Compact GaN-based optical inclinometer.

In this Letter, a compact optical inclinometer in sub-centimeter size is proposed and demonstrated. A 1×1 mm2 GaN-on-sapphire chip composed of a light-emitting diode and photodetector is fabricated through wafer-scale processes and integrated with a spherical glass cavity with a diameter of 5 mm, which contains ethanol as a liquid pendulum. When applying inclinations relative to the horizon, the extent to which the chip is immersed in ethanol changes, thereby altering the amount of light received by the on-chip detector. The underlying mechanisms of angle-dependent reflectance characteristics at the sapphire boundary are identified, and the measured photocurrent signal can be used as quantitative readouts for determining the angle of inclination from -60 to +60°. A linear response with a sensitivity of 19.4 nA/° and an estimated resolution of 0.003° is obtained over a wide linear range from -40 to +40°. Verified by a series of dynamic experiments, the developed inclinometer exhibits a high degree of repeatability and stability, which paves the way for its widespread usage and applications.

Chip-scale optical airflow sensor.

Airflow sensors are an essential component in a wide range of industrial, biomedical, and environmental applications. The development of compact devices with a fast response and wide measurement range capable of in situ airflow monitoring is highly desirable. Herein, we report a miniaturized optical airflow sensor based on a GaN chip with a flexible PDMS membrane. The compact GaN chip is responsible for light emission and photodetection. The PDMS membrane fabricated using a droplet-based molding process can effectively transform the airflow stimuli into optical reflectance changes that can be monitored by an on-chip photodetector. Without the use of external components for light coupling, the proposed sensor adopting the novel integration scheme is capable of detecting airflow rates of up to 53.5 ms-1 and exhibits a fast response time of 12 ms, holding great promise for diverse practical applications. The potential use in monitoring human breathing is also demonstrated.

Compact integration of GaN-based photonic chip with microfluidics system.

This Letter reports a demonstration of integrating a tiny GaN-based photonic chip with a PDMS microfluidics system. The photonic chip containing InGaN/GaN quantum wells is responsible for light emission and photodetection and fabricated through standard microfabrication techniques. The PDMS-enclosed chip is formed adjacent to the fluidic channel and operates in reflection mode, enabling the optical signals coupled into and out of the fluidic channel without the aid of external optics. The luminescence and photo-detecting properties are thoroughly characterized, confirming that the chip is capable of tracking the continuously flowing microdroplets with the changes of absorbance, length, and flow rate. The novel, to the best of our knowledge, photonic integration presented in this Letter is a significant step forward in the development of compact, miniature, and self-contained on-chip sensing systems, which are of great value in portable lab-on-a-chip applications.

Ultracompact Chip-Scale Refractometer Based on an InGaN-Based Monolithic Photonic Chip.

Optical refractometer constitutes the core element for many applications, from determining the purity and concentration of pharmaceutical ingredients to measuring the sugar content in food and beverages, and the analysis of petroleum. Here, we demonstrated the monolithic integration of light-emitting diodes (LEDs) and photodetectors (PDs) to fabricate ultracompact refractometers with a chip size of 475 × 320 µm2. The light emission and photodetection properties of the devices containing the same InGaN/GaN multi-quantum wells have been characterized, confirming that the PD can respond to the emission of the LED. The flip-chip assembly of the chip enables the exposed sapphire substrate to be in direct contact with the solution, and the refractive index sensing capability governed by the change of critical angle and Fresnel reflection at the sapphire/solution interface has been investigated. The processing of the optically smooth surface of sapphire and the integration of high-reflectance distributed Bragg reflector beneath the devices facilitate the amount of light received by the PD. The monolithic chip is capable of detecting solutions with a refractive index ranging from 1.3325 to 1.5148 RIU and exhibits a sensitivity of 7.77 µA/RIU and a resolution of 6.4 × 10-6 RIU at the LED current of 10 mA. Rapid real-time responses of 33.9 ms for rise time and 34.7 ms for fall time are obtained in the detected photocurrent, thereby verifying the feasibility of the chip-scale refractometer.

Achieving Ultrahigh Capacity with Self-Assembled Ni(OH)2 Nanosheet-Decorated Hierarchical Flower-like MnCo2O4.5 Nanoneedles as Advanced Electrodes of Battery-Supercapacitor Hybrid Devices.

Self-assembled Ni(OH)2 nanosheet-decorated hierarchical flower-like MnCo2O4.5 nanoneedles were synthesized via a cost-effective and facile hydrothermal strategy, aiming to realize a high-capacity advanced electrode of a battery-supercapacitor hybrid (BSH) device. It is demonstrated that the as-synthesized hierarchical flower-like MnCo2O4.5@Ni(OH)2-nanosheet electrode exhibits a high specific capacity of 318 mAh g-1 at a current density of 3 A g-1 and still maintains a capacity of 263.5 mAh g-1 at a higher current density of 20 A g-1, with an extremely long cycle lifespan of 87.7% capacity retention after 5000 cycles. Moreover, using the unique core-shell structure as the cathode and hollow Fe2O3 nanoparticles/reduced graphene oxide as the anode, the BSH device delivers a high energy density of 56.53 Wh kg-1 when the power density reaches 1.9 kW kg-1, and there is an extraordinarily good cycling stability with the capacity retention rate of 90.4% after 3000 cycles. It is believed that the superior properties originate from desirable core-shell structures alleviating the impact of volume changes as well as the existence of two-dimensional Ni(OH)2 nanosheets with more active sites, thereby improving the cycle stability and achieving ultrahigh capacity. These results will provide more access to the rational material design of diverse nanostructures toward high-performance energy storage devices.

Preparation of LiFePO4/C Cathode Materials via a Green Synthesis Route for Lithium-Ion Battery Applications.

In this work, LiFePO4/C composite were synthesized via a green route by using Iron (III) oxide (Fe2O3) nanoparticles, Lithium carbonate (Li2CO3), glucose powder and phosphoric acid (H3PO4) solution as raw materials. The reaction principles for the synthesis of LiFePO4/C composite were analyzed, suggesting that almost no wastewater and air polluted gases are discharged into the environment. The morphological, structural and compositional properties of the LiFePO4/C composite were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), Raman and X-ray photoelectron spectroscopy (XPS) spectra coupled with thermogravimetry/Differential scanning calorimetry (TG/DSC) thermal analysis in detail. Lithium-ion batteries using such LiFePO4/C composite as cathode materials, where the loading level is 2.2 mg/cm², exhibited excellent electrochemical performances, with a discharge capability of 161 mA h/g at 0.1 C, 119 mA h/g at 10 C and 93 mA h/g at 20 C, and a cycling stability with 98.0% capacity retention at 1 C after 100 cycles and 95.1% at 5 C after 200 cycles. These results provide a valuable approach to reduce the manufacturing costs of LiFePO4/C cathode materials due to the reduced process for the polluted exhaust purification and wastewater treatment.