David Surman

Shimadzu/Kratos Analytical Inc., Chestnut Ridge, NY

Abstract

The evolution and application of nanomaterials has increased rapidly over the last few years and with this increase has come a need to analyze and study their surface properties. X-ray Photoelectron Spectroscopy is an ideal tool as it provides both elemental and chemical information simultaneously as well as information on their spatial distribution.

During the last 5 years developments in both the technology used and application methods have opened up new capabilities for the analysis of surfaces with nanometer resolution. With regard to technology developments these have arisen from new detector systems and improved x-ray sources that have generated improved sensitivity as well as the evolution of direct Photoelectron imaging systems. The application of improved mathematical analysis methods has enabled a better understanding of the distribution of elements within the top 10nm of the material. This presentation will focus on the instrumentation and application developments that have led to a greater understanding of the surface properties of materials within the nanometer range.

Biographical Sketch

David received hisB.Sc. and Ph.D. degrees in chemistry from the University of Bath in 1975, and 1978, respectively (Mentor: Prof. Frank Stone). Subsequently he worked as a Post Doctoral Fellow at the University of Manchester Institute of Science and Technology (1979-1981); (Mentor: Dr. John Vickerman). He is presently president of Kratos Analytical Inc., a manufacturer of surface analysis instrumentation. His interests remain in the development of instrumentation for the advancement of the understanding of surface chemical processes.

Introduction

Surface Analytical techniques have evolved utilizing a wide range of probe species with a correspondingly wide range of detected species. These techniques produce a variety of information related to the surface properties of the materials under investigation, of these techniques, X-ray Photoelectron Spectroscopy (XPS, also known as ESCA) has a unique capability to identify not only the type of elemental species at the surface but also its local chemical environment generating information on oxidation state. The technique probes only the outermost 10nm of material making an ideal technique to study surface properties. This presentation will focus on the development of enhanced capabilities that enable increased sensitivity and spatial distribution as well as discuss recent enhancements and implementation of mathematical models for improved quantification and image analysis.

 
Discussion

We have shown that it has required developments and enhancements within a wide range of different aspects of the instrumentation to achieve the total performance gains required for successful quantitative analysis of surfaces. Typical XPS data represents information averaged over the first 10nm of material, with the application of mathematical models and angle resolved XPS, information at the nm scale can be identified within the surface region. The application of imaging technology to XPS has resulted in the extension of this nm information into lateral distribution information at the micron level. The evolution of "gentle" sputter techniques for the removal of small volumes of material has further enhanced this nanometer information content into true 3D distributions. The development of the delay line detection system provided the necessary breakthrough to quickly and easily generate lateral quantitative information to complement the quantitative depth information previous available. The application of mathematical techniques (such as PCA) subsequently enabled high quality quantitative data to be generated from single image pixels.

The enhancement of traditional designs to improve X-ray source performance, both from a power and X-ray flux density perspective, have resulted in an increase in the information retained at the surface and therefore substantially more reliable models that reflect the true surface chemistry. These changes have also provided substantial improvements in the data acquisition rates, enabling a larger volume of samples to be analyzed in a given time period. This increased speed has paved the way for XPS to become a potential screening technique within the biochemical field where large numbers of samples are routinely generated in the search for solutions to pressing medical challenges.

The development of the polyatomic Ion source for the sputter depth profiling of organic surfaces has proven to be a substantial enhancement over the previous use of inert gas molecules. The unique ability of the polyatomic species to remove organic material while allowing the retention of the underlying chemistry has for the first time allowed a true understanding of chemical changes taking place in the surface region. This region most readily influences the chemical properties of the material and its ability to interact with its environment whether that is biological or chemical.

Conclusions

X-ray Photoelectron Spectroscopy has become an invaluable tool for the analysis of the surface chemistry of nanomaterials. Its ability to provide quantitative elemental and chemical information both in 2 and 3 dimensions has resulted from significant enhancements to the instrumentation for sensitivity and improvements in the application of mathematical models for noise reduction and resolution enhancements. Finally the advent of polyatomic ion sources for the retention of organic surface species during sputter depth profiles has proven invaluable.

Acknowledgements

The author wishes to thank Simon Hutton, Adam Roberts, Sarah Coultas, Chris Blomfield, Simon Page, Gautam Mishra and Peter Butler, of Kratos Analytical Ltd., for their contributions and assistance with this paper. In addition thanks to the entire Kratos Analytical Ltd R&D group for their continued efforts for the enhancement of the instrumentation and technology that enable breakthroughs in the understanding of the surface chemistry.

Thanks to NPL (UK) for the use of the slide for the comparison of Surface Analytical techniques.

Thanks to Neal Fairly of CASAXPS for assistance with the mathematical models and PCA.