Probing the Mechanical Properties of 2D Materials via Atomic-Force-Microscopy-Based Modulated Nanoindentation.

As the field of low-dimensional materials (1D or 2D) grows and more complex and intriguing structures are continuing to be found, there is an emerging need for techniques to characterize the nanoscale mechanical properties of all kinds of 1D/2D materials, in particular in their most practical state: sitting on an underlying substrate. While traditional nanoindentation techniques cannot accurately determine the transverse Young's modulus at the necessary scale without large indentations depths and effects to and from the substrate, herein an atomic-force-microscopy-based modulated nanomechanical measurement technique with Angstrom-level resolution (MoNI/ÅI) is presented. This technique enables non-destructive measurements of the out-of-plane elasticity of ultra-thin materials with resolution sufficient to eliminate any contributions from the substrate. This method is used to elucidate the multi-layer stiffness dependence of graphene deposited via chemical vapor deposition and discover a peak transverse modulus in two-layer graphene. While MoNI/ÅI has been used toward great findings in the recent past, here all aspects of the implementation of the technique as well as the unique challenges in performing measurements at such small resolutions are encompassed.

Stiffness anisotropy coordinates supracellular contractility driving long-range myotube-ECM alignment.

In skeletal muscle tissue, injury-related changes in stiffness activate muscle stem cells through mechanosensitive signaling pathways. Functional muscle tissue regeneration also requires the effective coordination of myoblast proliferation, migration, polarization, differentiation, and fusion across multiple length scales. Here, we demonstrate that substrate stiffness anisotropy coordinates contractility-driven collective cellular dynamics resulting in C2C12 myotube alignment over millimeter-scale distances. When cultured on mechanically anisotropic liquid crystalline polymer networks (LCNs) lacking topographic features that could confer contact guidance, C2C12 myoblasts collectively polarize in the stiffest direction of the substrate. Cellular coordination is amplified through reciprocal cell-ECM dynamics that emerge during fusion, driving global myotube-ECM ordering. Conversely, myotube alignment was restricted to small local domains with no directional preference on mechanically isotropic LCNs of same chemical formulation. These findings reveal a role for stiffness anisotropy in coordinating emergent collective cellular dynamics, with implications for understanding skeletal muscle tissue development and regeneration.

Co-localizing Kelvin Probe Force Microscopy with Other Microscopies and Spectroscopies: Selected Applications in Corrosion Characterization of Alloys.

Kelvin probe force microscopy (KPFM), sometimes referred to as surface potential microscopy, is the nanoscale version of the venerable scanning Kelvin probe, both of which measure the Volta potential difference (VPD) between an oscillating probe tip and a sample surface by applying a nulling voltage equal in magnitude but opposite in sign to the tip-sample potential difference. By scanning a conductive KPFM probe over a sample surface, nanoscale variations in surface topography and potential can be mapped, identifying likely anodic and cathodic regions, as well as quantifying the inherent material driving force for galvanic corrosion. Subsequent co-localization of KPFM Volta potential maps with advanced scanning electron microscopy (SEM) techniques, including back scattered electron (BSE) images, energy dispersive spectroscopy (EDS) elemental composition maps, and electron backscattered diffraction (EBSD) inverse pole figures can provide further insight into structure-property-performance relationships. Here, the results of several studies co-localizing KPFM with SEM on a wide variety of alloys of technological interest are presented, demonstrating the utility of combining these techniques at the nanoscale to elucidate corrosion initiation and propagation. Important points to consider and potential pitfalls to avoid in such investigations are also highlighted: in particular, probe calibration and the potential confounding effects on the measured VPDs of the testing environment and sample surface, including ambient humidity (i.e., adsorbed water), surface reactions/oxidation, and polishing debris or other contaminants. Additionally, an example is provided of co-localizing a third technique, scanning confocal Raman microscopy, to demonstrate the general applicability and utility of the co-localization method to provide further structural insight beyond that afforded by electron microscopy-based techniques.

Impact of grain orientation and phase on Volta potential differences in an additively manufactured titanium alloy.

This work introduces a method for co-localized multi-modal imaging of sub-µm features in an additively manufactured (AM) titanium alloy. Ti-6Al-4V parts manufactured by electron beam melting powder bed fusion were subjected to hot isostatic pressing to seal internal porosity and machined to remove contour-hatch interfaces. Electron microscopy and atomic force microscopy-based techniques (electron backscatter diffraction and scanning Kelvin probe force microscopy) were used to measure and categorize the effects of crystallographic texture, misorientation, and phase content on the relative differences in the Volta potential of α-Ti and ß-Ti phases. Given the tunability of additive manufacturing processes, recommendations for texture and phase control are discussed. In particular, our findings indicate that the potential for micro-galvanic corrosion initiation can be regulated in AM Ti-6Al-4V parts by minimizing both the total area of {111} prior-ß grains and the number of contact points between {111} ß grains and α laths that originate from {001} prior-ß grains.

Elastic-plastic properties of mesoscale electrodeposited LIGA nickel alloy films: microscopy and mechanics.

The elastic-plastic properties of mesoscale electrodeposited LIGA Ni alloy specimens are investigated as a function of specimen size, strain rate, and material composition. Two material compositions are studied: a high-strength fine-grained Ni-Fe alloy and a high-ductility coarse-grained Ni-Co alloy. The specimens have thicknesses of approximately 200 µm and gauge widths ranging from 75 µm to 700 µm. Tensile tests are conducted at strain rates of 0.001/s and 1/s using tabletop loading apparatuses and digital image correlation (DIC) for strain measurement. For each test condition, the apparent Young's modulus, yield strength, ultimate tensile strength, and strain hardening exponent and strength coefficient are extracted from the tensile tests. The true strains to failure are also assessed from fractography. Size, rate, and composition effects are discussed. For most properties, the statistical scatter represented by the standard deviation exceeds the measurement uncertainty; the notable exceptions to these observations are the apparent Young's modulus and yield strength, where large measurement uncertainties are ascribed to common experimental factors and material microplasticity.

Injury-mediated stiffening persistently activates muscle stem cells through YAP and TAZ mechanotransduction.

The skeletal muscle microenvironment transiently remodels and stiffens after exercise and injury, as muscle ages, and in myopathic muscle; however, how these changes in stiffness affect resident muscle stem cells (MuSCs) remains understudied. Following muscle injury, muscle stiffness remained elevated after morphological regeneration was complete, accompanied by activated and proliferative MuSCs. To isolate the role of stiffness on MuSC behavior and determine the underlying mechanotransduction pathways, we cultured MuSCs on strain-promoted azide-alkyne cycloaddition hydrogels capable of in situ stiffening by secondary photocrosslinking of excess cyclooctynes. Using pre- to post-injury stiffness hydrogels, we found that elevated stiffness enhances migration and MuSC proliferation by localizing yes-associated protein 1 (YAP) and WW domain-containing transcription regulator 1 (WWTR1; TAZ) to the nucleus. Ablating YAP and TAZ in vivo promotes MuSC quiescence in postinjury muscle and prevents myofiber hypertrophy, demonstrating that persistent exposure to elevated stiffness activates mechanotransduction signaling maintaining activated and proliferating MuSCs.

Photo-tunable hydrogel mechanical heterogeneity informed by predictive transport kinetics model.

Understanding the three-dimensional (3D) mechanical and chemical properties of distinctly different, adjacent biological tissues is crucial to mimicking their complex properties with materials. 3D printing is a technique often employed to spatially control the distribution of the biomaterials, such as hydrogels, of interest, but it is difficult to print both mechanically robust (high modulus and toughness) and biocompatible (low modulus) hydrogels in a single structure. Moreover, due to the fast diffusion of mobile species during printing and nonequilibrium swelling conditions of low-solids-content hydrogels, it is challenging to form the high-fidelity structures required to mimic tissues. Here a predictive transport and swelling model is presented to model these effects and then is used to compensate for these effects during printing. This model is validated experimentally by photopatterning spatially distinct hydrogel elastic moduli using a single photo-tunable poly(ethylene glycol) (PEG) pre-polymer solution by sequentially patterning and in-diffusing fresh pre-polymer for further polymerization.

Hydrogen embrittlement in ferritic steels.

Hydrogen will be a crucial pillar in the clean-energy foundation, and therefore, the development of safe and cost-effective storage and transportation methods is essential to its success. One of the key challenges in the development of such storage and transportation methods is related to the interaction of hydrogen with structural materials. Despite extensive work, there are significant questions related to the hydrogen embrittlement of ferritic steels due to challenges associated with these steels, coupled with the difficulties with gauging the hydrogen content in all materials. Recent advancements in experimental tools and multi-scale modeling are starting to provide insight into the embrittlement process. This review focuses on a subset of the recent developments, with an emphasis on how new methods have improved our understanding of the structure-property-performance relationships of ferritic steels subjected to mechanical loading in a hydrogen environment. The structure of ferritic steels in the presence of hydrogen is described in terms of the sorption and dissociation processes, the diffusion through the lattice and grain boundaries, and the hydrogen-steel interactions. The properties of ferritic steels subjected to mechanical loading in hydrogen are also investigated; the effects of test conditions and hydrogen pressure on the tensile, fracture, and fatigue properties of base metal and welds are highlighted. The performance of steels in hydrogen is then explored via a comprehensive analysis of the various embrittlement mechanisms. Finally, recent insights from in situ and high-resolution experiments are presented and future studies are proposed to address challenges related to embrittlement in ferritic steels.

Dominant factors for fracture at the micro-scale in electrodeposited nickel alloys.

Two different LIGA electrodeposited nickel alloys displayed distinct fracture modes after meso-scale tensile testing. The Ni-Co alloy failed in a ductile manner, while the Ni-Fe alloy failed in a more brittle-appearing manner. Various factors affecting the fracture are discussed; it was determined that the fracture mode did not depend upon the strain rate but did depend upon the sample geometry. The difference in the microstructure is likely the cause of the difference in fracture mode, as the Ni-Co alloy is fine-grained, while the Ni-Fe alloy is nano-grained and likely failed by a creep-like mechanism.

Predicting strength distributions of MEMS structures using flaw size and spatial density.

The populations of flaws in individual layers of microelectromechanical systems (MEMS) structures are determined and verified using a combination of specialized specimen geometry, recent probabilistic analysis, and topographic mapping. Strength distributions of notched and tensile bar specimens are analyzed assuming a single flaw population set by fabrication and common to both specimen geometries. Both the average spatial density of flaws and the flaw size distribution are determined and used to generate quantitative visualizations of specimens. Scanning probe-based topographic measurements are used to verify the flaw spacings determined from strength tests and support the idea that grain boundary grooves on sidewalls control MEMS failure. The findings here suggest that strength controlling features in MEMS devices increase in separation, i.e., become less spatially dense, and decrease in size, i.e., become less potent flaws, as processing proceeds up through the layer stack. The method demonstrated for flaw population determination is directly applicable to strength prediction for MEMS reliability and design.

Operational and environmental conditions regulate the frictional behavior of two-dimensional materials.

The friction characteristics of single-layer h-BN, MoS2, and graphene were systematically investigated via friction force microscopy measurements at various operational (e.g., normal force and sliding speed) and environmental (e.g., relative humidity and thermal annealing) conditions. The low friction characteristics of these single-layer materials were clearly observed from the normal force-dependent friction results, and their interfacial shear strengths were further estimated using a Hertz-plus-offset model. In addition, speed-dependent friction characteristics clearly demonstrated two regimes of friction as a function of sliding speed - the first is the logarithmic increase in friction with sliding speed regime at sliding speeds smaller than the critical speed and the second is the friction plateau regime at sliding speeds greater than the critical speed. Fundamental parameters such as effective shape of the interaction potential and its corrugation amplitude for these single-layer materials were characterized using the thermally-activated Prandtl-Tomlinson model. Moreover, friction of single-layer h-BN, MoS2, and graphene was found to increase with relative humidity and decrease with thermal annealing; these trends were attributed to the diffusion of water molecules to the interface between the single-layer materials and their substrates, which leads to an increase in the puckering effect at the tip-material interface and interaction potential corrugation. The enhanced puckering effect was verified via molecular dynamics simulations. Overall, the findings enable a comprehensive understanding of friction characteristics for several classes of two-dimensional materials, which is important to elucidate the feasibility of using these materials as protective and solid-lubricant coating layers for nanoscale devices.

Monitoring Fast, Voxel-Scale Cure Kinetics via Sample-Coupled-Resonance Photorheology.

Photopolymerizable materials are the focus of extensive research across a variety of fields ranging from additive manufacturing to regenerative medicine. However, poorly understood material mechanical and rheological properties during polymerization at the relevant exposure powers and single-voxel length-scales limit advancements in part performance and throughput. Here, a novel atomic force microscopy (AFM) technique, sample-coupled-resonance photorheology (SCRPR), to locally characterize the mechano-rheological properties of photopolymerized materials on the relevant reaction kinetic timescales, is demonstrated. By coupling an AFM tip to a photopolymer and exposing the coupled region to a laser, two fundamental photopolymerization phenomena: (1) timescales of photopolymerization at high laser power and (2) reciprocity between photodose and material properties are studied. The ability to capture rapid kinetic changes occurring during polymerization with SCRPR is demonstrated. It is found that reciprocity is only valid for a finite range of exposure powers in the verification material and polymerization is highly localized in a low-diffusion system. After polymerization, in situ imaging of a single polymerized voxel is performed using material-appropriate topographic and nanomechanical modalities of the AFM while still in the as-printed environment.

Layer-by-layer thinning of MoS2 via laser irradiation.

Layer-by-layer thinning of molybdenum disulfide (MoS2) via laser irradiation was examined using Raman spectroscopy and atomic force microscopy. In particular, the effects of number of layers, laser conditions, and substrate were systematically identified. The results demonstrated the presence of nanoparticles on the MoS2 at sufficient laser treatment conditions prior to layer-by-layer thinning. The volume of nanoparticles was found to increase and then decrease as the number of MoS2 layers increased; the non-monotonic trend was ascribed to changes in the thermal conductivity of the film and interfacial thermal conductance between the film and substrate with number of layers. Moreover, the volume of nanoparticles was found to increase as the magnification of the objective lens decreased and as laser power and exposure time increased, which was attributed to changes in the power density with laser conditions. The effect of substrate on nanoparticle formation and layer-by-layer thinning was investigated through a comparison of freestanding and substrate-supported MoS2 subjected to laser irradiation; it was illustrated that freestanding films were thinned at lower laser powers than substrate-supported films, which highlighted the function of the substrate as a heat sink. For conditions that elicited thinning, it was shown that the thinned areas exhibited triangular shapes, which suggested anisotropic etching behavior where the lattice of the basal plane was preferentially thinned along the zigzag direction terminated by an Mo- or S-edge. High-resolution transmission electron microscopy of freestanding MoS2 revealed the presence of a 2 nm thick amorphous region around the laser-treated region, which suggested that the crystalline structure of laser-treated MoS2 remained largely intact after the thinning process. In all, the conclusions from this work provide useful insight into the progression of laser thinning of MoS2, thereby enabling more effective methods for the development of MoS2 devices via laser irradiation.

Identification of a mechanogenetic link between substrate stiffness and chemotherapeutic response in breast cancer.

Mechanical feedback from the tumor microenvironment regulates an array of processes underlying cancer biology. For example, increased stiffness of mammary extracellular matrix (ECM) drives malignancy and alters the phenotypes of breast cancer cells. Despite this link, the role of substrate stiffness in chemotherapeutic response in breast cancer remains unclear. This is complicated by routine culture and adaptation of cancer cell lines to unnaturally rigid plastic or glass substrates, leading to profound changes in their growth, metastatic potential and, as we show here, chemotherapeutic response. We demonstrate that primary breast cancer cells undergo dramatic phenotypic changes when removed from the host microenvironment and cultured on rigid surfaces, and that drug responses are profoundly altered by the mechanical feedback cells receive from the culture substrate. Conversely, primary breast cancer cells cultured on substrates mimicking the mechanics of their host tumor ECM have a similar genetic profile to the in situ cells with respect to drug activity and resistance pathways. These results suggest substrate stiffness plays a significant role in susceptibility of breast cancer to clinically-approved chemotherapeutics, and presents an opportunity to improve drug discovery efforts by integrating mechanical rigidity as a parameter in screening campaigns.

US-Czech conference strengthens bilateral and multidisciplinary collaborations in nanotechnology and chemistry.

There are several methods to encourage bilateral and multidisciplinary collaborations-the most oft-used and effective include technical conferences, workshops, and seminars with the optimal combination of people, agenda, and length. However, this balance is difficult to attain, especially for meetings across academic disciplines and geographic borders. For nearly two decades, the US National Academies have developed a series of bilateral meetings to bring together scientists in different countries, which appear to have addressed such challenges via concise meetings with select emerging leaders from disparate fields and sectors. The authors utilized a similar approach to promote US and Czech collaborations in nanotechnology, given their mutual investments in the area and interests in building relationships. The bilateral meeting was held in Prague and designed to focus on nanotechnology and chemistry. The goals were four-fold: (1) evaluate US and Czech capabilities and challenges in nanotechnology and chemistry, (2) identify likely research projects and funding sources for collaborations, (3) celebrate 100 years of US and Czech diplomatic relations, and (4) increase government and public support for bilateral scientific research. The two-day meeting was named the US-Czech Conference on Advanced Nanotechnology and Chemistry. A summary of the research presentations, funding opportunities, panel discussions, and potential joint projects are given herein. It is our hope that this perspective will facilitate similar bilateral meetings in other fields to promote more multidisciplinary research.

Surface Properties of Laser-Treated Molybdenum Disulfide Nanosheets for Optoelectronic Applications.

Transition metal dichalcogenide two-dimensional materials have attracted significant attention due to their unique optical, mechanical, and electronic properties. For example, molybdenum disulfide (MoS2) exhibits a tunable band gap that strongly depends on the numbers of layers, which makes it an attractive material for optoelectronic applications. In addition, recent reports have shown that laser thinning can be used to engineer an MoS2 monolayer with specific shapes and dimensions. Here, we study laser-thinned MoS2 in both ambient and vacuum conditions via confocal µ-Raman spectroscopy, imaging X-ray photoelectron spectroscopy (i-XPS), and atomic force microscopy (AFM). For low laser powers in ambient environments, there is insufficient energy to oxidize MoS2, which leads to etching and redeposition of amorphous MoS2 on the nanosheet as confirmed by AFM. At high powers in ambient, the laser energy and oxygen environment enable both MoS2 nanoparticle formation and nanosheet oxidation as revealed in AFM and i-XPS. At comparable laser power densities in vacuum, MoS2 oxidation is suppressed and the particle density is reduced as compared to ambient. The extent of nanoparticle formation and nanosheet oxidation in each of these regimes is found to be dependent on the number of layers and laser treatment time. Our results can shed some light on the underlying mechanism of which atomically thin MoS2 nanosheets exhibit under high incident laser power for future optoelectronic applications.

Interfacial Strength and Surface Damage Characteristics of Atomically Thin h-BN, MoS2, and Graphene.

Surface damage characteristics of single- and multilayer hexagonal boron nitride (h-BN), molybdenum disulfide (MoS2), and graphene films were systematically investigated via atomic force microscopy (AFM)-based progressive-force and constant-force scratch tests and Raman spectroscopy. The film-to-substrate interfacial strengths of these atomically thin films were assessed based on their critical forces (i.e., the normal force where the atomically thin film was delaminated from the underlying substrate), as determined from progressive-force scratch tests. The evolution of surface damage with respect to normal force was further investigated using constant-force tests. The results showed that single-layer h-BN, MoS2, and graphene strongly adhere to the SiO2 substrate, which significantly improves its tribological performance. Moreover, defect formation induced by scratch testing was found to affect the topography and friction force differently prior to failure, which points to distinct surface damage characteristics. Interestingly, the residual strains at scratched areas suggest that the scratch test-induced in-plane compressive strains were dominant over tensile strains, thereby leading to buckling in front of the scratching tip and eventually failure at sufficient strains. These trends represent the general failure mechanisms of atomically thin materials because of a scratch test. As the number of layers increased, the tribological performances of atomically thin h-BN, MoS2, and graphene were found to significantly improve because of an increase in the interfacial strengths and a decrease in the surface damage and friction force. In all, the findings on the distinctive surface damage characteristics and general failure mechanisms are useful for the design of reliable, protective and solid-lubricant coating layers based on these materials for nanoscale devices.

Determination of ceramic flaw populations from component strengths.

A procedure is outlined for determining the population of flaws in manufactured ceramics from strength measurements of sampled components. The broad applicability of the procedure is demonstrated in a quantitative manner, using strength measurements from a range of ceramic materials (eg, glass, glass-ceramic, single crystal, and polycrystal) with different flaw types (eg, bulk, surface, and edge). The deconvoluted flaw populations are mostly dominated by small flaws with extended large flaw tails and are all in domains of tens of micrometers. The procedure greatly extends the useful information to be gained by ceramics manufacturers and designers from strength distribution measurements and emphasizes the importance of identifying strength-limiting characteristics within a flaw population.

Indentation mapping revealed poroelastic, but not viscoelastic, properties spanning native zonal articular cartilage.

Osteoarthrosis is a debilitating disease affecting millions, yet engineering materials for cartilage regeneration has proven difficult because of the complex microstructure of this tissue. Articular cartilage, like many biological tissues, produces a time-dependent response to mechanical load that is critical to cell's physiological function in part due to solid and fluid phase interactions and property variations across multiple length scales. Recreating the time-dependent strain and fluid flow may be critical for successfully engineering replacement tissues but thus far has largely been neglected. Here, microindentation is used to accomplish three objectives: (1) quantify a material's time-dependent mechanical response, (2) map material properties at a cellular relevant length scale throughout zonal articular cartilage and (3) elucidate the underlying viscoelastic, poroelastic, and nonlinear poroelastic causes of deformation in articular cartilage. Untreated and trypsin-treated cartilage was sectioned perpendicular to the articular surface and indentation was used to evaluate properties throughout zonal cartilage on the cut surface. The experimental results demonstrated that within all cartilage zones, the mechanical response was well represented by a model assuming nonlinear biphasic behavior and did not follow conventional viscoelastic or linear poroelastic models. Additionally, 10% (w/w) agarose was tested and, as anticipated, behaved as a linear poroelastic material. The approach outlined here provides a method, applicable to many tissues and biomaterials, which reveals and quantifies the underlying causes of time-dependent deformation, elucidates key aspects of material structure and function, and that can be used to provide important inputs for computational models and targets for tissue engineering. STATEMENT OF SIGNIFICANCE: Elucidating the time-dependent mechanical behavior of cartilage, and other biological materials, is critical to adequately recapitulate native mechanosensory cues for cells. We used microindentation to map the time-dependent properties of untreated and trypsin treated cartilage throughout each cartilage zone. Unlike conventional approaches that combine viscoelastic and poroelastic behaviors into a single framework, we deconvoluted the mechanical response into separate contributions to time-dependent behavior. Poroelastic effects in all cartilage zones dominated the time-dependent behavior of articular cartilage, and a model that incorporates tension-compression nonlinearity best represented cartilage mechanical behavior. These results can be used to assess the success of regeneration and repair approaches, as design targets for tissue engineering, and for development of accurate computational models.