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Band alignment engineering of bilayer WS2/Ga2O3 heterostructures with interface-dependent photoluminescence  PDF

  • YANG Wan-Li 1,4
  • HUANG Tian-Tian 1
  • ZHANG Le-Peng 3
  • XU Pei-Ran 1
  • JIANG Cong 1
  • LI Tian-Xin 1
  • CHEN Zhi-Min 3
  • CHEN Xin 1,2,4
  • DAI Ning 1,2,4
1. State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China; 2. Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, Hangzhou 310024, China; 3. College of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450052, China; 4. University of Chinese Academy of Sciences, Beijing 100049, China

CLC: O472+.3

Updated:2023-04-19

DOI:10.11972/j.issn.1001-9014.2023.02.004

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Abstract

The hetero-interface induced anomalous photoluminescence (PL) emissions in the vertical WS2/Ga2O3 heterostructures was demonstrated. The WS2/Ga2O3 hetero-interface varies type-II band structure and brings subsequent PL decline in the bottom WS2 monolayer contacted with Ga2O3 layer. Such hetero-interlayer coupling interaction between oxides and 2D layered transition metal dichalcogenides (TMDs) in the stacked heterostructures impacts interlayer interaction between the bottom WS2 monolayer and the upper WS2 monolayer in a WS2 bilayer, which leads to an anomalous PL enhancement in the bilayer WS2. Stacked hetero-interface will benefit for controlling the optical or electronic behavior and modulating energy band structures by customizing transformative 2D heterostructures used in next-generation nanoscale optoelectronic detectors and photodetectors.

Introduction

Stacked van der Waals heterostructures have extended versatile electrical, optical and chemical properties of individual 2D materials, and recently drawn broad attentions in optoelectronic detection and photodetection fields

1-9. The interfaces and interlayer interactions have shown significant impact on the energy band structure, charge transfer and density distribution, and defect formation in 2D heterostructures3-410-14. While the underlying physical mechanism still needs be further explored, 2D vertical heterostructures and interfacial engineering have become a promising platform to artificially design and manipulate desired atomic layered heterostructures and photodetectors. Notably, 2D TMDs (e.g., MoS2, WS2) can emit pronounced photoluminescence (PL) by exciton recombination and release photons at room temperature1115. Monolayer WS2 possesses a direct band gap and abundant exciton behaviors for high PL quantum yield owing to strong light-matter coupling and thin dielectric screening in an atomic monolayer. However, the non-conservation of electron momentum will lead to a poor PL in a bilayer WS216-17. The PL behaviors in 2D TMDs are determined by the energy band structure and exciton energy related to interlayer interaction, defects or doping. For a 2D interlayer stacking, the defect energy levels and bound excitons will also change energy band structure, transition behaviors of electrons and photons, and the proportion of excitons in TMDs18-20.

In van der Waals heterostructure, the interfacial interaction is ubiquitous and vital to significantly modulate and alter the optical and optoelectronic properties of 2D materials

621. Hetero-interface strategies have provided a great opportunity to design and construct 2D stacked heterojunctions by band alignment engineering for the advanced microelectronic and optoelectronic detection devices922. It still remains challenging great to control complex and versatile interfaces in 2D homostructures and heterostructures. Various interface-engineering methods have been exploited to manipulate 2D heterostructures and their functions. Both CVD (i.e., chemical vapor deposition) growth and mechanical transferring/stacking have been exploited to design and realize 2D van der Waals heterostructures on the desired substrates. Especially, during a CVD process, clean surface and original interface coupling can be feasibly obtained in 2D heterostructures1023-24. Emerging 2D hetero-interfaces between TMDs and traditional semiconductors have sparked intensive interest in 2D heterostructures. Various Ga2O3 materials have been exploited to fabricate deep-ultraviolet photodetectors, functional FETs and high-power devices25-27. Excellent electronic-photonic properties and high temperature-stability of Ga2O3 makes it possible to design and directly fabricate TMDs/oxide heterostructures.

In this work, we demonstrate an anomalous PL in the bilayer WS2 induced by a hetero-interface between WS2 layers and Ga2O3 thin films. In virtue of CVD-grown WS2/Ga2O3 heterostructures on SiO2/Si substrates, we analyzed surface-dependent PL and the role of interfaces. Converse PL was found and anomalous in the region of bilayer-WS2 on the Ga2O3 thin films. The PL intensity in the bilayer WS2i.e., 2L-WS2) region is approximately 10 times stronger than that in the monolayer WS2i.e., 1L-WS2) region. Such anomalous PL behaviors in bilayer WS2 depend on hetero-interface and modified energy band structures in the WS2/Ga2O3 heterostructure. WS2/oxide hetero-interfaces provide an alternative route to understand and manipulate the optical and electronic behaviors of 2D vertical heterostructures and functional detection devices.

1 Materials and methods

The 2D WS2/Ga2O3 vertical heterostructures are directly fabricated by a CVD method. In brief, the Ga2O3 thin films were atomic-layer-deposited on the SiO2/Si substrates as we reported elsewhere

26-27. The layered WS2 was subsequently CVD-grown on the as-prepared Ga2O3 thin film, which may help for a clean hetero-interface in WS2/Ga2O3 vertical heterostructure. Thus, WO3 and S powders were used as precursors during the CVD-growth of 2D WS2 and WS2/Ga2O3 heterostructures grown at ~850 °C. High-resolution Raman/PL maps were obtained with 100 × objective, 1 800/300 G/mm grating, and a scanning step of 300 nm while the intensity of 532 nm laser is less than 1 mW. Raman/PL spectroscopy was performed at room temperature. To fabricate transferred-WS2/Ga2O3 heterostructure, the target WS2 flakes on the SiO2/Si substrate were pasted on a cut polyvinyl alcohol hydrogel sheet, and then transferred onto the Ga2O3 thin film on SiO2/Si substrate according to the processes reported elsewhere. Briefly, KPFM image was obtained on Veeco/DI multimode SPM while optical microscopy was performed on Leica DM4000M28-29.

2 Results and discussions

Figure 1(a) displays a schematic illustration of the 2D WS2/Ga2O3 vertical heterostructure on SiO2/Si substrates. Here, the bottom monolayer in bilayer-WS2 contacted Ga2O3 thin film was referred as first layer WS2i.e., 1stL-WS2) while the upper layer as second layer WS2i.e., 2ndL-WS2). Generally, the PL intensity of the 1stL-WS2 is much stronger than that of the 2ndL-WS2 in the bilayer-WS2 obtained on SiO2/Si substrates

16. Nonetheless, an entire converse PL phenomenon was found from PL intensity map (at 640 nm) of the bilayer-WS2/Ga2O3 heterostructure, as shown in Fig. 1(b). The PL intensity in the 2L-WS2 domain surrounded by the white dashed triangle is evidently much stronger than that in the 1L-WS2 domain. Figure 1(c)shows obvious contrast PL spectra in the 1L-WS2 and 2L-WS2 domains. The inset reveals that the PL intensity in the 2L-WS2 domain is approximately 10 times stronger than the intensity in the 1L-WS2 domain. Notably, the anomalous PL emissions were observed in at least six cases of CVD-grown WS2/Ga2O3 heterostructures, where the stronger PL intensity of 2L-WS2 is than that of 1L-WS2. Subsequently, we further focused on such anomalous PL enhancement and the roles of the hetero-interface between Ga2O3 and the 1stL-WS2, and the homo-interface between the 1stL-WS2 and the 2ndL-WS2 in the bilayer-WS2/Ga2O3 heterostructure.

Fig. 1  (a) Structural model schematic illustration of bilayer-WS2/Ga2O3 heterostructure, (b) PL intensity map (at a wavelength of 640 nm) of layered WS2 on Ga2O3 thin film, (c) PL spectra of the 1 L-WS2 and 2 L-WS2 domains in the heterostructure shown in (b)

图1  (a) 双层WS2/Ga2O3异质结的结构示意图,(b) Ga2O3薄膜上层状WS2的PL强度图(在640 nm),(c) 图(b)异质结中1 L-WS2和2 L-WS2的PL光谱

Interlayer interactions and interfaces affect and even determine PL emission of 2D materials

1730. Therefore, we thought that the hetero-interlayer coupling between the bottom Ga2O3-layer and the 1stL-WS2 layer might play an important role in the anomalous PL. The PL emission intensity of 1L-WS2 decreases as displayed in Fig. 1(c), which might relate with the energy band structure and changed crystal lattice of the 1stL-WS231. The WS2/Ga2O3 hetero-interface should be different from those in the WS2/SiO2 and 1stL/2ndL WS2 cases due to some possible changes in dielectric interfaces and interlayer spacing32. Such WS2/Ga2O3 hetero-interface might change the homo-interlayer coupling between the 1stL-WS2 and the 2ndL-WS2, which leads to a special 1L-WS2 and 2L-WS2 different from that in the cases of monolayer WS2 and bilayer-WS2 on the SiO2/Si substrates. As a consequence, all of these may lead to a stronger PL emission in the 2L-WS2 region while a weaker one in the 1L-WS2.

Figure 2 further shows optical image and Raman measurements of the 1st-layer and the 2nd-layer WS2 region in the bilayer WS2/Ga2O3 heterostructure. As represented in Fig. 2(a), 1L-WS2 and 2L-WS2 domains are easily distinguished from the optical contrast. In Raman spectra, the in-plane shear vibration (2LA and E2g1) and the out-plane layer breathing vibration (A1g) locate at ~350 cm-1 and ~420 cm-1, respectively

33. In addition, for the vibration peak at ~350 cm-1, the second-order 2LA mode is dominant due to the double-resonance process34. Moreover, varying with layer number, the intensity of A1g mode (Fig. 2(b)) is effectively discriminated in the Raman map in Fig. 2(b). The A1g peak intensity of 2L-WS2 (pink region) is stronger than that of 1L-WS2 (blue region). The ratio of the intensities of two characteristic peaks (i.e.,I2LA/IA1g) is calculated and approximately 4.38 for the 1L-WS2 while that is about 2.82 for the 2L-WS2 Fig. 2(c)). The value of I2LA/IA1g decreased as the layer number increased, and then was exploited to further check and distinguish 1L-WS2 and 2L-layer WS2 as reported elsewhere35.

Fig. 2  (a) Optical microscopy image of bilayer WS2/Ga2O3 heterostructure, (b) corresponding Raman A1g mode intensity map of bilayer-WS2 on Ga2O3 thin film in (a), the blue region is the 1 L region marked in (a) while the 2 L region is shown as the red, (c) Raman spectra of 1L-WS2 and 2L-WS2 in the heterostructure

图2  (a) 双层WS2/Ga2O3异质结的光学显微镜照片, (b) 对应图(a) Ga2O3薄膜上双层WS2的Raman A1g强度图,蓝色区域为1 L而红色区域为2 L, (c)异质结中1L-WS2和2L-WS2的Raman光谱

For further comparison, a trilayer-WS2/Ga2O3 heterostructure were checked and investigated as suggested in Fig. 3. Optical contrast in Fig. 3(a) changes and is different in the 1L, 2L, and 3L-WS2 domains. Raman intensity map in Fig. 3(c) also displays and manifests the corresponding 1L-WS2 (blue region), 2L-WS2 (pink region) and 3L-WS2 (white region) domains. As mentioned above, the intensity of A1g vibration increases with layer number. The value of I2LA/IA1g decreases with the increase of layer number, and is approximately 4.93, 2.94, and 2.58 for 1L-WS2, 2L-WS2 and 3L-WS2, respectively. Typically, the value of I2LA/IA1g is more than 5 for a WS2 monolayer on SiO2/Si substrate, but it is less than 5 for that of 1L-WS2 in WS2/Ga2O3 heterostructure, which implies that the vibration mode of 1L-WS2 may be changed by the hetero-interface between Ga2O3 and 1stL-WS2

35.

Fig. 3  (a) Optical image of trilayer-WS2/Ga2O3 heterostructure,(b) PL intensity map of WS2 on Ga2O3 thin film,(c) Raman A1g mode intensity map of WS2 on Ga2O3 thin film,(d) PL and (e) Raman spectra of 1 L-WS2, 2 L-WS2 and 3 L-WS2 in the heterostructure shown in (a)

图3  (a) 三层WS2/Ga2O3异质结的光学显微镜照片,(b) Ga2O3薄膜上WS2的PL强度图,(c)Ga2O3薄膜上WS2的Raman A1g强度图,图(a)异质结中1 L-WS2、2 L-WS2和3 L-WS2的(d) PL光谱和(e) Raman光谱

Furthermore, the PL intensity of trilayer-WS2 on Ga2O3 reveals the dark-bright-dark alternating arrangement from accordant outer-1L to inner-3L as indicated in the PL intensity map in Fig. 3(b). The respective PL spectra are extracted from PL intensity map, as shown in Fig. 3(d). We noted that the PL intensity of 2L-WS2 is higher than that of 3L-WS2 while the one of 1L-WS2 is the lowest. Moreover, a redshift of ~5 nm in the PL between 1L-WS2 and 2L-WS2 is shown in Fig. 3(d), and but such shifts do not occur in the case of trilayer-WS2 on the SiO2/Si substrate. All these convincingly suggest that the hetero-interface between 1stL-WS2 and underlying Ga2O3 thin film plays a critical role in changing and weakening PL of 1L-WS2. The hetero-interface between the bottom 1stL-WS2 and the underlying Ga2O3 thin film might reduce the homo-interlayer coupling between 1stL-WS2 and 2ndL-WS2.

Monolayer-WS2/Ga2O3 heterostructure in Fig. 4 is used to further check and certify the role of hetero-interface. The optical contrast of OM image (Fig. 4(a)) and intensity consistency of PL/Raman maps (Fig. 4(b) and 4(c)) display and confirm the monolayer-WS2/Ga2O3 heterostructure. Figure 4(e) implies that the I2LA/IA1g value of the WS2 monolayer in monolayer-WS2/Ga2O3 heterostructure is approximately 4.00 from Raman spectra, which also results from the WS2/Ga2O3 interface interaction as mentioned above. In addition, the PL intensity of monolayer-WS2 in the heterostructure in Fig. 4(d) is less than that of monolayer-WS2 on the SiO2/Si substrate. Notably, the PL intensity of 1L-WS2 in bilayer-WS2/Ga2O3 heterostructure (Fig. 1c) is less than that of the WS2 monolayer (Fig. 4(d)). In addition, the PL intensity of 2L-WS2 is stronger than both the one of 1L-WS2 in bilayer-WS2/Ga2O3 heterostructure (Fig. 1c) and that of the WS2 monolayer (Fig. 4(d)). All these results reveal that the presence of 2L-WS2 in bilayer-WS2/Ga2O3 heterostructure may further weaken the PL intensity of the 1L-WS2 in bilayer-WS2/Ga2O3 heterostructure. The varied energy band structure and possible interlayer charge transfer in both 1L-WS2 and 2L-WS2 may play an important role in an increasing PL intensity of 2L-WS2 and a weaken PL intensity in the bilayer-WS2/Ga2O3 heterostructure.

Fig. 4  (a) Optical image of monolayer-WS2/Ga2O3 heterostructure,(b) PL intensity map of WS2 on Ga2O3 thin film,(c) Raman A1g mode intensity map of WS2 on Ga2O3 thin film,(d) PL spectrum and (e) Raman spectrum of WS2 in monolayer WS2/Ga2O3 heterostructure

图4  (a) 单层WS2/Ga2O3 异质结的光学显微镜照片,(b) Ga2O3 薄膜上WS2的PL强度图,(c) Ga2O3 薄膜上WS2的Raman A1g强度图;单层WS2/Ga2O3 异质结中WS2的(d) PL光谱和(e) Raman光谱

It has been documented that different contacted materials means different interfaces and dielectric environments

1336-38. Figure 5 shows the PL variation in the trilayer-WS2 on SiO2/Si substrate, and further verifies the roles of different interfaces on the PL intensity distribution of layered WS2. Figure 5(a) displays the different layer-domains distribution and the optical contrast in the trilayer-WS2 on SiO2/Si substrate. As illustrated in Fig. 5(b), the 1L-WS2 on SiO2/Si substrate displays the strongest PL emission, which is far outweighing that of 2L-WS2 and 3L-WS2. Thus, it is hard to distinguish the 2L and 3L-WS2 domains in Fig. 5(b). Figure 5(c) shows the Raman map of trilayer-WS2 on the SiO2/Si substrate and the different layer-domains marked by dashed triangles.

Fig. 5  (a) Optical image of trilayer-WS2 on the SiO2/Si substrate,(b) PL intensity map of WS2 on the SiO2/Si substrate,(c) Raman A1g mode intensity map of WS2 on the SiO2/Si substrate,(d) PL and (e) Raman spectra of 1L-WS2, 2L-WS2 and 3L-WS2 shown in (a)

图5  (a) SiO2/Si衬底上三层WS2的光学显微镜照片,(b) SiO2/Si衬底上WS2的PL强度图,(c) SiO2/Si衬底上WS2的Raman A1g强度图,图(a)中1L-WS2、2L-WS2和3L-WS2的(d) PL光谱和(e) Raman光谱

PL spectra of each layer in the trilayer-WS2 on SiO2/Si substrate are taken from Fig. 5(b) and then represented in Fig. 5(d). The PL intensity in 1L region is much larger than that in 2L and 3L regions because the PL intensity of WS2 decreases sharply with the increase of the layer number. Monolayer WS2 possesses direct band gap while bilayer and few layers WS2 are generally indirect band gap. Notably, the PL peak position of each layer in the trilayer-WS2 on SiO2/Si substrate is near 630 nm. Figure 5(e) shows the values of I2LA/IA1g for 1L-WS2, 2L-WS2 and 3L-WS2 is approximately 6.59, 4.52, 1.74, respectively. Here, the I2LA/IA1g value of the 1L-WS2 in the trilayer-WS2 on SiO2/Si substrate is larger than 5. All these are different from those in the case of the trilayer-WS2/Ga2O3 heterostructure above, and further suggest that the hetero-interface between Ga2O3 thin film and WS2 definitely affects the optical band gap of WS2.

To further prove and understand the role of WS2/Ga2O3 hetero-interfaces on PL emission of WS2, we also constructed transferred-bilayer-WS2/Ga2O3 heterostructure (Fig. 6) similar with Fig. 1(a) by transferring a 2L-WS2 from SiO2/Si substrate to the annealed Ga2O3 thin film. Raman, PL and OM images were obtained on the transferred-bilayer-WS2/Ga2O3 heterostructure, and further used to understand the hetero-interlayer coupling in WS2/Ga2O3 heterostructures. The transferred-bilayer-WS2/Ga2O3 heterostructure can be observed from the OM image and PL/Raman maps. We noted that the PL intensity map (Fig. 6(b)) is similar to that in the case of the layered WS2 on SiO2/Si substrates. The intensity of 1L-WS2 domain is much stronger than that in 2L-WS2 domain in the transferred-bilayer-WS2/Ga2O3 heterostructure (Fig. 6(d)) while the PL behaviors of 1L-WS2 and even 2L-WS2 do not change. Notably, the anomalous PL emissions, that is the PL intensity of 2L-WS2 is stronger than that of 1L-WS2, were observed in at least 6 samples. All these confirm the critical effects of WS2/Ga2O3 hetero-interfaces on the anomalous PL behaviors of WS2 in the bilayer-WS2/Ga2O3 heterostructure. In addition, the I2LA/IA1g value for 1L-WS2 in the transferred-bilayer-WS2/Ga2O3 heterostructure is approximately 7.98 while the one for 2L-WS2 is about 4.69 (Fig. 6(e)). The unavoidable interface contaminations during transferring process affected the interfacial coupling

2039.

Fig. 6  (a) Optical image of transferred bilayer-WS2/Ga2O3 heterostructure,(b) PL intensity map of WS2 transferred on Ga2O3 thin film,(c) Raman A1g mode intensity map of WS2 transferred on Ga2O3 thin film,(d) PL and (e) Raman spectra of 1L-WS2 and 2L-WS2 in the heterostructure shown in (a)

图6  (a) 转移的双层WS2/Ga2O3异质结的光学显微镜照片,(b) 转移到Ga2O3薄膜上的WS2的PL强度图,(c) 转移到Ga2O3薄膜上的WS2的Raman A1g强度图,图(a)异质结中1L-WS2和2L-WS2的(d) PL光谱和(e) Raman光谱

Subsequently, two peaks can be fitted by Lorentz model and assigned to neutral excitons and negative trions, which help study the distinctive PL emission behaviors in bilayer WS2/Ga2O3 heterostructure. The PL spectra of 1L and 2L-WS2 on different substrates are displayed in Fig. 1(c) and Fig. 5(e). The intensity ratio of trions and excitons (Itrion/Iexciton) of the WS2 on Ga2O3 is greater than that on SiO2/Si. It means that trions dominate the PL emission of WS2 layer contacted with Ga2O3 thin film, which reveals that the charge density of WS2 in WS2/Ga2O3 heterostructure is higher. It is well-known that more electrons can be bonding with neutral excitons to form more trions, which may reduce PL emission. Then, the I2LA/IA1g value is weak in the case of WS2 grown on Ga2O3 because of the enhancement of A1g mode, and relates to the n-type doping and interfacial distance, where produce more trions. The interfacial distance affects charges doping and transferring between WS2 and Ga2O3 in WS2/Ga2O3 heterostructures

40-41, which reveals a stronger interfacial interaction in grown-WS2/Ga2O3 heterostructures.

We further exploited Kelvin probe force microscopy (KPFM) to check and identify varied surface potential in WS2/Ga2O3 heterostructures for understanding their optical behaviors (Fig. 7). The theoretical energy bands calculated by density function theory suggested the formation of type-II heterojunctions between layered WS2 and Ga2O3 thin film, which may result in an indirect-band WS2 monolayer in WS2/Ga2O3 heterostructures. Furthermore, the presence of type-II band alignments in heterojunctions leads to layer-separated electrons and hole carriers in two different materials, and then to a sudden fall of PL emission. We noted that the surface potential difference was approximately 148 mV between WS2 and Ga2O3, which helped electron-transferring from the WS2 layer to the Ga2O3 layer in the WS2/Ga2O3 heterostructure. All these processes depend on the interlayer charge transfer and intralayer recombination competition

42-43, and then cause consequent variations of Raman and PL spectra. In fact, the interfacial interaction affects and even dominates the optical and electrical behaviors (e.g., enhanced or reduced PL emission) in van der Waals heterostructures24. The strong PL suppression in Fig. 4 compared with that of Fig. 5 indicated the strong coupling in the WS2/Ga2O3 heterostructure.

Fig. 7  (a) Surface potential (KPFM) profile of WS2/Ga2O3 heterostructure, inset image is schematic diagram of WS2/Ga2O3 heterostructure,(b) schematic of the energy band structure of WS2/Ga2O3 heterostructure

图7  (a) WS2/Ga2O3异质结的表面电势分布,插图为WS2/Ga2O3异质结的原子结构示意图,(b) WS2/Ga2O3异质结的能带结构示意图

There are several synergistic determinants for the interlayer coupling, including interface composition, defects, the twist angle, the charge transfer, the interfacial charge traps, original internal stress and other possible factors

31-323644-48. For the WS2/Ga2O3 heterostructures, the anomalous PL behaviors in the WS2 bilayer depended on the hetero-interfaces between bilayer-WS2 and Ga2O3. The decreased interlayer spacing and the strong hetero-interfacial interaction between 1stL-WS2 and Ga2O3 were possibly caused by the interfacial defects and doping, the covalent bonding or enhanced van der Waals force23-24. Moreover, the WS2/Ga2O3 hetero-interfaces may change interlayer spacing and then effect the homo-interlayer coupling between two near monolayers in bilayer or trilayer WS2. All these may alter the energy band structure and PL behaviors of 2L-WS218-2034. Although the dynamics mechanism of the PL in WS2 hetero-interface needs further exploration, these investigations will extend an alternative prospect of understanding the function of the interfacial interaction and constructing vertical TMDs/oxide stacking with excellent optical and electronic performances.

3 Conclusions

In summary, we demonstrated the anomalous PL behaviors in CVD-grown bilayer WS2 in a 2D stacking WS2/Ga2O3 heterostructure. Various hetero-interfaces and interfacial interactions were explored and uncovered to determine unconditional PL emissions in WS2/Ga2O3 heterostructures. Strong WS2/Ga2O3 hetero-interfacial coupling affects 1stL-WS2/2ndL-WS2 interlayer interactions and weakens PL emissions of 1L-WS2 for a PL enhancement in bilayer WS2. Ongoing investigations focus on the interface-dependent PL dynamics in WS2/Ga2O3 heterostructures. Such heterointerface-dependent anomalous PL behaviors will provide more opportunities for modulating energy band structures and the optical or electronic properties of 2D stacked heterostructures, and may benefit for next-generation nanoscale TMDs/oxide-based optoelectronic detectors and photodetection.

Acknowledgements

The authors thank X. H. Zhou and X. Ge for their help with DFT calculations and discussions.

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