Atomic force microscopy (AFM) is a powerful and versatile microscopy technique that has been used across many scientific disciplines for decades. While it is a single technique, there are many ways in which it can be used. These are known as the different imaging modes and differ by how the cantilever beam and the tip (collectively known as the probe) interact with a sample. In this article, we look at AFM as a general microscopy technique and why these different modes are used.
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What is AFM?
Atomic force microscopy (AFM) is a microscopy technique that uses a tip attached to a cantilever beam (also known as a probe) to determine the topology of a surface. They do this by measuring the van der Waals forces between the tip and the surface, and this enables the position of the atoms to be deduced.
AFM is itself a stand-alone technique, but it has also given rise to many other variations that are more suited to measuring the topological properties of a material. However, standard AFM instruments and modes are not commonly used for these applications (although a few features can be deduced in some modes, or by using colloidal probes).
The Basic Principles of AFM
In AFM, the tip is attached to the end of a cantilever beam. The tip is placed above the sample at a given distance, and as the tip moves across the surface of the material, the van der Waals forces between the surface and the tip causes the cantilever beam to move towards the surface. There are many ways in which this movement can be manifested, and this is the reason why there are different imaging modes.
Regardless of the mode, the cantilever moves towards the surface using intermolecular attraction. As it does so, a laser which remains at a constant position on the cantilever beam becomes deflected towards a position-sensitive photodiode (PSPD). The position at which the deflected laser hits the PSPD is used to determine the atomic location of each atom at the surface of the material. It does so by recording both the lateral and vertical displacement of the cantilever. AFM employs a feedback loop system that enables a high-resolution topographic map to be generated using the data from each time the laser is deflected onto the PSPD.
AFM commonly employs an atomically sharp tip (although other types can be used). Most tips are usually less than 10 nm in diameter and are made of silicon-based materials or carbon-based composite tips. For different AFM variations, they can also be coated. The sharpness of the tip is essential as this ensures high accuracy. However, because the are atomically sharp, they are not suitable for use with softer materials in contact-type modes. This is the reason why many different imaging modes exist, as it enables AFM to be a versatile technique for both information output and the materials it can be used with.
The Different Analysis Modes of AFM
There are many different modes of AFM that are determined by how the cantilever moves, and therefore, how the tip interacts with the surface. There are different modes because some materials are not compatible with some of the modes because the force of the tip on the surface can destroy the sample. The most common mode used with AFM are tapping, contact and non-contact imaging modes. It should be noted that these modes apply to conventional AFM only, and not the different variations of AFM that are discussed within this AFM series (as they generally employ modes which can differ to those below).
Contact Mode
The contact AFM imaging mode is a widely used mode. It is also the most accessible mode to employ and has become the basis for some other variations of AFM, including scanning capacitance mode (SCM) and scanning spreading resistance mode (SSRM).
In contact mode, the tip is ‘dragged’ across the surface of a material, and the PSPD maps the relative changes in position towards the sample. It uses the feedback signal to keep the cantilever in a constant position. At the same time, the interaction with the sample creates a force curve which can be used to back out other relevant information such as the adhesive and compliance properties of the material.
The main disadvantage of contact mode is the effect that dragging the tip has on the sample, as it often leads to damage of the sample which reduces the amount of information that can be obtained (unless the material is very hard). Additionally, because the cantilever is static (i.e., it doesn’t oscillate), it is prone to noise. Cantilevers with a low spring constant (i.e., a low stiffness) can be used to minimize some of the noise by creating a larger deflection towards the sample. However, it is these drawbacks that led to the realization of the tapping mode.
Tapping Mode
The tapping mode is perhaps the most widely used AFM imaging mode nowadays. Tapping mode still contacts the sample, but only for a short (and intermittent) time. This is particularly useful for materials that are prone to developing a liquid meniscus layer under ambient conditions, as it prevents the probe from sticking to the material (something which can happen with contact mode under similar conditions).
It also does not exhibit the same lateral dragging forces that can often damage the sample. However, contact mode is a lot easier to control than tapping mode because the feedback in tapping mode can sometimes be unstable, which can restrict the information obtainable beyond the topography of the sample.
In tapping mode, the cantilever oscillates up and down at its resonance frequency (or near to it). The drive amplitude and energy of the cantilever are also kept constant throughout. As the tip oscillates along a sample, it can be brought close to the surface by intermolecular attraction, and the oscillation amplitude drops.
As the tip is brought closer, it touches the sample, where repulsive forces then come into play. These repulsive forces will cause the cantilever to shift to a higher oscillation amplitude, and the cantilever is then reset by the feedback loop which returns the cantilever amplitude to its original setpoint.
Non-Contact Mode
The non-contact mode is often used for material that is soft, such as polymers or biological materials, as they can become damaged by the sharp tip under both contact and tapping modes. In this mode, the tip of the cantilever does not touch the sample, hence its name, so that no physical damage can bestow the sample.
In a similar fashion to tapping mode, the cantilever oscillates at (or near) its resonance frequency, but the difference is that it is far enough away from the sample so that it doesn’t touch it; even when the cantilever moves towards the surface through an intermolecular attraction. However, there does need to be some care when employing this imaging mode, because it needs to be far enough away so that it won’t touch the sample, yet, it needs to be near enough to still interact with the sample. If it is placed too near, it will hit the sample; whereas, if it is too far away, there wouldn’t be enough deflection from the cantilever beam to register the position of an atom.
Overall, non-contact mode works using similar principles as the tapping mode, but at a greater distance away from the sample. One distinct advantage (aside from no sample damage) is that because the tip never touches a sample, it never degrades. This means that there will never be any inaccuracies that arise from worn out tips.
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