Sputter‐Deposited α‐MoO3 Interlayers for van der Waals Epitaxy and Film Transfer

Integration of functional thin films onto flexible substrates is driven by the need to improve the performance and durability of flexible electronic devices. 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 is presented. The α‐MoO3 thin films, consisting of weakly bonded 2D layers, grow epitaxially on SrTiO3 (001) substrates, exhibiting mosaic domains rotated by 90°. Metallic Au films grown on the α‐MoO3 are 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 are easily transferred to flexible substrates via mechanical exfoliation, and the magnetic anisotropy of the transferred CoFe2O4 films is modulated by bending.


Introduction
[3][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 DOI: 10.1002/adfm.202306909wearable 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.
[6][7][8][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.[12][13][14][15] Furthermore, the low stability of SAO layers in ambient air has required the insertion of a capping layer, such as SrTiO 3 (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 CoFe 2 O 4 (CFO) thin films from STO to a flexible polyimide (PI) substrate after removing an interlayer of epitaxial MgO in 10% (NH 4 ) 2 SO 4 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 watersoluble NaCl substrates to grow Ni and ZnO thin films, allowing their magnetic anisotropy and piezo-phototronic properties to be modified. [19,20]Nanostructured WO 3 and Sn-doped In 2 O 3 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 -MoO 3 thin films as an interlayer for van der Waals (vdW) epitaxy and transfer of metal and oxide films.The layered structure of -MoO 3 enables vdW epitaxy of oxide thin films and subsequent exfoliation by rupturing the weak bonding between the double layers of distorted edge-sharing MoO 6 octahedra.Prior work has shown that thermally evaporated -MoO 3 crystals with a side length of tens of micrometers yield nanosheets by mechanical exfoliation [23] which have been transferred to flexible substrates after synthesis on mica. [24]Other works have revealed the growth behavior and optical properties of the MoO 3 thin films grown at low temperature and their potential in transfer technology. [25,26]e first demonstrate epitaxial growth of sputter-deposited -MoO 3 thin films on STO.We show that -MoO 3 films are soluble in warm water, allowing dissolution of the -MoO 3 and lift-off of overlaid films.We exhibit the transfer of Au thin films both by dissolving the -MoO 3 interlayer and by peeling off the Au using tape.We then report the epitaxial growth of magnetoelastic CFO thin films on -MoO 3 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 -MoO 3 is a useful 2D 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 -MoO 3 by sputtering is a contrast to integration on graphene, which is easily damaged during plasma processing.

Results and Discussion
The -MoO 3 has a layered structure consisting of sheets of MoO 6 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 Figure 1a.-MoO 3 can therefore be exfoliated to form sheets with [010] normal.
The peak positions of -MoO 3 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 -MoO 3 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 -MoO 3 is larger.Figure 1a shows top view and side view schematics of the structural relationship between -MoO 3 and STO.We believe that the strain of the -MoO 3 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 fourfold symmetry of the STO, two variants of the -MoO 3 can form with [100] of -MoO 3 parallel to [100] and [010] of STO, respectively.
A flat surface morphology with protruding faceted edges is observed for the 100 nm thick -MoO 3 film in the top-view scanning electron microscopy (SEM) image in Figure 1c.The [001] and [010] directions for the STO are indicated.The large-area (010) facets of -MoO 3 are parallel to the surface of the STO, consistent with the strong (0k0) peaks in XRD.In contrast, -MoO 3 films grown under different sputter conditions consisted of crystals with a range of orientations and had low (0k0) peak intensities (Figure S1a-d, Supporting Information).
The XRD relative peak intensities of a 30 nm thick -MoO 3 film are the same as those of the 100 nm thick film, with dominant (0k0) peaks; however, the peak position yields an out-ofplane lattice parameter of 13.923 Å indicating a higher out-ofplane tensile strain (Figure 1d).Faceted rectangular crystallites on the flat surface are observed, although they are smaller than those in the 100 nm thick film (Figure 1e).Electron backscatter diffraction (EBSD) mapping identifies the crystalline orientations of grains, as shown in the right images of Figure 1c The feasibility of using the as-grown -MoO 3 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 roomtemperature water, whereas -MoO 3 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 (Figure S2a,b, Supporting Information), 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 Figure S2c,d (Supporting Information).
When the hydroxyl group in water interacts with the Mo cations in the -MoO 3 lattice at the surface of the films, a watersoluble MoO 2− 4 anion is formed. [27]Figure S3a,b (Supporting Information) demonstrates 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 -MoO 3 phase disappeared when the asgrown film was immersed for 24 h, while the STO peaks remained, Figure 2a,b.From the energy dispersive spectroscopy (EDS) data in Figure S4a (Supporting Information), Mo was detected in the -MoO 3 thin film etched for 30 min but was absent after 24 h etching.The chemical etching behavior and mechanism of -MoO 3 single crystals in NaOH aqueous and KOH buffer solutions have been previously reported, [28][29][30] but our work is the first report of the etching of an epitaxial -MoO 3 film in heated water.In contrast, amorphous or polycrystalline -MoO 3 phases did not etch in water.The selective removal of -MoO 3 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 Figure 2d.Polyethylene terephthalate (PET) tape was attached to an 800 nm thick Au-coated -MoO 3 layer on the STO substrate.After 24 h immersion in 45 °C wa-ter, the PET/Au floats to the surface, whereas the STO substrate sinks, Figure S4b (Supporting Information).Figure 2e shows the Au thin film on -MoO 3 before etching and Figure 2f is the transferred Au on PET after etching.The STO substrate after etching appears indistinguishable from the bare substrate as shown in Figure 2g.
We then redeposited -MoO 3 on the 24 h etched substrate.The regrown film was visually similar to that of the first grown film (Figure 2h), the XRD pattern indicated (0k0) reflections (Figure 2c) and the regrown film had a flat surface morphology with rectangular crystallites in the top-view SEM image (Figure S4c, Supporting Information), i.e., the regrown film is indistinguishable from the first as-grown thin film.This demon- strates the potential for recycling the single-crystalline STO substrates.
An alternative transfer process was demonstrated by peeling the Au off from the -MoO 3 with tape, Figure S4d (Supporting Information), similar to the transfer using scotch tape of films grown on graphene.The top-view SEM image in Figure S4e (Supporting Information) exhibited traces of -MoO 3 remaining on the substrate, and the EDS spectra of the transferred layer showed both Au and Mo (Figure S4f, Supporting Information), indicating that the thin-film transfer was achieved by rupturing the weak vdW bonding between the -MoO 3 nanosheets instead of separation between the Au and -MoO 3 interfaces.These two examples therefore demonstrate the versatility of -MoO 3 as a sacrificial layer, which can either be dissolved in warm water or split apart by mechanical forces.
We now apply -MoO 3 to the exfoliation of an oxide, spinelstructured 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 (Figure 3a). Figure S5a (Supporting Information) shows the -2 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, Figure 3a, 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 inplane 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 [31] implying that a thin film of CFO has a shape anisotropy field of 5.0 kOe.34] Comparing a CFO film on a 100 nm thick -MoO 3 layer (Figure 3b), we observe only the (004) CFO reflection in the XRD pattern, Figure S5b (Supporting Information); however, the surface morphology showed a higher density of crystallites, Figure 3b.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 -MoO 3 /STO versus CFO/STO.The strain relaxation in CFO/100 nm thick -MoO 3 /STO results in a decrease in magnetic anisotropy, but the easy axis remains in-plane.The out-ofplane 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 -MoO 3 layer.In Figure 3c,d EBSD maps of the CFO/30 nm thick -MoO 3 /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 (Figure 3e).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 S5c, Supporting Information) confirms the epitaxial growth.Figure S6a-f (Supporting Information) shows that -MoO 3 remained on both the PET/Au/CFO and the STO after exfoliation.A SEM image of the separated -MoO 3 /CFO/Au/PET shows steps parallel to the [100] and [001] directions of -MoO 3 (Figure S6b, Supporting Information).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 image of transferred film on PET substrate obtained from an optical microscopy in Figure S6c (Supporting Information) revealed fragments.However, the surface of the residue was flat as shown the height profiles by atomic force microscopy (AFM) analysis in Figure S6d (Supporting Information) suggests the adhesives on PET substrate did not affect the surface morphology of the transferred films in nano-micro scale.The step structure is also visible at the surface of the detached STO substrate (Figure S6e, Supporting Information).
TEM was used to image a cross-section of exfoliated -MoO 3 /CFO/Au/PET which included regions with different heights of residual -MoO 3 , Figure S7a-c (Supporting Information), after coating with carbon.Figure 4d indicates that the film consisted of a 100 nm thick CFO, a thin interlayer, and 50 nm thick blocks of -MoO 3 left when the -MoO 3 was split apart.Figure S7d  in -MoO 3 , and that bombardment (which occurs during sputtering) can generate oxygen vacancies and modify the structure of the oxide. [35,36]e now describe the magnetic behavior of a CFO film transferred on a flexible PET substrate using a 30 nm thick -MoO 3 layer.The magnetic hysteresis loops of the as-grown CFO/-MoO 3 /STO and the transferred CFO/Au/PET are similar, showing an in-plane easy axis Figures ( 3c and 5a), 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 Figure 5b, 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 versus the in-plane loop of the flat film, Figure 5c.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. [18,37]ur results show that mechanical exfoliation of a -MoO 3 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 -MoO 3 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.

Conclusions
We show that epitaxial -MoO 3 provides a platform for vdW epitaxy and transfer of complex oxide thin films.-MoO 3 is grown epitaxially with mosaic domains on a (001)-oriented STO substrate.The -MoO 3 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 inplane directions with -MoO CFO films grew epitaxially on the -MoO 3 with in-plane compressive strain, resulting in a magnetoelastic anisotropy favoring an in-plane easy axis.A ≈5 nm thick layer of modified -MoO 3 formed at the interface, likely due to bombardment of the -MoO 3 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 -MoO 3 in this work was grown using sputter deposition, which is compatible with semiconductor manufacturing processes.These results illustrate the potential -MoO 3 for production of membranes for flexible electronics applications.

Experimental Section
Target Preparation: Sputtering targets were produced in a box furnace using a conventional solid-state reaction method.Commercial MoO 3 powder with a purity of 99.5% (Kanto Chemical Co.Inc.) was pressed into cylindrical pellets and sintered for 5 h at 700 °C in air with a heating and cooling rate of 5 °C min −1 .The powder mixtures of Co 3 O 4 (Alfa Aesar, purity: 99.9%) and Fe 2 O 3 (Alfa Aesar, purity: 99.9%) for the CFO target were calcined at 1200 °C for 5 h.Uniaxial pressing was used to pelletize the synthesized powders into a 2 in.diameter disk before sintering at 1300 °C.
Thin Film Growth: The -MoO 3 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 -MoO 3 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.
Thin Film Transfer: The -MoO 3 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, -MoO 3 thin films were grown on substates, removed in 45 °C water for 24 h, then another -MoO 3 film was grown.Prior to thin film transfer, 800 nm of Au was grown on top of the -MoO 3 /STO and CFO/-MoO 3 /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 -MoO 3 thin film because the Au and CFO overlayers covered the sides of the film stacks.Au thin films on -MoO 3 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).
Characterizations: The structure of the films was characterized using XRD (XRD-7000, Shimadzu) with Cu K  radiation ( = 1.5406Å).The microstructures and surface morphologies of the sputtered and etched thin films were imaged with SEM (HITACHI, S-4800).The etched -MoO 3 was coated with Pt for conductivity.EDS in the SEM was used to analyze the composition of the -MoO 3 thin films.EBSD mapping results for 30 and 100 nm thick -MoO 3 thin films were obtained using a ZEISS Merlin high-resolution SEM equipped with an EBSD detector.The crystal structure of detached Au/CFO/-MoO 3 stacks on adhesive tape was examined using 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.The surface morphologies and thickness profiles of detached film on PET substrate were observed using an AFM (PSIA XE-150).
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 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.
,e.The two epitaxial relationships of -MoO 3 (010) || STO (001), -MoO 3 [100] || STO [100], and -MoO 3 (010) || STO (001), -MoO 3 [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 -MoO 3 film, Figure 1e, suggesting that the SEM contrast originates from the orientation of the domains.

Figure 1 .
Figure 1.a) Side and top view schematics of -MoO 3 thin film on STO substrate.Projection of orthorhombic -MoO 3 along the a-axis reveals a layered structure with van der Waals (vdW) bonding.The unit cell of the ab plane of STO and ac plane of -MoO 3 were projected along the STO [00 1] and -MoO 3 [0 10] directions.The green, sky blue, yellow, and red spheres represent the Sr, Ti, Mo, and O atoms, respectively.b) XRD pattern of -MoO 3 thin film grown at a substrate temperature of 600 °C, a working pressure of 20 mTorr, and an Ar to oxygen ratio of 10:1.c) Top-view SEM image and EBSD map of 100 nm thick -MoO 3 thin film.d) Comparison of the XRD peak positions of 30 and 100 nm thick films around the -MoO 3 (060) reflection.e) Top-view SEM and EBSD map of 30 nm thick -MoO 3 thin film.EBSD maps of the 100 and 30 nm thick -MoO 3 thin films on (001)-oriented STO substrate were recorded along the [100] direction.The image between (c) and (e) is the inverse pole figure color scale for crystallographic orientations.

Figure 2 .
Figure 2. Magnified XRD pattern around (001) and (002) reflections of a) as-grown -MoO 3 thin film on STO and b) after a 24 h etching in 45 °C water.c) XRD pattern of redeposited -MoO 3 thin film after etching for 24 h.d) Schematic diagram of the Au thin film transfer process by etching the -MoO 3 layer in water and redeposition of -MoO 3 on the remaining STO substrate.Photograph of e) Au thin film on -MoO 3 /STO before etching and f) transferred Au thin film on PET substrate.Photograph of a STO substrate g) after etching the -MoO 3 layer and h) after regrowing -MoO 3 thin film.

Figure 3 .
Figure 3. a) Schematic diagram of CFO film on STO (001) substrate (left), top-view SEM image (middle), and magnetic hysteresis loop (right) of a 100 nm thick CFO film.b) Schematic diagram of CFO/100 nm thick -MoO 3 /STO stack (left), top-view SEM image (middle), and magnetic hysteresis loop (right) of 100 nm thick CFO film.c) Schematic diagram of CFO/30 nm thick -MoO 3 /STO stack (left), top-view SEM image (middle), and magnetic hysteresis loop (right) of 100 nm thick CFO film.d) Comparison of XRD pattern of the CFO thin film on -MoO 3 layers with different thickness.e) EBSD orientation mapping of CFO thin film on 30 nm thick -MoO 3 layer.
, CFO/30 nm thick -MoO 3 /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 -MoO 3 /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 -MoO 3 /STO.Furthermore, the CFO/30 nm thick -MoO 3 /STO exhibited fewer crystallites than the CFO/100 nm thick -MoO 3 /STO, consistent with these surface features being associated with strain relaxation.

Figure 4 .
Figure 4. a) Schematic of the CFO thin film transfer process from Au/CFO/-MoO 3 /STO onto a sticky adhesive layer-coated flexible PET substrate via mechanical exfoliation.b) Optical images of the separated STO and PET substrates.c) Comparison of XRD patterns of the transferred CFO thin film on PET after growing on -MoO 3 layers with different thickness.d) Low magnification cross-sectional TEM image of the partially detached -MoO 3 on CFO and supporting Au.The TEM sample was cut along STO [100].e) (Top left) HRTEM image of the CFO--MoO 3 interface supported on Au layer.(Top middle) An enlarged HRTEM image of the -MoO 3 and (Top right) Fast Fourier transform (FFT) of selected area projected from [100] direction of -MoO 3 .(Bottom left) Magnified HRTEM image of the CFO--MoO 3 interface (Bottom middle and right).An enlarged HRTEM image of the CFO and the FFT of selected area projected along [100] of CFO.

Figure
Figure4ashows the transfer process of CFO thin films onto flexible PET substrates.An Au layer was deposited over the CFO/-MoO 3 /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 -MoO 3 nanosheets to break the vdW force.The PET/Au/CFO/-MoO 3 /STO was separated into two parts: the STO substrate part, which was dark in color compared to the bare STO or -MoO 3 coated STO, and the PET part, which exhibited a maroon color, as shown in Figure4b.The out-of-plane lattice parameters of the CFO layer did not change after exfolia-tion, as shown by a comparison of the magnified XRD pattern in Figure4c.The CFO films were detached from the substrates regardless of the thickness of the -MoO 3 ; however, exfoliation was unsuccessful for CFO grown on polycrystalline or amorphous -MoO 3 .FigureS6a-f (Supporting Information) shows that -MoO 3 remained on both the PET/Au/CFO and the STO after exfoliation.A SEM image of the separated -MoO 3 /CFO/Au/PET shows steps parallel to the [100] and [001] directions of -MoO 3 (FigureS6b, Supporting Information).When the tape is lifted, the nanosheets separate along the (010) vdW planes, but the sheets also break

Figure 5 .
Figure 5. a) In-plane and out-of-plane magnetic hysteresis loops of transferred CFO on PET after peeling apart the PET/Au (800 nm)/ CFO (100 nm)/-MoO 3 (30 nm)/STO stack.b) Magnetic hysteresis loops of CFO thin film with a 1 cm bending radius.c) Comparison of magnetic hysteresis loops of unbent and bent CFO thin films when magnetic field is applied in in-plane directions.Insets present schematics of the applied magnetic field along in-plane and out-of-plane directions.
(Supporting Information) shows steps in the side of the residual -MoO 3 block where the -MoO 3 separated.A high-resolution transmission electron microscopy (HRTEM) image along the [100] zone axis of STO, -MoO 3 , and CFO is shown in Figure 4e.Considering the two possible orientational variants of -MoO 3 , we expect to see regions of -MoO 3 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 -MoO 3 shown by the gray square displayed two orthogonal planes with spacing of 6.94 and 3.70 Å, which correspond to (020) and (001) of -MoO 3 , respectively.The (020), (001), and (021) spots observed in the fast Fourier transform (FFT) pattern in Figure 4e are consistent with the orthorhombic -MoO 3 phase, in which the (021) planes formed an angle of 62°w ith the (020) plane.The bottom middle HRTEM image shows perpendicular (02 2) and (0 22 ) 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 -MoO 3 extends across the entire sample and showed bright contrast with a thickness of 5 nm, and a different crystal structure from the -MoO 3 , Figure 4e lower left.The EDS line scan in Figure S7e (Supporting Information) indicates Mo, Co, and Fe are present at the interface.The layer most likely results from intermixing of the CFO into the MoO 3 .It has been reported that Co and Fe are soluble 3 [001] or [100] parallel to the STO [100].Metals such as Au, and oxides such as CoFe 2 O 4 (CFO), were grown on the -MoO 3 and subsequently transferred, either by dissolving the -MoO 3 in 45 °C water, or by mechanically exfoliating the -MoO 3 which separates along the vdW-bonded (010) planes.Residual -MoO 3 was removed in water, enabling reuse of the STO substrate.