller – It’s a box. But this is where the wizardry happens! Without the controller the AFM wouldn’t be able to do what it does. Cool electronics and cool maths enable the AFM to respond very quickly to changes in the sample surface but the good news is you don’t need to understand it in detail to operate the AFM (and for more info ask an electrical engineer...)
Add ons – There are a number of extra bits you can add onto your instrument depending on the application. For example the tip holder will be different if you want to image biological samples as the electronics need to be protected (e.g. liquid chamber add on). For electrical modes such as conductive AFM you might need an extra electrical module, to enable you to apply voltage across your probe/sample. Or there might be a heating stage add on, a special stage to change the temperature of the sample.
Various parts of an AFM system
The parts you can’t see (and what they do)
Optical Detection System - Laser + probe + mirrors + photodetector – collectively these detect changes in the movement of the probe on the sample surface. This needs to be correctly aligned for the system to function. The information detected in the photodetector about the movement of the cantilever on the surface is sent to the controller (where the wizardry happens) with information sent back to track the probe on the surface via the all-important Z Scanner piezo…
The setup of these components varies significantly between instruments. We have included a schematic from Park Systems showing the components of the optical detection system (minus the mirrors!), along with the sample and scanners/piezos. If you look closely at the AFM head photo, you may be able to see the mirrors, and if you picked it up in real life I would be able to show you where the laser is emitted and where the photodetectors are located. It is system dependent, sometimes these parts are more or less hidden, but it was easier to categorise as small parts that are not easy to see for this explanation.
Optical Detection System Schematic (Laser, Cantilever and Photodetector) - taken from Park Systems Applications Note #21
Piezos – The Piezo crystals are made of ceramic material that you can apply a certain voltage across to make them oscillate a set amount. This is a key parameter as AFM works by being able to controllably put small voltages in to enable nanometre distance movements. Without this we wouldn’t be able to track nanoscale features.
Scanner
Z piezo – works with the controller, via laser/probe/photodetector - to track across the surface
XY piezos – scan the probe across the surface so we can see a square area (called a Raster scan)
With a regular camera we see every part of an image simultaneously, but with AFM the probe must physically touch and scan over every part of the sample. The 3 piezo crystals enable movement in 3 dimensions.
Shake piezo – This oscillates the probe holder in order to oscillate the cantilever. This is required if you’re doing tapping mode AFM – the most common form of AFM. Some AFMs don’t use a shake piezo anymore but use photoacoustic oscillation instead, which uses a second laser on the probe to heat it up and oscillate the cantilever.
Environment
The AFM is a sensitive instrument and when you’re looking to capture perfect images it’s important to be aware of how noise and the environment can impact the work you are doing. Below are a list of parts and factors that need to be taken into consideration when using and getting to know your AFM:
Acoustic hood – for noise isolation
Noise Isolation Table – An air table (nitrogen) or an active vibration isolation table (electronic)
The room – Noise? Windows? Doors? People? Trains? Plumbing? It might pay to be aware of these things if you want super low noise and high resolution – normally best achieved in a basement
You – try not to sneeze!
Hopefully this guide will be a useful introduction to the different parts of the AFM and will help you get to know yours better. The more familiar you are with your AFM the more awesome the images you can produce using it. And the more you get to know this instrument, the more you come to understand the possibilities for your work are almost endless...Enjoy! And drop me a line (jamie@nunano.com) if you have any questions.
