Alkali resistant nanocomposite gel beads as renewable adsorbents for water phosphate recovery.

Hydrous zirconium oxide (HZO) encapsulated alginate gel beads were synthesized for phosphate recovery from water. Importantly, we find that HZO/alginate gel beads (ZrA) crosslinked with Ca2+, Mg2+, Fe3+, Al3+, and Zr4+ are unstable under an intense alkali regeneration condition. Only Sr2+-crosslinked ZrA can endure a high alkali solution. ZrA possesses a high specific surface area (80.84 m2·g-1) and a mesoporous structure (15.3 nm and 0.196 cm3·g-1), which endow them with a high Langmuir adsorption capacity of 52.5 mg-P/g. ZrA can be easily recycled, and the mass loss of HZO is prevented. Furthermore, the strontium alginate gel framework protects the encapsulated HZO nanoparticles from adverse humic acid contamination. ZrA can be regenerated for at least 5 adsorption/desorption cycles. Cost analysis indicates the potential scale application feasibility for ZrA. This study provides a novel, simple, and environmentally benign solution to immobilize HZO for efficient phosphate recovery.

Study of crystalline defect induced optical scattering loss inside photonic waveguides in UV-visible spectral wavelengths using volume current method.

In this work, we study the crystalline defect induced optical scattering loss inside photonic waveguide. Volume current method is implemented with a close form of dyadic Green's function derived. More specifically, threading dislocation induced scattering loss inside AlN waveguides in UV-visible spectrum wavelengths are studied since this material is intrinsically accompanied with high densities of dislocations (typically on order of 108-1010cm-2). The results from this study reveal that threading dislocations contribute significant amount of scattering loss when material is not MOCVD grown. Additionally, the scattering loss is strongly dependent on polarization and waveguide geometries: TM modes exhibit higher scattering loss compared with TE modes, and the multimode large core waveguides are more susceptible to threading dislocations compared with single mode waveguides and high-aspect-ratio waveguides. Conclusions from this work can be supported by several recently published investigations on III-N based photonic devices. The model derived from this work can also be easily altered to fit other material systems with other types of crystalline defects.

Steep-slope field-effect transistors with AlGaN/GaN HEMT and oxide-based threshold switching device.

We report the demonstration of a steep-slope field-effect transistor with AlGaN/GaN MIS-HEMTs employing SiO2-based threshold switching devices in series with the source. The SiO2-based threshold switching devices exhibited steep slope when changing resistance states. The integrated steep-slope transistor showed a low subthreshold swing of sub-5 mV/dec with a transition range of over 105 in the transfer characteristics in both sweep directions at room temperature, as well as the low leakage current (10-5 µA µm-1) and a high I ON/I OFF ratio (>107). Moreover, with the SiO2-based threshold switching devices we also observed a positive shift of threshold voltages of the integrated device. Results from more than 50 transfer characteristics measurements also indicate the good repeatability and practicability of such a steep-switching device, where the average steep slopes are below 10 mV/decade. This steep-slope transistor with oxide-based threshold switching devices can be further extended to various transistor platforms like Si and III-V and are of potential interest for the development of power switching and high frequency devices.

Facile defluoridation of drinking water by forming shell@fluorapatite nanoarray during boiling egg shell.

High fluoride water is one of the major problems against drinking water and are affecting millions of people all over the world. Refined adsorbents and water treatment plants aim at massive water supply but can't meet scattered household requirements, especially in the developing areas. Here, we developed a facile defluoridation method in which F- can be removed by boiling eggs or shell assisted by phosphate. 0.4 L of high fluoride water (10 mg/L) can be transformed to safe drinking water with F- concentration lower than 1.5 mg/L by boiling one egg at 80 °C for 10 min with the addition of 0.3 g/L of NaH2PO4 and 0.05 v% acetic acid. The mechanism study shows that F- is adsorbed onto the egg shell outer surface forming nanorod arrays of fluorapatite and/or F- substituted hydroxyapatite. Higher F- adsorption capacity can be obtained (Langmuir adsorption capacity, 47.9 mg/g) if using egg shell powder instead of whole eggs. Pilot scale defluoridation (2.5 L, 10 times) was successfully realized by boiling egg shell in the presence of phosphate and acetic acid. The boiling shell defluoridation technology has potential household applications by common people with little professional backgrounds.

Characterizations of the nonlinear optical properties for (010) and (2¯01) beta-phase gallium oxide.

We report, for the first time, the characterizations on optical nonlinearities of beta-phase gallium oxide (ß-Ga2O3), where both (010) ß-Ga2O3 and (2¯01) ß-Ga2O3 were examined for two-photon absorption coefficient, Kerr nonlinear refractive index, and their polarization dependence. The wavelength dependence of two-photo absorption coefficient and Kerr nonlinear refractive index were also estimated by a widely used analytical model. ß-Ga2O3 exhibits a two photon absorption (TPA) coefficient of 1.2 cm/GW for (010) ß-Ga2O3 and 0.6 cm/GW for (2¯01) ß-Ga2O3. The Kerr nonlinear refractive index is -2.1 × 10-15 cm2/W for (010) ß-Ga2O3 and -2.9 × 10-15 cm2/W for (2¯01) ß-Ga2O3. In addition, ß-Ga2O3 shows stronger in-plane nonlinear optical anisotropy on (2¯01) plane than on (010) plane. Compared with GaN, TPA coefficient of ß-Ga2O3 is 20 times smaller, and the Kerr nonlinear refractive index of ß-Ga2O3 is also found to be 4-5 times smaller. These results indicate that ß-Ga2O3 have the potential for ultra-low loss waveguides and ultra-stable resonators and integrated photonics, especially in UV and visible wavelength spectral range.

Low loss GaN waveguides at the visible spectral wavelengths for integrated photonics applications.

We perform comprehensive studies on the fundamental loss mechanisms in III-nitride waveguides in the visible spectral region. Theoretical analysis shows that free carrier loss dominates for GaN under low photon power injection. When optical power increases, the two photon absorption loss becomes important and eventually dominates when photon energy above half-bandgap of GaN. When the dimensions of the waveguides reduce, the sidewall scattering loss will start to dominate. To verify the theoretical results, a high performance GaN-on-sapphire waveguide was fabricated and characterized. Experimental results are consistent with the theoretical findings, showing that under high power injection the optical loss changed significantly for GaN waveguides. A low optical loss ~2 dB/cm was achieved on the GaN waveguide, which is the lowest value ever reported for the visible spectral range. The results and fabrication processes developed in this work pave the way for the development of III-nitride integrated photonics in the visible and potentially ultraviolet spectral range for nonlinear optics and quantum photonics applications.

Active tracking system for visible light communication using a GaN-based micro-LED and NRZ-OOK.

Visible light communication (VLC) holds the promise of a high-speed wireless network for indoor applications and competes with 5G radio frequency (RF) system. Although the breakthrough of gallium nitride (GaN) based micro-light-emitting-diodes (micro-LEDs) increases the -3dB modulation bandwidth exceptionally from tens of MHz to hundreds of MHz, the light collected onto a fast photo receiver drops dramatically, which determines the signal to noise ratio (SNR) of VLC. To fully implement the practical high data-rate VLC link enabled by a GaN-based micro-LED, it requires focusing optics and a tracking system. In this paper, we demonstrate an active on-chip tracking system for VLC using a GaN-based micro-LED and none-return-to-zero on-off keying (NRZ-OOK). Using this novel technique, the field of view (FOV) was enlarged to 120° and data rates up to 600 Mbps at a bit error rate (BER) of 2.1×10-4 were achieved without manual focusing. This paper demonstrates the establishment of a VLC physical link that shows enhanced communication quality by orders of magnitude, making it optimized for practical communication applications.

Optical properties of highly polarized InGaN light-emitting diodes modified by plasmonic metallic grating.

We implement finite-difference time-domain (FDTD) method to simulate the optical properties of highly polarized InGaN light emitting diodes (LEDs) coupled with metallic grating structure. The Purcell factor (Fp), light extraction efficiency (LEE), internal quantum efficiency (IQE), external quantum efficiency (EQE), and modulation frequency are calculated for different polarized emissions. Our results show that light polarization has a strong impact on Fp and LEE of LEDs due to their coupling effects with the surface plasmons (SPs) generated by metallic grating. Fp as high as 34 and modulation frequency up to 5.4 GHz are obtained for a simulated LED structure. Furthermore, LEE, IQE and EQE can also be enhanced by tuning the coupling between polarized emission and SPs. These results can serve as guidelines for the design and fabrication of high efficiency and high speed LEDs for the applications of solid-state lighting and visible-light communication.

Discrete Li-occupation versus pseudo-continuous Na-occupation and their relationship with structural change behaviors in Fe2(MoO4)3.

The key factors governing the single-phase or multi-phase structural change behaviors during the intercalation/deintercalation of guest ions have not been well studied and understood yet. Through systematic studies of orthorhombic Fe2(MoO4)3 electrode, two distinct guest ion occupation paths, namely discrete one for Li and pseudo-continuous one for Na, as well as their relationship with single-phase and two-phase modes for Na(+) and Li(+), respectively during the intercalation/deintercalation process have been demonstrated. For the first time, the direct atomic-scale observation of biphasic domains (discrete occupation) in partially lithiated Fe2(MoO4)3 and the one by one Na occupation (pseudo-continuous occupation) at 8d sites in partially sodiated Fe2(MoO4)3 are obtained during the discharge processes of Li/Fe2(MoO4)3 and Na/Fe2(MoO4)3 cells respectively. Our combined experimental and theoretical studies bring the new insights for the research and development of intercalation compounds as electrode materials for secondary batteries.