Simple Chemical Treatment to n-Dope Transition-Metal

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Simple Chemical Treatment to N-dope Transition-Metal Dichalcogenides and Enhance the Optical and Electrical Characteristics Guru P. Neupane, Minh Dao Tran, Seok Joon Yun, Hyun Kim, Changwon Seo, Jubok Lee, Gang Hee Han, Ajay K. Sood, and Jeongyong Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15239 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 18, 2017

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ACS Applied Materials & Interfaces

Simple Chemical Treatment to N-dope Transition-Metal Dichalcogenides and Enhance the Optical and Electrical Characteristics Guru P. Neupane,†,‡ Minh Dao Tran,†,‡ Seok Joon Yun,†,‡ Hyun Kim,†,‡ Changwon Seo,†,‡ Jubok Lee,†,‡ Gang Hee Han,†,‡ A. K. Sood,§ and Jeongyong Kim*,†,‡ †

Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon 440746, Republic of Korea ‡

Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea §

Department of Physics, Indian Institute of Science, Bangalore-560012, India *Email: [email protected]

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ABSTRACT:

Optical

and

electrical

properties

of

monolayer

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transition-metal

dichalcogenides (1L-TMDs) are critically influenced by two dimensionally confined exciton complexes. While extensive studies on controlling optical properties of 1L-TMDs through external doping or defect engineering have been carried out, the effects of excess charges, defects and the populations of exciton complexes on the light emission of 1L-TMDs are not yet fully understood. Here we present a simple chemical treatment method to n-dope 1LTMDs that also enhances their optical and electrical properties. We show that dipping 1Ls of MoS2, WS2 and WSe2, whether exfoliated or grown by chemical vapor deposition, into methanol for several hours can increase the electron density and also can reduce the defects, resulting in enhancement of their photoluminescence, light absorption and the carrier mobility. This methanol treatment was effective for both n-as well as p-type 1L-TMDs, suggesting that the surface restructuring around structural defects by methanol is responsible for the enhancement of optical and electrical characteristics. Our results have revealed a simple process for external doping that can enhance both the optical and electrical properties of 1LTMDs and help us understand how the exciton emission in 1L-TMDs can be modulated by chemical treatments. KEYWORDS: transition-metal dichalcogenides, MoS2, WS2, WSe2, methanol, n-doping, photoluminescence, mobility

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■ INTRODUCTION Two-dimensional (2D) monolayer transition-metal dichalcogenides (1L-TMDs) are atomically thin semiconductors having direct band gaps in visible and near-infrared spectral range.1-3 Optical properties of 1L-TMDs can be critcally controlled by manipulating population of their two dimensionally confined exciton complexes, including excitons, trions and biexcitons1,4 as demonstrated by electrostatic gating,5 surface plasmon excitation,6,7 chemical doping8,9 and strain engineering.10,11 Chemical doping, in particular, was shown to be an effective and convenient method, 8,9,12-15 where charge transfer between the dopant molecules and the 1L-TMDs was observed to induce a shift in the Fermi level and to modulate the optical and electrical properties of the 1L-TMDs.8,9,12 Significant enhancement (reduction) of photoluminescence (PL) of 1L-TMDs have been reported after treating with n(p) type dopants.8,9,12 For example, the increase in the electron density of 1L-TMDs by ndoping enhanced trion formation over the neutral excitons, and lowered the PL efficiency.4,9,12,16 However most experimental methods of chemical doping, applications of electric fields, plasma treatment, heating treatment and interfacing with metallic nanostructures that modulate the carrier density of 1L-TMDs are also likely to affect the physical conditions of 1L-TMDs by introducing strain, changing the dielectric environment or even creating structural defects.17-22 Such physical modifications of 1L-TMDs can have large effects on their optical properties including their PL strength, especially considering the extremely high surface-to-volume ratio of 1L-TMDs and the strong sensitivity of exciton transitions on the surface conditions. Recently, oxygen plasma treatment was shown to increase the overall PL intensity and also the spectral weight of the trion PL emission of 1LMoS2,23 and CVD-grown 1L-WS2.24 On the other hand, recent studies showed that organic 3



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super acid treatment can cause a giant PL increase in some 1L-TMDs, indicating an independent role of structural defects in determining their PL efficiency.25-27 In this report, we present a simple chemical treatment of dipping of various CVD-grown and exfoliated 1L-TMDs, such as 1L-MoS2, 1L-WS2 and 1L-WSe2, into methanol for many hours at room temperature. This treatment caused the 1L-TMDs to become n-doped and their PL to be enhanced. Characteristic spectral features of the n-doping effect such as an increase in the spectral weights of trions in PL spectra and the red-shift of Raman A1g modes were unambiguously observed in nanoscale PL and Raman spectral mappings. With a clear indication of the increase in electron density, the PL intensity of methanol-treated 1L-TMDs was commonly enhanced two-fold and the carrier mobility was also increased for all the 1LTMDs, whether CVD-grown or exfoliated, indicating that the methanol treatment was effective in restructuring of surface to anneal structural defects. Our results demonstrate that the increase of electron density and the enhancement of optical properties of 1L-TMDs can concurrently occur, suggesting diverse ways to engineer the optical properties of 1L-TMDs for promising optoelectronic applications.

■ RESULTS AND DISCUSSION

As shown in Figure 1a, PL intensity maps of CVD-grown 1L-MoS2 before and after 16 h of methanol treatment revealed a clear enhancement of PL after the methanol treatment. Averaged PL spectra taken from the pristine state and from 16 h-treated 1L-MoS2 shown in Figure 1b indicated that an overall 2.2-fold enhancement was obtained from this methanol treatment. Besides the increase of the PL intensity, a redshift of the A exciton peak was also 4


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observed after the methanol treatment. Maps of the positions of the A exciton peaks of pristine and 16 h-treated 1L-MoS2 shown in Figure 1c indicated a red-shift of A exciton peaks by ~11 nm throughout the region of the 1L-MoS2 grain. We also monitored the peak heights and the peak positions of A exciton peak of 1L-MoS2 every 4 hours of the methanol treatment, as shown in Figure 1d (see Figure S1 for the complete time series of PL intensity maps and corresponding PL spectra). Monotonic increases of the PL intensity and the peak wavelength of the A exciton peak up to 16 h were observed. The redshift of PL peak of 1LMoS2 has been attributed to an increase in the trion spectral weight, indicating that the methanol treatment caused an increase of the electron density in 1L-MoS2.9,12 Indeed, as shown in Figure 1e, the intensity ratio of trions (A-) to neutral excitons (A0) was found to increase from 1.1 to 2.3 as the methanol treatment time was increased from 0 to 16 h. (See Figure S2 for the fitting results for deconvoluting PL spectra into A- and A0 emissions and B exciton peak for total time series.) The varying positions of the A- and A0 peaks as a function of methanol treatment time are also shown in Figure 1f. Here A- showed a gradual shift from 680 to 687 nm and A0 from 664 to 668 nm as the methanol treatment time was increased. The redshift of the A- peak position and the resultant increase of the difference between the A- and A0 peak positions with increasing treatment time were direct indications of an increased trion dissociation energy caused by the increase of Fermi level, which was also consistent with an increased electron density suggested from the increased spectral weight of trion emission (Figure 1e).4,9,28,29 The slight increase of the A0 peak wavelength may have been due to the bandgap renormalization that could occur in high electron density environments.30 Raman spectroscopy has been used to investigate the doped state of MoS2 because electron doping leads to a significant change in the electron-phonon coupling, resulting in a

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modification of the A1g Raman mode.9,12 Comparison of the Raman spectra of the pristine and 16 h methanol-treated 1L-MoS2 shown in Figure 2a revealed a red shift of the A1g peak by ~1.5 cm-1 in the spectrum of the methanol-treated sample, corresponding to an increase of ~7 × 1012/cm2 in the electron density.31 According to the peak position maps of the Raman A1g mode of the pristine and methanol-treated 1L-MoS2 samples shown in Figure 2b, the peak shift of the A1g mode occurred uniformly throughout the sample. In addition, we observed an increase of the Raman intensity of the second order longitudinal acoustic (2LA) mode at 450 cm-1 for the methanol-treated 1L-MoS2 sample. This 2LA peak originated from an in-plane collective motion of atoms in the lattice32 and the increase in its Raman intensity indicates a reduction of structural defects.20,33 We also performed low temperature PL measurements of the pristine and the treated 1L-MoS2 sample. As the results shown in Figure 2c, at 125 K a broad PL peak at ~ 1.75 eV emerged due to the emission from localized defects.34 (See Figure S3 for whole series of PL spectra obtained in the range 77K - 300K). It is clearly displayed that this defect emission is largely quenched in treated samples compared to the pristine samples. The results of Raman spectroscopy and low temperature PL investigation therefore suggested that methanol treatment not only led to n-doping but also a reduction of defects in 1L-MoS2. We applied the similar investigation to exfoliated MoS2 films. Figure 3a shows PL intensity maps of exfoliated MoS2 before and after the 16 h of methanol treatment. Monolayer (1L), bilayer (2L) and trilayer (3L) domains (marked by dotted lines) were identified by measuring the frequency difference between the E12g and A1g Raman modes.2,3,35 Cleary enhanced PL was observed in each of these domains in the methanol-treated sample. Averaged PL spectra obtained from each domain shown in Figure 3b revealed an 6


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approximately 1.8-fold enhancement of the PL. We also observed a red shift of the PL peak in the methanol-treated sample; this shift ranged from ~4 nm in 3L-MoS2 to ~7 nm in 1LMoS2 (see Figure S4a for A exciton peak position maps). Raman spectra shown in Figure 3c displayed red shifts of the A1g Raman peaks by ~ 1.0, 0.7 and 0.5 cm-1 from 1L, 2L and 3L domains, respectively, after 16 h of methanol treatment; these results were also indicative of n-doping. (See Figure S4b for the Raman A1g maps of the same sample.) Note that the magnitudes of the peak shifts in the PL and Raman spectra observed after the 16 h methanol treatment were lower for the exfoliated 1L-MoS2 than for CVD-grown 1L-MoS2. We inspected the n-doping effect resulting from treating another CVD-grown 1L-MoS2 sample with methanol and also found that the values of the spectral redshifts were ~12 nm and ~1.5 cm-1 for the PL peak and Raman A1g mode, respectively (see Figure S5 in for PL and Raman spectral measurements of this other CVD-grown 1L-MoS2 sample), suggesting methanol treatment to be somewhat more effective for n-doping of CVD-grown 1L-MoS2 than exfoliated 1L-MoS2. The methanol treatment of CVD-grown 1L-MoS2 also yielded a slightly larger PL enhancement than did treatment of exfoliated MoS2. In recent studies, the defects of exfoliated 1L-MoS2 and CVD-grown 1L-MoS2 were shown to be mostly sulfur vacancies.36,37 and our result suggests that exfoliated 1L-MoS2 has less number of defects as compared to the CVD grown samples. As seen in Figure 3c, an increase of the Raman 2LA mode was observed in exfoliated 1LMoS2, in each of the regions with different thicknesses, after 16 h of methanol treatment, similar to the result for CVD-grown 1L-MoS2 and suggested that a reduction in the number of intrinsic defects also occurred in the exfoliated MoS2. We also applied the methanol treatment to the CVD-grown MoS2 having different numbers of layers and found similar PL 7



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enhancements and evidence of n-doping (see Figure S6 for PL intensity maps and PL and Raman spectra of pristine and methanol-treated CVD-grown MoS2 with various thickness values). These results indicated methanol to be effective in inducing n-doping and increasing PL for single or multi-layered MoS2, whether the MoS2 was exfoliated or CVD-grown. We studied the n-doping effect of methanol treatment of CVD-grown 1L-WS2, which is known to be also intrinsically n-type.38 As displayed in Figure 4a, PL intensity maps for 1LWS2 before and after 12 h of methanol treatment showed a 2-fold more intense PL in the methanol-treated sample. (See Figure S7a for PL intensity maps of 1L-WS2 with all treatment times and Figure S7b for PL intensity histogram with pixel frequencies of 1L-WS2 before and after the 12 h methanol treatment.) The averaged PL spectra in Figure 4b shows a clear red shift of the A exciton peak by ~ 8 nm after the 12 h-methanol treatment, indicative of the increase of the trion spectral weight described above. (See Figure S7c for A exciton peak position maps of 1L-WS2 before and after the 12 h methanol treatment) As shown in Figure 4c, averaged Raman spectra obtained from the same region before and after the methanol treatment showed that the A1g Raman mode red-shifted by ~ 1.2 cm-1. These results indicated that the n-doping effect occurred within 12 h of methanol treatment of 1L-WS2. An increase of the 2LA Raman mode, indicative of the reduction in the density of defects described above, was also observed after treating 1L-WS2 with methanol. In contrast to intrinsically n-type 1L-MoS2 and 1L-WS2, CVD-grown 1L-WSe2 is known to be p-type.39 We studied the effects of methanol treatment on CVD-grown 1L-WSe2. We first observed a 2-fold enhancement of PL with 12 h of methanol treatment throughout the region containing 1L-WSe2, as shown in Figure 4d. (See Figure S8a for PL intensity maps of 1L-WSe2 with all treatment times and Figure S8b for PL intensity histogram with pixel 8


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frequencies of 1L-WSe2 before and after the 12 h methanol treatment.) However, in contrast to the cases of 1L-MoS2 and 1L-WS2, which showed redshifts of their PL peaks with the methanol treatment, 1L-WSe2 displayed a blue shift of the A exciton peak, by ~ 8 nm, after the 12 h methanol treatment as shown in Figure 4e. (See Figure S8c for A exciton peak position maps of 1L-WSe2 before and after the 12 h methanol treatment) This result occurred because 1L-WSe2, being p-type, hosts positive trions, so an n-doping process would have caused the depletion of positive trions. Comparing the Raman spectra of pristine and 12 h methanol-treated 1L-WSe2 shown in Figure 4f revealed a red shift of ~ 1.2 cm-1 in the A1g Raman mode and a noticeable enhancement in the 2LA Raman mode intensity in the methanol-treated sample. Low temperature PL measurements of the pristine and the treated 1L-WSe2 were taken as the results shown in Figure S9, where substantial decreases of intensities of defect-oriented PL peaks were observed in methanol treated 1L-WSe2. These results taken together showed methanol treatment to be effective for p-type 1L-WSe2, enhancing its PL, and for reducing its density of defects. The reduction of defects in 1L-TMDs with methanol treatment was confirmed by timeresolved PL (TRPL) spectroscopy. Figure 5a and 5b display the maps of fluorescence lifetime and representative TRPL spectra of pristine 1L-WS2 and methanol treated 1L-WS2. In fluorescence lifetime maps, the brightness represents the PL intensity and the color represents the mean decay time estimated from the TRPL spectra at the pixel position. We note that the fluorescence lifetime show some variation over 1L-WS2, where the range of measured PL lifetime is 100-130 ps, and 128-200 ps for pristine 1L-WS2 and methanol treated 1L-WS2, respectively, indicating the lifetime is longer for methanol treated 1L-WS2 than pristine 1LWS2. In 1L-TMDs, the longer fluorescence lifetimes have been indication of lower defect 9



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density because exciton quenching in structural defects is known to be the major origin of low quantum yield.25,33 We performed the similar measurements for 1L-MoS2 and 1L-WSe2, with similar results (see Figure S10). Figure 6 shows the X ray photoelectron spectroscopy (XPS) results for pristine and 16 h methanol-treated CVD-grown 1L-MoS2. For the pristine sample, binding energy peaks associated with S 2p and S 2s core levels were found at 162.3 eV, 163.5 eV and 226.5 eV specifically corresponding to S 2p3/2, S 2p1/2 and S 2s1/2, respectively, and two peaks associated with Mo 3d core levels were found at 229.5 eV and 232.5 eV, corresponding to Mo 3d5/2 and Mo 3d3/2, respectively. All of these peaks were up-shifted by 0.55 eV after the methanol treatment. Because the binding energy value derived from an XPS spectrum is referenced to the Fermi level in the material, the upshift of the XPS spectrum of MoS2 after methanol treatment was attributed to the shift of the Fermi level toward the conduction band, indicating the existence of n-type doping.40 XPS results of 1L-WS2, 1L-WSe2 and exfoliated MoS2 also indicated the n-doping by methanol treatment (see Figure S11). We investigated the effect of the methanol treatment on the electrical transport of CVDgrown 1L-MoS2, 1L-WS2 and 1L-WSe2 in a back-gated field-effect transistor (FET) configuration. Figure 7a displays plots of source-drain current (Ids) versus gate bias (Vg) with a source-drain voltage of 1 V for pristine 1L-MoS2 and 1L-MoS2 treated with methanol for 16 h. A -15 V shift in the threshold voltage (Vth) was observed after treating 1L-MoS2 with methanol, consistent with the occurrence of n-doping. Figure 7b shows Ids -Vg characteristics for pristine 1L-WS2 and 1L-WS2 after 12 h of methanol treatment. The threshold voltage also shifted, by -17 V, when treating 1L-WS2 with methanol, again indicative of the n-doping caused by the methanol treatment. 1L-WSe2 in Figure 7c showed the shift of Vth by 13 V for 10


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the methanol treated sample, which is also indication of n-doping because 1L-WSe2 is intrinsically p-type. We extracted the field-effect electron mobility (µFE) values from these transport curves according to the equation µFE = dIds/dVds (L/WCiVds), where Ci is the capacitance between the MoS2 channel and the silicon layer per unit area, and L and W are the length and width of the channel, respectively.41 Using a Ci value of 1.15 x 10-4 F/m2 for a SiO2 layer with a thickness of 300 nm,42 we obtained µFE values of 0.13 and 4.8 cm2 V−1 s−1 for pristine and methanol-treated 1L-MoS2, respectively, indicating a significant improvement in the electron mobility after the methanol treatment. We obtained similar trends for 1L-WS2 and 1L-WSe2 where the obtained electron mobility increased from 0.0084 to 0.22 cm2 V−1 s−1 and from 5.9 x 10-5 to 1.28 x 10-4 V−1 s−1, respectively, after the 12 h methanol treatment. Such significant increases of electron mobility of 1L-MoS2, 1L-WS2 and 1L-WSe2 is likely due to the reduction of structural defects,43,44 and indicates that methanol treatment is effective to enhance the optical and electrical characteristics of both n-type and p-type 1L-TMDs. The PL, Raman, XPS and FET measurements all clearly showed signs of increased electron density of 1L-TMDs and reduction of defects after they were treated with methanol for several hours. We also treated CVD-grown 1L-MoS2 samples with other chemicals, namely hexane, acetone, isopropyl alcohol (IPA) and toluene, and investigated the spectroscopic effects of these treatments. In contrast to the results for methanol, dipping CVD-grown 1L-MoS2 for 16 h in any of these chemicals did not yield any change in its PL spectrum, neither in the peak intensity nor peak positions, and no noticeable changes in its Raman spectrum were observed either (see Figure S12). These results indicated that n-doping and PL enhancement of 1L-TMDs occurred specifically with methanol treatment.

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To further investigate the mechanism by which 1L-TMDs become n-doped and by which their PL levels increase when they are treated with methanol, we performed differential reflection spectral mappings of pristine and methanol-treated 1L-TMDs. The differential reflection of atomically thick layered TMDs on transparent substrates has been regarded as a measure of the absorption intensity.12 As shown in the absorption intensity maps in Figure 8a, we observed a distinct increase in the absorbance of 1L-MoS2 when treated with methanol. Averaged absorption spectra revealed that the increase in absorption was specifically due to increases in the intensities of the A, B and C exciton peaks, as shown in the absorption spectra of Figure 8b. Such increases in the intensities of the A and B exciton peaks were not observed when the 1L-MoS2 samples were treated with any of the four other chemicals, i.e., hexane, acetone, IPA and toluene, though a slight increase in the intensity of the C exciton peak with IPA treatment was observed (see Figure S13). Note that most of the previously reported external processes that used chemicals or an electric field to n-dope 1L-TMDs caused a distinct quenching of exciton peaks in the absorption spectra, and this quenching was interpreted as resulting from increased charge screening.9,16 In our case with methanol, an increase in the height and a decrease in the width of the A exciton peak were observed, which suggested that a significant improvement of crystal quality occurred, due to reduction of the density of defects. This result validated the effectiveness of the methanol treatment in simultaneously performing n-doping and enhancing the optical properties of the TMD. Similar results of increased exciton absorption peaks that implies the reduction of defects were obtained from 1L-WS2 and 1L-WSe2 (see Figure S14). Previously, chemical treatments using nonoxidizing organic acids were shown to be highly effective at increasing the intensity of the PL of exfoliated 1L-TMDs via a fixation of defects,

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mostly of the sulfur vacancies.25,27 Our methanol treatment seemed effective both for the exfoliated and CVD grown samples. Even though a further study of atomic-scale lattice inspection by TEM and theoretical analysis seem necessary for the complete understanding on how methanol was effective for reducing the defects and n-doping of 1L-TMDs, a possible mechanism of n-doping and reduction of defects by methanol treatment, we postulate, is the adsorption and dissociation of methanol occurring on the surface of TMDs. Previous a density function theory study reported that methanol would favorably adsorb and dissociate on edges of MoS2 clusters through O-H scission and then the pathway of CH3O → CH3 → CH2.45 We also note that in the previous result of PL enhancement of 1L-MoS2 by using organic superacid of bis(trifluoromethane) sulfonamide (TFSI), hydrogenation by TFSI was postulated to be the most responsible for the fixation of the defects.25 Based on such reports, we believe that consecutive hydrogen release occurring during methanol dissociation at the defect sites of 1L-MoS2 and other TMD samples may be responsible for the fixation of the defect states. In addition, CH3O and CH3 are known as electron-donating group,46 which may have induced the n-doping of our 1L-TMDs and the exfoliated MoS2 film samples.

■ CONCLUSIONS

We presented a simple methanol treatment that can both n-dope and enhance the optical and electrical properties of 1L-TMDs. The methanol treatment was effective for MoS2, WS2 and WSe2, in each case whether exfoliated or grown by CVD. Effects of n-doping and the PL enhancement were thoroughly visualized by using nanoscale PL, Raman, absorption spectroscopy and imaging, and the field-effect mobility values of 1L-MoS2, 1L-WS2 and 1L13



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WSe2 were also significantly improved after the methanol treatment, which suggests that methanol treatment has caused the reduction of structural defects in 1L-TMDs. Our results show that an increase of electron density and the enhancement of optical, electrical and structural properties of 1L-TMDs can be simultaneously achieved, which suggests diverse possibilities for engineering the functional properties of 1L-TMDs.

■ MATERIALS AND METHODS

1Ls of MoS2, WS2 and WSe2 were synthesized on SiO2/Si substrates by using seedingpromoter-assisted CVD techniques.41,47,48 The thicknesses of CVD-grown 1L-MoS2, 1L-WS2 and 1L-WSe2 and exfoliated MoS2 films were confirmed by Raman spectroscopy (Figure S15). We also prepared exfoliated MoS2 samples, where scotch tape49 was used to peel MoS2 from bulk natural 2H-MoS2 crystals (2D Semiconductor Supplies Corp.) and deposited onto silicon/polyvinyl alcohol (PVA)/polymethyl methacrylate (PMMA) substrates. All of the CVD-grown 1L/ML-TMDs were covered with spin-coated PMMA films and transferred onto clean thin (1 mm) glass substrates using the wet-transfer technique.50 We used a dilute HF solution for etching the SiO2 during the wet-transfer process. The exfoliated MoS2 sample was first kept in warm water for a few minutes to dissolve the PVA layers, and floating MoS2/PMMA was transferred onto the clean thin (1 mm) glass substrate by using a dry transfer technique.51 All of the PMMA residues were removed by repeatedly cleaning with acetone, IPA and ethanol. Transferred TMD samples were dipped in methanol for many hours at room temperature. Afterwards, they were dried with nitrogen air and subjected to optical characterizations and XPS analysis. For the electrical measurements, CVD-grown 1L14


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TMDs were transferred onto clean SiO2/Si substrates using wet transfer techniques50 and then cleaned with acetone, IPA and ethanol to remove the PMMA residues. The source and drain electrodes of the FET devices were fabricated by using the normal photolithography technique. For these electrodes, a 5 nm Cr/50 nm Au layer was deposited by applying the electron beam evaporation technique.25,41 After removal of the photoresist, the electrical properties of the fabricated 1L-TMD FETs were tested in ambient conditions. We used a lab-made inverted laser confocal microscope (LCM) system combined with a spectrometer for confocal PL, Raman and absorption spectral imaging measurements.12,52,53 A 514-nm-wavelength line of an Ar ion laser was focused on samples with an objective lens with a 0.9 numerical aperture. The same objective was used to collect the PL and Raman signals from the samples, which were guided to a 50-cm-long monochromator equipped with a cooled charge-coupled device. For absorption spectral mapping, we used the same microscope and spectrometer to focus the white light from a tungsten-halogen lamp and to collect the reflected light. The lateral resolution of the PL imaging and spectroscopy was estimated to be ∼500 nm54,55 and ∼1 µm for the absorption imaging and spectroscopy.12,56 The laser power values used for the PL measurements were 200 µW for MoS2 and 15 uW for the WS2 and WSe2 samples. For the Raman measurement, 1 mW of laser power was used for all samples. Each set of PL, Raman and absorption spectra with different methanol treatment times were obtained using exactly the same experimental parameters such as laser power, exposure time and image size. FET characteristics were measured using a four-probe station equipped with a current detector (SourceMeter 2400, Keithley) at room temperature in air.

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■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: PL spectral mapping results of CVD-grown 1L-MoS2 with various methanol treatment times (S1); deconvolution of PL spectra obtained from methanol-treated 1L-MoS2 (S2); temperature-dependent PL spectra in the range of 77-300 K for pristine and methanoltreated 1L-MoS2 (S3); peak positions maps of the A exciton peak and A1g Raman mode of pristine and methanol-treated exfoliated MoS2 (S4); Raman and PL spectral measurements of pristine and methanol-treated CVD-grown 1L-MoS2 (S5); Raman and PL spectral measurements of pristine and methanol-treated CVD-grown multilayer MoS2 (S6); PL intensity maps of 1L-WS2 with all treatment times and PL intensity histogram with pixel frequencies of 1L-WS2, A exciton peak position maps of 1L-WS2 before and after the 12 h methanol treatment (S7); PL intensity maps of 1L-WSe2 with all treatment times and PL intensity histogram with pixel frequencies of 1L-WSe2, A exciton peak position maps of 1LWSe2 before and after the 12 h methanol treatment (S8); Low temperature PL (77 K) and temperature dependent PL spectra in the range of 77-275 K for pristine and methanol-treated 1L-WSe2 (S9); TRPL map and spectra for pristine and methanol-treated CVD-grown 1LMoS2 and 1L-WSe2 (S10); XPS spectra of pristine and methanol treated multilayered exfoliated MoS2, and CVD-grown 1L-WS2 and 1L-WSe2 (S11); PL images and Raman spectra of CVD-grown 1L-MoS2 treated with hexane, acetone, IPA and toluene (S12); absorption image and spectra of CVD-grown 1L-MoS2 treated with hexane, acetone, IPA and toluene (S13); absorption images and spectra of CVD-grown 1L-WS2 and 1L-WSe2 treated 16


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with methanol (S14); and Raman spectral measurement of exfoliated

and CVD-grown

pristine MoS2 film and CVD-grown pristine WS2 and WSe2 for confirmation of layers thickness (S15) (PDF)

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was supported by IBS-R011-D1.

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Figure 1. (a) PL intensity maps of pristine and 16 h methanol-treated CVD-grown 1L-MoS2. (b) Averaged PL spectra obtained from pristine and 16 h-treated 1L-MoS2. (c) Peak position maps of A exciton peaks of pristine and 16 h-treated 1L-MoS2. (d) Plot of peak heights and peak positions of A exciton peak estimated from average spectra of 1L-MoS2 vs. methanol 26


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treatment times. (e) PL intensity ratio of trions to neutral excitons (A-/A0) and (f) positions of A- and A0 peaks for various durations of methanol treatment. Scale bars in panels a and c indicate 10 µm.

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Figure 2. (a) Averaged Raman spectra of pristine and 16 h methanol-treated CVD-grown 1LMoS2. Dotted line indicates the position of the A1g peak of the untreated sample. (b) Peak position maps of the A1g Raman mode of pristine and 16 h-treated 1L-MoS2. Scale bar indicates 10 µm. (c) PL spectra measured at 125 K, of pristine and 16 h-treated 1L-MoS2.

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Figure 3. (a) PL intensity maps of mechanically exfoliated MoS2 with various layer thicknesses before and after 16 h of methanol treatment. Scale bar indicates 10 µm. (b) Averaged PL spectra obtained from 1L-, 2L- and 3L-MoS2 (pristine and methanol-treated for 16 h). (c) Averaged Raman spectra of pristine and methanol-treated 1L-, 2L- and 3L-MoS2. Dotted lines indicate the positions of the A peaks and A1g peaks of the untreated sample.

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Figure 4. (a) PL intensity maps of pristine and 12 h methanol-treated CVD-grown 1L-WS2. (b) Averaged PL spectra and (c) Raman spectra obtained from pristine and 12 h-treated 1L-WS2. (d) PL intensity maps of pristine and 12 h methanol-treated 1L-WSe2. All scale bars indicate 10 µm.

(e) Averaged PL spectra and (f) Raman spectra obtained from pristine and 12 h-

treated 1L-WSe2. Dotted lines indicate the positions of the A peaks and A1g peaks of the untreated samples.

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Figure 5. (a) Fluorescence lifetime images, and (b) representative PL decay curve of pristine 1L-WS2 and methanol treated 1L-WS2.

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Intensity


S 2p

160

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0h

0h

16h

16h Mo 3d S 2s

162 164 166 168 225 228 231 234 237 Binding Energy (eV) Binding Energy (eV)

Figure 6. XPS spectra of the CVD-grown 1L-MoS2 before and after the 16 h methanol treatment.

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Figure 7. Electrical transport characteristics of pristine and methanol-treated CVD-grown (a) 1L-MoS2 and (b) 1L-WS2, and (c) 1L-WSe2. Insets show the optical images of the FET devices. All scale bars in the insets indicate 20 µm.

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Figure 8. (a) Confocal absorption spectral maps of pristine and 16 h methanol-treated CVDgrown 1L-MoS2. Scale bar indicates 10 µm. (b) Averaged absorption spectra obtained from pristine and 16 h-treated 1L-MoS2. (c) Schematic illustration of methanol dissociation and electron doping on 1L-MoS2.

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Table of Contents (TOC) graphic

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Simple Chemical Treatment to n-Dope Transition-Metal

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