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Research Paper

Applied Science and Convergence Technology 2023; 32(6): 165-167

Published online November 30, 2023


Copyright © The Korean Vacuum Society.

Tuning Coupling Behavior of WS2/WSe2/WS2 Trilayer Van Der Waals Heterostructures

Hyojung Kima , † , Bora Kimb , † , and Hye Min Ohc , *

aThe Institute of Basic Science, Kunsan National University, Gunsan 54150, Republic of Korea
bDepartment of Physics, Hanyang University, Seoul 04763, Republic of Korea
cDepartment of Physics, Kunsan National University, Gunsan 54150, Republic of Korea

Correspondence to:ohmin@kunsan.ac.kr

†These authors equally to this work.

Received: November 10, 2023; Revised: November 20, 2023; Accepted: November 21, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc-nd/4.0/) which permits non-commercial use, distribution and reproduction in any medium without alteration, provided that the original work is properly cited.

Van der Waals (vdW) heterostructures, composed of stacked transition metal dichalcogenides (TMDCs), are promising components for the advancement of state-of-the-art optoelectronic devices. Therefore, understanding and controlling interactions within these layered structures are essential. In this study, we fabricated a trilayer configuration consisting of stacked WS2/WSe2/WS2 and investigated the impact of annealing on the interlayer coupling. This structure, featuring a type-II band alignment, enables charge transfer between TMDC layers. The modulation of the WS2/WSe2/WS2 trilayer structure was confirmed using photoluminescence (PL) and Raman spectroscopy. Abnormal PL enhancement and quenching, as well as notable Raman peak shifts, demonstrate the essential role of annealing in controlling the carrier dynamics in vdW heterostructures.

Keywords: Van der Waals heterostructures, Transition metal dichalcogenides, Interlayer coupling, Photoluminescence, Raman spectroscopy

Traditional bulk Si or III-V semiconductors exhibit superior electronic and optoelectronic characteristics but have limited flexibility and transparency [1]. Consequently, the use of these materials in flexible displays and transparent solar cells is challenging. In contrast, two-dimensional (2D) transition metal dichalcogenides (TMDCs) exhibit outstanding optoelectronic properties while maintaining favorable flexibility and transparency [2]. A TMDC consists of an MX2 structure, where ‘M’ represents a transition metal (e.g., Mo, W, and Re) and ‘X’ denotes a chalcogenide element (S, Se, and Te) [3]. TMDCs have band gaps ranging from 1 to 2 eV, making them suitable for various applications, such as field-effect transistors, photodetectors, and solar cells [46]; thus, they are expected to play a crucial role in the future of optoelectronic devices as emerging semiconductors [7]. Furthermore, their intrinsic weak van der Waals (vdW) interlayer interactions allow for facile stacking of 2D TMDC layers. Recent research has focused on enhancing the optical characteristics or implementing new devices through strategic stacking of TMDC heterostructures such as MoS2/WSe2, MoS2/WS2, WSe2/MoSe2/WSe2, and WSe2/WS2/WSe2 [811]. These heterostructures have active band gaps with bound electrons and holes localized within individual monolayers (MLs). Notably, TMDC heterostructures can form p-n junctions based on their constituent materials, thereby facilitating type-I, type-II, and type-III band alignments [12]. The distinctive properties of these junctions render them exceptionally promising for optoelectronic device applications. Additionally, their energy and luminescence intensity can be finely tuned by adjusting the vertical gate voltage, laser intensity, and thermal treatment [11,13,14].

In this study, we investigated the effect of annealing on the interlayer coupling in a WS2/WSe2/WS2 vdW heterostructure. The WS2/ WSe2/WS2 trilayer was fabricated using a conventional wet transfer process, and it exhibited a type-II band alignment, resulting in the spatial separation and charge transfer of photogenerated electron-hole pairs between the TMDC layers. The interlayer interactions within the WS2/WSe2/WS2 vdW heterostructure were probed using photoluminescence (PL) and Raman spectroscopy. Interestingly, we observed an abnormal PL enhancement in the stacked WS2/WSe2/WS2 layers before annealing, which significantly decreased after annealing. Furthermore, various Raman peak shifts, including those of E2g1+A1g and A1g, provided clear evidence of interlayer coupling between WS2 and WSe2 after annealing. These findings emphasize the critical role of annealing in precisely controlling the interlayer carrier dynamics within vdW heterostructures.

ML WSe2 and WS2 films were synthesized through chemical vapor deposition (CVD), and the stacked WS2/WSe2/WS2 was fabricated on a SiO2/Si substrate using a wet transfer method. An additional annealing process was performed to control the vdW interactions between the TMDC layers. The furnace was purged with 1,000 sccm Ar for 15 min, and the as-prepared WS2/WSe2/WS2 sample was annealed at 300 °C for 3 h while being exposed to 200 sccm Ar and 50 sccm H2 to eliminate polymethyl methacrylate (PMMA) residue and enhance the vdW interaction between the layers. Confocal microscopy systems [Nanobase (NANOBASE) and NT-MDT (NTEGRA SPECTRA)] were used to acquire Raman and PL spectra at the nanoscale. A 532 nm excitation laser (2.33 eV) with a laser power of 0.3 mW was used for excitation, and 150- and 1,800-groove gratings were used for PL and Raman analyses, respectively.

First, we confirmed the optical properties of the CVD-grown ML WSe2 and ML WS2 using PL spectroscopy. Figure 1(a) shows the PL spectrum and optical image of ML WS2, with the main peak of ML WS2 appearing at 616 nm (2.01 eV), corresponding to neutral A exciton states [15,16]. Figure 1(b) presents the PL spectrum of ML WSe2, where the A exciton peak appears at approximately 745 nm (1.66 eV) [17]. The sizes of the triangular WS2 and irregular shaped WSe2 flakes are approximately 60 and 100 µm, respectively. We fabricated the WS2/WSe2/WS2 structure using these TMDCs, following the wet transfer process illustrated in Fig. 2(a). To extend the etching area, we attached tape to all edges of the as-grown TMDC sample and spin-coated PMMA C4 at 3,000 rpm for 1 min. For SiO2 etching, we detached the tape and floated the sample on a 1 M potassium hydroxide (KOH) solution heated at 80 °C. After 1 min, we removed the PMMA/TMDC film from the KOH solution using a polyester film and rinsed it several times in deionized (DI) water. We then removed the film from the DI water using a cleaned SiO2/Si substrate and dried it by blowing N2 gas. Finally, we removed the PMMA by rinsing the samples with acetone for 30 s. This sequence was repeated three times to fabricate the WS2/WSe2/WS2 structure. Optical images of each step are shown in Fig. 2(b).

Figure 1. PL spectra of (a) ML WS2 and (b) ML WSe2 (inset: optical images of each TMDC).

Figure 2. (a) Schematic illustration of wet transfer process. (b) Optical images of WS2, WSe2/WS2, and WS2/WSe2/WS2 samples following transfer repetition from the left.

Figure 3(a) shows the structure and band alignment of the WS2/W Se2/WS2 heterostructure. An optical image of the WS2/WSe2/WS2 trilayer is shown in Fig. 3(b). The triangular samples marked with white and black dotted lines correspond to the upper and lower WS2 flakes, respectively. The yellow dotted line represents the WSe2 layer in the middle of the WS2/WSe2/WS2 structure. Figure 4 presents the PL spectra obtained from ML WS2, ML WSe2, WS2/WSe2, and stacked WS2/WSe2/WS2 before and after annealing. As shown in Fig. 4(a), we observed significant PL quenching in the stacked WS2/WSe2 region compared to the case of the ML WS2. This is attributed to the charge transfer processes associated with the formation of a type-II band alignment in WS2/WSe2, as depicted in Fig. 3(a) [18,19]. However, the PL intensity of the stacked WS2/WSe2/WS2 region increased, even compared to that of ML WS2. The strong PL peak position of the stacked WS2/WSe2/WS2 was consistent with that of ML WS2; however, an additional shoulder peak was clearly observed at 630 nm, whereas the PL peak associated with WSe2 was significantly attenuated. Interestingly, after annealing, a substantial decrease in PL was observed for the stacked WS2/WSe2/WS2 as shown in Fig. 4(b). The abnormal increase in PL before annealing is attributed to the decoupling between WS2 and WSe2 as well as the transfer of electrons from the WSe2 middle layer to each WS2 layer. As a result, the WS2 trion peak at 630 nm also significantly increased while maintaining the WS2 neutral A exciton peak [20]. After the annealing process, the interlayer coupling between WS2 and WSe2 was strengthened, leading to charge separation from WS2 to WSe2, which eventually resulted in PL suppression [19,21,22].

Figure 3. (a) Schematic representation of the WS2/WSe2/WS2 structure and its band alignment. (b) Optical image of the WS2/WSe2/WS2 sample.

Figure 4. PL spectra of ML WS2, ML WSe2, WS2/WSe2, and WS2/WSe2/WS2 (a) before and (b) after annealing.

Figure 5 shows the Raman spectra of ML WSe2, ML WS2, and the stacked WS2/WSe2/WS2, both before and after annealing. The Si Raman peak at 520.1 cm−1 was used for equipment calibration. In the case of ML WSe2, two primary peaks were observed at 248.3 and 260.7 cm−1, corresponding to the E2g1+A1g and 2LA(M) modes, respectively [23]. For ML WS2, the prominent Raman peak was located at 354.6 cm−1, representing the E2g1 mode [24,25]. The Raman peaks of the stacked WS2/WSe2/WS2 incorporated all the individual peaks of WSe2 and WS2, as shown in the top graph in Fig. 5. This again suggests that the WS2/WSe2/WS2 vdW heterostructure was well constructed [11].

Figure 5. Raman spectra of ML WSe2, ML WS2, and WS2/WSe2/WS2 before (black lines) and after (red lines) annealing, respectively.

For a more detailed analysis, we focused on the portions of the graph ranging from 220−280 and 260−440 cm−1, which display the dominant peaks of WSe2 and WS2, respectively. Upon stacking the WS2/WSe2/WS2 layers, we observed a decrease in the Raman intensity of WSe2, accompanied by an increase in the WS2 peak. The stacking order and number of layers may influence the initial Raman intensity, and the overall intensity was found to increase after annealing owing to the enhancement of interlayer coupling. Interestingly, an additional peak was clearly observed at 415.9 cm−1, representing the A1g mode of WS2. Furthermore, the E2g1+A1g mode of WSe2 exhibited a slight blue shift after annealing. Generally, the A1g mode is highly affected by the carrier doping effect induced by charge transfer [22,26]; therefore, the changes in Raman peaks can be interpreted as being results of the movement of photogenerated carriers in the WS2/WSe2/WS2 heterostructure. As depicted in Fig. 3(a), electrons moved from WSe2 to WS2 and holes flowed in the opposite direction, from WS2 to WSe2, which is consistent with the PL results.

We further compared the Raman changes for an iso-bilayer (WS2/ WS2) and a hetero-bilayer (WS2/WSe2 and WSe2/WS2), as shown in Fig. 6. Generally, the frequency difference between the E2g1 and A1g modes increased with the number of layers, and was accompanied by an E2g1 peak increase. The WS2/WS2 iso-bilayer exhibited a shift in the E2g1 mode from 354.6 to 352.5 cm−1 and in the A1g mode from 415.9 to 417.9 cm−1, without a WSe2 peak. For the hetero-bilayer, instead of an increase in E2g1, a similar blue-shifted E2g1+A1g mode and an A1g mode were observed after annealing; these can be attributed to the charge transfer process. However, the changes in the Raman spectra of the hetero-bilayer were comparatively weaker than those of the WS2/WSe2/WS2 heterostructure owing to the absence of a top layer.

Figure 6. Raman spectra of WS2/WS2, WS2/WSe2, and WSe2/WS2 before (black lines) and after (red lines) annealing, respectively.

We successfully fabricated a trilayer configuration of stacked WS2/ WSe2/WS2 using the wet transfer method and explored the effects of annealing on interlayer coupling. The WS2/WSe2/WS2 heterostructure was characterized using PL and Raman spectroscopy. Significant PL enhancement and Raman mode changes were observed, indicating photogenerated carrier movement between the TMDC layers, facilitated by the type-II band alignment. Strengthening the interlayer coupling through thermal treatment resulted in PL quenching, underscoring the critical role of the annealing process in controlling carrier dynamics. Our findings highlight the potential for tuning carrier movement in vdW heterostructures through a straightforward annealing process, opening up various avenues for future applications.

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (Nos. 2022R1A4A1033358 and RS-2023-00214318).

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