Applications of OES and Plasma Diagnostics

Plasma is an ionized gas that has been put under pressure and subjected to electromagnetic fields or intense heating to the point that positive ions and electrons are unbound.

Image Credit: Romaset/Shutterstock.com

Plasma is one of the four fundamental states of matter. It is produced by applying heat and pressure as it does not exist on Earth naturally. Plasma is unique from other states of matter because of its behavior. The atoms in a plasma are move a lot quicker than than in a gas, creating an electric current within a magnetic field. is The charge is highly conductive although the overall charge of a plasma is usually neutral.

Plasma is said to be the main state of matter throughout the universe, even though they are rare at normal Earth conditions. For example, neon lighting is only partially ionized, while the sun and stars are examples of fully ionized plasma.

Plasmas are used for many applications in spectroscopy. Thin film deposition and photo-resist etching for semiconductors and solar collectors are the most commonly used, but plasmas also have applications in aerospace and biomedical, amongst other industries.

Avantes is known for its high-speed data capture and high resolution spectra that Plasma diagnostics need. Our devices can be found in plasma research and industrial environments all over the world.

Plasma Deposition Thin Films and Coatings

A thin film is one or more layers, usually nanometers to microns thick, which are deposited on a substrate material such as a silicon wafer. Thin Films and coatings are crucial supporting technology within the semiconductor and solar energy production industries.

Magnetron sputtering is a typical method of physical vapor deposition. It uses one of the noble gases such as Argon which is excited to create a plasma, this is in turn used to knock electrons from a cathode source to be deposited on the coated surface.

A certain class of plasmas called nonthermal plasmas, or atmospheric pressure plasmas in which the plasma is created at normal atmospheric pressure with the application of electromagnetic radiation are interesting. In this instance, because the gas is not under pressure the super energized (“heated”) electrons do not collide with “cold” non-energized ions or neutral particles enough to attain a thermal equilibrium. This technique of plasma creation has lower initial cost as well as other advantages such as enabling adjustment of the plasma jet dimensions so it does not need a vacuum chamber.

Surface modification of polypropylene thin films using an atmospheric pressure plasma jet generated in an Argon gas have been explored by researchers from the Russian Academy of Science and Ghent University, Belgium. The results of the polymer surface treatment were determined by the concentration of OH and H radicals generated in the discharge and measured using the AvaSpec-ULS3648 spectrometer with a resolution of 0.05 nm  in the 300-350 nm region.

It is well reported that UV radiation, specifically from 306 to 315 nm, is a chief generation mechanism for hydroxyl radicals like OH and H. These researchers introduced a 0.05% water vapor into the plasma jet which made the UV radiation more intense. This correlated to incorporation of oxygen into the polypropylene surface being much more evident.

The same scientists from the University of Ghent, Belgium, Changsha University, China and the Russian Academy of Science lead another joint research project. They investigated the use of Nitrogen and ordinary air (78% N₂/20% O₂) as a plasma medium. They found that their method eliminates the demand for precursor gases such as Argon or Helium which are more expensive.

Researchers undertook optical emission spectroscopy to characterize the plasma discharges and ascertain vibrational and rotational distribution functions of electronically excited states of molecules throughout the plasma regions by employing a multichannel spectrometer system from Avantes. This spectrometer system possesses the AvaSpec-ULS3648 high resolution spectrometer measuring from 300-350 with a resolution of 0.05 nm, alongside the AvaSpec-ULS2048 StarLine spectrometer measuring a broadband spectral range from 250-800 nm with a resolution of 0.5 nm. They calculated gas temperature in the active plasma zone by analyzing the rotational structure of a nitrogen molecule as shown by the nitrogen molecular band emitted at λ 337.1 nm.

This study could uphold the direct current plasma jet for each gas constituent in two distinct modes, at low average current (< 5 mA) with the plasma self-pulsing with oscillations of current and voltage, and at high average current (> 10 mA) in which the plasma acts as a glow discharge plasma.

Plasma Diagnostics are Essential to the Space Race

Engineers need to develop space craft capable of planet entry in varied atmospheric and gravitational conditions for space exploration in the future, and principally any prospective manned missions to Mars.

Gases around the space craft are super-heated and under extreme pressure during planetary entry, routinely generating plasma conditions. Thermal shielding against these conditions is required for any craft with entry capability. Developing a thermal protection system (TPS) starts with assessing design solutions using computer modeling, and finally in real world experiments in a plasma wind tunnel.

Several high-enthalpy plasma wind-tunnels (PWK1, PWK2, and PWK3, etc.) can be found at the Institute of Space Systems at the University of Stuttgart. These can simulate the atmospheric entry environment of a Venus or Mars entry. These particular ground-based simulations contribute hugely to how we understand the aerodynamics of spaceflight. They do this by producing a continuous stream of carefully controlled plasma of high specific enthalpy and velocity by employing a thermo- or magneto- plasma-dynamic generator (TPG or MPG). The PWK3 can also generate plasma tests in a variation of gasses, like high CO₂ which simulates Mars or Venus’ atmosphere, simplified conditions can be imitated to explore the reactions and effects in particular test cases of a single gas species. Furthermore, the test data that is gathered can be fed back into computational models to validate accuracy and modeling results.

The AvaSpec-ULS3068 and new AvaSpec-ULS4096CL-EVO  are our highest recommended spectrometers allowing resolution down to 0.05 nm. Optical emission spectroscopy is the perfect non-invasive measurement technique for plasma diagnostics, preventing contamination of the test conditions but requiring high resolution instrumentation in the hostile environment of a plasma reaction.

Plasma in Medicine

The treatment of chronic wound infections is a practical use for nonthermal atmospheric plasmas in medicine. suitability looked at several gas species for their effects on short term bacterial density, long-term inhibition of bacterial growth and safety for use on human tissues was studied by researchers from the Institute of Pathology at the Technical University of Munich and the Max-Planck Institute for Extraterrestrial Physics. The AvaSpec-ULS2048was used by this team to characterize the plasma emissions and give plasma diagnostic data.

This work examined argon plasma which emits UV radiation to issue a sterilizing effect which is short-term, but human tissue can be adversely affected. On the other hand, plasma generated reactive nitrogen or oxygen species demonstrate a longer-term after effect of impeding bacteria growth, a principle factor in preventing recolonization of the wound  after treatment, and with no negative effects on human cells.

The Future of Plasma Research

Research in this field will continue to lead to advancements in industry and the sciences across many fields because of the unique properties of plasma, and the many practical applications. Avantes is prepared to reinforce applications such as these with our AvaSpec instrument line and fiber optic assemblies.  

The EVO series ideally suited to support your plasma application. Discover the next EVOlution in spectrometers, the Avantes EVO Series AS7010 electronics feature faster communication via USB3.0 or native gigabit ethernet, precise triggering, and superior signal to noise.

References and Further Reading

1. Deng, X. L., et al. "Direct current plasma jet at atmospheric pressure operating in nitrogen and air." Journal of Applied Physics 113.2 (2013): 023305.
2. Endlich, Pia, et al. "The inductively heated plasma wind tunnel PWK3 as a means for emission experiments to rebuild radiation test cases." Radiation of High Temperature Gases in Atmospheric Entry. Vol. 533. 2003.
3. Nosenko, T., T. Shimizu, and G. E. Morfill. "Designing plasmas for chronic wound disinfection." New Journal of Physics 11.11 (2009): 115013.
4. Sarani, Abdollah, et al. "Surface modification of polypropylene with an atmospheric pressure plasma jet sustained in argon and an argon/water vapour mixture." Applied Surface Science 257.20 (2011): 8737-8741.

This information has been sourced, reviewed and adapted from materials provided by Avantes BV.

For more information on this source, please visit Avantes BV.