電場調控二維共價有機框架材料之磁性相變之理論計算研究

Abstract

本論文聚焦於二維共價有機框架（2D-COFs）的磁性相變行為。三角烯([3]Triangulene）的高自旋電子基態源於其子晶格之間的不對稱分佈（bipartite imbalance），此現象可由 Lieb 定理所預測。其衍生物，例如具有S = 1/2 自旋態的異原子三角烯（heterotriangulenes），被視為人造原子，並被應用於構築具有磁性的二維共價有機框架結構。二維共價有機框架是一種由有機分子構建單元所組成的週期性網狀結構，其電子能帶結構高度受到其構成單元前沿分子軌域（FMOs）對稱性的主導。本研究中，我們利用第一原理密度泛函理論（DFT）模擬，探討具有混合蜂巢籠目晶格（CHK lattice）之二維共價有機框架的磁性特性。該混合蜂巢-籠目晶格由具有三重對稱性的異原子三角烯單元構成蜂巢（Honeycomb）晶格點，並以具有雙重對稱性的奈米石墨烯片段作為連接單元填入籠目（Kagome）晶格點。透過在連接單元上引入推電子與拉電子官能基，使其分子平面性可受外加電場調控，進而操控整體系統的磁性與自旋色散行為。這些二維共價有機框架可視為磁性可調的有機分子機械，具備多元應用潛力，並為自旋電子元件的設計原則提供了嶄新見解。This thesis focuses on the magnetic phase transition of two-dimensional covalent organic frameworks (2D-COFs). The high-spin electronic ground state of [3]triangulene originates from bipartite imbalance of the sublattices, as predicted by Lieb’s theorem. Its derivatives, such as heterotriangulenes with S = 1/2 spin states, are regarded as artificial atoms and have been utilized to construct magnetic 2D-COFs. A 2D-COF is a periodic network composed of organic molecular building blocks, whose electronic band structure is strongly governed by the symmetry of the frontier molecular orbitals (FMOs) of its constituent units. In this work, we investigate the magnetic properties of 2D-COFs with a combined Honeycomb-Kagome (CHK) lattice using first-principles DFT simulations. The CHK lattice is constructed from 3-fold symmetric heterotriangulenes at the Honeycomb lattice points and 2-fold symmetric nanographene linkers at the Kagome lattice points. By functionalizing the linker units with electron-withdrawing and electron-donating groups, their planarity can be modulated by an external electric field, thereby maneuvering the magnetism and the spin dispersions of the system. These 2D-COFs act as magnetically controllable organic molecular machines with versatile applications, offering new insights into the design principles of spintronic devices.