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Herein, a prototype nonvolatile bent ferroelastic domain wall (DW) memory based on extension–retraction of DWs in a top electrode/bent ferroelastic DWs/bottom electrode architecture is demonstrated and the effects of mechanical condition, electrical condition, and the material parameter on ferroelastic DWs in PbTiO3 ferroelectric thin films are studied by phase field modeling. Misfit strain can be used to drive the bend of DWs in PbTiO3 thin film, resulting in a change of ferroelastic domain size, bending degree, and conductivity. Stable and reversible switching of DWs between the extendible state with high conductivity and the retractile state with low conductivity can be realized, resulting in an apparent resistance change with a large ON/OFF ratio of >102 and an excellent retention characteristic. The extension and retraction speed, corresponding to data writing speed, can be adjusted by the electric field magnitude and distributions. The memory speed increases by 5% under a homogeneous electric field and 6% under an inhomogeneous probing electric field, after the buildup of space charges in a ferroelectric thin film, and the fastest memory speed is obtained at tip potential φ = 1.8. Moreover, polarization orientations of a and c domains separated by bent ferroelastic DWs do not affect memory performance. This paper can guide the development of new ferroelectric domain wall memory.

Misfit dislocations at a heteroepitaxial interface produce huge strain and, thus, have a significant impact on the properties of the interface. Here, we use scanning transmission electron microscopy to demonstrate a quantitative unit-cell-by-unit-cell mapping of the lattice parameters and octahedral rotations around misfit dislocations at the BiFeO 3 /SrRuO 3 interface. We find that huge strain field is achieved near dislocations, i.e., above 5% within the first three unit cells of the core, which is typically larger than that achieved from the regular epitaxy thin-film approach, thus significantly altering the magnitude and direction of the local ferroelectric dipole in BiFeO 3 and magnetic moments in SrRuO 3 near the interface. The strain field and, thus, the structural distortion can be further tuned by the dislocation type. Our atomic-scale study helps us to understand the effects of dislocations in this ferroelectricity/ferromagnetism heterostructure. Such defect engineering allows us to tune the local ferroelectric and ferromagnetic order parameters and the interface electromagnetic coupling, providing new opportunities to design nanosized electronic and spintronic devices.

Room-temperature polar skyrmion bubbles that are recently found in oxide superlattice, have received enormous interests for their potential applications in nanoelectronics due to the nanometer size, emergent chirality, and negative capacitance. For practical applications, the ability to controllably manipulate them by using external stimuli is prerequisite. Here, we study the dynamics of individual polar skyrmion bubbles at the nanoscale by using in situ biasing in a scanning transmission electron microscope. The reversible electric field-driven phase transition between topological and trivial polar states are demonstrated. We create, erase and monitor the shrinkage and expansion of individual polar skyrmions. We find that their transition behaviors are substantially different from that of magnetic analogue. The underlying mechanism is discussed by combing with the phase-field simulations. The controllable manipulation of nanoscale polar skyrmions allows us to tune the dielectric permittivity at atomic scale and detailed knowledge of their phase transition behaviors provides fundamentals for their applications in nanoelectronics.

Topologically nontrivial polar structures are not only attractive for high-density data storage, but also for ultralow power microelectronics thanks to their exotic negative capacitance. The vast majority of polar structures emerging naturally in ferroelectrics, however, are topologically trivial, and there are enormous interests in artificially engineered polar structures possessing nontrivial topology. Here we demonstrate reconstruction of topologically trivial strip-like domain architecture into arrays of polar vortex in (PbTiO3)10/(SrTiO3)10 superlattice, accomplished by fabricating a cross-sectional lamella from the superlattice film. Using a combination of techniques for polarization mapping, atomic imaging, and three-dimensional structure visualization supported by phase field simulations, we reveal that the reconstruction relieves biaxial epitaxial strain in thin film into a uniaxial one in lamella, changing the subtle electrostatic and elastostatic energetics and providing the driving force for the polar vortex formation. The work establishes a realistic strategy for engineering polar topologies in otherwise ordinary ferroelectric superlattices., The majority of polar structures emerging naturally in ferroelectrics are topologically trivial. Here, the authors demonstrate reconstruction of topologically trivial strip-like domain architecture into arrays of polar vortex in (PbTiO3)10/(SrTiO3)10 superlattice.

Hierarchical polydomain nanostructures, usually formed in epitaxial films, serve to fully relax the misfits between the film and substrate, as well as the strains between the poly-twin variants of the film material. The minimization of the elastic energy endows this type of structure superior stability over other domain architectures. However, the existence of energetically degenerate polytwin variants and their intersecting domain boundaries often led to undesirable functional properties in non-engineered films. Here we show that such nanostructures in epitaxial perovskite ferroelectric films can be crystallographically engineered to eliminate intersecting domain boundaries, thereby achieving an improved ferroelectric performance. In addition to a reduced coercive field and increased dielectric and piezoelectric responses, this type of ferroelectric polydomain nanostructure shows an enhanced ferroelectric polarization with a substantially improved fatigue resistance, as opposed to non-engineered nanostructures with intersecting domain boundaries. These findings suggest an alternative route for the fabrication of highly-reliable nonvolatile ferroelectric memory devices, and may find broader applications in piezoelectrics and energy storage.

In multiferroics, electromagnons have been recognized as a noticeable topic due to their indispensable role in magnetoelectric, magnetodielectric, and magnetocapacitance effects. Here, the electromagnons of Bi1−xNdxFeO3 (x = 0–0.2) nanoparticles are studied via terahertz time-domain spectroscopy, and the impacts of doping concentrations on electromagnons have been discussed. We found that the electromagnons in Bi1−xNdxFeO3 nanoparticles are associated with their phase transition. The total coupling weight of electromagnons is gradually increased in polar R3c structures and then reduces in the antipolar Pbam phase, and the weight in the antipolar phase is less than that of the pure R3c phase. Interestingly, a colossal electromagnon is observed at polar–antipolar and antiferromagnetic–ferromagnetic phase boundaries. Our work offers an avenue for designing and choosing materials with better magnetodielectric and magnetocapacitance properties.

Nontrivial topological structures offer a rich playground in condensed matters and promise alternative device configurations for post-Moore electronics. While recently a number of polar topologies have been discovered in confined ferroelectric PbTiO3 within artificially engineered PbTiO3/SrTiO3 superlattices, little attention was paid to possible topological polar structures in SrTiO3. Here we successfully create previously unrealized polar antivortices within the SrTiO3 of PbTiO3/SrTiO3 superlattices, accomplished by carefully engineering their thicknesses guided by phase-field simulation. Field- and thermal-induced Kosterlitz–Thouless-like topological phase transitions have also been demonstrated, and it was discovered that the driving force for antivortex formation is electrostatic instead of elastic. This work completes an important missing link in polar topologies, expands the reaches of topological structures, and offers insight into searching and manipulating polar textures. While the role of PbTiO3 in PbTiO3/SrTiO3 superlattices has been widely studied, little attention is paid to possible polar topologies in SrTiO3. Here, the authors create atomic-scale vortex–antivortex pairs in the superlattices, expanding the understanding of topological structures.

The ability to controllably manipulate the complex topological polar configurations, such as polar flux-closure via external stimuli, enables many applications in electromechanical devices and nanoelectronics including high-density information storage. Here, by using the atomically resolved in situ scanning transmission electron microscopy, we find that a polar flux-closure structure in PbTiO3/SrTiO3 superlattices films can be reversibly switched to ordinary mono ferroelectric c domain or a domain under electric field or stress. Specifically, the electric field initially drives the flux-closure move and breaks them to form intermediate a/c striped domains, while the mechanical stress firstly starts to squeeze the flux-closures to convert into small vortices at the interface and form a continues dipole wave. After the removal of the external stimuli, the flux-closure structure spontaneously returns. Our atomic study provides valuable insights into understanding the lattice-charge interactions and the competing interactions balance in these complex topological structures. Such reversible switching between the flux-closure and ordinary ferroelectric domains also provides the foundation for applications such as memories and sensors., 22 pages, 5figures

Transparent flexible electronics have captured tremendous attention as the next-generation of electronic devices. Recently, flexible devices for wearing are in high demand in practical applications under special circumstances, such as aeronautics and astronautics, which require highly-transparent and large-scale flexibility, as well as stability at elevated temperature. Here, highly transparent, epitaxial Pb(Zr0.1Ti0.9)O3 (PZT) ferroelectric thin films were fabricated on Sn-doped In2O3 (ITO) transparent electrodes on yttria-stabilized zirconia (YSZ) bufferred mica substrate using pulsed laser deposition. The structural, optical, mechanical, and electrical properties of these all-oxide heterostructures of the PZT/ITO/YSZ/mica were studied. The heteroepitaxy exhibit a high transmittance and an excellent remarkable mechanical properties with robust operation in bending cycle tests. Moreover, the electrical properties are examined to show the preservation of the primitive properties of PZT. Various bending tests are performed to show the tunability of functionalities and to confirm that the heterostructures retain the physical properties under repeated cycles at high temperature. These results demonstrated that these high-performance transparent flexible capacitors would open a route for the applications in transparent flexible devices under harsh conditions.

The ability to electrically write magnetic bits is highly desirable for future magnetic memories and spintronic devices, though fully deterministic, reversible, and nonvolatile switching of magnetic moments by electric field remains elusive despite extensive research. In this work, we develop a concept to electrically switch magnetization via polarization modulated oxygen vacancies, and we demonstrate the idea in a multiferroic epitaxial heterostructure of BaTiO3/Fe3O4 fabricated by pulsed laser deposition. The piezoelectricity and ferroelectricity of BaTiO3 have been confirmed by macro- and microscale measurements, for which Fe3O4 serves as the top electrode for switching the polarization. X-ray absorption spectroscopy and X-ray magnetic circular dichroism spectra indicate a mixture of Fe2+ and Fe3+ at O h sites and Fe3+ at T d sites in Fe3O4, while the room-temperature magnetic domains of Fe3O4 are revealed by microscopic magnetic force microscopy measurements. It is demonstrated that the magnetic domains of Fe3O4 can be switched by not only magnetic fields but also electric fields in a deterministic, reversible, and nonvolatile manner, wherein polarization reversal by electric field modulates the oxygen vacancy distribution in Fe3O4, and thus its magnetic state, making it attractive for electrically written magnetic memories.

Stoichiometric gallium ferrite (GFO) is a promising single-phase multiferroic material at room temperature. Nevertheless, simultaneous magnetism and ferroelectricity in a single GFO at room temperature has not been demonstrated yet. In this work, single-phase GFO thin films with different amount of excessive Fe have been successfully grown using a dual target pulsed laser deposition (PLD) process, and the magnetic transition temperature is found to be above room temperature with excessive Fe distributed among the available cation sites of GFO unit cell. Ferroelectricity of GFO films have been confirmed by second harmonic generation (SHG), polarization hysteresis, pyroelectric reversal, and piezoresponse force microscopy (PFM), and magnetoelectric (ME) coupling has been demonstrated by increase in piezoresponse induced via external magnetic field applied in-plane, all measured at room temperature. From the data, it is theorized that the magnetic properties of GFO originate from the superexchange interaction mediated via the Fe1-O-Fe2 bond, and an additional Fe1-O-Fe* bond can form from excess Fe atoms. Under a magnetic field applied along c axis, the magnetic moment of Fe2 and Fe* increase while that of Fe1 decreases, and these changes in magnetic moment result in larger distortion of Fe2 and Fe* octahedrons along the b-axis, and thus enhanced polarization and piezoresponse. This series of studies thus confirm single-phase GFO with excessive Fe as a multiferroic material at room temperature with ME coupling, paving way for its potential functional applications in microelectronic and spintronic devices.

Ferroelectric films epitaxially grown on vicinal substrates have been widely researched in recent decades for their unprecedented performance tuned by the surface-step-terrace (SST) induced strains of substrates. Here, BaTiO3 films are epitaxially grown on miscut (001) SrTiO3 substrates with different miscut angles of 0°, 0.2°, 1°, and 2°, and the impacts of SST-induced strains on the second-order nonlinear optical coefficients are discussed by fitting the measured second harmonic generation (SHG) signals from the films. The results show that different miscut angles of the substrates can bring about different SST strain states in the BTO films, which make different contributions on the second-order nonlinear optical coefficients. These results disclose that the second-order nonlinear optical coefficients can be effectively controlled by SST induced strains, which provides a new way to tune the optical properties of functional oxide thin films.

Ferroelectric semiconductors splitting water reactions have presented a new paradigm in improving conversion efficient of solar energy into electricity or fuel. Here, we show the switchable photoelectrochemical response controlled by ferroelectric polarization in (101)-oriented Pb(Zr0.2Ti0.8)O3 (PZT) epitaxial films prepared on the (110) SrTiO3 substrate with SrRuO3 as bottom electrodes. Photoelectrochemical (PEC) measurements exhibit that the switchable photocurrent response can be observed in the PZT/SRO heterostructures photo-electrode. Moreover, under illumination, the switchable photocurrents of the PZT/SRO electrode present high repeatability and non-volatility, which only depend in the ferroelectric polarization direction of the PZT film. The result indicates that the PZT/SRO electrode may be as a photoanode or photocathode only through poling treatment with the external electric field. We suggest that the internal electric field and depletion region by ferroelectric polarization at the PZT film is mainly responsible for the switchable PEC effect. The study indicates that this ferroelectric photo-electrode material may have potential applications in solar water splitting, and specially in the PEC sensing and any other smart photo-responsive systems. Keywords: Switchable PEC effect, Ferroelectric polarization, Poling treatment, PZT thin film, Photo-electrode, Water splitting

In this paper, the epitaxial hexagonal LuFeO3 (h-LuFeO3) thin films with c-axis-oriented single phase, smooth surface were grown on YSZ (111) substrates by pulsed laser deposition method. Furthermore, a structural distortion of increased lattice constant of c is found in the epitaxial h-LuFeO3 thin films. Moreover, the epitaxial h-LuFeO3 thin films show room-temperature ferromagnetism. The coercive field and remnant magnetization of the epitaxial h-LuFeO3 thin film decrease with the increase in the test temperature from 50 to 300 K. The study would be of benefit to the room-temperature single-phase multiferroic materials.

Due to their excellent carrier mobility, high absorption coefficient and narrow bandgap, most 2D IVA metal chalcogenide semiconductors (GIVMCs, metal = Ge, Sn, Pb; chalcogen = S, Se) are regarded as promising candidates for realizing high-performance photodetectors. We synthesized high-quality two-dimensional (2D) tin sulfide (SnS) nanosheets using the physical vapor deposition (PVD) method and fabricated a 2D SnS visible-light photodetector. The photodetector exhibits a high photoresponsivity of 161 A⋅W−1 and possesses an external quantum efficiency of 4.45 × 104%, as well as a detectivity of 1.15 × 109 Jones under 450 nm blue light illumination. Moreover, under poor illumination at optical densities down to 2 mW⋅cm−2, the responsivity of the device is higher than that at stronger optical densities. We suggest that a photogating effect in the 2D SnS photodetector is mainly responsible for its low-light responsivity. Defects and impurities in 2D SnS can trap carriers and form localized electric fields, which can delay the recombination process of electron-hole pairs, prolong carrier lifetimes, and thus improve the low-light responsivity. This work provides design strategies for detecting low levels of light using photodetectors made of 2D materials.

Multilevel data ferroelectric storage memory is a breakthrough for addressing low density in ferroelectric random access memories. However, the application of the multilevel data ferroelectric storage memory is limited by high cost and difficulty to prepare high quality epitaxial films. Herein, we create a multilevel data ferroelectric storage memory with a non-epitaxial ferroelectric ultrathin film to overcome these issues. Through controlling the polarization switching and oxygen vacancy migration with the voltage pulses, we demonstrated that voltage-controlled barrier heights yield a memristive behavior with resistance variations. Moreover, we achieved eight logic states, which are written and read easily. Our results suggest new opportunities for ferroelectrics as high density non-volatile memories.

We report accurates and informative measurement with subunit cell resolution for local polarization distribution in ferroelectrics., Recently, several captivating topological structures of electric dipole moments (e.g., vortex, flux closure) have been reported in ferroelectrics with reduced size/dimensions. However, accurate polarization distribution of these topological ferroelectric structures has never been experimentally obtained. We precisely measure the polarization distribution of an individual ferroelectric vortex in PbTiO3/SrTiO3 superlattices at the subunit cell level by using the atomically resolved integrated differential phase contrast imaging in an aberration-corrected scanning transmission electron microscope. We find, in vortices, that out-of-plane polarization is larger than in-plane polarization, and that downward polarization is larger than upward polarization. The polarization magnitude is closely related to tetragonality. Moreover, the contribution of the Pb─O bond to total polarization is highly inhomogeneous in vortices. Our precise measurement at the subunit cell scale provides a sound foundation for mechanistic understanding of the structure and properties of a ferroelectric vortex and lattice-charge coupling phenomena in these topological ferroelectric structures.

The ability to electrically write magnetic bits is highly desirable for future magnetic memories and spintronic devices, though fully deterministic, reversible, and nonvolatile switching of magnetic moments by electric field remains elusive despite extensive research. In this work, we develop a concept to electrically switch magnetization via polarization modulated oxygen vacancies, and we demonstrate the idea in a multiferroic epitaxial heterostructure of BaTiO

Domain orientations and their volume ratios in ferroelectrics are recognized as a compelling topic recently for domain switching dynamics and domain stability in devices application. Here, an optimized second harmonic generation method has been explored for ferroelectric domain characterization. Combing a unique theoretical model with azimuth-polarization-dependent second harmonic generation response, the complex domain components and their distributions can be rigidly determined in ferroelectric thin films. Using the proposed model, the domain structures of rhombohedral BiFeO3 films with 71° and 109° domain wall, and, tetragonal BiFeO3, Pb(Zr0.2Ti0.8)O3, and BaTiO3 ferroelectric thin films are analyzed and the corresponding polarization variants are determined. This work could provide a powerful and all-optical method to track and evaluate the evolution of ferroelectric domains in the ferroelectric-based devices. An optimized optical method is developed to determine the ferroelectric domain structure and their distributions in several ferroelectric films. A team led by Xiangli Zhong and Yichun Zhou from Xiangtan University, and Jianmin Yuan from National University of Defense Technology in China, combined optimized second harmonic generation (SHG) measurements and theoretical analysis to determine the polarization components of domains and their distribution in rhobohedral BiFeO3, tetragonal BiFeO3, Pb(Zr0.2Ti0.8)O3 and BaTiO3 thin films. They first collect azimuth-polarization-dependent SHG signals with respect to both the polarization angle of the incident beam and the azimuth angle of sample relative to the beam. Then they fit the SHG signals with a theoretical model with good agreement. This work provides an effective and all-optical method to determine the domain structures and distributions for ferroelectric thin films.

5-inch diameter Bi 3.15 Nd 0.85 Ti 3 O 12 (BNT) ferroelectric thin films were prepared on Si wafers by a modified pulsed laser deposition method, in which two laser scanning rates and 6-inch diameter target were used. The investigation on the distribution of the film thickness reveals that the laser scanning technique used in conjunction with the large-diameter target can improve the uniformity of film thickness. Especially the reasonable setup of the laser scanning rate ablating the fringe area of the large target has a prominent effect on weakening the edge deviation of thickness distribution. Such phenomenon was fundamentally explained by the simulation results on the ablation tracks of the laser beam on target. Through optimizing the laser scanning rates, BNT thin film with uniform thickness was obtained on 5-inch diameter Si wafer and the maximum variation in film thickness was found to be less than ± 2.5%. The results of X-ray photoelectron spectroscopy and X-ray diffraction show that the prepared 5-inch BNT thin film has great uniformity in stoichiometry and crystallinity. In addition, the measurements of electrical properties indicated that the prepared BNT thin film exhibits good ferroelectric property and I – V characteristic.

Lanthanide doped NaYF 4 microcrystals were synthesized via a facile hydrothermal method. Multicolor upconversion luminescence was observed in NaYF 4 microcrystals doped with Yb 3+ /Er 3+ , Yb 3+ /Tm 3+ , and Yb 3+ /Er 3+ /Tm 3+ under the excitation of 980 nm infrared light. Importantly, the excitation power density dependence of upconversion emission intensity indicated clearly the energy transfer from Tm 3+ to Er 3+ ions under the excitation of low power density (510 2 −9×10 2 W/cm 2 ). Meanwhile, the inverse energy transfer from Er 3+ to Tm 3+ ions under the excitation of relatively higher power density (4.1×10 4 −4.9×10 4 W/cm 2 ) was also revealed. This was a direct evidence for reversible energy transfer between Er 3+ and Tm 3+ ions. Under the excitation of high power density (4.1×10 4 −4.9×10 4 W/cm 2 ), dark sensitizers were also motivated so that the bottleneck effect of high concentration Yb 3+ ion doping was broken. This was the main reason for realizing high upconversion efficiency of the samples with heavy doping of Yb 3+ ion.

Here, we show the simultaneous enhancement of fluorescent and paramagnetic properties in bifunctional NaGdF4:Yb3+/Er3+ nanocrystals by crystal field tuning. The energy level splitting calculation indicates, that lanthanide ionic pairs La3+/Lu3+ introduced into the NaGdF4 host can modify the crystal field around emitters (e.g., Er3+ and Tm3+) and sensitizers (e.g., Yb3+) that result in the broadening of crystal field splitting of energy levels and the abundant multi-site distribution of upconversion luminescence. The optimization of the paramagnetic properties in NaGdF4 doped with emitters and sensitizers is ascribed to the lowering of anti-ferromagnetic coupling.

Here, we show that co-activation in the Gd 3+ –Eu 3+ –Sm 3+ system can generate the intense red emission from Eu 3+ ion. Gd 2 O 3 :Sm 3+ ,Eu 3+ colloidal spheres were synthesized by combining a low temperature hydrothermal process and a higher temperature of pyrolysis. Under the excitation of 380 nm near UV light, Gd 2 O 3 :Sm 3+ ,Eu 3+ emits intense red light while the reference Gd 2 O 3 :Eu 3+ emits very weak one. The calculated polarizability α b μ based on the shift (16 nm) of Eu 3+ –O 2− charge transfer band and the crystallography analysis revealed that Sm 3+ ions codoping in Gd 2 O 3 :Eu 3+ has generated a coordinating environment of Eu 3+ ions with high polarizability and low symmetry which leads to high energy transfer rate not only between Sm 3+ and Eu 3+ pairs, but also between Eu 3+ ( S 6 ) and Eu 3+ ( C 2 ) pairs, ensuring the efficient excitation of Gd 2 O 3 :Eu 3+ ,Sm 3+ . The energy migration from Sm 3+ to Eu 3+ and a corresponding feedback were also experimentally evidenced by the fluorescent decay, which is in well agreement with the theoretical calculation. In addition, the thermal stability of the photoluminescence in Gd 2 O 3 :Eu 3+ ,Sm 3+ colloidal spheres was evaluated.

Ferroelectric-gate thin-film transistors (FGTs) with a stacked oxide structure of ZnO/Bi3.15Nd0.85Ti3O12 (BNT)/LaNiO3 (LNO) on Si substrates have been prepared and characterized. The FGT devices show good electrical properties, such as a large “on” current of 2.5 × 10−4 A and low threshold voltage of 1.1 V. These are mainly attributed to the coupling enhancement of the gate electric field to the channel layer due to a-axis preferential orientation of BNT ferroelectric-gate insulator thin films obtained by using the LNO buffer layer and the relatively good interface properties. The results suggest that ZnO/BNT/LNO/Si structures are well suited for thin-film transistors for future nonvolatile memory applications.

a b s t r a c t Aluminum-doped ZnO (AZO) films were fabricated on glass by pulsed-laser deposition based on ortho- gonal design, and influences of homo-buffer layer on structural, optical and electrical properties of AZO films were investigated. The experiment results demonstrate that the homo-buffer layer is very ben- eficial to improve the quality of AZO films. In addition, both deposition temperature of homo-buffer layer (Thomo-buffer) and deposition oxygen pressure of homo-buffer layer (Phomo-buffer) have a considerable influence on the properties of AZO films. By optimizing the deposition parameters of homo-buffer layer and AZO film synthetically, the AZO film with low resistivity (2.13 × 10 −4 � cm) and high transmittance (89.1%) has been obtained, which is a promising candidate material in transparent electrode applications.

Simultaneous phase- and shape-controlled NaLuF4 microcrystals co-doped with Yb3+/Er3+ have been synthesized via a facile hydrothermal method by only changing the precursor pH value. The UC emission efficiency of these obtained microparticles showed a strong phase- and shape-dependency. Under irradiation of 980 nm, the luminescence intensity of hexagonal NaLuF4 microcrystals presents an enhancement by at least 5.8 times relative to their cubic counterpart. Specially, an up to 168-fold enhancement of emission intensity from the NaLuF4 superstructures, compared with the microplates with identical hexagonal phase, was observed. Such improvement is likely to be dominated by the laser-cavity mirrors effect from the microcavities on their surface of the NaLuF4 superstructures. These results will permit a promising step to harness the upconversion phosphors in solid state lasers.

Lanthanide-doped Ba 2 LaF 7 nanocrystals have been successfully synthesized via a modified liquid–solid-solution process. The yielded nanocrystals are determined to be pure cubic Ba 2 LaF 7 phase and consist of nearly square-shaped nanoparticles with an average 14 nm in size. The Yb 3+ /Er 3+ (Tm 3+ ) co-doped Ba 2 LaF 7 nanocrystals emit bright multicolor upconversion luminescence following the excitation of 980 nm laser diode. Proposed mechanisms for the emission were analyzed based on the emission spectra and their dependence on pumping power, as well as corresponding energy level diagram.

Lanthanide-doped uniform pure cubic phase Y 2 O 3 hollow microspheres have been successfully synthesized via a facile, high yield urea-based coprecipitation route with assistant of carbon spheres templates. The diameter and shell thickness of the microspheres can be manipulated by adjusting carbon sphere templates. Under a 980 nm excitation, Yb 3+ /Er 3+ , Er 3+ , Yb 3+ /Tm 3+ -doped Y 2 O 3 hollow microspheres emit bright upconversion red, green, blue light with high purity, respectively, while Eu 3+ , Eu 3+ /Tb 3+ -doped Y 2 O 3 hollow microspheres exhibit intense downconversion red light under the excitation of 254 nm ultraviolet light. Especially, the 610 nm emission intensity of Eu 3+ in the Eu 3+ /Tb 3+ -codoped Y 2 O 3 hollow microspheres is almost 5 times of that in the Y 2 O 3 :Eu 3+ hollow microspheres indicating the occurring of the energy transfer from Tb 3+ to Eu 3+ ions.

Uniform Yb 3+ and Er 3+ -codoped Y 2 O 3 hollow microspheres were synthesized via urea co-precipitation using carbon spheres as templates. Intense red emission ( 4 F 9/2 → 4 I 15/2 ) and weak green emission ( 2 H 11/2 , 4 S 3/2 → 4 I 15/2 ) of Er 3+ were observed for the Yb 3+ and Er 3+ -codoped Y 2 O 3 hollow microspheres under 980 nm infrared excitation. The integrated intensity of visible emission and the ratio of red to green were found to be strongly dependent on the amount of carbon sphere templates and the concentration of Yb 3+ ions. The amount of carbon sphere templates also plays an important role in adjusting the size of crystallite. Multi-phonon relaxation resulted from the absorbents (OH − and CO 3 2− ) on the surface of the crystallite, and efficient occur of energy transfer processes and cross-relaxation between Er 3+ and Yb 3+ are responsible for the enhancement of intensity ratio of red to green emission. Interestingly, for higher concentration of Yb 3+ ions, the green emission is assigned to a three-phonon process in Y 2 O 3 :Yb/Er hollow microspheres, which also could result in the increase of the red to green emission ratio. An explanation to account for these behaviors was presented.

Lanthanide ions-doped NaLuF4 hexagonal-prismatic microtubes have been successfully synthesized via a facile hydrothermal approach for the first time. The as-grown microtubes are determined to be pure hexagonal (β-) NaLuF4 phase with an outermost diameter of about 3 μm and a length of approximately 40 μm. The Yb3+/Er3+(Tm3+)-codoped NaLuF4 microtubes emit bright three-primary-color (red, green, and blue) light under 980 nm laser diode excitation undergoing an upconversion. Mechanisms for the excitation and emission were analyzed based on the emission spectra, the plot of luminescence intensity to pump power and a simplified energy level diagram.

Mn-doped Bi 3.15 Nd 0.85 Ti 3 O 12 (BNT), i.e., Bi 3.15 Nd 0.85 Ti 3 − x Mn x O 12 (BNTM, x = 0, 0.005, 0.01, 0.03, 0.05, and 0.1) thin films with bismuth-layered perovskite structure were prepared on Pt/Ti/SiO 2 /Si(100) substrates by a chemical solution deposition method at 700 °C. The crystal structures of BNTM films were analyzed by X-ray diffraction and the surface morphologies were observed by field emission scanning electron microscopy. The effects of Mn contents on the microstructures and dielectric properties of BNTM films are investigated in detail. Among these BNTM films, it is found that the BNTM01 ( x = 0.01) film exhibits the highest dielectric tunability and dielectric constant but the lowest dielectric loss. Compared with BNT film, the BNTM01 film has lower leakage current density. Eventually, the mechanism involved in the Mn doping effect on the electrical properties of the BNT films is discussed.

The cubic Gd 2 O 3 :20% Yb 3+ , 2% Er 3+ nanotubes have been synthesized via a facile hydrothermal method and subsequently calcination. Field emission scanning electron microscopy images show that as-obtained Gd 2 O 3 entirely consists of uniform nanotubes with outermost diameters of ~ 250 nm and lengths of ~ 2 μm. The corresponding Gd 2 O 3 :20% Yb 3+ , 2% Er 3+ nanotubes exhibit completely paramagnetism and the magnetic mass susceptibility of them is about 0.28 × 10 − 4 emu g − 1 Oe − 1 at 300 K. Pure red light ranged from 645 nm to 685 nm in the nanotubes was observed and assigned to the 4 F 9/2 → 4 I 15/2 transition of Er 3+ under excitation of 980 nm undergoing an upconversion and the emission intensity was found to decrease in applied magnetic field.

Both the linear and nonlinear optical properties of Bi3.15Nd0.85Ti3O12 (BNT0.85) ferroelectric thin films deposited on quartz substrates were investigated. The fundamental optical constants were determined as a function of light wavelength by optical transmittance measurements. By performing single-beam Z-scan experiments with femtosecond laser pulses at a wavelength of 800 nm, the two-photon absorption (TPA) coefficient β and third-order nonlinear refraction index γ were measured to be 1.15 × 102 cm/GW and −8.15 × 10−3 cm2/GW, respectively. The large TPA is attributed to an indirect transition process via the intermediate energy levels and the large refractive nonlinearity is the result of the electronic polarization and ferroelectric polarization arisen from the femtosecond midinfrared radiation. The results indicate that the BNT0.85 thin film is a promising candidate for applications in nonlinear photonic devices.

Thin films of Nd3+∕Zr4+ cosubstituted Bi4Ti3O12 (BIT), i.e., Bi3.15Nd0.85Ti2.8Zr0.2O12 (BNTZ), were fabricated on Pt∕Ti∕SiO2∕Si(100) substrates by chemical solution deposition and annealed at different temperatures of 600, 650, 700, and 800°C. The effects of annealing temperature on the microstructure, leakage current, ferroelectric, and dielectric properties of the BNTZ films were investigated in detail. Significantly, compared with the Bi3.15Nd0.85Ti3O12 film, the BNTZ thin film has a lower coercive field (2Ec) and leakage current density and a slightly larger remnant polarization (2Pr). It shows that Nd3+∕Zr4+ cosubstitution in BIT film might be an effective way to improve ferroelectric properties of BIT.

Thin films of Nd and Mn cosubstituted bismuth titanate, i.e., Bi3.15Nd0.85(Ti3−xMnx)O12 (BNTM) (x=0, 0.005, 0.01, 0.03, 0.05, and 0.1), were fabricated on Pt∕Ti∕SiO2∕Si(100) substrates at 700°C by a chemical solution deposition technique. The structures of the films were analyzed using x-ray diffraction and Raman spectroscopy. These films possessed preferred (117) and (00l)-oriented polycrystalline structures. The ferroelectric properties of BNTM films were systematically investigated as a function of the Mn content. It is found that a low concentration substitution with manganese ions in Bi3.15Nd0.85Ti3O12 greatly enhances the remnant polarization (2Pr) and reduces the coercive field (2Ec) of the film. The 2Pr and 2Ec are 78μC∕cm2 and 205kV∕cm, respectively. No fatigue phenomenon is also observed for the BNTM film with x=0.01 up to 1.5×1010 switching cycles.