Symposium CM3 : Mechanics and Tribology at the Nanoscale---In Situ and In Silico Investigations

Aerographite is an ultra light porous material of carbon showing excellent elasticity to large deformations with several tens strain [1]. Its unique characteristics are derived from the three-dimensional interconnected structure of hollow carbon fibers. Conventional aerographite has tetrapod-type network, since the morphology is transferred from tetrapod-type ZnO used as the template in the fabrication process. Recently, urchin-like ZnO particles with micrometer order diameters, so-called ZnO nanorod-microsphere has been fabricated [2]. These particles consist of radially-arranged ZnO nanorods with 50nm diameter. Using this instead of tetrapod-type one, we have developed a new type of aerographite, spiky-shell microparticles. The spherical shell consisting of radially arranged hollow nanorods with 100nm diameters and 5-10nm thicknesses. The spiky-shell morphology can ensure the mechanical strengths of such thin shell particles. In this study, mechanical properties of the particles were evaluated by single-particle-level compressive tests by in-situ electron microscopy.
In a transmission electron microscope, a Si substrate supporting aerographite particles by Van der Waals force and a cantilevered probe for scanning probe microscopy were individually fixed to two stages of nano-manipulator system, one of which is movable driven by piezoelectricity. By operating the manipulator, a single particle on Si substrate contacted to the tip of the cantilevered probe. Structural changes in the particle were observed when stressed in the compressive direction. Relationship between stress and strain was also examined by similar experiments carried out in a scanning electron microscope. An aerographite particle was placed between two parallel cantilevers, and compressed by operating one cantilever with monitoring deflections of cantilevers and changes in shape of the particle simultaneously. Compressive force was estimated from the deflection of cantilevers.
As the result, an aerographite particle fabricated in this study showed excellent elastic behavior under large strain (73% in maximum). Stress-strain curves and in-situ observation suggested the two-step deformation process in the elastic compression; local deformation at contact region and compression of the whole particle. Since individual nanorods did not deform, elasticity of the particles may be derived from the flexible connections between nanorods at their bottom portions. Probability inducing crack increased above 30% strain, but the aerographite particles inducing cracks almost recovered its spherical shapes after unloading. Accumulation of residual strain during the repetitive compression was also evaluated. It indicated that improvement of the crystallinity of graphitic layers by annealing treatment well contributed to improve the performance as the ultraflexible microparticles.

[1] M. Mecklenburg et al. Adv. Mater. 24, (2012) 3486-3490.
[2] Y. Hirota et al. Chem. Lett. 43, (2014) pp. 360-362.

Nanodomain structures in ferroelectrics are attractive due to their applications in ultra-small electric, optical, actuator and memory devices [1-5]. Recently, atomic-resolution in-situ Transmission Electron Microscopy (TEM) has revealed numerous novel ferroelectric nanodomain structures such as the flux-closure array, the self-similar nested bundles and strongly charged domain walls, all of which exhibit extraordinary properties [1,6,7]. In this work we utilized an in-situ mechanical system in TEM to study the nanodomain structures in the free-standing BaTiO₃ nanostructures under both stress and electrical bias loading. Using a high speed direct electron detector on the aberration-corrected TEM, we were able to perform scanning nanobeam diffraction experiments during loading that revealed the domain evolution leading to superelastic deformation in the nanostructures. Here we will present our in situ observations coupled with the nanobeam strain mapping and high resolution investigation of the nanodomains formed during superelastic deformation BaTiO₃ nanostructures.

ACKNOWLEDGEMENTS
This work has been supported by the Natural Science Foundation of Jiangsu Province,China (Grant No.BK20151382) , and the Molecular Foundry, which is supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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The real-time observation of deformation of a nano junction under mechanical stress provides us fresh experimental knowledge toward understanding the mechanisms and tribology at the nanoscale [1]. A monolithic silicon MEMS (microelectromechanical system) was placed and operated in a TEM; this allowed us to measure the deformation and the size of 1-100 nm during tensile and shear testing. Also normal and shear forces were determined from the difference in the displacement of the supporting beam with and without the nano junction.
We prepared tips of bare Si and those coated with Ag, and Au films. Those tips were suspended by flexible beams and driven by electrostatic actuators. The longitudinal actuator brought the tips into contact and formed a junction. Tensile testing was performed by reducing the actuation voltage. An Au junction did not elongate much but a Si junction showed super plastic deformation as long as 2000 %. Molecular dynamic simulation based on a single-crystal/amorphous two-phase model could reproduce the behavior of the Si junction [2].
In the shear deformation testing of Si and Ag junctions, the lateral actuator applied shear force to the junction, whose size was typically 2-5 nm in diameter and 3-10 nm in length. The silicon junction elongated smoothly for 15 nm before breakage. The maximum shear force was 78 nN. Simulation based on molecular dynamics reproduced the behavior accurately [3]. On the other hand, a silver junction elongated for only 5 nm and was broken at 5 nN. Elongation was step-wise like a stick-slip motion. The size of steps (0.3 nm and 0.6 nm) corresponds well to the theoretical sliding distance of 0.29 nm calculated from the lattice spacing along the sliding plane. The mechanical work necessary to break the junction agreed well with the surface energy of newly created surfaces after breakage [4]. Whole shear testing processes of another Ag junction was in-situ observed by TEM from formation, and deformation to breakage with the measurement of normal and shear forces. The adhesion force was the major component in normal force; this results in an apparent negative friction coefficient. Although the contact area changed during the deformation process, the normal force and the shear force did not change much. Therefore, the usual assumption that the normal and shear forces are proportional to the real area of contact does not hold in the single nano junction.

References:
[1] T. Sato, L. Jalabert, H. Fujita, Microelectronic Engineering, vol.112, pp. 269–272, 2013
[2] T. Ishida, F. Cleri, K. Kakushima, M. Mit, T. Sato, M. Miyata, N. Itamura, J. Endo, H. Toshiyoshi, N. Sasaki, D. Collard and H. Fujita, Nanotechnology, 22 355704 (2011).
[3] Tadashi Ishida, Takaaki Sato, Masatsugu Oguma, Takahumi Ishikawa, Noriaki Itamura, Keisuke Goda, Naruo Sasaki and Hiroyuki Fujita, Nano Letters, vol.15, pp. 1476-1480 (2014)
[4] Takaaki Sato, Tadashi Ishida, Laurent Jalabert, Hiroyuki Fujita, Nanotechnology, Vol.23, p.505701, (2012).

Freestanding, amorphous TiAl (45 at.% Ti) thin films (150 nm thick) were subjected to in-situ TEM tensile straining using MEMS based testing stages. Micro strain along the longitudinal and transverse direction was calculated at different stages of loading by measuring geometric changes in the first ring of the selected area diffraction pattern. Simultaneously, the macroscopic stress-strain (σ-ε) response of the film was measured using in-built force and displacement gauges in the MEMS stage.
The micro strain along the longitudinal direction (e₁₁) followed a trend similar to that of the macroscopic strain (ε), but was consistently smaller. Thus, the Young’s modulus calculated using e₁₁ was significantly larger compared to the modulus obtained from the macroscopic σ-ε measurements. To investigate whether this deviation was a result of anelastic strain, which is not captured by the micro strain measurements, we carried out ex-situ deformation experiments at different strain rates. We found that the Young’s modulus was 7% higher when the film was loaded at a strain rate of 10^-2 sec^-1 compared 10^-6 sec^-1, revealing notable anelasticity in these metallic glass films.
In addition, we measured the full width at half maximum (FWHM) of peaks along all directions of the first diffraction ring during straining. The measurements showed that the averaged FWHM, which provides a measure of the variation in the nearest neighbor distances, decreased with increasing stress and did not recover its initial value upon unloading. This suggests that permanent atomic scale structural rearrangements are induced in the metallic glass films due to straining.
This in situ TEM technique gives us the unique capability to measure micro strains from extremely small regions (<1 μm in diameter) of thin film specimens, which is not possible using x-ray or neutron diffraction techniques which are typically used to probe bulk metallic glass specimens.

An understanding of how nanoscale defects nucleate, move within grains, and propagate through multiple grains at high strain rates (up to 10^4/s) in metals is needed in order to explain and predict dynamic behavior and spall strength. To enable in situ observation of dislocations and twins in a TEM at high strain rates, several challenges have been addressed. The time resolution of conventional TEM is usually limited to around 30 frames per second. To achieve the needed time resolution for high strain rates, we use the Dynamic TEM at the Lawrence Livermore National Laboratory which is able to record pictures every 70ns in movie mode. TEM stages for in situ mechanical testing are generally limited to quasi-static strain rates. We have designed a TEM holder capable of deforming samples at strain rates ranging from quasi-static to 10^4/s that utilizes two piezoelectric actuators working in bending to load samples. This system is calibrated and instrumented with strain gages to provide a time-resolved record of the net force and strain applied to the sample. Because of the piezo system’s limitations in terms of force, very small samples are required to achieve the desired strain rate. We have developed a new sample preparation procedure that combines mechanical polishing, femtosecond laser machining, and precision ion milling to form, from bulk samples, 300-µm-wide rectangular specimens with a 25-µm-wide gauge region where the electron transparent area is obtained by ion milling. We will present the latest results of high-strain-rate in situ mechanical tests conducted on copper and magnesium alloys specimens.

This talk will highlight recent advances with in situ Transmission Electron Microscopy (TEM) nanomechanical testing techniques that provide insight into small-scale plasticity and the evolution of defect structures in lightweight alloys and oxide nanostructures. In addition to measuring the strength of small-volumes, measuring the evolution of strain during plastic deformation is of great importance for correlating the defect structure with material properties. Here we demonstrate that strain mapping can be carried out during in-situ deformation in a TEM with the precision of a few nanometers without stopping the experiment. Our method of local strain mapping consists of recording large multidimensional data sets of nanodiffraction patterns during the test. This dataset can then be reconstructed to form a time-dependent local strain-map with sufficient resolution to measure the transient strains occurring around individual moving dislocations.

The development of nano-devices has aroused intensive investigation in the interfacial interaction (adhesion and friction) of two-body-contact at nano-scale. Here, via the joint results of in-situ high resolution transmission electron microscopy (HRTEM), we observe the nucleation and subsequent annihilation of “open stacking fault tetrahedron” (open-SFT), with only three of its four planes covered by staking faults, by deforming the single nano-contact between gold crystals. The direct visualization of dislocation behavior offers novel insights in the nano-scale tribological study.

Previous studies have shown that metal films with similar thickness and grain size but dissimilar texture show significant differences in their mechanical behavior. For instance, ultrafine-grained (UFG) Al films with no preferred texture show lower flow stress and more pronounced nonlinear behavior during unloading compared to films with a bicrystalline microstructure.
To understand the mechanisms of such texture-induced differences in mechanical behavior we performed quasi-static in situ TEM straining of non-textured and bicrystalline UFG Al films with automated crystal orientation mapping (ACOM). The ACOM results show that significant grain rotations, up to ~6, occur in the non-textured films even at small strains (~0.5%) during loading, whereas the grains in the bicrystalline film exhibited significantly smaller rotations. Furthermore, reverse rotation of the grains occurred upon unloading in the non-textured film, which provides a possible reason for the observed nonlinearity in the stress-strain response. Bright field TEM imaging showed that grain contrast changes (indicative of grain rotations) were time dependent, which suggests that diffusive processes could be active in addition to dislocation slip.
The results show that the combination of ACOM and in situ TEM straining can provide a more detailed picture of the complex deformation processes occurring in UFG and nanocrystalline metals.

In situ transmission electron microscopy (TEM) can provide valuable insights into the deformation behavior of nanostructured materials. However it is critical to understand and quantify the effects of the electron beam (e-beam) exposure on the deformation response of such materials. In this study, we investigated the effects of the e-beam on the stress-strain response of nanocrystalline and ultrafine-grained aluminum and gold thin films during in-situ tensile straining. The e-beam accelerating voltage, area and intensity were systematically varied to study the nature and extent of beam-induced artifacts at different beam conditions.

We found that e-beam exposure caused increased dislocation activation and marked stress relaxation in the Al and Au films that spanned a range of thicknesses (80-400 nm) and grain sizes (50-220 nm). The e-beam also caused an unusual necking along the width of the sample, with the extent of necking increasing with the area of the specimen exposed to the beam. Notably, these effects were observed at accelerating voltages well below the radiation damage threshold of these materials. And contrary to expectation, the beam-induced artifacts were more pronounced at lower accelerating voltages.

The experiments, performed on two metals with highly dissimilar atomic weights and properties, suggest that the e-beam can cause significant changes in the deformation behavior of a range of nanostructured materials during in situ TEM straining.

In situ indentation in a high-resolution transmission electron microscope (HRTEM) is used to observe dislocation nucleation and glide in single crystalline TiN thin films. The structural images acquired under load are used to measure lattice strains and the corresponding local stresses are inferred from first-principles computed non-linear elastic stress-strain response. This experimental approach is shown to estimate local resolved shear stresses corresponding to partial or full dislocation nucleation and motion of full dislocations in high strength materials, and validate the first-principles calculated Peierls stresses.

Heterogeneous catalysts often undergo dramatic changes in their structure as the mediate a chemical reaction. Multiple experimental approaches have been developed to understand these changes, but each has its particular limitations. Electron microscopy can provide analytical characterization with exquisite spatial resolution, but generally requires that the sample be imaged both ex situ and ex post facto. Photon probes have superior depth penetration and thus can be used to characterize samples in operando (i.e when they are actively working). But they generally lack spatial resolution and thus give only ensemble average information.

We have taken advantage of the recent developments in closed-cell microscopy methods to develop an approach that allows us to successfully combine electron, x-ray and optical probes to characterize supported nanoparticle catalysts in operando. By measuring the reaction products at each stage of the reaction, we can directly correlate the information that can be obtained from each approach, and thus gain a deep insight into the structural dynamics of the system.

In this work, we will show how a combination of x-ray absorption near edge (XANES) and scanning transmission electron microscopy (STEM) can be used to characterize the changes that occur in a model NiPt bimetallic catalyst during oxidation and reduction. Bimetallics are of broad interest in heterogeneous catalysis as the provide the opportunity to selectively tune reactivity and selectivity. However, the characterization of their structure by averaged probes such as x-ray absorption spectroscopy is comprised by the heterogeneity that such systems may proscribe.

The presentation will focus on the development and application of experimental methods used to describe the morphological changes that occur in this model bimetallic system. These will include high temperature atmospheric pressure electron microscopy, the direct measurement of reaction products using gas chromatography–mass spectrometry and the ability of a newly developed electron microscope for operando microscopy (based on the FEI Talos platform) to characterize bimetallic nanoparticles through energy dispersive x-ray spectroscopy.

Isolated single metal atoms dispersed on high-surface-area supports have recently demonstrated remarkable activity and selectivity for a plethora of catalytic reactions [1-4]. The interaction of the individual metal atoms with the support surface modifies the surface electronic structure of the metal-support ensembles and thus tunes the binding strength of the reactant molecules. Such an approach to engineering the surface electronic structure of high-surface-area support materials can be effectively utilized for developing new and better catalysts with broad applications in chemical transformations, energy and environment. Aberration-corrected scanning transmission electron microscopy (AC-STEM) techniques have proved to be critical in developing single atom catalysts (SACs) [1-4]. With subangstrom electron probe sizes, enhanced probe current, and efficient annular dark-field detectors the AC-STEM becomes a powerful tool for routinely determining the spatial dispersion of metal single atoms, their relative locations with respect to the positions of the surface atoms of crystalline supports, and, to a certain degree, their relative strength of binding to the surfaces of the support materials. It is hypothesized that metal single atoms located at the cation vacancy sites of the support material should be relatively stable even under electron beam irradiation while small metal clusters may be extremely unstable, a consequence of weak anchoring. The challenges in fully understanding the nature of supported metal single atoms include the electron transfer processes and the vertical location of the individual metal atoms with respect to the surface atoms of the support materials. Imaging of single atoms of metal (e.g., Au, Pt, Ir, Pd, etc.) on various types of supports (e.g., Fe₂O₃/Fe₃O₄, ZnO, NiO, Co₃O₄, graphene, etc.) and the catalytic performances of the corresponding SACs will be discussed [5].

References
[1] B. Qiao et al., Nat. Chem., 2011, 3, pp 634–641.
[2] J. Lin et al., J. Am. Chem. Soc., 2013, 135, pp 15314–15317.
[3] H. Wei et al., Nat. Commun., 2014, 5, Article # 5634.
[4] B. Qiao et al., ACS Catal., 2015, 5, pp 6249–6254.
[5] This work was supported by the start-up fund of the College of Liberal Arts and Sciences of Arizona State University and the National Science Foundation under CHE-1465057. The author acknowledges the use of facilities in the John M. Cowley Center for High Resolution Electron Microscopy at Arizona State University.

Linking catalyst structure with activity is a primary goal of much catalysis research. Observation of the catalyst structure at the atomic scale using environmental TEM (ETEM) while catalysis is taking place is a powerful technique for linking activity with structure. To do this well, it is essential to know the activity of the catalyst while it is being observed. The simultaneous observation of the catalytic activity and atomic structure of a catalyst during catalysis in the microscope is known as operando TEM.
Operando TEM can be accomplished by monitoring the gas composition using the complimentary techniques of mass spectrometry and electron energy loss spectroscopy. For CO oxidation over Ru, this capability has been previously demonstrated, but details of the atomic structure of the nanoparticle surfaces could not be obtained without the use of aberration corrected microscopes. Now, with an image-corrected Titan ETEM, it is possible to directly observe the surface structures present on Ru catalyst nanoparticles.
Surface layers were observed on some particles exposed to 2 Torr of a stoichiometric ratio of CO and O₂ at 200°C. Similar surface layers were observed in mixtures of H₂ and O₂ and have been attributed to oxidation of the Ru nanoparticle surfaces. While the surface layers are clearly visible in the image corrected TEM, the exact structure of the surface layers could not be directly interpreted without image simulation [1].
Though Ru for CO oxidation is a well-studied system, there is still debate in the literature regarding the most active state of the catalyst surface, though the general consensus is that some oxidation of the surface occurs. Several oxidized Ru surface structures have been proposed for the (0001) surface of Ru. In the present work, image simulations have been performed using models of these proposed structures. We will present results comparing the simulated images of these and other possible structures to operando images of Ru nanoparticle surfaces obtained during high catalytic activity, to determine the atomic structure of the active surface observed on Ru catalyst nanoparticles in the aberration corrected ETEM.
[1] N.P. Walker, B.K. Miller, & P.A. Crozier, Materials Research Society. (these proceedings)

Iron oxide catalysts were widely used for industrial water-gas-shift reaction, ethylbenzene dehydrogenation, etc. Their catalytic performances strongly depend on their shape and surface structure which may dynamically evolve during the catalyst activation or the catalytic reaction processes. Aberration-corrected environmental transmission electron microscopy (AC-ETEM) and related atomic scale imaging and analytical technqiues enable the investigation of dynamic structural changes of nanostructured catalysts with atomic scale resolution under close to working conditions. In order to understand the evolution of the surface atomic structures of the Fe₂O₃ nanostructured catalysts during the reductive activation process we specifically synthesized α-Fe₂O₃ nanoplates with predominantly {001} facets. The AC-ETEM data clearly revealed the nucleation and growth processes of α-Fe₂O₃ to γ-Fe₂O₃ and finally to Fe₃O₄. During the H₂ reduction process (2 Torr. H₂ at 300°C) the shape of the original α-Fe₂O₃ nanoplates was maintained during the phase transformations. Furthermore, initial atomic resolution images unambigously confirmed that the phase transformations initiate at the corners and atomic steps. Although we have not been able to directly observe the hydrogen atoms these experimetnal data suggests that H₂ preferentially adsorbed and dissociated at the coner and step sites of the α-Fe₂O₃ nanoplates. Such understanding of the detailed atomic scale structures of nanostrcutred catalysts during the activation and catalytic reaction processes signifciantly enhances our understanding of the nature of working catalysts.

We have developed _s correctors for both TEM imaging and STEM probe is retained, and in some aspects enhanced. The ESTEM system is fully compatible with improved high vacuum STEM and TEM, especially for hot stage studies (5) and with window specimen holders for higher gas pressures (6) or liquids (7). This is a variant of the established and widely adopted design of ECELL system [3] with regular microscope aperture disks mounted inside lens polepieces to separate the column into a series of differential pumping zones at gas pressures in the range from mbar to Pa at the specimen to 10^-10 mbar in the gun.
Initial experiments with the system have explored the presence and dynamics of single atoms, clusters and nascent nanoparticles on the support between larger features. The reliable detection of single atoms of Pt on a 4nm model system and on more practical carbon supports extends the scope for effective studies to include single atom tracking. Work has begun to revise existing theories of nanoparticle stability under reaction conditions with the new information reliably accessed here for the first time, extending the single atom knowledge from STM studies [8] into a wider context.
References
1. E D Boyes, M R Ward, L Lari and P L Gai, Ann Phys (Berlin), 6 (2013) 423
2. P L Gai, L Lari, M R Ward and E D Boyes, Chem. Phys. Letts. 592 (2014) 335
3. E D Boyes and P L Gai, Ultramicrosc. 67 (1997) 219
4. P L Gai et al MRS Bulletin, 32 (2007) 1044
5. J Sagar et al, Appl. Phys. Letts. 105 (2014) 032401
6. J Creemer et al, Ultramicrosc. 108 (2008) 993
7. R Kroger, Nature Materials, 14 (2015) 369
8. P Jiang, X Bao and M Salmeron, Acc. Chem. Res. 48 (2015) 1524
The Engineering and Physical Sciences Research Council (UK) is thanked for supporting the program with the strategic research grant EP/J018058/1.

In situ environmental transmission electron microscopy (E-TEM) has recently advanced with technological developments such as aberration correction of lenses, fast detection cameras, and miniaturized specimen containers with various functions. These advances have enabled observation of a variety of phenomena in materials and devices at higher spatial and temporal resolution especially in gases. It is now possible to investigate the essential static and dynamic characteristics of materials and devices by quantitative in situ E-TEM at the atomic scale [1].

We briefly summarize our recent in situ E-TEM studies of the catalysts of Au/CeO₂, Au/TiO₂, Pt/CeO₂ and others using Cs-corrected Titan ETEM G2. It is well-known that gold, the most stable metallic element, shows remarkable catalytic activity for CO oxidation even below room temperature. Gold nanoparticulate catalysts, prepared using the deposition precipitation method exhibited high catalytic activity at room temperature. Systematic acquisition along with both numerical and statistical analyses of the E-TEM imaging data led to the intrinsic catalyst structure in the reaction environments. The quantitative analyses [1] further indicated that the activation sites of oxygen molecules at room temperature are most likely to be at the perimeter interface between gold nanoparticles and metal oxide supports. During the reaction, the perimeter interface is not structurally rigid. Glimpse of gas molecules that interact with the surface of a gold nanoparticle is now possible with Cs corrected E-TEM. We will present in situ Cs-corrected E-TEM analyses of other gold catalysts.

We have observed the oxidation and reduction processes of the surface of Pt nanoparticles by Cs-corrected E-TEM. Atomic layers of Pt oxides, including α-PtO₂ and Pt oxides of other forms, started forming on the preferential facets of Pt nanoparticles at the early stage, entire oxidization on the whole surface of Pt nanoparticles then followed. The oxides were reduced promptly to Pt by adding a small amount of CO or H₂O vapor to the dominant O₂ gas. Electron irradiation during E-TEM observation activates the gases non-thermally, therefore promoting or suppressing the processes at room temperature [2]. We will present the effect of moisture on other catalysts.

It is now realized that a crucial era of in situ E-TEM has started. For quantitative in situ TEM of time-dependent phenomena, for instance dynamic atomic motions in a chemical reaction, quantitative evaluation and removal of the electron-irradiation-induced phenomena that may appear in the background in in situ E-TEM data is required. We think that the robust E-TEM apparatus combined with quantitative methodologies is definitely a necessary condition for the serious applications of in-situ E-TEM in materials and devices.

References.

[1] S. Takeda, Y. Kuwauchi, H. Yoshida, Ultramicroscopy, 151 (2015) 178.

[2] H. Yoshida, H. Omote, S. Takeda, Nanoscale, 6 (2014) 13113.

Photocatalytic water splitting has the potential of producing sustainable clean energy by converting and storing solar energy into H₂ fuel. Photocatalytic materials generally consist of semiconductors with bandgaps larger than 1.8 eV and metals that can be catalytically active to perform water oxidation/reduction reactions. It is now recognized that atomic level in-situ observations of these catalysts are critical for understanding structure-reactivity relationships and deactivation processes such as photocorrosion. This requires that the system be observed not only in presence of reactant and product species but also during in-situ light illumination. Here concerns and applications associated with building a “photo-reactor” inside an aberration-corrected ETEM are discussed. An optical fiber based in situ illumination system has been developed previously for an FEI Tecnai F20 ETEM with light intensity up to 10 suns [1]. Using this system, we have observed detailed structural evolution on TiO₂ photocatalysts during exposure to in situ light and gas environments [2]. Recently we have installed an optical fiber into an FEI Titan environmental transmission electron microscope with an image corrector providing sub-Angstrom resolution. The optical fiber is guided by a fiber holder using the objective aperture port. We will discuss the design and performance on applications related to solar water splitting. NaTaO₃ which is a highly active photocatalyst with a large bandgap semiconductor will be tested as a model material to evaluate the system. It is believed that with the capability of in-situ illumination and superior resolution, deeper understanding of reaction and deactivation mechanisms of various photocatalyst systems will be gained.

[1] B.K. Miller, P.A. Crozier, Microsc. Microanal. 19 (2013) 461–469
[2] L. Zhang, B.K. Miller, P.A. Crozier, Nano Lett. 13 (2) (2013) 679–684
[3] The support from US Department of Energy (DE-SC0004954) and the use of Titan microscope at John M. Cowley Center for High Resolution Microscopy at Arizona State University is gratefully acknowledged.

To move forward with creating novel nano-electronic devices there is a need to understand behavior of electrons in a wide range of materials whith dimensions of the nanometer scale. This is a common task for the industry dealing with electronic or magnetic memories, light-emitting, photovoltaic or multiferroic devices. The important role for development and production of such devices is the characterization of the local magnetic fields. Such characterization is also of high importance for biomedical applications like hyperthermia treatment or local drug delivery, paleomagnetism, environmental magnetism or biomagnetism.
The best tool for the local magnetic fields characterization (in terms of resolution and sensitivity) is the Transmission Electron Microscopy (TEM). There are several methods within TEM which have been developed and successfully used for visualization and quantification of the nanoscale magnetic fields: Lorenz microscopy, Electron Holography and Differential Phase Contrast (DPC). All these methods have their intrinsic constraints, like limited field of view or low resolution, and all of them depend on the presence of equipment additional to a conventional TEM (Lorentz lens, biprism, segmented detector).
The DPC method is the most useful for multiscale imaging, i.e. fast switching between studding of objects of about 10µm down to a few nm in size. However the conventional DPC requires specially designed position sensitive detector(s) and costly hardware solutions.
Here we report a simple generalization of the DPC imaging method to emidiatly extend the capabilities of the majority of existing TEM systems (without modifications) towards multiscale characterization of local magnetic properties of nanomaterials. Our method implies a usage of a unitary (non-segmented) virtual bright field detector in combination with a modified differential phase contrast approach.
The suggested method demonstrates high sensitivity to the local magnetic fields, provides a very large field of view, a few nanometers spatial resolution and in-focus condition. It also, in principle, allows direct quantification of nanomaterials magnetic fields.
The usability of our method both at micro- and nano- scale is tested on the investigation of 2 materials: a) cylindrical Co/Ni nanowires with a high aspect (length to diameter) ratio; b) ordered arrays of Co/Ni nanowires – promising candidates for 3D magnetic memory devices.

Recent developments in in situ liquid cell Transmission Electron Microscopy (TEM) techniques enable direct investigation of nano-systems in relevant environments. By encapsulating the liquid between two electron transparent silicon nitride membranes the sample can be introduced into the TEM column without compromising vacuum. This allows for dynamic nanoscale phenomena to be directly observed in situ at high spatial and temporal resolution under controlled electron dose conditions allowing for imaging and chemical analysis.

In this work we use magnetite (Fe₃O₄) nanoparticles as a platform and alter surface chemistry to systematically study their behavior and the effects of the electron beam in situ. Iron oxides are ubiquitous in natural systems, from biomineralization to the terrestrial carbon cycle, and serve as a platform for a range of engineered applications, for example, Magnetic Particle Imaging (MPI). Interactions between iron oxides, solvents, minerals, small molecules, and tissues are fundamental to these diverse systems. Magnetite nanoparticles are synthesized by thermal decomposition of Fe³⁺oleate, and are single crystalline, monodisperse, and phase-pure to ensure uniform physio-chemical and magnetic properties. As-synthesized particles are terminated with oleic acid and soluble in organic solvents, and are transferred to aqueous phase by coating with an ambiphillic co-polymer then functionalized with positive and negative charged species.

Here we demonstrate application of the in situ liquid (S)TEM cell to study behavior of magnetite nanoparticles with different surface chemistries in organic and aqueous solutions. We show that magnetite in the presence of organic solvent (1-octadecene) is stable up to relatively large electron beam doses (>50 e-/Ųs). However, the same magnetite nanoparticles undergo a dissolution process in the presence of water even under low dose conditions of <1 e-/Ųs. The dissolution process can be tuned, as it is proportional to the total e-beam dose delivered to the sample during the experiment, and dependent on surface chemistry of the functionalized magnetite nanoparticles. We show that nanoparticles with charged functional groups interact with reactive species in solution, accumulating ions at the particle surface, slowing dissolution, and enhancing particle interaction and agglomeration. In this presentation we discuss how to mitigate and utilize the reductive effects of the electron beam, both in the case of magnetite and more broadly for other iron oxide/hydroxide phases, beam sensitive oxide materials, and in general for in situ TEM experiments.

This work was supported by NSF-1334012, NIH 1R01EB013689-01/NIBIB, 1R41EB013520-01, 1R42EB013520-01, and the CII LDRD at PNNL. PNNL is a multi-program national laboratory operated by Battelle for the U.S. DOE under Contract DE-AC05-76RL01830. A portion of the research was performed at the EMSL user facility sponsored by DOE-BER and located at PNNL.

With the advent of touch-screen displays and portable handheld electronics, the need for highly durable scratch-resistant glass has risen. It is known that under small contacts, silicate glasses undergo a ductile to brittle transition with increasing load. Surprisingly, this occurs even under self-similar indenters where the stress distribution is nominally constant with load, despite the fact that the typical size effect scaling factors seen in crystals (e.g. geometrically necessary dislocations) are not present. In this study, we investigated the brittle to ductile transition with indentation size in calcium aluminosilicate glasses having tectosilicate compositions as a function of silica content using nanoindentation and microhardness tests. At low loads, glasses deformed plastically with no evidence of cracking, and the hardness varied non-monotonically due to a transition from shear deformation to densification with increasing silica content. At higher loads, cracks formed and hardness dropped, resulting in monotonically increasing hardness with silica content. Variations in the brittle to ductile transition with load and composition are explained in terms of plastic deformation mechanisms and energy dissipation due to plastic deformation and cracking. Implications for surface damage due to small contacts in both glasses and crystalline materials are discussed.

The mechanical response of glassy materials is important in numerous technological and natural processes, yet the link between the embryonic stages of plastic deformation and macroscopic mechanical failure remains elusive. The incipient inelastic rearrangements are believed to be highly cooperative and characterized by a scaling of yield strength and elastic constants. When driven beyond yield, many amorphous solids exhibit concentrated regions of large plastic strain. These regions – referred to as shear bands – are the result of the cooperative rearrangements of particles known as shear transformations (STs). STs are believed to be dilatory, resulting in an increase of free volume and local softening that eventually leads to concentrated plasticity. However, the complex spatio-temporal evolution of STs into a mature shear band remains poorly understood. Whether these characteristics of deformation transcend the nature of bonding is still an open question.

Here, experiments will be shown for both submicron scale metallic glass nanowires and disordered colloidal micropillars. We first show that the plastic behavior of metallic glasses is influenced strongly by the structural state of the glass, which can be modified by treatments such as ion irradiation and structural relaxation annealing well beyond the reach of traditional thermal treatments. We then compare these atomically disordered solids with colloidal glasses to elucidate the particle-level deformation responsible for plasticity by performing in situ compression experiments on amorphous colloidal micropillars with free surfaces. The micropillars, which are comprised of fluorescent ~3 µm PMMA particles, are imaged in 3D using laser scanning confocal microscopy, where the positions of ~50,000 individual particles are tracked during the duration of a compression experiment. Particle-level position information allows us to quantify the spatio-temporal evolution of structure with microscopic strain. We examine correlations between local deformation quantities and the evolution of mesoscopic shear bands. Finally, commonalities between our colloidal pillars, metallic glasses, and nanocrystalline alloys will be discussed.

Metallic glasses represent a unique class of structural materials offering a host of desirable properties including high strength, a large elastic limit, and excellent corrosion resistance. In bulk, they suffer from catastrophic failure upon mechanical loads; ductility emerges in metallic glasses with nanoscale dimensions. We investigate the mechanical behavior and atomic level microstructure of glassy Zr-Ni-Al nanopillars with widths of 75-215 nm. The nanopillars are fabricated by focused-ion beam milling of sputter deposited Zr-Ni-Al films. Glassy samples in two different energy states are investigated: (1) as-sputtered (2) annealed. In-situ tensile experiments conducted inside a scanning electron microscope reveal extreme ductility in metallic glass nanopillars, reaching >10% engineering plastic strains, >150% true plastic strains, and necking down to a point during tensile straining. We found the extent of ductility to depend on nanopillar size and annealing conditions. Using molecular dynamics simulations, transmission electron microscopy, and synchrotron x-ray diffraction, we explain the observed mechanical behavior through changes in free volume and short-range order. The extreme tensile ductility observed in these sputtered nano-samples is significantly larger than any previous reports, illustrating the key role that the presence and distribution of free volume plays in enhancing ductility without a significant sacrifice in strength.

This study employs computational fracture mechanics methodologies to evaluate microstructural toughening effects in epoxy reinforced with highly ordered nanoplatelets (achieved through magnetic field application during processing) to invoke crack redirection in addition to toughening. Despite a large amount of scientific study, much work remains to be done to understand and quantify nano-scale toughening mechanisms. Herein, simulations of active fracture mechanisms on the micron-scale identify and quantify various toughening or weakening effects due to nanoplatelet inclusions. This study develops successful strategies for determining critical strain energy release rates (G_C) from simulation and examines some of the challenges involved therein. Simulation (and experimentation) on the micron scale of fracture is critical to understanding different micron-scale toughening mechanisms and their relative contributions, but quantitative correlation to continuum level properties is deceptively challenging. The current work examines successful strategies for bridging macro-scale and micron-scale simulation, such as replacing artificial damping with dynamic simulation of inertial effects to maintain computational and physical fidelity. Methodologies are validated with experimental data at the macro-scale. Validated methodologies are applied to fracture simulations of highly ordered nanoplatelet inclusions in an epoxy matrix, and preliminary results quantifying the potential for toughening and crack redirection are obtained.

Single-asperity wear experiments and simulations have identied different regimes of wear including Eyring- and Archard- like behaviors. A multi-bond dynamics model based on Filippov et al. [Phys. Rev. Lett. 92, 135503 (2004)] captures both qualitatively distinct regimes of single-asperity wear under a unified theoretical framework. In this model, the interfacial bond formation, wearless rupture and transfer of atoms are governed by three competing thermally activated processes. The Eyring regime holds under the conditions of low load and low adhesive forces; few bonds form between the asperity and the surface and wear is a rare and rate-dependent event. As the normal stress increases the Eyring-like behavior of wear rate breaks down. A nearly rate-independent regime holds under high load or high adhesive forces; bonds form readily and the resulting wear is limited by the sliding distance. In restricted regimes of normal load and sliding velocity, the dependence of wear rate on normal load is nearly linear, as described by the Archard equation. Detailed comparisons to two experimental investigations and one set of molecular dynamics simulation data illustrate both regimes and the cross-over between the two described by this unifying theory.

Instrumented indentation technique (IIT) or nanoindentation is a method that is widely used in the study of the mechanical deformation of materials on small length scales. Raman spectroscopy is a technique that provides insight into the molecular or crystallographic level processes involved in the mechanical deformation of materials, such as strain build-up, phase transformations and variations in crystallinity. Typically these approaches have been used separately wherein the spectroscopic analysis of the material might take place prior to and after the end of a mechanical transformation. Of course, there is significant interest in in situ analyses of materials during mechanical transformation as such an approach promises a richer understanding of the underlying physics than is likely possible with analysis limited to pre- and post-transformation. For example, the ability to follow the path of phase transformations rather than just the endpoints is certainly desirable. Consequently, significant effort has been directed toward the coupling of indentation instruments with various in situ analysis capabilities.
This talk describes the design and operation of a nanoindentation instrument that is coupled with a laser scanning Raman microscope to conduct in situ spectroscopic analyses of mechanically deformed regions of materials under contact loading. The capabilities of this novel instrument will be demonstrated by in situ studies of the generation and evolution of strain fields and the resulting mechanical deformation behavior of semiconductors, in particular crystalline and amorphous silicon, in both a microspectroscopy and a laser scanning Raman imaging configuration. The experimental findings will be compared to simulations and contact models relevant to this field of research.

Tapping mode AFM imaging, also known as amplitude-modulated (AM) atomic force microscopy (AFM) is fast, gentle and provides the high spatial resolution necessary for imaging nanoscale features. However, until recently, mechanical characterization with tapping mode was limited to only qualitative results. In AM-FM mode, a bimodal (dual-frequency) technique, the first resonant mode is operated in AM, whereas a higher resonant mode is frequency modulated (FM). AM-FM mode delivers high resolution topographical images, and additionally, it provides quantitative contact stiffness data, from which elastic modulus can be calculated with appropriate models for the tip-sample contact mechanics. One of the remaining challenges is the calibration of the cantilever’s higher mode spring constant and sensitivity. Here, we will present a calibration procedure that delivers higher mode cantilever oscillation amplitude in nanometers, which allows for the exact same experimental settings from one experiment to another. Experimental results on various samples such as metals, alloys and polymers will be presented to demonstrate the applicability of AM-FM mode for materials with a wide range of modulus (MPa-GPa). Furthermore, recent advances in AM-FM imaging will be discussed, such as the use of photothermal excitation to achieve molecular-level resolution on semi-crystalline polymers. With the growing demand for mechanical characterization of materials at the nanoscale, the AM-FM technique provides quantitative nanomechanical information while simultaneously offering all the familiar advantages of tapping mode.

One of the ongoing challenges in the field of piezoresponse force microscopy (PFM) is the accurate quantification of the piezoelectric coefficients. Conventional PFM systems almost exclusively use an optical beam deflection (OBD) system where a laser is focused on the back of the cantilever and the angle of the reflected light is used to deduce the cantilever normal and lateral tip motion. However, non-desirable buckling and torsion of the cantilever may be misinterpreted as cantilever tip motion. This is a shortcoming of the OBD method which measures the angle of the cantilever, rather than the displacement of the tip.
Here, we describe results on highly sensitive PFM imaging and spectroscopic studies of ferroelectrics (LiNbO₃ crystals and Pb(Zr,Ti)O₃ and BaTiO₃ thin films) performed with an interferometric AFM. This AFM is based on a commercial Cypher S AFM and combines the existing OBD system with a separate quantitative interferometric Laser Doppler Vibrometer (LDV) system to enable accurate measurements of the displacement and velocity of the cantilever tip [1]. This combined instrument allows a host of quantitative measurements to be performed including measuring a variety of in-situ PFM cantilever oscillation modes, as well as accurately measuring the cantilever spring constant prior to making contact with the surface. Importantly, the piezoelectric coefficients extracted from several LDV measurements showed an order of magnitude less variability compared to the error-prone OBD measurements acquired simultaneously. By performing simultaneous LDV and OBD measurements, we were able to conclude that most of the measurement error and variability in PFM measurements thus far can be attributed to the shortcoming of the OBD method.
We present a systematic methodology for accurate PFM measurement of d₃₃ and d₁₅ coefficients. In this context, the notable differences between the OBD and LDV results are demonstrated and discussed. Even though the interferometer provides an intrinsically quantitative measurement of the cantilever motion, there are additional requirements for quantification of the tip-sample electromechanical response that prevent cantilever dynamics and stray electrostatic interactions from overwhelming the PFM signal. Further considerations about the effects of finite loading forces that may reduce the apparent piezoelectric sensitivity are also discussed. In addition, quantitative lateral PFM results, determined from sequential LDV measurements at various LDV spot positions, are also presented.
[1] A. Labuda and R. Proksch, Appl. Phys. Lett. 106, 253103 (2015)

Simultaneous topography and mechanical property measurements have been a long-standing goal of AFM, especially obtaining complementary mechanical information during gentle tapping mode imaging. Bimodal force microscopy is a dynamic atomic force microscopy (AFM) method that excites two eigenmodes of a cantilever simultaneously [1]. The additional information provided by a second eigenmode allows the separation of topographic from mechanical properties. Combined with the benefits of operating the cantilever on resonance, bimodal force microscopy enables high-speed quantitative nanomechanical mapping across six orders of magnitude in modulus.
We present a new mathematical framework for the extraction of indentation depth and Young’s modulus from bimodal AFM observables that avoids the use of fractional calculus, Laplace transforms, and Bessel functions, which are used by existing theories [2]. The simplicity of our proposed mathematical framework leads to an intuitive interpretation of bimodal AFM data in the context of Hertzian contact mechanics and is more transparent to the approximations required to reach analytical solutions. The proposed framework can be applied to any combination of amplitude modulation (AM) and frequency modulation (FM) for driving both eigenmodes, as in AM-AM, AM-FM, FM-AM, and FM-FM. The pros and cons of these variations will be discussed with respect to different imaging conditions.
Data acquired with the AM-FM technique, which offers robust and stable imaging while maintaining accurate nanomechanical mapping, will be presented. Furthermore, these AM-FM measurements, ranging from soft polymers to stiff metals, will be compared to bimodal cantilever dynamic simulations that were performed to provide insight into data interpretation and better understanding of the sources of variability. The benefits of using photothermal excitation [3] to drive the cantilever as well as recent efforts in accurately calibrating the stiffness and amplitude of the second eigenmode will be presented, and their impact on AM-FM accuracy will be discussed.

[1] R. Garcia, R. Proksch, European Polymer Journal 49, 1897-1906 (2013)
[2] E.T. Herruzo, R. Garcia, Beilstein J Nanotechnology 3, 198-206 (2012)
[3] A. Labuda et al, Rev. Sci. Instrum. 83, 053702 (2012)

Materials near phase transitions often display enhanced properties, as a result of competition between phases and shallow energy landscapes. As devices are scaled down, the detection of phase transitions is more important for a variety of energy-related materials, although existing methods to detect phase transitions are often not directly scalable to length scales that are relevant for functional materials such as ferroics. Here, we introduce a method utilizing band-excitation piezoresponse force microscopy in order to study the field-induced phase transition in the prototypical relaxor-ferroelectric single crystal, 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3. Results indicate that there is a clear softening of the material (as detected by cantilever resonance shift), as well as enhanced piezoresponse during hysteresis loop acquisition, indicating a change from rhombohedral to tetragonal or monoclinic state. This field-induced phase transition is isolated to areas with radius 100-200nm, possibly due to compositional or stress fluctuations from the unpolished surface, and is directly mapped. The mapping of phase transitions in nanoscale volumes with resolution ~10nm is corroborated by thermodynamic Landau and Phase-field modeling, which confirms the stability of the tetragonal phase under applied field. These studies are directly relevant for many systems with field-induced structural phase transitions.

NBG would like to acknowledge helpful discussions with Prof. Susan Trolier-McKinstry, as well as funding from the US National Science Foundation under grant number DMR-1255379. This research was in part sponsored by the Division of Materials Sciences and Engineering, BES, DOE (RKV, AT, SVK). Research was conducted at the Center for Nanophase Materials Sciences, which also provided support (MBO, SJ) and which is a DOE Office of Science User Facility.

Electrically-assisted deformation (EAD) is an emerging technique to increase formability of metals by applying an electric current through them. Despite its importance in manufacturing applications, there is still an unresolved debate on the fundamental mechanism underlying EAD in between thermal (resistive heating) effects and non-thermal (electroplasticity) effects on dislocation mobility. This is mainly due to two critical challenges in experimental studies of electromechanical behavior in metals: (1) intrinsic coupling effect between an electric current and Joule heating, and (2) a lack of experimental techniques to directly observe atomic scale mechanisms during EAD. Here we present a novel microdevice-based testing method to overcome these challenges by utilizing in situ electromechanical characterization of a nanoscale metal specimen in TEM.
The microdevice consists of a dog-bone shaped metal specimen, a SiO₂-coated Si frame, and silver wires for application of an electric current to the specimen. For ease of handling and electrical isolation, individual specimens with the smallest feature size in the range of 20μm-40μm are fabricated using laser cutting and assembled onto the Si frame using silver epoxy. The Si frame is designed such that it provides sufficiently large mechanical rigidity to prevent any mechanical loading on the specimen during handling and assembly, and fits onto the electromechanical TEM holder for in situ characterization. Before TEM experiments, focused ion beam milling is used to create a nanoscale gauge section in the assembled specimen.
Unlike conventional TEM specimens, e.g., dimpled or wedge-shaped specimens, the metal specimen has a uniform cross-section (100nmx10μm) to prevent geometry-dependent electric current, mechanical loading, and temperature profiles. In addition, owing to the nanoscale cross-section, the effect of temperature due to Joule heating on material deformation can be eliminated. As an example, to achieve 100A/mm², 100μA needs to be applied to the cross-section of 10μmX100nm. Note that the corresponding electric power consumption is in the order of 10^-9 watt, which is unlikely to induce any significant temperature increase. Our numerical simulation confirms that is indeed the case (within 1°C increase) because of direct physical connections between the nanospecimen and the macroscopic heat sinks (the Si frame and TEM holder). To demonstrate the unique capabilities of the in situ electromechanical testing method, we prepared and tested single crystal copper (SCC) specimens. Our in situ uniaxial tension experiments with an applied current density of up to 500A/mm² have revealed that at least for SCC electroplasticity may not play a key role as the application of the electric current does not result in increased dislocation activities such as depinning and movement.

The interface between the adhesive toes of geckos and a substrate consists of an array of regularly sized, densely packed, and elastically coupled nanoscopic contacts. The velocity-dependent friction exhibited by this system hints at a convolution of various material and structural effects. We explore the dynamics of frictional sliding in these materials using models based on arrays of coupled masses driven by external forces that can become pinned and unpinned to a potential energy landscape. The model system is driven at normalized velocities spanning 6 orders of magnitude, and the output of this model captures both the low-V and high-V behavior of the actual gecko materials. We explore modifications to the essential model that incorporate features more representative of the structure and behavior of the natural gecko system. These results have implications in the design of materials with custom frictional properties.

Application of force on biomolecules has been instrumental in understanding biofunctional behaviour from single molecules to complex collections of cells. Current approaches, for example, those based on atomic force microscopy or magnetic or optical tweezers, are powerful but limited in their applicability as integrated biosensors. Here we describe a new force-based biosensing technique based on the quartz crystal microbalance. By applying centrifugal forces to a sample, we show it is possible to repeatedly and non-destructively interrogate its mechanical properties in situand in real time. We employ this platform for the studies of micron-sized particles, viscoelastic monolayers of DNA and particles tethered to the quartz crystal microbalance surface by DNA. Our results indicate that, for certain types of samples on quartz crystal balances, application of centrifugal force both enhances sensitivity and reveals additional mechanical and viscoelastic properties.
Nature Communications 5, Article number: 5284 doi:10.1038/ncomms6284

Expanding capabilities of Atomic Force Microscopy (AFM) provide different means for the examination of elastic, plastic and viscoelastic properties of polymers. Initially, the deflection-versus-distance curves, which were recorded at single sample locations, have been used for the extraction of quantitative elastic modulus and work of adhesion. This approach is now broadly expanded to the real-time nanoscale mapping of these properties with high spatial resolution. The quantitative maps of elastic modulus of semicrystalline polymers and block copolymers are routinely recorded with the 10-20 nm resolution comparable with a size of individual lamellae. Several results of such studies, in which the calculations are based on the common solid state deformation models (Hertz, DMT, JKR, etc), will be presented and discussed. We will introduce the experimental and theoretical studies of viscoelastic response of polymers with AFM. They also include the examination of dynamic behavior at different frequencies and temperatures. We have applied the stress relaxation and phase response in the oscillatory non-resonant mode (Hybrid mode) to examine viscoelastic properties of polymer films of neat poly(vinyl acetate) - PVAC and its blend with polystyrene - PS in the 30-90C range (below and above glass transition of PVAC) and in the frequency range from 1 Hz to 2 kHz. The results are also compared with quantitative maps of dielectric permittivity of these materials, which were obtained in single-pass AFM-based electric mode at similar frequencies and temperatures.

The exceptional mechanical properties of polymer nanocomposites are achieved through intimate mixing of the polymer and inorganic phases, which leads to spatial confinement of the polymer phase. The nature and degree of this confinement varies considerably, from macroscopic constraint in multilayer laminate systems to true nanoscale confinement in polymer nanocomposites. In this study we probe the mechanical and fracture properties of polymers in the extreme limits of molecular confinement, where a stiff inorganic phase confines the polymer chains to dimensions far smaller than their bulk radius of gyration [1]. We show that polymers confined at molecular length scales dissipate energy through a novel, confinement-induced molecular bridging mechanism in which individual confined polymer chains pull out from a nanoporous matrix. This mechanism contrasts with toughening processes in bulk and weakly-confined polymers and describes behavior that cannot be explained by existing entanglement-based theories of polymer deformation and fracture. We support the molecular bridging mechanism with a model that captures the associated nanomechanical processes, including the sliding friction of chain pullout, the deformation and stretching of confined polymer chains, and the eventual backbone scission of polymer molecules under extreme loads. This study provides new insight into the mechanical behavior of polymer chains under nanoscale confinement and suggests potential routes for increasing the cohesive strength of multifunctional nanocomposites, where the traditional bulk toughening mechanisms may be absent.

[1] S. Isaacson, K. Lionti, W. Volksen, T. Magbitang, Y. Matsuda, R. Dauskardt, G. Dubois, Nature Materials, in review (accepted).

We have used atomic force microscopy (AFM) in friction mode and the surface forces apparatus (SFA) to investigate the effects of uniaxial stretching on nanoscale adhesion and friction of glassy polymer substrates. Polymer substrates with a built-in capability for alignment of nanometer-sized objects are of interest for the development, performance, and large-scale production of robust, flexible devices. Controlled adhesion and release or molecular alignment on an anisotropic polymer substrate is expected to depend on its orientation with respect to the interacting objects or molecules. Directionality or chain orientation can be induced in polymer films through uniaxial stretching (“drawing”), which typically modifies materials properties both in the bulk and at the surface of the film. Examples will be shown of the different friction responses of semi-crystalline and amorphous polymers (polyethylene, poly(ethylene-co-norbornene), and polystyrene) along and across the stretching direction, and how this friction response is altered as the strength of adhesion between the sliding surfaces is deliberately changed.

Recent measurements suggest the possibility to exploit ionic liquids (ILs) as smart lubricants for nano-contacts, tuning their tribological and rheological properties by charging the sliding interfaces. Following our earlier theoretical study of charging effects on nanoscale confinement and squeezout of a model IL, we present here molecular dynamics simulations of the frictional and lubrication properties of that model under charging conditions.First we describe the case when two equally charged plates slide while being held together to a confinement distance of a few molecular layers.The shear sliding stress is found to rise as the number of IL layers decreases stepwise. However the shear stress shows, within each given number of layers, only a weak dependence upon the precise value of the normal load, a result in agreement with data extracted from recent experiments.We subsequently describe the case of opposite charging of the sliding plates, and follow the shear stress when the charging is slowly and adiabatically reversed in the course of time, under fixed load. Despite the fixed load, the number and structure of the confined IL layers changes with changing charge, and that in turn drives strong friction variations. The latter involve first of all charging-induced freezing of the IL film, followed by a discharging-induced melting, both made possible by the nanoscale confinement. Another mechanism for charging-induced frictional changes is a shift of the plane of maximum shear from mid-film to the plate-film interface, and viceversa. While these occurrences and results invariably depend upon the parameters of the model IL and upon its specific interaction with the plates, the present study helps identifying a variety of possible behavior, obtained under very simple assumptions, while connecting it to an underlying equilibrium thermodynamics picture.

Mechanical studies at the level of a single asperity – typically tens of nanometers in size – provide a powerful approach to discoveries of fundamental mechanisms that control friction, adhesion and wear. By isolating specific contributions to these phenomena it is becoming possible to develop predictive theories of friction for well-defined contacts and to design tribological interfaces from first principles. While knowledge learned from single asperity studies can be often transferred and utilized in rough micro- and macro-scale contacts, it is also possible that entirely new phenomena emerge when the contact size is decreased to the nanometer regime. In this talk, I will discuss studies of new phenomena and size effects in friction and wear that arise from decreasing the contact size and/or the grain size to the nanometer regime. The results were obtained using molecular dynamics (MD) simulations and atomic force microscopy (AFM) experiments. One example is our wear study of silicon carbide and silicon. By performing scratch experiments with a nanoindenter (with tip radius of curvature in the range of hundreds of nanometers) we found the wear volume of SiC to be smaller than that of Si. This finding is consistent with previously published micro and macroscopic studies of wear of these two materials. It is also consistent with the Archard law, since hardness of SiC (~38 GPa as measured in our experiments) is significantly higher than that of Si (~13 GPa). However, surprisingly we found that when the wear tests are performed with AFM (tip radius of curvature of about 20 nm), the wear volume of SiC becomes larger than that of Si. In all cases contact pressures are large enough to induce plastic deformation. The trend observed in AFM is opposite to that observed in micro and macroscopic studies and it is inconsistent with the fact that SiC is harder than Si. In addition, we find that in our AFM studies the grain size has a limited effect on scratch resistance of SiC, even in the regime where plastic zone is well developed. I will discuss mechanisms underlying these effects, as discovered in our detailed characterization studies and MD simulations. I will also present results of MD simulations on Cu-based alloys, demonstrating further that the effects of contact size can be coupled to the effects of grain size, leading to trends that are in contrast to wear trends reported at larger length scales.

Understanding the adhesion and low-load behavior of contacts between nanoscale bodies is critical for probe-based microscopy, nanomanufacturing, and other applications whose function or accuracy is determined by nanocontacts. However, many of the assumptions of continuum contact models are violated at these length scales. Therefore, contact properties such as the adhesion force between the surfaces, deformation of the near-surface material, and “true” size of the contact region are difficult to predict and control. Here, we investigate these properties, as a function of applied load, using molecular dynamics simulations of nanoscale bodies composed of silicon and silicon oxide. These simulations are complemented by experimental contact tests performed with in situ transmission electron microscopy, such that the two investigations have matching nanometer-scale dimensions and nanonewton-scale loads. The simulations are validated by direct comparison to measurable quantities in the experiments. Then they are used to provide atomic-scale detail about phenomena occurring within the materials and inside the perimeter of the contact, which are not visible experimentally. Finally, results are analyzed in terms of competing mechanics models to describe nanoscale contact, with the goal of providing mechanistic information and predictive insight into the fundamentals of small-scale contact.

Understanding adhesion forces in dry contact is of particular importance for the study of both technical and biological micro- and nanoelectromechanical systems. Even in their simplest manifestation -- the pull-off force necessary to break the contact between a nano-scale indenter and a rough surface -- they are poorly understood. We here use a boundary element method in combination with an empirical interaction potential the contact of a stiff spherical indenter of varying size acting on an elastic rough substrate. The model is compared to a series of atomic-force microscopy (AFM) pull-off measurements performed with silicon tips with tip radii varying between 14 nm and 100 nm on an ultrananocrystalline diamond (UNCD) substrates. Without any fitting parameters, we find good agreement between the experiment and our simulations. We use our simulations to analyze the link between distribution of pull-off forces and statistics of surface roughness of the substrate. This enables in particular extraction of small scale features of the rough topography not accessible by standard AFM measurements, such as the root mean square slope of surface roughness.

Adhesive contact of a rigid flat surface with an elastic substrate having Weierstrass surface profile is numerically analyzed using the finite element method. In this work, we investigate the relationship between load and contact area spanning the limits of non-adhesive normal contact to sticky contact for various substrate material properties, adhesive strength and roughness parameters. In the limit of non-adhesive normal contact, our results are consistent with (Ciavarella et al. 2000 Proc. R. Soc. Lond. A 456, 387-405 and Hyun et al. 2004 Phys. Rev. E 70, 026117) - contact area is linear with load and has a fractal character. For the adhesive contact problem, we employ Lennard-Jones type of interaction model with numerical regularization to resolve unstable pull-off. We study the transition from full contact to partial contact and the scaling of pull-off force with non-dimensional parameters to quantify the effect of roughness parameters, substrate material properties as well as adhesive strength and range.

Here we present a powerful and multifunctional dry adhesive film based on ultra-flat vertically aligned carbon nanotubes (VA-CNTs). This unique VA-CNT film has been fabricated by employing multi-step transfer processes, converting ultra-flat bottom ends of vertically aligned CNTs, directly grown on atomically flat silicon wafer substrates, into the top surface of adhesive layer. Unlike as-grown VA-CNTs that have a non-uniform surface, randomly entangled CNT arrays, and weak interface between CNTs and substrates, this ultra-flat VA-CNT film shows an extremely high static friction coefficient, 40 on the polished Si wafer under a small preloading of 0.2N/cm². This static friction coefficient is 20 times higher than a conventional as-grown VA-CNT film and zero adhesion force has been observed with a preloading of 11mN/cm² and maximum 100µm displacement in a piezo scanner. Using this unique structural feature and properties, we also demonstrate effective removal and assembly of nanoparticles into micrometer-scale circular and line patterns by a single brushing of this ultra-flat VA CNTs. This serves as enablers for a multifunctional platform for robot hands, reusable tapes, brushes for cleaning nano/micro scale contaminations, organized assembly of nanoparticles, MEMs and etc.

An in-situ transmission electron microscopy study was conducted at room temperature in order to understand an underlying mechanism on room temperature ductility of TiAl alloys. From in-situ straining transmission electron microscopy experiments, it was revealed that the crack path is different between the TiAl alloys with/without room temperature ductility. The crack in TiAl alloys having room temperature ductility interacted with lamellae by forming bridging ligaments between the two α₂ lamellae and the γ lamellae. In contrast, the cracks in TiAl alloys without room temperature ductility propagated along grain (colony) boundaries showing brittle intergranular fracture. The deformation behavior of γ lamellar of TiAl alloys were also investigated using advanced quantitative in-situ experiments. From the quantitative in-situ TEM experiements of the samples with [0-11] orientation parallel to the tensile direction, it was found that the γ lamellar of TiAl alloys having room temperature ductility was deformed by slip. However, the γ lamellar of TiAl alloys without room temperature ductility was deformed by deformation twin. Also the role of orientation and dimension was investigated using the samples of [11-2] and [00-1] orientations parallel to the tensile direction as well as the samples with 100 ~ 500 nm width. Finally, we proposed the important microstructural factors to have room temperature ductility of TiAl alloys.

Polyethylenimine ethoxylated (PEIE) is coated on the surface of ITO to make high output triboelectric nanogenerator. The dipole interaction between PEIE and ITO surface increases the surface potential of ITO surface and the potential difference between ITO and polymer makes much higher voltage and current. The change of surface potential is measured by ultraviolet photoemission spectroscopy(UPS) and Kelvin probe force microscope(KPFM). The output power density of nanogenerator increases 50% with both increased voltage and current. Using this method, we increase the efficiency of TENG without any change of materials or structure.

Adhering, ploughing and cutting frictions are predicted as the three typical friction regimes while never reported in a singular experiment. In this study, full character friction stages were displayed during nanoscratch by using a homemade nanoscale CeO2 indenter, which was purposely chosen to mimic the CeO2 abrasive in chemical mechanical polishing (CMP). In adhering regime, the copper film mainly undergoes an elastic deformation and the coefficient of friction (COF) decreases quickly with the increasing of applied normal force (Fn). In contrast, the scratch starts to initiate and the COF linearly increases with the elevating of Fn in ploughing regime. In cutting regime, the scratch becomes deeper and wider and the variation of COF generally adheres to the well-known proportional law. To quantitatively describe the variation of COF with Fn during the three character friction regimes, a standard model was proposed based on the Hertz contact theory and the classical friction models. Our findings suggest that an optimal CMP condition should be set within the ploughing region due to the effectively materials removing, tunable COF and relatively gentle scratches.

Tuning the surface properties of a material for low friction and little wear has long been a goal of tribological research for better energy efficiency and higher reliability in almost all engineering systems with moving components. From a materials science point of view, the subsurface microstructure of the material under the sliding contact strongly influences tribological performance, and therefore the ability to control this microstructure is of key importance. However, there is a significant lack of knowledge about the elementary mechanisms of microstructure evolution under tribological load.
To cover different stages of this microstructure evolution, high-purity copper was investigated as a model material after increasing numbers of sliding cycles against a sapphire sphere without lubrication in reciprocating motion. Scanning electron and focused ion beam microscopy were applied to monitor the microstructure changes. Scanning transmission electron microscopy and electron backscatter diffraction results are presented as well. A thin tribologically deformed layer which grew from tens of nanometers to several micrometers with increasing number of cycles was observed in cross-sections in the middle of the wear track and a surface texture was found. By analyzing dislocation structures and local orientation changes in the cross-sectional areas, dislocation activity, the occurrence of a distinct dislocation trace line and the emergence of new subgrain boundaries could be observed at different depths. These results strongly suggest that dislocation nucleation and self-organization are among the key elementary mechanisms for the microstructure evolution under a tribological load. Additionally, the formation of amorphous or nanocrystalline oxidation clusters on the surface at higher cycle numbers clearly introduces a new phase in the contact area. The three distinct elementary processes identified here will be essential for future modelling of the evolution of tribological contacts, which will allow for further tailoring the surface for low friction and little wear.

Graphene is a good candidate as a self-lubricating material due to its chemical inertness, strong mechanical properties and low shear strength on its densely packed and atomically smooth surface. Since it is ultrathin even with multilayers, it can be transferred on to micro-scale devices at the oscillating, rotating and sliding contacts to reduce stiction, friction and wear. By now, many other properties of graphene have drawn attention, including thermal, chemical, optical and mechanical properties. However, little work has been done on the tribological properties of graphene. Furthermore, the counter materials of the friction measurements were restricted by AFM tip materials such as Si and Si3N4. The aim of this study is to investigate the friction properties of single-layer graphene by using AFM and particularly, we focused on the effects of counter materials and environments on the friction properties. The friction measurements were conducted in the air and nitrogen under applied loads of several tens of nN. The counter materials were a glass ball and glass ball coated with hydrogenated or hydrogen-free amorphous carbon film. Also, the friction properties of HOPG were measured for comparison. In a result, the frictional force of graphene was larger than that of HOPG. The friction force of graphene slid against a glass ball was larger than those of graphenes slid against glass balls coated with amorphous carbon films. The effects of composition (i.e., hydrogen) of amorphous carbon films coated on the glass balls and environments (i.e., air and nitrogen) on the friction properties of graphene will be also discussed.

Nanoindentation is a common measurement technique used to measure mechanical properties and plastic deformation events. However, difficulties arise when investigating size effects due to the size of the resulting indent imprints. Recently, a combined imaging technique utilizing atomic force microscopy and scanning electron microscopy has been developed that can overcome any size limitations. With the two imaging techniques combined into one system nano-sized indents can be located with the SEM while the AFM was employed to resolve the actual depth and resulting surface deformation with nanometer resolution. The AFM-SEM technique will be demonstrated through the evolution of slip steps emanating around nanoindents in a single crystal brass. It will be shown how the load-displacement curve corresponds to the resulting deformation, what slip planes are activated, and the number of dislocations will be estimated using the heights of slip steps and pop-in events.

Graphite and its 2D counterpart graphene have exhibited unusual and remarkable tribological properties¹. In particular the ultra low resistance to sliding between contacting lattices when rotated out of commensurability has been reported on a number of systems. Micron sized graphite blocks have been shown to “self retract” when sheared out of their equilibrium position^2-3 also nano sized graphene flakes have been observed to diffuse across an underlying lattice by thermal motion when rotated out of commensurability⁴. We have discovered that surface forces of incommensurately folded graphene sheets can be sufficient to cause large scale interfacial sliding and fracture leading to self assembled folded structures ⁵. These observations are explained by a simple fracture-mechanics inspired model that shows how thermodynamic forces drive formation of graphene-graphene interface in lieu of substrate contact.
Here we demonstrate the incommensurability between the self-assembled graphene sheets by Raman spectroscopy, in conjuncture with AFM and optical microscopy. We suggest that the incommensurability is a necessary condition for the easy sliding between sheets which leads to large scale interfacial sliding.
We show the capability for scalable parallel self-assembly of graphene ribbons on mechanically exfoliated and also by extension, CVD graphene. The use of CVD graphene paves the way for more extensive, large scale investigation of this effect.

1 Lee, C. et al. Frictional Characteristics of Atomically Thin Sheets. Science 328, 76-80, doi:10.1126/science.1184167 (2010).
2 Liu, Z. et al. Observation of Microscale Superlubricity in Graphite. Physical Review Letters 108, doi:205503
10.1103/PhysRevLett.108.205503 (2012).
3 Yang, J. et al. Observation of High-Speed Microscale Superlubricity in Graphite (vol 110, 255504, 2013). Physical Review Letters 111, doi:029902
10.1103/PhysRevLett.111.029902 (2013).
4 Feng, X. F., Kwon, S., Park, J. Y. & Salmeron, M. Superlubric Sliding of Graphene Nanoflakes on Graphene. ACS Nano 7, 1718-1724, doi:10.1021/nn305722d (2013).
5 James Annett & Graham L.W. Cross. Dewetting of Graphene. manuscript submitted (2015).

Generally, wettability of solid materials is affected by the surface roughness. Variation of apparent contact angle caused by roughness has been the subject of controversy [1]. Especially, nanometer-scale surface morphology strongly affects the surface potential, which may make the true contact angle modulate. Here, there is an interesting question how a single carbon nanotube (CNT) gets wet. CNT is described as a rolled-up graphene sheet, so that we can investigate the relationship the potential modulation due to extremely high curvature and wettability, by pursuing this question. It may offer the key to understand the wettability of nanometer-scale engineered surface. With respect to curvature effect of CNTs, it is well known that electronic state strongly depends on curvature especially when diameter is less than 2nm. A theoretical study has predicted that contact angle gets greater as diameter decreases less than 10 nm [3]. Existence of water layer surrounding CNTs with 1nm diameter in humid atmosphere has also reported [3], although CNTs and graphene usually exhibit hydrophobicity. In addition, it has been experimentally confirmed that wettability of CNTs with diameters larger than 20 nm can be described by the macroscale law [4]. In this study, we experimentally evaluate the correlation between wettability and curvature of graphene sheets consisting of isolated CNTs with diameter ranging from 1.4 to 22 nm by using an atomic force microscope (AFM). A single CNT was attached in advance to a tip of cantilevered probe for AFM, by electron beam induced deposition of amorphous carbon in a scanning electron microscope. The tip of CNT was then contacted to liquid surface and the force due to wetting was measured on the balance, based on Wilhelmy balance method. In this method, force applied to a cylindrical solid material drawn into liquid F is defined as F = πdγcosθ, where d, γ and θ are diameter of cylinder, liquid surface tension, and contact angle, respectively. According to this balance equation, the value of Fd/π is equal to cosθ and therefore should be constant on macroscale. Experimental results, however, showed significant deviation when diameter of the CNT is less than 10 nm. In case of diameter ranging from 4.5 to 10 nm, it corresponded to the theoretical prediction [2], suggesting the modulation of surface potential. However, the value of Fd/π measured for CNTs with diameters
[1] K. J. Kubiak et al. Wear 271 (2011) 523-528.
[2] A. V. Neimark, J. Adhes, Sci. Technol. 13, (1999) 1137-1154.
[3] Y. Homma et al. Phys. Rev. Lett. 110, (2013) 157402.
[4] A. H. Barber et al. Phys. Rev. Lett. 92, (2004) 186103.

Carbon nanotubes (CNTs) exhibit very high strength and high modulus (50-150 GPa and 1TPa, respectively) [1]. However, only a few real life applications have been developed over the past decade. Mechanical properties of macro size assemblies of CNT, such as CNT yarn, often demonstrate only a fraction of the strength compared to their individual CNT constituents (1.8 GPa) [2]. It has been found that CNT yarn fails by relative sliding between the CNTs. This is because Van der Waals (VdW) forces between the CNT tubes or bundles are very low compared to the covalent bonding between carbon atoms. Clustering and bundling of CNTs due to such interactions have also been found to reduce effective load transfer between the CNT bundles [3-4].

It has been previously reported that the molten high molecular weight, high density polyethylene (HDPE) molecules can nucleate in the vicinity of the individual CNTs and form disk shaped crystallites when a blend of HDPE/CNT in solution is slowly cooled down to the HDPE crystallization temperature [5,6]. However, this work has been performed on HDPE nucleating off single nanotubes rather than CNT bundles or yarns.

In this work, we used solutions of HDPE at low concentration with which we penetrate the CNT yarn, and attempt to crystallize out disks that span multiple nanotubes. We observed such entities using SEM and TEM analysis and propose that the formation of such crystalline disks around the CNTs can act as clamps and fasten the individual tubes or bundles together, thus potentially increasing the stress transfer between them. After treatment with HDPE, infused CNT bundles were gripped by HDPE disks which were formed along the length of bundles consisting of 10-50 individual CNTs. An inverse proportionality was found between cooling rate and the disk’s dimensions, while a direct proportionality was found between the HDPE concentration in the solution and the disk diameter. Such firm compression of the bundles can in principal increase mechanical strength of the HDPE/CNT yarn.

References:
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The influence of crystallographic orientation, contact size and surface roughness effects on incipient plasticity in tungsten have been investigated by nanoindentation with a range of indenter radii (150, 350, 800 and 2800 nm) into single crystal samples with (100) and (111) orientation. The results on the single crystals have been compared to those on a reference sample of polycrystalline tungsten tested under the same conditions. Surface roughness measurements showed the R_a ~ 2, 4, and 6 nm on the (100), (111) and polycrystalline samples respectively. A strong size effect was observed, with the stress for incipient plasticity increasing as the indenter radii decrease. The maximum shear stress approached the theoretical shear strength when W(100) was indented by the tip with smallest radius. The higher roughness and greater dislocation density on the W(111) sample contributed to pop-ins occurring at lower stresses.

Superlubricity is a state of ultra-low friction due to geometrically imposed force cancellations between rigid incommensurate crystalline surfaces. Elastic deformations may lead to local interlocking and avert this cancellation, but they are difficult to treat analytically in finite and 3D systems. Large atomic-scale simulations of spherical asperities on deformable crystalline substrates show that elasticity affects the friction once the contact radius a exceeds a characteristic length set by the core width of interfacial dislocations b_core. As a increases past b_core, the frictional stress for both incommensurate and commensurate surfaces saturates to a constant value. This plateau corresponds to a Peierls stress that drops exponentially with increasing b_core but remains finite.

Nanoindention experiments on materials with high dislocation densities (~1e9 cm2) show a large range of plastic responses where plasticity can be initiated at the onset or there is a sudden elastic-plastic yielding resulting in pop-in. The high density of dislocations and wide range of forces at pop-in would indicate that under these conditions, pop-in is not due to dislocation nucleation but is instead caused by dislocation source activation. However, the type of dislocation source activated and the mechanisms initially limiting dislocation mobility is still unknown.

In this work we use dislocation dynamics (DD) simulations of nanoindentation to identify the underlying dislocation properties that lead to experimental observations of pop-in in a GaN crystal containing a high density of dislocations. The DD simulations model displacement controlled indentation where the displacement field of the indenter is directly coupled to the plastic deformation of the dislocations. In these simulations, a quasistatic indenter displacement field is applied to the film surface at the point of contact and as dislocations begin moving, the measured force produced by the indenter is reduced by the plastic deformation. This allows for a sudden drop in the force when yielding occurs, enabling us to capture pop-in.

The simulations are carried out for multiple randomly seeded dislocation configurations at several dislocation density levels in order to obtain a statistical relevant measure of the indenter’s response. Our simulations evaluate the role of common barriers to dislocation motion during pop-in such as Peierls stress and dislocation junction formation. We also identify alternative mechanisms that could attribute to pop-in that are unique to the hcp crystallography.

Our simulations use the DD algorithm of van der Geissen and Needleman [1] where the linear elastic fields due to dislocations in an infinite bulk crystal are superimposed onto the linear elastic fields produced by an auxiliary boundary value problem (BVP) whose boundary conditions consist of the corrective displacement and traction fields created by constrained or free surfaces. This simulation protocol is accomplished by coupling the DD simulator ParaDiS [2] to the corrective fields obtained from a BVP solved with a parallel finite element code [3]. This work also includes a novel algorithm for computing the displacement field of a dislocation based on dislocation segment data which works efficiently with node based dislocation dynamics simulators like ParaDiS [2].


[1] E. van der Giessen and A. Needleman, Model. Simul. Mater. Sci. Eng. 3, 689 (1995).
[2] A. Arsenlis et al., Model. Simul. Mater. Sci. Eng. 15, 553 (2007).
[3] J.C. Crone et al., Model. Simul. Mater. Sci. Eng. 22, 3 (2014).

Copper has been substituted for aluminum in electronic devices and microelectro-mechanical system (MEMS) materials because that has several advantage such as high resistance to electro-migration and high thermal and electric conductivity. Copper is generally deposited by electroplating which is restricted to lithographic processing of vapor deposition. Electroplating method can control microstructure such as grain size, texture, initial dislocation density and twin density by varying the electroplating conditions. Mechanical properties of electroplated copper is affected by microstructure. Many researchers investigated correlation between microstructure and mechanical properties using various method such as multi-scale uni-axial tensile testing and nanoindentation to measure mechanical properties.
Uni-axial tensile testing provides important mechanical properties such as elastic modulus, yield strength, ultimate strength and failure elongation of materials. When external size of micro- and nano-scales, sample fabrication and conducting tensile testing is challenging. On the contrary, nanoindentation required relatively-simple sample preparation and experimental method to measure elastic modulus and hardness regardless of scale. However hardness measured is not quantitative materials property, unlike strength measured by uniaxial tensile testing. Nanoindentation does not directly provide tensile properties such as yield strength, failure elongation and strain hardening exponent.
In this research, we introduce method of evaluate tensile properties of electroplated copper thin films by single nanoindentation test using a Berkovich indenter. We fabricated 10μm copper films by electroplating and produced microstructure with three different grain size of 1 to 10μm by post heat treatment. Tensile samples were separated from substrate by selectivity etching. We performed micro tensile test and nanoindentation test for three samples. We analyzed correlation between strain hardening exponent and characteristic length for indentation size effect (ISE) using constitutive equation and ISE theory suggested by Nix and Gao. From this relation we can evaluate tensile stress-strain curve using ISE parameters measured by nanoindentation.

Molybdenum disulfide (MoS2) has extraordinarily low friction and high wear life in dry and inert environments. However, the friction and wear of MoS2 are environmentally sensitive. Oxygen poses problems as temperatures rise and MoS2 oxidizes, increasing break-loose friction. In low earth orbit, atomic oxygen can easily oxidize MoS2. In humid environments, a disruption to low shear strength interfaces between MoS2 lamellae presumably increases friction. The mechanisms associated with this environmental poisoning of MoS2 coatings have largely remained a mystery. Utilizing High Sensitivity Low Energy Ion Scattering (HS-LEIS), the first few layers of MoS2 coatings were investigated across a range of temperatures and environments to understand interactions between MoS2 and oxygen, and atomic oxygen. This is combined with environmentally controlled microtribology experiments to link surface chemistry to tribological performance.

Graphene and related 2D materials have shown unusual and exceptional mechanical properties¹, with similarities to origami-like paper folding^2-3 and kirigami-like cutting^4-5. For paper analogues, a critical difference between macroscopic sheets and a 2D solid is the atomic scale of the thin dimension of the latter, allowing thermal activation of significant out-of-plane motion. To date, thermal activity has been shown to produce local wrinkles in a free graphene sheet that, for example, help in the understanding of its theoretical stability⁶ and give rise to unexpected long range bending stiffness⁴.
Here we show that thermal activation can have an even more dramatic effect on the behaviour of 2D solids, leading to self-driven sliding, tearing, and peeling from a surface on scales approaching the macroscopic⁷. We demonstrate that scalable nanoimprint-style contact techniques can nucleate and direct parallel self-assembly of graphene ribbons on length scales ranging from 100’s of nm’s up to 10’s of microns.
These observations are interpreted by a simple fracture-mechanics inspired model that shows how thermodynamic forces drive formation of graphene-graphene interface in lieu of substrate contact with sufficient strength to peel and tear single and multi-layer graphene sheets.
Our results can be viewed as observation of liquid-like character of 2D materials, where ultra-low friction due to structural lubricity combines with easy out-of-plane bending to form a kind of de-wetting flow, constrained to a 2D plane, driven by the dominance of surface forces in low dimensional objects.

1 Rasool, H. I., Ophus, C., Klug, W. S., Zettl, A. & Gimzewski, J. K. Measurement of the intrinsic strength of crystalline and polycrystalline graphene. Nature communications 4 (2013).
2 Cranford, S., Sen, D. & Buehler, M. J. Meso-origami: folding multilayer graphene sheets. Applied physics letters 95, 123121 (2009).
3 Ebbesen, T. W. & Hiura, H. Graphene in 3-dimensions: Towards graphite origami. Advanced Materials 7, 582-586, doi:10.1002/adma.19950070618 (1995).
4 Blees, M. K. et al. Graphene kirigami. Nature 524, 204-207, doi:10.1038/nature14588 (2015).
5 Castle, T. et al. Making the Cut: Lattice \textit{Kirigami} Rules. Physical Review Letters 113, 245502 (2014).
6 Fasolino, A., Los, J. H. & Katsnelson, M. I. Intrinsic ripples in graphene. Nat Mater 6, 858-861, doi:http://www.nature.com/nmat/journal/v6/n11/suppinfo/nmat2011_S1.html (2007).
7 James Annett & Graham L.W. Cross. Dewetting of Graphene. manuscript submitted (2015).

We report on a computational study of the fracture properties of graphene and polycrystalline graphene (PCG) upon tensile loading using molecular dynamics simulations. The recently introduced SED-REBO potential [1] used in this work, avoiding cut-off artifacts, allows the accurate investigation of fracture up to the graphene melting point. We show that pristine graphene undergoes brittle fracture, whatever the temperature. However, the spontaneous formation of Stone-Wales defect, observed at the highest temperatures, almost immediately induces fracture, thus significantly lowering the fracture strength and ultimate strain.

The room-temperature fracture properties of PCGs with nanosized grains are close to, though clearly lower, those of graphene. A particularly counter-intuitive result is that the most disordered PCG models investigated here are those with the highest fracture strains and stresses. This paradox, often termed "pseudo Hall-Petch effect"[2-3] is explained by the formation in the most disordered systems of several non-propagating and reversible - upon unloading - single-bond cracks. Conversely, in more ordered systems the load transfer following a bond rupture has a much more local character and quickly activate fracture propagation [4].

The reduction of fracture properties with increasing temperature in PCGs is evidenced. Despite an essentially brittle rupture, some signs of ductility can be clearly identified at very high temperatures.

[1] Perriot et al., Phys. Rev. B 88, 064101 (2013).
[2] Song et al. Nano Lett. 13, 1829 (2013).
[3] Sha et al. Sci Rep. 4, 5991 (2014).
[4] Gamboa et al. Science Adv. in Press (2015).

The exact mechanisms for the microstructure evolution under tribological loading are of great importance for both tribology and materials science, but not yet fully revealed and understood.
In this contribution, brass alloys with zinc contents between five and 36% in contact with a silicon carbide sphere were used to run the friction tests, which were carried out with a sphere-on-plate tribometer. Between these alloys, the stacking fault energy (SFE) varied by more than a factor of five. Starting with an annealed microstructure, we systematically varied the number of sliding cycles from one to 5000 cycles and followed the evolution of the microstructure by scanning electron and focused ion beam microscopy.
A layer of subgrains developed due to the high amount of plastic deformation near the surface, as well as a nanocrystalline layer at the sliding interface, were observed after 10 to 100 cycles. The depth of this changed microstructure increased with the sliding duration. A small amount of silicon found in the wear particles indicated a material transfer from the Si₃N₄ sphere to the sample surface of CuZn5 after 5000 cycles, whereas no generation of loose wear particles was seen during the sliding of a Si₃N₄ sphere on CuZn15 and CuZn36. The obtained results reveal a decreasing amount of generated loose wear particles with an increasing amount of zinc and therefore a decreasing SFE. Our results also demonstrate that high stacking fault energies favor a continuous transition from a nanocrystalline into a microcrystalline microstructure, whereas low stacking fault energies favor sharp transitions.
One of the goals of this study is the formulation of a model description of the microstructural changes based on energetic and mechanistic considerations in order to understand these changes and their influence on tribological properties. This might allow for materials with tailored ultrafine microstructures, combining low friction forces and very small wear rates.

Dislocation mediated plastic deformation is an expected consequence of indentation or scratching into a metal surface. However, the mechanisms by which dislocations are generated, how they evolve and to which dislocation microstructures this leads is still unclear. These questions are however of utmost importance for the understanding of the evolution of surface topology and the onset of wear. We have therefore undertaken a combined experimental and discrete dislocation dynamics (DDD) simulation study of the dislocation structure underneath sliding contacts at very small normal loads and therefore very mild wear conditions.

DDD simulations demonstrate that preexisting dislocations multiply underneath the indent and form hexagonal spirals or hexagonal loops that have also been found in several previous experiments. Under a sliding contact these dislocations are partially carried with the contacting sphere in DDD simulations.

Scanning transmission electron microscopy (STEM) analysis of a copper surface after a single sliding pass of a sapphire sphere shows the emergence of a distinct dislocation trace line at a depth of less than 100nm underneath the surface. This trace line evolves after multiple passes and is the prime characteristic of initial defect accumulation.

The large fraction of material residing at interfaces in nanocrystalline (NC) metals and alloys is responsible for their ultrahigh strength, but also undesirable microstructural instability and limited damage tolerance. While solute elements that segregate to grain boundaries (GBs) can have a stabilizing effect due to thermodynamic or kinetic factors, mechanical stresses sometimes still lead to grain growth in otherwise thermally stable NC materials. Despite observations of this phenomenon, the underlying mechanism of stress-driven microstructural evolution, and especially the role of GB doping by alloying, are still poorly understood and preclude rational alloy design. Here, we employ quantitative in situ electron microscopy to link GB solute excess in NC thin films of Al, Cu, and Ni dilute alloys with the critical stress required for GB migration and its associated kinetics. Site-specific in situ nanoindentation testing inside TEM reveals stress-driven migration of individual GBs and enables direct quantification of the chemically modulated critical stress, while 3D atom-probe tomography methods provide quantitative and spatially-resolved details of solute segregation along GBs. Transmission Kikuchi diffraction in SEM further provides statistical insight to the nature of stress-driven microstructural evolution in a complex and confined NC microstructure. Our results show that a critical GB excess of impurities is required to stabilize NC materials against mechanical driving forces, providing a new avenue for controlling deformation mechanisms and tailoring mechanical properties apart from grain size alone.

Molybdenum disulfide (MoS₂) has emerged in recent decades as an important solid lubricant due to its exceptional lubricity and the fact that it can easily be coated onto objects by physical vapor deposition (PVD) or chemical vapor deposition (CVD). These processes provide great flexibility in tuning the chemistry and/or structure, and therefore the properties, of the solid lubricant film. However, defining a clear set of design criteria for optimizing lubricity in MoS₂ remains challenging since the system evolves significantly during high cycle wear. This work investigates the evolution of the structure and properties of MoS₂ tribological coatings during wear. Tribo-indentation scratch experiments are performed on undoped and Cr-doped MoS₂ to investigate the evolution of the wear rates and coefficients of friction with increased wear. Site-specific test samples were fabricated from the wear tracks in order to perform nanomechanical testing in-situ in the TEM. In-situ imaging reveals the deformation modes and in-situ electron diffraction is used to characterize rotation of the MoS₂ crystallites during straining. The changes in mechanical properties and wear behavior were correlated with changes in structure as imaged by high-resolution TEM. The results provide new insights into the design of wear-resistant MoS₂ films.

We used molecular dynamics (MD) to study Cu_1-xAg_x|Ni and Cu|Au multilayer nanopillars with 5 nm layer width with (111) cube-on-cube interfaces. We find that Cu|Ni as well as Cu|Au multilayers form a semi-coherent interface with a network of partial dislocations arranged in a regular triangular pattern that is susceptible to easy shear. The Cu|Ni system is additionally alloyed with Ag to form Cu_1-xAg_x|Ni multilayers with x ranging from 0 to 10% to control the lattice misfit between the Cu and the Ni phase. We additionally use Monte Carlo (MC) simulations to initially equilibrate Cu_1-xAg_x|Ni (x=0%, 5% and 10%) system towards its thermal equilibrium. Compression simulations are then carried out at strain rates of 10⁸ s^-1 to study the influence of the angle θ between the (111) interfaces and pillar axis on the interface strength, with θ ranging from 0° to 30°. We find that in most cases deformation occurs predominantly in the softer layer and by sliding along the interface between the layers. For some of the rotation angles we find shear bands that evolve along (111) planes transversal to the semi-coherent interfaces and carry most of the deformation.

Harnessing the unique properties of 2D materials and heterostructures for nanoscale device applications requires precise control over the van der Waals epitaxy. The interface to the growth substrate, the layer number, the lateral arrangement of different domains, and the nature of subsurface structure elements, are all important variables to control. This calls for new characterization approaches to investigate subsurface features. Here, we demonstrate that contact resonance atomic force microscopy (CR-AFM) can yield a quantitative, subsurface-structure sensitive, mechanical fingerprint of 2D layered materials. To deconvolute the experimentally measured, aggregate contact stiffness and to quantify the nano-mechanical stiffness contributions from each material layer, we developed a new method that combines ab initio and continuum modeling approaches to predict the aggregate contact stiffness. We demonstrate the power of our method on epitaxially monolayer (MLG), bilayer (BLG), trilayer graphene (TLG), grown on silicon carbide, i.e., a system for which well-defined structure models exist. The experimental method can clearly distinguish, nanometer-sized, subsurface domains of oxygen-intercalated graphene grown on SiC(0001). A growing database of mechanical properties of 2D materials will allow, in combination with our method, unprecedented and quick structural characterization of 2D materials and heterostructures by unravelling the origin of subsurface-structure “fingerprints.”

Planar, micron-scale strain mapping of metallic, semiconductor, and polymer interfaces is an important step in predicting the reliability of semiconductor devices. Cracks, buckling, and voids often develop in close proximity to material interfaces and account for approximately 40% of all device failures.Today, Moiré interferometry and digital image correlation (DIC) are the most common micro-strain field mapping techniques. DIC provides high resolution strain mapping but is limited by a narrow field of view, as optical or electron microscopy is needed for image acquisition. Moiré interferometry provides a larger field of view than DIC, but possesses less mapping resolution and has only limited ability to detect micro-strain across material interfaces.

Strain mapping using a new technique based on laser diffraction, which affords submicron strain resolution and a wide field of view, will be reported. This technique has demonstrated considerable promise based on previous research, single-point strain mapping of thin, buckled, PDMS bonded to silicon and copper substrates results in a strain sensitivity of .001% [1]. Additionally, planer diffraction grating pitch mapping of silicon/epoxy/copper interfaces shows a local pitch detection sensitivity of approximately 3nm [2,3]. By fabricating a thin diffraction grating on the cross-sectional surface of an electronics package, 2D mapping of the grating pitch was realized. The local grating pitch differs as the microstructures present on the cross-section expand or contract with temperature changes, due to different thermal expansion coefficients of Si, Cu, solder, and dielectric materials. This local pitch shift provides a means of measuring in-plane strain.

Grating pitch and planer strain measurements will be described in detail, including sample preparation, measurement details, and results at different temperatures. Additionally, the strain sensing resolution will be evaluated and future work discussed.

[1] Teng Ma, Hanshuang Liang, George Chen, Benny Poon, Hanqing Jiang, and Hongbin Yu, "Micro-strain sensing using wrinkled stiff thin films on soft substrates as tunable optical grating," Opt. Express, 21, 11994 (2013)
[2] Hanshuang Liang, Teng Ma, Cheng Lv, Hoa Nguyen, George Chen, Hao Wu, Rui Tang, Hanqing Jiang, Hongbin Yu, High Sensitivity In- Plane Strain Measurement Using a Laser Scanning Technique, IEEE Electronic Components and Technology Conference, Orlando, FL, May 2014.
[3] Hanshuang Liang, Todd Houghton, Zeming Song, Teng Ma, Hoa Nguyen, George Chen, Hanqing Jiang, Hongbin Yu, Two-dimensional (2D) In-Plane Strain Mapping Using A Laser Scanning Technique on the Cross-Section of a Microelectronics Package, IEEE Electronic Components and Technology Conference, San Diego, Ca, May 2015.