Reaction of Nitrogen Dioxide with Ice Surface at Low Temperature (≤170 K)

We studied the adsorption and reaction of nitrogen dioxide gas on the surface of an ice film at temperatures of 100–170 K under ultrahigh vacuum (UHV) conditions. Cs⁺ reactive ion scattering (RIS) and low-energy sputtering (LES) techniques were used to identify and quantify the reactants and products on the surface of the ice film, in conjunction with the use of temperature-programmed desorption (TPD) to monitor the species desorbed. Temperature-ramping experiments were performed to examine the changes in the populations of these species as a function of temperature. Adsorption of NO₂ gas on the ice film at <110 K produced physisorbed species that may possibly possess negative charge character (NO₂^δ‑), as deduced from the NO₂ and NO₂^– signals in the RIS and LES experiments. At 110–130 K, NO₂^δ‑ species were either desorbed as NO₂ gas or converted to nitrous acid (HONO), NO₃^–, and H₃O⁺ on the surface. Nitrous acid gas was desorbed at 140–160 K. The efficiency of conversion of NO₂ to surface nitrous acid was about 40%, and that to nitrous acid gas was about 7%. The efficiency of the reaction of NO₂ on the ice surface may be higher than that at the gas/liquid water interface. The reaction efficiency increased with a decrease of the NO₂ coverage and was inversely correlated with the N₂O₄ coverage, which favors the mechanistic interpretation that an isolated NO₂ molecule reacts with water. However, NO₂ can diffuse on the ice surface to form clusters at ≥120 K. Under these conditions, the possibility that dimerization of NO₂ contributes to the hydrolysis reaction of NO₂ may not be excluded

Design of Nanowire Optical Cavities as Efficient Photon Absorbers

Recent investigations of semiconductor nanowires have provided strong evidence for enhanced light absorption, which has been attributed to nanowire structures functioning as optical cavities. Precise synthetic control of nanowire parameters including chemical composition and morphology has also led to dramatic modulation of absorption properties. Here we report finite-difference time-domain (FDTD) simulations for silicon (Si) nanowire cavities to elucidate the key factors that determine enhanced light absorption. The FDTD simulations revealed that a crystalline Si nanowire with an embedded 20-nm-thick amorphous Si shell yields 40% enhancement of absorption as compared to a homogeneous crystalline Si nanowire, under air-mass 1.5 global solar spectrum for wavelengths between 280 and 1000 nm. Such a large enhancement in absorption results from localization of several resonant modes within the amorphous Si shell. A nanowire with a rectangular cross section exhibited enhanced absorption at specific wavelengths with respect to a hexagonal nanowire. The pronounced absorption peaks were assigned to resonant modes with a high symmetry that red-shifted with increasing size of the rectangular nanowire. We extended our studies to investigate the optical properties of single- and multilayer arrays of these horizontally oriented nanowire building blocks. The absorption efficiency of a nanowire stack increases with the number of nanowire layers and was found to be greater than that of a bulk structure or even a single nanowire of equivalent thickness. Lastly, we found that a single-layer nanowire array preserves the structured absorption spectrum of a single nanowire and ascribed this result to a diffraction effect of the periodic nanowire array. The results from these provide insight into the design of nanowire optical cavities with tunable and enhanced light absorption and thus, could help enable the development of ultrathin solar cells and other nanoscale optoelectronic devices

Broadband Omnidirectional Diffuse Mirrors with Hierarchically Designed All-Dielectric Surfaces

An electromagnetic wave with a single wave vector can be converted into multiple partial ones, with discrete or continuum wave vectors, by means of diffraction or scattering elements; this phenomenon is called optical diffusion. Optical diffusion is a crucial light–matter interaction problem, particularly for lighting applications that require uniform illumination. However, omnidirectional diffuse mirrors with minimal absorption loss have not been reported thus far. Here, we demonstrate the high-diffusivity, low-absorption reflecting surfaces, on which hexagonally arranged Al₂O₃ cones, with a pitch of 3 μm, are conformally covered with HfO₂/Al₂O₃ multilayers. Spectrally resolved far-field measurements reveal that the hierarchically patterned surface diffuses reflected light uniformly over the entire range of azimuthal and polar angles at broadband wavelengths (λ = 400–800 nm), distinct to two-dimensional Al₂O₃ or Al patterned surfaces. Such omnidirectional optical diffusion is clearly identified by means of the momentum space representation; the hierarchical pattern allows all of the available diffraction modes to possess nearly equal amplitudes, which is strongly supported by near-to-far-field Fourier analysis. The degree of diffusivity is quantitatively evaluated with respect to different angular ranges (Δθ = 3°, 12°, and 24°) around a specular reflection angle. Under all of the considered metrics, the hierarchical pattern yields a relatively large diffusivity compared to the reference two-dimensional patterns. Measurements of reflectance spectra, together with full-vectorial electromagnetic simulations, suggest that the hierarchically patterned surface with a backside reflector serves as a high-reflectance diffuse mirror, contrasting with a patterned Al mirror that inevitably suffers from plasmonic absorption loss. These experimental and numerical findings studied herein will provide a fundamental platform for achieving omnidirectional optical diffusers

Doubling Absorption in Nanowire Solar Cells with Dielectric Shell Optical Antennas

Semiconductor nanowires (NWs) often exhibit efficient, broadband light absorption despite their relatively small size. This characteristic originates from the subwavelength dimensions and high refractive indices of the NWs, which cause a light-trapping optical antenna effect. As a result, NWs could enable high-efficiency but low-cost solar cells using small volumes of expensive semiconductor material. Nevertheless, the extent to which the antenna effect can be leveraged in devices will largely determine the economic viability of NW-based solar cells. Here, we demonstrate a simple, low-cost, and scalable route to dramatically enhance the optical antenna effect in NW photovoltaic devices by coating the wires with conformal dielectric shells. Scattering and absorption measurements on Si NWs coated with shells of SiN_<i>x</i> or SiO_<i>x</i> exhibit a broadband enhancement of light absorption by ∼50–200% and light scattering by ∼200–1000%. The increased light–matter interaction leads to a ∼80% increase in short-circuit current density in Si photovoltaic devices under 1 sun illumination. Optical simulations reproduce the experimental results and indicate the dielectric–shell effect to be a general phenomenon for groups IV, II–VI, and III–V semiconductor NWs in both lateral and vertical orientations, providing a simple route to approximately double the efficiency of NW-based solar cells

Facet-Selective Growth on Nanowires Yields Multi-Component Nanostructures and Photonic Devices

Enhanced synthetic control of the morphology, crystal structure, and composition of nanostructures can drive advances in nanoscale devices. Axial and radial semiconductor nanowires are examples of nanostructures with one and two structural degrees of freedom, respectively, and their synthetically tuned and modulated properties have led to advances in nanotransistor, nanophotonic, and thermoelectric devices. Similarly, developing methods that allow for synthetic control of greater than two degrees of freedom could enable new opportunities for functional nanostructures. Here we demonstrate the first regioselective nanowire shell synthesis in studies of Ge and Si growth on faceted Si nanowire surfaces. The selectively deposited Ge is crystalline, and its facet position can be synthetically controlled <i>in situ</i>. We use this synthesis to prepare electrically addressable nanocavities into which solution soluble species such as Au nanoparticles can be incorporated. The method furnishes multicomponent nanostructures with unique photonic properties and presents a more sophisticated nanodevice platform for future applications in catalysis and photodetection

High-Responsivity Deep-Ultraviolet-Selective Photodetectors Using Ultrathin Gallium Oxide Films

Wavelength-selective photodetectors responding to deep-ultraviolet (DUV) wavelengths (λ = 200–300 nm) are drawing significant interest in diverse sensing applications, ranging from micrometer biological molecules to massive military missiles. However, most DUV photodetectors developed thus far have suffered from long response times, low sensitivity, and high processing temperatures, impeding their practical use. Here, we report fast, high-responsivity, and general-substrate-compatible DUV photodetectors based on ultrathin (3–50 nm) amorphous gallium oxide (GaO_X) films grown by low-temperature (∼<250 °C) atomic layer deposition (ALD) for the first time. ALD-grown GaO_X films on glass substrates display a typical amorphous nature, which is identified by electron beam diffraction and X-ray diffraction measurements, while their band gap is sharply featured at ∼4.8 eV. Metal–semiconductor–metal photodetectors (active area of 30 × 30 μm²) using the 30-nm-thick GaO_X films work reliably only for DUV wavelengths; the responsivity is maximized to 45.11 A/W at λ = 253 nm, which dropped off at λ ≈ 300 nm (i.e., a cutoff wavelength). The dark current measured at 10 V is as low as 200 pA and the signal-to-noise ratio reaches up to ∼10⁴, underpinning the pristine material quality of the ALD-grown GaO_X films. In addition, the rise time (i.e., the time interval for photocurrent to increase from 10% to 90%) is as quick as 2.97 μs at λ = 266 nm. Such a reliable and fast photoresponse is achieved for even atomically thin (∼3 nm) devices. The substrate-compatible and low-temperature ALD growth permits the demonstration of flexible DUV photodetectors using amorphous GaO_X films grown on polyimide substrates, suggesting their facile integration into other curved optoelectronic systems. We believe that photodetectors developed herein will provide an economically viable solution for high-performance DUV detection and create a variety of sensing applications

Enhancement of Light Absorption in Silicon Nanowire Photovoltaic Devices with Dielectric and Metallic Grating Structures

We report the enhancement of light absorption in Si nanowire photovoltaic devices with one-dimensional dielectric or metallic gratings that are fabricated by a damage-free, precisely aligning, polymer-assisted transfer method. Incorporation of a Si₃N₄ grating with a Si nanowire effectively enhances the photocurrents for transverse-electric polarized light. The wavelength at which a maximum photocurrent is generated is readily tuned by adjusting the grating pitch. Moreover, the electrical properties of the nanowire devices are preserved before and after transferring the Si₃N₄ gratings onto Si nanowires, ensuring that the quality of pristine nanowires is not degraded during the transfer. Furthermore, we demonstrate Si nanowire photovoltaic devices with Ag gratings using the same transfer method. Measurements on the fabricated devices reveal approximately 27.1% enhancement in light absorption compared to that of the same devices without the Ag gratings without any degradation of electrical properties. We believe that our polymer-assisted transfer method is not limited to the fabrication of grating-incorporated nanowire photovoltaic devices but can also be generically applied for the implementation of complex nanoscale structures toward the development of multifunctional optoelectronic devices

Facet-Selective Epitaxy of Compound Semiconductors on Faceted Silicon Nanowires

Integration of compound semiconductors with silicon (Si) has been a long-standing goal for the semiconductor industry, as direct band gap compound semiconductors offer, for example, attractive photonic properties not possible with Si devices. However, mismatches in lattice constant, thermal expansion coefficient, and polarity between Si and compound semiconductors render growth of epitaxial heterostructures challenging. Nanowires (NWs) are a promising platform for the integration of Si and compound semiconductors since their limited surface area can alleviate such material mismatch issues. Here, we demonstrate facet-selective growth of cadmium sulfide (CdS) on Si NWs. Aberration-corrected transmission electron microscopy analysis shows that crystalline CdS is grown epitaxially on the {111} and {110} surface facets of the Si NWs but that the Si{113} facets remain bare. Further analysis of CdS on Si NWs grown at higher deposition rates to yield a conformal shell reveals a thin oxide layer on the Si{113} facet. This observation and control experiments suggest that facet-selective growth is enabled by the formation of an oxide, which prevents subsequent shell growth on the Si{113} NW facets. Further studies of facet-selective epitaxial growth of CdS shells on micro-to-mesoscale wires, which allows tuning of the lateral width of the compound semiconductor layer without lithographic patterning, and InP shell growth on Si NWs demonstrate the generality of our growth technique. In addition, photoluminescence imaging and spectroscopy show that the epitaxial shells display strong and clean band edge emission, confirming their high photonic quality, and thus suggesting that facet-selective epitaxy on NW substrates represents a promising route to integration of compound semiconductors on Si

Microstructured Air Cavities as High-Index Contrast Substrates with Strong Diffraction for Light-Emitting Diodes

Two-dimensional high-index-contrast dielectric gratings exhibit unconventional transmission and reflection due to their morphologies. For light-emitting devices, these characteristics help guided modes defeat total internal reflections, thereby enhancing the outcoupling efficiency into an ambient medium. However, the outcoupling ability is typically impeded by the limited index contrast given by pattern media. Here, we report strong-diffraction, high-index-contrast cavity engineered substrates (CESs) in which hexagonally arranged hemispherical air cavities are covered with a 80 nm thick crystallized alumina shell. Wavelength-resolved diffraction measurements and Fourier analysis on GaN-grown CESs reveal that the high-index-contrast air/alumina core/shell patterns lead to dramatic excitation of the low-order diffraction modes. Large-area (1075 × 750 μm²) blue-emitting InGaN/GaN light-emitting diodes (LEDs) fabricated on a 3 μm pitch CES exhibit ∼39% enhancement in the optical power compared to state-of-the-art, patterned-sapphire-substrate LEDs, while preserving all of the electrical metrics that are relevant to LED devices. Full-vectorial simulations quantitatively demonstrate the enhanced optical power of CES LEDs and show a progressive increase in the extraction efficiency as the air cavity volume is expanded. This trend in light extraction is observed for both lateral- and flip-chip-geometry LEDs. Measurements of far-field profiles indicate a substantial beaming effect for CES LEDs, despite their few-micron-pitch pattern. Near-to-far-field transformation simulations and polarization analysis demonstrate that the improved extraction efficiency of CES LEDs is ascribed to the increase in emissions via the top escape route and to the extraction of transverse-magnetic polarized light

Association between humidifier disinfectant exposure-related characteristics and HDLI risk.

<p>Association between humidifier disinfectant exposure-related characteristics and HDLI risk.</p