According to the World Bank’s World Development Report (WDR) 2021 on Internet Data Flow: „By 2022, traffic is expected to reach 150,000 GB of traffic per second, a 1,000-fold increase compared to the 156 GB in 2002, 20 years earlier. Ten years before that, in 1992, global internet traffic was 100 GB per day, which roughly equates to just 10 households binge watching a Netflix series for 10 hours.”  These values are growing rapidly every year. A significant part of this transfer is generated by mobile devices, which is not surprising in light of changes related to the style of work (remote work) and entertainment (streaming movies and online games). Also, the development of augmented and virtual reality and the Internet of Things (IoT) pose new challenges to telecommunications service providers. Already, 4G telecommunication networks (0.6–2.5 GHz channels) in large agglomerations are approaching their upper capacity limits during peak hours, forcing the transition to the higher standard of 5G networks (30–300 GHz channels). As we plan to leap to the next generation of 6G (300–3000 GHz or 0.3–3 THz), offering up to 125 GB/s per user, we must make great strides in developing suitable materials with absorption or emission in the sub-THz range (300–1000 GHz). In addition, it is required not only to develop them in practice but also to understand how to modify them and how external factors (e.g., temperature, pressure, humidity) affect their operating parameters.
Development of cutting-edge materials
In response to this demand, the team led by Professor Shin-ichi Ohkoshi from the University of Tokyo in Japan has launched research that may bring us closer to the practical implementation of these ambitious goals. In the last decade, Prof. Ohkoshi and members of the Solid State Physical Chemistry Laboratory have presented a series of absorption materials based on metal-substituted epsilon-Fe2O3 nanoparticles with absorption in the 20–230 GHz range, suitable for 5G technology. [2–4] However, further development of these materials to achieve absorption in the higher frequency range was impossible due to the physical limitations of these oxides.
We are currently conducting systematic research on the detection and determination of absorption characteristics in the sub-THz range for alternative materials. The original idea was based on previous research on the non-contact detection of cesium radioactive ions using terahertz time-domain spectroscopy (THz-TDS), which showed an absorption of around 1.4 THz.  As previously established, heavy metal ions trapped inside the crystal lattice of the coordination polymer, with properly selected structural parameters, can be an excellent platform for the construction of sub-THz absorbers. Despite obtaining materials with characteristics in the range of 0.7–1.4 THz, studies [6, 7] have shown that the development of such systems is complicated due to the difficulties in designing and synthesizing samples with very specific structural parameters. Using theoretical calculations and crystal engineering of coordination compounds, new directions of research were selected and implemented. Particular attention was paid to isolated complexes of heavy lanthanide ions with thiocyanates and selenocyanates. This qualitative change allowed the absorption to shift to the range of 0.59–0.68 THz, although it required the use of expensive chemical reagents. 
Results of the research
In the latest article published in the journal Angewandte Chemie International Edition, we presented a series of new materials based on simple to synthesize and cheap coordination polymers containing complexes of iron(II), tetra(thiocyanates)mercury(II) and tetra(selenocyanates)mercury(II) showing absorption in in the range of 0.63–0.96 THz.  In addition, we proposed a prototype scheme of an absorber based on these chemical compounds. We are currently working to eliminate the toxic component of mercury from our material.
Further development of 6G terahertz communication materials seems obvious, but their practical commercialization is still far away. First, we need to face many engineering, economic, and environmental challenges. Despite this, we can anticipate that this type of system will be used in our mobile devices in the coming years.
- World Bank “Crossing borders” World Development Report 2021: Data for Better Lives, The World Bank Group, 2021, https://wdr2021.worldbank.org/stories/crossing-borders/.
- S. Ohkoshi, S. Kuroki, S. Sakurai, K. Matsumoto, K. Sato, S. Sasaki „A Millimeter-Wave Absorber Based on Gallium-Substituted ε-Iron Oxide Nanomagnets” Angewandte Chemie International Edition, 2007, 46, 8392–8395. https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.200703010
- A. Namai, S. Sakurai, M. Nakajima, T. Suemoto, K. Matsumoto, M. Goto, S. Sasaki, S. Ohkoshi „Synthesis of an Electromagnetic Wave Absorber for High-Speed Wireless Communication” Journal of the American Chemical Society, 2009, 131, 1170–1173. https://pubs.acs.org/doi/10.1021/ja807943v
- A. Namai, M. Yoshikiyo, K. Yamada, S. Sakurai, T. Goto, T. Yoshida, T. Miyazaki, M. Nakajima, T. Suemoto, H. Tokoro, S. Ohkoshi „Hard magnetic ferrite with a gigantic coercivity and high frequency millimetre wave rotation” Nature Communications, 2012, 3, 1035. https://www.nature.com/articles/ncomms2038
- S. Ohkoshi, M. Yoshikiyo, A. Namai, K. Nakagawa, K. Chiba, R. Fujiwara, H. Tokoro „Cesium ion detection by terahertz light” Scientific Reports, 2017, 7, 8088. https://www.nature.com/articles/s41598-017-08551-4
- T. Yoshida, K. Nakabayashi, H. Tokoro, M. Yoshikiyo, A. Namai, K. Imoto, K. Chiba, S. Ohkoshi „Extremely low-frequency phonon material and its temperature- and photo-induced switching effects” Chemical Science, 2020, 11, 8989–8998. https://pubs.rsc.org/en/content/articlelanding/2020/sc/d0sc02605k
- S. Ohkoshi, K. Shiraishi, K. Nakagawa, Y. Ikeda, O. Stefanczyk, H. Tokoro, A. Namai „Reversible photoswitchable ferromagnetic thin film based on a cyanido-bridged RbCuMo complex” Journal of Materials Chemistry C, 2021, 9, 3081–3087. https://pubs.rsc.org/en/content/articlelanding/2021/tc/d1tc00583a
- K. Kumar, O. Stefanczyk, K. Nakabayashi, Y. Mineo, S. Ohkoshi „Development of Nd (III)-Based Terahertz Absorbers Revealing Temperature Dependent Near-Infrared Luminescence” International Journal of Molecular Sciences, 2022, 23, 6051. https://www.mdpi.com/1422-0067/23/11/6051
- G. Li, O. Stefanczyk, K. Kumar, Y. Mineo, K. Nakabayashi, S. Ohkoshi „Low-Frequency Sub-Terahertz Absorption in HgII−XCN−FeII (X=S, Se) Coordination Polymers” Angewandte Chemie International Edition, 2023, 62, e202214673. https://onlinelibrary.wiley.com/doi/10.1002/anie.202214673