Contact Resonance Atomic Force Microscopy Research Articles

Mechanical Mapping of Nanoblisters Confined by Two-Dimensional Materials Reveals Complex Ridge Patterns.

Understanding the mechanics of blisters confined by two-dimensional (2D) materials is of great importance for either fundamental studies or for their practical applications. In this work, we investigate the mechanical properties of nanoscale 2D material blisters using contact-resonance atomic force microscopy (CR-AFM). From the measurement results at the blister centers, the blisters' internal pressures are characterized, which are shown to be inversely proportional to the blisters' sizes. Our measurements agree considerably well with values predicted by theoretical mechanic analyses of the blisters. In addition, high-resolution mechanical mapping with CR-AFM reveals fine, complex ridge patterns of the blisters' confining membranes, which can hardly be distinguished from their topographies. The pattern complexity of a blister system is shown to increase with an increase in its bendability.

Contact resonance atomic force microscopy using long elastic tips

In this work, a new theoretical model for contact resonance atomic force microscopy, which incorporates the elastic dynamics of a long sensing tip is presented. The model is based on the Euler–Bernoulli beam theory and includes coupling effects from the two-beam structure, also known as an ‘L-shaped’ beam in the literature. Here, high-accuracy prediction of the sample stiffness, using several vibration modes with a relative error smaller than 10% for practical working ranges, is demonstrated. A discussion on the model’s capability to predict the dynamic phenomena of eigenmode veering and crossing, as the force applied to the sample increases, is presented. The L-shaped beam model presented here is also applicable for structural applications such as: micro-electro-mechanical systems, energy harvesting, and unmanned aerial vehicle landing gear.

Probing Relative Humidity Impact on Biological Protein Bovine Serum Albumin and Bovine Submaxillary Gland Mucin by Using Contact Resonance Atomic Force Microscopy.

In biomaterials, a substantial amount of research has been placed on the mechanical properties of biomolecules and their interactions with body fluids. Bovine serum albumin (BSA) is a widely studied model protein, while bovine submaxillary gland mucin (BSM) is another cow-derived protein frequently employed in research. Films were examined with contact resonance atomic force microscopy (CR-AFM), and the results showed that the mechanical characteristics of the films were affected by the relative humidity. We quantitatively analyze the viscoelasticity of these proteins after they have been subjected to humidity by measuring the resonance frequency and quality factor. The findings indicate that prolonged humidity exposure has a different effect on the mechanical properties of BSA and BSM films. The results show that after exposure to humidity, the resonance peaks of BSA shift to the left, indicating stiffness, while those of BSM shift to the right, indicating hydration. Moreover, BSM's hydration is caused by relative humidity, leading to a constant increase in resonance frequency and material softness. Contrarily, BSA showed a decrease in contact resonance frequency due to ongoing strain-induced deformation, indicating increased material stiffness. The findings have significance for the design and development of biomaterials for a variety of applications, such as the delivery of drugs, the engineering of tissue, and the development of biosensors. Our research demonstrates that CR-AFM has the potential to become a non-invasive and sensitive method that can be used to characterize the mechanical characteristics of biomolecules and their interactions with bodily fluids.

Mechanical properties and abnormal mechanical behaviors of ferroelectric hexagonal manganites

Hexagonal manganites (h-RMnO3, R=Y, Ho-Lu) are emerging as advantageous multiferroic materials with structural ferroelectric topological defects. Strain has been proven as an effective means to control topological vortex cores. To further understand and utilize the mechanical manipulation mechanism, mechanical properties such as the Young’s modulus are important; however, accurate values are still to be obtained for single-crystalline specimens. In this study, by combining contact resonance atomic force microscopy, nanoindentation, and density functional theory calculations, we systematically studied the Young’s moduli of YMnO3, ErMnO3, and LuMnO3 single crystals. The Young's moduli of LuMnO3 obtained by the different methods were consistent, whereas for ErMnO3 and YMnO3, the abnormal mechanical behavior observed in the nanoindentation experiments deviated from the theoretical and experimental calculations. Through in-situ transmission electron microscopy, it was confirmed that lattice distortion plays a key role in resisting deformation, which is the origin of the abnormal mechanical behavior. Our work provides insights into an important mechanical parameter and the origin of the difference in the Young’s modulus of h-RMnO3, which is important for further theoretical investigations and potential applications.

Experimental validation of contact resonance AFM using long massive tips

In this work, we present an experimental validation of a new contact resonance atomic force microscopy model developed for sensors with long, massive tips. A derivation of a new technique and graphical method for the identification of the unknown system parameters is presented. The technique and contact resonance model are experimentally validated. The agreement between our contact resonance experimental measurements and values obtained from nanoindentation show a minimal error of 1.4%–4.5% and demonstrate the validity of the new contact resonance model and system parameter identification technique.

Inner-paddled atomic force microscopy cantilever for rapid mechanical mapping

Mechanical characterization methods at the nanoscale are of critical importance for many fields including nanomaterials, micro/nano devices and nanomechanics. As a key tool in nanotechnology, atomic force microscopy (AFM) is widely used due to its high-resolution topography imaging capabilities, and is also recognized as a useful platform for nanoscale mechanical characterization. Contact-resonance AFM, which modulates the tip-sample contact with ultrasonic frequencies and then analyzes the cantilever’s resonance responses, is an important AFM method for viscoelastic characterization. However, contact-resonance AFM requires the cantilever’s contact-resonance frequency and quality factor values to quantify the elastic modulus and loss tangent of the sample. This requires time-consuming frequency sweep, which makes a quantitative scanning impractical, therefore only single-point quantitative measurement and qualitative single-frequency scanning are usually applied. To address this issue, here we present an AFM cantilever design with an integrated inner-paddle substructure, which provides an eigenmode whose resonance keeps consistent, but whose resonance amplitude varies with varying contact stiffness. With this probe, it is hoped to quantify the elastic properties of the sample with fast single-frequency amplitude imaging.

Resolving the Subsurface Structure and Elastic Modulus of Layered Films via Contact Resonance Atomic Force Microscopy.

Since its discovery, atomic force microscopy (AFM) has become widely used for surface characterization, evolving from a tool for probing surface topography to a versatile method for characterizing mechanical, electrical, chemical, magnetic, and electro-optical properties of surfaces at the nanoscale. Developments of several AFM-based techniques have enabled even subsurface imaging, which is routinely being carried out at the qualitative level of feature detection for localized subsurface inhomogeneities. We surmise, however, that a quantitative three-dimensional (3D) subsurface characterization can emerge from the AFM mechanical response of flat buried interfaces, and present here a methodology for determining the depth of a film and its mechanical properties. Using load-dependent contact resonance atomic force microscopy (CR-AFM) and accurate modeling of the contact between the AFM tip and a layered sample, we determine the relationship between the measured resonance frequency of the AFM probe and the contact stiffness. Our subsequent statistical analysis reveals an intrinsic and sample-specific interdependence between the depth and modulus sensitivities of CR-AFM. This interdependence prevents the simultaneous accurate determination of both depth and modulus from measurements on a single-layered sample. If the elastic moduli of the sample components are predetermined from separate investigations of bulk samples (or otherwise known), then this methodology accurately yields the location of the interface between the layers of the sample; as such, it can serve as a nondestructive and robust technique for probing layer thickness, subsurface features, and elastic properties of materials used in semiconductor electronics, additive manufacturing, or biomaterials.

Measurement of Sub-Surface Microstructures Based on a Developed Ultrasonic Atomic Force Microscopy

Accurate and non-destructive technology for detection of subsurface defect has become a key requirement with the emergence of various ultra-precision machining technologies and the application of ultra-precision components. The combination of acoustic technique for sub-surface detection and atomic force microscopy (AFM) for measurement with high resolution is a potential method for studying the subsurface structure of workpiece. For this purpose, contact-resonance AFM (CR-AFM) is a typical technique. In this paper, a CR-AFM system with a different principle from commercially available instruments is set up and used for the detection of sub-surface Si samples with grating structures and covered by different thickness of highly oriented pyrolytic graphite (HOPG). The influence of subsurface burial depth on the detection capability is studied by simulations and experiments. The thickest HOPG film allowing for sub-surface measurement by the proposed method is verified to be about 30 μm, which is much larger than the feature size of the subsurface microstructure. The manuscript introduces the difference between this subsurface topography measurement principle and the commercially available AFM measurement principle, and analyzes its advantages and disadvantages. The experimental results demonstrates that the technique has the capability to reveal sub-surface microstructures with relatively large buried depth and is potential for engineering application in ultra-precision technologies.

Linearizing the frequency-stiffness relation in contact resonance atomic force microscopy for facilitated mechanical characterization.

To facilitate mechanical characterization by contact resonance atomic force microscopy (CR-AFM), a cantilever that could enable a near linear relation between CR-frequency and contact stiffness is designed. The optimized structure is fabricated through removing a rectangular slot from a conventional cantilever as a prototype, and the dimensions of the removed slot are determined by finite element simulations. We validate the target CR-characteristics on a silicon substrate with circular cavities covered by highly oriented pyrolytic graphite flakes, on which the contact stiffness can be well modeled. Elastic modulus measurements on different materials demonstrate that the linear relation provides a convenient yet accurate way to evaluate sample's elasticity. It could thus bring great convenience to various CR-AFM relevant applications such as mechanical characterization and subsurface nano-imaging. RESEARCH HIGHLIGHTS: A cantilever that could enable a near linear relation between CR-frequency and contact stiffness is designed to facilitate contact resonance atomic force microscopy. The CR-characteristics are verified on a silicon substrate with circular cavities covered by highly oriented pyrolytic graphite flakes. Elastic modulus measurements on different materials demonstrate that the linear relation provides an easy yet accurate way to evaluate sample's elasticity.

Integrated analysis of chain orientation induced anisotropy in nanoimprinted PVDF based copolymers

Poly(vinylidene fluoride) (PVDF) and its copolymers are electrically active materials that have the ability to harvest mechanical energy through mechanical vibrations and structural flexing. Due to the inherent nanoscale dependency of the molecular structure of PVDF copolymer, efficient methods for investigating the effects of chain orientation on its localized nanomechanical properties are very limited. In this manuscript, we use infrared atomic force microscope (IR-AFM), contact-resonance atomic force microscopy (CR-AFM), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (TEM), to reveal the correlations between localized molecular vibration, anisotropy, and the corresponding dynamic nanomechanical properties. IR-AFM is able to probe the chain orientation of the PVDF copolymer and CR-AFM is able to resolve the in-plane anisotropy along the axis of the polymer chain. The combination of these techniques can provide a quick, effective, nondestructive method to reveal the formation mechanisms, which could lead to optimization of the electroactive polymer organization.

Cantilever signature of tip detachment during contact resonance AFM.

Contact resonance atomic force microscopy, piezoresponse force microscopy, and electrochemical strain microscopy are atomic force microscopy modes in which the cantilever is held in contact with the sample at a constant average force while monitoring the cantilever motion under the influence of a small, superimposed vibrational signal. Though these modes depend on permanent contact, there is a lack of detailed analysis on how the cantilever motion evolves when this essential condition is violated. This is not an uncommon occurrence since higher operating amplitudes tend to yield better signal-to-noise ratio, so users may inadvertently reduce their experimental accuracy by inducing tip–sample detachment in an effort to improve their measurements. We shed light on this issue by deliberately pushing both our experimental equipment and numerical simulations to the point of tip–sample detachment to explore cantilever dynamics during a useful and observable threshold feature in the measured response. Numerical simulations of the analytical model allow for extended insight into cantilever dynamics such as full-length deflection and slope behavior, which can be challenging or unobtainable in a standard equipment configuration. With such tools, we are able to determine the cantilever motion during detachment and connect the qualitative and quantitative behavior to experimental features.

Characterization of Eocene flint

Eocene flint 48–56.0 million years old (mya) from the Negev desert (Israel) was characterized using a suite of analytical techniques. High-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) of the inorganic component showed the texture, morphology, size, and distribution of two silica polymorphs: α–quartz and moganite. While euhedral forms were attributed to α-quartz, moganite crystals were comprised of spherulitic grains. An electron less-dense amorphous material (no scattering under SAED) was found between the siliceous crystallites. Energy dispersive X-rays (EDS) and electron energy loss spectroscopy (EELS) demonstrated that this electron less-dense amorphous material is composed solely of carbon. Low vacuum, low energy backscattered environmental scanning electron microscopy (BSE-eSEM) imaging of flint surfaces showed the presence of micrometer-sized organic inclusions randomly distributed throughout the siliceous matrix. Energy-dispersive X-ray studies (EDS) demonstrated that these organic micro-inclusions were composed of carbon, sulfur, and nitrogen with a C/N ratio attributed to marine sources. These micro-inclusions were not directly associated with hard-shell fossils. BSE-eSEM imaging conditions allowed the identification of entrapped carbon-rich organic material, which is not possible when applying commonly used electron microscopy conditions that require carbon coating and high acceleration voltages, rendering carbon-rich features electron-transparent. Phase contrast-enhanced micro-computed tomography (PC-μCT) showed that these organic micro-inclusions were randomly distributed throughout the siliceous matrix.Time-of-flight secondary ion mass spectrometry (ToF-SIMS), nano-Fourier transform infrared spectroscopy (nano-FTIR), and scanning probe microscopy (SPM) were used to further characterize these organic micro-inclusions. These three in situ analytical techniques with nanometer resolution provided complementary information on the chemical composition and structure of the organic material. Specifically, ToF-SIMS analysis revealed amino acid and hydrocarbon mass spectra fingerprints inside the organic micro-inclusions. While the former were exclusively found in the organic micro-inclusions, the mass spectral fingerprints for hydrocarbons were also found in the siliceous matrix in agreement with the HR-TEM/EDS/EELS results, where pure carbon was found between the siliceous nanocrystals. While ToF-SIMS provides chemical information, it does not provide structural information. Nano-FTIR analysis showed the presence of amide I and II infrared vibrations exclusively on the organic micro-inclusions. The scanning probe microscopy (SPM) techniques Peak Force Quantitative Nanomechanics (PF-QNM) and Contact Resonance Atomic Force Microscopy (CR-AFM) were used to assess the mechanical properties. PF-QNM measurements on the organic micro-inclusions, under dry and liquid conditions, demonstrated that the organic micro-inclusions swell upon hydration and soften, pointing toward the presence of hydrophilic molecules in agreement with nano-FTIR and ToF-SIMS results. CR-AFM allows in situ determination of the mechanical properties of materials with high stiffness at nanometer resolution. This technique, rarely used in a geological context, revealed that the organic micro-inclusions had an unusually high stiffness atypical for modern organic material, which was attributed to molecular cross-linking promoted by diagenesis.This work provided a comprehensive view of the inorganic and organic components of Eocene flint from the Negev desert with implications for paleontology and archaeology. It offers a roadmap of novel complementary techniques that can be used in the exploration of entrapped organic material in flint.

Layer dependent out-of-plane elastic modulus of graphene

Contact resonance atomic force microscopy (CR-AFM) was used to spatially map variations in the out-of-plane elastic modulus of graphene covered surfaces of a silicon wafer having a native oxide. A modulus of 0.973 ± 0.001 TPa was measured for one layer of graphene covering the silicon substrate, consistent with many experimental and theoretical studies. A monotonic decrease in the measured contact resonance frequency with increasing layers of graphene covering the silicon substrate was observed, resulting in a decrease in the measured modulus by 4.95% with four layers of graphene covering the substrate. The interplay between the hydrophobic graphene layers covering the hydrophilic silicon substrate, the interlayer spacing of graphene, and the range of adhesion from the substrate are proposed to influence the stiffness of the graphene–graphene interaction, which was probed with the sub-Ångström displacements with CR-AFM measurements.

Thermal noise in contact atomic force microscopy

As one of the fundamental sources of noise in atomic force microscopy (AFM), thermal fluctuations of the cantilever have been studied for the case of a free tip but not much for cantilevers in contact. In this paper, using the equipartition theorem, we calculated the thermal deflection amplitude for all normal modes of an elastically supported AFM cantilever, including the free cantilever as a special case. With increasing contact stiffness, the mean thermal fluctuation amplitude decreases for all cantilever modes when in the elastic contact. In addition, considering the optical lever detection scheme used in most AFMs, we calculated the corresponding output thermal noise amplitude. The experiments validated our theoretical calculations. Our investigation facilitates a more comprehensive understanding of the thermal noise in AFM. It provides guidance for thermally excited contact-resonance AFM, which is promising for quantitative viscoelastic measurements.

Multifunctional cantilevers for simultaneous enhancement of contact resonance and harmonic atomic force microscopy

To enhance contact resonance atomic force microscopy (CR-AFM) and harmonic AFM imaging simultaneously, we design a multifunctional cantilever. Precise tailoring of the cantilever’s dynamic properties is realized by either mass-removing or mass-adding. As prototypes, focused ion beam drilling or depositing is used to fabricate the optimized structures. CR-AFM subsurface imaging on circular cavities covered by a piece of highly oriented pyrolytic graphite validates the improved CR frequency to contact stiffness sensitivity. The detectable subsurface depth and cavity radius increase accordingly by using the multifunctional cantilever. At the same time, the free resonance frequency of the second mode is tuned to an integer multiple of the fundamental one. Harmonic AFM imaging on polystyrene and low-density polystyrene mixture shows the improved harmonic amplitude contrast and signal strength on the two material phases. The multifunctional cantilever can be extended to enhance other similar AFM operation modes and it has potential applications in relevant fields such as mechanical characterization and subsurface imaging.

Hardened core of bilayer shear bands in a Zr-based metallic glass

Bilayer shear bands with a spacing of hundreds of nanometers were prepared in a Zr-based metallic glass. Elastic properties of the bands were detected using a contact-resonance atomic force microscope. Compare with an elastic modulus reduction in each single shear band, the core of the bilayer shear bands is hardened. One possible reason for the forming of the bilayer shear bands is the load releasing and position adjustment during the severe plastic deformation process. Our results reveal a new plastic deformation mechanism of the shear bands in the form of layer by layer, and provide a promising strategy to control the stability of shear bands and enhance the plasticity of metallic glasses.

Application of contact-resonance AFM methods to polymer samples.

Contact-resonance AFM (CR-AFM) has been used in recent years for the measurement of mechanical properties of rather stiff materials, such as ceramics or metals, but also of some polymers. Compared with other techniques providing information on the mechanical properties of a sample, notably force–distance curves, CR-AFM has a much shorter acquisition time. This compensates in part the incomplete theoretical understanding of the underlying physical phenomena and of factors influencing the measurements. A commonly used method to analyze CR data requires the determination of the relative position of the tip, the calculation of the normalized contact stiffness, and the use of a calibration sample for the calculation of the elastic modulus of the sample. In the present paper, we propose an alternative procedure, based on approximations of the equations describing the system, which allows one to determine the elastic modulus of the sample as a parameter of the fit of the CR frequency as a function of the load. After showing that CR modes including scanning under continuous contact wear and damage the sample and/or alter the surface roughness, the results of point CR measurements on bulk and thin films are presented. Though Young’s moduli of bulk polystyrene and poly(methyl methacrylate) could be determined through the presented analysis, it is concluded that CR measurements are not appropriate for polymer samples. Major drawbacks are the bad resolution for moduli lower than ca. 10 GPa and the lack of a comprehensive physical model accounting for many factors affecting the dynamic response of a cantilever in contact with a sample.

A new insight into the vibrational modeling of contact mode for atomic force microscope beams in various immersion ambiances.

In this article, the vibrations of V-shaped beam as the most usable atomic force microscope (AFM) beam in contact mode has been modeled using finite element method. This work is a fundamental issue for contact resonance AFM. We have considered carbon tetrachloride (CCL4 ), methanol, acetone, water, and vacuum the immersion ambiences. For modeling of the beam dynamics, both of the Timoshenko and Euler-Bernoulli beam principles have been used. For presenting the best theoretical model all specifications for AFM beam and sample surface has been assumed. The outcomes show that by raising the interaction force between AFM beam and sample surface, the resonant frequency increases. By diluting the liquid surroundings, the resonant frequency and amplitude of frequency response functions (FRF) of vertical movement increase. The amplitude of FRF increases by raising the beam thickness and rectangular part breadth, but reduces by increasing the rectangular and tapered part lengths. The resonant frequency reduces by boosting the rectangular and tapered parts lengths and rectangular part width, but raises by increasing the beam thickness. Theoretical model in attendance and lack of tip-sample interaction force have been evaluated with experimental works for vacuum and water as the immersion ambiances. Results show good agreement.

Atomic force microscopy for nanoscale mechanical property characterization

Over the past several decades, atomic force microscopy (AFM) has advanced from a technique used primarily for surface topography imaging to one capable of characterizing a range of chemical, mechanical, electrical, and magnetic material properties with subnanometer resolution. In this review, we focus on AFM as a nanoscale mechanical property characterization tool and examine various AFM contact and intermittent contact modes that add mechanical contrast to an imaged surface. Through detailed analysis of the tip-sample contact mechanics, this contrast can be converted into quantitative measurements of various nanomechanical properties including elastic modulus, shear modulus, wear rate, adhesion, and viscoelasticity. Different AFM modes that provide such measurements are compared and contrasted in this work on a wide range of materials including ceramics, metals, semiconductors, polymers, and biomaterials. In the last few years, considerable improvements have been made in terms of fast imaging capabilities, tip preservation, and quantitative mechanics for multifrequency measurements as well as well-known AFM modes like amplitude modulation and peak-force tapping. In line with these developments, a major highlight of this review is the discussion of the operation and capabilities of one such mode, namely, intermittent contact resonance AFM (ICR-AFM). The applications of ICR-AFM to nanoscale surface and subsurface quantitative mechanical characterizations are reviewed with specific examples provided for thin polymeric films and patterned nanostructures of organosilicate dielectric materials. The combination of AFM-based mechanical characterization with AFM-based chemical spectroscopy to allow nanoscale structure-property characterization is also discussed and demonstrated for the analysis of low-k dielectric/copper nanoelectronic interconnect structures and further highlights synergistic advances in the AFM field.

Local probe of the interlayer coupling strength of few-layers SnSe by contact-resonance atomic force microscopy

The interlayer bonding in two-dimensional (2D) materials is particularly important because it is not only related to their physical and chemical stability but also affects their mechanical, thermal, electronic, optical, and other properties. To address this issue, we report the direct characterization of the interlayer bonding in 2D SnSe using contact-resonance atomic force microscopy (CR-AFM) in this study. Site-specific CR spectroscopy and CR force spectroscopy measurements are performed on both SnSe and its supporting SiO2/Si substrate comparatively. Based on the cantilever and contact mechanic models, the contact stiffness and vertical Young’s modulus are evaluated in comparison with SiO2/Si as a reference material. The interlayer bonding of SnSe is further analyzed in combination with the semi-analytical model and density functional theory calculations. The direct characterization of interlayer interactions using this non-destructive methodology of CR-AFM would facilitate a better understanding of the physical and chemical properties of 2D layered materials, specifically for interlayer intercalation and vertical heterostructures.