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Observation of Charge Transfer in Heterostructures Composed of MoSe Quantum Dots and a Monolayer of MoS or WSe 2

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Shrawan Roy, Guru P. Neupane, Krishna P. Dhakal, Jubok Lee, Seok Joon Yun, Gang Hee Han, and Jeongyong Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11778 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 4, 2017

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The Journal of Physical Chemistry

Observation of Charge Transfer in Heterostructures Composed of MoSe2 Quantum Dots and a Monolayer of MoS2 or WSe2 Shrawan Roy,a,b Guru P. Neupane,a,b Krishna P. Dhakal,a,b Jubok Lee, a,b Seok Joon Yun,a,b Gang Hee Han,a and Jeongyong Kima,b* a

Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Suwon 16419, Republic of Korea b

Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea

ABSTRACT: Monolayer transition metal dichalcogenides (TMDs) are atomically thin semiconductor films that are ideal platforms for the study and engineering of quantum heterostructures for optoelectronic applications. We present a simple method for the fabrication of TMD heterostructures containing MoSe2 quantum dots (QDs) and a MoS2 or WSe2 monolayer. The strong modification of photoluminescence and Raman spectra that includes the quenching of MoSe2 QDs and the varied spectral weights of trions for the MoS2 and WSe2 monolayers were observed suggesting the charge transfer occurring in these TMD heterostructures. Such optically active heterostructures, which can be conveniently fabricated by dispersing TMD QDs onto TMD monolayers, are likely to have various nanophotonic applications because of their versatile and controllable properties.

ACS Paragon Plus Environment


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█ INTRODUCTION Transition metal dichalcogenides (TMDs) are members of the class of layered materials held together by van der Waals interactions; TMDs undergo a distinct electronic transition: the bandgap is indirect in the bulk structure but becomes direct when thinned down to a monolayer.16

Monolayer (1L) two-dimensional (2D) TMDs of molybdenum disulfide (MoS2), tungsten

disulfide (WS2), molybdenum diselenide (MoSe2), and tungsten diselenide (WSe2) are direct bandgap semiconductors with strong photoluminescence (PL) caused by exciton formation at the K point of the Brillouin zone.1–6 These materials have been intensively investigated because of their potential applications, e.g., in photodetectors,7,8 transistors,9 sensors, and light-emitting devices.10 The extremely large Coulomb interaction in these confined 2D monolayers creates stable excitonic states even at room temperature. 1L-MoS2 is the most extensively studied 1LTMD; 1L-MoS2 is intrinsically n-type and have a direct bandgap of ~1.85 eV.1,2 1L-WSe2 is another interesting 1L-TMD, which is an intrinsic p-type semiconductor with a direct bandgap of ~1.65 eV, strong spin–orbit coupling, and a high quantum yield.6,11 A convenient and effective method for tuning the optical properties of 1L-TMDs is chemical doping; doping of 1L-TMDs results in interesting many-body phenomena and can thus be used to tailor their optical properties.4,12,13 In n-type 1L-TMDs, when they are n-doped and the electron density increases, the two electrons–one hole recombination (negative trion emission) process dominates the exciton recombination process and the total PL intensity was generally reduced, which was attributed to the increased charge screening.4 In contrast, p-type dopant molecules that deplete electrons hindering the process of trion formation resulted in enhancements to the neutral exciton PL and the overall PL efficiency of n-type 1L-TMDs.13,14 In previous studies, organic chemical solutions

of

benzyl

viologen,

2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane,

and

nicotinamide adenine dinucleotide, etc., have been used to tailor the optical properties of 1L-


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TMDs.4,13 Vertically stacked heterostructures consisting of 1L-TMDs of MoS2 and WS2 or MoS2 and MoSe2 have also been studied, and the charge exchange provided by the appropriate band alignment at their heterostructure interfaces means that their optical properties are tunable.15–17 MoS2 and WS2 QDs are analogous to graphene quantum dots (QDs) and have been synthesized by using simple sonication and solvothermal processes from bulk powders; such QDs have found applications in the bio-imaging of living cells because of their good cell permeability and low cytotoxicity, and exhibit high performance in hydrogen evolution reactions (HER) because of their large number of free and defective active edge sites.18, 19 In a previous study, MoS2 QDs were synthesized with an electrochemical etching process and found to exhibit excellent HER activity with an onset potential of ~210 mV.20 Nanodots of other TMDs (MoS2, WS2, ReS2, TaS2, WSe2, MoSe2, and NbSe2) have been synthesized with top-down methods and used as active layers in memory devices by mixing them with polyvinylpyrrolidone (PVP) to prepare, e.g., PVP–MoSe2 composites.21 Heterodimensional hybrid nanostructures of MoS2 QDs interspersed in few-layer MoS2 nanosheets have also been synthesized by using the liquid-phase exfoliation technique, and found to exhibit enhanced performance in a HER due to their high number of active sites.22 In heterostructures composed of graphene QDs and 1L-MoS2, n-type doping is observed because of the charge transfer from the QDs to 1L-MoS2.23 However, the optical properties of TMD QD heterojunctions with 1L-TMDs that could provide vertical heterojunctions and convenient platforms for the engineering of the optoelectronic properties of TMD nanostructures have not previously been reported. In this study, we synthesized TMD heterostructures by dispersing MoSe2 QDs prepared with sonication and solvothermal processes directly onto a MoS2 or WSe2 monolayer grown by chemical vapor deposition (CVD). Charge transfer in the heterostructures resulted in PL



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quenching of the QDs, and in either increased or decreased trion spectral weights in the PL spectra of the MoS2 and WSe2 monolayers depending on the type of the majority carrier in the 1L-TMD. These charge transfer phenomena were comprehensively visualized by using nanoscale confocal PL and Raman spectral imaging. █ MTERIALS AND METHODS MoSe2 QDs synthesis. The MoSe2 QDs were synthesized from MoSe2 powder by using combination of sonication and solvothermal processes.19 1 g MoSe2 powder (Sigma Aldrich) was mixed with 100 mL of N-methyl pyrrolidone (NMP) (Sigma Aldrich) in a 150 mL bottle and tip-sonicated (JEIO TECH-VTRC-20) for 24 h with an output power of 450 W to exfoliate the nanosheets. The exfoliated nanosheets dispersed in NMP were treated solvothermally for 12 h with vigorous stirring at 140°C. The resulting suspension was then centrifuged for 30 min at 12000 rpm and the upper supernatant containing the well-dispersed MoSe2 QDs was extracted for use in the subsequent experiments. Growth of monolayer TMDs and transfer onto cover glass substrate. Monolayers of MoS2 and WSe2 were grown on SiO2/Si substrates by using the CVD method.14,17,24 For each MoS2 monolayer, perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (2D semiconductors) was spin coated at 2600 rpm for 1 min onto the SiO2/Si substrate as a promotor, which was then suspended above an Al2O3 crucible boat containing MoO3 powder. Then, 200 mg of S (Sigma Aldrich) and 10 mg of MoO3 were placed in the middle of the furnace. The upstream side was heated to 210°C at a ramping rate of 42°C/min for 15 min, whereas the temperature of the downstream side was ramped up to 780°C. The whole procedure


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was carried out under 500 sccm of N2 delivered for 15 min. For each WSe2 monolayer, 200 mg of Se (Sigma Aldrich) and 10 mg of WO3 were used as the precursors, and the whole process was carried out following the same procedure as in 1L-MoS2 growth except that the upstream temperature was ramped to 410°C and the whole procedure was carried out under 500 sccm of N2 and 5 sccm of H2 delivered over 15 min. The monolayers of MoS2 and WSe2 were then transferred to cleaned cover class with the wet transfer method.14,17 Aqueous HF solution was used to etch out the SiO2 layers. A thin film of poly (methyl methacrylate) (PMMA) was deposited onto the as-grown samples by using the spin coating method. Then, the PMMA/TMDs film was transferred onto cleaned cover glass. The PMMA layer covering each sample was removed with acetone and isopropyl alcohol, and finally the samples were baked at 80°C for 30 min to improve the adhesion between the samples and the cover glass substrates. The MoSe2 QDs were dispersed on the transferred 1L-TMDs by using the spin coating method and optical characterizations were carried out after drying out the solvent from the samples. Confocal PL, Raman spectral mapping, absorption measurements, SEM and HRTEM measurements. For the confocal PL and Raman spectral mappings, a lab-made laser confocal microscope with a spectrometer and a 1.3 NA oil-immersion objective lens was used. Scattered light was collected by using the same objective and guided through an optical fiber to a 50 cm long monochromator equipped with a cooled charge-coupled device (CCD). Excitation was provided by a 488 nm Ar-ion gas laser with typical powers of 15 µW for PL mapping and 300 µW for Raman measurements, and the acquisition time was 0.5 s per pixel for the spectral images. The ultraviolet/visible (UV/Vis) absorption spectra of the exfoliated MoSe2 nanosheets and QDs were recorded in NMP at room temperature. The field emission scanning electron microscopy



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(FESEM, JEM-2100F, JEOL, Corp.) images of the MoSe2 QDs/MoS2 and WSe2 heterostructures were captured with field voltages from 5 to 20 keV. The high resolution transmission electron microscopy (HR-TEM, JEM-ARM200F, JEOL Corp.) images of the MoSe2 QDs were obtained at 200 kV.

█ RESULTS AND DISCUSSION The schematic diagram shown in Fig. 1 depicts the synthesis of the QDs and the dispersion of the QDs on a MoS2 or WSe2 monolayer to make our TMD heterostructures (Details in Materials and Methods). The HR-TEM image in Fig. 2a of the synthesized MoSe2 QDs shows that the diameters of the MoSe2 QDs are in the range 2–6 nm with an average of 4 ± 0.4 nm (see the size distribution in the inset). Crystal formation is evident in the MoSe2 QD in the magnified HR-TEM image, and its fast Fourier transform pattern is shown in Fig. 2b. The lattice parameter in the (104) direction of the synthesized QDs was measured to be 0.21 nm, which is consistent with the known values of MoSe2.21 To assess the QD dispersion and size distribution, large area FESEM images of QD-decorated 1L-MoS2 and 1L-WSe2 were obtained and are shown in Figs. 2c and d respectively. The well-dispersed decoration of the QDs on the surfaces of 1L-MoS2 and 1L-WSe2 is evident. Due to the limited resolution of the FESEM images, only the aggregations of QDs with diameters in the range 10–50 nm are visible. Optical absorption spectra of the MoSe2 nanosheets and QDs in NMP are shown in Fig. 3a. The characteristic A and B exciton peaks of MoSe2 nanosheets are evident at 802 nm and 695 nm respectively, as in previous studies.25–28 (See the UV/Vis spectra and TEM images of the nanosheet prepared with different sonication times details in Supporting Information (SI) Fig. S1)


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In contrast, the A and B peaks in the absorption spectrum of the synthesized QDs completely disappeared, as shown in the inset in Fig. 3a (red curve) and instead an absorption band around at 350 nm was observed. Such observation of blue-shifted excitonic features in MoSe2 QDs is consistent with previous studies of MoS2 QDs which displayed the higher exciton energy with the smaller size of the QD, originating from the quantum confinement effect.19,29 We also confirmed the PL emission of these MoSe2 QDs with fluorescence imaging after dispersing them on cleaned cover glass. The CCD image in Fig. 3b shows the uniform dispersion of the MoSe2 QDs. Considering the low light-emission of single QDs, the emissive features in the fluorescence image are believed to be due to aggregations of QDs, as are also visible in the FESEM images in Fig. 2. We measured the PL spectra of the MoSe2 QDs dispersed in NMP at varying excitation wavelength from 300 nm to 520 nm. We found that PL peak position monotonically increased from 470 nm to 570 nm with the increasing excitation wavelength from 300 nm to 520 nm (See SI Fig.S2). The red-shift of PL peak position with the increasing wavelength of excitation light is consistent with the previously results of MoS2 or WS2 QDs.19 Figure 4 shows the results of the confocal PL and Raman measurements for 1L-MoS2 decorated with MoSe2 QDs. For the MoSe2 QDs/MoS2 heterostructure, the PL of the QDs is strongly quenched, as is clear in the integrated PL intensity mapping image in the wavelength range 500–600 nm (the PL emission range of the MoSe2 QDs) in Fig. 4a; the intensity of PL emission from the MoSe2 QDs on the surface of MoS2 is much lower than that from the QDs on the bare substrate (the dotted lines indicate the 1L-MoS2 flakes). A representative PL spectrum of pristine 1L-MoS2 (black curve) is shown in Fig. 4b; the A and B exciton peaks are evident at 656 nm (1.88 eV) and 611 nm (2.02 eV) with a separation of 140 meV, which agree with the characteristic PL features of 1L-MoS2.1,2,4 The MoSe2 QDs produce green PL emission with a



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peak at ~550 nm, as shown in Fig. 4b (blue curve). Note that the peak position of the MoSe2 QDs arises ~250 nm lower than the known PL peaks of 1L-MoSe2.3 Similar PL features with a broader peak and blue-shifted peak position have also been observed for other WS2 and MoS2 QDs synthesized in a similar manner19 and MoS2 QDs fabricated by electron beam lithography,30 which confirms that the observed PL spectra for the MoSe2 QDs originate from the quantum size effect. The observed 250 nm (0.70 eV) blue-shift of MoSe2 QD PL peak is in a reasonable agreement with the predicted increase of the bandgap due to quantum confinement, which is in the range 1.1-0.71 eV using the QD size of 4-5 nm and the formulae suggested by Wei et al.30 We note that the PL intensity of 1L-MoS2 is reduced in the heterostructure, and the PL peak is red-shifted by 14 nm, as shown in the averaged PL spectrum in Fig. 4b (red curve). The PL intensity and peak position mapping images shown in Fig. 4c confirm the reduction in the PL intensity and the red shift in the MoS2 PL peak position for the MoSe2 QDs/MoS2 heterostructure is uniform over the 1L-MoS2 flake; Figures 4c(i) and (ii) show the integrated PL intensity mapping images of MoS2 for the wavelength range 650–690 nm (the PL emission range of 1LMoS2) for the MoSe2 QDs/MoS2 heterostructure and pristine 1L-MoS2 respectively and Figs. 4c(iii) and (iv) show the peak position mapping images of the MoSe2 QDs/MoS2 heterostructure and pristine 1L-MoS2 respectively. This intensity decrease and a red-shift in the peak in the PL spectrum are commonly observed in n-doped MoS2, and indicate that the spectral weight of the negative trion in the PL spectrum has increased due to an increase in the number of excess electrons.4,12,13,23 Indeed, we fitted the MoS2 PL spectra in Fig. 4b with three Lorentzian peaks of A exciton peak (A0), trion peak (A-) and B exciton peak and found that the spectral weight of Aincreased from 63% to 74% while that of A0 decreased from 29% to 14% with hybridization with MoSe2 QDs as shown in Fig. 4d. We also found peak difference between A0 peak and A- peak


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has increased from 13 nm (36 meV) to 18 nm (51 meV). (See the fitted values of peak positions and spectral weights of A0, A- and B peaks of pristine 1L-MoS2 and the MoSe2 QDs/MoS2 heterostructure in SI Table 1.) The peak difference between A0 peak and A- peak corresponds to the trion dissociation energy, the energy required to dissociate one electron from a trion, which suggests with the increasing electron density and the rise of Fermi energy the trion dissociation energy should increase as well.31 Therefore the observed increase of A0 and A- peak difference is well consistent with the n-doping effect. The n-doping effect of the MoSe2 QDs on 1L-MoS2 also manifests in the Raman peak shift in Fig. 4e. Whereas the position of the E2g1 Raman mode is unchanged, the A1g Raman mode of 1L-MoS2 decorated with MoSe2 QDs (the red curve in Fig. 4e) is softened by ~2 cm−1, which confirms the n-doping effect.4,23 The PL quenching of MoSe2 QDs and the observed n-doping of 1L-MoS2 in our heterostructure indicate that electron transfer occurred from the MoSe2 QDs to 1L-MoS2 as illustrated schematically in Fig. 5a. This process of charge transfer can be understood from the band alignment between the MoSe2 QDs and 1L-MoS2 as depicted in Fig 5b. We used the known energy levels of 1L-MoS2 (and 1L-WSe2) from the literature

32,33

and by using ultra-violet

photoemission spectra and X-ray photoemission spectroscopy estimated the valence band maximum (VBM) and the Fermi level of MoSe2 QD to be 2.8 eV and 4.1 eV, respectively. (See the details of measurement results in Fig. S3 in SI). We also assumed that the bandgap of MoSe2 QD is ~2.3 eV based on the PL peak wavelength of MoSe2 QDs. As shown in the band alignment, because of higher-lying Fermi level and conduction band minimum (CBM) of MoSe2 QD than those of 1L-MoS2, electrons of MoSe2 QD can effectively transfer to 1L-MoS2. Such charge transfer and the resultant modification of the PL emission of 1L-MoS2 are only expected to occur at the locations of the QDs on 1L-MoS2; therefore, the degree of the resultant



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modification of the PL emission of the MoSe2 QDs/MoS2 heterostructure can be conveniently adjusted by controlling the density of the dispersion of MoSe2 QDs on 1L-MoS2. This possibility is an advantage of our TMD heterostructure design over vertically-stacked TMD heterostructures. Band alignment shown in Fig. 5b also suggests that electron doping to 1L-WSe2 will occur between MoSe2 QDs and 1L-WSe2. Figure 6 shows the results of the confocal PL spectral measurements for 1L-WSe2 decorated with MoSe2 QDs. The PL spectrum of pristine 1L-WSe2 is shown in Fig. 6a. The sharp PL of A exciton arises at a peak position of 757 nm (1.63 eV), which is consistent with previously reported PL spectra of 1L-WSe2. The Raman spectrum also confirms the monolayer nature of the WSe2 film (see Fig. S4 in the SI).3,11,34,35 Note that as in the case of the MoSe2 QDs/MoS2 heterostructure, the PL of the MoSe2 QDs is strongly quenched when dispersed on 1L-WSe2. The quenching of the PL emission from the MoSe2 QDs is also evident in the averaged PL spectrum in Fig. 6a (a magnified spectrum is shown in the inset for the range 500– 650 nm). The PL intensity of WSe2 is also reduced and blue-shifted by 7 nm, as shown in the averaged PL spectra in Fig. 6a (the red curve is that of the heterostructure and the black curve is that of pristine 1L-WSe2). The PL intensity and peak position mapping images are shown in Figs. 6b and c respectively, and demonstrate the quenching of the emission of the MoSe2 QDs, the reduction in the PL intensity, and the blue-shift of the PL peak of 1L-WSe2 in the heterostructure. The quenching of the PL emission of the MoSe2 QDs is clearly illustrated in the integrated PL intensity mapping image for the wavelength range 500–600 nm in Fig. 6b(i), where the intensity of the emission from the QDs on the surface of 1L-WSe2 is much lower than that of the QDs on the bare substrate which produce significant PL emission (the dotted lines indicate the 1L-WSe2 flake). Figures 6b(ii) and (iii) show the integrated PL intensity mapping


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images of 1L-WSe2 in the wavelength range 730–775 nm (the PL emission range of 1L-WSe2) for the MoSe2 QDs/WSe2 heterostructure and pristine 1L-WSe2 respectively; the reduction in the intensity of the WSe2 PL of the heterostructure is clear. Figures 6c (i) and (ii) show the peak position mapping images for the MoSe2 QDs/WSe2 heterostructure and pristine 1L-WSe2 respectively; there is a clear blue-shift in the WSe2 A exciton peak of the heterostructure. The PL emission peak is blue-shifted by ~7 nm in the MoSe2 QDs/WSe2 heterostructure, which is in contrast to the red-shift in the PL peak of the MoSe2 QDs/MoS2 heterostructure. CVD-grown 1L-WSe2 is known to be intrinsically p-type 34, 35 so the trion emission in the PL spectrum of pristine 1L-WSe2 should be interpreted as emission due to positive trions. The observed blue shift in the PL peak of 1L-WSe2 is thus the result of a reduction in the trion spectral weight. By peak fitting process with WSe2 PL curves in Fig. 6a we found that the spectral weight of the exciton peak (A0) increased from 39% to 58% and the trion peak (A+) decreased from 61% to 42% as shown in Fig. 6d. This result suggests again that electron doping by MoSe2 QDs has occurred and has depleted the positive trions of 1L-WSe2. We also note that peak difference between A0 and A+ was reduced by 5 nm with hybridization with MoSe2 QDs. (See the fitted values of peak positions and spectral weights of A0 and A- peaks of pristine 1LWSe2 and the QDs/WSe2 heterostructure in SI Table 1.) In a similar manner as discussed above, this observation, i.e. reduction of the trion (positive in this case) dissociation energy is again consistent with n-doping effect. Here the overall PL intensity of 1L-WSe2 was reduced with hybridization with MoSe2 QDs, which is rather contradict to the previous model that decreasing hole density in p-type 1L-TMDs should lead to increasing PL.36 However, when the p-type 1LTMDs, the doping effect to PL of which were relatively less studied than n-type TMDs, is ndoped, the total PL tends to decrease, similarly to our observation.34 We explain the observed



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discrepancy as following. The PL efficiency of 1L-TMDs is largely affected not only by doping state also by defect formations37 because the structural defects of 1L-TMDs are the major sources of non-radiative recombination of photo-excited electron-hole pairs. In case of n-type 1L-MoS2 of which the quantum yield is very low, the doping state would be the major parameter that determines the PL efficiency of 1L-MoS2, which would be little influenced by additional defect formation possibly introduced by external n-doping processes. On the other side, 1L-WSe2 is known to have intrinsically high quantum yield and therefore the formation of defects caused by n-doping process could have the stronger effect to PL intensity than does the decrease of the hole density to the PL intensity of 1L-WSe2.

█ CONCLUSIONS We have synthesized all-TMD heterostructures consisting of MoSe2 QDs and a monolayer of MoS2 or WSe2, and observed the charge transfer occurring between the MoSe2 QDs and 1L-MoS2 and 1L-WSe2. The local electron doping effect due to the MoSe2 QDs and the resultant modifications of the optical spectra of intrinsically n-type 1L-MoS2 and p-type 1LWSe2 were unambiguously visualized with nanoscale PL and Raman spectral imaging. Our results suggest that these TMD heterostructures are convenient platforms for the study of the charge transfer phenomena of TMD quantum objects and for the promising modulation of the optical properties of 1L-TMDs for optoelectronic applications.

█ AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected]. Notes The authors declare no competing financial interests

█ ACKNOWLEDGEMENTS This study was supported by IBS-R011-D1.

Supporting Information Available: UV/Visible spectra and TEM images of MoSe2 nanosheets (S1); PL spectra of MoSe2 QDs in NMP under excitation range 300 nm to 520 nm (S2); UPS and XPS spectra of MoSe2 QDs (S3); Raman spectrum of pristine WSe2 monolayer films (S4); Fit parameters of neutral exciton and trions of pristine 1L-TMDs and MoSe2 QDs/TMDs (Table 1). This information is available free of charge via the Internet at http://pubs.acs.org.



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Fig. 1 Schematic illustration of the synthesis of MoSe2 quantum dots (QDs) and their decoration onto a MoS2 or WSe2 monolayer.


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Fig. 2 (a) HR-TEM image of the synthesized MoSe2 QDs. The inset shows the distribution of the diameters of the MoSe2 QDs. (b) Magnified HR-TEM image of a single MoSe2 QD. The inset is the FFT image. (c, d) FESEM images of (c) 1L-MoS2 and (d) 1L-WSe2 decorated with MoSe2 QDs. The magnified FESEM images show the well-dispersed decoration of the QDs. Scale bars are 50 nm in (a), 2 nm in (b), 10 µm in (c) and (d), and 1 µm in the magnified images in (c) and (d).



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Fig. 3 (a) UV/Visible spectra of the MoSe2 nanosheets and QDs. The inset shows a magnified comparison of the absorption spectra in the range 650–850 nm. Note the quenching of peaks A and B in the absorption spectrum of the MoSe2 QDs. (b) Fluorescence image of the MoSe2 QDs dispersed on the glass substrate. Scale bar is 5 µm.


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Fig. 4 (a) PL intensity map integrated in the wavelength range 500–600 nm (the wavelength range of PL for the MoSe2 QDs). (b) Representative PL spectra of pristine 1L-MoS2, MoSe2 QDs, and MoSe2 QDs/MoS2 heterostructure. For the MoSe2 QDs/MoS2 heterostructure, the PL of the MoSe2 QDs is completely quenched and the PL intensity of the 1L-MoS2 is reduced and its peak red-shifted. (c) PL intensity maps integrated in the wavelength range 650–690 nm (the wavelength range of MoS2 PL) of the (i) MoSe2 QDs/MoS2 heterostructure and (ii) pristine 1LMoS2; PL peak position maps of (iii) the MoSe2 QDs/MoS2 heterostructure and (iv) pristine 1LMoS2. (d) Deconvolutions of PL spectra of pristine 1L-MoS2 and MoSe2 QDs/MoS2 heterostructure with corresponding spectral weights of B, A0 and A- respectively, which depicts the increase in spectral weight of A- in heterostructure due to electron transfer from MoSe2 QDs to 1L-MoS2. (e) Raman spectra of pristine 1L-MoS2 and the MoSe2 QDs/MoS2 heterostructure, which show the softening of the A1g Raman band that indicates the n-doping of the MoSe2



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QDs/MoS2 heterostructure. The positions of the 1L-MoS2 flakes are indicated by dotted lines and all scale bars are 5 µm.


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Fig. 5 (a) Schematic illustration of the charge transfer between a MoSe2 QD and 1L-MoS2 in the heterostructure. Due to the depletion of electrons in the MoSe2 QD, the PL of the QD is quenched and 1L-MoS2 becomes n-doped. (b) Band diagram of 1L-MoS2, MoSe2 QD and 1LWSe2, shows the Fermi level of MoSe2 QD lying above the Fermi levels of both 1L-TMDs and depicts the possibilities of charge transfer mechanism from MoSe2 QD to both 1L-TMDs.



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Fig. 6 (a) Representative PL spectra of pristine 1L-WSe2, MoSe2 QDs, and the MoSe2 QDs/WSe2 heterostructure. For the MoSe2 QDs/WSe2 heterostructure, the PL of the MoSe2 QDs is completely quenched (see the magnified inset), and the PL intensity of WSe2 is reduced and the peak blue-shifted. (b) PL intensity maps integrated in the wavelength ranges (i) 500–600 nm (the PL emission range of MoSe2 QDs), (ii) 730–775 nm (the wavelength range of WSe2 PL) for the MoSe2 QDs/WSe2 heterostructure, and (iii) 730–775 nm for pristine 1L-WSe2. (c) PL peak position maps for (i) the MoSe2 QDs/WSe2 heterostructure and (ii) pristine 1L-WSe2, which show the clear blue shift of the PL peak of the heterostructure. The 1L-WSe2 flakes are indicated by dotted lines and all scale bars are 5 µm. (d) Deconvolution of PL spectra of pristine 1L-WSe2 and Mose2 QDs/1L-WSe2 with A0 and A+ peaks, which displays the decrease in spectral weight of A+ peak in heterostructure.


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TOC


Observation of Charge Transfer in Heterostructures ... - ACS Publications

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