Research: Terahertz Waves Influence Nanoconfined Water Dynamics

Abstract

Nanoconfined waters exhibit low static permittivity mainly due to interfacial effects that span about one nanometer. The characteristic length scale may be much longer in the terahertz (THz) regime where long-range collective dynamics occur; however, the THz dynamics have been largely unexplored because of the lack of a robust platform. Here, we use metallic loop nanogaps to sharply enhance light-matter interactions and precisely measure real and imaginary THz refractive indices of nanoconfined water at gap widths ranging from 2 to 20 nanometers, spanning mostly interfacial waters all the way to quasi-bulk waters. We find that, in addition to the well-known interfacial effect, the confinement effect also contributes substantially to the decrease in the complex refractive indices of the nanoconfined water by cutting off low-energy vibrational modes, even at gap widths as large as 10 nanometers. Our findings provide valuable insights into the collective dynamics of water molecules which is crucial to understanding water-mediated processes.

Professor Hyeong-Ryeol Park and his research team in the Department of Physics at UNIST, in collaboration with researchers from Kangwon National University, Chungbuk National University, and Seoul National University, has made a groundbreaking discovery regarding the behavior of water molecules confined within nanostructures. The study, published in the online version of Science Advances on April 24, sheds light on how terahertz waves can influence the dynamics of water molecules trapped in two-dimensional spaces within nanoresonators.

This study has been participated by Professor Jeeyoon Jeong from Kangwon National University, along with a multidisciplinary group of scientists, including Professors Dai-Sik Kim, Noejung Park, and Joonwoo Jeong from UNIST, Professor Kyungwan Kim from Chungbuk National University, and Professor Yun Daniel Park from Seoul National University. In this study, the research team employed innovative techniques to investigate the collective dynamics of water molecules at the nanoscale level.

Using metallic loop nanogaps to enhance light-matter interactions, the team was able to measure the real and imaginary refractive indices of nanoconfined water across varying gap widths, ranging from 2 to 20 nanometers. This comprehensive analysis offered insights into the interplay between interfacial effects and confinement effects on the complex refractive indices of nanoconfined water, highlighting the suppression of low-energy vibrational modes even at larger gap widths.

Lead author Hyosim Yang from UNIST emphasized the significance of the study, stating, "While previous research focused on low-frequency electrical measurements of water properties, our study delves into the dynamics of water molecules confined in narrow gaps at high terahertz frequencies, revealing novel phenomena."

The team's innovative use of atomic layer lithography technology enabled the fabrication of nanoresonators with gap widths as narrow as one nanometer, significantly enhancing the sensitivity of molecular motion measurements. The experimental findings not only confirmed the suppression of picosecond dynamics of water molecules by interfacial effects in sub-2-nanometer gaps but also unveiled the reduction of clustering motion at 10-nanometer gaps, further suppressing dynamics.

Co-author Gangseon Ji from UNIST highlighted the implications of the research, stating, "This study uncovers the dual effects of interfacial and confinement mechanisms on water dynamics in nanoconfined spaces, offering new perspectives on solid-like behaviors exhibited by confined water molecules."

Professor Park underscored the broader implications of the study, noting its potential applications in investigating superionic phases of two-dimensional water molecules and studying molecular dynamics in solvents like DNA and RNA. The research opens avenues for extending these investigations to the mid-infrared and visible light regions by adjusting the size of nano resonators.

The study marks a significant advancement in understanding the collective dynamics of water molecules at the nanoscale, paving the way for future explorations in related fields. This work was supported by the National Research Foundation (NRF) of Korea, the Ministry of Science and ICT (MSIT), the Institute for Information & Communication Technology Planning & Evaluation (IITP), UNIST, and the Gangwon Technopark (GWTP).

Journal Reference

HyoSim Yang, GangSeon Ji, Min Choi, et al., "Suppressed terahertz dynamics of water confined in nanometer gaps," Science Advances, (2024).