In this article, nanomechanical measurements and their importance for the nanomechanical characterization of materials, specifically soft materials, are discussed.
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Nanomechanical Measurements and their Importance
Nanotechnology has provided significant opportunities for developing advanced devices with exceptional economic benefits and quality of life. These devices are used in several applications, including infrastructural remote sensors, environmental toxicity detectors, and biomedical implantable actuators. Nanoscale mechanical properties of materials play a critical role in enabling these applications.
Thus, the development of nanomechanical measurements is a crucial requirement for the innovation and commercialization of these devices. Nanomechanical measurements are performed to determine the local stress and deformation states and the fracture, viscous, plastic, and elastic properties of materials with a nanometer spatial resolution in quantitative detail.
Nanomechanical Measurement Tools
Several nanomechanical measurement tools are based on contact probes. Instrumented indentation testing (IIT) platforms or atomic force microscopes (AFM) are utilized to manipulate the probes and determine the mechanical properties of the material surfaces with nanoscale accuracy.
For instance, AFM probes with a 10-nanomechanical radius are utilized to measure the humidity effects on the adhesion of contacts with a one-nanomechanical radius at the smallest length scales. Conducting-probe AFM can be used at similar length scales in ultra-high vacuum (UHV), to measure the metal-insulator-metal tunnel junction properties formed by self-assembled monolayers or small molecules on gold surfaces.
Contact resonance AFM (CR-AFM) techniques are employed to map and measure elastic moduli with more than 10 nanomechanical spatial resolution at slightly larger scales using 2-3 nanomechanical contacts and 20-40 nanomechanical AFM probes. Diamond probes with a large included angle can be used for IIT measurements to determine the onset of yield in single crystals with 10 nanomechanical indentation depths.
Non-contact beam-based tools, such as electron backscatter diffraction (EBSD) and confocal Raman microscopy (CRM), are also employed for nanoscale stress mapping.
Nanomechanical Measurements on Soft Materials
Soft materials are referred to as materials that are deformed easily by thermal fluctuations or thermal stresses at room temperature. Soft materials typically include polymers, liquids, foams, colloids, gels, granular materials, and several soft biological materials.
Nanomechanical measurements are performed on different soft materials for high-resolution, quantitative, and fast characterization of mechanical behavior at the nanoscale. For instance, contact resonance, AFM phase imaging, bimodal AFM, and force-volume (F-V), are used extensively for the nanomechanical mapping of soft materials.
F-V mapping is utilized to measure the mechanical properties of soft materials at the nanoscale. The feature is included in several commercial AFM application modules. An F-V map combines a topographic image of the surface by acquiring one/multiple force–distance curves (FDCs) on each pixel/point of the surface.
Several parameters, such as energy dissipation, adhesion force, stiffness, Young’s modulus, and height, can be extracted from an FDC. Thus, multiparametric mechanical property maps of polymer blends/films and block co-polymer thin films can be obtained easily using this mapping method.
In molecular biology, F-V mapping can be used for the stiffness characterization of liposomes, protein-coated lipid nanotubes, or protein complexes. F-V mapping is also used to characterize various aspects of the interactions and structure of living cells.
Bimodal AFM involves the simultaneous detection and excitation of two cantilever modes to determine material properties such as deformation, topography, dissipation, viscosity coefficients, and elastic modulus or long-range interaction parameters in one imaging step.
Bimodal AFM is used to characterize the organic electrochemical transistor performance by measuring the stiffness of the organic electronic material. The method has been also used extensively to investigate the local inelastic and elastic properties of flexible crystals, polymer-based materials, and polymer films.
Nanomechanical maps of biological systems such as cells, virus-like particles, and bone elements were produced successfully using the bimodal AFM method. CR-AFM is used for imaging the subsurface features in plant cells, bacteria, or polymer blends in a non-invasive manner. AFM phase-imaging is used extensively to characterize heterogeneous soft materials at the nanoscale.
Recently, a new multifrequency force microscopy method has been developed for non-destructive, high spatial resolution, and quantitative mapping of interfaces and surfaces of soft matter at the nanoscale.
The multifrequency force microscopy method combined bimodal detection and excitation for simultaneous mapping of multiple properties and frequency modulation feedbacks for fractional and sensitivity calculus to relate observables to nanomechanical properties.
Simultaneous mapping of the topography, indentation, viscous/damping coefficients, and Young’s modulus of several polymer/soft matter surfaces was performed successfully using the new method.
Additionally, the method was non-invasive, provided the indentation and peak force, did not restrict the spatial resolution and data acquisition speed of the force microscope, and minimized the tip radius influence on measurements.
Recent Developments
In a paper recently published in the journal IFAC-PapersOnLine, researchers developed an adaptive discrete nanomechanical mapping technique for time-varying nanomechanical property mapping of soft samples using AFM.
Mapping the time-varying nanomechanical properties is crucial to test samples undergoing dynamic evolutions. However, the existing continuous scanning-based approach is only limited to small-area mapping or extremely slow to capture the time-elapsing variations over an entire sampled area.
In this study, researchers proposed to extend a discrete nanomechanical mapping scheme where a set of points of interests (POIs)/discrete locations were visited by adaptively adjusting the measurement time distribution between the POIs through a batch process. The new approach successfully mapped the nanomechanical evolution of a poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) polymer undergoing the crystallization process.
Conclusion and Future Outlook
To summarize, nanomechanical measurements play an important role in characterizing the nanomechanical properties of soft materials. In the future, the incorporation of advanced data analysis tools and improved theoretical models, along with the introduction of new hardware architectures and components, will significantly increase the application of nanomechanical mapping techniques on soft materials.
Moreover, a new generation of instruments can be developed in the long term that will integrate correlative microscopy, nanoscale spatial resolution, quantitative accuracy, and automatization, enabling large-scale characterization of soft materials in their native state.
References and Further Reading
Wang, J., Zou, Q., Guo, S. (2022). Adaptive Discrete Mapping of Dynamic Nanomechanical Property of Soft Materials on Atomic Force Microscope. IFAC-PapersOnLine, 55(27), pp. 293-298. https://doi.org/10.1016/j.ifacol.2022.10.528
Garcia, R., Proksch, R. (2013). Nanomechanical mapping of soft matter by bimodal force microscopy. European Polymer Journal, 49(8), pp. 1897-1906. https://doi.org/10.1016/j.eurpolymj.2013.03.037
Herruzo, E.T., Perrino, A. P., Garcia, R. (2014) Fast nanomechanical spectroscopy of soft matter. Nature Communications, 5, p. 3126. https://doi.org/10.1038/ncomms4126
Cook, R.F. (2010). Nanomechanical Measurements and Tools [Online] Available at https://www.azonano.com/article.aspx?ArticleID=2498 (Accessed on 16 February 2023)
Garcia, R. (2020). Nanomechanical mapping of soft materials with the atomic force microscope: methods, theory and applications. Chemical Society Reviews, 49(16).
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