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Composition-Tunable Synthesis of Large-Scale Mo WS Alloys with Enhanced Photoluminescence 1-x

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Juhong Park, Min Su Kim, Bumsu Park, Sang Ho Oh, Shrawan Roy, Jeongyong Kim, and Wonbong Choi ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b03408 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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Composition-Tunable Synthesis of Large-Scale Mo1-xWxS2 Alloys with Enhanced Photoluminescence Juhong Park†,#, Min Su Kim‡,#, Bumsu Park§,¶, Sang Ho Oh¶, Shrawan Roy‡,¶, Jeongyong Kim‡,¶,*, and Wonbong Choi†,§,* †

Department of Materials Science and Engineering, University of North Texas, Denton, Texas, United States

§Department of Mechanical and Energy Engineering, University of North Texas, Denton, Texas, United States ‡

Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Sungkyunkwan University, Suwon, Republic of Korea

§

Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Republic of Korea ¶

Department of Energy Science, Sungkyunkwan University, Suwon, Republic of Korea

*Corresponding Author: [email protected] and [email protected] #Juhong Park and Min Su Kim contributed equally to this work.

ABSTRACT Alloying two-dimensional transition metal dichalcogenides (2D TMDs) is a promising avenue for band gap engineering. In addition, developing a scalable synthesis process is essential for the practical application of these alloys with tunable band gaps in optoelectronic devices. Here, we report the synthesis of optically uniform and scalable single-layer Mo1-xWxS2 alloys by a two-step chemical vapor deposition (CVD) method followed by a laser thinning process. The amount of W content (x) in the Mo1-xWxS2 alloy is systemically controlled by the cosputtering technique. The post-laser process allows layer-by-layer thinning of the Mo1-xWxS2 alloys down to a single-layer; such a layer exhibits tunable properties with the optical band gap ranging from 1.871 to 1.971 eV with the variation in the W content, x = 0 to 1. Moreover, the predominant exciton complexes, trions, are transitioned to neutral excitons with increasing W concentration; this is attributed to the decrease in excessive charge carriers with an increase in the W content of the alloy. Photoluminescence (PL) and Raman mapping analyses suggest that the laser-thinning of the Mo1-xWxS2 alloys is a self-limiting process caused via heat dissipation to the substrate, resulting in spatially uniform single-layer Mo1-xWxS2 alloy films. Our findings present a promising path for the fabrication of large-scale single-layer 2D TMDs alloys and the design of versatile optoelectronic devices.

KEYWORDS: two-dimensional transition-metal dichalcogenide, single-layer, alloy, band gap, laser thinning, exciton complexes

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Two-dimensional transition metal dichalcogenides (2D TMDs) exhibit unique electrical and optical properties such as high carrier mobility, high photoluminescence, and stable band gap at atomic thickness.1 Thus, 2D TMDs are considered the next-generation optoelectronic and nanoelectronic materials. For instance, singlelayer molybdenum disulfide (MoS2) is an appealing alternative to graphene because of its band gap of 1.8 eV,2 high quantum efficiency,3 and excellent electron mobility.4 Thus, 2D MoS2 has potential applications in solar cells,5 photo-detectors,6 and flexible field effect transistors (FETs).7 Further, a single-layer tungsten disulfide (WS2) exhibits a high photoluminescence yield with a band gap of 2.0 eV.8,9 It is essential to tune the band gap of the TMDs in order to take full advantage of their desirable spectral wavelengths; in addition, the 2D TMDs film should be uniform and have single-layer structure throughout the spatial dimension. Recently, several posttreatment processes have been reported to tune the optical properties of 2D TMDs. For example, McConney et al.10 reported the laser-annealing of amorphous MoS2 on polydimethylsiloxane to enable its phase transformation to a hexagonal crystal. Another example is the post-laser thinning of multi-layer 2D TMDs down to a single layer; as a result, the photoluminescence (PL) intensities of 2D TMDs are enhanced in defined micropatterns.11 Alloying binary 2D TMDs is another efficient strategy to tune the optical properties with great flexibility. Recently, Song et al.12 reported atomic layer deposition (ALD) for the fabrication of Mo1-xWxS2 alloys, revealing the variation of the PL peak position by randomly mixing molybdenum (Mo) and tungsten (W) atoms. Liu et al.13 synthesized few-layer Mo1-xWxS2 film comprising ~50 nm crystallites (lateral scale) on sapphire substrates using the vapor-phase growth process. Zheng et al.14 reported the fabrication of single-layer Mo1xWxS2 heterostructures

via chemical vapor deposition (CVD). However, the synthesized alloys were small flakes

and inhomogeneous. Since the first theoretical report on the 2D TMDs alloys in 2013,15 the synthesis of scalable single-layer 2D TMDs alloys with high spatial uniformity and well-controlled composition is rarely explored. In addition, the optical properties of 2D TMDs are determined by the behavior of the exciton complexes,16 which are known as neutral excitons, trions (bound states of two electrons and one hole, or two holes and one electron), and biexcitons (bound states of two excitons) generated by Coulomb attraction between electrons and holes in the conduction and valence band, respectively.17 Although strong excitonic effects have very recently been observed in 2D TMDs at room temperature,9,18 the variation of the exciton complexes in 2D TMDs alloys remains unclear. Here, we report the synthesis of optically uniform single-layer Mo1-xWxS2 alloys by a two-step CVD method followed by a laser thinning process and investigations on their excitonic behavior with compositional changes. We fabricated optically uniform single-layer alloy films by the laser thinning of as-fabricated few-layer


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Mo1-xWxS2 films, and the results are confirmed by PL mapping and scanning transmission electron microscopy (STEM). The optical band gap of the Mo1-xWxS2 alloys is systematically modulated by changing x from 0 to 1; additionally, the dominant exciton observed in the PL emission changes from a trion to neutral exciton with increasing W content (x). These outcomes set the stage for further development of 2D TMDs alloys and their application in optical and optoelectronic devices.

RESULTS AND DISCUSSION Figure 1a shows the overall schematic of the synthesis process. Uniform single-layer Mo1-xWxS2 alloys were fabricated by a two-step CVD process and a post-laser thinning process. This process allows systematic control over the W content (x) in the alloy from 0 to 1. The composition of the alloy was controlled by the power applied to the individual Mo and W targets during the sputtering process. As-deposited, large-scale Mo1-xWx alloy films were sulfurized to transform the metal alloys into Mo1-xWxS2 films using a low pressure chemical vapor deposition (LPCVD) furnace, as explained in the experimental section. The thickness of the assynthesized Mo1-xWxS2 film is 3.8 ± 0.8 nm (Figure 1c and e), which is estimated to correspond to 4 – 7 layers, as reported previously.19

Figure 1. Synthesis and structural characterization of the Mo1-xWxS2 alloy. (a) Schematic illustration of the synthesis of scalable and uniform single-layer Mo1-xWxS2 alloys. Mo1-xWx metal films deposition by cosputtering followed by sulfurization to transform metals into the Mo1-xWxS2 alloys; post-laser processing to thin the few-layer alloys down to a single-layer. (b) Optical image of the as-synthesized few-layer Mo1-xWxS2 (x = 0.48) alloy and bare SiO2/Si substrate. (c) 3D atomic force microscopy (AFM) height profile of the few-layer Mo1-xWxS2 (x = 0.48) alloy for the red dotted square area marked in (b). The average thickness is measured to be 3.8 ± 0.8 nm. (d) Optical image of the laser-irradiated Mo1-xWxS2 (x = 0.48) alloy. The laser is irradiated for 7 s over a 5 µm × 5 µm area (scale bar, 5 µm). (e) 2D AFM height profile of the laser-irradiated Mo1-xWxS2 (x = 0.48) alloy for the yellow dotted square marked in (d), showing the film is thinned by ~2.9 nm by the laser treatment.

To fabricate single-layer 2D TMDs without compromising the uniformity of the film, post-laser-thinning was carried out. Figure 1d displays the optical image of Mo1-xWxS2 (x = 0.48) after laser irradiation over a 5 µm × 5 µm area. The laser process shaves the as-synthesized 3.8 ± 0.8 nm-thick Mo1-xWxS2 film down to 0.88 ± 0.04 nm.


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It is noted that the laser-irradiated region exhibits a relatively smooth and uniform surface (Figure 1e), but the edges of the sidewalls remain unetched, which is attributed to the Gaussian profile of the laser-beam intensity. The sidewall could be removed by overlapping the irradiated laser spots, thereby producing a uniform single layer Mo1-xWxS2, as shown in the later section. We conducted X-ray photoelectron spectroscopy (XPS) to confirm the variation of the Mo1-xWxS2 alloy composition with respect to the film deposition condition. Figure 2a – c show the XPS spectra of the as-synthesized Mo1-xWxS2 alloys with the W content (x) varying from 0 to 1. The XPS peaks were calibrated with respect to the C1s peak at 284.8 eV. When the W content (x) increases, the intensities of the Mo3d peaks decrease, whereas the intensities of the W4f peaks increase. Moreover, the Mo3d peaks at 229.5 and 232.6 eV of the Mo1-xWxS2 (x = 0) alloy, that is MoS2, are monotonically shifted to lower binding energies of 229.1 and 232.2 eV, respectively, in case of the Mo1-xWxS2 (x = 0.63) alloy. The W4f peaks (32.7 and 34.8 eV) show a small peak shift of 0.1 – 0.2 eV towards lower binding energies. In addition, the S2p peaks are slightly shifted to higher binding energies. These small peak shifts cannot be overlooked since a small change in the binding energy at the core levels is associated with a variation of the attractive force between the elements and electrons.13 As per previous studies,12,13 the observed shift in peak positions is attributed to the enhanced electron attraction strength of Mo with an increase in the W content (x) because the electronegativity of W (2.36) is larger than that of Mo (2.16).20,21 It is noted that the Mo6+ 3d3/2 peak, which is related to the Mo-O bonding, is not observed in the XPS spectra, suggesting the absence of oxygen contamination in the Mo1-xWxS2 alloys. Table 1 summarizes the relative ratios of Mo, W, and S estimated from the Mo3d, W4f, and S2p peak areas in the XPS spectra. The W content (x) value is calculated from the atomic percentages of W and Mo, as W/(Mo + W)); x = 0.38 indicates 21.2% of Mo, 13.3% of W, and 65.4% of S. Raman spectra (λexc = 514 nm of Ar laser) of the few-layer Mo1-xWxS2 alloys are shown in Figure 2d.

Figure 2. XPS and Raman characterizations of as-synthesized Mo1-xWxS2 alloys. XPS spectra for the (a) Mo3d, (b) W4f, and (c) S2p core levels of the Mo1-xWxS2 alloys with different compositions. Whereas the binding energies of Mo3d and W4f are shifted to lower energies, the binding energy of S2p is shifted to a higher energy, with an increase in the W content (x). (d) Raman spectra of the as-synthesized Mo1-xWxS2 alloys as a function of W content, x = 0 to 1. The A1g and E12g (MoS2) modes are linearly shifted with respect to the change in the alloy composition.

Table 1. The compositions of as-synthesized Mo1-xWxS2 alloys estimated by XPS spectral analysis


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Composition of few-layer Mo1-xWxS2 alloys Sputtering condition Mo (%)

W (%)

S (%)

W composition (x)

P1

21.2

13.3

65.4

0.38

P2

17.8

17.3

64.8

0.48

P3

12.9

21.5

65.6

0.63

The Raman spectrum of WS2 shows phonon modes of E12g (Г) at 368.9 cm-1 and A1g (Г) at 433.7 cm-1, and a second-order phonon mode of 2LA (M) at 363.3 cm-1 (See Figure S1a). The frequency difference (△k) between the E12g (Г) and A1g (Г) modes is 64.8 cm-1, which is consistent with that previously reported for few-layer WS2.22 With decreasing W content (x), the A1g mode shifts to a lower frequency whereas the E12g (Г) mode emerges from x=0.63 and shifts to a higher frequency. However, the E' mode does not shift considerably. The frequency of the A1g mode shows a linear dependence on the W content (x), which agrees well with the estimation of the alloy composition by the XPS spectral analysis. A summary of the specific peak position dependency on the W content (x) is shown in Figure S1b. The structural characteristics of the as-synthesized Mo1-xWxS2 (x = 0.48) atomic layers is clearly verified by the STEM image and selected area electron diffraction (SAED) pattern. Figure 3a shows the low-magnification top-view image of the Mo1-xWxS2 (x = 0.48) alloy transferred onto a TEM grid, showing the few-layer film.

Figure 3. Characterization of the Mo1-xWxS2 (x = 0.48) alloy by scanning transmission electron microscopy (STEM). (a) Low magnification STEM image of the Mo1-xWxS2 (x = 0.48) alloy (scale bar, 0.5 µm). (b) Crosssectional STEM image of the folded region indicated by L in (a), where 4 layers of the alloy are shown. (c – f) Energy dispersive spectroscopy (EDS) mapping of the annular dark-field (ADF) image for the region M in (a). (c) Brighter areas show a higher elemental content, and the uniform distribution of (d) molybdenum, (e) tungsten, and (f) sulfur atoms is demonstrated (scale bar, 50 nm). (g) STEM image of the single-layer Mo1xWxS2

(x = 0.48) alloy at a selected region N in (a) (scale bar, 1 nm). (h) Close-up image of the region marked

by a yellow square in (g), where the single-layer film has a six-fold coordination symmetry and honeycomb-like structure. (i) Intensity profiles of W and S atoms from both simulation and experiment. (j) Intensity profiles of


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Mo and W atoms. (k) Normalized intensity distribution map for Mo (blue) and W (yellow) atoms (scale bar, 1 nm).

The number of layers in the folded region L is 4 ± 2 layers (Figure 3b), which is in good agreement with the atomic force microscopy (AFM) height profiles (Figure 1c). The as-synthesized few-layer Mo1-xWxS2 (x = 0.48) alloy is confirmed to be polycrystalline, according to the diffraction pattern shown in Figure S2a. Further, the crystal domain size of the polycrystalline Mo1-xWxS2 (x = 0.48) alloy ranges from 20 to 60 nm, as measured in the cross-sectional STEM images (Figure S2b – d). In order to confirm the elemental distribution in the Mo1xWxS2

(x = 0.48) alloy, STEM-energy dispersive X-ray spectroscopy (EDS) mapping was carried out (Figure 3c

– f), indicating that Mo, W, and S atoms are uniformly dispersed in the film. The high-resolution STEM structural analysis reveals that the single-layer Mo1-xWxS2 (x = 0.48) alloy has a six-fold coordination symmetry and honeycomb-like structure (Figure 3g), where S atoms have low contrast while W and Mo atoms exhibit relatively bright spots with different intensities. The magnified image (Figure 3h) and intensity profiles (Figure 3i and j)) confirm that W and Mo atoms are uniformly distributed by sharing adjacent S atoms. Figure 3k shows the high-resolution HAADF STEM Z-contrast image of the single-layer Mo1-xWxS2 (x = 0.48). Yellow and blue spots indicate the higher intensity W atoms and lower intensity Mo atoms, respectively. The calculated Mo: W intensity ratio is 52.7: 47.3, which is consistent with the atomic stoichiometry determined from the XPS spectral analysis (Figure 2a – c). Recently, laser treatment of 2D TMDs materials has been reported to control the number of layers and atomic crystallinity.10 For the first time, we demonstrate the thinning of the Mo1-xWxS2 alloys down to single layers by the laser-thinning process; we also present the excitonic behavior of the laser-thinned single-layer Mo1xWxS2

alloys as a function of W content (x). Figure 4a – c depict the PL spectra of the Mo1-xWxS2 (x = 0.48) alloy

with varying laser irradiation time ranging from 0 to 18 s. The PL intensity increases as a function of the laser irradiation time reaching the maximum value at 7 s. Then, the intensity decreases gradually (Figure 4b). Meanwhile, the PL peak position shifts from 1.871 to 1.897 eV, corresponding to the laser irradiation time of 0 to 7 s; the PL peak position remains unchanged with continuous laser irradiation for times exceeding 7 s. The highest PL peak position (1.897 eV) at 7 s is considered single-layer Mo1-xWxS2 (x = 0.48) alloy. This result is consistent with a previous report12 and the AFM height analysis (Figure 1e). In addition, the PL spectra of the Mo1-xWxS2 alloys with other compositions (x = 0, 0.38, 0.63, and 1) show a similar behavior during the laserthinning process. Figure 4c shows that PL intensity decreased continuously after reaching the maximum peak


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intensity at 7 s of laser irradiation, which is attributed to the formation of sulfur vacancies in the single-layer alloy film by the continuous laser irradiation.23 Figure 4d displays the PL spectra of the alloy before and after laser-thinning.

Figure 4. PL and Raman spectra of the Mo1-xWxS2 alloy. Experimental PL spectra as a function of the laser irradiation time, (a) 1 – 7 s and (b) 8 – 18 s under ambient condition. (c) The summarized PL intensity and peak position of the Mo1-xWxS2 (x = 0.48) alloy. The PL peak intensity increases until 7 s before diminishing. The PL peak position is shifted to a higher energy up to 7 s and unchanged with further irradiation. (d) Comparison of PL spectra before and after laser irradiation showing ~5 times increase in peak intensity and ~0.03 eV peak shift after 7 s irradiation. (e) Raman spectra obtained at laser irradiated region (7 s) in comparing with non-irradiated region showing that the gap between E12g (Г) and A1g (Г) modes is reduced from 34 cm-1 to 31 cm-1 after laser irradiation.

Evidently, the PL spectrum collected from the laser-irradiated region shows ~5 times higher peak intensity than that of the non-irradiated region. The corresponding Raman spectra are shown in Figure 4e, where the gap between the E12g (Г) and A1g (Г) modes is reduced from 34 to 31 cm-1 after the laser irradiation. Raman spectra of the single-layer Mo1-xWxS2 alloy with varying W content (x) are shown in Figure S3a. The gaps between the E12g (Г) and A1g (Г) modes for MoS2 and WS2 are 19.8 and 62.5 cm-1, respectively (see Figure S3b), which indicates the single-layer Mo1-xWxS2 alloys.12, 24 It is obvious that single-layer Mo1-xWxS2 alloys are successfully fabricated by the laser-thinning method, as confirmed by the results of PL and Raman spectroscopy as well as the AFM height analysis, as described above. STEM characterization of the laser-irradiated Mo1-xWxS2 (x = 0.48) alloy (Figure S4) demonstrates the successful fabrication of single-layer Mo1-xWxS2 (x = 0.48) film. The single-layer Mo1-xWxS2 (x = 0.48) film shows 2H phase with no Mo and W vacancies. However, sulfur vacancies emerged from the laser-irradiated regions with ~1.16 atoms per nm2, which is less number of vacancies compared to other CVD-grown singlelayer TMDs.25-27 To investigate the optical uniformity of the laser-irradiated single-layer Mo1-xWxS2 alloys, PL and Raman mappings were carried out on the laser-irradiated region (5 µm × 5 µm for 7 s). The PL intensity and peak position mappings of the Mo1-xWxS2 (x = 0.38, 0.48, and 0.63) alloys (Figure 5a and b) reveal high uniformity of the PL emissions at 1.865, 1.897, and 1.916 eV, respectively. These PL analyses confirm the formation of optically uniform, single-layer Mo1-xWxS2 films by the laser-thinning process. Recent studies have


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revealed that the SiO2/Si substrate acts as a heat sink when single-layer graphene28 and MoS229 are irradiated by high-power laser beams; thus, it is speculated that the top layer of the Mo1-xWxS2 film was removed by the accumulated heat of the absorbed laser light; however, the bottom layer was unchanged without further thinning after the formation of the single-layer alloy. In this respect, detailed temperature profile of single- to few-layer Mo1-xWxS2 (x = 0.48) alloys was analyzed by a numerical simulation (Figure S5). In case of the single-layer film, the temperature increases up to 407 ℃ at the center of the laser spot and gradually decreases from the center following a Gaussian distribution (Figure S5a), whereas the temperature for the 5-nm-thick film (~6 layers) reaches 604 ℃ (Figure S5b). The higher temperature of the profile for the few-layer film is due to the lower thermal conductivity in the vertical direction than planar direction30,31 that support the formation of single-layer by the laser-thinning. For additional characterization of the crystalline structure of the laser-irradiated region in the Mo1-xWxS2 (x = 0.48) alloy, Raman mappings of the E12g (MoS2) and A1g modes are employed, as shown in Figure 5c and d, respectively.

Figure 5. Confocal PL and Raman mapping of laser-irradiated Mo1-xWxS2 alloys. (a) PL intensity mappings and (b) PL peak position mappings (5 µm × 5 µm) of laser-irradiated Mo1-xWxS2 (x = 0.38, 0.48, and 0.63) alloys. The PL intensity mappings show high spatial uniformity with emissions at 1.865, 1.897, and 1.916 eV for W content (x) of 0.38, 0.48, and 0.63 for the Mo1-xWxS2 alloy, respectively. Raman mapping of (c) ~388 cm-1 peak for E12g (MoS2) mode and (d) ~419 cm-1 peak for A1g mode in the laser-irradiated Mo1-xWxS2 (x = 0.48) alloy (scale bar, 3 µm). (e) PL intensity patterns on the Mo1-xWxS2 (x = 0.48) film (scale bar, 3 µm).

As seen in Figure 5c and d, highly uniform area of the E12g (~388 cm-1) mode and the A1g (419 cm-1) mode are demonstrated, respectively. Figure 5e shows the PL intensity map of the Mo1-xWxS2 (x = 0.48) alloy with representative letters demonstrating the production of complex shapes of the single-layer Mo1-xWxS2 alloys by the laser treatment. Based on the results of the PL and Raman mappings, it can be inferred that the laser irradiation method enabled the fabrication of uniform single-layer Mo1-xWxS2 alloys. Further, we investigated the interaction of the exciton complexes (neutral exciton (AX), trion (AT), and biexciton (AA)) giving rise to PL emissions with respect to the W content (x) in the Mo1-xWxS2 alloy. It is noted that the binding energies of the exciton complexes are defined as, Eb (AX) = EG – AX, Eb (AT) = AX – AT, and Eb (AA) = AX – AA (EG is the band gap of the material).32 A schematic illustrating a typical PL emission, the transition energies of the exciton complexes, and the corresponding binding energies are presented in Figure S6.


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The relative intensities and the positions of the exciton complexes significantly affect the PL emissions of 2D TMDs.33 Thus, we investigated the relative PL emissions of the exciton complexes in the single-layer Mo1-xWxS2 alloy as a function of the W content (x). Figure 6a shows the deconvoluted PL spectra for the laser-irradiated single-layer Mo1-xWxS2 alloys. B exciton is generated by the valence band splitting, owing to strong spin-orbit coupling in the single-layer MoS2;34 thus, a relatively high peak intensity at 2.02 eV is observed. However, other single-layer alloys show relatively very low B exciton intensities. As W content (x) increases, the intensity of AX increases whereas that of AT decreases. Particularly, the predominant source of the PL emission of single-layer MoS2 is AT, whereas AX is dominant in single-layer WS2. To confirm the transition and binding energies of the exciton complexes, the PL emissions of the single-layer Mo1-xWxS2 (x = 0.48) alloy as a function of the laser power were deconvoluted. The estimated transition energies of AX, AT, and AA are 1.911, 1.874, and 1.847 eV, respectively (Figure S7a). The intensities slopes (m) of the exciton complexes in the PL emissions show that different logarithmic values observed in the power-dependency plot (See Figure S7b). Thus, the powerdependent data were fitted by I ~ Pm (I and P represent the emission intensity and laser power, respectively). The exponential value of the AX should be half the value of AA35; in addition, the typical exponent for AA is above 1.2 owing to the kinetics of excitation recombination and formation.36,37

Figure 6. Deconvoluted PL spectra of the single-layer Mo1-xWxS2 alloys. (a) The PL spectra obtained from the laser-irradiated single-layer Mo1-xWxS2 (x = 0, 0.38, 0.48, 0.63, and 1) alloys. The spectra are fitted with peaks corresponding to neutral excitons (AX) and trions (AT). (b) The peak positions of PL, AX, and AT spectra vs. alloy composition. (c) The intensity ratios of AX to AT (top panel), AX to total excitons (middle panel), and the excessive charge density (bottom panel) calculated by both the mass action model and three-level model. Based on the PL spectrum, the predominant components for MoS2 and WS2 change to AT and AX, respectively; in addition, the number of excessive charge carriers decreases when the W content (x) increases.

Therefore, the superliner increase (m ~1.29) in the PL emission and the half value of AA (m ~0.65) indicates a typical signature of AA. Initially, we also found that the peak positions of AX and AT in the PL curve decrease from x = 0 to x = 0.38; subsequently, the positions gradually increase up to x = 1 (Figure 6b). This peak shift is regarded as the “bowing effect,” which was observed in exfoliated single-layer Mo1-xWxS2 alloys and some other semiconducting alloys.38 The expected trend of the PL peak position is described by Equation (1) after fitting the experimental data (as shown in Figure 6b).


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EPL, Mo1-xWxS2 = (1 – x)EPL(MoS2) + xEPL(WS2) – bx(1 – x)

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(1)

where, b is the so-called bowing parameter. The obtained b value is 0.23 ± 0.05 eV, which is comparable to that obtained for Mo1-xWxS2 alloys grown by ALD (b = 0.25 ± 0.03 eV)12 and simulation data (b = 0.28 ± 0.04 eV).24 To investigate the variation in the intensities of both the AX and AT spectra in accordance with the content of W (x), the integrated intensity ratios of AT/AX and AT/Total in the PL spectra were studied (Figure 6c). Single-layer MoS2 (x = 0) shows the highest AT/AX and AT/Total ratio, indicating that the population of AT is higher than that of AX. In contrast, single-layer WS2 (x = 1) shows the lowest AT/AX and AT/Total ratio. The results demonstrate that the single-layer MoS2 has the highest excessive charge carriers, and the number of excessive charge carriers are reduced with an increase in the W content (x) because the intensity of the trions is generated by the association of an extra electron (negative trions) or a hole (positive trions) to a neutral exciton.32 The different numbers of charge carriers between single-layer MoS2 and WS2 is caused by the difference in the Coulombic attraction between the 2D MoS2 and WS2 films.39,40 According to previously reported calculation, sulfur atom is −0.52e charged in single-layer MoS2 and −0.58e in single-layer WS2;41 thus, the Coulombic attraction in WS2 is stronger than in MoS2. The excessive charge carriers are potently generated by electron transfer from or to the SiO2/Si substrate,42 because the substrate has a potentially higher charge trap density of 1010 – 1014 cm-2.43 Therefore, we assume that a higher number of electrons is transferred from the 2D MoS2 to the substrate than that of the 2D WS2, resulting in more excessive charge carriers (holes) in the 2D MoS2 than 2D WS2. In this respect, the charge carrier number is calculated using two models (i) mass action model and (ii) three-level model (see the section on excessive charge density calculation in the Supplementary Materials for more details); the result is shown in the bottom panel of Figure 6c. Single-layer MoS2 exhibits the highest number of excessive carriers; moreover, their density in the single-layer Mo1-xWxS2 decreases when the W content (x) increases. This demonstrates that single-layer MoS2 is most affected from the trap states on the SiO2/Si substrate, resulting in the strongest intensity ratios for both AT/AX and AT/Total.

CONCLUSIONS We successfully synthesized spatially uniform, single-layer Mo1-xWxS2 alloys via a two-step CVD method followed by a post-laser thinning process. The chemical composition of the alloy was systemically controlled by the co-sputtering process and verified by Raman and XPS analyses. STEM results show that the Mo1-xWxS2 alloys have uniform distribution of Mo, W, and S atoms and a honeycomb-like structure with six-fold coordination symmetry in the single-layer. The post-laser treatment thins down the as-synthesized few-layer


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Mo1-xWxS2 alloys to single-layer alloys with high optical uniformity; moreover, the single-layer alloys reveal controlled optical band gaps ranging from 1.871 to 1.971 eV with the W content (x) varying from 0 to 1. Furthermore, we found that the number of excessive charge carriers decreases as x increases, resulting in the change in the predominant component of the PL emission from trions for single-layer MoS2 to neutral excitons for single-layer WS2. The successful process of the uniform and scalable fabrication of single-layer Mo1-xWxS2 alloys with a tunable band gap can be readily applied to other TMD alloys. We expect that our developed 2D alloy process is a promising path for the fabrication and design of versatile optoelectronic devices based on 2D TMDs.

MATERIALS AND METHODS Synthesis of Wafer-Scale 2D Mo1-xWxS2 Alloys. Wafer-scale few atomic layer Mo1-xWxS2 alloys were synthesized by a two-step method,4,44 i.e., metal co-sputtering followed by sulfurization. In the first step, Mo1xWx

metal alloys were synthesized at room temperature on p-type Si substrate (Boron-doped, 0.001–0.005

Ω·cm) with a 300-nm-thick SiO2 layer. Initially, the substrates were cleaned thoroughly with acetone using an ultrasonic bath, followed by cleaning with ethanol, methanol, and deionized water; subsequently, the substrates were fastened to the sample holder in a sputtering chamber. The compositions of the Mo1-xWx metal alloys were controlled by varying the power (P) applied to each of the Mo and W target. The power ratio, P(W)/(P(Mo) + P(W) was increased from 0–1 to fabricate the Mo1-xWxS2 alloys with different compositions. Before each sputtering run, the chamber was evacuated to a vacuum level of 10-7 Torr without plasma. Then, the Mo and W targets (99.99%, Plasmaterials) were pre-sputtered for 5 min to stabilize the deposition process. The second step involved the use of a low pressure chemical vapor deposition (LPCVD system to sulfurize the Mo1-xWx metal alloys at 600 °C. The LPCVD system (Graphene Square CVD) was equipped with 4 inch diameter quartz tube furnace. Pure sulfur in a ceramic boat and the Mo1-xWx metal alloys were placed in the upstream and downstream of the quartz tube, respectively. Argon was employed as a carrier gas to transport sulfur vapor to the Mo1-xWx metal alloys. The furnace was heated to 600 °C for 1 h at a working pressure of 5 Torr with 100 sccm Ar gas flow. Characterization of the Mo1-xWxS2 Alloys. The observation of the surface morphology and thickness analysis of all Mo1-xWxS2 alloys were performed using an atomic force microscope (Parks system, NX-10 model). An Xray photoelectron spectroscope (Thermo Scientific, ESCALAB250 model) was utilized for analyzing the chemical binding energies of Mo, W, and S orbitals in the Mo1-xWxS2 alloys. After recording the XPS, the


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composition of the alloy (x) was calculated using the Thermo Scientific Avantage software. Annular bright-field (ABF) and HAADF STEM imaging were carried out using a field-emission TEM (JEM-ARM 200F, JEOL) equipped with a spherical aberration corrector (ASCOR, CEOS) operated at 80 kV. The angular ranges from the optical axis of ABF and annular dark-field (ADF) detector were 17 – 34 and 68 – 280 mrad, respectively. The image simulations were performed by quantitative STEM simulation. A custom-made spectrometer combined with a solid-state confocal laser microscope (514 nm wavelength and 200 µW power) was used for both confocal PL and Raman spectroscopy.9 Laser Thinning Process. A custom-made laser confocal microscope (2.5 mW power) was utilized to thin the as-synthesized few-layer Mo1-xWxS2 alloys. To study the interactions of the exciton complexes in the Mo1-xWxS2 alloys as a function of x, we employed the same laser and varied the exposure time between 0 and 25.6 s. The individual PL spectrum was recorded every 0.2 s to obtain 128 counts in total. For PL-intensity and peakposition mapping, the laser irradiation on the few-layer Mo1-xWxS2 alloys was performed by moving the laser beam laterally under ambient conditions. A total of 625 irradiations were carried out for the 5 µm × 5 µm area. A 0.95 NA objective lens was employed to focus the laser light with a lateral resolution of ~300 nm. The scattered light was gathered by a 0.95 NA objective lens and directed to a 50-cm-long monochromator equipped with a cooled charge-coupled device. ACKNOWLEDGMENTS We gratefully thank E. Cha for his help on the schematic illustration. J. K. acknowledges financial support from the Institute for Basic Science (IBS-R011-D1) and BK21PLUS Integrated Center for Fostering Global Creative Researcher. W. C. acknowledges partial support from the UNT SEED fund.

ASSOCIATED CONTENT Supporting Information The density calculation of excessive charge carriers; calculation of temperature profiles; Raman spectrum; scanning transmission electron microscopy images of films; temperature profiles for laser-thinned films; schematic illustration of transition energies and binding energies of excitons; and PL peak positions and peak intensities of excitons.


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REFERENCES 1.

Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and

Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699–712. 2.

Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: a New Direct-Gap

Semiconductor. Phys. Rev. Lett. 2010, 105, 136805. 3.

Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim, C.-Y.; Galli, G.; Wang, F. Emerging

Photoluminescence in Monolayer MoS2. Nano Lett. 2010, 10, 1271-1275. 4.

Choudhary, N.; Park, J.; Hwang, J. Y.; Choi, W. Growth of Large-Scale and Thickness-Modulated

MoS2 Nanosheets. ACS Appl. Mater. Interfaces 2014, 6, 21215-21222. 5.

Tsai, M. L.; Su, S. H.; Chang, J. K.; Tsai, D. S.; Chen, C. H.; Wu, C. I.; Li, L. J.; Chen, L. J.; He, J. H.

Monolayer MoS2 Heterojunction Solar Cells. ACS Nano 2014, 8, 8317-8322. 6.

Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors

Based on Monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497-501. 7.

Choi, W.; Choudhary, N.; Han, G. H.; Park, J.; Akinwande, D.; Lee, Y. H. Recent Development of

Two-Dimensional Transition Metal Dichalcogenides and Their Applications. Mater. Today 2017, 20, 116-130. 8.

Gutiérrez, H. R.; Perea-López, N.; Elías, A. L.; Berkdemir, A.; Wang, B.; Lv, R.; López-Urías, F.;

Crespi, V. H.; Terrones, H.; Terrones, M. Extraordinary Room-Temperature Photoluminescence in WS2 Monolayers. arXiv preprint arXiv:1208.1325 2012. 9.

Kim, M. S.; Yun, S. J.; Lee, Y.; Seo, C.; Han, G. H.; Kim, K. K.; Lee, Y. H.; Kim, J. Biexciton

Emission from Edges and Grain Boundaries of Triangular WS2 Monolayers. ACS Nano 2016, 10, 2399-2405. 10.

McConney, M. E.; Glavin, N. R.; Juhl, A. T.; Check, M. H.; Durstock, M. F.; Voevodin, A. A.; Shelton,

T. E.; Bultman, J. E.; Hu, J.; Jespersen, M. L. Direct Synthesis of Ultra-Thin Large Area Transition Metal Dichalcogenides and Their Heterostructures on Stretchable Polymer Surfaces. J. Mater. Res. 2016, 31, 967-974. 11.

Castellanos-Gomez, A.; Barkelid, M.; Goossens, A.; Calado, V. E.; van der Zant, H. S.; Steele, G. A.

Laser-Thinning of MoS2: on Demand Generation of a Single-layer Semiconductor. Nano Lett. 2012, 12, 31873192. 12.

Song, J. G.; Ryu, G. H.; Lee, S. J.; Sim, S.; Lee, C. W.; Choi, T.; Jung, H.; Kim, Y.; Lee, Z.; Myoung, J.

M. Controllable Synthesis of Molybdenum Tungsten Disulfide Alloy for Vertically Composition-Controlled Multilayer. Nat. Commun. 2015, 6, 7817.


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

13.

Liu, H.; Antwi, K. A.; Chua, S.; Chi, D. Vapor-Phase Growth and Characterization of Mo1− xWx S2(0≤

x≤ 1) Atomic Layers on 2-Inch Sapphire Substrates. Nanoscale 2014, 6, 624-629. 14.

Zheng, S.; Sun, L.; Yin, T.; Dubrovkin, A. M.; Liu, F.; Liu, Z.; Shen, Z. X.; Fan, H. J. Monolayers of

WxMo1− xS2 Alloy Heterostructure with In-Plane Composition Variations. Appl. Phys. Lett. 2015, 106, 063113. 15.

Kang, J.; Tongay, S.; Li, J.; Wu, J. Monolayer Semiconducting Transition Metal Dichalcogenide

Alloys: Stability and Band Bowing. J. Appl. Phys. 2015, 113, 143703. 16.

Ye, Z.; Cao, T.; O’Brien, K.; Zhu, H.; Yin, X.; Wang, Y.; Louie, S. G.; Zhang, X. Probing Excitonic

Dark States in Single-Layer Tungsten Disulphide. Nature 2014, 513, 214-218. 17.

Lee, H. S.; Kim, M. S.; Kim, H.; Lee, Y. H. Identifying Multiexcitons in MoS2 Monolayers at Room

Temperature. Phys. Rev. B 2016, 93, 140409. 18.

Mak, K. F.; He, K.; Lee, C.; Lee, G. H.; Hone, J.; Heinz, T. F.; Shan, J. Tightly Bound Trions in

Monolayer MoS2. Nat. Mater. 2013, 12, 207-211. 19.

Yu, Y.; Li, C.; Liu, Y.; Su, L.; Zhang, Y.; Cao, L. Controlled Scalable Synthesis of Uniform, High-

Quality Monolayer and Few-Layer MoS2 Films. Sci. Rep. 2013, 3, 1866. 20.

Jolly, W. L.; Perry, W. B. Calculation of Atomic Charges by an Electronegativity Equalization

Procedure. Inorg. Chem. 1974, 13, 2686-2692. 21.

Gordy, W. A New Method of Determining Electronegativity from Other Atomic Properties. Phys. Rev.

1946, 69, 604-607. 22.

Cong, C.; Shang, J.; Wu, X.; Cao, B.; Peimyoo, N.; Qiu, C.; Sun, L.; Yu, T. Synthesis and Optical

Properties of Large℃Area Single℃Crystalline 2D Semiconductor WS2 Monolayer from Chemical Vapor Deposition. Adv. Opt. Mater. 2014, 2, 131-136. 23.

Plechinger, G.; Castellanos-Gomez, A.; Buscema, M.; van der Zant, H. S.; Steele, G. A.; Kuc, A.;

Heine, T.; Schüller, C.; Korn, T. Control of Biaxial Strain in Single-Layer Molybdenite Using Local Thermal Expansion of the Substrate. 2D Materials 2015, 2, 015006. 24.

Chen, Y.; Xi, J.; Dumcenco, D. O.; Liu, Z.; Suenaga, K.; Wang, D.; Shuai, Z.; Huang, Y.-S.; Xie, L.

Tunable Band Gap Photoluminescence from Atomically Thin Transition-Metal Dichalcogenide Alloys. ACS Nano 2013, 7, 4610-4616. 25.

Feng, Q.; Zhu, Y.; Hong, J.; Zhang, M.; Duan, W.; Mao, N.; Wu, J.; Xu, H.; Dong, F.; Lin, F. Growth

of Large℃Area 2D MoS2(1–x)Se2x Semiconductor Alloys. Adv. Mater. 2014, 26, 2648-2653. 26

Lee, Y. H.; Zhang, X. Q.; Zhang, W.; Chang, M. T.; Lin, C. T.; Chang, K. D.; Yu, Y. C.; Wang, J. T. W.;


Page 14 of 23

Page 15 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Chang, C. S.; Li, L. J. Synthesis of Large℃Area MoS2 Atomic Layers with Chemical Vapor Deposition. Adv. Mater. 2012, 24, 2320-2325. 27.

Najmaei, S.; Liu, Z.; Zhou, W.; Zou, X.; Shi, G.; Lei, S.; Yakobson, B. I.; Idrobo, J.-C.; Ajayan, P. M.;

Lou, J. Vapour Phase Growth and Grain Boundary Structure of Molybdenum Disulphide Atomic Layers. Nat. Mater. 2013, 12, 754-759. 28.

Han, G. H.; Chae, S. J.; Kim, E. S.; Gunes, F.; Lee, I. H.; Lee, S. W.; Lee, S. Y.; Lim, S. C.; Jeong, H.

K.; Jeong, M. S. Laser Thinning for Monolayer Graphene Formation: Heat Sink and Interference Effect. ACS Nano 2010, 5, 263-268. 29.

Lu, J.; Lu, J. H.; Liu, H.; Liu, B.; Chan, K. X.; Lin, J.; Chen, W.; Loh, K. P.; Sow, C. H. Improved

Photoelectrical Properties of MoS2 Films after Laser Micromachining. ACS Nano 2014, 8, 6334-6343. 30.

Yalon, E.; Aslan, O. z. r. B.; Smithe, K. K.; McClellan, C. J.; Suryavanshi, S. V.; Xiong, F.; Sood, A.;

Neumann, C. M.; Xu, X.; Goodson, K. E. Temperature-Dependent Thermal Boundary Conductance of Monolayer MoS2 by Raman Thermometry. ACS Appl. Mater. Interfaces 2017, 9, 43013-43020. 31.

Pisoni, A.; Jacimovic, J.; Gaál, R.; Náfrádi, B.; Berger, H.; Révay, Z.; Forró, L. Anisotropic Transport

Properties of Tungsten Disulfide. Scr. Mater. 2016, 114, 48-50. 32.

Lee, H. S.; Kim, M. S.; Kim, H.; Lee, Y. H. Identifying Multiexcitons in MoS2 Monolayers at Room

Temperature. Phys. Rev. B 2016, 93, 140409. 33.

Hu, P.; Ye, J.; He, X.; Du, K.; Zhang, K. K.; Wang, X.; Xiong, Q.; Liu, Z.; Jiang, H.; Kloc, C. Control

of Radiative Exciton Recombination by Charge Transfer Induced Surface Dipoles in MoS2 and WS2 Monolayers. Sci. Rep. 2016, 6, 24105. 34.

Cao, T.; Wang, G.; Han, W.; Ye, H.; Zhu, C.; Shi, J.; Niu, Q.; Tan, P.; Wang, E.; Liu, B. Valley-

Selective Circular Dichroism of Monolayer Molybdenum Disulphide. Nat. Commun. 2012, 3, 887. 35.

Shang, J.; Shen, X.; Cong, C.; Peimyoo, N.; Cao, B.; Eginligil, M.; Yu, T. Observation of Excitonic

Fine Structure in a 2D Transition-Metal Dichalcogenide Semiconductor. ACS Nano 2015, 9, 647-655. 36.

Plechinger, G.; Nagler, P.; Kraus, J.; Paradiso, N.; Strunk, C.; Schüller, C.; Korn, T. Identification of

Excitons, Trions and Biexcitons in Single℃layer WS2. phys. status solidi Rapid Res. Lett. 2015, 9, 457-461. 37.

Gourley, P.; Wolfe, J. Thermodynamics of Excitonic Molecules in Silicon. Phys. Rev. B 1979, 20,

3319-3327. 38.

Van Vechten, J.; Bergstresser, T. Electronic Structures of Semiconductor Alloys. Phys. Rev. B 1970, 1,

3351-3358.


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

39.

Ross, J. S.; Wu, S.; Yu, H.; Ghimire, N. J.; Jones, A. M.; Aivazian, G.; Yan, J.; Mandrus, D. G.; Xiao,

D.; Yao, W. Electrical Control of Neutral and Charged Excitons in a Monolayer Semiconductor. Nat. Commun. 2013, 4, 2498. 40.

Fan, W.; Zhu, X.; Ke, F.; Chen, Y.; Dong, K.; Ji, J.; Chen, B.; Tongay, S.; Ager, J. W.; Liu, K.

Vibrational Spectrum Renormalization by Enforced Coupling Across the van Der Waals Gap Between MoS2 and WS2 Monolayers. Phys. Rev. B 2015, 92, 241408. 41.

Chernikov, A.; Berkelbach, T. C.; Hill, H. M.; Rigosi, A.; Li, Y.; Aslan, O. B.; Reichman, D. R.;

Hybertsen, M. S.; Heinz, T. F. Exciton Binding Energy and Nonhydrogenic Rydberg Series in Monolayer WS2. Phys. Rev. Lett. 2014, 113, 076802. 42.

Kay, N. D.; Robinson, B. J.; Fal’ko, V. I.; Novoselov, K. S.; Kolosov, O. V. Electromechanical Sensing

of Substrate Charge Hidden under Atomic 2D Crystals. Nano Lett. 2014, 14, 3400-3404. 43.

Dolui, K.; Rungger, I.; Sanvito, S. Origin of the N-Type and P-Type Conductivity of MoS2

Monolayers on a SiO2 Substrate. Phys. Rev. B 2013, 87, 165402. 44.

Park, J.; Choudhary, N.; Smith, J.; Lee, G.; Kim, M.; Choi, W. Thickness Modulated MoS2 Grown by

Chemical Vapor Deposition for Transparent and Flexible Electronic Devices. Appl. Phys. Lett. 2015, 106, 012104.


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Synthesis and structural characterization of the Mo1-xWxS2 alloy. (a) Schematic illustration of the synthesis of scalable and uniform single-layer Mo1-xWxS2 alloys. Mo1-xWx metal films deposition by co-sputtering followed by sulfurization to transform metals into the Mo1-xWxS2 alloys; post-laser processing to thin the few-layer alloys down to a single-layer. (b) Optical image of the as-synthesized few-layer Mo1-xWxS2 (x = 0.48) alloy and bare SiO2/Si substrate. (c) 3D atomic force microscopy (AFM) height profile of the few-layer Mo1-xWxS2 (x = 0.48) alloy for the red dotted square area marked in (b). The average thickness is measured to be 3.8 ± 0.8 nm. (d) Optical image of the laser-irradiated Mo1-xWxS2 (x = 0.48) alloy. The laser is irradiated for 7 s over a 5 µm × 5 µm area (scale bar, 5 µm). (e) 2D AFM height profile of the laser-irradiated Mo1-xWxS2 (x = 0.48) alloy for the yellow dotted square marked in (d), showing the film is thinned by ~2.9 nm by the laser treatment. 313x119mm (300 x 300 DPI)


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XPS and Raman characterizations of as-synthesized Mo1-xWxS2 alloys. XPS spectra for the (a) Mo3d, (b) W4f, and (c) S2p core levels of the Mo1-xWxS2 alloys with different compositions. Whereas the binding energies of Mo3d and W4f are shifted to lower energies, the binding energy of S2p is shifted to a higher energy, with an increase in the W content (x). (d) Raman spectra of the as-synthesized Mo1-xWxS2 alloys as a function of W content, x = 0 to 1. The A1g and E12g (MoS2) modes are linearly shifted with respect to the change in the alloy composition. 266x62mm (300 x 300 DPI)


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Characterization of the Mo1-xWxS2 (x = 0.48) alloy by scanning transmission electron microscopy (STEM). (a) Low magnification STEM image of the Mo1-xWxS2 (x = 0.48) alloy (scale bar, 0.5 µm). (b) Cross-sectional STEM image of the folded region indicated by L in (a), where 4 layers of the alloy are shown. (c – f) Energy dispersive spectroscopy (EDS) mapping of the annular dark-field (ADF) image for the region M in (a). (c) Brighter areas show a higher elemental content, and the uniform distribution of (d) molybdenum, (e) tungsten, and (f) sulfur atoms is demonstrated (scale bar, 50 nm). (g) STEM image of the single-layer Mo1xWxS2 (x = 0.48) alloy at a selected region N in (a) (scale bar, 1 nm). (h) Close-up image of the region marked by a yellow square in (g), where the single-layer film has a six-fold coordination symmetry and honeycomb-like structure. (i) Intensity profiles of W and S atoms from both simulation and experiment. (j) Intensity profiles of Mo and W atoms. (k) Normalized intensity distribution map for Mo (blue) and W (yellow) atoms (scale bar, 1 nm). 253x157mm (300 x 300 DPI)


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PL and Raman spectra of the Mo1-xWxS2 alloy. Experimental PL spectra as a function of the laser irradiation time, (a) 1 – 7 s and (b) 8 – 18 s under ambient condition. (c) The summarized PL intensity and peak position of the Mo1-xWxS2 (x = 0.48) alloy. The PL peak intensity increases until 7 s before diminishing. The PL peak position is shifted to a higher energy up to 7 s and unchanged with further irradiation. (d) Comparison of PL spectra before and after laser irradiation showing ~5 times increase in peak intensity and ~0.03 eV peak shift after 7 s irradiation. (e) Raman spectra obtained at laser irradiated region (7 s) in comparing with non-irradiated region showing that the gap between E12g (Г) and A1g (Г) modes is reduced from 34 cm-1 to 31 cm-1 after laser irradiation. 272x126mm (300 x 300 DPI)


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Confocal PL and Raman mapping of laser-irradiated Mo1-xWxS2 alloys. (a) PL intensity mappings and (b) PL peak position mappings (5 µm × 5 µm) of laser-irradiated Mo1-xWxS2 (x = 0.38, 0.48, and 0.63) alloys. The PL intensity mappings show high spatial uniformity with emissions at 1.865, 1.897, and 1.916 eV for W content (x) of 0.38, 0.48, and 0.63 for the Mo1-xWxS2 alloy, respectively. Raman mapping of (c) ~388 cm-1 peak for E12g (MoS2) mode and (d) ~419 cm-1 peak for A1g mode in the laser-irradiated Mo1-xWxS2 (x = 0.48) alloy (scale bar, 3 µm). (e) PL intensity patterns on the Mo1-xWxS2 (x = 0.48) film (scale bar, 3 µm). 283x117mm (300 x 300 DPI)


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Deconvoluted PL spectra of the single-layer Mo1-xWxS2 alloys. (a) The PL spectra obtained from the laserirradiated single-layer Mo1-xWxS2 (x = 0, 0.38, 0.48, 0.63, and 1) alloys. The spectra are fitted with peaks corresponding to neutral excitons (AX) and trions (AT). (b) The peak positions of PL, AX, and AT spectra vs. alloy composition. (c) The intensity ratios of AX to AT (top panel), AX to total excitons (middle panel), and the excessive charge density (bottom panel) calculated by both the mass action model and three-level model. Based on the PL spectrum, the predominant components for MoS2 and WS2 change to AT and AX, respectively; in addition, the number of excessive charge carriers decreases when the W content (x) increases. 169x134mm (300 x 300 DPI)


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