Synthesis of uniform single layer WS2 for tunable photoluminescence

Two-dimensional transition metal dichalcogenides (2D TMDs) have gained great interest due to their unique tunable bandgap as a function of the number of layers. Especially, single-layer tungsten disulfides (WS2) is a direct band gap semiconductor with a gap of 2.1 eV featuring strong photoluminescence and large exciton binding energy. Although synthesis of MoS2 and their layer dependent properties have been studied rigorously, little attention has been paid to the formation of single-layer WS2 and its layer dependent properties. Here we report the scalable synthesis of uniform single-layer WS2 film by a two-step chemical vapor deposition (CVD) method followed by a laser thinning process. The PL intensity increases six-fold, while the PL peak shifts from 1.92 eV to 1.97 eV during the laser thinning from few-layers to single-layer. We find from the analysis of exciton complexes that both a neutral exciton and a trion increases with decreasing WS2 film thickness; however, the neutral exciton is predominant in single-layer WS2. The binding energies of trion and biexciton for single-layer WS2 are experimentally characterized at 35 meV and 60 meV, respectively. The tunable optical properties by precise control of WS2 layers could empower a great deal of flexibility in designing atomically thin optoelectronic devices.

energies ranging from 0.3 to 1.0 eV, which is attributed to their strong Coulomb attraction between charged particles 17,18 . In particular, photoexcitation in 2D TMDs leads to the formation of multi-carrier bound states because excitons can interact with free electrons 19,20 . Such interaction forms the exciton complexes including trions, a localized excitons consists of three charged quasiparticles (e.g., a negative trion consists of two electrons and one hole and a positive trion consists of two holes and one electron). The interaction of charged carriers and excitons controls the optical properties of TMDs 21 . Hence, the first step is to understand the behavior of exciton complexes in WS 2 for practical applications in opto-electronic devices as well as for the fundamental physics of emergent new materials. Furthermore, the exact values for the binding energy of the excitons in WS 2 are still debated and the behavior of exciton complexes with respect to the WS 2 layers remains unexplored.
In this regard, we have employed high-power laser processing to fabricate a uniform single-layer WS 2 from the few-layer WS 2 synthesized by a scalable two-step CVD method. Atomically uniform single-layer WS 2 was successfully synthesized by the two-step CVD method followed by a laser thinning method. The behavior of exciton complexes with the number of WS 2 layer was investigated during the laser thinning process. We found that the PL intensity has increased linearly (up to 6 times higher) as the number of WS 2 layer decreased. In particular, the dominant exciton in single-layer WS 2 is neutral exciton, while trion is dominant in few-layer WS 2 as analyzed by PL spectrum. These changes of exciton intensity contribute largely to the PL spectra of WS 2 film; such phenomenon is invaluable to engineer the opto-electronic properties of 2D WS 2 .

Results
Synthesis and characterization of few-layer WS 2 . The synthesis of uniform few atomic layer 2D TMDs was presented in our previous report by the two-step method 1,22 . Here we introduce a large-scale single-layer WS 2 film synthesis by using the two-step method followed by a laser thinning process. Schematic of the overall process and optical images of the sample in each step are presented in Fig. 1.
The synthesized WS 2 film was characterized by AFM, Raman spectroscopy, PL, and XPS. The optical image of as-synthesized few-layer WS 2 film on a SiO 2 /Si substrate indicates uniform and large-scale growth of WS 2 film (Fig. 2a). The thickness of WS 2 film is estimated to be ~3.78 nm (Fig. 2b), which is 4-5 layers of WS 2 , as confirmed by the AFM in the previous studies 6,23 . Figure 2c presents the Raman spectrum of as-synthesized few-layer WS 2 film (measured at 514 nm excitation laser line). The Raman spectrum is governed by the first-order modes: E 1 2g (Г) at 369.1 cm −1 and A 1g (Г) at 434.0 cm −1 ; however, the intensity of the second order mode of 2LA (M) at 363.5 cm −1 is also very high for WS 2 . Even though the 2LA (M) mode is overlapped with the E 1 2g mode, the peak is separated their individual contributions by the Lorentzian fitting. The frequency difference (△k) between 2LA (M) and A 1g as well as E 1 2g and A 1g modes are 70.5 cm −1 and 64.9 cm −1 , respectively. Thus, it is confirmed that the as-synthesized film is 4-5 layers of WS 2 6,24 . We also characterized the WS 2 film using XPS (Fig. 2d). The 4 f core-level spectrum represents three peaks at 32.6, 34.8, 38.2 eV corresponding to the W 4f 7/2 , W 4f 5/2 , and W 5p 3/2 state, respectively. The S 2p core-level shows two peaks at 162.1 and 163.3 eV, which match with the S 2p 3/2 and S 2p 1/2 states, respectively. Based on the XPS data, an excellent stoichiometry of the atomic WS 2 film is realized by the calculated S (66.7%) to W (33.3%) ratio of 2 25 .
Characterization of single-layer WS 2 fabricated by laser thinning method. The as-synthesized WS 2 film was irradiated by a scanning laser to thin the few-layer WS 2 down to a single-layer. Inset of Fig. 3a shows the optical image of WS 2 film after laser irradiation for 10 seconds; the image represents the laser covering 5 μm × 5 μm area. Color contrast is observed for the laser irradiated region (A) showing uniform and reduced thickness. The relative thickness difference after the laser irradiation (Fig. 3a) is ~2.88 nm; thus, the thickness of laser irradiation region is ~0.9 nm which corresponds to a single-layer WS 2 . The detailed surface profile of laser irradiated region is investigated by AFM (Fig. 3b). Remarkably, the laser beam can etch up to ~2.92 nm uniformly over the as-synthesized ~3.78 nm thick WS 2 film. It should be noted that the edge of the sidewalls is not flat. This is attributed to the Gaussian intensity profile of the confocal laser. The sidewall could be removed by overlapping the irradiated laser spots. After laser irradiation, Raman and PL measurements were performed with a reduced laser power of 200 μW to the irradiated area of 5 μm × 5 μm (Fig. 3c). The Raman spectra show the 2LA (M), E 1 2g (Г), and A 1g (Г) modes at 366.6 cm −1 , 369.6 cm −1 , and 432.1 cm −1 , respectively. Schematics illustrating the two-step method for few-layer WS 2 film growth and the laser thinning process for single-layer WS 2 fabrication (Insets show the optical images of as-deposited W film, WS 2 film after sulfurization of W film, and laser thinned WS 2 film on SiO 2 /Si substrate).
SCIENtIfIC REPORTS | 7: 16121 | DOI:10.1038/s41598-017-16251-2 Figure 3d presents the 2LA (M) and A 1g modes of both laser irradiated (A) and non-irradiated (B) regions on WS 2 film. The frequency differences (△k) between two modes changed from 70.5 cm −1 to 65.5 cm −1 ; also, the relative intensity of I 2LA /I A1g increased from 0.77 to 1.1. Berkdemir et al. 6 reported that frequency differences (△k) and the intensity ratios investigated by Raman spectra (λ exc = 514 nm) for thin WS 2 film are modifiable by the number of layers. Particularly, single-layer WS 2 shows △k of 65.2 cm −1 and I 2LA /I A1g of 2.1; those values are slightly different from the results of our single-layer WS 2 fabricated by laser irradiation. The differences are attributed to the unetched few-layer WS 2 as illustrated in the height profile (Fig. 3b). Figure 3e indicates PL intensities for both regions A (laser-irradiated) and B (non-irradiated) in Fig. 3a. After laser irradiation, PL intensity increases substantially up to 6 times; in addition, the PL peak position is shifted from 1.92 eV to 1.97 eV. The 1.97 eV PL peak position corresponds to single-layer WS 2 26,27 . Based on this approach, wafer scale laser-thinned single-layer WS 2 film could be readily fabricated by using a scanning laser beam irradiation. We employed in situ confocal PL and Raman spectroscopy to investigate the variation in the PL and Raman spectra with respect to the laser irradiation time. Figure 4a depicts the PL spectra of the WS 2 film measured with laser irradiation times (from 0 to 7 seconds) under ambient conditions. The PL intensity increases steadily as a function of the laser irradiation time of up to 7 seconds in which the PL intensity reaches maximum; as a result, the PL peak position is shifted from 1.92 eV to 1.97 eV, (dotted line in Fig. 4a). Thus, the precise thinning of the as-synthesized few-layer WS 2 film by the laser irradiation is evidenced by the increase in PL peak intensity and the shift in position. Figure 4b indicates Raman spectra of WS 2 film recorded at different laser irradiation time. The relative intensity (I 2LA /I A1g ) is gradually changed from 0.8 for 1 second to 1.1 for 7 seconds; also, the frequency difference between 2LA (M) and A 1g (Г) modes is reduced to 65.5 cm −1 for 7 seconds of irradiation time. The constant change of PL and Raman spectra with laser irradiation time reveals that the few-layer WS 2 film is continuously thinned with the laser time. The Raman peak position shift in A 1g mode shows an interesting phenomenon with regard to self-limited etching behavior. Figure 4c shows the A 1g mode shift as a function of laser irradiation time (from 0 to 25 seconds). It is noted that there exist two regimes: the region exposed for 0-7 seconds shows larger slope than the region exposed for 8-25 seconds. As a result, it is expected that the top WS 2 layers are etched entirely by the laser irradiation of 7 seconds, whereas the bottom single-layer WS 2 remains unetched even after 7 seconds laser irradiation. The lower slope of the A 1g mode after 7 seconds is attributed to the local strains or disorders caused by the continuous laser irradiation 28,29 . PL and Raman spectra ( Fig. 4a and b) verify that the single-layer WS 2 film is already achieved after laser irradiation for 7 seconds; thus, the etching rate of WS 2 film is ~0.42 nm/sec calculated by etched thickness (2.92 nm in Fig. 3b) and laser irradiation time (7 seconds).
To demonstrate the fabrication of single-layer WS 2 film with high uniformity, PL mapping with a PL peak position is carried out on the laser irradiated area (Fig. 4d-f). It is observed that the PL peak variation is uneven for 2 and 5 second irradiation time. However, the spatial uniformity increases significantly after laser irradiation for 10 seconds as shown in Fig. 4f. This uniform PL peak position demonstrates that the formation of uniform single-layer WS 2 film was achieved by the laser irradiation; also, further undesirable damage after forming the single-layer WS 2 film was prevented 30 . Once the normally incident laser is absorbed on WS 2 layers, the WS 2 layers produce local heating on the plane. Gastellanos-Gomez et al. 12 reported that the generated thermal energy mostly dissipate through the planer direction to the TMDs layers rather than the perpendicular direction which is bonded by weak van der Waals forces. Similarly, as reported by Han et al. 30 , the generated heat by light absorption is mainly accumulated on the upper graphene layers when a laser is induced on the film, while the SiO 2 /Si substrate plays an important role as a heat reservoir for the single-layer graphene to remain unetched. It is also reported that the heat conduction across 2D crystals-substrate interface is not negligible 31 . Initially, the heat propagates mostly along the basal plane of WS 2 film due to higher thermal conductivity of the basal plane (124 W/ mK) than the c-axis of WS 2 (1.7 W/mK) 32 . When the thickness is getting reduced by the laser irradiation, the heat conduction across WS 2 -SiO 2 /Si substrate becomes dominant, and the SiO 2 /Si substrate plays a role as a heat sink. Therefore, the flat and uniform single-layer WS 2 film could be produced by laser irradiation.
Investigation on behavior of exciton complexes. The change of PL spectra was reported to be associated with a transition from indirect band gap (few-layer WS 2 ) to direct band gap (single-layer WS 2 ). Because the origins of variation in relative contributions of exciton complexes (i.e. neutral exciton (A X ), trion (A T ), and biexciton (AA)) to PL emission have not been studied, we investigate the behavior of the exciton complexes depending on the numbers of WS 2 layer. First, we analyze the PL intensities of exciton complexes as a function of laser power to determine the transition and binding energies of the exciton complexes ( Fig. 5a and b) as per the definition of the energies 21 . These plots reveal that the estimated transition energies for A X , A T , and AA are 1.971, 1.936, and 1.911, respectively; also, the binding energies for A T , and AA are 35 meV and 60 meV, respectively. Those binding energies are consistent with the computationally simulated results of WS 2 33-35 . Each excitons indicates different values of slope (m) regarding the increase of PL intensity with respect to laser power (Fig. 5b). Based on previous studies, the exponent of m = 1.2-1.9 is a typical value for AA 36,37 ; in other words, AA has super-linear slopes because of the kinetics of excitation recombination and formation 38 . It is also important to note that there is an indication for the exponent of the A X (m ~0.66) is half the value of the AA (m ~1.26) 39 . Therefore, the sub-linear values of m ~0.66 and m ~0.91 for A X and A T , respectively, and the super-linear value of m ~1.26 for AA can verify the types of exciton for WS 2 40 . Figure 5c and d indicates the PL spectra obtained from representative irradiation times of 1-7 seconds by decomposing them into A X , A T , and AA. For the increased laser irradiation of up to 7 seconds, the predominant exciton of the PL spectrum is changed from A T to A X (Fig. 5c). It is noted that the steady intensity of AA disappeared at 3 seconds of irradiation time; thus, only A T and A X affects the PL spectra of laser thinned WS 2 film, whereas both A T and A X increase as the number of WS 2 layers decrease by the longer laser irradiation time. The increased rate in the intensity of A X is higher than A T (Fig. 5d); therefore, the PL spectrum is shifted up to 1.97 eV for the single-layer.
We expect that the intensity variation for both A T and A X is affected by the addition of charge carriers (either by an electron or hole) absorbed by oxygen molecule during laser irradiation; also, it is assumed that the variation of charge carriers can directly modify the intensity of both A T and A X 41,42 . Oh et al. 43 reported the intensity of exciton complexes for single-layer MoS 2 with various laser irradiation time under ambient conditions; here, the intensity of A X increases dramatically in the first few minutes of laser irradiation due to the charge transfer of MoS 2 to the adsorbed oxygen group induced by laser irradiation. Another report also presented a single-layer MoS 2 treated by oxygen plasma that shows much improved PL spectrum because of the charge transfer from MoS 2 to oxygen molecule on the sulfur vacancy 44 .
To confirm the charge transfer effect, full width at half-maximum (FWHM) of the A 1g mode is measured with laser irradiation time (0-25 seconds) (Fig. 6a). The reduced FWHM of the A 1g mode during the WS 2 film thinning is the evidence of p-doping (electron is moved from WS 2 to oxygen molecule) 45 ; otherwise, FWHM of the E 1 2g mode is not widely variable as a function of laser irradiation time (Fig. 6b). Therefore, the charge transfer from WS 2 to oxygen molecule induced by laser irradiation contributes to the increased ratio of A X to A T with the thinning of WS 2 . Interestingly, we observed the blue-shift of ~50 meV for the exciton peak as the thickness is reduced from few-to single-layer (Fig. 5c). Based on the previous experimental and theoretical studies, the quasiparticle band gap in 2D WS 2 is expected to increase with thinning of 2D WS 2 ; also, the binding energy of exciton complexes is predicted to increase with the thinning of WS 2 film due to the enhanced electron-hole interaction by weak dielectric screening [46][47][48][49][50] . The increase in quasiparticle band gap and the exciton binding energy affects the exciton peak position; thus, the shift of exciton complexes is negligible with the decreasing number of WS 2 layer. The blue-shifting of the exciton peak (measured ~50 meV) during the laser thinning process is consistent with the previous report 15 .

Discussion
Precise control of WS 2 layers is achieved by using two-step CVD synthesis method followed by a laser-thinning. The scalable synthesis of uniform single-layer WS 2 film is confirmed by AFM thickness measurement (~0.9 nm), Raman frequencies difference (65.5 cm −1 ), and PL peak position (1.97 eV). Based on the variation of PL and Raman spectra with respect to laser irradiation time, the optimum laser irradiation time for the synthesis of a single-layer WS 2 film is found to be 7 seconds. It is also realized that the single-layer WS 2 is retained with additional laser irradiation time, which is attributed to the thermal energy dissipated through the substrate. The intensity of both neutral exciton and trion increases with the reduction of WS 2 thickness; however, the biexciton appears to have no noticeable change. In particular, dominant exciton component in PL spectrum for the few-layer and single-layer WS 2 is trion and neutral exciton, respectively. The observed binding energies of the exciton complexes for single-layer WS 2 are 35 meV (trion) and 60 meV (biexciton). The tunable optical properties  by precise control of WS 2 layers and the understanding of their exciton complexes would lead to the design of novel optoelectronic devices.

Methods
Preparation of wafer scale thin WS 2 film. Large-scale few-layer WS 2 film was synthesized on a p-type silicon substrate (Boron doped; 0.001-0.005 Ω·cm) with 300 nm thick SiO 2 by using two-step method involving magnetron sputtering for deposition of tungsten (W) film and chemical vapor deposition (CVD) for sulfurization of W film. For the first step, 99.99% purity W target (Plasmaterials) was used to deposit thin film of W. Sputtering W film was carried out for 10 seconds at room temperature. Subsequently, the W film was sulfurized in the CVD for 1 hour at 600 °C to transform the film into WS 2 . During sulfurization, the sulfur powder was heated separately at ~250 °C, and argon was used as a carrier gas to convey sulfur (S) vapor species toward the W films.
Characterization of the WS 2 film. Thickness and surface analysis of as-synthesized WS 2 film were performed by atomic force microscopy (AFM) (Parks system, NX-10 model). X-ray photoelectron spectroscopy (XPS) (Thermo Scientific, ESCALAB250 model) was used for the chemical binding energies of W and S orbitals. A lab-made spectrometer combined with 514 nm wavelength and 200 μW power of a solid-state confocal laser microscope was used for PL and Raman spectroscopy measurements 40,51 . A 0.9 NA objective was used to focus the laser light which the lateral resolution was set at approximately 300 nm. Scattered light was gathered by the 0.9 NA objective and directed to a 50 cm long monochromater equipped with a cooled CCD.
Condition of laser thinning process. A scanning laser from a lab-made spectroscope (laser wavelength of 514 nm with 2.5 mW power and 300 nm lateral resolution) was used to thin few-layer WS 2 film down to single-layer by moving the laser over the WS 2 film laterally by 300 nm for the next exposure. The total number of exposures is 256 times for 5 × 5 μm area, and the exposure time is 15 seconds for each laser irradiation. For a detailed study of exciton complexes variation depending on layer numbers, we used the same laser with increased laser exposure time from 0 to 25.6 seconds and measured the individual PL spectrum per each 0.2 second (total 128 measurements) under ambient conditions.