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.