Water-Soluble Sacrificial Layer of Sr₃Al₂O₆ for the Synthesis of Free-Standing Doped Ceria and Strontium Titanate

Abstract

Epitaxial layers of water-soluble Sr₃Al₂O₆ were fabricated as sacrificial layers on SrTiO₃ (100) single-crystal substrates using the Pulsed Laser Deposition technique. This approach envisages the possibility of developing a new generation of micro-Solid Oxide Fuel Cells and micro-Solid Oxide Electrochemical Cells for portable energy conversion and storage devices. The sacrificial layer technique offers a pathway to engineering free-standing membranes of electrolytes, cathodes, and anodes with total thicknesses on the order of a few nanometers. Furthermore, the ability to etch the SAO sacrificial layer and transfer ultra-thin oxide films from single-crystal substrates to silicon-based circuits opens possibilities for creating a novel class of mixed electronic and ionic devices with unexplored potential. In this work, we report the growth mechanism and structural characterization of the SAO sacrificial layer. Epitaxial samarium-doped ceria films, grown on SrTiO3 substrates using Sr₃Al₂O₆ as a buffer layer, were successfully transferred onto silicon wafers. This demonstration highlights the potential of the sacrificial layer method for integrating high-quality oxide thin films into advanced device architectures, bridging the gap between oxide materials and silicon-based technologies.

1. Introduction

In recent decades, there has been growing interest in integrating functional oxides with semiconductor devices. This interest is driven by the increasing demand for combining functional layers produced through different techniques. It is crucial to fabricate and interconnect layers with different functionalities and integrate them into devices. However, achieving the integration of optimal materials presents significant challenges, as high crystal quality and minimal interfacial defects are often necessary to enhance device performance.

Epitaxial oxide layers can only be fabricated when the substrate’s crystal symmetry provides a compatible lattice parameter. The lattice mismatch between functional layers limits the integration of different materials and their applications. Moreover, the challenges of producing integrated devices become even more significant when attempting to combine dissimilar layers. Oxides and semiconductors are just two examples of materials whose integration can be particularly difficult.

A promising solution to these challenges is the production of free-standing oxide membranes using the sacrificial layer method. These membranes can be manipulated, bent, stretched, and folded to tailor their properties. More importantly, free-standing membranes can be transferred onto otherwise incompatible substrates. This enables the design of a new generation of devices assembled by stacking free-standing oxide membranes [1,2,3,4,5].

The ability to engineer and fabricate high-quality functional oxide membranes is essential for advancing this new generation of devices that integrate functional layers into multilayer stacks. The stacking capability and strain tunability of these membranes can significantly enhance physical properties in engineered silicon-based devices.

A particularly effective method for producing free-standing membranes involves using water-soluble tristrontium dialuminum hexaoxide (Sr₃Al₂O₆, SAO) as a sacrificial layer. The lattice of SAO (cubic, a = 15.844 Å [1]) matches the equivalent of four unit cells of common functional perovskite oxides, such as SrTiO₃, La_xSr_1-xMnO₃, BiFeO₃, and LaNiO₃, facilitating epitaxial growth [2,3,4,5,6,7].

It is worth noting that the silicon micromachining technique can also be employed to fabricate free-standing membranes. After the deposition of the functional oxide layer, silicon can be etched to produce the free-standing membrane [8]. However, silicon micromachining often results in poor crystallographic quality of the oxide thin film deposited on silicon substrates. Furthermore, PLD growth conditions, such as oxygen partial pressure and deposition temperature, can significantly affect the properties of the silicon substrate. Recently, the epitaxial lift-off technique has emerged as another approach to obtaining free-standing membranes. This method allows epitaxial layers to be transferred and rotated at different twist angles. When these layers are periodically stacked with varying orientations, they can form a new class of super-lattices with novel and tunable properties arising from the interfaces between the twisted layers [5,9].

Recently, the interest in sacrificial layers, coupled with the growing demand for miniaturized devices in energy storage and conversion, has made the advancement of thin-film technologies strategically important for the development of micro-electrochemical devices such as micro-Solid Oxide Fuel Cells (m-SOFCs) and micro-Solid Oxide Electrochemical Cells (m-SOECs) [8,9,10,11,12,13]. As a result, strontium aluminate Sr₃Al₂O₆ (SAO) has gained significant attention as an effective sacrificial layer in the fabrication of free-standing thin-film membranes. Improving the manufacturing of complex oxide thin films and their realization as free-standing membranes could substantially impact the performance and the field of utilization of materials like samarium-doped ceria (SDC) and gadolinium-doped ceria (GDC).

Indeed, non-stoichiometric doped ceria (CeO_2-δ), with Ce⁺⁴ replaced by trivalent cations such as Gd⁺³ or Sm⁺³, is an attractive redox-active material due to its high ionic and electronic conductivity and stability [14,15,16,17,18].

For this reason, doped ceria-based materials represent a very promising class of electrocatalysts for CO₂ reduction in solid oxide electrolysis cells (SOECs) due to their high stability and efficiency in converting CO₂ to CO with remarkable carbon tolerance [10,11,19,20,21].

However, fabricating high-quality thin-film membranes of SDC and GDC remains a significant challenge. This process necessitates using a sacrificial support layer that can be removed post-deposition without compromising the structural integrity of the film [9,12]. In particular, reducing electrochemical components such as cathodes, electrolytes, and anodes to two-dimensional configurations with nanometer-scale thicknesses can enhance device performance by increasing energy density, lowering operating temperatures, and enabling integration into compact systems. To produce complex oxide thin films with excellent crystallographic quality, Pulsed Laser Deposition (PLD) is a widely used technique for the fabrication of thin films with precise control over their structural, chemical, and physical properties.

The PLD process employs a high-energy pulsed laser to ablate material from a solid target, generating a plasma plume containing ions, atoms, and molecules ejected from the target. This material is deposited onto a substrate under precisely controlled conditions, such as temperature and ambient gas pressure, forming a thin film. A key advantage of PLD is its ability to maintain the stoichiometry of complex materials, making it especially effective for fabricating oxide thin films, superconductors, and multilayer heterostructures [9,12,13]. Additionally, it supports the growth of films with smooth surfaces and sharp interfaces, which are critical for applications in microelectronics and nanotechnology. Reflection High-Energy Electron Diffraction in the last decades has become an in situ diagnostic tool in PLD, providing real-time monitoring of film growth at the atomic scale. By directing a high-energy electron beam at a grazing incidence angle to the substrate surface and analyzing the resulting diffraction pattern, RHEED enables the precise assessment of surface morphology, crystallinity, and layer-by-layer growth dynamics during deposition. One of the key advantages of integrating RHEED into PLD systems is its ability to monitor the evolution of thin-film surfaces without interrupting the deposition process. This real-time feedback allows the optimization of growth parameters, such as laser fluence, substrate temperature, and ambient gas pressure, ensuring high-quality film deposition. Furthermore, RHEED facilitates the study of growth modes, distinguishing between layer-by-layer (2D), step-flow, or island (3D) growth mechanisms [22], which are crucial for tailoring film properties to specific applications. RHEED is particularly valuable for achieving epitaxial growth, where precise control of crystallographic alignment and interface sharpness is required. For instance, the oscillations observed in the RHEED intensity during layer-by-layer growth can be used to determine deposition rates with sub-monolayer precision, enabling the fabrication of complex heterostructures and super-lattices [2,9,22]. Beyond its role in film characterization, RHEED also aids in detecting deviations from ideal growth conditions, such as surface roughening, amorphization, or phase transitions. The combination of RHEED with PLD enhances the reliability and reproducibility of the deposition process and expands the range of materials and applications accessible from oxide electronics to spintronics and advanced photonic devices.

An emerging area of interest in thin films and their application is the detachment of the films from their original substrates for transfer onto alternative materials. This step is essential for integrating films with flexible, lightweight, or unconventional substrates that are incompatible with the high temperatures and oxygen partial pressures (10⁻³ to 1 mbar) typically required during deposition. Techniques such as sacrificial layer etching have been developed to facilitate this transfer, broadening the functionality and applicability of PLD-grown films. The nature of the substrate plays a crucial role in determining the quality of PLD-produced films. For epitaxial thin films free of cracks and grain boundaries, single-crystal substrates with lattice parameters closely matching those of the deposited film are generally required. However, the deposition conditions—temperatures ranging from 400 °C to 800 °C—often preclude direct deposition onto substrates such as integrated circuits. To overcome this limitation, films are commonly transferred to alternative substrates more suitable for subsequent applications. PLD remains an indispensable technique for exploring novel materials and advancing next-generation technologies, especially in fields demanding precise control of thin-film properties and the ability to integrate films onto diverse substrates.

SAO has been shown considerable interest due to its favorable physical and chemical properties that make it an ideal sacrificial material. One of the primary advantages of SAO is its high solubility in water, allowing an easy removal process that is gentle on SDC and GDC films [12]. Upon water immersion, SAO rapidly dissolves without leaving residues, which simplifies the fabrication of defect-free, free-standing oxide membranes. Additionally, SAO possesses high thermal stability and is compatible with elevated temperatures used during the deposition of ceramic films. This stability ensures that the sacrificial layer can endure the high-temperature processing required for oxide film sintering and densification without significant decomposition or degradation. Another critical factor in the suitability of SAO as a sacrificial layer is its minimal chemical interaction with doped ceria films. Unlike other potential sacrificial materials that might introduce unwanted side reactions, impurities, or residual stresses, SAO remains chemically inert concerning both SDC and GDC during high-temperature processing. This inertness helps preserve the microstructural integrity and ionic conductivity of the ceria films, both of which are essential for their performance in electrochemical applications.

2. Materials and Methods

In this work, we report the growth mechanism of the SAO sacrificial layer, the realization of STO, SDC, and GDC free-standing membranes onto a PDMS Gel-pack polymer (Gel-Pak, Hayward, CA, USA), and the subsequent transfer onto a silicon substrate. The film depositions were performed using an excimer laser charged with KrF, generating UV radiation (248 nm wavelength, pulses of 25 ns width; Coherent Corp., Saxonburg, PA, USA). The laser beam was focused to an energy density of 3 J/cm² on the slowly rotating target at an incidence angle of 45°. The pulse repetition rate was fixed at 10 Hz. The substrate was glued by silver paint to the heater. The distance between the substrate and the target was fixed at about 5 cm. The base pressure in the deposition chamber was 1 × 10⁻⁷ mbar. During the deposition, an oxygen partial pressure ranging between 10⁻² and 10⁻¹ mbar was maintained in the chamber. After deposition, the oxygen partial pressure was raised to 1 atm, and films were cooled to room temperature in about half an hour. The targets used were prepared by solid-state reaction starting from high-purity powders, which were successively calcinated in air. Powders were then re-grinded, pressed to form a disk, and partially reacted at 1450 °C in air.

The surface quality of the deposited heterostructures was evaluated using the RHEED diagnostics. When the deposition process was conducted at oxygen pressures below 10⁻³ mbar, RHEED provided real-time feedback on the crystallinity and surface morphology during the deposition. However, at oxygen pressures higher than 10⁻³ mbar, RHEED analysis was performed only after cooling the sample to room temperature, with the oxygen pressure reduced to values below 10⁻³ mbar. The RHEED was used at 15 kV and with a filament current of 1.5 A.

To optimize the deposition conditions of the SAO layer, an epitaxial sacrificial layer of SAO was grown using PLD on an STO (100)-oriented substrate. Subsequently, an STO film was deposited onto the SAO layer using a multitarget carousel, which allows the fabrication of heterostructures.

A bilayer structure consisting of SAO (sacrificial layer) and SDC was fabricated. The SAO layer was first deposited onto the STO substrate, followed by the growth of the SDC layer on top of the SAO. The PLD growth conditions for both materials are reported elsewhere [12]. The surface area of the SDC-buffered SAO on STO (100) measured 0.5 cm × 0.5 cm. A PDMS Gelpack adhesive polymer was then applied to the SDC surface of the SDC/SAO/STO heterostructure.

The SAO sacrificial layer was subsequently dissolved by immersing the PDMS/SDC/SAO/STO structure in distilled water at room temperature. After a few hours, the SDC layer was successfully transferred to the PDMS. For further transfer to a Si wafer, the PDMS/SDC structure was placed directly onto the Si wafer’s surface.

Structural characterization of the films and membranes was performed using XRD, allowing us to analyze the crystallographic quality and identify any strain or defects induced by the substrate. Rocking curve analysis and θ-2θ X-ray scans were carried out using a Rigaku diffractometer (Rigaku, Tokyo, Japan) in Bragg–Brentano configuration equipped with a Co anode as the X-ray source, operating at 30 kV and 30 mA.

Furthermore, we studied the structural evolution, morphological characteristics, and surface integrity of the free-standing membranes through scanning electron microscopy (SEM), observing changes that occurred post-transfer and during dissolution of the sacrificial layer. The long-term stability of the SDC free-standing membrane was assessed during electrochemical impedance spectroscopy (EIS) measurements (see reference [12]). The EIS measurements were repeated daily, and the membranes demonstrated stability in air for two weeks.

3. Results and Discussion

Initially, to optimize the deposition parameters of SAO, only the SAO layer was deposited. However, it was necessary to prevent its exposure to ambient humidity, as the SAO layer degraded rapidly once removed from the deposition chamber. To mitigate this issue, a crystalline STO cap layer was deposited on top of the SAO layer to protect it from degradation caused by humidity. SAO has a cubic structure with a lattice constant of a_SAO = b_SAO = c_SAO = 15.844 Å [1]. This lattice parameter is almost equal to four times the STO lattice parameter (c_STO = 3.905 Å): c_SAO = 4 × c_STO. Figure 1a presents the XRD patterns in θ-2θ scan mode for the STO/SAO/STO (001) heterostructure. The SAO peaks observed in the θ-2θ scan confirm the c-axis orientation of the sacrificial SAO layer in the STO/SAO bilayer. The diffraction peak of the STO layer deposited on the SAO sacrificial layer is superimposed on the STO (001) substrate single-crystal peak, indicating that the STO layer replicates the orientation of the underlying substrate. The insets in Figure 1a provide a closer view of the XRD patterns, highlighting the SAO (004) and SAO (008) peaks, which further confirm the high crystallographic quality of the SAO layer. The c-lattice parameter obtained by these diffraction peaks is c = 15.82 Å [12]. This value is in agreement with the values found in the literature. These results suggest that the epitaxial relationship between the SAO sacrificial layer and the STO substrate is well maintained. A PDMS Gelpack adhesive polymer was applied onto the STO film surface of the heterostructure STO/SAO/STO. The heterostructure was then immersed in water to dissolve the SAO to obtain an STO free-standing membrane. Figure 1b displays the XRD patterns in θ-2θ scan mode for the STO membrane transferred onto a PDMS wafer. The presence of the (001) and (002) diffraction peaks corresponding to the STO layer confirms the successful transfer process. Additionally, the calculated c-lattice parameter for STO on PDMS (c = 3.92 Å) is slightly larger compared to the bulk value (c = 3.905 Å), likely due to compressive strain induced by the non-flat nature of the PDMS substrate. These results highlight the structural integrity of the STO layer and demonstrate the successful transfer of high-quality free-standing membranes onto a different support. Furthermore, they confirm the feasibility of using SAO as a sacrificial layer for the subsequent transfer of SDC and GDC thin films. Following this initial step of optimizing the deposition parameters for SAO, we proceeded with the fabrication of the heterostructure GDC/SAO on an STO (100) substrate. The GDC has a fluorite crystal structure with a lattice parameter of a_GDC = b_GDC = c_GDC = 5.412 Å [23]. Figure 2 shows the XRD patterns obtained in θ-2θ scan mode for the GDC/SAO/STO (100) heterostructure. The θ-2θ scan reveals the SAO (00l) diffraction peaks, confirming the c-axis orientation of the SAO sacrificial layer within the heterostructure. Additionally, the diffraction pattern in Figure 2a indicates two orientations for the GDC layers (111) and (002), highlighting the coexistence of different crystalline domains during growth. The presence of (111) reflection in the GDC/SAO/STO (100) heterostructure can be attributed to the strain induced by the underlying SAO layer and/or the STO substrate. In contrast, the XRD θ-2θ scan for the GDC layer transferred onto the PDMS substrate shows only the (001) orientation (Figure 2b), indicating a single crystallographic orientation after transfer. This could be attributed to the relaxation of residual stresses or the influence of the flexible PDMS.

Figure 3a,b present the X-ray diffraction patterns in θ-2θ scan mode for the SDC/SAO/STO (001) heterostructure and the SDC layer transferred onto PDMS, respectively.

The SDC has a fluorite structure with a lattice parameter of a_SDC = b_SDC = c_SDC = 5.422 Å [23], slightly larger than that of the GDC.

In the θ-2θ scan of the heterostructure, the presence of distinct SDC, STO, and SAO (00l) diffraction peaks confirms the c-axis orientation of the entire multilayer structure. This result indicates that the sacrificial layer (SAO) and the underlying STO substrate played a critical role in guiding the epitaxial growth of the SDC layer. The high degree of orientation is essential for ensuring the desired functional properties of the heterostructure, as it minimizes defects that could impact performance.

For the SDC layer, the observation of the (001) diffraction peak in the XRD pattern highlights its unique and superior crystallographic alignment compared to GDC. This suggests that the deposition process and epitaxial relationships in the multilayer system effectively promoted the formation of a well-ordered SDC structure. Such alignment is particularly important for applications that rely on anisotropic properties, such as ionic conductivity or electronic transport.

After transferring the SDC layer to the flexible PDMS substrate, the XRD patterns demonstrate that the structural integrity and c-axis crystalline orientation of the SDC layer are largely preserved. This preservation is crucial for maintaining the layer’s functionality in potential applications, such as flexible electronics, energy devices, or sensors. The successful retention of crystalline orientation suggests that the detachment process, which involves sacrificial layer etching and redeposition onto PDMS, was conducted with minimal mechanical or chemical damage to the SDC layer.

The absence of secondary phases or additional orientations in the transferred SDC layer further validates the effectiveness of the transfer process. This result confirms that the multilayer system maintained its phase purity and avoided significant structural degradation, which could otherwise compromise its performance. The findings underscore the reliability of this approach for producing high-quality free-standing oxide layers with potential integration into flexible or unconventional substrates, enabling broader applications for epitaxial oxide materials.

Figure 4 displays the RHEED patterns for the STO single-crystal substrate (Figure 4a), the SAO layer deposited onto the STO substrate (Figure 4b), and the SDC layer deposited onto the SAO/STO structure (Figure 4c) during PLD growth. The RHEED pattern of the SAO layer exhibits a streaky pattern, confirming the formation of a smooth, two-dimensional (2D) surface for the sacrificial layer (Figure 4b). This indicates that the deposition conditions were optimized to achieve a layer-by-layer growth mode, which is crucial for maintaining the structural integrity of the heterostructure and ensuring high-quality interfaces during subsequent processing steps.

Additionally, in Figure 4b, the presence of superstructure streaks is evident, with these additional streaks spaced at one-quarter of the distance between the primary diffraction streaks corresponding to the STO lattice parameter. These superstructure lines indicate the formation of an in-plane lattice with a periodicity four times larger than that of the STO substrate. This observation is consistent with the lattice parameter of the SAO layer, which is approximately four times the STO lattice parameter. The appearance of these superstructure features demonstrates the epitaxial relationship between the SAO layer and the STO substrate, indicating a well-ordered lattice alignment and confirming the high crystalline quality of the SAO sacrificial layer.

Moreover, the observation of these superstructure lines provides valuable insight into the lattice matching and strain accommodation mechanisms at the SAO/STO interface. This alignment is critical for ensuring that subsequent layers, such as the SDC layer, allow the high crystallinity and smooth morphology necessary for advanced functional devices. These results are indicative of the optimized growth parameters to achieve precise surface quality control and ensure the application of this technique to the realization of complex heterostructures.

In contrast, the initial growth of the SDC layer displayed characteristics of three-dimensional (3D) island growth, as expected for the first 20–30 unit cells due to the lattice mismatch or differences in surface energy. However, as the deposition progressed, the RHEED pattern transitioned to streaky lines, signifying the evolution from a 3D to a 2D growth mode (Figure 4c). The ability of the SDC layer to achieve a 2D growth mode after the initial 3D growth phase is critical for ensuring a high-quality crystalline film suitable for subsequent applications. The RHEED observations indicate the precise control over growth parameters needed to achieve high-quality surfaces and interfaces within the heterostructure, which are essential for its functional performance.

Figure 5 presents the FE-SEM images of the SDC membrane transferred onto the PDMS substrate. Although the SDC film was successfully and entirely transferred onto the flexible PDMS, cracks are apparent in the FE-SEM images. These cracks are likely caused by mechanical stresses during the transfer process or strain relaxation as the film detaches from the rigid substrate. Such defects could influence mechanical robustness and limit the potential applications of the membrane in devices that demand structural continuity and reliability. At higher magnification, as shown in the inset of Figure 5b, single-crystal SDC flakes with a distinctive square shape are observed. The square morphology reflects the cubic symmetry of the SDC crystal structure, highlighting its well-preserved crystallographic features. This observation suggests that, despite the presence of cracks, the crystalline quality of the SDC layer remains largely unaffected by the transfer process. The retention of these structural characteristics shows the effectiveness of the growth and transfer protocols in maintaining the film’s intrinsic properties, which is essential for its performance in applications such as ionic conductors, solid oxide fuel cells, or thin-film-based electronic devices. To further enhance the quality of the transferred membranes, improvements in the transfer process could focus on minimizing cracking. Such optimization would enable the production of high-quality, flexible membranes tailored for advanced technological applications. The sacrificial layer method gives the possibility to evaluate the transport properties of the materials in a free-standing configuration without any interfacial contribution due to the presence of the substrate and or the bottom layer in a heterostructure configuration. Figure 6 illustrates step by step a possible procedure to make electrochemical devices with the sacrificial layer method based on a free-standing membrane:

• Step 1: PLD deposition of the water-soluble sacrificial Sr₃Al₂O₆ (SAO) onto STO single crystals.
• Step 2: PLD deposition of the electrolyte layer, which is a pure O₂- conductor with the scope to separate the current collector from the bottom of the cell, preventing a shortcut in the electrochemical cell [10,24,25].
• Step 3: PLD deposition of the cathode layer (SDC or GDC).
• Step 4: PDMS is glued onto the surface of the film, and the sacrificial layer SAO is etched in distilled water solution to detach the STO substrate.
• Step 5: Transfer of the active layers onto a porous substrate.

4. Conclusions

In this work, we showed the procedure to fabricate free-standing membranes with remarkable crystallographic quality by using water-soluble SAO as a sacrificial layer. As an example, we reported on the preparation of STO/SAO/STO, GDC/SAO/STO, and SDC/SAO/STO heterostructures and then on the procedure to dissolve the SAO layer by sinking the heterostructure in distilled water at room temperature. Furthermore, the STO, GDC, and SDC layers detached from the STO substrate were transferred onto the polymer PDMS Gelpack, maintaining a good crystallographic quality.

Free-standing membranes fabricated using SAO as a sacrificial layer offer a transformative pathway for transferring thin films onto diverse substrates. This approach not only preserves the crystalline structure and electrical properties of the films—even in the presence of minor imperfections such as cracks or flakes—but also demonstrates remarkable robustness and reliability. Such advancements allow the integration of high-crystallinity oxide thin films grown via PLD into established technologies, including silicon-based platforms, which are central to microelectronics and energy systems.

For ionic and electronic conductors like SDC, this method provides a solid foundation for developing advanced electrochemical devices, such as micro-SOFCs and other miniaturized energy conversion systems. The retention of high-quality crystalline properties in transferred films is critical for achieving superior ionic conductivity, mechanical stability, and long-term reliability. Furthermore, the ability to fabricate free-standing membranes extends beyond SDC, enabling the exploration of a broad spectrum of multifunctional materials, including ferroelectric, piezoelectric, and magnetic oxides. These materials can be leveraged in next-generation MEMS sensors, actuators, and energy-harvesting devices, addressing key challenges in emerging technologies, such as wearables, IoT devices, and autonomous systems.

A particularly promising avenue is the integration of ultrathin oxide films onto surfaces with tailored oxide terminations. This strategy enables precise control over interfacial properties, which are critical in designing heterostructures with novel functionalities. For instance, strain engineering at the interface can significantly enhance the transport properties of oxide thin films, such as ionic conductivity, electronic conductivity, and superconductivity. Such improvements are vital for applications in ionotronics, spintronics, and quantum devices. Additionally, the ability to create free-standing membranes expands opportunities for investigating phenomena such as flexoelectricity and strain-induced phase transitions, which are challenging to study in conventional substrate-bound systems.

Importantly, this technique paves the way for integrating oxide thin films into silicon-based technologies, bridging the gap between traditional semiconductors and functional oxides. This integration could drive the development of a new generation of micro-Solid Oxide Electrochemical Cells, micro-Solid Oxide Fuel Cells, and other multifunctional devices. These innovations hold the potential to revolutionize fields such as micro-scale energy harvesting, portable power systems, and environmental sensing.

Moreover, the scalability and versatility of the sacrificial layer approach make it a promising candidate for industrial adoption. The costs associated with the sacrificial layer technique are primarily due to the fabrication of the SAO layer and the functional layers. These costs are comparable for various high-quality films. Moreover, the etching process is straightforward and does not require any specific solvents or equipment.

Moreover, future research should focus on optimizing the transfer process to further minimize structural defects and improve scalability, as well as exploring its applicability to emerging material systems. By preserving intrinsic properties and enabling integration onto unconventional substrates, this method sets the stage for groundbreaking advancements in multifunctional oxide-based devices and materials engineering.