鐵基二維材料的空位輔助剝離及二維約束下氯化鐵的化學反應

Abstract

在不斷發展的材料科學領域中，二維（2D）材料，如石墨烯、矽烯和六方氮化硼（h-BN），因其強大的面內鍵結和層間弱凡德瓦相互作用而受到廣泛關注。嵌入反應不僅是製備單層、亞單層和多層2D材料的關鍵方法，還為奈米反應器的設計提供了新的見解和理論基礎。 傳統的剝離方法利用鍵結或組成特性的異向性，來剝離具有大橫向尺寸和原子厚度的二維材料。然而，這種方法限制了對具有特定組成的層狀主體晶體的選擇。在這裡，我們展示了沿著有序空位的平面剝離晶體，作為達到先前難以實現的二維晶體結構的新途徑。磁黃鐵礦(Pyrrhotite)是一種非化學計量的硫化鐵，由於其複雜的空位超結構而被用作原型系統。通過氣體輔助的塊材轉換，合成了磁黃鐵礦晶體塊材，其繞射圖顯示晶胞內帶有3個空位界面的4C超結構。電化學嵌入和隨後的剝離產生具有大橫向範圍的超薄二維薄片。原子力顯微鏡證實，剝離發生在所有三個超晶胞界面，形成了亞晶胞厚度為1/2和1/4單層的二維結構分離。研究了將二維材料的形貌，控制在單層極限以下對二維磁性質的影響。磁黃鐵礦塊材表現出亞鐵磁有序性，其與理論預測一致，並在剝離後仍具有亞鐵磁有序性。複雜的磁疇結構和空位界面對磁化強度的增強影響，強調了我們的合成方法在未來電子學和自旋電子學中調節磁性質的潛力。 另一方面，蝕刻技術在二維材料圖案化中起著關鍵作用。從傳統的濕蝕刻發展出來，先進的方法如反應離子蝕刻已被采用，以精確控制層間、層內和材料圖案輪廓。乾式和濕式蝕刻之間的選擇考慮了選擇性和環境安全等因素，迫切需要開發安全且高選擇性的蝕刻方法。本研究報導了透過石墨烯與二氧化矽之間的凡德瓦壓力實現的選擇性蝕刻。通過傳統的封管嵌入法，氯化鐵(FeCl3)粉末吸濕後與轉印在矽基板上的氟化石墨烯樣品真空封管進入安剖瓶中。經升溫退火後，使用原子力顯微鏡(AFM)確認石墨烯下的SiO2層實現了選擇性蝕刻。拉曼光譜及高解析氣相層析質譜儀(HRGCMS)證實了蝕刻後的氟化石墨烯仍然存在，並檢測到蝕刻反應產物Iron oxychloride(FeOCl)、氫氯酸(HCl)和蝕刻反應衍生物Si-O環狀物及Si-OH化合物的訊號。QuantumATK-Atomistic Simulation Software的計算顯示，在蝕刻實驗的退火溫度下，凡德瓦壓力下的自發性大於僅考慮HCl分壓的情況，證實了氟化石墨烯選擇性蝕刻源於凡德瓦力。通過調整FeCl3粉末的含水量和在固定含水量下改變退火時間，AFM確認到對於不同層數間的蝕刻深度差異的漸變過程，佐證了凡德瓦力與石墨烯層數呈正相關的相關理論。最後，通過在石墨烯生長過程中形成的皺紋，演示了共形石墨烯裝飾的奈米尺度通道陣列，同時確認了人工轉印多層石墨烯和自然生長多層石墨烯都具有選擇性蝕刻的能力。這些演示不僅顯示了這項技術是一種安全且高選擇性的蝕刻方法，同時也為二維材料下的奈米反應器提供了最佳實踐。

In the continually advancing field of materials science, two-dimensional (2D) materials, such as graphene, silicene, and hexagonal boron nitride (h-BN), have garnered widespread attention due to their robust in-plane bonding and weak van der Waals interactions between layers. Intercalation reactions not only serve as crucial methods for preparing single-layer, sub-monolayer, and multilayer 2D materials but also offer novel insights and theoretical foundations for the design of nanoreactors Conventional exfoliation exploits the anisotropy in bonding or compositional character to delaminate 2D materials with large lateral size and atomic thickness. This approach, however, limits the choice to layered host crystals with a specific composition. Here, we demonstrate the exfoliation of a crystal along planes of ordered vacancies as a novel route toward previously unattainable 2D crystal structures. Pyrrhotite, a non-stoichiometric iron sulfide, was utilized as a prototype system due to its complex vacancy superstructure. Bulk pyrrhotite crystals were synthesized by gas-assisted bulk conversion, and its diffraction pattern revealed a 4C superstructure with 3 vacancy interfaces within the unit cell. Electrochemical intercalation and subsequent delamination yield ultrathin 2D flakes with a large lateral extent. Atomic force microscopy confirms that exfoliation occurs at all three supercell interfaces, resulting in the isolation of 2D structures with sub-unit cell thicknesses of ½ and ¼ monolayers. The impact of controlling the morphology of 2D materials below the monolayer limit on 2D magnetic properties was investigated. Bulk pyrrhotite was shown to exhibit ferrimagnetic ordering that agrees with theoretical predictions and that is retained after exfoliation. A complex magnetic domain structure and an enhanced impact of vacancy planes on magnetization emphasize the potential of our synthesis approach as a powerful platform for modulating magnetic properties in future electronics and spintronics. On the other hand, etching technology plays a pivotal role in the imaging of 2D materials. Evolving from traditional wet etching, advanced methods like reactive ion etching have been adopted for precise control over interlayer, intralayer, and material pattern contours. The choice between dry and wet etching considers factors such as selectivity and environmental safety, prompting the urgent development of safe and highly selective etching methods. This study reports on the achievement of selective etching through van der Waals pressure between graphene and silicon dioxide. Using the conventional encapsulation method, FeCl3 powder, after absorbing moisture, was vacuum-sealed with fluorinated graphene samples transferred onto silicon substrates and placed in an autopsy bottle. After annealing, atomic force microscopy (AFM) confirmed the selective etching of the SiO2 layer beneath graphene. Raman spectroscopy and high-resolution gas chromatography-mass spectrometry (HRGCMS) verified the persistence of fluorinated graphene post-etching, detecting etching byproducts such as Iron oxychloride (FeOCl), hydrochloric acid (HCl), Si-O cyclic compounds, and Si-OH compounds. Calculations using QuantumATK-Atomistic Simulation Software demonstrated that, at the annealing temperature of the etching experiment, the spontaneous effect under van der Waals pressure exceeded that under consideration of only HCl partial pressure, confirming the origin of selective graphene etching in van der Waals forces. By adjusting the moisture content of FeCl3 powder and varying annealing times under fixed moisture content, AFM confirmed a gradient process of etching depth differences between different layers, corroborating the positive correlation between van der Waals forces and graphene layers as per theoretical expectations. Lastly, through the formation of wrinkles during graphene growth, a demonstration of nanoscale channel arrays decorated with conformal graphene was showcased. It was confirmed that both artificially transferred and naturally grown multilayer graphene possessed the capability for selective etching. These demonstrations not only highlight the safety and high selectivity of this technique but also provide optimal practices for nanoreactors in the realm of 2D materials.