Thermal Properties of the Binary-Filler Composites with Few-Layer Graphene and Copper Nanoparticles

The thermal properties of an epoxy-based binary composites comprised of graphene and copper nanoparticles are reported. It is found that the "synergistic" filler effect, revealed as a strong enhancement of the thermal conductivity of composites with the size-dissimilar fillers, has a well-defined filler loading threshold. The thermal conductivity of composites with a moderate graphene concentration of ~15 wt% exhibits an abrupt increase as the loading of copper nanoparticles approaches ~40 wt%, followed by saturation. The effect is attributed to intercalation of spherical copper nanoparticles between the large graphene flakes, resulting in formation of the highly thermally conductive percolation network. In contrast, in composites with a high graphene concentration, ~40 wt%, the thermal conductivity increases linearly with addition of copper nanoparticles. The electrical percolation is observed at low graphene loading, less than 7 wt.%, owing to the large aspect ratio of graphene. At all concentrations of the fillers, below and above the electrical percolation threshold, the thermal transport is dominated by phonons. The obtained results shed light on the interaction between graphene fillers and copper nanoparticles in the composites and demonstrate potential of such hybrid epoxy composites for practical applications in thermal interface materials and adhesives.Comment: 25 pages, 4 figure

Efficient Photon Upconversion Enabled by Strong Coupling Between Organic Molecules and Quantum Dots

Hybrid structures formed between organic molecules and inorganic quantum dots can accomplish unique photophysical transformations by taking advantage of their disparate properties. The electronic coupling between these materials is typically weak, leading photoexcited charge carriers to spatially localize to a dot or a molecule at its surface. However, we show that by converting a chemical linker that covalently binds anthracene molecules to silicon quantum dots from a carbon-carbon single bond to a double bond, we access a strong-coupling regime where excited carriers spatially delocalize across both anthracene and silicon. By pushing the system to delocalize, we design a photon upconversion system with a higher efficiency (17.2%) and lower threshold intensity (0.5 W/cm^2) than that of a corresponding weakly-coupled system. Our results show that strong coupling between molecules and nanostructures achieved through targeted linking chemistry provides a new route for tailoring properties in materials for light-driven applications.Comment: 33 pages (20 in main text, 13 in supporting information), 12 figures (5 in main text, 7 in supporting information

Harnessing Plasma Environments for Ammonia Catalysis: Mechanistic Insights from Experiments and Large-Scale Ab-initio Molecular Dynamics

By combining experimental measurements with ab initio molecular dynamics simulations, we provide the first microscopic description of the interaction between metal surfaces and a low-temperature nitrogen-hydrogen plasma. Our study focuses on the dissociation of hydrogen and nitrogen as the main activation route. We find that ammonia forms via an Eley-Rideal mechanism where atomic nitrogen abstracts hydrogen from the catalyst surface to form ammonia on an extremely short timescale (a few picoseconds). On copper, ammonia formation occurs via the interaction between plasma-produced atomic nitrogen and the H-terminated surface. On platinum, however, we find that surface saturation with NH groups is necessary for ammonia production to occur. Regardless of the metal surface, the reaction is limited by the mass transport of atomic nitrogen, consistent with the weak dependence on catalyst material that we observe and has been reported by several other groups. This study represents a significant step towards achieving a mechanistic, microscopic-scale understanding of catalytic processes activated in low-temperature plasma environments

A Non-Thermal Plasma Route to Plasmonic TiN Nanoparticles

In this contribution, we present a high-throughput method for the synthesis of titanium nitride nanoparticles. The technique, based on a continuous-flow nonthermal plasma process, leads to the formation of free-standing titanium nitride particles with crystalline structures and below 10 nm in size. Extinction measurements of the as-synthesized particles show a clear plasmonic resonance in the near-infrared region, with a peak plasmon position varying between 800 and 1000 nm. We have found that the composition can be controllably tuned by modifying the process parameters and that the particle optical properties are strongly dependent upon composition. XPS and STEM/EDS analyses suggest that nitrogen-poor particles are more susceptible to oxidation, and the extinction spectra show a decrease and a red-shift in plasmon peak position as the degree of oxidation increases. The role of oxidation is confirmed by real-time, time-dependent density functional tight binding (RT-TDDFTB) calculations, which also predict a decrease in the localized surface plasmon resonance energy when a single monolayer of oxygen is added to the surface of a titanium nitride nanocrystal. This study highlights the opportunity and challenges presented by this material system. Understanding the processing-properties relationships for alternative plasmonic materials such as titanium nitride is essential for their successful use in biomedical, photocatalytic, and optoelectronic applications

Colloidal Synthesis of Silicon-Carbon Composite Material for Lithium-Ion Batteries.

We report colloidal routes to synthesize silicon@carbon composites for the first time. Surface-functionalized Si nanoparticles (SiNPs) dissolved in styrene and hexadecane are used as the dispersed phase in oil-in-water emulsions, from which yolk-shell and dual-shell hollow SiNPs@C composites are produced via polymerization and subsequent carbonization. As anode materials for Li-ion batteries, the SiNPs@C composites demonstrate excellent cycling stability and rate performance, which is ascribed to the uniform distribution of SiNPs within the carbon hosts. The Li-ion anodes composed of 46 wt % of dual-shell SiNPs@C, 46 wt % of graphite, 5 wt % of acetylene black, and 3 wt % of carboxymethyl cellulose with an areal loading higher than 3 mg cm-2 achieve an overall specific capacity higher than 600 mAh g-1 , which is an improvement of more than 100 % compared to the pure graphite anode. These new colloidal routes present a promising general method to produce viable Si-C composites for Li-ion batteries

Achieving spin-triplet exciton transfer between silicon and molecular acceptors for photon upconversion.

Inorganic semiconductor nanocrystals interfaced with spin-triplet exciton-accepting organic molecules have emerged as promising materials for converting incoherent long-wavelength light into the visible range. However, these materials to date have made exclusive use of nanocrystals containing toxic elements, precluding their use in biological or environmentally sensitive applications. Here, we address this challenge by chemically functionalizing non-toxic silicon nanocrystals with triplet-accepting anthracene ligands. Photoexciting these structures drives spin-triplet exciton transfer from silicon to anthracene through a single 15 ns Dexter energy transfer step with a nearly 50% yield. When paired with 9,10-diphenylanthracene emitters, these particles readily upconvert 488-640 nm photons to 425 nm violet light with efficiencies as high as 7 ± 0.9% and can be readily incorporated into aqueous micelles for biological use. Our demonstration of spin-triplet exciton transfer from silicon to molecular triplet acceptors can critically enable new technologies for solar energy conversion, quantum information and near-infrared driven photocatalysis

Colloidal Synthesis of Silicon–Carbon Composite Material for Lithium‐Ion Batteries

We report colloidal routes to synthesize silicon@carbon composites for the first time. Surface-functionalized Si nanoparticles (SiNPs) dissolved in styrene and hexadecane are used as the dispersed phase in oil-in-water emulsions, from which yolk-shell and dual-shell hollow SiNPs@C composites are produced via polymerization and subsequent carbonization. As anode materials for Li-ion batteries, the SiNPs@C composites demonstrate excellent cycling stability and rate performance, which is ascribed to the uniform distribution of SiNPs within the carbon hosts. The Li-ion anodes composed of 46 wt % of dual-shell SiNPs@C, 46 wt % of graphite, 5 wt % of acetylene black, and 3 wt % of carboxymethyl cellulose with an areal loading higher than 3 mg cm-2 achieve an overall specific capacity higher than 600 mAh g-1 , which is an improvement of more than 100 % compared to the pure graphite anode. These new colloidal routes present a promising general method to produce viable Si-C composites for Li-ion batteries