Tokyo, Japan – Scientists from Tokyo Metropolitan University have found a way to make plasma more effective for its wide-ranging uses, from anti-microbial applications to surface conditioning in the semiconductor industry. They mapped the production of atomic oxygen, a key ingredient of oxygen plasma, while a high voltage was put across oxygen gas. They discovered that the amount of atomic oxygen available for reactions was boosted at pressures moderately lower than atmospheric conditions. Their findings promise more energy-efficient, higher performance plasma technologies.
Plasma is often called the fourth state of matter alongside the gas, liquid, and solid states. Made of charged atoms and electrons, some estimates say that plasma makes up 99.9% of all matter in the universe. While rarely encountered on Earth except during lightning storms, artificial plasmas can be generated by applying high electrical voltages across gases, making so-called nonthermal plasmas. This is where only the electrons in a material are driven to a high temperature, leaving the atoms in a highly reactive "radical" state.
Nonthermal plasmas are widely used to kill microorganisms, chemically modify materials, and break down contaminants; ways to make them more effective would have a major impact on industry and society. Oxygen plasma is particularly popular for the powerful oxidizing ability of atomic oxygen, the key radical species generated when oxygen gas is broken down. While widely used, a key challenge has been the short lifetime of the plasma at atmospheric pressure; frequent collisions with gas molecules mean that less oxygen radicals react with the intended target.
A team led by Associate Professor Yusuke Nakagawa from Tokyo Metropolitan University has shown how reducing pressure helps make oxygen plasma treatments more effective. They studied pulsed electrical discharges across oxygen gas at slightly lower pressures than atmospheric conditions. Using lasers to make atomic oxygen fluoresce at a specific wavelength of light, they could map how much atomic oxygen is made at different locations around the positive and negative electrodes either side of the gap. While the radical lifetime was extended by the reduced pressure, the team discovered that the amount of radicals remained at a level comparable to that at atmospheric pressure, contrary to expectations. Further analysis showed that this led to several times more radicals available for reactions with a target. Their observations also showed that atomic oxygen was not produced exclusively in parts of the plasma that exhibited a visible glow, but in dark regions close to the electrodes. This means that electrons with moderate energy are also involved in radical production, helping to explain why more radicals are available at moderately lower pressure as well as illuminating a new way by which radicals are made.
The team's findings highlight a practical way to make oxygen plasma treatments more effective, a core innovation for industries ranging from biomedical sectors and agriculture to semiconductor manufacture.
This work was supported by JSPS KAKENHI Grant Numbers JP23K22746, JP24H02248, and JP25K01220.
