Carbon-based single-atom catalysts: impacts of atomic coordination on the oxygen reduction reaction.

Slow oxygen reduction reaction (ORR) kinetics is the main factor restricting the development of fuel cells and metal-air batteries. Carbon-based single-atom catalysts (SACs) have the advantages of high electrical conductivity, maximal atom utilization, and high mass activity, thus showing great potential in exploring low-cost and high-efficiency ORR catalysts. For carbon-based SACs, the defects in the carbon support, the coordination of non-metallic heteroatoms, and the coordination number have a great influence on the adsorption of the reaction intermediates, thus significantly affecting the catalytic performance. Consequently, it is of vital importance to summarize the impacts of atomic coordination on the ORR. In this review, we focus on the regulation of the central atoms and coordination atoms of carbon-based SACs for the ORR. The survey involves various SACs, from noble metals (Pt) to transition metals (Fe, Co, Ni, Cu, etc.) and major group metals (Mg, Bi, etc.). At the same time, the influence of defects in the carbon support, the coordination of non-metallic heteroatoms (such as B, N, P, S, O, Cl, etc.), and the coordination number of the well-defined SACs on the ORR were put forward. Then, the impact of the neighboring metal monomers for SACs on the ORR performance is discussed. Finally, the current challenges and prospects for the future development of carbon-based SACs in coordination chemistry are presented.

Thermal gradient induced tweezers for the manipulation of particles and cells.

Optical tweezers are a well-established tool for manipulating small objects. However, their integration with microfluidic devices often requires an objective lens. More importantly, trapping of non-transparent or optically sensitive targets is particularly challenging for optical tweezers. Here, for the first time, we present a photon-free trapping technique based on electro-thermally induced forces. We demonstrate that thermal-gradient-induced thermophoresis and thermal convection can lead to trapping of polystyrene spheres and live cells. While the subject of thermophoresis, particularly in the micro- and nano-scale, still remains to be fully explored, our experimental results have provided a reasonable explanation for the trapping effect. The so-called thermal tweezers, which can be readily fabricated by femtosecond laser writing, operate with low input power density and are highly versatile in terms of device configuration, thus rendering high potential for integration with microfluidic devices as well as lab-on-a-chip systems.

Tunable double resonance of silver nanodecahedron on the insulator/conductor film.

The generation of double resonance in a nanostructure, thus permitting the modulation of optical field at two frequencies simultaneously, offers new application opportunities for surface enhanced Raman scattering (SERS) and surface enhanced fluorescence (SEF). Here, we present a simple composite nanostructure of silver nanodecahedron (Ag ND)/silica spacer/gold film/glass substrate for achieving double resonance under the normal incidence of polarized light. The optical responses of the composite structure have been theoretically studied by varying the thickness of silica spacer layer from 5 nm to 35 nm for mediating the interaction between Ag ND and gold film. Results indicate that the extinction spectrum of the composite system is strongly dependent on the separation between Ag ND and gold film. The electric field and charge distribution during resonance have been investigated in order to obtain a detailed understanding on the coupling between these two objects. More importantly, due to the anisotropic geometry of Ag ND, double resonance with two plasmonic modes (dipole and gap modes) whose responses can be adjusted through varying the size of Ag ND and mediating its coupling with the gold film respectively, has been achieved in the composite structure under the excitation with polarization parallel to the Ag ND edge adjacent to the spacer surface. The knowledge gained through this work will benefit the development of applications based on local field enhancement.

Surface-enhanced Raman scattering via entrapment of colloidal plasmonic nanocrystals by laser generated microbubbles on random gold nano-islands.

Surface-enhanced Raman scattering (SERS) typically requires hot-spots generated in nano-fabricated plasmonic structures. Here we report a highly versatile approach based on the use of random gold nano-island substrates (AuNIS). Hot spots are produced through the entrapment of colloidal plasmonic nano-crystals at the interface between AuNIS and a microbubble, which is generated from the localized plasmonic absorption of a focused laser beam. The entrapment strength is strongly dependent on the shape of the microbubble, which is in turn affected by the surface wetting characteristics of the AuNIS with respect to the solvent composition. The laser power intensity required to trigger microbubble-induced SERS is as low as 200 µW µm(-2). Experimental results indicate that the SERS limit of detection (LOD) for molecules of 4-MBA (with -SH bonds) is 10(-12) M, R6G or RhB (without -SH bonds) is 10(-7) M. The proposed strategy has potential applications in low-cost lab-on-chip devices for the label-free detection of chemical and biological molecules.

Electronic Properties of MoS2-WS2 Heterostructures Synthesized with Two-Step Lateral Epitaxial Strategy.

Formation of heterojunctions of transition metal dichalcogenides (TMDs) stimulates wide interest in new device physics and technology by tuning optical and electronic properties of TMDs. TMDs heterojunctions are of scientific and technological interest for exploration of next generation flexible electronics. Herein, we report on a two-step epitaxial ambient-pressure CVD technique to construct in-plane MoS2-WS2 heterostructures. The technique has the potential to artificially control the shape and structure of heterostructures or even to be more potentially extendable to growth of TMD superlattice than that of one-step CVD technique. Moreover, the unique MX2 heterostructure with monolayer MoS2 core wrapped by multilayer WS2 is obtained by the technique, which is entirely different from MX2 heterostructures synthesized by existing one-step CVD technique. Transmission electron microscopy, Raman and photoluminescence mapping studies reveal that the obtained heterostructure nanosheets clearly exhibit the modulated structural and optical properties. Electrical transport studies demonstrate that the special MoS2 (monolayer)/WS2 (multilayer) heterojunctions serve as intrinsic lateral p-n diodes and unambiguously show the photovoltaic effect. On the basis of this special heterostructure, depletion-layer width and built-in potential, as well as the built-in electric field distribution, are obtained by KPFM measurement, which are the essential parameters for TMD optoelectronic devices. With further development in future studies, this growth approach is envisaged to bring about a new growth platform for two-dimensional atomic crystals and to create unprecedented architectures therefor.

Lateral Built-In Potential of Monolayer MoS2-WS2 In-Plane Heterostructures by a Shortcut Growth Strategy.

Lateral WS2-MoS2 heterostructures are synthesized by a shortcut one-step growth recipe with low-cost and soluble salts. The 2D spatial distributions of the built-in potential and the related electric field of the lateral WS2-MoS2 heterostructure are quantitatively analyzed by scanning Kelvin probe force microscopy revealing the fundamental attributes of the lateral heterostructure devices.

Plasmonic random nanostructures on fiber tip for trapping live cells and colloidal particles.

We demonstrate optical trapping on a gold-coated single-mode fiber tip as excited by 980-nm laser radiation. The trapping force here is not due to common plasmonic localization, but dominated by the combined effect of thermophoresis and thermal convection. The reported scheme only requires simple thin-film deposition. More importantly, efficient broadband plasmonic absorption of the gold random nanostructures, aided by purely Gaussian excitation profile from the fiber core, has led to very low trapping-power threshold typically in hundreds of microwatts. This highly versatile fiber-based trapping scheme clearly offers many potential application possibilities in life sciences as well as engineering disciplines.

Optofluidic guiding, valving, switching and mixing based on plasmonic heating in a random gold nanoisland substrate.

We present a versatile optofluidic flow manipulation scheme based on plasmonic heating in a random gold nanoisland substrate (Au-NIS). With its highly efficient conversion of optical power to hydrodynamic actuation, the reported substrate is used for laser-controlled optofluidic manipulation. It is the first time that microfluidic flow guiding, valving, and mixing within the same functional substrate has been realised. Plasmonic heating provides power for guiding the sample flow inside a microfluidic channel at controlled speed and transport of small particles or living cells is demonstrated. We have also made a laser-actuated microfluidic valve through controlling the surface wettability of the sample/Au-NIS interface. When the laser power density is sufficiently high to generate a bubble, localized convection around the bubble can lead to efficient sample mixing within a microfluidic chamber. The reported Au-NIS scheme practically offers a programmable functional surface on which users have the freedom to control the wetting characteristics with a focused laser beam. We have verified that this optofluidic approach induces insignificant degradation in cell viability. The reported scheme therefore offers a wide range of application possibilities in microfluidics and biomedical engineering, particularly those operated under a low Reynolds number.

Trapping and assembling of particles and live cells on large-scale random gold nano-island substrates.

We experimentally demonstrated the use of random plasmonic nano-islands for optical trapping and assembling of particles and live cells into highly organized pattern with low power density. The observed trapping effect is attributed to the net contribution due to near-field optical trapping force and long-range thermophoretic force, which overcomes the axial convective drag force, while the lateral convection pushes the target objects into the trapping zone. Our work provides a simple platform for on-chip optical manipulation of nano- and micro-sized objects, and may find applications in physical and life sciences.

Experimental and theoretical investigation of macro-periodic and micro-random nanostructures with simultaneously spatial translational symmetry and long-range order breaking.

Photonic and plasmonic quasicrystals, comprising well-designed and regularly-arranged patterns but lacking spatial translational symmetry, show sharp diffraction patterns resulting from their long-range order in spatial domain. Here we demonstrate that plasmonic structure, which is macroscopically arranged with spatial periodicity and microscopically constructed by random metal nanostructures, can also exhibit the diffraction effect experimentally, despite both of the translational symmetry and long-range order are broken in spatial domain simultaneously. With strategically pre-formed metal nano-seeds, the tunable macroscopically periodic (macro-periodic) pattern composed from microscopically random (micro-random) nanoplate-based silver structures are fabricated chemically through photon driven growth using simple light source with low photon energy and low optical power density. The geometry of the micro-structure can be further modified through simple thermal annealing. While the random metal nanostructures suppress high-order Floquet spectra of the spatial distribution of refractive indices, the maintained low-order Floquet spectra after the ensemble averaging are responsible for the observed diffraction effect. A theoretical approach has also been established to describe and understand the macro-periodic and micro-random structures with different micro-geometries. The easy fabrication and comprehensive understanding of this metal structure will be beneficial for its application in plasmonics, photonics and optoelectronics.

Plasmonic graded nano-disks as nano-optical conveyor belt.

We propose a plasmonic system consisting of nano-disks (NDs) with graded diameters for the realization of nano-optical conveyor belt. The system contains a couple of NDs with individual elements coded with different resonant wavelengths. By sequentially switching the wavelength and polarization of the excitation source, optically trapped target nano-particle can be transferred from one ND to another. The feasibility of such function is verified based on the three-dimensional finite-difference time-domain technique and the Maxwell stress tensor method. Our design may provide an alternative way to construct nano-optical conveyor belt with which target molecules can be delivered between trapping sites, thus enabling many on-chip optofluidic applications.

Quantitative determination of scattering mechanism in large-area graphene on conventional and SAM-functionalized substrates at room temperature.

In this letter, the different scattering mechanisms of triphenylene-derived graphene on conventional SiO2/Si substrates and octadecyltrimethoxysilane (OTMS) self-assembled monolayer (SAM) functionalized SiO2/Si substrates were systematically studied at room temperature. In comparison with the devices on conventional SiO2/Si substrates, triphenylene-derived GFETs with OTMS-SAM modified SiO2/Si substrate exhibit the marked carrier-density-dependent field-effect mobility. Quantitative analyses reveal that at ambient temperature, the predominant scattering sources affect the carrier mean free path for graphene devices on bare SiO2 substrates and for those on OTMS passivated SiO2 substrates are charged impurity induced long-range scattering (~5.34 × 10(11) cm(-2) in carrier density) and resonant scattering (short-range scattering ~9.77 × 10(10) cm(-2) carrier in density), respectively. Our findings elucidate the underlying dominant factors for achieving significantly improved device performance of GFETs at room temperature.

Plasmonic optical trap having very large active volume realized with nano-ring structure.

The feasibility of using gold nano-rings as plasmonic nano-optical tweezers is investigated. We found that at a resonant wavelength of λ=785 nm, the nano-ring produces a maximum trapping potential of ~32k(B)T on gold nanoparticles. The existence of multiple potential wells results in a very large active volume of ~10(6) nm(3) for trapping the target particles. The report nano-ring design provides an effective approach for manipulating nano-objects in very low concentration into the high-field region and is well suited for integration with microfluidics for lab-on-a-chip applications.

Coupled metal gap waveguides as plasmonic wavelength sorters.

We propose a coupled metal gap waveguide structure for realizing plasmonic wavelength sorters. Theoretical analysis from the coupled-wave theory reveals that wavelength dependent coupling length of guided surface plasmon polaritons contributes to the routing of different wavelengths to different output ports with reasonable high extinction ratio. The analytical results are confirmed by the finite-difference time-domain numerical simulations. Our result may provide an alternative way to construct nanoscale frequency multiplexers, routers, and sorters for nanophotonic integration and optical communication.