Concentration Of Centers Research Articles

How Many Electron Traps are formed in Persistent Phosphors?

Persistent luminescence is caused by a charge carrier detrapping process from the carrier traps filled by excitation light. Thus, the carrier storage capacity is an important factor in determining the persistent luminescence intensity. Here, the electron storage capacity was investigated in the YAGG:Ce3+-Yb3+ transparent ceramic persistent phosphor, in which the Ce3+ ion is the luminescence center and the Yb3+ ion acts as the electron trap. The number density of the Yb2+ electron trapping center was estimated to be approximately 1.6 × 1018 ions cm−3 from the absorption coefficient spectrum of Yb2+:4f–5d photochromic absorption center and the Yb-LIII edge XANES spectrum, which means that approximately 12% of the Yb3+ ions in the sample were changed to the divalent state after charging. Although the maximum energy density of 0.61 J cm−3 was calculated as a storage property of persistent phosphors from the Yb2+ number density and the photon energy of Ce3+:5d1−4f luminescence at 520 nm, the actual energy density which was detected as persistent luminescence was 0.011 J cm−3. It is suggested that the recombination efficiency of the detrapped electrons from the Yb2+ ions with the hole-trapped Ce3+ ions is approximately a few percent.

Variations in the Near‐Infrared Spectral Properties of Ferrous Mineral Mixtures With Different Relative Abundances

AbstractIn near‐infrared spectral studies, the relationship between the 1‐μm absorption (Band I) center and the band area ratio (BAR, the area ratio of 2–1‐μm absorption features) is useful in compositional and mineralogical analyses of ferrous mineral‐bearing mixtures. Zhang and Cloutis (2020), https://doi.org/10.1029/2020ea001153, investigated various lunar ferrous iron‐bearing rocks and minerals and found that pyroxene‐bearing materials rich in ilmenite (Ilm), plagioclase (Pl), or glass are offset from the lunar olivine‐clinopyroxene‐orthopyroxene (Ol‐Cpx‐Opx) mixing line in the plot of the BAR versus the Band I center. To analyze the variation trends of the spectral properties of these mixtures with different components, this study presents a systematic evaluation of laboratory spectra of terrestrial and synthetic ferrous iron‐bearing mineral mixtures based on published databases. In general, the mixing trends of the Pl‐pyroxene mixtures, the glass‐pyroxene mixtures, and the Ilm‐basalt mixtures are consistent with the findings of Zhang and Cloutis (2020), https://doi.org/10.1029/2020ea001153. Moreover, this study also finds that the BAR of the Pl‐pyroxene mixtures varies nonlinearly with different relative abundances and that the BAR is generally not sensitive to Pl abundances below 60%. For glass‐pyroxene mixtures, the corresponding data points are usually appreciably offset from the Ol‐Cpx‐Opx mixing line at glass abundances above 20%. The BAR of the Ilm‐basalt mixtures increases with increasing Ilm content, mainly due to the weakening of 1‐μm absorption generally being greater than that of 2‐μm absorption.

Defects formation in YSZ ceramics with different Y2O3 content irradiated with 0.25 MeV electrons energy

This work aims to study the effect of electron irradiation and Y2O3 content on the optical and luminescent properties of translucent cubic yttrium-stabilized zirconia (YSZ) ceramics fabricated by free sintering technique. Commercial nanopowders TZ-8YS and TZ-10YS TOSOH were used for sintering ceramics. It was shown that irradiation leads to the formation of centers of induced absorption, which are deconvoluted by the set of three Gaussian bands with maxima at 2.9, 2.5, and 2.1 eV, respectively. An increase in the Y2O3 content from 8 mol.% to 10 mol.% significantly affect the formation of induced absorption centers. The cathodoluminescence (CL) spectrum can be satisfactorily approximated by the sum of three Gaussian bands (2.8, 2.28 and 2.04 eV). The CL intensity band at 2.8 eV (F+-center) growth when the Y2O3 content leads to an increase.

Ultra-broadband THz absorber with doped silicon based on periodic T-shaped arrays

A polarization-independent ultra-broadband T-type absorber with doped silicon in the terahertz band is proposed, which consists of T-shaped Si/SiO2 arrays. The results show that the absorption is more than 95% at 1.0–5.3 THz. Such an absorber can have a center frequency and relative bandwidth ratio of 3.15 THz and 137%. In addition, the absorption of 90% can be achieved at 1.0–5.3 THz with the incident angle of 0–44°. The energy distribution of the electromagnetic field at the peak of the ultra-broadband absorber is clearly explained by finite element method. Furthermore, compared with the proposed reference planar absorber and the reported broadband absorber, the T-type absorber in this work has better absorption bandwidth, center frequency, and relative bandwidth ratio. The impedance of T-type terahertz absorber is demonstrated well by impedance matching theory. It is believed that the ultra-wideband terahertz absorber with silicon doped based on periodic T-shaped arrays has great potential in the wideband terahertz absorber field and can be applied in the biomedical field.

Buckling on Fe‐Doped AlGaN/GaN High Electron Mobility Transistor Films after Laser Liftoff Process: Phenomena and Mechanism

The influence of Fe in the buffer layer on the laser liftoff (LLO) of GaN high electron mobility transistors is presented herein. LLO is performed on Fe‐doped GaN films using a KrF excimer LLO system. The morphology, crystal characteristics, and 2D electron gas (2DEG) characteristics of the GaN films after LLO are characterized using a 3D optical microscope, X‐ray diffraction, and Hall methods. The present work is different from previous methods because of the large‐area buckling instead of cracks on the Fe‐doped GaN films. The results show that the crystal quality and 2DEG characteristics of the buckled parts are significantly affected. Furthermore, a modified simulation model of the temperature and thermal‐induced stress fields in the films is used to explain the buckling of the films, whose results are consistent with the characterization results. The additional absorption centers induced by the Fe impurity energy levels increase the gradient of temperature field, thereby increasing the compressive stresses in the films at the edges of the laser spots. Moreover, a higher temperature is observed at the interface between the film and the bonding glue, which results in the debonding. Therefore, Fe‐doped GaN films are more prone to buckling than unintentionally doped GaN films.

Effects of γ irradiation on the structure, physical and mechanical properties of a series of tetrafunctional epoxies modified DGEBA/DDS

In some nuclear applications, epoxy materials have become increasingly appealing. However, radiation-induced structural changes often deteriorate properties. Thus, the long-term stability of epoxies after exposure to radiation has to be evaluated. Irradiation induced changes in three tetrafunctional epoxy modified DGEBA/DDS were reported. Irradiation leads to changes in chemical structure and appearance, i.e., darkening in color and reduction in transparency. Irradiated samples show two absorption centers, and both the bending and tensile strengths of samples, which are modified by the tetrafunctional epoxy that has a diphenylsulfone spacer, maintain high values even after 2.8 MGy irradiation. γ irradiation leads to chain scission and decreases Tg; however, Tgs of irradiated samples modified with tetrafunctional epoxies were still higher than 220 °C. Results show that addition of tetrafunctional epoxies improves Tg of DGEBA/DDS after exposure to radiation. Irradiation-induced chain scissions also increase the heterogeneity of the cured network. Although ray reduces the thermal stability, the thermal and mechanical properties are adequate for many applications.

Near‐infrared Spectra of Lunar Ferrous Mineral Mixtures

AbstractIron‐bearing minerals are a major component of materials on the lunar surface, and many of them can be distinguished based on the diagnostic absorption features in visible and near‐infrared reflectance spectra. The relationship between the 1 µm absorption (Band I) center and the band area ratio (BAR), defined as the ratio of 2–1 µm absorption areas, provides a sensitive way to estimate the relative abundance of olivine (Ol) and pyroxene. In the plot of the BAR value versus the Band I center, the Ol‐orthopyroxene (Opx) mixing line (derived from terrestrial materials) is strictly applicable only to Ol‐Opx mixtures. Based on published database of laboratory spectra and compositional data for lunar rocks and mineral separates, this study investigated the spectral characteristics of a variety of common Fe2+‐bearing lunar minerals and rocks, such as clinopyroxene (Cpx), ilmenite‐rich basalt, pyroxene‐bearing anorthosite, and glass‐rich impact melt. The lunar Ol‐Cpx‐Opx mixing line for rocks and minerals is determined, which contains less curvature than the Ol‐Opx mixing line, consistent with both the higher Fe2+ content of lunar mafic silicates and the presence of appreciable Cpx. This study suggests that some of the pyroxene‐bearing lunar materials that are rich in ilmenite, glass, or plagioclase can also be distinguished by this method. These results indicate that the plot of the BAR value versus the Band I center is a useful tool for spectral analysis of lunar composition and mineralogy, especially for those having pyroxene‐dominated spectra.

Metal-Based Ionic Liquids in Oxidative Desulfurization: A Critical Review.

Ionic liquids (ILs) as novel functional desulfurization materials have attracted increasing attentions. Metal-based ionic liquids (MILs) are classified into three types of metal chloride ILs, metal oxide ILs, and metal complex ILs based on the definition and basic structure of MILs in this critical review. On the basis of the properties of ILs such as structure designability, super dissolution performance, good thermal and chemical stability, nonflammability, and wide electrochemical window, MILs exhibit unique advantages on hydrophobicity, oxidation performance, and Brönsted-Lewis acidity. Therefore, MILs possess both the absorption and oxidation centers for the intramolecular adsorption and oxidation to improve the oxidative desulfurization (ODS) process. During the novel nonaqueous wet oxidative desulfurization process (Nasil), H2S can be oxidized into elemental sulfur with hydrophobic MILs, which can be regenerated by oxygen for recycle, to solve the problems of low sulfur capacity, low sulfur quality, and severe secondary pollution in the aqueous Lo-Cat wet oxidative desulfurization process. Another outstanding feature of MILs in ODS is biomimetic catalysis, which has the function of activating molecular oxygen and improving the oxidation performance. Metal oxide ILs and metal complex ILs are used in combination with hydrogen peroxide or oxygen with the existing water to generate a Fenton-like reaction to convert hydrophobic organic sulfur or SO2 into hydrophilic sulfoxide/sulfone or sulfur acid, respectively. However, the corrosion of Cl- to the equipment and emulsification phenomenon in the extraction process of sulfoxide/sulfone separation still need further study. Furthermore, the promising strategies to construct highly efficient and green desulfurization processes for large-scale applications are provided.

Optical Properties of Aluminum- and Silicon-Nitride Films and Al–Si–N Nanocomposite Coatings Deposited by Reactive Magnetron Sputtering

In this paper, we study the optical properties of aluminum- and silicon-nitride films and Al–Si–N coatings with variable atomic composition deposited by reactive magnetron sputtering on glass, silicon, and steel substrates. The absorption and luminescence characteristics are determined by the composition of the coatings and microstructure and depend on the physical properties of the substrate. The absorption and luminescence centers are associated with intrinsic defects in the nitrides and their simplest complexes. The relationships between the accumulation of growth defects, their interaction, the type of distribution of localized states, the band gap, and the stability of the optical properties are established. At an increase in the silicon content in the coatings, the degree of static induced disorder increases, and the contribution of the continuous distribution of the defect levels and interband absorption increases. Silicon-containing defects stabilize the optical properties of the coatings.

Effect of the Organic Semiconductor Side Groups on the Structural and Electronic Properties of Their Interface with Dopants.

Two derivatives of [1]benzothieno[3,2-b][1]benzothiophene (BTBT), namely, 2,7-dioctyl-BTBT (C8-BTBT) and 2,7-diphenyl-BTBT (DPh-BTBT), belonging to one of the best performing organic semiconductor (OSC) families, have been employed to investigate the influence of the substitutional side groups on the properties of the interface created when they are in contact with dopant molecules. As a molecular p-dopant, the fluorinated fullerene C60F48 is used because of its adequate electronic levels and its bulky molecular structure. Despite the dissimilarity introduced by the OSC film termination, dopant thin films grown on top adopt the same (111)-oriented FCC crystalline structure in the two cases. However, the early stage distribution of the dopant on each OSC film surface is dramatically influenced by the group side, leading to distinct host-dopant interfacial morphologies that strongly affect the nanoscale local work function. In this context, Kelvin probe force microscopy and photoelectron emission spectroscopy provide a comprehensive picture of the interfacial electronic properties. The extent of charge transfer and energy level alignment between OSCs and dopant are debated in light of the differences in the ionization potential of the OSC in the films, the interface nanomorphology, and the electronic coupling with the substrate.

Size Effect on Optical Properties of Silicon Dioxide Hollow Particles

The optical properties of silicon dioxide hollow particles with different size were investigated in UV/visible/near-IR region, as well as X-ray photoelectron spectra were analyzed. Synthesis of SiO2 hollow particles was carried out using a template method. It was established that hollow particle reflectance lower than bulk microparticles. Absorptance in the red and near infrared spectral ranges increases with decreasing size of hollow particles, but in the UV-region conversely. This is due to different absorption centers.

Effect of irradiation with continuous 28 MeV He2+ ion beam and short-pulsed 200 keV C+ ion beam on optical properties of multilayer Al-Si-N coatings

The article reports on radiation defect formation parameters and radiation resistance of multilayer coatings from thin layers of aluminum and silicon nitrides deposited on sodium-calcium-silicate glass and monocrystalline silicon substrates by reactive magnetron sputtering. The samples were irradiated with helium ions of 28 MeV on a cyclotron and carbon ions 200 keV on an accelerator in the mode of short-pulse implantation. The characteristics of local absorption and luminescence centers before and after irradiation and their probable nature were determined. The optical centers had been identified as point intrinsic defects of a growth and radiation nature. The accumulation of radiation defects in layers of amorphous silicon nitride a-Si3N4 prevailed over the accumulation in crystalline c-AlN layers due to the diffusion of defects in amorphous layers and the formation of secondary defects in them. Changes in optical properties led to the conclusion about the high radiation resistance of the coatings. The main reasons for the resistance of coatings to ion irradiation were the high concentration of growth defects, their strong interaction and the wide band gap of the nitrides. Coatings deposited on silicon substrates had a higher radiation resistance compared to the same coatings deposited on glass substrates.

Low-dose X-ray-stimulated LaGaO3:Sb,Cr near-infrared persistent luminescence nanoparticles for deep-tissue and renewable in vivo bioimaging

X-ray-stimulated near-infrared persistent luminescence nanoparticles (NIR-PLNPs) offer attractive capabilities for autofluorescence free deep-seated imaging and depth-independent treatment, but still suffer from adverse effects caused by high-dose X-ray irradiation (>5 Gy). From the viewpoint of two centers of NIR-PLNPs, i.e., X-ray photon absorption center and defects center, we here propose a top-down composition design strategy for the development of low dose X-ray-stimulated NIR-PLNPs with improved X-ray photon absorption efficiency and optimized persistent luminescence (PersL) performance. Successfully, hypersensitive X-ray-stimulated NIR-PLNPs LaGaO3:Sb3+,Cr3+ with a super-long PersL emission (>500 h) at ∼750 nm are prepared. The nature of high Z atomic constituents of the host LaGaO3 endows these NIR-PLNPs with strong X-ray absorption capacity. Simply by codoping with size-mismatched ions Sb3+, the concentration of oxygen vacancies in the host is rationally optimized, leading to the enhanced NIR PersL performance of Cr3+ ions. In vivo bioimaging demonstrates that the designed X-ray-stimulated NIR-PLNPs LaGaO3:Sb3+,Cr3+ can be readily reactivated under X-ray irradiation with even a lower dose to 0.37 Gy, showing the advantages of these nanoparticles on deep-seated imaging and treatment. More importantly, we anticipate that our proposed top-down composition design strategy can be applied to develop much low dose X-ray-stimulated NIR-PLNPs in the future.

Efficient Co@Co3O4 core-shell catalysts for photocatalytic water oxidation and its behaviors in two different photocatalytic systems

Co@Co 3 O 4 photocatalyst was prepared in situ to investigate its water splitting performance and mechanism in two photocatalytic systems. • Photocatalytic activity of catalyst 1 in two systems were compared for the first time. • Catalyst 1 exhibited excellent photocatalytic performances. • Core-shell structure of Catalyst 1 can promote light capture and electron transmission. The photocatalytic performances of water oxidation were usually carried out in two different systems, photosensitizer and non-photosensitizer systems. There is few report about the same catalyst used in two systems and therefore it is of great significant to compare its role of the same catalyst in two systems and explore its different reaction mechanisms. In this work, first 4 kinds of metallic Co microparticles were obtained by different reduction methods through hydrothermal processes, and Co@Co 3 O 4 core-shell microparticles (1–4) were obtained from these metallic Co microparticles oxidized in air or in the reacting solution in situ. The core-shell structure of 1 was characterized by a series of analytical techniques. 1–4 exhibited excellent activities and stabilities in the [Ru(bpy) 3 ] 2+ /S 2 O 8 2− /light system when they were used as catalysts for the photocatalytic water oxidation. The maximum O 2 evolution of 1 after 20 min’s illumination was 98.2 μmol, the O 2 yield was 65.5%, the initial turnover frequency was 6.6 × 10 −3 , the initial quantum efficiency ( Φ QY initial ) was 15.0% higher than Co (8.3%), CoO (11.7%), Co 2 O 3 (1.2%), Co 3 O 4 (2.8%) and 5 (2.2%). Even after the sixth run, the catalytic activity of recovered 1 still remained 85.1% of initial activity. In addition, the photocatalytic performances of 1 in the [Ru(bpy) 3 ] 2+ /S 2 O 8 2− /light system and S 2 O 8 2− /light system were compared for the first time. In the non-photosensitizer system, 1 shows bifunctional roles and acts as optical absorption center and active catalytic site, and oxygen evolution rate is lower and it takes longer time. In the photosensitizer system, 1 only acts as a catalyst, the photosensitizer enhances the light absorption and promotes water oxidation reaction with higher O 2 yield and QE, meanwhile the photosensitizer brings the defect of high cost and instability into the system. Based on the results the two different reaction mechanisms were deeply discussed.

Influence of Fe in the buffer layer on the laser lift-off of AlGaN/GaN HEMT film: phenomena and mechanism

In this paper, a study is presented of the effect of Fe in the buffer layer on the laser lift-off (LLO) of GaN high electron mobility transistors (HEMTs). AlGaN/GaN HEMTs grown on Fe-doped and unintentionally doped buffer layers were separated from sapphire substrates using a 248 nm KrF excimer laser lift-off system. We analyzed the variations of the crystal characteristics, two-dimensional electron gas (2DEG) characteristics and the strain state of AlGaN/GaN HEMT films before and after LLO by the x-ray diffraction, Hall and Raman methods. The measurement of the distribution of elemental Fe in the GaN buffer layer was performed by secondary ion mass spectroscopy. The results show that a peak Fe concentration of 9.56 × 1018 cm−3 appears at the interface. Moreover, the crystal quality and 2DEG characteristics of the Fe-doped GaN film obviously degenerated, with the formation of micro-cracks under the threshold laser separation energy density (Es). In this case, the threshold laser damage energy density (ED) was used to measure the laser damage tolerance of GaN in LLO. The analysis considers that additional laser absorption centers, induced by the Fe impurity energy level, coupled with a large residual stress of up to 1.0763 GPa in Fe-doped GaN, reduced the threshold laser damage energy density (ED).

Study of the Modified Gaussian Model on olivine diagnostic spectral features and its applications in space weathering experiments

The absorption features of olivine in visible and near-infrared (VNIR) reflectance spectra are the key spectral parameters in its mineralogical studies. Generally, these spectral parameters can be obtained by exploiting the Modified Gaussian Model (MGM) with a proper continuum removal. However, different continua may change the deconvolution results of these parameters. This paper investigates the diagnostic spectral features of olivine with diverse chemical compositions. Four different continuum removal methods with MGM for getting the deconvolution results are presented and the regression equations for predicting the Mg-number (Fo#) are introduced. The results show that different continua superimposed on the mineral absorption features will make the absorption center shift, as well as the obvious alterations in shape, width and strength of the absorption band. Additionally, it is also found that the logarithm of a second-order polynomial continuum can match the overall shape of the spectrum in logarithmic space, and the improved regression equations applied to estimate the chemical composition of olivine-dominated spectra also have a better performance. As an application example, the improved approach is applied to pulse laser irradiated olivine grains to simulate and study the space weathering effects on olivine diagnostic spectral features. The experiments confirm that space weathering can make the absorption band center shift toward longer wavelength. Therefore, the Fo# estimated from remote sensing spectra may be less than its actual chemical composition. These results may provide valuable information for revealing the difference between the spectra of olivine grains and olivine-dominated asteroids.

Down-conversion luminescence of Yb3+ in novel Ba4Y3F17:Yb:Ce solid solution by excitation of Ce3+ in UV spectral range

Abstract The down-conversion luminescence under UV excitation was demonstrated in single-phase Ba4Y3F17 solid solution nanoparticles codoped with Ce3+/Yb3+. The complex photodynamic processes occurred in the samples including excited state absorption, excitons and color centers formation. The Yb doping suppressed the solarization effect. The efficiency of down-conversion from UV to near IR luminescence of Yb3+ appeared to be high due to Ce4+-Yb2+ charge transfer mechanism responsible for energy transfer between Ce3+ and Yb3+ ions. The direct measurements of down-conversion luminescence quantum yield by means of integrating sphere and calculations of energy transfer efficiency from luminescence spectra data were carried out. The down-conversion luminescence quantum yield reached the maximum value of about 1% for the sample with 0,1 mol.% of Ce3+ and 5,0 mol.% of Yb3+ ions contents, whereas energy transfer coefficient reached its maximum of about 6,3% for 0,05 mol.% of Ce3+ and 5,0 mol.% of Yb3+ ions.

Influence of Quenching on the Opto-Electronic Properties of F:SnO2 Layers.

For many opto-electronic applications, F:SnO2 materials must benefit from high transparency, high conductivity, and high mechanical strength even after quenching. The purpose of this study was to investigate the influence of quenching on the opto-electronic properties of the F:SnO2 layers synthesized at high temperature on SixCyO-coated soda-lime glass by atmospheric chemical vapor deposition. The morphology, structure, and composition of the layers were studied before and after quenching in air- and oxygen-rich atmospheres at 670 °C. The free carrier concentration was reduced by oxygen vacancy (VO) passivation, as well as by F and Na diffusion, with all effects scaling up with quenching time in air. The transmittance also decreased with quenching time as Na impurities acted as absorption and electron recombination centers. In an oxygen-rich atmosphere, the VO passivation was even more emphasized, with however a moderate contribution to conductivity loss. The F:SnO2 layer microstructure and composition were rather fringed through high-temperature deposition. The almost invariable free carrier concentration and transmittance of the F:SnO2 samples quenched in O2 versus air were related to a moderation in Na diffusion. For long quenching times (>20 min) in air, Na and F diffusion prevailed explaining the conductivity drop.

Low-Temperature Luminescent Studies of Emissive Guanine Substitute for the Detection of Biopolymers

The optical absorption at 300 K and the fluorescence and phosphorescence at 78 K of the emissive guanine substitute, deoxythienoguanosine, (dthG) were investigated in aqueous and TRIS-HCl-buffer solutions. Two optical absorption and fluorescence centers at room temperature were attributed to two keto-enol tautomers of dthG, which confirms previously obtained results. In contrast to room temperature, only one emission band was observed at 78 K in fluorescence spectra that was close to the long-wave fluorescence band at room temperature and could be associated with the tautomer with long-wave absorption. This phenomenon can be explained by the energy transfer by excitations in a frozen solution between two types of the optical centers mentioned above. The similar conclusion is drawn for the phosphorescence: only one tautomer phosphorescence band is observed. The spectral positions of this band maximum are essentially different for aqueous and buffer solutions (∼50 nm).

One kind of new Ti3+ luminous center in Ti:Al2O3 crystals

In the optical absorption spectra of Ti:Al2O3 crystal doped with higher concentration of titanium and annealed with hydrogen, in addition to the main absorption band of TiAl3+ (490 nm, 540 nm), a small absorption band of 390 nm and a concomitant small bump on the short-wave side of the 490 nm absorption band was often observed. In this study, an independent optical absorption center that also has doublet structured optical absorption bands at 388 and 460 nm was separated from the above spectra by means of spectral decomposition, and its polarization property and annealing effect are consistent with the main absorption band of TiAl3+. It was supposed to be another kind of Ti3+ center related to interstitial Ti3+ locating at the intrinsic vacant octahedral in the corundum structure. Moreover, this Ti3+ center also produced a wide emission band centered at 760 nm, with weaker intensity, which indicated that there were two kinds of Ti3+ luminous centers in Ti:Al2O3 crystal: one is TiAl3+ in the lattice site of A1, the other is Tii3+ in the interstitial site.