AVS2009 Session AS1-ThM: Advances in Surface Analysis

G-SIMS^[1] is a powerful method for the identification of organics and complex molecules at surfaces. For complex molecules, evaluating the molecular structure can be key to correct identification. We have shown^[2,3] that the molecular structure may be reassembled from fragment ions by studying the evolution of G-SIMS intensities as the surface plasma, with effective temperature T_p, is varied, known as G-SIMS-FPM. Recently, we have developed a novel approach^[4], based on SMILES^[5] (Simplified Molecular Input Line Entry Specification), to assist the reassembly process in an automated way through evaluation of the fragmentation pathways for given molecular structures. A computer program takes a parent structure and goes through every possible fragmentation to provide a tree structure of fragmentation products and simulated fragmentation pathways. For any fragment it is then possible to identify the molecular structure, its mass and a pathway to the parent. We find that there is a good correlation with peak evolution in G-SIMS-FPM data and simulated pathways for two amino acids and a simple peptide. This significantly enhances the application of G-SIMS-FPM to unknown materials. Once fragmentation pathways have been calculated for a molecule they are added to a library.

We have now developed an informatics database system with a web-based front-end that allows analysts to generate fragmentation pathways for any molecule in SMILES format. For new molecules, the pathways are added to the library. Analysts may then explore the fragment pathways for comparison with the G-SIMS spectra. The free web-based facility allows the library to grow rapidly as use by the community grows. Recent developments in G-SIMS including G-tip technology and the use of cluster ions will be discussed. Examples of the use of G-SIMS and SMILES to identify the structure for different molecules are provided as well as examples of how to use the web-based system.

[1] I. S. Gilmore and M. P. Seah, Appl. Surf. Sci., 161 (2000), 465.

[2] I. S. Gilmore and M. P. Seah, Appl. Surf. Sci., 231-232 (2004) 224.

[3] I. S. Gilmore, F. M. Green and M. P. Seah, Appl. Surf. Sci., 252 (2006) 6601.

[4] F.M. Green, E.A. Dell, I. S. Gilmore, M.P. Seah, Int J Mass Spectrom 272 (2008) 38

[5] SMILES, Daylight Chemical Information Systems, http://www.daylight.com/smiles/ .

Characteristic X-ray emission from a well grounded metal samples using standard 30 keV Ga⁺ focused ion beams is demonstrated. X-ray yields are found to be on the order of 10^-10 per incident ion, consistent with previous studies of low energy, high mass ion – solid interactions. X-ray yields were found to be highest for soft X-rays, i.e., low energy transitions or low atomic number target atoms. Bremstrahhlung X-ray emission was found to be minimal, possibly increasing the detectability limits compared with electron beam induced X-rays. The generation of heavy ion induced X-rays is consistent with a molecular-orbital level crossing model where velocity coupling between the primary ion beam and target atom electrons is not necessary and the majority of X-rays are in fact generated due to recoil effects within the ion – solid interaction cascade.

The temperature dependence in the molecular desorption of coronene films stimulated by 20-keV Au₁⁺, Au₃⁺, and C₆₀⁺ projectiles were experimentally investigated by means of strong field laser photoionization coupled with time-of-flight secondary neutral mass spectrometry (ToF-SNMS) at 300 K and 77 K. The sputtering dynamics of highly energetic ion beams with surfaces have always been assumed to be temperature independent. However, recent data indicate this might not be the case for high energy cluster ion beams.

At 300 K, the kinetic energy distributions of ion beam desorbed coronene neutral molecules are projectile independent. The molecular desorption events stimulated by either projectile are ejected with a most probable translational energy of approximately 0.1 eV and show negligible differences in the high-energy component of the distribution when comparing the projectiles. However, at 77 K the most probable translational energy of ejection is shifted to higher energies as a result of cluster ion bombardment. A most probable translational energy of 0.5 eV is observed when using cluster projectiles such as 20-keV C₆₀⁺. Fragments created by the C₆₀ cluster ion sputtering dynamics are detected as low mass fragments at 300 K and exhibit a Maxwell-Boltzmann distribution. This behavior is not evident at 77 K or induced by the other projectiles.

This work continues a previous study¹ on thin overcoat effects on ion attenuation, and the change the proximity of the underlying metal layer has on the spectra of organic materials on top of the carbon. In this study, the overcoat thickness was varied over a larger range than in the previous study, down to bare metal. The samples were measured before and after application of a thin polymer film, a perfluoropolyether commonly used as a lubricant in the hard disk drive industry (Z-Tetraol). The spectra of the lubricant coated samples were explored both for the effect of the lubricant coating on the metal attenuation and for the effect of the varying carbon layer thickness on the lubricant spectra. For this series of samples, the total ion count is expected to vary from sample to sample as it reflects the differences between samples as much as relative individual ion intensities does. Also, the carbon itself produces little signal in TOF-SIMS spectra. The use of a system involving external standards will be described that allows the comparison of spectra from day to day without normalization.

1. Spool, A.; White, R., “Probing Thin Over Layers with Variable Energy / Cluster Ion Beams”, Appl. Surf. Sci. Volume: 252 Issue: 19 (2006) 6517-6520

We show that simultaneous molecular orientation and bond chemistry of large area (18 mm x 13 mm) planar chemically heterogeneous surfaces can be obtained by combining near edge X-ray absorption fine structure (NEXAFS) spectroscopy, a new parallel process magnetic field electron yield optics detector, and a full field incident soft X-ray beam on the sample. The rapid parallel process magnetic field electron yield optics detector (LARIAT:Large ARea Imaging Analytical Tool) produces a series of two-dimensional NEXAFS spatial images as the incident soft X-ray energy is scanned above a K or L absorption edge. The image stack reveals information about the chemistry (including bond concentration) and orientation of the surface-bound molecules with 50-micron planar spatial resolution and sub-monolayer molecular sensitivity. The power of the combinatorial imaging NEXAFS method is illustrated by simultaneously probing the concentration and molecular orientation of single-strand DNA micro array sensors, semifluorinated molecular gradients, and organic electronic combinatorial device arrays. Other possible applications described include the surface orientation and chemistry of continuously graded polymer films and graded or patterned self-assembled monolayers that exhibit tunable surface properties of potential use in nanotechnology. We also envision combinatorial imaging NEXAFS as an insitu probe for catalyst discovery using micro arrays to directly image catalytic chemical activity of thousands of catalysts simultaneously under reaction conditions.