Structural Analysis and Electronic Properties of Negatively Charged TCNQ: 2D Networks of (TCNQ)2Mn Assembled on Cu(100)

A compound two-dimensional monolayer mixing Mn atoms and 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules was synthesized by supramolecular assembly on a Cu(100) surface under ultrahigh-vacuum conditions. The interactions in the Mn(TCNQ)2 network and in the full system are analyzed from a molecular orbital perspective and in the light of scanning tunneling microscopy (STM) imaging and simulations. Structural, electronic, and magnetic properties are studied in detail using density functional theory (DFT) calculations. In the absence of Cu and depending on the theoretical method used, the TCNQ species can be formally described as either dianions TCNQ2− interacting with Mn4+ cations according to ligand field theory (using GGA calculations) or, alternatively, as radical monoanions interacting with Mn2+ cations in a high spin state (using GGA+U calculations). In the complete system including a Cu substrate, whatever theoretical method is used, TCNQs appear as dianions interacting with both Mn2+ cations in a high spin state (d5) and charged copper surface atoms.

一种由 Mn 原子与 7,7,8,8-四氰基醌二亚甲基 (TCNQ) 分子组成的二维单层化合物混合物，通过在超高压真空中 Cu(100) 表面进行超分子组装而合成。从分子轨道的角度，结合扫描隧道显微镜 (STM) 成像和模拟，对 Mn(TCNQ)2 网络及整个系统的相互作用进行了分析。利用密度泛函理论 (DFT) 计算对结构、电子和磁性质进行了深入研究。在缺乏 Cu 的情况下，根据所用的理论方法，TCNQ 物种可以形式上描述为根据配位场理论（使用 GGA 计算）与 Mn4+ 阳离子相互作用的二负离子 TCNQ2−，或者，作为选择，描述为与 Mn2+ 阳离子在高自旋态（使用 GGA+U 计算）中相互作用的自由基单负离子。在包括 Cu 衬底在内的完整系统中，无论使用何种理论方法，TCNQs 均表现为与高自旋态（d5）的 Mn2+ 阳离子以及带电铜表面原子相互作用的二负离子。