Published by Nanosurf on Jul 26, 2023 1:45:00 PM

Does the Material of the Cantilever Affect the Image Quality?

Atomic Force Microscopy (AFM) is a powerful tool that allows researchers to investigate the surface properties of various materials at the nanoscale. The image quality obtained through AFM imaging is crucial for accurate analysis and interpretation of the data. While the cantilever material itself may not significantly affect image quality, it is important to consider the material of the AFM tip and its maintenance to ensure optimal imaging results. In this article, we will explore the relationship between cantilever material and image quality in AFM, emphasizing the role of the tip material and its impact on imaging.

The Significance of Cantilever Material

The cantilever is a fundamental component of an AFM system. It serves as a mechanical spring that deflects as it interacts with the sample surface, allowing the measurement of various surface properties. The cantilever material, in most cases silicon or silicon nitride but sometimes also quartz-like material, is typically chosen based on the mechanical properties, such that the desired cantilever spring constants and resonance frequencies are achieved. These parameters determine for which application a cantilever will be typically used.Nanosurf brand cantilever close up Most cantilevers are equipped with a reflective coating on the side opposite to the laser beam's focus point. This coating enhances the sample's reflectivity, allowing for better detection of the cantilever's deflection when the laser beam is directed onto it. Increased reflectivity in contrast to uncoated cantilevers increases the signal-to-noise ratio and helps improve operation. Coating materials are typically either aluminum or gold. While aluminum works well in an air environment, it is less suitable in liquid, aqueous environments. Aluminum coatings may delaminate in these environments causing a loss or at least significant decrease in the light intensity on the photodetector. Gold coatings on the other hand are well suited for such applications. Any coating may, however, introduce stress to the cantilever structure that in turn can be a source of cantilever drift, i.e. bending of the cantilever while equilibrating inside the AFM, or even significant bending of the cantilever.

Understanding the Role of the AFM Tip

While the cantilever material may not directly influence image quality, the material of the AFM tip plays a critical role in the imaging process. The tip is the part of the cantilever that interacts with the sample surface and detects the surface features during scanning. The choice of the tip material can affect the imaging results in several ways, e.g. through its wear resistance, surface chemistry or charge, and tip geometry. All these parameters can influence how the tip interacts and “feels” the surface and thus they have an impact on the resulting image and its quality.

Tip Geometry

AFM cantilevers are available with different tip geometries. Most cantilevers have pyramidal tips, but there are also tips with hyperbolic or conical geometries. Most of these overall geometries provide very sharp tips, which are well suited for imaging purposes as they terminate in a very sharp apex with a radius of a few nanometers. Despite having a very sharp tip, the opening angle and thus the aspect ratio of the tip can vary significantly in different types of cantilevers. Some cantilevers are specially made with high aspect ratio tips that allow entering narrow trenches, which are not accessible by regular cantilever tips. The material or surface coating of the tip may have an impact on the tip wear properties. Worn tips have different local geometries, basically small plateaus, which have a strong impact on the way the tip interacts with the sample surface and how well the tip can resolve surface structures spatially. For some applications, spherical probes with radii ranging from a few tens of nanometers up to a several tens of micrometers are available. Such so-called spherical or colloidal probes are often available in different materials, such as gold, polystyrene, or glass. The choice of the material in these cases depends on whether the probes are used to analyze the mechanical properties of surfaces or if these probes should be chemically modified to alter the surface chemistry of the sphere, e.g. by introducing specific chemical groups or attaching proteins or DNA molecules.

Wear and Contamination

Certain tip materials may wear off more quickly than others. Silicon tips for example can be produced with a sharper tip compared to silicon nitride, but they also wear off more quickly. As the tip wears off, its geometry changes, potentially leading to a degradation in image quality. Moreover, contamination of the tip surface can occur during imaging, negatively impacting the image resolution and fidelity.

Tips for Choosing the Right Tip Material

When selecting the tip material, it is important to consider the specific requirements of the imaging experiment and the nature of the sample. Some commonly used tip materials include silicon, silicon nitride, and diamond-like carbon (DLC). Silicon tips are widely used and offer good imaging capabilities for various samples. They also provide very sharp tips, which is required for high-resolution imaging. Silicon nitride tips typically wear off less than silicon tips but generally exhibit a higher tip radius. DLC-coated tips provide enhanced durability and hardness as well as reduced wear, making them suitable for applications like nanolithography or nanoindentation on hard materials; the DLC coating typically causes larger tip radii. For some applications, special tip coatings are required. Electrical modes require a conductive tip. In such cases, the coating itself may have an impact on the obtained results. Some coating materials might wear off quickly when in contact with the surface thus losing their conductive properties. For some samples, highly wear-resistant tips are required, as isolating native oxide layers must be penetrated to obtain a stable measurement signal at all. In such cases, wear-resistant conductive diamond or platinum silicide tips are better suited compared to tips with a pure platinum or gold coating.

Handling and Care

Cantilevers are consumables and often replaced with new ones if degradation in the image quality is observed. However, gentle handling and storage in a clean environment is mandatory to reduce the risk of tip damage or contamination prior to usage. For example, handling of sharp cantilevers may require ESD protection measures to prevent immediate tip damage. For some applications, it might also be possible to try to clean cantilevers e.g. by immersing them in solutions that remove possible contaminants from the tip. Depending on the possible contamination such solutions could be acetone, ethanol, or aqueous solutions containing detergent. Organic contaminants could be removed using an UV ozone cleaner.

Conclusion

While the cantilever material itself may not have a significant impact on image quality in AFM, the choice of the tip material can play a crucial role in obtaining high-quality results. The tip material affects interactions with the sample surface or the tip’s wear characteristics, all of which can influence image fidelity and resolution. By carefully considering the tip material, conducting regular maintenance, and adhering to best practices, researchers can ensure optimal imaging quality and enhance the accuracy of their AFM experiments.