Showing papers on "Absorption band published in 2017"

Ab Initio Site Occupancy and Far-Red Emission of Mn4+ in Cubic-Phase La(MgTi)1/2O3 for Plant Cultivation

Abstract: Mn4+-activated oxide phosphors La(MgTi)1/2O3 (LMT) with far-red emitting were prepared via a sol–gel route. The structures of samples were determined by X-ray diffraction (XRD) and Reitveld refinement. The occupied sites of Mn4+ (d3 electronic configuration) in host La(MgTi)1/2O3 were confirmed by ab initio calculations in which the system has the lower formation energy, stable lattice structure, and strong bonding state as Mn4+ enters into Ti site. The luminescent properties of Mn4+-doped samples were investigated; the samples emit far-red light centered at 708 nm with ultraviolet light (345 nm) or blue light (487 nm) excitation. According to the photoluminescence (PL) and excitation (PLE) spectra, the crystal field strength of the Mn4+-occupied environment was estimated. The thermal stability of phosphor was also evaluated through temperature-dependent PL intensity in a heating and cooling cycle process. The emission band is well-matched with the absorption band of phytochrome PFR under the excitation o...

Near-IR Absorbing J-Aggregate of an Amphiphilic BF2-Azadipyrromethene Dye by Kinetic Cooperative Self-Assembly

Abstract: A new amphiphilic BF2 -azadipyrromethene (aza-BODIPY) dye 1 has been synthesized using a CuI -catalyzed "click" reaction. For this dye, two self-assembly pathways that lead to different type of J-aggregates with distinct near-infrared optical properties have been discovered. The metastable off-pathway product displays a broad, structureless absorption band while the thermodynamically stable on-pathway aggregate exhibits the characteristic spectral features of a J-aggregate, that is, red-shifted intense absorption band with significantly narrowed linewidth. The morphology and structure of the aggregates were studied by atomic force microscopy, transmission and scanning electron microscopy. The aggregation processes of 1 were investigated by temperature- and concentration-dependent UV/Vis spectroscopy and evaluated by models for cooperative self-assembly.

Chemical Ni-C Bonding in Ni-Carbon Nanotube Composite by a Microwave Welding Method and Its Induced High-Frequency Radar Frequency Electromagnetic Wave Absorption.

Abstract: In this work, a microwave welding method has been used for the construction of chemical Ni–C bonding at the interface between carbon nanotubes (CNTs) and metal Ni to provide a different surface electron distribution, which determined the electromagnetic (EM) wave absorption properties based on a surface plasmon resonance mechanism. Through a serial of detailed examinations, such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectrum, the as-expected chemical Ni–C bonding between CNTs and metal Ni has been confirmed. And the Brunauer–Emmett–Teller and surface zeta potential measurements uncovered the great evolution of structure and electronic density compared with CNTs, metal Ni, and Ni–CNT composite without Ni–C bonding. Correspondingly, except the EM absorption due to CNTs and metal Ni in the composite, another wide and strong EM absorption band ranging from 10 to 18 GHz ...

Numerical study of an ultra-broadband near-perfect solar absorber in the visible and near-infrared region.

Abstract: We propose and numerically investigate a novel ultra-broadband solar absorber by applying iron in a 2D simple metamaterial structure. The proposed structure can achieve the perfect absorption above 95% covering the wavelength range from 400 to 1500 nm. The average absorption reaches 97.8% over this wavelength range. The broadband perfect absorption is caused by the excitation of localized surface plasmon resonance and propagating surface plasmon resonance. We first propose and demonstrate that the iron is obviously beneficial to achieve impedance matching between the metamaterial structure and the free space over an ultra-broad frequency band in the visible and near-infrared region, which play an extremely important role to generate an ultra-broadband perfect absorption. In order to further broaden the absorption band, we also demonstrate the perfect absorption exceeding 92% for the 400–2000 nm range by adding the number of metal-dielectric pairs and using both gold and iron simultaneously in the proposed structure. The average absorption of the improved absorber reaches 96.4% over the range of 400–2000 nm. The metamaterial absorbers using iron are very promising for many applications, which can greatly broaden the perfect absorption band in the solar spectrum and, meanwhile, can enormously reduce the cost in the actual production.

Photocatalytic improvement of Mn-adsorbed g-C3N4

Abstract: This study employed experimental results and theoretical calculations to investigate Mn-adsorbed g-C3N4 as a potential photocatalyst with high efficiency. Mn was chosen as the incorporating element, because among the 3d transition metals it exhibits the highest binding energy and most suitable band edge positions. The photocatalytic efficiency of Mn-adsorbed g-C3N4 is 3 times higher that of pristine g-C3N4. Although small variations in the phase and surface morphology were observed, which were confirmed to not be the determining factors to improve efficiency. The factors that affect the high photocatalytic efficiency are therefore the electronic structure, optical absorption, and band edge variations after Mn-adsorption. The Mn atoms stably are bonded with N atoms, due to the strong absorption energy and ionic bond. Moreover, reduction of the g-C3N4 band gap after Mn-adsorption results in a red shift of the absorption band edge. The half-filled Mn 3d state introduces impurity states into the forbidden band gap, which will increase the life time of charge carriers. In addition, the up-shifting of band edges of Mn-adsorbed g-C3N4 leads to inhibition of the electron-hole recombination. As a consequence, the photocatalytic efficiency of Mn-adsorbed g-C3N4 is enhanced due to the combination of the aforementioned effects.

Carbazole-Linked Near-Infrared Aza-BODIPY Dyes as Triplet Sensitizers and Photoacoustic Contrast Agents for Deep-Tissue Imaging

Abstract: Four new N-ethylcarbazole-linked aza-boron-dipyrromethene (aza-BODIPY) dyes (8 a,b and 9 a,b) were synthesized and characterized. The presence of the N-ethylcarbazole moiety shifts their absorption and fluorescence spectra to the near-infrared region, λ≈650–730 nm, of the electromagnetic spectrum. These dyes possess strong molar absorptivity in the range of 3–4×104 m−1 cm−1 with low fluorescence quantum yields. The triplet excited state and singlet oxygen generation of these dyes were enhanced upon iodination at the core position. The core-iodinated dyes 9 a,b showed excellent triplet quantum yields of about 90 and 75 %, with singlet oxygen generation efficiency of about 70 and 60 % relative to that of the parent dyes. Derivatives 8 a,b showed dual absorption profiles, in contrast to dyes 9 a,b, which had the characteristic absorption band of aza-BODIPY dyes. DFT calculations revealed that the electron density was spread over the iodine and dipyrromethene plane of 9 a,b, whereas in 8 a,b the electron density was distributed on the carbazole group and dipyrromethene plane of aza-BODIPY. The uniqueness of these aza-BODIPY systems is that they exhibit efficient triplet-state quantum yields, high singlet oxygen generation yields, and good photostability. Furthermore, the photoacoustic (PA) characteristics of these aza-BODIPY dyes was explored, and efficient PA signals for 8 a were observed relative to blood serum with in vitro deep-tissue imaging, thereby confirming its use as a promising PA contrast agent.

Tungsten based anisotropic metamaterial as an ultra-broadband absorber

Abstract: We show theoretically that an array of tungsten/germanium anisotropic nano-cones placed on top of a reflective substrate can absorb light at the wavelength range from 0.3 μm to 9 μm with an average absorption efficiency approaching 98%. It is found that the excitation of multiple orders of slow-light resonant modes is responsible for the efficient absorption at wavelengths longer than 2 μm, and the anti-reflection effect of tapered lossy material gives rise to the near perfect absorption at shorter wavelengths. The absorption spectrum suffers a small dip at around 4.2 μm where the first order and second order slow-light modes get overlapped, but we can get rid of this dip if the absorption band edge at a long wavelength range is reduced down to 5 μm. The parametrical study reflects that the absorption bandwidth is mainly determined by the filling ratio of tungsten as well as the bottom diameter of the nano-cones and the interaction between neighboring nano-cones is quite weak. Our proposal has some potential applications in the areas of solar energy harvesting and thermal emitters.

A Band-Notched Absorber Designed With High Notch-Band-Edge Selectivity

Abstract: In this paper, a band-notched absorber with high notch-band-edge selectivity is proposed and investigated It is formed by introducing a narrower and full reflectance band into a wider absorption band For each absorbing unit cell, two independently magnetic and electric resonances with adjacent resonant frequencies are employed to develop the narrower and full reflectance band, while a dipole-shaped metal strip with a resister printed on one side of the supporting substrate and a ground plane is used to realize the wider absorption band By retrieving relative equivalent circuits of each cell, the performance of the proposed absorber can be qualitatively analyzed Simply through tuning the frequencies of the independently magnetic and electric resonances, the performance of the notched band can be adjusted with high notch-band-edge selectivity A sample of an absorber with the notched band frequency from 82 to 98 GHz, good absorption band from 48 to 80 GHz and 102 to 16 GHz has been designed, fabricated, and measured Good agreement among circuit analysis, simulation results, and measurement results is finally obtained

Luminescence studies on Dy 3+ doped calcium boro-tellurite glasses for White light applications

Abstract: A new series of Dy 3+ doped calcium boro-tellurite glasses have been prepared by melt quenching technique and their spectroscopic properties were studied through FTIR, absorption luminescence and lifetime spectral measurements. FTIR studies have been made to explore the presence of various stretching and bending vibrations of different borate and tellurite groups in the prepared glasses. The bonding parameter values were estimated from the absorption band positions using Nephelauxetic ratios to examine the nature of the metal-ligand bond. The optical band gap and Urbach energy (ΔE) values were obtained from the absorption spectra to explore the electronic band structure of the studied glasses. Judd-Ofelt (JO) theory have been used to determine the JO intensity parameters (Ω 2 , Ω 4 , Ω 6 ) following the least square fitting procedure between the experimental and calculated oscillator strength values. The luminescence spectra of the Dy 3+ doped calcium boro-tellurite glasses exhibit two intense emission bands corresponding to the 4 F 9/2 → 6 H 15/2 and 4 F 9/2 → 6 H 13/2 transitions. Further, they exhibit less intense emission band due to the 4 F 9/2 → 6 H 11/2 transition. Luminescence spectra were characterized through CIE 1931 chromaticity diagram to obtain the dominant emission color of the prepared glasses. The JO intensity parameters and refractive index values have been used to calculate the radiative parameters such as transition probabilities (A R ), branching ratios (β R ) and stimulated emission cross-section ( σ P E ) values for the observed transitions in the luminescence spectra. The decay curves of all the studied glasses found to exhibit non-exponential behavior and further to understand the energy transfer process takes place between the Dy 3+ ions, the decay curves were fitted to the Inokuti-Hirayama (IH) model. The structural and optical properties of the Dy 3+ doped calcium boro-tellurite glasses have been studied as a function of different metal cations (Zn, Cd, Pb and Bi) and the obtained results were discussed and compared with the similar reported glasses.

Sub-band-gap absorption in Ga 2 O 3

Abstract: β-Ga2O3 is a transparent conducting oxide that, due to its large bandgap of 4.8 eV, exhibits transparency into the UV. However, the free carriers that enable the conductivity can absorb light. We study the effect of free carriers on the properties of Ga2O3 using hybrid density functional theory. The presence of free carriers leads to sub-band-gap absorption and a Burstein-Moss shift in the onset of absorption. We find that for a concentration of 1020 carriers, the Fermi level is located 0.23 eV above the conduction-band minimum. This leads to an increase in the electron effective mass from 0.27–0.28 me to 0.35–0.37 me and a sub-band-gap absorption band with a peak value of 0.6 × 103 cm–1 at 3.37 eV for light polarized along the x or z direction. Both across-the-gap and free-carrier absorption depend strongly on the polarization of the incoming light. We also provide parametrizations of the conduction-band shape and the effective mass as a function of the Fermi level.

Structural and Visible-Near Infrared Optical Properties of Cr-Doped TiO 2 for Colored Cool Pigments

Abstract: Chromium-doped TiO2 pigments were synthesized via a solid-state reaction method and studied with X-ray diffraction, SEM, XPS, and UV-VIS-NIR reflectance spectroscopy. The incorporation of Cr3+ accelerates the transition from the anatase phase to the rutile phase and compresses the crystal lattice. Moreover, the particle morphology, energy gap, and reflectance spectrum of Cr-doped TiO2 pigments is affected by the crystal structure and doping concentration. For the rutile samples, some of the Cr3+ ions are oxidized to Cr4+ after sintering at a high temperature, which leads to a strong near-infrared absorption band due to the 3A2 → 3 T1 electric dipole-allowed transitions of Cr4+. And the decrease of the band gap causes an obvious redshift of the optical absorption edges as the doping concentration increases. Thus, the VIS and near-infrared average reflectance of the rutile Ti1 − x Cr x O2 sample decrease by 60.2 and 58%, respectively, when the Cr content increases to x = 0.0375. Meanwhile, the color changes to black brown. However, for the anatase Ti1 − x Cr x O2 pigments, only the VIS reflection spectrum is inhibited by forming some characteristic visible light absorption peaks of Cr3+. The morphology, band gap, and NIR reflectance are not significantly affected. Finally, a Cr-doped anatase TiO2 pigment with a brownish-yellow color and 90% near-infrared reflectance can be obtained.

Thermochromic VO2 thin films on ITO-coated glass substrates for broadband high absorption at infra-red frequencies

Abstract: Thin films of vanadium dioxide (VO2) are deposited on indium tin oxide (ITO), stainless steel (SS), and glass substrates using chemical vapour deposition. X-ray diffraction and Raman spectroscopy measurements confirmed the single phase nature of the VO2, which showed a phase transition from a low conducting state at low temperature( 68 °C). This was confirmed by electrical conductance and infra-red reflectance measurements. X-ray photoelectron spectroscopy was used to measure the charge states of vanadium species. The optical constants of VO2 were determined using visible and near-infra red(NIR) reflectivity and show that the VO2 film on ITO has a lowered plasma frequency compared with VO2 on glass substrates. The thin films of VO2 enable a broadband of ultra-high absorption at mid-wave infra-red frequencies due to a Fabry-Perot (F-P) like resonance due to the dielectric properties of ITO, SS, or glass. The tunability of this absorption band via VO2 ...

Unprecedented low energy losses in organic solar cells with high external quantum efficiencies by employing non-fullerene electron acceptors

Abstract: In order to realize high photocurrent generation in the low-energy region of the solar spectrum, two conjugated A–D–A oligomers, PY-1 and DCI-2 comprising a central dithieno[3,2-b:2′,3′-d]pyrrole donor (D) unit and end-capping acceptors (A) 1-butyl-4-methyl-2,6-dioxopyridine-3-carbonitrile (PY) or 3-dicyanomethyleneindan-1-one (DCI) have been synthesized and characterized. The oligomers showed strong absorptions in the red and near-IR region in solution and in the solid state. As a consequence of the strong electron-accepting character, the absorption band of DCI-2 in thin films is significantly red-shifted compared to that of PY-1 resulting in a low optical energy gap of 1.23 eV. In optimized solution-processed bulk-heterojunction solar cells using a polymeric donor P1, the new non-fullerene acceptor DCI-2 provides an excellent power conversion efficiency of 6.94% which is noticeably higher than that of PY-1-based devices (PCE = 4.89%). Most importantly, a high open-circuit voltage (VOC) of ∼0.8 V with unprecedented energy losses, between the optical energy gap and the open-circuit voltage, between 0.39 and 0.43 eV concomitant with excellent external quantum efficiencies of 69%@880 nm in the NIR-regime have been achieved for DCI-2-based devices.

Effects of cobalt doping on structural, morphological, and optical properties of Zn2SiO4 nanophosphors prepared by sol-gel method

Abstract: In this paper, undoped and cobalt (II) (3, and 5 mol %) doped zinc silicate nanophosphors prepared by a sol-gel method which were heated at 1000 °C were studied in detail. From the XRD result, undoped and Co2+ doped zinc silicate sample yields α- and β-Zn2SiO4. This also shows that α-Zn2SiO4 acts as the primary phase in the crystal structure of the sample. For FESEM, the grain size decreases as the dopant increases where undoped sample indicates average grain size of 181 nm while 3 and 5 mol % exhibits containing ones 136.89 nm and 176.22 nm respectively. With FTIR transmission a major peak at range of 1100 cm−1, 880, 550, and 350 assigning as Si-O-Si asymmetric stretching, SiO4 asymmetric stretching, ZnO4 symmetric stretching, and Si-O asymmetric deformation vibration were observed respectively. The absorption band of doped Zn2SiO4 exhibited a higher absorbance intensity in the UV region compared to undoped Zn2SiO4. Besides, all the samples depict that the sample was having a red shift in which the band shifted to the longer wavelength. The energy band gap value of undoped sample shows 3.07 eV and when Co2+ dopant (3 and 5 mol %) was introduced, the energy band gap started to decrease (2.46 and 2.42 eV respectively). PL emission presents two peaks at blue emissions (420 and 480 nm) and a green emission (525 nm). The result present that Co2+:Zn2SiO4 is potentially good to use as blue and green phosphors for luminescent optical material.

Ultrafast Charge and Triplet State Formation in Diketopyrrolopyrrole Low Band Gap Polymer/Fullerene Blends: Influence of Nanoscale Morphology of Organic Photovoltaic Materials on Charge Recombination to the Triplet State

Abstract: Femtosecond transient absorption spectroscopy of thin films of two types of morphologies of diketopyrrolopyrrole low band gap polymer/fullerene-adduct blends is presented and indicates triplet state formation by charge recombination, an important loss channel in organic photovoltaic materials. At low laser fluence (approaching solar intensity) charge formation characterized by a 1350 nm band (in ~250 fs) dominates in the two PDPP-PCBM blends with different nanoscale morphologies and these charges recombine to form a local polymer-based triplet state on the sub-ns timescale (in ~300 and ~900 ps) indicated by an 1100 nm absorption band. The rate of triplet state formation is influenced by the morphology. The slower rate of charge recombination to the triplet state (in ~900 ps) belongs to a morphology that results in a higher power conversion efficiency in the corresponding device. Nanoscale morphology not only influences interfacial area and conduction of holes and electrons but also influences the mechanism of intersystem crossing (ISC). We present a model that correlates morphology to the exchange integral and fast and slow mechanisms for ISC (SOCT-ISC and H-HFI-ISC). For the pristine polymer, a flat and unstructured singlet-singlet absorption spectrum (between 900 and 1400 nm) and a very minor triplet state formation (5%) are observed at low laser fluence.

Fluorescence properties and energy level structure of Ce-doped silica fiber materials

Abstract: A Ce-doped silica fiber material is prepared using powder-in-tube technique via sol-gel method. Its absorption and emission spectra are investigated from experimental and theoretical perspectives. The experimental results show an absorption band at 320 nm and a broad emission band at 430 nm, which comprises of two bands at 420 and 470 nm, respectively. In addition, a local microstructure model of the Ce-doped silica fiber material is developed and its structure parameters and fluorescence spectra are calculated using density functional theory. The theoretical results indicate a strong absorption peak at 336 nm and an emission peak at 486 nm. Furthermore, the spontaneous emission lifetime is also calculated to be approximately 64.82 ns. This confirms theoretically that the Ce-doped silica materials possess the scintillator characteristics. Moreover, an energy level diagram for the Ce-doped silica fiber material is constructed. It is significant for understanding the fluorescence properties of the Ce-doped silica fiber material at the microstructural level.

Rewriting the Story of Excimer Formation in Liquid Benzene

Abstract: Formation of benzene excimer following UV excitation of the neat liquid is monitored with femtosecond spectroscopy. A prompt rise component in excimer transient absorption, which contradicts the classical scenario of gradual reorientation and pairing of the excited monomers, is observed. Three-pulse experiments in which the population of evolving excimers is depleted by a secondary dump pulse demonstrate that the excimer absorption band is polarized along the interfragment axis. The experiments furthermore prove that the subsequent 4-fold increase in excimer absorption over ∼50 ps is primarily due to an increase in the transition dipole of pairs which are formed early on, and not to excited monomers forming excimers in a delayed fashion due to unfavorable initial geometry. Results are analyzed in light of recent studies of local structure in the liquid benzene combined with advanced electronic structure calculations. The prompt absorption rise is ascribed to excited states delocalized over nearby benzene ...

Phantom Electrons in Mesoporous Nanocrystalline SnO2 Thin Films with Cation-Dependent Reduction Onsets

Abstract: Mesoporous thin films comprised of ∼15 nm diameter SnO2 nanocrystallites were synthesized and characterized in acetonitrile electrolytes by electrochemical and spectroscopic techniques. Spectroelectrochemical reduction of the thin films resulted in broad, non-superimposable UV/vis absorption changes. Simultaneous analysis of potential-dependent spectra, by a process termed “potential associated spectra”, resulted in the identification of three unique absorption spectra for reduced SnO2, while only one spectrum was identified for TiO2. Reduction of SnO2 resulted in the appearance of (1) a broad absorption that spans across the visible and near-IR regions, (2) a blue-shifted fundamental absorption, and (3) an absorption band in the blue region. The absorption onsets were dependent on the electrolyte cation, present as the perchlorate salt of Li+, Na+, Mg2+, Ca2+, and TBA+, where TBA+ is tetrabutylammonium. Correlations between the charge within the thin film and the absorbance intensity revealed that signif...

Synthesis and Photoluminescence Properties of a Blue-Emitting La3Si8N11O4:Eu2+ Phosphor.

Abstract: Eu2+-doped La3Si8N11O4 phosphors were synthesized by the high temperature solid-state method, and their photoluminescence properties were investigated in this work. La3Si8N11O4:Eu2+ exhibits a strong broad absorption band centered at 320 nm, spanning the spectral range of 300–600 nm due to 4f7 → 4f65d1 electronic transitions of Eu2+. The emission spectra show a broad and asymmetric band peaking at 481–513 nm depending on the Eu2+ concentration, and the emission color can be tuned in a broad range owing to the energy transfer between Eu2+ ions occupying two independent crystallographic sites. Compared to the Ce3+-doped La3Si8N11O4, the Eu2+-doped one shows a larger thermal quenching, predominantly owing to photoionization. Under 320 nm excitation, the internal and external quantum efficiencies are 44 and 33%, respectively.