Impact of Pressure and Temperature on the Broadband Dielectric Response of the HKUST‑1 Metal–Organic Framework

Research on the broadband dielectric response of metal–organic frameworks (MOFs) is an emergent field that could yield exciting device applications, such as smart optoelectronics, terahertz sensors, high-speed telecommunications, and microelectronics. Hitherto, a detailed understanding of the physical mechanisms controlling the frequency-dependent dielectric and optical behavior of MOFs is lacking because a large number of studies have focused only on static dielectric constants. Herein, we employed high-resolution spectroscopic techniques in combination with periodic ab initio density functional theory (DFT) calculations to establish the different polarization processes for a porous copper-based MOF, termed HKUST-1. We used alternating current measurements to determine its dielectric response between 4 Hz and 1.5 MHz where orientational polarization is predominant, while synchrotron infrared (IR) reflectance was used to probe the far-IR, mid-IR, and near-IR dielectric response across the 1.2–150 THz range (ca. 40–5000 cm–1) where vibrational and optical polarizations are principal contributors to its dielectric permittivity. We demonstrate the role of pressure on the evolution of broadband dielectric response, where THz vibrations reveal distinct blue and red shifts of phonon modes from structural deformation of the copper paddle-wheel and the organic linker, respectively. We also investigated the effect of temperature on dielectric constants in the MHz region pertinent to microelectronics, to study temperature-dependent dielectric losses via dissipation in an alternating electric field. The DFT calculations offer insights into the physical mechanisms responsible for dielectric transitions observed in the experiments and enable us to explain the frequency shifts phenomenon detected under pressure. Together, the experiments and theory have enabled us to glimpse into the complex dielectric response and mechanisms underpinning a prototypical MOF subject to pressure, temperature, and vast frequencies.金属有机框架（metal–organic frameworks, MOFs）的宽带介电响应研究是新兴前沿领域，有望催生智能光电子、太赫兹传感器、高速通信及微电子等极具应用价值的器件。迄今为止，由于多数研究仅聚焦于静态介电常数，学界对调控MOFs频率依赖性介电与光学行为的物理机制仍缺乏系统深入的认知。本文中，我们结合高分辨率光谱技术与周期性从头算密度泛函理论（density functional theory, DFT）计算，对多孔铜基MOF——HKUST-1的各类极化过程展开解析。我们通过交流测量获取了4 Hz至1.5 MHz频段的介电响应，该频段以取向极化为主；同时利用同步辐射红外（infrared, IR）反射光谱，在1.2~150 THz（约40~5000 cm⁻¹）范围内探测了远红外、中红外及近红外波段的介电响应，该频段内振动极化与光学极化是介电常数的主要贡献来源。我们还揭示了压力对宽带介电响应演化的调控作用：太赫兹振动光谱显示，铜桨轮结构与有机配体的结构形变分别导致声子模式出现显著的蓝移与红移。此外，针对微电子领域相关的MHz频段，我们研究了温度对介电常数的影响，并通过交变电场中的耗散过程分析了介电损耗的温度依赖性。密度泛函理论计算为实验中观测到的介电跃迁物理机制提供了理论阐释，并帮助我们解释了压力下观测到的频率位移现象。综上，本研究结合实验与理论手段，深入剖析了受压力、温度及宽频率范围调控的典型MOF的复杂介电响应与内在机制。