Supporting Information

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1 Supporting Information 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 76207, United States Department of Mechanical and Energy Engineering, University of North Texas, Denton, Texas 76207, United States Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Sungkyunkwan University, Suwon 16419, Republic of Korea Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea *Corresponding Author: and

2 1. The Density Calculation of Excessive Charge Carriers First, the excessive charge carriers in Mo 1-xW xs 2 alloys were estimated from an analysis of neutral excitons and trions in the PL emissions. Based on the mass action model which is on the basis of the dynamic equilibrium between neutral excitons, trions, and free electrons, the following relationship is observed. 1-3 where E b is the trion binding energy of single-layer MoS 2 and WS 2 film, k B is Boltzman constant, and T is temperature (k). The effective masses of neutral excitons, trions, and electrons are expressed as m A 0 (0.8m 0), m A - (1.15m 0), and m e (0.35m 0), respectively; m 0 is mass of free electrons. 4 Moreover, N A 0 and N A - are populations of neutral excitons and trions, respectively; n el is the electron density. We employed a three level model including neutral excitons, trions, and the ground state 5 to find the relationship between the PL intensity and populations of the neutral excitons and trions. According to this model, the PL intensity weight is related to the populations of neutral excitons and trions, as indicated below. Where γ A 0 and γ A - are the relative decay rates and I A 0 and I A - are the integrated intensity of the neutral excitons and trions, respectively. Based on Mouri et al., 5 the value of is ~ The electron density (n el) is obtained after combining the three level model and mass action model, as expressed below.

3 2. Temperature Profiles of Single- and Few-Layer Mo1-xWxS2 Alloys under Laser Irradiation For the physics conditions of the simulation, the Heat Transfer in Solids model was used with heat source implied in a Gaussian function in the heat transfer equation, ρc p = (k T) + Q, where ρ, C p, and k represent the mass density, specific heat capacity, and thermal conductivity, respectively. Q is the total heat source, and the laser beam with the heat source using the Gaussian function was calculated by the formula, Q = exp, where t is the thickness of the Mo 1-xW xs 2 film, r 0 represents the radius of the Gaussian laser beam, and P is the incident laser power. In the simulation, the configuration of the samples is organized with a 5 μm-thick silicon substrate, a 300 nm SiO 2 layer, and a 0.7 nm-thick for single-layer and 5 nm-thick for few-layers with an area of 50 X 50 μm. We employed the detail physical properties of silicon and SiO 2 contained in the material library of Comsol Multiphysics. For the properties of single-layer Mo 1-xW xs 2 film (x = 0.48), the density, 6 thermal conductivity, 7,8 and specific heat capacity 9,10 are deduced to be 6.28 g/cm 3, W/mK, and 327 J/kg K, respectively. Furthermore, for properties of the few-layer film, we assumed the value 44.8 W/mK for the thermal conductivity, 11 and the values of density and heat capacity of single-layer were utilized for few-layer film because the irradiated laser enable to etch layer-bylayer from the top of the few-layer Mo 1-xW xs 2 film. We expected that the temperature at the top face of silicon domain was fixed at the ambient temperature (300 K). In addition, the computational optimization was focused on meshing to generate extra fine; thus, highly accurate temperature profiles were created.

4 Intensity (arb. unit) Raman frequency (cm -1 ) a b 430 2LA(M) E 1 2g(Γ) A 1g (Γ) A 1g E 1 2g (MoS 2 ) Raman shift (cm -1 ) W composition (x) Figure S1. (a) Raman spectrum from the as-synthesized few-layer WS 2 film including Lorentzian peak fits. The frequency difference between E 1 2g (Г) and A 1g (Г) modes is 64.8 cm -1. (b) W content (x)-dependent Raman frequencies of A 1g and E 1 2g (MoS 2) modes of the few-layers Mo 1-xW xs 2. The A 1g and E 1 2g (MoS 2) modes shift to higher and lower frequencies, respectively, as decreasing W content (x).

5 a b Folded Mo 1-x W x S 2 (x = 0.48) Thickness Measured region c d c d unknown unknown Figure S2. (a) Selected area electron diffraction (SAED) pattern of the as-prepared few-layer Mo 1-xW xs 2 (x = 0.48) alloy. The diffraction pattern represents a polycrystalline thin film. (b) Low magnification STEM image highlighting the number of alloy layers (scale bar, 500 nm). (c) and (d) are zoomed-in two folded regions in highlighted in (b) to give a closer look into the number of layers. 6 ± 2 layers were observed in the regions (scale bar, 20 nm).

6 Intensity (arb. unit) Intensity (arb. unit) a E (WS 2 ) A 1g b WS 2 x = 1 x = LA(M) E 1 2g(Γ) A 1g (Γ) x = 0.48 x = 0.38 E 1 2g (MoS 2 ) x = Ramans shift (cm -1 ) Raman shift (cm -1 ) Figure S3. (a) Raman spectra for the single-layer Mo 1-xW xs 2 as the W content (x) changed from 0 to 1. (b) Raman spectrum for the single-layer WS 2 film including Lorentzian peak fits. The frequency difference between E 1 2g (Г) and A 1g (Г) modes is 62.5 cm -1.

7 a b c Non-irradiated region d e Laser-irradiated region Figure S4. (a) STEM image of the Mo 1-xW xs 2 (x = 0.48) alloy showing laser-irradiated (right side) and nonirradiated regions (left side) (scale bar, 2 μm). (b e) Close-up images of laser-irradiated single-layer Mo 1-xW xs 2 (x = 0.48) alloy (scale bar, 1 nm) indicating that the Mo 1-xW xs 2 (x = 0.48) alloy is 2H semiconducting phase without having Mo and W vacancy, but sulfur vacancies of 48 out of 451 sulfur sites are identified. The calculated concentration of sulfur vacancies is about ~1.16 atoms per nm 2.

8 a b Figure S5. Calculated temperature profiles for (a) laser-irradiated single-layer Mo 1-xW xs 2 (x = 0.48) alloy and (b) asprepared few-layer Mo 1-xW xs 2 (x = 0.48) alloy on SiO 2/Si substrate (2.5 mw laser irradiation). Both temperature profiles show that the highest temperature at center part is 407 for single-layer and 604 for few-layer alloy, and the temperature gradually decrease following a Gaussian distribution.

9 hν E b (A X ) A X A T E b (A T ) AA E b (AA) CB PL E G VB Figure S6. Schematic illustrating the transition energies (A X, A T, and AA) and binding energies (E b (A X ), E b (A T ), and E b (AA)) for neutral exciton, trion, and biexciton, respectively.

10 Peak wavelength (ev) PL intensity (counts) a b 10 4 Neutral excitons Trions Biexcitons m ~ ev m ~ ev 1.84 Neutral excitons Trions Biexcitons ev m ~ Excitation laser power ( W) Excitation laser power ( W) Figure S7. (a) Peak positions and (b) peak intensities of neutral excitons, trions, biexcitons from laser irradiated single-layer Mo 1-xW xs 2 (x = 0.48) alloy. The dashed lines in (a) and (b) are guides for visualization. The m values in (b) indicate logarithmic values of each slope.

11 REFERENCES 1. Siviniant, J.; Scalbert, D.; Kavokin, A.; Coquillat, D.; Lascaray, J. Chemical Equilibrium Between Excitons, Electrons, and Negatively Charged Excitons in Semiconductor Quantum Wells. Phys. Rev. B 1999, 59, 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 Two-Dimensional Neutral and Charged Excitons in a Monolayer Semiconductor. arxiv preprint arxiv: Ron, A.; Yoon, H.; Sturge, M.; Manassen, A.; Cohen, E.; Pfeiffer, L. Thermodynamics of Free Trions in Mixed Type GaAsAlAs Quantum Wells. Solid State Commun. 1996, 97, Cheiwchanchamnangij, T.; Lambrecht, W. R. Quasiparticle Band Structure Calculation of Monolayer, Bilayer, and Bulk MoS 2. Phys. Rev. B 2012, 85, Mouri, S.; Miyauchi, Y.; Matsuda, K. Tunable Photoluminescence of Monolayer MoS 2 via Chemical Doping. Nano Lett. 2013, 13, Worsley, M. A.; Shin, S. J.; Merrill, M. D.; Lenhardt, J.; Nelson, A. J.; Woo, L. Y.; Gash, A. E.; Baumann, T. F.; Orme, C. A. Ultralow Density, Monolithic WS 2, MoS 2, and MoS 2/graphene Aerogels. ACS Nano 2015, 9, Yan, R.; Simpson, J. R.; Bertolazzi, S.; Brivio, J.; Watson, M.; Wu, X.; Kis, A.; Luo, T.; Hight Walker, A. R.; Xing, H. G. Thermal Conductivity of Monolayer Molybdenum Disulfide Obtained from Temperature-Dependent Raman Spectroscopy. ACS Nano 2014, 8, Peimyoo, N.; Shang, J.; Yang, W.; Wang, Y.; Cong, C.; Yu, T., Thermal Conductivity Determination of Suspended Mono-and Bilayer WS 2 by Raman Spectroscopy. Nano Res. 2015, 8, Volovik, L.; Fesenko, V.; Bolgar, A.; Drozdova, S.; Klochkov, L.; Primachenko, V. Enthalpy and Heat Capacity of Molybdenum Disulfide. Powder Metall. Met. Ceram. 1978, 17, O'hare, P.; Hubbard, W.; Johnson, G.; Flotow, H. Calorimetric Measurements of the Low-Temperature Heat Capacity, Standard Molar Enthalpy of Formation at K, and High-Temperature Molar Enthalpy Increments Relative to K of Tungsten Disulfide (WS 2), and the Thermodynamic Properties to 1500 K. J. Chem. Thermodyn. 1984, 16,

12 11. Gu, H.; Lu, Y.; Zhu, D.; Li, K.; Zheng, S.; Wang, J.; Ang, K.-W.; Xu, K.; Liu, X. High Temperature Study on the Thermal Properties of Few-layer Mo 0.5W 0.5S 2 and Effects of Capping Layers. Results phys. 2017, 7,