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 qualit

Continuous Shape Tuning of Nanotetrapods: Toward Shape-Mediated Self-Assembly

We describe a surfactant-driven method to synthesize highly monodisperse CdSe-seeded CdS tetrapods with differing arm lengths and diameters in order to examine their effects on self-assembly. We exploited the phenomena of weak- and strong-binding capping groups to tune the arm length and diameter with uniform shape and achieved >95% yield. Afterward, we utilize these particles to overcome some of the key problems in the assembly of anisotropic shaped particles. Intriguingly, we found that tetrapods with certain arm lengths pack like fishbone chains, which was greatly dependent on particle shape and size. These ordered assembly phenomena were understood with the assistance of computer simulations, which strongly support our experimental observations. Importantly, this work presents a synthetic route toward shape tuning in CdSe-seeded CdS tetrapod structures, which has great influence on their self-assembly behavior at the solution/substrate interface

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

Protein Induces Layer-by-Layer Exfoliation of Transition Metal Dichalcogenides

Here, we report a general and facile method for effective layer-by-layer exfoliation of transition metal dichalcogenides (TMDs) and graphite in water by using protein, bovine serum albumin (BSA) to produce single-layer nanosheets, which cannot be achieved using other commonly used bio- and synthetic polymers. Besides serving as an effective exfoliating agent, BSA can also function as a strong stabilizing agent against reaggregation of single-layer nanosheets for greatly improving their biocompatibility in biomedical applications. With significantly increased surface area, single-layer MoS₂ nanosheets also exhibit a much higher binding capacity to pesticides and a much larger specific capacitance. The protein exfoliation process is carefully investigated with various control experiments and density functional theory simulations. It is interesting to find that the nonpolar groups of protein can firmly bind to TMD layers or graphene to expose polar groups in water, facilitating the effective exfoliation of single-layer nanosheets in aqueous solution. The present work will enable to optimize the fabrication of various 2D materials at high yield and large scale, and bring more opportunities to investigate the unique properties of 2D materials and exploit their novel applications