A fluid model of pulsed direct current planar magnetron discharge.

We simulated a pulsed direct current (DC) planar magnetron discharge using fluid model, solving for species continuity, momentum, and energy transfer equations, coupled with Poisson equation and Lorentz force for electromagnetism. Based on a validated DC magnetron model, an asymmetric bipolar potential waveform is applied at the cathode at 50-200 kHz frequency and 50-80% duty cycle. Our results show that pulsing leads to increased electron density and electron temperature, but decreased deposition rate over non-pulsed DC magnetron, trends consistent with those reported by experimental studies. Increasing pulse frequency increases electron temperature but reduces the electron density and deposition rate, whereas increasing duty cycle decreases both electron temperature and density but increases deposition rate. We found that the time-averaged electron density scales inversely with the frequency, and time-averaged discharge voltage magnitude scales with the duty cycle. Our results are readily applicable to modulated pulse power magnetron sputtering and can be extended to alternating current (AC) reactive sputtering processes.

First-principles study of coadsorption of Cu2+ and Cl- ions on the Cu (110) surface.

Motivated by the importance of Cl- in the industrial electrolytic Cu plating process, we study the coadsorption of Cl- and Cu2+ on the Cu (110) surface using first-principles density functional theory (DFT) calculations. We treat the solvent implicitly by solving the linearized Poisson-Boltzmann equation and evaluate the electrochemical potential and energetics of ions with the computational hydrogen electrode approach. We find that Cl- alone is hardly adsorbed at sufficiently negative electrochemical potentials µ Cl but stable phases with half and full Cl- coverage was observed as µ Cl is made more positive. For Cl- and Cu2+ coadsorption, we identified five stable phases for electrode biases between -2V < U SHE < 2V, with two being Cl- adsorption phases, two being Cl- + Cu2+ coadsorption phases and one being a pure Cu2+ adsorption phase. In general, the free energy of adsorption for the most stable phases at larger |U SHE| are dominated by the energy required to move electrons between the system and the Fermi level of the electrode, while that at smaller |U SHE| are largely dictated by the binding strength between Cl- and Cu2+ adsorbates on the Cu (110) substrate. In addition, by studying the free energy of adsorption of Cu2+ onto pristine and Cl- covered Cu (110), we conclude that the introduction of Cl- ion does not improve the energetics of Cu2+ adsorption onto Cu (110).

An all-atom kinetic Monte Carlo model for chemical vapor deposition growth of graphene on Cu(1 1 1) substrate.

Various graphene morphologies (compact hexagonal, dendritic, and circular domains) have been observed during chemical vapor deposition (CVD) growth on Cu substrate. The existing all-atom kinetic Monte Carlo (kMC) models, however, are unable to reproduce all these graphene morphologies, suggesting that some crucial atomistic events that dictate the morphology are missing. In this work, we propose an all-atom kMC model to simulate the graphene CVD growth on Cu substrate. Besides the usual atomistic events, such as the deposition and diffusion of carbon species on the substrate, and their attachments to the edge, we further include three other important events, that is, the edge attachment of carbon species to form a kink, the diffusion of carbon species along the edge, and the rotation of dimers to form kinks. All the energetic parameters of these events are obtained from first-principles calculations. With this new model, we successfully predict the growth of various graphene morphologies, which are consistent with the morphology phase diagram. In addition to confirming that carbon dimers are the dominant feeding species, we also find that the dominance level depends on the growth flux and temperature. Therefore, the proposed model is able to capture the growth kinetics, providing a useful tool for controlled synthesis of graphene with desired morphologies.

Oxygen-Promoted Chemical Vapor Deposition of Graphene on Copper: A Combined Modeling and Experimental Study.

Mass production of large, high-quality single-crystalline graphene is dependent on a complex coupling of factors including substrate material, temperature, pressure, gas flow, and the concentration of carbon and hydrogen species. Recent studies have shown that the oxidation of the substrate surface such as Cu before the introduction of the C precursor, methane, results in a significant increase in the growth rate of graphene while the number of nuclei on the surface of the Cu substrate decreases. We report on a phase-field model, where we include the effects of oxygen on the number of nuclei, the energetics at the growth front, and the graphene island morphology on Cu. Our calculations reproduce the experimental observations, thus validating the proposed model. Finally, and more importantly, we present growth rate from our model as a function of O concentration and precursor flux to guide the efficient growth of large single-crystal graphene of high quality.

pH-dependent evolution of five-star gold nanostructures: an experimental and computational study.

Dendritic structures, such as snowflakes, have been observed in nature in far-from-equilibrium growth conditions. Mimicking these structures at the nanometer scale can result in nanomaterials with interesting properties for applications, such as plasmonics and biosensors. However, reliable production and systematic fine-tuning morphologies of these nanostructures, with novel hierarchical or complex structures, along with theoretical understanding of these processes, are still major challenges in the field. Here, we report a new method of using pH to control HAuCl4 reduction by hydroxylamine for facile production of gold nanostructures with morphologies in various symmetries and hierarchies, both in solution and on solid surface. Of particular interest is the observation of five-star-like dendritic and hierarchical gold nanostructures under certain reaction conditions. Phase-field modeling was used to understand the growth and formation dynamics of the five-star and other gold complex nanostructures, and the results not only explained the experimental observations, but also predicted control of the nanostructural morphologies using both pH and hydroxylamine concentrations. In addition to revealing interesting growth dynamics in forming fascinating complex gold nanostructures, the present work provides a pH-directed morphology control method as a facile way to synthesize and fine-tune the morphology of hierarchical gold nanostructures.