Low-Loss Organic Hyperbolic Materials in the Visible Spectral Range: A Joint Experimental and First-Principles Study

Hyperbolic media strengthen numerous attractive applications in optics such as super-resolution imaging, enhanced spontaneous emission, and nanoscale waveguiding. Natural hyperbolic materials exist at visible frequencies; however, implementations of these materials suffer substantial compromises resulting from the high loss in the currently available candidates. Here, the first experimental and theoretical investigation of regioregular poly(3-alkylthiophenes) (rr-P3ATs), a naturally low-loss organic hyperbolic material (OHM) in the visible frequency range, is shown. These hyperbolic properties arise from a highly ordered structure of layered electron-rich conjugated thiophene ring backbones separated by insulating alkyl side chains. The optical and electronic properties of the rr-P3AT can be tuned by controlling the degree of crystallinity and alkyl side chain length. First-principles calculations support the experimental observations, which result from the rr-P3AT's structural and optical anisotropy. Conveniently, rr-P3AT-based OHMs are facile to fabricate, flexible, and biocompatible, which may lead to tremendous new opportunities in a wide range of applications.     

Facilitation of consumer loyalty toward branded applications: The dual-route perspective

Although many brands develop mobile applications (apps) to build relationships with consumers, most branded apps fail to retain consumers’ loyalty. This study examines the facilitation of consumer loyalty toward branded apps (continuance intention, in-app purchase intention, and word-of-mouth intention) from the dual-route perspective. One route is the affective (relationship) route, where brand benefits (functional benefits, experiential benefits, symbolic benefits, and monetary benefits) drive parasocial interactions between consumers and the brand, which, in turn, influences branded app loyalty. The other route is the utility route, where system characteristics (system quality and information quality) affect perceived usefulness, which, in turn, facilitates branded app loyalty. An online survey was conducted, and the research model was empirically tested using partial least squares structural equation modeling. The findings support the dual-route perspective according to which both affective and utilitarian paths facilitate branded app loyalty. The key theoretical contribution of this study is that it moves beyond the utilitarian path and finds the affective (relationship) path to give a more complete picture of the facilitation of consumer loyalty in the branded app context. A strategy is provided to suggest to practitioners how to design branded apps to facilitate consumer loyalty.     

Antimicrobial-resistant Staphylococcus aureus and MRSA prevalence among Korean families and household items

Food Science and Biotechnology - One-hundred-and-seventy-nine Staphylococcus aureus strains, collected from a total of 825 resident and household item samples in Korean homes nationwide, were...     

Determination and identification of titanium dioxide nanoparticles in confectionery foods,marketed in South Korea,using inductively coupled plasma optical emission spectrometry and transmission electron microscopy

Food-grade titanium dioxide (TiO₂) is a common and widespread food additive in many processed foods, personal care products, and other industrial categories as it boosts the brightness and whiteness of colours. Although it is generally recognised as safe for humans, there is a growing interest in the health risks associated with its oral intake. This study quantified and identified TiO₂ nanoparticles present in confectionery foods, which are children’s favourite foods, with inductively coupled plasma optical emission spectrometry (ICP-OES) and transmission electron microscopy (TEM). A reliable digestion method using hot sulphuric acid and a digestion catalyst (K₂SO₄:CuSO₄ = 9:1) was suggested for titanium analysis. Validations of the experimental method were quite acceptable in terms of linearity, recoveries, detection limits, and quantification limits. Of all the 88 analysed foods, TiO₂ was detected in 19 products, all except three declared TiO₂ in their labelling. The mean TiO₂ content of candies, chewing gums, and chocolates were 0.36 mg g^?1, 0.04 mg g^?1, and 0.81 mg g^?1, respectively. Whitish particles isolated from the confectionery foods were confirmed as TiO₂ nanoparticles via TEM and energy dispersive X-ray spectroscopy (EDX), in which nanosized particles (<100 nm) were identified.     

Copper-doped flower-like molybdenum disulfide/bismuth sulfide photocatalysts for enhanced solar water splitting

A facile synthesis approach to fabricate Cu-doped MoS₂/Bi₂S₃ (Cu-MoS₂/Bi₂S₃) photocatalysts is reported. The photocatalyst samples with varying amounts of Cu are applied in the photocatalytic splitting of water to produce H₂ under the irradiation of simulated solar light. The Cu-MoS₂/Bi₂S₃ photocatalysts with an optimum Cu loading of 20 mol% exhibited high photocatalytic performance, achieving a total H₂ yield of 32.4 μmol/h after 6 h of reaction. The photoactivity of the Cu-doped sample was shown to have risen more than 40% than that of pure MoS₂/Bi₂S₃. The improved performance is attributed to the impurity states generated within Cu-doped MoS₂, which serve as trapping sites for photogenerated electrons. The effective charge transfer mechanism achieved was evidenced by photoelectrochemical measurements. Based on the experimental results obtained, a plausible mechanism for the photocatalytic process associated with Cu-MoS₂/Bi₂S₃ was proposed.     

Synergistic Improvement of Long-Term Plasticity in Photonic Synapses Using Ferroelectric Polarization in Hafnia-Based Oxide-Semiconductor Transistors

A number of synapse devices have been intensively studied for the neuromorphic system which is the next-generation energy-efficient computing method. Among these various types of synapse devices, photonic synapse devices recently attracted significant attention. In particular, the photonic synapse devices using persistent photoconductivity (PPC) phenomena in oxide semiconductors are receiving much attention due to the similarity between relaxation characteristics of PPC phenomena and Ca²⁺ dynamics of biological synapses. However, these devices have limitations in its controllability of the relaxation characteristics of PPC behaviors. To utilize the oxide semiconductor as photonic synapse devices, relaxation behavior needs to be accurately controlled. In this study, a photonic synapse device with controlled relaxation characteristics by using an oxide semiconductor and a ferroelectric layer is demonstrated. This device exploits the PPC characteristics to demonstrate synaptic functions including short-term plasticity, paired-pulse facilitation (PPF), and long-term plasticity (LTP). The relaxation properties are controlled by the polarization of the ferroelectric layer, and this polarization is used to control the amount by which the conductance levels increase during PPF operation and to enhance LTP characteristics. This study provides an important step toward the development of photonic synapses with tunable synaptic functions.     

Direct optical-structure correlation in atomically thin dichalcogenides and heterostructures

Atomically thin transition metal dichalcogenides (TMDs) have distinct opto-electronic properties including enhanced luminescence and high on-off current ratios, which can be further modulated by making more complex TMD heterostructures. However, resolution limits of conventional optical methods do not allow for direct nanoscale optical-structural correlation measurements in these materials, particularly of buried interfaces in TMD heterostructures. Here we use, for the first time, electron beam induced cathodoluminescence in a scanning transmission electron microscope (CL-STEM) to measure optical properties of monolayer TMDs (WS₂, MoS₂ and WSSe alloy) encapsulated between layers of hBN. We observe dark areas resulting from localized (~ 100 nm) imperfect interfaces and monolayer folding, which shows that the intimate contact between layers in this application-relevant heterostructure is required for proper inter layer coupling. We also realize a suitable imaging method that minimizes electron-beam induced changes and provides measurement of intrinsic properties. To overcome the limitation of small electron interaction volume in TMD monolayer (and hence low photon yield), we find that encapsulation of TMD monolayers with hBN and subsequent annealing is important. CL-STEM offers to be a powerful method to directly measure structure-optical correspondence in lateral or vertical heterostructures and alloys.

     

Characterization of a Tryptophan 6-Halogenase from Streptomyces albus and Its Regioselectivity Determinants

Tryptophan halogenases are found in diverse organisms and catalyze regiospecific halogenation. They play an important role in the biosynthesis of halogenated indole alkaloids, which are biologically active and of therapeutic importance. Here, a tryptophan 6-halogenase (SatH) from Streptomyces albus was characterized by using a whole-cell reaction system in Escherichia coli. SatH showed substrate specificity for chloride and bromide ions, leading to regiospecific halogenation at the C6-position of l -tryptophan. In addition, SatH exhibited higher performance in bromination than that of previously reported tryptophan halogenases in the whole-cell reaction system. Through structure-based protein mutagenesis, it has been revealed that two consecutive residues, A78/V79 in SatH and G77/I78 in PyrH, are key determinants in the regioselectivity difference between tryptophan 6- and 5-halogenases. Substituting the AV with GI residues switched the regioselectivity of SatH by moving the orientation of tryptophan. These data contribute to an understanding of the key residues that determine the regioselectivity of tryptophan halogenases.