Boosting electrocatalytic hydrogen evolution via partial oxidation of rhenium through cobalt modification in nanoalloy structure.

Although the theoretical electrocatalytic activity of rhenium (Re) for the hydrogen evolution reaction is comparable to that of platinum, the experimental performance of reported rhenium-based electrocatalysts remains unsatisfactory. Herein, we report a highly efficient and stable electrocatalyst composed of rhenium and cobalt (Co) nanoalloy embedded in nitrogen-doped carbon film (Re3Co2@NCF). The Re3Co2@NCF electrocatalyst exhibited remarkable hydrogen evolution performance, with an overpotential as low as 30 ± 3 mV to reach a current density of 10 mA cm-2. In addition, the Re3Co2@NCF demonstrated exceptional stability over several days at a current density of 150 mA cm-2. Theoretical calculations revealed that alloying cobalt with rhenium altered the electronic structure of the metals, causing partial oxidation of the superficial metal atoms. This modification provided a balance for various intermediates' adsorption and desorption, thereby boosting the intrinsic activity of rhenium for hydrogen evolution reaction. This work improves the electrocatalytic performance of rhenium to its theoretical activity, suggesting a promising future for rhenium-based electrocatalysts.

Sidewall Corrugation-Modulated Phase-Apodized Silicon Grating Filter.

In this work, phase-apodized silicon grating filters with varying sidewall corrugation width and location were investigated, while the resonance wavelength, extinction ratio, and rejection bandwidth were tuned flexibly. The grating filters with a waveguide width of 500 nm and grating period of 400 nm were fabricated and characterized as a proof of concept. The resonance wavelength of the device can be shifted by 4.54 nm by varying the sidewall corrugation width from 150 to 250 nm. The corresponding rejection bandwidth can be changed from 1.19 to 2.03 nm by applying a sidewall corrugation location offset from 50 to 200 nm. The experimental performances coincide well with the simulation results. The presented sidewall corrugation-modulated apodized grating can be expected to have great application prospects for optical communications and semiconductor lasers.

Gallium Oxide for Gas Sensor Applications: A Comprehensive Review.

Ga2O3 has emerged as a promising ultrawide bandgap semiconductor for numerous device applications owing to its excellent material properties. In this paper, we present a comprehensive review on major advances achieved over the past thirty years in the field of Ga2O3-based gas sensors. We begin with a brief introduction of the polymorphs and basic electric properties of Ga2O3. Next, we provide an overview of the typical preparation methods for the fabrication of Ga2O3-sensing material developed so far. Then, we will concentrate our discussion on the state-of-the-art Ga2O3-based gas sensor devices and put an emphasis on seven sophisticated strategies to improve their gas-sensing performance in terms of material engineering and device optimization. Finally, we give some concluding remarks and put forward some suggestions, including (i) construction of hybrid structures with two-dimensional materials and organic polymers, (ii) combination with density functional theoretical calculations and machine learning, and (iii) development of optical sensors using the characteristic optical spectra for the future development of novel Ga2O3-based gas sensors.

Femtosecond Laser Modification of Silica Optical Waveguides for Potential Bragg Gratings Sensing.

The optimum femtosecond laser direct writing of Bragg gratings on silica optical waveguides has been investigated. The silica waveguide has a 6.5 × 6.5 µm2 cross-sectional profile with a 20-µm-thick silicon dioxide cladding layer. Compared with conventional grating inscribed on fiber platforms, the silica planar waveguide circuit can realize a stable performance as well as a high-efficiency coupling with the fiber. A thin waveguide cladding layer also facilitates laser focusing with an improved spherical aberration. Different from the circular fiber core matching with the Gaussian beam profile, a 1030-nm, 400-fs, and 190-nJ laser is optimized to focus on the top surface of the square silica waveguide, and the 3rd-order Bragg gratings are inscribed successfully. A 1.5-mm long uniform Bragg gratings structure with a reflectivity of 90% at a 1548.36-nm wavelength can be obtained. Cascaded Bragg gratings with different periods are also inscribed in the planar waveguide. Different reflection wavelengths can be realized, which shows great potential for wavelength multiplexing-related applications such as optical communications or sensing.

Optical Bistability in a Tunable Gourd-Shaped Silicon Ring Resonator.

In this study, a tunable gourd-shaped ring resonator is demonstrated to generate optical bistability. The system consists of two sub-rings for a gourd shape configuration with a U-shaped wave guiding pathway. The transfer matrix method and FDTD simulation are used to acquire the spectral characteristics of the system. For the fabricated device, the spectra profile and extinction ratio can be effectively tuned by the microheater above the U-shaped waveguide, which matches with the theoretical results. Due to the gourd structure of the resonator, the light waves in two rings can be cross-coupled with each other, and the optical bistability could come out effectively with the change in the input optical power around 6 mW. The presented optical bistability devices have great application potential in optical information processing such as optical storage, switch and logic operation.

Universal dimension reduced phase compensation algorithm for an optical phased array.

Optical phased arrays (OPAs) can achieve non-mechanical beam deflection. Many types of OPA face the problem of low deflection efficiency due to the phase distortion induced by mutual coupling between nearby channels. In this Letter, a universal optimization algorithm is proposed to compensate for this structural phase distortion, in which the adjacent sampling principal component analysis (AS-PCA) method is introduced to reduce the dimension of the solution space. Simulations and experimental results on different classes of OPA verified that this method can considerably optimize the deflection beam with a rapid convergence speed, irrespective of the scale of OPA, and maintain the universal feature, laying the foundation for large-scale, high-density OPA in-line optimization. We envision it to become a general method on different platforms.

Silicon Nanowire-Assisted High Uniform Arrayed Waveguide Grating.

Determining how to improve the non-uniformity of arrayed waveguide grating (AWG) is of great significance for dense wavelength division multiplexing (DWDM) systems. In this work, a silicon nanowire-assisted AWG structure is proposed, which can achieve high uniformity with a low insertion loss. The article compares the effect of nanowire number and shape on uniformity and insertion loss, finding that double nanowires provide the best performance. Double nanowires with a width of 230 nm and length of 3.5 µm can consist of a slot configuration between arrayed waveguides, both connecting to the star coupler and spacing 165 nm from the waveguides. Compared with conventional 8- and 16-channel AWGs with channel spacing of 200 GHz, the non-uniformity of the presented structure can be improved from 1.09 and 1.6 dB to 0.24 and 0.63 dB, respectively. The overall footprint of the device would remain identical, which is 276 × 299 or 258 × 303 µm2 for the 8- or 16-channel AWG. The present high uniformity design is simple and easy to fabricate without any additional insertion loss, which is expected to be widely applied in the highly integrated DWDM systems.

Tilted Nano-Grating Based Ultra-Compact Broadband Polarizing Beam Splitter for Silicon Photonics.

An ultra-compact broadband silicon polarizing beam splitter is proposed based on a tilted nano-grating structure. A light cross coupling can be realized for transverse-magnetic mode, while the transverse-electric light can almost completely output from the through port. The length of the coupling region is only 6.8 µm, while an extinction ratio of 23.76 dB can be realized at a wavelength of 1550 nm. As a proof of concept, the device was fabricated by a commercial silicon photonic foundry. It can realize a 19.84 dB extinction ratio and an 80 nm working bandwidth with an extinction ratio of larger than 10 dB. The presented device also shows a good fabrication tolerance to the structure deviations, which is favorable for its practical applications in silicon photonics.

A graphene oxide coated tapered microfiber acting as a super-sensor for rapid detection of SARS-CoV-2.

COVID-19 is a new strain of highly contagious coronavirus, and at present, more than 221.4 million people have been infected with this virus, and the death toll exceeds 2793398. Early and fast detection of COVID-19 from infected individuals is critical to limit its spreading. Here, we report an innovative approach to detect the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid (N) protein by combining DNA/RNA oligomers as aptamers and a graphene oxide (GO) coated optical microfiber as a sensor system. The DNA/RNA aptamers can effectively capture the SARS-CoV-2 N protein in vitro, with the GO coated optical microfiber aptasensor for real-time monitoring of the SARS-CoV-2 N protein. Due to the extremely high surface-to-volume ratio and excellent optical and biochemical properties of the GO surface layer, the fixing effect of the microfiber surface is significantly improved and the lowest limit of detection (LOD) is 6.25 × 10-19 M. Furthermore, in order to prove the feasibility of this sensing method in clinical applications, we use this sensor to detect the N protein mixed in fetal bovine serum (FBS) samples. The experimental results show that the biosensor can quickly and effectively detect the N protein (1 × 10-9 M) in a complex sample matrix within 3 minutes. These findings suggest that this approach can be utilized for quantitative monitoring of coronavirus particles due to its high sensitivity, which can help to quickly exclude patients who do not have the infection. Collectively, the optical microfiber sensor system could be expected to become an important platform for the diagnosis of coronavirus due to its simple detection scheme and easy miniaturization.

Voltammetric determination of ofloxacin by using a laser-modified carbon glassy electrode.

A novel electrochemical sensor is described for the determination of ofloxacin (OFL) in environmental water samples. A laser-modified glassy carbon electrode (LGCE) was structured and characterized by scanning electron microscopy, atomic force microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy. The increase in electrochemical activity is due to a moderate increase in the surface roughness and to the presence of functional groups on the LGCE. Under optimal conditions (viz. a pH value of 5.5, a laser power of 1.8 W and an action time of 40 s), the sensor is capable of detecting OFL by differential pulse voltammetry at a working potential of +0.91 V (versus Ag/AgCl). Response is linear from 0.25 to 200 µM for OFL concentration range, and the detection limit is 75 nM (at S/N = 3). Removal of oxygen from samples is not required. The sensor was successfully applied to the determination of OFL in spiked groundwater, tap water and wastewater samples, with apparent recoveries from 94.0 to 108.0% and a relative standard deviation of less than 4.8%." Graphical abstractSchematic representation of a method for determination of ofloxacin (OFL) by differential pulse voltammetry. It is making use of a laser modified glassy carbon electrode (LGCE), which increases the number of active functional groups and the surface area compared to a conventional GCE.

Systematic Study of the SiOx Film with Different Stoichiometry by Plasma-Enhanced Atomic Layer Deposition and Its Application in SiOx/SiO2 Super-Lattice.

Atomic scale control of the thickness of thin film makes atomic layer deposition highly advantageous in the preparation of high quality super-lattices. However, precisely controlling the film chemical stoichiometry is very challenging. In this study, we deposited SiOx film with different stoichiometry by plasma enhanced atomic layer deposition. After reviewing various deposition parameters like temperature, precursor pulse time, and gas flow, the silicon dioxides of stoichiometric (SiO2) and non-stoichiometric (SiO1.8 and SiO1.6) were successfully fabricated. X-ray photo-electron spectroscopy was first employed to analyze the element content and chemical bonding energy of these films. Then the morphology, structure, composition, and optical characteristics of SiOx film were systematically studied through atomic force microscope, transmission electron microscopy, X-ray reflection, and spectroscopic ellipsometry. The experimental results indicate that both the mass density and refractive index of SiO1.8 and SiO1.6 are less than SiO2 film. The energy band-gap is approved by spectroscopic ellipsometry data and X-ray photo-electron spectroscopy O 1s analysis. The results demonstrate that the energy band-gap decreases as the oxygen concentration decreases in SiOx film. After we obtained the Si-rich silicon oxide film deposition, the SiO1.6/SiO2 super-lattices was fabricated and its photoluminescence (PL) property was characterized by PL spectra. The weak PL intensity gives us greater awareness that more research is needed in order to decrease the x of SiOx film to a larger extent through further optimizing plasma-enhanced atomic layer deposition processes, and hence improve the photoluminescence properties of SiOx/SiO2 super-lattices.

Surface-enhanced Raman scattering using nanoporous gold on suspended silicon nitride waveguides.

A hybrid integration of nanoporous gold with silicon nitride waveguide has been realized for surface-enhanced Raman spectroscopy (SERS) at 633-nm wavelength. The SERS signal is excited through 580-nm-thick T-shape suspended waveguides and collected through an objective lens. Raman spectra for different mesa width at either transverse electric (TE) or transverse magnetic (TM) mode are measured and compared. The localized surface plasmon resonance of the nanoporous gold can result in a waveguide and polarization-dependent SERS enhancement. The presented miniaturized SERS chips can work from visible to near-infrared wavelength and a wide application prospect could be expected.

Single-walled carbon nanotubes assisted THz silicon grating modulator.

Single-walled carbon nanotubes (SWCNTs) are applied to realize an enhanced frequency modulation for a suspended THz silicon grating, which is fabricated by a nanosecond laser direct writing and coated with the synthetic SWCNTs/polyacrylic emulsion composite. With terahertz time domain spectroscopy system, the transmission spectra of the bare and SWCNTs coated silicon grating are measured and compared. The SWCNTs coated silicon grating can realize an improved extinction ratio and quality factor, which is due to the SWCNTs caused local field enhancement and can be explained by the theoretical simulation with finite element method. Besides the effective modulation of the grating transmittance, SWCNTs can also be integrated with other platforms and applied in future THz imaging and communication systems.

Vertically integrated waveguide self-coupled resonator based tunable optical filter.

A vertically integrated waveguide self-coupled resonator based tunable optical filter was demonstrated. Unlike the conventional U-bend self-coupled waveguide structure, a top-layer S-bend waveguide was cross-coupled with the racetrack resonator on a bottom layer. The different waveguide coupling effect was compared with the same resonance structure, which can realize the same free spectral range as well as a high quality factor. Spectrum response can be designed separately by varying the coupling coefficient between waveguide and resonator. A heater attached on the top of the resonator can be utilized for the resonance wavelength tuning, while a heater on the top of cross-coupled waveguide has little influence on the device performance, which can help to improve the stability. The presented device can also be applied as a tunable modulator/switch.

All-optical XOR logic gate using intersubband transition in III-V quantum well materials.

A monolithically integrated all-optical exclusive-OR (XOR) logic gate is experimentally demonstrated based on a Michelson interferometer (MI) gating device in InGaAs/AlAsSb coupled double quantum wells (CDQWs). The MI arms can convert the pump data with return-to-zero ON-OFF keying (RZ OOK) to binary phase-shift keying (BPSK) format, then two BPSK signals can interfere with each other for realizing a desired logical operation. All-optical format conversion from the RZ OOK to BPSK is based on the cross-phase modulation to the transverse electric (TE) probe wave, which is caused by the intersubband transition excited by the transverse magnetic (TM) pump light. Bit error rate measurements show that error free operation for both BPSK format conversion and XOR logical operation can be achieved.

An organic coprecipitation route to synthesize high voltage LiNi0.5Mn1.5O4.

High-voltage cathode material LiNi0.5Mn1.5O4 has been prepared with a novel organic coprecipitation route. The as-prepared sample was compared with samples produced through traditional solid state method and hydroxide coprecipitation method. The morphology was observed by scanning electron microscopy, and the spinel structures were characterized by X-ray diffraction and Fourier transform infrared spectroscopy. Besides the ordered/disordered distribution of Ni/Mn on octahedral sites, the confusion between Li and transition metal is pointed out to be another important factor responsible for the corresponding performance, which is worthy further investigation. Galvanostatic cycles, cyclic voltammetry, and electrochemical impedance spectroscopy are employed to characterize the electrochemical properties. The organic coprecipitation route produced sample shows superior rate capability and stable structure during cycling.

Band edge tailoring of InGaAs/AlAsSb coupled double quantum wells for a monolithically integrated all-optical switch.

We demonstrate a compact all-optical Michelson interferometer (MI) gating switch with monolithic integration of two different bandgap energies. Based on the ion-induced intermixing in InGaAs/AlAsSb coupled double quantum wells, the blueshift of the band edge can be tailored. Through phosphorus ion implantation with a dose of 5 × 10(14) cm(-2) and subsequent annealing at 720 °C for 60 s, an implanted sample can acquire a high transmittance compared with the as-grown one. Meanwhile, the cross-phase modulation (XPM) efficiency of a non-implanted sample undergoing the same annealing process decreases little. An implanted part for signal propagation and a non-implanted section for XPM are thus monolithically integrated for an MI switch by an area-selective manner. Full switching of a π-rad nonlinear phase shift is achieved with pump pulse energy of 5.6 pJ at a 10-GHz repetition rate.

Ultrafast all-optical switch with cross-phase modulation by area-selective ion implantation in InGaAs/AlAsSb coupled double quantum wells.

We have developed a compact gate switch with monolithic integration of all-optical cross-phase modulation (XPM) in a Mach-Zehnder interferometer (MZI). XPM is caused by intersubband transition (ISBT) in InGaAs/AlAsSb coupled double quantum wells (CDQWs) by area-selective silicon ion implantation and rapid thermal annealing (RTA). While injecting pump light through a transverse electric/transverse magnetic (TE/TM) beam combiner, XPM is induced in one MZI arm and gating operation can be realized. The RTA condition is optimized, and the sample is annealed at 780 °C for 8 s with an implantation dose of 5 × 10(13) cm(-2). Dependence of XPM efficiency on the length of the implanted mesa is also analyzed, and there exists an optimum implantation length to fulfill both high efficiency of ISBT modulation and low loss of the probe and pump signals.

High-performance LiNi0.5Mn1.5O4 spinel controlled by Mn3+ concentration and site disorder.

The complex correlation between Mn(3+) ions and the disordered phase in the lattice structure of high voltage spinel, and its effect on the charge transport properties, are revealed through a combination of experimental study and computer simulations. Superior cycling stability is achieved in LiNi(0.45)Cr(0.05)Mn(1.5)O(4) with carefully controlled Mn(3+) concentration. At 250th cycle, capacity retention is 99.6% along with excellent rate capabilities.

Effect of the methylation of uracil and/or glycine on their mutual interaction.

In order to simulate the hydrogen bonding and proton transfer (PT) in protein-DNA/RNA interactions, a series of simplified models were employed and investigated in the gas phase. These models included various neutral, anionic and cationic glycine-uracil dimers, and their methylated derivatives generated by the mono- or dimethylation of glycine and/or uracil moieties of the dimer. The results reveal that the only process that can occur in the neutral complexes is a double-PT process leading to proton exchange between the two moieties (i.e., point mutation). The first methyl substitute can reduce the activation energy of the PT process and thus promote the isomerization of the two moieties; further methylation can reduce the isomerization in only some of the cases. In the anionic complexes, only the one-way PT (i.e., amino acid → nucleic acid base) process is energetically favorable, and this PT process is an interesting barrier-free one (BFPT), with the attached electron locating itself at the base moiety. Methylation will disfavor BFPT, but it cannot alter the nature of BFPT. In the cationic complexes, three different PT processes can occur. These processes can transform mutually by adjusting either or both of the methylated sites and methyl number, indicating that the methylation can regulate the dynamics of these PT processes.