Solid-State Far-Ultraviolet C Light Sources for the Disinfection of Pathogenic Microorganisms Using Graphene Nanostructure Field Emitters.

The ongoing global outbreak of coronavirus disease has necessitated the use of ultraviolet (UV) disinfection techniques to reduce viral transmission in public places. The previously used UV wavelength is harmful to the human body, the wavelength range from 200 to 235 nm, often referred to as far-UVC light, has attracted attention as a novel disinfection wavelength range that can be used in a safe manner. However, the currently used light sources have practical problems, such as an expensive cost, a low efficiency, and short lifetimes. Therefore, environmentally friendly solid-state light sources with a lower cost, higher efficiency, and longer lifetimes are demanded. Here, an efficient mercury-free far-UVC solid-state light source is presented. This light source demonstrates intense 230 nm emission with a narrow spectral width of 30 nm and a long lifetime of more than 1000 h. These characteristics can be achieved by graphene nanostructure field emitters and wide-bandgap magnesium aluminate phosphors. By using this light source, the efficient disinfection of Escherichia coli is demonstrated. The light sources presented here facilitate future technologies for preventing the spread of infectious diseases in a safe and convenient manner.

Low-Temperature Direct Synthesis of Multilayered h-BN without Catalysts by Inductively Coupled Plasma-Enhanced Chemical Vapor Deposition.

Low-temperature direct synthesis of thick multilayered hexagonal-boron nitride (h-BN) on semiconducting and insulating substrates is required to produce high-performance electronic devices based on two-dimensional (2D) materials. In this study, multilayered h-BN with a thickness exceeding 5 nm was directly synthesized on quartz and Si at low temperatures, between 400 and 500 °C, by inductively coupled plasma-enhanced chemical vapor deposition using borazine as the precursor material. The quality and thickness of the h-BN crystals were investigated with respect to synthesis parameters, namely, temperature, radio frequency power, N2 flow rate, and H2 flow rate. Introducing N2 and H2 carrier gases critically affected the deposition rate, and increasing the carrier gas flow rate enhanced the h-BN crystal quality. The typical optical band gap of synthesized h-BN was approximately 5.8 eV, consistent with that of previous studies. The full width at half-maximum of the h-BN Raman peak was 32-33 cm-1, comparable to that of commercially available multilayered h-BN on Cu foil. These results are expected to facilitate the development of 2D materials for electronics applications.

Precise Deposition of Carbon Nanotube Bundles by Inkjet-Printing on a CMOS-Compatible Platform.

Carbon nanotubes (CNTs) are ultimately small structures, attractive for future nanoelectronics. CNT-bundles on Si nanostructures can offer an alternative pathway to build hybrid CMOS-compatible devices. To develop a simple method of using such CNT-bundles as transistor channels, we fabricated semiconductor single-walled CNT field-effect transistors using inkjet printing on a CMOS-compatible platform. We investigated a method of producing stable CNT solutions without surfactants, allowing for CNT debundling and dispersion. An inkjet-printing system disperses CNT-networks with ultimately low density (down to discrete CNT-bundles) in Al source-drain gaps of transistors. Despite the small number of networks and random positions, such CNT-bundles provide paths to the flow current. For enhanced controllability, we also demonstrate the manipulation of CNT-networks using an AFM technique.

A large piezoelectric response in highly-aligned electrospun poly(vinylidene fluoride/trifluoroethylene) nanofiber webs for wearable energy harvesting.

In this study, highly-aligned and molecularly oriented poly(vinylidene fluoride/trifluoroethylene) [P(VDF/TrFE)] nanofiber webs were fabricated and their piezoelectric response was investigated. Using systematic post-treatments under appropriate conditions, a significant enhancement of the piezoelectric response in the P(VDF/TrFE) nanofiber webs was observed for the first time. The high-quality nanofibers exhibited a large output voltage of 0.4 V. The short-circuit current of post-treated nanofibers was found to be 731.25 µA, which increased about 330 times and the surface electric charge density was found to be 0.64 nC cm-2, which was about 2.7 times higher than those of the as-spun nanofibers. The large enhancement of piezoelectric response of the nanofibers is attributed to the additional stretching, annealing and poling of the as-spun nanofibers under the appropriate post-treatment conditions. The results highlight the potential of the high-quality P(VDF/TrFE) nanofibers to be used as wearable piezoelectric energy harvesters and other flexible self-powered portable devices.

Observation of CO Detection Using Aluminum-Doped ZnO Nanorods on Microcantilever.

An oscillating piezoresistive microcantilever (MC) coated with an aluminum (Al)-doped zinc oxide (ZnO) nanorods was used to detect carbon monoxide (CO) in air at room temperature. Al-doped ZnO nanorods were grown on the MC surface using the hydrothermal method, and a response to CO gas was observed by measuring a resonant frequency shift of vibrated MC. CO gas response showed a significant increase in resonant frequency, where sensitivity in the order of picogram amounts was obtained. An increase in resonant frequency was also observed with increasing gas flow rate, which was simultaneously followed by a decrease in relative humidity, indicating that the molecular interface between ZnO and H2O plays a key role in CO absorption. The detection of other gases of carbon compounds such as CO2 and CH4 was also performed; the sensitivity of CO was found to be higher than those gases. The results demonstrate the reversibility and reproducibility of the proposed technique, opening up future developments of highly sensitive CO-gas detectors with a fast response and room temperature operation.

Giant and highly reflective Goos-Hänchen shift in a metal-dielectric multilayer Fano structure.

We experimentally demonstrated a giant Goos-Hänchen (GH) shift in a metal-dielectric multilayer Fano structure. The observed GH shift was 0.176 mm, which corresponded to (GH shift/λ) = 493, where λ is the incident wavelength. A unique feature of this giant GH shift was that it occurred without attenuation, i.e., reflectivity ∼1, due to Fano interference between surface plasmon polariton and high-Q dielectric waveguide mode. The Q-value is determined by the coupling loss. Therefore, we can enhance the GH shift to an arbitrarily large value by controlling the coupling strength. The unique feature whereby the giant GH shift occurs without attenuation has great potential for real-world applications, such as optical switching, optical filters, and sensors, where the reduction of reflected beam intensity is currently a major drawback.

Effects of Seeding Layer Annealing Temperature on Physical and Morphological Structure of Ga/F Co-Doped ZnO Nanostructures.

In this work, effects of annealing temperature of seeding layer on structural properties and morphologies of Ga/F co-doped ZnO nanostructures synthesized by hydrothermal process were investigated by varying the annealing temperature of seed layer as 300-500 °C. The ZnO seeding layers were deposited onto cleaned glass substrates by dip-coating technique using zinc acetate dehydrate (CH3COO)2Zn·2H2O as starting coating precursor. The Ga/F co-doped ZnO nanostructures were then grown on these seed layers by conventional hydrothermal process using Zn(NO3)2, NH4F, GaN3O9 and hexamethyltetramine as Zn, F and Ga sources, respectively. Effect of seed layer annealing temperature on morphologies, structural and Photoluminescence properties was investigated by X-ray diffraction (XRD), Field emission scanning electron microscope (FE-SEM), and Photoluminescence spectra, respectively. Variation of annealing temperature of seed layers can significantly result to the difference in morphological, structure and shape of the as-synthesized nanostructure products. It is found that the increase in annealing temperature leads to alternation in their shape from vertically-aligned nanosheets to nanorods with their average size ranging from 50 to 200 nm. Furthermore, the luminescence could be ascribed to the different contributions of the defect emissions, such asthe increase in the oxygen vacancy (VO) emissionor the decrease of the Zinc vacancy (Vzn). However, it can be speculated from the photoluminescence that the incorporated Ga and F substitute into ZnO.

Tailored plasmon-induced transparency in attenuated total reflection response in a metal-insulator-metal structure.

We demonstrated tailored plasmon-induced transparency (PIT) in a metal (Au)-insulator (SiO2)-metal (Ag) (MIM) structure, where the Fano interference between the MIM waveguide mode and the surface plasmon polariton (SPP) resonance mode induced a transparency window in an otherwise opaque wavenumber (k) region. A series of structures with different thicknesses of the Ag layer were prepared and the attenuated total reflection (ATR) response was examined. The height and width of the transparency window, as well as the relevant k-domain dispersion, were controlled by adjusting the Ag layer thickness. To confirm the dependency of PIT on Ag layer thickness, we performed numerical calculations to determine the electric field amplitude inside the layers. The steep k-domain dispersion in the transparency window is capable of creating a lateral beam shift known as the Goos-Hänchen shift, for optical device and sensor applications. We also discuss the Fano interference profiles in a ω - k two-dimensional domain on the basis of Akaike information criteria.

Transformation from plasmon-induced transparence to -induced absorption through the control of coupling strength in metal-insulator-metal structure.

Photonic structures created by coupling a narrow resonance to a broad resonance can significantly improve the sensitivity of optical sensors. We investigated a planar metal-insulator-metal (MIM) multilayered structure using attenuated total reflection to couple surface plasmon polaritons with the waveguide (WG) mode. A plasmon-induced transparency (PIT) to plasmon-induced adsorption (PIA) transformation was realized by controlling the coupling strength between the incident light and the WG mode. The results indicated that PIT and PIA have differing coupling strength and reflectance phase at surface plasmon resonance. Moreover, Fano resonance was realized by adjusting the center of the absorption band of the WG mode.

Attenuated total reflection response to wavelength tuning of plasmon-induced transparency in a metal-insulator-metal structure.

We experimentally demonstrated a plasmon-induced transparency in a metal-insulator-metal (MIM) structure based on the attenuated total reflection (ATR) response. Here, the MIM waveguide (MIMWG) mode and the surface plasmon polariton (SPP) resonance mode acted as low- and high-Q resonance modes, respectively. The dependence of the resonance angles of SPP and MIMWG mode resonances on the incident wavelength differed, which allowed the coupling condition between the two modes to be tuned via the wavelength. When the resonance angles of the two modes coincided, the ATR response showed a symmetric plasmon-induced transparency spectrum; in contrast, when the resonance angles were detuned, the ATR exhibited a sharp asymmetric spectrum characteristic to off-resonance Fano interference.

Determining the physisorption energies of molecules on graphene nanostructures by measuring the stochastic emission-current fluctuation.

A method of determining the physisorption energy of molecules on carbon nanostructures using field emission current fluctuation measurements is presented. A stochastic model, broken into birth and death processes, was applied to analyze the current fluctuation and determine the physisorption energy. This method yields a highly sensitive, precise determination of the physisorption energy of molecules, and we include the physisorption energies for various molecules on a graphene nanostructure.