Active Metamaterials with Negative Static Electric Susceptibility

Although well‐established textbook arguments suggest that static electric susceptibility χ(0) must be positive in “all bodies,” it has been pointed out that materials that are not in thermodynamic equilibrium are not necessarily subject to this restriction. Media with inverted populations of atomic and molecular energy levels have been predicted theoretically to exhibit a χ(0) < 0 state, however the systems envisioned require reduced temperature, reduced pressure, and an external pump laser to maintain the population inversion. Further, the existence of χ(0) < 0 has never been confirmed experimentally. Here, a completely different approach is taken to the question of χ(0) < 0 and a design concept to achieve “true” χ(0) < 0 is proposed based on active metamaterials with internal power sources. Two active metamaterial structures are fabricated that, despite still having their power sources implemented externally for reasons of practical convenience, provide evidence in support of the general concept. Effective values are readily achieved at room temperature and pressure and are tunable throughout the range of stability −1 < χ(0) < 0, resulting in experimentally‐determined magnitudes that are over one thousand times greater than those predicted previously. Since χ(0) < 0 is the missing electric analog of diamagnetism, this work opens the door to new technological capabilities such as stable electrostatic levitation

Tunable cavity coupling of the zero phonon line of a nitrogen-vacancy defect in diamond

We demonstrate the tunable enhancement of the zero phonon line of a single nitrogen-vacancy color center in diamond at cryogenic temperature. An open cavity fabricated using focused ion beam milling provides mode volumes as small as 1.24 $\mu$m$^3$. In-situ tuning of the cavity resonance is achieved with piezoelectric actuators. At optimal coupling of the full open cavity the signal from individual zero phonon line transitions is enhanced by about a factor of 10 and the overall emission rate of the NV$^-$ center is increased by 40% compared with that measured from the same center in the absence of cavity field confinement. This result is important for the realization of efficient spin-photon interfaces and scalable quantum computing using optically addressable solid state spin qubits.Comment: 11 pages Main Article + 4 pages Supplementary Info Typos fixed from v

High Performance PbS Quantum Dot/Graphene Hybrid Solar Cell with Efficient Charge Extraction.

Hybrid colloidal quantum dot (CQD) solar cells are fabricated from multilayer stacks of lead sulfide (PbS) CQD and single layer graphene (SG). The inclusion of graphene interlayers is shown to increase power conversion efficiency by 9.18%. It is shown that the inclusion of conductive graphene enhances charge extraction in devices. Photoluminescence shows that graphene quenches emission from the quantum dot suggesting spontaneous charge transfer to graphene. CQD photodetectors exhibit increased photoresponse and improved transport properties. We propose that the CQD/SG hybrid structure is a route to make CQD thin films with improved charge extraction, therefore resulting in improved solar cell efficiency

Monodisperse PbS nanocrystals synthesized in a conducting polymer

A novel method for synthesizing monodisperse 10 nm Lead Sulphide (PbS) nanocrystals in the conducting polymer poly-(3-hexylthiophene-2,5-diyl) (P3HT) at elevated temperatures is reported. Transmission electron microscopy shows that nanocrystals grow rapidly to 10 nm with a monodisperse size distribution. Selected area electron diffraction shows rings that are matched to the PbS rock-salt crystal structure. Steady-state optical spectroscopy and energy dispersive X-ray spectroscopy are also used to confirm the formation of PbS nanocrystals. © 2006 Elsevier B.V. All rights reserved

Controlling PbS nanocrystal aggregation in conducting polymers

PbS nanocrystals were synthesized directly in the conducting polymer, poly (3 -hexylthiophene-2,5-diyl). Transmission electron microscopy shows that the PbS nanocrystals are faceted and relatively uniform in size with a mean size of 10 nm. FFT analysis of the atomic lattice planes observed in TEM and selected area electron diffraction confirm that the nanocrystals have the PbS rock salt structure. The synthesis conditions are explored to show control over the aggregation of PbS nanocrystals in the thiophene conducting polymer

Flexography printing for organic thin film transistors

High throughput manufacture is of key importance for flexible electronics based on functional organic thin films. One challenge in particular is patterning at high speeds. In this paper results are presented of in-vacuum patterning of an organic dielectric, which has been used in organic thin film transistors. This combines an industry standard patterning method with resolution good enough to create functional devices. The viability of flexography is critiqued from the literature and the compatibility of the diacrylate dielectric monomer with printing system is investigated. It is found that flexography printing of a diacrylate monomer is viable and compatible and initial results from print trials are described. The results show that good pattern fidelity can be achieved with capacitance measured to be between 11 and 13nF.cm-2 with a thickness of 40nm, but that the curing step of the process leads to high surface roughness, although without pin hole defects. The surprisingly thin acrylate layers are attributed to the low viscosity of the monomer and the high capacitance measured being a result of the large surface area of the rough acrylate surface, probably resulting from the charged particle curing step. This paper concludes that this in-vacuum patterning technique could have many applications for organic electronic devices

Thermally deposited lead oxides for thin film photovoltaics

Lead oxide is demonstrated for the first time as the active layer in a Schottky junction photovoltaic device. Thin films of lead were thermally deposited and oxidised to produce polycrystalline films of lead oxide. Different heat treatments yield variations in the ratio of orthorhombic to tetragonal lead oxide that lead to different device performances, where devices with a higher content of orthorhombic PbO show higher power conversion efficiencies of up to 0.17%. Efficiencies are limited by low short circuit currents and further improvements are expected through increased film homogeneity and grain size as well as the implementation of suitable blocking layers to stop current leakage. © 2011 Elsevier B.V. All rights reserved

Flexography printing for organic thin film transistors

High throughput manufacture is of key importance for flexible electronics based on functional organic thin films. One challenge in particular is patterning at high speeds. In this paper results are presented of in-vacuum patterning of an organic dielectric, which has been used in organic thin film transistors. This combines an industry standard patterning method with resolution good enough to create functional devices. The viability of flexography is critiqued from the literature and the compatibility of the diacrylate dielectric monomer with printing system is investigated. It is found that flexography printing of a diacrylate monomer is viable and compatible and initial results from print trials are described. The results show that good pattern fidelity can be achieved with capacitance measured to be between 11 and 13nF.cm-2 with a thickness of 40nm, but that the curing step of the process leads to high surface roughness, although without pin hole defects. The surprisingly thin acrylate layers are attributed to the low viscosity of the monomer and the high capacitance measured being a result of the large surface area of the rough acrylate surface, probably resulting from the charged particle curing step. This paper concludes that this in-vacuum patterning technique could have many applications for organic electronic devices

Layer-by-layer spray deposition and unzipping of single-wall carbon nanotube-based thin film electrodes for electrochemical capacitors

Spray deposited single-wall carbon nanotube (SWCNT) film electrodes comprising a randomly interconnected meso-porous network with high electrical conductivity and ionic mobility when immersed in an electrolyte have been investigated for applications in electrochemical capacitors. Layer-by-layer (LbL) spray deposition assembly of functionalised, oppositely charged (carboxylic COO- or amine NH3+) SWCNTs has been used to control the density and structure of sprayed electrodes, followed by vacuum and hydrogen heat treatment, and their electrochemical performance as a supercapacitor electrode assessed. Sprayed LbL SWCNT electrodes had faster charging and discharging kinetics than SWCNT-COOH electrodes, with a capacitance of 94 F g-1 compared with 63 F g-1 at 2 mV s-1 in a 1 M H2SO4 electrolyte; and LbL electrode capacitance was maintained to 91 F g-1 at 200 mV s-1. The capacitance of LbL SWCNT electrodes increased to 151 F g-1 at 2 mV s-1 after vacuum and H2 heat treatment that removed NH2 and COOH functional groups, and resulted in a hybrid microstructure of SWCNTs and multi-layer graphene sheets from unzipped SWCNTs. © 2013 Elsevier Ltd. All rights reserved

Crystallization and Self-Assembly of Calcium Carbonate Architectures

Shape-controlled crystallization and self-assembly of high-ordered CaCO3 architectures were realized by using soluble starch as structure directing agents in aqueous solution. Mushrooms, dumbbells, spheres, and truncated-octahedron CaCO3 crystals composed of spindly microcrystals were obtained by tuning the experimental parameters, such as the concentration of starch, pH value, and the concentration of Ca2+ cations. The phases, morphologies, and structures of the products were characterized by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, and infrared spectrophotometry. The surface properties of CaCO3 crystals were also successfully modified to be hydrophobic using n-dodecanethiol as a modifier. The contact angle changed from 23° to 137° and 148° after surface treatment. The hydrophobic properties of the resulting products may have potential applications as a filler in composite materials. © 2008 American Chemical Society