Enhanced Operational Characteristics Attained by Applying HfO2 as Passivation in AlGaN/GaN High-Electron-Mobility Transistors: A Simulation Study.

This study investigates the operating characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs) by applying HfO2 as the passivation layer. Before analyzing HEMTs with various passivation structures, modeling parameters were derived from the measured data of fabricated HEMT with Si3N4 passivation to ensure the reliability of the simulation. Subsequently, we proposed new structures by dividing the single Si3N4 passivation into a bilayer (first and second) and applying HfO2 to the bilayer and first passivation layer only. Ultimately, we analyzed and compared the operational characteristics of the HEMTs considering the basic Si3N4, only HfO2, and HfO2/Si3N4 (hybrid) as passivation layers. The breakdown voltage of the AlGaN/GaN HEMT having only HfO2 passivation was improved by up to 19%, compared to the basic Si3N4 passivation structure, but the frequency characteristics deteriorated. In order to compensate for the degraded RF characteristics, we modified the second Si3N4 passivation thickness of the hybrid passivation structure from 150 nm to 450 nm. We confirmed that the hybrid passivation structure with 350-nm-thick second Si3N4 passivation not only improves the breakdown voltage by 15% but also secures RF performance. Consequently, Johnson's figure-of-merit, which is commonly used to judge RF performance, was improved by up to 5% compared to the basic Si3N4 passivation structure.

Enhancing catalytic efficiency of carbon dots by modulating their Mn doping and chemical structure with metal salts.

Nanozymes are emerging materials in various fields owing to their advantages over natural enzymes, such as controllable and facile synthesis, tunability in catalytic activities, cost-effectiveness, and high stability under stringent conditions. In this study, the effect of metal salts on the formation and catalytic activity of carbon dots (CDs), a promising nanozyme, is demonstrated. By introducing Mn sources that possess different counter anions, the chemical structure and composition of the CDs produced are affected, thereby influencing their enzymatic activities. The synergistic catalytic effect of the Mn and N-doped CDs (Mn&N-CDs) is induced by effective metal doping in the carbogenic domain and a high proportion of graphitic and pyridinic N. This highly enhanced catalytic effect of Mn&N-CDs allows them to respond sensitively to the interference factors of enzymatic reactions. Consequently, ascorbic acid, which is an essential nutrient for maintaining our health and is a reactive oxygen scavenger, can be successfully monitored using color change by forming oxidized 3,3',5,5'-tetramethylbenzidine with H2O2 and Mn&N-CDs. This study provides a basic understanding of the formation of CDs and how their catalytic properties can be controlled by the addition of different metal sources, thereby providing guidelines for the development of CDs for industrial applications.

Improvement in Acid Resistance of Polyimide Membranes: A Sustainable Cross-Linking Approach via Green-Solvent-Based Fenton Reaction.

In this study, we present a facile surface modification method using green solvents for a commercial polyimide (PI) nanofiltration membrane to exhibit good acid stability. To enhance acid stability, the PI organic solvent nanofiltration membrane was modified using Fenton's reaction, an oxidative cross-linking process, using environmentally friendly solvents: water and ethanol. The surface properties of the pristine and modified PI membranes were investigated and compared using various analytical tools. We studied the surface morphology using scanning electron microscopy, performed elemental analysis using X-ray photoelectron spectroscopy, investigated chemical bonds using attenuated total reflectance-Fourier transform infrared spectroscopy, and studied thermal stability using thermogravimetric analysis. The acid resistances of the pristine and modified membranes were confirmed through performance tests. The pristine PI nanofiltration membrane exposed to a 50 w/v% sulfuric acid for 4 h showed an increase in the normalized water flux to 205% and a decrease in the MgSO4 normalized rejection to 44%, revealing damage to the membrane. The membrane modified by the Fenton reaction exhibited a decline in flux and improved rejection, which are typical performance changes after surface modification. However, the Fenton-modified membrane exposed to 50 w/v% sulfuric acid for 4 h showed a flux increase of 7% and a rejection increase of 4%, indicating improved acid resistance. Furthermore, the Fenton post-treatment enhanced the thermal stability and organic solvent resistance of the PI membrane. This study shows that the acid resistance of PI membranes can be successfully improved by a novel and facile Fenton reaction using green solvents.

Optimization of Gate-Head-Top/Bottom Lengths of AlGaN/GaN High-Electron-Mobility Transistors with a Gate-Recessed Structure for High-Power Operations: A Simulation Study.

In this study, we propose an optimized AlGaN/GaN high-electron-mobility transistor (HEMT) with a considerably improved breakdown voltage. First, we matched the simulated data obtained from a basic T-gate HEMT with the measured data obtained from the fabricated device to ensure the reliability of the simulation. Thereafter, to improve the breakdown voltage, we suggested applying a gate-head extended structure. The gate-head-top and gate-head-bottom lengths of the basic T-gate HEMT were symmetrically extended by 0.2 µm steps up to 1.0 µm. The breakdown voltage of the 1.0 µm extended structure was 52% higher than that of the basic T-gate HEMT. However, the cutoff frequency (fT) and maximum frequency (fmax) degraded. To minimize the degradation of fT and fmax, we additionally introduced a gate-recessed structure to the 1.0 µm gate-head extended HEMT. The thickness of the 25 nm AlGaN barrier layer was thinned down to 13 nm in 3 nm steps, and the highest fT and fmax were obtained at a 6 nm recessed structure. The fT and fmax of the gate-recessed structure improved by 9% and 28%, respectively, with respect to those of the non-gate-recessed structure, and further improvement of the breakdown voltage by 35% was observed. Consequently, considering the trade-off relationship between the DC and RF characteristics, the 1.0 µm gate-head extended HEMT with the 6 nm gate-recessed structure was found to be the optimized AlGaN/GaN HEMT for high-power operations.

Analysis of Operational Characteristics of AlGaN/GaN High-Electron-Mobility Transistor with Various Slant-Gate-Based Structures: A Simulation Study.

This study investigates the operational characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs) by applying a slant-gate structure and drain-side extended field-plate (FP) for improved breakdown voltage. Prior to the analysis of slant-gate-based HEMT, simulation parameters were extracted from the measured data of fabricated basic T-gate HEMTs to secure the reliability of the results. We suggest three different types of slant-gate structures that connect the basic T-gate electrode boundary to the 1st and 2nd SiN passivation layers obliquely. To consider both the breakdown voltage and frequency characteristics, the DC and RF characteristics of various slant-gate structures including the self-heating effect were analyzed by TCAD simulation. We then applied a drain-side extended FP to further increase the breakdown voltage. The maximum breakdown voltage was achieved at the FP length of 0.4 µm. Finally, we conclude that the slant-gate structures can improve breakdown voltage by up to 66% without compromising the frequency characteristics of the HEMT. When the drain-side FP is applied to a slant-gate structure, the breakdown voltage is further improved by up to 108%, but the frequency characteristics deteriorate. Therefore, AlGaN/GaN HEMTs with an optimized slant-gate-based structure can ultimately be a promising candidate for high-power and high-frequency applications.

Ultrasound-mediated triboelectric nanogenerator for powering on-demand transient electronics.

On-demand transient electronics, technologies referring subsequent material disintegration under well-defined triggering events and programmed time lines, offer exceptional clinical experiences in diagnosis, treatment, and rehabilitation. Despite potential benefits, such as the elimination of surgical device removal and reduction of long-term inimical effects, their use is limited by the nontransient conventional power supplies. Here, we report an ultrasound-mediated transient triboelectric nanogenerator (TENG) where ultrasound determines energy generation and degradation period. Our findings on finite element method simulation show that porous structures of the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) play an essential role in the triggering transient process of our device under high-intensity ultrasound. Besides, the addition of polyethylene glycol improves triboelectric output performance; the voltage output increased by 58.5%, from 2.625 to 4.160 V. We successfully demonstrate the tunable transient performances by ex vivo experiment using a porcine tissue. This study provides insight into practical use of implantable TENGs based on ultrasound-triggered transient material design.

Highly Enhanced Enzymatic Activity of Mn-Induced Carbon Dots and Their Application as Colorimetric Sensor Probes.

Nanomaterial-based enzyme mimetics (nanozymes) have attracted significant interest because of their lower cost and higher stability compared to natural enzymes. In this study, we focused on improving the enzymatic properties of metal induced N-doped carbon dots (N-CDs), which are nanozymes of interest, and their applications for sensory systems. For this purpose, Mn(acetate)2 was introduced during the synthetic step of N-doped carbon dots, and its influence on the enzymatic properties of Mn-induced N-CDs (Mn:N-CDs) was investigated. Their chemical structure was analyzed through infrared spectroscopy and X-ray photoelectron spectrometry; the results suggest that Mn ions lead to the variation in the population of chemical bonding in Mn:N-CDs, whereas these ions were not incorporated into N-CD frameworks. This structural change improved the enzymatic properties of Mn:N-CDs with respect to those of N-CDs when the color change of a 3,3',5,5'-tetramethylbenzidine/H2O2 solution was examined in the presence of Mn:N-CDs and N-CDs. Based on this enhanced enzymatic property, a simple colorimetric system with Mn:N-CDs was used for the detection of γ-aminobutyric acid, which is an indicator of brain-related disease. Therefore, we believe that Mn:N-CDs will be an excellent enzymatic probe for the colorimetric sensor system.

Lead-Free AE Sensor Based on BZT-BCT Ceramics.

In this study, an acoustic emission (AE) sensor was fabricated using lead-free Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) ceramics. The acoustic and electromechanical properties of the AE sensor were determined by the shapes of the piezoelectric ceramics. To optimize the AE sensor performance, the shapes of the ceramics were designed according to various diameter/thickness ratios (D/T) = 0.5, 1.0, 1.5, 2.0, 2.5, 3.0. The BZT-BCT ceramic with D/T = 1.0 exhibited excellent values of a piezoelectric charge coefficient (d33), piezoelectric voltage coefficient (g33), and electromechanical coupling factor (kp), which were 370 (pC/N), 11.3 (10-3 Vm/N), and 0.58, respectively. Optimum values of resonant frequency (fr) = 172.724 (kHz), anti-resonant frequency (fa) = 196.067 (kHz), and effective electromechanical coupling factor (keff) = 0.473 were obtained for the manufactured BZT-BCT ceramic with D/T = 1.0. The maximum sensitivity and frequency of the AE sensor made of the BZT-BCT ceramic with a D/T ratio of 1.0 were 65 dB and 30 kHz, respectively.

Synthesis of high quality 2D carbide MXene flakes using a highly purified MAX precursor for ink applications.

The practical application of 2D MXenes in electronic and energy fields has been hindered by the severe variation in the quality of MXene products depending on the parent MAX phases, manufacturing techniques, and preparation parameters. In particular, their synthesis has been impeded by the lack of studies reporting the synthesis of high-quality parent MAX phases. In addition, controllable and uniform deposition of 2D MXenes on various large-scale substrates is urgently required to use them practically. Herein, a method of pelletizing raw materials could synthesize a stoichiometric Ti3AlC2 MAX phase with high yield and processability, and fewer impurities. The Ti3AlC2 could be exfoliated into 1-2-atom-thick 2D Ti3C2T x flakes, and their applicability was confirmed by the deposition and additional alignment of the 2D flakes with tunable thickness and electrical properties. Moreover, a practical MXene ink was fabricated with rheological characterization. MXene ink exhibited much better thickness uniformity while retaining excellent electrical performances (e.g., sheet resistance, electromagnetic interference shielding ability) as those of a film produced by vacuum filtration. The direct functional integration of MXenes on various substrates is expected to initiate new and unexpected MXene-based applications.