Yijie Jiang

Atomic-Scale Wear of Amorphous Hydrogenated Carbon during Intermittent Contact: A Combined Study Using Experiment, Simulation, and Theory

In this study, we explore the wear behavior of amplitude modulation atomic force microscopy (AM-AFM, an intermittent-contact AFM mode) tips coated with a common type of diamond-like carbon, amorphous hydrogenated carbon (a-C:H), when scanned against an ultra-nanocrystalline diamond (UNCD) sample both experimentally and through molecular dynamics (MD) simulations. Finite element analysis is utilized in a unique way to create a representative geometry of the tip to be simulated in MD. To conduct consistent and quantitative experiments, we apply a protocol that involves determining the tip-sample interaction geometry, calculating the tip-sample force and normal contact stress over the course of the wear test, and precisely quantifying the wear volume using high-resolution transmission electron microscopy imaging. The results reveal gradual wear of a-C:H with no sign of fracture or plastic deformation. The wear rate of a-C:H is consistent with a reaction-rate-based wear theory, which predicts an exponential dependence of the rate of atom removal on the average normal contact stress. From this, kinetic parameters governing the wear process are estimated. MD simulations of an a-C:H tip, whose radius is comparable to the tip radii used in experiments, making contact with a UNCD sample multiple times exhibit an atomic-level removal process. The atomistic wear events observed in the simulations are correlated with under-coordinated atomic species at the contacting surfaces.

Structure-property relationships from universal signatures of plasticity in disordered solids

Behavioral universality across size scales Glassy materials are characterized by a lack of long-range order, whether at the atomic level or at much larger length scales. But to what extent is their commonality in the behavior retained at these different scales? Cubuk et al. used experiments and simulations to show universality across seven orders of magnitude in length. Particle rearrangements in such systems are mediated by defects that are on the order of a few particle diameters. These rearrangements correlate with the materials softness and yielding behavior. Science, this issue p. 1033 A range of particle-based and glassy systems show universal features of the onset of plasticity and a universal yield strain. When deformed beyond their elastic limits, crystalline solids flow plastically via particle rearrangements localized around structural defects. Disordered solids also flow, but without obvious structural defects. We link structure to plasticity in disordered solids via a microscopic structural quantity, “softness,” designed by machine learning to be maximally predictive of rearrangements. Experimental results and computations enabled us to measure the spatial correlations and strain response of softness, as well as two measures of plasticity: the size of rearrangements and the yield strain. All four quantities maintained remarkable commonality in their values for disordered packings of objects ranging from atoms to grains, spanning seven orders of magnitude in diameter and 13 orders of magnitude in elastic modulus. These commonalities link the spatial correlations and strain response of softness to rearrangement size and yield strain, respectively.

Tuning the mechanical properties of vertical graphene sheets through atomic layer deposition.

We report the fabrication and characterization of graphene nanostructures with mechanical properties that are tuned by conformal deposition of alumina. Vertical graphene (VG) sheets, also called carbon nanowalls (CNWs), were grown on copper foil substrates using a radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) technique and conformally coated with different thicknesses of alumina (Al2O3) using atomic layer deposition (ALD). Nanoindentation was used to characterize the mechanical properties of pristine and alumina-coated VG sheets. Results show a significant increase in the effective Youngs modulus of the VG sheets with increasing thickness of deposited alumina. Deposition of only a 5 nm thick alumina layer on the VG sheets nearly triples the effective Youngs modulus of the VG structures. Both energy absorption and strain recovery were lower in VG sheets coated with alumina than in pure VG sheets (for the same peak force). This may be attributed to the increase in bending stiffness of the VG sheets and the creation of connections between the sheets after ALD deposition. These results demonstrate that the mechanical properties of VG sheets can be tuned over a wide range through conformal atomic layer deposition, facilitating the use of VG sheets in applications where specific mechanical properties are needed.

Enhancing the stiffness of vertical graphene sheets through ion beam irradiation and fluorination

Many applications of graphene can benefit from the enhanced mechanical robustness of graphene-based components. We report how the stiffness of vertical graphene (VG) sheets is affected by the introduction of defects and fluorination, both separately and combined. The defects were created using a high-energy ion beam while fluorination was performed in a XeF2 etching system. After ion bombardment alone, the average effective reduced modulus (E r), equal to ∼4.9 MPa for the as-grown VG sheets, approximately doubled to ∼10.0 MPa, while fluorination alone almost quadrupled it to ∼18.4 MPa. The maximum average E r of ∼32.4 MPa was achieved by repeatedly applying fluorination and ion bombardment. This increase can be explained by the formation of covalent bonds between the VG sheets due to ion bombardment, as well as the conversion from sp2 to sp3 and increased corrugation due to fluorination.

Nanoporous Polymer-Infiltrated Nanoparticle Films with Uniform or Graded Porosity via Undersaturated Capillary Rise Infiltration

In this work, we present the fabrication of nanoporous polymer-infiltrated nanoparticle films (PINFs) with either uniform or graded porosity based on undersaturated capillary rise infiltration (UCaRI) and study the processing-structure-property relationship of these nanoporous PINFs. The UCaRI process involves first generating a bilayer film of a randomly packed nanoparticle layer atop a polymer layer, such that the volume of the polymer is less than the void volume in the nanoparticle packing. Subsequently, the bilayer film is annealed above the glass transition temperature of the polymer to induce polymer infiltration into the voids of the nanoparticle packing. Using in situ spectroscopic ellipsometry and molecular dynamics simulations, we observe that the polymer transport occurs in two stages: capillarity-induced infiltration, followed by gradual spreading, likely via surface diffusion. By varying the annealing time, UCaRI enables the generation of graded or uniform nanoporous PINFs. We also show that these nanoporous PINFs have tunable optical and mechanical properties, which can be tailored simply by changing the nanoparticle to polymer layer thickness ratio in the initial bilayer. The UCaRI approach is versatile and widely applicable to various polymers, which allows generation of nanoporous PINFs for multiple applications.

Multibond Model of Single-Asperity Tribochemical Wear at the Nanoscale

Single-asperity wear experiments and simulations have identified different regimes of wear including Eyring- and Archard-like behaviors. A multibond dynamics model has been developed based on the friction model of Filippov et al. [Phys. Rev. Lett. 92, 135503 (2004)]. This new model 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 behavior of wear rate breaks down. A nearly rate-independent regime arises under high load or high adhesive forces, in which wear becomes very nearly, but not precisely, proportional to sliding distance. In this restricted regime, the dependence of wear rate per unit contact area is nearly independent of the normal stress at the point of contact. In true contact between rough elastic surfaces, where contact area is expected to grow linearly with normal load, this would lead to behavior very similar to that described by the Archard equation. Detailed comparisons to experimental and molecular dynamics simulation investigations illustrate both Eyring and Archard regimes, and an intermediate crossover regime between the two.

Correcting for Tip Geometry Effects in Molecular Simulations of Single-Asperity Contact

Molecular simulation is a powerful tool for studying the nanotribology of single-asperity contacts, but computational limits require that compromises be made when choosing tip sizes. To assess and correct for the finite size effects, complementary finite element (FE) and molecular statics (MS) simulations examining the effects of tip size (height and radius) on contact stiffness and stress were performed. MS simulations of contact between paraboloidal tips and a flat, rigid diamond substrate using the 2B-SiCH reactive empirical bond-order potential were used to generate force–displacement curves and stress maps. Tips of various radii and heights, truncated by a rigid boundary, were formed from carbon- and silicon-containing materials so that they possessed differing elastic properties. Results were compared to FE simulations with matching geometries and elastic properties. FE analysis showed that the rigid boundary at the back of the tip influences the contact stiffness strongly, deviating from the Hertz model for small tip heights and radii. By examining the relationships between force, tip height, tip radii, and elastic properties obtained with FE simulations, a map interpolation method is presented that accounts for the effect of tip size and enables the extraction of Young’s modulus from MS force–displacement data. Furthermore, the FE results show that the effect of the finite size of the tip on contact stress is less pronounced than its effect on stiffness. The MS simulations also demonstrate that stress propagation within the tip is significantly impacted by the structure of the tip.

Tribochemical Wear of Diamond-Like Carbon-Coated Atomic Force Microscope Tips

Nanoscale wear is a critical issue that limits the performance of tip-based nanomanufacturing and nanometrology processes based on atomic force microscopy (AFM). Yet, a full scientific understanding of nanoscale wear processes remains in its infancy. It is therefore important to quantitatively understand the wear behavior of AFM tips. Tip wear is complex to understand due to adhesive forces and contact stresses that change substantially as the contact geometry evolves due to wear. Here, we present systematic characterization of the wear of commercial Si AFM tips coated with thin diamond-like carbon (DLC) coatings. Wear of DLC was measured as a function of external loading and sliding distance. Transmission electron microscopy imaging, AFM-based adhesion measurements, and tip geometry estimation via inverse imaging were used to assess nanoscale wear and the contact conditions over the course of the wear tests. Gradual wear of DLC with sliding was observed in the experiments, and the tips evolved from initial paraboloidal shapes to flattened geometries. The wear rate is observed to increase with the average contact stress, but does not follow the classical wear law of Archard. A wear model based on the transition state theory, which gives an Arrhenius relationship between wear rate and normal stress, fits the experimental data well for low mean contact stresses (<0.3 GPa), yet it fails to describe the wear at higher stresses. The wear behavior over the full range of stresses is well described by a recently proposed multibond wear model that exhibits a change from Archard-like behavior at high stresses to a transition state theory description at lower stresses.

Ultrasensitive, Mechanically Responsive Optical Metasurfaces via Strain Amplification

Optical metasurfaces promise ultrathin, lightweight, miniaturized optical components with outstanding capabilities to manipulate the amplitude, phase, and polarization of light compared to conventional, bulk optics. The emergence of reconfigurable metasurfaces further integrates dynamic tunability with optical functionalities. Here, we report a structurally reconfigurable, optical metasurface constructed by integrating a plasmonic lattice array in the gap between a pair of symmetric microrods that serve to locally amplify the strain created on an elastomeric substrate by an external mechanical stimulus. The strain on the metasurface is amplified by a factor of 1.5-15.9 relative to the external strain by tailoring the microrod geometry. For the highest strain amplification geometry, the mechano-sensitivity of the optical responses of the plasmonic lattice array is a factor of 10 greater than that of state-of-the-art stretchable plasmonic resonator arrays. The spatial arrangement and therefore the optical response of the plasmonic lattice array are reversible, showing little hysteresis.

Shear Adhesion of Tapered Nanopillar Arrays

Tapered nanopillars with various cross sections, including cone-shaped, stepwise, and pencil-like structures (300 nm in diameter at the base of the pillars and 1.1 μm in height), are prepared from epoxy resin templated by nanoporous anodic aluminum oxide (AAO) membranes. The effect of pillar geometry on the shear adhesion behavior of these nanopillar arrays is investigated via sliding experiments in a nanoindentation system. In a previous study of arrays with the same geometry, it was shown that cone-shaped nanopillars exhibit the highest adhesion under normal loading while stepwise and pencil-like nanopillars exhibit lower normal adhesion strength due to significant deformation of the pillars that occurs with increasing indentation depth. Contrary to the previous studies, here, we show that pencil-like nanopillars exhibit the highest shear adhesion strength at all indentation depths among three types of nanopillar arrays and that the shear adhesion increases with greater indentation depth due to the higher bending stiffness and closer packing of the pencil-like nanopillar array. Finite element simulations are used to elucidate the deformation of the pillars during the sliding experiments and agree with the nanoindentation-based sliding measurements. The experiments and finite element simulations together demonstrate that the shape of the nanopillars plays a key role in shear adhesion and that the mechanism is quite different from that of adhesion under normal loading.