Bimodal AFM Research Articles

Visualization and Quantification of State of Charge-Dependent Young's Modulus of LiMn2O4 Nanosized Particles by Bimodal AFM.

Mechanical damage of LiMn2O4 active material caused by volume change, phase transition, and lithium diffusion-induced stress is the main degradation mechanism in lithium-ion batteries. Young's modulus is a key parameter of mechanical property, and its variation with lithium content x or state of charge (SOC) at the nanoscale is an important issue because such variation may have influences on the stress level and lithium-ion transport. In this study, we successfully developed bimodal atomic force microscopy (bimodal AFM) and related approaches to carry out surface topography imaging and Young's modulus mapping of LixMn2O4 nanosized particles. It was validated that the size of particles decreased with decreasing SOC due to delithiation during the charging cycle. The variation in Young's modulus with SOC was quantitatively determined using the silicon material as a reference, and the trend of the variation is consistent with the reported results of molecular dynamics simulation. Furthermore, spatially nonuniform distribution of Young's modulus on the nanosized particle surface was found even upon completion of charging. This phenomenon could be attributed to the coexistence of two phases during the charging process. Our experimental study reveals the correlation between Young's modulus of LiMn2O4 and SOC at the nanosized particle level, and we believe that the bimodal AFM will be widely used in the nanocharacterization of the electrode materials because lithium content- or SOC-dependent mechanical properties are common in battery electrode materials.

Fast and high-resolution mapping of van der Waals forces of 2D materials interfaces with bimodal AFM.

High-spatial resolution mapping of van der Waals forces is relevant in several fields ranging from nanotechnology to colloidal science. The emergence of two-dimensional heterostructures assembled by van der Waals interactions has enhanced the interest of those measurements. Several AFM methods have been developed to measure the adhesion force between an AFM probe and the material of interest. However, a reliable and high-resolution method to measure the Hamaker constant remains elusive. We demonstrate that an atomic force microscope operated in a bimodal configuration enables fast, quantitative, and high-resolution mapping of the Hamaker constant of interfaces. The method is applied to map the Hamaker constant of monolayer, bilayer and multilayer MoS2 surfaces. Those interfaces are characterized with Hamaker constant and spatial resolutions of, respectively, 0.1 eV and 50 nm.

Stability and contrast in bimodal amplitude modulation atomic force microscopy for different mode combinations in ambient air

Bimodal amplitude modulation atomic force microscopy (AM-AFM) is widely used in nanoscale topography and mechanical property imaging for a variety of materials. In this paper, the stability of the amplitude/phase spectroscopy curves and the imaging contrast in bimodal AM-AFM for different mode combinations are investigated computationally in ambient air. The results show that with the second mode amplitude used for topography feedback on a stiff material, the amplitude/phase spectroscopy would probably undergo volatile fluctuation, leading to unstable imaging. With the third mode amplitude set for topography imaging, it would be difficult for the feedback to maintain the prescribed amplitude since a large cantilever position variation is required for different sample moduli. With the first mode amplitude set for topography feedback, the amplitude and the phase of the second mode vary monotonically with sample modulus or viscosity in comparison with the third or the fourth mode, which is suitable for compositional contrast imaging. These results would provide useful guidelines for optimum imaging in bimodal AFM measurements.

In operando nanomechanical mapping of PEDOT:PSS thin films in electrolyte solutions with bimodal AFM.

Understanding the interplay between the nanomechanical properties of organic electronic materials and their electronic properties is central to developing sensors and transducers for applications ranging from immunosensing to e-skin. Controlling organic device operations in aqueous electrolyte solutions and their mechanical compliance with the host tissue or living systems, as for instance in active implants for the recording or stimulation of neural signals, is still largely unexplored. Here, we implemented bimodal AFM to map the nanomechanical and structural properties of thin films made from poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS), the most widely used conducting polymer blend, during operation as a microelectrode in an electrolyte solution. Nanomechanical maps showed the film consisting of a granular structure made from PEDOT:PSS regions embedded in the PSS matrix. The film swelled upon immersion in an aqueous solution. In operando bimodal AFM data obtained by applying a sequence of doping/de-doping bias cycles showed a significant decrease in the modulus (70%) that saturated after about 10 cycles. A similar sequence of biases at the opposite polarity did not significantly influence the mechanical behaviour of PEDOT:PSS. The decrease in the modulus was explained by the development of persistent hydration, which was enhanced by the cations trapped inside the organic electronic material.

Quantitative nanomechanical properties evaluation of a family of β-sheet peptide fibres using rapid bimodal AFM

Self-assembling peptides have become important building blocks for materials design (e.g. hydrogels) and play a crucial role in a range of diseases including Alzheimer and Parkinson. In this context, accessing the nanomechanical properties of ubiquitous β-sheet rich nanofibres (e.g.: amyloids) is key to the formulation of materials and design of therapies. Although the bulk mechanical properties of hydrogels can easily be accessed using common techniques and equipment, the mechanical properties of their constituent fibres, in particular if with radii in the nanometre scale, are more challenging to measure and estimate. In this work we show for the first time how the rapid nanomechanical mapping technique: amplitude modulation-frequency modulation (AM-FM), can be used to determine the heights, Young's moduli and viscosity coefficients of a series of β-sheet peptide nanofibres with high statistical confidence. Our results show how peptide sequence and in particular length, charge and interaction with the substrate affect the viscoelastic properties of the peptide fibres.

Quantitative mapping of magnetic properties at the nanoscale with bimodal AFM.

We demonstrate that a force microscope operated in a bimodal configuration enables the mapping of magnetic interactions with high quantitative accuracy and high-spatial resolution (∼30 nm). Bimodal AFM operation doubles the number of observables with respect to conventional magnetic force microscopy methods which enables to determine quantitatively in a single processing step several magnetic properties. The theory of bimodal AFM provides analytical expressions for different magnetic force models, in particular those characterized by power-law and exponential distance dependences. Bimodal AFM provides a self-evaluation protocol to test the accuracy of the measurements. The agreement obtained between the experiments and theory for two different magnetic samples support the application of bimodal AFM to map quantitatively long-range magnetic interactions.

Effects of laser spot positioning with optical beam deflection method on tapping mode and bimodal AFM

This work focuses on the importance of laser location and its effect on contact mode, tapping mode, and bimodal AFM both theoretically and experimentally. It is found that the current guidelines in the field might lead to mischaracterization of matter especially in bimodal AFM. A numerical study is done for a cantilever with its tip located at the end while interacting with two different polymers of Polystyrene (PS) and Low-density polyethylene (LDPE). Different observables are recorded at the end of the cantilever, 80% of its length, and 60% of its length. These results are verified by experiments in contact mode, tapping mode and bimodal AFM. Bimodal AFM observables are converted to energy quantities (i.e., virial and dissipated power) which are used to characterize soft matter in this field. Similar to simulation, for each of these measurements three different laser locations are selected. Finally, it is found there are certain locations on the cantilever that should be avoided for placing the laser while exciting higher eigenmodes. Hence, locating the laser around 80% of its length while performing bimodal AFM with the first and second eigenmodes on polymer surfaces can decrease phase contrast. It is concluded that: (1) contact mode AFM on stiff surfaces might not be effected by change of laser location, (2) misalignment of laser can cause up to 40% of mischaracterization in tapping mode and (3) 10% of phase contrast reduction in addition to 20% percent of reduction in amplitude oscillations in bimodal AFM. It is shown this can lead to mischaracterization of material. By positioning the laser location closer to the clamped end of the cantilever, surface indentation can occur. Although this is considered as a surface damage in soft matter imaging, it can be considered as a new capability of multifrequency AFM for surface modification.

Nanoscale chemical and mechanical heterogeneity of human dentin characterized by AFM-IR and bimodal AFM

Human dentin, as an important calcified tissue in the body, plays significant roles in withstanding masticatory forces and has a complex hierarchical organization. Understanding the composition and ultrastructure of dentin is critical for elucidating mechanisms of biomineralization under healthy and pathological states. Here, atomic force microscope infrared spectroscopy (AFM-IR) and AFM-based amplitude modulation-frequency modulation (AM-FM) techniques were utilized to detect the heterogeneity in chemical composition and mechanical properties between peritubular and intertubular dentin at the nanoscale. AFM-IR spectra collected from peritubular and intertubular dentin contained similar vibrational bands in the amide regions (I, II and III), suggesting that collagen may exist in both structures. A distinctive band at 1336 cm−1 indicative of SO stretching vibrations was detected only in peritubular dentin. AFM-IR imaging showed an uneven distribution of chemical components at different locations, confirming the heterogeneity of dentin. The Young’s modulus of peritubular dentin was higher, and was associated to a higher mineral content. This study demonstrated distinctive chemical and mechanical properties of peritubular dentin, implying the different development and mineralization processes between peritubular and intertubular dentin. AFM-IR is useful to provide compositional information on the heterogeneity of human dentin, helping to understand the mineral deposition mechanisms of dentin.

Direct Visualization of Local Spin Transition Behaviors in Thin Molecular Films by Bimodal AFM.

Thin films of the molecular spin-crossover complex [Fe(HB(1,2,4-triazol-1-yl)3 )2 ] undergo spin transition above room temperature, which can be exploited in sensors, actuators, and information processing devices. Variable temperature viscoelastic mapping of the films by atomic force microscopy reveals a pronounced decrease of the elastic modulus when going from the low spin (5.2 ± 0.4 GPa) to the high spin (3.6 ± 0.2 GPa) state, which is also accompanied by increasing energy dissipation. This technique allows imaging, with high spatial resolution, of the formation of high spin puddles around film defects, which is ascribed to local strain relaxation. On the other hand, no clustering process due to cooperative phenomena was observed. This experimental approach sets the stage for the investigation of spin transition at the nanoscale, including phase nucleation and evolution as well as local strain effects.

Power transfer in bimodal amplitude modulation atomic force microscopy in liquids: A numerical investigation

Bimodal amplitude modulation atomic force microscopy (AM-AFM) is an emerging technique for compositional imaging in liquids. In this work, we investigate the power transfer in bimodal AM-AFM in liquids by a numerical analysis. Power items are calculated by direct numerical integral and the corresponding amplitude and phase response is presented. Results show power balance is satisfied for each mode. The power transfer in each mode is significantly small compared to the external input power and most of the power is dissipated into the surrounding medium, especially for a large setpoint or cantilever-sample separation. The power transfer among different modes is complex and strongly depends on the cantilever and imaging parameters. Power transfer between different modes goes up with increasing free amplitude of the second mode. In addition, a stiffer sample will produce a more complex force spectra, which perturbs the cantilever oscillation more heavily compared to a compliant sample. Besides, the non-driven higher mode of a softer cantilever is more likely to be momentarily excited. The power items and cantilever response during imaging are also provided, revealing the phases in bimodal AFM in liquids may not be utilized to characterize the sample elasticity due to the non-monotonic trends. Instead, the amplitude of the second mode could be used to characterize the elasticity of the sample with moderate to high moduli.

Fast, quantitative and high resolution mapping of viscoelastic properties with bimodal AFM.

Quantitative mapping of viscoelastic properties of soft matter with a nanoscale spatial resolution is an active and relevant research topic in atomic force microscopy (AFM) and nanoscale science characterization. The AFM has demonstrated its accuracy to measure the energy dissipated on a sample surface with an atomic-scale resolution. However, the transformation of energy dissipation values associated with viscoelastic interactions to a material property remains very challenging. A key issue is to establish the relationship between the AFM observables and some material properties such as viscosity coefficient or relaxation time. Another relevant issue is to determine the accuracy of the measurements. We demonstrate that bimodal atomic force microscopy enables the accurate measurement of several viscoelastic parameters such as the Young's modulus, viscosity coefficient, retardation time or loss tangent. The parameters mentioned above are measured at the same time that the true topography. We demonstrate that the loss tangent is proportional to the viscosity coefficient. We show that the mapping of viscoelastic properties neither degrades the spatial resolution nor the imaging speed of AFM. The results are presented for homogeneous polymer and block co-polymer samples with Young's modulus, viscosity and retardation times ranging from 100 MPa to 3 GPa, 10 to 400 Pa s and 50 to 400 ns, respectively. Numerical simulations validate the accuracy of bimodal AFM to determine the viscoelastic parameters.

Sensing in-plane nanomechanical surface and sub-surface properties of polymers: local shear stress as function of the indentation depth.

Dynamic force microscopy (DFM) is an excellent tool for the study of the compositional and nanomechanical properties of polymers that exploits the flexural eigenmodes of a cantilever comprising a sharp tip vibrating perpendicular to the sample surface. However, the in-plane nanomechanical properties of a specimen cannot be detected by this technique. Here, a bimodal approach was developed where flexural and torsional eigenmodes are driven simultaneously. The corresponding vibrational amplitude and phase shift of the vertical tip motion were utilized for topographical feedback and out-of-plane dissipative interaction acquisition, respectively, whereas the frequency shift and the drive amplitude of the lateral tip motion mapped the in-plane conservative and dissipative interactions of two heterogeneous polymers: an elastomeric polypropylene (ePP) and a polystyrene-block-polybutadiene diblock copolymer (SB). The shear stress at different sub-surface levels revealed an amorphous cover layer as well as a "slipping" plane on the ePP crystallites. The imaging of SB supported by dynamic force spectroscopy experiments showed that SB exhibits considerably different in- and out-of-plane nanomechanical properties at certain areas due to the complex polymer conformation of this diblock copolymer accompanied by inter- and intramolecular interactions that give rise to its viscoelastic behavior.

Analysis and modification of defective surface aggregates on PCDTBT:PCBM solar cell blends using combined Kelvin probe, conductive and bimodal atomic force microscopy.

Organic photovoltaic systems comprising donor polymers and acceptor fullerene derivatives are attractive for inexpensive energy harvesting. Extensive research on polymer solar cells has provided insight into the factors governing device-level efficiency and stability. However, the detailed investigation of nanoscale structures is still challenging. Here we demonstrate the analysis and modification of unidentified surface aggregates. The aggregates are characterized electrically by Kelvin probe force microscopy and conductive atomic force microscopy (C-AFM), whereby the correlation between local electrical potential and current confirms a defective charge transport. Bimodal AFM modification confirms that the aggregates exist on top of the solar cell structure, and is used to remove them and to reveal the underlying active layer. The systematic analysis of the surface aggregates suggests that the structure consists of PCBM molecules.

Distinguishing between Mechanical and Electrostatic Interaction in Single Pass Multi Frequency Electrostatic Force Microscopy Measurements on a Molecular Material.

Single-pass electrostatic force microscopy is postulated as one of the most advanced techniques in terms of spatial resolution and fastness in data acquisition for the study of electrostatic phenomena at the nanoscale. However, crosstalk anomalies, in which mechanical interactions combine with tip-sample electrostatic forces, are still a major issue to overcome, specifically in soft and biological samples. In this paper we propose a novel method based on bimodal-atomic force microscopy to distinguish mechanical crosstalk from electrostatic images. The method is based in the comparison of bimodal AFM images with electrostatic ones, where pure mechanical interaction can be discerned from a mixture of mechanical and electrostatic interactions. The proposed method is optimized and demonstrated using a supramolecular charge transfer material. Finally, the method is used as a tool to depict different crosstalk levels in tetrathiafulvalene-based (TTF) assemblies, discerning between electrical and mechanical interactions. This kind of observation is important for obtaining accurate descriptions of charge distribution in samples made from organic and molecular layers and materials.

Nucleation and mechanical enhancements in polyethylene-graphene nanoplate composites

Abstract Graphene nanoplatelets (GNP) dispersed in high-density polyethylene (HDPE) were found to significantly enhance nucleation of PE crystallization, and thermal/mechanical properties of HDPE-GNP nanocomposites. Excellent dispersion of GNPs was achieved by combining solution blending and twin-screw extrusion homogenization. The dispersion state was confirmed by bimodal AFM and also by the resulting improvements in thermal, mechanical, and transport properties. The crystallization rates were shown to markedly increase at low GNP content (0.1 wt%) and continued to increase with GNP loading up to 15 wt%. The size of secondary structures, or spherulites, decreased with GNP loading. Smaller spherulite sizes and faster kinetics suggested enhanced nucleation with GNP addition. 30–40% reductions in permeability and up to 60% higher Young’s moduli were found in these HDPE-GNP nanocomposites. Restricted polyethylene mobility and enhanced thermal stability with increasing GNP loading were also indicated in these HDPE-GNP nanocomposites.

High-bandwidth multimode self-sensing in bimodal atomic force microscopy.

Using standard microelectromechanical system (MEMS) processes to coat a microcantilever with a piezoelectric layer results in a versatile transducer with inherent self-sensing capabilities. For applications in multifrequency atomic force microscopy (MF-AFM), we illustrate that a single piezoelectric layer can be simultaneously used for multimode excitation and detection of the cantilever deflection. This is achieved by a charge sensor with a bandwidth of 10 MHz and dual feedthrough cancellation to recover the resonant modes that are heavily buried in feedthrough originating from the piezoelectric capacitance. The setup enables the omission of the commonly used piezoelectric stack actuator and optical beam deflection sensor, alleviating limitations due to distorted frequency responses and instrumentation cost, respectively. The proposed method benefits from a more than two orders of magnitude increase in deflection to strain sensitivity on the fifth eigenmode leading to a remarkable signal-to-noise ratio. Experimental results using bimodal AFM imaging on a two component polymer sample validate that the self-sensing scheme can therefore be used to provide both the feedback signal, for topography imaging on the fundamental mode, and phase imaging on the higher eigenmode.

Rapid quantitative chemical mapping of surfaces with sub-2 nm resolution.

We present a theory that exploits four observables in bimodal atomic force microscopy to produce maps of the Hamaker constant H. The quantitative H maps may be employed by the broader community to directly interpret the high resolution of standard bimodal AFM images as chemical maps while simultaneously quantifying chemistry in the non-contact regime. We further provide a simple methodology to optimize a range of operational parameters for which H is in the closest agreement with the Lifshitz theory in order to (1) simplify data acquisition and (2) generalize the methodology to any set of cantilever-sample systems.

Optimization of the excitation frequency for high probe sensitivity in single-eigenmode and bimodal tapping-mode AFM

The cantilever excitation frequency and tip free oscillation amplitude are two critical imaging parameters in amplitude-modulation atomic force microscopy (AM-AFM, often referred to as tapping-mode AFM). In general, the excitation frequency is selected to be ‘near’ the measured resonance frequency of the probe, but there is no established systematic approach for making that choice. In this work we show that the choice of excitation frequency can play a very significant role in the characterization of viscoelastic materials, even when considering small deviations with respect to the resonance frequency. Additionally, we offer an analytical expression, verified through experiments and numerical simulations, which offers guidance for selecting the drive frequency that maximizes probe sensitivity. Our approach is illustrated experimentally through single-eigenmode and bimodal AFM measurements performed on spin-coated Nafion® proton exchange thin films. We find that very often, the phase contrast channel is optimized by selecting an excitation frequency that is not necessarily at or near the free resonance frequency.

Dynamic force microscopy simulator (dForce): A tool for planning and understanding tapping and bimodal AFM experiments.

We present a simulation environment, dForce, which can be used for a better understanding of dynamic force microscopy experiments. The simulator presents the cantilever–tip dynamics for two dynamic AFM methods, tapping mode AFM and bimodal AFM. It can be applied for a wide variety of experimental situations in air or liquid. The code provides all the variables and parameters relevant in those modes, for example, the instantaneous deflection and tip–surface force, velocity, virial, dissipated energy, sample deformation and peak force as a function of time or distance. The simulator includes a variety of interactions and contact mechanics models to describe AFM experiments including: van der Waals, Hertz, DMT, JKR, bottom effect cone correction, linear viscoelastic forces or the standard linear solid viscoelastic model. We have compared two numerical integration methods to select the one that offers optimal accuracy and speed. The graphical user interface has been designed to facilitate the navigation of non-experts in simulations. Finally, the accuracy of dForce has been tested against numerical simulations performed during the last 18 years.