Toward Exploring the Structure of Monolayer to Few-layer TaS2 by Efficient Ultrasound-free Exfoliation

Tantalum disulfide nanosheets have attracted great interest due to its electronic properties and device applications. Traditional solution-ased ultrasonic process is limited by ultrasound which may cause the disintegration into submicron-sized flake. Here, an efficient multi-step intercalation and ultrasound-free process has been successfully used to exfoliate 1T-TaS2. The obtained TaS2 nanosheets reveal an average thickness of 3 nm and several micrometers in size. The formation of few-layer TaS2 nanosheets as well as monolayer TaS2 sheets is further confirmed by atomic force microscopy images. The few-layer TaS2 nanosheets remain the 1T structure, whereas monolayer TaS2 sheets show lattice distortion and may adopt the 1H-like structure with trigonal prism coordination. Electronic supplementary material The online version of this article (10.1186/s11671-018-2439-z) contains supplementary material, which is available to authorized users.

As one of the typical TMD isomorphism, tantalum disulfide (TaS 2 ) shows a unique combination of valuable structural, mechanical, chemical, and electronic properties, which has been studied for decades [10,12,15,[20][21][22]. Its lamellar structure and electronic properties are beneficial for high-power applications such as microelectromechanical system, device applications [18,21,23], and superconductor [10,12]. 1T and 2H phases are the most representative structures. The distinction between two phases can be the S-Ta-S coordinated situations, which are recognized as octahedral coordination (1T) and prismatic coordination (2H). The two phases are quite different in electronic structures and other properties. For example, bulk 1T-TaS 2 shows charge density wave (CDW) transition and such a transition is found to vanish measured at (001) oriented single-crystal nanoflake (< 5 nm thickness) [18,21,23]. Whereas, the 2H phase shows superconductivity and its critical temperature can be raised by intercalation [24][25][26].
TMDs including TaS 2 can be successfully exfoliated by mechanical method [22] or ultrasound-assisted intercalation process [1, 4-9, 14, 15, 19]. The exfoliated TaS 2 obtained by mechanical method is only used to explore its microscopic properties [20,23,27]. All the other methods have not got rid of the physical ultrasound assistance [15][16][17]19], which may suffer the size of the exfoliated nanoflakes [15] and limits their wide applications. Therefore, it is necessary for us to explore an efficient alternative exfoliation route.
Our search leads us to a multi-step intercalation method using alkali metal-ammonia as the first step intercalation reaction. In sodium liquid ammonia solution, for example, the metal transfers an electron to the solution to produce a radical solvated electron. The reaction introduces both alkali metal and ammonia molecule into the interlayer of TaS 2 . Moreover, the ammonia can be replaced by hydroxyl-contained water [28]. Hydroxyl-contained glucose was ever used to intercalate perovskite [29,30] which can produce very large layer spacing. It is interesting to consider whether the hydroxyl-contained glucose can get into the TaS 2 layer just like water and achieve the exfoliation of TaS 2 . And water-organic mixed solvent was ever employed to exfoliate high-quantity two-dimensional material [31,32].
Herein, using a multi-step ultrasound-free intercalation process, we report efficient exfoliation of 1T-TaS 2 . The exfoliated TaS 2 nanosheets show 1T structure with several micrometers in size and an average thickness of about 3 nm. Monolayer TaS 2 can also be obtained, and some work was done to explore its structure features.

Experimental
Ta, Na, and S power are available from Aladdin reagent. All reagents were used as received without further purification.
First, 1T-TaS 2 was prepared by sealing a stoichiometric amount of Ta and S in vacuum quartz tube, then quenching the tube after heating to 900°C for 1 day.
The exfoliation mechanism can be attributed to a multi-step intercalation process. Na x (NH 3 ) y TaS 2 was synthesized through liquid ammonia procedure. First, a sealed reaction bulb with certain amounts of sodium was placed under − 80°C to obtain the ammonia solution of sodium. Then, the TaS 2 powder was added into the deep-blue solution with a molar ratio of TaS 2 :Na being 1:0.8. After a 10-20-min reaction, liquid ammonia evaporated and Na x (NH 3 ) y TaS 2 was available.
Fifty milligrams of Na x (NH 3 ) y TaS 2 was exposed in air to transfer to Na x (H 2 O) y TaS 2 , after this, the Na x (H 2 O) y TaS 2 was ground and added to 20 ml 0.2 g/ml aqueous glucose solution. The mixing solution was stirred for 5 min to disperse in the air and then bubbled with nitrogen to expel the oxygen for avoiding the oxidation. Na x (C 6 H 12 O 6 ) y TaS 2 can be separated from the reaction solution after about 2 h. The above dispersion was sealed and kept stirring for 6 h. Then, the solution was centrifuged at 12000 rpm for 30 min to obtain the product (marked as sample A) for further characterization. For contrast, part of the solution was standing for 1 h and the supernatant solution was centrifuged (marked as sample B). The water bubbled with nitrogen for more than 30 min (anaerobic water) could be utilized to wash off glucose and then alcohol was used to wash the sample to move water.
X-ray diffraction (XRD) patterns were recorded on a Philips X'pert Pro Super diffractometer with Cu Kα radiation at room temperature. XRD pattern of Na x (NH 3 ) y TaS 2 was measured in air-free capillary tube, differently, and all the other intercalated samples were measured in air. The scanning electron microscopy (SEM) images were taken on a JEOL JSM-6700F field emission scanning electron microscope (FESEM, 20 kV). Transmission electron microscopy (TEM) images were obtained on a JEOL-2010 with an accelerating voltage of 200 kV. Atomic force microscopy (AFM, DI Innova Multimode SPM platform) was used for detecting the thickness. Microscopic structure investigations were carried via high-angle annular dark field (HAADF) imaging mode in an advanced JEM-ARM 200F aberration-corrected high-resolution TEM facility.
Results and Discussion Figure 1 shows the schematic diagram of the multi-step intercalation reactions. The alkali metal-ammonia has reducibility and can be introduced simultaneously into the 1T-TaS 2 interlayer gallery to obtain Na x (NH 3 ) y TaS 2 within a short time. Na x (NH 3 ) y TaS 2 is not stable and easily transfers to Na x (H 2 O) y TaS 2 in water, along with the de-intercalation of Na + and the generation of hydrogen [33,34]. After transferring Na x (H 2 O) y TaS 2 into the aqueous glucose solution, Na x (C 6 H 12 O 6 ) y TaS 2 can form and be separated after 2 h. Then, after a total of 6 h, exfoliated TaS 2 forms eventually owing to the constant de-intercalation of Na + and the weak interaction between glucose molecule and TaS 2 layer. The flame reaction of reaction solution further confirms the deintercalation of Na + , and the reaction formulas are shown (Additional file 1: Figure S1).
It is found that full dispersal of Na x (H 2 O) y TaS 2 in glucose solution is a key factor to exfoliate TaS 2 . Additional file 1: Figure S2 shows the disperse status of Na x (H 2 O) y TaS 2 in aerobic water, i.e., operating in air, and anaerobic water, i.e., dispelling the oxygen of water, respectively. If oxygen is bubbled before adding Na x (H 2 O) y TaS 2 , it will never be dispersed in aqueous glucose solution and exfoliated TaS 2 cannot be obtained. In contrary, Na x (H 2 O) y TaS 2 can be fully dispersed in aerobic water, which may be due to the hydroxylation process of the layer edge [17]. Then, the full dispersal leads to the effectively exfoliation. All products can be centrifuged and used for the characterization.
XRD patterns (Fig. 2a) show the interlayer spacing changes of each stage. The 001 peak of pristine TaS 2 is shifted completely toward lower angles, indicating expansion of the lattice along the c-axis. The interlayer spacing of Na x (NH 3 ) y TaS 2 , Na x (H 2 O) y TaS 2 , and Na x (C 6 H 12 O 6 ) y TaS 2 are 9.1, 11.7, and 14 Å, respectively. Comparing with the 5.86 Å of TaS 2 , the interlayer spacing of Na x (C 6 H 12 O 6 ) y TaS 2 has increased by 8.1 Å, which would greatly weaken the Van der Waals' force of interlayer and is of advantage to the exfoliation of TaS 2 .
Figure 2b (A) shows the XRD pattern of sample A. The 001, 002, 100, 101, and 003 peaks are completely in accordance with 1T-TaS 2 . The sharp 100 peak, and wide and strong 00 l reflections indicate the ab extensional orientation of the nanosheets. The average thickness is estimated to be 3 nm by Scherrer equation based on the FWHM of 001 peak. Besides, The XRD pattern (Fig. 2b (B)) of sample B shows that the 001 peak vanishes completely, i.e., the period of c-axis disappears, meaning the formation TaS 2 monolayers. Besides, as the same structure, 1T-TiS 2 can also be exfoliated by our method. The XRD pattern of exfoliated TiS 2 is shown in Additional file 1: Figure S3. The average thickness of exfoliated TiS 2 is estimated to be 4 nm by Scherrer equation. This shows the potential of our method to expand into other TMDs.
Additional file 1: Figure S4 shows the Raman spectra of bulk TaS 2 and exfoliated nanosheets. There are two  Raman features that can be determined, which are E 1g and A 1g modes [35,36]. From bulk to nanosheet, the E 1g and A 1g modes show some shifts. These shifts are probably attributed to the decrease of the force constant resulted from the weakening of the interlayer Van der Waals force between layers. There is also a new mode of vibration which may come from structure changes. The FWHM of exfoliated TaS 2 is larger than that of bulk, which also indicates the decrease of thickness The FESEM images in Fig. 3a, b clearly reveal flakelike morphology with several micrometers in size. One remarkable result is the case that most of the TaS 2 sheets have micro-sized proportion. TEM images of Fig. 3c, d further show the morphology of the sample. Figure 4 shows the AFM analysis performed on samples which are deposited on muscovite substrate. We measured three sheets (0.45, 0.45, and 2.1 nm, respectively). The thickness of 0.45 nm is within the permissible range of the 001 interlayer spacing of 5.86 Å, and the polyhedron size of 2.93 Å formed by Ta and S atoms (more evidence can be seen in HADDF image of Additional file 1: Figure S5), which is the direct evidence of the formation of monolayer TaS 2 . Meanwhile, the 2.1-nm-thick nanosheet stands for three to four layers of TaS 2 .
Our approach can produce micro-sized sheets (maximum lateral size is 5 μm) and monolayer TaS 2 (minimum thickness is 0.45 nm). Ultrasound is not necessary and may suffer the size of the exfoliated nanoflakes. More details about the differences between different exfoliation techniques are shown in Additional file 1: Table S1.   We also carried out atomic structure investigations via HAADF imaging mode in an advanced aberrationcorrected high-resolution STEM facility. Figure 6a recorded at a few-layer TaS 2 sheet shows a 1T atomic structure which can be clearly seen in the HAADF image. The distance between two Ta atoms is 3.35 Å, and the FFT pattern shows the normal 2.9 Å spot of 100. To further explore the structure, we have utilized the Z-contrast in HAADF-STEM images (Fig. 6a). The contrasts contributed by S atoms distribute symmetrically along the AB line (Fig. 6c). The results are consistent with the octahedral coordination of S-Ta-S in 1T-TaS 2 structure (Additional file 1: Figure S6a) [37]. S atoms are symmetrically distributed around the Ta sites (Fig. 6c).
When we further characterized the monolayer nanosheet, an interesting phenomenon is found that the atomic distance between two Ta atoms from three directions are 3.4, 3.45, and 3.53 Å, respectively (Fig. 6b). The corresponding FFT pattern shows a distorted hexagon (2.93, 2.98, and 3.07 Å, respectively) which indicates the lattice distort in ab plane (Fig. 6b). The intensity profile recorded along CD line (Fig. 6e) differs from the case of few-layer TaS 2 (Fig. 6c). The contrasts contributed by S atoms distribute asymmetrically along the CD line, and its relative intensities are much stronger than that in  Fig. 6c, which means another S-Ta-S coordination in the monolayer structure. Note that there are two S-Ta-S coordinated situations in TaS 2 , the results are consistent with the 1H single layer structure with trigonal prism coordination of S-Ta-S (Additional file 1: Figure S6b) [38]. In this case, the S atoms are asymmetrically distributed around Ta atom along the CD line, and the overlap of two S atoms along c-axis (the electron beam direction) result in the enhancement of the signal from the S atoms (Fig. 6f ). Therefore, 1H structure with trigonal prism coordination of S-Ta-S in monolayer TaS 2 sheet is proposed. Such a single layer structure may be unstable, and the lattice distortion would probably occur. This structure may be responsible for the new vibration mode in Raman result. Of course, the essence of lattice distortion in monolayer TaS 2 should be further explored. Meanwhile, different structural phases of TaS 2 sheets may be exploited for different device applications.

Conclusions
In summary, we have developed an efficient exfoliation method for the exfoliation of tantalum disulfide and obtained micron-sized monolayer to few-layer sheet. This is a pure chemical exfoliation method in which ultrasound is not necessary. Monolayer and few-layer sample have all been characterized in detail, and coordination change from octahedron to triangular prism is found on monolayer TaS 2 . This really warns everyone to pay attention to the point of property variation caused by structural changes in exploring the finite-size effect, while few-layer TaS 2 still keeps the 1T structure.
Briefly, the present work not only provides an efficient ultrasound-free strategy for synthesizing high-exfoliated functional nanosheets but also explores the structure of monolayer and few-layer TaS 2 . The present work will provide insights to better understanding 2D materials and their physical and chemical properties, and offer more opportunities for their applications.