Interaction of oxygen with samarium on Al2O3 thin film grown on Ni3Al(111) Dingling Cheng, Qian Xu, Yong Han, Yifan Ye, Haibin Pan, and Junfa Zhu Citation: The Journal of Chemical Physics 140, 094706 (2014); doi: 10.1063/1.4867387 View online: http://dx.doi.org/10.1063/1.4867387 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/140/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Vacuum sealing using atomic layer deposition of Al2O3 at 250°C J. Vac. Sci. Technol. A 32, 01A101 (2014); 10.1116/1.4820240 Al2O3 e-Beam Evaporated onto Silicon (100)/SiO2, by XPS Surf. Sci. Spectra 20, 43 (2013); 10.1116/11.20121102 Variation in the threshold voltage of amorphous-In2Ga2ZnO7 thin-film transistors by ultrathin Al2O3 passivation layer J. Vac. Sci. Technol. B 31, 061205 (2013); 10.1116/1.4827276 Characterization of Al2O3/GaAs interfaces and thin films prepared by atomic layer deposition J. Vac. Sci. Technol. B 31, 04D111 (2013); 10.1116/1.4813436 Growth and electronic structure of Sm on thin Al2O3/Ni3Al(111) films J. Chem. Phys. 136, 154705 (2012); 10.1063/1.4704676
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THE JOURNAL OF CHEMICAL PHYSICS 140, 094706 (2014)
Interaction of oxygen with samarium on Al2 O3 thin film grown on Ni3 Al(111) Dingling Cheng, Qian Xu,a) Yong Han, Yifan Ye, Haibin Pan, and Junfa Zhua) National Synchrotron Radiation Laboratory and Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230029, People’s Republic of China
(Received 9 January 2014; accepted 20 February 2014; published online 7 March 2014) The interaction between oxygen and samarium (Sm) on the well-ordered thin Al2 O3 film grown on Ni3 Al(111) has been investigated by X-ray photoelectron spectroscopy and synchrotron radiation photoemission spectroscopy. At Sm coverage higher than one monolayer, exposure of oxygen to the Sm films at room temperature leads to the formation of both samarium peroxide (O2 2− ) states and regular samarium oxide (O2− ) states. By contrast, when exposing O2 to Sm film less than one monolayer on Al2 O3 , no O2 2− can be observed. Upon heating to higher temperatures, these metastable O2 2− states dissociate, supplying active O atoms which can diffuse through the Al2 O3 thin film to further oxidize the underlying Ni3 Al(111) substrate, leading to the significant increase of the Al2 O3 thin film thickness. Therefore, it can be concluded that Sm, presumably in its peroxide form, acts as a catalyst for the further oxidation of the Ni3 Al substrate by supplying the active oxygen species at elevated temperatures. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4867387] I. INTRODUCTION
Rare-earth metals have been the focus of many investigations in recent years, owing to their characteristic activity and selectivity for a range of catalytic reactions.1–9 Lanthanide oxides, particularly ceria and samaria, have been used as components in various catalytic applications for a number of chemical processes.10–15 Especially, Sm2 O3 has been reported to be an effective promoter to the Al2 O3 catalysts in oxidative coupling of methane.16 However, unlike ceria, the catalytic properties of samarium and samaria have not been investigated thoroughly. Less is known about the interaction of samarium with oxygen on oxides at an atomic level although a thin layer of samaria was believed to be an active catalyst or an effective catalytic promoter in many reactions. Up to now, only a few studies addressing on the adsorption and oxidation of samarium on metal and semiconductor surfaces can be found in the literature, providing limited information about surface structures and electronic properties.17, 18 Recently we have investigated the growth and electronic structure of Sm on the Al2 O3 ultrathin films which were grown on a Ni3 Al(111) substrate by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and synchrotron radiation photoemission spectroscopy (SRPES).19 It has been found that at room temperature Sm grows in a layer-by-layer fashion up to at least 1 ML, followed by a three-dimensional growth. The strong interaction of Sm with the Al2 O3 thin films leads to an initial oxidation of Sm, accompanied by a parallel reduction of the Al2 O3 substrate. Both the oxidation states of Sm2+ and Sm3+ are found at low coverages ( 800 K should be associated with the diffusion of samaria into the Al2 O3 /Ni3 Al(111) substrate to form SmAlO3 -like species. Recently, thin SmAlO3 films was reported by co-depositing of samarium and aluminum on SiO2 in diluted O2 at 350 K followed by rapid thermal annealing at high temperatures (873 K, 973 K, and 1073 K), and it was found that the SmAlO3 film annealed at 1073 K has better crystalline than annealed at 873 K.46 Moreover, in a number of previous studies, where formation of mixedoxide phase such as Bax Tiy Oz ,47 BaAl2 O4 -like species,48 and CeAlO3 49 were also reported at elevated temperatures. However, it should be pointed out here that even after annealing to 1000 K, we did not observe any ordered structure in the LEED pattern, indicating that if SmAlO3 is formed, it has polycrystalline structure. The Al 2p region in the SRPES spectra of Figure 4(b) also reveals important information regarding the changes during the annealing steps. As mentioned above, three typical Al 2p features at 75.0, 73.6, and 72.4 eV associated with Als 3+ , Ali 3+ , and Al0 states, are respectively, visible at 300 K. Before annealed to 500 K, no significant changes can be observed
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FIG. 4. Photoemission spectra of (a) O 1s, (b) Al 2p, and (c) Sm 3d5/2 after subsequent annealing the O2 -saturated Sm/Al2 O3 (θ Sm = 1.5 ML) surface to different temperatures in UHV. (d) The intensity evolution for total Sm 3d5/2 as a function of annealing temperatures.
in Al 2p, similar to those in O 1s spectra. When the annealing temperature is higher than 500 K, the Al3+ 2p feature increases in intensity and shifts gradually to lower BE. Meanwhile, a detectable intensity decrease of the Al0 2p signal is seen. These observations indicate that annealing the oxygensaturated 1.5 ML Sm films on Al2 O3 /Ni3 Al(111) can promote the further oxidation of the Ni3 Al(111) substrate. In connection with the observation in the O 1s spectra, this process can be probably attributed to the oxygen atoms from the decomposition of the peroxide species penetrating through the thin Al2 O3 film and oxidizing the Ni3 Al(111) substrate during thermal treatment. A similar behavior has been observed for O2 adsorbed on Pd/Al2 O3 /NiAl(110) at 400–500 K50 as well as vanadium deposited on the alumina film at elevated temperatures.51 It should be mentioned that the further oxidation of Ni3 Al(111) substrate due to reaction of Al0 atoms with the oxygen containing background gases such as H2 O, as reported in the case of Ba/Al2 O3 /NiAl(110),48 can be excluded here, because the thickening of the alumina film was not observed when the Sm/Al2 O3 /Ni3 Al(111) was heated in the absence of a gaseous oxidizing agent.52 When the annealing temperature is higher than 800 K, an intensified Als 3+ 2p with a symmetric line shape is observed probably due to the formation of SmAlO3 phase. This observation is consistent with the observations in the O 1s and Sm 3d5/2 spectra discussed above.
C. O2 adsorption on 0.3 ML Sm on Al2 O3 at 300 K and subsequent annealing in UHV
As mentioned above, both exposure of O2 to the 1.5 ML Sm/Al2 O3 film at 300 K and subsequent annealing of the oxygen-saturated 1.5 ML Sm/Al2 O3 film lead to further oxidation of Al0 in the Ni3 Al(111) substrate. To obtain more information about the interface interaction, the Al2 O3 /Ni3 Al(111) substrate with a low coverage of Sm (θ Sm = 0.3 ML) was exposed to O2 at 300 K. Figure 5(a) presents the O 1s spectra taken from the clean and oxygen-saturated 0.3 ML Sm/Al2 O3 surface. The total oxygen uptake curve for this 0.3 ML Sm covered Al2 O3 surface, obtained by integrating the O 1s intensity, versus O2 exposure is shown in the inset of Figure 5(a). Before O2 exposure, the O 1s spectrum shows an asymmetric peak dominated at 532.2 eV, in consistent with our previous work.19 After exposing 0.4 L at 300 K, the O2 uptake is saturated. The O 1s peak slightly shifts to lower binding energy and settles at 531.9 eV with an enhanced intensity. This is associated with the further oxidation of Sm2+ to Sm3+ , as evidenced by the evolution of Sm 3d spectra shown in Figure 5(b). However, compared with the O2 adsorption experiments on Sm films with higher coverage presented above, no evidence for the formation of peroxide states (O2 2− ) is found here, indicating that the peroxide states only appear on the multilayer Sm films. It should be recalled that
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FIG. 5. XPS spectra of (a) O 1s and (b) Sm 3d5/2 after exposing Sm/Al2 O3 (θ Sm = 0.3 ML) to given oxygen exposures; (c) O 1s and (d) Sm 3d5/2 after subsequent annealing in UHV to various temperatures. The inset in (a) shows the integrated intensity evolution of O 1s total with increasing O2 exposure.
the appearance of different oxygen species on different coverages of metallic adsorbates has also been observed previously for oxygen adsorption on the cerium covered silver surface.53 Shown in Figure 5(b) is the Sm 3d5/2 spectra from the 0.3 ML Sm/Al2 O3 film at different O2 exposure. Before O2 exposure, two peaks at 1075.4 eV and 1084.8 eV, attributed to ionic Sm2+ and Sm3+ ,19 respectively, are observed. With increasing O2 exposure, the peak at 1084.8 eV grows gradually in intensity at the expense of the peak at 1075.4 eV. At oxygen exposure above 0.4 L, the Sm2+ feature completely disappears and only a single peak of Sm3+ at 1084.8 eV is observed, indicating that all the ionic Sm2+ are further oxidized to Sm3+ by oxygen adsorption. This observation is in agreement with the results obtained from the O 1s. Figures 5(c) and 5(d) present the O 1s and Sm 3d5/2 spectra after subsequent annealing of the oxygen-saturated Al2 O3 surface covered with 0.3 ML Sm at different temperatures. When the annealing temperature below 550 K, there are no observable changes in both O 1s and Sm 3d spectra. However, when the sample is annealed to 723 K, the shoulder peak of O 1s at higher binding energy side almost disappears and the O 1s spectrum becomes more symmetric. Meanwhile, the dominant peak shifts by 531.7 eV. Concurrently, the Sm 3d intensity attenuates at 723 K. Apparently, these observations imply that at temperature higher than 720 K, Sm2 O3 starts to diffuse into the Al2 O3 substrate, and presumably forms the SmAlO3 phase. It is worth mentioning that the Al 2p spectra (not shown), which are simultaneously monitored in parallel with the spectra given in Figure 5, do not show any evidence of the further
oxidation of Al0 during oxygen uptake on the 0.3 ML Sm covered Al2 O3 /Ni3 Al(111) surface and subsequent annealing. This is in contrast to the case for the 1.5 ML Sm covered Al2 O3 /Ni3 Al(111) surface. Since the O2 2− species is not observed for oxygen adsorption on 0.3 ML Sm/Al2 O3 , no further oxidation of the Ni3 Al(111) substrate here suggests that the O2 2− states play an important role in this reaction. IV. CONCLUSIONS
In this work, we have investigated the interaction of oxygen with the Al2 O3 /Ni3 Al(111) surface covered with Sm for three different coverages at 300 K and the thermal stability of oxygen-saturated Sm/Al2 O3 films using XPS and SRPES. The results can be summarized as follows: For the Sm coverage higher than 1 ML, the adsorption of O2 at 300 K leads to the observation of two types of oxygen species, O2 2− and O2− , on the Sm/Al2 O3 surface. These metastable peroxide (O2 2− ) states convert to regular oxide (O2− ) states via dissociation at elevated temperatures, supplying active O atoms which diffuse through the Al2 O3 film to further oxidize the Ni3 Al(111) substrate, leading to the thickening of the Al2 O3 film. However, for the Sm coverage less than one monolayer, only the regular oxide species, O2− , can be observed upon oxygen exposure. Moreover, upon annealing this surface to higher temperatures, no further oxidation of the Ni3 Al(111) substrate occurs. On all Sm/Al2 O3 surfaces saturated with oxygen, when they are annealed to the temperature higher than 700 K, the diffusion of Sm2 O3 into the Al2 O3 substrate is observed.
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ACKNOWLEDGMENTS
We greatly acknowledge the National Basic Research Program of China (2010CB923302, 2013CB834605) and Natural Science Foundation of China (Grant No. U1232102) for the financial support of this work. Q.X. specially thanks the Fundamental Research Funds for the Central Universities for the support (WK2310000027). 1 Q.
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THE JOURNAL OF CHEMICAL PHYSICS 140, 094706 (2014)
Interaction of oxygen with samarium on Al2 O3 thin film grown on Ni3 Al(111) Dingling Cheng, Qian Xu,a) Yong Han, Yifan Ye, Haibin Pan, and Junfa Zhua) National Synchrotron Radiation Laboratory and Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230029, People’s Republic of China
(Received 9 January 2014; accepted 20 February 2014; published online 7 March 2014) The interaction between oxygen and samarium (Sm) on the well-ordered thin Al2 O3 film grown on Ni3 Al(111) has been investigated by X-ray photoelectron spectroscopy and synchrotron radiation photoemission spectroscopy. At Sm coverage higher than one monolayer, exposure of oxygen to the Sm films at room temperature leads to the formation of both samarium peroxide (O2 2− ) states and regular samarium oxide (O2− ) states. By contrast, when exposing O2 to Sm film less than one monolayer on Al2 O3 , no O2 2− can be observed. Upon heating to higher temperatures, these metastable O2 2− states dissociate, supplying active O atoms which can diffuse through the Al2 O3 thin film to further oxidize the underlying Ni3 Al(111) substrate, leading to the significant increase of the Al2 O3 thin film thickness. Therefore, it can be concluded that Sm, presumably in its peroxide form, acts as a catalyst for the further oxidation of the Ni3 Al substrate by supplying the active oxygen species at elevated temperatures. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4867387] I. INTRODUCTION
Rare-earth metals have been the focus of many investigations in recent years, owing to their characteristic activity and selectivity for a range of catalytic reactions.1–9 Lanthanide oxides, particularly ceria and samaria, have been used as components in various catalytic applications for a number of chemical processes.10–15 Especially, Sm2 O3 has been reported to be an effective promoter to the Al2 O3 catalysts in oxidative coupling of methane.16 However, unlike ceria, the catalytic properties of samarium and samaria have not been investigated thoroughly. Less is known about the interaction of samarium with oxygen on oxides at an atomic level although a thin layer of samaria was believed to be an active catalyst or an effective catalytic promoter in many reactions. Up to now, only a few studies addressing on the adsorption and oxidation of samarium on metal and semiconductor surfaces can be found in the literature, providing limited information about surface structures and electronic properties.17, 18 Recently we have investigated the growth and electronic structure of Sm on the Al2 O3 ultrathin films which were grown on a Ni3 Al(111) substrate by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and synchrotron radiation photoemission spectroscopy (SRPES).19 It has been found that at room temperature Sm grows in a layer-by-layer fashion up to at least 1 ML, followed by a three-dimensional growth. The strong interaction of Sm with the Al2 O3 thin films leads to an initial oxidation of Sm, accompanied by a parallel reduction of the Al2 O3 substrate. Both the oxidation states of Sm2+ and Sm3+ are found at low coverages ( 800 K should be associated with the diffusion of samaria into the Al2 O3 /Ni3 Al(111) substrate to form SmAlO3 -like species. Recently, thin SmAlO3 films was reported by co-depositing of samarium and aluminum on SiO2 in diluted O2 at 350 K followed by rapid thermal annealing at high temperatures (873 K, 973 K, and 1073 K), and it was found that the SmAlO3 film annealed at 1073 K has better crystalline than annealed at 873 K.46 Moreover, in a number of previous studies, where formation of mixedoxide phase such as Bax Tiy Oz ,47 BaAl2 O4 -like species,48 and CeAlO3 49 were also reported at elevated temperatures. However, it should be pointed out here that even after annealing to 1000 K, we did not observe any ordered structure in the LEED pattern, indicating that if SmAlO3 is formed, it has polycrystalline structure. The Al 2p region in the SRPES spectra of Figure 4(b) also reveals important information regarding the changes during the annealing steps. As mentioned above, three typical Al 2p features at 75.0, 73.6, and 72.4 eV associated with Als 3+ , Ali 3+ , and Al0 states, are respectively, visible at 300 K. Before annealed to 500 K, no significant changes can be observed
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FIG. 4. Photoemission spectra of (a) O 1s, (b) Al 2p, and (c) Sm 3d5/2 after subsequent annealing the O2 -saturated Sm/Al2 O3 (θ Sm = 1.5 ML) surface to different temperatures in UHV. (d) The intensity evolution for total Sm 3d5/2 as a function of annealing temperatures.
in Al 2p, similar to those in O 1s spectra. When the annealing temperature is higher than 500 K, the Al3+ 2p feature increases in intensity and shifts gradually to lower BE. Meanwhile, a detectable intensity decrease of the Al0 2p signal is seen. These observations indicate that annealing the oxygensaturated 1.5 ML Sm films on Al2 O3 /Ni3 Al(111) can promote the further oxidation of the Ni3 Al(111) substrate. In connection with the observation in the O 1s spectra, this process can be probably attributed to the oxygen atoms from the decomposition of the peroxide species penetrating through the thin Al2 O3 film and oxidizing the Ni3 Al(111) substrate during thermal treatment. A similar behavior has been observed for O2 adsorbed on Pd/Al2 O3 /NiAl(110) at 400–500 K50 as well as vanadium deposited on the alumina film at elevated temperatures.51 It should be mentioned that the further oxidation of Ni3 Al(111) substrate due to reaction of Al0 atoms with the oxygen containing background gases such as H2 O, as reported in the case of Ba/Al2 O3 /NiAl(110),48 can be excluded here, because the thickening of the alumina film was not observed when the Sm/Al2 O3 /Ni3 Al(111) was heated in the absence of a gaseous oxidizing agent.52 When the annealing temperature is higher than 800 K, an intensified Als 3+ 2p with a symmetric line shape is observed probably due to the formation of SmAlO3 phase. This observation is consistent with the observations in the O 1s and Sm 3d5/2 spectra discussed above.
C. O2 adsorption on 0.3 ML Sm on Al2 O3 at 300 K and subsequent annealing in UHV
As mentioned above, both exposure of O2 to the 1.5 ML Sm/Al2 O3 film at 300 K and subsequent annealing of the oxygen-saturated 1.5 ML Sm/Al2 O3 film lead to further oxidation of Al0 in the Ni3 Al(111) substrate. To obtain more information about the interface interaction, the Al2 O3 /Ni3 Al(111) substrate with a low coverage of Sm (θ Sm = 0.3 ML) was exposed to O2 at 300 K. Figure 5(a) presents the O 1s spectra taken from the clean and oxygen-saturated 0.3 ML Sm/Al2 O3 surface. The total oxygen uptake curve for this 0.3 ML Sm covered Al2 O3 surface, obtained by integrating the O 1s intensity, versus O2 exposure is shown in the inset of Figure 5(a). Before O2 exposure, the O 1s spectrum shows an asymmetric peak dominated at 532.2 eV, in consistent with our previous work.19 After exposing 0.4 L at 300 K, the O2 uptake is saturated. The O 1s peak slightly shifts to lower binding energy and settles at 531.9 eV with an enhanced intensity. This is associated with the further oxidation of Sm2+ to Sm3+ , as evidenced by the evolution of Sm 3d spectra shown in Figure 5(b). However, compared with the O2 adsorption experiments on Sm films with higher coverage presented above, no evidence for the formation of peroxide states (O2 2− ) is found here, indicating that the peroxide states only appear on the multilayer Sm films. It should be recalled that
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FIG. 5. XPS spectra of (a) O 1s and (b) Sm 3d5/2 after exposing Sm/Al2 O3 (θ Sm = 0.3 ML) to given oxygen exposures; (c) O 1s and (d) Sm 3d5/2 after subsequent annealing in UHV to various temperatures. The inset in (a) shows the integrated intensity evolution of O 1s total with increasing O2 exposure.
the appearance of different oxygen species on different coverages of metallic adsorbates has also been observed previously for oxygen adsorption on the cerium covered silver surface.53 Shown in Figure 5(b) is the Sm 3d5/2 spectra from the 0.3 ML Sm/Al2 O3 film at different O2 exposure. Before O2 exposure, two peaks at 1075.4 eV and 1084.8 eV, attributed to ionic Sm2+ and Sm3+ ,19 respectively, are observed. With increasing O2 exposure, the peak at 1084.8 eV grows gradually in intensity at the expense of the peak at 1075.4 eV. At oxygen exposure above 0.4 L, the Sm2+ feature completely disappears and only a single peak of Sm3+ at 1084.8 eV is observed, indicating that all the ionic Sm2+ are further oxidized to Sm3+ by oxygen adsorption. This observation is in agreement with the results obtained from the O 1s. Figures 5(c) and 5(d) present the O 1s and Sm 3d5/2 spectra after subsequent annealing of the oxygen-saturated Al2 O3 surface covered with 0.3 ML Sm at different temperatures. When the annealing temperature below 550 K, there are no observable changes in both O 1s and Sm 3d spectra. However, when the sample is annealed to 723 K, the shoulder peak of O 1s at higher binding energy side almost disappears and the O 1s spectrum becomes more symmetric. Meanwhile, the dominant peak shifts by 531.7 eV. Concurrently, the Sm 3d intensity attenuates at 723 K. Apparently, these observations imply that at temperature higher than 720 K, Sm2 O3 starts to diffuse into the Al2 O3 substrate, and presumably forms the SmAlO3 phase. It is worth mentioning that the Al 2p spectra (not shown), which are simultaneously monitored in parallel with the spectra given in Figure 5, do not show any evidence of the further
oxidation of Al0 during oxygen uptake on the 0.3 ML Sm covered Al2 O3 /Ni3 Al(111) surface and subsequent annealing. This is in contrast to the case for the 1.5 ML Sm covered Al2 O3 /Ni3 Al(111) surface. Since the O2 2− species is not observed for oxygen adsorption on 0.3 ML Sm/Al2 O3 , no further oxidation of the Ni3 Al(111) substrate here suggests that the O2 2− states play an important role in this reaction. IV. CONCLUSIONS
In this work, we have investigated the interaction of oxygen with the Al2 O3 /Ni3 Al(111) surface covered with Sm for three different coverages at 300 K and the thermal stability of oxygen-saturated Sm/Al2 O3 films using XPS and SRPES. The results can be summarized as follows: For the Sm coverage higher than 1 ML, the adsorption of O2 at 300 K leads to the observation of two types of oxygen species, O2 2− and O2− , on the Sm/Al2 O3 surface. These metastable peroxide (O2 2− ) states convert to regular oxide (O2− ) states via dissociation at elevated temperatures, supplying active O atoms which diffuse through the Al2 O3 film to further oxidize the Ni3 Al(111) substrate, leading to the thickening of the Al2 O3 film. However, for the Sm coverage less than one monolayer, only the regular oxide species, O2− , can be observed upon oxygen exposure. Moreover, upon annealing this surface to higher temperatures, no further oxidation of the Ni3 Al(111) substrate occurs. On all Sm/Al2 O3 surfaces saturated with oxygen, when they are annealed to the temperature higher than 700 K, the diffusion of Sm2 O3 into the Al2 O3 substrate is observed.
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ACKNOWLEDGMENTS
We greatly acknowledge the National Basic Research Program of China (2010CB923302, 2013CB834605) and Natural Science Foundation of China (Grant No. U1232102) for the financial support of this work. Q.X. specially thanks the Fundamental Research Funds for the Central Universities for the support (WK2310000027). 1 Q.
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