Probing the Charge Build-Up and Dissipation on Thin PMMA Film Surfaces at the Molecular Level by XPS

Abstract

Electrets are well-known and utilized materials that develop a permanent electrostatic potential or a dipole moment. However, the nature of this “electrification” at the atomic and molecular scale is poorly understood. A better understanding of the fundamental processes that lead to electret formation could allow more intelligent utilization of these materials. Kelvin-probe atomic force microscopy (KP–AFM) has been the most advanced tool for probing, mapping, and quantifying the development of charge at submicrometer length scales. However, as in most electrical-based measurements, it lacks chemical specificity. In contrast, spectroscopic techniques such as IR, Raman, or NMR spectroscopy have excellent chemical specificity, but are not sensitive to charge accumulation. In this respect, ESR and EPR techniques have been quite successful in analysis of trapped charges. However, the use of these techniques is also limited to only radicals and paramagnetic species. Unlike optical techniques, X-ray photoelectron spectroscopy (XPS) is a charged-particle-based technique and is also very sensitive to the presence of electrical potentials on the analyzed surfaces. Moreover, the photoelectron emission process itself leads to the creation of positive potentials in nonconductive samples as a result of uncompensated charges, and elaborate charge compensation methods have been developed using low-energy electrons or ions to eliminate sample charging. However, complete removal, that is, achieving the point of zero charge (PZC), is only an ideal. Besides, the measurement of the sign and the extent of the potentials developed can reveal significant information. Herein, we describe a contactless analysis technique to investigate the nature of the charging process of polymer surfaces at the molecular level, using XPS, whereby poly(methyl methacrylate) (PMMA) films are analyzed either in their pristine state or deliberately charged using a flood gun as an external electron source, and by applying external bias to control the extent of charging resulting from a combination of the photoemission process and the compensating electrons from the flood gun. Insulating materials such as polymers, salts, metal oxides, and nitrides have large band-gap values, and electrons are localized, leading to extremely low conductivities. In these materials, other electronic states, such as interface and impurity states, as well as defect sites, completely dominate their electrical properties. In addition, the electrical properties of these materials are influenced by external stresses, such as exposure to light, energetic particles, mechanical distortions, slicing, and ball milling, which is attributed to insertion of localized electrons or ions at interfaces, grain boundaries, cracks, or in bulk sites such as cavities. This charge insertion can even lead to chemical oxidation–reduction reactions. Contact electrification has recently been in focus. Two different mechanisms were proposed as its cause, one being electron transfer and the other ions or materials transfer, and sound experimental findings support both mechanisms. PMMA, with an average molecular weight of 120000 (Aldrich) was used to prepare films from 0.4% (w/w) solution in chlorobenzene by spin coating onto conducting Si wafers. XPS measurements were carried out using a Thermo Fischer K-Alpha spectrometer, which was modified for the introduction of external bias to the substrate in the form of directcurrent (d.c.) or square-wave potential pulses with varying frequencies (10 3 to 10 Hz), as described previously. The instrument also provides a facility to record a narrow region of the spectrum in the snapshot mode with less than 50 ms steps. Different modes of data gathering are used to probe the sign, the extent, and the dynamics of charging/discharging in both the C1s and O1s regions. As prolonged exposure to Xrays causes decomposition of the PMMA films, and because of the long-lasting nature of charging (several hundreds of seconds), extreme care was exercised to always probe a pristine region of the PMMA films for each and every measurement with an approximately 400 mm X-ray spot size. Note also that, although the nature of charging is consistent, the measured potentials exhibit strong fluctuations from one film to the other and also across each film. Therefore, our results should be considered as qualitative findings for a proof of the principle. Figure 1 displays the O1s and C1s regions of XP spectra that were recorded at three different charged states of a PMMA film, as judged by their positions referenced to the Si2p3/2 peak of the substrate, and using the tabulated peak positions given in Table 1. Accordingly, if the recorded binding energy (BE) positions are less than the reference values, the kinetic energy of the photoelectron is increased with respect to neutral (PZC) state, and hence the sample is negatively charged (and vice versa). As can be gathered from the figure, in addition to the overall shift of peaks, measurable [*] Dr. E. Yilmaz, Dr. H. Sezen, Prof. S. Suzer Department of Chemistry, Bilkent University Bilkent, TR-06800 Ankara (Turkey) E-mail: suzer@fen.bilkent.edu.tr