Controllable Synthesis of Nonlayered Ultrathin Mn3O4 with High Dielectric Constant

doi:10.21203/rs.3.rs-4833727/v1

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Controllable Synthesis of Nonlayered Ultrathin Mn₃O₄ with High Dielectric Constant

https://doi.org/10.21203/rs.3.rs-4833727/v1

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Two-dimensional (2D) materials have been identified as promising candidates for future electronic devices. However, high dielectric constant (κ) materials, which can be integrated with 2D semiconductors, are still very rare. This study employed a hydrate-assisted thinning chemical vapor deposition (CVD) technique combined with a strategy of minimizing substrate lattice mismatch, achieving precise control over the growth kinetics, enabling the controllable array growth of ultrathin manganese oxide (Mn₃O₄), demonstrating an extraordinary dielectric constant of 237, an equivalent oxide thickness (EOT) as low as 0.57 nm, and a breakdown field strength (E_bd) exceeding 10 MV/cm, marking it as a superior gate dielectric material. MoS₂ field-effect transistors (FETs) integrated with ultrathin Mn₃O₄ through mechanical stacking method operate under ultra-low voltages (<1 V), achieving a near 10⁸ I_on/I_off ratio and a subthreshold swing (SS) as low as 84 mV/dec. The MoS₂ FET exhibit nearly zero hysteresis (~5 mV) and an extremely low drain-induced barrier lowering (~20 mV/V). This work further expands the family of 2D high-κ dielectric materials and provides a feasible exploration for the epitaxial growth of single-crystal thin films of non-layered materials.

Physical sciences/Nanoscience and technology/Nanoscale materials/Two-dimensional materials

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

Two-dimensional (2D) materials, with their atomic-level thin layers and unique electronic properties, have shown tremendous potential in surpassing the performance limitations of traditional silicon materials.^1-3 However, significant challenges remain in selecting and integrating high-κ materials to optimize the performance of 2D material-based field-effect transistors (FETs) and to further scale down device sizes.⁴ Traditional high-κ insulating materials such as Al₂O₃, and HfO₂, despite being widely used in modern semiconductor processes, often introduce charge scattering and trap states due to their amorphous structures, negatively impacting the electronic transport characteristics of 2D FETs.^5-7 Therefore, the research community has started exploring high-κ single crystal materials with atomically smooth surfaces as gate dielectrics, aiming to improve the interface quality and enhance device performance.⁸ Materials such as LaOCl (ε_r ≈ 10.8),⁹ SrTiO₃ (ε_max ≈ 105),¹⁰ Bi₂SiO₅ (ε_r ≈ 30),¹¹ Bi₂GeO₅ (ε_r ≈ 40)¹² and CaF₂ (ε_r ≈ 8.4)¹³ show significant potential by forming smoother dielectric/semiconductor interfaces compared to traditional amorphous dielectrics.^9-11 Transferring mechanically exfoliated ultrathin high-κ dielectrics, such as h-BN (ε_r ≈ 3.8),¹⁴ 2D perovskite oxide Sr₂Nb₃O₁₀ (ε_r ≈ 24.6),¹⁵ Bi₂SeO₅ (ε_r ≈ 15.6)¹⁶ and onto 2D materials has demonstrated its potential for damage-free integration to optimize the performance of 2D devices.^10,17,18 High-κ materials, due to their high dielectric constants, enable the use of thicker dielectric layers while preserving high capacitance. This effectively addresses the leakage current and reliability challenges posed by thinner dielectric layers

Recently, chemical vapor deposition (CVD) has emerged as a promising technology capable of directly growing high-quality 2D nanosheets on various substrates.^19-21 However, the challenge of further extending the growth of high-quality ultrathin dielectric single-crystal films remains.¹¹ This underscores the urgent need for high-κ, high-quality single-crystal films that can be controllably synthesized via CVD, which is crucial for the continuation of Moore's Law. Bulk Mn₃O₄, with its high static permittivity (ε_bulk ≈ 1703 at room temperature), presents itself as a potential gate dielectric for electrostatic modulation of silicon, graphene, or 2D electron gases at complex oxide heterointerfaces.²² Despite its potential, the controlled synthesis of ultrathin Mn₃O₄ films remains a significant challenge. Overcoming this obstacle is essential to leverage the advantages of high-κ materials in next-generation electronic devices.

In this work, by employing strategies of hydrate-assisted thinning and reducing substrate lattice mismatch, the single-oriented growth of non-layered ultrathin Mn₃O₄ on mica was successfully customized. The ultrathin Mn₃O₄ single-crystal film is the 2D gate insulating material with the highest dielectric constant (ε_r ≈ 237) discovered to date. It can achieve an EOT of 0.57 nm. Compared to the more challenging thinning strategies required for existing high-κ (ε_r < 100) 2D gate dielectrics, the selection of high-κ Mn₃O₄ offers a new solution for integrating and optimizing FET performance and further miniaturizing device dimensions. Electrical performance tests further demonstrated the excellent properties of ultra-high κ Mn₃O₄ as a gate dielectric in MoS₂ FETs. These performance advantages stem from the atomically smooth surface of Mn₃O₄ single crystals, ensuring high compatibility with MoS₂ and excellent vdW interface formation.

2.1 Synthesis of high-quality ultrathin Mn₃O₄ arrays

Mn₃O₄ is a magnetic oxide known for its non-layered tetragonal spinel structure.²³ At room temperature, it adopts the stable tetragonal hausmannite structure with the I41/amd (141) space group. In this structure, Mn³⁺ and Mn²⁺ ions occupy octahedral and tetrahedral sites, respectively. As illustrated in Fig. 1a, ultrathin Mn₃O₄ exhibits a standard non-layered tetragonal crystal system structure (a = 5.762 Å, b = 5.762 Å, c = 9.439 Å, α = β = γ = 90°) and presents a top view of the atomic hexagonal arrangement along the [111] zone axis.

Supplementary Fig. 1 depicts the experimental setup used to synthesize ultrathin Mn₃O₄ array single crystals on a mica (KMg₃(AlSi₃O₁₀) F₂) substrate through CVD technique. In brief, NaCl and MnCl₂·4H₂O were utilized as precursors, and the growth process took place in an Ar gas atmosphere to facilitate the formation of the Mn₃O₄ array structure. Figure 1b displays the optical micrograph (OM) image of the resulting Mn₃O₄ arrays, showcasing uniform geometric morphologies. The consistent size and unidirectional arrangement of Mn₃O₄ domains highlight the precise control achieved in the synthesis process. To confirm the uniformity and phase purity of the ultrathin Mn₃O₄ arrays across different thicknesses, the Raman spectra presented in Fig. 1c validate the structural integrity of ultrathin Mn₃O₄. By using the characteristic peak of mica at 263 cm^-1 for calibration, three significant Raman peaks corresponding to the hexagonal phase of Mn₃O₄ are observed, indicating the characteristic lattice vibration modes, which align with previous findings.^24,25 Specifically, the A_1g mode peak at 658 cm^-1 signifies the Mn-O breathing vibrations of Mn²⁺ in tetrahedral coordination, while the two weak peaks at 317 cm^-1 and 371 cm^-1 are attributed to the T_2g mode of oxygen atom vibrations.

The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image revealed the precise hexagonal geometry of the ultrathin Mn₃O₄ nanosheets, which exhibited exceptional crystal quality with uniformity at the atomic level and minimal discontinuities or defects (Fig. 1d). The lattice spacing measurement of 0.312 nm confirmed the presence of the (112) crystal plane (Fig. 1e). The selected area electron diffraction (SAED) pattern demonstrated perfect in-plane sixfold symmetry, confirming the high-quality single-crystal structure of the hexagonal phase Mn₃O₄ (Fig. 1f). As shown in Supplementary Fig. 2, the EDS analysis exhibited a uniform distribution of Mn and O elements, highlighting the consistency of the chemical composition. Furthermore, EELS analysis provided detailed information on the electronic structure of the nanosheets, particularly the Mn L-edge and O K-edge spectra, which revealed distinct "white lines" corresponding to transitions of Mn ions and pre-edge structures consistent with previous data (Supplementary Fig. 3).^26–28 The Mn XPS analysis confirmed the presence of Mn 2p_1/2 and Mn 2p_3/2 spin-orbit states at 652.9 and 641.3 eV, respectively, corresponding to Mn²⁺ and Mn³⁺ valence states (Fig. 1g).^24,25,29 The full spectrum analysis further supported the presence of these valence states (Supplementary Fig. 4).

Ultrathin Mn₃O₄ nanosheets show ferromagnetic behavior at low temperatures, transitioning from paramagnetic to ferrimagnetic states near 48 K, influenced by size and surface effects (Supplementary Fig. 5).³⁰ These properties suggest potential applications in electronic and spintronic devices, advancing data storage, sensing, and spin manipulation technologies.

2.2 High-quality ultrathin Mn₃O₄ arrays

The oriented growth of 2D nanosheet arrays is of significant importance for the synthesis of single-crystal thin films.^8,31,32 On the single crystal substrate such as sapphire, wafer-scale growth of 2D transition metal dichalcogenides (such as MoS₂ and WS₂), has been achieved. This method relies on designing specific c/a ratios and sapphire planes to confine monolayer nucleation at the substrate step edges, thereby achieving unidirectional orientation and single-crystal growth.^33,34 Nonetheless, maintaining the orientation and height of sapphire step edges with high precision across a wafer scale remains a challenge.³³

We have developed a hydrate-assisted thinning CVD method. By pre-calculating the lattice mismatch between Mn₃O₄ and various substrates, including mica and sapphire, we found that

Mn₃O₄ has the smallest lattice mismatch with mica (1.9%). This enabled the precise prediction of the synthesis of controllable ultrathin Mn₃O₄ nanosheet arrays on mica without any pretreatment of the mica substrate. The change in free energy for vertical growth (ΔE_ver) primarily arises from the binding forces at the upper interface subunits, while the loss in edge energy dominates lateral growth (ΔE_lat). Therefore, their difference (ΔE) can serve as a criterion for evaluating growth modes (Supplementary Fig. 6).³⁵ In fact, besides the material itself, the surrounding microscale atomic environment may also influence these energies during real growth processes. When ΔE is negative, it favors lateral growth of the material. Additionally, interface adsorption or passivation can suppress vertical growth of the material. Choosing hydrates as precursors, water molecules adsorb on the material surface, lowering ΔE to promote the material's lateral growth. The transition of ultrathin Mn₃O₄ nanosheets from isolated islands to continuous films on a mica substrate has been demonstrated through a CVD growth strategy, revealing the microscopic mechanisms behind macroscopic orientation control of 2D materials (Fig. 2a-c, Supplementary Fig. 7). Notably, nearly 100% uniformly oriented ultrathin Mn₃O₄ triangular nanosheets were observed on the mica substrate, a stark contrast to the antiparallel orientations of previously reported 2D TMDCs grown on the same substrate.³⁶ This unique unidirectional alignment not only demonstrates the orientation selectivity of ultrathin Mn₃O₄ nanosheets during the crystal growth process but also highlights the strong interaction with the mica substrate. Further statistical analysis, as illustrated in Fig. 2d, validates the consistency of nanosheet orientation, attributed to the growth of ultrathin Mn₃O₄ nanosheets being influenced by a dual-coupling guided mechanism.³⁸ Initially, the interaction between Mn₃O₄ and the substrate induces epitaxial growth. Subsequently, the interaction between nanosheets determines the preferential growth in a single direction. As shown in Fig. 2e and Supplementary Fig. 8, the lattice constant of mica (a₁ ≈ 5.3 Å) equals $\:\sqrt{3}\:$a₂ of Mn₃O₄ (a₂ ≈ 3.12 Å), with a mismatch rate of 1.9%. The lattice constant of sapphire (a₃ ≈ 4.81 Å) equals $\:1.5$a₂ of Mn₃O₄ (a₂ ≈ 3.12 Å), with a mismatch rate of 3.3%. This perfect lattice matching achieves strong interaction between mica and Mn₃O₄, inducing the epitaxial growth of ultrathin Mn₃O₄.

The surface roughness of ultrathin Mn₃O₄ nanosheets of different thicknesses grown on the same mica piece has been measured, and their flatness is confirmed by the root mean square roughness (R_q) of 0.13 nm and the height profile shown in Supplementary Fig. 9 Mn₃O₄ synthesized via CVD exhibits outstanding air stability. Even after being exposed to air for over a year, its surface morphology remains nearly unchanged, which is critical for the fabrication of devices using high-κ gate dielectrics (Supplementary Fig. 10). The reduction of surface defects in dielectric materials significantly improves the performance of 2D semiconductor electronic devices. By precisely controlling the growth conditions of CVD, ultrathin Mn₃O₄ single crystals of various thicknesses can be obtained, as shown in Fig. 2f. At a relatively suitable growth temperature (923 K), ultrathin Mn₃O₄ nanosheets demonstrating controllable atomic thinness down to 2.8 nm are presented. Figure 2g shows a schematic illustration of the fusion of two isolated single crystals. As shown in Supplementary Fig. 11 presents AFM images of ultrathin Mn₃O₄ nanosheet arrays grown at different times, and the height profile further demonstrates the thickness consistency among different islands on the same mica (all around 11.2 nm), providing a feasible solution for synthesizing reliable and uniform non-layered ultrathin single-crystal films.

2.3 Dielectric properties of ultrathin Mn₃O₄ single crystals

To further explore the potential of ultrathin Mn₃O₄ as a dielectric material in sophisticated electronic devices, a graphene dual-gate FET is utilized to measure the dielectric properties. This evaluation method has been validated in previous studies, effectively measuring the dielectric capabilities of materials.^15,43 In Fig. 3a, a dual-gate graphene FET is fabricated by integrating few-layer graphene with ultrathin Mn₃O₄ nanosheets (~ 50 nm), employing advanced transfer techniques. In this article, ultrathin Mn₃O₄ and 285 nm SiO₂ function as the dielectric layers for the top and back gates, respectively, with the device channel spanning 4.5 µm in width. This configuration facilitates independent modulation of channel carriers via separate adjustments of top and back gate voltages. The R-V_tg characteristics exhibits the graphene channel's bipolar conductivity under back gate voltage modulation in Fig. 3b. The alteration in the Dirac point voltage signifies shifts in the charge neutrality point, illustrating the capability of back gate voltage adjustments to effectively modulate the channel charge.⁴⁴ The observation of a minor shift in the top gate voltage corresponding to the graphene Dirac point is attributed to the competing capacitance effects arising from simultaneous voltage applications to both the ultrathin Mn₃O₄ top gate and SiO₂ back gate dielectrics. The back-gate V_Dirac shows a linear dependence on the V_tg (Fig. 3c). This relationship hinges on the capacitance ratio of the top gate to the back gate (C_TG/C_BG), encapsulated in the following formula:

$$\:-\frac{{\varDelta\:\text{V}}_{\text{T}\text{G}}}{{\varDelta\:\text{V}}_{\text{D}\text{i}\text{r}\text{a}\text{c},\text{B}\text{G}}}=\frac{{\text{C}}_{\text{B}\text{G}}}{{\text{C}}_{\text{T}\text{G}}}=\frac{{{\epsilon\:}}_{{\text{S}\text{i}\text{O}}_{2}}{\text{t}}_{{\text{M}\text{n}}_{3}{\text{O}}_{4}}}{{{\epsilon\:}}_{{\text{M}\text{n}}_{3}{\text{O}}_{4}}{\text{t}}_{{\text{S}\text{i}\text{O}}_{2}}}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:$$

where C, ε, and t stand for capacitance, effective dielectric constant, and thickness of Mn₃O₄, respectively. Given the characteristics of the back gate dielectric SiO₂—C_BG at 11.6 nF cm^− 2, $\:{\epsilon\:}_{{\text{S}\text{i}\text{O}}_{2}}$ at 3.9, and thickness at 285 nm—the dielectric constant for 50 nm Mn₃O₄ calculates to 171. For a refined measurement of Mn₃O₄'s effective dielectric constant, systematic analyses were performed on ultrathin Mn₃O₄ nanosheets with thicknesses ranging from 20–66 nm, as detailed in Supplementary Fig. 12. This comprehensive analysis confirmed a correlation between the dielectric constant and the thickness of ultrathin Mn₃O₄.

As shown in Fig. 3d, a peak dielectric constant of 237 was measured for 39 nm Mn₃O₄. A decline in the dielectric constant with reduced Mn₃O₄ nanosheet thickness aligns with trends

observed in other 2D dielectrics. Such behavior is convincingly explained by the interface effects in nanoscale dielectric layers, particularly where the interface capacitance (C_i) at the electrode/dielectric interfaces manifests a dielectric constant lower than the material’s intrinsic

bulk value, a phenomenon known as the "dead layer" effect.^45,46 These "dead layers" act as additional series capacitors, reducing the overall dielectric constant relative to bulk material values. In alignment with the International Roadmap for Devices and Systems (IRDS),⁴⁷ which stipulates that the most cutting-edge MOSFETs (5 nm node FinFET) necessitate an EOT below 1 nm, the findings illustrated in the inset of Fig. 3d reveal that Mn₃O₄ thicknesses below 39 nm meet this EOT criterion, with the EOT for 21 nm thick nanosheets dropping to as low as 0.57 nm. As shown in Supplementary Fig. 13, a dual-gate MoS₂ FET was fabricated to further confirm the ultra-high dielectric constant of 149 for the 48 nm Mn₃O₄.

Beyond the dielectric constant, parameters such as optical band gap, leakage current density, and breakdown field strength are critical in high-κ dielectric materials research.⁴⁸ The optical band gap of ultrathin Mn₃O₄ nanosheets was accurately determined through ultraviolet-visible absorption spectroscopy, which revealed a pronounced absorption edge at a wavelength of approximately 300 nm (Fig. 3e). Analysis utilizing the Tauc plot method yielded an optical band gap of 3.89 eV (inset of Fig. 3e). This is close to the theoretically calculated band gap of 4.21 eV (Supplementary Fig. 14).Additionally, the band gap was corroborated by examining the low-loss region in the electron energy loss spectrum (EELS) of ultrathin Mn₃O₄ crystals, affirming a value of approximately 3.9 eV as shown in Supplementary Fig. 15. The leakage current characteristics of a metal/Mn₃O₄/graphene dual-gate structure under operational voltages, revealing an exceptionally low leakage current of 10^–14 A, equating to a current density of approximately 10^− 7 A/cm² (Supplementary Fig. 16). These findings emphasize the high insulating performance of ultrathin Mn₃O₄, attributed to its wide band gap.

Comparative optical imaging conducted before and after the breakdown of devices with disparate thicknesses substantiated the electrical stability of the ultrathin Mn₃O₄ nanosheets, which demonstrated E_bd exceeding 13.2 MV/cm (Supplementary Fig. 17, Supplementary Fig. 18). This performance aligns with the benchmarks set by the IRDS for breakdown field strength (> 10 MV/cm).⁴⁷ Remarkably, for 66 nm Mn₃O₄ nanosheets, an increase in voltage leads to a significant escalation in leakage current to the µA range. Despite this, no breakdown-induced short circuiting or current disorder was observed, with the E_bd consistently maintained at 9.5 MV/cm. (Supplementary Fig. 17d). This resilience is ascribed to the high dielectric constant of ultrathin Mn₃O₄. As shown in Fig. 3f, the large E_bd (13.2 MV/cm) and low EOT (0.57 nm) of the ultrathin Mn₃O₄ outperform those of other advanced dielectric materials. It provides an important physical foundation for further scaling of micro and nanoscale devices and for providing a stable dielectric environment.

2.4 High performance of MoS₂ FET with Mn₃O₄ dielectric

The high dielectric constant and atomically flat surface of ultrathin Mn₃O₄ nanosheets suggest their significant potential for integration with 2D semiconductor devices. Few-layer MoS₂ was prepared on the SiO₂/Si substrate through mechanical exfoliation to construct Mn₃O₄/MoS₂ top gate FETs. Reportedly, vdW gaps effectively prevent carrier tunneling, significantly reducing gate leakage current.^10,15,18 This feature stems from the chemical inertness and lack of dangling bonds on vdW material surfaces, allowing stable and definitive vdW interfaces with two-dimensional materials.⁵⁶ As shown in Fig. 4a, observation of the cross-section with TEM confirmed the existence of an accurate vdW gap of approximately 6.5 Å between Mn₃O₄ and MoS₂. This clear interface ensures no interface disorder above the MoS₂ channel, with the ultrathin Mn₃O₄ gate dielectric forming an ideal coupling. Furthermore, EDS analysis revealed a high degree of uniformity in the elemental distribution at the interface, proving the formation of a high-quality vdW interface between Mn₃O₄ and MoS₂. This discovery not only highlights the potential of using Mn₃O₄ as a top gate dielectric in optimizing the performance of 2D FETs but also provides an essential physical basis for designing 2D electronic devices with low leakage current and high stability.

Mn₃O₄/MoS₂ top-gate FET was fabricated on a SiO₂/Si substrate, as shown in Fig. 4b, where the SiO₂/Si also served as the back gate dielectric and electrode roles when necessary. Through I_ds-V_g characteristic curve measurements conducted at various drain-source voltages (V_ds), as shown in Fig. 4c, the excellent gate control capability of 22.9 nm Mn₃O₄ as a gate dielectric for MoS₂ FETs was demonstrated, achieving an I_on/I_off ratio of nearly 10⁸ at operational voltages from − 0.8 V to -0.2 V. Further electrical performance analysis showed a linear I_ds-V_ds relationship in the low V_ds region and gradual saturation in the high V_ds region for the MoS₂ transistor, indicating good ohmic contact performance and sufficient gate modulation effect (Fig. 4d). During both forward and reverse top-gate scans, for different orders of magnitude of I_ds, the SS remained below 100 mV/dec (Fig. 4e), highlighting the efficient dielectric modulation capability of ultrathin Mn₃O₄.

Furthermore, the extremely low gate leakage current (10^–14 A) (Fig. 4f), equivalent to a current density of 10^− 7 A/cm², corresponds to near the detection limit of the measurement system, significantly below the low-power limit value (10^− 2 A/cm²), demonstrating tremendous application potential in MOSFETs.⁵⁷ Minimal gate hysteresis of about 5 mV and a very low DIBL of approximately 20 mV/V further confirm the perfect vdW interface formed between Mn₃O₄ and MoS₂ (Supplementary Fig. 19). These measurement results not only highlight the high efficiency and low power consumption characteristics of the device but also confirm the effectiveness of Mn₃O₄ as a top gate dielectric in improving interface quality and overall device performance.

As shown in Fig. 4g, ultrathin Mn₃O₄ is a material with a suitable wide bandgap and ultra-high dielectric constant. The dielectric constant of 237 for ultrathin Mn₃O₄ is among the highest reported for ultrathin dielectric materials, far surpassing other non-layered oxides and composite oxides (exemplified by ε_r ≈ 105 for SrTiO₃ and ε_r ≈ 117 for TiO₂.) The search for ultra-high κ dielectrics provides a viable solution for the further scaling of future transistors Various important performance parameters of the FET with Mn₃O₄ as the gate material are detailed in Table S1.

High-κ dielectric environments are generally considered to improve MoS₂ mobility, as it helps reduce charged impurity scattering, providing a good encapsulation effect.⁵⁸As shown in Fig. 5, the encapsulation by Mn₃O₄ increased the MoS₂ FET mobility from 11.3 cm²/Vs to 22.6 cm²/Vs. The performance of the FET has nearly doubled, enhancing the excellent conductivity of the transistor. The mobility of MoS₂ FETs before and after the transfer of Mn₃O₄ using dual-probe field-effect measurements is detailed in Table S2. Correctly utilizing high-quality monocrystals and 2D materials to construct vdW interfaces is crucial for optimizing device performance.

In summary, this study achieved the growth of ultrathin Mn₃O₄ nanosheet arrays through CVD. Through a hydrate-assisted thinning strategy and the calculation of ultra-low lattice mismatch between single-crystal Mn₃O₄ and various substrates, Mn₃O₄ achieved tunable growth from single orientation to thin films on mica. The Mn₃O₄ nanosheets exhibit an exceptionally high dielectric constant of 237 (at 39 nm), surpassing any gate dielectric materials currently discovered, while maintaining superior gate modulation and h-BN-like encapsulation capabilities. This work not only opens new avenues for the practical application of ultra-high κ materials but also offers an ideal dielectric solution for the fabrication of next-generation highly integrated, high-performance 2D transistors.

Synthesis and Transfer of Mn ₃ O ₄ Nanosheet Arrays

Within a 2-inch diameter quartz tube, ultrathin Mn₃O₄ nanosheet films were successfully prepared on mica substrates using in situ atmospheric pressure chemical vapor deposition (APCVD) technology. Pure KCl (Aladdin, 99.9%) and MnCl₂·4H₂O (Aladdin, 99.9%) were used as the solvent and reactant, placed at the core of the heating zone. The substrate material, freshly cleaved fluorphlogopite mica [KMg₃(AlSi₃O₁₀)F₂], was positioned above the mixture. Prior to the reaction, the furnace was purged with argon gas at a flow rate of 200 sccm for 15 minutes to remove residual gases. During the growth process, argon gas was used as the carrier gas at a flow rate of 50 sccm. The temperature was ramped up to 660°C and maintained for 10 minutes to allow for the growth of the nanosheet arrays. After growth, the system was naturally cooled to room temperature, resulting in high-quality Mn₃O₄ nanosheet arrays on the mica substrate. To transfer the grown nanosheets to the target substrate, a polystyrene (PS) assisted layer transfer method was employed. Firstly, 10 grams of PS pellets were dissolved in 200 mL of toluene, and then this solution was evenly spin-coated on the Mn₃O₄ nanosheets/mica and cured on a hot plate at 80°C for 5 minutes. Subsequently, the PS film deposited with Mn₃O₄ was carefully lifted in deionized water and transferred to a clean SiO₂/Si wafer. Finally, after air drying at room temperature for 10 minutes, the PS layer was removed by treating with toluene for 5 minutes, completing the transfer of the nanosheets.

Material Characterization

The morphology and thickness characteristics of the two-dimensional nanosheets were finely observed using an optical microscope (Nikon instrument ECLIPSE LV150N) and an atomic force microscope (Bruker Dimension Icon). Furthermore, Raman spectroscopy of the Mn₃O₄ nanosheets was conducted at room temperature using a micro-Raman spectrometer (LabRAM HORIBA) equipped with a 532 nm laser source. Chemical composition and elemental analysis were performed using X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy (250Xi ESCALAB). The bandgap of Mn₃O₄ was characterized using a micro-ultraviolet-visible absorption spectrometer (Mstarter ABS). The atomic structure and elemental composition were characterized using a field-emission transmission electron microscope (JEM-F200, EELS). Magnetic measurements were conducted using a physical property measurement system and a vibrating sample magnetometer (PPMS-9T and VSM, Quantum Design).

First-principles calculations

All calculations were based on DFT, executed by Vienna Ab-initio Simulation Package (VASP). The whole three-dimensional (3D) periodic models were constructed by Materials Studio (MS). The Perdew-Burke-Ernzerhof (PBE) of generalized gradient approximation (GGA) was used to describe the exchange-correlation effect. The core interactions were treated using the pseudo-potential of projector-augmented-wave (PAW) method. In the study, the cutoff of 500 eV, 3x3x1 (Mn₃O₄) k-points for entire geometry calculation. All the geometry structures and atomic

positions were fully relaxed by a conjugate gradient (CG) method with convergence criteria of-0.01 eV/Å, and 10^− 5 eV/atom for force and energy, respectively.

Device Fabrication and Testing

Bulk MoS₂ crystals were dispersed on blue tape, and then the dispersed MoS₂ crystals on blue tape were pressed gently against PDMS. After slowly separating PDMS and blue tape, the thin layers of two-dimensional MoS₂ were peeled off onto PDMS. The MoS₂ was then transferred to a SiO₂/Si substrate using a dry transfer platform for two-dimensional materials, and the grown Mn₃O₄ nanosheets were wet transferred onto MoS₂. All devices were defined using electron beam lithography (Raith) for electrode patterning, and Cr/Au (10/50 nm) contact electrodes were deposited on the SiO₂/Si substrate using electron beam evaporation. Electrical measurements were performed at room temperature in a vacuum probe station coupled with a semiconductor parameter analyzer (Agilent 4155B).

Data availability

Relevant data supporting the key findings of this study are available within the article and the Supplementary Information file. All raw data generated during the current study are available from the corresponding authors upon request.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 52202323, 52372240, U23A2094), the Natural Science Foundation of Fujian Province of China (Nos. 2021J05096, 2021J01520), China Postdoctoral Science Foundation 2023M733495, and the Self-deployment Project Research Program of Haixi Institutes, Chinese Academy of Sciences, CXZX-2022-GH09.

Author contributions

Competing interests

The authors declare no competing interest.

Additional information

Supplementary information The online version contains supplementary material available at

Correspondence and requests for materials should be addressed to Wei Liu.

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Controllable Synthesis of Nonlayered Ultrathin Mn₃O₄ with High Dielectric Constant

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Version 1

Abstract

Figures

1. Introduction

2. Results

2.1 Synthesis of high-quality ultrathin Mn₃O₄ arrays

2.2 High-quality ultrathin Mn₃O₄ arrays

2.3 Dielectric properties of ultrathin Mn₃O₄ single crystals

2.4 High performance of MoS₂ FET with Mn₃O₄ dielectric

3. Conclusion

4. Methods

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

Controllable Synthesis of Nonlayered Ultrathin Mn3O4 with High Dielectric Constant

Status:

Version 1

Abstract

Figures

1. Introduction

2. Results

2.1 Synthesis of high-quality ultrathin Mn3O4 arrays

2.2 High-quality ultrathin Mn3O4 arrays

2.3 Dielectric properties of ultrathin Mn3O4 single crystals

2.4 High performance of MoS2 FET with Mn3O4 dielectric

3. Conclusion

4. Methods

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

Controllable Synthesis of Nonlayered Ultrathin Mn₃O₄ with High Dielectric Constant

2.1 Synthesis of high-quality ultrathin Mn₃O₄ arrays

2.2 High-quality ultrathin Mn₃O₄ arrays

2.3 Dielectric properties of ultrathin Mn₃O₄ single crystals

2.4 High performance of MoS₂ FET with Mn₃O₄ dielectric