Digital cameras with designs inspired by the arthropod eye.

In arthropods, evolution has created a remarkably sophisticated class of imaging systems, with a wide-angle field of view, low aberrations, high acuity to motion and an infinite depth of field. A challenge in building digital cameras with the hemispherical, compound apposition layouts of arthropod eyes is that essential design requirements cannot be met with existing planar sensor technologies or conventional optics. Here we present materials, mechanics and integration schemes that afford scalable pathways to working, arthropod-inspired cameras with nearly full hemispherical shapes (about 160 degrees). Their surfaces are densely populated by imaging elements (artificial ommatidia), which are comparable in number (180) to those of the eyes of fire ants (Solenopsis fugax) and bark beetles (Hylastes nigrinus). The devices combine elastomeric compound optical elements with deformable arrays of thin silicon photodetectors into integrated sheets that can be elastically transformed from the planar geometries in which they are fabricated to hemispherical shapes for integration into apposition cameras. Our imaging results and quantitative ray-tracing-based simulations illustrate key features of operation. These general strategies seem to be applicable to other compound eye devices, such as those inspired by moths and lacewings (refracting superposition eyes), lobster and shrimp (reflecting superposition eyes), and houseflies (neural superposition eyes).

Two-dimensional material membranes: an emerging platform for controllable mass transport applications.

Two-dimensional materials provide an ideal platform for studying the fundamental properties of atomic-level thickness systems, and are appropriate for lots of engineering applications in various fields. Although 2D materials are the thinnest membranes, they have been revealed to have high impermeability even to the smallest molecule. By the virtue of this high impermeability of the 2D materials in combination with their other unique properties, 2D materials open up a variety of applications that are impossible for conventional membranes. In this review, the latest applications based on high impermeability and selective permeation of these 2D material membranes are overviewed for different fields, including environmental control, chemical engineering, electronic devices, and biosensors. The working mechanism for each kind of application is described in detail. A summary and outlook is then provided on the challenges and new directions in this emerging research field.

Self-Driven Electrochromic Window System Cu/WOx-Al3+/GR with Dynamic Optical Modulation and Static Graph Display Functions.

Electrochromic devices with unique advantages of electrical/optical bistability are highly desired for energy-saving and information storage applications. Here, we put forward a self-driven Al-ion electrochromic system, which utilizes WOx films, Cu foil, and graphite rod as electrochromic optical modulation and graph display electrodes, coloration potential supplying electrodes, and bleaching potential supplying electrodes, respectively. The inactive Cu electrode can not only realize the effective Al3+ cation intercalation into electrochromic WOx electrodes but also eliminate the problem of metal anode consumption. The electrochromic WOx electrodes cycled in Al3+ aqueous media exhibit a wide potential window (∼1.5 V), high coloration efficiency (36.0 cm2/C), and super-long-term cycle stability (>2000 cycles). The dynamic optical modulation and static graph display function can be achieved independently only by switching the electrode connection mode, thus bringing more features to this electrochromic system. For a large-area electrochromic system (10 × 10 cm2), the absolute transmittance value in its color-neutral state can reach about 41% (27%) at 633 nm (780 nm) by connecting the Cu and WOx electrodes for 140 s. The original transparent state can be readily recovered by replacing the Cu foil with the graphite rod. This work throws light on next-generation electrochromic applications for optical/thermal modulation, privacy protection, and information display.

Nanocrystalline ZnSnN2 Prepared by Reactive Sputtering, Its Schottky Diodes and Heterojunction Solar Cells.

ZnSnN2 has potential applications in photocatalysis and photovoltaics. However, the difficulty in preparing nondegenerate ZnSnN2 hinders its device application. Here, the preparation of low-electron-density nanocrystalline ZnSnN2 and its device application are demonstrated. Nanocrystalline ZnSnN2 was prepared with reactive sputtering. Nanocrystalline ZnSnN2 with an electron density of approximately 1017 cm-3 can be obtained after annealing at 300 °C. Nanocrystalline ZnSnN2 is found to form Schottky contact with Ag. Both the current I vs. voltage V curves and the capacitance C vs. voltage V curves of these samples follow the related theories of crystalline semiconductors due to the limited long-range order provided by the crystallites with sizes of 2-10 nm. The I-V curves together with the nonlinear C-2-V curves imply that there are interface states at the Ag-nanocrystalline ZnSnN2 interface. The application of nanocrystalline ZnSnN2 to heterojunction solar cells is also demonstrated.

Temperature effect on electrospinning of nanobelts: the case of hafnium oxide.

Electrospinning is a convenient and versatile method for fabricating different kinds of one-dimensional nanostructures such as nanofibres, nanotubes and nanobelts. Environmental parameters have a great influence on the electrospinning nanostructure. Here we report a new method to fabricate hafnium oxide (HfO(2)) nanobelts. HfO(2) nanobelts were prepared by electrospinning a sol-gel solution with the implementation of heating and subsequent calcination treatment. We investigate the temperature dependence of the products by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy-dispersive x-ray (EDX) spectroscopy. The heating temperature of spinning ambient is found to be crucial to the formation of HfO(2) nanobelts. By tuning the temperature, the morphological transformation of HfO(2) from nanowires to nanobelts was achieved. It was found that the rapid evaporation of solvent played an important role in the formation process of HfO(2) nanobelts. It is shown that nanobelts can only be obtained with the temperature higher than 50 °C and they are in the high quality monoclinic phase. A possible growth mechanism of the nanobelts based on phase separation is proposed. The enhanced photoluminescence (PL) of HfO(2):Eu(3+) nanobelts is also illustrated.

Unprecedented Electrochromic Stability of a-WO3-x Thin Films Achieved by Using a Hybrid-Cationic Electrolyte.

With large interstitial space volumes and fast ion diffusion pathways, amorphous metal oxides as cathodic intercalation materials for electrochromic devices have attracted attention. However, these incompact thin films normally suffer from two inevitable imperfections: self-deintercalation of guest ions and poor stability of the structure, which constitute a big obstacle toward the development of high-stable commercial applications. Here, we present a low-cost, eco-friendly hybrid cation 1,2-PG-AlCl3·6H2O electrolyte, in which the sputter-deposited a-WO3-x thin film can exhibit both the long-desired excellent open-circuit memory (>100 h, with zero optical loss) and super-long cycling lifetime (∼20,000 cycles, with 80% optical modulation), benefiting from the formation of unique Al-hydroxide-based solid electrolyte interphase during electrochromic operations. In addition, the optical absorption behaviors in a-WO3-x caused by host-guest interactions were elaborated. We demonstrated that the intervalence transfers are primarily via the "corner-sharing" related path (W5+ ↔ W6+) but not the "edge-sharing" related paths (W4+ ↔ W6+ and/or W4+ ↔ W5+), and the small polaron/electron transfers taking place at the W-O bond-breaking positions are not allowed. Our findings might provide in-depth insights into the nature of electrochromism and provide a significant step in the realization of more stable, more excellent electrochromic applications based on amorphous metal oxides.

Layered coating of ultraflexible graphene-based electrodes for high-performance in-plane quasi-solid-state micro-supercapacitors.

To meet the demand of rapid development of portable and wearable electronic devices, in-plane quasi-solid-state micro-supercapacitors (QSS MSCs) have great potential as miniaturized energy storage devices. However, their ultralow areal capacitance and poor flexibility limit their practical applications. Here, we demonstrate a new strategy for the fabrication of ultraflexible MnO2@reduced graphene oxide (rGO) films (MGFs) for high-performance planar QSS MSCs through a facile layer-by-layer coating and a laser engraving method. Benefiting from conductive and flexible rGO films reduced by HI, the MGF based symmetrical QSS MSC exhibits a high areal capacitance (31.5 mF cm-2 at 0.2 mA cm-2), excellent flexibility (no capacity degradation at a bending radius from ∞ to 0 cm), and outstanding cycling stability (retaining 77.0% of its initial capacity after 6000 cycles). Most importantly, the electrochemical performance of QSS MSCs can be multiplied by simply adding more MGF layers. By adding up to 5 MGF layers, the MSC can deliver an ultrahigh areal capacitance of 144.3 mF cm-2 at 0.3 mA cm-2, and a superior energy density of 13.9 mW h cm-3 at 34.7 mW cm-3. Therefore, this work offers versatile quasi-solid-state MSCs and provides an impressive strategy to enhance electrochemical performance which will greatly enrich the design and fabrication of MSCs.

Structure Shift of GaN Among Nanowall Network, Nanocolumn, and Compact Film Grown on Si (111) by MBE.

Structure shift of GaN nanowall network, nanocolumn, and compact film were successfully obtained on Si (111) by plasma-assisted molecular beam epitaxy (MBE). As is expected, growth of the GaN nanocolumns was observed in N-rich condition on bare Si, and the growth shifted to compact film when the Ga flux was improved. Interestingly, if an aluminum (Al) pre-deposition for 40 s was carried out prior to the GaN growth, GaN grows in the form of the nanowall network. Results show that the pre-deposited Al exits in the form of droplets with typical diameter and height of ~ 80 and ~ 6.7 nm, respectively. A growth model for the nanowall network is proposed and the growth mechanism is discussed. GaN grows in the area without Al droplets while the growth above Al droplets is hindered, resulting in the formation of continuous GaN nanowall network that removes the obstacles of nano-device fabrication.

Mass transport mechanism of cu species at the metal/dielectric interfaces with a graphene barrier.

The interface between the metal and dielectric is an indispensable part in various electronic devices. The migration of metallic species into the dielectric can adversely affect the reliability of the insulating dielectric and can also form a functional solid-state electrolyte device. In this work, we insert graphene between Cu and SiO2 as a barrier layer and investigate the mass transport mechanism of Cu species through the graphene barrier using density functional theory calculations, second-ion mass spectroscopy (SIMS), capacitance-voltage measurement, and cyclic voltammetry. Our theoretical calculations suggest that the major migration path for Cu species to penetrate through the multiple-layered graphene is the overlapped defects larger than 0.25 nm2. The depth-profile SIMS characterizations indicate that the "critical" thickness of the graphene barrier for completely blocking the Cu migration is 5 times smaller than that of the conventional TaN barrier. Capacitance-voltage and cyclic voltammetry measurement reveal that the electrochemical reactions at the Cu/SiO2 interface become a rate-limiting factor during the bias-temperature stressing process with the use of a graphene barrier. These studies provide a distinct roadmap for designing controllable mass transport in solid-state electrolyte devices with the use of a graphene barrier.

Synthesis and White-Light Emission of ZnO/HfO(2): Eu Nanocables.

ZnO/HfO(2):Eu nanocables were prepared by radio frequency sputtering with electrospun ZnO nanofibers as cores. The well-crystallized ZnO/HfO(2):Eu nanocables showed a uniform intact core-shell structure, which consisted of a hexagonal ZnO core and a monoclinic HfO(2) shell. The photoluminescence properties of the samples were characterized. A white-light band emission consisted of blue, green, and red emissions was observed in the nanocables. The blue and green emissions can be attributed to the zinc vacancy and oxygen vacancy defects in ZnO/HfO(2):Eu nanocables, and the yellow-red emissions are derived from the inner 4f-shell transitions of corresponding Eu(3+) ions in HfO(2):Eu shells. Enhanced white-light emission was observed in the nanocables. The enhancement of the emission is ascribed to the structural changes after coaxial synthesis.