AVS2013 Session AS-WeA: Developments in Electron Spectroscopies for Non-Ideal Samples 2

Synchrotron sources of x-rays have revolutionized many areas of materials science. From the 1^st generation synchrotrons of the 1970s to the 3^rd generation of storage ring-based machines of today, the brightness has increased by around 6 orders of magnitude. Measurements that were formerly done at the mm spatial scale have become routine at sub 100 nm spatial resolution. New methods such as ptychography are taking us now to nm resolution. Storage rings can be pushed further, and sources with full transverse coherence are planned, increasing the brightness again b y 2 - 3 orders of magnitude. Over the last 10 years, there has been a parallel development of Free Electron Lasers (FELs), that push the peak brightness up again by a further factor of 10¹⁰ and reduce pulse length to the few fsec range. These machines complement quasi cw synchrotrons and are the tool of choice for studying ultrafast dynamics. I will briefly review some of these historical developments, and then focus on a key enabling technology for MHz repetition rate FELs, the production of ultrafast pulses of electrons from photocathodes. I will review our current work in the synthesis of high efficiency alkali antimonide based cathodes, and cathodes based on non-linear photoemission from plasmonic systems.

The concept of high energy X-ray sources for laboratory based XPS analysis has been around for a long time. The often cited work by Yates and West was published thirty years ago [1]. However, it is still rare to find data in the literature recorded using X-ray sources other than Al Kα or Mg Kα. In this paper we attempt to illustrate the usefulness of the Ag Lα source for a variety of sample types.

After Al Kα, Ag Lα is probably the most practical monochromatic source for standard laboratory based XPS instrumentation as it utilises the second order diffraction of the same quartz crystals as used for Al Kα. This allows it to be easily accommodated on a modern spectrometer.

The photon energy of Ag Lα is 2984.2 eV, approximately twice that of Al Kα (1486.6 e). This greater energy leads not only to the excitation of additional, higher binding energy, core lines for some elements (Table 1) but also the possibility of analysis of deeper layers due to the decrease in attenuation length with increasing photoelectron energy [2]. For example, the ineleastic mean free path (IMFP) of C 1s, binding energy 285 eV, using Al Kα X-rays is 3.14 nm at a kinetic energy (KE) 1201 eV, but for Ag Lα this is increased to 5.89 eV at KE 2699 eV [3].

In this work we present examples of sample analysis where the consequences of using a higher energy excitation source have been exploited. Examples of using higher binding energy core lines, such as Si 1s and Hf 3d, are presented together with applications utilising the greater information depth available from the source. We have studied samples using both spectroscopy and imaging and explored the depth dimension by ARXPS and depth profiling.

Table 1: Elements with core line binding energies in the range of Ag Lα X-rays but not Al Kα (*at least one half of doublet)[4]

References

[1] K Yates & RH West, Surf. Interface Anal., 1983, 4, 5

[2] MP Seah & WA Dench, Surf. Interface Anal., 1979, 1, 2

[3] www.nist.gov/srd/nist71.cfm

[4] X-Ray Data Booklet LBNL/PUB-490 Rev.2 Jan 2001

Auger Electron Spectroscopy (AES) is one of the most widely used techniques for quantitative surface analysis. AES is the dominant technique in numerous applications in industries. The advantage of high spatial resolution in AES makes it an attractive technique over XPS in solving new types of problems in a diverse range of technologies that are growing in importance. The presentation gives an emphasis on unique advances in surface analysis using AES, including the application of TEM specimen preparation techniques (FIB, microtome and ion-milling). We also demonstrate high energy resolution applications for chemical states analysis by the concept of the sample biasing method [1].

Keywords: AES, energy resolution, chemical states

[1] D.S. Watson, P.E. Larson, D.F. Paul and R.E. Negri, Surf. Interface Anal. 44 (2012) 121.

As surface analytical methods, Auger electron and x-ray photoelectron spectroscopies (AES and XPS) are now approximately fifty years old and both are in wide use. Based on the rapidly growing number of publications, XPS has become the most widely applied method for surface chemical analysis. Although analysis is now mostly routine and quantitative as applied to many types of samples, it has taken considerable effort by many in the research and vendor communities to establish the instrument quality and implement new technologies and to establish the fundamental understanding and relevant physical data needed to develop the accurate approaches to analysis that are now available. However, many modern materials have significant characterization requirements that extend beyond those available using many of the standard measurement and analysis approaches in routine use and as supplied by instrument vendors. In this talk we will look at the types of advances that have taken place over the past four decades and explore some of the issues that were identified that needed to be addressed along the way. Although many of these are now transparent to instrument users, some of them highlight the types of unexpected challenges that can arise when careful analysis is required. The roles that AES, XPS and other methods may play in meeting the significant analysis challenges presented by a new generations of organic, composite and other nanomaterials will be discussed. Among the issues that need to be considered are: sample preparation, mounting and analysis damage; in situ and time-dependent analysis; three dimensional structure information; rapid imaging; multimodal and functional characterization; coating uniformity and thickness; surface functionality; analysis speed and cost, and the nature and presence of very minor amounts of contamination. Many of these issues are not new, but the advanced materials now capable of being synthesized exaggerate some of the challenges and raise additional problems still to be addressed.

This talk has evolved from research programs, research conducted as part of the EMSL User Program (www.EMSL.PNL.gov), and interactions with colleagues from around the world. Portions of this work were performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s (DOE) Office of Biological and Environmental Research (BER) and located at PNNL. Aspects of the work have been supported by the DOE’s Offices of Basic Energy Science (BES) and Biological and Environmental Research (BER) and the NIEHS under Center grant U19 ES019544.

Elastomers made of organic polymers are used in forms such as O-rings, gaskets and electrical wire insulation. Changes in oxidation can lead to significant performance changes as mechanical properties are affected; therefore it is important to be able to directly measure oxidation of these materials. Elastomers have been found to exhibit diffusion limited oxidation whereby the edges are preferentially oxidized over the interior of the part. Much work has been done to characterize ideal elastomer systems; that is, elastomers with no fillers. Fillers, such as oxides and carbon black, confound most chemical analyses. While studies with non-filled elastomers reveal much about the chemistry and mechanical properties in an aged state, the measurement of the oxidation in “real” systems has often involved modulus profiling but much less chemical analyses. We are developing a method that promises to be effective for measuring the oxidation of filled polymer systems based on intrinsic chemical signatures.

Imaging Near Edge X-ray Absorption Fine Structure (NEXAFS) has been found to be a technique that overcomes the major limitations encountered in filled systems. Located at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratories (BNL), imaging NEXAFS is uniquely capable of measuring the local bonding of elements over large length scales. Thus, the oxidation of these polymers can be directly measured using this technique. We are developing the methods to acquire and process data from samples, ultimately providing a direct chemical measure of oxidation.

In this talk, the results from multivariate analysis of the data are used to calculate oxidation. Specific materials, aging conditions, sample preparation, data acquisition, multivariate analysis and results will be discussed.

**Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Surface analysis using multiple techniques is becoming increasingly common in materials characterisation, where the desire for more detailed chemical and structural information is complemented by increasing functionality and automation in instrumentation. It is now often possible to use a single instrument to analyse a sample using four or more techniques in quick succession.

X-ray Photoelectron Spectroscopy (XPS) has become routine in many laboratories, while related techniques such as Auger Electron Spectroscopy (AES), Ultraviolet Photoelectron Spectroscopy (UPS), Ion Scattering Spectroscopy (ISS) and Reflected Electron Energy Loss Spectroscopy (REELS) are being used more frequently to supplement XPS analyses.

Any sample that has been exposed to the ambient environment will typically have a thin (1-2 nm or more) covering of hydrocarbons and other contaminants, and may also have oxidised, hydrated or undergone other reactions. These surface deposits can have a significant effect on the results of surface analytical techniques.

The aforementioned techniques can provide a wealth of complementary information from the surface and near-surface regions of a material, but they have differing susceptibilities to the common surface modifications and contaminants. This presentation evaluates samples including polymers, metals and semiconductors, and examines methods of overcoming the contamination issues with particular focus on the use of monatomic and gas cluster ion beams for surface cleaning.