Applied Science and Convergence Technology 2023; 32(6): 165-167
Published online November 30, 2023
Copyright © The Korean Vacuum Society.
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
†These authors equally to this work.
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 . 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 . 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) . 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 [4–6]; thus, they are expected to play a crucial role in the future of optoelectronic devices as emerging semiconductors . 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 [8–11]. 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 . 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
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) . 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 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 . 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 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
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
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
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).
The authors declare no conflicts of interest.