Atomic Force Acoustic Microscopy Research Articles

Thickness-Dependent Nanoscale Elastic Stiffening of Chemical Vapor Deposited Atomically Thin 2H-MoS2 Films.

Understanding the nanoscale elastic-size-effects of atomically thin transition-metal dichalcogenides (TMDs) as a function of thickness underpins the avenue of flexible 2D electronics. In this work, we employed the atomic force acoustic microscopy (AFAM) technique to investigate the thickness-dependent elastic properties of CVD grown 2H-MoS2 films. The monolayer MoS2 exhibited a Young's modulus of 273 ± 27 GPa. Our systematic analysis from bulk to monolayer suggests that the 2H-MoS2 phase exhibits nanoscale elastic-stiffening behavior with decreasing number of layers (thickness). The Young's modulus increased by a factor of ∼2.7 for monolayer MoS2 when compared with the bulk. First-principle DFT calculations affirm the nanoscale elastic-stiffening behavior of MoS2 with decreasing number of layers. Our findings suggest that the observed elastic stiffening is due to the interlayer sliding, which may be facilitated by defects in MoS2 layers. The observed elastic stiffening may be of potential importance for understanding TMD based nanomechanical devices.

Emerging multi-frequency surface strain force microscopy

During the past decade, Scanning Probe Microscopy (SPM) based surface strain detection techniques have been extensively used in the characterization of functional materials, structures, and devices. Here, we refer these techniques as Surface Strain Force Microscopy (SSFM), which mainly includes the Piezoresponse Force Microscopy, Atomic Force Acoustic Microscopy, Atomic Force Microscopy-Infrared spectroscopy (or photothermal induced resonance), Piezomagnetic Force Microscopy, and Scanning Joule Expansion Microscopy. The inception of SSFM opens up a pathway to study the nanoscale physical properties by using a sharp tip to detect the local field-induced surface strain. Through measuring the signals of the surface strain, multiple physical properties, such as the electromechanical, mechanical, photothermal, magnetic, thermoelastic properties, can be characterized with an unprecedented spatial resolution. In order to further develop and overcome the fundamental issues and limitations of the SSFM, the multi-frequency SPM technology has been introduced to the SSFM-based techniques, leading to the emerging of multi-frequency SSFM (MF-SSFM). As a technical breakthrough of the SSFM, MF-SSFM has demonstrated substantial improvements in both performance and capability, resulting in increased attentions and numerous developments in recent years. This Perspective is, therefore, aimed at providing a preliminary summary and systematic understanding for the emerging MF-SSFM technology. We will first introduce the basic principles of conventional SSFM and multi-frequency SPM techniques, followed by a detailed discussion about the existing MF-SSFM techniques. MF-SSFM will play an increasingly important role in future nanoscale characterization of the physical properties. As a result, many more advanced and complex MF-SSFM systems are expected in the coming years.

Influence of plasma ionization on the elastic modulus and tribology behavior of carbon films deposited by the HiPIMS technique

This work reveals the influence of discharge current on carbon-ion energies of plasma, elastic modulus, and friction coefficient at the nano- and macroscale of carbon films deposited via high-power impulse magnetron sputtering. Three applied discharge current conditions in deposition processes were employed to obtain three-carbon films of interest. The number of carbon ions with their energies was obtained via mimic tests of the deposition process using three similar discharge currents through a quadrupole mass spectrometer detector based on the time-averaged ion energy distribution function. The bonding structure of the films was evaluated using Raman spectroscopy, fitting the Diamond and Graphite peaks to obtain a semiquantitative analysis. The elastic modulus of the carbon films was determined from atomic force acoustic microscopy measurements avoiding the influence of the substrates. The friction coefficient was analyzed at the nanoscale via atomic force microscopy and at the macroscale via tribometry. Significant alterations were observed in the number and energy level of the carbon ions with the variation of discharge current. These alterations significantly influenced the bonding properties, elastic modulus, and tribology behavior. A higher elastic modulus and higher sp3 bond content were observed for the film with a lower number of carbon ions and less energy.

Cell spreading behaviors on hybrid nanopillar and nanohole arrays

Although nanopillars (NPs) provide a promising tool for capturing tumor cells, the effect of mixing NPs with other nanopatterns on cell behavior remains to be further studied. In this paper, a method of fabricating silicon nanoscale topographies by combining laser interference lithography with metal assisted chemical etching was introduced to investigate the behaviors and pseudopodia of A549 cells on the topologies. It was found that cells had a limited manner in spreading with small cell areas on the silicon nanopillar (SiNP) arrays, but a good manner in spreading with large cell areas on the silicon nanohole (SiNH) arrays. When on the hybrid SiNP/SiNH arrays, cells had medium cell areas and they arranged orderly along the boundaries of SiNPs and SiNHs, as well as 80% of cells displayed a preference for SiNPs over SiNHs. Furthermore, the lamellipodia and filopodia are dominant in the hybrid SiNP/SiNH and SiNP arrays, respectively, both of them are dominant in the SiNH arrays. In addition, the atomic force acoustic microscopy was also employed to detect the subsurface features of samples. The results suggest that the hybrid SiNP/SiNH arrays have a targeted trap and elongation effect on cells. The findings provide a promising method in designing hybrid nanostructures for efficient tumor cell traps, as well as regulating the cell behaviors and pseudopodia.

Band Excitation Piezoresponse Force Microscopy Adapted for Weak Ferroelectrics: On-the-Fly Tuning of the Central Band Frequency.

New interest in microscopic studies of ferroelectric materials with low piezoelectric coefficient, $d_{33}^\ast$, has emerged after the discovery of ferroelectric properties in HfO2 thin films, which are the main candidate for the next generation of nonvolatile ferroelectric memory. The study of the microscopic structure of ferroelectric HfO2 capacitors is crucial to get insights into the device behavior and performance. However, a small $d_{33}^\ast$ of ferroelectric HfO2 films leads to a low piezoresponse, especially in band excitation piezoresponse force microscopy (BE-PFM). In this work, we have implemented the BE-PFM technique with an increased scanning rate, thus improving this versatile tool for weak ferroelectrics. The acceleration of measurement was achieved by focusing excitation into a narrow frequency band and tuning the central frequency on-the-fly using an online real-time model estimation by fitting a complex BE response. The tracking of the contact resonance frequency was implemented using a pure mechanical cantilever response acquired in BE atomic force acoustic microscopy. To obtain optimal excitation parameters, we perform statistical analysis by minimizing estimator variance. The measurement precision of several PFM techniques was compared both by the simulation and experimentally using a Hf0.5Zr0.5O2-based ferroelectric capacitor.

Acoustic subsurface-atomic force microscopy: Three-dimensional imaging at the nanoscale

The development of acoustic subsurface atomic force microscopy, which promises three-dimensional imaging with single-digit nanometer resolution by the introduction of ultrasound actuations to a conventional atomic force microscope, has come a long way since its inception in the early 1990s. Recent advances provide a quantitative understanding of the different experimentally observed contrast mechanisms, which paves the way for future applications. In this Perspective, we first review the different subsurface atomic force microscope modalities: ultrasonic force microscopy, atomic force acoustic microscopy, heterodyne force microscopy, mode-synthesizing atomic force microscopy, and near-field picosecond ultrasonic microscopy. Then, we highlight and resolve a debate existing in the literature on the importance of the chosen ultrasound excitation frequencies with respect to the resonance frequencies of the cantilever and the observed contrast mechanisms. Finally, we discuss remaining open problems in the field and motivate the importance of new actuators, near-field picosecond ultrasonics, and integration with other techniques to achieve multi-functional non-destructive three-dimensional imaging at the nanoscale.

An Atomic Force Acoustic Microscopy Image Fusion Method Based on Grayscale Inversion and Selection of Best-Fit Intensity

Atomic force acoustic microscopy (AFAM) can provide surface morphology and internal structures of the samples simultaneously, with broad potential in non-destructive imaging of cells. As the output of AFAM, morphology and acoustic images reflect different features of the cells, respectively. However, there are few studies about the fusion of these images. In this paper, a novel method is proposed to fuse these two types of images based on grayscale inversion and selection of best-fit intensity. First, grayscale inversion is used to transform the morphology image into a series of inverted images with different average intensities. Then, the max rule is applied to fuse those inverted images and acoustic images, and a group of pre-fused images is obtained. Finally, a selector is employed to extract and export the expected image with the best-fit intensity among those pre-fused images. The expected image can preserve both the acoustic details of the cells and the background’s gradient information well, which benefits the analysis of the cell’s subcellular structure. The experiments’ results demonstrated that our method could provide the clearest boundaries between the cells and background, and preserve most details from the morphology and acoustic images according to quantitative comparisons, including standard deviation, mutual information, Xydeas and Petrovic metric, feature mutual information, and visual information fidelity fusion.

A Fusion Method for Atomic Force Acoustic Microscopy Cell Imaging Based on Local Variance in Non-Subsampled Shearlet Transform Domain

Atomic force acoustic microscopy (AFAM) is a measurement method that uses the probe and acoustic wave to image the surface and internal structures of different materials. For cellular material, the morphology and phase images of AFAM reflect the outer surface and internal structures of the cell, respectively. This paper proposes an AFAM cell image fusion method in the Non-Subsampled Shearlet Transform (NSST) domain, based on local variance. First, NSST is used to decompose the source images into low-frequency and high-frequency sub-bands. Then, the low-frequency sub-band is fused by the weight of local variance, while a contrast limited adaptive histogram equalization is used to improve the source image contrast to better express the details in the fused image. The high-frequency sub-bands are fused using the maximum rule. Since the AFAM image background contains a lot of noise, and improved segmentation algorithm based on the Otsu algorithm is proposed to segment the cell region, and the image quality metrics based on the segmented region will make the evaluation more accurate. Experiments with different groups of AFAM cell images demonstrated that the proposed method can clearly show the internal structures and the contours of the cells, compared with traditional methods.

Indentation modulus and microstructural properties of Zirconia – Alumina – Magnesia composite thin films deposited by electron beam evaporation under varying oxygen pressure

Zirconia - Alumina – Magnesia (ZrO2Al2O3MgO) ternary composite thin films are used in the fabrication of interference multilayer thin film devices for lasers and photo-physical applications. Optical, structural and morphological properties of such thin films deposited under varying deposition parameters, are reported by researchers. Such thin films possess fascinating spectral stability and optical properties. However, reports on elastic properties that are crucial parameter to assess mechanical stability, are scanty for such thin films. In the present work, ZrO2Al2O3MgO thin films have been deposited by electron beam evaporation under varying oxygen (O2) pressure. Atomic force acoustic microscopy, which is rather new technique, has been employed to estimate indentation modulus of the films. Indentation modulus exhibits a decreasing trend with deposition O2 pressure. Film density measured by grazing incidence X-ray reflectivity, decreases monotonically with O2 pressure. The variation of indentation modulus as well as density has been explained in terms of varying microstructure and mutual concentration of different components of ternary composite thin films. Correlation length, root mean square surface roughness and roughness exponents have been obtained through analysis of height-height correlation function measured by atomic force microscopy. Variation of correlation length corroborates the variation of indentation modulus and film density.

Investigating effects of silicon nanowire and nanohole arrays on fibroblasts via AFAM

Understanding the cell–substrate interactions has great significance in tissue regeneration therapies. However, the cell–substrate interactions are not well understood because the interface of cell–substrate is typically buried beneath the cells. This research investigated the subsurfaces of fibroblasts cultured on hybrid nanoarrays using atomic force acoustic microscopy (AFAM). We fabricated hybrid silicon nanowires (SiNWs) and silicon nanoholes (SiNHs) on Si substrates to serve as biomimetic nanoarrays by employing laser interference lithography and the metal-assisted chemical etching (MacEtch) method. After the L929 cells were cultured on the nanoarrays, scanning electron microscopy (SEM) and AFAM were employed to investigate the surface and subsurface of L929 cells. It was suggested that fibroblasts could sense the morphology of the hybrid nanoarrays and membrane damage of fibroblasts on the hybrid nanoarrays were related to the nanostructures. This study can help guide the design of biointerfaces and provide a useful tool for the study of cell subsurfaces in diverse biological fields.

Local mechanical properties of an ultrastable metallic glass

Recently, ultrastable glasses gained growing attention due to the revelation of the mechanism leading towards the understanding of glasses and glass transition in general. We report the adaptation of vapor deposition techniques to create ultrastable amorphous phases of Cu50Zr50 with different mechanical properties. Investigations using atomic force acoustic microscopy reveal a connection between an enhanced elastic modulus, its low potential energy and its homogeneity throughout the sample. Here, higher stability is always accompanied by high homogeneity. We relate the preparation conditions to the resulting mechanical properties, potentially opening a path to systematically engineer ultrastability in metallic glasses. Furthermore, we give a qualitative explanation for our results in the framework of the potential energy landscape, providing insights to the origin of ultrastability in metallic glasses in general.

Stochastic excitation for high-resolution atomic force acoustic microscopy imaging: a system theory approach.

In this work, a high-resolution atomic force acoustic microscopy imaging technique is developed in order to obtain the local indentation modulus at the nanoscale level. The technique uses a model that gives a qualitative relationship between a set of contact resonance frequencies and the indentation modulus. It is based on white-noise excitation of the tip–sample interaction and uses system theory for the extraction of the resonance modes. During conventional scanning, for each pixel, the tip–sample interaction is excited with a white-noise signal. Then, a fast Fourier transform is applied to the deflection signal that comes from the photodiodes of the atomic force microscopy (AFM) equipment. This approach allows for the measurement of several vibrational modes in a single step with high frequency resolution, with less computational cost and at a faster speed than other similar techniques. This technique is referred to as stochastic atomic force acoustic microscopy (S-AFAM), and the frequency shifts of the free resonance frequencies of an AFM cantilever are used to determine the mechanical properties of a material. S-AFAM is implemented and compared with a conventional technique (resonance tracking-atomic force acoustic microscopy, RT-AFAM). A sample of a graphite film on a glass substrate is analyzed. S-AFAM can be implemented in any AFM system due to its reduced instrumentation requirements compared to conventional techniques.

Mechanobiology Analysis of Manifold Live Cells in Vitro with Atomic Force Acoustic Microscopy.

Mechanobiology analysis has been considered one of the important parameters to explore the physiological behavior inside cells. However, the morphology and mechanical characterization of live cells by employing atomic force acoustic microscopy (AFAM) has rarely been investigated to date. In this paper, the morphological and mechanical properties of five various live cells in vitro based on AFAM is detected for the first time. Combining AFAM with inverted optical microscopy and scanning electron microscopy (SEM), it has enabled the imaging of various live cells. According to the AFAM experimental results, the mechanical properties of live cells were obtained by fitting the force-depth curves with Hertz-Sneddon model. The conducted experiments and obtained results verified that it is viable to furnish insights into the multifunctional applicability of the AFAM-based characterization in mechanobiology, which has the advantages of making a contribution to diagnosing diseased cells.

Study of SU‐8 photoresist cross‐linking process by atomic force acoustic microscopy

In this paper, a method is presented to detect the different phases of epoxy cross-linking process and the subsurface structures of SU-8 thin films by atomic force acoustic microscopy (AFAM). The AFAM imaging of SU-8 thin films was investigated under different exposure and bake conditions. Optimized conditions were obtained for the cross-linking of SU-8 thin film at the exposure does of eight laser pulses with the laser fluence 10mJcm-2 per pulse and the post exposure bake (PEB) time at 90 s. The subsurface structures of undeveloped SU-8 thin films were visible in the AFAM images. This method provides an effective and low-cost way for the determination of different phases of epoxy cross-linking process in nanostructured compounds, for the non-destructive testing of subsurface defects, and for the evaluation of the quality of patterned structures.

Atomic force acoustic microscopy reveals the influence of substrate stiffness and topography on cell behavior.

The stiffness and the topography of the substrate at the cell–substrate interface are two key properties influencing cell behavior. In this paper, atomic force acoustic microscopy (AFAM) is used to investigate the influence of substrate stiffness and substrate topography on the responses of L929 fibroblasts. This combined nondestructive technique is able to characterize materials at high lateral resolution. To produce substrates of tunable stiffness and topography, we imprint nanostripe patterns on undeveloped and developed SU-8 photoresist films using electron-beam lithography (EBL). Elastic deformations of the substrate surfaces and the cells are revealed by AFAM. Our results show that AFAM is capable of imaging surface elastic deformations. By immunofluorescence experiments, we find that the L929 cells significantly elongate on the patterned stiffness substrate, whereas the elasticity of the pattern has only little effect on the spreading of the L929 cells. The influence of the topography pattern on the cell alignment and morphology is even more pronounced leading to an arrangement of the cells along the nanostripe pattern. Our method is useful for the quantitative characterization of cell–substrate interactions and provides guidance for the tissue regeneration therapy in biomedicine.

Noninvasive Subcellular Imaging Using Atomic Force Acoustic Microscopy (AFAM).

We report an imaging approach applying the atomic force acoustic microscopy (AFAM), which has unique potential for nondestructive imaging of cell internal structures. To obtain high spatial resolution images, we optimized the significant imaging parameters, including scanning speeds, feedback configurations and acoustic frequencies of an AFAM system, to increase the amplitude of the acoustic signal and to stabilize the morphological signals. We also combined the acoustic amplitude and phase signals, and generated pseudo-color figures for better illustration of subcellular features such as pseudopodia, membranes and nucleus-like. The subcellular structural image atlas can describe nanoscale details of multiple samples and provide clearer images of the subcellular features compared to other conventional techniques. This study builds a strong basis of transmission AFAM for cell imaging, which can help researchers to clarify the cell structures in diverse biological fields and push the understanding of biology evolution to a new stage.

Investigating the detection limit of subsurface holes under graphite with atomic force acoustic microscopy.

The subsurface imaging capabilities of atomic force acoustic microscopy (AFAM) was investigated by imaging graphite flakes suspended over holes in a silicon dioxide substrate. The graphite thickness and the hole size were varied to determine the detection limit on the maximum graphite thickness and the smallest detectable hole size. Parameters including operating frequency, eigenmode, contact force, and cantilever stiffness were investigated for their influence of defect detection. AFAM was reliably able to detect 2.5 μm diameter holes through a maximum graphite thickness of 570 nm and sub 100 nm holes through 140 nm of graphite. The smallest detectable defect size was a 50 nm hole covered by an 80 nm thick graphite flake. Increasing the graphite thickness and decreasing the hole size both resulted in a decrease in subsurface contrast. However, the non-linear trend observed from increasing the graphite thickness indicates thickness has a greater effect on subsurface defect detection than variations in defect size. Through investigating various parameters, we have found certain cases to increase the observed contrast of the embedded subsurface holes, however the smallest detectable defect size remained the same. This technique's ability to reveal sub 100 nm defects buried under graphite has previously only been demonstrated in much softer polymer systems.

Nanoscale studies of magnetoelectric coupling in multiferroic BTO–CFO composite

We report on the existence of interacting ferromagnetic and ferroelectric nanodomains in multiferroic 0.65BaTiO3–0.35CoFe2O4 (BTO–CFO) composite synthesized using a conventional solid-state route. X-ray diffraction study shows the coexistence of individual phases (BTO and CFO) of the composite material. Nanometric studies on the sample pellet of BTO–CFO composite are carried out using Scanning Probe Microscopy (SPM) to probe local ferroelectric and magnetic ordering. Importantly, the interactive nanodomains of individual phases are distinctly seen in the Atomic Force Acoustic Microscopy (AFAM) and Piezo-Force Microscopy (PFM) studies, which are done to investigate the magnetoelectric coupling in the BTO–CFO composite. Elasticity maps and piezoresponse images show local inhomogeneities with respect to the distribution of magnetoelectric coupling in the composite.

Depth sensitivity of subsurface imaging using atomic force acoustic microscopy: FEA Study

Atomic force acoustic microscopy makes it possible to image objects below the surface of a sample. Such subsurface imaging capabilities are of great interest in electronics, semiconductors, (bio)materials and manufacturing, polymers and microbiology. In this article, we report a numerical method to study the subsurface depth sensitivity. We calculated the depth sensitivity of atomic force microscopy and atomic force acoustic microscopy for subsurface objects of different heights and elasticity, as well as for different probe radii, applied forces and indentation depths. We also conducted experiments to validate the simulations. The results indicate that atomic force acoustic microscopy has higher subsurface depth sensitivity than atomic force microscopy.

Nanomechanical characterization of porous materials by atomic force microscopy

AbstractAtomic force microscopy (AFM) and nanoindentation were used to characterize poly (methyl methacrylate) (PMMA) films with a wide distribution of pores. Pores with diameters ranging from tens of nanometers to few micrometers were measured by AFM and cross-section scanning electron microscopy (SEM). Atomic force acoustic microscopy (AFAM) mapping of the elastic modulus were correlated with the samples topography and pore distribution. The elastic moduli of the samples were additionally measured by nanoindentation.