Silicon as a ubiquitous contaminant in graphene derivatives with significant impact on device performance

Silicon-based impurities are ubiquitous in natural graphite. However, their role as a contaminant in exfoliated graphene and their influence on devices have been overlooked. Herein atomic resolution microscopy is used to highlight the existence of silicon-based contamination on various solution-processed graphene. We found these impurities are extremely persistent and thus utilising high purity graphite as a precursor is the only route to produce silicon-free graphene. These impurities are found to hamper the effective utilisation of graphene in whereby surface area is of paramount importance. When non-contaminated graphene is used to fabricate supercapacitor microelectrodes, a capacitance value closest to the predicted theoretical capacitance for graphene is obtained. We also demonstrate a versatile humidity sensor made from pure graphene oxide which achieves the highest sensitivity and the lowest limit of detection ever reported. Our findings constitute a vital milestone to achieve commercially viable and high performance graphene-based devices

Photonic fractal metamaterials: a metal–semiconductor platform with enhanced volatile‐compound sensing performance

Advance of photonics media is restrained by the lack of structuring techniques for the 3D fabrication of active materials with long‐range periodicity. A methodology is reported for the engineering of tunable resonant photonic media with thickness exceeding the plasmonic near‐field enhancement region by more than two orders of magnitude. The media architecture consists of a stochastically ordered distribution of plasmonic nanocrystals in a fractal scaffold of high‐index semiconductors. This plasmonic‐semiconductor fractal media supports the propagation of surface plasmons with drastically enhanced intensity over multiple length scales, overcoming the 2D limitations of established metasurface technologies. The fractal media are used for the fabrication of plasmonic optical gas sensors, achieving a limit of detection of 0.01 vol% at room temperature and sensitivity up to 1.9 nm vol%−1, demonstrating almost a fivefold increase with respect to an optimized planar geometry. Beneficially to their implementation, the self‐assembly mechanism of this fractal architecture allows fabrication of micrometer‐thick media over surfaces of several square centimeters in a few seconds. The designable optical features and intrinsic scalability of these photonic fractal metamaterials provide ample opportunities for applications, bridging across transformation optics, sensing, and light harvesting

Hybrid crystalline-ITO/metal nanowire mesh transparent electrodes and their application for highly flexible perovskite solar cells

Here, we propose crystalline indium tin oxide/metal nanowire composite electrode (c-ITO/metal NW-GFRHybrimer) films as a robust platform for flexible optoelectronic devices. A very thin c-ITO overcoating layer was introduced to the surface-embedded metal nanowire (NW) network. The c-ITO/metal NW-GFRHybrimer films exhibited outstanding mechanical flexibility, excellent optoelectrical properties and thermal/chemical robustness. Highly flexible and efficient metal halide perovskite solar cells were fabricated on the films. The devices on the c-ITO/AgNW- and c-ITO/CuNW-GFRHybrimer films exhibited power conversion efficiency values of 14.15% and 12.95%, respectively. A synergetic combination of the thin c-ITO layer and the metal NW mesh transparent conducting electrode will be beneficial for use in flexible optoelectronic applications

Gold nanoparticles to boost the gas sensing performance of porous sol-gel thin films

In this paper we review our research work of the last few years on the synthesis and the gas sensing properties of nanocomposite thin films of sensitive materials with a large specific surface area, which consist of porous matrices containing functional nanocrystals of metal oxides and gold. The film porosity provides a path for the gas molecules to reach the active reaction sites on the nanoparticles surface undergoing chemical reactions which nature depends on the nature of the active material. The introduction of Au nanoparticles affects the reactions mechanism improving the sensing process, moreover the Au Surface Plasmon Resonance peak can be used for the realization of selective optical gas sensor. Two different synthetic approaches will be described, each of them characterized by a peculiar control of the final materials morphology, structure and micro-structure. © 2011 Springer Science+Business Media, LLC

Flash-assisted processing of highly conductive zinc oxide electrodes from water

Fabricating high-quality transparent conductors using inexpensive and industrially viable techniques is a major challenge toward developing low cost optoelectronic devices such as solar cells, light emitting diodes, and touch panel displays. In this work, highly transparent and conductive ZnO thin films are prepared from a low-temperature, aqueous deposition method through the careful control of the reaction chemistry. A robotic synthetic platform is used to explore the wide parameter space of a chemical bath system that uses only cheap and earth abundant chemicals for thin film deposition. As-deposited films are found to be highly resistive, however, through exposure to several millisecond pulses of high-intensity, broadband light, intrinsically doped ZnO films with sheet resistances as low as 40 Ω □-1 can be readily prepared. Such values are comparable with state-of-the-art-doped transparent conducting oxides. The mild processing conditions (<150 °C) of the ZnO electrodes also enable their deposition on temperature sensitive substrates such as PET, paving the way for their use in various flexible optoelectronic devices. Proof-of-concept light emitting devices employing ZnO as a transparent electrode are presented. Highly conductive, intrinsically doped ZnO electrodes are prepared using a low-temperature aqueous deposition and a flash light postprocessing. The aqueous bath chemistry favors the formation of oxygen deficient ZnO films, which are then flashed with millisecond light pulses to achieve record conductivity and high transparency. Deposition on PET substrates and indium-tin-oxide-free optoelectronic devices is demonstrated

Photonic sintering of copper through the controlled reduction of printed CuO nanocrystals

The ability to control chemical reactions using ultrafast light exposure has the potential to dramatically advance materials and their processing toward device integration. In this study, we show how intense pulsed light (IPL) can be used to trigger and modulate the chemical transformations of printed copper oxide features into metallic copper. By varying the energy of the IPL, CuO films deposited from nanocrystal inks can be reduced to metallic Cu via a Cu2O intermediate using single light flashes of 2 ms duration. Moreover, the morphological transformation from isolated Cu nanoparticles to fully sintered Cu films can also be controlled by selecting the appropriate light intensity. The control over such transformations enables for the fabrication of sintered Cu electrodes that show excellent electrical and mechanical properties, good environmental stability, and applications in a variety of flexible devices

Selective near-perfect absorbing mirror as a spatial frequency filter for optical image processing

Spatial frequency filtering is a fundamental enabler of information processing methods in biological and technical imaging. Most filtering methods, however, require either bulky and expensive optical equipment or some degree of computational processing. Here, we experimentally demonstrate real-time, on-chip, all-optical spatial frequency filtering using a thin-film perfect absorber structure. We experimentally demonstrate edge enhancement of an amplitude image and conversion of phase gradients to intensity modulation in an image. The device is used to demonstrate enhancement of an image of pond algae

Directing Energy into a Subwavelength Nonresonant Metasurface across the Visible Spectrum

Group 10 metals (i.e., Ni, Pd, Pt) catalyze a wide range of chemical transformations, but the weak interaction of their nanoparticles with light hinders their development for photocatalytic applications. Conversely, coinage metal nanoparticles (particularly Ag and Au) exhibit intense localized surface plasmon resonances in the visible spectrum but are relatively unreactive, limiting the scope and efficiency of their photochemical processes. Here we demonstrate the design, fabrication, and characterization of a new structure containing a single layer of Pd nanoparticles that absorbs up to >98% of visible light. Furthermore, the wavelength of absorption is controlled throughout the visible range of the electromagnetic spectrum by modulating the thickness of a supporting metal oxide film. We show that the absorbed energy is concentrated in the nanoparticle layer, crucial for energy conversion applications, including photocatalysis and photothermal processes