Single-crystal Membranes Research Articles

Gas-Liquid Interface Route to Hybrid Copper Bromine Perovskite Single-Crystal Membrane with Dielectric Transitions and Ferromagnetic Exchanges.

Single-crystal membranes (SCMs) show great promise in the fields of sensors, light-emitting diodes, and photodetection. However, the growth of a large-area single-crystal membranes is challenging. We report a new organic-inorganic SCMs [HCMA]2CuBr4 (HCMA = cyclohexanemethylamine) crystallized at the gas-liquid interface. It also has low-temperature ferromagnetic order, high-temperature dielectric anomalies, and narrow band gap indirect semiconductor properties. Specifically, the reversible phase transition of the compound occurs at 350/341 K on cooling/heating and exhibits dielectric anomalies and stable switching performance near the phase transition temperature. The ferromagnetic exchange interaction in the inorganic octahedra and the organic layer enables ferromagnetic ordering at low-temperature 10 K. Finally, the single crystal exhibits an indirect semiconducting property with a narrow band gap of 0.99 eV. Such rich multichannel physical properties make it a potential application in photodetection, information storage and sensors.

Long-Term Conductivity Stability of Electrolytic Membranes of Scandia Stabilized Zirconia Co-Doped with Ytterbia.

The effect of high-temperature aging for 4800 h at a temperature of 1123 K on the crystal structure and the conductivity of (ZrO2)0.90(Sc2O3)0.09(Yb2O3)0.01 and (ZrO2)0.90(Sc2O3)0.08(Yb2O3)0.02 single-crystal membranes were studied. Such membrane lifetime testing is critical to the operation of solid oxide fuel cells (SOFCs). The crystals were obtained by the method of directional crystallization of the melt in a cold crucible. The phase composition and structure of the membranes before and after aging were studied using X-ray diffraction and Raman spectroscopy. The conductivities of the samples were measured using the impedance spectroscopy technique. The (ZrO2)0.90(Sc2O3)0.09(Yb2O3)0.01 composition showed long-term conductivity stability (conductivity degradation not more than 4%). Long-term high-temperature aging of the (ZrO2)0.90(Sc2O3)0.08(Yb2O3)0.02 composition initiates the t″ → t' phase transformation. In this case, a sharp decrease in conductivity of up to 55% was observed. The data obtained demonstrate a clear correlation between the specific conductivity and the change in the phase composition. The (ZrO2)0.90(Sc2O3)0.09(Yb2O3)0.01 composition can be considered a promising material for practical use as a solid electrolyte in SOFCs.

Giant energy storage of flexible composites by embedding superparaelectric single-crystal membranes

Flexible organic-based composites embedding nanosheet-like inorganics with high energy storage density (U) are imperatively demanded for applications in portable electronics and sensors. However, the breakdown phases can easily bypass the discontinuous nanosheets, leading to the failure of conduction barriers. Here, continuous flexible Sm-doped BiFeO3-BaTiO3 (Sm-BFBT) membranes with superparaelectric characteristics were synthetized via etching a water-soluble Sr3Al2O6 buffer layer. With the single-crystal Sm-BFBT membranes embedded into poly(vinylidene fluoride), a giant U of 46.4 J/cm3 at 770 MV/m is achieved in the sandwich-structured composites, attributed to a formation of depolarized field induced by interfacial ions. Meanwhile, the composites exhibit a stable U of ∼20 J/cm3 at 550 MV/m during bending process, owing to the excellent flexibility of Sm-BFBT membranes originating from nanodomain switching. The proposed composites containing flexible 2D inorganic membranes offer unprecedented structural insights into the integration of high energy storage and stability of bending, and suggest potential uses in flexible energy storage devices.

Luminescence properties related anti-phase domain of alpha-Ga2O3

This work investigates the relationship between atomic arrangement and luminescence properties in a high-quality alpha-Ga2O3 thin film grown on an Al2O3 single-crystal membrane. The strain induced by merging domain boundaries shows more significant variability in annular darkfield images even though there is no additional gallium concentration confirmed. The bandgap energy of alpha-Ga2O3 is estimated to be 5.56 eV from the CL measurement in a transmission electron microscope. A peak at 320 nm was observed within the domain, while the domain boundary showed spectrum peaks with 380–480 nm. The anti-phase domain (APD) is formed by the instabilities of Al–O bonding templates provided by the Al2O3 substrate. The APD boundary gives a characteristic wavelength of 350 nm, which is the result of the merging boundary of in-phase and anti-phase domains.

Directional growth of quasi-2D Cu2O monocrystals on rGO membranes in aqueous environments.

The preparation technology of unconventional low-dimensional Cu2O monocrystals, which exhibit specific crystal planes and present significantly unique interfacial and physicochemical properties, is attracting increasing attention and interest. Herein, by integrating a high-temperature oxidation process under vacuum and a pure-water incubation process under ambient conditions, we propose the self-assembled growth and synthesis of quasi-two-dimensional Cu2O monocrystals on reduced graphene oxide (rGO) membranes. The prepared Cu2O crystals have a single (110) crystal plane, regular rectangular morphology, and potentially well conductivity. Experimental and theoretical results suggest that this assembly is attributed to the pre-nucleation clusters aggregation and directional attachment of Cu and O on the rGO membranes in aqueous environment and cation-π interactions between the (110) crystal plane of Cu2O and rGO surface. Our findings offer a potential avenue for the discovery and design of advanced low-dimensional single-crystal materials with specific interfacial properties in a pure aqueous environment.

Graphene nanopattern as a universal epitaxy platform for single-crystal membrane production and defect reduction.

Heterogeneous integration of single-crystal materials offers great opportunities for advanced device platforms and functional systems1. Although substantial efforts have been made to co-integrate active device layers by heteroepitaxy, the mismatch in lattice polarity and lattice constants has been limiting the quality of the grown materials2. Layer transfer methods as an alternative approach, on the other hand, suffer from the limited availability of transferrable materials and transfer-process-related obstacles3. Here, we introduce graphene nanopatterns as an advanced heterointegration platform that allows the creation of a broad spectrum of freestanding single-crystalline membranes with substantially reduced defects, ranging from non-polar materials to polar materials and from low-bandgap to high-bandgap semiconductors. Additionally, we unveil unique mechanisms to substantially reduce crystallographic defects such as misfit dislocations, threading dislocations and antiphase boundaries in lattice- and polarity-mismatched heteroepitaxial systems, owing to the flexibility and chemical inertness of graphene nanopatterns. More importantly, we develop a comprehensive mechanics theory to precisely guide cracks through the graphene layer, and demonstrate the successful exfoliation of any epitaxial overlayers grown on the graphene nanopatterns. Thus, this approach has the potential to revolutionize the heterogeneous integration of dissimilar materials by widening the choice of materials and offering flexibility in designing heterointegrated systems.

High-temperature ferromagnetic metallic phase in LaMnO3/Sr3Al2O6 heterostructure

• Oxygen migration occurs at the interface due to the stronger oxygen affinity of Al. • Continuously tunable strain can be generated by adjusting the thickness of SAO. • In-plane tensile strain would stabilizes d x 2 -y 2 occupancy and increases T c . To achieve a flexible single-crystal multifunctional membrane, the freestanding process of a rigid epitaxial transition metal oxide thin film via a buffered water-dissolution sacrificial layer has attracted reasonable attentions. Owing to the difference in chemical potential, specific element affinity, and lattice constant between the target membrane and the sacrificial layer, the freestanding process may cause an indelible change of physics property once the target thin film is sensitive to the above factors. Here, the heterostructures composed of the generally adopted sacrificial layer Sr 3 Al 2 O 6 (SAO) and LaMnO 3 (LMO) have been systematically investigated. The electrical and magnetic properties of LMO show extreme sensitivity to the thickness of SAO ( t SAO ). Then we have also found that LMO/SAO heterostructures can exhibit the coexistence of two ferromagnetic phases, the significantly enhanced Curie temperature ∼ 342 K, and the large magnetoresistance -23.3% at 300 K, which is similar to the optimal-doped manganite such as La 2/3 Sr 1/3 MnO 3 . X-ray diffraction results show that continuously tunable strain from out-of-plane tension to relaxation and then to compression can be generated by adjusting t SAO . This strain can stabilize the migrated oxygen from LMO to SAO, which is induced by the large oxygen affinity difference between B-site Mn and Al. It is believed that these unexpected electrical/magnetic phenomena are originated from the combined effects of interfacial element diffusion and strain. Our study provides a strategy for designing new magnetic phases, and a reference for the fundamental understanding of strongly correlated transition metal oxide systems in the freestanding process.

Strain Modulation of Perpendicular Magnetic Anisotropy in Wrinkle-Patterned (Co/Pt)5/BaTiO3 Magnetoelectric Heterostructures.

The rapid development of spintronics requires the devices to be flexible, to be used in wearable electronics, and controllable, to be used with magnetoelectric (ME) structures. However, the clamping effect inevitably leads to a decreased ME effect on the rigid substrate, and it remains challenging to directly prepare high-quality ferroelectric (FE) membranes on the widely used flexible substrate such as MICA or polyimide (PI). Here, periodic wrinkle-patterned flexible (Co/Pt)5/BaTiO3 (BTO) perpendicular magnetic anisotropy (PMA) heterostructures were prepared using the water-soluble method. The high-quality single-crystal BTO membrane ensures that intricate wrinkles do not fracture and a high ME coefficient is achievable. The transferred sample that is released from the clamping effect shows an enhanced ME effect in both in-plane and out-of-plane directions, with the ME coefficient reaching up to 68 Oe °C-1. The ferromagnetic resonance (FMR) field of the flexible sample can be tuned by tensile strain up to 272 Oe. The finely controlled wrinkle shows periodic strain variations at peak and valley regions that switch the PMA magnetic domain motion as an effective control method. The proposed ultraflexible wrinkle sample shows great potential for combining multiple magnetization tuning approaches, allowing it to potentially serve as a tunable high-density 3D storage prototype.

Atomic layer-by-layer etching of graphene directly grown on SrTiO3 substrates for high-yield remote epitaxy and lift-off

Epitaxial lift-off techniques, which aim to separate ultrathin single-crystalline epitaxial layers off of the substrate, are becoming increasingly important due to the need of lightweight and flexible devices for heterogeneously integrated ultracompact semiconductor platforms and bioelectronics. Remote epitaxy is a relatively newly discovered epitaxial lift-off technique that allows substrate-seeded epitaxial growth of ultrathin films through few layers of graphene. This universal epitaxial lift-off technique allows freestanding single-crystal membrane fabrication very quickly at low cost. However, the conventional method of remote epitaxy requires transfer of graphene grown on another substrate to the target single-crystalline substrate, which results in organic and metallic residues as well as macroscopic defects such as cracks and wrinkles, significantly reducing the yield of remote epitaxy. Here, we show that direct growth of thick graphene on the target single-crystalline substrate (SrTiO3 for this study) followed by atomic layer etching (ALE) of the graphene layers create a defect- and residue-free graphene surface for high yield remote epitaxy. We find that the ALE efficiently removes one atomic layer of graphene per cycle, while also clearing multi-dots (clumps of carbon atoms) that form during nucleation of the graphene layers. Our results show that direct-grown graphene on the desired substrate accompanied by ALE might potentially be an ideal pathway toward commercialization of remote epitaxy.

Stress-strain engineering of single-crystalline silicon membranes by ion implantation: Towards direct-gap group-IV semiconductors

The introduction of strain into semiconductors offers a well-known route to modify their band structure. Here, we show a single-step procedure for generating such strains smoothly and deterministically, over a very wide range, using a simple, easily available, highly scalable, ion implantation technique to control the degree of amorphization in and around single-crystal membranes. The amorphization controls the density of the material and thus the tension in the neighboring crystalline regions. We have demonstrated up to 3.1% biaxial tensile strain and 8.5% uniaxial strain in silicon, based on micro-Raman spectroscopy. This method achieves strain levels never previously reached in mesoscopic defect-free, crystalline silicon structures. The flexible, gently controllable, single-step process points toward very high mobility complementary metal-oxide-semiconductor devices and easy fabrication of direct-bandgap germanium for silicon-compatible optoelectronics.

Towards a Large-Area Freestanding Single-Crystal Ferroelectric BaTiO3 Membrane

The fabrication and transfer of freestanding single-crystal ferroelectric membranes deserve intensive investigations as to their potential applications in flexible wearable devices, such as flexible data storage devices and varied sensors in E-skin configurations. In this report, we have shown a comprehensive study approach to the acquisition of a large-area freestanding single-crystal ferroelectric BaTiO3 by the Sr3Al2O6 scarification layer method. By controlling the thickness of the BaTiO3 and Sr3Al2O6, the exposed area of the Sr3Al2O6 interlayer, and the utilization of an additional electrode La2/3Sr1/3MnO3 layer, the crack density on the freestanding BaTiO3 can be dramatically decreased from 24.53% to almost none; then, a more than 700 × 530 μm2 area high-quality freestanding BaTiO3 membrane can be achieved. Our results offer a clear and repeatable technology routine for the acquisition of a flexible large-area ferroelectric membrane, which should be instructive to other transition metal oxides as well. Our study can confidently boost flexible device fabrication based on single-crystal transition metal oxides.

Super-elastic ferroelectric single-crystal membrane with continuous electric dipole rotation.

Ferroelectrics are usually inflexible oxides that undergo brittle deformation. We synthesized freestanding single-crystalline ferroelectric barium titanate (BaTiO3) membranes with a damage-free lifting-off process. Our BaTiO3 membranes can undergo a ~180° folding during an in situ bending test, demonstrating a super-elasticity and ultraflexibility. We found that the origin of the super-elasticity was from the dynamic evolution of ferroelectric nanodomains. High stresses modulate the energy landscape markedly and allow the dipoles to rotate continuously between the a and c nanodomains. A continuous transition zone is formed to accommodate the variant strain and avoid high mismatch stress that usually causes fracture. The phenomenon should be possible in other ferroelectrics systems through domain engineering. The ultraflexible epitaxial ferroelectric membranes could enable many applications such as flexible sensors, memories, and electronic skins.

Raman Spectra Studies of Inner “Anode | Electrolyte” Interface on ESC and ASC SOFCs

In this work we present the results of in-situ case studies of fuel oxidation at the SOFC anode for anode-supported cells with thin-film electrolyte deposited by magnetron sputtering technique in comparison with studies carried out on optically transparent single-crystal membrane based electrolyte-supported model SOFC. Changes of the valence state of Ce ions were studied under different current loads applied to the cell, these changes were indicated in the Raman spectra obtained from inner interface of electrolyte membrane and anode electrode. Area of the corresponding peak in Raman spectra of ceria was studied for different working conditions for both cells constructions. Special processing technique gave opportunity to obtain more relevant data.

Germanium-on-Nothing for Epitaxial Liftoff of GaAs Solar Cells

Summary Solar cells from III-V materials offer outstanding light conversion efficiency and power densities and have a proven reliability record. Nevertheless, the utilization of III-V devices has been hindered by high production costs that partially stem from expensive substrates for the growth of III-V materials. Here, we present an ultrathin epitaxially ready single-crystal Ge membrane, formed by germanium-on-nothing (GON) technology, which employs morphological evolution of an arrayed porous Ge during hydrogen annealing. This new process, inspired by a silicon-on-nothing (SON) process, significantly improves the reformed Ge surface compared with previous porous Ge studies such that low-defect-density heteroepitaxy of GaAs is achievable. We demonstrated the growth of a 14.44% efficient GaAs solar cell on GON with nearly identical open circuit voltage to a control cell grown on a bulk Ge and successfully transferred it onto an inexpensive handle by employing the plate-like void under the Ge film as a release layer.

Gas permeation through single-crystal ZIF-8 membranes

Grain boundaries are an unavoidable microstructural feature in intergrown polycrystalline metal-organic framework (MOF) membranes. They have been suspected to be less size-selective than a MOF's micropores, resulting in suboptimal separation performances – a speculation recently confirmed by transmission electron microscopy of MOF ZIF-8. Single-crystal membranes, without grain boundaries, should confine mass transport to micropores and reflect the intrinsic selectivity of the porous material. Here, we demonstrate the feasibility of fabricating single-crystal MOF membranes and directly measuring gas permeability through such a membrane using ZIF-8 as an exemplary MOF. Our single-crystal ZIF-8 membranes achieved ideal selectivities up to 28.9, 10.0, 40.1 and 3.6 for gas pairs CO2/N2, CO2/CH4, He/CH4 and CH4/N2 respectively, much higher than or reversely selective to over 20 polycrystalline ZIF-8 membranes, unequivocally proving the non-selectivity of grain boundaries. The permeability trend obtained in single-crystal membranes aligned with a force field that had been validated against multiple empirical adsorption isotherms.

The Electrochemical Conversion of Carbon Dioxide on Cu−Ni Stacked Bilayer Electrodes

The electrochemical reduction of carbon dioxide (CO2) has gained popularity as a novel technique to realize a sustainable carbon cycle owing to improvements in power generation using renewable energy sources, such as solar and wind energy [1−2]. Recent progress of CO2 conversion utilizing the crystalline copper surfaces offers a promising approach to produce the useful hydrocarbon products. In our previous works, we demonstrated the fabrication of the single-crystal copper membranes by sputtering copper onto the specific support substrates, such as c-plane sapphire (Al2O3) and magnesium oxide (MgO). The electrochemical reduction of CO2 on these copper surfaces was produced selective hydrocarbons as compared with the commercially available copper plates [3]. In addition, these sputtered single-crystal copper membranes have been successfully peeled away from the support substrates, and thus it is able to fabricate repeatedly onto the identical support substrate [4]. The electrochemical reduction of CO2 on these crystalline copper electrodes is an effective route toward the practical use of the CO2 conversion process, however this methodology of homogeneous copper membrane deposition is essential to the complicated process. On the other hand, it has been demonstrated that the electrochemical reduction of CO2 to hydrocarbon on a copper surface is due to moderate surface adsorption of a carbonate intermediate by theoretically calculations [5]. Binary metal electrodes, such as copper and another metal, are expected to allow to the carbonate intermediate to adsorb to the appropriate surface and achieve the more effective CO2 conversion to hydrocarbon production as compared with pristine copper electrodes. In this work, we study the electrochemical conversion of CO2 on Cu−Ni stacked bilayer electrodes. Hydrocarbon emissions for the electrochemical reduction of CO2 were strongly dependent on the amount of nickel deposited on the copper electrodes. However, excess deposition of nickel on the copper electrodes suppressed the CO2 conversion. This is suggested that the surface adsorption of the carbonate intermediate is improved on the Cu−Ni stacked bilayer electrodes, compared with pristine copper electrodes, due to the interactions between copper and nickel. Additionally, we also show the selectivity of the hydrocarbon products formed via CO2 conversion on the Cu−Ni stacked bilayer electrodes in terms of the crystal structure underneath the copper electrodes. Reference [1] Y. Hori et al., J. Mol. Catal. A Chem., 199, 39–47 (2003). [2] K. J. P. Schouten et al., ACS Catal., 3(6), 1292–1295 (2013). [3] N. Yoshihara et al., ECS Trans., 66(24), 83-89 (2015). [4] N. Yoshihara et al., ECS Trans., 75(43), 33-38 (2017). [5] A. A. Peterson et al., J. Phys. Chem. Lett., 3, 251−258 (2012).

A quantitative strain analysis of a flexible single-crystalline silicon membrane

This study presents a quantitative strain analysis of a single-crystal Si membrane for high performance flexible devices. Advanced thinning and transfer methods were used to make flexible single-crystal Si devices. Two Si membrane strain gauges, each with a different stack, were fabricated on a polydimethylsiloxane/polyimide film using a silicon-on-insulator wafer. One gauge contains a 10-μm-thick handling Si layer, whereas the handling Si layer was completely removed for the other case. Although the Si membrane with the 10-μm-thick handling Si layer is flexible, the strain applied to the active Si layer (0.127%) is three times higher than the strain applied to the Si membrane without the handling Si layer (0.037%) at a bending radius of 5 mm. This leads to the more reliable electrical and mechanical performance of the device fabricated on the Si membrane without the handling Si layer. The experimental results were verified through a finite element method simulation and analytical modeling. The quantitative strain analyses for flexible devices suggested here can expedite the realization of high performance flexible electronics using a single crystal silicon active layer.

In-situ Raman spectroscopy analysis of the interfaces between Ni-based SOFC anodes and stabilized zirconia electrolyte

A novel experimental approach for in-situ Raman spectroscopy of the electrode | solid electrolyte interfaces in controlled atmospheres, based on use of the optically transparent single-crystal membranes of stabilized cubic zirconia, was proposed and validated. This technique makes it possible to directly access the electrochemical reaction zone in SOFCs by passing laser beam through the single-crystal electrolyte onto the interface, in combination with simultaneous electrochemical measurements. The case study centered on the analysis of NiO reduction in standard cermet anodes under open-circuit conditions, demonstrated an excellent agreement between the observed kinetic parameters and literature data on nickel oxide. The porous cermet reduction kinetics at 400–600°C in flowing H2-N2 gas mixture can be described by the classical Avrami model, suggesting that the reaction rate is determined by metal nuclei growth limited by Ni diffusion. The advantages and limitations of the technique are briefly addressed.

Electrochemical Conversion of Carbon Dioxide on the Exfoliated Single-Crystal Copper Membranes

Single-crystal copper electrodes have gained significant interest in the conversion of carbon dioxide (CO2) to hydrocarbons that are widely utilized as fuels and as industrial feedstock in the manufacture of important chemicals. In this study, we have demonstrated the CO2 electrochemical conversion on single-crystal copper membranes exfoliated from the support substrates. Copper membranes that were deposited on the c-plane sapphire substrate for several hours were peeled away from the support substrate. The formation of methane (CH4) and ethylene (C2H4) in CO2 electrochemical conversion was more favorable on copper surfaces that faced the support substrate than on deposited surfaces. This study offers a promising route to mass production of smooth and large-area single-crystal copper electrodes from only one support substrate for efficient conversion of CO2 to hydrocarbons.

Electrochemical Conversion of Carbon Dioxide on the Exfoliated Single-Crystal Copper Membranes

The electrochemical conversion of carbon dioxide (CO2) enables to produce hydrocarbons, which are widely utilized as fuels or industrial feedstock, at ambient pressure and room temperature. In addition, due to the improved power generation technology using renewable energy sources such as solar and wind, CO2 electrochemical conversion has attracted a great deal of interest as a novel technique to realize a sustainable carbon cycle [1-2]. In our previous work, single-crystal copper membranes were prepared by sputtering on specific support substrates such as c-plane sapphire or magnesium oxide [3]. CO2 conversion on our crystalline copper membranes evolved hydrocarbons more favorably than on commercial copper plates. However, a surface of such epitaxially grown copper membranes is no atomically flat. For CO2conversion into hydrocarbons effectively, the electrode of high quality and smooth single-crystal copper is required. In this work, CO2electrochemical conversion was studied on the single-crystal copper membranes peeled off from a support substrates.In the experiments, single-crystal copper membranes used as the working electrode (WE) were deposited onto c-plane sapphire substrates with a power of 200 W at 200 °C by radio frequency magnetron sputtering (RFS-200S, ULVAC Corp., Japan) for 5 hr. The copper membranes deposited onto c-plane sapphire were identified as Cu(111) by XRD measurement. The crystalline copper membrane is able to be peeled away from a support substrate. Figure 1 shows cyclic voltammograms on the exfoliated single-crystal copper membranes in the CO2-saturated 0.25 M K2HPO4+KH2PO4 electrolyte at a scan rate of 10 mV/s. In topside of the exfoliated copper membrane faced on a surface of c-plane sapphire, a significant decrease in current density started from −0.5 V vs. RHE, while those on backside started from −0.6 V vs. RHE. This suggests that an activity of CO2electrochemical conversion was contributed the surface smoothness of a copper membrane in addition to that of the crystallinity. We also show hydrocarbon emissions for CO2conversion on the exfoliated single-crystal copper membranes, and the electrochemical mechanism based on carbonate ions concentration in the electrolyte is discussed. Acknowledgement This work is supported by the Shiraishi Scientific Research Grant. Reference [1] Y. Hori, I. Takahashi, O. Koga, and N. Hoshi, Journal of Molecular Catalysis A: Chemical, 199, 39–47 (2003). [2] K. J. P. Schouten, E. P. Gallent, and M. T. M. Koper, ACS Catal., 3(6), 1292–1295 (2013). [3] N. Yoshihara, M. Arita, and M. Noda, ECS Transactions, 66(24), 83-89 (2015). Fiugre 1 Cyclic voltammograms for CO2 conversion on topside and backside of the single-crystal copper membranes peeled off from the c-plane sapphire substrates in the CO2-saturated 0.25 M K2HPO4+KH2PO4electrolyte. Figure 1