The absorption properties of ZrO₂ nanoparticles in the THz and sub-THz frequency ranges

As terahertz (THz) and sub-THz region electromagnetic waves are becoming vital for industrial applications such as 5G wireless communication, so too are THz and sub-THz wave absorbing materials. Herein, we report the optical properties of monoclinic zirconia (m-ZrO₂) nanoparticles in these frequency regions, with different crystalline sizes. The crystalline sizes of the three samples, measured by transmission electron microscopy, are 93 ± 23 nm (denoted 1), 28 ± 14 nm (denoted 2) and 2.6 ± 0.7 nm (denoted 3). X-ray diffraction and Raman spectra show that 1 and 2 have high crystallinity whereas 3 shows peak broadening due to its small crystalline size. Terahertz time-domain spectroscopy (THz-TDS) measurements of pelletised samples show that the small crystalline size sample exhibits larger absorption, e.g., the absorbance value at 300 GHz is 0.18 mm^-1 (1), 0.04 mm^-1 (2) and 1.11 mm^-1 (3), and the related dielectric loss value (ε'') is 0.04 (1), 0.01 (2) and 0.82 (3), respectively. This is considered to be due to the proportional increase in surface water molecules for the small particle size sample due to the relative increase in surface area and under-coordinated atoms, shown by IR spectra. These results show that small crystalline size m-ZrO₂ nanoparticles have potential as THz and sub-THz wave absorbing materials, which are crucial for noise reduction in THz and sub-THz wave technologies.