Influence of Concentration Gradients on Electroconvection at a Cation-Exchange Membrane Surface.

Membrane-based systems, such as electrodialysis, play an important role in desalination and industrial separation processes. Electrodialysis uses alternating anion- and cation-exchange membranes with a perpendicular electric field to generate concentrated and diluate streams from a feed solution. It is known that under overlimiting current conditions, reduced charge and mass transfer at the membrane interface leads to regions of high ion depletion generating instability and vortices termed electroconvection. While electroconvective mixing is known to directly impact the separation efficiency of electrodialysis, the influence of ion concentration gradients across the membrane experienced in a functional electrodialysis system is not known. Here, we report the influence of ion concentration gradients across a cation exchange membrane (Nafion) that is both aligned with and opposed to the applied electric field. Experiments were conducted by coflowing NaCl solutions of different concentrations (0.1-100 mM) on each side of the membrane, and electroconvection was visualized with a fluorescence dye (Rhodamine 6G). We obtained concentration profiles from fluorescence image data and systematically measured the thickness of the depletion boundary layer dBL under different conditions. We found smaller dBL values at a higher flow rate both with and without concentration gradients. Our results show that electroconvection is enhanced when the electric field is opposite to the direction of the concentration gradient.

A Universal Approach to Determine the Atomic Layer Numbers in Two-Dimensional Materials Using Dark-Field Optical Contrast.

Two-dimensional (2D) materials possess unique properties primarily due to the quantum confinement effect, which highly depends on their thicknesses. Identifying the number of atomic layers in these materials is a crucial, yet challenging step. However, the commonly used optical reflection method offers only very low contrast. Here, we develop an approach that shows unprecedented sensitivity by analyzing the brightness of dark-field optical images. The brightness of the 2D material edges has a linear dependence on the number of atomic layers. The findings are modeled by Rayleigh scattering, and the results agree well with the experiments. The relative contrast of single-layer graphene can reach 70% under white-light incident conditions. Furthermore, different 2D materials were successfully tested. By adjusting the exposure conditions, we can identify the number of atomic layers ranging from 1 to over 100. Finally, this approach can be applied to various substrates, even transparent ones, making it highly versatile.

Bobbing chemical garden tubes: oscillatory self-motion from buoyancy and catalytic gas production.

Chemical reactions can induce self-propulsion by the production and ejection of gas bubbles from micro-rocket like cylindrical units. We describe related micro-submarines that change their depth in response to catalytic gas production. The structures consist of silica-supported CuO and are produced by utilizing the self-assembly rules of chemical gardens. In H2O2 solution, the tube cavity produces O2(g) and the resulting buoyancy lifts the tube to the air-solution interface, where it releases oxygen and sinks back down to the bottom of the container. In 5 cm deep solutions, the resulting bobbing cycles have a period of 20-30 s and repeat for several hours. The ascent is characterized by a vertical orientation of the tube and a constant acceleration. During the descent, the tubes are oriented horizontally and sink at a nearly constant speed. These striking features are quantitatively captured by an analysis of the involved mechanical forces and chemical kinetics. The results show that ascending tubes increase their oxygen-production rate by the motion-induced injection of fresh solution into the tube cavity.

Actively Controllable Terahertz Metal-Graphene Metamaterial Based on Electromagnetically Induced Transparency Effect.

A metal-graphene metamaterial device exhibiting a tunable, electromagnetically induced transparency (EIT) spectral response at terahertz frequencies is investigated. The metamaterial structure is composed of a strip and a ring resonator, which serve as the bright and dark mode to induce the EIT effect. By employing the variable conductivity of graphene to dampen the dark resonator, the response frequency of the device shifts dynamically over 100 GHz, which satisfies the convenient post-fabrication tunability requirement. The slow-light behavior of the proposed device is also analyzed with the maximum group delay of 1.2 ps. The sensing performance is lastly studied and the sensitivity can reach up to 100 GHz/(RIU), with a figure of merit (FOM) value exceeding 4 RIU-1. Therefore, the graphene-based metamaterial provides a new miniaturized platform to facilitate the development of terahertz modulators, sensors, and slow-light applications.

Shape-preserving conversion of calcium carbonate tubes to self-propelled micromotors.

The self-assembly of inorganic structures beyond the euhedral shape repertoire is a powerful approach to grow hierarchically ordered materials and mesoscopic devices. The hollow precipitate tubes in chemical gardens are a classic example, which we produce on Nafion membranes separating a CaCl2-containing gel from a Na2CO3 solution. The resulting CaCO3 microtubes are conical and consist of either pure vaterite or calcite. The process also forms branched T- and Y-shaped structures. The metastable vaterite polymorph can be converted to Mn-based structures without loss of the macroscopic shape. In H2O2 solution, the resulting tubes self-propel by the release of O2 bubbles, which for branched structures causes rotation. The tubes can contain multiple bubbles which are ejected in a quasi-periodic fashion (e.g. in groups of four). The addition of surfactants causes the accumulation of bubble trails and bubble rafts that interact with the moving tubes and give rise to distinct motion patterns.

Front-like expansion and arrest of programmed cell death in brown banana spots.

The spot patterns on bananas are a striking case of biological pattern formation and-as a qualitative ripeness indicator-linked to 50 million tons of wasted food per year. Ripening bananas develop these senescent spots as phenolic compounds are enzymatically oxidized and cellular integrity is lost. We characterize the dynamics of the spot expansion and their nucleation rates based on time-lapse movies. Spots nucleate for about 2 days yielding a typical density of 8 spots/cm2. The expansion is initially diffusion controlled and the effective diffusion coefficient decreases with nucleation time from 1.3 to 0.4 mm2d-1. During and after expansion, the browning fronts maintain a steep and constant intensity gradient. We quantitatively reproduce these features by a reaction-diffusion model that considers the local oxygen concentration and browning degree of the peel. All model parameters are based on measurements and front stalling is explained by decreasing oxygen levels in the nucleation sites.

Collective motion of self-propelled chemical garden tubes.

In H2O2 solutions, manganese-containing chemical garden tubes can self-propel due to the catalytic production and ejection of oxygen bubbles. Here, we investigate the collective behavior of these self-assembled precipitate tubes. In thin solution layers, the tubes show definite autonomous dynamics with only weak interactions that result from fluid motion around the moving units and directional changes during collisions. In thick solution layers with convex menisci forcing spatial confinement, the tubes undergo cycles of self-assembly and dispersion. This collective motion results from the rhythmic creation of a large master bubble around which the tubes align tangentially.

Flexible Liquid Crystal Polymer Technologies from Microwave to Terahertz Frequencies.

With the emergence of fifth-generation (5G) cellular networks, millimeter-wave (mmW) and terahertz (THz) frequencies have attracted ever-growing interest for advanced wireless applications. The traditional printed circuit board materials have become uncompetitive at such high frequencies due to their high dielectric loss and large water absorption rates. As a promising high-frequency alternative, liquid crystal polymers (LCPs) have been widely investigated for use in circuit devices, chip integration, and module packaging over the last decade due to their low loss tangent up to 1.8 THz and good hermeticity. The previous review articles have summarized the chemical properties of LCP films, flexible LCP antennas, and LCP-based antenna-in-package and system-in-package technologies for 5G applications, although these articles did not discuss synthetic LCP technologies. In addition to wireless applications, the attractive mechanical, chemical, and thermal properties of LCP films enable interesting applications in micro-electro-mechanical systems (MEMS), biomedical electronics, and microfluidics, which have not been summarized to date. Here, a comprehensive review of flexible LCP technologies covering electric circuits, antennas, integration and packaging technologies, front-end modules, MEMS, biomedical devices, and microfluidics from microwave to THz frequencies is presented for the first time, which gives a broad introduction for those outside or just entering the field and provides perspective and breadth for those who are well established in the field.

Self-Propelled Chemical Garden Tubes.

Synthetic autonomous locomotion shows great promise in many research fields, including biomedicine and environmental science, because it can allow targeted drug/cargo delivery and the circumvention of kinetic and thermodynamic limitations. Creating such self-moving objects often requires advanced production techniques as exemplified by catalytic, gas-forming microrockets. Here, we grow such structures via the self-organization of precipitate tubes in chemical gardens by simply injecting metal salts into silicate solutions. This method generates hollow, cylindrical objects rich in catalytic manganese oxide that also feature a partially insulating outer layer of inert silica. In dilute H2O2 solution, these structures undergo self-propulsion by ejecting streams of oxygen bubbles. Each emission event pushes the tube forward by 1-2 tube radii. The ejection frequency depends linearly on the peroxide concentration as quantified by acoustic measurements of bursting bubbles. We expect our facile method and key results to be applicable to a diverse range of materials and reactions.

Multi frequency multi bit amplitude modulation of spoof surface plasmon polaritons by schottky diode bridged interdigital SRRs.

Multi-frequency multi-bit programmable amplitude modulation (AM) of spoof surface plasmon polaritons (SPPs) is realized at millimeter wave frequencies with interdigital split-ring resonators (SRRs) and In-Ga-Zn-O (IGZO) Schottky diodes. Periodic SRRs on a metal line guide both SRR mode and spoof SPP mode, the former of which rejects the spoof SPP propagation at the SRR resonant frequencies. To actively modulate the amplitude of spoof SPPs, IGZO Schottky diodes are fabricated in the SRR gaps, which continuously re-configure SRRs to metallic loops by applying bias. Interdigital gaps are designed in SRRs to increase the capacitance, thus red shifting the resonant frequencies, which significantly broadens the operation bandwidth of multi-frequency AM. Thus, cascading different kinds of interdigital SRRs with Schottky diodes enables multi-frequency multi-bit AM programmable. As a demonstration, a dual-frequency device was fabricated and characterized, which achieved significant multi-bit AM from -12.5 to -6.2 dB at 34.7 GHz and from -26 to -8.5 dB at 50 GHz independently and showed programmable capability.

Chemical Garden Membranes in Temperature-Controlled Microfluidic Devices.

Thin-walled tubes that classically form when metal salts react with sodium silicate solution are known as chemical gardens. They share similarities with the porous, catalytic materials in hydrothermal vent chimneys, and both structures are exposed to steep pH gradients that, combined with thermal factors, might have provided the free energy for prebiotic chemistry on early Earth. We report temperature effects on the shape, composition, and opacity of chemical gardens. Tubes grown at high temperature are more opaque, indicating changes to the membrane structure or thickness. To study this dependence, we developed a temperature-controlled microfluidic device, which allows the formation of analogous membranes at the interface of two coflowing reactant solutions. For the case of Ni(OH)2, membranes thicken according to a diffusion-controlled mechanism. In the studied range of 10-40 °C, the effective diffusion coefficient is independent of temperature. This suggests that counteracting processes are at play (including an increased solubility) and that the opacity of chemical garden tubes arises from changes in internal morphology. The latter could be linked to experimentally observed dendritic structures within the membranes.

Flow-Assisted Self-Organization of Hybrid Membranes.

Microfluidic flows are a powerful tool to drive reactions far from equilibrium and, thus, induce chemical selforganization. Studies of membrane formation in microfluidic devices have been limited to non-redox and purely inorganic reactions. Here, the formation of hybrid membranes at the interface of AgNO3 and 3,3',5,5'-tetramethylbenzidine solutions, which are steadily co-injected into a microfluidic device, is reported. The membrane thickening occurs in both directions and reveals oscillatory dynamics. The hybrid membrane mainly consists of hair-like Ag microstructures, Ag nanowires, and unbranched TMB-TMB2+ microfibers. Branched dendrite-like fibers form on the TMB side when the flow is stopped. These components were characterized with techniques including micro-Raman and energy dispersive X-ray spectroscopy as well as scanning electron microscopy. The effects of initial concentration ratios on the membrane thickening speed and its opaqueness were also studied.

Microfluidic Production of Pyrophosphate Catalyzed by Mineral Membranes with Steep pH Gradients.

Pyrophosphate might have functioned as an energy storage/currency molecule on early Earth, essential for the emergence of life. Here we synthesized mineral membranes involving iron(II), iron(III), and other divalent metal cations (calcium, manganese, cobalt, copper, zinc, and nickel) and tested their ability to catalyze the formation of pyrophosphate from phosphate and acetyl phosphate across steep pH gradients in microfluidic devices. We studied the chemical compositions of the precipitate membranes (which included vivianite, goethite, and green rust) using in situ and ex situ micro-Raman spectroscopy. The yields of pyrophosphate were determined by aqueous 31 P NMR spectroscopy. We found that Fe2+ and Ca2+ were the best catalysts for pyrophosphate synthesis among investigated ions; Fe3+ and mixed-valence iron membranes were also able to promote pyrophosphate formation. In addition, the pH gradients across the membranes affected the pyrophosphate yields and the smallest pH gradient resulted in the highest yield. These results suggest a possible route of substrate phosphorylation in prebiotic hydrothermal systems.

A Sputtered Silicon Oxide Electrolyte for High-Performance Thin-Film Transistors.

Low operating voltages have been long desired for thin-film transistors (TFTs). However, it is still challenging to realise 1-V operation by using conventional dielectrics due to their low gate capacitances and low breakdown voltages. Recently, electric double layers (EDLs) have been regarded as a promising candidate for low-power electronics due to their high capacitance. In this work, we present the first sputtered SiO2 solid-state electrolyte. In order to demonstrate EDL behaviour, a sputtered 200 nm-thick SiO2 electrolyte was incorporated into InGaZnO TFTs as the gate dielectric. The devices exhibited an operating voltage of 1 V, a threshold voltage of 0.06 V, a subthreshold swing of 83 mV dec-1 and an on/off ratio higher than 105. The specific capacitance was 0.45 µF cm-2 at 20 Hz, which is around 26 times higher than the value obtained from thermally oxidised SiO2 films with the same thickness. Analysis of the microstructure and mass density of the sputtered SiO2 films under different deposition conditions indicates that such high capacitance might be attributed to mobile protons donated by atmospheric water. The InGaZnO TFTs with the optimised SiO2 electrolyte also showed good air stability. This work provides a new pathway to the realisation of high-yield low-power electronics.

Efficient diode end-pumped acousto-optically Q-switched Nd:YAG/BaTeMo2O9 Raman laser.

BaTeMo2O9 (BTM) is employed to achieve efficient stimulated Raman scattering conversion in a diode end-pumped acousto-optically Q-switched Nd:YAG laser. With an incident diode power of 8.6 W, 732 mW of 1179 nm first-Stokes average output power was generated at a pulse repetition rate of 10 kHz, corresponding to a diode-to-Raman conversion efficiency of 8.5%.

Extremely Sensitive Dependence of SnOx Film Properties on Sputtering Power.

An extremely sensitive dependence of the electronic properties of SnOx film on sputtering deposition power is discovered experimentally. The carrier transport sharply switches from n-type to p-type when the sputtering power increases by less than 2%. The best n-type carrier transport behavior is observed in thin-film transistors (TFTs) produced at a sputtering power just below a critical value (120 W). In contrast, at just above the critical sputtering power, the p-type behavior is found to be the best with the TFTs showing the highest on/off ratio of 1.79 × 104 and the best subthreshold swing among all the sputtering powers that we have tested. A further increase in the sputtering power by only a few percent results in a drastic drop in on/off ratio by more than one order of magnitude. Scanning electron micrographs, x-ray diffraction spectra, x-ray photoelectron spectroscopy, as well as TFT output and transfer characteristics are analyzed. Our studies suggest that the sputtering power critically affects the stoichiometry of the SnOx film.

Graphene ballistic nano-rectifier with very high responsivity.

Although graphene has the longest mean free path of carriers of any known electronic material, very few novel devices have been reported to harness this extraordinary property. Here we demonstrate a ballistic nano-rectifier fabricated by creating an asymmetric cross-junction in single-layer graphene sandwiched between boron nitride flakes. A mobility ∼200,000 cm(2) V(-1) s(-1) is achieved at room temperature, well beyond that required for ballistic transport. This enables a voltage responsivity as high as 23,000 mV mW(-1) with a low-frequency input signal. Taking advantage of the output channels being orthogonal to the input terminals, the noise is found to be not strongly influenced by the input. Hence, the corresponding noise-equivalent power is as low as 0.64 pW Hz(-1/2). Such performance is even comparable to superconducting bolometers, which however need to operate at cryogenic temperatures. Furthermore, output oscillations are observed at low temperatures, the period of which agrees with the lateral size quantization.

Passively Q-switched Nd:YAG ceramic laser with V(3+):YAG saturable absorber at 1357 nm.

1357 nm single-wavelength continuous-wave and passively V(3+):YAGQ-switched Nd:YAG ceramic crystal lasers have been experimentally demonstrated for the first time to our knowledge. The output power of the continuous-wave laser was as high as 2.4 W under the incident pump power of 14.8 W. The corresponding optical to optical efficiency was about 16.2% and the slope efficiency was about 18.6%. The Q-switched laser remained single-wavelength output with a maximum average output power of 628 mW at the incident pump power of 12.4 W. The corresponding pulse width was 21 ns and the repetition frequency was 15 kHz.

High-power dual-wavelength eye-safe ceramic Nd:YAG/SrWO(4) Raman laser operating at 1501 and 1526 nm.

By employing SrWO(4) as a Raman active medium and a ceramic Nd:YAG laser operating at 1319 and 1338 nm as a pump source, an efficient eye-safe dual-wavelength Raman laser operating at 1501 and 1526 nm has been demonstrated. A maximum total output power as high as 3.36 W is obtained under a pump power of 33.3 W and a pulse repetition frequency (PRF) of 30 kHz. The highest optical-to-optical conversion efficiency of 11.2% is achieved at the pump power of 27.3 W. To our knowledge, 3.36 W is the highest output power of a 1.5 µm eye-safe Raman laser.

Raman operation around 1.2 µm within a diode-pumped actively Q-switched ceramic Nd:YAG/SrWO4 laser.

By employing SrWO4 as Raman active media and ceramic Nd:YAG as laser gain media, a diode-pumped actively Q-switched Raman laser is demonstrated. Single or multiple wavelength first-stokes Raman generations at 1238 and 1252 nm are achieved. To our knowledge, this is the first Raman laser demonstration based on fundamental wavelengths around 1.1 µm. Maximum output powers are 1.02 W at single 1252 nm wavelength, 1.08 W at single 1238 nm wavelength, and 442 mW at multiple wavelengths of 1252 and 1238 nm, corresponding to optical-to-optical conversion efficiencies of 4.78%, 4.95%, and 2.54%, respectively.