Biexciton Emission from Edges and Grain Boundaries of Triangular

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Biexciton Emission from Edges and Grain Boundaries of Triangular WS Monolayers 2

Min Su Kim, Seok Joon Yun, Yongjun Lee, Changwon Seo, Gang Hee Han, Ki Kang Kim, Young Hee Lee, and Jeongyong Kim ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b07214 • Publication Date (Web): 13 Jan 2016 Downloaded from http://pubs.acs.org on January 15, 2016

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Biexciton Emission from Edges and Grain Boundaries of Triangular WS2 Monolayers Min Su Kim†, Seok Joon Yun†,#, Yongjun Lee†,#, Changwon Seo†,#, Gang Hee Han†, Ki Kang Kim†,⊥, Young Hee Lee†,#, Jeongyong Kim*,†,#

†

Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Sungkyunkwan University, Suwon 440-746, Republic of Korea

#

Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea

⊥Department

of Energy and Materials Engineering, Dongguk University, Seoul 100-715, Republic of Korea *

Email: [email protected]

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Abstract

Monolayer tungsten disulfides (WS2) constitute a high quantum yield two-dimensional (2D) system, and can be synthesized on a large area using chemical vapor deposition (CVD), suggesting

promising

nanophotonics

applications.

However,

spatially

non-uniform

photoluminescence (PL) intensities and peak wavelengths observed in single WS2 grains have puzzled researchers, with the origins of variation in relative contributions of excitons, trions, and biexcitons to the PL emission not well understood. Here we present nanoscale PL and Raman spectroscopy images of triangular CVD-grown WS2 monolayers of different sizes, with these images obtained under different temperatures and values of excitation power. Intense PL emissions were observed around the edges of individual WS2 grains and the grain boundaries between partly merged WS2 grains. The predominant origin of the main PL emission from these regions changed from neutral excitons to trions and biexcitons with increasing laser excitation power, with biexcitons completely dominating the PL emission for the high-power condition. The intense PL emission and the preferential formation of biexcitons in the edges and grain boundaries of monolayer WS2 were attributed to larger population of charge carriers caused by the excessive incorporation of growth promoters during the CVD, suggesting positive roles of excessive carriers in the PL efficiency of TMD monolayers. Our comprehensive nanoscale spectroscopic investigation sheds light on the dynamic competition between exciton complexes occurring in monolayer WS2, suggesting a rich variety of ways to engineer new nanophotonic functions using 2D transition metal dichalcogenide monolayers.

Keywords: tungsten disulfide, monolayer, photoluminescence, biexciton, chemical vapor deposition 2


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Transition-metal dichalcogenide (TMD) monolayers that have direct band gaps exhibit strong photoluminescence (PL) emissions in the visible and near-infrared spectral range.1,2 Monolayer tungsten disulfide (WS2) in particular is known for its high quantum yield, amounting to ~6 % (in contrast to, for example, ~0.1 % for monolayer MoS2),3 suggesting promising applications of TMD monolayers in optoelectronic and nanophotonic applications.4 With the development of chemical vapor deposition (CVD) techniques, largearea monolayer WS2 crystals are available nowadays,5,6 but spatial non-uniformities in the PL intensity and emission wavelength from single WS2 grains are frequently observed.5,7,8 These non-uniformities have usually been attributed either to the existence of localized electronic states or lattice defects,7-9 but the detailed physical reasons for these non-uniformities have not yet been uncovered, hindering the practical applications of WS2 monolayers as quantum light-emitting devices. TMD monolayers are known for strong excitonic effects that entirely account for their PL emission, and large exciton binding energies of 0.9 eV and 0.5 eV have been reported for monolayer MoS2 and monolayer WS2, respectively.10-12 Besides the neutral exciton, trions and biexcitons consisting of three and four charge carriers, respectively, have been observed even at room temperature, originated from strong Coulombic interactions between charge carriers.7-9,13-19 While trions in monolayer MoS2, which dominate the PL emission in highly doped states, are relatively well understood,18,19 the existence of trions and biexcitons in monolayers WS2 and WSe2 have only been recently observed16,17 and there is much to be discovered with regards to the formation of trions and biexcitons and their contributions to PL emission. Improving our understanding of these processes is required for the practical application of the WS2 monolayer as a light source.

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The effect of lattice defects in TMD monolayers on their PL emission is an interesting issue too. Lattice defects in conventional semiconductor systems mainly act as non-radiative recombination centers for excitons, which result in reduced PL emission intensities. In recent observations of TMD monolayers, however, the presence of lattice defects — whether intrinsic or induced — and the charge carriers associated to them can contribute to enhanced PL emissions through formation of bound exciton states or charge-induced doping effects13,14, suggesting that engineering lattice defects and controlling the local charge-carrier population in TMD monolayers may be a way to effect efficient PL emission. Modification of band structures due to the presence of lattice defects also could influence the efficiency of optical transition in TMD monolayers.20,21 Here, we performed nanoscale confocal spectral imaging of CVD-grown WS2 monolayers at various temperatures and excitation power values, and found distinct spatial patterns of PL intensities and peak wavelengths. We found intense PL emissions at the edges and grain boundaries of the WS2 monolayers. Moreover, these regions showed a continuous change in the distribution of the types of exciton complexes formed as the laser excitation power was changed, where biexcitons completely dominated the PL emission at high excitation power. Our results also suggested the positive role of excessive charge carriers for efficient PL emission.

RESULTS AND DISCUSSION In Figure 1a, we show a series of PL images of triangular WS2 monolayers of different lateral sizes, from about 6 to 45 µm. These triangular WS2 grains are believed to be single crystals, as judged from the equilateral triangular shape,7 and the thickness of the WS2 grains were confirmed to be that of a monolayer by inspection of the wavenumber difference and intensity ratio between the E12g and A1g peaks in the Raman spectra (see Figure S1a).7,22,23 4


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The PL profile derived from PL images of our CVD-grown WS2 grains exhibited a distinct and regular pattern; in contrast to the uniform view in the optical microscope and the Raman peak intensity ratio (IE12g/IA1g) map (Figure S1), all of the triangular WS2 grains displayed a much stronger PL intensity around the edges of the WS2 grains than in its inner regions (Figure 1). The PL intensities from the edges were observed to be about twice those from the inner regions for the small WS2 grains, and up to seven times greater for the larger grains. A plot of PL intensity ratio between the edge and the inner regions (Iedge/Iinner) vs. grain size is displayed as inset of Fig. 1a. We conducted the confocal absorption spectral mapping of a selected WS2 grain,19 which showed no distinct variation of absorption spectra between the inner region and the edge region, indicating the stronger PL is the direct result of higher quantum yield in the edge region (see the Figure S2). Interestingly, a relatively strong PL emission extended laterally from the very edge to about ~2 µm at nearly all locations, independent of the grain size, while the PL intensity in the edge regions showed a monotonic increase with increasing size of the WS2 grain. For a parallel display of all PL images of different-sized WS2 grains on the same intensity scale in Figure 1a, the image contrast of the smallest WS2 grain (in the dotted box) was increased ten folds. The PL spectra also showed clear differences between the inner region and the edge region; the peak positions of the PL spectra for the edge regions were generally observed to be redshifted by ~ 20 meV compared to the inner region, and the widths of the peaks in the PL spectra also were a few meV larger for the edge region, as shown in Figure 1b. This figure shows representative normalized PL spectra obtained from the edge region and the inner region of the 20 µm WS2 grain (indicated by the arrow in Figure 1a). The redshift of the PL peak position and the increase of the PL intensity were spatially well correlated, according to the peak position map shown in Figure 1c (the same 20 µm WS2 grain). These analyses 5


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suggest that the strength, redshift, and broadening of the PL peak all derived from the same physical origin. We also observed a similar enhancement of PL at the grain boundaries (GBs) between two triangular WS2 grains. The PL at the line between two WS2 grains was clearly enhanced, as shown in the examples of Figure 1d and 1e (PL intensity scales in Figure 1d and 1e were adjusted separately for the best contrast). In CVD growth of triangular TMD monolayers, the formation of the boundary between two merging monolayer grains occurs in a predictable way and thus the exact location of the GB can be drawn,24-26 as shown in the insets of Figure 1d and 1e. We found the PL enhancement from the inner regions of the WS2 grains occurred exactly along the predicted locations of the GBs, as indicated by the arrows in Figure 1d and 1e. Note that a relative increase in PL intensity along the GB was observed only in the inner regions; perhaps the PL was already saturated in the edge region and its intensity could not increase any more. A relatively strong PL and red-shift of the PL spectra, observed at the edges of triangular WS2 grains, were also observed along these GB locations (see Figure S3), suggesting similar origins of PL enhancement for the grain edges and grain boundaries, which will be discussed below. We investigated the dependence of PL images and spectra on the laser excitation power. Figure 2a shows the spatial profiles of the PL intensity of WS2 grains obtained with three different values of laser power, from 0.3 µW to 100 µW. Representative normalized PL spectra obtained from the edge region and the inner regions are displayed in Figure 2b and Figure 2c for 0.3 µW and 100 µW of laser excitation, respectively. For 100 µW of laser power, the PL peak energy of the edge region was lower by 12 meV than in the inner region, similar to the results obtained with a laser power of 500 µW (see Figure 1). In contrast, with a 0.3 µW laser power, the PL peak energy in the edge region was higher by 15 meV than in the 6


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inner region. The maps of the PL peak positions for each laser excitation power (insets in Figure 2a) confirmed this trend. Figure 2d shows the plot of PL peak position vs. laser power. Upon increasing the laser power from 0.3 µW to 100 µW, the total shift of the peak position in the edge region was observed to be ~54 meV, about twice as large as the ~26 meV shift resulting from the inner regions. To determine the origin of the observed dependence of spectral properties on laser power, we deconvoluted the representative PL spectra obtained from the inner region and the edge region for each laser power by fitting them with exciton peaks of neutral excitons (A0), trions (A‒) and biexcitons (AA), as shown in Figure S4, and plotted their peak positions and the integrated intensities vs. the laser power, as shown in Figure 3. In these plots we included the data extracted from a laser excitation power of 500 µW (Figure 1). For each of the laser power values tested, only the use of two peaks, i.e. the A0 and A‒ peaks, were needed to fit the PL curves from the inner region, but for the edge region, all three peaks, i.e., A0, A‒ and AA, were required for fitting (see Figure S4). By inspecting the peak positions of the different types of excitons, as displayed in Figure 3a, we estimated the binding energies of trions and biexcitons to be ~20 meV and ~55 meV, respectively; these values are consistent with previous observations.8,15 We fitted the PL intensities of each exciton type vs. laser power with the dependence I~Pm, where I, P and m represent the PL intensity, the laser power and the numeric power, respectively, as shown in Figure 3b. In both the inner and edge regions, we found different values of the slope relating the increases of PL and laser power for the different types of excitons: sublinear ~0.7 and 0.9 values of m for A0 and A‒, and a superlinear ~1.3 value of m for the AA peak from the edge region. Such a superlinear increase in PL emission is a typical signature of biexcitons,16,27 and the power dependence of the AA peak being twice that of the A0 peak is also indicative of biexcitons.28 We found that the full 7


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width at half maximum (FWHM) of AA peak is approximately twice of that of A peak, which is an additional signature of biexciton.28 (see Figure S5) Note that in the edge region, for laser power values above 100 µW, the PL emission of the monolayer WS2 was dominated by biexciton emission. Such a predominant biexciton emission has not yet been observed from any TMD monolayer at room temperature to the best of our knowledge. Note that with the same 0.3 µW to 500 µW range of excitation power values, the PL emission from the inner region was governed by A0 and A‒, but the predominant origin of the PL emission from the edge region changed from A0 to A‒ and then to AA as the power was increased, while maintaining a much higher PL intensity than from the inner region. We attribute the relatively stronger PL intensity and larger variation of exciton-type contribution occurred in the edge regions of our triangular WS2 grains to the larger population of charge carriers (here electrons) available for the formation of various exciton complexes of neutral excitons, trions and biexcitons in the edge region. We note that in Fig. 2a the lateral extension of strong PL from the very edge increased from ~2 µm to ~3 µm with the increased laser power, which also suggests a strong correlation between the PL efficiency and the local charge carrier population. We also inspected the effect of heating the WS2 monolayers on their PL characteristics. The results of annealing at 800 oC for 15 min in sulfur environment are given in Figure 4, with the PL intensity maps before and after the heat treatment shown in Figures 4a and 4b. We observed an overall reduction of PL intensity with the heat treatment both from the edge region and the inner region. The larger reduction of PL intensity occurred for the edge, decreasing by a factor of 3.5 with the heat treatment, while the reduction was only by a factor of ~2.0 for the inner region. The peak position and the FWHM of the PL curves also changed with the heat treatment. According to the average of PL spectra from the edge region shown 8


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in Figure 4c, the peak position blue shifted and the FWHM was reduced. These spectral changes were more distinct in the edge region, and as a result the heat treatment reduced the differences in PL intensity, peak position and the FWHM between the edge region and inner region. This result suggests that heating the WS2 monolayers reduced the number of types of excitons in these monolayers. We found by peak deconvolution that the decrease in PL intensity with the heat treatment was mainly due to the quenching of A‒ and AA peaks while the spectral weight of A0 peaks for both the edge and inner regions were increased by the heat treatment (see Figure S6). The narrowing of the PL curves with heat treatment is also explained by the resultant dominance of A0 peaks. The effect of heat treatment on the Raman spectra, shown in Figure 4d, was similar to the effect on PL spectra, showing the larger changes in the peak position and FWHMs in the edge region; here, the peak got narrower and the peak intensity increased. The strongest Raman peak of the WS2 monolayer is a combination of 2LA(M) and E12g modes.22,23 The multi-peak Lorentzian fitting shown in Figure S7 clearly distinguished the presences of the 2LA(M) and E12g modes. The FWHM of the E12g mode at the edge region decreased from 7.6 to 5.4 cm-1 upon heating, while at the inner region it decreased from 6.7 to 5.6 cm-1. The increase of 2LA(M) peak and the decrease of FWHMs of the E12g peak are usually indications of better crystallinity by the reduction of the lattice imperfections in WS2 monolayers,23 Our heat treatment results also suggest that higher PL intensity is not necessarily the indication of better crystallinity in TMD monolayers.14 We conjecture that the heat treatment somehow caused, in addition to the reduction of lattice imperfections, the reduction of charge carrier population in WS2 monolayers resulting in the decreased PL emission. We also obtained PL and Raman spectra on WS2 monolayers transferred to a TEM copper grid. First, the distinct spatial profiles of PL intensity, peak position and Raman peak FWHM 9


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shown in Figure 1 and 2, were preserved in the WS2 monolayer prepared on a copper grid as shown in the PL image displaying stronger PL in the edges of the triangular WS2 grain in Figure 5a, excluding the possibility of the substrate causing non-uniform PL and Raman properties. Figure 5b shows energy-dispersive X-ray spectra (EDS) obtained from the edge region and the inner region (specifically from the red spot and the blue spot in Figure 5a, respectively). The S peak from the edge region was observed to be weaker than that from the inner region. While ESD may not reflect the exact stoichiometry of Mo and S, it suggests that sulfur concentration in the edge regions may be lower than the inner region, possibly due to the larger number of S vacancies in the edge region. The PL spectra measured at 77 K showed more distinct differences between the edge and the inner regions than did the spectra measured at 300 K as shown in Figure 6a. The PL spectrum obtained at the inner region was determined to be composed of five peaks: at 2.075 eV (A0), 2.045 eV (A-), 2.011 eV (AA/D), 1.971 eV and 1.897 eV (defect-bound excitons: D).16 At the edge region, however, only two peaks were derived: at 2.009 eV (AA/D) and 1.971 eV (D). At 300 K, for the inner region all peaks except for A‒ were strongly quenched leaving A‒ as the main emission origin, and for the edge region AA emission was still dominant (Figure S8). These results confirmed that the main contributors to the PL spectra were the A‒ emission and AA emission for the inner region and the edge region, respectively. The dependence of the PL intensity on the laser excitation power was investigated at 77K as shown in Figure 6b. The PL emission of the inner region showed an almost linear relationship between PL intensity and excitation power, with m~1.0 as indicated by blue solid line fit, as is reasonable for an excitonic or trionic feature. In contrast, the PL intensity at the edge region exhibited a sublinear power dependence at low excitation power — but a super-linear dependence of m~1.7 for excitation power values greater than 50 µW, reasonable for the 10


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dominant presence of biexcitons. We believe that the different behaviors of this particular peak of AA for low and high values of excitation power may have been due to two different sources of emission being responsible for the observed AA peak, i.e., with the main contribution to the PL stemming from defect-bound excitons at low excitation power and from biexcitons at high excitation power.14,16 Now we discuss the origin of the spatially non-uniform charge population to which we attributed the observed non-uniform PL properties within single WS2 grains. Figures 7a and 7b display confocal PL images of partly merged triangular WS2 grains. While relatively strong PL appeared around the edges of individual WS2 triangular grains, even stronger PL was observed in edges adjacent to another WS2 grain (regions in the figure indicated with white dotted rectangles). These observations suggest that the PL at the edges was influenced by the proximity of the adjacent WS2 grains. From this observation we suggest that the local increase of promoter concentration played a major role in inducing larger charge population at the edges of WS2 grains. For the CVD growth of the WS2 monolayer, sodium cholate promoters were sprinkled by spin-coating on the growth substrate prior to the CVD process,29 as illustrated in the top panel of a schematic in Figure 7c. These promoters, which were not completely consumed during the growth, were to some extent pushed across from the grain center during the lateral growth of the monolayer WS2 grains, resulting in an increase in the local density of the promoters near the edges of the WS2 grains as illustrated in the bottom panel of Figure 7c. Excessive amount of promoters may have induced structural imperfections with the high electron affinity that resulted in increasing the local charge population.18,30 With this model, the stronger PL around the GBs between two merging WS2 grains are also explained, with promoters accumulating between the WS2 grains during the growth and these regions eventually forming GBs between two merged grains. Figure 7d 11


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displays a magnified nanoscale PL image obtained by using a near-field scanning optical microscope (NSOM) in the GB area indicated by the white dotted box in Figure 7e.25 While the PL was observed to be clearly enhanced around the GB, the nanoscale NSOM PL image revealed a reduced PL directly on the location of the GB, probably due to local defects that developed along the GB.25 This result suggests that the presence of the GB itself, by the formation of localized electronic states around the GB, were not responsible for the observed enhancement of the PL around the GB. Our findings of stronger PL emissions at the edges and grain boundaries of WS2 grains with a favored formation of biexcitons at relatively higher excitation power are somewhat surprising, because previously for monolayer MoS2 the dominance of trions with excess charge carriers that were produced by doping or electric-gating usually led to weaker PL emissions due to exciton-exciton annihilation or charge screening.18,19 However, our experimental results clearly indicate the positive role of excessive charges in PL emission of WS2 monolayers and that the PL emission mechanism determined by the formation of exciton complexes vastly differs for different kinds of TMD monolayers .

CONCLUSIONS Here we have reported the results of a comprehensive nanoscale PL and Raman spectroscopy investigation of triangular CVD-grown WS2 monolayers. We visually identified distinct patterns of PL emissions in single WS2 grains, which showed strong PL emissions from the edges and grain boundaries, and we found that these regions with strong PL emissions were very efficient in generating biexcitons at high excitation power. We attributed strong PL emission and the favored formation of biexcitons in the edge region to the larger local population of charge carriers available for the formation of various exciton complexes. 12


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Our observation that preferential formation of trions and biexcitons can result in enhanced PL emission suggests a rich variety of ways to engineer specialized optical properties by using WS2 monolayers and other TMD monolayers.

METHODS Monolayer WS2 synthesis: Triangular WS2 monolayers were synthesized by using the CVD method. Sulfur chips (Sigma-Aldrich, 344621) and ammonium metatungstate hydrate (AMT) (Sigma-Aldrich, 358975) were used as a tungsten precursor. A mass of 200 mg of S was loaded in zone 1 (upstream) of a two-inch-diameter furnace. The SiO2/Si substrate was spincoated using a solution of 3.1 mM AMT and 0.02 g/ml sodium cholate promoter and was placed in zone 2 (downstream). All growth experiments were conducted at atmospheric pressure. To initiate growth of WS2, the temperature of zone 1 was elevated from room temperature to 230 °C at a rate of 23 °C/min. At the same time, zone 2 was also heated to 800 °C for 10 min. When zone 2 reached this temperature, H2 was introduced at a flow rate of 2 sccm for 5 min while maintaining the 800 °C temperature. After 5 min, the flow of H2 was stopped and the furnaces were then allowed to naturally cool to room temperature. Confocal and NSOM spectral mapping measurements: A lab-made laser confocal microscope combined with a spectrometer was used for the confocal PL and Raman spectroscopy measurements. The laser light was focused with a 0.9 NA objective and the lateral resolution was estimated to be ~700 nm (see the Figure S9). Scattered light was collected using the same objective and guided to a 50-cm-long monochromator equipped with a cooled CCD. A 532 nm laser line of a diode-pumped solid-state laser was used for the photo-excitation with a typical acquisition time of 5 ms per pixel in the spectral images. A more detailed description of the spectral analysis is provided elsewhere.26 Near-field scanning 13


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optical microscopy (NSOM) images were collected using a commercial NSOM instrument with a cantilever-style NSOM probe (Alpha-300S, WITec Instrument GmbH).25

Conflict of Interest: The authors declare no competing financial interests.

Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI:.

Acknowledgment. This work was supported by IBS-R011-D1.

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References 1. 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. 2. 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. 3. Yuan, L.; Huang, L. Exciton Dynamics and Annihilation in WS2 2D Semiconductors. Nanoscale 2015, 7, 7402-7408. 4. Lee, H. S.; Kim, M. S.; Jin. Y.; Han, G. H.; Lee, Y. H.; Kim, J. Efficient ExcitonPlasmon Conversion in Ag Nanowire/Monolayer MoS2 Hybrids: Direct Imaging and Quantitative Estimation of Plasmon Coupling and Propagation. Adv. Opt. Mater. 2015, 9, 943-947. 5. 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. 6. Yun, S. J.; Chae, S. H.; Kim, H.; Park, J. C.; Park, J.-H.; Han, G. H.; Lee, J. S.; Kim, S. M.; Oh, H. M.; Seok, J.; Jeong, M. S.; Kim, K. K.; Lee, Y. H. Synthesis of CentimeterScale Monolayer Tungsten Disulfide Film on Gold Foils. ACS Nano 2015, 9, 5510-5519. 7. Gutiérrez, H. R.; Perea-López, N.; Elías, A. L.; Berkdemir, A.; Wang, B.; Lv, R.; LópezUrías, F.; Crespi, V. H.; Terrones, H.; Terrones, M. Extraordinary Room-Temperature Photoluminescence in Triangular WS2 Monolayers. Nano Lett. 2013, 13, 3447-3454. 8. Peimyoo, N.; Shang, J.; Cong, C.; Shen, X.; Wu, X.; Yeow, E. K. L.; Yu, T. Nonblinking, Intense Two-Dimensional Light Emitter: Monolayer WS2 Triangles. ACS Nano 2013, 7, 10985-10994. 9. Wang, X. H.; Ning, J. Q.; Zheng, C. C.; Zhu, B. R.; Xie, L.; Wu, H. S.; Xu, S. J. 15


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Photoluminescence and Raman Mapping Characterization of WS2 Monolayers Prepared Using Top-Down and Bottom-Up Methods. J. Mater. Chem. C 2015, 3, 2589-2592. 10. 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. 11. Ye, Z.; Cao, T.; O’Brien, K.; Zhu, H.; Yin, X.; Wang, Y.; Louie, S. G.; Zhang, X. Probing Exciton Dark States in Single-Layer Tungsten Disulphide. Nature 2014, 513, 214-218. 12. Zhu, B.; Chen, X.; Cui, X. Exciton Binding Energy of Monolayer WS2. Sci. Rep. 2015, 5, 9218. 13. Chow, P. K.; Jacobs-Gedrim, R. B.; Gao, J.; Lu, T.-M.; Yu, B.; Terrones, H.; Koratkar, N. Defect-Induced Photoluminescence in Monolayer Semiconducting Transition Metal Dichalcogenides. ACS Nano 2015, 9, 1520-1527. 14. Tongay, S.; Suh, J.; Ataca, C.; Fan, W.; Luce, A.; Kang, J. S.; Liu, J.; Ko, C.; Raghunathanan, R.; Zhou, J.; Ogletree, F.; Li, J.; Grossman, J. C.; Wu, J. Defects Activated Photoluminescence in Two-Dimensional Semiconductors: Interplay Between Bound, Charged, and Free Excitons. Sci. Rep. 2013, 3, 2657. 15. Mitioglu, A. A.; Plochocka, P.; Jadczak, J. N.; Escoffier, W.; Rikken, G. L. J. A.; Kulyuk, L.; Maude, D. K. Optical Manipulation of the Exciton Charge State in Single-Layer Tungsten Disulfide. Phys. Rev. B 2013, 88, 245403. 16. 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. Stat. Sol. RRL 2015, 9, 457-461. 17. You, Y.; Zhang, X.-X.; Berkelbach, T. C.; Hybertsen, M. S.; Reichman, D. R.; Heinz, T. 16


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F. Observation of Biexcitons in Monolayer WSe2. Nat. Phys. 2015, 11, 477-481. 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. Dhakal, K. P.; Duong, D. D. ; Lee, J. ; Nam, H.; Kim, M. S.; Kan, M.; Lee, Y. H.; Kim, J. Confocal Absorption Spectral Imaging of MoS2: Optical Transitions Depending on the Atomic Thickness of Intrinsic and Chemically Doped MoS2. Nanoscale 2014, 6, 1302813035. 20. Huang, Y. L.; Chen, Y.; Zhang, W.; Quek, S. Y.; Chen, C.-H.; Li, L.-J.; Hsu, W.-T.; Chang, W.-H.; Zheng, Y. J.; Chen, W.; Wee, A. T. S. Bandgap Tenability at Single-Layer Molybdenum Disulphide Grain Boundaries. Nat. Commun. 2014, 6, 6298. 21. Kormányos, A.; Burkard, G.; Gmitra, M.; Fabian, J.; Zólyomi, V.; Drummond, N. D.; Fal’ko, V.; Corrigendum: k.p Theory for Two-Dimensional Transition Metal Dichalcogenide Semiconductors. 2D Mater. 2015, 2, 022001. 22. Zhao, W.; Ghorannevis, Z.; Amara, K. K.; Pang, J. R.; Toh, M.; Zhang, X.; Kloc, C.; Tan, P. H.; Eda, G. Lattice Dynamics in Mono- and Few-Layer Sheets of WS2 and WSe2. Nanoscale 2013, 5, 9677-9683. 23. Berkdemir, A.; Gutiérrez, H. R.; Botello-Méndez, A. R.; Perea-López, N.; Elías, A. L.; Chia, C.-I.; Wang, B.; Crespi, V. H.; López-Urías, F.; Charlier, J.-C.; Terrones, H.; Terrones, M. Identification of Individual and Few Layers of WS2 Using Raman Spectroscopy. Sci. Rep. 2013, 3, 1755. 24. Cheng, J.; Jiang, T.; Ji, Q.; Zhang, Y.; Li, Z.; Shan, Y.; Zhang, Y.; Gong, X.; Liu, W.; Wu, S. Kinetic Nature of Grain Boundary Formation in As-Grown MoS2 Monolayers. Adv. Mater. 2015, 27, 4069-4074. 25. Lee, Y.; Park, S.; Kim, H.; Han, G. H.; Lee, Y. H.; Kim, J. Characterization of the 17


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Structural

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MoS2

Using

Near-Field

Photoluminescence Imaging. Nanoscale 2015, 7, 11909-11914. 26. Park, S.; Kim, M. S.; Kim, H.; Lee, J.; Han, G. H.; Jung, J.; Kim, J. Spectroscopic Visualization of Grain Boundaries of Monolayer Molybdenum Disulfide by Stacking Bilayers. ACS Nano 2015, 9, 11042-11048. 27. Gourley, P. L.; Wolfe, J. P. Thermodynamics of Excitonic Molecules in Silicon. Phys. Rev. B 1979, 20, 3319-3327. 28. 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. 29. 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. 30. Zhou, W.; Zou, X.; Najmaei, S.; Liu, Z.; Shi, Y.; Kong, J.; Lou, J.; Ajayan, P. M.; Yakobson, B. I.; Idrobo, J.-C. Intrinsic Structural Defects in Monolayer Molybdenum Disulfide. Nano Lett. 2013, 13, 2615-2622.

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ACS Nano

Figure 1. (a) PL intensity maps of triangular WS2 monolayers of different lateral sizes, from about 6 to 45 µm. All of the WS2 grains were measured with the same 500 µW laser power and integration time of 5 ms per pixel, and with the same optical set-up. Five separate images of the WS2 grains were merged into one image and the vertical scale represents the PL intensity in photon counts. The inset is the plot of PL intensity ratio between the edge and the inner regions (Iedge/Iinner) vs. grain size. The arrow indicates the WS2 grain used for the image shown in Figures 1b and 1c. (b) Representative PL spectra obtained from the edge region and the inner region of the selected WS2 monolayer. (c) The PL peak position map of the selected WS2 monolayer. (d) and (e) PL intensity maps of polycrystalline WS2 monolayers showing an enhancement of PL along expected grain boundary (GB) locations between two grains. Expected locations of GBs are displayed in the insets.22-24 All scale bars are 10 µm.

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Figure 2. (a) PL intensity maps of WS2 monolayers obtained with different excitation laser power levels. The insets show the corresponding PL peak position maps. (b) and (c) Representative normalized PL spectra obtained from the edge region and the inner region for excitation laser power levels of 0.3 µW and 100 µW, respectively. (d) Plots of PL peak position vs. laser powers. The red and blue lines are guides for the eye.

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Figure 3. (a) Peak positions and (b) intensities of deconvoluted PL peaks of neutral excitons (A0), trions (A-) and biexcitons (AA) as a function of excitation laser power. The solid lines are power fits and m values represent the numeric power.

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Figure 4. Effects of heat treatment on PL and Raman characteristics of the WS2 monolayer. (a) and (b) are PL intensity maps before and after the heat treatment, respectively. Note the especially reduced PL intensities in the edge region after heat treatment. (c) and (d) are, respectively, representative PL and Raman spectra obtained from the edge region before and after the heat treatment. All scale bars are 10 µm.

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ACS Nano

Figure 5. (a) PL intensity map of a WS2 monolayer grain prepared on a TEM copper grid. The inset shows a bright-field TEM image. (b) Energy-dispersive X-ray spectra obtained from the edge region (red spot in Figure 5a) and the inner region (blue spot in Figure 5a).

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Figure 6. Investigations of the origin of PL emission of the edge region and the inner region of the WS2 monolayer at 77 K. (a) Representative PL spectra and deconvoluted exciton peaks obtained from the inner region and the edge region. (b) Plot of PL peak intensities vs. laser power. The solid lines are the power fits to these plots, with m representing numeric powers.

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Figure 7. (a) and (b) Confocal PL images of partly merged triangular WS2 monolayers. The white dotted boxes indicate brighter edge regions due to the presence of an adjacent WS2 grain. (c) Schematic of the CVD process for WS2 growth explaining a higher promoter concentration between the WS2 grains, resulting in a stronger PL emission. (d) The magnified PL image obtained by using a near-field scanning microscope in the grain boundary area indicated by the white dotted box in Figure 7 (e). All scale bars are 20 µm, except 5 µm in Fig. 7d.

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Biexciton Emission from Edges and Grain Boundaries of Triangular

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