By Taha KhanReviewed by Lexie CornerMay 14 2024

In material science, powerful microscopy tools are of utmost importance as they provide detailed insights into the intricate nature of material at micro scales. Among such tools, atomic force microscopy (AFM) stands out as a powerful technique that transcends the limitations of traditional methods by offering high-resolution imaging and characterization at the nanoscale.¹

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This article discusses the application of AFM in surface topography and morphology analysis, mechanical properties assessment, and chemical composition mapping.

Introduction to AFM

AFM differs significantly from traditional microscopy techniques as it does not project light or electrons on the sample's surface to create its image. Instead, AFM utilizes a sharp probe while scanning the sample's surface, measuring the minute forces exerted between the probe and the sample's surface. This measurement translates into information about the sample's properties.

AFM can analyze a wide range of materials, both conductive and non-conductive, in various environments, including air, liquids, and even extreme temperatures.¹

Surface Topography and Morphology Analysis

AFM helps investigate surface features by creating high-resolution, three-dimensional (3D) images that reveal surface roughness, topography, and morphology. This is critical for understanding a material's interaction with its environment, such as its potential for wear and tear, adhesion properties, and ability to self-assemble into complex structures.

For instance, AFM can be used to analyze the surface texture of implants or drug delivery devices, aiding in the design of biocompatible materials with optimal performance.^{2, 3}

Analyzing Polymer-DNA Morphologies using AFM: A Case Study

A 2022 study investigated the morphology of conductive polymers (polyimidazole, polyindole, and polypyrrole) templated with DNA using AFM. The researchers found that polyimidazole/DNA exhibited globular and agglomerate nanostructures, while polyindole/DNA showed dense networks of nanowires in concentrated form and individual single wires in diluted samples.³

Polypyrrole/DNA displayed different morphologies, including bare DNA strands, well-aligned polymeric nanomaterials, and high-density films. Statistical analysis revealed predominant nanowire diameters of 3-4 nm. Despite differences, all polymers showed potential for applications in nanoelectronic devices and chemical sensors due to their uniform, smooth, and continuous morphologies, which are suitable for alignment on nanoelectrodes.

This comparative AFM study enhances understanding of polymer/DNA templated nanowires' morphology and their potential applications.³

Mechanical Properties Assessment

High-speed AFM provides surface imaging and can assess the mechanical properties of the material at the nanoscale. Different modes of AFM are used to measure properties like elasticity, stiffness, and adhesion forces, which help in understanding how a material responds to stress and strain at the nanoscale and in predicting its bulk behavior.^{4, 5}

AFM-Based Indenter Characterization

In a recent study, researchers investigated a method for assessing nanoindenter area profiles using AFM to analyze indentation-induced plastic footprints. The study focused on utilizing indium as a calibration material and conducted nanoindentation tests with AFM imaging on an indium sample.

Results showed that AFM-based methodology provides a low-cost and efficient means of evaluating nanoindenter area profiles, particularly at heights below 200 nm. Researchers successfully characterized the indenter geometry shape at the nanoscale by indirectly calculating effective footprint projected areas.⁵

This approach offers a promising alternative to electron microscopy-based techniques, with advantages in cost-effectiveness, ease of implementation, and rapid data processing. The study highlights the potential of AFM for accurate mechanical properties assessment, emphasizing its significance in nanomechanics research.⁵

Chemical Composition Mapping

AFM can be integrated with other techniques to extend its applications further.

For instance, scanning probe microscopy (SPM) combined with spectroscopic methods creates a powerful tool for chemical analysis, allowing researchers to visualize the topography and identify different materials present within the sample at nanoscale resolution.⁶

Other techniques, such as scanning tunneling microscopy (STM) or spectroscopy with scanning probe microscopy (S-SPM), can be coupled with AFM to map the chemical composition of a sample's surface.

To extend the applications of AFM even further, researchers are constantly working on developing new methods that overcome the limitations of conventional AFM.^{7, 8}

For instance, in a recent study, researchers developed coated active scanning probes for AFM to enable topography imaging in opaque liquid environments. Traditional AFM systems rely on optical beam deflection, limiting imaging to transparent environments. The newly developed probes feature piezoresistive deflection sensing and thermomechanical actuation, eliminating the need for an optical system and enabling imaging in opaque liquids.⁹

These probes withstand harsh chemical conditions and have been successfully tested in various opaque liquid environments. The study demonstrates the potential of coated active probes for observing samples in their native environments, opening new avenues for AFM applications in biology, chemistry, and material science. ⁹

Future Directions and Innovations

Innovations in AFM instrumentation, such as faster scanning speeds and higher sensitivity probes, will further enhance AFM's capabilities for studying dynamic processes. For instance, coupling AFM with advanced spectroscopy methods could provide real-time information on a material's chemical composition and response to external stimuli.

AFM can also assist in designing biocompatible materials for implants and prosthetics by providing details of their interaction with biological systems at the nanoscale.

AFM could also be utilized in 3D printing; by analyzing the surface morphology and mechanical properties of printed materials, researchers can refine the printing process and ensure high-quality, functional structures.¹⁰

As these innovations continue to develop, AFM's role in material science is set to become even more integral.

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References and Further Reading

1. Cubillas, P., Anderson, MW. (2014). Atomic force microscopy. Multi Length‐Scale Characterisation. doi.org/10.1002/9781118683972.ch3
2. Sarkar, A. (2022). Biosensing, characterization of biosensors, and improved drug delivery approaches using Atomic Force Microscopy: A review. Frontiers in Nanotechnology. doi.org/10.3389/fnano.2021.798928
3. Ibrahim, YM. (2022). Comparative AFM studies on the morphology of conducting polymers/DNA templated nanowires. JMESR.  doi.org/10.55455/jmesr.2022.003
4. Ganser, C., Uchihashi, T. (2024). Measuring mechanical properties with high-speed atomic force microscopy. Microscopy. doi.org/10.1093/jmicro/dfad051
5. Roa, S., Sirena, M. (2022). AFM imaging analysis of nanoindentation-induced plastic strain in indium surface for calibrating nanoindenters area profiles. Physica B: Condensed Matter. doi.org/10.1016/j.physb.2022.413773
6. Flores, SM., Toca-Herrera, JL. (2009). The new future of scanning probe microscopy: Combining atomic force microscopy with other surface-sensitive techniques, optical microscopy and fluorescence techniques. Nanoscale. doi.org/10.1039/B9NR00156E
7. Pürckhauer, K., Maier, S., Merkel, A., Kirpal, D., Giessibl, FJ. (2020). Combined atomic force microscope and scanning tunneling microscope with high optical access achieving atomic resolution in ambient conditions. Review of Scientific Instruments. doi.org/10.1063/5.0013921
8. Poggi, MA., Gadsby, ED., Bottomley, LA., King, WP., Oroudjev, E., Hansma, H. (2004). Scanning probe microscopy. Analytical chemistry. doi.org/10.1021/ac0400818
9. Xia, F., et al. (2019). Lights out! nanoscale topography imaging of sample surface in opaque liquid environments with coated active cantilever probes. Nanomaterials. doi.org/10.3390/nano9071013
10. Nguyen, KQ., Vuillaume, PY., Robert, M., Elkoun, S. (2022). AFM Analysis of 3D Printing PEI for Automotive Applications. Proceedings of the 8^th International Conference on Mechanical, Automotive and Materials Engineering. doi.org/10.1007/978-981-99-3672-4_10

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