Intrinsic Lattice Defects Research Articles

Influence of {Ce3+ – Mg2+} pairs on the photo- and thermally stimulated luminescence of Mg2+ co-doped Gd3(Ga,Al)5O12:Ce single crystals

Single crystals of Gd3(Ga,Al)5O12:Ce,Mg with different concentration of Mg2+ ions varying from 0 to 0.029 at.% are investigated by the X-ray diffraction, steady-state and time-resolved photoluminescence, and thermally stimulated luminescence (TSL) methods in a wide temperature range (77–510 K). The origin and structure of the luminescence centers are considered. The influence of Mg2+ ions on the structure of the Ce3+-related excited state, the processes taking place in this state, and the TSL characteristics is clarified. At low temperatures, the photoluminescence optical quenching in close {Ce3+ - Mg2+} pairs is found to be responsible for the light yield reduction accompanied with the acceleration of the luminescence decay kinetics. At room temperature, the luminescence quenching can be additionally caused by the thermal ionization of the 5d1 excited state of Ce3+ ion, whose probability increases with the increasing Ga content. The conclusion is made that in order to create the fast scintillator with the maximum light yield, the concentration of intrinsic crystal lattice defects in Gd3(Ga,Al)5O12:Ce,Mg should be minimized, the optimum composition of the host should be chosen, and the balance between the concentrations of the doped Mg2+ and Ce3+ ions should be optimised.

Aerosol assisted-chemical vapour deposition of tetrahedrite copper antimony sulphide thin films: the effect of zinc(II) impurities on optical properties

Non-stoichiometric semiconducting copper antimony sulphide (CAS) thin films, both undoped and with Zn2+ ions incorporated, were deposited at 500, 550 and 600 °C onto glass substrates by aerosol-assisted chemical vapour deposition (AACVD) using metal diethyldithiocarbamate precursors. Data from powder X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and scanning electron microscopy – energy dispersive X-ray spectroscopy suggest a correlation of composition of the non-stoichiometric sulphur-deficient tetrahedrite phase (cubic structure) microcrystalline CAS (Cu12Sb4S13) thin films with particle sizes ranging from 0.1 to 4 µm. For undoped thin films, visible optical absorption with a bandgap of ∼2.1 eV is likely associated with compositional variations involving intrinsic lattice defects, including shallow electronic states such as copper interstitials and vacancies of sulphur as deep-lying donors, and copper-antimony anti-sites and antimony vacancies as deep-lying acceptors. Additionally, tetrahedrite phase CAS (tCAS) thin films with Zn2+ ions incorporated (Znx-tCAS) exhibit tunable composition-driven electronic structure for small Zn2+ content influencing narrower bandgaps between 1.7 and 1.9 eV. The Raman data suggest that the phase purity is affected by small fractions of the famatinite (tetragonal) phase. Overall, the thin films display broad emission of fast dual radiative recombination, and an additional recombination pathway exhibited for Znx-tCAS associated with zinc-related point defects. Though further studies are required to explore in more detail the defect chemistry, these results show the utility of AACVD in tuning compositional and optical properties of undoped tCAS and Znx-tCAS thin films.

Tailoring stress relaxation for dopant-free ZnO thin films with high thermoelectric power factor

In this study, the effects of stress relaxation on the thermoelectric properties (carrier concentration n, Hall mobility μH, weighted mobility μW, density-of-state mass md*, Seebeck coefficient S, and thermopower factor PF) of undoped ZnO films were rationalized in terms of native defects (VO-related defects and Zni-related donors) induced through the deposition temperature (TD) during the sputtering process. All investigated ZnO films exhibited compressive stress and tended to become less compressive with increasing TD. The stress relaxation at high TD resulted in improved film crystallization and decreased native defect concentration, thus significantly enhancing md* through the reduction of intrinsic lattice defects, while less carriers were trapped and scattered by defects. Therefore, n and μ increased simultaneously (by 28 times and one order of magnitude, respectively), markedly enhancing the PF of dopant-free ZnO films.

Blue light emission enhancement and robust pressure resistance of gallium oxide nanocrystals.

Exploration of pressure-resistant materials largely facilitates their operation under extreme conditions where a stable structure and properties are highly desirable. However, under extreme conditions, such as a high pressure over 30.0 GPa, fluorescence quenching generally occurs in most materials. Herein, pressure-induced emission enhancement (PIEE) by a factor of 4.2 is found in Ga2O3 nanocrystals (NCs), a fourth-generation ultrawide bandgap semiconductor. This is mainly attributed to pressure optimizing the intrinsic lattice defects of the Ga2O3 nanocrystals, which was further confirmed by first-principles calculations. Note that the bright blue emission could be stabilized even up to a high pressure of 30.6 GPa, which is of great significance in the essential components of white light. Notably, after releasing the pressure to ambient conditions, the emission of the Ga2O3 nanocrystals can completely recover, even after undergoing multiple repeated pressurizations. In addition to stable optical properties, synchrotron radiation shows that the Ga2O3 nanocrystals remain in the cubic structure described by space group Fd3m upon compression, demonstrating the structural stability of the Ga2O3 nanocrystals under high pressure. This study pays the way for the application of oxide nanomaterials in pressure anti-counterfeiting and pressure information memory devices.

Multi-colour emission from ZnO[SO4]:Eu2+ nanophosphors: Effects of SO42- and Eu2+ doping concentration

Blue-emitting ZnO-SO4:Eu2+ powder phosphors were synthesized via solid state reaction method. Based on the X-ray diffraction (XRD) data, hexagonal wurtzite structures of ZnO were formed. Further structural elucidation via Raman spectroscopy showed a peak at 439 cm−1 associated with E2hgh optical mode that is a distinctive characteristic band of hexagonal wurtzite ZnO phase. The ZnO powders were composed of distorted spherical nanoparticles, which were agglomerated together as confirmed from scanning electron microscopy images. After incorporation of SO42- and Eu2+ into the ZnO lattices sites, the particle shapes became irregular. When excited at 351 nm with Xenon lamp, our samples emitted UV and blue-green visible light associated, respectively, with excitonic recombination and intrinsic structural lattice defects in ZnO. Both the photoluminescence (PL) peak intensity and positions of these emissions were influenced by SO42- and Eu2+ ions. The blue emission was dependent on the Eu2+ content with the largest intensity measured from the sample doped with 1.0 mol% of Eu2+ ions. Mechanisms of energy transfer and concentration quenching effect are discussed.

Enhanced thermoelectric figure-of-merit in ‘defective’ half-Heusler Nb0.8CoSb

The half Heusler alloys are potential mid-to-high temperature range thermoelectrics due to their high power factor, solid structural stability and high mechanical strength. However, the high lattice thermal conductivity inherent to these materials reduces the zT, rendering them less useful for practical applications. The ‘defective’ half-Heuslers provide an attractive alternative to mitigate these issues. In these materials, the intrinsic lattice defects present in the form of vacancies lower the lattice thermal conductivity substantially. Here, we study the effect of excess Nb and Sn doping in the defective half Heuslers Nb0.8+δCoSb1−xSnx. We show that Sn doping allows for: (i) optimizing the carrier concentration with a much finer control than if only δ is increased, and (ii) incorporation of a higher concentration of Nb in the structure. A combination of optimized carrier concentration and synergistic changes in the band structure, including the appearance of a new flat band near 50 meV above the Fermi energy due to excess Nb, suppresses the bipolarity and enhances the thermopower further at high temperatures as the average band effective mass of the carriers increases. We, therefore, obtain a high zT exceeding 1 at 1100 K for Nb0.85CoSb0.95Sn0.05. This value is ≈ 15% higher than the highest zT previously reported.

Narrowband Ultraviolet-B Persistent Luminescence in an Indoor-Lighting Environment through Energy Transfer from Host Excitons to Gd3+ Emitters in ScPO4.

Narrowband ultraviolet-B (NB-UVB) luminescent materials are characterized by high photon energy, narrow spectral width, and visible-blind emission, thus holding great promise for photochemistry and photomedicine. However, most NB-UVB phosphors developed so far are photoluminescent, where continuous external excitation is needed. Herein, we realize NB-UVB persistent luminescence (PersL) in an indoor-lighting environment by exploiting the interaction between self-trapped/defect-trapped excitons and Gd3+ emitters in ScPO4. The phosphor shows a self-luminescing feature with a peak maximum at 313 nm with a time duration of >24 h after ceasing X-ray irradiation, which can be clearly imaged by an UVB camera in a bright environment. Spectroscopic and theoretical approaches reveal that thermo- and photo-stimulations of energies trapped at intrinsic lattice defects followed by energy transfer to Gd3+ emitters account for the emergence of the afterglow. The present results can initiate more exploration of NB-UVB PersL phosphors for emerging applications in secret optical tagging and phototherapy.

Dual-salt assisted synergistic synthesis of Prussian white cathode towards high-capacity and long cycle potassium ion battery

Prussian blue analogues (PBAs) are considered as superior cathode materials for potassium-ion batteries (PIBs) because of their three-dimensional open framework structure, high stability, and low cost. However, the intrinsic lattice defects and low potassium content typically results in poor rate and cycling performance, thus limited their practical applications. In this work, high-quality K1.64FeFe(CN)6 (PW-HQ) material with less crystalline water (6.21%) and high potassium content (1.64 mol−1) was successfully synthesized by a novel coprecipitation method with potassium citrate (K-CA) and potassium chloride (KCl) addition. Specifically, the electrode delivers a reversible capacity of 113.1 mA h g−1 at the current rate of 50 mA g−1 with ∼100% coulombic efficiency. Besides, the electrode retained 90% reversible capacity at 500 mA g−1 current density after 1000 cycles, indicating only 0.01% capacity decay per cycle. Moreover, we have revealed that the introduction of K-CA controlled the chelating rate of Fe(II) and the addition of KCl increased the K+ content, hence improving the capacity and stability of the as-prepared electrodes. Structural evolution and potassium storage mechanism were further investigated by detailed ex-situ X-ray diffraction and in-situ Raman measurements, which demonstrated reversible potassiation/depotassiation behavior and negligible volume change during the electrochemical process. In general, this work provides an efficient strategy to eliminate water contents in Prussian blue cathode and improve its electrochemical performance, which plays a key role in promoting the industrialization of potassium ion batteries.

Dynamic domain boundaries: chemical dopants carried by moving twin walls.

Domain walls and specifically ferroelastic twin boundaries are depositaries and fast diffusion pathways for chemical dopants and intrinsic lattice defects. Ferroelastic domain patterns act as templates for chemical structures where the walls are the device and not the bulk. Several examples of such engineered domain boundaries are given. Moving twin boundaries are shown to carry with them the dopants, although the activation of this mechanism depends sensitively on the applied external force. If the force is too weak, the walls remain pinned while too strong forces break the walls free of the dopants and move them independently. Several experimental methods and approaches are discussed.

Enhanced thermoelectric performance of hot-pressed n-type Ag2Se nanostructures by controlling the intrinsic lattice defects

Figure (a) Power factor (S2σ), (b) zT of sintered Ag2Se (S1), Ag1.99Se (S2), Ag1.96Se, and Ag1.93Se (S4) samples.

Robust Electrocatalytic Li2 S Redox of Li-S Batteries Facilitated by Rationally Fabricated Dual-Defects.

The commercialization of lithium-sulfur batteries with ultra-high theoretical energy density is restricted mainly by the notorious polysulfides "shuttle effect" and slow Li2 S redox reaction kinetics. A sulfur host material with high catalytic activity and high conductivity is greatly desired to improve its electrochemical performance. Herein, a sulfur host material, etched cotton@petroleum asphalt carbon (eCPAC), with high specific surface area and excellent catalytic activity, is demonstrated based on a synergistic strategy of introducing intrinsic lattice defects and composite carbon structure. Benefiting from in situ coupling of amorphous and crystalline materials, eCPAC exhibits high conductivity and high sulfur adsorbability. Furthermore, eCPAC containing dual intrinsic defect sites can catalyze the bidirectional sulfur chemistry of Li2 S and capture polysulfides, which is also demonstrated by systematic density functional theory calculations and the potential intermittent titration technique. S@eCPAC/Li cells exhibit excellent cycling stability and rate performance, with an average capacity decay rate of only 0.05% over 1000 cycles at 0.5 C and even 0.03% over 600 cycles at 5 C. Meanwhile, the practicality of eCPAC is proven in high-load batteries and pouch batteries. eCPAC provides a reliable strategy for achieving a win-win situation of capturing polysulfides and accelerating Li2 S redox kinetics.

Novel (Zr, Ti)(C, N)–SiC ceramics via reactive hot-pressing at low temperature

Novel (Zr, Ti)(C, N)–SiC ceramics were fabricated by reactive hot-pressing at 1500–1700 °C using ZrC, TiC0.5N0.5, and Si powders as raw materials for the first time. The effects of Si addition on the microstructures and mechanical properties were investigated. The reaction completely proceeded to generate SiC with an Si addition of 5 mol%. The grain refinement by SiC and solid solution strengthening of (Zr, Ti)(C, N) improve the mechanical properties. A high flexural strength of 522 ± 20 MPa was obtained in the Z5T5S ceramics (ZrC with 4.75 mol% TiC0.5N0.5 and 5 mol% Si additional). When the Si addition increases to 10 mol%, the residual Ti-stabilized β-ZrSi phase appears, which significantly reduces the flexural strength. The Vickers hardness and fracture toughness monotonically increase with increasing Si addition. The fine-grained microstructure without a residual phase and abundant intrinsic lattice defects in the Z5T5S composite endow it with the potential to obtain excellent radiation resistance.

Ru/Rh Cation Doping and Oxygen-Vacancy Engineering of FeOOH Nanoarrays@Ti3 C2 Tx MXene Heterojunction for Highly Efficient and Stable Electrocatalytic Oxygen Evolution.

Oxyhydroxides hold promise as highly-efficient non-noble electrocatalysts for the oxygen evolution reaction (OER), but their poor conductivity and structural instability greatly impede their progress. Herein, the authors develop a cation-doping and oxygenvacancy engineering strategy to fabricate Ru/Rh-doped FeOOH nanoarrays with abundant oxygen-vacancies in situ grown on Ti3 C2 Tx MXene (Ru/Rh-FeOOH@Ti3 C2 Tx ) as highly-efficient OER electrocatalysts. Benefiting from Ru/Rh-cation regulation, oxygenvacancy engineering, and heterojunction synergy between MXene and modulated FeOOH, the optimized Rh/Ru-FeOOH@Ti3 C2 Tx electrocatalysts exhibit excellent OER activities and remarkable stabilities with 100 h. Particularly, 3%Rh-FeOOH@Ti3 C2 Tx electrocatalyst only needs a 223 mV overpotential at 10 mA cm-2 and 306 mV to reach 100 mA cm-2 , which is superior to commercial IrO2 catalyst and most reported oxyhydroxide-based electrocatalysts. Further, systematically theoretical caculation, kinetics, thermodynamics, and microstructural analysis verify that the integration of Ru/Rh-cation doping and oxygen vacancy obviously enhances the intrinsic conductivity and lattice defects of FeOOH and expose more active sites, thereby decreasing the adsorption/desorption energy barrier and activation energy, and improving the specific activity and catalytic kinetics of electrocatalysts, whereas in situ hybridization with MXene strengthens the structural stability. This work clearly confirms that cationdoping and oxygen-vacancy engineering offers a joint strategy for the electronic structure modulation and design of highly-efficient inexpensive OER electrocatalysts.

Thermostimulated luminescence mechanisms of UV irradiated zirconium dioxide nanotubes

A nanotubular layer of zirconium dioxide was synthesized with anodic oxidation. Curves of spectrally resolved thermostimulated luminescence were studied in the 390-550 nm range after 300 nm UV irradiation. Energetic and kinetic parameters of thermostimulated luminescence curves were estimated. The mechanism of thermally-activated processes involving intrinsic lattice defects is proposed. Keywords: Spectrally resolved thermostimulated luminescence, zirconium dioxide, nanotubes, electron and hole traps.

Effect of Li+ co-doping on the luminescence and defects creation processes in Gd3(Ga,Al)5O12:Ce scintillation crystals

Photo- and thermally stimulated luminescence characteristics, as well as photostimulated defects creation processes are investigated in the 85–510 K temperature range for the Li+ co-doped single crystal of Gd3(Ga,Al)5O12:Ce (GAGG:Ce). The results for GAGG:Ce,Li are compared with the corresponding characteristics of the GAGG:Ce and GAGG:Ce, Mg single crystals of similar host composition. The influence of Li+ and Mg2+ ions on the Ce3+ - related emission band position, temperature dependence of the photoluminescence intensity, efficiency of the Gd3+ → Ce3+ energy transfer, intensity and decay kinetics of the afterglow, and thermally stimulated luminescence characteristics is revealed. The presence of the Ce3+ → Ce4+ conversion in GAGG:Ce,Li is confirmed. The conclusion is made that, unlike GAGG:Ce,Mg, the compensation of the Li+ excess negative charge in GAGG:Ce,Li occurs due to both the Ce3+ → Ce4+ conversion and the effective formation of intrinsic crystal lattice defects. The origin of these defects as well as the origin of the electron and hole trapping centers in the GAGG:Ce,Li crystal are discussed.

Spectroscopy and Theoretical Modeling of Phonon Vibration Modes and Band Gap Energy of Cu2ZnSn(S x Se1-x )4 Bulk Crystals and Thin Films.

Semiconductor Cu2ZnSn(SxSe1–x)4 (CZTSSe) solid solution is considered as a perspective absorber material for solar cells. However, during its synthesis or deposition, any modification in the resulting optical properties is hardly predicted. In this study, experimental and theoretical analyses of CZTSSe bulk crystals and thin films are presented based on Raman scattering and absorption spectroscopies together with compositional and morphological characterizations. CZTSSe bulk and thin films are studied upon a change in the x = S/(S + Se) aspect ratio. The morphological study is focused on surface visualization of the solid solutions, depending on x variation. It has been discovered for the first time that the surface of the bulk CZTSSe crystal with x = 0.35 has pyramid-like structures. The information obtained from the elemental analysis helps to consider the formation of a set of possible intrinsic lattice defects, including vacancies, self-interstitials, antisites, and defect complexes. Due to these results and the experimentally obtained values of the band gap within 1.0–1.37 eV, a deviation from the calculated band gap values is estimated in the range of 1.0–1.5 eV. It is suggested which defects can have an influence on such a band gap change. Also, on comparing the experimental Raman spectra of CZTSSe with the theoretical modeling results, an excellent agreement is obtained for the main Raman bands. The proposed theoretical approach allows to estimate the values of concentration of atoms (S or Se) for CZTSSe solid solution directly from the experimental Raman spectra. Thus, the visualization of morphology and the proposed theoretical approach at various x values will help for a deeper understanding of the CZTSSe structure to develop next-generation solar cells.

Influence of the Defect Stability on n-Type Conductivity in Electron-Doped α- and β-Co(OH)2 Nanosheets.

Electronic doping of transition-metal oxides (TMOs) is typically accomplished through the synthesis of nonstoichiometric oxide compositions and the subsequent ionization of intrinsic lattice defects. As a result, ambipolar doping of wide-band-gap TMOs is difficult to achieve because the formation energies and stabilities of vacancy and interstitial defects vary widely as a function of the oxide composition and crystal structure. The facile formation of lattice defects for one carrier type is frequently paired with the high-energy and unstable generation of defects required for the opposite carrier polarity. Previous work from our group showed that the brucite (β-phase) layered metal hydroxides of Co and Ni, intrinsically p-type materials in their anhydrous three-dimensional forms, could be n-doped using a strong chemical reductant. In this work, we extend the electron-doping study to the α polymorph of Co(OH)2 and elucidate the defects responsible for n-type doping in these two-dimensional materials. Through structural and electronic comparisons between the α, β, and rock-salt structures within the cobalt (hydr)oxide family of materials, we show that both layered structures exhibit facile formation of anion vacancies, the necessary defect for n-type doping, that are not accessible in the cubic CoO structure. However, the brucite polymorph is much more stable to reductive decomposition in the presence of doped electrons because of its tighter layer-to-layer stacking and octahedral coordination geometry, which results in a maximum conductivity of 10-4 S/cm, 2 orders of magnitude higher than the maximum value attainable on the α-Co(OH)2 structure.

Effect of W and Mo co-doping on the photo- and thermally stimulated luminescence and defects creation processes in Gd3(Ga,Al)5O12:Ce crystals

Photo- and thermally stimulated luminescence characteristics of Gd3(Ga,Al)5O12:Ce single crystals co-doped with W and Mo are investigated in the 85 - 510 K temperature range and compared with the corresponding characteristics of the undoped and Ce3+ - doped Gd3(Ga,Al)5O12 single crystals of similar composition. A strong effect of the W and Mo impurity ions appears in the photoluminescence spectra and temperature dependences of the photoluminescence intensity, afterglow intensity and decay kinetics, thermally stimulated luminescence (TSL) intensity and TSL glow curves, excitation spectra of the TSL glow curve peaks and activation energy of their creation. The obtained results are explained by the enhancement of the intrinsic emission contribution into the luminescence spectrum of Gd3(Ga,Al)5O12:Ce,W and Gd3(Ga,Al)5O12:Ce,Mo due to a large concentration of various W - and Mo - related electron traps in these crystals and, consequently, the O- - type hole centers, as well as intrinsic crystal lattice defects (e.g., cation vacancies needed for the excess positive charge compensation of the W6+ and Mo6+ ions). The influence of the co-doping with W and Mo ions on the scintillation characteristics of Gd3(Ga,Al)5O12:Ce is discussed.

Механизмы термостимулированной люминесценции в УФ-облученных нанотрубках диоксида циркония

A nanotubular layer of zirconium dioxide was synthesized with anodic oxidation. Curves of spectrally resolved thermostimulated luminescence were studied in the 390-550 nm range after 300 nm UV irradiation. Energetic and kinetic parameters of thermostimulated luminescence curves were estimated. The mechanism of thermally-activated processes involving intrinsic lattice defects is proposed.

Luminescence of YAG:Ce Phosphors Excited by UV Laser Radiation

Spectral and kinetic characteristics of photoluminescence of commercial YAG:Ce phosphors excited both by laser radiation in the range of 4–6.415 eV and by beams of electrons with energy of 250 keV are investigated. It is shown that electron-hole (e-p) pairs are formed in the matrix by electron beam and upon exposure to laser radiation at 193.3 nm (6.415 eV) as well. All excitation methods initiate the band in the range of 560 nm caused by Ce3+ ions and the UV bands at 320 nm and 360–380 nm. UV luminescence is due to luminescence of intrinsic lattice defects. Possible processes of energy transfer to luminescence centers under optical excitation in the range of 4–6.415 eV are discussed.