Anomalous insulator metal transition in boron nitride-graphene hybrid atomic layers

The study of two-dimensional (2D) electronic systems is of great fundamental significance in physics. Atomic layers containing hybridized domains of graphene and hexagonal boron nitride (h-BNC) constitute a new kind of disordered 2D electronic system. Magneto-electric transport measurements performed at low temperature in vapor phase synthesized h-BNC atomic layers show a clear and anomalous transition from an insulating to a metallic behavior upon cooling. The observed insulator to metal transition can be modulated by electron and hole doping and by the application of an external magnetic field. These results supported by ab-initio calculations suggest that this transition in h-BNC has distinctly different characteristics when compared to other 2D electron systems and is the result of the coexistence between two distinct mechanisms, namely, percolation through metallic graphene networks and hopping conduction between edge states on randomly distributed insulating h-BN domains.Comment: 9 pages, 15 figure

Quasi-Molecular Fluorescence from Graphene Oxide

Aqueous dispersions of graphene oxide (GO) have been found to emit a structured, strongly pH-dependent visible fluorescence. Based on experimental results and model computations, this is proposed to arise from quasi-molecular fluorophores, similar to polycyclic aromatic compounds, formed by the electronic coupling of carboxylic acid groups with nearby carbon atoms of graphene. Sharp and structured emission and excitation features resembling the spectra of molecular fluorophores are present near 500 nm in basic conditions. The GO emission reversibly broadens and red-shifts to ca. 680 nm in acidic conditions, while the excitation spectra remain very similar in shape and position, consistent with excited state protonation of the emitting species in acidic media. The sharp and structured emission and excitation features suggest that the effective fluorophore size in the GO samples is remarkably well defined

Two-step fabrication of nanoporous copper films with tunable morphology for SERS application

peer-reviewedIt is important to design and fabricate nanoporous metals (NPMs) with optimized microstructures for specific applications. In this contribution, nanoporous coppers (NPCs) with controllable thicknesses and pore sizes were fabricated via the combination of a co-sputtering of Cu/Ti with a subsequent dealloying process. The effect of dealloying time on porous morphology and the corresponding surface enhanced Raman scattering (SERS) behaviors were systematically investigated. Transmission electron microscopy (TEM) identified the presences of the gaps formed between ligaments and also the nanobumps on the nanoparticle-aggregated ligament surface, which were likely to contribute as the “hot spots” for electromagnetic enhancement. The optimal NPC film exhibited excellent SERS performance towards Rhodamine 6G (R6G) with a low limiting detection (10−9 M), along with good uniformity and reproducibility. The calculated enhancement factor of ca. 4.71 × 107 was over Au substrates and comparable to Ag systems, promising the proposed NPC as a cheap candidate for high-performance SERS substrate

Self‐optimizing Cobalt Tungsten Oxide Electrocatalysts toward Enhanced Oxygen Evolution in Alkaline Media

Self‐optimizing mixed metal oxides represent a novel class of electrocatalysts for the advanced oxygen evolution reaction (OER). Here, we report self‐assembled cobalt tungsten oxide nanostructures on a lab‐synthesized copper oxide substrate through a single‐step deposition approach. The resulting composite exhibits remarkable self‐optimization behavior, shown by significantly reduced overpotentials and enhanced current densities, accompanied with substantial increase in OER kinetics, electrocatalytically active surface area, surface wettability, and electrical conductivity. Under operating conditions, interfacial restructuring of the electrocatalyst reveals the in situ formation of oxidized cobalt species as the true active site. Complementary density functional theory (DFT) calculations further demonstrate the formation of *OOH intermediate as the rate‐determining step of OER, and highlight the adaptive binding of oxygen intermediates, which transitions from tungsten to cobalt site during OER process. Our study provides a fundamental understanding of the self‐optimization mechanism and advances the knowledge‐driven design of efficient water‐splitting electrocatalysts

Nanocarbon Composites for Energy Storage Applications

In my talk, I will introduce our recent work on energy storage nanocarbon composite materials. Several structural designs of graphene and nano silicon anode composites were prepared for high energy density lithium ion battery; We also produced double-shell of graphene and artificial solid state electrolytes, such as Li7P3S11(LPS), Li4SiO4, LiAlO2, encapsulated commercial Si nanoparticles structure for LIB anode with excellent cycling stability and superior rate capability. A simple ball milling method was found to be a suitable method to produce nano-sized LiMn2O4and nanocarbon (graphene, carbon nanotubes, or super P) composites for high performance hybrid supercapacitor application. I will also introduce our work using porous nanocarbons as Li host materials for Lithium metal battery application. Ref: Ai Q., Ci L., et al., Diamond and Related Materials 88, 60 (2018) Chen L., Ci L., et al., Journal of Power Sources 392, 116 (2018) Xu X., Ci L., et al., Electrochimica Acta 276, 325 (2018) Hou G., Ci L., et al., Journal of Power Sources 386,77 (2018) Ma X., Ci L., et al., Journal of Materials Chemistry A 6, 1574 (2018) Ma X., Ci L., et al., Scientific Reports 7, 9642 (2017) Zhai W., Ci L., et al., Nano Research 10, 4274 (2017) Feng J., Ci L., et al., Journal of Power Sources 287, 177 (2015) </jats:p