Highly tunable band structure in ferroelectric R-stacked bilayer WSe_2

Domain: physics
Published: 2026-02-06
Editorial outcome: major_revision

Abstract
Transition metal dichalcogenide homobilayers unite two frontiers of quantum materials research: sliding ferroelectricity, arising from rhombohedral stacking, and moiré quantum matter, emerging from small-angle twisting. The spontaneous polarization of ferroelectric rhombohedral stacked homobilayers produces a highly tunable band structure, which, together with strain-induced piezoelectricity, governs the topology and correlated electronic phases of twisted bilayers. Here we present a systematic low-temperature optical spectroscopy study of rhombohedral stacked bilayer WSe2 to quantitatively establish its fundamental electronic and ferroelectric properties. Exciton and exciton-polaron spectroscopy under doping reveals a pronounced electron-hole asymmetry that confirms type-II band alignment, with the conduction and valence band edges located at the Λ and K valleys, respectively. Through distinct excitonic responses and tunable interlayer-intralayer exciton hybridization under displacement fields, we uncover the coexistence of AB and BA ferroelectric domains. Using exciton-polarons as a probe, we directly measure the intrinsic polarization field and extract the interlayer potential. Finally, we demonstrate electric-field-driven symmetric switching of the valence band maximum, attributed to ferroelectric domain switching. These results provide a complete experimental picture of the band alignment, spontaneous polarization field, and domain dynamics of rhombohedral stacked WSe2, establishing key parameters to understand twisted bilayers and enabling new ferroelectric and excitonic device opportunities.

Introduction
Transition metal dichalcogenide (TMD) homobilayers exhibit a notable duality: rhombohedral (R) stacking generates intrinsic sliding ferroelectricity, while small twist angles give rise to moiré quantum phenomena. In R-stacked bilayers, two mirror-related layer registries—AB and BA (Fig. 1a)—break inversion symmetry and generate a spontaneous out-of-plane polarization1. The permanent dipole orientation can be reversed by lateral sliding of one layer with respect to the other, giving rise to sliding ferroelectricity2,3. By exploiting both charge and layer degrees of freedom, this unique switching mechanism enables the design of new multiferroic devices for ultrafast, non-volatile memory, distinguishing it from conventional ferroelectrics, where atoms move along the field direction4–6. Recent experiments have demonstrated robust polarization, domain formation, and excitonic signatures of ferroelectric order in R-stacked MoS2 and WSe2 homobilayers7–11. Moreover, the combination of ferroelectric, piezoelectric, and strain fields at AB/BA domain boundaries and intersections can create confinement potentials for excitons, effectively forming arrays of quantum-dot-like states under appropriate conditions12, opening opportunities to harness switchable ferroelectric materials for quantum optoelectronics.

While the intrinsic ferroelectricity of R-stacked homobilayers represents a striking phenomenon in its own right, it also plays a defining role in the emergent physics of twisted bilayers. Among homobilayer platforms, small-angle twisted bilayer WSe2 (tWSe2) has recently emerged as a system of profound interest13–17. The discovery of superconductivity in tWSe2 has placed it as a key material to explore correlated physics beyond the graphene family16,17. A crucial feature of tWSe2 at near-zero twist angles is that the lattice reconstructs into a periodic superlattice of nanometer-scale ferroelectric domains, providing the building blocks for moiré superlattices with flat bands and correlated states18,19. In the 0-degree (untwisted) limit, which is the focus of our study, these domains further relax into large, micrometer-scale regions (see schematic in Fig. 1b). The spontaneous polarization of these nanometer-scale domains and the electrostatic fields at their boundaries imprint a highly nontrivial potential landscape that shapes the moiré flat bands. Recent theoretical work suggests that the interplay of intrinsic ferroelectricity with strain-induced piezoelectricity governs both the topology and the correlated electronic phases in tWSe2 and related materials20. This highlights a critical point: a quantitative and comprehensive understanding of the untwisted R-stacked bilayer WSe2—including the precise band alignment, the magnitude of the built-in polarization field, and its tunability—is an indispensable prerequisite for understanding the complex interplay of correlation, topology, and superconductivity observed in its twisted counterpart. However, these fundamental properties of R-WSe2 remain largely unexplored.

In this work, we address this critical knowledge gap by performing a systematic low-temperature optical spectroscopy study on a dual-gated, untwisted R-stacked WSe2 bilayer. We leverage the sensitivity of excitons and exciton-polarons to the local electronic environment to map the material’s fundamental properties with high precision. We first uncover a distinct asymmetry in the system’s response to electron and hole doping, which unambiguously establishes a type-II band alignment at the K-valleys and identifies the conduction and valence band edges to be at the Λ and K points, respectively. By applying a small external electric field and tracking the doping-dependent evolution of excitonic features, combined with the observation of interlayer exciton hybridization, we provide direct optical evidence for the coexistence of AB and BA ferroelectric domains.

Furthermore, we employ exciton-polarons as a sensitive probe to quantitatively determine the magnitude of the intrinsic built-in electric field arising from spontaneous polarization. Strikingly, we demonstrate precise control over the band structure, observing a layer symmetric valence band maximum (VBM) switch under a strong electric field, which we attribute to field-induced ferroelectric domain switching. Our work provides a complete electronic band picture and a set of crucial experimental parameters—including the band-gap difference (δ), interlayer potential (ϕ0), and valence band offset (Δv)—that lay the essential foundation for understanding the emergent correlated and superconducting phases in twisted WSe2 systems.

Results
Charge- and field-tunable electronic structure of R-stacked WSe2
Our experimental platform is an electrically contacted 0∘ R-stacked bilayer WSe2 fully encapsulated in ~40nm thick hexagonal boron nitride (hBN) with graphite top and bottom gates (Fig. 1b, Fig. S1), enabling a dual-gate geometry that allows independent tuning of carrier density (n) and perpendicular displacement field (D). The bilayer was fabricated using the tear-and-stack technique21, which reliably produces micrometer-scale ferroelectric domains (Fig. S2). Additional control experiments confirm that these ferroelectric domains form robustly at the interface, even in the presence of encapsulation bubbles (see Fig. S3). To probe the electronic structure of this device, we employ low-temperature optical reflectance contrast spectroscopy at 4K. This technique sensitively probes interband transitions, providing direct access to excitonic resonances that reflect the underlying band structure. As shown in Fig. 1c, while monolayer WSe2 exhibits a single neutral exciton resonance, the R-stacked bilayer displays two distinct neutral exciton peaks. We label these the low-energy (LE) exciton at 1.6757 eV and the high-energy (HE) exciton at 1.6899 eV (see Fig. S4 for more data from other positions), which correspond to intralayer transitions in the two non-equivalent WSe2 layers11. The measured energy difference between these excitons, which reflects the intrinsic band-gap difference δ between the two layers, is 14.2 meV. Full-width at half-maximum (FWHM) values of 7.2 meV and 9.8 meV are observed for the LE and HE peaks, respectively.

The electronic asymmetry between the AB and BA stacking configurations in R-stacked WSe2 (Fig. 1a) lifts the layer degeneracy and produces a type-II band alignment (Fig. 1d). This structure can be quantitatively described by two key contributions10,22,23:

1. Direct band-gap difference (δ): The in-plane lateral displacement of one layer relative to the other places tungsten and selenium atoms in non-equivalent local registries (e.g., metal over chalcogen vs. hollow site), resulting in slightly different intrinsic band gaps for the two layers.

2. Interlayer potential (ϕ0): The interlayer coupling in the AB/BA configuration generates a spontaneous ferroelectric polarization, which creates a built-in electrostatic potential between the layers.

Layer-dependent hopping processes between conduction and valence bands (including higher-energy states) further renormalize the effective band edges. We account for this complexity with an asymmetric interlayer coupling coefficient α that modifies the direct band-gap difference δ. The combination of these effects determines the conduction- and valence-band offsets between the two layers, which can be expressed as23:

Δc = αδ + eϕ0, (1)

Δv = (α+1)δ + eϕ0. (2)

These band offsets provide a useful framework, but their precise values and tunability must be determined experimentally. To this end, we measure the differential reflectivity while sweeping both top (Vtg) and bottom (Vbg) gate voltages from −12 V to 12 V. As a representative probe, we track the reflectance contrast at the LE exciton energy (1.6757 eV), which captures the interplay between carrier doping, band filling, and the underlying ferroelectric order of the bilayer. The resulting 2D map (Fig. 1e) reveals a rich electronic landscape with multiple sharp boundaries that demarcate regions of distinct electronic character (labeled I–VI). In the following sections, we systematically deconstruct this landscape to unveil the underlying physics.

Asymmetric carrier doping and band alignment at zero field
To understand the origin of the complex excitonic landscape in Fig. 1e, we first investigate the intrinsic band structure by studying the system’s response to carrier doping at zero displacement field (D = 0). The voltage scanning path is shown in Fig. S5. Figure 2a shows the doping-dependent reflectance contrast map. In the intrinsic region (n ≈ 0), two prominent neutral exciton peaks, corresponding to the LE and HE transitions, are clearly visible. We label these as X0L and X0H, respectively.

Upon electron doping (n > 0), the spectra evolve significantly. The neutral exciton peaks X0L and X0H blueshift and form repulsive polarons, while two new attractive polaron branches (AP−L and AP−H) emerge at lower energies24. As the electron concentration increases, both attractive polarons exhibit a pronounced redshift. Notably, AP−H redshifts more rapidly than AP−L, eventually merging with it at a density of n > 3.0 × 1012 cm−2. A redshift of attractive polarons is a key signature that the doped carriers do not occupy the K-valley states associated with the excitons25,26. This observation is consistent with theoretical predictions that the conduction band minimum in this system is located at the Λ valley27 (also known as Q valley in literature28,29). This conclusion is further corroborated by g-factor measurements (Fig. 2f, Figs. S6, S7), where the g-factors of AP− are found to be identical to those of the neutral excitons, confirming that electrons are not doped into the K-valley. The g-factor for AP−, X0L and X0H are −5.04 ± 0.03, −4.89 ± 0.04 and −5.26 ± 0.08, respectively. The different redshift rates of the two polarons suggest that the electrons in the Λ valley are not symmetrically distributed between the layers, but are instead polarized towards the HE layer, likely due to the built-in electric field.

The more complex response under hole doping (n < 0) reveals the staggered nature of the valence bands at the K-points. Initially, as holes are introduced, the X0L peak loses oscillator strength and forms a repulsive polaron (RP+) and a corresponding attractive polaron (AP+L). The AP+L branch blueshifts with increasing hole density - a hallmark of phase-space filling as carriers are doped directly into the associated K-valley band. This is confirmed by g-factor measurements (Fig. 2f), which show a much larger g-factor for AP+L (24.39 ± 1.70) than for the neutral exciton25. During this initial doping stage, the X0H peak maintains its oscillator strength while exhibiting a slight redshift, likely due to changes in the effective dielectric environment. As hole doping is further increased, a second threshold is crossed at n ≈ 9.1 × 1012 cm−2. Beyond this point, the X0H peak also begins to quench, forming RP+H and AP+H. This sequential filling process provides direct evidence that the K-valley valence bands of the two layers are staggered in a type-II alignment.

From Fig. 2a, we can extract the binding energies of the various polaron states. For the electron-doped side, the binding energies of AP−L and AP−H are 8.7 meV and 16.4 meV, respectively. We attribute this significant difference primarily to the built-in electric field, which localizes the doped electrons closer to the HE layer, thereby enhancing the exciton-carrier interaction. For the hole-doped side, the binding energies for AP+L and AP+H are 17.6 meV and 10.5 meV. This disparity stems from varied dielectric screening, as the total carrier concentration is substantially higher when AP+H forms compared to when AP+L forms.

The schematics in Fig. 2b–e summarize the band alignment and carrier filling sequence derived from our doping-dependent spectroscopy. This comprehensive picture allows us to directly map the electronic configurations to the distinct regions observed in the gate map of Fig. 1e, with regions I-IV corresponding to the sequential filling of the hole and electron bands as described above.

Optical signatures of mixed AB/BA ferroelectric domains
To further validate the influence of ferroelectric domains on the band structure, we apply a small out-of-plane electric field of D = −0.025 V nm−1. Since the AB and BA domains possess opposite stacking structures (see Fig. 1a), the response of their respective LE and HE layers to the external field will also be inverted. Figure 3a displays the reflectance spectrum as a function of doping concentration under this field. On the electron-doped side, the spectrum remains largely unchanged compared to the zero-field case. The low-concentration hole-doped regime also shows similar behavior to that shown in Fig. 2a, where only the LE layer is populated.

However, significant differences emerge once the hole concentration is high enough to begin populating the HE layer. Specifically, two key changes are observed. First, the doping threshold to populate the HE layer is reduced from n ≈ 9.1 × 1012 cm−2 (at D = 0) to n ≈ 7.3 × 1012 cm−2. Second, after the formation of the attractive polaron AP+H,D1 and repulsive polaron RP+H,D1, where the D1 subscript is used to label Domain 1, the oscillator strength of the neutral exciton X0H does not quench to zero as it did in Fig. 2a. Instead, it maintains a considerable intensity.

These phenomena can be understood by considering the effect of the applied field on the two different domain types within the optical focus, as depicted in the schematic in Fig. 3b. In Domain 1, the external field raises the valence band of the HE layer, making it easier to populate and thus lowering the doping threshold. Conversely, in Domain 2, the field lowers the HE layer’s valence band, requiring a larger change in the Fermi level to achieve doping into the layer. This explains why the X0H oscillator strength persists even after doping commences in the HE layer of Domain 1. We plot the intensity of X0H as a function of doping in Fig. 3c. At zero field, the two domains are degenerate, and the X0H intensity drops sharply to a baseline signal as the HE layer is doped. Under a field of D = −0.025 V nm−1, the intensity drops as Domain 1 is doped but then stabilizes at a plateau. Based on the relative intensity change, we can estimate that the laser spot covers an area with an AB to BA domain ratio of approximately 1:1 for this particular sample position. See Figs. S8, S9 for data at another spatial position, which has only a single ferroelectric domain. We note that the data from the mixed-domain region not only provides a powerful, simultaneous demonstration of the opposite AB/BA response but is also more representative of typical R-stacked samples where domain sizes are often smaller than the optical spot.

The presence of mixed domains is further confirmed by the hybridization of interlayer excitons (IX) with intralayer excitons under an applied field, shown in Fig. 3d. An interlayer exciton species is observed to hybridize with an intralayer exciton at an electric field of D ≈ 0.02 V nm−1. Crucially, this anti-crossing behavior exists for both positive and negative polarities of the applied field, which is a definitive signature of the coexistence of both AB and BA domains, as they exhibit opposite Stark shifts. The corresponding energy level schematic is shown in Fig. 3e. At the same time, we note that due to the coupling between interlayer and intralayer excitons, the actual energy offset between the two layers is slightly larger than that at zero displacement field. By extracting the intralayer exciton energy splitting away from the hybridization region and averaging around D ≈ −0.07 V nm−1, we obtain a more accurate estimate of δ ≈ 16.0 meV.

Quantifying the ferroelectric built-in field
Having confirmed the existence of mixed domains, we next seek to determine the precise magnitude of the built-in electric field. As established earlier, the intrinsic field polarizes the doped Λ-valley electrons, pulling them closer to the HE layer. By applying an external out-of-plane electric field, we can counteract this effect and, by finding the point of cancellation, directly measure the internal field’s strength. To avoid averaging of this effect over both domains, we perform measurements on a sample area identified as having a single ferroelectric domain within our optical focus spot (see Figs. S8–S10). We apply a constant electron doping concentration of n = 2.0 × 1012 cm−2 and sweep the external displacement field D. The key insight is that when the external field exactly cancels the internal field, the Λ-valley electrons are symmetrically distributed between the two layers. At this symmetric point, the Λ electron gas interacts identically with both the HE and LE excitons, leading to an equal energy redshift for both polaron states. Consequently, the energy difference between the attractive polarons, AP−L and AP−H, becomes equal to the energy difference of the neutral excitons, X0L and X0H.

Figure 4a shows the evolution of the AP−L and AP−H peaks as a function of the applied field D (see Fig. S11 for the whole scan range). As the field strength increases, the AP−L peak redshifts while the AP−H peak blueshifts. This behavior is a clear signature of the electron wavefunction being transferred from the vicinity of the HE layer toward the LE layer, as illustrated schematically in Fig. 4b.

We extract the peak positions and plot the energy difference between them, ΔE, as a function of the applied field in Fig. 4c. The energy difference ΔE exhibits a nearly linear dependence on the field. The critical point is where this energy difference matches that of the neutral excitons (the dashed line in the plot). This condition is met at an applied field of D ≈ 0.102 V nm−1. This value directly corresponds to the magnitude of the intrinsic built-in electric field, Ebuilt-in. We find that this field strength is nearly independent of the doping concentration. Field-dependent ΔE for other doping levels is also summarized in Fig. 4c, which yields the same built-in electric field value.

Based on this measured field, we can calculate the corresponding interlayer potential, ϕ0, using the relation ϕ0 = Ebuilt-in × d0, where d0 is the interlayer distance. Using a typical value of d0 ≈ 0.65 nm for bilayer TMDs, we obtain:

ϕ0 = 0.102 V nm−1 × 0.65 nm = 66.3 mV (3)

This value for the interlayer potential is comparable in magnitude to earlier measurements on R-stacked WSe2 using alternative techniques30, as well as to values reported in other R-stacked TMDs such as MoS210, establishing a quantitative measure of the spontaneous polarization in bilayer WSe2.

Highly tunable valence band and domain switching
Next, we manipulate the relative positions of the LE and HE layer valence band maxima by applying an electric field to determine the valence band offset, Δv. To begin, we fix the hole doping concentration at n = 2.0 × 1012 cm−2, a density at which only the LE layer’s valence band is populated at zero field. As we gradually increase the external field, the spectrum undergoes an abrupt transformation at a critical field of D ≈ 0.086 V nm−1, as shown in Fig. 5a. At this point, the X0H oscillator strength suddenly decreases as it forms a repulsive polaron (RP+H), while the AP+L peak vanishes, accompanied by the re-emergence of the neutral X0L exciton.

This sudden spectral change indicates that a crossover has occurred, and the HE layer’s valence band has become the new valence band maximum (VBM). The switching behavior is further supported by doping dependence measurement near the switching field (Fig. S12). The underlying band structure evolution is depicted schematically in Fig. 5b,c. Due to the spectral overlap between the newly formed AP+H and the reappeared X0L, we cannot resolve them as distinct peaks. From this VBM switching experiment, we determine the valence band offset to be:

Δv = e × Dswitch × d0
= e × 0.086 V nm−1 × 0.65 nm (4)
= 55.9 meV

Surprisingly, we observe a similar VBM switching event at the symmetrically opposite field strength of D ≈ −0.086 V nm−1. This is unexpected, as a negative field should theoretically drive the LE and HE valence bands further apart. This observation strongly implies that the domain itself has undergone a ferroelectric switch at a field strength below −0.086 V nm−1, converting its stacking configuration to the opposite type (e.g., AB→BA). The system then behaves as a new domain with a reversed built-in field. This dynamic domain switching is independently verified through IX photoluminescence experiments (Fig. S13).

To confirm this band structure reconfiguration, we fix the field in the VBM switched state at D = −0.100 V nm−1 and study the doping dependence (Fig. 5d). On the electron-doped side, because the external field is now close in magnitude to the built-in field, electrons are more evenly distributed than in the scenario of Fig. 3a, causing the AP−L and AP−H peaks to redshift nearly in parallel. On the hole-doped side, because the filling sequence is now inverted: the X0H peak quenches first, forming RP+H. Although the corresponding AP+H is obscured by the X0L peak, its presence can be inferred from the change in the X0L lineshape. At higher hole concentrations, the LE layer also begins to fill, forming AP+L and RP+L.

Methods
Device fabrication
High-quality bulk crystals of WSe2 and hBN were used. Monolayer WSe2 was mechanically exfoliated onto a Si/SiO2 substrate. An R-stacked bilayer was created using the “tear-and-stack" method under an optical microscope21. The heterostructure was assembled using a standard dry-transfer technique, resulting in the bilayer WSe2 being encapsulated between two hBN flakes, with thin graphite flakes serving as top and bottom gates. A topmost thick hBN (≈50 nm) flake was used to cover and protect the whole sample. Pre-patterned electrical contacts were defined by laser lithography followed by evaporation of Cr/Au (5 nm/50 nm).

Optical spectroscopy
All measurements were performed in a closed-cycle optical cryostat (Attodry 1000) at a base temperature of 4K. Reflectance contrast spectra were acquired using a home-built microscope setup. White light from a tungsten-halogen lamp was focused on the sample using an apochromatic objective (NA = 0.82), and the reflected light was collected and analyzed by a spectrometer equipped with a liquid-nitrogen-cooled CCD camera. Photoluminescence measurements were performed using a 532 nm continuous wave laser with a power of 30 nW.

Data availability
The data that support the findings of the study are included in the main text and supplementary information files. Raw data can be obtained from the corresponding author upon request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors present a detailed investigation of untwisted R-stacked WSe₂ bilayers and successfully identify the coexistence of AB and BA ferroelectric domains. The results are scientifically intriguing and contribute meaningfully to the field of twistronics. However, I believe the current version of the manuscript requires further clarification and supporting data before it can be considered for publication. My concerns are outlined below:\n1. The reliability of the band alignment analysis critically depends on the quality of the R-stacked bilayer. While the authors mention the use of the “tear-and-stack” method, the manuscript lacks sufficient characterization data to confirm sample uniformity. Given the nature of the stacking process, the presence of bubbles or other inhomogeneities is likely. Therefore, comprehensive data such as AFM topography across the entire sample, PL/Raman mapping, and representative spectra should be provided to validate the sample quality.\n2. As illustrated in Fig. 1b, domains induced by stacking are expected to exhibit symmetry or regular patterns. However, the KPFM data shown in Fig. S1 does not clearly demonstrate such regularity. The authors are encouraged to explain this discrepancy and discuss possible reasons for the observed domain distribution.\n3. It is unclear which specific region of the sample was selected for the measurements presented in Fig. S1. Can identical optical properties be consistently observed across all domains that exhibit similar contrast in Fig. S1? If so, please provide supporting evidence. Furthermore, is the data reproducible even within the same domain? Clarification and corresponding data are requested. Since multiple domains are visible, a more detailed justification for the choice of measurement location would significantly enhance the credibility of the experimental results.\n4. The abstract mentions “establishing key parameters to understand twisted bilayers and enabling new ferroelectric and excitonic device opportunities,” and a similar statement appears in the conclusion. To strengthen the impact of the manuscript, the authors should elaborate on how their findings could be applied in practical device contexts, with specific examples or mechanisms.\nIn summary, while the manuscript focuses on deep scientific insights, additional explanation and supporting data are necessary to improve its accessibility and impact. In particular, given the experimental complexity, the quality of the sample plays a crucial role. I strongly recommend the inclusion of more thorough sample characterization to ensure the reliability of the reported results.", "reviewer_2": "The manuscript entitled \"Highly tunable band structure in ferroelectric R-stacked bilayer WSe2\" by Zhe Li et al. presents interesting results on the analysis of key parameters affecting the electronic band structure of untwisted R-stacked bilayer WSe2 using optical measurements. The device scheme involving a dielectric-sandwiched R-stacked bilayer WSe2 to understand doping via the same polarity gating and ferroelectric built-in potential via opposite polarity gating is a novel approach to understanding the complex type-II band alignment in R-stacked bilayer WSe2. Their experimental results and the proposed model for understanding the band-gap difference, interlayer potential, and band offset in R-stacked bilayer WSe2 are informative and convincing. Thus, I suggest minor revisions before this manuscript can be considered suitable for publication in Nature Communications.\ni) The authors present KPFM data in Fig. S1 to confirm that the device stacking is R with either BA or AB stacking order in bilayer WSe2. However, the surface potential values for both stackings are positive. Could the authors explain why both stackings show positive values, given that the built-in potential for BA and AB R-stacked WSe2 should be opposite? Also, is KPFM data alone sufficient to prove that the WSe2 bilayer is untwisted R-stacked?\nii) Given the broad readership of Nature Communications, the authors should provide more detailed information on key experimental parameters and the calculation process. Specifically, the manuscript should present detailed information on the reflectance contrast (ΔR/R0 and normalized ΔR/R0). Please also include the calculation process for evaluating the doping concentration in R-stacked bilayer WSe2 using dual gating, as well as the calculation for the g-factor. Additionally, the g-factor for AP- is slightly different from that of the neutral excitons. Can these values be treated as identical, as mentioned on page 3?\niii) On page 6, the authors suggest a method for measuring the internal potential in R-stacked WSe2 by sweeping the external electric field D. How can the internal potential be determined by finding the external electric field D value that results in an equal energy difference between the attractive polarons and the neutral excitons of both layers?\niv) Regarding ferroelectric switching in R-stacked bilayer WSe2, why does only the p-doped R-stacked bilayer WSe2 show switching behavior in contrast to the n-doped R-stacked bilayer WSe2?\nv) In Fig. 1(b), the device schematic for untwisted R-stacked WSe2 with two domains could mislead readers since it looks like a twisted R-stacked WSe2 with a moiré structure. Please clarify that the orange or white domains represent opposite stacking orders, not a moiré structure.\nvi) Physical quantities should be italicized. For example, in Fig. 1(e), V should be italicized. Please correct any typos throughout the manuscript.\nvii) While I agree with the authors’ claim that untwisted R-stacked bilayer WSe2 exhibits LE and HE type-II band structures regardless of stacking order, it seems that optical measurements cannot determine which layer is LE or HE. Is there an efficient method to distinguish the LE layer from the HE layer in untwisted R-stacked bilayer WSe2?", "reviewer_3": "The manuscript entitled \"Highly tunable band structure in ferroelectric R-stacked bilayer WSe2” by Li et al. reveals the exciton complex in the R-stacked bilayer WSe2. The data quality is high standard. The switching between the AB and BA stacks was detected in the photoluminescence spectra. Nevertheless, there are several concerns that the authors should address before it can be recommended for publication in Nature Communications.\n1. The size of the AB and BA domains is several micrometers. It indicates that the authors can investigate in AB and BA domains. The data shown in Figs. S4 and S5 were claimed in a single domain. Why did the author choose the data from mixed domains to analyze in the main manuscript? This makes the data analysis more complicated, and it is difficult for the reader to follow.\n2. Compare the data of Fig. 3a (multidomain), Fig. S4b (single domain), and Fig. S5 (single domain); it is difficult to see the difference. Could the author provide more analysis to make the claim of multidomain more convincing? Could the authors provide the spectra at that specific multidomain and compare them with the corresponding spectra of the single domain?\n3. Could the authors provide a similar analysis of Fig. 3c for Fig. S4 and Fig. S5?\n4. In Figure 3b, the appearance of the Q valley is the lowest energy point. Is the indirect exciton from the K-K valley in Fig. 3e correct? Is it possible from the Q-K valley?\n5. Could the author provide the data of AB and BA PL, as shown in Fig. 1c? It is interesting to see how it is different.\n6. Could the author provide the data for the positive D field, as in Fig. 4a for the negative D field?\nIn addition, I have other comments:\n- The introduction part is not convincing to me. The relaxation at the micrometer AB and BA deteriorates the flat band. The citation in this part is for STM, with AB and BA at the nanometer scale. That part of the introduction should be revised.\n- Fig. 1a: Adding a top view of the structure will be useful.\n- Figure 1d: Exaggerate bandgap differences, make the figure more informative.\n- Single spectrum (e.g., Fig. 1c) with different regions?"}, "author_response": "Reply to Reviewers' Comments\nReviewer #1 (Remarks to the Author):\nThe authors present a detailed investigation of untwisted R-stacked WSe₂ bilayers and\nsuccessfully identify the coexistence of AB and BA ferroelectric domains. The results are\nscientifically intriguing and contribute meaningfully to the field of twistronics. However, I\nbelieve the current version of the manuscript requires further clarification and supporting data\nbefore it can be considered for publication. My concerns are outlined below:\nResponse 1:\nWe thank the reviewer for the positive assessment of our work and for the suggestions that have\nhelped to improve our manuscript.\n1. The reliability of the band alignment analysis critically depends on the quality of the R-\nstacked bilayer. While the authors mention the use of the “tear-and-stack” method, the\nmanuscript lacks sufficient characterization data to confirm sample uniformity. Given the\nnature of the stacking process, the presence of bubbles or other inhomogeneities is likely.\nTherefore, comprehensive data such as AFM topography across the entire sample, PL/Raman\nmapping, and representative spectra should be provided to validate the sample quality.\nResponse 2:\nWe thank the reviewer for this important comment. We agree that the uniformity of the bilayer\nis critical for our analysis.\nFor this type of TMD homobilayer device, we argue that the quality of the buried interface is\nmore critical than the topography of the outermost surface. In our device structure, the bilayer\nWSe2 is fully encapsulated, buried beneath two layers of thick hBN with a combined thickness\nexceeding 100 nm. In this configuration, the information regarding the critical WSe2/WSe2\ninterface that can be reliably obtained from AFM topography is limited.\nTherefore, to validate the sample quality, we provide comprehensive optical characterization,\nincluding bright-field (BF), dark-field (DF), and photoluminescence (PL) images. These data\nare now included in the Supplementary Information as Figure R1.\nFrom the BF and DF images, the boundaries of the individual monolayers are clearly visible.\nWithin the bilayer region of interest, only a few, very small bubbles can be observed.\nMore significantly, the PL mapping provides crucial insight into the interlayer coupling. As\nshown in Figure R1(c), the monolayer WSe2 region exhibits strong and uniform PL, indicating\nthe high intrinsic quality of the material. In sharp contrast, the PL in the entire bilayer region is\nalmost completely quenched. This strong and highly uniform quenching is a clear signature of\nrobust and homogeneous interlayer coupling across the stacked area.\nFurthermore, we note that the few bubbles visible in the BF/DF images do not appear as local\nbright spots in the PL map. This strongly suggests that these bubbles are not located at the\ncritical WSe2/WSe2 interface (which would disrupt the quenching), but are likely located at\nother interfaces, such as between the graphene back-gate and the bottom hBN layer.\nThese optical data collectively demonstrate the high uniformity and excellent interface quality\nof our bilayer WSe2 sample. We have added these images and discussion to the Supplementary\nInformation (new Figure S1) to validate the sample quality for all readers.\nFigure R1: Optical characterization and schematic of the bilayer WSe2 device. (a) Bright-field (BF)\nand (b) Dark-field (DF) optical images of the device. The boundaries of the monolayer (ML) and bilayer\n(BL) WSe2 regions are clearly visible. (c) Photoluminescence (PL) intensity image of the sample. The\nML region shows strong, uniform PL, whereas the BL region exhibits strong and highly uniform PL\nquenching, indicating robust and homogeneous interlayer coupling. (d) Device schematic illustrating the\nML and BL areas, top and bottom gates, and bubbles. The experimental data were collected from the\ndual-gated BL region, which these optical data collectively demonstrate is highly uniform and possesses\na high-quality interface.\n2. As illustrated in Fig. 1b, domains induced by stacking are expected to exhibit symmetry or\nregular patterns. However, the KPFM data shown in Fig. S1 does not clearly demonstrate such\nregularity. The authors are encouraged to explain this discrepancy and discuss possible reasons\nfor the observed domain distribution.\nResponse 3:\nWe thank the reviewer for this insightful question.\nThe reviewer is correct that homobilayers with a small twist angle typically exhibit periodic,\nregular reconstruction patterns. However, this behavior changes dramatically as the twist angle\napproaches exactly 0° and the interlayer strain is minimized. In this near-ideal alignment limit,\nthe domain size is expected to increase significantly, and the domain structure becomes less\nperiodic and more irregular. In the most idealized scenario of a perfectly aligned, strain-free\nbilayer, the entire sample would form a single domain. These large irregular domains are often\nobserved in naturally 3R-stacked MoS2 (e.g., Nature 612, 465–469 (2022)).\nIn artificially stacked homostructures, some degree of residual strain or a minimal twist angle\nis often unavoidable, which typically results in the formation of periodic reconstruction patterns\nreported in many studies. Therefore, the observation of very large and irregular domains in our\nKPFM data (new Figure S2) is not a discrepancy, but rather strong evidence for the\nexceptionally high quality of our sample, indicating a near-perfect 0° twist angle and minimal\nstrain.\nWe also thank the reviewer for pointing out that our original schematic in Figure 1b could be\nmisleading. We have now revised Figure 1b to depict two larger, more distinct domains, which\nmore accurately reflects the physical reality of our high-quality, 0° twisted sample.\n3. It is unclear which specific region of the sample was selected for the measurements presented\nin Fig. S1. Can identical optical properties be consistently observed across all domains that\nexhibit similar contrast in Fig. S1? If so, please provide supporting evidence. Furthermore, is\nthe data reproducible even within the same domain? Clarification and corresponding data are\nrequested. Since multiple domains are visible, a more detailed justification for the choice of\nmeasurement location would significantly enhance the credibility of the experimental results.\nResponse 4:\nWe thank the reviewer for raising these important points regarding reproducibility and\nmeasurement location. We apologize for the initial lack of clarity.\nWe must first clarify a critical experimental detail. The KPFM measurements (shown in new\nFigure S2) must be performed on a sample surface and thus were taken on a non-encapsulated\nbilayer. In contrast, the low-temperature optical measurements, which are central to our work,\nrequire the sample to be fully encapsulated in hBN to protect the material and enable\nelectrostatic gating. Therefore, the optical measurements were necessarily performed on a\nseparate, fully encapsulated device. It is not feasible to perform KPFM on the actual\nencapsulated device used for the optical experiments.\nThe purpose of new Figure S2 is to provide independent, supporting evidence that our R-\nstacked bilayer WSe2 material inherently forms ferroelectric domains (AB vs. BA stacking)\nwith distinct, measurable surface potentials. This is a known characteristic of this material\nsystem. To prevent this confusion for future readers, we have revised the caption for new Figure\nS2 to make the sample difference and the figure's purpose explicit. We have added two\nsentences to the caption \"This measurement was performed on a separate sample from the main\nencapsulated (gated) device used for optical measurements, as KPFM requires an exposed\nsurface. The purpose of this figure is to provide independent validation of intrinsic ferroelectric\ndomain formation. \"\nHowever, to thoroughly address the reviewer's valid concerns about uniformity and\nreproducibility on the device used for our main results, we have performed extensive 4K\nreflectance mapping experiments on the same device in the main text.\nFigure R2(a) shows a 4K reflectance map of the device, generated by integrating the spectral\nintensity from 1.65 eV to 1.70 eV (a range influenced by both WSe2 and graphite absorption).\nThe map, which includes a schematic of the device structure, confirms that the optical\nproperties across the entire bilayer (BL) WSe2 area are highly uniform.\nTo further demonstrate this consistency, Figure R2(b) presents reflectance contrast spectra from\nnine different locations along the red and white star direction. While there are, as expected,\nminor variations in peak position, intensity, and full-width at half-maximum (FWHM), all nine\nspectra clearly exhibit the two distinct absorption peaks corresponding to the LE and HE\nexcitons.\nFurthermore, to explicitly confirm the reproducibility of the core physical phenomena, we\nperformed complete doping-dependent reflectance measurements at four additional locations\n(presented in Figure R2(c-f)), separate from the data in the main text. Although the overall data\nquality varies between spots (for instance, the signal in Figure R2(e) is visibly weaker than in\nother locations), the spectral evolution as a function of doping is completely consistent across\nall measured locations. This robustly demonstrates that the exciton-polaron physics we report\nis a general and reproducible property of the device, not an artifact of a single, specially chosen\nlocation.\nThe data presented in the main manuscript was collected from the locations marked by the red\nand white stars in Figure R2(a). We selected these specific points because they exhibit the\nnarrowest and strongest reflectance peak, which allows the underlying physical phenomena to\nbe presented with the greatest clarity for the reader. Furthermore, these spots are highly\nrepresentative: the red star is in a region with multiple domains, while the white star is in a large\nsingle-domain region, confirming our conclusions hold across different local domain\nenvironments.\nWe have added these data the Supplementary Information (new Figure S3) to validate the data\nreproducibility for all readers.\nFigure R2 | Optical uniformity and reproducibility of the encapsulated device. (a) 4K reflectance\nmap of the device, generated by integrating the spectral intensity between 1.65 eV and 1.70 eV. A\nschematic of the device stack is overlaid for clarity. The reflectance signal is influenced by both the\nWSe2 bilayer (BL) and the graphite gates. The map demonstrates the high spatial uniformity of the BL\nWSe2 area. The red and white stars indicate the measurement locations for the data presented in the main\nmanuscript, corresponding to a mixed-domain and a large single-domain region, respectively. (b)\nReflectance contrast spectra from nine locations along the red and white star direction. Despite minor\nvariations in peak characteristics (position, intensity, and width), all spectra clearly resolve the distinct\nLE and HE exciton peaks. (c-f) Doping-dependent reflectance contrast spectra collected from four\nadditional locations on the device (distinct from those in the main text). While the signal-to-noise ratio\nvaries (e.g., weaker intensity in (e)), the spectral evolution trend with doping is identical across all\nlocations, confirming the reproducibility of the reported exciton-polaron physics.\n4. The abstract mentions “establishing key parameters to understand twisted bilayers and\nenabling new ferroelectric and excitonic device opportunities,” and a similar statement appears\nin the conclusion. To strengthen the impact of the manuscript, the authors should elaborate on\nhow their findings could be applied in practical device contexts, with specific examples or\nmechanisms.\nResponse 5:\nWe thank the reviewer for this valuable suggestion to strengthen the manuscript's impact. We\nagree that elaborating on the practical device applications of our findings is crucial. To address\nthis point effectively, we believe the conclusion section is the most appropriate place for this\ndetailed discussion and future outlook, as the abstract must remain a concise summary of our\nprimary results.\nTherefore, we have expanded the final paragraph of our conclusion to provide specific\nexamples and mechanisms. The added text is as follows:\n\"Specifically, the electric-field control over coexisting ferroelectric domains, which we\ndemonstrate, is a key mechanism for developing ultra-low power sliding ferroelectric memories.\nAs recent work has highlighted, these systems show great promise for applications such as\nneuromorphic computing (e.g., as artificial synapses) [Li, Xiuzhen, et al. Nature\nCommunications 15.1 (2024): 10921.] and sequential logic-in-memory circuits [Niu, Jingjie, et\nal. ACS nano 19.5 (2025): 5493-5502.]. Furthermore, coupling these controllable ferroelectric\ndomains with other intrinsic 2D properties, such as the spin-valley locking in WSe2, could\nenable novel spintronic devices like spin field-effect transistors or spin logic gates. This\nplatform also offers a new route to electrically switch or tune quantum phases, such as\nsuperconductivity, in related moiré twisted bilayers.”\nWe believe this addition directly addresses the reviewer's comment by connecting our\nfundamental findings to practical device concepts, thereby strengthening the manuscript's\nimpact.\nIn summary, while the manuscript focuses on deep scientific insights, additional explanation\nand supporting data are necessary to improve its accessibility and impact. In particular, given\nthe experimental complexity, the quality of the sample plays a crucial role. I strongly\nrecommend the inclusion of more thorough sample characterization to ensure the reliability of\nthe reported results.\nResponse 6:\nWe thank the reviewer again for the constructive comments on improving our manuscript.\nReviewer #2 (Remarks to the Author):\nThe manuscript entitled \"Highly tunable band structure in ferroelectric R-stacked bilayer\nWSe2\" by Zhe Li et al. presents interesting results on the analysis of key parameters affecting\nthe electronic band structure of untwisted R-stacked bilayer WSe2 using optical measurements.\nThe device scheme involving a dielectric-sandwiched R-stacked bilayer WSe2 to understand\ndoping via the same polarity gating and ferroelectric built-in potential via opposite polarity\ngating is a novel approach to understanding the complex type-II band alignment in R-stacked\nbilayer WSe2. Their experimental results and the proposed model for understanding the band-\ngap difference, interlayer potential, and band offset in R-stacked bilayer WSe2 are informative\nand convincing. Thus, I suggest minor revisions before this manuscript can be considered\nsuitable for publication in Nature Communications.\nResponse 7:\nWe thank the reviewer for appreciating our experimental results and model.\ni) The authors present KPFM data in Fig. S1 to confirm that the device stacking is R with either\nBA or AB stacking order in bilayer WSe2. However, the surface potential values for both\nstackings are positive. Could the authors explain why both stackings show positive values,\ngiven that the built-in potential for BA and AB R-stacked WSe2 should be opposite? Also, is\nKPFM data alone sufficient to prove that the WSe2 bilayer is untwisted R-stacked?\nResponse 8:\nWe thank the reviewer for the valuable comments. The reviewer correctly notes that the built-\nin potential for AB and BA R-stacked domains should theoretically have opposite signs. The\npositive potentials observed in our new Figure S2 are due to a significant systematic offset\ninherent in the KPFM measurement. KPFM accurately measures the relative potential\ndifference between the domains, which is sufficient to confirm the existence of two distinct\nelectronic states (AB and BA). However, the absolute potential value is highly susceptible to\nexternal factors such as stray electric fields from the substrate, ambient contaminants, and the\nprecise condition of the tip. In our case, these factors introduced a large positive offset, shifting\nthe entire potential map. We chose to present the raw, measured data rather than apply an\narbitrary zero-point shift to force the values to be opposite. We have also included this\ndiscussion into new Figure S2.\nRegarding the second point, the reviewer is correct that KPFM data alone is insufficient to\ndefinitively prove the WSe2 bilayer is R-stacked. Our conclusion is based on the combination\nof KPFM data and our fabrication method. The KPFM images, by showing large-scale domains\n(micrometers) rather than a small-period moiré pattern, strongly indicate that the twist angle\nbetween the layers is very close to 0°.\nThis near-zero twist angle is a direct consequence of our \"tear-and-stack\" fabrication technique\n[Kim, K. et al. Nano Letters 16, 1989–1995 (2016).]. This method involves tearing a single\nWSe2 monolayer flake and then precisely stacking one piece back onto the other. Because both\nlayers originate from the exact same parent crystal, their relative orientation can be controlled\nwith high precision. By stacking them without intentional rotation, we ensure their crystal axes\nare aligned (parallel), which defines the R-stacking configuration (near 0° twist angle).\nTherefore, the KPFM data and our fabrication method confirms the untwisted R-stacking.\nii) Given the broad readership of Nature Communications, the authors should provide more\ndetailed information on key experimental parameters and the calculation process. Specifically,\nthe manuscript should present detailed information on the reflectance contrast (ΔR/R0 and\nnormalized ΔR/R0). Please also include the calculation process for evaluating the doping\nconcentration in R-stacked bilayer WSe2 using dual gating, as well as the calculation for the g-\nfactor. Additionally, the g-factor for AP- is slightly different from that of the neutral excitons.\nCan these values be treated as identical, as mentioned on page 3?\nResponse 9:\nWe thank the reviewer for these constructive suggestions, which help improve the clarity and\ncompleteness of the manuscript for a broad readership.\nWe have addressed all points by adding a detailed methods section to the Supplementary\nInformation (Section S1). This new section provides:\n1. Reflectance Contrast (ΔR/R ): A detailed explanation of how the differential\n0\nreflectance contrast ΔR/R0 = (R -R )/R is calculated, where R is the background\nsample 0 0 0\nreflectance from a substrate area without the WSe2 bilayer.\n2. Normalized Reflectance Contrast: A clarification of the normalization procedure\nused in Figure 3c. We explain that the integrated intensity of the X0 (which serves as\nH\na proxy for oscillator strength) is normalized to its maximum value. This normalization\nallows for the direct comparison of the relative contributions from the coexisting AB\nand BA domains.\n3. Doping Concentration Calculation: A step-by-step description of the calculation\nused to evaluate the doping concentration n induced by the dual-gate structure, based\non a parallel-plate capacitor model.\n4. g-factor Calculation: The full procedure for extracting the g-factors. We show the\nhelicity-resolved spectra in the new Figure S5 and detail the fitting process for the\nZeeman splitting, which is linearly fitted to extract g (new Figure S6).\nRegarding the second point on the AP- g-factor: The reviewer is correct that its g-factor is\nslightly different from that of the neutral excitons, and our original text was imprecise. We have\nrevised the main text (page 3) to state that the values are \"similar\" rather than \"identical\".\nThe g-factors of WSe2 excitons are known to be sensitive to their dielectric environment [Liu,\nErfu, et al. Physical Review B 99.20 (2019): 205420. Förste, Jonathan, et al. Nature\nCommunications 11.1 (2020): 4539. Koperski, Maciej, et al. 2D Materials 6.1 (2018): 015001.\nRobert, Cedric, et al. Physical Review Letters 126.6 (2021): 067403.]. Since AP- is an attractive\npolaron (an exciton dressed by a Fermi sea), its g-factor is expected to be further modified by\nthe doping level. This small discrepancy does not invalidate our main conclusion that in the\nAP- case, the electrons are not doped into the K valley.\nWe believe these additions significantly strengthen the manuscript by providing the necessary\nexperimental and computational details.\niii) On page 6, the authors suggest a method for measuring the internal potential in R-stacked\nWSe2 by sweeping the external electric field D. How can the internal potential be determined\nby finding the external electric field D value that results in an equal energy difference between\nthe attractive polarons and the neutral excitons of both layers?\nResponse 10:\nWe thank the reviewer for pointing out the lack of clarity in our description. We apologize for\nthe confusion and have revised this section in the manuscript (Page 6) to provide a more detailed\nexplanation based on the underlying physical mechanism.\nOur method is based on tuning the spatial distribution of the doped Λ valley electrons using the\nexternal displacement field, D. The key physics is as follows:\nAt finite electron doping, the neutral excitons (X0 and X0) interact with the Λ valley electron\nL H\ngas. This many-body interaction (e.g., screening) renormalizes the exciton energy, forming the\nattractive polaron (AP) states. This process manifests as an energy redshift (E = E - E ).\nshift X AP\nThe magnitude of this redshift E is directly related to the strength of the exciton-electron\nshift\ninteraction, which in turn depends on the spatial overlap between the exciton wavefunction\n(localized in the HE or LE layer) and the Λ valley electron gas wavefunction.\nDue to the intrinsic E in R-stacked WSe2, the Λ valley electrons are pulled asymmetrically,\nbuilt-in\nlocating them closer to the HE layer. This results in a stronger interaction and thus a larger\nenergy redshift for the HE polaron compared to the LE polaron (E > E ).\nshift, H shift, L\nBy applying an external field D, we can counteract the internal field and \"push\" the Λ electron\ngas. The internal potential is determined by finding the exact external field D that precisely\ncancels E . At this specific cancellation point (D = E ), the Λ electron wavefunction\nbuilt-in built-in\nbecomes symmetrically distributed between the two layers. At this unique symmetric point, the\nelectron gas interacts identically with both the HE and LE excitons. Consequently, their energy\nredshifts become equal: E = E .\nshift, H shift, L\nTherefore, if we look at the polaron energy splitting,\nΔE = E(AP- )- E(AP-) = [E(X0) - E ] - [E(X0) - E ]\nAP H L H shift, H L shift, L\n= ΔE X0 - [E shift, H - E shift, L ].\nWhen we reach the symmetric point, E = E , the second term becomes zero. The\nshift, H shift, L\nequation simplifies to ΔE\nAP\n= ΔE X0. Thus, the D field at which the polaron energy splitting\nequals the neutral exciton splitting is, by definition, the value of the internal E .\nbuilt-in\nWe have revised the main text on Page 6 to clarify this mechanism:\n\"We apply a constant electron doping concentration of n = 2.0 × 10¹² cm⁻² and sweep the\nexternal displacement field D. The key insight is that when the external field exactly cancels\nthe internal field, the Λ-valley electrons are symmetrically distributed between the two layers.\nAt this symmetric point, the Λ electron gas interacts identically with both the HE and LE\nexcitons, leading to an equal energy redshift for both polaron states. Consequently, the energy\ndifference between the attractive polarons, AP- and AP- , becomes equal to the energy\nL H\ndifference of the neutral excitons, X0 and X0. \"\nL H\niv) Regarding ferroelectric switching in R-stacked bilayer WSe2, why does only the p-doped\nR-stacked bilayer WSe2 show switching behavior in contrast to the n-doped R-stacked bilayer\nWSe2?\nResponse 11:\nWe thank the reviewer for this question, which allows us to clarify a key point. We apologize\nif our manuscript gave the impression that this behavior was unique to p-doping. In fact, we\nobserve robust ferroelectric switching in n-doping as well.\nTo demonstrate this explicitly, we have performed additional D field dependent experiments\nwith n-doping as shown in Figure R3.\n• Figure R3a shows the forward sweep, where D is scanned from -0.12 V/nm to +0.12\nV/nm (indicated by the red arrow). As the field becomes positive and its magnitude\nincreases, we observe two distinct, abrupt jumps in the attractive polaron energies.\n• Figure R3b shows the backward sweep, from -0.12 V/nm to -0.12 V/nm. Similarly,\nas the field becomes negative and its magnitude increases, we again observe two sharp\ndiscontinuities.\nThese abrupt jumps are the clear signature of ferroelectric domain switching. The fact that we\nobserve two jumps in each sweep direction, rather than a single one, likely indicates that the\ndomain boundary moves across our optical spot in two discrete steps.\nThese results unequivocally confirm that ferroelectric switching occurs in n-doped samples.\nThis suggests that the type of charge carrier (electrons versus holes) does not significantly\nimpact the fundamental mechanism of domain switching in this system. We have added this\nfigure and a corresponding discussion to the Supplementary Information (new Figure S9).\nFigure R3 | Ferroelectric switching in n-doped R-stacked bilayer WSe2. (a, b) Reflectance spectrum\nas a function of the displacement field D. The energies of the attractive polarons (AP) are tracked. (a)\nThe forward sweep of the D field, from -0.12 V/nm to +0.12 V/nm. (b) The backward sweep of the D\nfield, from +0.12 V/nm to -0.12 V/nm. The sweep direction is indicated by the red arrows. In both panels,\ntwo abrupt, discontinuous jumps in the polaron energies are observed, which are the characteristic\nsignatures of ferroelectric domain switching. The presence of two jumps in each direction suggests a\nmulti-step process for the domain wall to traverse the optical spot. This data confirms that robust\nswitching is also present with n-doping.\nv) In Fig. 1(b), the device schematic for untwisted R-stacked WSe2 with two domains could\nmislead readers since it looks like a twisted R-stacked WSe2 with a moiré structure. Please\nclarify that the orange or white domains represent opposite stacking orders, not a moiré\nstructure.\nResponse 12:\nWe thank the reviewer for this valuable comment. We agree that our previous schematic in\nFigure 1(b) could potentially be misleading. As the reviewer correctly points out, the different\ndomains (previously depicted as periodic orange and white regions) are intended to represent\nthe two coexisting, opposite stacking orders (e.g., AB and BA stacking) in our high-quality, 0°\ntwisted sample, and not a moiré structure.\nTo address this ambiguity and make the schematic clearer, we have revised Figure 1(b) in the\nnew manuscript. The updated figure now depicts two larger, more distinct domains, clearly\nillustrating the presence of both AB and BA stacking configurations. We believe this revised\nrepresentation more accurately reflects the physical reality of our device while avoiding any\npotential confusion with moiré patterns.\nvi) Physical quantities should be italicized. For example, in Fig. 1(e), V should be italicized.\nPlease correct any typos throughout the manuscript.\nResponse 13:\nWe thank the reviewer for the comments. We have now corrected typos throughout the\nmanuscript.\nvii) While I agree with the authors’ claim that untwisted R-stacked bilayer WSe2 exhibits LE\nand HE type-II band structures regardless of stacking order, it seems that optical measurements\ncannot determine which layer is LE or HE. Is there an efficient method to distinguish the LE\nlayer from the HE layer in untwisted R-stacked bilayer WSe2?\nResponse 14:\nWe thank the reviewer for this insightful question.\nThe reviewer is correct that for intralayer optical measurements, such as our reflectance\nspectroscopy, it is challenging to assign the LE and HE signals to the specific physical (e.g.,\ntop or bottom) layer.\nHowever, an efficient optical method to make this distinction does exist by measuring the Stark\nshift of the interlayer exciton (IX) photoluminescence (PL) (as shown in new Figure S11).\nBased on the established type-II band structure of R-stacked bilayer WSe2, the conduction band\nminimum is at the Λ valley (spatially located between the two layers), while the valence band\nmaximum is at the K valley of the LE layer. Therefore, the lowest-energy IX transition consists\nof an electron in the Λ valley and a hole physically localized within the LE layer.\nThis inherent charge separation of the IX allows us to use an external electric field (D) to\npinpoint the LE layer's physical location. If we apply a D field pointing, for example, from the\ntop layer to the bottom layer:\n1. If the top layer is the LE layer: The hole is in the top layer. The applied field will\nincrease the hole's potential energy (relative to the electron in the middle). This will\ncause the IX energy to increase (a blueshift).\n2. If the bottom layer is the LE layer: The hole is in the bottom layer. The applied field\nwill decrease the hole's potential energy. This will cause the IX energy to decrease (a\nredshift).\nThus, by observing the sign of the IX Stark shift (a blueshift or redshift) in response to a known\nfield direction, one can unambiguously determine whether the top or bottom physical layer is\nthe LE layer.\nAs shown in new Figure S11, we clearly observe these linear Stark shifts. The sign of this shift\nunambiguously determines whether the top or bottom physical layer is the LE layer within the\nmeasured ferroelectric domain. We have added the following discussion into the caption of\nnew Figure S11:\n\"Furthermore, this field-dependent measurement provides an unambiguous method to identify\nthe physical location of the LE and HE layers. The IX consists of an electron in the Λ valley\n(spatially located between the layers) and a hole localized in the K valley of the LE layer. A\nredshift (blueshift) under positive D indicates the hole is in the bottom (top) layer, meaning the\nbottom (top) layer is the LE layer. \"\nReviewer #3 (Remarks to the Author):\nThe manuscript entitled \"Highly tunable band structure in ferroelectric R-stacked bilayer WSe2”\nby Li et al. reveals the exciton complex in the R-stacked bilayer WSe2. The data quality is high\nstandard. The switching between the AB and BA stacks was detected in the photoluminescence\nspectra. Nevertheless, there are several concerns that the authors should address before it can\nbe recommended for publication in Nature Communications.\nResponse 15:\nWe thank the reviewer for appreciating our data quality.\n1. The size of the AB and BA domains is several micrometers. It indicates that the authors can\ninvestigate in AB and BA domains. The data shown in Figs. S4 and S5 were claimed in a single\ndomain. Why did the author choose the data from mixed domains to analyze in the main\nmanuscript? This makes the data analysis more complicated, and it is difficult for the reader to\nfollow.\nResponse 16:\nWe thank the reviewer for the comments and this important question regarding our data\nselection.\nWe chose to present the data from the mixed-domain region (Figure 3) in the main manuscript\nprecisely because it offers a powerful, side-by-side demonstration of ferroelectricity. This\ndataset uniquely illustrates the simultaneous and opposite response of adjacent AB and BA\ndomains to the same external electric field. This is the direct hallmark of their inverted band\nstructures and spontaneous polarization, providing compelling evidence in a single\nmeasurement.\nFurthermore, we believe this data is more representative and of greater value to the wider\nresearch community. While our high-quality sample exhibits large, micrometer-scale domains\n(as shown in KPFM in new Figure S2) that allow us to isolate single domains (data in new\nFigure S7, S8), this is not the typical case. In many R-stacked samples, the domain sizes are on\nthe order of several hundred nanometers. For any far-field optical measurement (which\ntypically uses a laser spot of ~ 1 um), probing such a sample will inevitably result in averaging\nover a mixed-domain region.\nTherefore, our analysis of the mixed-domain data in the main text provides a crucial framework\nfor other researchers to interpret their own optical data, which will most likely originate from\nmixed-domain areas. We have now added a sentence to the main text to clarify this rationale:\n\"Based on the relative intensity change, we can estimate that the laser spot covers an area with\nan AB to BA domain ratio of approximately 1:1 for this particular sample position. See new\nFigure S7 and S8 for data at another spatial position which has only a single ferroelectric\ndomain. We note that the data from mixed-domain region not only provides a powerful,\nsimultaneous demonstration of the opposite AB/BA response but is also more representative of\ntypical R-stacked samples where domain sizes are often smaller than the optical spot. \"\n2. Compare the data of Fig. 3a (multidomain), Fig. S4b (single domain), and Fig. S5 (single\ndomain); it is difficult to see the difference. Could the author provide more analysis to make\nthe claim of multidomain more convincing? Could the authors provide the spectra at that\nspecific multidomain and compare them with the corresponding spectra of the single domain?\nResponse 17:\nWe thank the reviewer for the comments.\nTo make the distinction between the multi-domain and single-domain regions more clear, we\nhave created a new comparison figure (presented as Figure R4 in this response) that directly\ncompares the reflectance maps at the same fixed field of D = -0.025 V/nm.\n• Figure R4a shows the multi-domain region (from main text Figure 3a).\n• Figure R4b shows the Type 1 single domain (from new Figure S7).\n• Figure R4c shows the Type 2 single domain (from new Figure S8).\nTo help the reader quickly focus on the critical area of interest, we have added red dashed boxes\nto highlight the high hole-doping regime in all three panels.\nBy comparing these highlighted regions, the difference is now stark:\n1. In the single domain (b), the red box shows that the neutral exciton (X0) retains its\nH\noscillator strength. This means the external field pushed the HE layer valence band\ndown, making it harder to populate with holes, so no polarons form.\n2. In the other single domain (c), the red box shows that the X0 oscillator strength has\nH\nvanished, and is completely replaced by the attractive and repulsive polarons. This is\nbecause the external field pushed the HE layer valence band up, making it easier to\npopulate.\n3. In the multi-domain region (a), the red box clearly shows a superposition of both\nbehaviors: the X0 peak persists (like domain b) with a reduced oscillator strength, but\nH\nat the same time, the attractive and repulsive polaron branches also clearly appear (like\ndomain c).\nThis simultaneous observation of two opposite responses is the definitive proof that the area in\nFigure 3a contains both AB and BA domains. To ensure this is clear for all readers, we have\nalso added these red dashed boxes to the original figures in the Supplementary Information\n(new Figure S7 and S8).\nFigure R4 | Direct comparison of multi-domain and single-domain responses at a fixed electric\nfield. Doping-dependent reflectance maps for (a) the multi-domain region, (b) a single domain (Type 1),\nand (c) the other single domain (Type 2). The red dashed boxes highlight the high hole-doping regime.\nThe single domains show a clear, opposite response: in (b), the X0 peak persists (doping suppressed),\nH\nwhile in (c), it is replaced by polarons (doping enhanced). The multi-domain region in (a) clearly shows\na superposition of both signatures, confirming the coexistence of AB and BA domains.\n3. Could the authors provide a similar analysis of Fig. 3c for Fig. S4 and Fig. S5?\nResponse 18:\nWe thank the reviewer for the comments.\nAs requested, we have performed the same analysis as in Figure 3c (plotting the X0 oscillator\nH\nstrength versus doping) for the single-domain data taken at the same electric field of D = -0.025\nV/nm (data from new Figure S7b and Figure S8b).\nThe results are now plotted in Figure R5. The two curves show a starkly different behavior:\n• For the first single domain (from Figure S7b), the X0 oscillator strength decreases\nH\nslowly and gradually as the hole concentration increases. This minor decrease is likely\ndue to the changing dielectric environment as the entire sample is doped. Crucially,\neven at a high doping of n ≈ -10.0 × 10¹² cm⁻², the X0 strength remains at a high level,\nH\nconfirming that holes are not doping into the HE layer in this domain.\n• In sharp contrast, for the second single domain (from Figure S8b), the X0 strength\nH\ndrops rapidly at a threshold of n ≈ -7.0 × 10¹² cm⁻², quickly falling to a low level. This\nis the clear signature of holes doping into the HE layer, causing the X0 exciton to\nH\nconvert into attractive and repulsive polarons.\nThis direct, quantitative comparison of the two single domains provides definitive proof of their\nopposite response to the applied electric field. It further validates our analysis of the multi-\ndomain region (Figure 3c), which showed a partial, plateaued drop in intensity consistent with\na superposition of these two behaviors.\nFigure R5 | Quantitative comparison of X0 oscillator strength in single domains. Oscillator strength\nH\nof the high-energy neutral exciton (X0), plotted as a function of hole doping concentration (n). The data\nH\nis extracted from the single-domain maps in new Figure S7b and S8b, both at D = -0.025 V/nm.\n4. In Figure 3b, the appearance of the Q valley is the lowest energy point. Is the indirect exciton\nfrom the K-K valley in Fig. 3e correct? Is it possible from the Q-K valley?\nResponse 19:\nWe thank the reviewer for this very sharp observation.\nAs shown in our band diagram (Figure 3b), the Q(Λ) valley is indeed the conduction band\nminimum (CBM). Consequently, the lowest-energy interlayer exciton (IX) is the Q-K transition,\nwhich involves an electron at the Q-valley and a hole at the K-valley.\nThis is precisely what we observe in our photoluminescence (PL) spectra in new Figure S11.\nAs shown there, the Q-K exciton emission appears at a much lower energy (around 1.5 eV at\nD = 0) than the intralayer excitons.\nHowever, the data presented in Figure 3d is from reflectance spectroscopy, which differs\nfundamentally from PL. While PL spectroscopy is dominated by emission from the lowest-\nenergy relaxed state (the Q-K exciton) due to efficient carrier relaxation, reflectance\nspectroscopy can probe other, higher-energy excitonic states with larger oscillator strength.\nThe IX feature observed in Figure 3d (which anti-crosses with the intralayer excitons) has an\nenergy that is very close to the intralayer transitions. Given that the Q-K exciton's energy is\nsignificantly lower (~1.5 eV), the transition we observe in the reflectance spectrum in Figure\n3d cannot be the Q-K exciton. Therefore, we identify it as a well-known, higher-energy\ninterlayer exciton: the K-K transition.\n5. Could the author provide the data of AB and BA PL, as shown in Fig. 1c? It is interesting to\nsee how it is different.\nResponse 20:\nWe thank the reviewer for the comments. We have now extracted multiple PL spectra from\nnew Figure S11 and plotted as Figure R6.\n• Figure R6a shows the PL spectra at zero displacement field (D=0). As can be seen, the\ntwo spectra are nearly identical. This is because although the AB and BA domains\npossess opposite built-in electric fields, their electronic states are degenerate at D=0.\nTheir respective interlayer excitons (IX) have oppositely oriented dipoles, but their\nenergies are the same, resulting in overlapping emission peaks.\n• Figure R6b shows the spectra under an applied external field of D = 0.03 V/nm. Here,\nthe difference is clear. The external field breaks the degeneracy. Because the intrinsic\nIX dipoles in the two domains point in opposite directions, the applied field induces an\nopposite Stark shift: one peak shifts to a lower energy (redshift) while the other shifts\nto a higher energy (blueshift).\nThis new PL data provides further direct confirmation of the opposite ferroelectric nature of the\nAB and BA domains.\nFigure R6 | Photoluminescence (PL) spectra of AB and BA domains. (a) At zero displacement field\n(D = 0). The interlayer exciton (IX) emission peaks from both domains are degenerate, resulting in nearly\nidentical spectra. (b) Under an applied displacement field of D = 0.03 V/nm. The external field lifts the\ndegeneracy. Due to the oppositely oriented dipoles of the interlayer excitons in the AB and BA domains,\ntheir emission peaks exhibit an opposite Stark shift (one blueshift and one redshift), confirming their\ndistinct ferroelectric nature.\n6. Could the author provide the data for the positive D field, as in Fig. 4a for the negative D\nfield?\nResponse 21:\nWe thank the reviewer for this suggestion. We have performed this measurement (Figure R7)\nand included the data in the Supplementary Information (now Figure S9).\nThis figure shows a full sweep of the displacement field D from -0.12 V/nm to +0.12 V/nm\n(and back). This dataset provides the data for the positive D field range as requested. It also\nclearly captures the ferroelectric domain switching in the n-doped sample, which is visible as\nabrupt jumps in the polaron energies. This figure is now available in the Supplementary\nInformation.\nFigure R7 | Ferroelectric switching in n-doped R-stacked bilayer WSe2. (a, b) Reflectance spectrum\nas a function of the displacement field D. The energies of the attractive polarons (AP) are tracked. (a)\nThe forward sweep of the D field, from -0.12 V/nm to +0.12 V/nm. (b) The backward sweep of the D\nfield, from +0.12 V/nm to -0.12 V/nm. The sweep direction is indicated by the red arrows. In both panels,\ntwo abrupt, discontinuous jumps in the polaron energies are observed, which are the characteristic\nsignatures of ferroelectric domain switching. The presence of two jumps in each direction suggests a\nmulti-step process for the domain wall to traverse the optical spot. This data confirms that robust\nswitching is also present with n-doping.\nIn addition, I have other comments:\n- The introduction part is not convincing to me. The relaxation at the micrometer AB and BA\ndeteriorates the flat band. The citation in this part is for STM, with AB and BA at the nanometer\nscale. That part of the introduction should be revised.\nResponse 22:\nWe thank the reviewer for the comments. The reviewer correctly notes that the moiré flat bands\ndiscussed in Refs. [18, 19] arise from the periodic superlattice formed by nanometer-scale AB\nand BA domains, which occurs in small-angle twisted bilayers. In contrast, our work focuses\non the 0-degree (untwisted) limit. In this regime, the lattice fully relaxes into large, micrometer-\nscale domains. As the reviewer rightly implies, this configuration does not host moiré flat bands,\nas it lacks the periodic moiré potential.\nOur intention in citing Refs. [18, 19] was to establish the critical importance of the R-stacked\nferroelectric domain as the fundamental \"building block\" whose properties are essential in both\nphysical regimes:\n1. In the twisted case, the periodic potential landscape created by these nm-scale\nferroelectric blocks is what generates the flat bands.\n2. In our untwisted case, the large micrometer-scale of the domains allows us to isolate\nand study the intrinsic physical properties of this \"building block\" itself (e.g., its built-\nin field and band alignment) directly, without the complexities of a moiré superlattice.\nWe have revised this paragraph in the introduction (Page 1) to make this distinction clear. The\nfollowing sentence in the introduction have been changed from:\n\"A crucial feature of tWSe2 at near-zero twist angles is that the lattice reconstructs into\nextended R-stacked ferroelectric domains (see schematic in Fig. 1b), providing the building\nblocks for moir´e superlattices with flat bands and correlated states [18, 19]. \"\nTo:\n\"A crucial feature of tWSe2 at near-zero twist angles is that the lattice reconstructs into a\nperiodic superlattice of nanometer-scale ferroelectric domains, providing the building blocks\nfor moiré superlattices with flat bands and correlated states [18, 19]. In the 0-degree (untwisted)\nlimit, which is the focus of our study, these domains further relax into large, micrometer-scale\nregions (see schematic in Fig. 1b). \"\n- Fig. 1a: Adding a top view of the structure will be useful.\nResponse 23:\nWe thank the reviewer for the comments. A top view is added in Figure 1a.\n- Figure 1d: Exaggerate bandgap differences, make the figure more informative.\nResponse 24:\nWe thank the reviewer for this suggestion. We have now exaggerated bandgap differences in\nFigure 1d to make the figure more informative.\n- Single spectrum (e.g., Fig. 1c) with different regions?\nResponse 25:\nWe thank the reviewer for the comments.\nWe have performed extensive spatial-dependent measurements to validate this, as shown in\nFigure R8.\n1. Our sample is highly uniform. As shown in the 4K reflectance map (Figure R8a), the\noptical properties across the entire bilayer WSe2 area are consistent.\n2. Spectra from different regions are nearly identical. We collected reflectance spectra\nfrom nine different spot along the red and white star direction. (Figure R8b). All nine\nspectra are exceptionally similar, clearly resolving the same Low-Energy (LE) and\nHigh-Energy (HE) exciton peaks.\nThis high degree of similarity is expected, even when probing different ferroelectric domains\n(AB vs. BA) at zero external field (D = 0). There are two key physical reasons for this:\nFirst, as we demonstrate with PL measurements in Response 19, the electronic states of the AB\nand BA domains are degenerate at D = 0. Although they possess opposite built-in fields, their\nenergy levels are identical, leading to overlapping spectral peaks.\nSecond, the main features in our reflectance spectrum (Figure 1c) are the intralayer excitons.\nThese excitons are charge-neutral, with their electron and hole wavefunctions confined within\nthe same physical layer. This makes them, by nature, highly insensitive to electric fields (i.e.,\nthey exhibit a negligible Stark shift). Therefore, the opposite internal built-in fields of the AB\nand BA domains do not cause a measurable energy shift in these intralayer reflectance peaks.\nIn summary, the spectrum in Figure 1c is highly representative of the entire device.\nFigure R8 | Optical uniformity and reproducibility of the encapsulated device. (a) 4K reflectance\nmap of the device, generated by integrating the spectral intensity between 1.65 eV and 1.70 eV. A\nschematic of the device stack is overlaid for clarity. The reflectance signal is influenced by both the\nWSe2 bilayer (BL) and the graphite gates. The map demonstrates the high spatial uniformity of the BL\nWSe2 area. (b) Reflectance contrast spectra from nine locations along the red and white star direction.\nDespite minor variations in peak characteristics (position, intensity, and width), all spectra clearly resolve\nthe distinct LE and HE exciton peaks."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Most of my questions have been addressed with fairly thorough answers. However, I have found a serious problem in the interpretation of the experimental results. Specifically, the two different domains, which are the core of this paper, must be clearly distinguished. The domains were clearly identified in Figure S2 using KPFM, and that part is relatively clear. However, the optical analysis was conducted on an entirely different sample, not the one confirmed by KPFM. Figure R8 presents such a sample image.\nThe problem lies in the method used to distinguish the two different domains. The authors attempted to differentiate the domains by using reflectance data, marking the positions of the two domains with white and red stars. Yet, based on the presented results, it is difficult to objectively distinguish the domains. In other words, there is no direct evidence that the positions marked by the red and white stars correspond to different domains.\nFigure R8(b) shows changes in the reflectance contrast spectra, but these signals can also vary depending on the sample quality (e.g., interface contaminations, bubbles, wrinkles). In fact, Figure S1 BF/DF images reveal that many bubbles are present in the sample. Figure S1(d) marks the positions of bubbles, and the authors argue in Figure S3 that measurements were taken at clean locations. However, microscopic bubbles or contamination cannot be identified solely with an optical microscope. This means that there could be subtle impurities at the positions marked by the red and white stars that are not distinguishable optically.\nTherefore, more definitive experimental evidence is required. Unless direct proof is provided that the key results discussed in the main text were indeed measured from different domains, the experimental results cannot be considered reliable at all.", "reviewer_2": "The revised manuscript entitled \"Highly tunable band structure in ferroelectric R-stacked bilayer WSe2\" by Zhe Li et al. have satisfactorily addressed all my previous comments and made corresponding changes to the manuscript and supplementary information. Now I recommend the study \"Highly tunable band structure in ferroelectric R-stacked bilayer WSe2\" for publication in Nature communications.", "reviewer_3": "My concerns in the original manuscript were nicely resolved by the authors in the revised version. Nevertheless, before I can accept the manuscript for publication in Nature Communications, the authors should incorporate the additional data from the reviewers' response into the revised version, at least in the supplementary information. For example, the data in Figures R5 and R6. These will help readers better understand the quality of the sample and the scientific conclusion drawn from the study."}, "author_response": "Reply to Reviewers' Comments\nReviewer #1 (Remarks to the Author):\nMost of my questions have been addressed with fairly thorough answers. However, I have\nfound a serious problem in the interpretation of the experimental results. Specifically, the two\ndifferent domains, which are the core of this paper, must be clearly distinguished. The domains\nwere clearly identified in Figure S2 using KPFM, and that part is relatively clear. However, the\noptical analysis was conducted on an entirely different sample, not the one confirmed by KPFM.\nFigure R8 presents such a sample image.\nThe problem lies in the method used to distinguish the two different domains. The authors\nattempted to differentiate the domains by using reflectance data, marking the positions of the\ntwo domains with white and red stars. Yet, based on the presented results, it is difficult to\nobjectively distinguish the domains. In other words, there is no direct evidence that the positions\nmarked by the red and white stars correspond to different domains.\nFigure R8(b) shows changes in the reflectance contrast spectra, but these signals can also vary\ndepending on the sample quality (e.g., interface contaminations, bubbles, wrinkles). In fact,\nFigure S1 BF/DF images reveal that many bubbles are present in the sample. Figure S1(d)\nmarks the positions of bubbles, and the authors argue in Figure S3 that measurements\nwere taken at clean locations. However, microscopic bubbles or contamination cannot be identified\nsolely with an optical microscope. This means that there could be subtle impurities at the\npositions marked by the red and white stars that are not distinguishable optically.\nTherefore, more definitive experimental evidence is required. Unless direct proof is provided\nthat the key results discussed in the main text were indeed measured from different domains,\nthe experimental results cannot be considered reliable at all.\nResponse 1:\nWe thank the reviewer for this critical observation. We understand the concern regarding the\nobjective identification of domains in the optical measurement sample, especially given the\npresence of bubbles. To address this, we have compiled a comprehensive rebuttal combining 1)\nnew experimental data (KPFM on a control sample) confirming that bubbles in a few\nspecific locations do not impede the formation of ferroelectric domains, 2) comparison with\nstate-of-the-art literature, and 3) detailed spectroscopic analysis proving the effects of the\ndomains is distinct from artifacts.\n1. Integrity of the WSe2 Interface and New Control Experiment (Figure R1)\nRegarding the bubbles: Our device consists of 8 layers (Topmost hBN, Top Graphite Gate, Top\nhBN, Graphite Contact, WSe2 Layer 1, WSe2 Layer 2, Bottom hBN, Bottom Graphite Gate).\nThe critical interface is the WSe2/WSe2 interface, fabricated using the mature \"tear-and-stack\"\ntechnique, widely proven to produce high-quality homostructures [1-3]. The bubbles observed\nin optical images are statistically highly likely to be located at non-critical interfaces (e.g.,\nbetween graphite and hBN) rather than the twisted WSe2 interface.\nTo verify this experimentally, we fabricated a new control sample (Figure R1) consisting of 2\nnm top hBN / WSe2 / WSe2 / Graphite contact / Bottom hBN / Bottom Graphite gate.\n• Optical Inspection: As shown in Figure R1a and R1b, bubbles are clearly visible in\nthis sample, with a density and size distribution similar to the device used in the main\ntext.\n• KPFM Measurement: Despite the visible bubbles, KPFM measurements on this sample\nreveal clear, well-defined ferroelectric domains (Figure R1c).\nFigure R1 | Verification of ferroelectric domain formation in a control sample. (a) Bright and (b)\ndark microscope image of the control heterostructure (2 nm top hBN / R-bilayer WSe2 / graphite contact\n/ bottom hBN / bottom graphite gate). Similar to the device in the main text, bubbles are clearly visible\nacross the sample. (c) Kelvin Probe Force Microscopy (KPFM) surface potential map of the red dashed\nboxes in (a) and (b). Despite the presence of bubbles visible in optical image, the KPFM scan reveals\nclear, well-defined ferroelectric domains with distinct potential contrast. This confirms that the bubbles\ngenerally reside at non-critical interfaces (e.g., hBN/graphite) and do not disrupt the formation of the\nferroelectric moiré interface created by the tear-and-stack technique.\nThis result is crucial: it demonstrates that the presence of optically visible bubbles does not\nprevent the formation of ferroelectric domains at the WSe2 interface. The \"tear-and-stack\"\nmethod remains robust against bubbles in the encapsulation layers.\n2. Comparison with State-of-the-Art Literature (Figure R2)\nWe further note that bubbles are often inevitable in complex, multi-layer van der Waals\nheterostructures. Recent high impact works, for instance ref [2] and [4]. utilizing similar multi-\nlayer stacks also exhibit comparable density of bubbles (often less visible due to specific hBN\nthickness interference and lack of dark-field images that highlight bubbles more effectively).\nThe community consensus is that while bubbles exist, they do not invalidate the physics\nobserved in the clean regions of the device.\n[Figure Redacted]\nFigure R2 | Bubbles in state-of-the-art van der Waals heterostructures. Optical micrographs of high-\nperformance devices reported in recent literature: (a) [2] and (b) [4]. These studies represent the current\nstate-of-the-art in 2D material research. Note that despite the high quality of the physical results reported,\nbubbles (indicated by yellow arrows) are inevitably formed during the assembly of complex, multi-layer\nstacks. This comparison indicates that the presence of bubbles in the encapsulation layers does not\npreclude the existence of pristine interfaces and reliable physics in the active regions of the device.\nIn addition to visual inspection, we emphasize that in spectroscopic studies of 2D materials, the\nlinewidth of the reflection spectrum serves as a more rigorous and quantitative standard for\nassessing sample quality. Our devices exhibit exceptionally narrow linewidths: the Full Width\nat Half Maximum (FWHM) of the reflection spectra reaches 3.6 meV for the monolayer, and\n7.2 meV and 9.8 meV for the bilayer. These values match the highest standards reported in the\nliterature for corresponding materials [5,6], providing solid spectroscopic evidence of the high\ncrystalline quality and pristine interfaces of our samples, independent of the localized bubbles.\n3. Spectroscopic Evidence: Distinct Domain Responses (Figure R3)\nThe reviewer asks for direct evidence that the red and white stars correspond to different\ndomains rather than local impurities. We respectfully submit that the reflectance spectra\nthemselves provide this definitive proof – a key finding of our manuscript. Due to the specific\nstacking orders (AB vs. BA), the two domains must exhibit opposite responses to the same\nexternal electric field.\nFigure R3 | Direct comparison of multi-domain and single-domain responses at a fixed electric\nfield. (a) single domain Type 1, (b) single domain Type 2 and (c) multi-domain region. The red dashed\nboxes highlight the high hole-doping regime. The simultaneous observation of two opposite responses\nin (c) serves as definitive proof that this area contains both AB and BA domains. (d) Schematic showing\nthe opposite response of AB and BA domains to an external electric field. (e) Normalized peak intensity\nof the high-energy neutral exciton, plotted as a function of hole doping concentration (n).\nIn Figure R3 (above), we directly compare the doping-dependent reflectance maps of the multi-\ndomain region against single-domain regions. To facilitate the comparison, we have added red\ndashed boxes to highlight the high hole-doping regime.\n• Single Domain (Type 1, Panel a): In the red box, the neutral exciton (X0) retains its\nH\noscillator strength. This indicates the external field pushes the high energy layer\nvalence band down, suppressing hole population (no polarons form). The schematic of\nthe band structure change and doping-dependent intensity change are shown in Fig.\nR3d and R3e, respectively.\n• Single Domain (Type 2, Panel b): In the red box, the X0 oscillator strength vanishes\nH\nand is replaced by attractive and repulsive polarons. Here, the field pushes the valence\nband up, facilitating hole population.\n• Multi-domain Region (Panel c): The red box clearly shows a superposition of both\nbehaviors. The X0 peak persists (characteristic of Type 1) but with reduced intensity,\nH\nwhile the polaron branches (characteristic of Type 2) appear simultaneously.\nThis simultaneous observation of two opposite physical responses is the \"fingerprint\" of a\nmulti-domain spot. This clear fingerprint is enabled by very clean optical spectroscopy of a R-\nWSe2 with a pristine (ferroelectric) interface. This effect cannot be replicated if bubbles or\ncontamination is present at the interface: spectroscopically, this would typically broaden\nlineshapes or reduce overall intensity. Rather, two distinct doping-dependent behaviors (Fig\nR3a and R3b) are superimposed in the mixed domain situation. We have added these\nclarifications and the marked figures to the Supplementary Information as new Figure S9.\nIn summary, the specific \"superposition\" optical signal combined with the new KPFM control\ndata confirms that our results arise from ferroelectric domains, not sample contamination.\nReferences:\n[1] Y. Cao et al., Nature 556, 43 (2018).\n[2] J. Cai et al., Nature 622, 63 (2023).\n[3] Y. Xia et al.et al., Nature 637, 833 (2025).\n[4] R. Qi et al., Science 390, 299 (2025).\n[5] F. Cadiz et al., Phys. Rev. X 7, 021026 (2017).\n[6] KM McCreary, et al., Nanoscale 14, 147 (2022).\nReviewer #2 (Remarks to the Author):\nThe revised manuscript entitled \"Highly tunable band structure in ferroelectric R-stacked\nbilayer WSe2\" by Zhe Li et al. have satisfactorily addressed all my previous comments and\nmade corresponding changes to the manuscript and supplementary information. Now I\nrecommend the study \"Highly tunable band structure in ferroelectric R-stacked bilayer WSe2\"\nfor publication in Nature communications.\nResponse 2:\nWe sincerely thank the reviewer for their positive evaluation and their recommendation for\npublication.\nReviewer #3 (Remarks to the Author):\nMy concerns in the original manuscript were nicely resolved by the authors in the revised\nversion. Nevertheless, before I can accept the manuscript for publication in Nature\nCommunications, the authors should incorporate the additional data from the reviewers'\nresponse into the revised version, at least in the supplementary information. For example, the\ndata in Figures R5 and R6. These will help readers better understand the quality of the sample\nand the scientific conclusion drawn from the study.\nResponse 3:\nWe thank the reviewer for their constructive suggestion. We agree that including these figures\nimproves the transparency and clarity of the study.\nAs requested, we have incorporated the data from Figures R5 and R6 (from the previous\nresponse letter) into the revised Supplementary Information. They can now be found in new\nFigure S9 and Figure S12, respectively. We believe these additions significantly strengthen\nthe manuscript."}, {"round": 3, "reviewer_comments": {"reviewer_1": "In Response 1 to Reviewer #1’s question, the Figure R1 data is considered to be very important. However, this data cannot be found in the revised manuscript or the revised supplementary materials. If Figure R1 and the related discussion are added to the revised manuscript or supplementary, I would recommend the publication of this paper.", "reviewer_3": "The authors addressed all of my concerns. I recommend publishing the current version of the manuscript in Nature Communications."}, "author_response": "Reply to Reviewers' Comments\nReviewer #1 (Remarks to the Author):\nIn Response 1 to Reviewer #1’s question, the Figure R1 data is considered to be very important.\nHowever, this data cannot be found in the revised manuscript or the revised supplementary\nmaterials. If Figure R1 and the related discussion are added to the revised manuscript or\nsupplementary, I would recommend the publication of this paper.\nResponse 1:\nWe thank the reviewer for the comment. We have now included the data and discussion from\nFigure R1 of the previous rebuttal into the Supplementary Information as Figure S3.\nReviewer #3 (Remarks to the Author):\nThe authors addressed all of my concerns. I recommend publishing the current version of the\nmanuscript in Nature Communications.\nResponse 2:\nWe sincerely thank the reviewer for their recommendation for publication."}, {"round": 4, "reviewer_comments": {"reviewer_1": "I have received sufficient answers to all the questions I asked. I recommend that this paper be published."}, "author_response": "Response 1:\nWe sincerely thank the reviewer for their recommendation for publication."}]

Decision letter