Intrinsic carrier multiplication in layered Bi2O2Se avalanche photodiodes with gain bandwidth product exceeding 1 GHz

Emerging layered semiconductors present multiple advantages for optoelectronic technologies including high carrier mobilities, strong light-matter interactions, and tunable optical absorption and emission. Here, metal-semiconductor-metal avalanche photodiodes (APDs) are fabricated from Bi₂O₂Se crystals, which consist of electrostatically bound [Bi₂O₂]²⁺ and [Se]²⁻ layers. The resulting APDs possess an intrinsic carrier multiplication factor up to 400 at 7 K with a responsivity gain exceeding 3,000 A/W and bandwidth of ~ 400 kHz at a visible wavelength of 515.6 nm, ultimately resulting in a gain bandwidth product exceeding 1 GHz. Due to exceptionally low dark currents, Bi₂O₂Se APDs also yield high detectivities up to 4.6 × 10¹⁴ Jones. A systematic analysis of the photocurrent temperature and bias dependence reveals that the carrier multiplication process in Bi₂O₂Se APDs is consistent with a reverse biased Schottky diode model with a barrier height of ~ 44 meV, in contrast to the charge trapping extrinsic gain mechanism that dominates most layered semiconductor phototransistors. In this manner, layered Bi₂O₂Se APDs provide a unique platform that can be exploited in a diverse range of high-performance photodetector applications.

     

Microstructure and multifunctional properties of liquid + polymer bicomponent structural electrolytes: Epoxy gels and porous monoliths

Multifunctional structural batteries and supercapacitors have the potential to improve performance and efficiency in advanced lightweight systems. A critical requirement is a structural electrolyte with superior multifunctional performance. We present here structural electrolytes prepared by the integration of liquid electrolytes with structural epoxy networks. Two distinct approaches were investigated: direct blending of an epoxy resin with a poly(ethylene‐glycol) (PEG)‐ or propylene carbonate (PC)‐based liquid electrolyte followed by in‐situ cure of the resin; and formation of a porous neat epoxy sample followed by backfill with a PC‐based electrolyte. The results show that in situ cure of the electrolytes within the epoxy network does not lead to good multifunctional performance due to a combination of plasticization of the structural network and limited percolation of the liquid network. In contrast, addition of a liquid electrolyte to a porous monolith results in both good stiffness and high ionic conductivity that approach multifunctional goals. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42681.     

Melt‐Centrifuged (Bi,Sb)2Te3: Engineering Microstructure toward High Thermoelectric Efficiency

Microstructure engineering is an effective strategy to reduce lattice thermal conductivity (κ_l) and enhance the thermoelectric figure of merit (zT). Through a new process based on melt‐centrifugation to squeeze out excess eutectic liquid, microstructure modulation is realized to manipulate the formation of dislocations and clean grain boundaries, resulting in a porous network with a platelet structure. In this way, phonon transport is strongly disrupted by a combination of porosity, pore surfaces/junctions, grain boundaries, and lattice dislocations. These collectively result in a ≈60% reduction of κ_l compared to zone melted ingot, while the charge carriers remain relatively mobile across the liquid‐fused grains. This porous material displays a zT value of 1.2, which is higher than fully dense conventional zone melted ingots and hot pressed (Bi,Sb)₂Te₃ alloys. A segmented leg of melt‐centrifuged Bi_0.5Sb_1.5Te₃ and Bi_0.3Sb_1.7Te₃ could produce a high device ZT exceeding 1.0 over the whole temperature range of 323–523 K and an efficiency up to 9%. The present work demonstrates a method for synthesizing high‐efficiency porous thermoelectric materials through an unconventional melt‐centrifugation technique.     

Glassy phases in organic semiconductors

Organic semiconductors may be processed from fluids using graphical arts printing and patterning techniques to create complex circuitry. Because organic semiconductors are weak van der Waals solids, the creation of glassy phases during processing is quite common. Because structural disorder leads to electronic disorder, it is necessary to understand these phases to optimize and control the electronic properties of these materials. Here we review the significance of glassy phases in organic semiconductors. We examine challenges in the measurement of the glass transition temperature and the accurate classification of phases in these relatively rigid materials. Device implications of glassy phases are discussed. Processing schemes that are grounded in the principles of glass physics and sound glass transition temperature measurement will more quickly achieve desired structure and electronic characteristics, accelerating the exciting progress of organic semiconductor technology development.     

Additive Manufacturing of Batteries

Additive manufacturing, i.e., 3D printing, is being increasingly utilized to fabricate a variety of complex‐shaped electronics and energy devices (e.g., batteries, supercapacitors, and solar cells) due to its excellent process flexibility, good geometry controllability, as well as cost and material waste reduction. In this review, the recent advances in 3D printing of emerging batteries are emphasized and discussed. The recent progress in fabricating 3D‐printed batteries through the major 3D‐printing methods, including lithography‐based 3D printing, template‐assisted electrodeposition‐based 3D printing, inkjet printing, direct ink writing, fused deposition modeling, and aerosol jet printing, are first summarized. Then, the significant achievements made in the development and printing of battery electrodes and electrolytes are highlighted. Finally, major challenges are discussed and potential research frontiers in developing 3D‐printed batteries are proposed. It is expected that with the continuous development of printing techniques and materials, 3D‐printed batteries with long‐term durability, favorable safety as well as high energy and power density will eventually be widely used in many fields.     

Removal of emerging contaminants of concern by alternative adsorbents

The effective removal of emerging contaminants of concern (ECCs) such as endocrine-disrupting chemicals, pharmaceutically active compounds, personal care products, and flame retardants is a desirable water treatment goal. In this study, one activated carbon, one carbonaceous resin, and two high-silica zeolites were studied to evaluate their effectiveness for the removal of an ECC mixture from lake water. Adsorption isotherm experiments were performed with a mixture of 28 ECCs at environmentally relevant concentrations (200–900 ng/L). Among the tested adsorbents, activated carbon was the most effective, and activated carbon doses typically used for taste and odor control in drinking water (<10 mg/L) were sufficient to achieve a 2-log removal for most of the tested ECCs. The carbonaceous resin was less effective than the activated carbon because this adsorbent had a smaller volume of pores in the size range required for the adsorption of many ECCs (6–9 Å). For the removal of ECC mixture constituents, zeolites were less effective than the carbonaceous adsorbents. Because zeolites contain pores of uniform size and shape, a few of the tested ECCs with matching pore size/shape requirements were well removed, but the adsorptive removal of others was negligible, even at zeolite doses of 100 mg/L. The results of this study demonstrate that effective adsorbents for the removal of a broad spectrum of ECCs from water should exhibit heterogeneity in pore size and shape and a large pore volume in the 6–9 Å size range.     

Strategies to improve electrode positioning and safety in cochlear implants

An injection-molded internal supporting rib has been produced to control the flexibility of silicone rubber encapsulated electrodes designed to electrically stimulate the auditory nerve in human subjects with severe to profound hearing loss. The rib molding dies, and molds for silicone rubber encapsulation of the electrode, were designed and machined using AutoCad and MasterCam software packages in a PC environment. After molding, the prototype plastic ribs were iteratively modified based on observations of the performance of the rib/silicone composite insert in a clear plastic model of the human scala tympani cavity. The rib-based electrodes were reliably inserted farther into these models, required less insertion force and were positioned closer to the target auditory neural elements than currently available cochlear implant electrodes. With further design improvements the injection-molded rib may also function to accurately support metal stimulating contacts and wire leads during assembly to significantly increase the manufacturing efficiency of these devices. This method to reliably control the mechanical properties of miniature implantable devices with multiple electrical leads may be valuable in other areas of biomedical device design.     

The expectation-maximization algorithm for symbol unsynchronizedsequence detection

The expectation-maximization (EM) algorithm for maximizing likelihood functions, combined with the Viterbi algorithm, is applied to the problem of sequence detection when symbol timing information is not present. Although the EM algorithm is noncausal, results obtained using the algorithm on the problem of nonsynchronized sequence detection indicate that it converges most of the time in three iterations, making it both of theoretical and of practical interest. A practical algorithm based on the EM algorithm is introduced. It reduces the computational burden and improves performance by making use of timing estimates in previous observation windows     

Identification and purification of diphosphoinositol pentakisphosphate kinase, which synthesizes the inositol pyrophosphate bis(diphospho)inositol tetrakisphosphate

Diphosphoinositol pentakisphosphate (PP-IP5) and bis(diphospho)inositol tetrakisphosphate (bis-PP-IP4) were recently identified as inositol phosphates which possess pyrophosphate bonds. The molecular mechanisms that regulate the cellular levels of these compounds are not yet characterized. To pursue this question, we have previously purified an inositol hexakisphosphate (IP6) kinase from rat brain supernatants [Voglmaier, S. M., et al. (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 4305-4310]. We now report the identification and purification of another novel kinase, diphosphoinositol pentakisphosphate (PP-IP5) kinase, which uses PP-IP5 as a substrate to form bis(diphospho)inositol tetrakisphosphate (bis-PP-IP4) in soluble fractions of rat forebrain. The purified protein, a monomer of 56 kDa, displays high affinity (Km = 0.7 microM) and selectivity for PP-IP5 as a substrate. The purified enzyme also can transfer a phosphate from bis-PP-IP4 to ADP to form ATP. This ATP synthase activity is an indication of the high phosphoryl group transfer potential of bis-PP-IP4 and may represent a physiological role for PP-IP5 and bis-PP-IP4.