393-barr-jvsta-1995-1239 - Nature of the use of...

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Unformatted text preview: Nature of the use of adventitious carbon as a binding energy standard Tery L. Barr and Sudipta Seal a) Materials Department and Laboratory for Surface Studies, University of Wisconsin – Milwaukee, Milwaukee, Wisconsin 53201 ~ Received 24 October 1994; accepted 16 January 1995 ! It has become common practice to employ, as a binding energy reference for x-ray photoelectron spectroscopy studies on nonconductive materials, the C(1 s ) spectra of the ubiquitous ~ adventitious ! carbon that seems to exhibit an instantaneous presence on all air exposed materials. Despite this commonality, surface scientists, including many practitioners, have expressed substantial concerns about the validity of this approach. A detailed discussion of the method is presented including consideration of the types of materials and the electronic energy states involved, e.g., Fermi edges, vacuum levels, etc., and the couplings that must exist for the referencing method to be correctly applied. A number of other surface environments for which the carbon referencing method may be fallacious are also presented. This leads to a consideration of the electron spectroscopy for chemical analysis results for different types of adventitious species and how the presence of some of these may confuse the use of the method. In this regard, we will also discuss the use of other methods to establish binding energy scales, such as Fermi edge coupling and select doping ~ e.g., the Au dot approach ! . © 1995 American Vacuum Society. I. INTRODUCTION Perhaps the most maligned process in surface analysis is also one that is used as much today as it was in the past. The employment of adventitious carbon to establish binding en- ergy scales has been the standard in more electron spectros- copy for chemical analyses ~ ESCAs ! than all other referenc- ing methods combined. In order to define the use of adventitious carbon, 1 we must first try to describe what it is and how it behaves. The material has been observed on every surface that has not been carefully cleaned and maintained in ultrahigh vacuum ~ UHV ! and even in UHV this cleanliness will soon ‘‘deteriorate’’ and begin to realize an adventitious carbon deposition unless the process is periodically repeated. As a result, the detection of some adventitious carbon is a product of nearly every x-ray photoelectron spectroscopy ~ XPS ! analysis. Although each of these studies exhibits slight differences, the striking feature of the many thousands of XPS observations of adventitious carbon is their apparent chemical and physical similarity. We present a partial list of many of these features. ~ 1 ! Adventitious carbon ~ AC ! deposits on all air exposed surfaces. Although it is enhanced by such features as the presence of vacuum pump oil, the latter is not a necessity for the presence of adventitious carbon....
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393-barr-jvsta-1995-1239 - Nature of the use of...

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