Coupled Charge Transfer Dynamics and Photoluminescence Quenching in Monolayer MoS2 Decorated with WS2 Quantum Dots

Herein, we have investigated the tunability of the photoluminescence (PL) of the monolayer MoS2 (1L-MoS2) by decorating it with WS2 quantum dots (WS2 QD). The direct bandgap 1L-MoS2 and WS2 QDs are grown by chemical vapor deposition and liquid exfoliation methods, respectively. The room temperature PL spectrum of bare 1L-MoS2 is systematically quenched with its decoration with WS2 QDs at different concentrations. A decrease in the work function of 1L-MoS2 with the decoration of WS2 QDs was established from the Kelvin probe force microscopy analysis. A detailed quantitative analysis using the four-energy level model involving coupled charge transfer was employed to explain the redshift and the systematic decrease in the intensity of the PL peak in 1L-MoS2/WS2 QD heterostructure. The modulation of the PL in the heterostructure is attributed to the increase in the formation of negative trions through the charge transfer from WS2 QD to the 1L-MoS2 and thus making the 1L-MoS2 heavily n-type doped, with increase in the electron density by ~1.5 × 1013 cm−2. This study establishes the contribution of defects in the coupled charge transfer dynamics in 1L-MoS2, and it lays out a convenient strategy to manipulate the optical and electrical properties of 1L-MoS2 for various optoelectronic applications.

neutral exciton and charged exciton (trion) population resulting in the modulation in the photoluminescence (PL) of 1L-MoS 2 . Similarly, Roy et al. 17 fabricated a heterostructure composed of MoSe 2 QDs and 1L-MoS 2 or WSe 2 and studied the charge transfer mechanism. However, in these studies, the role of defects in PL quenching of the 1L-MoS 2 has not been addressed. To our knowledge, there is no report on the charge transfer from WS 2 QDs to 1L-MoS 2 and the resulting doping and PL quenching effect. It is interesting to study the role of defects in the charge transfer dynamics in the 1L-MoS 2 layers through PL spectroscopy and its implications for future applications. In the literature, the studies on heterostructures have been usually performed on chemically grown 2D layers, which are often multilayered and crystalline quality of layer is inferior to that grown by chemical vapor deposition (CVD) techniques.
Herein, we report a study on the tunability of the PL emission spectrum through charge transfer at the 1L-MoS 2 /WS 2 QD HS interface. The HS was synthesized by WS 2 QDs prepared by the liquid exfoliation method onto the CVD grown 1L-MoS 2 . The PL intensity of 1L-MoS 2 is reduced after the formation of the 1L-MoS 2 /WS 2 QD HS. This quenching of the PL is traced to the charge transfer from the WS 2 QD to 1L-MoS 2 resulting in the conversion of the neutral exciton to trion, thus making the 1L-MoS 2 n-type doped. Additionally, the presence of defects may be another dominant factor that alters the PL emission. We show that by solving the carrier dynamics based on the coupled rate equations, we can have a better understanding of the contribution of the defects in the recombination dynamics of the hybrid structure.

Results and Discussion
Morphology studies. Figure 1(a) displays the optical image of monolayer MoS 2 film grown with triangular-shaped MoS 2 grains towards the edge of the sapphire substrate. These triangular shaped MoS 2 regions merge to form a large continuous monolayer film with millimeter-scale uniformity, as evident from Fig. 1(a). The layer uniformity is evident from the small difference in contrast over the whole film. Details of the growth conditions for monolayer MoS 2 film over a large area have been discussed in our previous work 18 . Figure 1(b) shows the AFM image of the triangular-shaped monolayer MoS 2 . It reveals that the triangular-shaped MoS 2 have a tendency to interconnect with each other rather than overlap when they grow to form a continuous film as seen by the homogeneous color contrast, which further indicates a good uniformity. The AFM height profile taken along the black line in Fig. 1(c) indicates a thickness of ~0.7 nm, which corresponds to monolayer thickness. The AFM image of the 1L-MoS 2 /WS 2 QD HS is shown in Fig. S1(a) (Supporting Information). The height profile of MoS 2 layer and the QDs decorated over it clearly revealed the growth of monolayer MoS 2 and monolayer WS 2 QDs, as shown in Fig S1(b) (Supporting Information).
The typical morphological and structural properties of the as-prepared WS 2 QDs were studied using TEM. Figure 2(a) shows the TEM image of the WS 2 QDs. The selected area electron diffraction (SAED) pattern (top right inset of Fig. 2(a)) shows the presence of diffused rings, which indicates the polycrystalline nature of the QD. The WS 2 QDs size ranges from 3-11 nm with an average diameter of 4.5 ± 0.2 nm, as shown in Fig. 2(b). The high-resolution TEM (HRTEM) image of the WS 2 QD (Fig. 2 (c)) displays ordered lattice fringes. The inset in Fig. 2(c) shows the inverse fast Fourier transform (IFFT) of the lattice fringes with an interplanar spacing of 0.22 nm, which corresponds to the (103) plane of WS 2 . To examine the coverage of the WS 2 QDs on the 1L-MoS 2 , TEM imaging of the 1L-MoS 2 /WS 2 QD HS was carried out. Figure 2(d) shows a low magnification TEM image of the QD decorated on large area 1L-MoS 2 film. A higher magnification TEM image is depicted in Fig. 2(e), where a uniform surface coverage of WS 2 QDs is clearly observed over the MoS 2 layer. The corresponding SAED pattern shows the polycrystallinity of the WS 2 QDs. In addition, hexagonally aligned diffraction spots are attributed to the (101) plane of MoS 2 (inset of Fig. 2(e)). Thus, the as-grown 1L-MoS 2 is highly crystalline in nature and is uniformly decorated with WS 2 QDs. The HRTEM image of the 1L-MoS 2 /WS 2 QD HS is displayed in Fig. 2(f), which shows distinct lattice planes. The top-left inset shows the IFFT of the atomic planes of the MoS 2 film. The lattice d-spacing is 0.27 nm that corresponds to (101) plane of MoS 2 . Additional ordered domains are observed with a lattice spacing of 0.22 nm, which can be assigned to the (101) plane of WS 2 (top right inset of Fig. 2(f)).
Structural and optical analysis. The chemical composition of the 1L-MoS 2 and WS 2 QDs was confirmed from the XPS analysis. Figure 3 shows the XPS spectra of the core level Mo 3d, W 4 f and S 2p bands for the 1L-MoS 2 and WS 2 QDs samples. Figure 3(a) confirms the elemental composition of 1L-MoS 2 with the presence of the peaks of Mo and S. In Fig. 3(b), several Mo 3d 5/2 and 3d 3/2 peaks fitted for Mo (3d) envelope, indicating that more than one Mo species were present. The first peak, centered at 226.4 eV, agrees well with that of the 2 s binding energy of elemental S. The strongest Mo 3d doublet peaks for 1L-MoS 2 detected at 229.1 eV (3d 5/2 ) and 232.0 eV (3d 3/2 ) correspond to the +4 oxidation state of Mo, confirming the formation of MoS 2 19 . Additional Mo peaks were observed at 232.8 eV and 235.2 eV corresponding to the oxides of Mo metal (Mo 6+ ) probably due to the presence of traces of MoO 3 in the sample after CVD growth and post-synthesis exposure to air. Figure 3(c) exhibits the S 2p XPS spectra of 1L-MoS 2 with peaks at ∼161.8 eV (S 2p 3/2 ) and ∼162.9 eV (S 2p 1/2 ) corresponding to the divalent sulfide ions (S 2− ). Additionally, a peak at 162.1 eV (S 2p 3/2 ) (with 8.1% spectral weight) is present that could be due to the presence of surface defects introduced during the CVD growth. These defect sites are the S vacancies as there are fewer S atoms around the Mo atoms at such sites 20 . The survey scan XPS spectrum of WS 2 QDs shows the presence of W, S, C, N and O peaks (Fig. 3(d)). The high-resolution XPS spectrum for carbon (C 1 s) is shown in Fig. S2 (Supporting Information). The deconvoluted spectrum consists of three main components centered at 284 eV, 285.2 eV and 286.7 eV that correspond to sp 2 hybridized carbon, sp 3 carbon and C-O bonds, respectively 18 . It is well known that carbon dots are composed mainly of sp 3 hybridized carbon bonds, which in www.nature.com/scientificreports www.nature.com/scientificreports/ our case, constitute merely of 5.8% of the high-resolution C 1 s spectrum. In contrast, the sp 2 hybridized carbon accounts for 73.9%. These results rule out the possible presence of any carbon dots in the WS 2 QDs samples. For the as-synthesized WS 2 QD, the peaks at 32.5 eV and 34.8 eV are identified to be from W 4f 7/2 and W 4f 5/2 , respectively, corresponding to the 4 + oxidation state of W, as shown in Fig. 3(c), which are consistent with those reported for 2H-WS 2 21 . Figure 3(d) shows the S 2p XPS of the WS 2 QD with peaks at ∼161.8 eV (S 2p 3/2 ), and ∼162.9 eV (S 2p 1/2 ), which are similar to that of the 1L-MoS 2 sample. The existence of surface defects (S vacancies) in the WS 2 QD is shown by the presence of the peak at 162.1 eV (S 2p 3/2 ) (with 13.5% spectral weight), which may be created during the synthesis by liquid exfoliation method. Additionally, there is a small peak at 167.5 eV corresponding to SO 2 which suggests the minor presence of oxidized sulfur edges.
Raman spectroscopy has been widely used for the determination of the number of layers 22 , the strain, the external field and doping effects 16,23,24 in 2D TMDs. Figure 4(a) shows the comparative Raman spectra for 1L-MoS 2 and 1L-MoS 2 /WS 2 QD HS at room temperature. Two characteristic Raman modes E 2g and A 1g corresponding to the in-plane vibration of Mo and S atoms and out-of-plane vibration of S atoms respectively can be clearly seen 22 . The frequency difference (Δk) between E 2g and A 1g modes has been used to identify the number of layers in MoS 2 22 . For 1L-MoS 2 sample, the measured Δk is ~19.6 cm −1 confirming the monolayer growth 22 , which is consistent with the AFM result. WS 2 QDs also show the presence of two characteristic Raman modes E 2g and A 1g of WS 2 , which confirms the crystallinity of the QDs 25 . A comparative Raman analysis of the WS 2 QDs and WS 2 nanosheets shows a red shift in the E 2g mode and a blue shift in the A 1g mode in the QDs with respect to that of the nanosheets (see Fig. S3, Supporting Information). This shift in the Raman modes is attributed to the decrease in the number of layers of the WS 2 QD compared to the WS 2 nanosheets 25 . Interestingly, after the formation of the 1L-MoS 2 /WS 2 QD HS, the position of the Raman modes of MoS 2 , A 1g is red-shifted by 1.2 cm −1 , while that of E 2g is not influenced (See Table 1). This shift occurs due to the fact that the A 1g mode couples much more strongly with electrons than the E 2g mode 23 . The redshift of the A 1g mode indicates an effective n-type doping effect in the MoS 2 layer due to the strong electron-phonon coupling 16 . Crystallinity of the WS 2 QDs is further confirmed from the XRD analysis (see Fig. S4, Supporting Information) that shows a strong peak at 14.3° corresponding to the (002) plane and multiple weak peaks corresponding to (004), (101), (103), (006) and (105) lattice planes of 2H-phase of crystalline WS 2 (JCPDS 08-0237) 26 . Figure 4(b) shows the UV-vis absorption spectra of the samples. The 1L-MoS 2 exhibits three excitonic absorption peaks A, B and C at 1.85, 2.00 and 2.74 eV, respectively. The excitonic A and B peaks originate from the transitions between the spin-orbit split valence band and the minimum of the conduction band at the K and K′ points of the Brillouin zone 7 . The C absorption peak is assigned to the direct transition from the deep valence band to the conduction band 27 . The absorption spectrum of WS 2 QDs (see Fig. 4(b)) shows low absorbance in the visible range and no distinct excitonic features in contrast to that of the monolayer WS 2 reported in the literature 28 . Since the QDs are mostly monolayer, the bandgap is expected to be direct type and the optical bandgap calculated from the Tauc plot is 3.45 eV (see Fig. S5, Supporting Information), which is much higher than that of the monolayer WS 2 28 . In case of 1L-MoS 2 /WS 2 QD HS, three absorption peaks (A, B, C) were observed, which is consistent with the spectra of 1L-MoS 2 . A marginal enhancement in the absorbance of 1L-MoS 2 /WS 2 QD HS compared to that of individual absorbance of 1L-MoS 2 and WS 2 QDs is observed in the spectral range 2.48 to 4.59 eV. The www.nature.com/scientificreports www.nature.com/scientificreports/ enhancement of the absorbance of the 1L-MoS 2 /WS 2 QD heterostructure compared to that of the pristine monolayer MoS 2 and WS 2 QDs may be due to the combined effect of the increase in the number of layers as well as the enhanced light-material interaction in the heterostructure 29 . To determine the absorption peaks of spin-orbit split B and A excitons in the 1L-MoS 2 and 1L-MoS 2 /WS 2 QDs, we have taken the first derivative of the absorption spectra (see the inset of Fig. 4(b)). The A and B excitonic peaks for 1L-MoS 2 are located at 1.844 eV and 1.990 eV, respectively. For 1L-MoS 2 /WS 2 QD HS, there is only ~4 meV redshift in the A excitonic peak with respect to the 1L-MoS 2 . This small redshift in the A peak may be due to the n-type doping of 1L-MoS 2 after the formation of the HS due to the charge transfer from the WS 2 QDs to the 1L-MoS 2 . In contrast to our case of charge transfer, the shift in the excitonic peaks in the absorption spectra has been more prominent in chemically doped 1L-MoS 2 30 .

Raman modes
Relative weightage of PL peaks www.nature.com/scientificreports www.nature.com/scientificreports/ The as-synthesized WS 2 QDs are highly fluorescent in nature with a quantum yield (QY) of ~15%. The PL emission spectra usually depend on the wavelength of excitation due to the contribution from multiple states and size distribution 31 . Figure 4(c) displays the normalized PL emission spectra of the WS 2 QDs for various excitation wavelengths. As the excitation wavelength is increased systematically from 300 to 480 nm, the emission peak position systematically redshift from 2.52 eV to 2.31 eV. The excitation wavelength-dependent PL shift in WS 2 QDs is poorly understood in the literature. The broadening in PL peak usually results from the polydispersity in the WS 2 QD size, which is attributed to the colloidal synthesis process 32,33 . To explain the broad PL spectrum in WS 2 QDs under 300 nm excitation, we have deconvoluted the spectrum with three Gaussian peaks: the B exciton, the neutral A exciton, and the defect bound exciton X, as shown in Fig. 4(d). The A and B-excitons centered at 3.1 eV and 3.5 eV arise from the giant spin-orbit splitting of the valence band in the K-K′ point 21 . The B and A excitons arise from the splitting of the valence band at the K point due to strong spin-orbit coupling in the W atom of WS 2 7,34 . The energy difference between these two peaks is found to be ~400 meV, which is similar to that of monolayer WS 2 35 . The contribution from the A and B exciton is gradually reduced with increasing excitation wavelength and hence the spectrum is narrower than that with low wavelength excitation. The X band in the fitting at 2.54 eV is associated with the surface defect bound exciton X, and at higher excitation wavelength (>380 nm), the PL emission arises only from the bound exciton transition (Fig. 4(d)). Thus, the PL peak position is dictated by the excitation energy; lower the excitation energy lower will be the emission energy due to the selective excitation of energy levels. This explains the wavelength-dependent shift in the PL emission peaks in WS 2 QDs. Note that the PL peak assignments are based on the measured bandgap and the energy band relationship: 36 , where E b is the exciton binding energy (~0.3 eV for monolayer WS 2 ). E SO is the energy difference arising due to splitting of the valence band due to strong spin-orbit coupling (~0.4 eV) in the W atom of WS 2 35 . Thus, based on the measured bandgap, E B is expected to be ~3.5 eV. Likewise, the A exciton peak is expected at ~3.1 eV. The deconvoluted peaks positions in Fig. 4(d) closely match with the above. Note that the defect contribution to the PL intensity is very significant in all the spectra. Figure 5(a) displays representative PL spectra of pristine 1L-MoS 2 , WS 2 QD and 1L-MoS 2 /WS 2 QD HS, measured with 488 nm laser excitation. The PL emission peak for the WS 2 QD is broad due to the size distribution of QDs and it is much weak compared that of the 1L-MoS 2 . The PL peak position (~2.28 eV) is consistent with the result presented in Fig. 4(c) 37 . Interestingly, this peak is at a much higher energy than that of 1L-WS 2 28 . The broadening and blue shifting of the PL peak of the WS 2 QD originate from the quantum size effect as well as the surface defect states 31 . For 1L-MoS 2, we observe a PL peak at 1.86 eV with 488 nm excitation. However, after the formation of the 1L-MoS 2 /WS 2 QD HS the PL peak position is redshifted by ~30 meV and the intensity is also partially quenched. Such a redshift and quenching of the PL is an indication of the charge transfer and n-doping effect due to the specific band alignment at the interface. This is consistent with the Raman analysis discussed earlier.
To further interpret the possible origin of the PL evolution, a deconvolution analysis was carried out by fitting each spectrum with four Gaussian peaks: the neutral exciton (A 0 ), negative trion (A − ), B exciton, and the defect bound exciton (X). Figure 5(b) shows the fitted PL spectra of the sample 1L-MoS 2 and 1L-MoS 2 /WS 2 QD HS, respectively. The A 0 and B exciton peaks are associated with the direct bandgap transition at the K point in the Brillouin Zone, with energy split from the strong valence-band spin-orbit coupling 8 . It has been reported that the A − trion peak arises from charged impurities in the 1L-MoS 2 grown by a CVD method on accounts of unintentional n-type doping 38 , and the X exciton peak is assigned to the radiative recombination of bound excitons from the defect trap states 39 . Note that in the fitting process, we have fixed only the peak positions of the A 0 (1.88 eV), B (1.98 eV), A − (1.83 eV) and the X (1.78 eV) bands and the rest are kept as free parameters. With the decoration of the WS 2 QDs, the PL spectral weight of the A 0 exciton peak decreased from 53% to 20.6%, while that of the A − trion peak increased from 23.2% to 37.2% (see Fig. 4(b) and Table 1). This increase in the spectral weight of the negative trion in 1L-MoS 2 /WS 2 QD HS is due to an increase in the number of excess electrons in the 1L-MoS 2 . This is an indication that electrons are transferred from the WS 2 QDs to the 1L-MoS 2 . Upon illumination (at 488 nm) with photon energy lesser than the bandgap (E g ) of the WS 2 QDs, only electrons in the defect states of the QDs absorb the photons and these electrons are excited to the conduction band. Some of these generated electrons are transferred to the 1L-MoS 2 resulting in n-type doping, as can be understood from the schematic of the band alignment of the 1L-MoS 2 and WS 2 QD depicted in Fig. 5(c). DFT calculations on the MoS 2 /WS 2 HS from previous studies show charge transfer from 1L-WS 2 to 1L-MoS 2 40 . The spectral weight of the defect bound excitons X increases from 10.6% to 26.7% after the formation of HS. (see Fig. 4(b) and Table 1).
To provide evidence in support of the proposed charge transfer process, the change in the work function of 1L-MoS 2 before and after the decoration of WS 2 QD was estimated by KPFM (Kelvin probe force microscopy). Figure 6(a,c) show the AFM topography of 1L-MoS 2 and 1L-MoS 2 /WS 2 QD HS, while Fig. 6(b,d) show the surface potential image of 1L-MoS 2 and 1L-MoS 2 /WS 2 QD HS, respectively. Before measurement, the work function of the tip (Φ t , in eV) was calibrated (∼4.52 eV). The overall contact potential difference (V CPD , in V) values of the measured samples were provided by the KPFM measurements. The measured V CPD between the sample and the tip can be expressed as, e × V CPD = Φ t − Φ s , where e is the elementary charge and Φ s is the work function of the sample. The contact potential difference for 1L-MoS 2 is ~85 mV, while that for 1L-MoS 2 /WS 2 QD HS is ~120 mV. So, the work functions of 1L-MoS 2 , Φ 1L-MoS2 ~ 4.435 eV, which is similar to previously reported values 41 and Φ 1L-MoS2/WS2 ~ 4.400 eV, respectively. Thus, there is a distinct decrease in the work function of the 1L-MoS 2 /WS 2 QD HS by 35 meV compared to 1L-MoS 2 . The reduction in the work function of the HS suggests the favorable band bending for the charge transfer from the WS 2 QDs to the 1L-MoS 2 .
To further understand the change of the PL intensity of the 1L-MoS 2 with the addition of the WS 2 QDs (concentration 4 to 36 mg/L), PL intensity was measured for the HS system. Figure 7(a) shows the variation of the PL spectra of the 1L-MoS 2 with different concentrations of WS 2 QDs. The PL intensity of the 1L-MoS 2 decreases systematically and PL peak broadens and red-shifts as the concentration of the WS 2 QDs is increased. The total PL intensity of the 1L-MoS 2 decreases dramatically after the formation of the 1L-MoS 2 /WS 2 QD HS even at very low concentration (4 mg/L), as shown in Fig. 7(b). Note that attachment of WS 2 QDs to 1L-MoS 2 surface is limited by the specific surface area of the 1L-MoS 2 and beyond a certain concentration, WS 2 QDs are not directly attached to the MoS 2 surface sites and hence further charge transfer is restricted at high concentration. To have a better understanding of the spectral changes in PL, we have considered the contribution of the neutral exciton, trion and defect bound exciton in the spectral deconvolution of PL peaks, as shown in Fig. 7(c). We believe that with increasing concentration of WS 2 QDs, charge carrier density increases in 1L-MoS 2 . These doped electrons easily form trions and restrain the electron-hole pair recombination and as a result, the PL intensity quenches systematically and the PL peak is redshifted. Therefore, the neutral excitons are gradually converted to trions resulting in the change of the spectral weight of the individual component. It is evident from the fitting shown in Fig. 7(c), for low concentrations of the WS 2 QD (<12 mg/L), the PL emission is dominated by the neutral exciton peak (A 0 ). At higher concentration of WS 2 QDs, the contribution of the trions becomes higher than the neutral exciton and hence induces quenching of the PL intensity and a redshift of the PL peak position. Figure 8(a) shows a plot of the integrated PL intensity of neutral excitons I A 0, negative trions − I A and the bound excitons I X as a function of the concentration of WS 2 QDs. We notice that the intensity of the neutral excitons I A 0 decreases gradually and then almost saturates at high concentration of the WS 2 QD (>24 mg/L). However, there is a very small change in the integrated intensity of the trions. This is because the trion emission saturates after a certain doping level due to Pauli blocking effect 15 . Thus, the excess electrons that are transferred from the WS 2 QDs to the 1L-MoS 2 will www.nature.com/scientificreports www.nature.com/scientificreports/ further move to the defect trap states. It is interesting to note that despite the systematic decrease in the integrated PL intensity of A 0 and A − peaks, the defect-related X peak intensity does not decrease with doping, which is essentially due to the charge transfer from the A − level to X level. In the absence of defect, one would expect an increase in trion population with increasing doping (electron) concentration, which is contrary to our experimental data. On the other hand, the total integrated PL intensity I Total decreases in a similar way as that of I A 0. Figure 8(b) shows the change of the PL spectral weight of the neutral exciton (I A 0/I Total ) with the increase in the concentration of the WS 2 QDs. For pristine 1L-MoS 2 the spectral weight is ~0.61, whereas, with doping at higher concentration (>24 mg/L), the spectral weight decreases up to ~0.29. This is an indication of the transition from neutral exciton to trion with the increase in the doping.
For a quantitative understanding of the relative change in the PL intensity of the neutral exciton I A 0, trion − I A and defect bound exciton I X , we discuss the exciton and trion relaxation dynamics with rate equations based on a four-energy level model, as shown in Fig. 8(c) 42 . Here, G represents the generation rate of excitons, Γ 1 and Γ 2 represent the decay rates of the exciton and trions, respectively. k tr (δ) is the formation rate of trion from the exciton, which is dependent on the doping concentration (δ) of the WS 2 QDs. To better model our experimental observation, we have assumed Γ 1 to be dependent of δ and it is taken as proportional to doping concentration δ, without which the trion population would not decay with increasing δ, which will be evident from the solution of the rate equations discussed below. In case of high doping density, carrier-density-dependent recombination dynamics of excitons is rational and it has been reported for InGaN/GaN quantum wells 43 . Thus the dependence of Γ 1 on δ is reasonable in the present case. The trions also decay through the defect trapping state at the rate Γ 3 . Lastly, Γ 4 represents the decay rate of the defect bound excitons. Thus, based on the evolution of the three peaks with different doping concentrations, the electronic transitions are shown in Fig. 8(c). The corresponding rate equations for the population of neutral excitons N A 0, trions − N A and the defect bound excitons N X can be expressed as: www.nature.com/scientificreports www.nature.com/scientificreports/ where the parameter α in Eq. (4) represents the WS 2 QD adsorption probability and β in Eq. (5) is a proportionality constant. Considering that the rate of adsorption of WS 2 QDs obeys the Langmuir's law, the formation rate of trions with doping concentrations can be described as k tr (δ) and s (∼85% for our best-fitted data) reflects the ability of charge transfer from WS 2 QD to 1L-MoS 2 . Doping concentration δ is increased in steps for 4 mg/L in our experiment. By solving the above rate equations analytically within the framework of the four-level model (see Section S1, Supporting Information, for the full derivation), under steady-state condition, the equations reduce to A tr The steady-state PL intensities of neutral exciton (I A 0), trion ( − I A ) and defect bound exciton (I X ) can be represented as follows: www.nature.com/scientificreports www.nature.com/scientificreports/ A ex A tr tr where A is the collection efficiency of luminescence, γ ex , γ tr and γ X are the radiative decay rates of neutral exciton, trion and defect bound exciton, respectively. The calculated/fitted PL intensities °− I I , A A and I X in Eqs. (9)(10)(11), are in excellent agreement with the experimental results, as shown in Fig. 8(a). The parameters used in this analysis are Γ 1 (0) = 0.002 ps −1 , Γ 2 = 0.02 ps −1 ,Γ 3 = 0.05 ps −1 , and k tr (0) = 0.5 ps −1 , which are based on previously reported data 42,44 . We have assumed an intermediate decay rate from the defect trap state, Γ 4 = 0.01 ps −1 for a good fit to the carrier recombination dynamics. The fitting parameters of AGγ tr /AGγ ex and AGγ X /AGγ ex to match the experimental data are 0.38 and 0.01, respectively, which implies that γ tr < γ ex and γ X ≪ γ ex , consistent with their relative PL intensities observed experimentally. Note that our value of γ tr /γ ex is nearly double of the reported value (γ tr / γ ex = 0.15) 12 , due to the specific band alignment for favorable charge transfer and formation of trions. Due to the higher bandgap of WS 2 QDs than that of monolayer WS 2 , the band bending is higher in our case resulting in more efficient charge transfer. Our results further imply that the defect (X) contribution to the PL evolution is smaller than the trion (A − ) contribution. However, it is significant enough and necessary to consider it in the rate equation to match with the experimental data.
Assuming the validity of the law of mass action here, the relationship between the population of the neutral exciton (N A 0), trions ( − N A ) and the charge density n e in the 1L-MoS 2 is expressed as , and the E b and T are taken as 25 meV and 300 K, respectively. The γ tr /γ ex and γ X /γ ex values as obtained from the fitting are substituted here. Thus, the charge density n e is calculated from the exciton spectral weight using Eq. (13) and is shown in Fig. 8(d). For pristine 1L-MoS 2 , the charge density is ~ 5.5 × 10 12 cm −2 owing to its unintentional n-doping attributes 45 . After WS 2 QD doping, in the saturation region, the calculated electron density of the 1L-MoS 2 /WS 2 QD HS increases to 20.5 × 10 12 cm −2 . It is important to note that the difference in the electron density before and after the formation of the HS is, Δn e ~ 1.5 × 10 13 cm −2 , which is significant. This change in the electron density signifies the approximate density of doped electrons in 1L-MoS 2 . The inset in Fig. 8(d) shows the gradual increase in the charge density n e in the 1L-MoS 2 with the increase in the WS 2 QD concentration. Thus, these results demonstrate effective control of doping/electron density in 1L-MoS 2 about one order of magnitude by the decoration of WS 2 QDs. We believe that the electron density in the 2D materials can be effectively tuned by decorating with QDs of other 2D materials with high bandgap and thus, enable suitable control of the electrical and optical properties of the 2D materials, which is very significant for the ensuing applications.

conclusion
In conclusion, we have demonstrated the tunability in the light emission of the 1L-MoS 2 by decorating it with the WS 2 QD. KPFM analysis revealed a decrease in the work function of 1L-MoS 2 with the decoration of WS 2 QDs. Systematic quenching of the PL intensity of 1L-MoS 2 with the decoration of WS 2 QDs was explained on the basis of charge transfer from WS 2 QDs to 1L-MoS 2 . A detailed analysis using coupled charge transfer among four-energy levels was employed to explain the redshift and the decrease in the PL intensity of the 1L-MoS 2 after decoration with the WS 2 QDs. An analytical solution to the coupled rate equations for change in the population of different excitonic emissions including bound excitonic transition was successfully employed to quantitatively understand the quenching process. The contribution of defects in the charge transfer induced quenching of PL and the carrier-density-dependent recombination dynamics of excitons were established through the quantitative analysis of the spectral evolution. Charge transfer induced increase in electron density in 1L-MoS 2 leads to the transition of the neutral excitons to trions. The change in the electron density up to Δn e ~ 1.5 × 10 13 cm −2 indicates high n-type doping in the 1L-MoS 2 by a simple decoration approach. Our results suggest an effective way to manipulate the electron density through decoration/doping technique, which is advantageous to tune the optical and electrical properties of monolayer TMDs for optoelectronic applications.

Methods
Synthesis of WS 2 quantum dots. High purity WS 2 powder (Sigma Aldrich, 99%) was dispersed in 80 ml N-methyl-2-pyrrolidinone (NMP) (Alfa Aesar, HPLC grade, 95%) and tip-sonicated using an ultrasonic homogenizer (Sonic Ruptor 250, Omni International) for 15 hours. Subsequently, the suspension was allowed to settle for 12 hours and was centrifuged for 45 minutes at 12000 rpm. The top 2/3 rd of the solution (supernatant) contains the WS 2 quantum dots, while the bottom 1/3 rd (centrifugate) comprises of the bigger WS 2 quantum dots and the nanosheets (See Fig. S6, Supporting Information). The excess solvent from the centrifugate was evaporated with constant stirring and the resultant residue was dispersed in Milli-Q water at various concentrations (4, 8, 12, 16, 20, 24, 28, 32,  Sapphire substrates by the CVD method using a two-zone horizontal muffle furnace. 15 mg of MoO 3 powder (99.5%, Sigma-Aldrich) and 200 mg of sulfur powder (99.95%, Sigma-Aldrich) in separate quartz boats were placed inside the 2" diameter quartz tube at the center of their respective zones for the CVD growth of MoS 2 , as reported previously 46 . The substrates were placed face down on top of the quartz mask with a circular opening and then placed on the boat containing MoO 3 . Then, the quartz tube was flushed with high purity argon gas at 300 sccm for 30 minutes prior to the growth. The sources temperatures were gradually increased from room temperature to 700 °C and 150 °C at the rates 15 and 3.5 °C/min for MoO 3 and Sulphur, respectively, and kept at this temperature for 5 minutes at an argon flow rate of 10 sccm. Afterward, the furnace is allowed to cool down to room temperature. It was observed that the 1L-MoS 2 film was deposited only on the portions of the substrate which were covered by the quartz mask. The unmasked regions of the substrate were found to be deposited with few-layer and multilayer MoS 2 . We observed that in both the SiO 2 /Si and sapphire substrates, large-area monolayer MoS 2 film was grown as reported in our previous work 47  www.nature.com/scientificreports www.nature.com/scientificreports/ For the formation of the heterostructure, WS 2 QDs were spin-coated onto the 1L-MoS 2 and are dried before optical characterizations were carried out (see Fig. S6, Supporting Information). characterization techniques. The 1L-MoS 2 grown over various substrates, WS 2 QDs, and their heterostructure were studied by high-resolution micro-Raman spectroscopy (LabRam HR800, Jobin Yvon). Both Raman and PL spectra were acquired sequentially from the same spot on the sample through a 100X objective lens with a spot size ∼1 μm and laser power ~1.5 mW to avoid laser-induced sample damage. The signal was then collected by a charge-coupled device (CCD) using a backscattering geometry sent through a multimode fiber grating (1800 grooves mm −1 ). Atomic force microscopy (AFM) (Cypher, Oxford Instruments) images were acquired to confirm the layer thickness of CVD-grown MoS 2 and WS 2 quantum dots. In order to carry out the surface potential (SP) analysis of the samples, the Kelvin probe force microscopy (KPFM) measurements were done. Conducting platinum (Pt)/iridium (Ir)-coated tips were used for KPFM studies, having the optimum frequency of operation ~72 kHz. To avoid the noise between the topographical and the surface potential measuring images, the measurements were carried out in the dual-pass lift mode. The calculation of the work function for the sample (Φ s ) was obtained from the AFM by using Pt/Ir tips in the KPFM mode. The morphology, size and structural properties of the as-prepared WS 2 QDs were studied by a transmission electron microscope (TEM) (JEOL-JEM 2010 operated at 200 kV). Samples for TEM analysis were prepared on a carbon-coated Cu grid of 400 mesh size (Pacific Grid, USA). TEM imaging was used to examine the decoration of WS 2 QD on 1L-MoS 2 . For this purpose, the CVD grown 1L-MoS 2 was transferred from the SiO 2 substrates to carbon-coated Cu-grids. To transfer as-grown MoS 2 film, the sample was coated with polymethylmethacrylate (PMMA) by spin coating at 1500 rpm for 60 s, and then baked at 140 °C for 10 min. The PMMA-coated sample was then treated with 6 M NaOH solution for one hour to etch out the PMMA supported MoS 2 film, which was then repeatedly washed with DI water. Then, the film was fished out onto a Cu grid and allowed to dry at low temperature (50 °C). The PMMA was removed from the MoS 2 film by the addition of acetone dropwise. WS 2 QDs of the concentration 4 mg/L was then drop cast on the sample for TEM imaging. A commercial spectrophotometer (PerkinElmer, Lambda 950) was used to study the UV−vis absorption spectra of the 1L-MoS 2 /WS 2 QD HS as well as its individual counterparts.