Structure evolution during deposition and thermal annealing of amorphous carbon ultrathin films investigated by molecular dynamics simulations.

The evolution of the structure of amorphous carbon (a-C) films during deposition and thermal annealing is of significant interest from both the materials science and application perspectives. However, despite the voluminous literature of studies dealing with the deposition and physical properties of a-C films, basic understanding of the structure evolution due to phase change during film growth and heating is fairly sparse and empirical, presumably due to the lack of high-resolution instruments that can probe structural changes at the atomic and molecular levels in real time. Molecular dynamics (MD) is a powerful computational method for studying atomic/molecular-scale movement and interactions. Thus, the objective of this study was to perform MD simulations that provide insight into changes in the structure of ultrathin a-C films during deposition and annealing. Simulation results reveal a multi-layer film structure, even for a-C films as thin as ~20 Å, the existence of a deposition energy that yields a-C films with the highest sp3 content, the transient and steady-state stages of the structure evolution during annealing at different temperatures, and the changes in the hybridization state (mainly in the bulk layer) encountered during annealing at elevated temperatures. The MD results of this study are of particular importance to applications where the deposition conditions and operation temperature affect the structure and, in turn, the physical properties of ultrathin a-C films used as protective overcoats

Highly flexible, foldable, and rollable microsupercapacitors on an ultrathin polyimide substrate with high power density.

The design and functionality of extremely flexible, foldable, and rollable microsupercapacitors (MSCs) with in-plane interdigital electrodes that consist of single-walled carbon nanotube (SWCNT) networks on an ultrathin polyimide substrate are demonstrated through experiments and finite element simulations. The all-solid-state MSCs can be reversibly bent, folded, and rolled purely elastically without degradation of their electrical performance. The simulation results confirm that the deformation in bent, folded, and rolled MSCs is purely elastic. The high power density (1125 W cm-3) and small time constant (1 ms) of the present MSCs are comparable to those of aluminum electrolytic capacitors. The MSCs operate at scan rates of up to 1000 V s-1, are characterized by a volumetric capacitance of 18 F cm-3 and an energy density of 1.6 mWh cm-3, and exhibit superior electrochemical stability with 96% capacity retention even after 100,000 charge/discharge cycles. The developed MSCs demonstrate high potential for integration in flexible and wearable electronic systems

Sum Frequency Generation Vibrational Spectroscopy of Pyridine Hydrogenation on Platinum Nanoparticles

Pyridine hydrogenation in the presence of a surface monolayer consisting of cubic Pt nanoparticles stabilized by tetradecyltrimethylammonium bromide (TTAB) was investigated by sum frequency generation (SFG) vibrational spectroscopy using total internal reflection (TIR) geometry. TIR-SFG spectra analysis revealed that a pyridinium cation (C{sub 5}H{sub 5}NH{sup +}) forms during pyridine hydrogenation on the Pt nanoparticle surface, and the NH group in the C{sub 5}H{sub 5}NH{sup +} cation becomes more hydrogen bound with the increase of the temperature. In addition, the surface coverage of the cation decreases with the increase of the temperature. An important contribution of this study is the in situ identification of reaction intermediates adsorbed on the Pt nanoparticle monolayer during pyridine hydrogenation

Failure mechanisms of single-crystal silicon electrodes in lithium-ion batteries

Long-term durability is a major obstacle limiting the widespread use of lithium ion batteries (LIBs) in heavy-duty applications and others demanding extended lifetime. As one of the root causes of degradation and failure of battery performance, the electrode failure mechanisms are still unknown. Here, we reveal the fundamental fracture mechanisms of single-crystal silicon electrodes over extended lithiation/delithiation cycles, using electrochemical testing, microstructure characterization, fracture mechanics, and finite element analysis. Anisotropic lithium invasion causes crack initiation perpendicular to the electrode surface, followed by growth through the electrode thickness. The low fracture energy of the lithiated/unlithiated silicon interface provides a weak microstructural path for crack deflection, accounting for the crack patterns and delamination observed after repeated cycling. Based on this physical understanding, we demonstrate how electrolyte additives can heal electrode cracks and provide strategies to enhance the fracture resistance in future LIBs from surface chemical, electrochemical, and material science perspectives.Comment: 8 pages, 5 figure

A Multiscale Theoretical Analysis of the Mechanical, Thermal, and Electrical Characteristics of Rough Contact Interfaces Demonstrating Fractal Behavior

The highly complex contact interface phenomena require analysis at different length scales ranging from nanometer up to nearly centimeter scales. When two nominally smooth surfaces are brought into contact, solid-solid interaction across their contact interface is confined at multiple protrusions (asperities) of various shapes and sizes. The deformation mechanisms encountered at the asperity level control the surface conformity, which, in turn, influences the transmission of traction, heat, and electric current across the contact interface. Thus, the multiscale roughness of real surfaces necessitates the advance of methodologies and contact models that bridge the spectrum of relevant length scales. Rough surfaces have been traditionally characterized by statistical parameters, which cannot be uniquely determined because they depend on the sampling interval and the resolution of the measuring device. On the contrary, the scale-invariant parameters employed in fractal geometry provide an unbiased representation of the surface topography. This article provides an appraisal of the multiscale mechanical, thermal, and electrical characteristics of rough contact interfaces demonstrating fractal behavior. Theoretical treatments of elastic, elastic-plastic, and fully plastic deformation, heat conduction, temperature rise, and electrical contact resistance are presented for contact interfaces characterized by fractal geometry, providing a fundamental basis for developing multiscale thermo-electro-mechanics analytical treatments for contacting solid bodies

Dynamics of Tribofilm Formation in Boundary Lubrication Investigated Using In Situ Measurements of the Friction Force and Contact Voltage.

The complex dynamics of tribofilm formation on boundary-lubricated steel surfaces were investigated in real time by combining in situ measurements of the temporal variation of the coefficient of friction and contact voltage. Sliding experiments were performed with various blends consisting of base oil, zinc dialkyl dithiophosphate (ZDDP) additive, and two different dispersants at an elevated oil temperature for a wide range of normal load and fixed sliding speed. The evolution of the transient and steady-state coefficient of friction, contact voltage, and critical sliding distance (time) for stable tribofilm formation were used to evaluate the tribological performance of the tribofilms. The blend composition affected the load dependence of the critical sliding distance for stable tribofilm formation. Tribofilm friction was influenced by competing effects between the additive and the dispersants. Among various formulations examined, the tribofilm with the best friction characteristics was found to be the blend consisting of base oil, a small amount of ZDDP, and a bis-succinimide dispersant treated with ethylene carbonate. The results of this study demonstrate the effectiveness of the present experimental approach to track the formation and removal of protective tribofilms under boundary lubrication conditions in real time

Diamond nucleation and growth from submicron amorphous carbon clusters containing randomly oriented diamond nanocrystallites

Diamond nucleation and growth from submicron clusters consisting of an amorphous carbon phase with predominantly sp 3 atomic hybridization and randomly oriented diamond nanocrystallites was investigated by various microanalysis techniques. The carbon clusters were created by exposing a highly sp 3 hybridized carbon thin film, deposited on a smooth silicon substrate by a vacuum arc method, to a low-temperature, methane-rich hydrogen plasma in a microwave plasma-enhanced chemical vapor deposition system. Diamond nanocrystallites inside the carbon clusters produced by the pretreatment acted as diamond nucleation sites. Microanalysis results provided insight into the structure and composition of the carbon clusters, the diamond nanocrystallites, and the amorphous ultrathin interlayers at the interfaces of the clusters and the grown diamond film with the silicon substrate. The physical phenomena responsible for the enhancement of diamond nucleation and growth on smooth substrates by the present method are interpreted in the context of the obtained results. Graphical abstract: [Figure not available: see fulltext.

A molecular dynamics analysis of the effect of surface passivation on the adhesion, deformation behavior and structure stability of amorphous carbon ultrathin films

Basic knowledge of the interfacial interactions, contact deformation, and structure changes in amorphous carbon (a-C) ultrathin films due to irreversible deformation is of critical importance to the protective effectiveness of these films. This paper presents a molecular dynamics (MD) analysis that reveals the role of surface passivation on the contact deformation and adhesion characteristics of ultrathin (<30 Å) a-C films possessing a layered structure consisting of intermixing, bulk, and surface layers. MD simulations reveal much higher interfacial adhesion, destabilization of the film structure resulting in partial sp3-to-sp2 rehybridization, and film delamination in the intermixing layer for unpassivated compared to hydrogen-passivated diamond surfaces. The results of this study illuminate the adhesion and contact deformation behaviors of ultrathin a-C films with layered structures, which are impossible to track experimentally due to the extremely small spatiotemporal scales