Humidity-driven modulation of ferroelectricity in hafnia–zirconia membranes

Abstract

Ferroelectric hafnia-based compounds, known for exhibiting strong ferroelectricity in films of sub-5 nm thickness, hold significant potential for being integrated into complementary metal-oxide-semiconductor devices. Due to the polymorphic nature of hafnia, their ferroelectric properties can be modulated through various mechanisms, including defects, strain, and electrochemical states. In this study, we fabricated ultrathin freestanding hafnia membranes, free from substrate and electrode-capping effects, to explore the relationship between their intrinsic ferroelectricity and surface electrochemical state by modulating humidity conditions during scanning probe microscopy measurements. Our results demonstrate enhanced ferroelectricity in hafnia under low-humidity conditions without requiring a wake-up process. This enhancement is attributed to reduced adsorption of water molecules on the membrane surface, which helps preserve oxygen vacancies that stabilize the ferroelectric phase in hafnia under an applied electric field. These findings suggest that beyond electrical control via field-cycling-induced phase transitions, electrochemical modulation through humidity provides an effective approach for tuning the ferroelectric properties of hafnia-based compounds, optimizing their performance in flexible nanoelectronics applications.

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New concepts

In this manuscript, we present a novel strategy for enhancing ferroelectricity in freestanding hafnia membranes by controlling ambient humidity, demonstrating that low-humidity conditions significantly improve ferroelectric properties without requiring a wake-up process. This enhancement is attributed to the reduced adsorption of water molecules, which helps preserve oxygen vacancies crucial for stabilizing the ferroelectric phase in hafnia. Unlike previous studies, which were influenced by substrate, electrode-capping effects, or minimal surface contributions due to large film thicknesses, we fabricated ultrathin freestanding hafnia membranes completely free from these constraints. By varying humidity conditions during scanning probe microscopy measurements, we investigated the intrinsic relationship between ferroelectricity and the surface electrochemical state. This work introduces electrochemical modulation via humidity as a novel alternative to conventional field-cycling methods for tuning hafnia-based ferroelectrics, paving the way for improved performance in flexible nanoelectronic devices.

Introduction

Ferroelectricity in fluorite-structure oxides, e.g. hafnia and zirconia, was first reported by Böscke et al.,^1,2 marking the initial reliable observation of polarization in a binary oxide. In contrast to traditional single-crystal ferroelectric ABO₃ perovskites, hafnium-based thin films can display robust ferroelectricity with polycrystallinity fabricated by atomic layer deposition or sputtering, being compatible with modern semiconductor manufacturing workflows. This capability paves the way for large-scale applications of ferroelectric non-volatile random-access memories (FeRAM),^3,4 energy harvesters,^5,6 electrostatic supercapacitors,^7,8 ferroelectric field-effect transistors (FeFETs)^9–12 and ferroelectric tunnel junctions (FTJ).^13–16 Additionally, this discovery has spurred interest in alternative emerging polar materials, such as Al_xSc_1−xN¹⁷ and Mg_xZn_1−xO.^18–21

Hafnia-based thin films exhibit more complex ferroelectric behavior than traditional ferroelectrics due to their polymorphic nature, which involves the coexistence of stable and metastable phases.^22–24 The switchable polarisation in hafnia is primarily attributed to the non-centrosymmetric orthorhombic phase (o-phase).²³ However, previous studies have shown that alternative mechanisms, such as uneven charge defect distribution, chemical dipole formation, strain effects, and doping, also contribute to its ferroelectricity,^25–28 and lead to some critical issues, such as the wake-up effect.²⁹ Furthermore, a key advantage of hafnia-based systems over conventional perovskite-structure ferroelectrics is their superior scalability. In hafnia-based thin films, the ferroelectric o-phase becomes more stable as the crystal size decreases or when interlayers are introduced to distort the lattice.³⁰ In contrast, polarization in classic ferroelectric films typically diminishes below a critical thickness in very thin films.³¹ This highlights the crucial role of surface and interface energy in stabilizing the ferroelectric phase in hafnia-based thin films. Kelley et al. recently demonstrated that the surface electrochemical state can influence the ferroelectric phase transition and stability of Hf_0.5Zr_0.5O₂ through variations in gas environmental (pressure) and temperature.³² Therefore, developing different strategies to modulate the electrochemical boundary conditions is essential for understanding and controlling the ferroelectricity of hafnia-based systems.

Extensive research has been conducted on interacting ferroelectric material surfaces with water molecules through mechanisms such as physical adsorption, dissociation, or chemisorption.^33,34 Among these, TiO₂-terminated perovskite oxides, including BaTiO₃^35,36 and Pb(Zr_0.2Ti_0.8)O₃,³⁷ have been the primary focus, largely due to TiO₂-mediated surface catalysis and the ferroelectric polarization screening effect caused by ionic species dissociated from water molecules.³⁴ More recently, studies have also expanded to hafnia-based thin films, demonstrating that the ferroelectric imprint effect is influenced by the incorporation of moisture.^38,39 Wei et al. highlight the disadvantage of water adsorption, which negatively impacts domain retention and polarization stability.³⁹ However, the strain from the substrate,^40,41 bottom and top electrodes have a significant impact on the ferroelectric properties of HZO.^42,43 Therefore, investigating the ferroelectric properties of hafnia membranes without substrate and electrode-capping effects, particularly with respect to surface electrochemical states influenced by humidity, is essential.

In this study, we investigate the impact of humidity on the ferroelectricity of ultrathin freestanding Hf_0.5Zr_0.5O₂ (HZO) membranes. The HZO thin film was fabricated using pulsed laser deposition (PLD) followed by membrane exfoliation and transfer to the conductive substrate. This approach ensures the formation of the orthorhombic ferroelectric phase during the film growth process on SrTiO₃ (STO) substrates while preserving its ferroelectric property after exfoliation, free from the substrate-induced strain. The macroscopic ferroelectricity of HZO was confirmed in both epitaxial films and freestanding membranes through capacitance–voltage (C–V), polarization–electric field hysteresis (P–E), and current vs. electric field (I–E) measurements. Plan-view atomic-resolution observation via scanning transmission electron microscopy (STEM) reveals the coexistence of ferroelectric (orthorhombic) and non-ferroelectric (monoclinic) phases within the HZO membranes. Microscale polarization electrical switching behaviour and the wake-up effect are characterized using high-resolution piezoresponse force microscopy (PFM) mapping and spectroscopic switching PFM (SSPFM) measurements. Additionally, humidity-dependent ferroelectricity is demonstrated using SSPFM with an integrated humidity control system in the range of 3% to 30% relative humidity (RH). A low-humidity atmosphere enhances the ferroelectricity of HZO nearly 30-fold, resulting in a “wake-up-free” state. This effect is attributed to modifications in the surface electrochemical state, as reduced water absorption helps maintain a high concentration of oxygen vacancies, which stabilize the ferroelectric o-phase under an applied electric field.

Results and discussion

Fig. 1(a) illustrates the preparation of a HZO freestanding membrane. Initially, the HZO thin film was epitaxially grown on a (100) SrTiO₃ substrate with a La_0.7Sr_0.3MnO₃ (LSMO) bottom electrode using PLD. Then, the sacrificing LSMO layers of the sample were selectively etched in a hydrochloric acid-based solution to exfoliate the top layer of the HZO thin film, which was subsequently transferred onto a Pt/Au-coated silicon substrate, as shown in the left panel of Fig. 1(b). Following this, top electrodes of varying diameters, ranging from 10 μm to 30 μm, were deposited for subsequent macroscopic electrical measurements, depicted as a matrix of circular electrodes in the right panel of Fig. 1(b). Both the epitaxial thin film and the freestanding membrane of HZO were characterized by X-ray diffraction measurements to confirm the crystal structure. As shown in Fig. 1(c), the main diffraction peak for the as-grown 5-nm-thick HZO appears around 29.8°, which is slightly smaller than the reported value for the orthorhombic phase (111) reflection.⁴⁴ This shift suggests the presence of in-plane compressive strain in the HZO thin film. However, after releasing the HZO film from the SrTiO₃ substrate, the o-(111) peak of the membrane shifts back to 30.1° in the absence of strain, as shown in Fig. 1(c) (yellow curve). This result demonstrates that the polar o-(111) phase formed during the epitaxial growth of HZO films on SrTiO₃ (STO) substrates is preserved even after the HZO membrane is exfoliated.

Fig. 1 Preparation of freestanding HZO films and their ferroelectricity. (a) Schematic of transferring HZO membrane followed by deposition of Au top electrodes via photolithography. (b) Optical microscopy of HZO freestanding membranes on substrate (left) and its Au top electrodes (right). (c) X-ray 2θ/θ diffraction patterns of the HZO epitaxial thin film on LSMO/STO substrate and its freestanding membranes on Pt/Au-coated Si substrate. (d) Macroscopic C–V characterization of ferroelectric capacitance peaks with calculated relative permittivity for HZO epitaxial film (top panel) and its membranes (bottom panel). (e) P–E hysteresis loops (red) and I–E curves (blue) of both epitaxial and transferred film.

To demonstrate the macroscopic ferroelectricity of HZO and its potential application in capacitors, we performed C–V measurements at room temperature for both the thin film and membrane with a measurement AC frequency of 10 kHz. Fig. 1(d) presents the macroscopic C–V characterization of ferroelectric capacitance peaks, showing relative permittivity (ε_r) curves for as-grown and freestanding HZO. The butterfly-shaped ε_r curves, observed in the film before and after exfoliation, are characteristic of ferroelectric capacitors and can be attributed to the increased movement of domain walls near the coercive voltage (E_c).⁴⁵ The higher E_c observed in the transferred HZO membrane is attributed to the release of in-plane strain, while the presence of out-of-plane tensile strain facilitates the switching of ferroelectric polarization.⁴⁶ Additionally, we measured the room-temperature P–E loop for both the transferred and as-grown thin films at 1 kHz. A distinct hysteresis loop was observed for both samples, further validating the survival of ferroelectricity in freestanding HZO membranes after the exfoliation. The I–E curves, which demonstrate the ferroelectric switching current, are shown in Fig. 1(e). The presence of two switching current peaks under positive and negative electric fields reveals the coercive voltage of the HZO films, with the transferred film exhibiting a higher value. This observation is consistent with the coercive voltage derived from C–V curves, taking into account the membrane thickness. Please note that the higher electric field required to switch the polarization in the HZO membrane can be explained by strain-influenced switching behavior in fluorite-based materials.⁴⁶ The epitaxial HZO subjected to in-plane compressive strain by the STO substrate, which induces expansion strain along the out-of-plane direction and facilitate ferroelectric switching in the out-of-plane direction.⁴⁷ Additionally, all the electrical measurements didn’t involve any “wake-up” procedure.

Further electrical measurements were conducted on the HZO membrane to assess its ferroelectric performance for practical applications. A remanent polarization (P_r) of 10 μC cm⁻² was obtained for the freestanding membranes using the positive-up-negative-down (PUND) method (Fig. S1a, ESI†). Furthermore, leakage current, polarization switching endurance, and switching time were investigated (Fig. S1b–d, ESI†), showing values comparable to those reported for ferroelectric hafnium oxide devices (ESI,† S1).

In conclusion, the present method first ensures the development of a well-oriented (111) crystalline structure of HZO thin films with an orthorhombic ferroelectric phase through the epitaxial growth of HZO on STO substrates, benefiting from favourable clamping effects during the film growth process.^48,49 Subsequently, the film is released onto different substrates to form a freestanding membrane, effectively eliminating substrate-induced strain while preserving its ferroelectric properties. It is important to note that direct deposition of HZO on conductive substrates without exfoliation may induce undesirable strain due to mismatches in lattice parameters and thermal expansion coefficients between HZO and the substrate.^50–55

For the identification of the crystal structure of HZO, we conduct plan-view high-angle annular dark-field (HAADF) STEM characterization, which visualizes cation arrangements directly, by transferring the freestanding HZO membrane onto a holey carbon film supported by a Cu grid. Fig. 2(a) shows the HAADF STEM image, visualizing HZO with three different types of crystallographic phase and grain sizes ranging from 5 to 20 nm. Each type of phase can be identified as (111)-oriented orthorhombic (o) phase in Fig. 2(b), (001)-oriented orthorhombic phase in Fig. 2(c) and (−111)-oriented monoclinic (m) phase in Fig. 2(d) with their fast Fourier transform (FFT) patterns and structural models on the side (hafnium/zirconium atom in blue and oxygen atom in red). The presence of m-phase and o-phase provided the possibility of phase transformation, which could be the origin of “wake up” and “fatigue” behaviour.⁵⁶ To conclude, we identified that the dominant phase of HZO film is the ferroelectric orthorhombic phase and its polymorphic nature.

Fig. 2 Identification of crystallographic phases of HZO membrane. (a) Plan-view HAADF-STEM image. Orthorhombic phases oriented along the (b) (111), (c) (001) direction, and (d) monoclinic phases oriented along the (−111) direction with corresponding FFT patterns and crystal structures on the right.

The freestanding HZO membrane, with a thickness below 5 nm and free from substrate-induced strain, allows its ferroelectric properties to be closely coupled with the surface electrochemical state. In this study, we microscopically examined the ferroelectricity of HZO freestanding membranes and explored the influence of environmental conditions. To achieve this, we performed a series of PFM measurements under both ambient (∼50% RH) and ultra-low humidity conditions (3% RH), as shown in Fig. 3. A well-defined domain wall and 180° phase contrast were observed after box-in-box poling with a −8 V (0.8 μm × 0.8 μm) and +8 V (1.5 μm × 1.5 μm) dc bias, demonstrating the polarization's electrical switchability under 3% RH (Fig. 3(a) and (b)). Notably, the contrast between the unpoled and −8 V poled areas is similar, indicating a pristine upward-oriented polarization of the HZO. The atomic force microscopy has also been conducted in a 2 μm area, showing a smooth surface with an RMS roughness of 400 pm (Fig. S2a, ESI†). Additionally, local bias-dependent spectroscopic switching PFM (SSPFM) measurements further confirm that the ferroelectric properties of the HZO film remain well-preserved after exfoliation under low humidity conditions. This is evidenced by the robust butterfly-shaped amplitude loop and 180° phase hysteresis loop shown in Fig. 3(c) and Fig. S3 (ESI†) for 50 cycles, without the need for a “wake-up” process, consistent with the results from macroscopic measurements. The surface conditions in this case are analogous to those in macroscopic measurements, where the surface is covered by an electrode and free from water molecules in the surrounding air. In contrast, under ambient conditions (RH 60%), the same area cannot be poled, indicating weak polarization switching behaviour (Fig. S2b and c, ESI†). Additionally, a wake-up process was observed through SSPFM measurements, where the development of ferroelectricity was found to depend on the number of cycles. This result is consistent with previous studies showing that high-humidity environments commonly induce a wake-up effect.³⁸ As shown in Fig. 3(e), the remanent d^eff₃₃ value from the hysteresis loops increased approximately 20-fold, reaching a peak around cycle no. 35, followed by a subsequent decline. This behaviour is consistent with previous findings, where cycle no. 1–35 corresponds to the wake-up effect driven by a field-cycling-induced phase transition from the non-ferroelectric monoclinic phase to the ferroelectric orthorhombic phase.^3,57,58 Cycles no. 35–50, however, mark the onset of fatigue, attributed to the increase in defect density.⁵⁸ It is important to note that the accelerated wake-up and fatigue effects can be attributed to the microstructural field cycling applied through the tip to the freestanding membrane, which lacks the clamping strain from the substrate that would otherwise constrain the phase transformation.

Fig. 3 Electric polarization switching in freestanding HZO membranes and wakeup phenomenon. Out-of-plane PFM (a) amplitude and (b) phase images after ±8 V box-in-box poling. (c) PFM butterfly loop for amplitude (blue) and hysteresis loop (red) for phase. (d) Remanent d^eff₃₃ as a function of the cycles number. (e) PFM hysteresis loops for cycles no. 1, 10, 20, 30, 40 and 50.

Next, to gain deeper insight into the impact of humidity on the ferroelectric properties of HZO, we varied the atmospheric humidity during measurements. Fig. 4(a) demonstrates that as the humidity level increases from 3% to 30%, the amplitude of the hysteresis loop gradually decreases. The remanent d^eff₃₃ values, extracted from the PFM hysteresis loops at each humidity level, are plotted in Fig. 4(b). Initially, the piezoresponse decreases linearly with increasing humidity, from ∼9 pm V⁻¹ at 3% RH to ∼2 pm V⁻¹ at 8% RH, before gradually declining further to 0.3 pm V⁻¹ at 30% RH. This indicates that an ultra-low humidity environment enhances the ferroelectricity by nearly 30-fold. To exclude the influence of extrinsic effects such as ionic motion, electrostatic forces, or Vegard strains in the humidity-driven changes, we conducted the first and second harmonic frequency response measurements of HZO using a small AC across a broad frequency range. The results in Fig. 4(c) indicate that ferroelectricity remains the dominant intrinsic effect under both 3% and 30% RH conditions. At ultra-low humidity levels (3% RH), the amplitude of the first harmonic response shows a marked increase, aligning with the higher d^eff₃₃ values presented in Fig. 4(a). This behaviour can be attributed to the effect of humidity on the electrochemical state of the HZO surface. Kelley et al. found that in a pristine upward-polarization HZO thin film (similar to ours), a low-oxygen-pressure environment—akin to low humidity conditions—reduces compensatory mechanisms by limiting the adsorption of surface adsorbates or ionic species from the atmosphere. This reduction in surface oxygen content increases the negative surface charge, thereby enhancing the ferroelectric stability of HZO.³² Moreover, previous studies³⁸ indicate that polarization is more sensitive to humidity levels than to oxygen conditions, as the incorporation of hydroxide ions from humid atmospheres is more efficient than that of oxygen. This suggests that low-humidity environments may offer comparable or even more superior ferroelectric stability for HZO relative to Kelley's findings regarding oxygen.

Fig. 4 Humidity-dependent ferroelectric behaviour. (a) PFM hysteresis loops at 3% to 30% RH atmosphere and their (b) remanent d^eff₃₃ as a function of humidity. (c) The first and second harmonic response for HZO membranes at 3% and 30% RH atmosphere.

In addition, oxygen vacancies, as a predominant intrinsic defect in HfO₂-based thin films, are unavoidable and commonly present at relatively high concentrations, around 1.7 × 10²¹ cm⁻³.^28,59 Surface oxygen vacancies are known to promote water dissociation with water acting as an oxidizing agent that fills and deactivates these vacancies.^60,61 Specifically, an OH⁻ occupies one oxygen vacancy (), while a proton bonds to a neighbouring oxygen site (O_O), resulting in a pair of positively charged OH˙ ions () within the lattice, as illustrated in the following equation:^38,62,63

Thus, low-humidity conditions maintain a higher concentration of active oxygen vacancies. These vacancies have been shown to stabilize or promote the polar orthorhombic phase transition under an electric field while suppressing the formation of the monoclinic (m) phase, thereby enhancing the ferroelectric properties of HfO₂-based materials.^64–68

In a low-humidity environment (RH 2.5–10%), hydroxyl (OH) groups strongly chemisorb onto the surface of metal oxides, forming a discontinuous sub-monolayer of water.^33,34,69 Given the limited availability of water molecules within this low RH range, water absorption by the material positively correlates with humidity levels, resulting in the gradual occupation of oxygen vacancies as humidity increases, which ultimately reduces the ferroelectricity of HZO.^70,71 However, when humidity exceeds 10% RH, additional water layers have limited influence on the sample surface once the first monolayer is complete. This phenomenon may give rise to the two-phase behaviour of HZO d^eff₃₃ as a function of RH, with distinct regimes observed between 3–8% RH and 8–30% RH, as shown in Fig. 4(b).

Conclusion

In summary, we have identified a humidity-dependent ferroelectric behaviour in 5 nm ultrathin freestanding HZO membranes, free from substrate and electrode-capping effects. These membranes were fabricated using pulsed laser deposition (PLD) on an STO substrate to form the polar o-(111) phase, then exfoliated and transferred onto a Si substrate while retaining their ferroelectric properties. Macroscopic ferroelectricity in the membranes was confirmed through electrical measurements (P–E, I–E, and C–V), highlighting the potential of HZO for capacitor applications. Crystallographic analysis using plan-view atomic-resolution STEM observation revealed the coexistence of orthorhombic and monoclinic phases, which supports the wake-up behaviour arising from phase transitions. Further investigation of nanoscale polarization switching through PFM techniques and spectroscopic piezoresponse measurements on a free surface demonstrated a negative correlation between ferroelectricity and relative humidity (RH). This behaviour is attributed to the effect of humidity on the electrochemical conditions of the HZO surface. By reducing surface water adsorption, a higher concentration of oxygen vacancies can be maintained, promoting the stability of the ferroelectric phase under an electric field.

Overall, this study presents a novel approach to tuning surface electrochemical conditions while mitigating the wake-up effect in HZO. We demonstrate the coexistence of two key mechanisms—field-cycling-induced phase transitions and humidity-modulated surface electrochemical state that can be leveraged to optimize the ferroelectric properties of HZO membranes. These findings offer valuable insights for the development and optimization of other binary oxide-based devices.

Methods

Sample preparation

The freestanding hafnia membranes were prepared by selective etching of the LSMO buffer layer in STO/LSMO/HZO epitaxial heterostructures, which were prepared by pulsed laser deposition (PLD). The detailed deposition conditions were reported previously.⁷² The etchant contains 5 wt% hydrochloric acid and 0.05 M potassium iodide, which is known to effectively etch the Mn-based perovskite layer.⁷³ The Si substrate was coated with sputter-deposited Pt/Au layers for better conductivity prior to the conventional dry transfer method. The whole transfer process of freestanding membranes to conductive substrates is detailed in our previous work.⁷⁴

Structural characterization

XRD 2θ/θ diffraction patterns were measured with a four-circle diffractometer (X’Pert MRD, PANalytical) using Cu Kα1 radiation. The HAADF STEM image was obtained using a JEOL JEM-ARM300F2 equipped with aberration correctors. The accelerating voltage and probe forming half-angle were set to be 300 keV and 24 mrad, respectively.

Electrical measurement

The electrical measurements were performed on metal–insulator–metal (MIM) structures by depositing gold pads (∅ 10–30 μm) on top of HZO through photolithography and thermal evaporation. The LSMO and p-type silicon layers served as bottom electrodes for epitaxial and freestanding HZO samples, respectively. Polarization hysteresis loops and PUND (positive-up-negative-down) measurements were conducted by using ferroelectric testers (TOYO Tech, FCE-10) with a measurement frequency of 1 kHz at room temperature. The capacitance butterfly curves were measured by the LCR meter and later converted to the dielectric constant.

Scanning probe microscopy

Scanning probe microscopy measurements were conducted using an AIST-NT Smart SPM 1000 under ambient conditions with a built-in humidity controller. Conductive platinum-coated tips (Mikromasch HQ: NSC35/Pt) with a force constant of ∼5.4 N m⁻¹ were employed for the measurements. During PFM measurements, a modulating voltage of 1 V (peak-to-peak) at a frequency of 750 kHz was applied. Spectroscopic PFM hysteresis loops were recorded by introducing a triangular DC bias waveform along with small-signal AC modulation. For all SSPFM measurements, the tip has been calibrated using the thermal noise method.⁷⁵ Throughout all SPM experiments, the bias was applied to the conductive nanoscale tip, with the sample grounded on a conductive metal plate to complete the electrical circuit.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

We acknowledge the financial support provided by the UNSW-MAHE collaboration seed grant (RG194284-D). We further acknowledge support from ARC discovery grants and the ARC Centre of Excellence in Future Low-Energy Technologies (FLEET). This work was partly supported by Grants-in-Aid for Scientific Research (no. 23KJ1239, 23H05457, and 24H01190) and by grants for the Integrated Research Consortium on Chemical Sciences and the International Collaborative Research Program of the Institute for Chemical Research in Kyoto University from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. The work was also supported by the Japan Science and Technology Agency (JST) as part of PRESTO, Grant No. JPMJPR24H3, and the Adopting Sustainable Partnerships for Innovative Research Ecosystem (ASPIRE), grant no. JPMJAP2312 and JPMJAP2314. STEM images were acquired using a transmission electron microscope installed through the Innovative Science and Technology Initiative for Security, Grant Number JPJ004596, from the Acquisition, Technology, and Logistics Agency (ATLA), Japan. The work was also supported by The Samco Foundation.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5mh00397k

‡ These authors contributed equally.

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