We report a method in making transmission electron microscopy sample for both CVD-grown and exfoliated 2D materials without etching process, thus gentle to those 2D materials that are sensitive to water and reactive etchants. Large-scale WS2 monolayer grown on glass, NbS2 atomic layers grown on exfoliated h-BN flakes, and water-sensitive exfoliated TiS2 flakes are given as representative examples. We show that the as-transferred samples not only retain excellent structural integrity down to atomic scale but also have little oxidations, presumably due to the minimum contact with water/etchants. This method paves the way for atomic scale structural and chemical investigations in sensitive 2D materials.

The rich electronic properties in the family of two-dimensional (2D) materials make them attractive for fundamental research. Besides the well-studied semi-metallic graphene and semiconducting monolayer MoS2 for nano-electronic applications,1–4 an emerging new group of monolayer materials, in which the group IV and V transition metal serves as cations such as TiS2 and NbSe2, maintains novel fundamental physical phenomenon like charge-density-wave (CDW) and superconducting transition when cooled down to a critical temperature.5–7 Transmission electron microscopy (TEM) or scanning TEM (STEM) serve as one of the most important techniques in characterizing the atomic structure of the 2D materials, including the pristine lattice and complicated defect structures within the monolayer matrix.2,8 In order to achieve high resolution images, the grown or exfoliated monolayer flakes have to be transferred in free-standing configuration onto the TEM grid. Furthermore, methods in making the atom-thin flakes to be free-standing are important not only for TEM sample preparation but also for free-standing monolayer device fabrication, such as monolayer piezoelectricity devices9,10 and electromechanical resonator.11 However, conventional methods typically involve the wet etching process of the underlying substrate, which requires the use of toxic (highly reactive) chemicals.9,12–14 Besides the toxicity which is environmental unfriendly and needs to be handled carefully, these reactive chemicals may also facilitate the oxidation of some unstable 2D materials and lead to their quality degradation. To avoid the etching process, polydimethylsiloxane (PDMS) has been proposed as an effective supporting scaffold in many dry-transfer methods for transferring 2D materials, including graphene and transition-metal dichalcogenide monolayers, between different substrates and for layer-by-layer device fabrications.15–19 However, the current existing dry transfer techniques are not applicable in free-standing TEM sample preparations due to the fragile supporting layer on the delicate TEM grid. Therefore, a facile and gentle free-standing TEM sample preparation method is highly desired, especially for the water- and air-sensitive materials in the 2D material family.

Here, we reported a facile and promising free-standing TEM sample preparation method without the use of any etching agent such as acid/alkali. This method uses the poly(bisphenol A carbonate) (abbreviated as PC) coating on the monolayer flakes which can be directly lifted up by adhesive PDMS sheet. The PC coating film can be picked up on the PDMS film in a droplet of isopropyl alcohol (IPA) which reduces the adhesive force between the PDMS and the PC coating film. We examined the as-transferred samples made from large-scale WS2 monolayer grown on glass and NbS2 atomic layers grown on h-BN flakes exfoliated onto SiO2/Si wafer by high resolution scanning transmission electron microscopy (STEM). Both of the samples show intact lattice structures without any obvious impurities or structural damage due to the transfer process. Furthermore, since the method minimizes the use of water in all the procedures, we successfully demonstrate for the first time that non-oxidized TiS2 atomic layers with sharp edges, a well-known water (or humidity) sensitive 2D material, are observable, as evidenced by electron energy loss spectrum. These results may open a new way for the fabrication of free-standing monolayer device made of environment-sensitive novel 2D materials as well as the high quality TEM/STEM characterizations.

Figure 1 shows the schematic of the developed method step-by-step along with the representative optical photos. First of all, the substrate (in the photo it is SiO2/Si) with the targeted atomic layers on top is spin-coated with a thin layer of PC film, where the PC grains are dissolved in chloroform with 5% concentration beforehand,20 as shown in Figs. 1(a) and 1(b). The thickness of the PC coating film varies with the rotational speed and coating time of the spin coater, which is around 23μm in our case according to the standard recipe. We then carefully removed the extra coating film around the corner of the transferred region by scratching it with tweezers, leaving a region that is slightly larger than a typical TEM grid, as shown in Fig. 1(c). We noticed that removing the coating film around the edges of the substrate increases the film integrity during the lift up process by PDMS.17 A PDMS thin sheet is then pressed onto the modified coating region with moderate pressure for several minutes in order to fully adhere to the underlying PC film. Since the flakes are atomic layers attaching to the substrate through weak van der Waals interactions, most of the flakes can be easily peeled off the substrate along with the coating film on PDMS, as illustrated in Fig. 1(d). Now the targeted flakes are sitting on top of the PC coating film which adheres to the PDMS sheet, and then the next step is to “fish” the coating PC film by a TEM grid. This is fulfilled by placing a TEM grid onto the PC film with a carbon supporting layer (in this case a Mo TEM grid with carbon quantifoil as an example, but should be applicable to other types of grids with perforated supporting layer) facing towards the sample (Fig. 1(e)). Then we drop cast a droplet of IPA onto the TEM grid. Since IPA, a gentle polar organic solvent, dissolves little of the PC coating film and changes the surface adhesion of PDMS, the coating film with the TEM grid is therefore floating on the PDMS sheet spontaneously as shown in Fig. 1(f). Such configuration makes the TEM grid with the coating film be easily picked up with tweezers on the lateral side. It is important to pick up the TEM grid while the coating film is still floating in the IPA droplet, i.e., before the complete evaporation of the IPA droplet. Once the IPA evaporates, the coating film will adhere back to the PDMS again, which is hard to preserve the integrity of the coating film and the TEM grid if picking it up at this stage. Finally, we “fish” the coating film out of the PDMS sheet with the TEM grid and then dry it in a vacuum chamber after several rounds of rinsing using IPA solvent, as shown in Fig. 1(g). The evaporation of the IPA droplet makes the PC coating film to fully stick to the carbon supporting layer on the TEM grid, as reported in the previous literature.21 The PDMS sheets are, therefore, reusable. The dried TEM sample is then immersed in chloroform for 30 min to wash away the PC coating film, then dried and stored in a vacuum chamber for further characterizations.

FIG. 1.

Step-by-step schematic and optical images of the transfer method. ((a)–(g)) Cartoon schematics and representative optical images in each stage of the transfer process. (a) Substrate with 2D material flakes on top. (b) Sample after poly(bisphenol A carbonate) (PC) coating. (c) Sample after the extra coating PC film being scratched which leaves a region around the targeted transfer region. (d) Direct peeling off the PC coating film by PDMS. (e) TEM grid is placed down on the PC coating film with the carbon supporting layer facing the sample. (f) TEM grid sitting on top of the PC coating film in the droplet of IPA. (g) Dried TEM grid sample with the PC coating film sticking after being fished by tweezers.

FIG. 1.

Step-by-step schematic and optical images of the transfer method. ((a)–(g)) Cartoon schematics and representative optical images in each stage of the transfer process. (a) Substrate with 2D material flakes on top. (b) Sample after poly(bisphenol A carbonate) (PC) coating. (c) Sample after the extra coating PC film being scratched which leaves a region around the targeted transfer region. (d) Direct peeling off the PC coating film by PDMS. (e) TEM grid is placed down on the PC coating film with the carbon supporting layer facing the sample. (f) TEM grid sitting on top of the PC coating film in the droplet of IPA. (g) Dried TEM grid sample with the PC coating film sticking after being fished by tweezers.

Close modal

Figure 2 shows optical images of two examples that we transferred using this method. Figure 2(a) displays large-scale NbS2 atomic layers grown on top of exfoliated h-BN flakes on SiO2/Si wafer, in which part of the NbS2 was grown directly on the wafer substrate. Figure 2(b) shows the PC coating film with the NbS2/h-BN flakes on top after peeling off the Si wafer by PDMS. One can see that most of the flakes were successfully isolated from the substrate. The final TEM product after all the procedures described in Fig. 1 is shown in Fig. 2(c). Most of the quantifoil carbon layer remained intact and the NbS2/h-BN flakes were successfully transferred onto the TEM grid. Figs. 2(d)2(f) show another example of transferring large scale monolayer WS2 grown directly on a glass via CVD method. Figures 2(d) and 2(e) show the initial and final stage of the transfer process. No obvious difference in the shape of the monolayer was observed between the images, suggesting no massive structural damage introduced during the transfer. Figure 2(f) shows a magnified optical image of monolayer WS2 on the carbon quantifoil, confirming the sample as a free-standing configuration.

FIG. 2.

Optical images of two CVD-grown samples that are transferred by the developed method. ((a)–(c)) Optical images of the CVD-grown NbS2 atomic layers grown on exfoliated h-BN flakes on Si/SiO2 substrate (a), after transferred onto the PC film (b), and on the Mo TEM grid after washing the PC film (c). ((d)–(f)) Optical images of large-scale WS2 monolayers grown on glass (d), transferred to the Mo TEM grid using the developed method ((e) and (f)).

FIG. 2.

Optical images of two CVD-grown samples that are transferred by the developed method. ((a)–(c)) Optical images of the CVD-grown NbS2 atomic layers grown on exfoliated h-BN flakes on Si/SiO2 substrate (a), after transferred onto the PC film (b), and on the Mo TEM grid after washing the PC film (c). ((d)–(f)) Optical images of large-scale WS2 monolayers grown on glass (d), transferred to the Mo TEM grid using the developed method ((e) and (f)).

Close modal

The quality of the as-transferred samples was further characterized by STEM in atomic scale. Figure 3(a) shows a low magnified Z-contrast STEM image of monolayer WS2 on the quantifoil film, showing the flat and continuous morphology over a large scale. Figure 3(b) shows a magnified region of the continuous film, indicating no apparent structural defects where the hexagonal lattice alternating in bright (W column) and less bright (S2 column) spots is preserved (shown in inset). Electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (EDS) were further employed to examine the chemical composition in the transferred sample. The results are summarized in Fig. 3(c), where only characteristic peaks of W O2,3 (∼43 eV) and N6,7 edges (∼53.5 eV) and S L2,3 edge (∼165 eV) were found in the EELS. EDS is collected in a region much larger than EELS, which shows consistent results with EELS, while the Si signal may come from the residual contaminations since the monolayer was grown on the glass surface. Figures 3(d)3(f) show the results of the transferred NbS2 atomic layers grown on exfoliated h-BN. Vertical stacking structures of a monolayer NbS2 on top of multilayer h-BN are clearly visualized in Fig. 3(d). The NbS2 monolayer maintained lattice integrity in 1H phase (inset in Fig. 3(d)), a hexagon lattice with threefold symmetry. Similar EELS measurement was performed across a sharp step edge of the NbS2 monolayer, as indicated by the white dashed line in Fig. 3(d). We examined a wide spectral region in the EELS and only found the characteristic peaks of B-K, N-K S-L, and Nb-M edges, as indicated in Fig. 3(e). No other impurities (such as oxygen) were present which suggests that our newly developed etchant- and water-free transfer process can efficiently preclude the surface oxidation and contaminations. The chemical mapping for elemental identification shown in Fig. 3(f) along the line is consistent with the integrated intensity of the annual dark-field (ADF) image which confirms that the vertically stacked heterostructure model is preserved with little oxidation.

FIG. 3.

Structural characterizations of the two transferred samples. (a) Low-magnified STEM image of the CVD-grown WS2 monolayer. (b) Atomic resolution STEM image showing the structural integrity of the as-transferred film. The inset shows the hexagonal atomic arrangement of WS2 monolayer, where each atomic column can be clearly resolved. No apparent defects are seen. (c) EELS and EDS spectrum of the WS2 monolayer collected in the same region, which shows merely strong signal of W and S. No other impurity except Si is observed in the EDS but not in EELS, which may be due to the surrounding contaminations on the flakes. (d) Atomic resolution STEM image of monolayer NbS2 on h-BN flakes, showing the vertically stacked heterostructure with excellent structural integrity. Inset: magnified image of the region highlighted by red square showing the pristine lattice of NbS2. (e) EEL spectra collected along the highlighted dashed line shown in (d), confirming the chemical composition of the flakes with little oxidation. (f) Chemical distribution profile along the dashed line shown in (a).

FIG. 3.

Structural characterizations of the two transferred samples. (a) Low-magnified STEM image of the CVD-grown WS2 monolayer. (b) Atomic resolution STEM image showing the structural integrity of the as-transferred film. The inset shows the hexagonal atomic arrangement of WS2 monolayer, where each atomic column can be clearly resolved. No apparent defects are seen. (c) EELS and EDS spectrum of the WS2 monolayer collected in the same region, which shows merely strong signal of W and S. No other impurity except Si is observed in the EDS but not in EELS, which may be due to the surrounding contaminations on the flakes. (d) Atomic resolution STEM image of monolayer NbS2 on h-BN flakes, showing the vertically stacked heterostructure with excellent structural integrity. Inset: magnified image of the region highlighted by red square showing the pristine lattice of NbS2. (e) EEL spectra collected along the highlighted dashed line shown in (d), confirming the chemical composition of the flakes with little oxidation. (f) Chemical distribution profile along the dashed line shown in (a).

Close modal

Since our method has the minimum use of water and acid/alkali during the transfer, it also helps to reduce the damage to 2D materials which are sensitive to humidity or reactive chemicals during the transfer process. We used exfoliated TiS2 atomic layers to demonstrate the effectiveness of our method. The bulk TiS2 is metallic in the 1T phase, which is proposed to be a promising candidate for the anode materials for high energy density in battery research.22 However, bulk TiS2 is also well-known to be unstable in ambient conditions, not to mention its monolayer form, which would be quickly oxidized presumably due to the oxygen and water vapor in the air.7,23 In order to fully develop the potential use of TiS2 for any single-layer device, transferring the intact quality of 2D crystal and direct observing its pristine lattice structure without oxidation are prerequisite.

We exfoliated fresh TiS2 atomic layers from the bulk crystal on Si/SiO2 wafer, which are then quickly transferred to the TEM grid using the developed method. We also transferred these fresh exfoliated TiS2 flakes onto the TEM grid using the conventional wet-etching method. Here, methods that involve the use of reactive chemical including all kinds of acid or alkali to remove the substrate are typical “conventional” methods. We presented the results based on the use of the hydrofluoric acid (HF) to etch away the underlying SiO2 layer as a controlled experiment. The results are summarized in Fig. 4. Fig 4(a) shows Z-contrast STEM images of typical TiS2 flakes transferred by our newly developed method. A large area of clean TiS2 layered structure with its intact monolayer at the edges (pointed by arrow) was clearly resolved with atomic precision (inset in Fig. 4(a)). Sharp S-L and Ti-L edges with no O signal in EELS shown in Fig. 4(b) prove that no oxidization occurs on the film during our newly developed transfer process. In contrast, the TiS2 flakes transferred by the conventional wet-etching method show disordered amorphous structure with severe oxidization (the bright clusters in Fig. 4(c)). Though some crystalline TiS2 flakes still remained after wet-etching, non-negligible oxygen signal (Fig. 4(d)) found on these flakes suggests that the oxidization starts from the surface or edge of the TiS2 atomic layers, which may be caused by the interaction with water or acid during the transfer. Besides, inevitable metal contamination such as calcium, a common impurity from water, was also found in the specimen prepared by using the conventional transfer method. These results indicate that this etchant- and water-free transfer method indeed helps to preserve the intrinsic lattice structure of monolayer TiS2 against oxidation and contamination.

FIG. 4.

Comparison of the oxidation of exfoliated TiS2 flakes using the as-developed and conventional wet-etching method. (a) Atomic resolution image of TiS2 atomic layers transferred without contact with water and acid. The intact lattice structure and sharp edges indicate that little oxidation occurs during the transfer process. Inset: magnified image of the region highlighted by red square with the overlaid atomic model, showing the pristine lattice of TiS2. (b) EEL spectra collected along the dashed line shown in (a). No oxygen signal is detected, indicating that the pristine edge structure of TiS2 atomic layers is preserved. (c) Atomic resolution image of the exfoliated TiS2 flake that transferred using conventional wet-etching method. Oxidation is found everywhere on the film and along the edge. (d) EEL spectra of the highlighted dashed line shown in (c). Strong oxygen signal is observed. The Ca signal may come from the water rinsing. Both samples show the carbon K edge (starting at 284 eV) in the spectra, which comes from the hydrocarbon contaminations that are commonly seen in thin TEM samples.

FIG. 4.

Comparison of the oxidation of exfoliated TiS2 flakes using the as-developed and conventional wet-etching method. (a) Atomic resolution image of TiS2 atomic layers transferred without contact with water and acid. The intact lattice structure and sharp edges indicate that little oxidation occurs during the transfer process. Inset: magnified image of the region highlighted by red square with the overlaid atomic model, showing the pristine lattice of TiS2. (b) EEL spectra collected along the dashed line shown in (a). No oxygen signal is detected, indicating that the pristine edge structure of TiS2 atomic layers is preserved. (c) Atomic resolution image of the exfoliated TiS2 flake that transferred using conventional wet-etching method. Oxidation is found everywhere on the film and along the edge. (d) EEL spectra of the highlighted dashed line shown in (c). Strong oxygen signal is observed. The Ca signal may come from the water rinsing. Both samples show the carbon K edge (starting at 284 eV) in the spectra, which comes from the hydrocarbon contaminations that are commonly seen in thin TEM samples.

Close modal

In conclusion, we develop a facile transfer method to make free-standing TEM samples of exfoliated and CVD-grown 2D materials without the degradation of crystal quality. This method does not involve the etching process and all major components in the transfer process are reusable, which can greatly reduce the cost of chemical consumption and is much more environmental friendly than the conventional wet-etching method. The structural integrity of the as-transferred samples is characterized by high resolution STEM to ensure the effectiveness of this method. Moreover, since the whole process did not involve acid/alkali and water, it also helps to reduce the oxidation to the water- or acid/alkali-sensitive 2D materials. This method will shed light on future exploration of free-standing monolayer device made of sensitive 2D materials and their high resolution structural TEM/STEM characterizations.

NbS2 synthesis. NbS2 monolayers and few-layers were synthesized by the reaction of NbCl5 (99.95%, Alfa Aesar) and S (99.98%, Sigma-Aldrich) in a CVD furnace. SiO2/Si with h-BN flakes was put in the downstream of the furnace. The reaction temperature was 650 °C at 75 Torr. Through all processes, 60 sccm Ar and 20 sccm H2 were used as carrier gases.

PDMS thin film preparation. The PDMS films are prepared from the Sylgard 184 silicone elastomer kit with the standard receipt. The base and the curing agent are mixed in 15:1 ratio. The thickness of the final PDMS films is around 1 mm.

STEM and EELS experiments. STEM imaging and EELS chemical analysis were performed by JEOL 2100F equipped with Delta correctors and GIF quantum spectrometer, operated at 60 kV. The inner and outer collection angles for the STEM image (β1 and β2) were 62 and 129–140 mrad, respectively, with a convergence semi-angle of 35 mrad. The Gatan Quantum GIF spectrometer was modified for low primary energy operation (15–60 keV) with high stability.

We thank Dr. Lain-Jong Li and Dr. Chien-Chi Tseng at King Abdullah University of Science and Technology (KAUST) for providing the WS2 monolayer sample. Authors from AIST acknowledge JST-ACCEL and JSPS KAKENHI JP16H06333 (J.L., Y.C.L., and K.S.) and P16382 (J.L. and K.S.) for financial support.

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