Accounting for Auger Yield Energy Loss for Improved Determination of Molecular Orientation Using Soft X-Ray Absorption Spectroscopy
Jan Genzer, E K. Kramer, Daniel A. Fischer
Partial (Auger) yield near edge X-ray absorption fine structure (NEXAFS) is a structural analytical technique that has been primarily used to measure thespatial orientation and chemical bonding of small molecules on solid (i.e., inorganic or semiconductor) surfaces. In most instances, the so-called building block (BB) model [Outka, D.A. et al., Phys. Rev. Lett. 59 1321(1987)] has been applied to deduce the molecule orientation from the experimental NEXAFS data.However, recently the BB model has been claimed to be inadequate for describing the data from larger molecules [H hner, G. et al., Phys. Rev. Lett. 67, 851(1991)]. In this paper we demonstrate that the building block scheme can be applied to model the molecular orientation of larger molecules, provided oneaccounts properly for kinetic energy losses of the Auger electrons traversing through the sample and hence the attenuation in measured Auger yield. We testthe applicability of the proposed modified building block (MBB) model by measuring the orientation of a self-assembled monolayer of -O1.5Si-(CH2)2-(CF2)8F, (SF-SAM (SiOx), deposited on top of SiOx-covered silicon wafer as a function of the entrance grid bias (EGB) of the channeltron photoelectron detector. Our measurements of the EGB-dependent electron escape depth revealed that a crude depth profiling within the top 5 nm of the sample is possible by increasing the negative EGB on the channeltron detector, at the highest bias thus selecting only the Auger electrons, which have suffered negligible energy loss. In addition, we discuss how the order parameter method introduced recently [St hr, J. and Samant, M. G., J. Electron Spectrosc. 98-99, 189 (1999)] can be used to determine the molecular orientation of large organic molecules on surfaces. We also show that by accounting for energy losses of the NEXAFS Auger electrons (attenuation of measured Auger yield), the corrected order parameter (COP) approach gives good estimates of the orientation of molecules. We present a comparison between the MBB and COP models using experimental data collected from NEXAFS experiments from semifluorinated (SF) mesogens, -(CH2)x(CF2)yF, which are attached to: 1) the isoprene backbone of polyisoprene or a styrene-isoprene diblock copolymer, and 2) a SiOx- covered solid substrate. We show that on both surfaces, the SF groups are oriented and on average are tilted by an angle F-helix> from the sample normal. We show that at higher <τF-helix> the results from the COP approach agree almost quantitatively with those extracted using the MBB model. However, for molecules standing almostperpendicular to the sample surface (<τF-helix>. This deviation is explained by the limitations of the more simpler COP model as compared to the more detailed, but also more complicated, MBB approach.
, Kramer, E.
and Fischer, D.
Accounting for Auger Yield Energy Loss for Improved Determination of Molecular Orientation Using Soft X-Ray Absorption Spectroscopy, Journal of Applied Physics
(Accessed December 2, 2023)