Photochemical Reaction in Monolayer MoS2 via Correlated

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Photochemical Reaction in Monolayer MoS2 via Correlated Photoluminescence, Raman Spectroscopy and Atomic Force Microscopy Hye Min Oh, Gang Hee Han, Hyun Kim, Jung Jun Bae, Mun Seok Jeong, and Young Hee Lee ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b00895 • Publication Date (Web): 25 Apr 2016 Downloaded from http://pubs.acs.org on April 26, 2016

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Photochemical Reaction in Monolayer MoS2 via Correlated Photoluminescence, Raman Spectroscopy and Atomic Force Microscopy Hye Min Oh†,‡, Gang Hee Han†, Hyun Kim†,‡, Jung Jun Bae†, Mun Seok Jeong*,†,‡, Young Hee Lee*,†,‡ †

Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS),

Sungkyunkwan University, Suwon 440-746, Republic of Korea. ‡

Department of Energy Science, Department of Physics, Sungkyunkwan University, Suwon

440-746, Republic of Korea

*

Corresponding author E-mail: [email protected], [email protected]

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ABSTRACT Photoluminescence (PL) from monolayer MoS2 has been modulated using plasma treatment or thermal annealing. However, a systematic way of understanding the underlying PL modulation mechanism has not yet been achieved. By introducing PL and Raman spectroscopy, we analyze that the PL modulation by laser irradiation is associated with structural damages and associated oxygen adsorption on the sample from ambient. Three distinct behaviors were observed according to the laser irradiation time: i) slow photooxidation at the initial stage, where the physisorption of ambient gases gradually increases the PL intensity, ii) fast photo-oxidation at a later stage, where chemisorption increases the PL intensity abruptly, and iii) photo-quenching, complete reduction of PL intensity. The correlated confocal Raman spectroscopy confirms that no structural deformation is involved in slow photo-oxidation stage but the structural disorder is invoked during fast photooxidation stage and severe structural degradation is generated during photo-quenching stage. The effect of oxidation is further verified by repeating experiments in vacuum where the PL intensity is simply degraded with laser irradiation in vacuum owing to a simple structural degradation without involving oxygen-functional groups. The charge scattering by oxidation is further explained by emergence/disappearance of neutral excitons and multi-excitons during each stage. KEYWORDS:

Molybdenum

disulfide,

photo-oxidation,

photoluminescence, Raman, AFM

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photo-quenching,

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Recently, layered transition-metal dichalcogenide (TMdC) materials with the chemical structure MX2 (M = Mo, W, Ti, V, Ta, Hf, Pt and X = S, Se,Te) have attracted considerable interest in the fundamental sciences and for technology applications.1-5 Among them, monolayer (1L) molybdenum disulfide (MoS2) has exhibited strong photoluminescence (PL) and received significant attention regarding potential application in optoelectronic devices, such as light emitting diodes and solar cells.6-9 However, because all the atoms are exposed to the environment, the optical and electronic properties of TMdCs are generally easily modified by environmental changes.10, 11 Therefore, investigating the optical and electronic properties of monolayer TMdCs modulated with various parameters, including environmental effects, is required. Several attempts have been made to modulate the optical properties of 1L-MoS2, including enhancing or quenching the PL intensity by engineering structural defects (such as vacancies, point defects, dislocations, and grain boundaries) that are known to play a significant role in the interaction between TMdCs materials and other molecules.11-14 Some previous works have shown that the enhancement of PL intensity by annealing or weak plasma irradiation is closely related to strong oxygen bonding at the defect sites of 1L-MoS2,10, 12 although the PL enhancement was explained by the removal of adsorbates with acid treatment,15 and the quenching of PL intensity by strong plasma irradiation is associated with the lattice distortion or creation of MoO3 defects.16 A significant peak shift of the E12g and A1g modes in the Raman spectra has also been observed, whereas in some cases, such Raman modes remained unchanged.11-13 Therefore, a systematic study that involves careful control of the parameters and measurements of the related structural properties is required to understand the underlying mechanism of PL enhancement or reduction. Nevertheless, no effort has been done in

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understanding emergence/disappearance of exciton and multiexciton contributions under such parameter variances. In this work, we demonstrate a simple method to control the optical properties of a selected area of 1L-MoS2 by laser irradiation. PL intensity measurements revealed different behaviors according to the laser irradiation time: the PL intensity of 1L-MoS2 sheets gradually increased at initial stage and PL quenching occurred at a later stage, which was attributed to the structural degradation accompanying severe oxidation, confirmed by confocal Raman spectroscopy and atomic force microscopy (AFM). To study the origin of photo-oxidation, the laser irradiation on the chemical vapor deposition (CVD)-grown 1L-MoS2 was investigated in air and vacuum environments. Under the vacuum condition, the PL intensity was gradually reduced as the laser irradiation time was prolonged, whereas in air, the PL intensity was enhanced under the same irradiation conditions. This clearly demonstrates that oxygen-related functional groups attached to 1L-MoS2 are the primary origin of PL enhancement. The influence of oxidation to multiexcitons is also examined.

RESULTS AND DISCUSSION Large triangular flakes (30–100 µm in length) of 1L-MoS2 were synthesized on SiO2/Si by chemical vapor deposition (CVD).17 The sample was transferred onto quartz using the wet transfer method (see Methods for details). Figure 1 shows an illustration of the laser irradiation process, in which a focused laser beam illuminates the 1L-MoS2 on quartz along the normal direction of the sample. A diode-pumped solid state laser of 532 nm wavelength was used for the excitation. A fixed power of 3.7 mW was delivered to a focal spot size of ~ 900 nm, obtaining a laser beam power density of 5.8 × 105 W cm-2. Ambient gases, such as 4


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O2 and H2O, were expected to be considerably involved during laser irradiation. Therefore, we used the controlled environment of a vacuum chamber. At initial stage oxidation, gas adsorbates are physisorbed (they can be removed in high vacuum). At a later stage of oxidation, laser irradiation creates damages on the MoS2 surface and forms defects such as S vacancies, which can be stabilized by strong chemisorptions of gas adsorbates (they cannot be removed in high vacuum), as schematically drawn in the atomic model of Fig. 1. Figure 2a depicts PL spectra of 1L-MoS2 measured with various laser irradiation times under ambient conditions. The figure shows that the PL intensity increases gradually as a function of laser irradiation time until it reaches a maximum value in 1 h. Meanwhile, the PL peak position is blueshifted from 1.856 eV to ∼1.906 eV, as indicated by the dotted line in Figure 2a, which clearly demonstrates that the PL spectra are strongly influenced by laser irradiation under ambient conditions. The inset shows the PL image of 1L-MoS2 after a laser irradiation time of 55 min. The bright-colored spot corresponds to the enhanced PL intensity of the laser-irradiated region. The integrated intensity is increased two times and a large peak shift of +50 meV is observed. However, the PL intensity gradually decreases after a laser irradiation time of 55 min, as shown in Figure 2b. In this region, the peak positions are relatively unaffected by irradiation time. The dark-colored spot corresponds to the laserirradiated site. Figure 2c shows the peak position and PL intensity of the 1L-MoS2 versus the laser irradiation time. After an irradiation time of 55 min, the distinct blue shift of the PL spectra observed at the earlier stage is not so obvious and remains flat. On the other hand, the PL intensity is reduced at the later stage after an abrupt enhancement at the initial stage. These phenomena were uniquely observed in our laser irradiation experiments and are distinctly different from other reported results.12,16 Such distinct stages of oxidation are ubiquitous in a wide range of powers (0.1 - 4.3 mW) except low power below 10 µW (see 5


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Supporting Information (SI), Figure S1). To interpret these spectral changes, the peaks were deconvoluted into several components, according to methodology used in previous works.1820

In general, A exciton in PL spectra are strong for 2D materials owing to reduced charge

screening. The exciton binding energies are of the order of 0.3–0.6 eV.21-23 Multiexciton peaks, biexcitons or trions depending on the sample conditions and laser power used, are predominant in 2D materials due to reduced charge screening, unlike in bulk materials.24, 25 The trion binding energies are 30–40 meV,26, 27 and biexciton binding energies are located 30–70 meV 28, 29 below the trion peak, which are usually observed even at room temperature. The trion peak is dominant in highly doped samples whereas the biexciton peak is dominant in neutral samples with high excitation laser power.30-32 The A peak in the PL spectrum is positioned near 1.86 eV with an additional B peak near 2.03 eV, as shown in Figure 2d. The peak shape is asymmetric with a bump at the low-energy side. Emergence of B-exciton and asymmetric contribution in A peak implies strongly the emergence of multiexcitons. Since relatively high laser power of 3.7 mW is used in our work, it is likely to observe the contribution of multiexciton peaks in PL spectra.29,33 For this, the peaks were deconvoluted into three peaks; neutron exciton peak (X0), multiexciton peak (XX), and B neutron exciton peak (B) due to spin-orbit splitting of the valence band.31 Because the 1L-MoS2 sample was intrinsically n-doped (but not heavily doped),34 it is reasonable to fit the A peak of the pristine sample into the neutral exciton (X0) (navy curve) near 1.91 eV, multiexciton peak XX near 1.86 eV, as shown in Figure 2d. The asymmetric large multiexciton peak in the pristine sample implies the presence of biexciton peak in addition to trion peak. For the simplicity, we still limit ourselves not to differentiate the peak further. Multiexciton peaks, such as trion and biexciton peaks, usually emerge simultaneously with the B peak (blue curve). At 55 min laser irradiation, the A peak position of the pristine sample was blueshifted from 1.86 eV to 1.91 6


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eV (Figure 2(d)). Because the peak shape was severely altered, the A peak was fitted only with two peaks. Note that in this case, not only multiexciton peak was missing but also the B peak almost disappeared. No further significant changes were observed in the peak position and shape after 55 min. We confirmed that the two-peak fitting of the A exciton peak was reasonable with an acceptable R2 = 0.99 (see SI, Figure S2). Figure 2e summarizes the time evolution of the variations of the integrated PL intensity and the peak positions of each exciton. In the pristine sample, the multiexciton intensity is larger than that of the neutral exciton owing to the use of high-power laser (3.7 mW). At initial stage (region I), the multiexciton intensity decreases slightly, while that of the neutral exciton intensity remains almost unchanged. After 40 min of irradiation (region II), the neutral exciton intensity increase drastically. After 50 min (region III), both intensities are reduced. The neutral and multiexciton peak positions remain nearly constant throughout the irradiation process. On the other hand, the full width at half maximum (FWHM) of the neutral exciton was sharply increased up to 50 min and gradually reduced to saturate at longer irradiation times (Figure 2f). The FWHM of multiexciton was not changed appreciably at initial stage but rapidly reduced after 40 min. We presume that the sample could have been chemically modified during persistent laser irradiation, although the detailed mechanism of such a process is still unclear. To understand how the PL intensities are modified with laser irradiation time, confocal PL mapping with a high spatial resolution (~ 300 nm) was performed. Figure 3a shows the PL image of the pristine 1L-MoS2 sample, which exhibits a uniform PL intensity for the entire 1L-MoS2 flake except the edge areas. The PL spectra of several points (A–D) are almost identical (Figures 3c) and independent of their positions on the sample. Figure 3b displays the PL intensity map after 205 min of laser irradiation focused on area B, which shows a severely 7


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degraded intensity in area B. The PL spectra of points A–D are also provided in Figure 3d (see SI, Figure S3). In this case, both the peak positions and the intensities vary drastically from position to position; namely, the PL intensity is severely reduced and the peak position is upshifted in area B. Interestingly, the PL intensities and peak positions at areas C and D are enhanced compared to the unchanged position (area A). To further understand the change of PL of 1L-MoS2 during laser irradiation associated with structural deformation, confocal Raman spectra were measured for 1L-MoS2 according to laser irradiation time. Figure 4a presents Raman spectra of 1L-MoS2 for various laser irradiation times under ambient conditions. At 5 min laser irradiation, the intensity of A1g mode increases and the peak position is blueshifted and FWHM is reduced compared to those of pristine sample, while the intensity and peak position of E12g mode is almost unchanged. These changes are clearly visualized in the normalized intensity (Fig. 4b). It has been reported that the blueshift of A1g peak and the reduced FWHM is an evidence of p-doping and the E12g peak is shifted by strain.35, 36 Our results imply that the sample is p-doped and no strain is involved during the laser irradiation at 5 min. With prolonged laser irradiation time, no further blueshift is observed in A1g peak, while the intensity keeps decreasing. Meanwhile, the peak shift in E12g is nearly negligible, although the peak intensity is reduced gradually. At early stage of laser irradiation, laser induces physisorption of oxygen-functional groups and induced charge transfer from MoS2 to oxygen-functional groups. At a later stage of laser irradiation, the samples are damaged by severe laser irradiation and the generated defects form strong chemical bonds with gas adsorbates. The similar blueshift of A1g was observed in other excitation wavelength (see SI, Figure S4). Figure 4c and 4d show the Raman mapping images for the integrated intensity of both E12g and A1g mode of 1L-MoS2 before and after laser irradiation. All other areas except the laser8


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irradiated spot show similar intensities. The reduced Raman intensity was observed in laserirradiated spot, which is caused by the structural degradation by laser irradiation. The intensity for E12g mode after laser irradiation demonstrates negligible change with laser irradiation, implying again no strain effect (see SI, Figure S5c and S5d). The maximum blueshift in A1g peak is observed in the middle of the laser spot (Fig. 4e and 4f). To verify the structural evolution with laser irradiation, atomic force microscope (AFM) topological images were provided in Fig. 4g and 4h. The laser-irradiated region was damaged and oxidized. The marked area B in Fig. 3 was strongly corrugated after laser irradiation. The height profile across the red line in Figure 4h reveals a wide height distribution of approximately 20 nm. This strongly suggests that the sample was damaged (negative region) or oxidized (protruded region) (see SI, Figure S6). The effect of ambient gases was further tested by repeating our experiment in a vacuum chamber (basal pressure 2.3 × 10-5 Torr). For this experiment, a 633-nm laser with a power of 1.67 mW was used (see SI, section I). Figure 5a shows the PL spectra of 1L-MoS2 after laser irradiation for 30 min in vacuum and air. The same flake but a different laser spot position was used in these measurements. The PL spectrum at 633 laser excitation before irradiation (black curve) showed a peak position near 1.86 eV, taken as a reference spectrum before irradiation, which was similar to that of the 532-nm excitation. After 30 min of laser irradiation in air, the peak position was blueshifted with enhanced PL intensity (navy curve), similarly to the previous observations. However, a different behavior was observed when the laser irradiation was performed in the vacuum chamber for 30 min. The PL intensity was significantly reduced and the peak was shifted closer (red curve) to the position that it had without irradiation. Figure 5b shows the time evolution of the PL position and the intensity for the sample irradiated in vacuum and demonstrates that they both gradually decreased with 9


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time. We clearly observe that the adsorption of ambient gases enhances the PL intensity and induces blue shift. Without such adsorption, the sample becomes more n-typed, has a dominant electron concentration. In this case, multiexciton formation prevails, neutral exciton population decreases and, consequently, the A peak intensity and position decrease (see SI, Figure S7). As discussed above, laser irradiation under ambient conditions induces adsorption of ambient gases, enhancing PL intensity and blueshifting the PL peak position. However, structural degradation occurs at prolonged laser irradiation times. This raises the question of how the structural degradation occurs in 1L-MoS2 at the applied laser power. To clarify this issue, the heat equation was solved numerically for the performed laser irradiation. Using the finite element method (FEM) (see SI, section II and Figure S8), we obtained the numerical simulation results of Figure 6. The temperature profile around the center of the laser spot is displayed in Figure 6b. The temperature increased to 253 °C at the center of the laser spot at a power of 3.7 mW and gradually decreased, following a Gaussian distribution when moving away from the center. This was caused by the heat dissipation to the quartz substrate and along the radial direction of the 1L-MoS2 layer. To simulate the occurrence of the structural degradation at the peak temperature (~260 °C), the pristine sample was annealed in air at 260 °C for various times (see SI, Figure S9). Figure 6d shows the optical image of the sample annealed for 180 min, which clearly indicates a higher degradation than in the other samples. The Raman mapping image also revealed structural degradation, as displayed in Figure 6e. The peak intensity at the dark spot (circle II in Figure 6e) clearly shows the reduced Raman intensity compared to the bright spot (circle I), implying the damage of the sample. A similar trend was also exhibited by the PL intensity increase in the same positions in the bright spot (see SI, Figure S10). 10


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The observed time evolution of the gradual structural degradation of 1L-MoS2 morphology with thermal annealing has also been observed in previous AFM studies of thermal annealing in air.37 Although no obvious structural change of the 1L-MoS2 film was observed during short-time annealing, the PL intensity increased. The enhanced PL intensity has been attributed to physisorption or chemisorption of O2 and H2O molecules on MoS2.12, 37 At a very early stage of adsorption, the adsorption rate or photo-oxidation rate is low; thus, the PL intensity increases slowly, as displayed in region I of Figure 2c. Fast adsorption or fast photooxidation can occur at a later stage with smaller activation barrier heights, after the adsorption is initiated to create chemisorption; hence, PL increases rapidly, as described by region II. No structural damage occurs until this stage, as evidenced by the unmodulated Raman intensity. During this process, sulfur atoms can be desorbed and replaced by oxygen atoms, producing minimum structural deformation. After prolonged annealing, the damage of the strongly chemisorbed MoS2 surface can create structural defects, as evidenced by the reduced Raman intensity, and often etch the surface; thus, the PL intensity decreases or photo-quenching occurs, as shown in region III.

However, the structural degradation is limited only to the center of the spot, whereas the periphery maintains its original structure owing to severe heat dissipation. In contrast, the adsorption of ambient gases around this region can be enhanced by the increased temperature because of the reduced activation barrier heights and, therefore, PL enhancement is expected, as shown in Figure 3.

CONCLUSION We have investigated the photochemical reaction of 1L-MoS2 under laser irradiation via correlated PL, Raman and AFM spectroscopy. We observed three distinct regions of 11


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modification behaviors according to the laser irradiation time under ambient conditions: slow photo-oxidation involving physisorption, fast photo-oxidation involving chemisorption, and photo-quenching involving structural degradation. During photo-oxidation, the neutral exciton peak intensity increased while multiexciton intensity decreased gradually. This was explained by the de-doping effect of n-type MoS2 by the adsorption of oxygen-related functional groups. During photo-quenching, the intensities of neutral excitons and multiexcitons were reduced significantly. While our photo-oxidation phenomena are observed at relatively high laser power, the similar trend of the modification on PL and structure deformation via three steps of oxidation will be observed even at different laser powers.

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METHODS Wet-Etching Transfer Method A PMMA C4 (Micro Chem, 4 wt% in chlorobenzene) solution was spin-coated on a MoS2/SiO2/Si substrate at 3000 rpm for 60 s and dried in air. The PMMA/MoS2 film was floated on the surface of the potassium hydroxide (1 M) solution that was used as the etchant for the SiO2 layer. The PMMA/MoS2 film was rinsed by distilled water to remove the residual etchant. The washed PMMA/MoS2 was transferred onto the quartz substrate. After drying in air, the PMMA was removed with acetone

Thermal Annealing The thermal annealing was performed on a digital hot plate in air. The samples were heated to 260 °C for different time periods. After annealing, the samples were removed from the hot plate and then optical measurements were immediately performed.

Characterizations Confocal micro-Raman/PL measurements were performed using commercial equipment (NT MDT, NTEGRA spectra PNL). A 100 × objective lens with a numerical aperture of 0.7 was used. The excitation source was a 532-nm laser (2.33 eV) with an optical power of 3.7 mW. The topography of the sample surfaces was obtained using a commercial AFM in contact mode (Anasys Instruments). A commercial spectrophotometer (V-670, JASCO) was used for measurements of UV-Vis absorption spectra.

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ASSOCIATED CONTENT Supporting Information Experimental method, simulation, supporting figures S1-S10, and supporting information references. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected], [email protected]

Conflict of Interest The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was supported by IBS-R011-D1 and the Human Resources Development program (No. 20124010203270) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), funded by the Ministry of Trade, Industry and Energy of the Korean government.

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17. Han, G. H.; Kybert, N. J.; Naylor, C. H.; Lee, B. S.; Ping, J.; Park, J. H.; Kang, J.; Lee, S. Y.; Lee, Y. H.; Agarwal, R.; Johnson, A. T. C. Seeded Growth of Highly Crystalline Molybdenum Disulphide Monolayers at Controlled Locations. Nat. Commun. 2015, 6, 6128. 18. Mouri, S.; Miyauchi, Y.; Matsuda, K. Tunable Photoluminescence of Monolayer MoS2 via Chemical Doping. Nano Lett. 2013, 13, 5944-5948. 19. Lin, Y.; Ling, X.; Yu, L.; Huang, S.; Hsu, A. L.; Lee, Y.-H.; Kong, J.; Dresselhaus, M. S.; Palacios, T. Dielectric Screening of Excitons and Trions in Single-Layer MoS2. Nano Lett. 2014, 14, 5569-5576. 20. Li, Y.; Qi, Z.; Liu, M.; Wang, Y.; Cheng, X.; Zhang, G.; Sheng, L. Photoluminescence of Monolayer MoS2 on LaAlO3 and SrTiO3 Substrates. Nanoscale 2014, 6, 15248-15254. 21. Hill, H. M.; Rigosi, A. F.; Roquelet, C.; Chernikov, A.; Berkelbach, T. C.; Reichman, D. R.; Hybertsen, M. S.; Brus, L. E.; Heinz, T. F. Observation of Excitonic Rydberg States in Monolayer MoS2 and WS2 by Photoluminescence Excitation Spectroscopy. Nano Lett. 2015, 15, 2992-2997. 22. Cheiwchanchamnangij, T.; Lambrecht, W. R. L. Quasiparticle Band Structure Calculation of Monolayer, Bilayer, and Bulk MoS2. Phys. Rev. B 2012, 85, 205302. 23. Ramasubramaniam, A. Large Excitonic Effects in Monolayers of Molybdenum and Tungsten Dichalcogenides. Phys. Rev. B 2012, 86, 115409. 24. Zhang, D. K.; Kidd, D. W.; Varga, K. Excited Biexcitons in Transition Metal Dichalcogenides. Nano lett. 2015, 15, 7002-7005. 25. You, Y.; Zhang, X.-X.; Berkelbach, T. C.; Hybertsen, M. S.; Reichman, D. R.; Heinz, T. F. Observation of Biexcitons in Monolayer WSe2. Nature Phys. 2015, 11, 477-481. 26. Berkelbach, T. C.; Hybertsen, M. S.; Reichman, D. R. Theory of Neutral and Charged Excitons in Monolayer Transition Metal Dichalcogenides. Phys. Rev. B 2013, 88, 045318. 17


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27. Ross, J. S.; Wu, S.; Yu, H.; Ghimire, N. J.; Jones, A. M.; Aivazian, G.; Yan, J.; Mandrus, D. G.; Xiao, D.; Yao, W.; Xu, X. Electrical Control of Neutral and Charged Excitons in a Monolayer Semiconductor. Nat. Commun. 2013, 4, 1474. 28. Mai, C.; Barrette, A.; Yu, Y.; Semenov, Y. G.; Kim, K. W.; Cao, L.; Gundogdu, K. ManyBody Effects in Valleytronics: Direct Measurement of Valley Lifetimes in Single-Layer MoS2. Nano Lett. 2013, 14, 202-206. 29. Sie, E. J.; Frenzel, A. J.; Lee, Y.-H.; Kong, J.; Gedik, N. Intervalley Biexcitons and ManyBody Effects in Monolayer MoS2. Phys. Rev. B 2015, 92, 125417. 30. Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M.; Chhowalla, M. Photoluminescence from Chemically Exfoliated MoS2. Nano Lett. 2011, 11, 5111-5116. 31. 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. 32. Ji, Q.; Zhang, Y.; Gao, T.; Zhang, Y.; Ma, D.; Liu, M.; Chen, Y.; Qiao, X.; Tan, P.-H.; Kan, M.; Feng, J.; Sun, Q.; Liu, Z. Epitaxial Monolayer MoS2 on Mica with Novel Photoluminescence. Nano Lett. 2013, 13, 3870-3877. 33. 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 34. Ghatak, S.; Pal, A. N.; Ghosh, A. Nature of Electronic States in Atomically Thin MoS2 Field-Effect Transistors. ACS Nano 2011, 5, 7707-7712. 35. Wang, Y.; Cong, C.; Qiu, C.; Yu, T. Raman Spectroscopy Study of Lattice Vibration and Crystallographic Orientation of Monolayer MoS2 under Uniaxial Strain. Small 2013, 9, 28572861. 36. Chakraborty, B.; Bera, A.; Muthu, D.; Bhowmick, S.; Waghmare, U. V.; Sood, A. 18


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Symmetry-Dependent Phonon Renormalization in Monolayer MoS2 Transistor. Phys. Rev. B 2012, 85, 161403. 37. Wei, X.; Yu, Z.; Hu, F.; Cheng, Y.; Yu, L.; Wang, X.; Xiao, M.; Wang, J.; Wang, X.; Shi, Y. Mo-O Bond Doping and Related-Defect Assisted Enhancement of Photoluminescence in Monolayer MoS2. AIP Adv. 2014, 4, 123004.

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Figures:

Figure 1. Schematic illustration of laser irradiation on MoS2/quartz under ambient conditions. The beam size was smaller than 900 nm. The laser power was 3.7 mW.

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Figure 2. Time evolution of PL spectra of 1L-MoS2 obtained with an excitation energy of 2.3 eV (532 nm) and a power of 3.7 mW. Measured PL spectra as a function of laser irradiation time (a) from 30 to 55 minutes and (b) from 55 to 205 minutes under ambient conditions. The insets show the confocal PL intensity mapping images after laser irradiation for 55 and 205 minutes, respectively. (c) Experimental PL peak position and intensity of A exciton as a function of laser irradiation time. (d) Analysis of the PL spectral shapes of the pristine and laser-irradiated samples (for 50 and 205 minutes). The peaks were deconvoluted into three peaks (neutral exciton (Xo), multexciton (XX), and B exciton (B)) using Lorentzian curves. (e) The PL intensity and peak position and (f) the FWHM of multiexciton emission (XX), neutral exciton emission (Xo).

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Figure 3. Confocal PL intensity images of 1L-MoS2 (a) before and (b) after laser irradiation for 205 minutes under ambient conditions. Corresponding local PL spectra of (c) and (d) obtained at points A through D in (a) and (b), respectively.

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Figure 4. (a) Raman spectra of 1L-MoS2 according to laser irradiation time. (b) Raman spectra of pristine 1L-MoS2, and laser-irradiated MoS2 for 5 minutes. Confocal Raman intensity mapping of 1L-MoS2 (c) before and (d) after laser irradiation for 100 minutes under ambient conditions. Raman peak position images of the A1g mode of the 1L-MoS2 (e) before and (f) after laser irradiation, respectively. (g) AFM topography of the 1L-MoS2 after laser irradiation. The inset shows the optical image of MoS2 after laser irradiation. (h) The height profile across the line with arrow direction.

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Figure 5. (a) PL spectra of 1L-MoS2 after laser irradiation for 30 min with an excitation energy of 633 nm in air and vacuum. (b) The peak position and PL intensity of the A exciton as a function of laser irradiation time under vacuum.

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Figure 6. (a) Schematic illustration of the laser irradiation on the 1L-MoS2/quartz sample. (b) Calculated temperature profile of the 1L-MoS2 with 3.7-mW laser excitation and (c) calculated temperature distribution profile along the x-axis of laser irradiation on MoS2/quartz. (d) Dark-field optical image and (e) confocal Raman intensity mapping of 1LMoS2 after annealing for 180 min at 260 °C in air and (f) the local Raman spectra obtained at points I and II of (e).

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Photochemical Reaction in Monolayer MoS2 via Correlated

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