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 heterostructures^{［3-4，10-14］}. 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 temperature^{［11，15］}. 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 WS₂^［16-17］. 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 TMDs^［18-20］.

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^［6，21］. 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 devices^［9，22］. 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 heterostructures^{［10，23-24］}. 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 devices^［25-27］. 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 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 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 DM4000M^［28-29］.

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 WS₂（i.e.，1stL-WS₂）while the upper layer as second layer WS₂（i.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.

Interlayer interactions and interfaces affect and even determine PL emission of 2D materials^{［17，30］}. 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-WS₂^［31］. 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 spacing^［32］. 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 andE2g1）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 process^［34］. 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 elsewhere^［35］.

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/IA1gis 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］.}

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.

It has been documented that different contacted materials means different interfaces and dielectric environments^{［13，36-38］}. 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.

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^{［20，39］}.

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（I_trion/I_exciton）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 A_1g 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 heterostructures^［24］. The strong PL suppression in Fig. 4 compared with that of Fig. 5 indicated the strong coupling in the WS₂/Ga₂O₃ heterostructure.

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^{［31-32，36，44-48］}. 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 force^［23-24］. 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-WS₂^{［18-20，34］}. 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.

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.