Simultaneous Hosting of Positive and Negative Trions and the

May 17, 2016 - Heterostacking of layered transition-metal dichalcogenide (LTMD) monolayers (1Ls) offers a convenient way of designing two-dimensional ...
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Simultaneous Hosting of Positive and Negative Trions and the Enhanced Direct Band Emission in MoSe2/MoS2 Heterostacked Multilayers Min Su Kim,† Changwon Seo,†,‡ Hyun Kim,†,‡ Jubok Lee,†,‡ Dinh Hoa Luong,†,‡ Ji-Hoon Park,† Gang Hee Han,† and Jeongyong Kim*,†,‡ †

Center for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea ‡ Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea S Supporting Information *

ABSTRACT: Heterostacking of layered transition-metal dichalcogenide (LTMD) monolayers (1Ls) offers a convenient way of designing two-dimensional exciton systems. Here we demonstrate the simultaneous hosting of positive trions and negative trions in heterobilayers made by vertically stacking 1L MoSe2 and 1L MoS2. The charge transfer occurring between the 1Ls of MoSe2 and MoS2 converted the polarity of trions in 1L MoSe2 from negative to positive, resulting in the presence of positive trions in the 1L MoSe2 and negative trions in the 1L MoS2 of the same heterostacked bilayer. Significantly enhanced MoSe2 photoluminescence (PL) in the heterostacked bilayers compared to the PL of 1L MoSe2 alone suggests that, unlike other previously reported heterostacked bilayers, direct band transition of 1L MoSe2 in heterobilayer was enhanced after the vertical heterostacking. Moreover, by inserting hexagonal BN monolayers between 1L MoSe2 and 1L MoS2, we were able to adjust the charge transfer to maximize the MoSe2 PL of the heteromultilayers and have achieved a 9-fold increase of the PL emission. The enhanced optical properties of our heterostacked LTMDs suggest the exciting possibility of designing LTMD structures that exploit the superior optical properties of 1L LTMDs. KEYWORDS: molybdenum disulfide, molybdenum diselenide, heterobilayer, positive trions, negative trions, interlayer charge transfer

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Excitons predominantly govern the optical properties of 1L LTMDs due to the strong Coulombic interactions between charge carriers confined in 1L LTMDs, which also lead to the stable formation of charged excitons known as trions. Positive (negative) trions are formed by two holes (electrons) and an electron (hole) when there is an excess of one charge or the other.13−15 Such an excess of charge may develop during the synthesis of 1L LTMDs or may be intentionally induced by carrying out electric back-gating,14 photoionization of impurities,16 or chemical doping.17 Photoluminescence,14,18 electroluminescence,19 and absorption spectroscopy18 can be used to detect trions in LTMDs since the energy levels of trions are ∼30 meV lower than those of neutral excitons, due to the finite binding energies of trions. Trions in 1L LTMDs, in addition to

ayered transition-metal dichalcogenides (LTMDs) are intriguing two-dimensional (2D) electronic systems that display a large variety of physical properties with numerous possible combinations of transition metals and dichalcogenide atoms.1,2 Engineering the physical properties of these 2D systems is being further expanded by having them form heterostructures achieved by stacking various kinds of monolayer (1L) LTMDs.3−5 These heterostacked LTMDs have been interesting theoretical subjects for designing unique properties in 2D exciton systems that can modulate electronic band structures3,6 and enhance optical properties.4 Experiments have shown the quenching of photoluminescence (PL),5 formation of interlayer excitons,7 spectral broadening,8 and the emergence of charged excitons9 originating from the charge exchange occurring between stacked LTMD 1Ls5,9−11 or due to the formation of hybrid band structures.12 However, an experimental result of enhancement of optical properties, such as increase of PL emissions, by stacking LTMDs has not yet been reported. © 2016 American Chemical Society

Received: April 1, 2016 Accepted: May 17, 2016 Published: May 17, 2016 6211

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Figure 1. (a) Integrated PL intensity map of the HB region and 1L MoSe2 and 1L MoS2 regions. Brighter contrast in the images represents the higher PL intensity. (b) Deconvoluted PL spectra of the 1L MoSe2, 1L MoS2, and the HB region fitted with a neutral A exciton peak (A0), trion peaks (A+ or A−) and B exciton peaks (B). PL peak positions of 1L MoSe2 and 1L MoS2 are indicated with dotted lines to show the redshift of the PL peaks in the HB. (c) Integrated PL intensity map of multiple HB regions. (d) Map of PL peak positions of the same area as shown in (a). All scale bars are 5 μm.

being of basic scientific interest, can also find practical applications in the electrical control of light emission.15,20 So far, manipulation of trion states such as controlling the population of trions in LTMDs has mostly been carried out in 1L LTMDs of MoS2, MoSe2, WS2, and WSe2.14,21−23 In particular, in 1L MoSe2, investigators were able to switch positive and negative trions on and off by using an external electrical control.21 However, the engineering of trion states in heterostructures has not been actively explored. There has only been a recently published report about heterostacked MoSe2/ WS2 bilayers displaying the emergence of negative trions, which was attributed to electron transfer from MoSe2 to WS2 in the heterostacked bilayer.9 Here we report the simultaneous observation of positive and negative trions in vertically stacked MoSe2/MoS2 heterostructures at room temperature. By using spatially resolved PL and Raman spectroscopy and imaging, we showed that the polarity of trions in 1L MoSe2 can change from negative to positive when heterostacking the 1L MoSe2 with 1L MoS2. We also demonstrated that inserting a hexagonal boron nitride (hBN) 1L in between the 1Ls of MoSe2 and MoS2 resulted in a PL emission nine times greater than that of 1L MoSe2 alone, apparently by changing the extent of charge transfer between the stacked 1L MoS2 and 1L MoSe2.

spectrum to the A exciton peak of MoS2. We found, as seen in the PL image and the spectra in Figure 1a,b, that the PL intensity of 1L MoSe2 was slightly higher than that of 1L MoS2, consistent with previous reports.11,12 Interestingly, the overall PL intensity of the HB region was significantly higher than that of either the 1L MoSe2 or 1L MoS2 region. This enhancement of the PL was due to a greater MoSe2 PL in the HB region than in 1L MoSe2, while the PL of MoS2 was somewhat reduced. All HB regions that we inspected displayed an overall increased PL, as shown in a PL image of multiple HB regions (Figure 1c). In addition to the increase of PL intensity, the A exciton peaks of MoSe2 and MoS2 in the HB region were observed to be lower in energy. They were red-shifted by ∼30 meV from 1.575 eV in the 1L MoSe2 region to 1.548 eV in the HB region and from 1.876 eV in the 1L MoS2 region to 1.848 eV in the HB region. The peak positions of the original (unstacked) 1L MoSe2 or 1L MoS2 are indicated by dotted lines in the PL spectra of the HB. The map of the PL peak positions in Figure 1d showed the redshift of the A exciton peak of MoSe2 to be uniformly present throughout the HB region. We note that ∼30 meV shift of MoSe2 PL peak is significantly larger than the reported effects of dielectric environment change to PL of 1L LTMDs,26 indicating other major mechanisms are responsible for observed modification of PL characteristics in HB. To investigate the origin of the observed increase and redshift of the A exciton peaks of MoS2 and MoSe2 in the HB region, we deconvoluted the PL spectra of the 1L MoSe2, 1L MoS2, and the HB regions by fitting them with neutral A exciton peaks (A0), trion peaks (A− or A+) and B exciton peaks, as shown in Figure 1b. We found the spectral weights of A0 and A− in 1L MoS2 and MoSe2 to be comparable in strength, and we assigned the trion peaks in both of 1L MoS2 and MoSe2 to negative trions (A−) based on previous reports indicating chemical vapor deposition (CVD)-grown MoS2 and MoSe2 to be intrinsically n-type.2,20,25 In the HB region, the MoS2 A exciton peak was completely dominated by A−. Also, the trion peak occupied a much larger portion of the spectral weight of the MoSe2 PL peak in the HB region than in the 1L MoSe2 region, with the spectral weight of the A0 peak in the former strongly reduced. We also noticed the presence of an additional

RESULTS AND DISCUSSION The results of PL spectroscopic imaging of the MoSe2/MoS2 heterobilayer (HB) made by stacking 1L MoSe2 onto 1L MoS2 are shown in Figure 1. An integrated PL intensity map of the HB region and 1L regions is displayed in Figure 1a. Representative PL spectra obtained from the 1L MoSe2, 1L MoS2, and HB regions are displayed in Figure 1b. These PL spectra were not normalized, and their PL intensities can be directly compared. In the 1L MoS2 region and 1L MoSe2 region, we observed characteristic A exciton and B exciton peaks,2,20,24,25 and we were able to assign A exciton peaks and B exciton peaks of MoSe2 and MoS2 for the PL spectrum from the HB region as well. The B exciton peak of MoSe2 was not clearly distinguished in the PL spectrum of the HB region, due to the relatively weak peak intensity and its proximity in the 6212

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Figure 2. (a) Schematic of the band alignment in the HB describing the transfer of electrons (holes) from MoSe2 (MoS2) to MoS2 (MoSe2). (b) Representative absorption spectra of the 1L MoSe2, 1L MoS2, and the HB regions. The inset shows an absorption intensity image of the HB region and 1L region. The absorption spectra were not normalized and are directly comparable in intensity. The sum of the absorption spectra of 1L MoSe2 and 1L MoS2 (MoSe2 + MoS2) is displayed for comparison. The A exciton peak position of 1L MoS2 is indicated with a dotted line to show the redshift of A exciton peaks in the HB. (c) Representative Raman spectra of the 1L MoSe2, 1L MoS2, and the HB regions. The insets show the maps of Raman A1g peak positions of MoSe2 and MoS2 with the contrast representing the local peak position. The A1g peaks of MoS2 and MoSe2 from the HB regions were red-shifted and slightly blue-shifted, respectively, compared to those from the corresponding 1L regions. Scale bars in insets indicate 5 μm.

Figure 3. (a) PL spectra of the HB region for different gate bias values. Normalized PL spectra of the MoSe2 and MoS2 regions of the spectra are displayed in separate panels. (b) Plot of the positions of the MoSe2 and MoS2 PL peaks from the HB as a function of the gate bias voltage. (c) PL spectra of 1L MoSe2 for different gate bias values. Note the gradual redshift of the PL peak here with increasing gate bias, in contrast to the behavior of the MoSe2 PL peak in the HB.

electrostatic tunability of A− and A+ in their gate-tuned PL spectroscopy studies,20 where the polarity of trions in 1L MoSe2 was switched from negative to positive by supplying

peak ∼40 meV lower in energy than the trion peak, which matched the energy position of the biexciton (AA) of MoSe2.3,27−29 Ross et al. reported the observation and 6213

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Figure 4. (a) Normalized deconvoluted PL spectra of the MoSe2 (left panel) and MoS2 (right panel) peaks of the spectra, obtained from the HB region of our product. (b) The peak position of each exciton complex, including that of neutral excitons (A0), trions (A+ or A−), and biexcitons (AA) in the HB as a function of the excitation laser power. (c) The emission intensity of the each exciton complex vs excitation laser power. Colored lines are the power fits to the integrated intensities of the peaks for A0, A+, and AA, and the slopes (m values) are fitted values of numeric powers.

hole charges via electric gating. Based on the reported switchability of the trion polarity of 1L MoSe2 and the effective charge exchange reported in other heterostacked bilayer structures,5,7,30 we attributed the observed trion peak of MoSe2 in the HB region to positive trions (A+) that originated from the charge transfer occurring between 1L MoS2 and MoSe2 in the HB region. Schematic in Figure 2a describes such charge transfer occurring between 1L MoS2 and MoSe2 and the formations of positive trions and negative trions in the HB. Contact between 1L MoSe2 and 1L MoS2 is known to result in the socalled type II band alignment, as illustrated in the band diagram shown in the inset of Figure 2a.11,12 In this configuration, electrons from 1L MoSe2 are expected to transfer to 1L MoS2, while the holes in 1L MoS2 to 1L MoSe2, resulting in effective n-doping of MoS2 and p-doping of MoSe2.5,9,10 The charge transfer effect in the HB was also indicated by the absorption spectral imaging and Raman spectral imaging. Differential reflectance spectral imaging was performed on the 1L MoS2, 1L MoSe2, and HB regions, and the averaged spectrum from each region is displayed in Figure 2b. In the approximation of a thin absorbing material on a transparent substrate, the differential reflectance is proportional to the absorption intensity; therefore the observed differential spectra were regarded as the absorption spectra.31 The inset in Figure 2b shows the total integrated absorption intensity map. First, note that the absorption spectrum of the HB almost represents the simple sum of the absorption spectra of 1L MoS2 and 1L MoSe2, as shown in Figure 2b. However, similar to the case of PL spectra obtained from the HB region, the absorption spectra from the HB displayed significantly red-shifted exciton peaks of MoS2 and MoSe2, confirming the increase in the population of trions

in MoS2 and MoSe2 layers of the HB. Figure 2c displays the representative Raman spectra obtained from the 1L MoSe2, 1L MoS2, and HB regions, which were normalized in order to compare them. The insets show the maps of the Raman A1g peak positions of MoSe2 and MoS2. In the HB region, characteristic Raman E12g and A1g mode peaks of MoS2 and MoSe2 were clearly observed, and the A1g peak of MoS2 was observed to be red-shifted by ∼3 cm−1 compared to the corresponding peak of 1L MoS2. This relative shift of the A1g peak can be interpreted as resulting from enhanced electron− phonon interaction with increased electron density.31,32 The E12g peak at 390 cm−1 was also red-shifted by 2.7 cm−1 in the HB region, indicating the presence of the tensile strain in MoS2 due to the lattice mismatch with MoSe2 in the HB region.6 In contrast, the A1g peak of MoSe2 in the HB region was blueshifted by 0.8 cm−1, indicating the p-doping effect in the MoSe2 layer.33 The polarity of trions in 1L MoSe2 and the HB region was further identified by gate-dependent PL spectroscopy. By carrying out back gate-induced charging, tuning of the charge density in LTMD layers can be carried out.14,20,21 With the sample area grounded to a metal contact, PL spectra were obtained from the 1L MoS2, 1L MoSe2, and HB regions, while the backgate bias was applied by varying the voltage from −100 V to +100 V. Figure 3a shows PL spectra obtained from the HB region that displayed the variation of PL peaks corresponding to the MoS2 PL peak and the MoSe2 PL peak. We note that the intensity of the MoS2 PL peak gradually decreased and its position became more red-shifted as the gate bias was increased, which may be interpreted as an increase of the trion spectral weight in the MoS2 PL curve. This result confirmed the n-type nature of the MoS2 layer in the HB 6214

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emission of MoSe2 in the heterostacked multilayers (HMs). Figure 5a,b displays PL images and representative PL spectra,

region. In contrast, the position of the PL peak corresponding to MoSe2 became more blue-shifted with increasing gate bias. These completely opposite effects of gate bias voltage on the positions of the PL peaks of MoS2 and MoSe2 observed in the HB are shown in Figure 3b. The blue shift of the MoSe2 PL peak may be interpreted as a decrease of trion spectral weight with positive gate bias, suggesting the p-type nature of the MoSe2 layer in the HB. In contrast to the PL peak of MoSe2 in the HB, PL spectra obtained from the 1L MoSe2 region clearly showed the redshift of the peak position with increasing gate bias (Figure 3c), indicative of the intrinsic n-type nature of 1L MoSe2. This opposite effect of increasing gate bias on the behavior of the PL peak position of MoSe2 in HB compared to that in 1L MoSe2 confirmed the conversion of the trion polarity of 1L MoSe2 from negative to positive by its heterostacking with 1L MoS2, which we believe was caused by the charge carrier exchange with the MoS2 layer in the HB. The presence of trions in the MoS2 and MoSe2 layers in the HB region was further investigated by determining the effect of laser excitation power on the contributions of the exciton complexes to the PL emission (Figure 4a, see left panel for spectral region of MoSe2 PL and right panel for MoS2 PL). Only two peaks, an A− peak at 1.835 eV and B peak at 1.996 eV, were found to constitute the MoS2 PL peak at all excitation powers, which can be explained by the complete dominance of negative trions (A−) in heavily n-doped 1L MoS2. Considering the comparable spectral weights of A0 and A− in intrinsic 1L MoS2 (see Figure 1b), the effect of charge transfer between MoSe2 and MoS2 in the HB depicted in Figure 2a is clearly discernible. The PL peak of MoSe2 in the HB was found to consist of three peaks corresponding to A0, A+, and AA, as shown in Figure 4a (left panel), and the relative spectral weights of A+ and AA increased with increasing excitation power. The peak positions of the A− (in MoS2 layer), A0, A+, and AA exciton complexes (in MoSe2 layer) in the HB at the various excitation powers tested are displayed in Figure 4b, which indicates the binding energies of A+ and AA in the HB to be ∼30 meV and ∼60 meV, respectively. These values are consistent with previously reported binding energies of A+ and AA of 1L MoSe2.16,21,27 We also plotted the PL emission intensity of each exciton complex vs excitation laser power, as shown in Figure 4c. The power fits with I ∼ Pm, where I, P, and m represent the PL intensity, laser power, and numeric power, are shown for each exciton complex in the HB. The integrated intensity of A0 peak was found to increase linearly, with m ∼ 1.0, at low excitation laser powers and then plateau at excitation laser powers >60 μW. In contrast, the integrated PL intensities of A+ and AA peaks showed superlinear behaviors throughout the tested range, which are reasonable features for these exciton complexes.22,27,28 We observed a ∼2-fold increase of MoSe2 PL in the HB, compared to 1L MoS2 or 1L MoSe2, as shown in Figure 1. In most previous studies, charge separation between 1Ls in HB led to the quenching of direct band PL of the constituent 1L LTMDs and/or the emergence of interlayer excitons in the HB.5,9−11 In our observations, however, the supply of holes from the MoS2 layer to MoSe2 layer and the electron transfer from the MoSe2 layer to the MoS2 layer seemed to help increase the PL efficiency of the MoSe2 layer in the HB. Moreover, we found that a subtle adjustment of the charge transfer, accomplished by inserting h-BN 1Ls between 1L MoSe2 and 1L MoS2, was able to induce an even higher PL

Figure 5. (a) PL intensity maps of the MoSe2/MoS2 HB and MoSe2/h-BNs/MoS2 HMs as well as of the 1Ls. The insets are schematics of the heterostacked structures where one or two 1L hBNs were stacked between 1L MoSe2 and 1L MoS2 to form HMs. Scale bars are 5 μm. (b) Representative PL spectra of 1L MoS2, 1L MoSe2, the HB, HM1 (1 h-BN), and HM2 (2 h-BN). Vertical dotted line indicates the PL peak position of the A exciton of 1L MoSe2. (c) Representative absorption spectra obtained from the HB and HM1. The sum of the absorption spectra of 1L MoSe2 and 1L MoS2 (MoSe2 + MoS2) is displayed for comparison. Vertical dotted line indicates the peak position of the A exciton absorption peak in the 1L MoS2 absorption spectrum.

respectively, of the MoSe2/MoS2 HB, MoSe2/h-BN/MoS2 heterostacked multilayer (HM1), and MoSe2/h-BN/h-BN/ MoS2 heterostacked multilayer (HM2). The insets in Figure 5a show the schematics of sample structures, where one or two hBN 1Ls were present between 1L MoSe2 and 1L MoS2 to form HM1 or HM2, respectively. In Figure 5a (center and lower panels) h-BN layers cover the whole area in the images. Therefore, MoS2 and MoSe2 layers are either on 1L h-BN (center panel) or 2L h-BN (lower panel). The effect of h-BN below 1L of MoS2 and MoSe2 to the PL spectra was shown to be negligible (see Figure S1). The presence and number of inserted h-BN layers were confirmed by the detection of h-BN characteristic E12g Raman peaks34 (see Figure S2). The PL intensity of HM1 was found to be nine times greater than that of 1L MoSe2, as shown in Figure 5a,b. HM1 displayed an almost identical PL peak position as did 1L MoSe2, implying that the level of charge transfer that occurred in HM1 was less than that in the MoSe2/MoS2 HB. These results suggest that the insertion of the h-BN layer between MoSe2 and MoS2 suppressed the charge-transfer process compared to the HB where MoSe2 and MoS2 made a direct contact. Additional evidence of reduced charge transfer in HM1, i.e., the HM with one h-BN inserted, was provided by analyzing an 6215

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Figure 6. (a) Color map of the PL spectrum of the MoSe2/MoS2 HB as a function of temperature ranging from 77 to 300 K. (b) Representative PL spectrum obtained at different temperature of the MoSe2/MoS2 HB. (c) The emission intensity of the A exciton PL peaks vs excitation laser power. (d) Plot of the exciton PL peak positions of the MoSe2/MoS2 HB as a function of the temperature. Lines show the fitting results using Varshni’s empirical formula.

between our observation and others is attributed to the coexistence of two opposite effects of interlayer charge transfer to the PL of MoSe2 in HB or HMs, where the charge depletion in MoSe2 layer by the interlayer charge transfer will result in pdoping, emergence of positive trions, and enhanced PL of MoSe2, however the excitons of MoSe2 layer can also be dissociated by the interlayer charge transfer weakening the PL. Indeed, when we increased a heat treatment time in MoSe2/ MoS2 stacking process (from 2 h normally used in our study) to 12 hours for the purpose of making stronger (perhaps shorter) contact between MoSe2 layer and MoS2 layer, significant quenching of the PL was observed in the HB (see Figure S3). Our results indicate that the former effect occurs through the longer range of interlayer distance and the latter occurs only for much shorter interlayer distance. One possible explanation is that p-doping to MoSe2 layer is mostly due to depletion of majority carriers of MoSe2 layer which are relatively free compared to charge carriers consisting of neutral excitons bound with the large dissociation energy. The different effective ranges of the interlayer distance for competing effects of increasing PL and quenching PL suggest a possibility of tuning the optimum conditions for enhanced PL of MoSe2 in vertical heterostructures. Such a tuning ability for maximizing PL of heteromultilayers was demonstrated in this work with four different interlayer conditions of direct contacts with 2 or 12 h heat treatment and insertion of one or two h-BN layer between MoSe2 and MoS2 layers. Along the line of similar phenomena, we present an interesting thermal behavior of the PL spectra of the HB we observed in temperature-dependent PL spectroscopy conducted together on 1L MoS2, 1L MoSe2, and MoS2/MoSe2 HB. Figure 6a shows the map of PL spectra of HB obtained with the varying temperature from 77 to 300 K. Three exemplary PL spectra selected at 77, 200, and 300 K are also

absorption spectrum obtained from HM1 together with the absorption spectrum obtained from the HB, as shown in Figure 5c. A simple sum of the 1L MoSe2 and 1L MoS2 absorption spectra (MoSe2 + MoS2) is also shown again in Figure 5c. The peak positions of the A excitons in HM1 are almost the same as those of the 1L MoSe2 and 1L MoS2 without the large redshifts observed in the absorption spectra of the HB, again indicating the lower level of charge transfer occurring in HM1 than in the HB. As indicated above, we also stacked two layers of 1L h-BN on 1L MoS2 before stacking 1L MoSe2 to form MoSe2/h-BN/ h-BN/MoS2 (HM2) in order to further suppress the interlayer charge transfer and acquired the PL image and spectra of HM2 as shown in Figure 5a,b. HM2 yielded a lower PL intensity than did HM1 but a greater PL intensity than did 1L MoSe2, indicating that interlayer charge transfer leading to an increase in PL in the MoSe2 layer of the HM2 still occurred. In our experiments, a MoSe2/MoS2 HB and MoSe2/h-BN/ MoS2-type heterostacked multilayers that we fabricated displayed greater PL efficiency than did 1L MoSe2 and 1L MoS2. This difference is a direct result of the greater quantum yields of the HB and HMs, as their optical absorption spectra did not show an increase as marked as the increase of PL; i.e., their absorption intensities were merely similar to the sum of the MoSe2 absorption and MoS2 absorption intensities (see Figures 2 and 5). Our observation is consistent with the previous theoretical predictions for heterostacked structures that the valence-band maximum in the MoS2/MoSe2 bilayer is strongly localized, and optical transitions of monolayers could be retained.3,4 However, in previous experiments, LTMD HBs with the type II band alignment, including MoSe2/MoS2 HB, showed quenching of the overall PL emission, which was attributed to fast interlayer charge transfer and the resulting nonradiative dissociation of excitons followed by slow (∼μs) radiative decay of the interlayer excitons.5,9−12 Discrepancy 6216

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ACS Nano shown in Figure 6b. One striking observation is that MoSe2 PL of HB displayed a sudden increase of PL at ∼200 K. Figure 6c displays the variations of PL intensities of 1L MoS2, 1L MoSe2, and MoS2/MoSe2 HB as a function of temperature (see Figure S4 for the complete set of temperature-dependent PL spectra). We note that at temperatures under ∼200 K, the PL intensity of HB is much lower than 1L MoS2 or MoSe2, however above 200 K, the MoSe2 PL of HB starts to increase with the increasing temperature, while other PL peaks of 1L MoS2, 1L MoSe2 and HB MoS2 continue to decrease, resulting in the PL enhancement of HB compared to 1L MoSe2 or 1L MoS2 at 300 K, as we discussed so far. The variation of peak positions of MoSe2 A+ and A0 and MoS2 A− in HB as a function of temperature is shown in Figure 6d, which exhibits the trend of the typical Varshni’s empirical formula (see the detail in Supporting Information). The observed much weaker PL of HB compared to 1L MoS2 and 1L MoSe2 at low temperature indicates that at low temperature, the exciton quenching by interlayer charge transfer occurs in our HB structure, similar to previous observations of the suppressed PL in heterostacked LTMD bilayers due to exciton separation by interlayer charge transfer.5,9−11 However, observed increase of PL of the HB above ∼200 K suggests that the conditions of the interlayer contact in our HBs were modified (possibly weakened) at elevated temperature, and as a result the exciton separation by interlayer charge transfer must have been suppressed while pdoping of MoSe2 by interlayer charge transfer was still active, increasing the PL of MoSe2. Such a sudden modification of interlayer decoupling with the increasing temperature was previously reported in a few layer MoSe2 as well.34

1Ls, 200 mg of Se (Sigma-Aldrich) and a 10 mg of MoO3 were used as a precursor, and the procedure followed was the same as that for the MoS2 1Ls except that the temperature at the upstream side was 410 °C. The whole process was carried out under 500 sccm of N2 and 5 sccm of H2 delivered over 15 min. To grow 1L h-BN by CVD, a Pt foil was used as a substrate. The furnace was ramped up to 1100 °C for 30 min under a flow of 10 sccm of H2 gas and maintained there for 15 min in order to stabilize the temperature. 1L h-BN on a Pt foil was synthesized for 20 min with borazine and hydrogen at flow rates of 0.2 and 10 sccm, respectively. Details of the growth of h-BN are described in the previous report.35 Preparation of the Stacked Heterobilayer and the Heteromultilayer Samples. 1Ls of MoS2 and MoSe2 grown on a SiO2/Si wafers by CVD were transferred to a transparent cover glass substrate for confocal PL, Raman, and absorption measurements, using a typical wet transfer method.36 An aqueous KOH solution and an aqueous HF solution were used for the transfers of the 1L MoS2 and 1L MoSe2, respectively. To fabricate MoSe2/MoS2 heterostacked bilayers, the PMMA/ MoS2 film was transferred onto the cover glass. The PMMA layer covering the 1L MoS2 was removed by using acetone, and the sample was baked at 80 °C for 30 min to improve the adhesion between 1L MoS2 and the cover glass. Then, the PMMA/MoSe2 film was transferred onto the 1L MoS2/cover glass, resulting in the formation of MoSe2/MoS2 heterostacked bilayers on the cover glass. The PMMA layer covering the heterostacked bilayers was also removed by using acetone, and this sample was also baked at 80 °C for 30 min to improve the adhesion between 1L MoSe2 and 1L MoS2. Lastly, the heterostacked bilayers were baked at 180 °C for 2 h under an argon atmosphere to remove residue and to improve the adhesion between the 1Ls and the cover glass.36 To fabricate the MoSe2/h-BN/MoS2 and MoSe2/h-BN/h-BN/MoS2 heterostacked multilayers, 1L h-BNs grown on a Pt foil were detached from the Pt foil using a “bubbling-based” transfer method.35 The PMMA/h-BN films were transferred either onto 1L MoS2 or h-BN depending on the desired stacking order. The PMMA removal and baking processes after each transfer were performed in the same way as described above. For backgate-bias PL measurements, 50 nm-thick Au/5 nm-thick Cr electrodes were printed over the prepared MoSe2/MoS2 HB sample by applying electron-beam lithography and subsequent evaporation. Confocal PL, Raman, and Absorption Spectral Mapping Measurements. For the confocal PL, Raman, and absorption imaging and spectroscopy, a combination of a labmade laser confocal microscope and spectrometer was used.27,36−38 The laser light was focused with a 0.9 NA objective lens, and the lateral resolution of the PL imaging and spectroscopy was estimated to be ∼500 nm36,38 and ∼1 μm for the absorption imaging and spectroscopy.31,37 Scattered light was collected using the same objective and guided to a 50 cmlong monochromator equipped with a cooled CCD. The excitation laser was the 514 nm wavelength laser line of an argon laser for the PL measurements and the 532 nm line of a solid-state laser for the Raman measurements.

CONCLUSIONS Here we showed that the stacked heteromultilayers consisting of 1L MoSe2 and 1L MoS2 exhibited the simultaneous presence of positive trions and negative trions and a greatly enhanced PL emission compared to 1L MoSe2 and 1L MoS2. These interesting observations were due to the effective charge transfer between 1L MoSe2 and 1L MoS2 that enhanced the direct-band transition of 1L MoSe2. We also showed the ability to tune the level of charge transfer and thereby enhance the PL by inserting 1L h-BNs between 1L MoSe2 and 1L MoS2. Our results showed that vertically heterostacked LTMD structures provide a convenient and reliable platform to study intriguing phenomena of confined exciton complexes driven by interlayer charge exchanges, which can be tunable for the enhanced optical properties. METHODS Monolayer MoS2, MoSe2, and h-BN Synthesis. MoS2 and MoSe2 1Ls were separately grown on a SiO2/Si substrate by carrying out chemical vapor deposition (CVD). This growth was promoted by first spin-coating perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS, 2D semiconductors) onto a SiO2/Si wafer at 2600 rpm for 1 min. For MoS2 1Ls, 200 mg of S (Sigma-Aldrich) and 10 mg of MoO3 were placed in the middle of a furnace. The promoter-coated substrate was suspended above an Al2O3 crucible boat containing the MoO3 powder. The upstream side was heated to 210 °C at a ramping rate of 42 °C/min, whereas the temperature of the downstream side was ramped up to 780 °C. The whole process was carried out under 500 sccm of N2 delivered over 15 min. For MoSe2

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free to charge on the ACS Publications Web site at The Supporting Information is 6217

DOI: 10.1021/acsnano.6b02213 ACS Nano 2016, 10, 6211−6219

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available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02213. Experimental details and data (PDF)

AUTHOR INFORMATION Corresponding Author

*Email: [email protected]. Notes

The authors declare no competing financial interest.

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