Oxide Single Crystals Research Articles

Scalable Synthesis of Holey Deficient 2D Co/NiO Single-Crystal Nanomeshes via Topological Transformation for Efficient Photocatalytic CO2 Reduction.

Preparation of holey, single-crystal, 2D nanomaterials containing in-plane nanosized pores is very appealing for the environment and energy-related applications. Herein, an in situ topological transformation is showcased of 2D layered double hydroxides (LDHs) allows scalable synthesis of holey, single-crystal 2D transition metal oxides (TMOs) nanomesh of ultrathin thickness. As-synthesized 2D Co/NiO-2 nanomesh delivers superior photocatalytic CO2 -syngas conversion efficiency (i.e., VCO of 32460µmol h-1 g-1 CO and of 17840µmol h-1 g-1 H2 ), with VCO about 7.08 and 2.53 times that of NiO and 2D Co/NiO-1 nanomesh containing larger pore size, respectively. As revealed in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the high performance of Co/NiO-2 nanomesh primarily originates from the edge sites of nanopores, which carry more defect structures (e.g., atomic steps or vacancies) than basal plane for CO2 adsorption, and from its single-crystal structure adept at charge transport. Theoretical calculation shows the topological transformation from 2D hydroxide to holey 2D oxide can be achieved, probably since the trace Co dopant induces a lattice distortion and thus a sharp decrease of the dehydration energy of hydroxide precursor. The findings can advance the design of intriguing holey 2D materials with well-defined geometric and electronic properties.

Enhanced CO oxidation with cobalt-impregnated porous single-crystal manganese oxides

In this work, Mn2O3, Mn3O4 and MnO single crystals are prepared by using MnCO3 as the precursor. In addition, Co element is loaded on the three manganese oxide single crystals for the CO oxidation reaction.

Research progress of applications of freestanding single crystal oxide thin film

Flexible electronics have aroused great interest of researchers because of their wide applications in information storage, energy harvesting and wearable device. To realize extraordinary functionalities, freestanding single crystal oxide thin film is utilized due to its super elasticity, easy-to-transfer, and outstanding ferro/electric/magnetic properties. Using the state-of-art synthesis methods, functional oxide films of various materials can be obtained in freestanding phase, which eliminates the restrictions from growth substrate and is transferable to other flexible layers. In this work, we first introduce wet etching and mechanical exfoliation methods to prepare freestanding single crystal oxide thin film, then review their applications in ferroelectric memory, piezoelectric energy harvester, dielectric energy storage, correlated oxide interface, and novel freestanding oxide structure. The recent research progress and future outlooks are finally discussed.

A practical guide to pulsed laser deposition.

Nanoscale thin films are widely implemented across a plethora of technological and scientific areas, and form the basis for many advancements that have driven human progress, owing to the high degree of functional tunability based on the chemical composition. Pulsed laser deposition is one of the multiple physical vapour deposition routes to fabricate thin films, employing laser energy to eject material from a target in the form of a plasma. A substrate, commonly a single-crystal oxide, is placed in the path of the plume and acts as a template for the arriving species from the target to coalesce and self-assemble into a thin film. This technique is tremendously useful to produce crystalline films, due to the wide range of atmospheric conditions and the extent of possible chemical complexity of the target. However, this flexibility results in a high degree of complexity, oftentimes requiring rigorous optimisation of the growth parameters to achieve high quality crystalline films with desired composition. In this tutorial review, we aim to reduce the complexity and the barrier to entry for the controlled growth of complex oxides by pulsed laser deposition. We present an overview of the fundamental and practical aspects of pulsed laser deposition, discuss the consequences of tailoring the growth parameters on the thin film properties, and describe in situ monitoring techniques that are useful in gaining a deeper understanding of the properties of the resultant films. Particular emphasis is placed on the general relationships between the growth parameters and the consequent structural, chemical and functional properties of the thin films. In the final section, we discuss the open questions within the field and possible directions to further expand the utility of pulsed laser deposition.

Electronic and thermal properties of Ag-doped single crystal zinc oxide via laser-induced technique

The doping of ZnO has attracted lots of attention because it is an important way to tune the properties of ZnO. Post-doping after growth is one of the efficient strategies. Here, we report a unique approach to successfully dope the single crystalline ZnO with Ag by the laser-induced method, which can effectively further post-treat grown samples. Magnetron sputtering was used to coat the Ag film with a thickness of about 50 nm on the single crystalline ZnO. Neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was chosen to irradiate the Ag-capped ZnO samples, followed by annealing at 700 °C for two hours to form ZnO:Ag. The three-dimensional (3D) information of the elemental distribution of Ag in ZnO was obtained through time-of-flight secondary ion mass spectrometry (TOF-SIMS). TOF-SIMS and core-level x-ray photoelectron spectroscopy (XPS) demonstrated that the Ag impurities could be effectively doped into single crystalline ZnO samples as deep as several hundred nanometers. Obvious broadening of core level XPS profiles of Ag from the surface to depths of hundred nms was observed, indicating the variance of chemical state changes in laser-induced Ag-doped ZnO. Interesting features of electronic mixing states were detected in the valence band XPS of ZnO:Ag, suggesting the strong coupling or interaction of Ag and ZnO in the sample rather than their simple mixture. The Ag-doped ZnO also showed a narrower bandgap and a decrease in thermal diffusion coefficient compared to the pure ZnO, which would be beneficial to thermoelectric performance.

Limpet teeth microstructure unites auxeticity with extreme strength and high stiffness.

Materials displaying negative Poisson's ratio, referred to as auxeticity, have been found in nature and created in engineering through various structural mechanisms. However, uniting auxeticity with high strength and high stiffness has been challenging. Here, combining in situ nanomechanical testing with microstructure-based modeling, we show that the leading part of limpet teeth successfully achieves this combination of properties through a unique microstructure consisting of an amorphous hydrated silica matrix embedded with bundles of single-crystal iron oxide hydroxide nanorods arranged in a pseudo-cholesteric pattern. During deformation, this microstructure allows local coordinated displacement and rotation of the nanorods, enabling auxetic behavior while maintaining one of the highest strengths among natural materials. These findings lay a foundation for designing biomimetic auxetic materials with extreme strength and high stiffness.

Formation of optically-active disordered layers in (001) MgO single crystals

The formation of defective layers and their associated optical properties in (001) magnesium oxide (MgO) single crystals were examined in this study. To form defective layers, 100 keV beams with different doses (1015, 5 × 1015, and 1016 ions/cm2) of atomic nitrogen ions (N+) were used. Following the ion implantation, the surface roughness remained unaffected, while a structurally and chemically distinct surface layer was formed in the MgO single crystal. Spectroscopic methods indicated that defects in the layer consist of nitrogen ions and oxygen vacancies. Moreover, the resultant disordering triggered the softening of optical phonons. Upon using the magnetic resonance technique, it was observed that the F+ center created by oxygen vacancies did not contribute to this optical phenomenon.

Single-crystalline Mn-based oxide as a high-rate and long-life cathode material for potassium-ion battery

Mn-based oxides are promising cathode materials for potassium-ion batteries due to their high theoretical capacity and abundant raw materials. However, the anisotropic properties of their conventional polycrystalline structures lead to insufficient rate capability and cycle life. Here, a single-crystal Mn-based layered oxide, P3′-type K0.35Mn0.8Fe0.1Cu0.1O2 (KMFCO), is designed and synthesized through a bimetallic co-induction effect and used as a cathode for potassium-ion battery. Benefiting from a unique single-crystal structure that is devoid of grain boundaries, it achieves a higher K+ transport rate and a reduced volume change during the K+ intercalation/deintercalation process. Accordingly, the single-crystal P3′-type KMFCO delivers superior rate capability (52.9 ​mAh ​g−1 at 1000 ​mA ​g−1) and excellent cycling stability (91.1% capacity retention after 500 cycles at 500 ​mA ​g−1). A full cell assembled with the P3′-type KMFCO cathode and a graphite anode also exhibits a high reversible capacity (81.2 ​mAh ​g−1 at 100 ​mA ​g−1) and excellent cycling performance (97% capacity retention after 300 cycles). The strategy of developing single-crystal materials may offer a new pathway for maintaining structural stability and improving the rate capability of layered manganese oxide cathodes and beyond.

Ultrafast modulation of a high harmonic generation in a bulk ZnO single crystal.

Optical modulation of high harmonic generation (HHG) is of fundamental interest in science and technology, which can facilitate understanding of HHG generation mechanisms and expand the potential optoelectronic applications. However, the current established works have neither shown the advanced modulation performance nor provided a deep understanding of modulation mechanisms. In this work, taking wurtzite zinc oxide (ZnO) single crystal as a prototype, we have demonstrated an all-optical intensity modulation of high-order HHG with a response time of less than 0.2 ps and a depth of more than 95%, based on the pump-probe configuration with two different pumping wavelengths. Besides the achieved excellent modulation performance, we have also revealed that the modulation dynamics in ZnO single crystal highly depend on the excitation conditions. Specifically, the modulation dynamics with the near-bandgap or above-bandgap excitation are attributed to the non-equilibrium interband carrier relaxations, while for mid-gap excitation, the modulation dynamics are dominated by the nonlinear frequency mixing process. This work may enhance the current understanding of the HHG modulation mechanism and enlighten novel device designs.

(Digital Presentation) Understanding of Single-Crystal Fabrication of High-Nickel Layered Oxide through Variation of Heating Process

Single-crystal processing of layered oxide cathode has received great attention recently. It has been recognized that Ni-rich layered oxide with grain size in mm range tends to provide better cycling performance due to better structural integrity against volume changing during charge/discharge cycles.However, during heating/calcination process of oxide/carbonates/nitrates powder compacts, it may involve decomposition, chemical reactions, diffusion, oxidation, sintering, grain-growth.For grain growth to take place at final stage of sintering, the powder compacts need to be heated to higher temperatures for certain period of time. On the other hand, the formation of Ni-rich layered oxides need air/oxygen for Ni oxidation and better crystallization. Thus, it is crucial to investigate the processing of single-crystal Ni-rich layered oxide based on the sintering behavior, thermal gravimetric analysis, and phase transformation. Furthermore, the lithiation/oxidation of Ni-containing oxides during single-crystal processing may involve charge compensation resulted from defect reactions. In this study, the affected electrical, optical, structural, and electrochemical properties of layered oxides will be discussed based on defect chemistry consideration. The structural, microstructural and electrical properties of decomposed precursors and doped layered oxides will be examined using XRD, SEM and DC/AC impedance measurements. CV and charge/discharge tests from assembled coin cells will be used to evaluate electrochemical properties of single-crystal Ni-rich layered oxides.

Superior Room Temperature Compressive Plasticity of Submicron Beta‐Phase Gallium Oxide Single Crystals

AbstractBulk‐scale (01)‐oriented monoclinic beta‐phase gallium oxide (β‐Ga2O3) single crystals are brittle and fracture at low compressive strains. Here, it is reported that submicron β‐Ga2O3 pillars exhibit an exceptional room temperature plastic strain of up to ≈22% under compression. Deformation is observed in transition from brittle to superior plasticity with reduction of pillar size. The critical diameter for the brittle to ductile transition is ≈800 nm, attributed to the initiation of dislocation slip on the primary (400) slip planes. Below 500 nm, a second transition is reported to superior plastic deformation, achieved through the activation of secondary mechanisms due to both deconfinement and low crystalline symmetry of β‐Ga2O3, differentiating this finding from size‐effected plasticity of other brittle materials where plasticity is attributed to dislocation slip on primary slip planes. Molecular dynamics simulation supports the proposed mechanism of pillar deconfinement where plastic deformation in larger pillars is solely dominated by planar defects on (400) slip planes, while secondary defects are induced for sufficiently small pillars. No plasticity is observed for equally dimensioned pillars tested on a (010)‐oriented surface, highlighting the importance of presenting crystallography on submicron plasticity in this material.

Enhancing structure and cycling stability of Ni-rich layered oxide cathodes at elevated temperatures via dual-function surface modification

High-nickel single-crystal layered oxide material has become the most promising cathode material for electric vehicle power battery due to its high energy density. However, this material still suffers from structural degradation during cycling and especially the severe interfacial reactions at elevated temperatures that exacerbate irreversible capacity loss. Here, a simple strategy was used to construct a dual-function Li1.5Al0.5Ge1.5P3O12 (LAGP) protective layer on the surface of the high-nickel single-crystal (SC) cathode material, leading to SC@LAGP material. The strong Al–O bonding effectively inhibits the release of lattice oxygen (O) at elevated temperatures, which is supported by the positive formation energy of O vacancy from first-principal calculations. Besides, theoretical calculations demonstrate that the appropriate amount of Al doping accelerates the electron and Li+ transport, and thus reduces the kinetic barriers. In addition, the LAGP protective layer alleviates the stress accumulation during cycling and effectively reduces the erosion of materials from the electrolyte decomposition at elevated temperatures. The obtained SC@LAGP cathode material demonstrates much enhanced cycling stability even at high voltage (4.6 V) and elevated temperature (55 °C), with a high capacity retention of 91.3% after 100 cycles. This work reports a simple dual-function coating strategy that simultaneously stabilizes the structure and interface of the single-crystal cathode material, which can be applied to design other cathode materials.

Endotaxial α-Fe Nanoparticles in the High-Fluence Iron-Implanted Single-Crystal MgO

In this work, we report on the endotaxial growth of α-Fe nanoparticles in the near-surface layer under high-fluence iron ion implantation of the single-crystal magnesium oxide substrate. Comprehensive Mössbauer effect and magnetometry studies show that the implanted sample reveals a pronounced ferromagnetic response even at room temperature, and the α-Fe nanoparticles serve as its main source. The broad band at ~1000 Oe in the X-band magnetic resonance spectra originates from the α-Fe fraction. It manifests the properties of the easy-plane system with the four-fold in-plane anisotropy. The last indicates that the α-Fe nanoparticles are coherently incorporated into the host MgO matrix.

Effects of Au2+ irradiation induced damage in a high-entropy pyrochlore oxide single crystal

We report an ion-irradiation study of a compositionally complex (high-entropy) pyrochlore oxide. The damage produced from 4 MeV Au2+ ion irradiation on single crystal (Yb0.2Tm0.2Lu0.2Ho0.2Er0.2)2Ti2O7 aligned along the [100] direction is investigated at room temperature by Rutherford backscattering spectrometry in channeling mode (RBS/C). Damage profiles based on RBS/C are presented and compared to single-component pyrochlore titanate oxides to evaluate the relative resistance to irradiation-induced amorphization. The results show that this high-entropy pyrochlore goes amorphous at a dose of 0.13 dpa, which is comparable to that of single-component pyrochlores previously studied. Transmission electron microscopy images unveil the damaged surface layer, which is consistent with the RBS/C results.

Si integration of La0.7Sr0.3MnO3:BiFeO3 nanocomposite thin films with strong exchange bias coupling

Integrating functional oxide-based nanocomposite thin films on Si wafer is a large step toward their applications in electronics and spintronics. Here, to overcome the large lattice mismatch and potential inter-diffusion between the complex oxides and Si, a set of buffer layers of SrRuO3 (SRO)/TiN have been applied. The La0.7Sr0.3MnO3:BiFeO3 system has been selected to grow on the SRO/TiN buffered Si, and the nanocomposite films exhibit highly textured growth along the c direction for both phases. The microstructure study shows a typical vertically aligned nanocomposite structure of the films, which is similar to the same films on single crystal oxide substrates. Furthermore, magnetic measurements indicate the strong ferromagnetic–antiferromagnetic coupling in the nanocomposite system, which refers to the large exchange bias value HEB of ∼1800 Oe in the L0.5B0.5 sample under out-of-plane magnetic field. This work provides an effective approach for the Si integration of oxide-based nanocomposite thin films.

(Digital Presentation) Fabrication of Single-Crystal Layered Oxide Cathode through Variation on Heating Process

Since the announcement of a million-mile battery by Tesla, single-crystal processing of layered oxide cathode has received great attention recently. From single-crystal process, high Ni layered oxides with grain size in mm range tends to give better cycling performance due to better structural integrity against repeated charge/discharge cycles.However, during heating/calcination process of oxide/carbonates/nitrates powder compacts, it may involve decomposition, chemical reactions, diffusion, oxidation, sintering, grain-growth. For single-crystal process, the powder compacts need to be heated to higher temperatures (~950°C) for certain period of time. On the other hand, enough air/oxygen is need for Ni oxidation and better crystallization in Ni-rich layered structure. Therefore, it is important to investigate the processing of single-crystal Ni-rich layered oxide based on the sintering behavior, thermal gravimetric analysis, and phase transformation.Nevertheless, the lithiation/oxidation of Ni-containing oxides involves charge compensation resulted from defect reactions. As a result, the affected electrical, optical, structural, and electrochemical properties of layered oxides will be discussed based on defect chemistry consideration. The structural, microstructural and electrical properties of decomposed precursors and doped layered oxides will be examined using XRD, SEM and DC/AC impedance measurements. CV and charge/discharge tests from assembled coin cells will be used to evaluate electrochemical properties of single-crystal Ni-rich layered oxides.

Enhancement Mode Ga2O3 Field Effect Transistor with Local Thinning Channel Layer

β−Ga2O3 field−effect transistors (FETs) were fabricated with and without local thinning to change the threshold voltage. A 220 nm Ga2O3 layer was mechanically exfoliated from a Cr−doped gallium oxide single crystal. Approximately 45 nm Ga2O3 was etched by inductively coupled plasma to form the local thinning. The threshold voltage of the device with etched local thinning increased from −3 V to +7 V compared to the unetched device. The effect of the local thinning was analyzed by device simulation, confirming that the local thinning structure is an effective method to enable enhancement−mode Ga2O3 FETs.

Effects of Co/Mn Content Variation on Structural and Electrochemical Properties of Single-Crystal Ni-Rich Layered Oxide Materials for Lithium Ion Batteries.

The development of single-crystal nickel-rich layered LiNixCoyMn1-x-yO2 materials (S-NCMs) represents the most significant progress for the electrification applications of nickel-rich ternary materials. There has been prior research on the important role of transition metal elements in agglomerated materials, supplemented by surface and internal lattice optimization to drive the performance improvements. However, studies on S-NCMs, especially on the role of transition metals (TM, i.e., Co and Mn), have not been reported. In this study, we synthesized four kinds of S-NCMs with different Co/Mn contents and studied their structural, electrochemical, kinetic, and thermodynamic properties with different Co/Mn contents. The results were as follows: (1) Electrochemically, Co was more effective than Mn at 25 °C at enhancing the intercalation/deintercalation kinetics, which resulted in an increased discharge capacity, an improved rate capability, and a reduced energy loss. (2) Thermodynamically, Mn was more effective at maintaining a higher thermal stability than Co, especially at a low cutoff voltage, but at a high cutoff voltage, the difference between the action of Co and Mn decreased. The main finding of this work was the enhanced structural stability provided by Co, which could be attributed to the following: (i) the absence of the H2/H3 phase transformation when Co exceeded 15%, which inhibited the irreversible phase transformation and reduced the volume strain, and (ii) the lower degrees of decrease in the cell parameters a and c with higher contents of Co, which contributed to a low cracking degree along the (003) crystal plane. The current work provides an important reference for the single-crystallization strategy of nickel-rich materials.

Magnetism, spin-phonon coupling and Kitaev interaction in Mott insulator La2ZnIrO6 single crystal oxide

Due to the coexistence of spin-orbital coupling (SOC) and electron correlation interactions, double perovskite iridates are considered ideal material to investigate the Mott insulating states and Kitaev interactions. In this work, we studied the spin-orbital coupling (SOC), spin-phonon coupling (SPC), and Kitaev magnetism in antiferromagnetic La2ZnIrO6 single crystal by magnetometer and Raman scattering spectroscopy measurements. Based on the temperature dependence of the magnetization under different magnetic fields, we found that all the initial temperatures of the paramagnetic-ferromagnetic transition were basically concentrated around 8.8 K, while the temperature of ferromagnetic-antiferromagnetic transition continued to decrease from 7 K to 2 K. Anomalous modes of frequency softening at magnetic-order temperature were detected from the polarized Raman scattering spectra, indicating the presence of strong spin-phonon coupling and Kitaev interaction in La2ZnIrO6.

Radiation Resistance of Composite Scintillators Based on Grains of Oxide Single Crystals

Radiation Resistance of Composite Scintillators Based on Grains of Oxide Single Crystals