Sputter-deposited α-MoO3 interlayers for van der Waals epitaxy and film transfer

doi:10.21203/rs.3.rs-3006214/v1

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Sputter-deposited α-MoO₃ interlayers for van der Waals epitaxy and film transfer

https://doi.org/10.21203/rs.3.rs-3006214/v1

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Integration of functional thin films onto flexible substrates is driven by the need to improve the performance and durability of flexible electronic devices. We present a van der Waals epitaxy technology that accomplishes the transfer of oxide or metal thin films via exfoliation or dissolution of sacrificial α-MoO₃ layers produced by sputtering. The α-MoO₃ thin films, consisting of weakly bonded two-dimensional layers, grew epitaxially on SrTiO₃ (001) substrates, exhibiting mosaic domains rotated by 90°. Metallic Au films grown on the α-MoO₃ were transferred by mechanical exfoliation or by dissolving the α-MoO₃ in water at 45°C. Spinel-structured CoFe₂O₄ thin films grown on α-MoO₃ layers were easily transferred to flexible substrates via mechanical exfoliation, and the magnetic anisotropy of the transferred CoFe₂O₄ films was modulated by bending.

Physical sciences/Materials science/Nanoscale materials/Two-dimensional materials

Physical sciences/Materials science/Materials for devices/Electronic devices

Van der Waals epitaxy

α-MoO3 thin film

Epitaxial oxide thin films

Mechanical exfoliation

Large-area flexible devices

The manipulation of electronic, optoelectronic, and magnetic properties of oxide thin films by applying strain provides functionality for future electronic devices [1–4]. Substrate clamping limits the strain that can be introduced in films on substrates via lattice or thermal expansion mismatch, and modulation of film strain by mechanical deformation of rigid substrates is limited. This motivates the integration of functional oxides onto flexible substrates for the development of high-performance flexible or wearable electronics. However, the growth of oxides directly onto such substrates has been hampered by the limited thermal stability of flexible substrates compared to the high temperatures typically needed to form crystalline oxide films.

The thermal limitations of flexible substrates have been addressed by developing processes to lift-off epitaxial thin films such as oxides from other substrates using two-dimensional sacrificial layers, then transferring them to flexible substrates. [4–6] In particular, remote epitaxy using an atomically thin graphene layer, which is typically transferred from a foreign substrate to a growth substrate, has been developed for layer transfer of compound semiconductors, halide perovskites, and complex oxides. [4–9] Even though remote epitaxy using graphene has been successful, it has been challenging to scale the process to large-area substrates and to control defects in graphene such as cracks and wrinkles that occur during the transfer process; furthermore, graphene can be damaged in high temperature oxide growth. These factors encourage the investigation of novel sacrificial interlayer materials for layer transfer.

Sr₃Al₂O₆ (SAO) thin films, which can be dissolved in water, have drawn significant attention as an alternative sacrificial material for layer transfer because its lattice has a good in-plane match for many perovskite oxides and it can be grown with atomically flat surface morphology. [10–13] Epitaxial SAO thin films are typically fabricated by pulsed laser deposition (PLD) at substrate temperatures above 700°C. [10–15] Furthermore, the low stability of SAO layers in ambient air has required the insertion of a capping layer such as SrTiO₃ (STO). Large-area SAO layers have been prepared by sputtering, [16, 17] but they did not provide conclusive evidence of epitaxial growth of the SAO and overlying materials.

MgO is another example of a material used as a sacrificial interlayer. Zhang et al. reported the transfer of PLD-grown CoFe₂O₄ (CFO) thin films from STO to a flexible polyimide (PI) substrate after removing an interlayer of epitaxial MgO in 10% (NH₄)₂SO₄ solution at 80°C. [18] The magnetic properties of the CFO films were preserved after transfer and the magnetic anisotropy of the transferred films was varied by bending the flexible substrate. Other demonstrations of film transfer have used water-soluble NaCl substrates to grow Ni and ZnO thin films, allowing their magnetic anisotropy and piezo-phototronic properties to be modified. [19, 20] Nanostructured WO₃ and Sn-doped In₂O₃ have been deposited on NaCl by thermal evaporation then transferred onto large-area flexible substrates to fabricate devices. [21, 22] However, NaCl substrates and thin films are susceptible to moisture and require vacuum storage, and the high temperatures needed during deposition of epitaxial overlayers results in rough surface morphology and degraded film quality.

In this study, we demonstrate thermodynamically stable α-MoO₃ thin films as an interlayer for van der Waals (vdW) epitaxy and transfer of metal and oxide films. The layered structure of α-MoO₃ enables vdW epitaxy of oxide thin films and subsequent exfoliation by rupturing the weak bonding between the double layers of distorted edge-sharing MoO₆ octahedra. Prior work has shown that thermally evaporated α-MoO₃ crystals with a side length of tens of microns yield nanosheets by mechanical exfoliation [23] which have been transferred to flexible substrates after synthesis on mica. [24]

We first demonstrate epitaxial growth of sputter-deposited α-MoO₃ thin films on STO. We show that α-MoO₃ films are soluble in warm water, allowing dissolution of the α-MoO₃ and liftoff of overlaid films. We exhibit the transfer of Au thin films both by dissolving the α-MoO₃ interlayer and by peeling off the Au using tape. We then report the epitaxial growth of magnetoelastic CFO thin films on α-MoO₃ and layer transfer onto flexible substrates. Finally, we compare the structure, strain state, and magnetic properties of CFO thin films transferred onto flexible substrates with those of CFO films on rigid single-crystalline substrates, demonstrating bending effects on magnetic properties. These results indicate that α-MoO₃ is a useful two-dimensional material for vdW epitaxy and a promising sacrificial layer for transfer of perovskite or spinel thin films with well-matched lattice constants. Unlike SAO, NaCl, or MgO it can be separated by simple mechanical exfoliation as well as chemical etching. The integration of epitaxial oxide films on α-MoO₃ by sputtering is a contrast to integration on graphene, which is easily damaged during plasma processing.

Sputtering targets were produced in a box furnace using a conventional solid-state reaction method. Commercial MoO₃ powder with a purity of 99.5% (Kanto Chemical Co. Inc.) was pressed into cylindrical pellets and sintered for 3 h at 750°C in air with a heating and cooling rate of 5°C/min. The powder mixtures of Co₃O₄ (Alfa Aesar, purity: 99.9%) and Fe₂O₃ (Alfa Aesar, purity: 99.9%) for the CFO target were calcined at 1200 ℃ for 3 h. Uniaxial pressing was used to pelletize the synthesized powders into a 2 inch-diameter disk before sintering at 1300°C.

The α-MoO₃ thin films were sputter-deposited on (001)-oriented STO substrates at substrate temperatures of 560–640°C, working pressures of 5–30 mTorr, and Ar to oxygen ratio of 10:0, 10:1, and 10:2 after being evacuated to 5 × 10^− 6 Torr. There was no additional heat treatment after deposition. 30 and 100 nm-thick α-MoO₃ thin films were grown under deposition conditions selected to minimize surface roughness and optimize crystal quality. The effects of deposition parameters will be reported separately.

The α-MoO₃ thin films were immersed in water at temperatures ranging from 25 to 60°C for 0.5–24 h to observe the etching behavior. To investigate the reuse of STO substrates, α-MoO₃ thin films were grown on substates, removed in 45°C water for 24 h, then another α-MoO₃ film was grown. Prior to thin film transfer, 800 nm of Au was grown on top of the α-MoO₃/STO and CFO/α-MoO₃/STO stacks by sputtering using a high purity Au foil (99.99%) with a low direct current (DC) power using an ion coater (Eiko IB3, Japan). Four of the film edges were polished using sandpaper to provide a pathway for the water etch to reach the α-MoO₃ thin film because the Au and CFO overlayers covered the sides of the film stacks. Au thin films on α-MoO₃ were transferred by a wet etching process in 45°C water. Au/CFO films were transferred by mechanical exfoliation after attaching a 150 µm thick polyethylene terephthalate (PET) tape with adhesive acrylic glue (Coretech, Korea).

The structure of the films was characterized using X-ray diffraction (XRD; XRD-7000, Shimadzu) with Cu K_α radiation (λ = 1.5406 Å). The microstructures and surface morphologies of the sputtered and etched thin films were imaged with scanning electron microscopy (SEM, HITACHI, S-4800). The etched α-MoO₃ was coated with Pt for conductivity. Energy dispersive spectroscopy (EDS) in the SEM was used to analyze the composition of the α-MoO₃ thin films. Electron backscatter diffraction (EBSD) mapping results for 30 and 100 nm-thick α-MoO₃ thin films were obtained using a ZEISS Merlin high-resolution SEM equipped with an EBSD detector. The crystal structure of detached Au/CFO/α-MoO₃ stacks on adhesive tape was examined using high-resolution transmission electron microscopy (HRTEM; JEM-2100F) operating at 200 kV. A cross-sectional TEM sample was prepared by cutting a CFO thin film exfoliated onto a PET substrate using a focused ion beam (FIB; Helius 5 UX, ThermoFisher Scientific) after depositing a Pt/carbon layer. The sample was cut with its plane normal to the STO [100] axis.

Magnetic properties of CFO thin films on STO and PET substates were measured using a vibrating sample magnetometer (VSM; ADE model 1660) at room temperature with magnetic fields ranging from − 10 kOe to 10 kOe in both in-plane and out-of-plane directions. The magnetic hysteresis loops of transferred CFO thin films on PET substrates were also measured when the sample was bent with a radius of 1 cm.

The α-MoO₃ has a layered structure consisting of sheets of MoO₆ bipyramids with normal along the [010] direction. The sheets are bonded by weak vdW forces while the bonds within each layer are covalent and ionic as shown in the side view schematic in Fig. 1(a). α-MoO₃ can therefore be exfoliated to form sheets with [010] normal.

Figure 1(b) shows the X-ray diffraction (XRD) patterns of 100-nm-thick films of sputtered MoO₃ target on STO grown at a substrate temperature of 600°C, working pressure of 20 mTorr, and argon-to-oxygen ratio of 10:1. The orthorhombic α-MoO₃ peaks are located at 2θ = 12.7°, 25.5°, 38.7°, and 67.0°, which correspond to (020), (040), (060), and (0 10 0) of the Pbnm space group (JCPDS #05-0508, with a = 3.962 Å, b = 13.858 Å, c = 3.697 Å in bulk and α = β = γ = 90°). The strong (0k0) reflection indicates that the planes are parallel to the surface of the (001)-oriented STO substrates.

The peak positions of α-MoO₃ thin films yield an (010) interplanar spacing of 13.884 Å, larger than the bulk value implying tensile strain along the out-of-plane direction. The lattice constant of α-MoO₃ along the [001] direction is significantly smaller than that of STO (a = b = c = 3.905 Å) but along the [100] direction, the lattice constant of α-MoO₃ is larger. Figure 1(a) shows top view and side view schematics of the structural relationship between α-MoO₃ and STO. We believe that the strain of the α-MoO₃ layer is partially or completely relaxed along the [001] direction, whereas a compressive strain is generated along the [100] direction, increasing the out-of-plane lattice constant when the films grow coherently. From the 4-fold symmetry of the STO, two variants of the α-MoO₃ can form with [100] of α-MoO₃ parallel to [100] and [010] of STO respectively.

A flat surface morphology with protruding faceted edges is observed for the 100-nm-thick α-MoO₃ film in the top-view SEM image in Fig. 1(c). The [001] and [010] directions for the STO are indicated. The large-area (010) facets of α-MoO₃ are parallel to the surface of the STO, consistent with the strong (0k0) peaks in XRD. In contrast, α-MoO₃ films grown under different sputter conditions consisted of crystals with a range of orientations and had low (0k0) peak intensities (Figures S1(a)–(d)).

The XRD relative peak intensities of a 30-nm-thick α-MoO₃ film are the same as those of the 100-nm-thick film, with dominant (0k0) peaks; however, the peak position yields an out-of-plane lattice parameter of 13.923 Å indicating a higher out-of-plane tensile strain. Faceted rectangular crystallites on the flat surface are observed, although they are smaller than those in the 100-nm-thick film (Fig. 1(f)). EBSD mapping identifies the crystalline orientations of grains, as shown in the right images of Fig. 1(c) and (e). The two epitaxial relationships of α-MoO₃ (010) || STO (001), α-MoO₃ [100] || STO [100] and α-MoO₃ (010) || STO (001), α-MoO₃ [001] || STO [100] are visible in both 30- and 100-nm-thick films, which consist of the two orientational variants rotated in-plane by 90°. The size of the mosaic domains in EBSD mapping is comparable to that of the dark and bright grains in the SEM image for the 30-nm-thick α-MoO₃ film, Fig. 1(e), suggesting that the SEM contrast originates from the orientation of the domains.

The feasibility of using the as-grown α-MoO₃ thin films on STO substrates as a sacrificial layer for transferring epitaxial oxide thin films was investigated by etching in deionized water. No discernible change was observed when the films were in room-temperature water, whereas α-MoO₃ thin films were etched in water heated to 45°C on a hot plate, as verified by the top-view SEM images. Pits and islands with edges parallel to the (100) and (001) planes were observed in films dipped for 30 min and 2 h (Figures S2(a) and (b)), indicating anisotropic etching. Small-sized grains remained for a longer time (6 h), and a smooth surface was observed after etching for 24 h, as shown in Figures S2(c) and (d).

When the hydroxyl group in water interacts with the Mo cations in the α-MoO₃ lattice at the surface of the films, a water-soluble \({MoO}_{4}^{2-}\) anion is formed. [25] Figures S3(a) and (b) demonstrate the spread of the removed areas. The pits are first formed because the etching along the vertical direction is dominant due to the rapid etching rate along the [010] direction; the removed area then widens along the lateral direction.

The peaks from the α-MoO₃ phase disappeared when the as-grown film was immersed for 24 h while the STO peaks remained, Fig. 2(a) and (b). From the energy dispersive spectroscopy (EDS) data in Figure S4(a), Mo was detected in the α-MoO₃ thin film etched for 30 min but was absent after 24 h etching. The chemical etching behavior and mechanism of α-MoO₃ single crystals in NaOH aqueous and KOH buffer solutions have been previously reported, [26–28] but our work is the first report of the etching of an epitaxial α-MoO₃ film in heated water. In contrast, amorphous or polycrystalline α-MoO₃ phases did not etch in water. The selective removal of α-MoO₃ in warm water is promising for its use as a sacrificial layer.

Transfer of an Au film was demonstrated as indicated in the schematic illustration in Fig. 2(d). Polyethylene terephthalate (PET) tape was attached to an 800-nm-thick Au-coated α-MoO₃ layer on the STO substrate. After 24 h immersion in 45˚C water the PET/Au floats to the surface whereas the STO substrate sinks, Figure S4(b). Figure 2(e) shows the Au thin film on α-MoO₃ before etching and Fig. 2(f) is the transferred Au on PET after etching. The STO substrate after etching appears indistinguishable from the bare substrate as shown in Fig. 2(g).

We then redeposited α-MoO₃ on the 24 h etched substrate. The regrown film was visually similar to that of the first grown film (Fig. 2(h)), the XRD pattern indicated (0k0) reflections (Fig. 2(c)) and the regrown film had a flat surface morphology with rectangular crystallites in the top-view SEM image (Figure S4(c)), i.e., the regrown film is indistinguishable from the first as-grown thin film. This demonstrates the potential for recycling the single-crystalline STO substrates.

An alternative transfer process was demonstrated by peeling the Au off the α-MoO₃ with tape, Figure S4(d), similar to the transfer using scotch tape of films grown on graphene. The top-view SEM image in Figure S4(e) exhibited traces of α-MoO₃ remaining on the substrate, and the EDS spectra of the transferred layer showed both Au and Mo (Figure S4(f)), indicating that the thin-film transfer was achieved by rupturing the weak vdW bonding between the α-MoO₃ nanosheets instead of separation between the Au and α-MoO₃ interfaces. These two examples therefore demonstrate the versatility of α-MoO₃ as a sacrificial layer, which can either be dissolved in warm water or split apart by mechanical forces.

We now apply α-MoO₃ to the exfoliation of an oxide, spinel-structured CFO. First, for comparison, a CFO thin film was grown directly on a (001)-oriented STO substrate. The CFO is under compressive strain along the in-plane directions because the lattice parameter of CFO (a_bulk = 8.392 Å, JCPDS # 22-1086) is greater than the doubled lattice parameter of STO (Fig. 3(a)). Figure S5(a) shows the θ–2θ XRD scan of a 100-nm-thick CFO thin film, which reveals the (004) peak of CFO without additional reflections, consistent with epitaxial or textured growth. The out-of-plane lattice parameter of the sputtered CFO was 8.469 Å, which is larger than that of bulk CFO, due to Poisson expansion and possibly the presence of oxygen vacancies. Top-view SEM images of a CFO thin film, Fig. 3(a), show 30–50 nm diameter faceted crystals within a smooth matrix, which are assumed to form to relieve strain.

Room temperature magnetic hysteresis loops show an in-plane easy axis. The in-plane loop shows a coercivity of 3.3 kOe and a magnetic moment of 230 emu cm^− 3 at 10 kOe, the maximum field of the magnetometer, though the loop is not saturated at this field. The out-of-plane direction is a hard axis with an anisotropy field much greater than 10 kOe, based on the low magnetic moment compared to the easy axis loop. The in-plane easy axis is attributed to the combination of shape anisotropy and magnetoelastic anisotropy. Bulk CFO has a magnetization of 400 emu cm^− 3 [29] implying that a thin film of CFO has a shape anisotropy field of 5.0 kOe. The magnetoelastic anisotropy also favors an in-plane easy axis for a compressively strained film owing to the negative magnetostriction constant λ_100,CFO = (-250 to -590) × 10^− 6 [30–32].

Comparing a CFO film on a 100-nm-thick α-MoO₃ layer (Fig. 3(b)), we observe only the (004) CFO reflection in the XRD pattern, Figure S5(b); however, the surface morphology showed a higher density of crystallites, Fig. 3(b). The (004) peak position is at a lower angle than that of CFO/STO (out-of-plane CFO lattice parameter is 8.412 Å), suggesting greater strain relaxation of CFO/100-nm-thick α-MoO₃/STO vs. CFO/STO. The strain relaxation in CFO/100-nm-thick α-MoO₃/STO results in a decrease in magnetic anisotropy but the easy axis remains in-plane. The out-of-plane loop is still not saturated at 10 kOe but reaches a higher magnetic moment than that of CFO/STO.

The strain state of the CFO thin film depends on the thickness of the α-MoO₃ layer. In Fig. 3(c,d), CFO/30-nm-thick α-MoO₃/STO had an out-of-plane lattice parameter of 8.426 Å, intermediate between 8.469 Å for CFO/STO and 8.412 Å for CFO/30-nm-thick α-MoO₃/STO. The hysteresis loop follows the trend, indicating an anisotropy field between that of the other two samples attributed to the intermediate strain state of CFO/30-nm-thick α-MoO₃/STO. Furthermore, the CFO/30-nm-thick α-MoO₃/STO exhibited fewer crystallites than the CFO/100-nm-thick α-MoO₃/STO, consistent with these surface features being associated with strain relaxation.

EBSD maps of the CFO/30-nm-thick α-MoO₃/STO measured with respect to all three-coordinate axis verified the single crystallinity of CFO with a cube-on-cube epitaxial orientation over a large area (Fig. 3(e)). Some points in the map were not indexed (shown in black color) most likely due to local variations in morphology. A (001) pole figure (Figure S5(c)) confirms the epitaxial growth.

Figure 4(a) shows the transfer process of CFO thin films onto flexible PET substrates. An Au layer was deposited over the CFO/α-MoO₃/STO then an adhesive-coated PET substrate was applied to the surface of the Au layer. The PET was gently lifted to peel off the CFO layer from the STO substrate by mechanically exfoliating the α-MoO₃ nanosheets to break the vdW force. The PET/Au/CFO/α-MoO₃/STO was separated into two parts: the STO substrate part, which was dark in color compared to the bare STO or α-MoO₃ coated STO, and the PET part, which exhibited a maroon color, as shown in Fig. 4(b). The out-of-plane lattice parameters of the CFO layer did not change after exfoliation, as shown by a comparison of the magnified XRD pattern in Fig. 4(c). The CFO films were detached from the substrates regardless of the thickness of the α-MoO₃; however, exfoliation was unsuccessful for CFO grown on polycrystalline or amorphous α-MoO₃.

Figure S6(a,d) show that α-MoO₃ remained on both the PET/Au/CFO and the STO after exfoliation. A SEM image of the separated α-MoO₃/CFO/Au/PET shows steps parallel to the [100] and [001] directions of α-MoO₃ (Figure S6(b)). When the tape is lifted, the nanosheets separate along the (010) vdW planes, but the sheets also break along the in-plane [001] and [100] directions leaving steps in the surface consisting of multiple nanosheets. The step structure is also visible at the surface of the detached STO substrate (Figure S6(c)).

TEM was used to image a cross-section of exfoliated α-MoO₃/CFO/Au/PET which included regions with different heights of residual α-MoO₃, Figure S7(a-c), after coating with carbon. Figure 4(d) indicates that the film consisted of a 100 nm-thick CFO, a thin interlayer, and 50 nm-thick blocks of α-MoO₃ left when the α-MoO₃ was split apart. Figure S7(d) shows steps in the side of the residual α-MoO₃ block where the α-MoO₃ separated.

A high-resolution transmission electron microscopy (HRTEM) image along the [100] zone axis of STO, α-MoO₃ and CFO is shown in Fig. 4(e). Considering the two possible orientational variants of α-MoO₃, we expect to see regions of α-MoO₃ with [100] and [001] zone axes parallel to STO [100], but the area of sample we examined contained only the [100] variant. The region of α-MoO₃ shown by the gray square displayed two orthogonal planes with spacing of 6.94 Å and 3.70 Å, which correspond to (020) and (001) of α-MoO₃, respectively. The (020), (001), and (021) spots observed in the Fast Fourier Transform (FFT) pattern in Fig. 4(e) are consistent with the orthorhombic α-MoO₃ phase, in which the (021) planes formed an angle of 62° with the (020) plane. The bottom middle HRTEM image shows perpendicular (02\(\stackrel{-}{2}\)) and (0\(\stackrel{-}{2}\stackrel{-}{2}\)) planes with spacings of 2.97 Å. The bottom right panel shows the FFT corresponding to the CFO [100] direction.

The interface layer between CFO and α-MoO₃ extends across the entire sample and showed bright contrast with a thickness of 5 nm, and a different crystal structure from the α-MoO₃, Fig. 4(e) lower left. The EDS line scan in Figure S7(e) indicates Mo, Co and Fe are present at the interface. The layer most likely results from intermixing of the CFO into the MoO₃. It has been reported that Co and Fe are soluble in α-MoO₃, and that bombardment (which occurs during sputtering) can generate oxygen vacancies and modify the structure of the oxide. [33, 34]

We now describe the magnetic behavior of a CFO film transferred on a flexible PET substrate using a 30-nm-thick α-MoO₃ layer. The magnetic hysteresis loops of the as-grown CFO/α-MoO₃/STO and the transferred CFO/Au/PET are similar, showing an in-plane easy axis (Figs. 3(c) and 5(a)), though the transferred film loops are less anisotropic, consistent with strain relaxation. The magnetic anisotropy of the transferred film was modulated by bending the film through a 1 cm radius of curvature. In Fig. 5(b), the in-plane (tangential to the bend) and out-of-plane loops of the film become more similar as a result of the curved geometry, because each loop measurement samples regions of the film tilted at an angle to the applied field. The effect of bending can be seen from a comparison of the in-plane (axial) loop of the bent film vs. the in-plane loop of the flat film, Fig. 5(c). The bent film shows lower in-plane coercivity, remanence and loop area compared to the flat film. This is consistent with the bending-induced introduction of a tensile in-plane (tangential) strain, which reduces the in-plane anisotropy energy. The magnetic anisotropy modulation of the CFO/Au/PET by bending is larger than that reported for exfoliated CFO grown on a sacrificial layer of MgO or SAO by ALD or PLD. [18, 35]

Our results show that mechanical exfoliation of a α-MoO₃ sacrificial layer can be used to effectively transfer CFO thin films onto flexible substrates. The CFO thin film retains the majority of its magnetic anisotropy after transfer, but the magnetic anisotropy can be modulated by bending. The transfer process of epitaxial oxide thin films onto flexible substrates using vdW epitaxy described here is relevant to achieving high-performance flexible electronic devices. Furthermore, the solubility of epitaxial α-MoO₃ films in warm water provides a guideline for the growth of highly crystallized oxide thin films using sputtering, paving the way for large-area flexible electronics with low cost, good performance and durability.

We show that epitaxial α-MoO₃ provides a platform for vdW epitaxy and transfer of complex oxide thin films. α-MoO₃ is grown epitaxially with mosaic domains on a (001)-oriented STO substrate. The α-MoO₃ grows with [010] normal to the plane, and a rectangular surface net (its unit cell ratio a/c = 1.072 from bulk lattice parameters) that can be oriented in two orthogonal in-plane directions with α-MoO₃ [001] or [100] parallel to the STO [100]. Metals such as Au, and oxides such as CoFe₂O₄ (CFO), were grown on the α-MoO₃ and subsequently transferred, either by dissolving the α-MoO₃ in 45°C water, or by mechanically exfoliating the α-MoO₃ which separates along the vdW-bonded (010) planes. Residual α-MoO₃ was removed in water, enabling reuse of the STO substrate.

CFO films grew epitaxially on the α-MoO₃ with in-plane compressive strain, resulting in a magnetoelastic anisotropy favoring an in-plane easy axis. A ~ 5 nm thick layer of modified α-MoO₃ formed at the interface, likely due to bombardment of the α-MoO₃ film during CFO growth and substitution of Co and/or Fe. Transfer of the CFO onto a flexible PET substrate was demonstrated by mechanical exfoliation, and bending of the resulting CFO membrane modulated its magnetic anisotropy as expected from the change in strain.

The α-MoO₃ in this work was grown using sputter deposition, which is compatible with semiconductor manufacturing processes. These results illustrate the potential of α-MoO₃ for production of membranes for flexible electronics applications.

Acknowledgments

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2021R1F1A1053287), and by the Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2009–0082580).

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Sputter-deposited α-MoO₃ interlayers for van der Waals epitaxy and film transfer

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