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.
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 field
In van der Waals heterostructure, the interfacial interaction is ubiquitous and vital to significantly modulate and alter the optical and optoelectronic properties of 2D material
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 WS2 (i.e., 2L-WS2) region is approximately 10 times stronger than that in the monolayer WS2 (i.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.
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 elsewher

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 material

Fig. 2 (a) Optical microscopy image of bilayer WS2/Ga2O3 heterostructure, (b) corresponding Raman 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 (a) Optical image of trilayer-WS2/Ga2O3 heterostructure,(b) PL intensity map of WS2 on Ga2O3 thin film,(c) Raman 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
Monolayer-WS2/Ga2O3 heterostructure in

Fig. 4 (a) Optical image of monolayer-WS2/Ga2O3 heterostructure,(b) PL intensity map of WS2 on Ga2O3 thin film,(c) Raman 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 environment

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 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
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 (a) Optical image of transferred bilayer-WS2/Ga2O3 heterostructure,(b) PL intensity map of WS2 transferred on Ga2O3 thin film,(c) Raman 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
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 (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 factor
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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