Using aberration-corrected scanning transmission electron microscope and energy dispersive x-ray spectroscopy, single, isolated impurity atoms of silicon and platinum in monolayer and multilayer graphene are identified. Simultaneously acquired electron energy loss spectra confirm the elemental identification. Contamination difficulties are overcome by employing near-UHV sample conditions. Signal intensities agree within a factor of two with standardless estimates.
A calibration procedure for the detection efficiency of energy dispersive X-ray spectrometers (EDS) used in combination with scanning electron microscopy (SEM) for standardless electron probe microanalysis (EPMA) is presented. The procedure is based on the comparison of X-ray spectra from a reference material (RM) measured with the EDS to be calibrated and a reference EDS. The RM is certified by the line intensities in the X-ray spectrum recorded with a reference EDS and by its composition. The calibration of the reference EDS is performed using synchrotron radiation at the radiometry laboratory of the Physikalisch-Technische Bundesanstalt. Measurement of RM spectra and comparison of the specified line intensities enables a rapid efficiency calibration on most SEMs. The article reports on studies to prepare such a RM and on EDS calibration and proposes a methodology that could be implemented in current spectrometer software to enable the calibration with a minimum of operator assistance.
The technique of scanned-energy photoelectron diffraction has been used to obtain detailed information on the adsorption site and internal structure of propyne (CH3CCH) and 3,3,3-trifluoropropyne (CF3CCH) on Cu(111). Both molecules are bound to the surface via the acetylenic unit in a site analogous to that previously found for ethyne on Cu(111). In particular, the acetylenic unit is parallel to the surface in a cross-bridging position such that one of the C atoms is above a fcc hollow site while the other is above a hcp hollow site, giving a C–C bond length of 1.47 Å. The methyl/trifluoromethyl group is strongly tilted away from the surface and is attached with equal probability to the C atoms in the fcc and hcp hollow sites. The molecular plane is perpendicular to the surface. The fluoro group of CF3CCH is preferentially oriented in the cis conformation. From the LEED pattern and the calculated coverage of 0.25 ML, a possible overlayer arrangement for propyne and 3,3,3-trifluoropropyne on Cu(111) has been proposed. For this overlayer the orientation of the F3 group can be explained by intermolecular steric repulsion.
Scanned-energy mode C 1s photoelectron diffraction has been used to investigate the local adsorption geometry of benzene on Si(001) at saturation coverage and room temperature. The results show that two different local bonding geometries coexist, namely the 'standard butterfly' (SB) and 'tilted bridge' (TB) forms, with a composition of 58 ± 29% of the SB species. Detailed structural parameter values are presented for both species including Si-C bond lengths. On the basis of published measurements of the rate of conversion of the SB to the TB form on this surface, we estimate that the timescale of our experiment is sufficient for achieving equilibrium, and in this case our results indicate that the difference in the Gibbs free energy of adsorption, G(TB) − G(SB), is in the range −0.023 to +0.049 eV. We suggest, however, that the relative concentration of the two species may also be influenced by a combination of steric effects influencing the kinetics, and a sensitivity of the adsorption energies of the adsorbed SB and TB forms to the nature of the surrounding benzene molecules.
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