Vapor Phase Growth and Imaging Stacking Order of Bilayer

Apr 9, 2014 - Various stacking patterns have been predicted in few-layer MoS2, strongly influencing its electronic properties. Bilayer MoS2 nanosheets...
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Vapor Phase Growth and Imaging Stacking Order of Bilayer Molybdenum Disulfide Shengxue Yang,*,† Jun Kang,† Qu Yue,‡ and Kun Yao§ †

State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China ‡ College of Science, National University of Defense Technology, Changsha 410073, China § State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Changchun, Jilin 130022, China S Supporting Information *

ABSTRACT: Various stacking patterns have been predicted in few-layer MoS2, strongly influencing its electronic properties. Bilayer MoS2 nanosheets have been synthesized by vapor phase growth. It is found that both A-B and A-A′ stacking configurations are present in bilayer MoS2 nanosheets through optical images, and the different stacking patterns exhibit distinctive line shapes in the Raman spectra. By theory calculation, it is also concluded that the A-B and A-A′ stacking are the most stable and lowest-energy stacking in the five predicted stacking patterns of bilayer MoS2 nanosheets, which proves the experimental observations. In bilayer MoS2, two MoS2 monolayers can form five different stacking structures (as shown in Figure 1). For MoS2 bilayers, two stable crystallographic configurations are predicted by theory computations: A-B and A-A′ stacking order.12 For the A-B stacking, two different atoms are superimposed and the Mo

M

olybdenum disulfide (MoS2), with a band gap in the range of 1.2−1.8 eV, is a typical layered two-dimensional (2D) transition-metal dichalcogenide semiconductor, whose physical properties are significantly dependent on the thickness.1−5 MoS2-based materials have stimulated intense interests because of their potential for novel applications.6−9 A single molecular layer of MoS2 consists of one layer of Mo atoms sandwiched between two layers of S atoms by covalent bonds packed in a hexagonal arrangement. The bonding between the adjacent S−Mo−S sheets held together by van der Waals interactions is weak, resulting in the quasi-2D character of MoS2, and the weak interlayer interactions allow single- or fewlayer MoS2 nanosheets to be created through a mechanical cleavage technique or be grown by chemical vapor deposition (CVD).10,11 In addition, the strong interlayer covalent bonds enable thermal stability of MoS2 crystals. In few-layer MoS2 nanosheets, different crystallographic stacking orders have been predicted to strongly influence the electronic properties of the MoS2 nanosheets, including the band structure, interlayer distance, and stability.12 2H-MoS2 is a semiconductor with the stacking sequence of BaB AbA, whereas the 1T phase is metallic with the stacking sequence of AbC, and the stacking sequence of the 3-R phase is AbA/BcB/CaC.13,14 Experimentally, both 2H and 1T phases that existed on a single layer of MoS2 (SLMoS2) are recently demonstrated by direct observation of highresolution scanning transmission electron microscopy (STEM) imaging of chemically exfoliated SL-MoS2.13 The two phases exhibit substantially different electronic structures due to the distinct lattice symmetries. By atomic resolution TEM, a typical 3-R stacking order is revealed in MoS2 fullerene.15 © 2014 American Chemical Society

Figure 1. Five typical stacking configurations of bilayer MoS2: (a) A-A, (b) A-A′, (c) A′-B, (d) A-B′, and (e) A-B configurations. Received: January 2, 2014 Revised: April 9, 2014 Published: April 9, 2014 9203

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atoms of the top MoS2 layer are above the hexagon centers of the bottom MoS2 layer. However, for the A-A′ stacking, all the S atoms in the top MoS2 layer are overlaid on the Mo atoms of the bottom MoS2 layer.16 A-A′ stacking order rather than A-B stacking has generally been presumed in most studies of exfoliated 2H-MoS2 materials, as this stacking order is believed to be slightly more thermodynamically stable than the A-B pattern.17,18 However, in the experiment, it does not directly observe these two stacking structures and prove that they are the two most stable forms in the five different predicted structures of bilayer MoS2. Raman spectra have been regarded as an effective method for spatial imaging of stacking order and determining many physical properties of MoS2 layers.19 Resonance Raman spectra show that the coupling between electronic transition and phonon is found to become weaker when the layer number of MoS2 decreases.20 The ultralow frequency interlayer breathing and shear modes in few-layer MoS2 have been uncovered by Raman signature, importantly implicating for both mechanical and electrical properties.17 In this communication, we synthesize bilayer MoS2 nanosheets by CVD technology and demonstrate that different stacking orders in bilayer MoS2 can be directly identified by way of optical imaging and Raman spectroscopy. We find that both A-B and A-A′ stacking configurations are present in bilayer MoS2 nanosheets through optical images, and the different stacking patterns exhibit distinctive line shapes in the Raman spectra. By theory calculation, we also conclude that the A-B and A-A′ stacking are the most stable and lowest-energy stacking in the five stacking patterns of bilayer MoS 2 nanosheets. As shown in Figure 1, bilayer MoS2 has five different stacking orders. In the A-A and A-A′ configurations (Figure 1a,b), two MoS2 monolayers are aligned. For the A-A stacking, all S atoms of the top monolayer are above the S atoms of the bottom monolayer. However, all S atoms are on top of Mo atoms in the A-A′ pattern, which is mostly studied by theory calculations.21,22 In the A′-B and A-B′ stacking orders, two MoS2 monolayers are reversed (Figure 1c,d). For the A′-B (A-B′) configuration, the Mo (S) atoms are overlaid, and the S (Mo) atoms in the top MoS2 layer lie above the hexagon centers of the bottom MoS2 layer. Figure 1e shows the stacking pattern of the A-B configuration.12,23 In our experiment, we prepared bilayer MoS2 samples by CVD technology using Si covered with a 300 nm thick oxide layer (SiO2/Si) as substrates. We used MoO3 and sulfur powder (S) instead of H2S as the sources, where the use of S was much safer than H2S.24 The reaction between MoO3 and S is given in eq 1 2MoO3 + 7S → 2MoS2 + 3SO2

Figure 2. Schematic diagrams of CVD growth process (a) and asprepared samples (b).

Another ceramic boat with S powder (1.2 g) was placed with a distance to the boat filled with MoO3 powder. Before arriving at the growth temperature, the boat with MoO3 powder was in the furnace and the boat containing S powder was out of the furnace. Then, the furnace was heated to 680 °C in a N2 environment. With the seeding of PTAS, MoO3 seeds were firstly grown on the substrates. When the temperature arrived at 680 °C, the boat with S powder was pushed into the furnace; then, the S began to melt and produced S vapor, resulting in the starting of the reaction. MoO3 was then reduced by the S vapor and formed MoO3−x. Subsequently, the MoO3−x reacted with S vapor and MoS2 monolayer films were grown on the substrates.26 After that, under the action of a proper amount of PTAS seeds, the second MoS2 monolayers continued to grow and covered the first monolayers to form the bilayer MoS2 nanosheets. A possible stepwise reaction process of MoO3 and S is given in eqs 2 and 328 MoO3 + x /2S → MoO3 − x + x /2SO2

(2)

MoO3 − x + (7 − x)/2S → MoS2 + (3 − x)/2SO2

(3)

After the growth, the reaction tube was rapidly cooled down.29 A schematic diagram of as-prepared samples is shown in Figure 2b. We first examined the samples by optical microscope. At the edge of SiO2/Si substrates, we find the forward and reversed stacking of bilayer triangle MoS2 nanosheets at the same time (Figure 3a,c), and the top MoS2 monolayer is a small triangle nucleus that just grew. We propose that the forward stacking of bilayer MoS2 is a A-B stacking pattern and the reversed stacking of the bilayer sheet is A-A′ stacking. When the top small triangle continues to grow, the forward stacking MoS 2 nanosheet turns out to be a large bilayer triangle sheet and the reversed stacking MoS2 nanosheet changes to a snowshaped hexagonal bilayer sheet, as shown in Figure 3b,d. Then, we measured the atomic force microscope (AFM) of the bilayer MoS2 nanosheets shown in Figure 3a,c. This technique permits accurate determination of layer thickness of the MoS2 samples, according to the AFM images; we also observe the bilayer

(1)

The substrates were first treated with piranha solution (sulfuric acid and hydrogen peroxide). Figure 2a illustrates the schematic diagram of the experimental setup. Prior to the growth, a droplet of perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) solution was spin-coated on the SiO2/Si substrates, followed by blow-drying with nitrogen (N2). The substrates pretreated with PTAS could promote the MoS2 layer growth, where the presence of PTAS possibly increased the surface adhesive force of MoS2, and PTAS molecular aggregates acted as the seeds and offered the heterogeneous nucleation sites for the formation of MoS2 nuclei.25−27 The MoO3 powder (0.1 g) was uniformly tiled in a ceramic boat, and the substrates were put on the top of the boat with their surface facing down. 9204

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Figure 3. Optical images of forward (A-B) (a, b) and reverse (A-A′) (c, d) stacking of bilayer triangle MoS2 sheets. (e) AFM image of forward (A-B) stacking of bilayer triangle MoS2 sheets. (f) A height profile along the dashed line in (e). (g) AFM image of reverse (A-A′) stacking of bilayer triangle MoS2 sheets. (h) A height profile along the dashed line in (g).

We then performed Raman measurements on the two patterns of bilayer stacking MoS2 nanosheets. Raman spectra were measured by using linearly polarized laser radiation at a wavelength of 532 nm with a spot size of the laser beam ∼ 1 μm, which was focused on the MoS2 samples. We observe that the different line shapes of the Raman spectra appear in the

triangles (Figure 3e,g). A height profile along the dashed line in Figure 3e,g is depicted in Figure 3f,h. It is noted that the bottom monolayer of the bilayer triangle sheet has a thickness of ∼0.65 nm for both A-A′ and A-B stackings, indicating one layer of MoS2, and the thickness also shows that the top small triangle is a monolayer MoS2 too. 9205

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Figure 4. Raman spectra of forward (A-B) (a) and reversed (A-A′) (b) stacking of bilayer triangle MoS2 sheets. The photoluminescence (PL) spectra of monolayer and bilayer MoS2 in different stacking patterns: (c) forward (A-B) stacking and (d) reversed (A-A′) stacking.

samples between forward and reversed stackings.30,31 Figure 4 displays the Raman results of bilayer MoS2 samples, and E12g and A1g are the two prominent Raman-active modes of MoS2 crystals. The in-plane E12g mode ascribes to the opposite vibration between two S atoms and the Mo atom, whereas the out-of-plane A1g mode results from the opposite vibration only in S atoms.20 The frequency difference (Δ) between E12g and A1g modes is usually used in recognizing the layer number of MoS2. The Δ value of the bottom monolayer in both A-B stacking and A-A′ stacking is 18 cm−1, similar to the monolayer MoS2 grown by CVD.25 As shown in Figure 4a,b, the Δ value decreases stepwise with the decreasing of layer numbers of MoS2, where the layer number is confirmed by AFM thickness (Figure 3). These results agree well with the previous reported MoS2 layer.25,32 In Figure 4a,b, we show the Raman spectra of the bottom monolayer and that of the center bilayer in both forward stacking (A-B) and reversed stacking (A-A′), respectively. Raman spectra of the center bilayer of MoS2 both in A-A′ stacking and in A-B stacking are stronger than that of the bottom monolayer, due to the reduced amount of material.33 We compare the difference of the two stacking patterns in the Supporting Information (Figure S1), and there are two characteristics Raman modes, i.e., A1g (405 cm−1) and E12g (383 cm−1), for the forward A-B stacking with the full width at half-maximum (fwhm) values of 5.48 and 3.60 cm−1. For the A-A′ stacking pattern, the same Raman modes also exist (A1g: 407 cm−1; E12g: 383 cm−1), where the fwhm values are 4.81 and 3.22, respectively. However, the line shape of A-B bilayers exhibits more symmetry than that of A-A′ bilayers. Specially, in the A-B spectrum, we observe a sharp and high peak. In the Supporting Information, Figure S2b,d shows the mapping of the two stacking patterns constructed by plotting the integrated MoS2 Raman peak intensity (350−460 cm−1) in confocal measurements. It is seemed that the thickness distribution is corresponding to the optical images (Figure S2a,c, Supporting Information). Figure 4c,d shows the

photoluminescence (PL) spectra of the bottom MoS 2 monolayers and the bilayers with different stacking patterns. There are two pronounced emission peaks known as the A1 and B1 direct excitonic transitions both in monolayer and in bilayer MoS2; however, the bottom monolayers exhibit much stronger PL than the center bilayers in the two stacking patterns.2,34 The emission intensity decreases with the layer number because the optical band gap transforms from indirect to direct when the bulk MoS2 is reduced to a monolayer sheet.34 For the A-B stacking pattern shown in Figure 4c, this bilayer MoS2 gives a peak at ∼1.77 eV, whereas this peak shifts to ∼1.81 eV in A-A′ stacking (Figure 4d). It is interesting to note that, in the synthesized structures, the small and large triangle MoS2 sheets have a relative angle of 0° (A-B pattern) or 60° (A-A′ pattern). To explore the atomic configuration of the experimental observed structures, we have performed DFT calculations using the VASP code35 and PBE functional.36 van der Waals correction of Grimme37 is included in the calculations. First, we consider five typical stacking configurations of bilayer MoS2, as shown in Figure 1a−e. The calculated interlayer distances, adsorption energies, and band gaps of the different configurations are listed in Table 1. It can be seen that, when the S atoms of the upper layer locate on top of the hollow sites of the bottom layer, the layer distance is smaller and the adsorption energy is larger. According to the adsorption energies, the stability of different stackings follows b Table 1. Calculated Interlayer Distances (d), Adsorption Energies (Ea), and Band Gaps (Eg) of the Configurations (a)−(e) in Figure 1

d (Å) Ea (meV) Eg (eV) 9206

a (A-A)

b (A-A′)

c (A′-B)

d (A-B′)

e (A-B)

6.81 99.8 1.44

6.20 160.3 1.23

6.26 146.2 1.24

6.80 102.5 1.44

6.19 159.6 1.19

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clearly observe the existence of the two stackings. By Raman spectra, we found that these two typical stacking patterns exhibited different line shapes and that the line shape of A-B bilayers showed more symmetry and was sharper than that of A-A′ bilayers. The theory calculation also confirmed that the AB and A-A′ stackings were the most stable and lowest-energy stacking patterns of bilayer MoS2 sheets, which was similar to the experimental conclusion.

> e > c > d > a (A-A′ > A-B > A′-B > A-B′ > A-A) in Figure 1. This trend is in agreement with a previous study.12 Hence, it is likely that the two synthesized structures correspond to Figure 1b (A-A′) and 1e (A-B) stacking. The band gap of A-A′ stacking is higher than that of A-B stacking, which is in accord with the PL spectra results. To further demonstrate why the relative angle between the small and the large flakes is 0° or 60°, we have constructed five different structures, as shown in Figure 5. In these structures, a small triangle MoS2 flake is

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ASSOCIATED CONTENT

* Supporting Information S

Experimental section and density functional theory calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-10-82304982. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the China Postdoctoral Science Foundation (No. 2013M540127). REFERENCES

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Figure 5. Five structures with different relative angles between the triangle flakes.

placed on a larger triangle MoS2 flake. The flakes are all Moterminated. In different structures, the relative angles between the two flakes are 0, 15, 30, 45, and 60°, respectively. It can be seen that the 0° case corresponds to the (e) stacking (A-B) in Figure 1, and the 60° case corresponds to the (b) stacking (AA′) in Figure 1. By taking the total energy of the 0° case as zero, the relative total energies of the five structures are 0, 0.41, 0.65, 0.61, and 0.004 eV. The total energies of the 0° and 60° cases are very close, and much lower than others. This is also in accordance with the calculation of adsorption energies of stacking (a)−(e) in Figure 1, as the stacking (b) (A-A′) and stacking (e) (A-B) are predicted to be the most stable. Therefore, our calculations show that, when the relative angle between the triangle flakes is 0° or 60°, the structure has the (e) (A-B) or (b) (A-A′) stacking, which is the most stable. This explains the experimental observations. In summary, we have prepared bilayer MoS2 sheets through CVD technology with simultaneously existing both A-A′ and AB stacking patterns. We have demonstrated optical images to 9207

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