Linking energy loss in soft adhesion to surface roughness

A mechanistic understanding of adhesion in soft materials is critical in the fields of transportation (tires, gaskets, seals), biomaterials, micro-contact printing, and soft robotics. Measurements have long demonstrated that the apparent work of adhesion coming into contact is consistently lower than the intrinsic work of adhesion for the materials, and that there is adhesion hysteresis during separation, commonly explained by viscoelastic dissipation. Still lacking is a quantitative experimentally validated link between adhesion and measured topography. Here, we used in situ measurements of contact size to investigate the adhesion behavior of soft elastic polydimethylsiloxane (PDMS) hemispheres (modulus ranging from 0.7 to 10 MPa) on four different polycrystalline diamond substrates with topography characterized across eight orders of magnitude, including down to the Ångström-scale. The results show that the reduction in apparent work of adhesion is equal to the energy required to achieve conformal contact. Further, the energy loss during contact and removal is equal to the product of the intrinsic work of adhesion and the true contact area. These findings provide a simple mechanism to quantitatively link the widely observed adhesion hysteresis to roughness rather than viscoelastic dissipation

Combining TEM, AFM, and Profilometry for Quantitative Topography Characterization Across All Scales

Surface roughness affects the functional properties of surfaces, including adhesion, friction, hydrophobicity, biological response, and electrical and thermal transport properties. However, experimental investigations to quantify these links are often inconclusive, because surfaces are fractal-like and the values of measured roughness parameters depend on measurement size. Here we demonstrate the characterization of topography of an ultrananocrystalline diamond (UNCD) surface at the Ångström-scale using transmission electron microscopy (TEM), as well as its combination with conventional techniques to achieve a comprehensive surface description spanning eight orders of magnitude in size. We performed more than 100 individual measurements of the nanodiamond film using both TEM and conventional techniques (stylus profilometry and atomic force microscopy). While individual measurements of root-mean-square (RMS) height, RMS slope, and RMS curvature vary by orders of magnitude, we combine the various techniques using the power spectral density (PSD) and use this to compute scale-independent parameters. This analysis reveals that “smooth” UNCD surfaces have an RMS slope greater than one, even larger than the slope of the Austrian Alps when measured on the scale of a human step. This approach of comprehensive multi-scale roughness characterization, measured with Ångström-scale detail, will enable the systematic evaluation and optimization of other technologically relevant surfaces, as well as systematic testing of the many analytical and numerical models for the behavior of rough surfaces

Quantifying the Pressure-dependence of Work of Adhesion in Silicon-Diamond Contacts

Continuum mechanics models for contacting surfaces assume a constant interfacial energy, or work of adhesion, between materials. Recent studies have challenged this assumption, instead demonstrating that stress-dependent chemical reactions across the interface modify the work of adhesion. Here, we perform 77 adhesion tests on diamond-silicon contacts using in situ TEM and atomistic simulations to quantify how the adhesion changes as a function of applied pressure. The results show a 7-fold increase in work of adhesion (from approximately 1 to 7 J/m2) with an increase in mean applied pressure from 0 to 11 GPa, where the most significant increase occurs above 5 GPa. We rule out alternative explanations for the changing work of adhesion, such as electron-beam artifacts, bulk shape change by inelastic deformation, and time-dependent processes such as creep. Therefore, these results confirm the presence of stress-driven chemical reactions in the contact and quantify the resulting change in adhesion of these materials with applied pressure

A Method for Quantitative Real-Time Evaluation of Measurement Reliability When Using Atomic Force Microscopy-Based Metrology

In atomic force microscopy (AFM) and metrology, it is known that the radius of the scanning tip affects the accuracy of the measurement. However, most techniques for ascertaining tip radius require interruption of the measurement technique to insert a reference standard or to otherwise image the tip. Here we propose an inline technique based on analysis of the power spectral density (PSD) of the topography that is being collected during measurement. By identifying and quantifying artifacts that are known to arise in the power spectrum due to tip blunting, the PSD itself can be used to determine progressive shifts in the radius of the tip. Specifically, using AFM images of an ultrananocrystalline diamond, various trends in measured PSD are demonstrated. First, using more than 200 different measurements of the same material, the variability in the measured PSD is demonstrated. Second, using progressive scans under the same conditions, a systematic shifting of the mid-to-high-frequency data is visible. Third, using three different PSDs, the changes in radii between them were quantitatively determined and compared to transmission electron microscopy (TEM) images of the tips taken immediately after use. The fractional changes in tip radii were detected; the absolute values of the tip radii could be matched between the two techniques, but only with careful selection of a fitting constant. Further work is required to determine the generalizability of the value of this constant. Overall, the proposed approach represents a step towards quantitative and inline determination of the radius of the scanning tip and thus of the reliability of AFM-based measurements

Gold−Copper Nano-Alloy, “Tumbaga”, in the era of nano: phase diagram and segregation

Gold–copper (Au–Cu) phases were employed already by pre-Columbian civilizations, essentially in decorative arts, whereas nowadays, they emerge in nanotechnology as an important catalyst. The knowledge of the phase diagram is critical to understanding the performance of a material. However, experimental determination of nanophase diagrams is rare because calorimetry remains quite challenging at the nanoscale; theoretical investigations, therefore, are welcomed. Using nanothermodynamics, this paper presents the phase diagrams of various polyhedral nanoparticles (tetrahedron, cube, octahedron, decahedron, dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron, and icosahedron) at sizes 4 and 10 nm. One finds, for all the shapes investigated, that the congruent melting point of these nanoparticles is shifted with respect to both size and composition (copper enrichment). Segregation reveals a gold enrichment at the surface, leading to a kind of core–shell structure, reminiscent of the historical artifacts. Finally, the most stable structures were determined to be the dodecahedron, truncated octahedron, and icosahedron with a Cu-rich core/Au-rich surface. The results of the thermodynamic approach are compared and supported by molecular-dynamics simulations and by electron-microscopy (EDX) observations

CuS2‐Passivated Au-Core, Au3Cu-Shell nanoparticles analyzed by Atomistic-Resolution Cs-Corrected STEM

Au-core, Au3Cu-alloyed shell nanoparticles passivated with CuS2 were fabricated by the polyol method, and characterized by Cs-corrected scanning transmission electron microscopy. The analysis of the high-resolution micrographs reveals that these nanoparticles have decahedral structure with shell periodicity, and that each of the particles is composed by Au core and Au3Cu alloyed shell surrounded by CuS2 surface layer. X-ray diffraction measurements and results from numerical simulations confirm these findings. From the atomic resolution micrographs, we identified edge dislocations at the twin boundaries of the particles, as well as evidence of the diffusion of Cu atoms into the Au region, and the reordering of the lattice on the surface, close to the vertices of the particle. These defects will impact the atomic and electronic structures, thereby changing the physical and chemical properties of the nanoparticles. On the other hand, we show for the first time the formation of an ordered superlattice of Au3Cu and a self-capping layer made using one of the alloy metals. This has significant consequences on the physical mechanism that form multicomponent nanoparticles

In situ Mechanical Testing of Contacts Between Nanoscale Bodies: Measuring the Load-dependence of Contact Area.

Mechanical tests were performed on a silicon/diamond nanocontact. Using in situ transmission electron microscopy (TEM) and matched atomistic simulations, the contact area was measured during loading and unloading. The results agreed within uncertainty, and both experiment and simulation data showed significant hysteresis. While the unloading curves could be fit to a continuum model, yielding a realistic value for elastic modulus, this model overpredicted the contact area upon loading by an average of 40%. The implications of these results for real-world nanoscale contacts are that the contact area upon loading can deviate significantly from continuum predictions, even when the behavior upon unloading is well described by these models

Matching atomistic simulations and in situ experiments to investigate the mechanics of nanoscale contact

Many emerging devices and technologies rely on contacts between nanoscale bodies. Recent analytical theories, experiments, and simulations of nanocontacts have made conflicting predictions about the mechanical response as these contacts are loaded and separated. The present investigation combined in situ transmission electron microscopy (TEM) and molecular dynamics (MD) simulation to study the contact between a flat diamond indenter and a nanoscale silicon tip. The TEM was used to pre-characterize the materials, such that an atomistic model tip could be created with identically matched materials, geometry, crystallographic orientation, loading conditions, and degree of amorphization. A large work of adhesion was measured in the experiment and attributed to unpassivated surfaces and a large compressive stress applied before separation, resulting in covalent bonding across the interface. The simulations modeled atomic interactions across the interface using a Buckingham potential in order to reproduce the experimental work of adhesion without explicitly modeling covalent bonds, thereby enabling larger time- and length-scale simulations than would be achievable with a reactive potential. Then, the experimental and simulation tips were loaded under similar conditions with real-time measurement of contact area and deformation, yielding three primary findings. First, the results demonstrated that significant variation in the value of contact area can be obtained from simulations, depending on the technique used to determine it. Therefore, care is required in comparing measured values of contact area between simulations and experiments. Second, the contact area and deformation demonstrated significant hysteresis, with larger values measured upon unloading as compared to loading. Therefore, continuum predictions, in the form of a Maugis-Dugdale contact model, could not be fit to full loading/unloading curves. Third, the load-dependent contact area could be accurately fit by allowing the work of adhesion in the continuum model to increase with applied force from 1.3 to 4.3 J/m^2. The most common mechanisms for hysteretic behavior—which are viscoelasticity, capillary interactions, and plasticity—can be ruled out using the TEM and atomistic characterization. Stress-dependent formation of covalent bonds is suggested as a physical mechanism to describe these findings, which is qualitatively consistent with trends in the areal density of in-contact atoms as measured in the simulation. The implications of these results for real-world nanoscale contacts are that significant hysteresis may cause significant and unexpected deviations in contact size, even for nominally elastic contact

Trimetallic nanostructures: the case of AgPd–Pt multiply twinned nanoparticles

We report the synthesis, structural characterization, and atomistic simulations of AgPd–Pt trimetallic (TM) nanoparticles. Two types of structure were synthesized using a relatively facile chemical method: multiply twinned core–shell, and hollow particles. The nanoparticles were small in size, with an average diameter of 11 nm and a narrow distribution, and their characterization by aberration corrected scanning transmission electron microscopy allowed us to probe the structure of the particles at an atomistic level. In some nanoparticles, the formation of a hollow structure was also observed, that facilitates the alloying of Ag and Pt in the shell region and the segregation of Ag atoms on the surface, affecting the catalytic activity and stability. We also investigated the growth mechanism of the nanoparticles using grand canonical Monte Carlo simulations, and we have found that Pt regions grow at overpotentials on the AgPd nanoalloys, forming 3D islands at the early stages of the deposition process. We found very good agreement between the simulated structures and those observed experimentally

Characterization of small-scale surface topography using transmission electron microscopy

Multi-scale surface topography is critical to surface function, yet the very smallest scales are not accessible with conventional measurement techniques. Here we demonstrate two separate approaches for measuring small-scale topography in a transmission electron microscope (TEM). The first technique harnesses “conventional” methods for preparation of a TEM cross-section, and presents how these methods may be modified to ensure the preservation of the original surface. The second technique involves the deposition of the material of interest on a pre-fabricated substrate. Both techniques enable the observation and quantification of surface topography with Ångström-scale resolution. Then, using electron energy loss spectroscopy (EELS) to quantify the sample thickness, we demonstrate that there is no systematic effect of thickness on the statistical measurements of roughness. This result was verified using mathematical simulations of artificial surfaces with varying thickness. The proposed explanation is that increasing the side-view thickness of a randomly rough surface may change which specific features are sampled, but does not significantly alter the character (e.g., root-mean-square (RMS) values and power spectral density (PSD)) of the measured topography. Taken together, this work establishes a new approach to topography characterization, which fills in a critical gap in conventional approaches: i.e., the measurement of smallest-scale topography