Active machine learning model for the dynamic simulation and growth mechanisms of carbon on metal surface.

Substrate-catalyzed growth offers a highly promising approach for the controlled synthesis of carbon nanostructures. However, the growth mechanisms on dynamic catalytic surfaces and the development of more general design strategies remain ongoing challenges. Here we show how an active machine-learning model effectively reveals the microscopic processes involved in substrate-catalyzed growth. Utilizing a synergistic approach of molecular dynamics and time-stamped force-biased Monte Carlo methods, augmented by the Gaussian Approximation Potential, we perform fully dynamic simulations of graphene growth on Cu(111). Our findings accurately replicate essential subprocesses-from the preferred diffusion of carbon monomer/dimer, chain or ring formations to edge-passivated Cu-aided graphene growth and bond breaks by ion impacts. Extending our simulations to carbon deposition on metal surfaces like Cu(111), Cr(110), Ti(001), and oxygen-contaminated Cu(111), our results align closely with experimental observations, providing a practical and efficient approach for designing metallic or alloy substrates to achieve desired carbon nanostructures and explore further reaction possibilities.

Growth Control Strategy of Hydrogen-Containing Nanocrystalline Carbon Films during Plasma-Enhanced Chemical Vapor Deposition based on Molecular Dynamics-Monte Carlo Simulations.

Hydrogen-containing nanocrystalline carbon films (n-C:H) with amorphous-nanocrystalline hydrocarbon composite structures exhibit excellent properties in diverse applications. Plasma-enhanced chemical vapor deposition (PECVD) is commonly employed to prepare n-C:H films due to its ability to create an adjustable deposition environment and control film compositions. However, the atomic-scale growth mechanism of n-C:H remains poorly understood, obstructing the design of the appropriate deposition parameters and film compositions. This paper employs a state-of-the-art hybrid molecular dynamics-time-stamped force-biased Monte Carlo model (MD/tfMC) to simulate the plasma-assisted growth of n-C:H. Our results reveal that optimizing the energy of ion bombardments, deposition temperature, and precursor's H:C ratio is crucial for achieving the nucleation and growth of highly ordered n-C:H films. These findings are further validated through experimental observations and density functional theory calculations, which show that hydrogen atoms can promote the formation of nanocrystalline carbon through chemical catalytic processes. Additionally, we find that the crystallinity reaches its optimum when the H/C ratio is equal to 1. These theoretical insights provide an effective strategy for the controlled preparation of hydrogen-containing nanocrystalline carbon films.

Electronic Transport and Corrosion Mechanisms of Graphite-Like Nanocrystalline Carbon Films Used on Metallic Bipolar Plates in Proton-Exchange Membrane Fuel Cells.

Nanocrystalline carbon films, which consist of graphite-like nanocrystals within an amorphous carbon matrix, have recently attracted extensive theoretical and experimental attention. Understanding the electronic transport and corrosion mechanisms of graphite-like nanocrystalline carbon films (GNCFs) is essential for their application in proton-exchange membrane fuel cells (PEMFCs). So far, limited progress has been made on the electronic or atomistic understanding of how the degree of structural order and grain boundaries affect the electronic transport and corrosion behaviors of GNCFs. In this work, using the Landauer-Büttiker formula merged with first-principles density functional theory, the conductance of GNCFs is presented as a function of their crystallinity. As the crystallinity decreases, the electron states around the Fermi level are found to be more spatially localized, thus hindering the electronic transport of GNCFs. Additionally, a systemic picture of the chemical reactivity of nanostructured surface in GNCFs toward typical particles existing in PEMFCs is drawn by ab initio molecular dynamics simulations. Systemic experimental investigations on the corrosion mechanisms of GNCFs used in PEMFCs have been conducted in this work. Compared with pure amorphous carbon films, the GNCFs exhibit higher corrosion current densities due to the preferential corrosion in the larger slit pores at the grain boundaries, but their stability in interfacial contact resistance is significantly improved by the embedded graphite-like nanocrystals, which have high levels of resistance to oxygen chemical adsorptions and act as high-speed ways to transport electrons.

Controlling the Nucleation and Growth Orientation of Nanocrystalline Carbon Films during Plasma-Assisted Deposition: A Reactive Molecular Dynamics/Monte Carlo Study.

Nanocrystalline carbon films containing preferentially oriented graphene-based nanocrystals within an amorphous carbon matrix have attracted significant theoretical and experimental interest due to their favorable chemical and physical properties. At present, there are intense efforts to study the grain size and growth orientation of the graphene-based nanocrystals to achieve a controllable growth of nanocrystalline carbon films. However, despite the frequent use of plasma-assisted deposition techniques, the atomistic-scale mechanisms, including the effects of plasma density and energy on the nucleation process and growth orientation of the graphene-based nanocrystals, as well as associated dynamic processes involved in deposition processes, have not yet been thoroughly studied. In this paper, the plasma-assisted growth of nanocrystalline carbon thin films with preferentially oriented nanocrystals was systematically studied by hybrid molecular dynamics-Monte Carlo simulations using a recently developed force field, the charge-implicit ReaxFF. By combining the experimental data with the atomistic simulations, we reveal that plasma ion bombardments, in suitable ranges of energies and densities, allow the highest nucleation density in the nanocrystalline carbon films. Theoretically optimum windows of the plasma energy and density are first presented in the form of crystallization phase diagrams. Furthermore, to investigate the relationship between the growth orientation and the plasma ion energy, simulations of graphene irradiated with Ar ions from different incident angles were also performed. On the basis of the mechanism of "survival of the fittest", we proposed using the critical energy of generating the Stone-Thrower-Wales defects to design the growth orientation of graphite-like nanocrystals by controlling the plasma ion energy.

Large-Area Stable Superhydrophobic Poly(dimethylsiloxane) Films Fabricated by Thermal Curing via a Chemically Etched Template.

Inspired by nature, large-area stable superhydrophobic poly(dimethylsiloxane) (PDMS) films have generated extensive interest for various applications such as self-cleaning, corrosion protection, liquid transport, optical services, and flexible electronics. However, the current methods used to prepare such films are difficult to apply for efficient large-area fabrication. In this article, an effective technique for fabricating low adhesive superhydrophobic films based on the use of a chemically etched template followed by a thermal curing process is introduced. On the basis of this approach, the importance of chemical solution concentration as well as etching time is discussed to outline the specific rules required for forming different surface topographies of the templates. Then, PDMS films with varying wettabilities can be fabricated in which one can achieve CA > 160° and SA < 10°. Finally, for engineering needs and actual preparation, large-area PDMS films are obtained via a roll-to-roll (R2R) process, which show a superhydrophobic property even after high-intensity friction and have excellent acid and alkaline resistance, UV resistance, and optical transparency. The prepared large-area stable superhydrophobic PDMS films have the potential to be used in the aerospace field in the future because of their excellent anti-icing performance.

Effect of Epoxy Adhesive on Nugget Formation in Resistance Welding of SAE1004/DP600/DP780 Steel Sheets.

This study focused on the nugget formation in resistance welding of three dissimilar steel sheets influenced by different types and thicknesses of epoxy adhesive. An improved finite element model was employed to estimate the temperature distribution in three-sheet weld-bonding and was validated by the metallographic tests. Results showed that the weld initiation time and corresponding nugget size for weld-bonds would be earlier and larger than that of resistance spot welds in term of the same welding parameters. Compared to the adhesive Betamate Flex, the weld-bonding joint of three-sheets with adhesive Terokal 5089 would have a greater increment of the weld nugget sizes due to the increase of the static contact resistance brought by the interfaces between the steel sheets. However, the bond line thickness of the previously mentioned adhesive would take little effect on the weld sizes in weld-bonding of three dissimilar steel sheets.

Impact of Film Thickness on Defects and the Graphitization of Nanothin Carbon Coatings Used for Metallic Bipolar Plates in Proton Exchange Membrane Fuel Cells.

Metallic bipolar plates (BPPs) are considered promising alternatives to traditional graphite BPPs used in proton exchange membrane fuel cells (PEMFCs). Major auto companies, such as Toyota, GM, Ford, and BMW, are focusing on the development of metallic BPPs. Amorphous carbon (a-C) coating are widely known to be effective at enhancing the performance of metallic BPPs. However, a-C coatings prepared by sputtering are mostly micrometers thick, which can render mass production difficult due to their low deposition rates. In this study, we investigate effects of thickness on the formation of defects and the graphitization of nanothin a-C layers deposited by magnetron sputtering from scanning electron microscope (SEM) and transmission electron microscope (TEM) observations, internal stress measurements, X-ray diffractometer (XRD) data, Raman spectra, and X-ray photoelectron spectroscopy (XPS). Furthermore, corrosion and interfacial contact resistance (ICR) test results show that an approximately 69 nm a-C layer, with a deposition time of only 15 min, can meet ex situ technical targets of US Department of Energy. As the thickness of a-C layers increases, vacancy-like defects become more pronounced, which is accompanied by stress relaxation. Furthermore, the larger the graphite-like clusters, the more sp2-hybridization carbon atoms found in loose a-C films. The good properties of nanothin a-C layers are attributed to their limited defects and proper graphitization.

Enhanced Corrosion Resistance and Interfacial Conductivity of TiC x/a-C Nanolayered Coatings via Synergy of Substrate Bias Voltage for Bipolar Plates Applications in PEMFCs.

Proton-exchange membrane fuel cells are one kind of renewable and clean energy conversion device, whose metallic bipolar plates are one of the key components. However, high interfacial contact resistance and poor corrosion resistance are still great challenges for the commercialization of metallic bipolar plates. In this study, we demonstrated a novel strategy for depositing TiC x/amorphous carbon (a-C) nanolayered coatings by synergy of 60 and 300 V bias voltage to enhance corrosion resistance and interfacial conductivity. The synergistic effects of bias voltage on the composition, microstructure, surface roughness, electrochemical corrosion behaviors, and interfacial conductivity of TiC x/a-C coatings were explored. The results revealed that the columnar structures in the inner layer were suppressed and the surface became rougher with the 300 V a-C layer outside. The composition analysis indicated that the sp2 content increased with an increase of 300 V sputtering time. Due to the synergy strategy of bias voltage, lower corrosion current densities were achieved both in potentiostatic polarization (1.6 V vs standard hydrogen electrode) and potentiodynamic polarization. With the increase of 300 V sputtering time, the interfacial conductivity was improved. The enhanced corrosion resistance and interfacial conductivity of the TiC x/a-C coatings would provide new opportunities for commercial bipolar plates.

Flexible Semiconductor Technologies with Nanoholes-Provided High Areal Coverages and Their Application in Plasmonic-Enhanced Thin Film Photovoltaics.

Mechanical flexibility and advanced light management have gained great attentions in designing high performance, flexible thin film photovoltaics for the realization of building-integrated optoelectronic devices and portable energy sources. This study develops a soft thermal nanoimprint process for fabricating nanostructure decorated substrates integrated with amorphous silicon solar cells. Amorphous silicon (a-Si:H) solar cells have been constructed on nanoholes array textured polyimide (PI) substrates. It has been demonstrated that the nanostructures not only are beneficial to the mechanical flexibility improvement but also contribute to sunlight harvesting enhancement. The a-Si:H solar cells constructed on such nanopatterned substrates possess broadband-enhanced light absorption, high quantum efficiency and desirable power conversion efficiency (PCE) and still experience minimal PCE loss even bending around 180°. The PCE performance without antireflection coatings increases to 7.70% and it improves 40% compared with the planar devices. Although the advantages and feasibility of the schemes are demonstrated only in the application of a-Si:H solar cells, the ideas are able to extend to applications of other thin film photovoltaics and semiconductor devices.

Continuous fabrication of nanostructure arrays for flexible surface enhanced Raman scattering substrate.

Surface-enhanced Raman spectroscopy (SERS) has been a powerful tool for applications including single molecule detection, analytical chemistry, electrochemistry, medical diagnostics and bio-sensing. Especially, flexible SERS substrates are highly desirable for daily-life applications, such as real-time and in situ Raman detection of chemical and biological targets, which can be used onto irregular surfaces. However, it is still a major challenge to fabricate the flexible SERS substrate on large-area substrates using a facile and cost-effective technique. The roll-to-roll ultraviolet nanoimprint lithography (R2R UV-NIL) technique provides a solution for the continuous fabrication of flexible SERS substrate due to its high-speed, large-area, high-resolution and high-throughput. In this paper, we presented a facile and cost-effective method to fabricate flexible SERS substrate including the fabrication of polymer nanostructure arrays and the metallization of the polymer nanostructure arrays. The polymer nanostructure arrays were obtained by using R2R UV-NIL technique and anodic aluminum oxide (AAO) mold. The functional SERS substrates were then obtained with Au sputtering on the surface of the polymer nanostructure arrays. The obtained SERS substrates exhibit excellent SERS and flexibility performance. This research can provide a beneficial direction for the continuous production of the flexible SERS substrates.

Roll-to-roll hot embossing system with shape preserving mechanism for the large-area fabrication of microstructures.

Roll-to-roll (R2R) hot embossing is a promising approach to fulfilling the demands of high throughput fabrication of large-area polymeric components with micro-structure arrays which have been widely employed in the domains of optics, optoelectronics, biology, chemistry, etc. Nevertheless, the characteristic of continuous and fast forming for the R2R hot embossing process limits material flow during filling stage and results in significant springback during demolding stage. As a result, forming defects usually appear and the process window is very narrow which hinders the industrialization of this technology. This study developed a R2R hot embossing machine and proposed a shape preserving mechanism to extend the material filling time and realized the cooling effect during the demolding process. Comparative experiments were conducted on the R2R hot embossing process for micro-pyramid arrays to understand the effect of shape preserving mechanism. The influence of tension force and encapsulation angle to the forming quality was systematically analyzed. Furthermore, the influence of processing parameters has been investigated by using the one-variable-at-a-time method. Afterwards, a series of experiments based on the central composite design approach have been conducted for the analysis of variance and the establishment of empirical models of the R2R hot embossing process. As a result, the process window was extended by the shape preserving mechanism. More importantly, the feeding speed was improved from 0.5 m min-1 to 2.5 m min-1 for the large-area fabrication of micro-pyramid arrays, which is very attractive to the industrialization of this technology.