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Abstract
Lateral heterojunction (HJ) of two-dimensional transition metal dichalcogenides has various optoelectronic applications that utilize in-plane charge separation. However, it has been difficult to identify charge transfer characteristics at HJ due to the limited spatial resolution of optical spectroscopy. In this study, near-field scanning optical microscopy is used to directly image the exciton separation occurring at the lateral MoSe2/WSe2 HJ, which was found to be ∼370 nm in spatial width. Efficient charge separation at HJ was confirmed by inspecting local variations of trion and exciton emissions of MoSe2 and WSe2.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Monolayer transition metal dichalcogenides (1L-TMDs) are direct bandgap, two-dimensional (2D) semiconductors in which strong Coulomb interactions of 2D-TMDs result in extremely stable formations of bound excitons at room temperature [1–4]. Therefore, optical properties of 1L-TMDs, including strong photoluminescence (PL), originate mainly from excitons and exciton species such as trions (charged excitons), and biexcitons [5–7]. The spectral weights of exciton species and optical emissions of 1L-TMDs can be controlled by adjusting the charge density through electric gating, chemical doping, and defect treatments [3,8–15]. Controllability of the optical properties of 1L-TMDs is expanded by stacking various kinds of 1L-TMDs to form a vertical heterostructure (HS) utilizing weak van der Waal’s interaction between TMDs. Such HSs often form a staggered (type-II) band alignment that facilitates efficient interlayer charge transfer, providing interesting physical properties [16–19]. For example, experiments have shown ultrafast charge transfer in MoS2/WS2 HS [20], charge separation on MoS2/MoSe2 HS [21], long-lived indirect excitons in MoSe2/WSe2 HS [22], and PL quenching on MoS2/WSe2 HS [23]. Recently, lateral heterojunction (HJ) between different kinds of 1L-TMDs have been fabricated where two domains of different TMDs form a lateral HJ with atomically sharp interfaces [24]. Nowadays, in-plane TMDs HJ in various combinations of TMDs of large areas and controllable domain sizes are being fabricated [25–29]. Optical and structural properties of these lateral HJ have been extensively studied; however, direct investigation of exciton behaviors such as exciton diffusion and charge separation across the HJ have not been performed.
Near field scanning optical microscopy (NSOM) and spectroscopy can provide a direct view of exciton emission profiles of TMDs due to its nanoscale spatial resolution. For example, structural defects such as grain boundaries and nanosize defects of 1L-MoS2 were optically visualized [30,31], localized defect-bounded excitons of 1L-WS2 were identified, and spatial profiles of exciton species were found to be closely correlated with local defects and charge accumulation [32]. Near-field (NF) imaging of 1L-MoS2 revealed a defect healing effect by chemical treatment of 1L-MoS2 [33]. Also, tip-enhanced PL spectroscopy [34,35] was used for nanoscale imaging of lateral TMD HJs [36,37]. These previous results of nano-optical imaging and spectroscopy on 1L-TMDs and TMD HJ strongly suggest that near-field investigations could provide direct information on exciton behaviors at lateral HJ.
Here, we performed NF PL imaging and spectroscopy to visualize spatial profiles of the exciton distribution at lateral WSe2/MoSe2 HJ. PL quenching was observed at HJ, which we attributed to the exciton charge separation. Spectral analysis of lateral variation of neutral excitons and trions emission confirmed the charge separation process occurring at HJ.
2. Experimental details
2.1 Sample synthesis
Monolayer WSe2/MoSe2 lateral HJ was grown on a SiO2/Si wafer using an atmospheric chemical vapor deposition (CVD) process. To synthesize WSe2/MoSe2 HJ, liquid metal precursors of Mo and W mixed to a certain ratio and used. The precursor solution was then spin casted onto a SiO2/Si substrate. The precursor solution was prepared by mixing four types of water-based solutions defined as A, B, C, and D. A (Tungsten precursor) was prepared by dissolving 0.1 g of Ammonium metatungstate hydrate [(NH4)6H2W12O40·×H2O: Sigma-Aldrich, 463922] in 10 ml of DI water. B (Molybdenum precursor) was prepared by dissolving 0.1 g of Ammonium molybdate tetrahydrate [(NH4)6Mo7O24·4H2O: Sigma-Aldrich, 431346] in 10 ml of DI water. C (promoter) was prepared by dissolving 0.1 gram of sodium hydroxide (NaOH: Sigma-Aldrich, 795429) in 30 ml of DI water for enhancing the portion of monolayer. D (Medium solution) OptiPrep density gradient medium (Sigma-Aldrich, D1556, 60% (w/v) solution of iodixanol in water) was used as a medium to mix promoter and metal precursors. A, B, C, and D solutions were mixed in the ratio of 3: 1: 9:3. The mixed solution was spin-coated at 3000 r.p.m. for 1 min onto a SiO2/Si wafer. A two-zone furnace CVD system was introduced for individually controlling the temperature of selenium and the substrate zone [38]. While the solution coated substrate containing metal precursors was placed in substrate zone, 0.2 g of selenium (Sigma, 209643) was loaded in selenium zone. The temperature of the selenium zone was increased to 400 oC at a rate of 50 oC/min while the substrate zone was increased to 780 oC at a rate of 100 oC/min. At the same time, 600 sccm of Nitrogen and 10 sccm of Hydrogen gas were injected as a carrier gas and reactive agent, respectively, to reduce metal oxides.
2.2 Transfer of the sample
The WSe2/MoSe2 lateral HJ grown on a SiO2/Si substrate was transferred onto a glass substrate [17,32,33,39]. Polymethylmethacrylate (PMMA) was spin-coated onto the samples, and the PMMA/ WSe2-MoSe2 HJ film was separated from the substrate by etching away SiO2 in a 1 mol/L KOH solution. PMMA/ WSe2-MoSe2 HJ films were rinsed with distilled water to reduce the amount of KOH residue and then transferred onto glass substrates. Acetone was used to remove PMMA.
2.3 Optical measurement
NSOM PL images and spectra were obtained using a manufactured instrument (Alpha-300S, WITec GmbH) in transmission mode [31–33]. A frequency-doubled ND-YAG (532 nm) laser light was focused by a 20×, 0.4 numerical aperture (NA) objective lens (OL) to the back opening of the cantilever-type NF probe that had an apex aperture size of ∼90 nm. The transmitted PL signal was collected by an OL (60×, 0.8 NA) located below the sample and focused on an optical fiber with a 150 µm core diameter, which was guided to a 30-cm-long spectrometer equipped with cooled CCD for spectral imaging or to photomultiplier tubes (PMT) for PL imaging. Confocal PL and Raman images and spectra were obtained by using 100×, 0.9 NA OL in reflection mode. For confocal PL and NF PL imaging, all the emission above 536 nm wavelength were counted by using the long pass filter placed in front of the PMT or spectrometer. All optical measurements were performed at room temperature.
3. Results and discussion
Figure 1(a) shows the optical image of WSe2/MoSe2 lateral HJ of outer (WSe2) and inner (MoSe2) hexagonal domains. Here, the exact boundary between two crystal domains is not clearly distinguished in the optical image (the approximate boundary is indicated by a white arrow in Fig. 1(a)). To identify the location of lateral HJ, we performed confocal PL and Raman spectroscopy. Figure 1(b) displays the representative Raman spectra obtained from the outer (WSe2) region and inner (MoSe2) region, and out of plane Raman modes (A1g) of WSe2 and MoSe2 are observed at 250 cm−1 and 242 cm−1, respectively [40–42]. Raman images obtained by integrating A1g peaks of WSe2 and MoSe2 show a clear boundary between the two crystal domains (insets in Fig. 1(b)). Dark radial lines in Fig. 1(a) and insets of Fig. 1(b), where no Raman signals are found, are believed to be cracks.
Fig. 1. (a) Optical image of MoSe2/WSe2 lateral HJ made between outer (WSe2) and inner (MoSe2) crystal domains. Blue dashed line indicates domain boundary. (b) Raman spectra of A1g modes of WSe2 (upper) and MoSe2 (lower), respectively. Insets: Raman mapping image of A1g modes of WSe2 (upper) and MoSe2 (lower), respectively. The contrast of MoSe2 Raman signal is not completely dark in the WSe2 region because of spectral overlap between the Raman A1g modes of MoSe2 and WSe2, and relatively higher Raman intensity of MoSe2. (c) PL spectra obtained from P1 through P4 positions shown in Fig. 1(a). Vertical lines are visual guides.
Figure 1(c) displays the representative PL spectra obtained from P1 through P4 points indicated in Fig. 1(a). The PL spectra obtained at P1 and P4 showed typical exciton transitions of monolayer MoSe2 at 1.58 eV [42,43] and WSe2 at 1.66 eV [44,45]. P2 and P3, near the domain boundary, displayed exciton emissions of both MoSe2 and WSe2. We believe that our HJ is the atomically sharp interface [27–29]; however, finite spatial resolutions of our PL measurements near HJ allowed the excitation of both regions of MoSe2 and WSe2. Some reports observed a single broad PL emission of intermediate energy at the HJ region, which was explained as the formation of intralayer HS exciton [25] or localized defect trapping exciton [26]. However, this is likely not the case here because the observed PL peak positions of the WSe2 or MoSe2 regions were the same at the center of the each domain or near the HJ.
We performed NF imaging on the region that includes the lateral MoSe2/WSe2 HJ to obtain the nanoscale profile of spectral variation across the HJ. To estimate the spatial resolution of our NF PL imaging, we show the confocal PL image (Fig. 2(a)) and NF image (Fig. 2(b)) of the same area of monolayer WSe2 and a line profile across the large radial crack (Fig. 2(c)). Line profile across the abrupt edge of monolayer TMD can be used to estimate the spatial resolution of PL imaging, by measuring the distance between the points of the line profile corresponding to 12% and 88% of the intensity [31]. The spatial resolution of NF imaging and confocal imaging was estimated to be 140 nm and 600 nm, respectively, confirming the superior spatial resolution of NF imaging.
Fig. 2. Estimation of spatial resolution of the near-field (NF) PL and confocal PL imaging. (a) Confocal and (b) NF PL image of monolayer WSe2. (c) Line profiles of the same region obtained across the crack as indicated in (a, b) by dotted lines. The spatial resolutions of the NF PL and confocal PL images were estimated to be 140 nm and 600 nm, respectively.
Figure 3(a) shows a NF PL image of lateral MoSe2/WSe2 HJ obtained by NSOM. The PL intensity was found to be higher at the MoSe2 region (inner) than the WSe2 region (outer), suggesting a higher quantum yield of the MoSe2 domain than the WSe2 domain [15]. It is also noted that the PL intensity of the WSe2 region is somewhat higher at the edges and near HJ than the domain middle, which was also noted in previous observations and attributed to composition mixing or local strain development [27,29]. Interestingly, we found that PL is particularly low along the HJ (indicated by an arrow). This type of PL quenching at lateral HJ is observed for the first time in this study. We attribute observed PL quenching at HJ to the charge separation occurring at MoSe2/WSe2 HJ, as this process is schematically described in Fig. 3(b). Considering that the lateral HJ of MoSe2/WSe2 forms type-II band alignment where the conduction band minimum (CBM) and the valence band maximum (VBM) of MoSe2 are lower than those of WSe2 [16,19,26,28,29], electrons (holes) of WSe2 (MoSe2) tend to move to MoSe2 (WSe2); i.e., excitons may be dissociated into spatially separated electrons and holes across the HJ. This process will result in a PL quenching region at HJ due to the depleted density of excitons. Line profile of NF PL images across the HJ region (inset in Fig. 3(a)) displays that this depletion region is about 370 nm in spatial width, and this value is similar to that of previous observations in a scanning kelvin probe microscopy study that showed charge depletion width of lateral WSe2/MoS2 HJ was ∼320 nm [27]. Observed width of PL quenching of ∼370 nm is much smaller than the spatial resolution of our confocal PL imaging but is quite larger than that of NF imaging, indicating that superior NF imaging enabled the observation of PL depletion along the lateral TMD HJ.
Fig. 3. (a) NF PL image of MoSe2/WSe2 lateral heterojunction (HJ) (indicated by yellow arrow) composed of WSe2 (outer) and MoSe2 (inner) monolayer domains. Inset: line profile obtained across the HJ (black dash line). (b) Schematic of the sample structure and band alignment of MoSe2 and WSe2. CBM: conduction band minimum. VBM: valence band maximum.
We selected a 5 µm × 5 µm region with 100 × 100 pixels of another lateral WSe2/MoSe2 HJ and performed NF PL spectral imaging and the PL image is shown in Fig. 4(a). Across the domain boundary or HJ (indicated by blue dash line), left side is the MoSe2, and the right side is the WSe2 region. Again, NF spectral PL image exhibited PL quenching at HJ.
Fig. 4. (a) NF PL image of MoSe2/WSe2 lateral HJ. (b) Representative seven PL spectra obtained from white dashed line in Fig. 3(a), red: fitting curve, pink: A− of MoSe2, magenta: A0 of MoSe2, green: A− of WSe2, blue: A0 of WSe2. (c) A plot of the A−/A0 of WSe2 (upper) and MoSe2 (lower) calculated from the result shown in Fig. 3(b). The inset displays the band alignment of MoSe2/WSe2 lateral HJ.
Here, we selected a line from the MoSe2 region to the WSe2 region including the depletion region (white dash line in Fig. 4(a)) and fitted spectra obtained at seven points (P1 through P7) along the line using four peaks of the neutral exciton peaks (A0) and charged exciton (trion) peaks (A−) of MoSe2 and WSe2 based on the known peak positions of A0 and A− of MoSe2 and WSe2 as shown in Fig. 4(b) [43,45]. While it was possible to fit PL spectra obtained from P1 and P7 with two peaks of A0 and A− of either one of MoSe2 or WSe2, the intermediate points near the HJ (P2 through P5) required A0 and A− peaks of both MoSe2 and WSe2. First, we note that the overall PL intensity is low at the HJ as both exciton emissions of MoSe2 and WSe2 gradually decrease toward the center of HJ (P4), consistent with the observation shown in Fig. 3 wherein the exciton depletion region is present at the lateral HJ.
In more detail, as the PL spectra and fitting results are shown at Fig. 4(b), as going from P1 (MoSe2 region) to P7 (WSe2 region), A0 intensity of MoSe2 PL drops more rapidly than A− intensity. In contradict, if one goes from P7 (WSe2 region) to P1 (MoSe2 region), A− intensity of WSe2 drops more rapidly than A0 intensity. This peculiar behaviors of local intensities of excitons and trions of MoSe2 and WSe2 around the lateral HJ are quantitatively manifested in plots of A−/A0 vs. lateral distance across the HJ in Fig. 4(c). The A−/A0 of MoSe2 PL is 0.43 in the MoSe2 region (P1) and gradually increases as approaching HJ and finally reaches 1.33. Because A−/A0 is proportional to the local electron density of n-type 1L-TMDs such as MoS2 and MoSe2, the gradual increase of A−/A0 indicates that MoSe2 tends to be n-doped near the HJ. The increase of electron density of MoSe2 at HJ is consistent with the charge separation process suggested in Fig. 3 where electrons from WSe2 side would transfer to the MoSe2 side due to the particular band alignment between MoSe2 and WSe2 (Scheme is shown in the inset of Fig. 4(c)). The gradual decrease of A−/A0 of WSe2 can be similarly explained where the WSe2 tends to lose electrons at HJ due to charge transfer.
4. Conclusion
We performed NF PL imaging and spectroscopy to study exciton behaviors at lateral HJ MoSe2/WSe2. We observed specific PL quenching with a spatial width of ∼370 nm at HJ originating from the exciton charge separation. Local variation of A−/A0 ratio of MoSe2 and WSe2 confirmed the charge separation due to the band alignment of MoSe2 and WSe2. Our results directly revealed the exciton separation unique to the lateral HJ of TMDs and could be used to design optoelectronic devices using lateral TMD HJs.
Funding
Sungkyunkwan University (SKKU) (Samsung Research Fund, 2017).
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