W films

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The fabrication and tunable optical properties of 2D transition metal dichalcogenides heterostructures by adjusting the thickness of Mo/W films ⁎

Fei Chena, , Weitao Sua, Su Dinga, Li Fua a

College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: TMDC Heterostructures CVD Optical properties Growth mechanism

Feasible strategy developed to realize the fabrication of two-dimensional (2D) transition-metal dichalcogenides (TMDCs) vertical/lateral heterostructures is highly desirable and beneficial for the construction of ultrathin and novel optoelectronic devices. In this work, we report the fabrication of diverse 2D TMDCs structures, such as MoS2 monolayer, WS2 monolayer, Mo1−xWxS2 alloy, and WS2-Mo1−xWxS2 lateral/vertical heterostructure, through one-step chemical vapor deposition (CVD) approach via adjusting stacking sequence and thickness of Mo/W films. Systematical Raman and PL characterizations have elucidated that the obtained samples are 2H phase, and possess outstanding structural and optical modulations. The possible growth mechanism of the 2D TMDC crystals with versatile structures has been discussed based on adjustable vapor pressure and interdiffusion of Mo/W atoms. Our experiments confirm the possibility of the synthesis strategy to promote the progress of different 2D-TMDC-based structures, benefiting for various optoelectronic applications.

1. Introduction The realization of graphene via mechanical exfoliation [1], has greatly motivated the continuous development of two-dimensional (2D) layered semiconductors during the past few years, owing to the peculiar structure as well as the versatile and intriguing physical and chemical properties, which guarantee they will be the promising materials for multifarious applications in ultra-thin optoelectronic devices [2,3]. In particular, atomically thin 2D 2H-phase (trigonal prismatic coordination phase, semiconducting) transition-metal dichalcogenides (TMDCs) with chemical formula of MX2 (M = Mo or W, X = S, Se or Te), are one of the most extensively researched 2D materials in recent years because of many distinguishable characteristics compared with that of bulk counterparts, including tunable bandgaps (1–2 eV) from the visible to near infrared light regions [4,5], high yield of electron-hole generation under light irradiation [6], large exciton binding energy [7], roomtemperature valley polarization [8], and so forth. Therefore, 2D TMDCs materials have been widely utilized in next-generation optoelectronic devices, for example, field-effect transistors [9], light-emitting diodes [10], valleytronics [11], photodetectors [12,13], chemical-/bio-sensors [14,15], and so on. However, it is still challenging for TMDCs to absorb sufficient light due to ultrathin thickness, which will restrain the external quantum efficiency and detectivity of optoelectronic devices. 2D vertical/lateral heterostructures, constructed by two or more 2D

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atomically thin TMDCs, usually exhibit fascinating functionalities, which are often not observed in the single-component 2D TMDC. In general, such 2D hybrid nanostructures have been demonstrated to possess distinctive tunable optical bandgaps, enhanced photon absorption, photoluminescence (PL) quenching effect, significant interlayer exciton emissions, and ultrafast and high-efficiency charge transfer due to the strong light-matter interaction [5,16–23]. In addition, 2D-TMDC-based vertical heterostructures can have high-quality interfaces and cannot be influenced by the lattice-mismatch problem, originating from the dangling-band-free surfaces of the TMDC materials [24]. Accordingly, 2D-TMDC-based heterostructures supply a versatile platform to explore novel essential performance and enhance the development of novel applications in various optoelectronic devices [25]. Since the first demonstration of MoS2/WS2 vertical heterostructures constructed via the combination of chemical vapor deposition (CVD) technique and transferring [19], numerous attention has been paid to the fabrication of a great diversity of 2D TMDCs vertical/lateral heterostructures through either assembly of individual exfoliated [26] or CVD grown 2D TMDCs [27], and/or employing sequential CVD growth approach [28–32]. For example, Gong et al. realized the one-step CVD synthesis of vertical/lateral WS2/MoS2 heterostructures via adjusting growth temperature and two-step CVD synthesis of vertical WSe2/ MoSe2 heterostructure [28,33]. Bogeart et al. utilized a two-step CVD strategy to fabricate monolayer MoS2/WS2 lateral heterostructure and

Corresponding author. E-mail address: [email protected] (F. Chen).

https://doi.org/10.1016/j.apsusc.2019.144192 Received 24 July 2019; Received in revised form 11 September 2019; Accepted 26 September 2019 Available online 08 November 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Fei Chen, et al., Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.144192

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TMDCs-based structures. S powder with the weight of ∼200 mg and substrate with Mo/W films were loaded in two quartz boats, which were located at two separated temperature zones, and corresponding distance is ∼20 cm along gas flow direction. Vaporization temperature of S and growth temperature were 270 °C and 850 °C, respectively. When the growth temperature raised to 550 °C, S powder began to heat up. The two zones simultaneously reached the target values, and then kept for 20 min. Flow rate of Ar during the whole process was maintained at 60 sccm.

Mo1−xWxS2 alloy through a temperature-dependent in-plane diffusion [34]. Duan et al. manipulated the vapor sources in situ by mechanically shifting and gas flow directions during the CVD process to obtain WS2/ WSe2, MoS2/MoSe2 lateral heterostructures, various kinds of lateral heterostructures (WS2/WSe2, WS2/MoSe2), multiheterostructures (WS2/WSe2/MoS2, WS2/MoSe2/WSe2), and superlattices (WS2/WSe2/ WS2/WSe2/WS2) [29,35]. Lee et al. synthesized the monolayer WS2/ WSe2/MoS2 lateral multijunctions through a multistep CVD method [31]. As discussed above, CVD technique can be viewed as a primary strategy to fabricate 2D TMDC heterostructures with high yield and sharp interfaces in compared to mechanical exfoliation and restacking pathway, which are severely blocked by the complex preparation process, unavoidable interfacial contamination, low-yield, and uncontrolled dimensions of certain structures. Meanwhile, the CVDgrowth of 2D TMDCs vertical/lateral heterostructures usually requires multi-step process to regulate the type and amount of vapor sources, resulting in the sequential growth of heterostructures. However, it is still primary challenging to synthesize 2D TMDCs heterostructures with desired component, certain stacking/stitching sequence, and tunable compositions through a one-step CVD procedure. In this study, we developed a feasible single-step CVD growth procedure to achieve the fabrication of 2D 2H-TMDC-based vertical heterostructures and alloys, including WS2-Mo1−xWxS2 vertical/lateral heterostructures, alloyed Mo1−xWxS2 monolayer with varied composition ranges, via the reaction of sulfur and deposited Mo/W vertical films with tunable thicknesses, as described in Fig. 1. The thickness, structure, and compositions of the as-produced 2D-TMDCs heterostructures are systematically identified by AFM, Raman-PL spectra/mappings. The variation of 2D-TMDC structures is speculated at the basis of the effect of deposited Mo/W films with different thicknesses and stacking sequence on Mo/W concentrations in chamber.

2.2. Characterizations and measurements Optical microscope (OM) images of as-grown TMDC samples were obtained by optical microscopy (Nikon OPTIPHOT-100), the thickness of the samples were estimated through atomic force microscopy (AFM, Bruker Multimode 8), structure, optical performance, and intensity imaging of the obtained samples were systematically characterized via PL/Raman spectroscopy (RENISHAW invia, ∼532 nm excitation wavelength). 3. Results and discussion 3.1. Mo film covered W film with different sputtering power of Mo When the Mo film covered the W film, and corresponding sputtering power was 20 W and 60 W, respectively, WS2-Mo1−xWxS2 vertical heterostructures can be fabricated, and corresponding OM image of the sample is provided in Fig. 2(a). Obviously, the as-grown heterostructures are high yield, large scale, and consisted of two concentric triangles stacked in opposite directions. Meanwhile, the distinct color contrast between the center and outside regions of the heterostructures indicates the existence of two phases in a triangular flake. The lateral size of small triangle at center and peripheral area is about 4 μm and 40 μm, respectively. In order to figure out the composition difference in the heterostructure, the Raman and PL spectra of the heterostructure are collected at different positions from center to edge areas, as described in Fig. 2(b). Fig. 2(c) exhibits position-dependent Raman spectra of the flake. At the core of the flake (point 1), the Raman spectrum is fitted and can be divided into four characteristic Raman peaks centered at 355 cm−1, 382 cm−1, 406 cm−1, and 419 cm−1, which can be attributed to 2H-WS2-like E12g(Γ), 2H-MoS2-like E12g(Γ), 2H-MoS2-like A1g(Γ), and 2H-WS2-like A1g(Γ) [36–38]. The differential of A1g-E12g of MoS2- and WS2-like phases is 24 and 64 cm−1, respectively, inferring center region of the heterostructure may be consisted of Mo1−xWxS2 alloy and multilayered 2H-WS2. At point 2, which is located at the edge of the small triangle, typical Raman peaks of E12g(Γ) and A1g(Γ) modes from 2H-MoS2- and 2H-WS2-like phases are centered at 356 cm−1, 384 cm−1, 406.5 cm−1, and 417 cm−1, and corresponding frequency difference of E12g(Γ) and A1g(Γ) is 22.5 cm−1 and 61 cm−1, respectively. Form point 2 to 5, as presented in Fig. 2(d), 2HWS2-like A1g(Γ) gradually shifts from 406.5 cm−1 to 417.6 cm−1, and 2H-MoS2-like E12g(Γ) shows a blue-shifting from 384 cm−1 to 376 cm−1. Meanwhile, 2H-WS2-like E12g(Γ) exhibits a slight blue-shifting from 356 cm−1 to 354 cm−1. The variation of the Raman modes from the center to the edge is originated from the formation of 2H-Mo1−xWxS2 alloy with the gradual increase of W content toward the edge [39]. In addition, the intensity of MoS2-like E12g(Γ) is basically quenched towards the edge of the flake, meaning the high W composition at the edge region. The Raman intensity mappings of the heterostructures, as supplied in Fig. 2(e)–(h), are also used to deeply analyze the phase compositions in the heterostructure. Obviously, WS2-like E12g (Γ) and A1g (Γ) locate at center and peripheral regions, and the inner part is the MoS2-related modes, suggesting that top layer of the heterostructure is 2H-WS2 and bottom layer is 2H-Mo1−xWxS2 alloy. The thickness of the core and the edge of the flake is obtained by using AFM, and corresponding results are provided in Fig. 3(a)–(c). The

2. Experimental section 2.1. Sample preparation In this research, we realized the formation of 2D Mo1−xWxS2 alloys with different composition ranges and WS2-Mo1−xWxS2 vertical heterostructure by sulfuration of metallic Mo-W precursors from sputtered Mo/W films with different thickness. As depicted in Fig. 1, W (or Mo) nano-film covered Mo (or W) nano-film, served as Mo and W precursors, were prepared using mask-assisted magnetron sputtering. During sputtering process, the sputtering time and Ar flow rate are constant, and corresponding value is 15 s and 15 sccm, respectively. As the Mo (or W) film covered the W (or Mo) film, the sputtering power of W (or Mo) film is 60 W, and the sputtering power of Mo (or W) film is 20 W, 40 W, and 60 W, respectively. The Mo/W films cover half of the substrate, and other blank region is utilized for the growth of 2D-

Fig. 1. Schematic diagram of the preparation steps for the 2D TMDC heterostructures. 2

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Fig. 2. (a) OM image of large-scale WS2-Mo1−xWxS2 vertical heterostructures obtained at Mo (20 W)-W (60 W), inset showing an magnified flake; (b) schematic illustration of a single heterostructure; (c) position-dependent Raman spectra of the heterostructure from center to edge; (d) Raman vibration modes of the different points shown in panel b; (e)-(h) Raman intensity mappings of different modes from the heterostructures: (e) WS2-like E12g (330–360 cm−1), (f) WS2-like A1g (410–420 cm−1), (g) MoS2-like E12g (370–390 cm−1), and (h) MoS2-like A1g (400–410 cm−1).

height of the edge area, as exhibited in Fig. 3(b), is ∼0.9 nm, suggesting the bottom layer of the heterostructure is single layer. Meanwhile, the height of the center region in Fig. 3(c) gradually increases from ∼0.7 nm to 2.8 nm, meaning the upper layer of the heterostructure is single-layer WS2 with multilayered nucleation site at the very center [40]. The Raman and AFM analyses demonstrate that the bottom and upper layers of the heterostructures are alloyed 2HMo1−xWxS2 monolayer with increased W content from center to edge locations and 2H-WS2 monolayer with multilayered nucleation site at the very center, respectively. PL spectra and mappings, as given in Fig. 3(d)–(g), are further utilized to confirm the compositions of the heterostructure. At point 1, the PL spectrum, as supplied in Fig. 3(d), has two outstanding peaks of ∼622 nm and 673 nm, which may be originated from direct excitonic transition energies in WS2 monolayer and alloyed Mo1−xWxS2 monolayer with high Mo content. Noticeably, the intensity of peak at 622 nm from WS2 monolayer is much weaker than that of alloyed Mo1−xWxS2 monolayer, which attributes to interlayer relaxation between WS2 and Mo1−xWxS2. From point 2 to point 5, stronger emission peak continuously shifts from 666 to 644 nm, as plotted in the inset of Fig. 3(d), originating from increased W concentration in Mo1−xWxS2

alloy, and corresponding emission intensity decreases sharply from points 1 → 3, and then increases drastically from point 3 to point 5. The decrease of peak intensity may be originated from the low crystal quality as the increase of alloy disorder effect at a moderate composition [41]. The peak intensity mappings of MoS2-rich (660–680 nm) and WS2-rich (630–650 nm) phases are given in Fig. 3(e)–(f). Remarkably, the MoS2-rich phase related emission is just distributed at inner part, and emission intensity of WS2-rich phase is much stronger at peripheral area while the very center is almost dark, further demonstrating that the bottom layer of such flake is single-layer Mo1−xWxS2 alloy with WS2-rich phase at the edge and the upper layer at the very center is WS2. Fig. 3(g) describes variation of PL peak position and photon energy with measured spots, elucidating tunable light emission characteristic of Mo1−xWxS2 (0 < x < 1) alloy with increase of W concentration toward edge region at bottom. As the Mo film covered the W film, and the sputtering power of Mo was increased to 40 W and that of W was kept at 60 W, Mo1−xWxS2 alloy can be fabricated. Fig. 4(a) exhibits typical OM image of a single flake, which possesses nearly equivalent straight edges with lateral size of ∼22 μm. Obviously, homogeneous color distribution across whole flake suggests uniform thickness of the flake. Fig. 4(b) exhibits the

Fig. 3. (a)–(c) AFM image and the height profiles of the edge and center of WS2-Mo1−xWxS2 vertical heterostructure; (d) position-dependent PL spectra of the vertical heterostructure; (e)-(f) PL intensity mappings with the wavelengths of 630–650 nm (e) and 660–680 nm (f); (g) the variation of the PL peak position and photon energy with the locations along the flake. 3

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Fig. 4. (a) OM image of a representative alloyed Mo1−xWxS2 flake with low W concentration obtained at Mo (40 W)-W (60 W); (b) position-dependent Raman spectra of the alloyed flake along the red line in (a); (c) the variation of the Raman modes as the change of locations; (d)-(e) AFM image and the height profile of the alloyed flake; (f) position-dependent PL spectra of the alloyed flake; (g) evolution of the PL peak located at ∼680 nm from different positions along the red line in (a), inset showing the intensity mapping of 680 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

defects, compositions, or strain. In addition, the emission peak, as depicted in Fig. 4(g), displays a continuous blue-shifting from 680 nm to 677 nm, further verifying the alloyed Mo1−xWxS2 flake with x = 0 and slight increase of W concentration toward the edge. PL intensity mapping of 680 nm is exhibited in the inset of Fig. 4(g), indicating the center of the flake has weak intensity and intensity distribution is not homogeneous, which is consistent with the PL spectra in Fig. 4(f). When the Mo film covered the W film, and the sputtering power of both Mo and W was 60 W, only MoS2 crystals can be grown on the substrate. OM image in Fig. 5(a) shows numerous isolated triangular flakes with the average edge dimension of ∼24 μm. Fig. 5(a) presents the Raman spectrum of an individual MoS2 flake. Remarkably, the spectrum is consisted of two typical Raman peaks corresponding to inplane (E12g(Γ)) and out-of-plane (A1g(Γ)) vibrational modes, and position difference of E12g-A1g is approximately 19 cm−1, verifying the monolayer feature of the CVD-grown 2H-MoS2 flake [42]. The intensity mappings of E12g (385 cm−1) and A1g (404 cm−1) in Fig. 5(c) confirm high degree of uniformity within a single flake. AFM image and height profile of an isolated domain are given in Fig. 5(d)–(e). The observed configuration is consistent with the morphology in the OM image, and the height of the flake is ∼0.8 nm, which can be considered to be single layer of CVD-grown 2H-MoS2 crystal [43]. Fig. 5(f) presents the PL spectrum of the MoS2 sample. It can be seen that there are two emission peaks centered at ∼679 nm and 629 nm, analogous to CVD-grown 2HMoS2 monolayer [16], which are correlated to the A and B direct-gap optical transitions, respectively, and the energy difference of A and B excitons is originated from spin-orbit splitting of valence band [44,45].

Raman spectra of the as-produced sample from varied positions, as the red line marked in Fig. 4(a). At the right center position (point 1), the Raman spectrum is consisted of two characteristic modes centered at 383.8 cm−1 and 403.5 cm−1, attributing to E12g and A1g modes of 2HMoS2, and corresponding difference of A1g - E12g is ∼19.7 cm−1, implying the core of the triangle is single-layer MoS2. Formation of alloyed 2H-Mo1−xWxS2 crystal with small x value. For points 2–4, all spectra are consisted of two distinguishable Raman modes at 370–420 cm−1 and a weak mode at 340–360 cm−1, assigned to 2H-WS2-like E12g(Γ), 2H-MoS2-like E12g(Γ) and A1g(Γ) modes, indicating. For point 1 → 4, 2H-MoS2-like E12g mode displays a slight blue shifting, while 2H-MoS2-like A1g mode presents a gradual red shifting. Meanwhile, 2H-WS2-like E12g mode shows a significant red shifting from 349.2 cm−1 to 350.8 cm−1, verifying the slight increase of x in 2HMo1−xWxS2 toward outer region [17]. Fig. 4(c) describes the relationship of Raman peak positions and measured points from the center to the vertex. MoS2-like A1g mode presents a slight red shifting and E12g mode gradually shifts to high energy side from the center to the edge positions. Meanwhile, WS2-like E12g mode of points 2–4 exhibits a progressively red shifting toward the edge. As exhibited in Fig. S1, variation of intensity of WS2-like E12g (330–360 cm−1), MoS2-like E12g (370–390 cm−1), and MoS2-like A1g (400–410 cm−1) modes is well consistent with that of the Raman spectra in Fig. 4(b). The Raman analyses confirm that the as-produced flake is 2H-Mo1−xWxS2 alloy with the gradual increase of low W concentration from the center to the vertex. Thickness of the alloyed flake is about 0.9 nm, as depicted in Fig. 4(d)–(e), elucidating the flake is monolayer. The room-temperature PL spectra collected at different regions of the alloyed flake is shown in Fig. 4(f). All the spectra possess one predominant peak centered at ∼680 nm and a weak shoulder at 625 nm, similar to A and B excitons of CVD-grown MoS2 monolayer [42]. Moreover, the intensity of the emission peak at ∼680 nm gradually increases from point 1 to point 3 and then obviously decreases, which may be ascribed to the variation of

3.2. W Film covered Mo film with different sputtering power of W As the W film covered the Mo film, and the sputtering power of W and Mo was 20 W and 60 W, respectively, alloyed Mo1−xWxS2 flake with graded compositions can be synthesized. As OM image exhibited 4

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Fig. 5. (a) Typical OM image of large-scale MoS2 triangles obtained at Mo (60 W)-W (60 W), inset showing an magnified flake; (b) Raman spectrum of a MoS2 flake; (c) Raman intensity mappings of the E12g(MoS2) (385 cm−1) and A1g(MoS2) (404 cm−1); (d)–(e) AFM image and the height profile of an individual flake; (f) PL spectrum of the MoS2 flake.

Fig. 6. (a) OM image of large-scale alloyed Mo1−xWxS2 flake with the decrease of W concentration from center to edge obtained at W (20 W)-Mo (60 W); (b) positiondependent Raman spectra of the alloyed flake along the red line in (a); (c) the relationship of Raman modes and locations along the red line in (a); (d)-(e) AFM image and height profile of a single flake; (f) position-dependent PL spectra of the alloyed flake with the inset showing the enlarged PL spectra; (g) PL peak position and the corresponding photon energy with the locations from the center to the edge. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

E12g peak presents a slight red-shifting from 381.8 cm−1 to 383.6 cm−1, and frequency difference of A1g - E12g peak decreases from 24.2 cm−1 to 19.6 cm−1, as depicted in Fig. 6(c). Meanwhile, the WS2-like E12g mode displays a blue-shifting from 351.5 cm−1 to 349.7 cm−1, manifesting the as-grown flake is 2H-Mo1−xWxS2 alloy with decrease of x value (W content) from the center to the periphery. At point 5, only 2H-MoS2-like E12g (383.7 cm−1) and A1g (403.4 cm−1) Raman modes without 2HWS2-related phases can be observed, and the corresponding peak separation is ∼19.7 cm−1, confirming the edge of such triangular crystal

in Fig. 6(a), triangular-shaped crystals with no obvious color contrast are grown on the substrate, and the corresponding edge size is approximately 20 μm. Fig. 6(b) shows the Raman spectra from different positions of the alloyed flake, as the red line marked in Fig. 6(a). For points 1 → 4, all the spectra possess two remarkable Raman modes located at ∼380 cm−1 and 405 cm−1, and a weak broad Raman mode centered at ∼350 cm−1, which are corresponding to 2H-MoS2-like E12g and A1g modes, and 2H-WS2-like E12g mode. Noticeably, MoS2-like A1g peak shifts gradually from 406 cm−1 to 403.2 cm−1, whereas MoS2-like 5

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Raman spectra of the flake collected at five spots as the red line marked in Fig. 7(a). At the core (point 1), Raman peak at the position of 400–420 cm−1 is broad and asymmetrical, which can be divided into two peaks at 408.6 cm−1 and 416.1 cm−1 by Lorentz fitting, ascribed to WS2-like A1g modes. Meanwhile, the other peaks around 351.4 cm−1 and 381.7 cm−1 can be identified as 2H-WS2-like and 2H-MoS2-like E12g modes. The frequency difference of WS2-like A1g (416.1 cm−1) and E12g (351.4 cm−1) modes is 64.7 cm−1, analogous to that of CVD-2H-WS2 monolayer [46]. Moreover, peak separation of WS2-like A1g (408.6 cm−1) and MoS2-like E12g (381.7 cm−1) modes is 26.9 cm−1, which is distinctly larger than that of 2H-MoS2 bulk [47], indicating the existence of monolayer 2H-WS2 and alloyed 2H-Mo1−xWxS2 monolayer at the center of the flake. For points 2 → 5, all the Raman spectra possess the similar Raman modes located at ∼ 350 cm−1 and 420 cm−1, attributing to 2H-WS2-like E12g and A1g modes. Clearly, WS2like A1g peak presents a gradual shifting to lower energy side from 415.7 cm−1 to 418.4 cm−1, while WS2-like E12g peak displays a slight red-shifting from 355.3 cm−1 to 357.9 cm−1, as described in Fig. 7(c), implying the increase of W concentration in 2H-Mo1−xWxS2 crystal toward the edge. Intensity mappings of WS2-like E12g (330–360 cm−1), MoS2-like E12g (370–390 cm−1), and WS2-like A1g (410–420 cm−1) are supplied in Fig. S3(a)–(c). Noteworthily, intensity of WS2-like A1g at center is strong and seems to be a small triangular shape, while the outer area is almost dark with sporadic bright spots. WS2-like E12g has higher intensity at peripheral area and almost disappears at the core. The variation of WS2-like A1g and E12g modes is well consistent with the change tendency of the Raman spectra illustrated in Fig. 7(b). The intensity mapping of MoS2-like E12g mode in Fig. S3(c) displays a decreased intensity from the region close to the core to the edge, and exhibits an extremely weak intensity at the center. Therefore, the Raman analyses confirm that the triangular flake is composed of 2HWS2 monolayer at the very center and 2H-Mo1−xWxS2 crystal with x increasing toward edge position. AFM characterizations, as

is pure 2H-MoS2 monolayer. Fig. S2(a)–(c) provide intensity mappings of WS2-like E12g (330–360 cm−1), MoS2-like E12g (370–390 cm−1), and MoS2-like A1g (400–410 cm−1) modes, elucidating the triangular flake is Mo1−xWxS2 alloy. The Raman analyses demonstrate that triangular crystal is single-layer Mo1−xWxS2 alloy with gradual decrease of W concentration to 0 at the edge. The AFM characterizations are supplied in Fig. 6(d)–(e), and thickness of the flake is about 0.8 nm, suggesting the one-atom-thick nature of Mo1−xWxS2 alloy. The room-temperature PL spectra of the alloyed flake is measured to further analyze composition variation, as shown in Fig. 6(f). From the core (point 1) to the vertex (point 5), each spectrum has a predominant emission peak at around 670–690 nm, and a weak shoulder at 610–630 nm, originating from A and B excitons of MoS2 crystal. As provided in the inset of Fig. 6(f), emission peak slightly shifts to lower energy side from 671 nm to 680 nm as the position toward the edge because of the increase of Mo concentration, while the peak intensity exhibits a remarkable increase from point 1 to point 2 and then decreases sharply from point 2 to point 5, which may be ascribed to the low crystallinity at the core and increased defect content as the position moving to the vertex [38]. Intensity mappings of 670 nm, 675 nm, and 680 nm in Fig. S2(d)–(f) further confirm the decrease of W concentration in Mo1−xWxS2 monolayer toward the outer area and the edge is MoS2 monolayer. Fig. 6(g) describes the evolution of PL peak positions and photon energy of the alloyed flake with measured positions. Apparently, emission wavelength toward the edge displays a slight redshifting, which further elucidates the tunable bandgaps and increased Mo concentration of Mo1−xWxS2 monolayer along the red line in Fig. 6(a). When W film covered Mo film, and the sputtering power of W and Mo was 40 W and 60 W, respectively, WS2-Mo1−xWxS2 lateral heterostructure with varied x value can be realized. OM image in Fig. 7(a) represents that as-grown flake has perfect equilateral triangle configuration, and the mean edge dimension is about 13 μm. Fig. 7(b) supplies

Fig. 7. (a) Representative OM image of a single WS2-Mo1−xWxS2 lateral heterostructure with higher W concentration grown at W (40 W)-Mo (60 W); (b) Raman spectra of the alloyed flake from different regions along the red line in (a); (c) the evolution of Raman modes with the positions; (d)–(e) AFM image and height profile of a single flake; (f) PL spectra of the alloyed flake from different regions with the inset showing the enlarged PL spectra; (g) PL peak position and the corresponding photon energy with the positions from the center to the edge. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 6

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spectrum of the obtained flake is shown in Fig. 8(b), and two characteristic Raman peaks via Lorentz fitting are centered at ∼356 cm−1 and 618 cm−1, identified as E12g(Γ) and A1g(Γ) modes of 2H-WS2, and the peak separation of A1g - E12g is 62 cm−1, elucidating that as-grown crystal is single layer [16]. Intensity mappings of E12g(Γ) (356 cm−1) and A1g(Γ) (618 cm−1) modes in Fig. 8(a) manifest the high degree of uniformity within an individual flake. AFM image and height profile of an isolated flake in Fig. 8(d)–(e) exhibit the thickness of ∼0.9 nm, demonstrating monolayer feature of CVD-grown WS2 crystal. The PL spectrum of the WS2 flake in Fig. 8(f) possesses a unique and significant emission peak centered at ∼630 nm (∼1.97 eV), confirming that roomtemperature emission characteristic is dominated by neutral exciton [48].

shown in Fig. 7(d)–(e), are employed to identify the thickness of the flake, and the corresponding thickness is about 1.0 nm, further evidencing the single-layer nature of the CVD-grown WS2-Mo1−xWxS2 lateral heterostructure. PL characterizations of such lateral heterostructure are supplied in Figs. 7(f)–(g) and S3(d)–(f). At the inner part (points 1 and 2), as shown in Fig. 7(f), both of the PL spectra possess two distinct peaks at ∼630 nm and ∼650 nm, which may be attributed to WS2 and Mo1−xWxS2 alloy. From point 1 to 2, the peak at longer wavelength side exhibits a significant blue-shifting from 655 nm to 651 nm, and the peak located at ∼630 nm almost keeps the constant. At points 3–5, there is a unique emission peak between 620 nm and 650 nm, and the wavelength of the peak for point 3 → 5 displays a gradual blue-shifting from 635.2 nm to 632 nm, as provided in the inset of Fig. 7(f), suggesting increase of x value in Mo1−xWxS2 crystal from the core to the vertex. Remarkably, the peak intensity displays a sharp increase from point 3 to point 5, which may be originated from the decreased defects from the core to the vertex in Mo1−xWxS2 flake. The position-dependent emission wavelengths and photon bandgaps of the flake plotted in Fig. 7(g) further elucidate adjustable bandgaps of the heterostructure from the core to the vertex. In addition, Fig. S3(d)–(f) provide the peak intensity mappings of the measured flake with the wavelengths of 625–635 nm, 635–645 nm, and 645–655 nm. It can be seen that the emission peak at the short wavelength side (625–635 nm) displays the strong intensity at the peripheral region and the core is almost dark, which is agreement with the variation tendency of the PL peak intensity in Fig. 7(f). The peak intensity distributions of the wavelengths at 635–645 nm and 645–655 nm are well consistent with the PL results in Fig. 7(f), further verifying that the obtained flake is WS2-Mo1−xWxS2 lateral heterostructure with WS2 monolayer at the core and increased W composition toward the edge (0 < x ≤ 1). When the W film covered the Mo film, and the sputtering power of W and Mo were 60 W, single-layer WS2 domains with equilateral triangular shape can be found on the substrate, as OM image represented in Fig. 8(a). The color contrast across the entire flake can be ignored and the corresponding average edge size is about 21 μm. The Raman

3.3. Growth mechanism of diverse structures From above-mentioned characterizations and analyses, it can be concluded that such evolution of 2D TMDC structures from WS2Mo1−xWxS2 vertical heterostructure, alloyed Mo1−xWxS2 monolayer, MoS2 monolayer, WS2-Mo1−xWxS2 lateral heterostructure, to WS2 monolayer, as listed in Table 1, are realized by modulating stacking sequence and thicknesses of sputtered Mo/W films. During one-step CVD synthesis of 2D TMDC structures, the formation process is usually divided into two steps of nucleation and epitaxy growth. In this study, stacking forms and thicknesses of Mo/W films should be taken into consideration during the fabrication of diverse 2D-TMDC-based structures. First, stacking sequence of Mo (or W) film covered W (or Mo) film can govern the type of Mo/W precursors in chamber, benefiting for the control of growth sequence of pure TMDC phase in 2D vertical/lateral heterostructures. Second, the thickness of Mo/W films can manipulate Mo/W ratio in vapor and drive the in-plane microscale interdiffusion of metallic Mo/W atoms, resulting in the formation of alloys with different composition ranges. In this regard, the growth mechanism of the different 2D TMDC structures in this work can be proposed at the basis of stacking sequence and thicknesses of Mo/W films, which can regulate the type and ratio of precursors.

Fig. 8. (a) Typical OM image of the WS2 triangles obtained at W (60 W)-Mo (60 W); (b) pristine and fitted Raman spectrum of an individual WS2 flake; (c) Raman intensity mappings of the E12g(WS2) (356 cm−1) and A1g(WS2) (418 cm−1); (d)-(e) AFM image and the height profile of an individual flake; (f) PL spectrum of the WS2 flake. 7

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Table 1 The growth conditions (stacking sequence and sputtering power), structure and composition of diverse TMDC products. Stacking sequence

Sputtering power (Mo, W) Products Structure Composition

Mo⊥W

Mo⊥W

Mo⊥W

W⊥Mo

W⊥Mo

W⊥Mo

(20 W, 60 W)

(40 W, 60 W)

(60 W, 60 W)

(60 W, 20 W)

(60 W, 40 W)

(60 W, 60 W)

WS2Mo1−xWxS2 Vertical heterostructure WS2 as top layer Mo1−xWxS2 as bottom layer

Mo1−xWxS2

MoS2

Mo1−xWxS2

WS2

Single-layer alloy

Monolayer

Single-layer alloy

Increased x toward the edge x = 0 at center

x=0

Decreased x toward the edge x = 0 at edge

WS2Mo1−xWxS2 Lateral heterostructure WS2 at center Mo1−xWxS2 at outer region

Monolayer x=1

higher diffusion rate of Mo atoms compared with that of W precursor, leading to formation of Mo1−xWxS2 alloy with higher Mo content. With continuous increase of growth temperature to target value, Mo atoms are rapidly vaporized and become the predominant precursor above the substrate, while W film is almost depleted with time, causing the growth of MoS2-rich phase in the Mo1−xWxS2 with pure MoS2 phase at outer area. Moreover, the variation of Mo concentration from the core to the edge in this alloy is contrast to that of Mo1−xWxS2 alloy shown in Fig. 4. If sputtering power of W film increases to 40 W, the obtained sample is monolayered WS2-Mo1−xWxS2 lateral heterostructure with the increase of x value toward the edge. Because of higher growth temperature of WS2 crystals than that of MoS2 crystals, the formation of WS2 crystals may be started at the stage of temperature preservation, while the thick W film will restrain the diffusion of Mo atoms at the bottom through the W film. With extend of holding time, the W/Mo ratio in vapor increases gradually with time, leading to the formation of Mo1−xWxS2 with graded W composition stitched laterally to WS2 crystal at the very center. In future, we will pay more attention to study the relationship between the variation of Mo/W films at nanoscale level and TMDC structures.

In this work, as shown in Fig. S4, AFM characterizations of the pure Mo or W films, which are sputtered with the power of 40 W and the time of 15 s, indicate that the thickness of pure Mo and W films is measured to be 24 nm and 19 nm, respectively. In this regard, the thickness of Mo film obtained at 20–60 W is estimated to be 10–14 nm, 22–26 nm, and 34–38 nm, respectively, and W film is 7–11 nm, 17–21 nm, and 27–31 nm, respectively. When the thickest Mo or W film locates at the top of the sputtered film, large-scale MoS2 or WS2 monolayer with high quality can be fabricated on blank part of the substrate, because of that the thickest Mo or W film as the top layer can be gradually evaporated and hinder the evaporation and diffusion of W or Mo film as the bottom layer. When the sputtering power of Mo film, which covers on top of W film, is 20 W, WS2-Mo1−xWxS2 vertical heterostructures is fabricated. As growth temperature gradually increases, Mo precursor in the chamber increases rapidly and becomes the predominant metal component above substrate, resulting in the growth of monolayer MoS2 at the core of the triangular flake owing to lower growth temperature of MoS2 crystals. As growth temperature continuously increases, metallic W starts to be slowly volatilized and increases gradually in vapor. At this stage, the edges of small MoS2 crystals, which act as preferential nucleation sites, can absorb W/Mo/S atoms due to the coexistence of native defects and dangling bands, and the diffusion of W atoms from the peripherical region to MoS2 to substitute some Mo atoms, conducing to formation of Mo1−xWxS2 alloy with tunable compositions. During growth duration, W/Mo ratio dramatically increases as the Mo film is gradually depleted with time, and high growth temperature will accelerate the diffusion of W atoms toward the core and promote the disadsorption/diffusion of Mo atoms, benefiting for the growth of WS2 crystal as the upper layer at the very center of the alloyed flake and Mo1−xWxS2 alloy with the increase of W concentration from the core (x > 0) to the periphery (x < 1). If the sputtering power of Mo film as the top film increases to 40 W, alloyed Mo1−xWxS2 monolayer with x = 0 at the center can be fabricated. The increased sputtering power of Mo film will lead to the much thicker Mo film covered on the W film. During the heating-up period, the vapor in the chamber is just Mo precursor on the substrate, resulting in the formation of small MoS2 crystals. During insulation stage, metallic Mo film is rapidly evaporated and becomes much thinner. Meanwhile, W atoms from metallic W film at the bottom slowly diffuse through the Mo thin film and gradually increase in the vapor. Therefore, the variation of a small amount of W atoms with time in vapor induces the growth of Mo1−xWxS2 monolayer with pure MoS2 phase at the core and ultra-low W concentration at the peripherical region. As the sputtering power of W film, locating at the top of Mo film, is 20 W, as-produced sample is Mo1−xWxS2 monolayer with increased Mo concentration from the core to the periphery. As growth temperature gradually increases, metallic W film is slowly evaporated, while Mo atoms gradually diffuse through the thin W film and are volatilized into the vapor due to the lower melting temperature of metallic Mo film and

4. Conclusions In summary, we have developed a feasible strategy to synthesis various 2D-TMDC-based structures through the sulfurization reaction between sputtered Mo/W films and sulfur vapor. By regulating stacking sequence and thicknesses of Mo/W films as source materials, the asgrown structures can be gradually evolved from bilayer WS2Mo1−xWxS2 vertical/lateral heterostructure, single-layer Mo1−xWxS2 alloy with different composition ranges, MoS2 monolayer, to WS2 monolayer. Detailed structure and optical performance of these diverse TMDCs structures have been comprehensively characterized by OM, AFM, Raman/PL spectra and mappings. The possible growth mechanism of the 2D-TMDC-based structures is explained from the changed Mo/W ratio in atmosphere via adjusting stacking sequence and thicknesses of Mo and W films. The methodology in this research supplies an alternative approach to fabricate various TMDCs alloys, TMDCs/TMDCs or TMDCs/metal vertical/lateral heterostructures, and these 2D structures can exhibit novel phenomena and are the promising candidates for the applications in the next-generation ultra-thin optoelectronic devices. Acknowledgements This work is supported by National Natural Science Foundation of China (51705115), Zhejiang Provincial Natural Science Foundation of China (LY18F040006, LY19E020012), and Research Foundation from Hangzhou Dianzi University (KYS205619040). 8

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Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.144192.

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