REPORT
Characterization of Organic Surfaces
W
e live in a world full of useful organic materials and surfaces. In addition to those in biological systems, a growing number of polymeric materials, whose surfaces provide key mechanical and chemical properties, are being used in materials technology. Most liquids used as interfacial lubricants are mixtures of organic molecules, as are liquid crystal displays. Moreover, researchers in the rapidly expanding field of biotechnology are using organic surfaces for protein adsorption and molecular recognition. Despite the importance of organic surfaces, they have not yet been subjected to the same type of molecular-level scrutiny as have metal and oxide surfaces and interfaces. This Report reviews the surface science techniques that appear most suitable for characterizing organic surfaces. We discuss 13 techniques, many of them developed in recent years, that provide information about composition and structure on the molecular level: secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), lowenergy electron diffraction (LEED), highresolution energy electron loss spectros-
Scott S. Perry Gabor A. Somorjai University of California, Berkeley 0003-2700/94/0366-403A/$04.50/0 ©1994American Chemical Society
A variety ofsurface analysis techniques can be used to obtain composition and structural information copy (HREELS), Raman spectroscopy (RS), Fourier transform infrared spectroscopy (FT-IR), sum frequency generation (SFG), scanning tunneling microscopy (STM), atomic force microscopy (AFM), surface force measurements (SFM), contact angle measurements, and neutron reflectivity measurements. The first two techniques, SIMS and XPS, provide information about surface composition and the chemical environment and bonding of surface species. UPS probes the density of electronic states in the valence band of materials and can provide a spectroscopic fingerprint of many organic compounds. LEED and HREELS are electron-scattering techniques that are uniquely suited to yield the structure of the surface monolayer. RS, FT-IR, and SFG are optical techniques used to study solid-gas interfaces at high pressures along with solid-solid
and solid-liquid "buried" interfaces. STM, AFM, and SFM provide atomic-scale resolution of surface structure, the forces between molecules at surfaces and, in some cases, measurements of hardness and friction properties of the surface region. Contact angle measurement is a classical technique that provides information about surface thermodynamic properties. Finally, neutron reflectivity measurements probe the density of a surface region and can be used to determine concentration profiles in polymer blends. We will discuss the operating principles of each technique and the associated instrumentation, and then give examples of the types of data that can be gathered about organic surfaces. In addition, we provide references to more detailed reviews of the individual techniques for the interested reader. In this broad discussion of techniques suitable for the study of organic surfaces, it should be noted that such surfaces can take many different forms, ranging from monolayers of molecules adsorbed on inorganic or metallic substrates, to supported thin films of organic material, to the actual surface of bulk compounds. Not all techniques will be applicable to every type of surface. In addition, the necessity of vacuum conditions for several of the techniques will also restrict their application to some materials. Finally, the techniques discussed here have varying degrees of sur-
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REPORT
face sensitivity and can probe molecules at a range of depths into the surface region. With these considerations, it appears that combinations of techniques may be necessary to provide a wellrounded picture of the surface properties of organic systems. SIMS SIMS is a powerful probe of organic and polymer surface composition. Noble gas ions (typically argon) or atomic metal ions (e.g., Cs+, Ga+) impinge on the surface at well-defined energies in the range of 130 keV and eject secondary ions from the surface (Figure la). Of the particles sputtered from the surface, only a small fraction are ionized, either positively or negatively; these secondary ions are detected in a mass spectrometer. In turn, the fragments of a given mass are assigned to specific compositional fragments and, through careful calibration studies, are related to the molecular structure of the surface. A complete discussion of the operating principles of SIMS can be found in the introductory text by Werner (1). The positive-ion spectra of polypropy-
lene and polyacetylene shown in Figure lb are examples of SIMS studies of organic surfaces (2). Only the relative intensities of the common mass fragments are used to distinguish the two polymers containing only carbon and hydrogen. For organic materials containing additional elements (e.g., 0, Cl, F), mass fragments unique to these elements would be observed in the spectrum, providing additional means of identification. The use of standards and calibration in SIMS studies is essential for proper identification and quantification. The high sensitivity of the mass spectrometric detection scheme allows surface species to be detected at concentrations of - 109 molecules/cm2. Because the concentration of a surface monolayer is usually 1015 molecules/cm2, SIMS detection limits approach 1 ppm. In addition, SIMS provides extreme surface sensitivity because it detects charged ions; ionic fragments generated beneath the surface plane have a high probability of neutralization before escaping the surface and are therefore not detected. The sputtering nature of this tech-
Figure 1 . SIMS. (a) Instrumentation, (b) Positive-ion secondary ion mass spectra of polypropylene and polyacetylene. (Adapted from Reference 3.) 404 A
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nique creates advantages and disadvantages in studying organic surfaces. With high incident beam currents, material is rapidly removed from the surface and depth profiles of the surface region can be obtained. Although depth profiling is important in some studies, the high flux of ions can also lead to deleterious reaction and degradation of the organic or polymer surface itself. Sputtering effects can be minimized by reducing the incident beam current—a technique referred to as static SIMS. An additional advantage of sputtering is realized when the ion source is focused on a small spot and rastered across the surface. By selectively detecting mass fragments as a function of raster position, compositional maps of the surface can be generated. Tarlov and Newman have imaged a photopatterned, two-component, self-assembled monolayer at micrometer resolution, which demonstrates the mapping capabilities of SIMS (5). XPS In XPS, X-rays of suitable energy (in the range > 1000 eV) eject photoelectrons from the surface and near-surface regions. These photoelectrons include those emitted directly from core and valence levels of the organic material as well as electrons emitted indirectly through an Auger process. (XPS focuses on the emission of core-level electrons.) The emitted electrons can be detected as a function of energy and angle of emission. The energy of the photoemitted electrons is a function of the core-level energy state from which they were generated and therefore provides elemental specificity. Because core-level energy states often are perturbed by local bonding energy states, this process also provides a measure of the chemical environment of the elements. Furthermore, angle-resolved detection of the emitted electrons probes the depth distribution of species in the surface region. Electrons ejected within a dense material must escape to be detected. The farther the electrons must travel within the material, the lower the probability of their detection. In XPS, electrons detected along grazing angles to the surface are more likely to be ejected from the near-surface region or from the surface monolayer; therefore, they provide
more surface-specific information. Conversely, a relatively larger concentration of electrons will be emitted from deeper below the surface at exit angles closer to surface normal. This angular dependence has been quantitatively treated in detail by Fadley (4). Figure 2a shows the principle of X-ray photoeiectron spectroscopy and a schematic of a typical XPS instrument. XPS is usually carried out at low pressures (lfT6 torr) or in an ultrahigh vacuum. Some excellent books describe the principles of operation of XPS (5, 6). The applicability of XPS to the study of polymer surfaces is demonstrated in Figure 2b. In this study, a series of Teflon surfaces were irradiated in air with increasing laser power to produce defluorination, branching, and surface oxidation (7). The X-ray photoeiectron spectra reveal the appearance of intensity at lower binding energies (284-288 eV) with increasing laser irradiation. This intensity can be deconvoluted
in terms of species (CK, and COH groups) of known chemical shifts (dotted lines). The appearance of these features indicates pathways of chemical degradation of the fluoropolymer under laser irradiation.
trons; angle-resolved measurements are useful in increasing surface sensitivity. For example, this approach would allow the study of surface segregation of one component of a polymer blend. A more complete discussion of UPS and its applications can be found in Reference 9.
UPS
An analogous technique to XPS is UPS, in which a noble gas, typically helium, is excited by a high-voltage dc discharge to produce photons in the 20-40-eV range. Irradiation of a surface with these photons produces the emission of photoelectrons from the valence band of the solid and measures the density of states within these bands. The valence-band spectra of polymers can be a useful fingerprint of their identity, as shown in Figure 3 for poly (ethylene), poly (ethyl ether ketone), and poly(phenylene) sulfide (
