Excellent Spectral Selectivity Research Articles

Fabrication of Carbon Nanotube/Indium Tin Oxide “Inverse Tandem” Absorbing Coatings with Tunable Spectral Selectivity for Solar–Thermal Applications

AbstractWe report the fabrication of a new selective “inverse tandem” absorbing coating based on carbon nanotube (CNT)/indium‐tin oxide (ITO) on aluminium (Al) for mid‐temperature solar–thermal application. The CNT layer is formed by spraying and functions as an excellent solar absorber whereas the ITO layer produced on top of the CNTs by sputtering serves as an IR reflector. The effect of the thickness of the ITO on the spectral selectivity of the absorbing coating was investigated. Controlling the thickness of ITO allowed the spectral selectivity of the coating to be tuned. The CNT/ITO solar coatings with optimized thickness of ITO showed excellent spectral selectivity values of absorptance (α) of 0.927 and emittance (ε) of 0.2. The performance of the coatings at high temperature after heating in air in the range of 25–300 °C for different durations was also investigated. The performance and structure of the CNT/ITO coating was also compared with the wet deposition method in which the ITO coating was formed by spraying.

High-performance ultraviolet photodetectors based on solution-grown ZnS nanobelts sandwiched between graphene layers.

Ultraviolet (UV) light photodetectors constructed from solely inorganic semiconductors still remain unsatisfactory because of their low electrical performances. To overcome this limitation, the hybridization is one of the key approaches that have been recently adopted to enhance the photocurrent. High-performance UV photodetectors showing stable on-off switching and excellent spectral selectivity have been fabricated based on the hybrid structure of solution-grown ZnS nanobelts and CVD-grown graphene. Sandwiched structures and multilayer stacking strategies have been applied to expand effective junction between graphene and photoactive ZnS nanobelts. A multiply sandwich-structured photodetector of graphene/ZnS has shown a photocurrent of 0.115 mA under illumination of 1.2 mWcm−2 in air at a bias of 1.0 V, which is higher 107 times than literature values. The multiple-sandwich structure of UV-light sensors with graphene having high conductivity, flexibility, and impermeability is suggested to be beneficial for the facile fabrication of UV photodetectors with extremely efficient performances.

High-performance self-powered UV photodetectors based on TiO2 nano-branched arrays

Nano-branched TiO2 arrays were fabricated on fluorine-doped tin oxide (FTO) glass by a facile two-step chemical synthesis process. Self-powered UV photodetectors based on photoelectrochemical cells (PECs) were assembled using these TiO2 nano-branched arrays as photoanodes. These visible–blind self-powered UV photodetectors exhibit high sensitivity and high-speed photoresponse. Compared with photodetectors based on bare TiO2 nanorod arrays, TiO2 nano-branched arrays show drastically improved photodetecting performance as photoanodes. The photosensitivity increases from 0.03 to 0.22 A W−1 when optimized nano-branched TiO2 arrays are used, corresponding to an incident photon-to-current conversion efficiency higher than 77%. The UV photodetectors also exhibit excellent spectral selectivity and fast response (0.05 s decay time). The improved performance is attributed to a markedly enlarged TiO2/electrolyte contact area and good electron conductivity in the one-dimensional, well-aligned TiO2 nanorod trunk.

ZnO nanoneedle/H2O solid-liquid heterojunction-based self-powered ultraviolet detector

ZnO nanoneedle arrays were grown vertically on a fluorine-doped tin oxide-coated glass by hydrothermal method at a relatively low temperature. A self-powered photoelectrochemical cell-type UV detector was fabricated using the ZnO nanoneedles as the active photoanode and H2O as the electrolyte. This solid-liquid heterojunction offers an enlarged ZnO/water contact area and a direct pathway for electron transport simultaneously. By connecting this UV photodetector to an ammeter, the intensity of UV light can be quantified using the output short-circuit photocurrent without a power source. High photosensitivity, excellent spectral selectivity, and fast photoresponse at zero bias are observed in this UV detector. The self-powered behavior can be well explained by the formation of a space charge layer near the interface of the solid-liquid heterojunction, which results in a built-in potential and makes the solid-liquid heterojunction work in photovoltaic mode.

Monolayer Graphene/Germanium Schottky Junction As High-Performance Self-Driven Infrared Light Photodetector

We report on the simple fabrication of monolayer graphene (MLG)/germanium (Ge) heterojunction for infrared (IR) light sensing. It is found that the as-fabricated Schottky junction detector exhibits obvious photovoltaic characteristics, and is sensitive to IR light with high Ilight/Idark ratio of 2 × 10(4) at zero bias voltage. The responsivity and detectivity are as high as 51.8 mA W(-1) and 1.38 × 10(10) cm Hz(1/2) W(-1), respectively. Further photoresponse study reveals that the photovoltaic IR detector displays excellent spectral selectivity with peak sensitivity at 1400 nm, and a fast light response speed of microsecond rise/fall time with good reproducibility and long-term stability. The generality of the above results suggests that the present MLG/Ge IR photodetector would have great potential for future optoelectronic device applications.

A self-powered UV photodetector based on TiO2 nanorod arrays

Large-area vertical rutile TiO2 nanorod arrays (TNAs) were grown on F/SnO2 conductive glass using a hydrothermal method at low temperature. A self-powered ultraviolet (UV) photodetector based on TiO2 nanorod/water solid–liquid heterojunction is designed and fabricated. These nanorods offer an enlarged TiO2/water contact area and a direct pathway for electron transport simultaneously. By connecting this UV photodetector to an ammeter, the intensity of UV light can be quantified using the output short-circuit photocurrent without a power source. A photosensitivity of 0.025 A/W and a quick response time were observed. At the same time, a high photosensitivity in a wide range of wavelength was also demonstrated. This TNA/water UV detector can be a particularly suitable candidate for practical applications for its high photosensitivity, fast response, excellent spectral selectivity, uncomplicated low-cost fabrication process, and environment-friendly feature.

Large-area fabrication of high aspect ratio tantalum photonic crystals for high-temperature selective emitters

The authors present highly selective emitters based on two-dimensional tantalum (Ta) photonic crystals, fabricated on 2 in. polycrystalline Ta substrates, for high-temperature applications, e.g., thermophotovoltaic energy conversion. In this study, a fabrication route facilitating large-area photonic crystal fabrication with high fabrication uniformity and accuracy, based on interference lithography and reactive ion etching is discussed. A deep reactive ion etch process for Ta was developed using an SF6/C4F8 based Bosch process, which enabled us to achieve ∼8.5 μm deep cavities with an aspect ratio of ∼8, with very steep and smooth sidewalls. The thermal emitters fabricated by this method show excellent spectral selectivity, enhancement of the emissivity below cut-off approaching unity, and a sharp cut-off between the high emissivity region and the low emissivity region, while maintaining the low intrinsic emissivity of bare Ta above the cut-off wavelength. The experimental results show excellent agreement with numerical simulations.

An ultraviolet photo-detector based on TiO2/water solid-liquid heterojunction

In this study, an ultraviolet photodetector (UV-PD) based on TiO2/water solid-liquid heterojunction (SLHJ) is reported. The SLHJ UV-PD exhibits a high photosensitivity, excellent spectral selectivity, linear variations in photocurrent, and fast response. In addition, it is suggested that the spectral response can be tailored and that the performance can be improved through deliberate design of the active layer, electrolyte, and substrate of the SLHJ UV–PD. Moreover, we anticipate this work to be a starting point for more sophisticated commercial photon detection using an SLHJ device.

High performance solar-selective absorbers using coated sub-wavelength gratings

Spectral control of the emissivity of surfaces is essential for efficient conversion of solar radiation into heat. We investigated surfaces consisting of sub-wavelength V-groove gratings coated with aperiodic metal-dielectric stacks. The spectral behavior of the coated gratings was modeled using rigorous coupled-wave analysis (RCWA). The proposed absorber coatings combine impedance matching using tapered metallic features with the excellent spectral selectivity of aperiodic metal-dielectric stacks. The aspect ratio of the V-groove can be tailored in order to obtain the desired spectral selectivity over a wide angular range. Coated V-groove gratings with optimal aspect ratio are predicted to have thermal emissivity below 6% at 720K while absorbing >94% of the incident light. These sub-wavelength gratings would have the potential to significantly increase the efficiency of concentrated solar thermal systems.

Design of wide-angle solar-selective absorbers using aperiodic metal-dielectric stacks

Spectral control of the emissivity of surfaces is essential in applications such as solar thermal and thermophotovoltaic energy conversion in order to achieve the highest conversion efficiencies possible. We investigated the spectral performance of planar aperiodic metal-dielectric multilayer coatings for these applications. The response of the coatings was optimized for a target operational temperature using needle-optimization based on a transfer matrix approach. Excellent spectral selectivity was achieved over a wide angular range. These aperiodic metal-dielectric stacks have the potential to significantly increase the efficiency of thermophotovoltaic and solar thermal conversion systems. Optimal coatings for concentrated solar thermal conversion were modeled to have a thermal emissivity <7% at 720K while absorbing >94% of the incident light. In addition, optimized coatings for solar thermophotovoltaic applications were modeled to have thermal emissivity <16% at 1750K while absorbing >85% of the concentrated solar radiation.

The LHCD launcher for Alcator C-Mod—Design, construction, calibration and testing

MIT and PPPL have joined together to fabricate a high-power lower hybrid current drive (LHCD) system for supporting steady-state AT regime research on Alcator C-Mod. The goal of the first step of this project is to provide 1.5 MW of 4.6 GHz rf power to the plasma with a compact launcher which has excellent spectral selectivity and fits into a single C-Mod port. Some of the important design, construction, calibration and testing considerations for the launcher leading up to its installation on C-Mod are presented here.

Fast imaging of phosphocreatine in the normal human myocardium using a three-dimensional RARE pulse sequence at 4 Tesla.

To investigate the use of a three-dimensional rapid acquisition with relaxation enhancement (RARE) pulse sequence for direct acquisition of phosphocreatine (PCr) images of the human myocardium. A short elliptical birdcage radiofrequency (RF) body coil was constructed to produce a uniform flip angle throughout the chest cavity. In vivo images using a spectrally-selective RARE sequence with a spatial resolution of 1.2 cm x 1.2 cm x 2.5 cm (4 cm(3)) were acquired in nine minutes and 40 seconds. Scans of phantoms demonstrated excellent spectral selectivity. The signal-to-noise ratio in the myocardium ranged from 12.6 in the anterior wall to 5.3 in the mid septum. This study demonstrates that PCr data can be acquired using a three-dimensional RARE sequence with greater spatial and temporal resolution than spectroscopic techniques.

Selective Detection of Dibenzodioxin, Dibenzofuran and Some Small Polycyclic Aromatics

Resonance-enhanced multiphoton ionization/time-of-flight mass spectrometry (REMPI/TOFMS) has been employed for detection limit measurements of dibenzodioxin, dibenzofuran, 2,8-dichloro-dibenzodioxin, biphenyl, anthracene, phenanthrene and pyrene. REMPI/TOFMS detection limit data are currently available for 28 molecules. Nineteen of these, with detection limits at or below the part-per-billion level, are recommended for use as surrogates for continuous real-time monitoring of hazardous emissions from incinerators. The excellent spectral selectivity of the REMPl/TOFMS tecbnique, exhibited by single ring phenyl compounds, may be substantially reduced for many of the higher aromatics. Additional studies are needed of the one- and two-color spectral signatures for resonance two-photon ionization of PCBs and chlorinated polycyclic aromatics.

Characteristics of a multilayer mirror polarimeter for measurements at extreme ultraviolet wavelengths

We have developed and tested a multilayer mirror (MLM) polarimeter optimized for λ=304 Å using electron impact on helium. Specifically we present the first experimental results of the polarization of the extreme ultraviolet Lyman-α emission from He+ [He II (2p)2P°→(1s)2S] as a function of the electron impact energy for impact energies ranging from 150 to 980 eV. A major challenge to performing this measurement is the suppression of the dominant He I (1snp)1P°→(1s2)1S radiation originating from the excitation of helium. Excellent spectral selectivity has been achieved with a VYNS copolymer filter whose transmission ratio of He II to He I emission has been measured to be greater than 1200:1. Additionally, we have measured the instrumental polarization using two identical MLMs in a crossed polarizer configuration.

Laser-excited fluorescence detection of gas-phase chromatography eluates

A high-spectral-resolution laser-excited molecular fluorescence gas chromatographic (GC) detection system is evaluated. It utilizes a pulsed supersonic jet expansion to yield very-narrow-bandwidth (e.g. ⩽ 0.1 nm) fluorescence excitation spectra that may be rapidly scanned via a tunable dye laser. A microcomputer synchronizes the entire system, collects data, performs calculations, and creates visual displays of results. The laser-excited fluorescence detection system was interfaced to a gas chromatograph to exploit both the selectivity of the chromatography and the excellent spectral selectivity of the detector. Fluorescence excitation chromatograms were acquired by monitoring fluorecence emission from selected transition wavelengths characteristic of the GC eluates. The excitation wavelengths were also programmed to change at appropriate retention times to provide greatest selectivities for individual analytes as they eluted, and to allow multiple analytes to be determined in a single elution. Response factors for the system varied appreciably from run to run, which precluded the use of external standard quantitation procedures. However, excellent stabilites for within-run relative response factors were sufficient to allow for good quantitative measurements using internal standard techniques.

Ultrafine nickel particles for photothermal conversion of solar energy

Ni particles with mean diameters ≲10 nm were produced by inert-gas evaporation onto KBr plates. Heating in air to 400 °C for a few minutes led to the electrical decoupling of the grains by formation of NiO pellicles. Transmittance measurements in the interval 0.3–30 μm showed excellent spectral selectivity with high absorption over the solar spectrum and low absorption in the thermal infrared range. Heat treatments up to 600 °C in air did not affect the selectivity significantly. These properties prove that coatings with Ni grains are very useful for the photothermal conversion of solar energy. A theoretical analysis of the optical properties of aggregated spherical particles with dielectric shells was carried out by use of an extended effective-medium theory founded on the Maxwell Garnett approach. We obtained satisfactory agreement between computations and experiments.

Ultrafine chromium particles for photothermal conversion of solar energy

Individually isolated chromium particles were prepared by evaporation onto KBr plates in a mixture of argon and air. Such coatings were taken as a model substance simulating the properties of electrodeposited chromium black—a well-known material for efficient photothermal conversion of solar energy. Median diameters of the particles were 5–13 nm; their shapes were spherical or cubelike. Optical transmittance at normal incidence was recorded by spectrophotometry in the interval 0.3–25 μm. Excellent spectral selectivity was documented with high absorptance over the wavelength range for solar radiation and high transmittance further out in the infrared. The optical data were interpreted within the Maxwell Garnett theory, which was generalized so as to encompass dipole-dipole coupling among aggregated spheres, cubelike shapes, and oxide pellicles. Agreement between theory and experiments was achieved by considering aggregation of spherical particles into linear chains. The effects of having cubic particles, as well as oxide-coated spherical ones, were calculated and found to be rather unimportant for understanding the spectral selectivity.

Optical properties of selectively absorbing metal insulator composite films

The optical properties of Ni/Al2O3, V/Al2O3, V/SiO2, V/MgO, Fe/Al2O3 and Fe/MgO composite films have been studied with consideration given to their applicability as selective solar energy absorbers. Thin films were produced by coevaporation and the microstructure examined by electron microscopy. The optical properties of Ni/Al2O3 composites, measured over the range of the solar spectrum, are in good accord with the predictions of Maxwell–Garnett theory provided the Ni volume fraction is ≲0.2. The Ni/Al2O3 films have excellent spectral selectivity for the absorption of solar radiation with a solar absorptivity of 0.94 obtained for a film produced with a graded composition. Low-temperature emissivities of ∠0.1 have been obtained for Ni/Al2O3 on a highly reflecting metal backing. The composite Ni/Al2O3 is stable at high temperatures in air. The V and Fe composite studies have optical properties different in character from the predictions of Maxwell–Garnett theory and are not as suitable for solar energy applications as Ni composites.

Optical properties of selectively absorbing Ni/Al2O3 composite films

Composite films of Ni particles embedded in an Al2O3 matrix have been produced by controlled coevaporation. The optical properties of the Ni/Al2O3 composites measured over the range of the solar spectrum are in good accord with the predictions of Maxwell-Garnett theory provided the Ni volume fraction is ≲0.2. The composite films have excellent spectral selectivity for the absorption of solar radiation, with a solar absorptivity of 0.94 obtained for a film produced with a composition gradient. Low-temperature emissivities of ∼0.1 have been obtained with composite films evaporated on highly reflecting metal substrates.