Continuous Absorption Band Research Articles

A broadband low-profile microwave absorber based on ferromagnetic material doped hybrid stereo metamaterial

This study introduces a ferromagnetic material-doped hybrid stereo metamaterial absorber (HSMA) that has been developed to overcome the limitations in broadband absorption of electromagnetic wave. This metamaterial combines a ferromagnetic block with a blind via in the center and a standing resistive trapezoidal patch together in a stereo meta-atom to enable a continuous and broad absorption band for 90% absorptivity from 2.3 GHz to 40 GHz, with a thickness only as 0.079 times the maximum working wavelength. The hybrid absorber also exhibits angular stability under oblique incidence and polarization insensitivity. Furthermore, a sample of proposed HSMA was fabricated and measured. The broadband absorption properties of such hybrid stereo metamaterial were validated by both simulated and experimental results. Our study provides a promising implementation approach for broadband and low-profile microwave absorbers.

Electronic spectroscopy of trans-2-hexenal and 3-hexanol: Experimental and computational studies

Spectroscopic study of the absorption spectra of two green gases viz. trans-2-hexenal and 3-hexanol molecules by means of synchrotron radiation based photoabsorption and FTIR spectroscopy is presented. Quantum chemical calculations are used to compute the ground and excited state properties and correlate to the experimental results. From the IR spectra, vibrational assignments in the 600–3200 cm−1 region are made for the first time with the help of predicted fundamental frequencies at the optimized geometries for the molecule using DFT methodology. The electronic absorption spectra recorded in the 5.5–10 eV region for trans-2-hexenal consists of strong valence band at 6 eV followed by weak structures assigned to Rydberg series converging to the first two ionization potentials. While for 3-hexanol the electronic spectra lies entirely in the VUV region consisting of broad continuous absorption band above 7 eV. Vertical excited state energies are predicted using TDDFT calculations assist the assignments of the experimental electronic peaks to Rydberg and valence transitions. To get insights into the nature of the excited states potential energy curves of the first few excited states with respect to bond lengths and bond angles are studied. This work presents a first consolidated report of experimental and theoretical computation of photo absorption spectrum of trans-2-hexenal and 3-hexanol in the region IR and VUV absorption spectra.

Efficient Charge Separation Enabled by N-Doped Graphene Quantum Dots and PCDTBT for a High-Performance Silicon Nanowire Solar Cell

Light absorption by n-silicon nanowires (SiNWs) grown by a chemical etching method is augmented by tethering photoresponsive and highly luminescent nitrogen-doped graphene quantum dots (N-GQDs) and poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) to a SiNW array. The resultant N-GQDs@PCDTBT@SiNW photoanode affords a wide, continuous, and intense absorption band spanning from ∼350 to 1200 nm wavelength range, enabling maximum light uptake, with a work function of ∼4.74 eV, deep enough to not serve as a charge-trapping state. Under illumination, efficient charge separation in this ternary composite is ensured by the p-type semiconducting nature of N-GQDs with a shallow valence band (VB) that allows for rapid hole extraction from the VBs of SiNW and PCDTBT and their rapid relay to the bromide ions in the electrolyte. This is simultaneously accompanied by fast, excited electron injection from N-GQDs and PCDTBT to the SiNW via a cascade process, thus minimizing back electron transfer to the tribromide ions. When the N-GQDs@PCDTBT@SiNW photoanode is coupled with a highly electrocatalytic and electrically conductive multiwalled carbon nanotube (MWCNT)@C-fabric counter electrode and a bromine/bromide electrolyte, a power conversion efficiency of 13.18% is achieved for this liquid junction solar cell, which is significantly enhanced compared to that of the SiNW/HBr,Br2/C-fabric cell (4.37%). The roles of N-GQDs, PCDTBT, and MWCNTs are independently quantified to explain the observed solar cell performance.

Absorptive frequency selective surface with two alternately switchable transmission/reflection bands.

The traditional frequency selective surface (FSS) needs further improvement with the development of stealth technology, and the design of multifunctional FSSs is essential. In this letter, an active absorptive FSS (AFSS) has been designed based on the absorption structure of the spoof surface plasmon polariton (SSPP) and the switching activity of the active FSS. The active FSS embedded with PIN diodes realizes the shift of two transmission/reflection frequency bands by controlling the bias voltage of the feed network, which switches from one band-pass response (at around 3.06 GHz) to the other (at around 4.34 GHz). And when one of the transmission windows switches to the other, the original transmission window closes. The upper plasmonic structure achieves a continuous and efficient absorption band from 6.31 to 8.34 GHz. A sample was also fabricated and carried out to verify the numerical simulation, and the experimental and simulation results are consistent. This work provides new ideas for the design of active AFSS and promotes its application in common aperture radome, antenna isolation, and electromagnetic shielding.

Wideband low-frequency sound absorption by inhomogeneous multi-layer resonators with extended necks

This study develops a structure with thin thickness targeting for wideband low-frequency sound absorption, which consists of multi-layer Helmholtz resonators with extended necks (HRENs). An analytical prediction model is established based on the classic transfer matrix method to study the acoustic characteristics of the multi-layer HREN absorber. Results show that the added layer shifts the first absorption peak of the original single-layer HREN absorber to a lower frequency and induces additional absorption peak. Then, a particle swarm optimization (PSO) algorithm is coupled with the prediction model to design optimal sound absorbing structure for effective sound absorption in a prescribed wideband frequency range. For illustration, a single-layer, a double-layer and a triple-layer optimal structures composed of 9 inhomogeneous units at each layer are designed and they yield about 90% absorption in the frequency ranges of [550,700] Hz, [400,650] Hz and [300,550] Hz. The mechanism of the continuous absorption band is due to the overlapping of the absorption peaks induced by different units. The proposed structure offers an effective acoustic absorption in the target wideband frequency range while its size is in a compact scale, and thus holds great potential for noise attenuation in various realistic applications.

Semiconductor nanocrystals possessing broadly size‐ and facet‐dependent optical properties

AbstractCu2O cubes, octahedra, and rhombic dodecahedra have been shown to exhibit continuous light absorption and emission band shifts with increasing particle sizes from 10 nm to sub‐microcrystals. They also possess clear facet‐dependent optical properties. Ag3PO4, Ag2O, SrTiO3, and CeO2 crystals show similar optical size and facet effects. Thus, spectral shifts over a broad size range far beyond the quantum‐size regime should be generally observable in many semiconductor materials. Facet‐dependent optical properties of a semiconductor can be understood to arise from the presence of an ultrathin surface layer with subtle bond and orbital level variations for different crystal faces. Although these optical features seem unexpected, they should be the general behaviors of semiconductor crystals. As more examples of these optical effects are available, we will find that these intrinsic properties of semiconductors have been ignored in the past. Furthermore, if valence and conduction band positions are broadly tunable by particle size, the knowledge should have tremendous impacts on the applications of semiconductors, where band energies are important to efficient interfacial charge transfer.

Photocatalytic Activity of Zn x Mn3-x O4 Oxides and ZnO Prepared From Spent Alkaline Batteries.

Oxides with ZnxMn3−xO4 stoichiometries and ZnO were synthesized from the “black mass” material recovered from spent alkaline batteries. The oxides were characterized by XRF, XRD with Rietveld refinement, SEM, and TEM methods. Optical characterization included diffuse reflectance (DRS) and photoluminescence (PL) measurements. ZnO presented a clear band edge in the UV region, and PL signals were detected. The Zn/Mn oxides showed strong absorption in the UV region and a continuous absorption band in the Vis-IR regions. There is a non-detected PL signal due to excited charges being trapped on sub-band energy states and/or transfer by non-radiative paths. Photocatalytic activity under both irradiation conditions was evaluated using the resazurin dye test, terephthalic acid fluorescence probe method, and NOx air purification evaluation. In the three photoactivity tests, ZnO performed well under both UV and Vis irradiation, whereas no evidence of any appreciable photocatalytic activity was observed for the Zn/Mn oxides. The results are discussed in terms of the findings of previously reported optical measurements.

Ultra-broad band perfect absorption realized by phonon–photon resonance in periodic polar dielectric material based pyramid structure

In this research, a mid-infrared wide-angle ultra-broadband perfect absorber which composed of pyramid grating structure has been comprehensively studied. The structure was operated in the reststrahlen band of SiC and with the presence of surface phonon resonance(SPhR), the perfect absorption was observed in the region between 10.25 and 10.85 μm. We explain the mechanism of this structure with the help of PLC circuit model due to the independence of magnetic polaritons. Besides, by studying the resonance behavior of different wavelength, we bridged the continuous perfect absorption band and the discrete peak in 11.05 μm (emerge two close absorption band together) by modification of the geometry. The absorption band has been sufficiently broadened. More over, both 1-D and 2-D periodic structure has been considered and the response of different incident angles and polarized angles has been studied and a omnidirectional and polarization insensitive structure can be realized which may be a candidate of several sensor applications in meteorology. The simulation was conducted by the Rigorous Coupled Wave Method(RCWA).

Design of the Broadband Metamaterial Absorber Based on Dispersed Carbon Fibers in Oblique Incidence

In this paper, a broadband double-layer metamaterial absorber (MMA) based on carbon fibers (CFs) is proposed, which consists of three basic elements. Firstly, the reflectivity of CFs double-layer MMA in normal incidence is analyzed, and compared to the copper double-layer MMA with same size, the simulated results show that it obtains better absorbing effect. With the increase of incident angle, the multi-frequency resonance is activated, leading to larger effective bandwidth. Moreover, when incident angle is between 55 and 70 degrees, the continuous absorption band above 85% can totally cover X and Ku microwave band. Meanwhile, when the EM frequency is at 6-7.5GHz, the change of incident angle hardly affects the absorbing performances of CFs double-layer MMA. A specimen of the CFs double-layer MMA was fabricated by the vacuum bag molding process to verify its effectiveness and the measured results fitted well with the simulated results. In order to discuss the mechanism of EM absorption, the distribution of E-fields and power loss density were monitored. The results indicate that incident angle effects the distribution of E-fields and the way of absorption to change the absorbing performances. The proposed MMA has potential application prospects in the structural and functional integration design of CFs.

Experimental and theoretical studies on the absorption spectra of n-dodecane in the IR and VUV regions

We report here a comprehensive spectroscopic study of the absorption spectrum of the n-dodecane molecule using synchrotron radiation based photoabsorption and FTIR spectroscopy. Quantum chemical calculations using the DFT and TDDFT methodologies are used to predict relevant ground and excited state properties and correlate them with the experimental results. In the IR spectrum, assignment of the prominent CH stretching bands in the 2800–3000 cm−1 region are consolidated while a few new vibrational assignments are made in the 700–1500 cm−1 region. The electronic absorption spectrum of n-dodecane lies entirely in the vacuum ultraviolet (VUV) region and consists of a broad continuous absorption band starting from ∼7.5 eV, with no discernible structure. TDDFT calculations of vertical excited states predict that most of the excited states are Rydberg in nature. Potential energy curves of the first few excited states with respect to various bond lengths and bond angles are studied in order to gain additional insights into the nature of the excited states. The broad nature of the absorption is attributed partly to the effect of several conformers as well as hot bands from low frequency modes in the ground state and partly due to overlap of several Rydberg series converging to the first few ionization potentials (IPs). Energy separation between the first and second IP is theoretically predicted to be ∼0.65 eV, while successive separation between the second, third, fourth and fifth IPs is ∼0.03 eV. To the best of our knowledge this is the first report of the VUV absorption spectrum of n-dodecane in the region from 6 to 10 eV, as also the first report of theoretical calculations of its electronically excited states.

Ti as Mediator in the Photoinduced Electron Transfer of Mixed-Metal NH2–UiO-66(Zr/Ti): Transient Absorption Spectroscopy Study and Application in Photovoltaic Cell

A series of mixed-metal NH2–UiO-66(Zr/Ti) with different percentages of exchanged Ti have been prepared and studied by transient absorption spectroscopy (TAS). The photogenerated transients from mixed NH2–UiO-66(Zr/Ti) exhibit at short time scales two defined absorption bands, evolving to a continuous absorption band expanding from 300 to 700 nm at longer time scales. The observed spectral changes are compatible with an initial formation of Ti3+–O–Zr4+ and its further transformation to Ti4+–O–Zr3+ via metal–metal electron exchange, thus providing support to the role of substituted Ti as mediator to facilitate electron transfer from excited ligand to the (Zr/Ti)6O4(OH)4 nodes in mixed NH2–UiO-66(Zr/Ti). The slow recombination of photogenerated electrons and holes in the mixed NH2–UiO-66(Zr/Ti) has been advantageously used for the construction of a photovoltaic cell fabricated with the mixed NH2–UiO-66(Zr/Ti), reaching a higher photon-to-current efficiency than NH2–UiO-66(Zr).

Solvation-Dependent Latency of Photoacid Dissociation and Transient IR Signatures of Protonation Dynamics.

We elucidate the characteristic proton pathways and the transient infrared signatures of intermediate complexes during the first picoseconds of photoinduced protonation dynamics of a photoacid (N-methyl-6-hydroxyquinolinium) in aqueous solution from first-principles molecular dynamics simulations. Our results indicate that the typical latency time between photoexcitation and proton dissociation ranges from 1 ps to longer time time scales (∼100 ps). The rate-limiting step for the actual dissociation of the proton into the solvent is the solvation structure of the first accepting water molecule. The nature of the proton pathway in water (stepwise or concerted) is not unique but determined by the coordination number of the accepting water molecules along the hydrogen bond chain. We find a characteristic uncommon infrared mode at ∼1300 cm(-1) of the transient photobase-Eigen cation complex immediately after photodissociation that we predict to be observable experimentally in time-resolved IR spectroscopy. A broad continuous absorption band from 1500 to 2000 cm(-1) arises from the acidic proton imminently before dissociation.

Dark photovoltaic effects of Ti-rooted TiO2 nanopillars as the anode in CdSe-sensitised solar cell

TiO2 nanopillars (NPs) rooted in Ti were cultivated hydrothermally with the strongest organic base tetramethylammonium hydroxide (TMAOH) in aqueous solution in Ti substrate. TiCl4 was used to coat TiO2 NPs with a nanoscale compact thin TiO2 layer against charge recombination with oxidised species in electrolyte. A consecutive layer of CdSe nanoparticles was induced by successive ionic layer adsorption and reaction (SILAR) mode for sensitisation by light harvesting. J–V curves surprisingly indicate that untreated TiO2 NPs photoanode rooted in Ti produces considerable currents in the dark (thermal photovoltaic power generation) even higher compared to that under intense UV illumination, signifying TiCl4–CdSe treatment does not profit. Electrochemical impedance spectroscopy fitting parameters further reveal that the parameters associated with injection and recombination issues have been improved, as a comprehensive electrochemical result of TiCl4–CdSe treatment. Thus, the current-limiting problem probably arises from the remarkable continuous absorption bands in a wide spectrum range from visible to far-infrared, which is confirmed by UV–vis absorption spectra. However, the reason for the substantial photocurrent-limiting is that the intense irradiating UV light causes the innumerable electrons distributed in the inter-band-gap states to transit to much higher levels above the CB edge of TiO2 with excessive energy, then they collide with TiO2 crystall lattices to release thermal energy eventually to raise the transport resistance.

One pot solution combustion synthesis of highly mesoporous hematite for photocatalysis

Iron oxides were synthesized by solution combustion synthesis using glycine (fuel) and ferric nitrate (oxidizer) as raw materials. The effects of the fuel to oxidizer ratio, ϕ, on the combustion behavior, phase, morphology and surface area of the products were systematically studied. Pure hematite was synthesized directly by the combustion of the precursors under fuel-lean conditions in one step without further heat treatment, simply by selecting a proper fuel to oxidizer ratio, ϕ. The hematite has a highly mesoporous structure with specific surface area of 103m2 g-1 and an average size of about 20nm, which was obtained at ϕ=0.3. The hematite displays a continuous absorption band in the visible region. The photocatalytic activity of the hematite was evaluated by degrading the methylene blue pollutant in water at ambient temperature. The synthesized mesoporous hematite is a promising visible light photocatalysts for organics decomposition.

Graphene as a Quencher of Electronic Excited States of Photochemical Probes

Graphene sheets quench the singlet and triplet excited states of a series of six photochemical probes including pyrene, acridine orange, tris(2,2́-bipyridyl)ruthenium(II) dichloride, methylene blue, meso-tetrakis(phenylsulphonate)porphyrin, and 5,10,15,20-tetraphenyl-21H,28H-porphine zinc. It was found that Stern-Volmer fluorescence quenching can fit to one or two different quenching regimes depending on the probe. In addition, the quenching can be either static or dynamic depending on the fluorophore. The occurrence of several quenching regimes has been interpreted considering that quenching arises from the crowding of the fluorophore on both graphene faces, or site isolation on the graphene sheets. Laser flash photolysis has shown that the triplet lifetime of the probes generally decreases due to graphene quenching and that no new transients appear except in the case of methylene blue, where a new absorption spectrum characterized by a continuous absorption band is observed and attributed to graphene radical ion. This spectroscopic evidence suggests that the most general quenching mechanism is energy transfer from the singlet or triplet excited state of the dye to graphene. This raises the issue of determining the energy of the electronic excited states of graphene.

Carbon quantum dots embedded with mesoporous hematite nanospheres as efficient visible light-active photocatalysts

Carbon quantum dots (CQDs) and mesoporous hematite (α-Fe2O3) complex photocatalysts were successfully prepared using a facile solvent-thermal process in an aqueous solution. Mesostructured α-Fe2O3 clusters with a high surface area and a porous framework were an important consideration in the design of the photocatalysts because such structures enhance the absorption of photons and promote the decomposition of organic pollutants. More significantly, the CQDs in this catalyst play a pivotal role in improving the photocatalytic activity under visible light irradiation. The nanocomposites were characterized using X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, energy-dispersive spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, dynamic light scattering methods, ultraviolet-visible spectroscopy, and other techniques. The results confirmed the formation of CQD/mesoporous α-Fe2O3 hybrid clusters with a uniform size (about 700 nm), three-dimensional spherical morphologies, a large internal surface area (up to 187 m2 g−1), and a wormhole-like mesopore structure. Moreover, these novel composite catalysts displayed a continuous absorption band in the visible region. Photocatalytic studies of the CQD-embedded mesoporous α-Fe2O3 showed excellent photocatalytic efficiency (up to 97% capacity retention after three cycles) toward the degradation of organic compounds in aqueous media under visible light irradiation. The relationship between the physicochemical properties and the photocatalytic performance in our system is described and discussed on the basis of the results.

Optical frequency comb spectroscopy

Optical frequency combs offer enormous potential in the detection and control of atoms and molecules by combining their vast spectral coverage with the extremely high spectral resolution of each individual comb component. Sensitive and multiplexed trace gas detection via cavity-enhanced direct frequency comb spectroscopy has been demonstrated for various molecules and applications; however, previous demonstrations have been confined to the visible and near-infrared wavelength range. Future spectroscopic capabilities are created by developing comb sources and spectrometers for the deep ultraviolet and mid-infrared spectral regions. Here we present a broadband high resolution mid-infrared frequency comb-based Fourier transform spectrometer operating in the important molecular fingerprint spectral region of 2100-3600 cm(-1) (2.8-4.8 microm). The spectrometer, employing a multipass cell, allows simultaneous acquisition of broadband, high resolution spectra (down to 0.0035 cm(-1) of many molecular species at concentrations in the part-per-billion range in less than 1 min acquisition time. The system enables precise measurements of concentration even in gas mixtures that exhibit continuous absorption bands. The current sensitivity, 2 x 10(-8) cm(-1) Hz-1/2 per spectral element, is expected to improve by two orders of magnitude with an external enhancement cavity. We have demonstrated this sensitivity increase by combining cavity-enhanced frequency comb spectroscopy with a scanning Fourier transform spectrometer in the near-infrared region and achieving a sensitivity of 4.7 x 10(-10) cm(-1) Hz(-1/2). A cavity-enhanced mid-infrared comb spectrometer will provide a near real-time, high sensitivity, high resolution, precisely frequency calibrated, broad bandwidth system for many applications.

Infrared Spectrum of the Hydrated Proton in Water.

Reactive molecular dynamics simulations have been utilized to calculate the infrared (IR) spectra of acidic HCl solutions of varying concentration with the goal of achieving a better understanding of the spectral features of the hydrated excess protons in bulk water. To incorporate the essential physics of the hydrated proton, we carried out the simulations using the specialized self-consistent iterative multistate empirical valence bond (SCI-MS-EVB) method, which is a form of multiconfigurational (reactive) molecular dynamics. After the pure water absorption background was removed, the calculated difference spectra are in good agreement with prior experimental results. The continuous broad absorption band in the acidic IR spectrum is, for the first time, interpreted based on the concept of a dynamically distorted Eigen cation, H9O4(+), which has been shown to provide the most accurate description for the charge defect character of the hydrated excess proton in liquid water.

Mid-infrared Fourier transform spectroscopy with a broadband frequency comb

We present a first implementation of optical-frequency-comb-based rapid trace gas detection in the molecular fingerprint region in the mid-infrared. Near-real-time acquisition of broadband absorption spectra with 0.0056 cm(-1) maximum resolution is demonstrated using a frequency comb Fourier transform spectrometer which operates in the 2100-to-3700-cm(-1) spectral region. We achieve part-per-billion detection limits in 30 seconds of integration time for several important molecules including methane, ethane, isoprene, and nitrous oxide. Our system enables precise concentration measurements even in gas mixtures that exhibit continuous absorption bands, and it allows detection of molecules at levels below the noise floor via simultaneous analysis of multiple spectral features.

Synthesis, photophysical and electrophosphorescent properties of mononuclear Pt(II) complexes with arylamine functionalized cyclometalating ligands

Abstract Four cyclometalated Pt(II) complexes, i.e., [(L2)PtCl] (1b), [(L3)PtCl] (1c), [(L2)PtC CC6H5] (2b) and [(L3)PtC CC6H5] (2c) (HL2 = 4-[p-(N-butyl-N-phenyl)anilino]-6-phenyl-2,2′-bipyridine and HL3 = 4-[p-(N,N′-dibutyl-N′-phenyl)phenylene-diamino]-phenyl-6-phenyl-2,2′-bipyridine), have been synthesized and verified by 1H NMR, 13C NMR and X-ray crystallography. Unlike previously reported complexes [(L1)PtCl] (1a) and [(L1)PtC CC6H5] (2a) (HL1 = 4,6-diphenyl-2,2′-bipyridine), intense and continuous absorption bands in the region of 300–500 nm with strong metal-to-ligand charge transfer (1MLCT) (dπ(Pt) → π∗(L)) transitions (e ∼ 2 × 104 dm3 mol−1 cm−1) at 449–467 nm were observed in the UV–Vis absorption spectra of complexes 1b, 1c, 2b and 2c. Meanwhile, with the introduction of electron-donating arylamino groups in the ligands of 1a and 2a, complexes 1b and 2b display stronger phosphorescence in CH2Cl2 solutions at room temperature with bathochromically shifted emission maxima at 595 and 600 nm, relatively higher quantum yields of 0.11 and 0.26, and much longer lifetimes of 8.4 and 4.5 μs, respectively. An electrochromic film of 1b-based polymer was obtained on Pt or ITO electrode surface, which suggests an efficient oxidative polymerization behavior. An orange multilayer organic light-emitting diode with 1b as phosphorescent dopant was fabricated, achieving a maximum current efficiency of 11.3 cd A−1 and a maximum external efficiency of 5.7%. The luminescent properties of complexes 1c and 2c are dependent on pH value and solvent polarity, which is attributed to the protonation of arylamino units in the C^N^N cyclometalating ligands.