Surface Chemistry Modification

Harrick Plasma → Surface Chemistry Modification

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.

plasma cleaners for surface modification, see the Surface Modification and Surface Wettability categories in our Technical Library.

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Surface Chemistry Modification

Benefits of Surface Chemistry Modification

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.

 

Figure 1. Water droplet contact angle measurements on 3 different borosilicate glass surfaces: (a) halocarbon wax-coated (92°), (b) untreated (32°), and (c) argon plasma-cleaned using a Harrick Plasma cleaner (<10°). Source: Sumner, A. L., E. J. Menke, Y. Dubowski, J. T. Newberg, R. M. Penner, J. C. Hemminger, L. M. Wingen, T. Brauers, B. J. Finlayson-Pitts. “The Nature of Water on Surfaces of Laboratory Systems and Implications for Heterogeneous Chemistry in the Troposphere.” Phys. Chem. Chem. Phys. (2004) 6: 604-613 – Reproduced by permission of The Royal Society of Chemistry (http://www.rsc.org/pccp).
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. 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.

Example Applications

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:

  • Render surfaces hydrophilic by oxidation and formation of hydroxyl (OH) groups
  • Render surfaces hydrophobic with deposition of fluorine-containing groups (CF, CF2, CF3)
  • Pattern alternating hydrophilic/hydrophobic regions on surfaces for self-assembly studies
  • Graft functional polymers or end groups onto plasma-activated surfaces [Figure 3]
  • Promote adhesion of cells and cell proliferation on plasma-modified biomaterials or tissue scaffolds
  • Deposit polymer layers by plasma polymerization

Processing Methods

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):

  • Pressure: 100 mTorr to 1 Torr
  • RF power: MEDIUM or HIGH
  • Process time: 1-3 minutes
  • Surfaces should be used immediately after plasma treatment; plasma-treated surfaces may recover their untreated surface characteristics with prolonged exposure to air

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