A Non-Newtonian liquid metal enabled enhanced electrography.

Biopotential signals, like electrocardiography (ECG), electromyography (EMG), and electroencephalography (EEG), can help diagnose cardiological, musculoskeletal and neurological disorders. Dry silver/silver chloride (Ag/AgCl) electrodes are commonly used to obtain these signals. While a conductive hydrogel can be added to Ag/AgCl electrodes to improve the contact and adhesion between the electrode and the skin, dry electrodes are prone to movement. Considering that the conductive hydrogel dries over time, the use of these electrodes often creates an imbalanced skin-electrode impedance and a number of sensing issues in the front-end analogue circuit. This issue can be extended to several other electrode types that are commonly in use, in particular, for applications with a need for long-term wearable monitoring such as ambulatory epilepsy monitoring. Liquid metal alloys, such as eutectic gallium indium (EGaIn), can address key critical requirements around consistency and reliability but present challenges on low viscosity and the risk of leakage. To solve these problems, here, we demonstrate the use of a non-eutectic Ga-In alloy as a shear-thinning non-Newtonian fluid to offer superior performance to commercial hydrogel electrodes, dry electrodes, and conventional liquid metals for electrography measurements. This material has high viscosity when still and can flow like a liquid metal when sheared, preventing leakage while allowing the effective fabrication of electrodes. Moreover, the Ga-In alloy not only has good biocompatibility but also offers an outstanding skin-electrode interface, allowing for the long-term acquisition of high-quality biosignals. The presented Ga-In alloy is a superior alternative to conventional electrode materials for real-world electrography or bioimpedance measurement.

Electrically driven whispering-gallery-mode microlasers in an n-MgO@ZnO:Ga microwire/p-GaN heterojunction.

In emerging miniaturized applications, semiconductor micro/nanostructures laser devices have drawn great public attentions of late years. The device performances of micro/nanostructured microlasers are highly restricted to the different reflective conditions at various side surfaces of microresonators and junction interface quality. In this study, an electrically driven whispering-gallery-mode (WGM) microlaser composed of a Ga-doped ZnO microwire covered by a MgO layer (MgO@ZnO:Ga MW) and a p-type GaN substrate is illustrated experimentally. Incorporating a MgO layer on the side surfaces of ZnO:Ga MWs can be used to reduce light leakage along the sharp edges and the ZnO:Ga/GaN interface. This buffer layer incorporation also enables engineering the energy band alignment of n-ZnO:Ga/p-GaN heterojunction and manipulating the current transport properties. The as-constructed n-MgO@ZnO:Ga MW/p-GaN heterojunction device can emit at an ultraviolet wavelength of 375.5 nm and a linewidth of about 25.5 nm, achieving the excitonic-related recombination in the ZnO:Ga MW. The broadband spectrum collapsed into a series of sharp peaks upon continuous-wave (CW) operation of electrical pumping, especially for operating current above 15.2 mA. The dominant emission line was centered at 378.5 nm, and the line width narrowed to approximately 0.95 nm. These sharp peaks emerged from the spontaneous emission spectrum and had an average spacing of approximately 5.5 nm, following the WGM cavity modes. The results highlight the significance of interfacial engineering for optimizing the performance of low-dimensional heterostructured devices and shed light on developing future miniaturized microlasers.

Electrically driven single microwire-based single-mode microlaser.

Engineering the lasing-mode oscillations effectively within a laser cavity is a relatively updated attentive study and perplexing issue in the field of laser physics and applications. Herein, we report a realization of electrically driven single-mode microlaser, which is composed of gallium incorporated zinc oxide microwire (ZnO:Ga MW) with platinum nanoparticles (PtNPs, d ~ 130 nm) covering, a magnesium oxide (MgO) nanofilm, a Pt nanofilm, and a p-type GaN substrate. The laser cavity modes could resonate following the whispering-gallery mode (WGM) among the six side surfaces by total internal reflection, and the single-mode lasing wavelength is centered at 390.5 nm with a linewidth of about 0.18 nm. The cavity quality factor Q is evaluated to about 2169. In the laser structure, the usage of Pt and MgO buffer layers can be utilized to engineer the band alignment of ZnO:Ga/GaN heterojunction, optimize the p-n junction quality and increase the current injection. Thus, the well-designed device structure can seamlessly unite the electron-hole recombination region, the gain medium, and optical microresonator into the PtNPs@ZnO:Ga wire perfectly. Such a single MW microlaser is essentially single-mode regardless of the gain spectral bandwidth. To study the single-mode operation, PtNPs working as superabsorber can engineering the multimode lasing actions of ZnO:Ga MWs even if their dimensions are typically much larger than that of lasing wavelength. Our findings can provide a straightforward and effective scheme to develop single-mode microlaser devices based on one-dimensional wire semiconductors.

A Liquid Metal Artificial Muscle.

Artificial muscles possess a vast potential in accelerating the development of robotics, exoskeletons, and prosthetics. Although a variety of emerging actuator technologies are reported, they suffer from several issues, such as high driving voltages, large hysteresis, and water intolerance. Here, a liquid metal artificial muscle (LMAM) is demonstrated, based on the electrochemically tunable interfacial tension of liquid metal to mimic the contraction and extension of muscles. The LMAM can work in different solutions with a wide range of pH (0-14), generating actuation strains of up to 87% at a maximum extension speed of 15 mm s-1 . More importantly, the LMAM only needs a very low driving voltage of 0.5 V. The actuating components of the LMAM are completely built from liquids, which avoids mechanical fatigue and provides actuator linkages without mechanical constraints to movement. The LMAM is used for developing several proof-of-concept applications, including controlled displays, cargo deliveries, and reconfigurable optical reflectors. The simplicity, versatility, and efficiency of the LMAM are further demonstrated by using it to actuate the caudal fin of an untethered bionic robotic fish. The presented LMAM has the potential to extend the performance space of soft actuators for applications from engineering fields to biomedical applications.

Plasmon-enabled spectrally narrow ultraviolet luminescence device using Pt nanoparticles covered one microwire-based heterojunction.

Owing to great luminescent monochromaticity, high stability, and independent of automatic color filter, low dimensional ultraviolet light-emitting diodes (LEDs) via the hyperpure narrow band have attracted considerable interest for fabricating miniatured display equipments, solid state lighting sources, and other ultraviolet photoelectrical devices. In this study, a near-ultraviolet LED composed of one Ga-doped ZnO microwire (ZnO:Ga MW) and p-GaN layer was fabricated. The diode can exhibit bright electroluminescence (EL) peaking at 400.0 nm, with a line width of approximately 35 nm. Interestingly, by introducing platinum nanoparticles (PtNPs), we achieved an ultraviolet plasmonic response; an improved EL, including significantly enhanced light output; an observed blueshift of main EL peaks of 377.0 nm; and a reduction of line width narrowing to 10 nm. Working as a powerful scalpel, the decoration of PtNPs can be employed to tailor the spectral line profiles of the ultraviolet EL performances. Also, a rational physical model was built up, which could help us study the carrier transportation, recombination of electrons and holes, and dynamic procedure of luminescence. This method offers a simple and feasible way, without complicated fabricating technology such as an added insulating layer or core shell structure, to realize hyperpure ultraviolet LED. Therefore, the proposed engineering of energy band alignment by introducing PtNPs can be employed to build up high performance, high spectral purity luminescent devices in the short wavelengths.

Pt nanoparticles utilized as efficient ultraviolet plasmons for enhancing whispering gallery mode lasing of a ZnO microwire via Ga-incorporation.

Introducing nanostructured metals with ultraviolet plasmonic characters has attracted much attention for fabricating high performance optoelectronic devices in the shorter wavelength spectrum. In this work, platinum nanoparticles (PtNPs) with controlled plasmonic responses in ultraviolet wavelengths were successfully synthesized. To demonstrate the promising availability, PtNPs with desired sizes were deposited on a hexagonal ZnO microwire via Ga-doping (PtNPs@ZnO:Ga MW). Under ultraviolet illumination, typical near-band-edge emission of ZnO:Ga MW was considerably enhanced; meanwhile, the photocurrent is much larger than that of the bare MW. Thereby, the enhanced phenomena of a ZnO:Ga MW is related to localized surface plasmon resonances of the decorated PtNPs. A single MW with a hexagonal cross-section can be a potential platform to construct a whispering gallery mode (WGM) cavity due to its total inner wall reflection. Given this, the influence of PtNPs via ultraviolet plasmons on lasing features of the ZnO:Ga MW was tested. The lasing characteristics are significantly enhanced, including lasing output enhancement, a clear reduction of the threshold and the improvement of the quality factor. To exploit the working principle, PtNPs serving as powerful ultraviolet plasmons can couple with ZnO:Ga excitons, accelerating radiative recombination. Since fabricating stable, typical nanostructured metals with ultraviolet plasmons remains a challenging issue, the results illustrated in the work may offer a low-cost and efficient scheme for achieving plasmon-enhanced wide-bandgap semiconductor based ultraviolet optoelectronic devices with excellent performances.

Particle-Based Porous Materials for the Rapid and Spontaneous Diffusion of Liquid Metals.

Gallium-based room-temperature liquid metals have enormous potential for realizing various applications in electronic devices, heat flow management, and soft actuators. Filling narrow spaces with a liquid metal is of great importance in rapid prototyping and circuit printing. However, it is relatively difficult to stretch or spread liquid metals into desired patterns because of their large surface tension. Here, we propose a method to fabricate a particle-based porous material which can enable the rapid and spontaneous diffusion of liquid metals within the material under a capillary force. Remarkably, such a method can allow liquid metal to diffuse along complex structures and even overcome the effect of gravity despite their large densities. We further demonstrate that the developed method can be utilized for prototyping complex three-dimensional (3D) structures via direct casting and connecting individual parts or by 3D printing. As such, we believe that the presented technique holds great promise for the development of additive manufacturing, rapid prototyping, and soft electronics using liquid metals.

Hybrid quadrupole plasmon induced spectrally pure ultraviolet emission from a single AgNPs@ZnO:Ga microwire based heterojunction diode.

Ultraviolet light-emitting materials and devices with high-efficiency are required for many applications. One promising way to enhance the ultraviolet luminescence efficiency is by incorporating plasmonic nanostructures. However, a large energy mismatch between the plasmons and the light emitters greatly limits the direct realization of light enhancement. In this work, a single Ga-doped ZnO microwire prepared with large-sized Ag nanoparticle (the diameter d ∼ 200 nm) deposition (AgNPs@ZnO:Ga MW) was utilized to construct a high-performance heterojunction diode, with p-GaN serving as the hole injection layer. In addition to enhanced optical output, the emission spectra also revealed that typical near-band-edge (NBE) emission with higher wavelength stability centered around 378.0 nm was achieved, accompanied by narrowing of the spectral linewidth to around 10 nm. Thus, the interfacial and p-GaN emissions were successfully suppressed. The spectral profile of the emission spectra of the heterojunction diodes precisely matched the photoluminescence spectrum of the single ZnO:Ga MW, which indicates that the single ZnO:Ga MW can act as the active region for the radiative recombination of electrons and holes in the diode operation. In the emission mechanism, hybrid quadrupole plasmons induce the generation of hot electrons, which are then injected into the conduction band of the neighboring ZnO:Ga and are responsible for the NBE-type emission of the single MW based heterojunction diode. This novel emission enhancement and modulation principle can aid in the design and development of new types of luminescent materials and devices with high-efficiency, spectral stability and spectral purity.

Fabrication of NiO/NiCo2O4 Mixtures as Excellent Microwave Absorbers.

The NiO/NiCo2O4 mixtures with unique yolk-shell structure were synthesized by a simple hydrothermal route and subsequent thermal treatment. The elemental distribution, composition, and microstructure of the samples were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and scanning electron microscope (SEM), respectively. The microwave absorption property was investigated by using vector network analysis (VNA). The results indicated that the excellent electromagnetic wave absorption property of the NiO/NiCo2O4 mixtures was achieved due to the unique yolk-shell structure. In detail, the maximum reflection loss (RL) value of the sample reached up to - 37.0 dB at 12.2 GHz and the absorption bandwidth with RL below - 10 dB was 4.0 GHz with a 2.0-mm-thick absorber. In addition, the NiO/NiCo2O4 mixtures prepared at high temperature, exhibited excellent thermal stability. Possible mechanisms were investigated for improving the microwave absorption properties of the samples.

Wavelength-Tunable Waveguide Emissions from Electrically Driven Single ZnO/ZnO:Ga Superlattice Microwires.

Because of the superlattice structures comprising periodic and alternating crystalline layers, one-dimensional photon crystals can be employed to expand immense versatility and practicality of modulating the electronic and photonic propagation behaviors, as well as optical properties. In this work, individual superlattice microwires (MWs) comprising ZnO and Ga-doped ZnO (ZnO/ZnO:Ga) layers were successfully synthesized. Wavelength-tunable multipeak emissions can be realized from electrically driven single superlattice MW-based emission devices, with the dominant wavelengths tuned from ultraviolet to visible spectral regions. To illustrate the multipeak character, single superlattice MWs were selected to construct fluorescent emitters, and the emission wavelength could be tuned from 518 to 562 nm, which is dominated by Ga incorporation. Especially, by introducing Au quasiparticle film decoration, emission characteristics can further be modulated, such as the red shift of the emission wavelengths, and the multipeaks were strongly modified and split into more and narrower subbands. In particular, electrically pumped exciton-polariton emission was realized from heterojunction diodes composed of single ZnO/ZnO:Ga superlattice MWs and p-GaN layers in the blue-ultraviolet spectral regions. With the aid of localized surface plasmons from Au nanoparticles, which deposited on the superlattice MW, significant improvement of emission characteristics, such as enhancement of output efficiencies, blue shift of the dominant emission wavelengths, and narrowing of the spectral linewidth, can be achieved. The multipeak emission characteristics would be originated from the typical optical cavity modes, but not the Fabry-Perot mode optical cavity formed by the bilateral sides of the wire. The resonant modes are likely attributed to the coupled optical microcavities, which formed along the axial direction of the wire; thus, the emitted photons can be propagated and selected longitudinally. Therefore, the novel ZnO/ZnO:Ga superlattice MWs with a quadrilateral cross section can provide a potential platform to construct multicolor emitters and low-threshold exciton-polariton diodes and lasers.