Journal of Electron Spectroscopy and Related Phenomena 178–179 (2010) 415–432 Contents lists available at ScienceDirect Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec XPS analysis of nanostructured materials and biological surfaces D.R. Baer ∗ , M.H. Engelhard Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Box 999, Richland, WA 99352, USA article info Article history: Available online 22 September 2009 Keywords: Nanomaterial Nanoparticle XPS Biosurfaces Catalysis Particle size abstract This paper examines the types of information that XPS can provide about a variety of nanostructured materials. Although it is sometimes not considered a “nanoscale analysis method,” XPS can provide a great deal of information about elemental distributions, layer or coating structure and thicknesses, sur- face functionality, and even particles sizes on the 1–20 nm scale for sample types that may not be readily analyzed by other methods. This information is important for both synthetic nanostructured or nanosized materials and a variety of natural materials with nanostructure. Although the links between nanostruc- ture materials and biological systems may not at first be obvious, many biological molecules and some organisms are the sizes of nanoparticles. The nanostructure of cells and microbes plays a significant role in how they interact with their environment. The interaction of biomolecules with nanoparticles is important for medical and toxicity studies. The interaction of biomolecules is important for sensor function and many nanomaterials are now the active elements in sensors. This paper first discusses how nanostructures influences XPS data as a part of understanding how simple models of sample structure and data analysis can be used to extract information about the physical and chemical structures of the materials being analyzed. Equally important, aspects of sample and analysis limitations and challenges associated with understanding nanostructured materials are indicated. Examples of the application of XPS to nanostructured and biological systems and materials are provided. © 2009 Published by Elsevier B.V. 1. Introduction X-ray photoelectron spectroscopy (XPS) has become an increas- ingly available and powerful tool for understanding the nature of many different types of surfaces. Although advances in existing and newly developing tools with high spatial resolution receive a good deal of press if they improve the analysis quality of individual nanosized features of materials, XPS is an important, established and frequently essential tool for understanding several important aspects of nanostructured natural and synthetic materials that can- not easily be obtained using other techniques. In addition, it is finding increasing applications for analysis of the surfaces of biolog- ical systems as well as a more traditional role in the characterization of synthetic materials (biomaterials) designed to be used in biologi- cal environments. Microbe and cell surfaces might be appropriately considered as very complex nanostructured systems. Characterization of nanostructures using XPS is not new. From the earliest days, XPS or ESCA (electron spectroscopy for chemical analysis) was widely used to study many aspects of nanosized cat- alyst particles [1–3]. Over the past 30 years many researchers have explored important aspects of how particle size or other nanosized ∗ Corresponding author. E-mail address: don.baer@pnl.gov (D.R. Baer). sample features impact XPS data. In a web-based presentation Yang and Sacher describe many different aspects of XPS measurements associated with nanoparticles, including references from their own work and some of historical importance [4]. The increasing importance of XPS for many types of analysis arises for several reasons. First, XPS can provide information about the actual composition and chemical state of surfaces and interfaces that dominate properties of nanostructured materials. Surfaces are equally important for the function of biological organisms and to the performance of synthetic biomaterials. Second, although it is frequently not considered to be a tool with “nano” resolution, because the electrons associated with XPS only travel distances measured in nanometers these electrons can be used to provide a good deal of information about the structure of nanometer-sized features of a sample near a surface region, as described below. Although the possibilities for obtaining nanometer-scale information from samples with flat surfaces may be more apparent to many researchers, information from XPS spectra can be used to determine the size of nanoparticles (sometimes in circumstances for which other types of high spatial resolution measurements cannot be applied) as well as provide information about coatings and layers on particles. A wide variety of challenges are associated with the characteri- zation of nanostructured materials [5,6], some specific to XPS and others applicable to many types of measurements on nanostruc- 0368-2048/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.elspec.2009.09.003