Iron-doped Lithium Research Articles

Iron-doping regulated light absorption and active sites in LiTaO3 single crystal for photocatalytic nitrogen reduction

In contrast to research on active sites in nanomaterials, lithium tantalate single crystals, known for their exceptional optical properties and long-range ordered lattice structure, present a promising avenue for in-depth exploration of photocatalytic reaction systems with fewer constraints imposed by surface chemistry. Typically, the isotropy of a specific facet provides a perfect support for studying heteroatom doping. Herein, this work delves into the intrinsic catalytic sites for photocatalytic nitrogen fixation in iron-doped lithium tantalate single crystals. The presence of iron not only modifies the electronic structure of lithium tantalate, improving its light absorption capacity, but also functions as an active site for the nitrogen adsorption and activation. The photocatalytic ammonia production rate of the iron-doped lithium tantalate in pure water is maximum 26.95 μg cm−2 h−1, which is three times higher than that of undoped lithium tantalate. The combination of first-principles simulations with in situ characterizations confirms that iron doping promotes the rate-determining step and changes the pathway of hydrogenation to associative alternating. This study provides a new perspective on in-depth investigation of intrinsic catalytic active sites in photocatalysis and other catalytic processes.

Trifurcated Splitting of Water Droplets on Engineered Lithium Niobate Surfaces.

Controlled splitting of liquid droplets is a key function in many microfluidic applications. In recent years, various methodologies have been used to accomplish this task. Here, we present an optofluidic technique based on an engineered surface formed by coating a z-cut iron-doped lithium niobate crystal with a lubricant-infused layer, which provides a very slippery surface. Illuminating the crystal with a light spot induces surface charges of opposite signs on the two crystal faces because of the photovoltaic effect. If the light spot is sufficiently intense, millimetric water droplets placed near the illuminated spot split into two charged fragments, one fragment being trapped by the bright spot and the other moving away from it. The latter fragment does not move randomly but rather follows one of three well-defined trajectories separated by 120°, which reflect the anisotropic crystalline structure of Fe:LiNbO3. Numerical simulations explain the behavior of water droplets in the framework of the forces induced by the interplay of pyroelectric, piezoelectric, and photovoltaic effects, which originate simultaneously inside the illuminated crystal. Such a synergetic effect can provide a valuable feature in applications that require splitting and coalescence of droplets, such as chemical microreactors and biological encapsulation and screening.

Optically induced fabrication of three-dimensional photonic lattice microstructures by using cutting-combination lens

A convenient method for producing three-dimensional photonic lattice microstructures was experimentally demonstrated by using a cutting-combination lens. Three-dimensional photonic lattices are optically induced by (4 + 1) plane waves interference in an iron-doped lithium niobate photorefractive crystal. Induced three-dimensional lattice microstructures are observed and verified by planar guided wave intensity imaging and far-field diffraction pattern. This method is inexpensive and does not require complex adjustment devices. In addition, it also has flexibility and scalability. This method can be used to fabricate other more complex three-dimensional photonic lattice microstructures by designing the cutting-combination lens appropriately.

Running streams of a ferroelectric nematic liquid crystal on a lithium niobate surface

ABSTRACTSessile droplets of a ferroelectric nematic liquid crystalline material were exposed to surface electric fields produced by pyroelectric and photogalvanic (photovoltaic) effects in X-cut iron-doped lithium niobate crystals. The resulting dynamic processes were monitored by polarisation optical (video)microscopy (POM). During heating/cooling cycles, at first, the droplets change their shape from spherical to extended ellipsoidal. Then, they start to move rapidly along the surface electric field, i.e. along the crystal’s polar axis (c-axis). During this motion, several droplets merge into running streams (tendrils) extending towards the edges of the top surface area. Finally, practically all liquid crystalline material is transported from the top surface to the side surfaces of the crystal. At stabilised temperatures, laser illumination of the assembly causes dynamic processes that are localised to the illuminated area. Also, in this case, the LC droplets merge into several tendril-like formations that are preferentially oriented along the c-axis of the crystal. The pattern of tendrils fluctuates with time, but it persists as long as the illumination is present. In this case, the LC material is transported between the central and the edge regions of the illuminated area.

Light-induced dynamics of liquid-crystalline droplets on the surface of iron-doped lithium niobate crystals

We investigated the effect of a photovoltaic field generated on the surface of iron-doped lithium niobate crystals on sessile droplets of a ferroelectric nematic liquid crystalline and a standard nematic liquid crystalline material present on this surface. When such an assembly is illuminated with a laser beam, a wide range of dynamic phenomena are initiated. Droplets located outside the laser spot are dragged in the direction of the illuminated area, while droplets located inside the illuminated region tend to bridge each other and rearrange into tendril-like structures. In the ferroelectric nematic phase (NF), these processes take place via the formation of conical spikes evolving into jet streams, similar to the behavior of droplets of conventional dielectric liquids exposed to overcritical electric fields. However, in contrast to traditional liquids, the jet streams of the NF phase exhibit profound branching. In the nematic phase (N) of both the ferroelectric nematic and the standard nematic material, dynamic processes occur via smooth-edged continuous features typical for conventional liquids subjected to under-critical fields. The difference in dynamic behavior is attributed to the large increase of dielectric permittivity in the ferroelectric nematic phase with respect to the dielectric permittivity of the nematic phase.

Determination of the Dielectrophoretic Force Induced by the Photovoltaic Effect on Lithium Niobate.

The actuation of droplets on a surface is extremely relevant for microfluidic applications. In recent years, various methodologies have been used. A promising solution relies on iron-doped lithium niobate crystals that, when illuminated, generate an evanescent electric field in the surrounding space due to the photovoltaic effect. This field can be successfully exploited to control the motion of water droplets. Here, we present an experimental method to determine the attractive force exerted by the evanescent field. It consists of the analysis of the elongation of a pendant droplet and its detachment from the suspending syringe needle, caused by the illumination of an iron-doped lithium niobate crystal. We show that this interaction resembles that obtained by applying a voltage between the needle and a metallic substrate, and a quantitative investigation of these two types of actuation yields similar results. Pendant droplet tensiometry is then demonstrated to offer a simple solution for quickly mapping out the force at different distances from the crystal, generated by the photovoltaic effect and its temporal evolution, providing important quantitative data for the design and characterization of optofluidic devices based on lithium niobate crystals.

All-photonic synapse based on iron-doped lithium niobate double metal-cladding waveguides

In an artificial neural network composed of neuromorphic computing units, the resistance of the memristor can be modulated dynamically and repeatedly under external stimuli, such as electric fields, magnetic fields, and light illumination, leading to variations in local conductivity and memory effects. Here we show, using an all-photonic memristor, that the memristance arises naturally in an optical system in which solid-state ionic transport is realized under low-power external incident light. These results show an extremely stable multilevel storage weight and a high signal-to-noise ratio. In all-photonic memristors, the light signal is employed as the extra stimuli of the memristor devices to ensure a large memory window and a variation margin of multiple storage levels.

Laser inscribed waveguide optical isolators in iron-doped lithium niobate.

There is a growing need for optical isolators that do not require a magnetic field, especially for uses such as on-chip optical devices and cold atom physics. As one approach, we propose using waveguides in photorefractive materials, such as Fe:LiNbO3, as optical isolator devices due to their unique asymmetric transmission properties that allow low loss transmission in one crystal orientation and attenuation in the flipped orientation. We utilize ultrafast laser inscription to fabricate photorefractive depressed cladding waveguides in Fe:LiNbO3 along the crystal c axis to demonstrate the operation of Fe:LiNbO3 waveguide optical isolators. We show the ability to write transmission and reflection gratings into these waveguides that provide an isolation ratio of approximately 5000:1 per cm of path length.

Enhanced Li+ adsorption by magnetically recyclable iron-doped lithium manganese oxide ion-sieve: Synthesis, characterization, adsorption kinetics and isotherm

Abstract The Li+ adsorption from aqueous solution by lithium-ion sieve has become one of the most promising methods due to the high efficiency and selectivity towards lithium ion (Li+). However, the industrial application of manganese oxide ion-sieve is limited due to its difficult separation and decrease of adsorption capacity resulting from manganese dissolution loss. In this paper, the magnetically recyclable Fe-doped manganese oxide lithium ion-sieves with spinel-structure were proposed and prepared from LiMn2-xFexO4 synthesized by solid state reaction method. The effects of calcination temperature, calcination time and Fe doping amounts on the phase compositions, dissolution loss and adsorption performance of lithium ion-sieve precursors were systematically studied, and the influences of solution pH value, initial Li+ concentration and adsorption temperature on the adsorption performance were investigated. The adsorption mechanism was further discovered through adsorption kinetics and thermodynamics. The results show that the adsorption capacity of lithium ion-sieves could reach to 34.8 mg·g–1 when the calcination temperature, time and Fe doping content were controlled at 450 °C, 6 h, and 0.05, respectively. The Mn dissolution loss was reduced to 0.51%, much lower than the undoped lithium ion-sieve (2.48%), which is attributed to the inhibition of disproportionation reaction with the increasing proportion of Mn4+ in the skeleton. The adsorption process conformed to the pseudo-second-order kinetics equation and Langmuir isothermal adsorption model. Furthermore, the recycling performance of Fe-doped lithium ion-sieve showed that the adsorption capacity could remain 22.5 mg·g–1 (about 70%) after five cycles, which is greater than that of undoped lithium ion-sieve (about 50%), and the recovery of lithium ion-sieve can be realized by magnetic separation in an applying magnetic field.

Bulk piezo-photovoltaic effect in LiNbO3

The deformations arising in iron-doped lithium niobate crystals (LiNbO3:Fe) under laser illumination were registered by changes in the parameters of X-ray diffraction rocking curves. The piezoelectric deformation as a result of the bulk photovoltaic effect was separated from heating according to the different kinetics of these processes using a set of time-resolved X-ray diffraction techniques. The dependence of the peak shift on the diffraction order confirmed the manifestation of the bulk piezo-photovoltaic effect. Additionally, the measurements of an external electric field influence on the diffraction peaks parameters were carried out as well as the measurements of the electrophysical and bulk photovoltaic characteristics of the crystal.

Optical induction of two-dimensional photorefractive photonic microstructures using six wedge prisms array

We experimentally demonstrate a convenient method to fabricate two-dimensional photonic microstructures in photorefractive crystal. Three kinds of photonic microstructures have been fabricated in iron-doped lithium niobate crystal using six wedge prisms array. The experimental setup of our method does not require complicated optical alignment components. Induced two-dimensional triangular lattice photonic microstructures are analyzed and validated by plane wave guiding, far-field diffraction pattern imaging, Brillouin-zone spectroscopy, and angle-dependent transmission spectrum. The method can be easily extended to fabricate other complex photonic microstructures by designing different wedge prisms array and adjusting polarization and phase delay properly.

Optoelectronic generation of bio-aqueous femto-droplets based on the bulk photovoltaic effect.

The generation and manipulation of small aqueous droplets is an important issue for nano- and biotechnology, particularly, when using microfluidic devices. The production of very small droplets has been frequently carried out by applying intense local electric fields to the fluid, which requires power supplies and metallic electrodes. This procedure complicates the device and reduces its versatility. In this work, we present a novel and flexible, to the best of our knowledge, electrodeless optoelectronic method for the production of tiny droplets of biologically friendly aqueous fluids. Our method takes advantage of the photoinduced electric fields generated by the bulk photovoltaic effect in iron-doped lithium niobate crystals. Two substrate configurations, presenting the polar ferroelectric axis either parallel or perpendicular to the active surface, have been successfully tested. In both crystal geometries, small droplets on the femtoliter scale have been obtained, although with a different spatial distributions correlated with the symmetry of the photovoltaic fields. The overall results demonstrate the effectiveness of the optoelectronic method to produce femtoliter droplets, both with pure water and with aqueous solutions containing biological material.

Iron-Doped Lithium Tantalate Thin Films Deposited by Magnetron Sputtering: A Study of the Iron Role in the Structure and the Derived Magnetic Properties

Fe-doped LiTaO3 thin films with a low and high Fe concentration (labeled as LTO:Fe-LC and LTO:Fe-HC, respectively) were deposited by magnetron sputtering from two home-made targets. The dopant directly influenced the crystalline structure of the LiTaO3 thin films, causing the contraction of the unit cell, which was related to the incorporation of Fe3+ ions into the LiTaO3 structure, which occupied Li positions. This substitution was corroborated by Raman spectroscopy, where the bands associated with Li-O bonds broadened in the spectra of the samples. Magnetic hysteresis loops, zero-field cooling curves, and field cooling curves were obtained in a vibrating sample magnetometer. The LTO:Fe-HC sample demonstrates superparamagnetic behavior with a blocking temperature of 100 K, mainly associated with the appearance of Fe clusters in the thin film. On the other hand, a room temperature ferromagnetic behavior was found in the LTO:Fe-LC layer where saturation magnetization (3.80 kAm−1) and magnetic coercivities were not temperature-dependent. Moreover, the crystallinity and morphology of the samples were evaluated by X-ray diffraction and scanning electron microscopy, respectively.

Optical in-situ study of the redox processes in LiNbO3: Fe crystals

Resultsare reported of an in-situ investigation of optical absorption of iron-doped lithium niobate single crystals, LiNbO3:Fe, at high temperatures. LiNbO3:Fe has been exposed to treatments in reducing (95%Ar + 5%H2) and oxidizing (O2) atmospheres at temperatures up to 1100 K. The high temperature spectra have been discussed in respect to the electronic defects giving rise to absorption. In addition, the time-dependent evolution of optical spectra upon rapid replacement of gas atmospheres has been followed at fixed wavelength. The experimental reaction kinetics is discussed in terms of oxygen vacancy diffusion accounting for the associated changes in the point defect subsystem of the material.

Theoretical Prediction of Umbilics Creation in Nematic Liquid Crystals with Positive Dielectric Anisotropy.

Optically assisted electrical generation of umbilic defects, arising in homeotropically aligned nematic liquid crystal cells and known as topological templates for the generation of optical vortices, are reported in nematic liquid crystals with positive dielectric anisotropy in detail. It is shown that nematic liquid crystals with positive dielectric anisotropy can serve as a stable and efficient medium for the optical vortex generation from both linearly and circularly polarized input Gaussian beams. Hybrid cells made from a thin layer of nematic liquid crystal confined between a photoresponsive slab of iron-doped lithium niobate and a glass plate coated with an active material, i.e., indium tin oxide, were studied. Exposure to a laser beam locally induces a photovoltaic field in the iron-doped lithium niobate substrate, which can penetrate into the liquid crystal film and induce realignment of molecules. The photovoltaic field drives charge carrier accumulation at the interface of indium tin oxide with the liquid crystal, which effectively modifies the shape and symmetry of the electric field. The photovoltaic field has a continuous radial distribution in the transverse xy-plane, weakening with increasing distance from the light irradiation center, where the electric field is normal to the cell plane. Umbilics are created as a result of the liquid crystal tendency to realign parallel to the electric field. Numerical studies of the transmitted intensity profiles in between linear polarizers reveal optical vortex pattern (of four and eight brushes) characteristics for the umbilical defects. The application of crossed circular polarizers results in annular-shaped intensity patterns as a result of spin-to-orbital angular momentum conversions, which give rise to the optical vortices.

Recovery of Fe, Mn, Ni and Co in sulfuric acid leaching liquor of spent lithium ion batteries for synthesis of lithium ion-sieve and NixCoyMn1-x-y(OH)2

A comprehensive utilization process of Fe, Mn, Ni and Co in the acid leaching liquor of spent lithium ion batteries was put forward. Fe and part of Mn were extracted to synthesize Fe-doped lithium ion-sieve. Ni, Co and the rest of Mn were coprecipitated to prepare ternary lithium ion battery cathode material precursor NixCoyMn1-x-y(OH)2. After removing Cu2+ ions in acid leaching liquor by adding MnS, two-stage counter current extraction and one-stage stripping were used to extract all Fe3+ and 40.3% of Mn2+ ions in sulfuric acid leaching solution to obtain a MnSO4 stripping solution containing Fe3+ ions. Iron-doped lithium ion-sieve was synthesized by oxidation of the resulting MnSO4 solution with KMnO4 followed by roasting and acid leaching. Fe incorporates into the lattice of lithium ion-sieve and improves the structure stability. Fe-doped lithium ion-sieve shows Li adsorption capacity of 35.29 mg.g−1 and Mn dissolution loss of 1.66%. The Li+ adsorption in salt lake brine with the Fe-doped ion-sieve follows a pseudo-second-order kinetic model. NixCoyMn1-x-y(OH)2 is regular spherical particles with β-Ni(OH)2-like structure. This process provides a simple method for recovery and utilization of Mn, Fe, Ni and Co in spent lithium ion batteries.

Pulse-induced transient blue absorption related with long-lived excitonic states in iron-doped lithium niobate

Transient absorption is studied in Fe-doped lithium niobate single crystals with the goal to control and probe a blue absorption feature related with excitonic states bound to Fe$_\textrm {Li}$Li defect centers. The exciton absorption is deduced from the comparison of ns-pump, supercontinuum-probe spectra obtained in crystals with different Fe-concentration and Fe$^{2+/3+}_\textrm {Li}$Li2+/3+-ratio, at different pulse peak and photon energies as well as by signal separation taking well-known small polaron absorption bands into account. As a result, a broad-band absorption feature is deduced being characterized by an absorption cross-section of up to $\sigma ^\textrm {max}(2.85$σmax(2.85 eV$) = (4\pm 2)\cdot 10^{-22}$)=(4±2)⋅10−22 m$^{2}$2. The band peaks at about 2.85 eV and can be reconstructed by the sum of two Gaussians centered at 2.2 eV (width $\approx 0.5$≈0.5 eV) and 2.9 eV (width $\approx 0.4$≈0.4 eV), respectively. The appropriate build-up and decay properties strongly depend on the crystals’ composition as well as the incident pulse parameters. All findings are comprehensively analyzed and discussed within the model of $\textrm {Fe}^{2+}_{\textrm {Li}}-\textrm {O}^{-}-\textrm {V}_{\textrm {Li}}$FeLi2+−O−−VLi excitonic states.

A polaron approach to photorefractivity in Fe : LiNbO3

The thermally activated, incoherent hopping of small electron polarons generated by continuous illumination in iron-doped lithium niobate is simulated by a Marcus-Holstein model for which all the input parameters are known from literature. The results of the calculations are compared with a comprehensive set of data obtained from photorefractive, photogalvanic and photoconductive measurements under green light excitation on samples with different doping levels and stoichiometries in the temperature range between and room temperature. We show that the temperature and composition dependence of the photorefractive observables can be interpreted by a change in the abundance of the different hop types that a polaron performs before being captured by a deep Fe trap. Moreover, by a comparison between experimental and numerical data we obtain new insights on the initial photo-excitation part of the photorefractive process. In particular all results are consistent if a single value of the photogalvanic length is assumed for all the samples and all the temperatures. The photo-generation efficiency ϕ under green light excitation (somewhere denoted as quantum efficiency) is also estimated. It appears to decrease from 10%–15% at room temperature to about 5% at 150 K. This behavior is qualitatively interpreted in terms of a temperature-dependent re-trapping probability of the light-emitted particles from the initial Fe donor center.

Transient transmission oscillations in doped and undoped lithium niobate induced by near-infrared femtosecond pulses

Abstract

Fabrication of the photonic lattices with the method of multiple groups of double beam alternate interference

In this paper, we demonstrate an approach of fabricating photonic lattices by designing a rotating beam selector, and we name the approach as multiple groups of double beam alternate interference. This method simplifies the interference configuration of multiple beams. And the structure of light field induced by this method has more concise symmetry compared to that obtained directly by the interference of multiple beams. And we can also analyze the interference beams configuration that produce the lattices as long as we know the structure of the lattices. We experimentally fabricate two-dimensional square photonic lattices in self-defocusing iron-doped lithium niobate photorefractive crystal by two groups of double beam alternate interference. And we analyze the structure of two-dimensional square photonic lattices by plane wave guiding, Brillouin zone spectroscopy and far field pattern and we make computer simulation of the light field pattern. Besides, we design a rotating beam selector and obtain the computer simulation pattern of five groups of double beam alternate interference.