Plasma treatment can be applied to alter surface chemistry in materials through functional groups introduced by the plasma gas. This application note discusses the benefits of plasma treatment for controlling surface properties, plasma processing guidelines, and examples of the effect of surface chemistry and contact angle on plasma-treated materials.
With plasma treatment, surfaces can be modified by attachment or adsorption of functional groups to alter surface properties for specific applications. The functional groups introduced can be tailored depending on the process gas used and, in turn, the surface wettability may be altered to be hydrophilic [Figure 1] or hydrophobic [Figure 2] with the appropriate gas. Increased wettability prepares the surface for subsequent processing (e.g. film deposition or adsorption of molecules) by improving surface coverage and spreading of coatings and enhancing adhesion between two surfaces, while creating a more hydrophobic surface may be critical for self-cleaning or where water penetration is undesirable.
Surfaces can be plasma treated to alter surface chemistry without affecting the bulk properties of the material. As such, plasma treatment can be applied to a wide variety of materials as well as complex surface geometries. Below are examples applications and samples that have been treated in our plasma instruments:
Air or oxygen (O2) gas is typically used for plasma cleaning and surface activation. An air or O2 plasma removes organic contaminants by chemical reaction with highly reactive oxygen radicals and ablation by energetic oxygen ions. The plasma also promotes hydroxylation (OH groups) on the surface, rendering the surface more hydrophilic and increasing surface wettability.
Water vapor (H2O) can also be used to introduce hydroxyl groups and render surfaces more hydrophilic. Special gas delivery equipment and gas handling procedures would be required to use with the plasma system. For samples that are sensitive to moisture, H2O plasma would not be recommended.
Alternatively, an argon plasma may be preferred for surface activation to minimize further oxidation of surfaces (e.g. metals). Argon plasma cleans by ion bombardment and physical ablation of contaminants off the surface and can also increase surface hydrophilicity by reaction of the plasma activated surfaces upon exposure to ambient air.
Carbon tetrafluoride (CF4) plasma may be applied on surfaces to form a hydrophobic coating of fluorine-containing groups (CF, CF2, CF3). The fluorinated plasma decreases the number of hydrophilic polar end groups on surface and decreases surface wettability. Use of fluorinated gas requires replacing the standard pyrex chamber with a quartz chamber.
In addition, applications that are sensitive to potential contamination from trace impurities in borosilicate glass may also benefit from a quartz chamber substitution.
Below are suggested process conditions for plasma cleaning in a Harrick Plasma cleaner (some experimentation may be required to determine optimal process conditions):
Figure 1. Water droplet contact angle as a function of N2/O2 plasma treatment time, using a Harrick Plasma cleaner, on polyetheretherketone (PEEK). The PEEK surface is rendered hydrophilic after 20 seconds of plasma treatment. Data from Ha, S.-W., M. Kirch, F. Birchler, K.-L. Eckert, J. Mayer, E. Wintermantel, C. Sittig, I . Pfund-Klingenfuss, M. Textor, N. D. Spencer, M. Guecheva, H. Vonmont. "Surface Activation of Polyetheretherketone (PEEK) and Formation of Calcium Phosphate Coatings by Precipitation." J. Mater. Sci.- Mater. Med. (1997) 8: 683-690.
Figure 2. Water droplet contact angle as a function of O2 plasma treatment time, using a Harrick Plasma cleaner, on poly(tetrafluoroethylene) (PTFE), indicating increased hydrophobicity. Plasma treatment produces nanoscale roughness that increases hydrophobicity. Data from Lee, S.-J., B.-G. Paik, G.-B. Kim, Y.-G. Jang. "Self-Cleaning Features of Plasma-Treated Surfaces with Self-Assembled Monolayer Coating." Jpn. J. Appl. Phys. (2006) 45: 912-918.
Figure 3. Surface density of carboxyl (COOH) groups as a function of air plasma treatment time, using a Harrick Plasma cleaner, on 100 μm thick poly(caprolactone) (PCL) nanofiber mats. The COOH layer facilitates subsequent grafting of gelatin molecules onto the PCL nanofiber mats for potential use as tissue-engineering scaffolds. Data from Ma Z, He W, Yong T and Ramakrishna S. "Grafting of gelatin on electrospun poly(caprolactone) nanofibers to improve endothelial cell spreading and proliferation and to control cell orientation." Tissue Eng. (2005) 11: 1149-1158.