Paper AS-ThM3
Challenges of the XPS Analysis of the Ionic Liquid [BMIM][PF₆]

Thursday, November 3, 2011, 8:40 am, Room 102

Room temperature ionic liquids (ILs) are ionic compounds that are liquids at ambient conditions due to a bulky, asymmetric organic ion that inhibits crystalline packing. The inhibition of crystalline packing results in ILs having a vapor pressure that is negligibly low. The properties of ILs make them candidates for applications such as nonvolatile solvents, electrolytes, and lubricants. The ionic liquid pair 1-butyl-3-methylimidazolium hexafluorophospate [BMIM][PF₆] is being investigated for use as a lubricant in micro-electrical mechanical systems (MEMS). Surface properties are important in tribological applications like lubrication and the negligible vapor pressure of the [BMIM][PF₆] ionic liquid makes it compatible with traditional surface analysis tools like x-ray photoelectron spectroscopy (XPS).

XPS analysis of ILs is challenging due to the ability of the ionic molecules within the ILs to freely move within the sample. Also, since the ILS are comprised of charged molecules, the motion of the molecules and their relative position within the sample may be affected by changes in surface charging brought about by photoelectron emission or by the electron and ion beams used to neutralize surface charging.

We will present results of our XPS analyses of [BMIM][PF₆] ionic liquid drop-cast onto a silicon substrate. We dropped enough of the IL to form an approximately 150 nm thick film to mimic a film that would be used for lubricating a MEMS device. However, the IL drop-cast in this manner did not form a continuous film, but rather beaded up on the silicon substrate. Under certain analysis conditions we can force the IL droplets to coalesce into a continuous film.

Preliminary results show that when the surface is positively charged the PF₆ molecule segregates to the liquid/vacuum interface. This observation is consistent with previous studies. When the IL surface is negatively charged the C1s peak shape changes from that indicative of the imidazole ring C-N bonding to that indicative of C-C bonding, most likely from the butyl chains of the BMIM molecules. This change in the C1s spectrum suggests the BMIM molecules are segregating to the liquid/vacuum interface. A corresponding drop in the F1s peak intensity suggests that the PF₆ molecules are moving deeper into the sample.

Along with the XPS results we will propose a mechanism by which the charge and/or energy provided by the x-ray, electron and ion beams induce the change from dispersed droplets to a continuous film. We will also provide insight into how these results could be used in developing or using ILs for MEMS lubricants.