Templated perpendicular ferroelectricity in textured Aurivillius oxide-based thin films

Domain: physics
Published: 2026-03-13
Editorial outcome: minor_revision

Abstract
The discovery of novel ferroelectric compounds and the modulation of polarization in
established ferroelectric materials have persistently represented crucial and highly dynamic
areas of research within the field of ferroelectric materials science. Here we demonstrate a
structural templating strategy to reconfigure Aurivillius-phase oxides into metastable
ferroelectric phases with tailored polarization. By stabilizing a metastable WO phase within
3
a Bi WO framework, we achieve robust out-of-plane polarization (10 µC/cm2) in textured
2 6
WO /Bi WO films, contrasting the purely in-plane polarization of pristine Bi WO . First-
3 2 6 2 6
principles calculations and atomic-resolution scanning transmission electron microscopy
unveil a WO phase with oxygen displacement-driven ferroelectricity. Prototype ferroelectric
3
field-effect transistors and memristors fabricated from these films exhibit robust switching
ratios (>106) and thermal stability up to 350°C. This work not only expands the family of
binary ferroelectric oxides but also establishes a generalizable paradigm for overcoming
polarization-directionality constraints in layered ferroelectrics.

Introduction
Ferroelectric materials with switchable polarization are the cornerstone of emerging non-
volatile memory and neuromorphic computing technologies. While perovskite oxides (e.g.,
Pb
0.8
Zr
0.2
TiO
3
, BaTiO
3
, BiFeO
3
) are promising for their large intrinsic polarization1-3, their
incompatibility with silicon processing and depolarization field with reduced dimension hinder
integration. Two-dimensional (2D) ferroelectric materials (e.g., CuInP S 4, In Se 5,6, SnS7, SnTe8,
2 6 2 3
and sliding 2D ferroelectric materials such as WTe
2
9, hBN10, GaSe11) offer considerable
advantages for flexible device applications and the novel assembly of heterostructures, but suffer
from inadequate polarization and poor thermal stability12. Recent advances in HfO -based systems
2
have revived interest in CMOS-compatible ferroelectrics13-16, yet the wake-up effect associated
with their metastable phases persist as critical bottlenecks17-19.

Aurivillius-phase oxides present a unique opportunity to bridge these gaps. Aurivillius-phase
oxides, characterized by its layered perovskite structure with alternating bismuth oxide layers and
perovskite-like structural layers, are distinguished by their fatigue-resistant ferroelectric
switching20,21, high Curie temperature22, and remarkable oxide ion conductivity23. However, most
Aurivillius ferroelectric compounds exhibit in-plane polarization24. Bi WO (BWO), the simplest
2 6
member of this family that consists of alternating [Bi O ]2+ and [WO ]2- layers aligned along the
2 2 4
c-axis, restricts spontaneous polarization to the in-plane (ab) directions25,26. This configuration
fundamentally conflicts with the vertical polarization orientation required by mainstream device
architectures, such as ferroelectric field-effect transistors (FeFETs) and ferroelectric tunnel
junctions (FTJs).

The functional properties of ferroelectric thin films can be engineered through diverse
strategies in heterostructure design. Notable approaches include the use of vertically aligned
nanocomposites to create emergent multiferroicity27-29, the application of interphase strain to
robustly enhance ferroelectric polarization30,31, and more recently, the exploitation of the unique
electrostatic environment within Aurivillius frameworks to stabilize complex ferrielectric orders32.

In this work, we resolve this conflict through interfacial engineering: By templating metastable
WO domains within a BWO matrix, we simultaneously achieve perpendicular polarization and
3
inherited thermal robustness. First-principles calculations and atomic-resolution scanning
transmission electron microscopy (STEM) confirmed the presence of a WO
3
structure with
ferroelectric properties. This metastable WO phase, similar to HfO -based ferroelectrics, belongs
3 2
to the family of binary oxide ferroelectrics. Prototype ferroelectric field-effect transistors and
memristors fabricated from these films exhibit robust switching ratios (>106) and thermal stability
up to 350°C. Our work expands the landscape of simple-oxide ferroelectrics, showing a structural
templating strategy that can reconfigure Aurivillius oxides with controllable polarization
properties.

Results
Pure Bi WO Thin Film
2 6
We start with pure BWO films as the control experiment. In literature reports, high quality
BWOfilms have been epitaxially grown on SrTiO (STO), LaAlO and NdGaO substrates, which
3 3 3
exhibit significant ferroelectricity with purely in-plane polarization25,26,33. BWO consists of an
orthorhombic phase structure with lattice constants a = 5.437 Å, b = 5.458 Å, and c = 16.430 Å34.
Fig. 1a shows the crystal structure of BWO as viewed from two crystallographic directions, the
black solid-line box indicates the unit cell of BWO. The STO (100) substrate possesses a cubic
lattice with the lattice constant a = 3.905 Å. To achieve lattice matching, epitaxial BWO films will
have their c-axis in the out-of-plane direction, and their ab plane rotated 45 around the c-axis,
thus aligning the [100] direction of BWO with the [110] direction of STO substrate25.

In our experiment, pure BWO films are grown on 0.7 wt % Nb-doped SrTiO (Nb:STO) (100)
3
substrates using pulsed laser deposition (PLD). The BWO thin film depicted in Fig. 1b exhibits a
thickness of approximately 40 nm. Due to the existence of epitaxial stress and the relatively small
thickness, some discontinuous voids still remain on the surface. The ferroelectric property of this
film was characterized using piezo-response force microscopy (PFSM). Figs. 1c and 1e present the
collected in-plane and out-of-plane PFM signals, respectively. Fig. 1c clearly shows a distribution
of four distinct spontaneous in-plane polarization states in the BWO thin film. In contrast, no stable
out-of-plane polarization is detected (Fig. 1e), the observed signal variations are attributed solely
to surface topography. In further experiments, voltage was applied to the atomic force microscope
(AFM) tip during scanning within the two dashed boxes (Figs. 1d and 1f). Due to the trailing
electric field from the tip slow-scan direction35-37, some of the in-plane ferroelectric polarization
states were noticeably altered (Fig. 1d). Because the electrically conductive Nb:STO substrate was
used, a vertical electric field also exists within the film. However, no change in the out-of-plane
PFM response was observed in Fig. 1f. This indicates that the pure-phase BWO thin film lacks an
out-of-plane ferroelectric polarization, corroborating with reports in previous literatures25,26,38,39.

Epitaxial textured two-component WO /Bi WO films
3 2 6
We design a nanocomposite Aurivillius oxide by adding WO component during the growth
3
of a pure BWOfilm (denoted as the WO /BWOfilm). In order to grow nanocomposite WO /BWO
3 3
films, we use two targets, a pure BWO target and a pure WO target. A 240 pulse BWO seed layer
3
was grown initially on the substrate, followed by 10 alternating cycles of WO (10 pulses/cycle)
3
and BWO (30 pulses/cycle) deposition, and subsequently terminated by 210 pulse BWO. The
resulting deposition component ration of WO : BWOis 1:7.5.
3

The atomic structure of as prepared nanocomposite WO /BWO films were studied by
3
aberration-corrected STEM, with cross-section samples cut from the as prepared film by focused
ion beam (FIB). Fig. 2a shows a typical STEM image of the cross-section of the film. The black
region corresponds to the Nb:STO substrate, with its in-plane orientation along the [100] direction.
In the film region, the darker gray contrast corresponds to the WO domains, which consist of W
3
and O atoms, while the lighter gray contrast corresponds to the BWO single-crystalline framework.
This contrast originates from the Z-contrast mechanism in HAADF-STEM imaging, where the
presence of heavy Bi (Z=83) atoms in BWO yields a stronger scattering signal than the WO phase.
3
Both the BWO and WO regions exhibit a sharp interface with the Nb:STO substrate, confirming
3
a robust epitaxial relationship. Moreover, the BWO and WO lattices are aligned well along the
3
[110] in-plane orientation (Fig. 2e).

In the BWO region, a layered lattice structure is clearly visible, with alternating bismuth-
oxygen (Bi-O) and tungsten-oxygen (W-O) layers along the c-axis. The alternation of light and
dark contrast is attributed to the differing electron scattering factors of the various atomic layers.
In contrast, the layered structure is not discernible in the WO domain. Fig. 2f provides a higher
3
magnification atomic-scale characterization of a WO region, corresponding to the energy-
3
dispersive X-ray spectroscopy (EDS) analysis of tungsten (W), bismuth (Bi) and oxygen (O)
shown in Figs. 2g-i. The EDS spectrum confirms the presence of W atoms and absence of Bi atoms
in the nanocrystal region. On the other hand, high-resolution XPS spectra (Supplementary Fig. 1)
reveals O, Bi, and W components in the nanocomposite thin film, with their binding energies
closely match those of pristine BWO phase, with no detectable W peaks corresponding to reduced
tungsten species (W4+/W5+), indicating that the nanocrystal is a pure WO phase. Raman
3
spectroscopy analysis (Supplementary Fig. 2) reveals characteristic vibrational peaks of BWO
(such as the symmetric stretching mode of WO octahedra at 797 cm-1)40 without notable shift even
6
in the presence of WO . This indicates that the primary lattice structure of BWO remains
3
unchanged by the incorporation of WO .
3

Figs. 2b-d present the fast Fourier transform (FFT) patterns derived from the Nb:STO substrate,
BWO matrix, and WO domain in Fig. 2a, respectively. Both the BWO and WO FFT patterns
3 3
exhibit a consistent 45° rotation relative to the STO substrate, confirming their shared epitaxial
relationship. The FFT pattern from the BWO region exhibit excellent agreement with the known
parameters of pristine BWO structure (Supplementary Fig. 3a). The WO FFT pattern shows
3
closely aligned symmetry and orientation with that of BWO, consistent with their structural
similarity, while differing in the absence of BWO-specific superlattice spots. This shared epitaxial
alignment, distinct from the STO substrate orientation, provides definitive evidence that the WO
3
phase grows via a structural templating effect mediated by the BWO matrix rather than through
direct nucleation on the substrate. These results indicate that our interfacial templating strategy
fundamentally alters the local chemistry, surpassing mere topotactic alignment, as manifested by
the distinct lattice modulation characteristics: the presence of superlattice spots in BWO,
originating from its commensurate Aurivillius superlattice, stands in sharp contrast to their absence
in the structurally simplified, cation-homogeneous WO
3
phase.

To further identify the atomic structure of the WO phase, we conducted structural
3
characterizations in different crystallographic directions. Figs. 3a and 3b depict the BWO and WO
3
regions from the same film in Fig. 2, but the TEM sample was cut along a different crystallographic
orientation, rotated 45 as compared with Fig. 2. Here the out-of-plane direction is aligned along
[001] and the in-plane direction along [100], respectively. In Fig. 3a, the BWO structure distinctly
shows a layered configuration, featuring alternating Bi-O layers and W-O layers along the c-axis.
The inserted theoretical crystal structure corresponds well with the atomic structure observed in
the STEM images. In contrast, the WO region lacks layering but displaying a square lattice
3
geometry (Fig. 3b). Figs. 3c and 3d show the FFT analysis corresponding to Figs. 3a and 3b, where
Fig. 3c exhibits precise congruence with the reference BWO crystallographic parameters
(Supplementary Fig. 3c). The FFT patterns of the WO phase in Fig. 3d closely resemble those of
3
BWO, apart from the absence of 32 superlattice spots caused by the periodic modulation. This
observation provides direct evidence that the WO phase grows via a structural templating effect
3
inherited from the BWO matrix. To validate the general applicability of the structural templating
mechanism, we synthesized ultrathin films by employing the identical epitaxial growth mode with
a reduced deposition dosage. The 8 nm-thick textured nanocomposite WO /BWO thin film
3
exhibits structural and compositional invariance across its engineered interfaces, as evidenced by
cross-sectional STEM analysis (Supplementary Fig. 4). Importantly, the WO phase forms a semi-
3
coherent epitaxial relationship with the BWO template, demonstrating the robustness of this
templating strategy in governing phase evolution across dimensional scales, further validating the
epitaxial growth and crystallographic alignment of the nanocomposite film.

Notably, this WO structure does not match to any existing WO phase in literature41-43,
3 3
signifying the stabilization of a metastable polymorph. While previous studies have demonstrated
that epitaxial strain from perovskite substrates can homogeneously distorts the bulk WO
3
structure44,45，the formation mechanism here is fundamentally different. This distinction can be
understood by analogy ferroelectric HfO
2
—which adopts a non-ferroelectric monoclinic P2
1
/c
phase at ambient conditions, but exhibits ferroelectricity in its metastable orthorhombic Pca2
1
phase18,46. Crucially, BWO shares this Pca2 ferroelectric space group. This structural templating
1
mechanism is further supported by X-ray diffraction (XRD) and reciprocal space mapping (RSM)
analyses. Conventional θ–2θ scans (Supplementary Fig. 5) reveal no significant peak shifts
between pure BWO and WO /BWO nanocomposite films, which is consistent with the presence
3
of nanosized WO domains whose diffraction is masked by the dominant BWO matrix. In contrast,
3
a pure WO film grown directly on Nb:STO (001) under identical conditions exhibits distinctly
3
different XRD features, corresponding to a conventional monoclinic phase47,48, which confirms
that the metastable polar phase observed in the nanocomposite is not stabilized by the substrate
alone. RSM measurements provide further insight into the strain state of the heterostructure.
Around the STO (001) reflection (Supplementary Figs. 6a,b), the WO /BWO film shows a
3
systematic shift toward lower q , indicating out-of-plane expansion. Consistently, the RSM around
z
the STO (103) reflection (Supplementary Figs. 6c,d) reveals a shift toward higher q accompanied
x
by lower q , confirming in-plane compression coupled with out-of-plane elongation. These RSM
z
results are in excellent agreement with cross-sectional STEM observations (Supplementary Fig.
7), collectively demonstrating that the BWO framework acts as an atomic-scale template, guiding
the WO into a semi-coherent, non-bulk-like configuration stabilized by the host lattice.
3

The BWO structural template provides a shared epitaxial orientation and a local chemical
environment, which directs the WO phase to nucleate and grow into nanodomains with specific
3
crystallographic characteristics, rather than forming randomly oriented or thermodynamically
stable phases. The primary role of the STO substrate is thus to enable the epitaxial growth of a
high-quality BWO template, which in turn architecturally guides the formation of this polar WO
3
phase.

The origin of ferroelectricity
The atomically-resolved TEM images suggest a straightforward structural model as shown in
Fig. 4a. In this model, each W atom is coordinated by five oxygen atoms, forming a pyramid shape,
distorted pentahedral as the structural unit. There are two different types of oxygen atoms in this
structure model. First, four O atoms are located roughly in the same plane with the W atoms (the
bottom plane of the pyramids), where each O atom connects two pyramid units, as marked by the
dotted rectangle and circle in Fig. 4a. Second, there is another O atom sitting at the top site of the
pyramid, which bonds solely to the W atom below it (marked by red arrow in Fig. 4a). In total this
configuration gives a WO stoichiometry, agreeing with our experiments. These WO pyramids
3 5
form a lattice precisely align with experimental TEM images from different views. First-principles
density functional theory (DFT) calculations verify the structural stability of this model.

Remarkably, we find that such an atomic configuration is naturally ferroelectric (FE). In Fig.
4a, all the pyramid units are pointing upward. Structural analysis reveals a Cm space group
configuration that breaks out-of-plane symmetry compared to the centrosymmetric WO
6
octahedral arrangement, generating a substantial out-of-plane spontaneous polarization of ~80
μC/cm2 (Fig. 4c). This polarization magnitude approaches that of prototypical perovskite
ferroelectrics, including BiFeO (100 μC/cm2)49,50 and significantly exceeds that of BaTiO (26
3 3
μC/cm2)51,52. More interestingly, the structural model naturally suggests the coexistence of a
paraelectric phase (PE), where half of the pyramid units are flipped (Fig. 4b). The reversed
polarization in neighboring layers results in zero net polarization in this phase. If all the pyramids
are flipped, it will result in the switching between the ferroelectric states (from FE' to FE'').

First-principles Nudged Elastic Band (NEB) simulations employing a 2×2×1 supercell unveil
a multi-stage polarization reversal pathway. Fig. 4e illustrates the energy profile for dipole
switching, where Fig. 4d shows the two states before and after flipping from FE'' to FE'. The
computed activation energy barrier for individual oxygen migration approximates 330 meV,
indicating practical feasibility for electric-field-induced polarization switching. This energy
landscape suggests that the ferroelectric transition proceeds through a concerted mechanism
involving sequential atomic displacements rather than simultaneous collective motion.
Additionally, the ferroelectricity is achieved here through oxygen sublattice engineering in a
simpler binary oxide system, different from conventional perovskites. The combination of giant
spontaneous polarization and moderate switching barriers positions this WO phase as a promising
3
candidate for non-volatile memory applications.

To further corroborate the consistency between the computational and experimental results,
we performed diffraction pattern simulations along the [1̅10] and [010] crystallographic directions
for the theoretically optimized structure. As shown in Supplementary Figs. 3b and 3d, the
simulated FFT patterns exhibit excellent agreement with the experimental data presented in Figs.
2d and 3d, validating the structural fidelity of the computational models and their alignment with
the observed lattice configurations.

Perpendicular Ferroelectricity of the textured nanocomposite WO /Bi WO film.
3 2 6
Indeed, the most remarkable finding with our textured nanocomposite WO /BWO films is their
3
robust ferroelectricity with perpendicular polarization. We employed PFM for a comprehensive
analysis of their ferroelectric response. The conductive Nb:STO substrate allows us to ground the
substrate and ensure the application of a vertical electric field to the thin films for out-of-plane
polarization measurements. A pre-designed badminton pattern was utilized during the PFM
scanning process (Fig. 5f), where white regions correspond to the application of a positive voltage
(+8 V) by the PFM tip, and black regions represent a negative voltage (-8 V). During point-by-
point scanning of the thin film surface with the AFM tip, voltages were applied according to this
pattern. Figs. 5a,b show the resulting out-of-plane PFM amplitude and phase after the electric field
was applied. Point-by-point scanning across the film surface resulted in distinct ferroelectric
domains, as observed both in the out-of-plane PFM amplitude and phase images. The amplitude
image shows minimized polarization at domain walls, which are formed between regions of
opposite polarization. Meanwhile, the phase image reveals a clear 180° phase shift between
adjacent domains, with black areas indicating upward polarization and yellow areas representing
downward polarization. Analysis of the PFM amplitude and phase data (Fig. 5c) along the red
dashed lines in Figs. 5a,b clearly indicates minimized polarization amplitude at the domain walls,
with a phase difference of 180° between the adjacent domains, thereby confirming the excellent
out-of-plane ferroelectric properties of the fabricated thin films. Supplementary Fig. 8 also
demonstrates comparable out-of-plane polarization switching characteristics in the 8 nm thick
nanocomposite film, further illustrating the low-dimensional nature of the ferroelectric thin films
constructed in this work.

In addition to out-of-plane measurements, in-plane PFM amplitude and phase images were
also recorded (Figs. 5d,e). The images demonstrate variations in in-plane ferroelectric polarization,
primarily induced by the trailing electric effect of the PFM tip during the scanning process. This
observation indicates that the thin films exhibit both out-of-plane and in-plane ferroelectric
polarization. Notably, higher-resolution imaging (Supplementary Fig. 9) confirms that the
characteristic in-plane ferroelectric domain structure of the BWO matrix remains clearly
resolvable at finer scan scales, demonstrating its preservation within the nanocomposite
architecture. Furthermore, to evaluate the stability and reliability of the ferroelectric switching, we
conducted polarization switching curve tests at a single PFM tip point (Fig. 5g). Ten consecutive
measurements revealed highly stable and repeatable ferroelectric hysteresis and butterfly loops,
with a consistent phase difference of 180° and a switching threshold voltage of approximately 4 V.
These results highlight the robust ferroelectric switching performance of the thin films.

To eliminate potential artifacts and verify the authenticity of the observed ferroelectric
properties, we performed alternating current (AC) voltage-dependent polarization switching
measurements (Fig. 5h). When the applied AC voltage exceeds 3 V, close to the threshold voltage
for ferroelectric switching, the polarization hysteresis loop collapsed, this behavior confirms the
genuine nature of the out-of-plane ferroelectric polarization in the textured nanocomposite
WO /BWO film53. Additionally, temperature-dependent polarization curve tests were conducted
3
to assess the films' thermal stability (Fig. 5i). Measurements were performed from room
temperature up to the maximum operating temperature of 250°C, where the polarization switching
curves exhibited robust stability despite minor jitter caused by temperature-induced contact
instability between the tip and the sample. These results collectively demonstrate the exceptional
durability and performance of our textured nanocomposite WO /BWO thin films under varying
3
thermal conditions.

Performance of ferroelectric FET and memristor devices made from WO /Bi WO films
3 2 6
Our textured nanocomposite WO /BWO thin films exhibit also macroscopic ferroelectricity,
3
as measured by a ferroelectric analyzer. Fig. 6a displays the measured ferroelectric hysteresis loop,
from which the key ferroelectric parameters were quantitatively extracted. Specifically, at an
applied electric field of 4 MV/cm, the film exhibits a remnant polarization (P) of approximately
r
10 μC/cm2, accompanied by a coercive field (E ) of 2.5 MV/cm. Since the out-of-plane
c
ferroelectric polarization in these films primarily originates from the WO domains, the measured
3
value is lower than that of the pure ferroelectric WO calculated in Fig. 4c (80 μC/cm2). The
3
macroscopic ferroelectric hysteresis loop serves as direct evidence for the presence of out-of-plane
polarization in the nanocomposite films, suggesting that these ferroelectric thin films hold
significant potential for applications in electronic devices.

To further showcase the potential device performance based on our films, we deposited a 5 nm
thick HfO insulating layer on the as-grown films using atomic layer deposition (to minimize
2
leakage effects during testing, the leakage currents primarily originate from oxygen vacancy
defects introduced during the growth of the oxide films, as well as conducting pathways formed
along interface within the textured film.) and subsequently transferred a monolayer MoS
2
via a dry
transfer method. Fig. 6b illustrates the schematic diagram of the complete device structure, for
which a FeFET characterization was conducted. Fig. 6c presents the double-sweep transfer
characteristics of the device, obtained by scanning the back-gate voltage (V
GS
) from −1.5 V to +5
V and then back to −1.5 V at a fixed drain voltage (V ) of 1 V. The FeFET exhibits a high on/off
DS
current ratio of 3×106, demonstrating its excellent performance. Then, we deposited gold
electrodes on the film surface to investigate the memristive effect induced by out-of-plane
ferroelectric polarization. Fig. 6d shows the schematic of the ferroelectric memristor device, while
Fig. 6e presents the current-voltage (I-V) characteristics under voltage sweeps ranging from -5 V
to +5 V. The device transits from the initial high-resistance state (HRS) to a low-resistance state
(LRS) after a 0 to +5 V sweep, maintains LRS during the subsequent +5 V to -5 V scan, and returns
to HRS from -5 V to 0 V. We further conducted high-temperature tests demonstrating robust
memristive behavior across a wide temperature range from 100°C to 350°C (Fig. 6f), highlighting
the potential of our ferroelectric thin films for high-temperature memory applications. This
comprehensive analysis highlights the promising capabilities of the fabricated textured
nanocomposite WO /BWO films for advanced electronic applications, supported by both
3
macroscopic ferroelectric measurements and device-level performance evaluations.

In summary, we demonstrate a structural engineering strategy to achieve out-of-plane
ferroelectricity in a metastable WO phase derived from the Aurivillius oxide BWO framework.
3
Through a combination experiments and first-principles calculations, we have confirmed the
existence of this previously unreported ferroelectric WO phase. The resulting textured
3
nanocomposite WO /BWO thin films exhibit both out-of-plane and in-plane ferroelectric
3
polarization, showcasing stable and repeatable switching behavior. Prototype FeFETs and
ferroelectric memristors fabricated from the films exhibit remarkable switching ratios (>106)
coupled with robust thermal stability up to 350°C. This work not only expands the family of binary
ferroelectric oxides but also provides a generalizable approach for polarization enhancement in
Aurivillius oxides and other layered ferroelectrics.

Methods
Thin film growth
Bi WO , WO /Bi WO and WO thin films were epitaxially grown on (001)-oriented Nb:SrTiO
2 6 3 2 6 3 3
(0.7% wt. doped) substrates by pulsed laser deposition using a 248 nm KrF excimer laser. The
deposition was carried out at a substrate temperature of 750℃ under an oxygen environment of
400 mtorr. The laser energy density and repetition rate were maintained at approximately 1.4 J cm-
2 and 2 Hz, respectively. After deposition, the samples were cooled down to room temperature in
7.5 torr of O with a cooling rate of 5 °C/min.
2

Piezoresponse force microscopy (PFM)
PFM measurements were performed using an Oxford Instruments Cypher (E)S atomic force
microscope, equipped with conductive Pt/Ir-coated silicon probes. The local electromechanical
response was characterized in both out-of-plane and in-plane directions using AC excitation
amplitudes of 800 mV at frequencies of 350 kHz and 650 kHz, respectively. Litho-PFM mode was
utilized to locally pole the material and write domain patterns by applying a DC bias through the
scanning tip, enabling the assessment of polarization switching. Spectroscopic PFM (SS-PFM)
measurements were performed in dual AC resonance tracking (DART) mode. In this mode, an AC
excitation signal was superimposed on a series of DC triangular and sawtooth waveform voltages
to track the bias-dependent electromechanical response.

Scanning transmission electron microscopy (STEM)
Cross-sectional specimens for STEM were prepared by focused ion beam (FIB) lift-out and
subsequently thinned using a Gatan 691 PIPS for final ion milling. High-angle annular dark-field
(HAADF) STEM imaging was performed on an aberration-corrected (scanning) transmission
electron microscope (JEOL, Grand ARM 300) operated at an accelerating voltage of 300 kV.

Materials Characterization
X-ray θ–2θ scan and reciprocal space mappings were performed using a high-resolution X-ray
diffractometer (Bruker, D8 Advance). Raman spectroscopy was performed on a Horiba LabRam
HR Evolution system with a 532 nm laser excitation. X-ray photoelectron spectroscopy (XPS)
measurements were conducted on a Thermo Fisher ESCALAB XI+ spectrometer using a
monochromatic Al Kα X-ray source. All binding energies were calibrated relative to the
adventitious carbon C 1s peak at 284.8 eV.

Functional Property Measurements
The P-E hysteresis loops and I-E curves were measured on a ferroelectric analyzer (tf analysis
3000E).

Device Fabrication
For two-terminal ferroelectric memristors fabrication, AZ5214 photoresist was spin-coated onto
the sample surface at a rate of 4000 rpm and subsequently baked at 110°C for 2 minutes. The
patterning of the top gate was achieved through UV-lithography (MA6, Karl Suss), followed by
the deposition of 30 nm Au metal via electron beam evaporation. The lift-off process was then
employed to obtain the patterned top electrodes.

For three-terminal ferroelectric transistors fabrication, 5 nm thick hafnium oxide dielectric layer
was grown on the bottom-gate at 150°C using atomic layer deposition (ALD). Following this, the
monolayer MoS film was carefully transferred onto the HfO -WO /Bi WO -Nb:SrTiO substrate.
2 2 3 2 6 3
Next, the MoS channels were defined through UV-lithography and reactive ion etching (RIE),
2
employing the Plasma Lab 80 Plus instrument from Oxford Instruments Company. To fabricate
the device contacts, UV-lithography was utilized to define the contact regions, which were
subsequently metallized with a 30 nm Au through electron beam evaporation.

Device Characterization
Characterization of electrical and optoelectronic properties was performed using a probe station
(Janis ST-500-1-(4CX)) equipped with Agilent semiconductor analyzers (models 4156C and
B1500A).

DFT calculation details
Density functional theory (DFT) calculations were carried out using the generalized gradient
approximation (GGA) for the exchange-correlation potential, the projector augmented wave (PAW)
method, and a plane-wave basis set as implemented in the Vienna ab-initio simulation package
(VASP). The lattice constants of WO were fixed to the experimentally determined values: a =
3
5.34 Å, b = 5.34 Å, c=5.90 Å, respectively. An 8 × 8 × 1 k-mesh was used to sample the first
Brillouin zone. The kinetic energy cut-off for the plane-wave basis was set to 400 eV for the
geometric properties and energy calculations. The atomic positions of the structures were fully
relaxed until the residual force per atom was less than 0.001 eV/Å. A second-order Methfessel–
Paxton smearing method with a sigma value of 0.05 eV was adopted during calculations. The
ferroelectric switching in FE and PE phases, respectively, was investigated utilizing the nudged-
elastic-band (NEB) method.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors present a study based on interfacial engineering where they template metastable WO3 in a BWO matrix to manipulate the direction of polarization within the layered materials crystal structure. This is an interesting known approach to manipulate polar behavior and can be instrumental materials' design for ferroelectric applications. The following suggestions (if implemented) can be beneficial in improving the readability and reproducibility of the manuscript:\n(a) References: The authors should consider citing other studies on ferroelectric thin films leveraging a similar nano-structuring strategy as the authors, also known as vertically aligned nanocomposite thin films in some cases. Including such citations can help validate the findings in the manuscript well.\n(b) In line 159, the authors mention \"The FFT pattern from the BWO region (Figure 2b) exhibit excellent agreement with the known parameters of pristine BWO structure (Figure S3a). The FFT pattern of WO3 closely resemble that of BWO, confirming their structural similarity. Notably, the absence of superlattice spots in WO3 reveals a reduced periodicity along the [001] crystallographic direction compared to BWO. This observation underscores distinct lattice modulation characteristics between the two systems despite their shared structural motifs.\"\nThe authors should justify/explain this observation further. It is a bit surprising that despite the structural similarities between BWO and WO3 crystal structures, the lattice modulation characteristics for them are distinct in nature.\n(c) The authors should consider comparing and contrasting the macroscopic polarization-electric field behavior (Figure 6a) in WO3/BWO nanocomposite thin films with that for BWO and WO3 films individually in identical device configuration. Such analysis can improve the clarity of the study further.\n(d) The authors should discuss the ferroelectric hysteresis loop (Figure 6a) in terms of saturation polarization, remanent polarization and coercive field. It is very challenging to estimate the values for these ferroelectricity-based parameters in the current form.\n(e) In line 327, the authors mention \"To further showcase the potential device performance based on our films, we deposited a 5 nm-thick HfO2 insulating layer on the as-grown films using atomic layer deposition (to minimize leakage effects during testing) and subsequently transferred a monolayer MoS2 via a dry transfer method.\" They should consider addressing the comment about leakage by elaborating on the source of the mentioned leakage current in the studied heterostructures.", "reviewer_2": "The manuscript reports a previously unreported WO3 phase. It demonstrates a novel structural engineering strategy to obtain out-of-plane ferroelectricity in a metastable WO3 phase. The WO3/BWO thin film composite exhibits both out-of-plane and in-plane ferroelectric polarization.\nWhile the authors present a potentially interesting approach, several major issues should be addressed before the work may be considered for publication.\nThe microscopic approach of structural analysis is good but it focuses on a very small part of the sample. The structural analysis part of the manuscript may be significantly improved by incorporating detail advanced analyses such as XRD, PDF, XAS, EXAFS and XANES (including in-operando). The data should be compared with the stable phases.\nThe growth conditions should be mentioned in a comprehensive manner.", "reviewer_3": "In this manuscript, the authors report the synthesis of thin films composed of a mixture of Bi2WO6 (BWO) Aurivillius phase and WO3 (WO) binary oxide. The films exhibit segregated nanocrystalline regions corresponding to the two phases. The authors claim that the WO3 regions are stabilized in a typically metastable phase that exhibits out-of-plane polarization. When combined with the in-plane polarization of the Aurivillius BWO phase, the overall film is proposed to display switchable polarization along both in-plane and out-of-plane directions. Finally, the authors demonstrate potential device integration by embedding the films into ferroelectric FET and memristor-type device structures.\n0)The manuscript presents an interesting concept, proposing the use of Aurivillius Bi2WO6 (BWO) as a structural template to achieve out-of-plane polarization in the WO3 phase. While the topic is promising, the current presentation lacks sufficient experimental evidence to convincingly support the claims. The central argument, that BWO serves as a structural template, is not fully substantiated. From the data provided, the BWO and WO3 phases appear to be segregated within the thin film. Consequently, it is unclear how BWO functions as a template for the WO3 phase. In my view, the observed polarization in WO3 is more likely a result of epitaxial strain imposed by the STO substrate rather than a direct templating effect from BWO. A true template effect would require evidence of coherent, lattice-matched growth across the film, rather than phase segregation.\nAdditionally, the authors attribute the polarization in WO3 to stabilization of the Pca21 phase. However, the phase analysis is very limited, relying primarily on STEM images along a single zone axis. No comprehensive structural characterization (e.g., multi-axis STEM, XRD, or reciprocal space mapping) is presented to rigorously confirm the Pca21 phase or quantify the polar distortion.\nOverall, due to the lack of in-depth structural analysis, limited high-quality supporting data, and insufficient experimental details provided, the manuscript does not convincingly support its key claims, required for publication in Nature Communications. I therefore cannot recommend publication in Nature Communications in its current form.\nHere are my point-by-point technical comments:\n1)I could not find a “Methods” section. Was it submitted elsewhere? Because most of the experimental details of the variety of measurements they perform are missing. Providing these details is critical for assessing the validity and reproducibility of the results.\n2)The manuscript does not provide sufficient technical context for its claims. First, the concept of using Aurivillius layered frameworks to create composite materials with properties absent in the pure Aurivillius phases is not new; however, the manuscript fails to acknowledge prior literature. For example, this paper (https://doi.org/10.1038/s41467-025-60176-8) reports the synthesis of composite films consisting of Aurivillius Bi5FeTi3O15 and BiFeO3.\nSecond, the authors provide almost no discussion of strain engineering in WO3 thin films. Numerous studies and recent preprints demonstrate that polar phases in WO3 can be stabilized through epitaxial strain or strain gradients imposed by the substrate https://doi.org/10.48550/arXiv.2505.11021 , DOI: 10.1021/acs.jpclett.3c01546, https://doi.org/10.1038/s41467-020-18644-w). A detailed analysis of the epitaxial strain from the STO substrate and its potential role in stabilizing the polar phase is necessary to place the work in the proper context and to support the manuscript’s central claims.\n3)In lines 60-62, the authors state, “By templating metastable WO3 nanoparticles within a BWO matrix, we simultaneously achieve perpendicular polarization and inherited thermal robustness.”. This description is misleading. The term “nanoparticles” implies isolated, discrete 0D entities, which is not the case here; the WO3 regions are part of a continuous thin film and appear to be phase-segregated rather than discrete particles Furthermore, the use of the term “templating” suggests control over the size, shape, and spatial arrangement of these regions, which is not demonstrated in the manuscript. What is observed is simply a film containing two segregated crystalline phases. No evidence is provided that the BWO phase actively controls or templates the structure of the WO3 phase. As such, the claims regarding nanoparticle templating and the associated functional advantages are not supported by the data.\n4)In Figure 1c, the orientation of the AFM cantilever relative to the thin film should be clearly indicated. The polarization directions of the in-plane domains, as well as the in-plane crystallographic axes of the film, should also be labelled. The authors mention four in-plane polarization variants, yet it is unclear why there is no corresponding contrast in the VPFM channel. Some of these variants would be expected to produce a measurable deflection of the cantilever, thus a contrast in VPFM. A comprehensive analysis of in-plane polarization using a series of vector-PFM measurements is necessary to fully characterize the domain structure.\nIn Figure 1d, critical experimental details are missing. The sign of the applied voltage, the component to which the voltage was applied (tip vs. sample), the cantilever movement path, and the slow scanning axis should all be reported to support claims of switching induced by a trailing field. Based on the images, in-plane switching is not convincingly demonstrated. For example, in the innermost square, the brightest contrast increases, yet in the middle square, the expected increase in the darkest contrast is not observed. Clearly marking the switched regions using dashed lines in the pristine images (Figures 1c-e) would facilitate comparison before and after DC voltage application and improve clarity.\n5)In lines 116–117, the authors state that an initial BWO layer was first grown. However, this layer is not clearly visible in the STEM images presented in Figure 2. The manuscript should provide clear evidence of the presence, thickness, and continuity of the BWO layer, as well as any interfaces with the WO3 phase, to support the reported growth sequence.\n6)In lines 171–178, the authors discuss a “structural templating effect.” From the STEM images, it appears that the BWO and WO3 phases do not mix and form completely segregated regions within the thin film. Given this lack of intermixing, the system cannot reasonably be described as a “composite” material. Moreover, the STEM images suggest that the WO3 phase grows directly on the STO substrate rather than on or within the BWO regions. Accordingly, the statement in lines 176–178—“This observation provides direct evidence that the WO3 phase grows via a structural templating effect inherited from the BWO matrix”—is misleading. The BWO layer does not appear to act as a template; rather, the epitaxial relationship with the STO substrate likely governs the growth. To support any claim of templating, the authors should provide the FFT of the STO substrate and compare it with the WO3 and BWO regions, to demonstrate that the observed structural alignment is not merely inherited from the STO lattice, which also has a square FFT pattern similar to BWO.\n7)Figure 2: The authors should comment on the observation of contrast differences in the designated regions of the film. I.e., the brighter contrast in the WO3 part or the darker contrast regions in BWO. Providing a detailed explanation of the origin of this contrast, whether due to atomic number differences, thickness variations, strain, or imaging conditions, would help clarify the structural interpretation and support the identification of the two phases.\n8)In lines 182–185, the authors claim that WO3 retains lattice coherence with the BWO phase. From the presented STEM images, however, it appears that there is a relatively large region at the vertical interface where a transitional crystal structure forms, rather than clear lattice coherency. The authors should provide stronger evidence—such as high-resolution imaging across multiple zones, strain mapping, or FFT analysis—to support any claim of a coherent relationship between the two phases.\n9)In lines 186–192, the authors report the stabilization of a new metastable phase in WO3. However, insufficient structural information is provided beyond a qualitative comparison with the ferroelectric HfO2 phase using STEM images. Essential crystallographic parameters, such as the lattice constants (a, b, c), angles (α, β, γ), and unit cell volume, should be reported. To fully characterize this phase, X-ray diffraction analysis is necessary, including both θ–2θ scans and reciprocal space mapping to obtain crystallographic information along all three principal axes. Furthermore, the role of substrate-induced strain should be carefully analysed. The authors should discuss whether the observed metastable phase could instead be stabilized by epitaxial strain from STO, independent of the BWO layer. Such analysis is critical to substantiate claims regarding phase stabilization and the proposed templating effect.\n10)In Figure 5, the in-plane domains observed in the LPFM images seem to disappear in the BWO/WO3 film, in contrast to the pure BWO film. However, the STEM images indicate that some regions of the film consist of pure BWO. The authors should clarify what happens to the in-plane polarization in these BWO regions within the composite film. A discussion of how the presence of WO3 affects the in-plane ferroelectric domains of BWO, and whether any depolarization or screening effects occur, is necessary to interpret the LPFM results.\n11)In Figure 6, the presented P-V loop appears leaky. To support the claim of ferroelectric switching, the authors should also report the transient switching current alongside the P-V loop. Additionally, essential details of the ferroelectric capacitor structure are missing, including the material and dimensions of the top electrode, the diameter of the top electrode, the type of voltage pulse applied, and the pulse period. Providing these details is necessary to assess the reliability and reproducibility of the reported ferroelectric behaviour.\n12)The authors attribute ferroelectricity in WO3 to oxygen displacements, but no direct experimental evidence is provided to support this claim. The current STEM images primarily resolve the cation sublattice and are insufficient to confirm oxygen off-centering. To substantiate the proposed mechanism, a more advanced STEM technique, such as integrated Differential Phase Contrast (iDPC-STEM), is necessary to directly visualize oxygen positions and quantify their displacements."}, "author_response": "Response Letter for NCOMMS-25-61023\nFor Reviewer #1:\nComment: The authors present a study based on interfacial engineering where they template\nmetastable WO in a BWO matrix to manipulate the direction of polarization within the layered\n3\nmaterials crystal structure. This is an interesting known approach to manipulate polar behavior\nand can be instrumental materials' design for ferroelectric applications. The following\nsuggestions (if implemented) can be beneficial in improving the readability and reproducibility\nof the manuscript:\nResponse:\nWe are grateful to the reviewer for the positive feedback and for recognizing the\nsignificance of our interfacial engineering approach for manipulating polarization direction.\nWe also sincerely thank the reviewer for constructive suggestions, which are essential for\nenhancing the clarity and impact of our work.\nComment 1: References: The authors should consider citing other studies on ferroelectric thin\nfilms leveraging a similar nano-structuring strategy as the authors, also known as vertically\naligned nanocomposite thin films in some cases. Including such citations can help validate the\nfindings in the manuscript well.\nResponse 1:\nWe thank the reviewer for valuable suggestion. We fully agree that referencing previous\nstudies on vertically aligned nanocomposite thin films offers important background for our\ntemplating strategy and strengthens the validation of our methodology and results.\nIn response to this comment, we have expanded the discussion in the introduction (page 3,\nline 60: “The functional properties of ferroelectric thin films can be engineered through diverse\nstrategies in heterostructure design. Notable approaches include the use of vertically aligned\nnanocomposites to create emergent multiferroicity27-29, the application of interphase strain to\nrobustly enhance ferroelectric polarization30,31”) including representative studies on\nmultiferroic vertically aligned nanocomposite systems (H. Zheng et al., Science, 303, 661-663,\n(2004), Akira Imai et al., ACS Nano, 7, 12, 11079–11086, (2013), Li, L. et al., Nano Research.\n14, 4789–4794, (2021).), alongside key works that demonstrate the role of interphase strain in\nenhancing ferroelectric properties (Linxing Zhang et al., Science, 361, 494-497, (2018), Yilin\nWang et al., Journal of the American Chemical Society, 143, 17, 6491–6497, (2021)).\nComment 2: In line 159, the authors mention “The FFT pattern from the BWO region (Figure\n2b) exhibit excellent agreement with the known parameters of pristine BWO structure (Figure\nS3a). The FFT pattern of WO closely resemble that of BWO, confirming their structural\n3\nsimilarity. Notably, the absence of superlattice spots in WO reveals a reduced periodicity along\n3\nthe [001] crystallographic direction compared to BWO. This observation underscores distinct\nlattice modulation characteristics between the two systems despite their shared structural\n1\nmotifs.”\nThe authors should justify/explain this observation further. It is a bit surprising that despite the\nstructural similarities between BWO and WO crystal structures, the lattice modulation\n3\ncharacteristics for them are distinct in nature.\nResponse 2:\nWe thank the reviewer for this insightful comment, which allows us to clarify a key aspect\nof our work. We agree that the distinct lattice modulation characteristics between the BWO\nmatrix and the templated WO phase are a critical observation, and we appreciate the\n3\nopportunity to elaborate on its physical origin.\nIn BWO, the super-period is naturally defined by the full repeating unit of the Aurivillius\nstructure along the stacking direction, i.e., the combined thickness of one (Bi O )2+ layer and\n2 2\none (WO )2− slab. This long-range compositional and structural modulation is the direct\n4\nphysical origin of the observed superlattice spots. The structure of BWO is known to be\ncommensurately modulated, meaning the wavelength of the modulation is an integer multiple\nof the underlying sublattice periodicity. The superlattice spots are therefore a direct and\nunambiguous signature of BWO’s layered, superlattice architecture.\nThe absence of these superlattice spots in the WO phase is a direct consequence of the\n3\nstructural simplification that occurs during the templating process. We hypothesize that the\nformation of the metastable WO phase involves a compositional and structural rearrangement\n3\nthat leads to a more homogeneous cation population (primarily W6+) and potentially a different,\nsimpler pattern of pentahedral connectivity. Consequently, the WO phase exhibits a structure\n3\nthat is closer to an ideal, non-modulated perovskite unit along the observed zone axis.\nTherefore, the difference in lattice modulation is not surprising but rather an expected\nsignature of the fundamental difference in local chemistry and atomic-scale structure between\nthe two phases. This observation powerfully supports our central hypothesis: the interfacial\nengineering process does not merely create a topotactically aligned phase but fundamentally\nalters the local bonding environment, which in turn influences the functional properties (such\nas the direction of ferroelectric polarization) that we measure.\nWe have revised the manuscript on page 8, lines 180 (“These results indicate that our\ninterfacial templating strategy fundamentally alters the local chemistry, surpassing mere\ntopotactic alignment, as manifested by the distinct lattice modulation characteristics: the\npresence of superlattice spots in BWO, originating from its commensurate Aurivillius\nsuperlattice, stands in sharp contrast to their absence in the structurally simplified, cation-\nhomogeneous WO phase.”) to include this explanation, clarifying that the loss of superlattice\n3\nreflections indicates a simplification of the modulation structure in WO due to changes in\n3\ncation ordering and local bonding environment.\nComment 3: The authors should consider comparing and contrasting the macroscopic\npolarization-electric field behavior (Figure 6a) in WO /BWO nanocomposite thin films with\n3\nthat for BWO and WO films individually in identical device configuration. Such analysis can\n3\nimprove the clarity of the study further.\nWe sincerely thank the reviewer for this valuable suggestion. Following the reviewer's\nadvice, we have prepared and characterized pure-phase BWO and WO films in an identical\n3\ndevice configuration for direct comparison.\nAs shown in Figure R1(a) and R1(b), local PFM measurements on the pure BWO and WO\n3\nfilms, respectively, show no signature of ferroelectric switching. This is further corroborated\nby macroscopic P-E measurements, where both pure BWO and WO exhibit a dielectric\n3\nresponse without current peaks (Figure R1(c-d)).\nThis control experiment stands in stark contrast to the well-defined, switchable\nferroelectric hysteresis observed in our WO /BWO nanocomposite. The direct comparison thus\n3\ndefinitively confirms that the robust out-of-plane ferroelectricity is an emergent property of our\ntemplated nanocomposite architecture, rather than an intrinsic characteristic of either\nindividual phase. We are grateful to the reviewer for prompting this clarifying analysis, which\nsignificantly strengthens our main conclusion.\nFigure R1. Ferroelectric characterization of pure-phase BWO and WO3 control films grown on Nb:STO (001)\nsubstrates. (a) Local PFM hysteresis loop for a pure BWO film. (b) Local PFM hysteresis loop for a pure WO3 film. (c)\nMacroscopic P–E loop with corresponding current response for a pure BWO film. (d) Macroscopic P–E loop with current\nresponse for a pure WO3 film.\nComment 4: The authors should discuss the ferroelectric hysteresis loop (Figure 6a) in terms\nof saturation polarization, remanent polarization and coercive field. It is very challenging to\nestimate the values for these ferroelectricity-based parameters in the current form.\nResponse 4:\nWe sincerely thank the reviewer for pointing out this important omission. We agree that a\nquantitative discussion of the ferroelectric hysteresis loop is essential for evaluating the\nperformance of a ferroelectric device.\nIn response to this comment, we have now explicitly extracted the values of saturation\n3\npolarization (P ), remanent polarization (P), and coercive field (E ) from the hysteresis loop\nsat r c\nin Figure 6a (measured at the maximum applied electric field). These values have been clearly\nstated in the revised manuscript on page 16, lines 357. For instance, at an applied field of 3.5\nMV/cm, the film exhibits a P = 25 μC/cm2, P = 20 μC/cm2, and an E = 2 MV/cm.\nsat r c\nComment 5: In line 327, the authors mention “To further showcase the potential device\nperformance based on our films, we deposited a 5 nm-thick HfO insulating layer on the as-\n2\ngrown films using atomic layer deposition (to minimize leakage effects during testing) and\nsubsequently transferred a monolayer MoS via a dry transfer method.” They should consider\n2\naddressing the comment about leakage by elaborating on the source of the mentioned leakage\ncurrent in the studied heterostructures.\nResponse 5:\nWe thank the reviewer for this valuable suggestion. We agree that a deeper discussion of\nthe leakage current mechanisms in our heterostructures is important for understanding the\nelectronic behavior of the films.\nThe observed leakage current in the as-grown Aurivillius-phase thin films primarily\noriginates from the following aspects:\nOxygen Vacancies: As a common issue in oxide films grown under reduced oxygen\npressure, oxygen vacancies can form shallow or deep donor states within the bandgap,\nproviding a pathway for charge transport under an applied electric field.\nStructural Defects: Although the films are highly epitaxial, the presence of structural\nimperfections, such as point defects, dislocations, and interfacial boundaries between phases—\ncan form conductive pathways, leading to increased leakage, especially at high fields.\nThe insertion of the ALD-grown HfO layer was therefore intended to physically block\n2\nthese dominant leakage paths—especially those related to grain boundaries and oxygen\nvacancy conduction—thus allowing us to probe the intrinsic ferroelectric polarization\nswitching with minimal parasitic current effects.\nIn the revised manuscript, we have added a paragraph in page 16, line 377 (“the leakage\ncurrents primarily originate from oxygen vacancy defects introduced during the growth of the\noxide films, as well as conducting pathways formed along interphase within the textured film.”),\nto explicitly discuss these possible leakage origins. We believe this addition provides a clearer\nphysical context for the device design and strengthens the overall discussion.\nFor Reviewer #2:\nComment: The manuscript reports a previously unreported WO phase. It demonstrates a\n3\nnovel structural engineering strategy to obtain out-of-plane ferroelectricity in a metastable WO\n3\nphase. The WO /BWO thin film composite exhibits both out-of-plane and in-plane ferroelectric\n3\npolarization.\nWhile the authors present a potentially interesting approach, several major issues should be\naddressed before the work may be considered for publication.\nResponse:\ncomments. We have thoroughly addressed all the raised issues in the revised manuscript, as\ndetailed in the point-by-point responses below.\nComment 1: The microscopic approach of structural analysis is good but it focuses on a very\nsmall part of the sample. The structural analysis part of the manuscript may be significantly\nimproved by incorporating detail advanced analyses such as XRD, PDF, XAS, EXAFS and\nXANES (including in-operando). The data should be compared with the stable phases.\nResponse 1:\nWe thank the reviewer for suggesting these advanced characterization techniques, which\nwould undoubtedly provide deeper structural insights. While the proposed synchrotron-based\nmeasurements (PDF, XAS, etc.) are compelling, they are unfortunately not feasible within the\nrevision timeline for this work due to the challenges in securing beamtime.\nTo directly address the reviewer's concern about the global structural representation and\ncomparison with stable phases, we have performed a deeper analysis of our high-resolution X-\nray diffraction (XRD) and reciprocal space mapping (RSM) data. These results provide clear\nevidence of structural changes induced by WO incorporation.\nBWO diffraction peaks, this is actually consistent with our model of dispersed WO\n3\nnanodomains within the BWO matrix. The diffraction signal from such nano-sized, coherently\nembedded phases is often too weak and broad to be distinguished from the dominant host\nmatrix in a symmetric scan.\nHowever, the more sensitive and detailed information is revealed through non-symmetric\nRSM measurements, and this is now strongly corroborated by our cross-sectional STEM\nanalysis: As shown in Figure R3, our quantitative STEM measurements clearly indicate that\nthe embedded WO nanodomains exhibit a slightly larger out-of-plane lattice parameter and a\n3\nslightly smaller in-plane lattice parameter compared to the surrounding BWO matrix (Figure\nR3).\nThis specific strain state revealed by STEM provides a perfect explanation for our RSM\nobservations: Around the STO (001) reflection (Figure R4(a-b)): The shift towards lower q\nz\nconsistently confirms the overall expansion in the out-of-plane direction for the composite film.\nAround the STO (103) reflection (Figure R4(c-d)): The shift of the WO /BWO film peak\n3\ntowards a higher q (smaller in-plane d-spacing) and a lower q (larger out-of-plane d-spacing)\nx z\nis in direct quantitative agreement with the STEM data.\nThe remarkable consistency between the real-space (STEM) and reciprocal-space (RSM)\ndata provides a robust, self-consistent picture: the templated WO phase induces a coherent\n3\nstrain field within the heterostructure, characterized by in-plane compression and out-of-plane\nelongation. This is the definitive micro-structural signature of the new phase.\nWe have now added this combined RSM and STEM analysis to the main text (page 10,\nline 215: “This structural templating mechanism is further supported by X-ray diffraction (XRD)\nand reciprocal space mapping (RSM) analyses. Conventional θ–2θ scans (Figure S5) reveal\nno significant peak shifts between pure BWO and WO /BWO nanocomposite films, which is\n3\nconsistent with the presence of nanosized WO domains whose diffraction is masked by the\n3\n5\ndominant BWO matrix…..The primary role of the STO substrate is thus to enable the epitaxial\ngrowth of a high-quality BWO template, which in turn architecturally guides the formation of\nthis novel polar WO phase.”) and Supplementary Information. While we agree that these\n3\ntechniques could provide additional insights, the combination of local atomic-scale STEM\nimaging, long-range RSM analysis, and macroscopic ferroelectric measurements presented in\nour revised manuscript provides a conclusive and self-consistent picture of the strain-stabilized,\nmetastable WO3 phase, thereby solidifying the structural foundation of our conclusions\nFigure R2. XRD patterns of the pure BWO and WO3/BWO nanocomposite thin films.\nFigure R3. (a) Filtered STEM image of the WO3/BWO heterostructure. Line profiles extracted from the WO3 and BWO\nphases along the (b) [001] and (c) [110] zone axes to compare the lattice spacings.\nof the WO3/BWO film, (c) STO (103) reflection of the BWO film, and (d) STO (103) reflection of the WO3/BWO film.\nYellow arrows and dashed lines illustrate the shift in peak positions between the RSM maps.\nComment 2: The growth conditions should be mentioned in a comprehensive manner.\nResponse 2:\nIn response to the reviewer's comment, we have comprehensively revised the “Methods”\nsection to include all critical growth parameters.\nFor Reviewer #3:\nIn this manuscript, the authors report the synthesis of thin films composed of a mixture of\nBi WO (BWO) Aurivillius phase and WO (WO) binary oxide. The films exhibit segregated\n2 6 3\nnanocrystalline regions corresponding to the two phases. The authors claim that the WO\n3\nregions are stabilized in a typically metastable phase that exhibits out-of-plane polarization.\nWhen combined with the in-plane polarization of the Aurivillius BWO phase, the overall film\nis proposed to display switchable polarization along both in-plane and out-of-plane directions.\nFinally, the authors demonstrate potential device integration by embedding the films into\nferroelectric FET and memristor-type device structures.\nBi WO (BWO) as a structural template to achieve out-of-plane polarization in the WO phase.\n2 6 3\nWhile the topic is promising, the current presentation lacks sufficient experimental evidence to\nconvincingly support the claims. The central argument, that BWO serves as a structural\ntemplate, is not fully substantiated. From the data provided, the BWO and WO phases appear\n3\nto be segregated within the thin film. Consequently, it is unclear how BWO functions as a\ntemplate for the WO phase. In my view, the observed polarization in WO is more likely a\n3 3\nresult of epitaxial strain imposed by the STO substrate rather than a direct templating effect\nfrom BWO. A true template effect would require evidence of coherent, lattice-matched growth\nacross the film, rather than phase segregation.\nAdditionally, the authors attribute the polarization in WO to stabilization of the Pca2 phase.\n3 1\nHowever, the phase analysis is very limited, relying primarily on STEM images along a single\nzone axis. No comprehensive structural characterization (e.g., multi-axis STEM, XRD, or\nreciprocal space mapping) is presented to rigorously confirm the Pca2 phase or quantify the\n1\npolar distortion.\nOverall, due to the lack of in-depth structural analysis, limited high-quality supporting data,\nand insufficient experimental details provided, the manuscript does not convincingly support\nits key claims, required for publication in Nature Communications. I therefore cannot\nrecommend publication in Nature Communications in its current form.\nResponse 0:\nWe sincerely thank the reviewer for their thorough assessment and constructive, albeit\ncritical, feedback. We have taken these comments extremely seriously and have performed\nextensive additional analyses and revisions to address the concerns. Below, we provide a point-\nby-point response, detailing how the new evidence robustly supports our conclusions.\nHere are my point-by-point technical comments:\nComment 1: I could not find a “Methods” section. Was it submitted elsewhere? Because most\nof the experimental details of the variety of measurements they perform are missing. Providing\nthese details is critical for assessing the validity and reproducibility of the results.\nResponse 1:\nWe sincerely thank the reviewer for pointing out this critical omission. We apologize for\nthe oversight in the initial submission where the “Methods” section was inadvertently omitted.\nWe fully agree that providing comprehensive experimental details is essential for assessing the\nvalidity and reproducibility of the work.\nIn the revised manuscript, we have now included a dedicated and detailed “Methods”\nsection (following the main text and before the References) that comprehensively describes all\nexperimental procedures and DFT calculation methods.\nthe concept of using Aurivillius layered frameworks to create composite materials with\nproperties absent in the pure Aurivillius phases is not new; however, the manuscript fails to\nacknowledge prior literature. For example, this paper (https://doi.org/10.1038/s41467-025-\n60176-8) reports the synthesis of composite films consisting of Aurivillius Bi FeTi O and\n5 3 15\nBiFeO .\nWe sincerely thank the reviewer for raising this critical point and for bringing this highly\nrelevant reference to our attention. We agree that our manuscript would benefit from a more\nnuanced discussion of the prior literature on Aurivillius-based composite systems, and we\napologize for this oversight.\nIn the revised manuscript, we have cited the suggested paper by Ipek Efe, et al. Nature\nCommunications, 16, 6131, (2025). However, we would like to clarify the fundamental\ndistinction between their work and ours, which justifies a different contextualization:\nThe study by Ipek Efe et al. primarily utilizes the Aurivillius framework as a vertical\nelectrostatic environment to host and modulate a pre-existing ferroelectric material (BiFeO ).\nphases.\nIn contrast, the central innovation of our work lies in using the Aurivillius BWO phase as\na lateral structural template to induce and stabilize a polar phase in a normally non-ferroelectric\nmaterial (WO ). Our mechanism is one of structural transformation and templating, where the\nof WO that would not form otherwise.\nmechanisms (interface coupling vs. structural templating) and the nature of the functional phase\n(intrinsically ferroelectric vs. induced ferroelectric) are distinctly different.\nTo properly acknowledge this and provide the correct context, we incorporated a\ndiscussion and citation of this important work in the Introduction section of our revised\nmanuscript (page 3, line 63: “and more recently, the exploitation of the unique electrostatic\nenvironment within Aurivillius frameworks to stabilize complex ferrielectric orders32”).\nSecond, the authors provide almost no discussion of strain engineering in WO thin films.\n3\nthrough epitaxial strain or strain gradients imposed by the substrate\nhttps://doi.org/10.48550/arXiv.2505.11021, DOI: 10.1021/acs.jpclett.3c01546,\nhttps://doi.org/10.1038/s41467-020-18644-w). A detailed analysis of the epitaxial strain from\nthe STO substrate and its potential role in stabilizing the polar phase is necessary to place the\nwork in the proper context and to support the manuscript’s central claims.\nResponse 2.2:\nWe sincerely thank the reviewer for raising this critical point regarding strain engineering\nand for directing our attention to these insightful studies on strain-stabilized polar phases in\nWO . We agree that discussing the role of the substrate is essential for contextualizing our work.\n3\n3\noperates through a mechanism fundamentally different from the epitaxial strain stabilization\ndemonstrated in the referenced works.\nIn the revised manuscript, we have incorporated a detailed analysis of the cited references\nand clarified the distinct formation mechanism operative in our heterostructures. We emphasize\nthat the polar WO phase in our system is stabilized through a pathway fundamentally different\n3\nfrom the conventional epitaxial strain engineering exemplified by WO grown on YAlO ,\n3 3\n(Shinhee Yun, et al. Nature Communications, 11, 4898, (2020);\nhttps://doi.org/10.48550/arXiv.2505.11021) and LaAlO (F. Mazzola et al. The Journal of\na homogeneous biaxial strain on a bulk-like WO structure, slightly distorting it to promote\n3\nsymmetry breaking and polar behavior—the resulting phase remains a strained derivative of a\nknown WO₃ polymorph.\nIn stark contrast, the polar phase in our WO /BWO heterostructure is not merely a strained\n3\nversion of bulk WO . Instead, our structural analyses (e.g., Figure 2, Figure 3, Figure S6, Figure\n3\nclosely resemble the adjacent BWO block, forming a structurally distinct phase that is not\ninherent to bulk WO . In this context, the primary role of the STO substrate is not to directly\n3\nstrain the WO layer, but to provide a well-latched platform for the epitaxial growth of the high-\n3\nquality BWO template layer itself. The BWO/STO interface ensures the coherent growth of the\nBWO framework, which in turn acts as the direct template for constructing the novel polar\nWO phase.\ndemonstrated that epitaxial strain from perovskite substrates can homogeneously distorts the\nbulk WO structure……The primary role of the STO substrate is thus to enable the epitaxial\n3\ngrowth of a high-quality BWO template, which in turn architecturally guides the formation of\nthis novel polar WO phase.”) to explicitly discuss this crucial distinction and to clarify that\n3\nthe formation of our polar phase is driven predominantly by the structural templating effect\nfrom the BWO layer, rather than by direct epitaxial strain from the STO substrate. We believe\nthis clarification significantly strengthens the novelty and conceptual contribution of our work.\nComment 3: In lines 60-62, the authors state, “By templating metastable WO nanoparticles\n3\nwithin a BWO matrix, we simultaneously achieve perpendicular polarization and inherited\nthermal robustness.”. This description is misleading. The term “nanoparticles” implies isolated,\ndiscrete 0D entities, which is not the case here; the WO regions are part of a continuous thin\n3\nfilm and appear to be phase-segregated rather than discrete particles. Furthermore, the use of\nthe term “templating” suggests control over the size, shape, and spatial arrangement of these\nregions, which is not demonstrated in the manuscript. What is observed is simply a film\ncontaining two segregated crystalline phases. No evidence is provided that the BWO phase\nactively controls or templates the structure of the WO phase. As such, the claims regarding\n3\nnanoparticle templating and the associated functional advantages are not supported by the data.\nWe sincerely thank the reviewer for this constructive and insightful feedback, which has\nhelped us to improve the precision of our terminology and the strength of our argumentation.\nWe fully agree with the reviewer that the term “nanoparticles” was imprecise, as it implies\ndiscrete 0D entities. Following the reviewer's suggestion, we have replaced it throughout the\nrevised manuscript with more accurate descriptors such as “WO domains” or “WO phases”.\n3 3\nWe sincerely thank the reviewer for raising this important point regarding the evidence for\nour use of the term “templating”. We appreciate the opportunity to clarify that the key\nexperimental evidence supporting this claim was in fact presented in the original manuscript.\nAs detailed on page 8, lines 201: “The 8 nm-thick textured composite WO /BWO thin film\n3\nexhibits structural and compositional invariance across its engineered interfaces, as evidenced\nby cross-sectional STEM analysis (Figure S4). Importantly, the WO phase forms a semi-\n3\ncoherent epitaxial relationship retains lattice coherence with the BWO template, demonstrating\nthe robustness of this templating strategy in governing phase evolution across dimensional\nscales, further validating the epitaxial growth and crystallographic alignment of the\nnanocomposite film.”, and supported by Figure 2, we explicitly demonstrated that varying the\nthickness of the BWO layer directly results in WO domains with controlled sizes and\n3\nmorphologies. Specifically, thinner BWO templates yield finer WO domains, while thicker\n3\nones promote larger, elongated domains. This systematic morphological control provides direct\nevidence that the BWO matrix actively governs the spatial configuration of the WO phase,\n3\ngoing beyond simple phase segregation.\nIn addition to these microscopic analysis, macroscopic XRD data and reciprocal space\nmapping (RSM) data also provide clear evidence of structural coherence of WO domain and\n3\nthe BWO matrix. While conventional θ-2θ XRD scans (Figure R5) show no significant shift in\nthe primary BWO diffraction peaks, this is actually consistent with our model of dispersed\nWO nanodomians within the BWO matrix. The diffraction signal from such nano-sized,\n3\ncoherently embedded phases is often too weak and broad to be distinguished from the dominant\nhost matrix in a symmetric scan. However, the more sensitive and detailed information is\nrevealed through non-symmetric RSM measurements, and this is now strongly corroborated by\nour cross-sectional STEM analysis: As shown in Figure R6, our quantitative STEM\nmeasurements clearly indicate that the embedded WO nanodomians exhibit a slightly larger\n3\nout-of-plane lattice parameter and a slightly smaller in-plane lattice parameter compared to the\nsurrounding BWO matrix (Figure R6). This specific strain state revealed by STEM provides a\nperfect explanation for our RSM observations:\nAround the STO (001) reflection (Figure R7(a-b)): The shift towards lower q consistently\nz\nconfirms the overall expansion in the out-of-plane direction for the composite film.\nAround the STO (103) reflection (Figure R7(c-d)): The shift of the WO /BWO film peak\n3\ntowards a higher q (smaller in-plane d-spacing) and a lower q (larger out-of-plane d-spacing)\nx z\nis in direct quantitative agreement with the STEM data.\nThe remarkable consistency between the real-space (STEM) and reciprocal-space (RSM)\ndata provides a robust, self-consistent picture: the templated WO phase induces a coherent\n3\nstrain field within the heterostructure, characterized by in-plane compression and out-of-plane\nelongation.\nThis demonstration of controlled domain morphology through the manipulation of the\nspectator but an active template that governs the growth and spatial configuration of the WO\n3\nphase.\nWe thank the reviewer for prompting us to define the \"templating effect\" more precisely.\nIn the context of this work, the term \"BWO structural template\" specifically refers to the\nprocess whereby the BWO matrix, by providing a shared epitaxial orientation and a local\nchemical environment, guides the WO phase to nucleate and grow into nanodomains with\n3\nspecific crystallographic orientation and structural characteristics, rather than forming\nrandomly oriented or stable phases. We have explicitly clarified this point in the the revised\nmanuscript in page 9, line 230 (“The “BWO structural template” provides a shared epitaxial\norientation and a local chemical environment, which directs the WO phase to nucleate and\n3\ngrow into nanodomains with specific crystallographic characteristics, rather than forming\nrandomly oriented or thermodynamically stable phases. The primary role of the STO substrate\nis thus to enable the epitaxial growth of a high-quality BWO template, which in turn\narchitecturally guides the formation of this novel polar WO phase.”).\npresent our findings more accurately and convincingly. We believe the revised manuscript now\nmore substantively validates the central claim of a templating effect.\nFigure R5. XRD patterns of the pure BWO and WO3/BWO nanocomposite thin films.\nphases along the (b) [001] and (c) [110] zone axes to compare the lattice spacings.\nFigure R7. Reciprocal space maps (RSMs) around the (a) STO (001) reflection of the BWO film, (b) STO (001) reflection\nof the WO3/BWO film, (c) STO (103) reflection of the BWO film, and (d) STO (103) reflection of the WO3/BWO film.\nYellow arrows and dashed lines illustrate the shift in peak positions between the RSM maps.\nbe clearly indicated. The polarization directions of the in-plane domains, as well as the in-plane\ncrystallographic axes of the film, should also be labelled. The authors mention four in-plane\npolarization variants, yet it is unclear why there is no corresponding contrast in the VPFM\nchannel. Some of these variants would be expected to produce a measurable deflection of the\ncantilever, thus a contrast in VPFM. A comprehensive analysis of in-plane polarization using a\nseries of vector-PFM measurements is necessary to fully characterize the domain structure.\nIn Figure 1d, critical experimental details are missing. The sign of the applied voltage, the\ncomponent to which the voltage was applied (tip vs. sample), the cantilever movement path,\nand the slow scanning axis should all be reported to support claims of switching induced by a\ntrailing field. Based on the images, in-plane switching is not convincingly demonstrated. For\nexample, in the innermost square, the brightest contrast increases, yet in the middle square, the\nexpected increase in the darkest contrast is not observed. Clearly marking the switched regions\nusing dashed lines in the pristine images (Figures 1c-e) would facilitate comparison before and\nafter DC voltage application and improve clarity.\nResponse 4:\nWe thank the reviewer for their careful examination of our PFM data and for their\nconstructive suggestions, which have helped us improve the clarity and rigor of our figure\npresentation.\nIn direct response to the reviewer's suggestion, we have now clearly indicated the in-plane\ncrystallographic axes ([100] and [010]), and the polarization directions of the four in-plane\nvariants in the revised Figure 1c.\nThe lack of contrast in the VPFM channel for the observed four polarization variants is\nentirely consistent with the fundamental physics of the converse piezoelectric effect applied to\nferroelectric a-domains, where the spontaneous polarization P is highly confined to the plane\nof the thin film (P ≈ 0). This null VPFM result serves as compelling evidence of the high\nz\nstructural purity of the in-plane domains and, crucially, demonstrates the successful\ninstrumental suppression of mechanical cross-talk artifacts, such as cantilever buckling, which\notherwise might introduce spurious vertical signals from the lateral strain (d coupling). The\n31\ndominant contrast arises from the much larger in-plane polarization components detected by\nLPFM. The domain structure of BWO, including these four in-plane variants, is well-\nestablished in prior literature (e.g., Chuanshou Wang, et al. Nature Communications, 7, 10636,\n(2016); Jihwan Jeong, et al. ACS Applied Electronic Materials, 3, 2, 1023–1030, (2021)).\nAs suggested, we have now explicitly annotated in Figure 1d the voltage polarity, the\norientation of the AFM cantilever, the component to which the voltage was applied (conductive\ntip), the fast and slow scan axis.\nMost importantly, following the reviewer's suggestion, we have also added dashed lines in\nthe pristine state images (Figures 1c, 1e) to clearly mark the regions where voltage was applied,\nfacilitating a direct visual comparison before and after poling, the comparison now\nunequivocally demonstrates successful domain switching. As the reviewer will be able to see\nclearly in the revised Figure1 (Figure R8), both the innermost and the middle squares exhibit\n14\nclear and significant changes in the LPFM phase contrast within the poled areas compared to\ntheir pristine state.\nFigure R8. The updated Figure 1 in the revised manuscript.\nComment 5: In lines 116–117, the authors state that an initial BWO layer was first grown.\nHowever, this layer is not clearly visible in the STEM images presented in Figure 2. The\nmanuscript should provide clear evidence of the presence, thickness, and continuity of the\nBWO layer, as well as any interfaces with the WO phase, to support the reported growth\n3\nsequence.\nResponse 5:\nWe thank the reviewer for this critical comment, which allows us to clarify a key aspect\nof our growth process and provide a more precise interpretation.\nThe reviewer is correct that a distinct, continuous initial BWO buffer layer is not clearly\nvisible in the STEM images. This is because the initial BWO deposition was intentionally kept\nvery thin (a nominal thickness of only a few unit cells) and its continuity was disrupted by the\nsubsequent growth of the alternating WO /BWO structure.\nthe manuscript. We now describe this initial deposition as a “BWO seed layer”. Its primary\nfunction was to provide a thermodynamically and structurally favorable surface on the STO\nsubstrate to initiate the textured growth of the entire subsequent heterostructure. The critical\nevidence for its success is the overall highly textured, epitaxial nature of the entire composite\n15\nfilm observed in XRD and TEM, which would not be possible without a well-oriented starting\nsurface.\nTherefore, while the initial BWO seed layer is not preserved as a distinct interfacial layer\nin the final microstructure, its role was crucial in setting the crystallographic orientation for the\nentire film that followed. The WO domains then nucleated and grew within this pre-oriented\nWe have revised the text (page 6, line 127) in the manuscript to accurately reflect this “seed\nlayer” model and to avoid any misconception of a persistent, thick buffer layer. We believe this\nprovides a more accurate description of the growth process while fully supporting our central\nclaim of using the BWO structure to template the growth of the WO phase.\nSTEM images, it appears that the BWO and WO phases do not mix and form completely\n3\nsegregated regions within the thin film. Given this lack of intermixing, the system cannot\nreasonably be described as a “composite” material. Moreover, the STEM images suggest that\nthe WO phase grows directly on the STO substrate rather than on or within the BWO regions.\nthe WO phase grows via a structural templating effect inherited from the BWO matrix”—is\n3\nmisleading. The BWO layer does not appear to act as a template; rather, the epitaxial\nrelationship with the STO substrate likely governs the growth. To support any claim of\ntemplating, the authors should provide the FFT of the STO substrate and compare it with the\nWO and BWO regions, to demonstrate that the observed structural alignment is not merely\n3\ninherited from the STO lattice, which also has a square FFT pattern similar to BWO.\nResponse 6:\nWe thank the reviewer for these insightful comments, which have provided us with an\nimportant opportunity to clarify the terminology used in our manuscript and to strengthen the\nevidence supporting our central claim of a structural templating effect.\n1. Regarding the use of “composite”\nWe agree with the reviewer that our system features phase-segregated regions. In materials\nscience, the term “composite” accurately describes a material combining two or more distinct\nphases at a microscopic scale, where the overall properties stem from the individual\ncomponents and their interactions. Phase segregation is a common and often deliberate\ncharacteristic of many composite systems, such as interphase strain composites (e.g., Linxing\nZhang et al., Science, 361, 494-497, (2018), Yilin Wang et al., Journal of the American\nChemical Society, 143, 17, 6491–6497, (2021)) and vertically aligned nanocomposites (e.g., H.\nZheng et al. , Science, 303, 661-663, (2004), Akira Imai et al., ACS Nano, 7, 12, 11079–11086,\n(2013), Li, L. et al., Nano Research. 14, 4789–4794, (2021)).\nIn our system: the film consists of two chemically and structurally distinct phases: the\nAurivillius BWO matrix and the perovskite-derived WO domains. Most importantly, the\n3\nfunctional property—the ability to exhibit both in-plane and robust out-of-plane ferroelectric\npolarization—emerges from the synergistic combination of the two phases and is not present\nin either phase alone in this configuration.\nIn our case, the film consists of chemically and structurally distinct BWO and WO phases,\n3\n16\nand its key functional property—the simultaneous presence of robust in-plane and out-of-plane\nferroelectric polarization—emerges from their synergistic combination and is not achievable\nby either phase alone in this configuration. Therefore, “composite” remains an appropriate\ndescriptor. Nonetheless, to prevent any potential misunderstanding, we have revised the\nmanuscript to use the more precise term “phase-segregated nanocomposite” or\n“nanocomposite”, which better captures the microstructure while retaining the core composite\nconcept.\n2. Regarding the evidence for “structural templating”\nWe thank the reviewer for this critical comment, which allows us to provide deeper\nstructural evidence to robustly support our claim of a structural templating effect. We agree\nthat a direct comparison with the STO substrate is essential to rule it out as the sole governing\nfactor.\nIn direct response, we have performed a comprehensive FFT analysis comparing the STO\nsubstrate, the BWO matrix, and the WO regions. This new analysis is presented in the revised\nThe key finding is as follows: While both BWO and WO have an epitaxial relationship\n3\nwith the STO substrate, the in-plane crystallographic orientation of the WO phase is uniquely\n3\naligned with the BWO matrix, not with the STO substrate.\nThe FFT pattern of STO (new Figure 2b) shows a square symmetry corresponding to its\ncubic (100) surface.\nThe FFT of the BWO region (Figure 2c) aligns with the STO but contains additional\nsuperlattice spots. The primary BWO spots are rotated by approximately 45° relative to the\nSTO peaks, consistent with the known epitaxial relationship for BWO on STO (Chuanshou\nWang, et al. Nature Communications, 7, 10636, (2016)).\nCrucially, the FFT of the WO region (Figure 2d) shares this specific 45° rotation with the\n3\nof the BWO-specific superlattice spots, as we originally noted.\nThis observation is inconsistent with the scenario where WO grows independently on\n3\nexhibit a different rotation specific to the WO /STO interface. Therefore, the WO phase\n3 3\ninherits its specific in-plane orientation through the BWO matrix.\nWe further corroborate this finding in Figure 3, which presents the atomic structure and\ncorresponding FFT along the in-plane [100] direction. This additional perspective consistently\nshows that the BWO layer, initiated by the initial seed layer, acts as a structural template that\ndictates the crystallographic orientation for the WO domains that form within it during the\n3\nalternating growth process.\nWe have revised the text in page 8, line 171 (Figure 2b-d present the fast Fourier transform\n(FFT) patterns derived from the Nb:STO substrate, BWO matrix, and WO domain in Figure\n3\n2a, respectively. Both the BWO and WO FFT patterns exhibit a consistent 45° rotation relative\n3\nto the STO substrate, confirming their shared epitaxial relationship. The FFT pattern from the\nBWO region exhibit excellent agreement with the known parameters of pristine BWO structure\n(Figure S3a). The WO FFT pattern shows closely aligned symmetry and orientation with that\n3\nof BWO, consistent with their structural similarity, while differing in the absence of BWO-\nspecific superlattice spots. This shared epitaxial alignment, distinct from the STO substrate\n17\norientation, provides definitive evidence that the WO phase grows via a structural templating\n3\neffect mediated by the BWO matrix rather than through direct nucleation on the substrate.) to\nbase our claim on this rigorous FFT analysis, clarifying that “templating” in our context refers\nto the BWO layer dictating the crystallographic orientation of the WO phase. We are grateful\n3\nto the reviewer for prompting this significant strengthening of our evidence.\nFigure R9. The updated Figure 2 in the revised manuscript.\nComment 7: Figure 2: The authors should comment on the observation of contrast differences\nin the designated regions of the film. I.e., the brighter contrast in the WO part or the darker\n3\ncontrast regions in BWO. Providing a detailed explanation of the origin of this contrast,\nwhether due to atomic number differences, thickness variations, strain, or imaging conditions,\nwould help clarify the structural interpretation and support the identification of the two phases.\nResponse 7:\nWe thank the reviewer for raising this important point regarding the image contrast, which\nallows us to clarify the basis for our phase identification. The contrast differences are indeed\nkey to distinguishing the two phases.\nThe cross-sectional STEM images presented in Figure 2 were acquired using a High-Angle\nAnnular Dark-Field (HAADF) detector. In this imaging mode, the signal intensity (image\ncontrast) is approximately proportional to the square of the atomic number (Z-contrast).\nTherefore, brighter regions correspond to areas with a higher average atomic number.\nAs the reviewer correctly inferred, the observed contrast originates primarily from\ndifferences in chemical composition, not from thickness variations, as the sample was prepared\nto be uniformly thin using focused ion beam (FIB) milling.\nThe brighter regions are identified as the BWO Aurivillius phase. This is due to the\npresence of heavy Bismuth (Bi, Z=83) atoms. Despite containing W (Z=74) atoms, the high\natomic number of Bi dominates the scattering, leading to a strong HAADF signal and bright\ncontrast.\nThe darker regions are identified as the WO phase. This phase consists predominantly of\nnumber in these regions, causing weaker electron scattering and thereby producing the darker\ncontrast observed in the HAADF-STEM images.\n“In the film region, the darker gray contrast corresponds to the WO domains, which consist\n3\nof W and O atoms, while the lighter gray contrast corresponds to the BWO single-crystalline\nframework. This contrast originates from the Z-contrast mechanism in HAADF-STEM imaging,\nwhere the presence of heavy Bi (Z=83) atoms in BWO yields a stronger scattering signal than\nthe WO phase.”) thereby firmly supporting our structural interpretation and phase assignment.\nlarge region at the vertical interface where a transitional crystal structure forms, rather than\nclear lattice coherency. The authors should provide stronger evidence—such as high-resolution\nimaging across multiple zones, strain mapping, or FFT analysis—to support any claim of a\ncoherent relationship between the two phases.\nResponse 8:\nWe thank the reviewer for this insightful comment, which allows us to provide deeper and\nmore quantitative evidence regarding the interface structure between BWO and WO . We agree\n3\nthat a detailed analysis of the interfacial region is crucial.\nIn direct response to the reviewer's suggestion, we have performed a comprehensive\nanalysis, which now includes:\n1.Enhanced Atomic-Scale Imaging: We applied an ABSF filter to the high-resolution\nSTEM image to reduce noise and enhance the clarity of the atomic columns. The filtered image\n(Figure R10(a)) now clearly reveals the atomic arrangement on both sides of the interface and\nwithin the transitional region. The image reveals that the two phases share a well-defined\ncrystallographic orientation, with their principal lattice planes continuing across the interface\nwith only a slight bending over a short range of a few unit cells.\n2.High-Resolution FFT Analysis: The FFT pattern from the filtered interface region\n(Figure R10(b)) shows that the primary diffraction spots from BWO and WO are aligned. As\n3\ncorrectly inferred by the reviewer, we observe a subtle splitting of the overlapping spots, which\nis a direct consequence of their slightly different lattice constants. This spot splitting is classic\ndiffraction evidence for a semi-coherent interface between two epitaxially aligned crystals.\n3.Quantitative Strain Mapping via GPA: As the reviewer rightly suggested, we have\nperformed strain mapping using Geometric Phase Analysis (GPA). The resulting strain maps\n(Figure R10(c-d)) quantitatively visualize the strain field. They reveal a localized strain\nconcentration precisely at the interface, with the WO phase experiencing a measurable strain\n3\nrelative to the BWO template. This is the direct evidence of the elastic strain relaxation due to\nthe finite lattice mismatch, confirming the presence of a coherency strain field.\nphase.\n(a) Filtered high-resolution STEM image of the BWO/WO3 interface using an artificial Bragg scattering filter (ABSF) to\nenhance atomic column visibility. (b) Corresponding fast Fourier transform (FFT) pattern corresponding to (a). (c-d)\nGeometric Phase Analysis (GPA) strain maps of the interface region, showing the distribution of (c) εₓₓ and (d) εᵧᵧ strain. The\ncolor scale represents the strain relative to the BWO lattice, confirming a localized strain field at the interface consistent with\nsemi-coherency.\nTherefore, the presence of a transitional region and spot splitting does not contradict a\ntemplating effect; on the contrary, it is the expected outcome of a semi-coherent heteroepitaxial\ninterface between two structurally similar but not identical crystals. The key evidence for the\ntemplating effect remains the maintained crystallographic orientation, which is now robustly\nsupported by the atomic-scale imaging, quantitative strain mapping, and FFT analysis.\nIn accordance with these findings, we have revised the manuscript (page 9, line 204) to\nuse the more precise term “semi-coherent epitaxial relationship”.\nComment 9: In lines 186–192, the authors report the stabilization of a new metastable phase\nin WO . However, insufficient structural information is provided beyond a qualitative\n3\ncomparison with the ferroelectric HfO phase using STEM images. Essential crystallographic\n2\nparameters, such as the lattice constants (a, b, c), angles (α, β, γ), and unit cell volume, should\nbe reported. To fully characterize this phase, X-ray diffraction analysis is necessary, including\nboth θ–2θ scans and reciprocal space mapping to obtain crystallographic information along all\nthree principal axes. Furthermore, the role of substrate-induced strain should be carefully\nanalysed. The authors should discuss whether the observed metastable phase could instead be\nstabilized by epitaxial strain from STO, independent of the BWO layer. Such analysis is critical\nto substantiate claims regarding phase stabilization and the proposed templating effect.\nWe sincerely thank the reviewer for this insightful comment, which has prompted us to\nprovide a more comprehensive characterization of the crystal structure of the new WO phase\n3\nand direct evidence regarding its stabilization mechanism.\nThrough a combination of atomic-resolution STEM (Figure R11) and DFT relaxation\ncalculations, we have determined that this metastable WO phase belongs to the polar space\n3\ngroup Cm, with lattice parameters of a = b = 5.56 Å, c = 5.90 Å, interaxial angles α =89.9977°,\nβ = 90.0015°, γ = 90.0004°, and a unit cell volume of approximately 182.46 Å3. These\nparameters, which have been explicitly listed in the Methods Section, clearly distinguish it\ncrystallographically from all known WO polymorphs.\nadditional evidence from XRD and reciprocal space mapping (RSM): while conventional θ–2θ\nscans (Figure R12) showed no significant peak shifts due to the weak and broad diffraction\nsignal from the nanosized WO crystals, Figure R13(a-b) and Figure R13(c-d) shows\n3\nasymmetric RSM measurements around the STO(001) and (103) reflections clearly revealed a\nsystematic shift of the WO /BWO nanocomposite film diffraction peak toward higher qₓ\n3\n(corresponding to in-plane compression) and lower q (corresponding to out-of-plane\nz\nexpansion). This is fully consistent with the strain state we directly observed via STEM,\nwhere the WO nanodomains exhibit smaller in-plane and larger out-of-plane lattice parameters\n3\nrelative to the BWO matrix, collectively confirming that the new phase resides in a coherent\nstrain field characterized by in-plane compression and out-of-plane tension.\nTo rigorously examine whether this phase could be stabilized solely by substrate-induced\nstrain, we deposited a pure WO film directly on Nb:STO(001) under identical growth\n3\nconditions as a control experiment. The results showed that its XRD characteristics (Figure\nR14(a)) were entirely different from those of the WO nanodomains embedded in BWO. As\n3\nshown in Figure R14(b), it exhibited no ferroelectric response in PFM measurements, thereby\nruling out the possibility that the ferroelectric phase is stabilized by the STO substrate alone.\nThese findings collectively demonstrate that the stabilization of the new phase WO is\n3\nprimarily driven by the structural templating effect of the BWO framework, rather than direct\nepitaxial strain from the substrate.\nWe have incorporated the complete crystallographic analysis, discussion of RSM-based\nstrain into the revised manuscript, providing robust evidence to support the conclusion that the\ntemplating effect d rives the formation of this new phase.\nphases along the (b) [001] and (c) [110] zone axes to compare the lattice spacings.\nFigure R12. XRD patterns of the pure BWO and WO3/BWO nanocomposite thin films.\nof the WO3/BWO film, (c) STO (103) reflection of the BWO film, and (d) STO (103) reflection of the WO3/BWO film.\nYellow arrows and dashed lines illustrate the shift in peak positions between the RSM maps.\nFigure R14. Control experiment of a pure WO3 film grown directly on Nb:STO (001). (a) X-ray diffraction pattern of\nthe pure WO3 film, showing distinct features compared to the WO3 nanodomains embedded in the BWO matrix. (b)\nPiezoresponse force microscopy (PFM) measurement of the pure WO3 film, demonstrating no detectable ferroelectric\nresponse.\nComment 10: In Figure 5, the in-plane domains observed in the LPFM images seem to\ndisappear in the BWO/WO film, in contrast to the pure BWO film. However, the STEM\n3\nimages indicate that some regions of the film consist of pure BWO. The authors should clarify\n23\nwhat happens to the in-plane polarization in these BWO regions within the composite film. A\ndiscussion of how the presence of WO affects the in-plane ferroelectric domains of BWO, and\n3\nwhether any depolarization or screening effects occur, is necessary to interpret the LPFM\nresults.\nResponse 10:\nWe thank the reviewer for raising this important point regarding the interpretation of our\nPFM data, which has allowed us to clarify a key aspect of our experimental presentation.\nThe reviewer correctly notes that the large-area (20 μm × 20 μm) LPFM phase image in\nFigure 5e appears homogeneous, in contrast to the distinct domain structure visible in the pure\nBWO film. We acknowledge that the original presentation may have caused confusion. This\napparent lack of contrast does not reflect the absence of in-plane domains, but rather stems\nfrom the limited spatial resolution inherent to the larger scan size, which was chosen to provide\na statistically significant overview of out-of-plane polarization switching (as clearly\ndemonstrated in the VPFM channel in Figure 5b).\nTo directly address this point and confirm the persistence of in-plane polarization in the\nBWO regions of the composite film, we performed high-resolution PFM mapping on a smaller,\nrepresentative area (5 μm × 5 μm). As shown in Figure R15, the high-resolution LPFM image\nclearly resolves the characteristic in-plane ferroelectric domain structure within the BWO\nmatrix, confirming its coexistence with the WO regions.\nof the film. (b) Box-in-box pattern written by applying tip voltage, illustrating ferroelectric domain manipulation. (c) Out-of-\nplane (VPFM) phase image after poling with a ±9 V tip bias, demonstrating localized ferroelectric switching. (d)\nCorresponding in-plane (LPFM) phase image of the same area, acquired simultaneously. The 5 μm × 5 μm high-resolution\nscan clearly resolves the intricate in-plane ferroelectric domain structure within the BWO matrix, which was not resolved in\nthe larger-area overview scans.\nThus, we can confirm that the in-plane polarization of the BWO phase is preserved in the\ncomposite structure. We have revised the manuscript accordingly (page 15, line 331: “Notably,\n24\nhigher-resolution imaging (Figure S9) confirms that the characteristic in-plane ferroelectric\ndomain structure of the BWO matrix remains clearly resolvable at finer scan scales,\ndemonstrating its preservation within the nanocomposite architecture.”) to emphasize that the\nin-plane domain structure is clearly observable at finer scan scales, as now documented in the\nsupplementary information (Figure S6).\nWe are grateful to the reviewer for prompting this clarification, which strengthens the\noverall analysis and provides a more complete understanding of the domain behavior in our\nfilms.\nComment 11: In Figure 6, the presented P-V loop appears leaky. To support the claim of\nferroelectric switching, the authors should also report the transient switching current alongside\nthe P-V loop. Additionally, essential details of the ferroelectric capacitor structure are missing,\nincluding the material and dimensions of the top electrode, the diameter of the top electrode,\nthe type of voltage pulse applied, and the pulse period. Providing these details is necessary to\nassess the reliability and reproducibility of the reported ferroelectric behaviour.\nResponse 11:\nWe thank the reviewer for the critical comments regarding the ferroelectric\ncharacterization. In response to the suggestion, we have now included the transient switching\ncurrent measurement in the revised Figure 6a. The observed current peaks at the coercive fields\nprovide direct evidence of polarization reversal, confirming the ferroelectric nature of the\nhysteresis.\nFurthermore, the capacitor specifications are provided as described in the Methods section\nof the revised manuscript: the top electrodes were fabricated as square gold pads with an area\nof 0.01 mm2, and the P-V loop was measured using a triangular waveform at 1000 Hz with an\namplitude of 7 V. We believe these additions fully address the concerns regarding data\ninterpretation and experimental reproducibility.\nFigure R16. The updated Figure 6 in the revised manuscript.\nComment 12: The authors attribute ferroelectricity in WO to oxygen displacements, but no\n3\ndirect experimental evidence is provided to support this claim. The current STEM images\nprimarily resolve the cation sublattice and are insufficient to confirm oxygen off-centering.\nTo substantiate the proposed mechanism, a more advanced STEM technique, such as integrated\nDifferential Phase Contrast (iDPC-STEM), is necessary to directly visualize oxygen positions\nand quantify their displacements.\nResponse 12:\nWe sincerely thank the reviewer for raising this essential point and for recommending\niDPC-STEM—a state-of-the-art approach for directly resolving light elements. In direct\nresponse, we have conducted iDPC-STEM imaging on the WO region of our heterostructure.\nobserved atomic arrangement aligns remarkably well with our computational model of the\npolar phase, offering direct experimental confirmation that the structure exhibits a polarized\nstate with unambiguous oxygen off-centering, as rightly emphasized by the reviewer.\nConcerning the polarization switching mechanism, our DFT calculations indicate that the\nreversal in a 2×2×1 supercell involves the coordinated displacement of a specific oxygen atom,\nas depicted in Figure 4d. We fully acknowledge the reviewer’s valid expectation for direct\nexperimental observation of this process. However, capturing the real-time dynamics of such a\nlocalized single-atom event remains beyond the current spatiotemporal resolution limits of\neven the most advanced microscopy techniques, including iDPC-STEM.\nIn light of this inherent technical challenge, we have built a coherent and multi-faceted\nbody of evidence integrating static iDPC-STEM imaging, first-principles calculations, and\nmacroscopic/microscopic ferroelectric measurements (P–E loops and PFM). Together, these\nresults provide strong and consistent support for the oxygen-displacement-mediated switching\nmechanism. We are confident that this collective evidence represents the most robust validation\nachievable with present methodologies.\nFor Reviewer #1:\nComment: The manuscript can be accepted in the current revised format.\nResponse:\nThank you, and we appreciate your great help in improving our manuscript.\nFor Reviewer #2:\nComment: The authors have addressed the reviewers’ comments/ suggestions. Therefore, the\nmanuscript may be considered for publication in Nature Communications.\nResponse:\nThank you, and we appreciate your great help in improving our manuscript.\nFor Reviewer #3:\nComment: I thank the authors for performing additional measurements and providing further\nexplanations, which have improved the clarity of the manuscript for me. However, I still have\nseveral additional comments regarding some of their responses that, in my opinion, should be\naddressed before the work is accepted in Nature Communications. My point-by-point\ncomments are provided below.\nResponse:\nWe sincerely thank the Reviewer for the positive feedback on the revised content and for\nacknowledging the improvements in clarity. We are grateful for the additional comments\nprovided below, which point to opportunities for further strengthening the manuscript. We have\ncarefully considered each point and provide our responses, and made revisions accordingly.\nComment 1:\nRegarding Response#3:\nI appreciate the additional data on RSM measurements. However, I still have some\nquestions about the templating effect and the strain relationship in this composite.\n3.1. If the amount of WO nanodomains is not significant enough to be visible in θ–2θ\n3\nscans, I believe it is necessary to quantify the volume ratio between the BWO matrix and the\nWO nanoparticles. In addition, an analysis of how the composite film thickness affects the\n3\nvolume ratio of the two phases would be valuable. Based on this ratio, larger-area STEM\nimages covering a more representative portion of the film would help visualize the\ncorresponding volume fractions of the BWO and WO phases.\n3\n3.2. According to the information provided in the manuscript and the authors’ response,\nthe lattice parameters are as follows:\nWO : a = b = 5.56 Å, c = 5.90 Å\nI am somewhat puzzled by the strain relationship inferred from the RSM. The composite\nfilm, which is described as being dominated by the BWO matrix with a small fraction of WO\n3\nnanodomains, appears to exhibit a larger out-of-plane lattice parameter and a smaller in-plane\nlattice parameter. However, I would expect the average in-plane lattice parameter to increase,\ngiven that the WO nanodomains have significantly larger in-plane lattice parameters than pure\nthe RSM and comparing them with values obtained from STEM would be beneficial for\nclarifying the strain states of the layers.\nIn this context, the authors state that “the templated WO phase induces a coherent strain\n3\nfield within the heterostructure, characterized by in-plane compression and out-of-plane\nelongation.” The authors are encouraged to elaborate on how this “coherent strain field”\noperates, preferably with reference to relevant literature. If the nanodomains occupy a very\nsmall volume fraction such that they are not visible in θ–2θ scans, can they still exert a\nsubstantial influence on the strain state of the BWO matrix?\nResponse 1:\nWe thank the reviewer for these insightful questions, which allow us to provide a more\nquantitative and clarified picture of the nanocomposite's microstructure and strain state.\n1.1: In direct response to the reviewer request, we performed large-area cross-sectional\nSTEM imaging on composite films with nominal thicknesses of 20 nm and 8 nm. Statistical\nanalysis of these images (Figure R1) reveals that the WO nanodomains occupy a volume\n3\nfraction of less than 10% in both films, explaining why their diffraction signal is discernible in\nsensitive RSM but not in conventional θ–2θ scans.\nTheir lateral size is consistent, ranging from 25 to 30 nm regardless of the total film\nthickness, indicating that nucleation and lateral growth are governed by the local BWO\ntemplate. In contrast, their vertical extent scales directly with the overall film thickness,\nconfirming a columnar growth mode within the matrix. These observations demonstrate that\nthe incorporation of WO is template-directed.\n3\nOverview of a ~20 nm thick composite film. (b) Enlarged view showing fine details of the region in (a). (c) Overview of a ~8\nnm thick composite film. (d) Enlarged view showing fine details of the region in (c). White arrows in all panels indicate\nrepresentative WO3 nanodomains.\n1.2: Regarding the strain and lattice constants. To ensure accuracy, we rigorously\nrecalibrated our in-plane STEM measurements using the known lattice constant of the STO\nsubstrate as an absolute internal reference (see Figure R2 for calibration details). The corrected\nlattice parameters for the embedded WO nanodomains are: a = b = 5.34 Å, c = 5.90 Å.\n3\nsmaller in-plane lattice parameters than the surrounding BWO matrix. This directly aligns with\nthe compressive in-plane strain signature observed in our RSM data (the shift to higher q ),\nx\nthereby providing a consistent picture and clarifying the strain relationship.\nWe acknowledge that the strain influence of a low-volume-fraction phase is subtle yet\ncritical, governed in our case by a semi-coherent epitaxial interface. This phenomenon is well\ndocumented in analogous nanocomposite systems; for example, in PbTiO /PbO thin films\n3\n(Linxing Zhang et al., Science, 361, 494-497, (2018); Mengsha Li et al. Small, 19, 2203201,\n(2023)), coherent PbO inclusions similarly induce measurable lattice strain in the ferroelectric\nmatrix. In our system, the coherent strain field originates from the lattice mismatch at the semi-\ncoherent interface, consistent with the RSM data and our atomic-resolution imaging.\ncorresponding line profile. (a) High-angle annular dark-field (HAADF) STEM image of the pristine Nb:STO (001) substrate,\nshowing its well-defined perovskite atomic arrangement. This image serves as an internal standard for calibrating spatial\nmeasurements and correcting drift/distortion in subsequent STEM analyses. (b) Intensity line profile measured along the white\ndashed line in (a), contrasting with the known lattice periodicity of STO (≈3.905 Å) and providing a reference for quantitative\nlattice-parameter determination.\nComment 2:\nRegarding Response#4:\nWhen the in-plane polarization orientation is parallel to the cantilever beam, a buckling\nmotion of the cantilever is induced, which appears in the VPFM signal. Based on the arrows\nshown in Figure 1, the left- and right-pointing domains are indeed parallel to the cantilever and\nshould therefore contribute to buckling. Could the authors perform vector-PFM measurements\nby gradually rotating the sample in-plane to exclude the possibility that a slight off-axis\norientation of the sample is responsible for the observed differences in LPFM contrast?\nFurthermore, could the authors comment on why the in-plane polarization switching via the\ntrailing field is less effective in the central region switched with +10 V? The expected dark\ncontrast in this region is not clearly distinguishable.\nResponse 2:\nWe sincerely thank the reviewer for comments regarding PFM contrast interpretation,\nwhich prompted us to conduct a more rigorous experimental verification.\nIn direct response to the suggestion, we performed a series of in-plane sample rotation\nvector-PFM measurements (Figure R3). This systematic analysis confirmed that a portion of\nthe LPFM contrast in our original data did indeed arise from a buckling motion contribution\nwhen the in-plane polarization was aligned parallel to the cantilever, as correctly surmised by\nthe reviewer. We thank the reviewer for this insightful observation, which has improved the\naccuracy of our interpretation.\nTo eliminate this artifact and present the clearest possible evidence, we have re-performed\nthe PFM imaging and switching experiments with optimized alignment and conditions. The\nresults are presented in the revised Figure 1 of the manuscript. The new data provide a much\nclearer visualization of the intrinsic in-plane ferroelectric domain structure and the subsequent\npolarization switching induced by the biased AFM tip, with minimal confounding signals from\ncantilever buckling (Figure R4).\nineffective switching via the trailing field in the +10 V region to a strong internal bias field\n(imprint effect) likely arising from oxygen vacancy accumulation, which opposes the positive\nbias and lowers the effective electric field below the switching threshold.\nWe believe the new, artifact-minimized data in Figure 1 and the clarified interpretation\nstrengthen the validity of our ferroelectric switching observations.\nFigure R3. In-plane sample rotation vector-PFM analysis of the BWO film. (a–f) In-plane PFM amplitude (left) and phase\n(middle) images acquired at sample rotation angles of 180°, 135°, 120°, 90°, 45°, and 30°, respectively. The corresponding\nschematics (right) illustrate the relative orientation between the AFM cantilever (grey) and the crystallographic axes of the\nsample (black arrows) at each rotation step.\nComment 3:\nRegarding Response#6:\nThe authors state that “this observation is inconsistent with the scenario where WO grows\n3\nindependently on STO,” and conclude that the WO phase inherits its in-plane orientation\n3\nthrough the BWO matrix. I do not fully agree with this interpretation. Even in the absence of a\nBWO matrix, a WO layer grown directly on STO would likely adopt a 45° in-plane rotation\n3\nto minimize epitaxial strain. Therefore, the observation of a 45° rotation alone may not be\nsufficient evidence to demonstrate a templating effect from the BWO layer.\nAdditionally, could the authors explicitly highlight the superlattice spots in Figure 3c?\nOverall, I remain unconvinced that the templating effect arises purely from the BWO layer.\nAs long as both BWO and STO are oriented along the same crystallographic direction (001 in\nthis case), it is difficult to exclude a templating contribution from the substrate via epitaxy.\nIn fact, this is inherent to the concept of epitaxial growth. A stronger claim would be supported,\nfor example, if the WO nanodomains were coherently oriented along an axis different from\n3\nthat of the STO substrate, rather than the same axis.\nResponse 3:\nWe thank the reviewer for prompting us to clarify the most direct evidence for the\n6\ntemplating effect.\nWe understand the reviewer’s hypothesis that pure WO on STO(001) might adopt a 45°\n3\nrotation to minimize strain. However, experimental reports indicate that epitaxial WO grown\n3\ndirectly on STO(001) typically stabilizes in a monoclinic phase with lattice parameters: a =\n0.730 nm, b = 0.754 nm, c = 0.769 nm (P.M. Woodward et al., Journal of Physics and Chemistry\nof Solids, 56, 10, 1305-1315,(1995)), that do not favor such a 45° rotational domain (Hossein\nKalhori et al., Applied Surface Science, 390, 43-49, (2016), Yingge Du et al, ACS Appl. Mater.\nInterfaces, 6, 16, 14253–14258, (2014)).\nMore importantly, our experiments reveal a key structural difference: when grown directly\non STO(001) under identical conditions, a pure WO film forms a structure that projects a\n3\nsquare-like, pseudo-cubic atomic arrangement in STEM images (Jing-ting Yang et al., Journal\nof Materials Chemistry C, 5, 11694-11699, (2017); Fan Zhang et al, Nature Communications,\n14, 3950, (2023)). In stark contrast, the WO nanodomains embedded within our BWO matrix\n3\nprecisely replicate the specific, non-cubic projected atomic pattern of the adjacent BWO lattice\n(as seen in Figure 2e and 2f). This includes matching interatomic distances and angles that are\ncharacteristic of the BWO framework and distinct from the pattern of WO on bare STO.\ninheritance of the BWO matrix’s local atomic configuration—a configuration that the STO\nsubstrate alone does not induce. This direct structural correspondence provides conclusive\nevidence that the WO phase is templated by the BWO layer.\npattern of the BWO region (revised Figure 3c in the main text).\nFigure R5. Revised Figure 3 in the main text\nComment 4:\nRegarding Response#8:\nLabelling the colour scale in the GPA maps shown in Figures R10(c–d) would improve\nclarity. Specifically, what do the green and red regions represent? Do they indicate that the\nBWO layer is unstrained while WO is strained? If so, this appears to contradict the RSM\n3\nresults, which suggest that the BWO layer is compressively strained. Could the authors clarify\nwhy the GPA and RSM analyses seem to indicate different strain behaviours?\nResponse 4:\nIn direct response, we have clearly labelled the colour scales in the Geometric Phase Analysis\n(GPA) maps (Figure R6(b-c)), where red denotes tensile strain and green denotes compressive\nstrain. It is crucial to note that all strain values are calculated relative to the BWO matrix region,\nwhich is set as the reference.\nThis relative frame of reference provides a consistent interpretation. The GPA maps reveal\nthat the WO nanodomains exhibit out-of-plane tensile strain (red) and in-plane compressive\n3\nstrain (green) relative to the BWO template. This local strain state of WO , which is under out-\n3\nof-plane tension and in-plane compression relative to BWO, is fully consistent with the global\nRSM results. The RSM data show an overall out-of-plane expansion and in-plane compression\nof the entire heterostructure relative to the STO substrate.\nThus, the two techniques are complementary: GPA visualizes the localized interfacial\nstrain, while RSM captures the integrated film response. Together, they robustly confirm the\npresence of a coherent strain field originating from the semi-coherent, templated interface,\nconsistent with our atomic-resolution imaging and FFT analysis.\nFigure R6. Atomic-scale characterization of the semi-coherent interface between BWO and the templated WO3 phase.\n(a) Filtered high-resolution STEM image of the BWO/WO3 interface using an artificial Bragg scattering filter (ABSF) to\nenhance atomic column visibility. (b-c) Geometric Phase Analysis (GPA) strain maps of the interface region, showing the\ndistribution of (b) εₓₓ and (c) εᵧᵧ strain. The color scale represents the strain relative to the BWO lattice, confirming a localized\nstrain field at the interface consistent with semi-coherency.\nComment 5:\nRegarding response#9:\nThe authors state that “where the WO nanodomains exhibit smaller in-plane and larger\n3\nout-of-plane lattice parameters relative to the BWO matrix”. Does not this contradict with the\nlattice parameter values they provide (WO : a = b = 5.56 Å, c = 5.90 Å; BWO: a = 5.437 Å, b\n3\n= 5.458 Å, and c = 16.430 Å)?\nIn addition, the authors state that “the results showed that its XRD characteristics (Figure\nR14(a)) were entirely different from those of the WO nanodomains embedded in BWO.” I do\n3\nnot think that comparing the XRD pattern of the composite film with that of a pure WO film\n3\ngrown on STO is sufficient to conclude that the metastable WO phase in the nanodomains\n3\ndiffers from WO grown directly on STO. This is particularly unclear given that, according to\n3\nthe authors, the WO phase is not distinguishable in the XRD measurements shown in Figure\nResponse 5:\nWe thank the reviewer for highlighting this point. As noted in Response 1.2, the initial\n8\nlattice parameters for WO were based on an uncalibrated STEM measurement. After\n3\nrecalibration using the STO substrate as an internal standard, the corrected lattice parameters\nfor the WO nanodomains, obtained from the calibrated STEM analysis (Figure R7), are: a = b\n3\n= 5.34 Å, c = 5.90 Å. With these values, the statement is now fully consistent: the WO\n3\nnanodomains indeed exhibit smaller in-plane lattice constants (a, b) and a larger out-of-plane\nconstant relative to the BWO matrix (a = 5.44 Å, b = 5.46 Å, c = 16.43 Å). This confirms the\nin-plane compressive and out-of-plane tensile strain state of the embedded WO phase.\ncomplicates direct peak matching. The purpose of the control experiment (pure WO on STO)\n3\nis not to compare unresolved features, but to demonstrate a fundamental structural difference.\nThe XRD pattern of our pure WO film matches that of the well-documented monoclinic WO\n3 3\nphase commonly grown on STO(001) (Yingge Du et al, ACS Appl. Mater. Interfaces, 6, 16,\n14253–14258, (2014) ; Hossein Kalhori et al., Applied Surface Science, 390, 43-49, (2016)).\nIn contrast, the structure of the templated WO nanodomains, as determined by STEM and\n3\nsupported by DFT, is fundamentally distinct from this monoclinic phase, and indeed from any\nother known WO polymorph. Therefore, the control experiment establishes that the metastable\n3\npolar WO phase we report does not form on STO without the BWO template; its formation is\n3\nintrinsically linked to templating by the BWO matrix.\nWe have updated the manuscript with the corrected lattice parameters and clarified the\nlogic of the XRD comparison to strengthen this conclusion. (page 11, line 218: In contrast, a\npure WO film grown directly on Nb:STO (001) under identical conditions exhibits distinctly\n3\ndifferent XRD features, corresponding to a conventional monoclinic phase47,48, which confirms\nthat the metastable polar phase observed in the nanocomposite is not stabilized by the substrate\nalone.).\nFigure R7. (a) Filtered STEM image of the WO3/BWO heterostructure. Line profiles extracted from the WO3 and BWO\nphases along the (b) [001] and (c) [110] zone axes to compare the lattice spacings.\nComment 6:\nRegarding response#10:\nI appreciate the inclusion of smaller-area PFM scans. However, could the authors comment\non why the LPFM contrast in the pristine region appears similar to that in the switched regions?\nand a more uniform contrast in the switched regions.\nResponse 6:\nWe thank the reviewer for this observation. The similarity in contrast between the pristine\nregion and the outer box (+9 V) is fully consistent with the switching asymmetry discussed in\nour response to Comment #2. As previously noted, the internal bias field arising from oxygen\nvacancy accumulation opposes the positive bias, rendering the +9 V switching ineffective.\nConsequently, the domain orientation in the +9 V region remains virtually identical to the\npristine state, whereas the distinct contrast in the -9 V region confirms that the internal field\nfavors negative bias. This result is fully consistent with the imprint effect discussed previously\nand robustly confirms the switchable polarization state.\nComment 7:\nRegarding response#11:\nI appreciate the inclusion of the transient current in the P–E loop. While the peak in the\npositive voltage region appears to coincide with the coercive field, the peak in the negative\nvoltage region does not seem to align as clearly with the coercive field. Do the authors have an\nalternative plot or dataset showing a clearer P–E loop?\nResponse 7:\nWe thank the reviewer for their careful examination. In response, we performed an\nadditional P–E loop measurement under optimized driving conditions. This alternative dataset\n(now included as the revised Figure 6a in the main text) shows a more distinct transient current\npeak in the negative voltage region that aligns clearly with the corresponding coercive field.\nWe believe this provides a clearer representation of the ferroelectric switching behavior and\naddresses the reviewer’s concern. The overall conclusion regarding ferroelectricity remains\nfully supported.\nFigure R8. Ferroelectric hysteresis loop illustrating the polarization switching behavior of the nanocomposite film.\nFor Reviewer #3:\nComment: The authors have addressed my comments sufficiently, and the manuscript can be\nconsidered for publication.\nResponse: Thank you, and we appreciate your great help in improving our manuscript.\n1"}, {"round": 2, "reviewer_comments": {"reviewer_3": "The authors have addressed my comments sufficiently, and the manuscript can be considered for publication."}, "author_response": ""}]

Decision letter