Aggregation‐Induced Emission Luminogens for Direct Exfoliation of 2D Layered Materials in Ethanol

Aggregation‐induced emission (AIE) luminogens are an important type of advanced functional materials with fantastic optical properties and have found potential applications in organic electronics, biochemistry, and molecular imaging. Herein, this article presents a novel application of AIE luminogens (AIEgens) for efficient exfoliation of layered transition metal dichalcogenides (TMDs, such as MoS2 and WSe2). From the 1H NMR spectroscopic analysis, the designed AIEgens can insert into the space between layers of MoS2 in ethanol solution and the dynamic molecular rotation against the weak interactions affords large‐scale few‐layer MoS2 nanosheets (7–8 layers) with enhanced smoothness. The 3D AIEgens play a significant role in preserving the crystal lattice of MoS2 even at high pressure (>15 GPa). More importantly, the new approach can also be used for exfoliation of WSe2 to achieve large‐scale few‐layer nanosheets. The present work thus provides a facile and high yielding synthetic method for accessing on a large scale 2D layered materials with enhanced properties for high‐technology applications.

Besides solvent-assisted exfoliation techniques, additiveassisted techniques have been widely used for the exfoliation and dispersion of 2D materials in solution. This is typically employed for bulk transition metal dichalcogenides (TMDs) and graphite, and the important thing is that the additive@MoS 2 can be directly integrated for application without further treatment. [17] Huang and coworkers reported a facile and scalable method to achieve mono-and few-layer nanosheets of bulk transition metal dichalcogenides by employing poly(3,4-ethyle nedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), which can be utilized for high-performance organic solar cells as a hole extraction layer. [18] Furthermore, both the biomoleculeassisted and surfactant-assisted exfoliation of layered MoS 2 can play important roles in achieving stable dispersions of atomically 2D layered materials in water. [19,20] It has become clear that the solvent exhibits little effect on the exfoliation of layered MoS 2 in additive-assisted systems.
Tang discovered an abnormal optical phenomenon, known as aggregation-induced emission (AIE) in 2001, [21] where representative luminogens, such as tetraphenylethylene (TPE) and hexaphenylsilole (HPS), exhibited negligible emission in solution but enhanced emission in the solid state. Furthermore, the working mechanism for AIE can be visualized in terms of the restriction of intramolecular motion (RIM), in which the terminal phenyl ring groups were dynamically rotated against the central core as a rotor in solution, and the photonic energy transferred to thermal energy via a nonradiative process. In contrast, molecular rotation was restricted and thus it emitted fluorescence in the aggregation state. [22][23][24][25] This novel molecular rotation model based on AIEgens inspired us to develop potential applications in the 2D semiconductor field.
The 2D single-layer MoS 2 is bound by strong in-plane bonding (SMoS) with a thickness of 0.6-0.9 nm, [26] and the bulk MoS 2 is assembled by numbers of 2D single-layer MoS 2 via weak van der Waals forces with a considerable space between layers. [27] Zhang et al. reported that alkali metal, [28] alkali naphthalenide compounds [29] can insert into the interlayer spaces of the bulk TMDs for achieving mono-/fewer layer 2D TMDs. According to the RIM mechanism, is it possible that AIEgens also can insert into the interlayer spaces and break the weak intermolecular interactions and exfoliate the 2D layered material in solution through the molecular rotation process? With this in mind, a feasible and efficient AIEgens-assisted strategy for exfoliation of layers 2D layered materials is described herein. Indeed, our work indicates that the AIEgens play a crucial role in exfoliating layered MoS 2 and WSe 2 . Meanwhile, it can improve the yield on a large scale and afford high quality few layer MoS 2 /WSe 2 flakes. More importantly, the exfoliated MoS 2 containing the AIE was composed of a crystal lattice that was stable even at pressures as high as 15 GPa. We believe this methodology provides a strategy to develop a new generation of semiconductor devices for potential applications under hyperbaric environments.

Synthesis
According to the synthetic route shown in Figure 1, the compound TPENA (Figure 1a) was synthesized by the Suzuki-Miyaura coupling of 1,8-dibromonaphthalene and 4-(1,2,2-triphenylvinyl) phenylboronic acid under Pd-catalyzed conditions in 70% yield. The molecular structure of TPENA was fully characterized by 1 H/ 13 C NMR spectroscopy and high resolution mass spectrometry. The TPENA exhibits good solubility in common organic solvents (such as dichloromethane, tetrahydrofuran (THF), and toluene), and considerable solubility in ethanol, but is insoluble in water. The optimized geometries of the TPENA molecule was performed by density functional theory (DFT) using Gaussian 09 program package. [30] The two TPE moieties were introduced at the 1,8-positions of the naphthalene ring giving a crowded space configuration, and both TPE moieties are near perpendicular to each other; one of the TPE groups is parallel to the naphthalene ring. The dimensions of the TPENA are ≈6.71 Å, ≈15.20 Å, and 4.96 to 11.58 Å along the x-, y-, and z-axes, respectively ( Figure S5, Supporting Information).

AIE Properties
The photoluminescence (PL) spectrum of TPENA was investigated in a mixture of THF/water using different water fractions (f w ), and was found to emit a weak blue emission peak at 458 nm in solution with low quantum yield (Φ f < 0.05%), as shown in Figure 2a. On increasing the water fraction to 60% (Figure 2b), the PL intensity only changed slightly. Further, the emission was enhanced by continuously increasing the fraction of water, and the maximum emission intensity observed was at f w < 99% with an 80-fold enhancement and much higher quantum yield (Φ f = 0.67) versus that in pure THF solution. Thus the TPENA is clearly exhibiting features associated with AIE ( Figure S6, Supporting Information).

Exfoliation of 2D Layered Materials
According to previous reports, ethanol has been widely exploited to exfoliate 2D layered materials. Herein, 1 mL ethanol was added to a mixture of MoS 2 powder (0.5 g) and TPENA (2.5 mg), which was further grinded in a mortar and pestle for 30 min. After grinding, the sample was sonicated in ethanol (20 mL) for 1 h followed by centrifuging twice at 5000 rpm min −1 . The suspension containing MoS 2 nanosheets was then collected and characterized by UV-vis, scanning electron microscope (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and Raman spectroscopy. For further comparison of the effect of the AIE molecule on the exfoliation of layered MoS 2 , we also prepared three control samples without the AIEgens-assisted technique or without grinding process and the detailed procedures are illustrated in Figure 3 and Figures S7-S17 (Supporting Information).
Generally, the size of MoS 2 , as determined using a dynamic light scattering (DLS) technique, would limit its scope of application in semiconductor devices. In our experiments, the AIEgenassisted exfoliated layered MoS 2 nanosheets possessed a narrow size distribution (polydispersity index 0.371-0.462). The respective size dimensions are 198.2, 175.9, and 130.9 nm for TPENA@MoS 2 , grinding@MoS 2 , and sonication@MoS 2 (Figure 3a1-a3).
The TEM and tapping-mode AFM provided visual information of the microscopic morphology and size of the exfoliated MoS 2 nanosheets. The structure of three samples of AIEgen-assisted, grinding-assisted and sonication-assisted exfoliated MoS 2 nanosheets were further characterized by TEM and AFM. The TEM images ( Figure 3b) show that the majority of the MoS 2 nanosheets are thin 2D layered flakes, and Figure 3b,c confirmed that the size of the exfoliated nanosheets vary from 400 nm to several micrometers depending on the experimental conditions employed. Besides, no clear selected area electron diffraction (SAED) was observed in AIEgenassisted grinding MoS 2 sample (TPENA@MoS 2 ). In particular, the TEM images revealed that the sample of MoS 2 nanosheets obtained via AIEgen (TPENA@MoS 2 ) are large (>400 nm) with transparent ultrathin smooth morphology, whilst in the absence of the AIEgen, the exfoliated nanosheets (grinding@ MoS 2 ) exhibited folded edges with multi-layer MoS 2 .
The thickness of the exfoliated nanosheets was confirmed by AFM and illustrated in Figure 3c. The AFM image revealed that the thickness is ≈4.45-6.79 nm for the AIEgen-assisted exfoliated MoS 2 nanosheets, 19.4-49.9 nm for grinding-assisted exfoliated MoS 2 nanosheets, and 21.2-74.5 nm for sonicationassisted exfoliated MoS 2 nanosheets, respectively. As we know, the thickness of the monolayer MoS 2 was a constant value of ≈0.6-0.9 nm. [26,31] The exact values of the thickness was fitted according to the multiple scans and histogram using AFM on MoS 2 , and the height distribution histogram shows that the corresponding thickness of the AIE-assisted exfoliated, grindingassisted and sonication-assisted exfoliated MoS 2 nanosheets were uniformly dispersed with average layers ranging from 7-8 layers, 38-40 layers, and 50-53 layers, respectively.
Thus, both the TEM and the AFM images indicate that the AIEgen is favorable for exfoliating the bulk MoS 2 into high quality ultrathin nanosheets. While in the absence of the AIE molecule, the thickness of the exfoliation sample was found to be more than 40 layers Figure S10B   www.advmatinterfaces.de note here that the AIEgen can be used for the effective exfoliation of layered MoS 2 in ethanol, to achieve single-or few-layered MoS 2 nanosheets.
The UV-vis absorption spectra of TPENA and the exfoliated suspension of MoS 2 flakes via the three methods (TPENA@MoS 2 , grinding@MoS 2 and sonication@MoS 2 ) were also recorded. For TPENA, a maximum absorption band was observed at 337 nm in dilute THF (10 × 10 −6 m), which is redshifted by ≈28 nm compared to naked TPE (λ abs = 309 nm) (see Figure S18 in the Supporting Information); [32] this can be attributed to the expanding π-conjugation of the naphthyl group. [33] Whereas the dispersion of MoS 2 nanosheets only exhibited a large red-shift of the broad and excitonic peak at 723 nm, which is attributed to the K point of the Brillouim zone. [34] The redshift of the absorption peak edge is related to the size of the nanosheets. [35,36] In addition, the TPENA exhibited no obvious absorption behaviour beyond 337 nm, indicating that the absorption band at 723 nm for TPENA will have a limited effect on MoS 2 . The concentration of dispersed material is confirmed using the Lambert-Beer law and the extinction coefficient at 600 nm α 600 = 2104 mL (mg m) −1 , [37] with the concentration of the three samples being 0.03, 0.024, and 0.015 mg mL −1 for TPENA@MoS 2 , grinding@MoS 2 and sonication@MoS 2 , respectively. Obviously, grinding can crush the bulk MoS 2 and improve the yield of nanosheets, but more importantly, the AIEgen-assisted additive technique plays a significant role to exfoliate and produce highly dispersible MoS 2 flakes in ethanol with high efficiency and via a readily scalable method. Moreover, in the presence of TPENA, the duration of sonication required was shorten by 8-48 times without any reduction in yield compared to the reported method. [38]

Possible Exfoliation Mechanism
To gain insight into the exfoliation process, the interaction between molecular AIE and MoS 2 was further investigated by 1 H NMR spectroscopy. The 1 H NMR spectra of TPENA and TPENA@MoS 2 are illustrated in Figure 4. The 1 H NMR spectra of TPENA in d-methanol exhibited three groups of overlapping peaks between 7.10-7.20 ppm (triplet peak at δ = 7.10 ppm, a doublet at δ = 7.14 ppm, and a doublet of doublets at δ = 7.20 ppm). Interestingly, the 1 H NMR spectra of dispersions of TPENA@MoS 2 are very different in d-methanol in comparison to that of pure TPENA. Although there are no additional resonances in the range 7.10-7.20 ppm, there are two distinct downfield chemical shifts for the doublet proton peak and multiple proton peak which appear at δ = 7.34 ppm at δ = 7.47 ppm, respectively. Although it is difficult to identify the proton peaks of TPENA, there is no doubt that the downfield shifts of the proton signals (δ = 7.34 ppm at δ = 7.47 ppm) are the result of de-shielding of the phenyl rings of TPENA by the large

www.advmatinterfaces.de
MoS 2 nanosheets. Moreover, one upfield shifted proton peak δ = 7.01 ppm is observed, which corresponds to the shielding of the proton peak of the TPE fragment.
Thus, we infer that the TPENA is well dispersed in MoS 2 with small content (0.5 g MoS 2 powder and 2.5 mg TPENA) in the whole grinding process, and the TPENA has more opportunities to intercalate into the space of pristine MOS 2 ; on the other hand, due to the concentration of TPENA in ethanol solution (20 mL) is very low (1.58 × 10 −4 mol L −1 ), the TPENA is almost dissolved to be a single molecular with free motion, which is easy to insert into the sandwiched structure of layered-MoS 2 . Based on RIM mechanism, the phenyl group can freely rotate in ethanol solution, combining to the optimized molecular structure (DFT/B3LYP/6-31G*) and the 1 H NMR spectroscopic experimental observations, it is reasonable to propose that the naphthanyl ring and one of TPE moiety of TPENA have inserted into the gap between layered MoS 2 with a spontaneous molecular motion, which would play a significant role in disrupting the weak intermolecular interactions and thereby exfoliating layered MoS 2 . The possible mechanism for exfoliate layered MoS 2 as exploited herein is depicted as a schematic in Figure 4B.
Furthermore, to verify the feasibility of the above-mentioned method, we attempt to exfoliate the layered WSe 2 by following the same experimental procedure. Theoretically, the thickness of the monolayer WSe 2 is similar to MoS 2 at 0.6-0.9 nm. [39] From the AFM images of TPENA@WSe 2 in Figure 5, we found that the thickness of the WSe 2 nanosheets ranged from 5.86 to 6.17 nm with regular sizes in the micron range, which indicated that the TPENA@WSe 2 flasks also consisted of 7-8 layers with a regular smoothness in the dispersion ( Figure S13, Supporting Information). The TPENA@WSe 2 nanosheets were further characterized by TEM images ( Figure 5B) and confirmed by elemental mapping images ( Figure 5D,E), scanning electron microscope (SEM) ( Figure S14, Supporting Information), as well as Raman spectra ( Figure S15, Supporting Information). While the controlled samples WSe 2 without the AIEgensassisted technique consist of multilayer structures. (Figure S16, Supporting Information) In addition, without grinding, the thickness of AIEgens-assisted exfoliated WSe 2 nanosheets is in a range from 12.11 to 23.98 nm, meaning the grinding process would help intercalation. (Figure S17, Supporting Information)

Pressure-Dependent Lattice Vibrational Properties
Raman spectroscopy as a nondestructive diagnostic tool is widely applied to examine the phonon vibrations and crystalline structure of MoS 2 . The mono-/few-layer MoS 2 exhibited two clear Raman peaks, which correspond to the interlayer vibrations (E 2g 1 mode) of the Mo and S atoms, and the interlayer vibrations (A 1g mode) of the SS atoms against the Mo atom, respectively. [40] More importantly, the number of layers of MoS 2 can be determined using Raman spectra via measuring the distance between the A 1g and E 2g modes. [41] The Raman spectra of the TPENA@MoS 2 nanosheet was measured and bands at 380.8 and 405.6 cm −1 were observed, indicative of few-layer MoS 2 which is consistent with the AFM results.
The electronic structure and lattice vibrational dynamics were investigated under high pressure by applying a hydrostatic pressure using a diamond anvil cell (DAC), with results shown in Figure 6. As the pressure increased from ambient to 25 GPa, both vibration modes were red-shifted to high wavelength from 380.8 to 397.4 cm −1 for the E 2g 1 mode, and 405.6 to 437.4 cm −1 for the A 1g mode with an increased intensity ratio of A 1g /E 2g . In addition, the intensity of the E 2g 1 mode almost disappeared (406.3 cm −1 ) under large hydrostatic pressures (>25 GPa), due to the effect of normal compressive strain on the TPENA@MoS 2 nanosheet. However, the A 1g mode remained dominant (456.0 cm −1 ). Without TPENA, the intensity of the E 2g 1 mode diminished somewhat when the hydrostatic pressures applied was up to 9 GPa ( Figure S19, Supporting Information). A possible explanation is that the structural distortion would occur under high pressure, while the TPENA molecules between the two layers of exfoliated MoS 2 play a significant role to preserve the crystal lattice of MoS 2 even at the high pressure.

Conclusion
In this work, we present an efficient approach to exfoliate layered MoS 2 /WSe 2 in the presence of AIEgen in pure ethanol. The obtained MoS 2 nanosheets exhibited a large-scale few-layer (7-8 layers). The AIE molecule TPENA plays a crucial role in improving the yield of exfoliating bulk MoS 2 over a shorter sonication time. The 1 H NMR spectra indicates that part of the TPENA molecule inserts into the gaps in layered MoS 2 , and the nature of the AIE molecule with its spontaneous molecular motion can disrupt the weak intermolecular interactions present in layered MoS 2 . In addition, the MoS 2 nanosheets containing TPENA play a significant role to preserve the crystal lattice of MoS 2 even under high pressure. Thus, this report presents a common approach for preparing 2D layered material (such as MoS 2 , WSe 2 ) nanosheets on a large scale and as thinlayered flakes and holds great potential for the utilization in strained semiconductor device applications, and some related work also is ongoing in our laboratory.

Experimental Section
Materials and Methods: Unless otherwise stated, all reagents used were purchased from commercial sources and were used without further purification. 1 H/ 13 C NMR spectra were recorded on an AVANCE III HD 400 MHz NMR spectrometer and referenced δ = 7.26 and 77.0 ppm for chloroform-D solvent with SiMe 4 as an internal reference, respectively; J-values are given in Hz. High-resolution mass spectra (HRMS) were taken on a TSQ Endura mass spectrometer operating in a MALDI-TOF mode. The surface morphologies of prepared sample were investigated using scanning electron microscopy (SEM, FEI Nova230) and transmission electron microscopy (TEM, FEI, Talos F200S), and the thickness of exfoliated nano-sheet MoS 2 was analyzed at atomic force microscopy (AFM, Bruker Dimension Edge SPM). Raman spectra were recorded at LabRAM HR 800 using a 532 nm visible-range laser.
Synthetic Procedures-Synthesis of 1,8-bis(4-(1,2,2-triphenylvinyl)phenyl) naphthalene (TPENA): A mixture of 1,8-dibromonaphthalene (0.51 mmol, 1.0 eq) and 4-(1,2,2-triphenylvinyl)phenylboronic acid (1.53 mmol, 3 eq) in toluene (15 mL) and ethanol (4 mL) at room temperature was stirred under argon, then potassium carbonate (2.04 mmol, 4.0 eq) and tetrakis(triphenylphosphine)palladium(0.051 mmol, 0.1 eq) were added. After the mixture was stirred for 30 min at room temperature under argon, the mixture was heated to 90 °C for 48 h with stirring. After cooling to room temperature, the mixture was quenched with water, extracted with CH 2 Cl 2 (3 × 100 mL), washed with water and brine. The organic extracts were dried with MgSO 4 and evaporated. The residue was purified by column chromatography eluting with (CH 2 Cl 2 /hexane) to give target compound. 1  Synthetic Procedures-Exfoliation of Layered TMDs: According to reported procedure, [14] (TPENA@MoS 2 ) the general procedure for exfoliation of layered bulky TMDs solids followed the procedure below: 1 mL ethanol was added into 0.5 g MoS 2 powder (Sigma-Aldrich, ≈6 µm) and 2.5 mg dye TPENA, the mixture was grinded in a mortar and pestle for 30 min. (Grinding@MoS 2 ) 1 mL ethanol was added into 0.5 g MoS 2 powder (Sigma-Aldrich, ≈6 µm), the mixture was grinded in a mortar and pestle for 30 min. (Sonication@MoS 2 ) 1 mL ethanol was added into 0.5 g MoS 2 powder without grinding and was kept at room temperature 30 min, then the solvent was evaporated. The three samples were then redispersed in 20 mL of ethanol and then sonicated for 1 h, while the temperature was maintained at 25°C by use of a water-cooled bath. To completely remove the bulk powder, firstly, the solution was centrifuged at 5000 rpm (Boeco, C-28A) for 1 h and the residue removed, the remaining liquid was re-centrifuged at 5000 rpm (Boeco, C-28A) for 1 h, and the supernatant, containing the exfoliated nano-sheet MoS 2 was collected.
Synthetic Procedures-Exfoliation of Layered WSe 2 : 1 mL ethanol was added to a mixture of WSe 2 powder (0.5 g) and TPENA (2.5 mg), which was further grinded in a mortar and pestle for 30 min. After grinding, the sample was sonicated in ethanol (20 mL) for 1 h followed by centrifuging twice at 5000 rpm min −1 for 1 h and the residue removed, the remaining liquid was re-centrifuged at 5000 rpm (Boeco, C-28A) for 1 h, and the supernatant, containing the exfoliated nano-sheet WSe 2 was collected.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.