Plasma-Modified Polymer Surfaces - How to Identify their Structures

It can be difficult to characterize the chemical changes that result from plasma modification as they tend to only occur over several nm into a sample’s surface. This article explores how plasma modifications can be located and analyzed using XPS.

Introduction

New breakthroughs in flexible electronics are being made in response to global demand for electronic devices that are cheaper, lighter and more convenient to use. This demand, in addition to demand from the medical industry for low-cost, disposable measurement systems, means that the return on investment for developing improvements in these devices is very high.

Thermo Scientific ESCALAB Xi⁺

Whilst polymers like polystyrene (PS) are flexible, have a low dielectric strength (which is advantageous for electronics) and are well suited to mass production, they require surface tuning for use in electronic applications. Surface tuning is used to make PS more accepting towards material deposition and to alter its conducting properties. This surface modification must be able to take place without impacting or destroying the polymers bulk behavior.

Method

XPS provides unique, highly useful information that can be used to determine if surface tuning has been carried out successfully. XPS is a well-established method that measures the photoelectrons emitted from a surface following excitation with a soft x-ray. The technique is used to characterize the chemical and compositional states of a surface.

When working with small surface structures, i.e. those of a size less than 50 μm, Parallel Imaging XPS is preferred as it provides enhanced resolution.

Parallel Imaging XPS stores the positional information of any photoelectrons emitted from the surface being analyzed and then maps them onto a 2D detector following energy selection. This process is illustrated in Figure 1. Parallel Imaging XPS allows the simultaneous recording of the distribution and specific energy of photoelectrons on a surface.

Figure 1: Parallel imaging XPS

The following research concerns the collection of XPS image data of a patterned polystyrene surface, formed using plasma modification with oxygen in defined areas (via masking) using a Thermo Scientific™ ESCALAB™ Xi⁺.

Results

Both static (single kinetic energy) and quantified images (multiple discreet kinetic energies) from a 150 μm square area were recorded for the C1s and O1s regions. The results for the static imaging, which took around 3 minutes 20 seconds to record per experiment, showed that regions with higher oxygen (O1s) levels had a lower carbon (C-C) signal (Figure 2). This clear contrast shows that the polystyrene surface’s unmasked regions have been successfully and significantly modified by plasma treatment.

Figure 2: Static Images of Carbon and Oxygen regions, with overlaid comparison

Of key interest is determining if the masked section retains its native chemistry and the unmasked region is treated. The intensity data for each pixel in the quantified image can be taken to reconstruct spectra for the surface’s treated and untreated regions.

Two spectra taken from the reconstruction of the image’s highlighted areas are shown in Figure 3. The first shows an untreated region and contains a feature relating to a π-π* shake-up, something characteristic of pure PS. The second shows a treated region, which has no π-π* shake-up feature and includes additional oxygen bonding. This data indicates that the masked area has kept the same properties as pure PS, whereas the treated area has modified surface chemistry.

Figure 3: Reconstructing the C1s spectrum from specified areas in an image

Summary

Parallel Imaging XPS was used to characterize the chemical surface modifications of PS patterned using a plasma-masking technique. XPS allows researchers to determine the extent of modification of a surface and if the modification has been carried out as intended.

The combination of chemical and spatial information afforded by Parallel Imaging XPS makes it possible to establish just how successful production via plasma modification has been.

Acknowledgments

Thermo Scientific would like to thank the Rutgers University Laboratory for Surface Modification for providing the samples used to collect these data.

Produced from materials originally authored by Adam Bushell and Paul Mack, from Thermo Fisher Scientific, East Grinstead, West Sussex, UK.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).

For more information on this source, please visit Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).