Accounting for Auger yield energy loss for improved determination of molecular orientation using soft x-ray absorption spectroscopy

Genzer, Jan; Kramer, Edward J.; Fischer, Daniel A.
December 2002
Journal of Applied Physics;12/15/2002, Vol. 92 Issue 12, p7070
Academic Journal
Partial (Auger) yield near edge x-ray absorption fine structure (NEXAFS) is a structural analytical technique that has been primarily used to measure the spatial orientation and chemical bonding of small molecules on solid (i.e., inorganic or semiconductor) surfaces. In this article we demonstrate that the building block (BB) scheme proposed by Outka and co-workers [Phys. Rev. Lett. 59, 1321 (1987)] for analyzing NEXAFS spectra can be applied to model the molecular orientation of larger molecules, provided one accounts properly for kinetic energy losses of the Auger electrons traversing through the sample and hence the attenuation in measured Auger yield. We test the applicability of the proposed "modified" BB (MBB) model by measuring the orientation of a self-assembled monolayer (SAM) of -O[sub 1.5]Si-(CH[sub 2])[sub 2]-(CF[sub 2])[sub 8]F, SF-SAM (SiO[sub x]), deposited on top of SiO[sub x]-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 reveal 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 by Stöhr and Samant [J. Electron Spectrosc. Relat. Phenom. 98-99, 189 (1989)] 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, -(CH[sub 2])[sub x](CF[sub 2])[sub y]F, which...


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