鐵磁性材料與二維材料之異質結構分析： 結構，磁性和特性操控
No Thumbnail Available
Date
2023
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
在二維材料體系中，獨特的特性和穩定的單層對稱結構具有發展令人興奮的物理的巨大潛力。此研究專注於製造 2D/2D 或 2D/鐵磁材料所組合的異質結構，並分析各種測量結果以研究幾個關鍵因素，包括擴散和插層現象、界面交互作用、以及涉及電子注入的化學吸附和物理吸附過程。在石墨烯(Gr)/CoPd系統中，Gr的覆蓋可以保護 CoPd 層免受氧化和層間擴散。如果沒有Gr，當暴露在大氣環境中 64 天時，表面粗糙度會增加，克爾強度百分比會降低。這表明未受保護的 CoPd 層隨著時間氧化和克爾強度退化。另一方面，當Gr存在時，形態和克爾強度保持穩定，保持CoPd的初始狀態。Gr充當保護屏障，防止氧氣和其他可能導致 CoPd 層氧化和降解的物質擴散。高溫成長的CoPd在 MoS2 上的沉積方法產生了均勻且平坦的二維層，如AFM 圖像中所觀察到的。CoPd 層的形態顯著影響 MAE，其中 CoPd/MoS2 的不同方位角方向表現出不同的磁異向能(MAE)。克爾圖像和磁滯迴線測量表明，改變 CoPd 層中 Co 和 Pd 的百分比組成（例如 Co 50%和35%）會導致 MoS2 的不同方位角方向都有其獨立的 MAE。 這表明MoS2的磁性能和優選磁化方向可以通過設計CoPd層的成分來控制。採用接觸力AFM技術可以去除PMMA殘留物，減小層間距離，剝離Gr層。通過施加110 nN的接觸力，樣品的粗糙度降低，並且可以觀察到Gr層中的皺紋。然而，由於剝離過程中引入缺陷，樣品的 PL 強度也會降低。當接觸力增加到 220 nN 以上時，結構損傷變得更加明顯，從輕微且不連續的薄片到 Gr 層完全剝落，使 MoS2 表面暴露。接觸力的作用對於確定剝落程度和最終的表面形態至關重要。此外，我們亦探討了在 Pt 尖端上施加正偏壓或負偏壓以及摩擦 Gr/MoS2 表面的影響，導致異質結構發生物理和化學變化，稱為掃描探針蝕刻。 該過程可以誘導MoS2從2H到1T相的相變或導致Mo-O鍵的形成。除了機械磨損之外，樣品和鉑尖端之間的水橋中還會發生電化學反應。產生的內部電場可以促進水分子的分離並誘導HER或OER。 這會導致 MoS2 結構的扭曲或氧鍵的形成。SPL處理後，D和G拉曼峰強度的比值（I(D)/I(G)）和I(G)/I(2D)比值可以洞察Gr結構的變化，包括空位濃度、結構連續性和晶格應變。MoS2 的 PL 特性表現出半導體行為改變。透過以上多個二維異質結構的研究，更多的功能性及操控有機會應用在未來的二維元件之中。
The unique characteristics and stable single-layered symmetry structures in the two-dimensional material system hold great potential for developing exciting physics. My research focuses on fabricating heterostructures involving 2D/2D or 2D/ferromagnetic material combinations and analyzing various measurements to investigate several critical factors. These factors include the diffusion barrier and intercalation phenomena, the influence of interfacial interactions, and the chemisorption and physisorption processes involving electron injection. The coverage of Gr provides protection to the CoPd layer against oxidation and interlayer diffusion, as discussed in Chapter 4. Without Gr, when exposed to the atmospheric environment for 64 days, the roughness of the surface increases, and the Kerr intensity percentage decreases. This suggests that the unprotected CoPd layer is susceptible to oxidation and Kerr intensity degradation over time. On the other hand, when Gr is present, the morphology and Kerr intensity remain stable, maintaining the CoPd layer's initial state. Gr acts as a protective barrier, preventing the diffusion of oxygen and other species that can cause oxidation and degradation of the CoPd layer. The high-temperature deposition method of CoPd on MoS2 results in a uniform and flat 2D layer, as observed in the AFM images. The CoPd layer's morphology significantly impacts the MAE, where the resulting different azimuth orientations of CoPd/MoS2 exhibit distinct MAE values. Kerr images and hysteresis loop measurements show that varying the Co and Pd percentage composition in the CoPd layer (e.g., Co 50% and 35%) leads to independent MAEs for different azimuthal orientations of MoS2. This indicates that the magnetic properties and preferred magnetization direction of MoS2 can be controlled by engineering the composition of the CoPd layer. The contact force AFM technique was used to remove PMMA residue, reduce the interlayer distance, and exfoliate the Gr layer. By applying a contact force of 110 nN, the roughness of the sample decreases, and wrinkles in the Gr layer can be observed. However, the PL intensity of the sample also decreases due to the introduction of defects during the exfoliation process. When the contact force is increased to above 220 nN, structural damage becomes more apparent, ranging from slight and non-continuous flakes to complete exfoliation of the Gr layer, leaving the MoS2 surface exposed. The role of contact force is crucial in determining the extent of exfoliation and the resulting surface morphology. We also explored the effects of applying positive or negative bias voltage on a Pt tip and rubbing the Gr/MoS2 surface, which leads to physical and chemical changes in the heterostructure, known as the scanning probe lithography. This process can induce phase transformation of MoS2 from the 2H phase to the 1T phase or result in the formation of Mo-O bonds. In addition to the mechanical wear, an electrochemical reaction occurs in the water bridge between the sample and the Pt tip. The internal electric field generated can facilitate the separation of water molecules and induce the HER or OER. This leads to distortions in the MoS2 structure or the formation of oxygen bonds. After the SPL treatment, the ratio of the D and G Raman peak intensities (I(D)/I(G)) and the I(G)/I(2D) ratio can provide insights into the changes in the Gr structure, including vacancy concentration, structural continuity, and lattice strain. The PL properties of MoS2 exhibit semiconductor behavior alternation.
The unique characteristics and stable single-layered symmetry structures in the two-dimensional material system hold great potential for developing exciting physics. My research focuses on fabricating heterostructures involving 2D/2D or 2D/ferromagnetic material combinations and analyzing various measurements to investigate several critical factors. These factors include the diffusion barrier and intercalation phenomena, the influence of interfacial interactions, and the chemisorption and physisorption processes involving electron injection. The coverage of Gr provides protection to the CoPd layer against oxidation and interlayer diffusion, as discussed in Chapter 4. Without Gr, when exposed to the atmospheric environment for 64 days, the roughness of the surface increases, and the Kerr intensity percentage decreases. This suggests that the unprotected CoPd layer is susceptible to oxidation and Kerr intensity degradation over time. On the other hand, when Gr is present, the morphology and Kerr intensity remain stable, maintaining the CoPd layer's initial state. Gr acts as a protective barrier, preventing the diffusion of oxygen and other species that can cause oxidation and degradation of the CoPd layer. The high-temperature deposition method of CoPd on MoS2 results in a uniform and flat 2D layer, as observed in the AFM images. The CoPd layer's morphology significantly impacts the MAE, where the resulting different azimuth orientations of CoPd/MoS2 exhibit distinct MAE values. Kerr images and hysteresis loop measurements show that varying the Co and Pd percentage composition in the CoPd layer (e.g., Co 50% and 35%) leads to independent MAEs for different azimuthal orientations of MoS2. This indicates that the magnetic properties and preferred magnetization direction of MoS2 can be controlled by engineering the composition of the CoPd layer. The contact force AFM technique was used to remove PMMA residue, reduce the interlayer distance, and exfoliate the Gr layer. By applying a contact force of 110 nN, the roughness of the sample decreases, and wrinkles in the Gr layer can be observed. However, the PL intensity of the sample also decreases due to the introduction of defects during the exfoliation process. When the contact force is increased to above 220 nN, structural damage becomes more apparent, ranging from slight and non-continuous flakes to complete exfoliation of the Gr layer, leaving the MoS2 surface exposed. The role of contact force is crucial in determining the extent of exfoliation and the resulting surface morphology. We also explored the effects of applying positive or negative bias voltage on a Pt tip and rubbing the Gr/MoS2 surface, which leads to physical and chemical changes in the heterostructure, known as the scanning probe lithography. This process can induce phase transformation of MoS2 from the 2H phase to the 1T phase or result in the formation of Mo-O bonds. In addition to the mechanical wear, an electrochemical reaction occurs in the water bridge between the sample and the Pt tip. The internal electric field generated can facilitate the separation of water molecules and induce the HER or OER. This leads to distortions in the MoS2 structure or the formation of oxygen bonds. After the SPL treatment, the ratio of the D and G Raman peak intensities (I(D)/I(G)) and the I(G)/I(2D) ratio can provide insights into the changes in the Gr structure, including vacancy concentration, structural continuity, and lattice strain. The PL properties of MoS2 exhibit semiconductor behavior alternation.
Description
Keywords
二維材料, 鐵磁材料, 化學吸附, 物理吸附, 異質結構, 半導體行為, 接觸力, 掃描探針蝕刻, two-dimensional material, ferromagnetic material, chemisorption processes, physisorption processes, heterostructures, scanning probe lithography, semiconductor property, contact force