Atomic imaging and thermally induced dynamic structural evolution of two-dimensional non-layered Cr2S3

doi:10.21203/rs.3.rs-1830305/v1

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Atomic imaging and thermally induced dynamic structural evolution of two-dimensional non-layered Cr₂S₃

https://doi.org/10.21203/rs.3.rs-1830305/v1

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Two-dimensional non-layered materials have attracted considerable attention owing to their potential for innovative applications. Chromium sulfide (Cr2S3) is a ferrimagnetic material with special spin states and thickness-dependent conduction-type transition properties. However, a fundamental understanding of Cr2S3 is limited, and a comprehensive study of its structural properties is essential for practical applications. In this study, facile salt-assisted chemical vapor deposition (CVD) was used to synthesize ultrathin non-van der Waals Cr2S3 with a thickness of ~1.9 nm. The structural transformation of as-grown Cr2S3 was studied using advanced in situ heating techniques combined with transmission electron microscopy (TEM). Two-dimensional (2D) and quasi-one-dimensional (1D) samples were fabricated to investigate the connection between specific planes and the dynamic behavior of the structural variation. For the 2D specimen, we studied the transition from a disorder-Cr1/3 layer of R-Cr2S3 to trigonal-Cr2S3, and trigonal-Cr2S3 to monoclinic-Cr3S4; for the quasi-1D specimen, we studied the transition from a disorder-Cr1/3 layer of R-Cr2S3 to monoclinic-Cr3S4 and monoclinic-Cr3S4 to trigonal-Cr5S6. The rearrangement of atoms during the phase transition was driven by the loss of sulfur atoms at elevated temperatures, resulting in increased free energy. The effect of the ratio of (001) plane on the structural transformation temperature was also discussed. A decrease in the ratio of (001) plane led to an overall increase in surface energy, thus lowering the critical phase transition temperature. Our study provides detailed insight into the mechanism of structural transformation and the critical factors governing transition temperature, thus paving the way for future studies on intriguing Cr-S compounds.

Ultrathin two-dimensional (2D) transition metal chalcogenides (TMCs) have attracted considerable attention owing to their specific layer- and structural-dependent properties.^1–4 For example, molybdenum disulfide (MoS₂) exhibits a tunable bandgap from bulk to monolayer. Moreover, polymorphs of MoS₂ have divergent electronic properties, such as the semiconducting 2H phase and the metallic 1T phase, which are attributed to the quantum confinement effect (QCE) and electron density variation, respectively.^{5, 6} The unique properties of TMCs under a 2D limit make them suitable for many potential applications, such as catalysis, electronics, energy storage, photonics, and spintronics. ^7–11

Among 2D materials, non-layered materials are advantageous in terms of their catalytic, sensing, and carrier transfer capabilities. The abundant dangling bonds on the surfaces of 2D non-layered materials enable increased chemical activities on the surface, making them ideal for surface-active applications.^{12, 13} The intercalating atoms in the van der Waals gap can eliminate interlayer barriers and allow easier electron transfer, thus enhancing the electrocatalytic performance.¹⁴ Moreover, Cr-based non-layered materials exhibit fascinating layered-dependent conduction and tunable room-temperature ferromagnetic properties.^15–18 However, fabricating non-layered materials into atomically thin crystals by a simple top-down exfoliation method is challenging because of the strong chemical bonds in three dimensions. Therefore, reliable synthetic methods for constructing ultrathin 2D non-layered crystals are extremely desirable.

The salt-assisted chemical vapor deposition (CVD) method is a promising route for synthesizing non-layered materials.^{19, 20} Salt is considered to have two primary effects: lowering the melting point of the metal precursor and increasing the vapor pressure of the target metal.^{21, 22} The second effect lowers the activation energy of particular planes and induces the distortion effect, which prevents isotropic growth and promotes lateral growth.²³ He et al. obtained unit-cell-thick non-layered Cr₂S₃ nanosheets by a NaCl-assisted CVD method using a CrCl₃ metal precursor.²⁴ Subsequently, the intriguing variable conduction properties of rhombohedral Cr₂S₃ (R-Cr₂S₃) were discovered by Zhang et al¹⁵. As the thickness of R- Cr₂S₃ decreased from 7.6 to 2.6 nm, its conduction behavior varied from n-type to ambipolar and then to p-type. The Cr atoms in the Cr_1/3 layer play an important role as electron transport channels between the CrS₂ layers. Therefore, the electrical properties of Cr₂S₃ are closely related to the interlayer arrangement of Cr atoms. However, the microstructure of CVD-grown 2D Cr₂S₃ with CrS₂-Cr_1/3-CrS₂ stacking has only been observed in plane-view TEM and scanning transmission electron microscopy (STEM) images, which significantly limits the information on chromium’s atomic arrangement in the Cr_1/3 layer. However, the operation of Cr₂S₃-based devices generally involves Joule heating effects, which may induce structural transitions and affect device properties. To further explore these interesting physical properties and discuss the phase transfer mechanism of 2D Cr₂S₃, obtaining detailed information on the structure and thermally induced phase transition of Cr₂S₃ is essential.

In this study, we successfully synthesized Cr₂S₃ nanosheets of unit-cell thickness (~ 1.97 nm) using a facile CVD method. We employed annular dark-field (ADF) STEM to identify the atomic structure of CVD-grown Cr₂S₃ from both plane-view and cross-section images, which illustrated a six-fold symmetry in [001] and a CrS₂-disorder Cr_1/3 layer alternately stacked in [110]. TEM is a powerful technique for analyzing microstructures, providing atomic-resolution images and diffraction information.^25–27 Moreover, the development of in situ TEM provides insight into the dynamic structural evolution at the atomic scale, which yields direct evidence for atomic motion under different environmental conditions (i.e., annealing and bias).^28–32 The phase transition during the annealing process was studied for both 2D and quasi-one dimensional (1D) specimens using the advanced in situ TEM technique. When comparing quasi-1D Cr₂S₃ with 2D flakes, a decrease in the ratio of the (001) plane leads to a lower temperature for the structural transformation. Furthermore, we deduced the consecutive atomic motion trajectory during the phase transition by combining in situ heat STEM images and atomic-resolution images.

CVD is a facile and controllable method for synthesizing 2D materials. Generally, precursors, substrate, temperature, and gas flow rate are the decisive factors for a successful synthesis. CrCl₃ has a lower melting point (1,150°C) than Cr₂O₃ (melting point = 2,435°C), making it a suitable precursor for CVD synthesis. In addition, sodium chloride (NaCl) can significantly lower the melting point of reactants and enhance their volatilization.³³ A mixture of CrCl₃ and NaCl was utilized as the precursor to provide sufficient Cr. To further reduce the energy barrier for Cr₂S₃ nucleation, fluorophlogopite mica (KMg₃AlSi₃O₁₀F₂) with an atomically flat surface was chosen as the substrate for epitaxial growth (Fig. 1a). ³⁴ Fig. S1 shows the experimental setup, and the detailed growth parameters are presented in the Experimental Method section. Figure 1b and c show the crystal structure of rhombohedral Cr₂S₃ (R-Cr₂S₃), which can be regarded as an alternative stacking of CrS₂-Cr_1/3, indicating that Cr atoms in the octahedral site are sandwiched between the CrS₂ layers. Optical microscopy (OM) images of the as-grown Cr₂S₃ are shown in Fig. 1d, revealing a hexagonal geometry with edge lengths of ~ 40 µm. Atomic force microscopy (AFM) was performed to determine the thickness of the Cr₂S₃ nanosheet. The thickness of a typical unit-cell Cr₂S₃ is 1.96 nm, which agrees well with that of as-grown Cr₂S₃ (Fig. 1e). X-ray photoemission spectroscopy (XPS) was employed to determine the elemental composition of our samples. Based on the spectra shown in Fig. 1f and S2, the peaks at 574.8 and 584.3 eV are attributed to Cr 2p_3/2 and Cr 2p_1/2, respectively. The S 2p_3/2 and S 2p_1/2 peaks are observed at 160.7 and 161.9 eV, respectively. This result is in good agreement with those of previous studies on CVD-grown Cr₂S₃.^{15, 35} The elemental ratio of Cr to S was 1.4, further confirming the successful synthesis of Cr₂S₃. Figure 1g shows the Raman spectrum obtained using an argon laser (488 nm) as the excitation source. The distinct Raman peak observed at 253.3 cm^− 1 was derived from the in-plane E_g mode, and the peaks at 286.5 and 365.1 cm^− 1 show the characteristic A_g mode dominated by an out-of-plane motion.²⁴

Moreover, elemental mapping images (Fig. S3) obtained by energy-dispersive X-ray spectrometry (EDS) show a uniform distribution of Cr and S in the flakes. The EDS spectra revealed that the S/Cr ratio of the as-grown sample was 1.5, which is identical to the stoichiometric ratio of Cr₂S₃. Figure 2a shows the low-magnification TEM images of plane-view Cr₂S₃. The high-resolution (HR) TEM image (Fig. 2b) shows a hexagonal arrangement of atoms. The corresponding fast Fourier transform (FFT) image in the inset shows the perfect six-fold symmetry and single-crystal nature of the as-grown Cr₂S₃. Figure 2c shows the ADF-STEM image projected from the [001] orientation. The d-spacing of (110) is 2.97 Å, which is in agreement with the atomic structure of thermal dynamic stable phase, rhombohedral Cr₂S₃.³⁶ Different contrast of sulfur atoms in STEM mode originated from a discrepancy in the stacking sequence. As shown in the cross-sectional atomic model of Cr₂S₃ with even-numbered layers (Fig. 2d), the number of sulfur atoms was different in the adjacent rows. For example, there were three atoms on the pink line but only two on the purple line. The observations from the simulated ADF-STEM image of rhombohedral-Cr₂S₃ (Fig. 2e) is consistent with the experimental results. To unveil the detailed atomic structure of CVD-grown Cr₂S₃, a cross-sectional investigation was performed using TEM/STEM, the results of which are shown in Fig. 2f–j. A TEM lamella prepared using a focused ion beam (FIB) is shown in Fig. 2f. The HR-TEM/STEM images (Fig. 2g and h) demonstrate the lattice structure of Cr₂S₃ view down [110], and the intensity profile (Fig. 2g) shows that the d-spacing of the (003) plane was 5.55 Å, identical to that of the rhombohedral phase of Cr₂S₃. Notably, the Cr_1/3 layer is neither arranged in a typical ABC sequence of the rhombohedral phase nor in other ordered arrangements of Cr-S compounds (Fig. S4). In the intensity distribution, the intensity of the Cr I peak was much stronger than that of the Cr II peak (Fig. 2i), indicating that the number of Cr atoms under the projection of the Cr II position was less than that of the Cr I position, i.e., Cr II is a Cr-deficient layer. By combining the EDS result and the STEM image, the as-grown Cr₂S₃ can be considered as a CrS₂- disordered Cr_1/3- CrS₂ alternating stacking. As shown in Fig. S4, the atomic arrangement of the Cr-S compounds shows significant differences in the Cr-deficient layers, which is reflected in the diffraction points inside the blue dashed box shown in Fig. S5. Comparing the diffraction patterns shown in Fig. 2j with those shown in Fig. S5, some missing spots were noted in the CVD-grown Cr₂S₃, indicating that the Cr atoms in the Cr_1/3 layer have a disordered arrangement.

The thermal evolution of the 2D Cr₂S₃ structure was explored via in situ heating and recorded by TEM, which enabled real-time observation of the structural evolution at the atomic scale. Furthermore, a detailed analysis of the Cr₂S₃ phase transition and critical stages of the phase transition are provided. Figure 3a–h show a series of low-magnification plane-view images recording a continuous heating process; each image was captured after maintaining the temperature for five minutes. The morphology of the as-grown Cr₂S₃ flakes hardly changed during the heating. As for the atomic structure shown in Fig. 3a'–h', the atoms remained in a hexagonal configuration because of the consistent atomic arrangement of the series of Cr-S compounds in the view down [001] direction. Additionally, a similar interplanar spacing of Cr-S compounds presented in Fig. S6 implies a challenge in distinguishing the Cr₂S₃ phase from a plane-view TEM observation. We attempted to address this issue by comparing the FFT diagram with a simulated diffraction pattern. As shown in Fig. 3a"-g," the appearance of extra diffraction spots around the center helped us differentiate the rhombohedral and trigonal phases of Cr₂S₃. Furthermore, the single set of (010) diffraction spots shown in Fig. 3h" represents monoclinic Cr₃S₄ (M-Cr₃S₄). To summarize, the original CVD-grown phase of 2D-Cr₂S₃ (disordered arrangement of Cr_1/3 layer of R-Cr₂S₃) was maintained at 0–350°C, and transitioned to trigonal Cr₂S₃ (T-Cr₂S₃) at 350°C. Finally, M-Cr₃S₄ was formed when the temperature reached 600°C. If the temperature was maintained for 3 min, the coexisting phases of T-Cr₂S₃ and M-Cr₃S₄ were obtained at 600°C, indicating that the phase transition from T-Cr₂S₃ to M-Cr₃S₄ was completed within 3–5 minutes (Fig. S7).

After observing the thermally induced phase transition in 2D-Cr₂S₃, a quasi-1D sample was prepared to reveal the effect of the ratio of (001) plane on the structural transformation. Cr₂S₃ was fabricated using a FIB to produce a specimen of 4.56 µm length, 124 nm width, and 118 nm thickness, as illustrated in Fig. S8. The thickness was measured by electron energy loss spectroscopy (EELS) using the Kramers-Kronig sum method.³⁷ The (110) plane was chosen as the observation plane because the atomic arrangement of the Cr-deficient layer could be well visualized in the projection of R-Cr₂S₃ [110] in the Cr-S compounds. Fig.s 4a–c show the ADF-STEM images of cross-sectional Cr₂S₃ held at 25°C, 400°C, and 600°C, respectively. The alternating CrS₂ layers combined with disordered Cr_1/3 layers (CrS₂-Cr_1/3 arrangement) were observed at 25°C, as shown in Fig. 4a. A CrS₂-Cr_1/2 arrangement was observed at 400°C, indicating that Cr₂S₃ has been transformed into M-Cr₃S₄ (Fig. 4b). When the temperature reached 600°C, M-Cr₃S₄ transformed into a CrS₂-Cr_2/3-CrS₂-Cr_1/3 arrangement as Cr₃S₄ transition phase (green dashed box) and an alternating CrS₂-Cr_2/3 arrangement as T-Cr₅S₆ (yellow dashed box), as shown in Fig. 4c. Fig.s 4d–f show the SAED patterns, which provide insights into the phase transition. Fig.s 4d and e show the transition from Cr₂S₃ to single-crystal M-Cr₃S₄; the extra rows of diffraction spots are found at 400°C. In contrast, the coexisting phases of M-Cr₃S₄, Cr₃S₄ transition, and T-Cr₅S₆ were formed at 600°C (Fig. 4f). Comparing the thermal-induced phase transition result of the quasi-1D specimen with that of the 2D specimen, we determined that the critical temperature of the phase transition from Cr₂S₃ to Cr₃S₄ decreased. Vertical and horizontal intensity line profiles were used to measure the distance between Cr-Cr atoms under different temperatures, as shown in Fig. 4g and j. The corresponding line graphs are shown in Fig. 4h and j. Fig. S9a shows that the theoretical distance of the horizontal Cr-Cr increased from Cr₂S₃ to Cr₃S₄ and decreased from Cr₃S₄ to Cr₅S₆. There was a discrepancy between the theoretical value and distance measured in our experiment, as shown in Fig. 4h. The average atomic spacing of Cr increased from 400°C to 600°C, which is attributed to the thermal expansion in the horizontal direction. Further, the theoretical Cr-Cr spacing in the vertical direction illustrated in Fig. S9b, is also inconsistent with the experimental results (Fig. 4j), indicating that the mixed phase of Cr₃S₄ (transition) and T-Cr₅S₆ may have negative thermal expansion coefficients in the [001] orientation.

The reduction in the mole fraction of S induced by the thermal effect results in a series of transformations in the Cr-S compounds. As depicted in Fig. S10, along with the decrease in the mole fraction (Xs), the thermal-dynamic stable phase varied from R-Cr₂S₃ to T-Cr₂S₃, T-Cr₂S₃ to M-Cr₃S₄, and M-Cr₃S₄ to T-Cr₅S₆.³⁶ The atomic ratio of sulfur depicted in Fig. S11 decreases when the temperature increases, agreeing well with the variation in the stoichiometry of the phase transition sequence. The elimination of sulfur atoms increased the defect concentration and Gibbs free energy, which prompted the Cr atoms to move to the nearest neighboring sites to stabilize the structure. As illustrated in Fig. 5a, the Cr atoms in the CrS₂ layer migrated to the Cr_1/3 layer; the Cr atoms shifted horizontally, and subsequently, Cr atoms that are overlapped by the upper and lower layers are displaced as well owing to electron repulsion reasons³⁸ resulting in the conversion of the Cr_1/3 layer into a specific alternating Cr_1/2 layer. Fig.s 5b and c illustrate the continuous migration paths from M-Cr₃S₄ to Cr₃S₄ (transition) and Cr₃S₄ (transition) to T-Cr₅S₆. After the rearrangement of Cr atoms in the Cr-deficient layers, the original Cr_1/2 layer of M-Cr₃S₄ transformed into a transition phase with Cr_2/3 and Cr_1/3 interlayers, and further evolved into T-Cr₅S₆ with a Cr_2/3 interlayer. As illustrated in Figs. 3 and 4, the 2D and quasi-1D specimens with different critical transition temperatures were attributed to the discrepancy in the total surface energy. The ratio of the (001) plane of the 2D specimen, which is 99.6%, was much larger than that of the quasi-1D specimen (50%) as shown in Fig. S12, which led to a significant difference in the total surface energy. When the same kinetic energy was applied to the crystal at the same temperature, the quasi-1D sample with many more dangling bonds tended to remove more sulfur atoms. Therefore, it is easier for the M-Cr₃S₄ phase to form at a lower temperature in the quasi-1D specimen than in the 2D specimen.

In summary, we synthesized high-quality Cr₂S₃ nanosheets on a mica substrate using a facile CVD method with a unit-cell thickness. Further, we revealed the disordered arrangement of CVD-grown Cr₂S₃ in Cr-deficient layers using ADF-STEM. In situ TEM/STEM was used to investigate the thermal-induced structural transformation in 2D and quasi-1D Cr₂S₃ single crystals. We revealed the phase transition process, including disordered Cr_1/3 layers of Cr₂S₃ to T-Cr₂S₃, T-Cr₂S₃ to M-Cr₃S₄, and M-Cr₃S₄, to the coexisting phases of Cr₃S₄ and T-Cr₅S₆ with an increasing temperature. The driving force behind the structural transformation is the deficiency of sulfur atoms at high temperatures. Increasing Gibbs free energy results in the movement of atoms to form lower-energy phases. Remarkably, reducing the ratio of the (001) plane results in an overall increase in the surface energy, leading to the transition of Cr₂S₃ to Cr₃S₄ at lower temperatures. This study provides detailed structural transformation information of Cr₂S₃ induced by the thermal effect and critical factors of the transition, which can serve as a cornerstone for further investigations of intriguing Cr-S compounds.

CVD growth of Cr ₂ S ₃ nanoflake. Cr₂S₃ flakes were grown via CVD in a three-zone furnace with a 2-inch diameter quartz tube. 300 mg of sulfur powder (99.5%, Sigma-Aldrich) was placed in a ceramic boat at the center of the first heating zone and heated to 200°C. A mixed powder of 30 mg CrCl₃ (anhydrous, 99.9%, ALFA) and NaCl was put in a ceramic boat and placed at the center of the third heating zone, where the temperature was raised to 750°C and held for 15 min. Fluorophlogopite mica was used as the substrate and was placed directly above the mixed powder of CrCl₃ and NaCl. Before performing the synthesis, the system was pumped to 0.3 torr and flushed with Ar gas for 10 min to provide a relatively clean environment. The growth was conducted with 120-sccm Ar as the carrier gas.

Transfer of Cr ₂ S ₃ nanoflake. The as-grown Cr₂S₃ was transferred to substrates (SiO₂, TEM Cu-grid, and TEM heating chips) through a typical poly(methyl methacrylate)-assisted transfer method. First, the PMMA was spin-coated onto fluorophlogopite mica at 5,000 rpm for 60 s. The Cr₂S₃/mica specimen was then baked on a hot plate at 180°C for 1 min, and soaked in a 20 wt% hydrofluoric acid solution for 2 h. The PMMA /Cr₂S₃ could be scooped up after the mica/PMMA interface was etched using a hydrofluoric acid solution. The PMMA/Cr₂S₃ thin film was immersed in deionized (DI) water to eliminate residues, and then transferred to the target substrate. Finally, the PMMA was dissolved by soaking in acetone at 70°C for 3 h.

Materials characterizations. The morphology was observed using an optical microscope, and the thickness was examined by AFM. Raman spectroscopy (excitation wavelength of 488 nm) was used to identify the characteristics of Cr₂S₃ single-crystal bonding. The chemical composition was characterized using XPS and EDS.

TEM, STEM analysis. The TEM/ADF-STEM images were obtained using JEOL JEM-F200 TEM and JEOL ARM-200F Cs-TEM with an accelerating voltage of 200 keV. In situ heating experiments were conducted using a fusion holder and chips (photochips) at 9 × 10 − 6 Torr (TEM chamber). Real-time TEM images were captured using a 4 K-resolution OneView camera. ADF-STEM images were processed using the ImageJ software (developed by National Institutes of Health, U.S.) with LUT fire and LUT spectrum to enhance the visualization of atomic stacking. Atomic models were constructed using the Vesta software. The simulated STEM and diffraction pattern images were obtained by Dr. Probe STEM simulation and Single Crystal software, respectively.

Reporting summary. Further information on research design is available in the Nature Research Reporting Summary linked to this article.

Data availability

The data that support the results in this paper and other findings of the study are available from the corresponding authors upon reasonable request.

Acknowledgements

We thank the support from Ministry of Science and Technology (MOST) in Taiwan (MOST 107-2119-M-009-019, MOST 108-2221-E-009-036-MY3, MOST 109-2628-E-009-008-MY3, MOST 110-2731-M-009-001 and MOST 110-2119-M-007-002-MBK). This work was financially supported by the “Center for Semiconductor Technology Research” and “Center for Emergent Functional Matter Science of National Chiao Tung University” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. Also supported in part by the Ministry of Science and Technology, Taiwan, under Grant MOST-109-2634-F-009-029 and MOST-110-2634-F-009-027.

Author contributions

C.-L. L. and W.-W. W. designed the study. C.-L. L. grew the ultrathin Cr₂S₃ nanosheets using CVD and characterized the as-grown samples. H.-Y. L. and A.-Y. H. fabricated the TEM chips using FIB. Y.-T. T. and C.-H. W. performed the in situ TEM, STEM, and EDX analyses. C.-W. H. performed the EELS examinations. C.-L. L. analyzed the in situ TEM experiment, HRTEM/ADF-STEM images, and simulated data. C.-L. L., Y.-T. T., and W.-W. W. conceived the study. C.-L. L., Y.-T. T., and W.-W. W. wrote this paper.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary information The online version contains supplementary material available.

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Atomic imaging and thermally induced dynamic structural evolution of two-dimensional non-layered Cr₂S₃

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