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The discovery of ferroelectricity in HfO2 ultrathin films has gathered considerable interest from the microelectronic industry owing to their compatibility with complementary metal oxide semiconductor technology. However, a significant challenge in utilizing HfO2 thin films for commercial devices is the wakeup effect, which is an increase in polarization with the number of electric field cycles in HfO2-based capacitors. Despite efforts to develop wakeup-free HfO2 thin films, the root cause of this effect remains elusive. Some studies attribute it to the tetragonal (T) to orthorhombic (O) phase transformation, while others suggest it is due to the redistribution of oxygen vacancies within the HfO2 layer during electric field cycling. This study investigated the phase transformation dynamics and oxygen vacancy distributions in TiN/Hf0.5Zr0.5O2/TiN capacitors subjected to electric field cycling using in situ grazing incidence synchrotron x-ray diffraction and ex situ x-ray photoelectron spectroscopy. The as-grown HZO films were crystallized in a mixed phase consisting of monoclinic (M), tetragonal, and orthorhombic fractions. The T-phase volume fraction decreased continuously up to 10k electric field cycles. In contrast, the O-phase fraction increased within the first 100 cycles and then stabilized with further cycling. The redistribution of oxygen vacancies occurred continuously throughout the cycling process. The results revealed that continuous oxygen vacancy redistribution during electric field cycling resulted in phase transformation between the T-, O-, and M-phases. The study provides insight into the phase transformation dynamics and oxygen vacancy redistribution in TiN/Hf0.5Zr0.5O2/TiN capacitors during electric field cycling, shedding light on the underlying mechanisms of the wakeup effect. [ABSTRACT FROM AUTHOR]

Above‐bandgap femtosecond optical excitation of a ferroelectric/dielectric BaTiO3/CaTiO3 superlattice leads to structural responses that are a consequence of the screening of the strong electrostatic coupling between the component layers. Time‐resolved X‐ray free‐electron laser diffraction shows that the structural response to optical excitation includes a net lattice expansion of the superlattice consistent with depolarization‐field screening driven by the photoexcited charge carriers. The depolarization‐field‐screening‐driven expansion is separate from a photoacoustic pulse launched from the bottom electrode on which the superlattice is epitaxially grown. The distribution of diffracted intensity of superlattice X‐ray reflections indicates that the depolarization‐field‐screening‐induced strain includes a photoinduced expansion in the ferroelectric BaTiO3 and a contraction in CaTiO3. The magnitude of expansion in BaTiO3 layers is larger than the contraction in CaTiO3. The difference in the magnitude of depolarization‐field‐screening‐driven strain in the BaTiO3 and CaTiO3 components can arise from the contribution of the oxygen octahedral rotation patterns at the BaTiO3/CaTiO3 interfaces to the polarization of CaTiO3. The depolarization‐field‐screening‐driven polarization reduction in the CaTiO3 layers points to a new direction for the manipulation of polarization in the component layers of a strongly coupled ferroelectric/dielectric superlattice. [ABSTRACT FROM AUTHOR]

Ferroelectricity in HfO2 thin films can be utilized for fast, power‐efficient, and highly scalable non‐volatile memories. However, the required wake‐up process for inducing ferroelectricity/ achieving higher polarization is one of the major hurdles that hinder HfO2‐based thin films from developing reliable electronic devices. The wake‐up effect is believed to originate from i) phase transformation from non‐ferroelectric to ferroelectric, ii) movement of defect entities (mainly oxygen vacancy defects) near the film‐electrode interface, and iii) heterogeneity of the electrode interfaces. In the present study, an experimental strategy is designed to overcome these sources of the wake‐up process. A multi‐step deposition and annealing process is carried out to induce wake‐up‐free ferroelectricity in Yttrium doped HfO2 (Y:HfO2) thin film directly grown on Si‐substrate. Furnace annealing is utilized instead of the standard rapid thermal annealing process to reduce the oxygen deficiencies and stimulate the direct growth of the polar Y:HfO2. The oxygen‐vacancy‐related defects are found to be the dominating source of wake‐up effect in Y‐doped HfO2 films. The step‐wise deposition and annealing in the oxygen atmosphere facilitate direct growth of the polar phase, reduce the oxygen vacancies, and induce wake‐up‐free ferroelectricity in Y:HfO2. [ABSTRACT FROM AUTHOR]

A series of silicate materials with noncentrosymmetric structures, namely, Na3ScSi3O9 (NSSO), NSSO:Ce3+, NSSO:Eu2+, and NSSO:Tb3+, have been successfully synthesized by a microwave‐assisted solid‐state reaction. The materials reveal interesting second‐ and third‐harmonic generation and two‐photon absorption (2PA) properties attributable to the unusual connectivity and unique coordination environments around the cations, as evidenced by detailed structural analysis and maximum entropy method calculations. Upon irradiation with 980 nm laser light, the activator‐doped NSSO materials show an unusual upconversion emission property without using Yb3+ sensitizer ions, because of nonlinear absorption processes, as evidenced from 2PA spectroscopy results. The materials also show good chemical stability compared to NaYF4:Er3+, Yb3+. The findings of this work can stimulate the creation of a new scientific field devoted to upconversion luminescence in solid‐state materials. [ABSTRACT FROM AUTHOR]

A new mechanoluminescent phosphor was developed from a sodium (Na) superionic conductor (NASICON)-structured Na 3 Sc 2 (PO 4) 3 :Eu2+ phosphor that is known for its self-healing properties. The compound that crystalized assuming monoclinic C 2/ c symmetry was found to be blue-emitting, with no noticeable persistent luminescence. Thermoluminescence analysis showed that the phosphor had two prominent distinct thermal emission bands corresponding to the high-temperature β- and γ-phases of the composition. The mechanoluminescence properties of the material that does not have any persistent luminescence at room temperature were investigated by imparting impulsive strain. The compound on charging with 365 nm radiation was found to have significant mechanoluminescent emission originating from shallow defects present in the β-phase of composition that formed by the stress-induced phase transition process. Its emission characteristics and temporal behavior were investigated by varying the impact velocity. [ABSTRACT FROM AUTHOR]

Intense hard x-ray pulses from a free-electron laser induce irreversible structural damage in a perovskite oxide epitaxial heterostructure when pulse fluences exceed a threshold value. The intensity of x-ray diffraction from a 25-nm thick epitaxial BiFeO3 layer on a SrTiO3 (STO) substrate measured using a series of pulses decreases abruptly with a per-pulse fluence of 2.7 × 106 photons μm−2 at a photon energy of 9.7 keV but remains constant for 1.3 × 106 photons μm−2 or less. The damage resulted in the destruction of the BiFeO3 thin film within the focal spot area and the formation of a deep cavity penetrating into the STO substrate via the removal of tens of nanometers of material per pulse. The damage threshold occurs at a fluence that is insufficient to heat the absorption volume to the melting point. The morphology of the ablated sample is consistent with fracture rather than melting. Together, these results indicate that the damage occurs via a nonthermal process consistent with ultrafast ionization of the absorption volume. [ABSTRACT FROM AUTHOR]

An integrated layered–spinel material with a nominal composition of (1 −x)Li1.2Mn0.6Ni0.2O2·xLiMn1.5Ni0.5O4 (0.15 ＜ x ＜ 0.3) and crystal defects has been found to be a promising cathode material with a high capacity of 280 mA h g−1. However, capacity fading arising from Mn2+ dissolution occurred at low voltages and long cycling times. To improve the cycling stability while preserving the advantages of this cathode material, a synergic coating and doping approach was studied. This method yields a coating with a similar, but more stable, structure to that of the pristine sample. This coating is achieved by the bulk doping of the surface while maintaining the ratio of layered to spinel phases. The coating layer had a thickness of 12 to 18 nm, which increased with increasing Ti doping, and protected the sample at low voltages while maintaining the ion and charge transport channels on the surface. The Ti-doped sample enhanced the capacity retention by up to 97% after 100 cycles at C/10 and 89% after 200 cycles at 1C compared to 75% and 74% of the pristine sample, respectively. The optimized sample delivered a stable capacity of 270, 250, and 145 mA h g−1 at C/20, C/10, and 1C respectively. This study provides an effective approach to improve the cycling performance of integrated spinel-layered cathode materials. [ABSTRACT FROM AUTHOR]

Spinel-layered composites of Li 1.5 MnTiO 4+ δ were studied for their use as high-energy, low-cost, and environmentally benign cathode materials. The bulk particles showed an attractive specific capacity of up to 250 mAh g −1 at C/10. To improve the performance of this cathode at a high C-rate, a spinel-layered Li 1.5 MnTiO 4+ δ nanorod was successfully synthesized using a β-MnO 2 nanorod template. The nanorod, which had an average diameter of 200 nm and a length of 1 μm, showed specific capacity as high as the bulk particle at C/10. However, owing to a one-dimensional nanostructure with a large effective contact area for Li + diffusion, the nanorod sample exhibited enhanced capacities 11% (170 mAh g −1 ) and 167% higher (80 mAh g −1 ) at 1C and 10C rates, respectively, compared to the bulk particles. Moreover, both samples showed good cycle stability and capacity retention of over 85% after 100 cycles at 1C. [ABSTRACT FROM AUTHOR]

Uniform surface conductive layers with porous morphology-conserved MnCo2O4 microspheres are successfully synthesized, and their electrochemical performances are thoroughly investigated. It is found that the microwave-assisted hydrothermally grown MnCo2O4 using citric acid as the carbon source shows a maximum Li+ ion lithiation/delithiation capacity of 501 mA h g−1 at 500 mA g−1 with stable capacity retention. Besides, the given microsphere compounds are effectively activated as air cathode catalysts in Li–O2 batteries with reduced charge overpotentials and improved cycling performance. We believe that such an affordable enhanced performance results from the appropriate quasi-hollow nature of MnCo2O4 microspheres, which can effectively mitigate the large volume change of electrodes during Li+ migration and/or enhance the surface transport of the LiOx species in Li–air batteries. Thus, the rationally designed porous media for the improved Li+ electrochemical reaction highlight the importance of the 3D macropores, the high specific area and uniformly overcoated conductive layer for the promising Li+ redox reaction platforms. [ABSTRACT FROM AUTHOR]

A green phosphor, La0.4Ca13.3Eu0.3Mg2Si8O31.6+δN0.4−δ (La CMSN:Eu2+), was prepared by a solid-state reaction and an efficient green emission was observed at 506 nm under near-ultraviolet ( NUV) excitation. The structural and optical properties of La CMSN:Eu2+ phosphors as well as their thermal quenching were investigated. The partial substitution of La3+ and N3− in Ca13.7Eu0.3Mg2Si8O32 led to a considerable enhancement in the peak emission intensity by as much as 194%. This demonstrates not only that the total number of Eu2+ activators increased, but also that the probability of a nonradiative transition between Eu2+ and Eu3+ could be reduced as the increase in concentration of the former is at the expense of the later. The white light-emitting diode ( LED) was fabricated using phosphor with a NUV LED chip. The LED showed warm white light with an excellent color rendering index of 91. The La CMSN:Eu2+ is thus a potential green-emitting phosphor for white LEDs. [ABSTRACT FROM AUTHOR]

Blue and orange-emitting, KNa 3− x Si 4 Al 4 O 16 : x Ce 3+ and KNa 3− y Si 4 Al 4 O 16 : y Yb 2+ were synthesized through solid-state reaction. Analyses of structural properties of the phosphors were performed through Rietveld refinement of the XRD pattern, and the luminescence properties were analyzed by activation with Ce 3+ and Yb 2+ ions. On excitation with 397 nm source, photoluminescence (PL) spectra Ce 3+ activated compound was found to be a typical blue asymmetric emission band originating from 5d 1 → 4 f 1 ( 2 F 7/2 and 2 F 5/2 ) transitions of Ce 3+ ions. On exciting with 442 nm source, Yb 2+ activated samples showed an orange broad emission band, due to transitions in the 4 f 13 5 d 1 and 4 f 14 levels. The temperature dependence of PL was analyzed for both phosphors, and Yb 2+ doped compound showed a rapid thermal quenching of luminescence, compared to Ce 3+ doped compound, as is expected for higher stokes shift emission bands. [ABSTRACT FROM AUTHOR]

We report the optical and structural studies of Eu3+-doped 1-dimensional CaYAl3O7 nano-fiber phosphor. CaYAl3O7:Eu3+ phosphors were synthesized by electrospinning technique and the pristine nano-fibers were annealed at 900°C to form well crystallized uniform fibers. Under ultraviolet excitation, the CaYAl3O7:Eu3+ exhibited red emission, due to transitions in the 4f states of Eu3+. In order to explore the difference between the quantum efficiency of nano-fiber and bulk CaYAl3O7:Eu3+ phosphor, detailed structural and optical analyses were carried out. The structural analysis of the CaYAl3O7:Eu3+ nano-fibers indicates that the structural environment surrounding the dopant Eu3+ ion was more unstable in nano-fiber when compared to a bulk sample. Decay curves for both the samples when fitted with double exponential decay model indicate that the nano-fiber has shorter decay time, arising from the larger contribution from the non-radiative decay, due to defect levels introduced in the host lattice. [ABSTRACT FROM AUTHOR]

Abstract: A full-color-emitting phosphor, CaYAl3O7:Pr3+,Ce3+ was synthesized through conventional solid-state reaction. The structural properties of the phosphor were analyzed by Rietveld refinement, maximum entropy method, and luminescent properties were analyzed by co-activating with different rare earth ions. Photoluminescence spectra on excitation with 450nm source were dominated by green band at 490nm originating from 3P0→3H4 transition of Pr3+ ions and less intense band at 617nm due to 1D2→3H4 transition. On codoping with Ce3+, phosphor exhibits an additional broad band originating from 5d1→4f1 (2F7/2 and 2F5/2) of Ce3+, centered at around 540nm due to efficient energy transfer from Pr3+ levels to Ce3+ levels. With this emission property we recommend CaYAl3O7:Pr3+,Ce3+ as potential single-phase full-color-emitting phosphor for solid-state lighting applications. [Copyright &y& Elsevier]

The Eu2+, M-codoped( M = Ce3+, Mn2+) phosphor powders were prepared by a solid-state reaction. The addition of Ce3+ in the Eu2+ sites in partially nitridated bredigite-structure phosphor( CMSN) remarkably enhances the luminescent intensity by ~180% through sensitized luminescence. Dual band emission was observed for Eu, Mn-codoped CMSN through energy transfer from Eu2+ to Mn2+. Ce3+-Eu2+ and Eu2+-Mn2+ energy-transfer mechanism was investigated through decay profile analysis using Inokuti-Hirayama model and energy-transfer parameters are determined. Interaction mechanism was identified as dipole-dipole interaction. In addition, phosphor in glass plates was prepared using the phosphor and its feasibility in white LED application was studied and is presented. [ABSTRACT FROM AUTHOR]

Abstract: The possibility of using a solid solution as a light convertor in white LEDs was studied. A solid solution of Sr3AlO4F and Sr3SiO5 was prepared by solid-state reaction and was activated with Ce3+. Phosphor exhibits an asymmetric yellow–orange emission band which is broadened and red-shifted when coactivated with Tb3+. Decay profile analysis using the Inokuti–Hirayama model revealed that the mechanism involved was electric dipole–quadrupole interaction. The critical distance was calculated from concentration quenching data, multipolar interaction, and exchange interaction. The thermal stability of photoluminescence was investigated by assuming an Arrhenius temperature dependence, and the activation energy for thermal quenching was calculated. When analyzed in combination with an InGaN LED, the phosphor exhibited high color-rendering index and excellent correlated color temperature. These results demonstrate that this solid solution phosphor with its superior properties is highly suitable for use in white LEDs. [Copyright &y& Elsevier]

The low-temperature processability of molecular ferroelectric (FE) crystals makes them a potential alternative for perovskite oxide-based ferroelectric thin films. Quinuclidinium perrhenate (HQReO4) is one such molecular FE crystal that exhibits ferroelectricity when crystallized in an intermediate temperature phase (ITP). However, bulk HQReO4 crystals exhibit ferroelectricity only for a narrow temperature window (22 K), above and below which the polar phase transforms to a non-FE phase. The FE phase or ITP of HQReO4 should be stabilized in a much wider temperature range for practical applications. Here, to stabilize the FE phase (ITP) in a wider temperature range, highly oriented thin films of HQReO4 were prepared using a simple solution process. A slow evaporation method was adapted for drying the HQReO4 thin films to control the morphology and the temperature window. The temperature window of the intermediate temperature FE phase was successfully widened up to 35 K by merely varying the film drying temperature between 333 and 353 K. The strategy of stabilizing the FE phase in a wider temperature range can be adapted to other molecular FE materials to realize flexible electronic devices. [ABSTRACT FROM AUTHOR]

Bismuth telluride (Bi2Te3) and poly(3,4‐ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) are two major inorganic and organic thermoelectric (TE) materials that are widely investigated for TE device applications. An inorganic–organic composite, based on Bi2Te3 and PEDOT:PSS possessing a high TE figure of merit, has immense potential in fabricating highly efficient and flexible TE generators. In order to achieve an enhanced TE performance, the interdependent TE parameters such as electrical conductivity and Seebeck coefficient of the composite should be decoupled. In this study, the TE properties of the composites are enhanced through the creation of additional defects in Bi2Te3 via proton irradiation. The structural variations occurred in Bi2Te3 via proton irradiation are analyzed using X‐ray diffraction, X‐ray photoelectron spectroscopy, and transmission electron microscopy, which quantitatively reveal structural damages as well as deviations from the Bi2Te3 stoichiometry. The proton irradiation‐induced antisite defects in the Bi2Te3 crystal are found to be beneficial in decoupling the interdependent TE parameters of Bi2Te3/PEDOT:PSS composite thin films, through which enhanced TE performance is achieved. The findings demonstrate that proton irradiation of Bi2Te3 is an effective method to engineer the TE properties through inducing defects in Bi2Te3 and enhancing the interaction between Bi2Te3 and PEDOT:PSS. An enhancement in thermoelectric parameters is achieved through proton irradiation induced defects in flexible Bi2Te3/poly(3,4‐ethylenedioxythiophene) doped with poly(styrene sulfonate) inorganic–organic composite thin films. The irradiation induced antisite defects are found beneficial in decoupling the interdependent thermoelectric parameters and achieving a high figure of merit. [ABSTRACT FROM AUTHOR]