AVS2011 Session AS-ThM: Analysis of Insulators and Challenging Samples

TOF-SIMS has emerged as an important tool for imaging mass spectrometry of biological samples due to its unique capability to detect molecular ion fragments and elemental ions at sub-micron spatial resolution and without the sample treatments required by e.g. the MALDI technique. For many biological specimens, the ability to image samples having a large degree of surface topography is also highly desired. The resulting elemental and molecular images provide important information regarding the composition of biointerfaces, for example between plant tissues and their natural environment. This TOF-SIMS study involves the differentiation and characterization of epicuticular waxes present at the surfaces of Arabidopsis thaliana organs including the flower, stem, adaxial (top) leaf surface and the abaxial (bottom) leaf surface.

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.

X-ray Photoelectron Spectroscopy (XPS) and other surface analysis methods (e.g. AES and SIMS) are playing increasingly important roles in the analysis of many materials ranging from ancient artifacts, geological minerals and even biological specimens to modern very complex nanostructured materials. Samples that need to be analyzed can be made up of components with a wide variety of physical and chemical properties that impact how they can be handled, prepared, mounted and analyzed without destroying the information that is sought from the analysis. During this presentation a variety of the methods and approaches needed to characterize a range of sample and analysis challenges that we have faced in the EMSL, a US Department of Energy user facility, will be discussed. Analysis challenges discussed will include: inexperienced users and their expectations; sample handling, preparation (including extraction of particles form aqueous solution) and mounting (including anaerobic transfer and freezing); identifying and dealing with damage; identifying and controlling impacts of charging (which are not always obvious); and analysis approaches used to extract information about the nanostructure of complex materials. The materials and materials systems to be discussed will include the impact of processing on catalyst surface composition and chemistry, the oxidation state of species adsorbed on mineral surfaces, the nature and properties of organic layers on inorganic substrates, and the nature of the surfaces and intentional as well as inadvertent coatings (shells) on nanoparticles. Some of the methodology and special capabilities developed in EMSL for controlling sample environments, the value of more rapid and enhanced data analysis, and the importance of procedural guides and standards will be noted. Although many of the topics discussed relate to a variety of surface analysis and other methods, the talk will focus on XPS and use TOF-SIMS to provide an example of challenges associated with measurement of hydrogen. The importance of complementary measurements by a variety of methods (including XRD, TEM, APT, SEM and RBS) will be highlighted.

Aspects of the work have been supported by the Offices of Basic Energy Sciences and Biological and Environmental Research of the US DOE and by the National Institute of Environmental Health Science under grant NIH U19 ES019544. Portions of this work were conducted in the Environmental Molecular Sciences Laboratory (EMSL), a DOE user facility operated by Pacific Northwest National Laboratory for the Office of Biological and Environmental Research of the DOE.

Electron emissions from poorly conductive specimen stimulated by photon and electron injections result in charging phenomena. These can cause distortion of data in electron spectroscopy and image artifacts in scanning electron microscopy because surface potential which influence the motion of emitted electron is modified by charging and change time dependently. Therefore, time dependence of electron emission yields are important for understanding of radiation induced charging of resistive materials.

In this study, we present the transient characteristics of electron emission from SiO₂ using a new experimental facility (Pulsed Electron Microscope: PULSEM) built in the present work. Main feature of the PULSEM, as with electron beam testing system, is a beam blanking system attached below the condenser lens. The beam blanking pulses electron beam into the minimum number of 6 electrons under the condition of primary beam current I_p = 20 pA and beam-pulse-width t_p = 50 ns. The Everhart-Thornley detector with high response scintillator, afterglow time of which is 0.5 ns, and with a photomultiplier is synchronized to the beam blanking system. Transient SE signal as a function of the number of injected electrons is obtained.

Time-dependent electron emission yield (σ) with the resolution of 30 ns is investigated as a function of the primary electron dose which is controlled by pulse duration (Δt) as well as primary energy (Ep). Quasi static emission yield, which assumed to be intrinsic electron emission in the absence of charging, is obtained under low dose condition of below 60 electrons with Ip = 20 pA and Δt = 500 ns. The maximum yield (σ_m) and its corresponding energy (E_m) are 2.5 and 300 eV, respectively. With the increase in electron dose, s approaches unity due to a charging effect. Finally, we will present total electron emission yield curves in the range of Ep < 6 keV and 50 ns < Δt < 500 ms.