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+title: "What Can ABACUS Do Too? | First-Principles Calculation Elucidates the Mechanism of Two-Dimensional Materials Regulating Complex Oxides"
+date: 2025-1-20
+categories:
+- ABACUS
+---
+
+Recently, Dr. Junxiong Hu, a senior postdoctoral researcher at the National University of Singapore, and the research group of Professor Ariando, in collaboration with Professor Song Liu from Hunan University, Dr. Xudong Zhu, a postdoctoral researcher at the Institute of Artificial Intelligence, Hefei Comprehensive National Science Center, and others, have made important progress in the field of the interaction between two-dimensional materials and complex oxides. This study focused on the influence of the graphene layer on the formation of oxygen vacancies in SrTiO₃ (STO) during high-temperature annealing. Combined with Raman spectroscopy, X-ray photoelectron spectroscopy, and photoluminescence spectroscopy studies, it was proved that the graphene layer can effectively regulate the formation of oxygen vacancies in STO. The researchers used the open-source density functional theory software ABACUS (Atomic Calculator) to further reveal the principle that the multilayer graphene coating suppresses the formation of oxygen vacancies in STO by increasing the energy barrier for the outward diffusion of oxygen atoms. The research result "Tuning oxygen vacancies in complex oxides using 2D layered materials" has been published in the 2D Materials journal. This study proposes a new approach to regulating the physical properties of complex oxides by designing the interface of two-dimensional materials.
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+<center><img src=https://dp-public.oss-cn-beijing.aliyuncs.com/community/Blog%20Files/ABACUS_20_01_2025/pic1.png pic_center width="60%" height="60%" /></center>
+
+*Figure 1: Schematic diagrams of the generation of oxygen vacancies in the STO substrate and the suppression of oxygen vacancy formation by graphene: (a) without graphene (STO), (b) single-layer graphene (1l - G/STO), (c) double-layer graphene (2l - G/STO), (d) triple-layer graphene (3l - G/STO). As the number of graphene layers increases from (a) to (d), the generation of oxygen vacancies is gradually suppressed.*
+
+## Research Background
+
+The coupling interface between two-dimensional materials and complex oxides leads to a series of interesting physical phenomena and has always been a research hotspot in the field of materials science. Previous studies mainly focused on the influence of complex oxides on layered two-dimensional materials. However, this research team did the opposite and focused on how two-dimensional materials change the properties of complex oxides. The research selected SrTiO₃ (STO), a key material in the field of complex oxides with a high dielectric constant, as a representative and used graphene as the two-dimensional material for experiments, focusing on the important property of complex oxides - oxygen vacancies.
+
+## Research Results
+
+The researchers used the layer-by-layer transfer method to transfer chemical vapor deposition (CVD) graphene films of different layers onto the STO substrate to construct the graphene/STO (G/STO) heterostructure. Subsequently, the samples were annealed at high temperature in a high vacuum to induce the formation of oxygen vacancies in STO.
+
+Raman spectroscopy showed that with the increase in the number of graphene layers, the change in the peak ratio before and after annealing indicated a decrease in the degree of graphene defects, indirectly showing that the formation of oxygen vacancies in STO was suppressed. X-ray photoelectron spectroscopy (XPS) detection further confirmed that the generation of Ti 3+ particles decreased after graphene coating, directly indicating that the number of oxygen vacancies decreased with the increase in the number of graphene layers. However, regardless of the graphene coating situation, oxygen vacancies were successfully induced. Photoluminescence spectroscopy (PL) analysis showed that the graphene layer can adjust the in-gap state induced by oxygen vacancies in STO, weakening the intensity of the luminescence peak related to oxygen vacancies.
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+<center><img src=https://dp-public.oss-cn-beijing.aliyuncs.com/community/Blog%20Files/ABACUS_20_01_2025/pic2.png pic_center width="60%" height="60%" /></center>
+
+*Figure 2: Optical and Raman characterization of the G/STO heterostructure. (a) Optical images of 1l - G/STO, (c) 2l - G/STO, and (e) 3l - G/STO. The boundaries of the graphene films are marked with white dotted lines. Scale bar: 500 µm; (b), (d), (f) Raman spectra of graphene with different layers on STO before annealing (black, marked as BA) and after annealing (blue, marked as AA). The shaded areas highlight the changes in the D peak related to the defect density of graphene. (g) Changes in the ID/IG ratio and the defect density of graphene with different layers. (The defect density of graphene (indirectly characterizing the oxygen vacancy content) is proportional to (ID/IG)^2.)*
+
+<center><img src=https://dp-public.oss-cn-beijing.aliyuncs.com/community/Blog%20Files/ABACUS_20_01_2025/pic3.png pic_center width="60%" height="60%" /></center>
+
+*Figure 3: XPS characterization of oxygen vacancies in the G/STO heterostructure. (a) Ti 2p XPS spectra of STO, (b) single - layer, (c) double - layer, and (d) triple - layer graphene on STO. (e) Variation of the Ti 3+/Ti 4+ ratio (blue) and the oxygen vacancy ratio (red) with the number of graphene layers.*
+
+<center><img src=https://dp-public.oss-cn-beijing.aliyuncs.com/community/Blog%20Files/ABACUS_20_01_2025/pic4.png pic_center width="60%" height="60%" /></center>
+
+*Figure 4: PL characterization of oxygen vacancies in the G/STO heterostructure. (a) Lorentzian fitting of multiple peaks in the PL spectrum of STO (without graphene); (b) PL spectra of G/STO with different numbers of graphene layers (excitation wavelength: 325 nm); (c) Potential luminescence mechanisms at 2.79 eV and 2.98 eV; (d) Luminescence related to oxygen vacancies at 2.37 eV in STO.*
+
+Finally, to confirm the role of graphene in the formation of oxygen vacancies, DFT calculations were performed using the  first-principles software ABACUS based on the numerical atomic orbital basis set (LCAO) to explore the mechanism of graphene affecting the diffusion of oxygen atoms in the G/STO heterostructure. In the study, a diffusion process in which oxygen atoms migrated from the Ti-O surface of STO to the nearest neighbor surface of graphene to form a gradually flat carbon-oxygen-carbon (C - O - C) bridge bond was constructed, and the energy barrier related to the calculation process was calculated. In the single-layer, double-layer, and triple-layer G/STO structures, the energy barriers for oxygen atoms to pass through the first layer of graphene were 10.80 eV, 12.41 eV, and 13.52 eV, respectively, which were significantly increased compared with the 0.81 eV energy barrier for oxygen diffusion in bulk STO. This indicates that graphene can play a shielding role, making the outward diffusion of oxygen atoms more difficult and thus suppressing the formation of oxygen vacancies.
+
+<center><img src=https://dp-public.oss-cn-beijing.aliyuncs.com/community/Blog%20Files/ABACUS_20_01_2025/pic5.png pic_center width="60%" height="60%" /></center>
+
+*Figure 5: Theoretical illustration of oxygen diffusion in the G/STO heterostructure. (a)-(c) Atomic models of different stages of an oxygen atom penetrating the first layer of graphene in 1l - G/STO. (d)-(f) Energy barriers for the diffusion of an oxygen atom through the first layer of graphene in 1l - G/STO, 2l - G/STO, and 3l - G/STO, respectively. (g) Variation of the energy barrier with the number of graphene layers.*
+
+## Acknowledgments
+
+Jiangbo Luo, a jointly trained doctoral student at the National University of Singapore and Hunan University, and Dr. Xudong Zhu, a postdoctoral researcher at the Institute of Artificial Intelligence, Hefei Comprehensive National Science Center, are the co-first authors of this paper. Dr. Junxiong Hu, a senior postdoctoral researcher at the National University of Singapore, Professor Ariando, and Professor Song Liu of Hunan University are the co-corresponding authors. This research result was supported by the Singapore National Research Foundation, the Academic Research Fund of the Ministry of Education of Singapore, and the Agency for Science, Technology and Research of Singapore. It was also funded by the China Postdoctoral Science Foundation and the Anhui Postdoctoral Science Foundation.
+
+Paper link: https://iopscience.iop.org/article/10.1088/2053-1583/ada041