M-site dependent terahertz intrinsic absorption in MXenes

Abstract

Ultrathin terahertz (THz) absorbing films are critical as building blocks for THz devices and systems. Although few-layer Ti₃C₂T_x MXene assemblies have approached the terahertz (THz) intrinsic absorption limit, it remains important to explore the THz intrinsic absorbing properties of other MXenes, which may elucidate the mechanism of THz-matter interactions for the future guidance of material design. In this study, eight representative MXenes with different M-sites were systematically analyzed. Surprisingly, the Ti₂CT_x thin film with direct current (DC) conductivity 26 times lower than that of the Ti₃C₂T_x film possessed similar high THz absorbing properties. Due to the significantly lower electron concentration of Ti₂CT_x compared to that of Ti₃C₂T_x, we concluded that the exceptional THz intrinsic absorption of Ti₂CT_x stemmed from its high terahertz electron mobility (μ_THz), which was attributed to its low electron effective mass (m*). Because the THz intrinsic absorption was determined by THz conductivity, which was proportional to the ratio of electron density (n) to electron effective mass (m*), we proposed that optimizing n/m* was crucial for achieving high THz intrinsic absorption in MXenes. This study not only explored the underlying THz-matter interaction mechanism in MXenes but also provided guidance for designing high THz absorption materials.