Energetic Performance of Optically Activated Aluminum/Graphene Oxide Composites.

Optical ignition of solid energetic materials, which can rapidly release heat, gas, and thrust, is still challenging due to the limited light absorption and high ignition energy of typical energetic materials ( e.g., aluminum, Al). Here, we demonstrated that the optical ignition and combustion properties of micron-sized Al particles were greatly enhanced by adding only 20 wt % of graphene oxide (GO). These enhancements are attributed to the optically activated disproportionation and oxidation reactions of GO, which release heat to initiate the oxidization of Al by air and generate gaseous products to reduce the agglomeration of the composites and promote the pressure rise during combustion. More importantly, compared to conventional additives such as metal oxides nanoparticles ( e.g., WO3 and Bi2O3), GO has much lower density and therefore could improve energetic properties without sacrificing Al content. The results from Xe flash ignition and laser-based excitation experiments demonstrate that GO is an efficient additive to improve the energetic performance of micron-sized Al particles, enabling micron-sized Al to be ignited by optical activation and promoting the combustion of Al in air.

Conformal Electroless Nickel Plating on Silicon Wafers, Convex and Concave Pyramids, and Ultralong Nanowires.

Nickel (Ni) plating has garnered great commercial interest, as it provides excellent hardness, corrosion resistance, and electrical conductivity. Though Ni plating on conducting substrates is commonly employed via electrodeposition, plating on semiconductors and insulators often necessitates electroless approaches. Corresponding plating theory for deposition on planar substrates was developed as early as 1946, but for substrates with micro- and nanoscale features, very little is known of the relationships between plating conditions, Ni deposition quality, and substrate morphology. Herein, we describe the general theory and mechanisms of electroless Ni deposition on semiconducting silicon (Si) substrates, detailing plating bath failures and establishing relationships between critical plating bath parameters and the deposited Ni film quality. Through this theory, we develop two different plating recipes: galvanic displacement (GD) and autocatalytic deposition (ACD). Neither recipe requires pretreatment of the Si substrate, and both methods are capable of depositing uniform Ni films on planar Si substrates and convex Si pyramids. In comparison, ACD has better tunability than GD, and it provides a more conformal Ni coating on complex and high-aspect-ratio Si structures, such as inverse fractal Si pyramids and ultralong Si nanowires. Our methodology and theoretical analyses can be leveraged to develop electroless plating processes for other metals and metal alloys and to generally provide direction for the adaptation of electroless deposition to modern applications.

Electroless Deposition and Ignition Properties of Si/Fe2O3 Core/Shell Nanothermites.

Thermite, a composite of metal and metal oxide, finds wide applications in power and thermal generation systems that require high-energy density. Most of the researches on thermites have focused on using aluminum (Al) particles as the fuel. However, Al particles are sensitive to electrostatic discharge, friction, and mechanical impact, imposing a challenge for the safe handling and storage of Al-based thermites. Silicon (Si) is another attractive fuel for thermites because of its high-energy content, thin native oxide layer, and facile surface functionality. Several studies showed that the combustion properties of Si-based thermites are comparable to those of Al-based thermites. However, little is known about the ignition properties of Si-based thermites. In this work, we determined the reaction onset temperatures of mechanically mixed (MM) Si/Fe2O3 nanothermites and Si/Fe2O3 core/shell (CS) nanothermites using differential scanning calorimetry. The Si/Fe2O3 CS nanothermites were prepared by an electroless deposition method. We found that the Si/Fe2O3 CS nanoparticles (NPs) had a lower reaction onset temperature (∼550 °C) than the MM Si/Fe2O3 nanothermites (>650 °C). The onset temperature of the Si/Fe2O3 CS nanothermites is also insensitive to the size of the Si core NP. These results indicate that the interfacial contact quality between Si and Fe2O3 is the dominant factor for determining the ignition properties of thermites. Finally, the reaction onset temperature of the Si/Fe2O3 CS NPs is comparable to that of the commonly used Al-based nanothermites, suggesting that Si is an attractive fuel for thermites.

Activating and optimizing MoS2 basal planes for hydrogen evolution through the formation of strained sulphur vacancies.

As a promising non-precious catalyst for the hydrogen evolution reaction (HER; refs ,,,,), molybdenum disulphide (MoS2) is known to contain active edge sites and an inert basal plane. Activating the MoS2 basal plane could further enhance its HER activity but is not often a strategy for doing so. Herein, we report the first activation and optimization of the basal plane of monolayer 2H-MoS2 for HER by introducing sulphur (S) vacancies and strain. Our theoretical and experimental results show that the S-vacancies are new catalytic sites in the basal plane, where gap states around the Fermi level allow hydrogen to bind directly to exposed Mo atoms. The hydrogen adsorption free energy (ΔGH) can be further manipulated by straining the surface with S-vacancies, which fine-tunes the catalytic activity. Proper combinations of S-vacancy and strain yield the optimal ΔGH = 0 eV, which allows us to achieve the highest intrinsic HER activity among molybdenum-sulphide-based catalysts.

Optoelectronic crystal of artificial atoms in strain-textured molybdenum disulphide.

The isolation of the two-dimensional semiconductor molybdenum disulphide introduced a new optically active material possessing a band gap that can be facilely tuned via elastic strain. As an atomically thin membrane with exceptional strength, monolayer molybdenum disulphide subjected to biaxial strain can embed wide band gap variations overlapping the visible light spectrum, with calculations showing the modified electronic potential emanating from point-induced tensile strain perturbations mimics the Coulomb potential in a mesoscopic atom. Here we realize and confirm this 'artificial atom' concept via capillary-pressure-induced nanoindentation of monolayer molybdenum disulphide from a tailored nanopattern, and demonstrate that a synthetic superlattice of these building blocks forms an optoelectronic crystal capable of broadband light absorption and efficient funnelling of photogenerated excitons to points of maximum strain at the artificial-atom nuclei. Such two-dimensional semiconductors with spatially textured band gaps represent a new class of materials, which may find applications in next-generation optoelectronics or photovoltaics.

Towards a dynamic clamp for neurochemical modalities.

The classic dynamic clamp technique uses a real-time electrical interface between living cells and neural simulations in order to investigate hypotheses about neural function and structure. One of the acknowledged drawbacks of that technique is the limited control of the cells' chemical microenvironment. In this manuscript, we use a novel combination of nanosensor and microfluidic technology and microfluidic and neural simulations to add sensing and control of chemical concentrations to the dynamic clamp technique. Specifically, we use a microfluidic lab-on-a-chip to generate distinct chemical concentration gradients (ions or neuromodulators), to register the concentrations with embedded nanosensors and use the processed signals as an input to simulations of a neural cell. The ultimate goal of this project is to close the loop and provide sensor signals to the microfluidic lab-on-a-chip to mimic the interaction of the simulated cell with other cells in its chemical environment.