We report the results of a VAMAS (Versailles Project on Advanced Materials and Standards) interlaboratory study on the measurement of composition in organic depth profiling. Layered samples with known binary compositions of Irganox 1010 and either Irganox 1098 or Fmoc-pentafluoro-l-phenylalanine in each layer were manufactured in a single batch and distributed to more than 20 participating laboratories. The samples were analyzed using argon cluster ion sputtering and either X-ray photoelectron spectroscopy (XPS) or time-of-flight secondary ion mass spectrometry (ToF-SIMS) to generate depth profiles. Participants were asked to estimate the volume fractions in two of the layers and were provided with the compositions of all other layers. Participants using XPS provided volume fractions within 0.03 of the nominal values. Participants using ToF-SIMS either made no attempt, or used various methods that gave results ranging in error from 0.02 to over 0.10 in volume fraction, the latter representing a 50% relative error for a nominal volume fraction of 0.2. Error was predominantly caused by inadequacy in the ability to compensate for primary ion intensity variations and the matrix effect in SIMS. Matrix effects in these materials appear to be more pronounced as the number of atoms in both the primary analytical ion and the secondary ion increase. Using the participants' data we show that organic SIMS matrix effects can be measured and are remarkably consistent between instruments. We provide recommendations for identifying and compensating for matrix effects. Finally, we demonstrate, using a simple normalization method, that virtually all ToF-SIMS participants could have obtained estimates of volume fraction that were at least as accurate and consistent as XPS.
We present experimental data on the angular distribution of Irganox 1010 organic molecules sputtered by large argon gas cluster projectiles (E/n = 5 eV, 10 keV Ar2000). Ejection probability distributions as derived from deposit patterns on planar collector surfaces were recorded at various angles of incidence of the primary cluster ion beam. The sputtered material is ejected at polar angles, on average, greater than 45° from the surface normal. At normal incidence there is no azimuthal dependence in the ejecta distribution, but the ejecta are forward directed even at incidence angles as low as 15°. After this initial rapid change, the ejecta distribution shows a rather weak dependence on the incidence angle of the primary ion beam and the polar, and azimuthal angles of preferred ejection remain relatively constant. Ejecta distributions agree with previously published results from molecular dynamics simulations for organic molecules sputtered with large argon gas cluster projectiles and are consistent with the picture derived from experimental data for metal target species. The close chemical resemblance of collector and target materials as identified by secondary ion mass spectrometry, and the large total volume of deposits accounting for over 75% of the sputtered material as inferred from symmetry considerations, indicate that a large fraction of the sputtered material is intact molecules. Findings are discussed with respect to the utilization of large cluster projectiles as primary ion beams in secondary neutral mass spectrometry.
This paper provides a description of the transmission function of an X‐ray photoelectron spectroscopy (XPS) instrument operating with exchangeable Al Kα (1486.6 eV) and Ag Lα (2984.3 eV) sources. Both sources use the same quartz crystal monochromator and illuminate the same area of the sample. The transmission‐function–corrected data from sputter cleaned gold provides a useful reference material to calibrate other instruments of the same type. Sensitivity factors for Ag Lα and Al Kα are calculated from photoionisation cross sections and electron effective attenuation lengths. These compare well with previous experimental values and data acquired from ionic liquids. The intensity of the Ag Lα source is found to be approximately 50 times lower than the Al Kα source. This, coupled with generally lower photoemission efficiencies, results in noisier data or extended acquisition times. However, there are clear advantages to using the Ag Lα source to analyse certain elements where additional core levels can be accessed and for many technologically important elements where interference from Auger electron peaks can be eliminated. The combination of calibrated data from both sources provides direct and easily interpreted insight into the depth distribution of chemical species. This could be particularly important for topographic samples, where angle resolved experiments are not always helpful. We also demonstrate, using thin coatings of chromium and carbon, that the inelastic background in Ag Lα wide‐scan spectra has a significantly increased information depth compared with Al Kα.
The bulk-surface diffusion of carbon in Pd(110) has been investigated using molecular beams to examine the clean-off of carbon with gas-phase oxygen. The dominant product of the reaction above 600 K is CO; CO 2 is only produced at much lower temperatures where the lifetime of adsorbed CO is significant. The segregation from the bulk only occurs at a measurable rate at >750 K, while loss of carbon into the bulk can occur at much lower temperatures. The reaction of carbon at high temperatures shows two regimes: (i) a non steadystate evolution of CO due to the clean-off of surface carbon already present on the surface, followed by (ii) a near steady-state reaction due to the continuous diffusion of carbon to the surface.
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