Water evaporation is a ubiquitous natural process that harvests thermal energy from the ambient environment. It has previously been utilized in a number of applications including the synthesis of nanostructures and the creation of energy-harvesting devices. Here, we show that water evaporation from the surface of a variety of nanostructured carbon materials can be used to generate electricity. We find that evaporation from centimetre-sized carbon black sheets can reliably generate sustained voltages of up to 1 V under ambient conditions. The interaction between the water molecules and the carbon layers and moreover evaporation-induced water flow within the porous carbon sheets are thought to be key to the voltage generation. This approach to electricity generation is related to the traditional streaming potential, which relies on driving ionic solutions through narrow gaps, and the recently reported method of moving ionic solutions across graphene surfaces, but as it exploits the natural process of evaporation and uses cheap carbon black it could offer advantages in the development of practical devices.

Hyperbolic media have attracted much attention in the photonics community due to their ability to confine light to arbitrarily small volumes and their potential applications to super-resolution technologies. The two-dimensional counterparts of these media can be achieved with hyperbolic metasurfaces that support in-plane hyperbolic guided modes upon nanopatterning, which, however, poses notable fabrication challenges and limits the achievable confinement. We show that thin flakes of a van der Waals crystal, α-MoO3, can support naturally in-plane hyperbolic polariton guided modes at mid-infrared frequencies without the need for patterning. This is possible because α-MoO3 is a biaxial hyperbolic crystal with three different Reststrahlen bands, each corresponding to a different crystalline axis. These findings can pave the way toward a new paradigm to manipulate and confine light in planar photonic devices.

Strong light-matter coupling manifested by Rabi splitting has attracted tremendous attention due to its fundamental importance in cavity quantum-electrodynamics research and great potentials in quantum information applications. A prerequisite for practical applications of the strong coupling in future optoelectronic devices is an all-solid-state system exhibiting room-temperature Rabi splitting with active control. Here we realized such a system in heterostructure consisted of monolayer WS and an individual plasmonic gold nanorod. By taking advantages of the small mode volume of the nanorod and large transition dipole moment of the WS exciton, giant Rabi splitting energies of 91-133 meV can be achieved at ambient conditions, which only involve a small number of excitons. The strong light-matter coupling can be dynamically tuned either by electrostatic gating or temperature scanning. These findings can pave the way toward active nanophotonic devices operating at room temperature.

the reaction system have a significant influence on the formation of hollow Ni spheres. The shape of the original template was imprinted after the reaction. It is expected that this emulsion system can be extended to the synthesis of other hollow metal spheres. Such hollow metal spheres have a variety of promising applications, for example: as catalysts; as magnetic and gas storage materials; and as low-density materials. Currently, the preparation and properties of Ni and other hollow metal spheres are underway in our laboratories. ExperimentalMaterials: Nickel sulfate, boric acid, ammonium fluoride, sodium hypophosphite monohydrate, cyclohexane, and polyglycol (M w 20 000) were of commercial grade and were used without further purification.Preparation of the Hollow Ni Submicrometer-Size Spheres: In a typical experiment, 2.5 g of polyglycol was first dissolved in 20 mL of deionized water, and then 47 mg (67.5 mM) NH 4 F, 246 mg (200 mM) H 3 BO 3 , 112 mg (20.0 mM) NiSO 4 , and 212 mg (100 mM) NaH 2 PO 2´H2 O were added to the solution. After that, 0.5 mL of cyclohexane was injected into 5 mL of the above solution. This mixture first was treated under ultrasonic radiation for 30 min at 20 C to make an emulsion system, and then at 75 C to induce the reducing reaction. After a period of time, the mixture foamed and the color of the mixture turned from green to light black, meaning that nickel was formed. The color of the mixture gradually turned blacker. 5 or 10 min after the mixture turned from green to light black, the mixture was cooled to room temperature. Finally, the mixture was separated by centrifugation. The deposit was collected, washed with deionized water several times, and dried in air: black, hollow nickel spheres were obtained.Structural characterization was performed by means of X-ray diffraction using a D/Max-RA diffractometer with Cu Ka radiation, TEM (JEM-200CX TEM) working at an accelerating voltage of 100 kV, and SEM operating at an accelerating voltage of 20 kV. EDS was performed on the microscope with a PV9100 scanning electron microanalyzer. Room-temperature magnetic characterization of the hollow Ni spheres was performed by using a Lake Shore 7303-9309 vibrating sample magnetometer (VSM). There is growing interest in nanowires of metals and related oxides because of their potential applications in nanoscale electronic and optoelectronic devices. [1,2] The template-based method is one of the techniques used for synthesizing nanowires.[3±7] However, for large-area electronic applications, such as field-emission displays, new synthetic methods may COMMUNICATIONS

Aluminum nitride nanostructures are attractive for many promising applications in semiconductor nanotechnology. Herein we report on vapor-solid growth of quasi-aligned aluminum nitride nanocones on catalyst-coated wafers via the reactions between AlCl3 vapor and NH3 gas under moderate temperatures around 700 degrees C, and the growth mechanism is briefly discussed. The as-prepared wurtzite aluminum nitride nanocones grow preferentially along the c-axis with adjustable dimensions of the sharp tips in the range of 20-60 nm. The photoluminescence spectrum reveals a broad blue emission band with a fine photon structure while the field emission study shows a notable emission current with a moderate turn-on field as expected, suggesting their potential applications in light and electron emission nanodevices.

2D van der Waals (vdW) layered polar crystals sustaining phonon polaritons (PhPs) have opened up new avenues for fundamental research and optoelectronic applications in the mid-infrared to terahertz ranges. To date, 2D vdW crystals with PhPs are only experimentally demonstrated in hexagonal boron nitride (hBN) slabs. For optoelectronic and active photonic applications, semiconductors with tunable charges, finite conductivity, and moderate bandgaps are preferred. Here, PhPs are demonstrated with low loss and ultrahigh electromagnetic field confinements in semiconducting vdW α-MoO . The α-MoO supports strong hyperbolic PhPs in the mid-infrared range, with a damping rate as low as 0.08. The electromagnetic confinements can reach ≈λ /120, which can be tailored by altering the thicknesses of the α-MoO 2D flakes. Furthermore, spatial control over the PhPs is achieved with a metal-ion-intercalation strategy. The results demonstrate α-MoO as a new platform for studying hyperbolic PhPs with tunability, which enable switchable mid-infrared nanophotonic devices.

No abstract

Large-scale, uniform, vertically standing graphene with atomically thin edges are controllably synthesized on copper foil using a microwave-plasma chemical vapor deposition system. A growth mechanism for this system is proposed. This film shows excellent field-emission properties, with low turn-on field of 1.3 V μm(-1) , low threshold field of 3.0 V μm(-1) and a large field-enhancement factor more than 10 000.