In-situ TEM investigation of MoS2 wrinkles and its effects on electrical properties

https://doi.org/10.1016/j.matchemphys.2020.123797Get rights and content

Highlights

  • The formation and disappearance of wrinkles are reversible.

  • MoS2 nanosheets corrugated as the strain reached 3.3% for 15 layers.

  • The existence of wrinkles can affect the electrical properties.

Abstract

MoS2 has potential application in the fields of electronics and optics. However, thin films with non-uniform or local compressive strain can form winkles. The presence of wrinkles has great effects on the performance of the transition metal dichalcogenides (TMDs) materials. In this work, by in-situ transmission electron microscopy (TEM), we tracked the formation and disappearance of MoS2 wrinkles to reveal the microstructure of wrinkles and its effects on electrical properties. It is found that compressive stress can lead to the formation of wrinkles, and the wrinkles disappeared with removal of the stress. The εxx maps show that MoS2 nanosheets corrugated as the strain reached 3.3% for 15 layers, accompanied by new diffraction points in fast Fourier transform (FFT) image. In contrast, removal of the stress resulted in the disappearance of wrinkles and new diffraction points, and MoS2 nanosheets restored to its initial morphology. Moreover, wrinkles also affect the electrical properties of MoS2 nanosheets. The existence of wrinkles caused an increase of the current, and the disappearance of the wrinkles accompanied by the current decreased, which is due to the piezoresistive effect. This study is helpful to understand the properties of MoS2 nanostructures and is of great significance to the structural design in the electronic field.

Introduction

MoS2, as an important member of TMDs, has potential application in the fields of electronics [[1], [2], [3], [4], [5]], optoelectronics [6,7], catalysis [[8], [9], [10]], energy storage [[11], [12], [13]] due to its unique structure and properties. However, some properties of MoS2-based devices, such as transport, still cannot meet the needs of commercial applications. The properties of the materials can be adjusted over a wide range via strain engineering by shifting its structure (e.g., bond length, bond angle and the positions of atoms) [[14], [15], [16], [17], [18]]. TMDs materials have the great capacity of deformation tolerance, which makes it possible to develop their properties by strain engineering [[19], [20], [21], [22]]. Therefore, strain engineering is an effective method to adjust the properties of TMDs materials. Among them, wrinkling is a method to realize the strain in TMDs materials. More importantly, sometimes wrinkles are inevitably introduced during the preparation process. For example, for the mechanical exfoliation method, the interaction between the bending energy and the interfacial adhesion energy of the TMDs can cause thin layers partially separating from the substrate surface during the release of type, thus causing the TMDs to contract and form wrinkles. In turn, these wrinkles also cause uniaxial strain in TMDs [23]. Furthermore, chemical vapor deposition (CVD) is the most useful and promising method to produce two-dimensional TMDs [24]. Because of the mismatch thermal expansion coefficient and lattice constant of substrate and TMDs materials, non-uniform and localized strain exist in TMDs during the preparation process of CVD, which can also form the wrinkles [24]. Wrinkles can also be deliberately introduced into the TMDs by other methods. For example, the deformation of flexible substrate [25,26] and substrate surface topography modification [23,27].

Wrinkles can play a significant role on the properties of TMDs. For example, By engineering the substrate's surface morphology, Liu et al. [28] corrugated the monolayer MoS2 on the SiNx substrate and achieved an almost two orders of magnitude increase in carrier mobility of the MoS2 two-dimensional FET compared to standard devices at room temperature and atmospheric pressure, which greatly improves the application prospect of MoS2 in the field of electronics. Wu et al. [29] realized 5.08% mechanical-electrical conversion efficiency by applying tensile and compressive stresses to the flexible substrate covered by monolayer MoS2. It has great application prospects in electromechanical sensing, wearable technology and so on. Wang et al. [30] used different thermal expansion coefficients of patterned sapphire substrate to produced various kinds of wrinkles in monolayer MoS2. Compressive strain caused by wrinkles can lead to greater absorption and bandgap funnel-effect, and enhancing the photocurrent significantly.

Above all, wrinkles have great influences on the performance of MoS2. So far, there is no study on the microstructure change of producing wrinkles. Moreover, the traditional ex situ study has disadvantages due to the post-treatment process which might bring in unforeseeable factors. In-situ transmission electron microscopy (TEM) can provide real-time solution to analyze morphologies and structural changes of the materials at high spatial resolution. Here, by using W tip to stress the suspended MoS2 nanosheets in TEM, the real-time high-resolution images of the formation and disappearance of MoS2 wrinkles and the corresponding changes in electrical properties were obtained.

Section snippets

Sample characterization

The morphology of MoS2 nanosheets were characterized by employing field-emission scanning electron microscopy (JEOL). Raman spectra were measured using a JY Horiba HR800 equipped with liquid nitrogen cooled CCD, a Nd:YAG 532 nm solid state excitation laser and an optical microscope. The topography of the MoS2 nanosheets were probed by atomic force microscope (AFM). The scanning transmission electron microscopy images were acquired using an aberration-corrected (S)TEM (JEM-ARM 300F) operated at

Results and discussion

Fig. 1a displays the low magnification TEM image, the size of the MoS2 nanosheets were several microns and the anisotropic contrast of the TEM image indicates good crystallinity of MoS2 nanosheets [31]. Obviously, the MoS2 nanosheets were suspended. The insert shows the selected area electron diffraction (SAED) pattern of blue boxes in Fig. 1a. The schematic illustration of the experimental setup is shown in Fig. 1b. Once the W tip was in contact with MoS2 nanosheets, the MoS2 nanosheets would

Conclusion

The structure and the dynamic of the formation and disappearance of wrinkles in MoS2 nanosheets and the effects of wrinkles on the electrical properties of MoS2 nanosheets were systematically studied by in-situ TEM. When the strain reached about 3.3% for 15 layers, the MoS2 nanosheets can be corrugated, accompanied by new diffraction points in FFT. And, removal of the stress resulted in the disappearance of wrinkles and new diffraction points. Furthermore, the MoS2 nanosheets did not collapse

CRediT authorship contribution statement

Lei Xu: Formal analysis, Investigation, Writing - original draft. Zhenyun Zhang: Formal analysis. Chenchen Wu: Formal analysis. Huan Liu: Formal analysis. Tao Shen: Investigation, Resources. Yongwan Cao: Investigation, Resources. Junjie Qi: Funding acquisition, Project administration, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Beijing Natural Science Foundation (2202030), the National Foundation of China (No. 41422050303), the Program of Introducing Talents of Discipline to Universities (B14003), and the Fundamental Research Funds for Central Universities (FRF-GF-19-001A, FRF-GF-19-002B)

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