Article pubs.acs.org/JACS
Very Large-Sized Transition Metal Dichalcogenides Monolayers from Fast Exfoliation by Manual Shaking Jing Peng,†,§ Jiajing Wu,†,§ Xiaoting Li,†,‡ Yuan Zhou,† Zhi Yu,† Yuqiao Guo,† Junchi Wu,† Yue Lin,† Zejun Li,† Xiaojun Wu,†,‡ Changzheng Wu,*,† and Yi Xie† †
Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Hefei Science Center of Chinese Academy of Science (CAS), and CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei 230026, P.R. China ‡ CAS Key Lab of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science & Technology of China, Hefei 230026, P.R. China S Supporting Information *
ABSTRACT: For two-dimensional transition metal dichalcogenides (TMD) materials, achieving large size with high quality to provide a basis for the next generation of electronic device geometries has been a long-term need. Here, we demonstrate that, by only manual shaking within several seconds, very large-sized TMD monolayers that cover a wide range of group IVB-VIB transition metal sulfides and selenides can be efficiently harvested from intercalated single-crystal counterparts. Taking TaS2 as examples, monolayers up to unprecedented size (>100 μm) are obtained while maintaining high crystalline quality and the phase structure of the starting materials. Furthermore, benefiting from the gentle manual shaking, we unraveled the atomic-level correlation between the intercalated lattice-strain effects and exfoliated nanosheets, and that strong tensile strain usually led to very large sizes. This work helps to deepen the understanding of exfoliation mechanism and provides a powerful tool for producing large-sized and highquality TMD nanosheets appealing for further applications.
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INTRODUCTION Layered materials, especially transition metal dichalcogenides (TMD), with abundant electronic properties have attracted tremendous research interest and have been studied for a wide variety of applications, in which their thickness is reduced to the two-dimensional limit.1−5 In fact, two-dimensional (2D) TMD materials with large sizes and high quality offer an ideal opportunity to form a basis for next-generation electronic device geometries.6−9 However, despite great efforts, the production of large-sized and high-quality TMD nanomaterials is still a formidable challenge, especially through solution-phase routes. These routes are notable in the field of 2D nanomaterials because they allow for mass production of freestanding 2D nanostructures, thus allowing for diverse applicability in printable nanodevices and energy conversion and storage.10−17 Existing solution-phase exfoliation methods for TMD nanomaterials commonly involve violent mechanical driving forces, including sonication,10,18,19 shearing,12 and milling,13 to laminate layered bulk counterparts into nanomaterials. However, owing to the high-energy driving input, despite forming uniform dispersions, these methods still greatly suffer from disintegration into submicron-sized nanosheets. Generally, exfoliation can be considered as infinite swelling in layered materials.20 Intercalation of external ions or molecules is a crucial step during the production of large-sized nanosheets © 2017
in terms of solution exfoliation of the layered materials, with the help of significantly weakened neighboring layers arising from the interlayer expansion.20−24 For TMD materials, lithium atoms are universally developed to charge the TMD layer and be inserted into the interlayer spaces. However, lithiumintercalated TMD has only a small interlayer expansion of less than 10%, and as a result, assisted sonication is usually required for solution exfoliation.25−27 To exfoliate layered TMD materials, great efforts have been made to optimize the intercalation of external ions, molecules, and chemical reactions between the layers to effectively weaken the interlayer forces.28−31 However, demanding challenges still remain in realizing gigantic expansion in the precursors; hence, it has been difficult to achieve gentle exfoliation in TMD materials with large size. Here, we demonstrate a total single-crystal (TSC) exfoliation method for 2D TMD nanomaterials, which starts from pristine MX2 (M = group IVB-VIB, X = S, Se) single crystals, then generates lithium-intercalated single-crystals and subsequently exfoliated single-crystal nanosheets, successfully producing high-quality monolayers with very large size. Through deliberately controlling the lithium content, gigantic expansion (∼94%) in the LixMX2 single crystals was realized, enabling Published: June 12, 2017 9019
DOI: 10.1021/jacs.7b04332 J. Am. Chem. Soc. 2017, 139, 9019−9025
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Figure 1. Process of the TSC exfoliation. (a and b) Photographs of the 1T-TaS2 and LixTaS2 single crystals, respectively. (c) MX2 single crystals are partially lithiated to form LixMX2 crystals that can be fast exfoliated into single-layer nanosheets by only manual shaking. (d) Photograph of the TaS2 suspension. (e) XRD spectra of 1T-TaS2, Li0.5TaS2, and (Li,H2O)TaS2. The scale bars in (a) and (b) are 1 mm.
LixTaS2 (0 < x < 0.83) with various lithium concentrations was obtained (Figure S1 of the Supporting Information, SI). Notably, using only hand-shaking, the LixTaS2 single crystals were easily dispersed into a dense suspension and exfoliated into very large-sized TaS2 monolayer within several seconds (Figures 1c,d and S2 and Movie S1), especially when x was less than 0.55. The efficient and gentle exfoliation method prevented in-plane destruction and maintained the integrity of the in-plane layers, thereby maximizing the sizes of the single crystalline nanosheets to the submillimeter range. The structural characteristics of the intermediate states throughout the exfoliation process were investigated with X-ray diffraction (XRD) analysis. As seen in Figure 1e, the (001) facet of the 1T-TaS2 single crystals shifted significantly toward lower angles after insertion of lithium into the interlayer to form Li0.5TaS2, with the interlamellar spacing increasing from 5.87 to 6.41 nm. Notably, only an approximately 0.079° shift of the full-width at half-maximum of the (001) reflection from Li0.5TaS2 indicated that good crystallinity was retained in the lithium-intercalated TaS2 single crystals. The XRD pattern at the bottom of Figure 1e showed the Li0.5TaS2 results after submersion and removal from water, and additional peaks with large interlayer spacing of 11.36 and 8.68 nm were formed. Gigantic expansions (∼94%) due to hydration effects were expected to significantly weaken the interactions between neighboring TaS2 layers,32,33 thus offering feasibility for gentle manual shaking exfoliation routes, in contrast to other highenergy input driving forces, such as sonication. In contrast, from Figure S3, no expansion occurred in the case of LixTaS2 (x > 0.55) placed in an aqueous solution, and manual shaking did not work, thus demonstrating that the water molecules played a crucial role during manual shaking. We propose that, owing to the partial lithiation with x < 0.55, unoccupied spaces between the TaS2 layers would be occupied by the water molecules. As a consequence, the water intercalant not only tremendously increases the interlamellar distance of the TaS2 layers but also reacts with lithium and released hydrogen gas, thereby causing further expansion and fast exfoliation by simply using hand driving forces. Morphology of the Exfoliated TMD. Owing to the manual shaking in our TSC exfoliation, the lithium-intercalated
exfoliation by gentle driving force. As a result, by only simple manual shaking within several seconds, these expanded LixMX2 crystals were enabled to exfoliate into homogenetic monolayers with unprecedented submillimeter scale sizes and high crystallinity. Furthermore, benefiting from the gentle force, the lattice strain in the LixMX2 crystals was unraveled to strongly affect the size of the exfoliated nanosheets, showing that a decreased compressive strain was favorable for increasing their sizes. This method should be a powerful tool to effectively produce large-sized and high-quality nanosheets for designing electronic devices.
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RESULTS AND DISCUSSION TSC Exfoliation Using Manual Shaking. In our process, single crystals of transition metal dichalcogenides were used as hosts of lithium intercalants for further lamination. Of note, regarding top-down exfoliation routes, the predicted size of exfoliated nanosheets strongly depends on the size of the bulk materials, and thus, precursors with large crystalline sizes are a prerequisite for producing large-sized nanosheets. In fact, polycrystalline TMD powders have usually been used in previous chemical exfoliation techniques,25−27 despite of producing monolayer, resulting in the limited sizes of the exfoliated nanosheets. In addition, because of slow, thermodynamically driven lithium diffusion when organolithium is used as the lithiation reagent, long lithiation times are typically required before solution exfoliation. Here, through our TSC exfoliation, we determined that solvothermal reactions provide an effective approach to intercalate lithium atoms into single crystalline TMD materials readily and completely within 2 h, a much shorter time than that has been required for previously reported organolithium intercalation. Compared with conventional lithiation routes, such as refluxing, solvothermal lithiation efficiently and rapidly inserts lithium atoms into MX2 (M = group IVB, VB, VIB; X = S, Se) counterparts, despite the large crystal sizes up to 1 mm, thus leading to well-prepared and homogeneous LiMX2 single crystals for subsequent exfoliation. Taking 1T-TaS2 as an example, very large single-crystalline 1TTaS2, larger than one millimeter, was reacted with n-BuLi at 100 °C, and the color distinctly changed from golden to black brown, as seen in Figure 1a,b. By controlling the reaction time, 9020
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Figure 2. Morphology of the exfoliated TaS2 monolayers. (a−c) Typical optical images of the exfoliated TaS2 monolayers on SiO2/Si substrate. (d) Statistical analysis of the lateral sizes of the TaS2 monolayers.
TaS2 single crystals were efficiently exfoliated into TaS2 nanosheets with very large sizes. As seen in Figure 2a−c, high-coverage ultrathin TaS2 nanosheets with unprecedentedly large sizes ranging from several tens of micrometers to more than 100 μm were yielded, which had areas hundreds of times larger than the exfoliated flakes previously reported through solution routes.25−27,33 From the statistics shown in Figure 2d, an average size of 21.9 μm was produced, thus enabling promising applications for designing nanodevices. Atomic force microscopy (AFM) was used to characterize the thickness of the exfoliated TaS2 nanosheets. Figure 3a showed a large-area scan AFM image in tapping mode of a representative large area of a TaS2 nanosheet deposited on a Si/ SiO2 substrate. The topographic height was approximately 0.85 nm, which was consistent with the height of a monolayer of TaS2.34 Moreover, the statistical analysis shown in Figures S4− S6 revealed that 90% of the exfoliated TaS2 nanosheets were single layer, demonstrating homogeneous single-layer thickness by manual shaking exfoliation. Raman mapping was applied to further identify the structural characteristics of the exfoliated monolayer. According to Figure 3b, the Raman spectra of our exfoliated monolayers corresponded well with the 1T phase structure, indicating phase retention of bulk TaS2 arising from partial lithiation33 (the comparison of excess lithiation can be seen in Figure S7). For 1T-TaS2, the in-plane E1g and E12g peaks occurred in the vicinity of 240 and 303 cm−1, respectively, and the out-of-plane A1g peak appeared at 374 cm−1.35 Notably, compared with its optical morphology in Figure 3c, a peak intensity map of the out-of-plane A1g mode, as an initial assessment of the homogeneity of the thickness, was presented in Figure 3d. The highly uniform intensity distribution of the whole nanosheet indicated the uniform 1T structure and the presence of a single layer throughout the entire nanosheet. Transmission electron microscopy (TEM) and corresponding selected-area electron diffraction (SAED) measurements were performed to verify the microscopic quality of the exfoliated nanosheets, and the pattern displayed in the inset of Figure 3e indicated only one set of 6-fold rotational symmetric diffraction spots, which was responsible for a single-crystalline hexagonal structure with large domain size up to several tens microns. Furthermore, high-angle annular dark field (HAADF) TEM provided more structural details. From Figure 3f, a homogeneous and almost defect-free structure was revealed across the
Figure 3. Homogeneity of the large nanosheets. (a) AFM image of an exfoliated single layer of TaS2. (b) Raman spectra of the TaS2 monolayer deposited on Si/SiO2 substrate (red line) and bulk crystal (black line), with distinct peaks of in-plane E1g and E12g mode and outof-plane A1g mode. (c) Optical image of an individual single layer nanosheet. (d) Raman mapping (out-of-plane A1g mode) of the single layer nanosheet. (e) TEM image of a TaS2 nanosheet. The inset is the corresponding SAED pattern. (f) HAADF images of a TaS2 nanosheet. The inset is the magnified image.
entire single-crystal domain, further confirming the high quality of the sheets. Moreover, the TaS2 was clearly stacked in a triangular lattice projection, which corresponded to the 1T structure36 (inset of Figure 3f and S8) and suggested that the phase is retained during our TSC exfoliation method. Thus, the 9021
DOI: 10.1021/jacs.7b04332 J. Am. Chem. Soc. 2017, 139, 9019−9025
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Journal of the American Chemical Society above results indicated that the single-layer nanosheets, which originated from their single-crystalline bulk counterparts through hand shaking, were of high quality and had very large sizes. Importantly, the TSC exfoliation can be adapted to a wide range of group IVB-VIB transition metal sulfides and selenides (Figures 4, S9, and S10), such as HfS2, NbSe2, and TiSe2.
Figure 5. Periodic lateral size distribution of TSC method exfoliated nanosheets. (a) and (b) ⟨L⟩ distributions of 13 kinds of TMD materials by the periodic law. Figure 4. Morphology of the exfoliated TMD nanosheets.(a−c) Typical optical images of the HfS2, NbSe2 and TiSe2 nanosheets, respectively. The corresponding suspensions are shown in the insets of (a−c).
Figure 4 displays optical images of the HfS2, NbSe2, and TiSe2 nanosheets. All of these species were exfoliated by manual shaking within several seconds and exhibited large sizes with lateral dimensions from several to tens of microns. In addition, our TSC method formed high concentrations of dispersed suspensions, demonstrating the potential for scalable production. And therefore, the general method shows promising feasibility for promoting the production of large-sized monolayers through solution-phase routes. Mechanism of TSC Exfoliation Method. Accordingly, liquid exfoliation is usually affected by the surface tension of the solvent with the assistance of sonication, in which appropriately matched solvent and sonication conditions are critical to optimize and enlarge the size of the nanosheets.10,37,38 In fact, even with well-controlled experimental conditions, fragmentation of the in-plane layers is usually inevitable, and the exfoliated sizes in the various layered precursors are indistinguishable, owing to the high-input energy of the driving forces, such as sonication. In our case, gentle manual shaking with very weak hand driving forces enabled in-plane structural integrity and formed large-sized TMD nanosheets. As a result, in the case of exfoliating a series of TMD materials, our gentle method produced distinguishable average lateral size (⟨L⟩) distributions. The size differences provided evidence that the internal factors of the material itself determine the final lateral sizes of the exfoliated monolayers. Specifically, by optimizing the lithiation conditions, the optimal monolayer size distributions of 13 kinds of TMD materials were summarized and presented in Figure 5. Intriguingly, the ⟨L⟩ decreased with the metal atom along each row of the periodic table and enhanced along each column. And the significant variations in the ⟨L⟩ of the different TMDs were observed, with sizes ranging from several tens of micrometers to less than one micrometer. Thus, the ⟨L⟩ was unraveled to be dependent on the atomic properties in the periodic table. To understand the lateral size correlation within the periodic table, the lithium-intercalated compounds were systematically characterized using scanning electron microscopy (SEM). As presented in Figure 6 and S11, typical microscopic structures were formed on the flat surfaces of all the TMDs after inserted with lithium atoms. We observed cracks formed on the flat substrate surfaces of HfS2 and TaS2 (Figure 6a,b), with layered crystal domain sizes of several tens of microns; in contrast,
Figure 6. Microscopic structures of lithium intercalated TMD crystals. SEM images of (a) HfS2, (b) TaS2, (c) NbSe2, and (d) MoS2 after liuthium intercalation.
wrinkles grew on the crystal surfaces of NbSe2 and MoS2 (Figure 6c,d) and were arranged in a meshed network. These wrinkles formed dense microstructures with a much smaller mesh size. The formation of cracks or wrinkles is known to be a sign of tensile or compressive strain, respectively, in the MX2 lattice because the lattice parameters of the intercalated layers changed owing to the charge transfer from the intercalant to TMD layers.39,40 Therefore, the different microscopic structures reflected the distinct types of lattice strain that existed in these TMD materials. Intriguingly, from statistics analysis as shown in Figure 7a, the sizes of the exfoliated monolayers showed a proportional relationship with the domain or mesh sizes in the corresponding LixMX2 crystals, manifesting a close relationship between the lattice strains and the sizes of the monolayers. Indeed, during the exfoliation process, because the hydration effect in LixMX2 resulted in large degrees of swelling and released gas to further expand the interlayer spacing, the inplane structure in the MX2 layers disintegrated along with cracks or wrinkles. Therefore, the specific size of the TMD was strongly correlated with the domain or mesh size in the LxMX2 crystal. First-principles calculations based on density function theory (DFT) were performed in the Vienna Ab Initio Simulation 9022
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CONCLUSIONS
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EXPERIMENTAL SECTION
We report a TSC exfoliation method for 2D TMD nanomaterials that cover a wide range of group IVB−VIB transition metal sulfides and selenides and successfully harvests highquality monolayers with large sizes reaching the submillimeter scale. Through regulating the lithium insertion ratio, the watermolecular assisted swelling enabled lattice-strain controlled TMD nanosheet exfoliation. By only slight manual shaking for a very short time, the amount of large-sized high-quality monolayers were produced that retained the structural phase of the starting material, exhibiting promising feasibility for the production of large-sized monolayer through solution-phase routes. Furthermore, we concluded that the lattice-strain effects in the LixMX2 crystals strongly affected the exfoliated nanosheet size, and that weak compressive strains usually led to very large sizes. We expect the TSC exfoliation by gentle manual shaking to provide a powerful route to effectively produce large-sized and high-quality nanosheets for designing electronic devices. Figure 7. Strain modulation toward lateral sizes of exfoliated monolayers. (a) The relationship between the mesh (domain) size and strain in the LixMX2 crystal and ⟨L⟩ in the corresponding monolayers. The blue line shows the mesh (domain) size variations. The red line shows ⟨L⟩ distributions, and the dark line presents the calculated lattice strain in their lithium-intercalated counterparts. The compressive strain is defined by positive values and the tensile strain by negative values. (b) Schematic model of the microscopic structures in the LixMX2 surface. (c) The surface distortion is illustrated for the case when lithium atoms are inserted into the TMD, and tensile (compressive) strain is generated due to lattice shrinkage (expansion). The dashed models represent the two-dimensional structure after treatment due to lithium insertion.
Preparation of LixMX2 Single Crystals. TMD single crystals with sizes up to 1 mm were synthesized by chemical vapor transport.41 The crystals mixed with 1.6 M n-BuLi in hexane were then sealed in a quartz-lined autoclave and kept at 100 °C for 30 min to 2 h. In order to prevent n-BuLi from being oxidized, the whole processes were put into an argon-filled atmosphere. It can be observed that the lithium intercalated products are well separated from solvent after reaction. The LixMX2 products were then washed with n-hexane several times and dried by Ar gas, and the excess of n-BuLi was consumed by ethyl alcohol. Exfoliation of LixMX2 Single Crystals. Distilled water was added to the LixMX2 crystals, just by slightly manual shaking, the mixture was formed into a uniform dispersion. Then, the dispersion was centrifuged at 1000 r.p.m. for 10 min to remove the very thick nanosheets and at 8000 r.p.m. for 10 min to remove excess impurity. The sediment was finally redispersed into aqueous solution for other applications. Sample Characterizations. XRD patterns were measured using a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.541 78 Å). The optical images were obtained on an Olympus BX51M. Raman spectra and mapping were recorded at room temperature with a Renishaw Raman System, of which the excitation wavelength is 532 nm. AFM was conducted by AFM (Bruker, Demension Icon) using contact mode. TEM and HAADF were obtained on a JEM 2100F transmission electron microscopes (200 kV, field-emission gun) equipped with an Oxford INCA x-sight EDS Si (Li) detector. The SEM images were taken on a JEOL JSM-6700F SEM. DFT Calculations. The density functional theory (DFT) simulations of all structures were implemented by the Vienna ab initio simulation package (VASP).42 To describe the electron−ion interactions, the projector augmented wave (PAW)43 method was used. Electron exchange and correlations were treated with the generalized gradient approximation (GGA) in Perdew, Burke and Ernzerhof (PBE) functional.44The convergences of force per atom and total energy for optimizations were set to 0.01 eV/Å and 10−6 eV, respectively. The kinetic cutoff energy with Plane wave basis set was 500 eV. The K-point grids setting with Brillouin zone integrations are 12 × 12 × 6 and 12 × 12 × 3 for the tetragonal and hexagonal symmetry cell, respectively. Due to the effect of van der Waals force in the bulk phase, the DFT-D3 method is applied to the whole calculation.45
Package (VASP) to stimulate the lattice variations and corresponding strains of the TMDs when inserted with lithium atoms. As shown in Table S1, the calculation revealed that lattice distortions along the in-plane direction occurred in the LiMX2 materials and changed the lattice parameter. Owing to the in-plane structural shrinkage (expansion), tensile (compressive) stresses were generated and further resulted in cracks (wrinkles) on the MX2 layers, as shown in Figure 7b,c. For example, lattice parameter a of HfS2 decreased from 3.60 to 3.49 Å, and a 2.97% tensile strain was induced as the lithium intercalated, thus resulting in evident cracks in the HfS2 layers. In MoS2, the value of parameter “a” increased by 0.105 Å with a corresponding compressive strain of 3.33%, which was accompanied by a dense wrinkle network. As seen in Figure 7a, the ⟨L⟩ of the monolayers exhibited an inversely proportional dependence on the compressive strain value, thus demonstrating that the exfoliated monolayer size was largely determined by the lattice strain. According to Figure S12, the sizes of the monolayer from our TSC exfoliation method were nearly consistent with the different Δa values of the 13 kinds of TMD materials used during the intercalation process. These results further demonstrated that the lattice strains induced by the structural distortions of the LixMX2 crystals played an essential role in the periodic changes of the exfoliated monolayer size. Therefore, larger compressive strains in the lithium-intercalated compounds usually led to smallersized monolayer, and the largest ⟨L⟩ was obtained with the largest tensile strain. 9023
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b04332. XRD patterns, optical images, exfoliation process, thickness statistics, strain parameter and additional structural variation data (PDF) Movie S1 (AVI)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Xiaojun Wu: 0000-0003-3606-1211 Changzheng Wu: 0000-0002-4416-6358 Yi Xie: 0000-0002-1416-5557 Author Contributions §
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Prof. Yong Ni at USTC for valuable advice. This work was financially supported by the National Basic Research Program of China (2015CB932302), the National Natural Science Foundation of China (U1432133, 11621063, and J1030412), National Young Top-Notch Talent Support Program, the Chinese Academy of Sciences (XDB01020300), the Fok Ying-Tong Education Foundation, China (Grant Nos. 141042 and 151008), the China Postdoctoral Science Foundation (Grant No. 2016M600483) and the Fundamental Research Funds for the Central Universities (WK2060190027 and WK2340000065). This work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication.
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DOI: 10.1021/jacs.7b04332 J. Am. Chem. Soc. 2017, 139, 9019−9025
