Spectroscopy NEXAFS

Near edge x-ray absorption fine stmcture spectroscopy (nexafs) and x-ray photoelectron spectroscopy (xps) have been used to study SAMs of OTS, octadecyltrimethoxysilane (OTMS), CH2(CH2) ySi(OCH2)3, and (17-aminoheptadecyl)-trimethoxysilane (AHTMS), H2N(CH2) ySi(OCH3)3 (149). A number of important observations have been reported. First, the chains in OTS SAMs are practicaUy perpendicular to the substrate surface (tilt angle... 

Case 1 Mn < M. The structure of uniaxially aligned LMW PF2/6 has been recently studied by combination of optical spectroscopy, NEXAFS and GIXRD, Fig. 22 exhibits a representative NEXAFS data and the subsequent analysis. These data support a structural model in which there is a graded morphology such that the top and bottom surfaces exhibit extensive planar, uniaxial alignment while the film interior is less well oriented and includes both planar and tilted (i.e., non-planar) PF2/6 chains [114]. 

There exists a large number of literature on SAM formation and characterization in general [1,3-5] and on functionalized alkane thiols in particular [6-8], The most frequently used methods to analyze the SAMs are Scanning Tunneling Microscopy (STM), Infra-red spectroscopy (IRS), X-ray absorption spectroscopy (NEXAFS) and Low Energy Electron Diffraction (LEED). In this contribution we will focus on the application of Thermal Desorption Spectroscopy (TDS). This method allows... 

Deposition of a5T from a Knudsen cell in high vacuum (HV) or ultra-high vacuum (UHV) at room temperature leads to highly ordered layers on SiOa and other oxidic substrates. The a5T molecules are oriented with their long axes mainly perpendicular to the substrate plane. This can be shown with polarized UV-Vis absorption and FT-IR spectroscopy, NEXAFS, and SFM (scanning force microscopy) measurements. 

A Millipore Milll-Q purification system was used for subphase preparation, and a constant temperature bath was used to control the subphase temperature. The mixed spreading solutions were dispersed at the air-water Interface and then slowly compressed at speeds of about 5 A2 mol min-" to surface pressures of 10-15 mN/m prior to deposition. Monolayers were transferred onto electron microscope grids for transmission electron microscopy and electron diffraction, using both the horizontal and vertical dipping techniques. Multilayer assemblies were prepared onto platinum-coated substrates using the vertical dipping technique for Near Edge X-Ray Absorption Fine Structure Spectroscopy (NEXAFS). 

In addition to these characterization tools, surface analytical techniques such as surface matrix-assisted laser desorption ionization (Surface MALDI) mass spectrometry, Rutherford backscattering spectroscopy (RBS), and near-edge X-ray absorption fine stmcture spectroscopy (NEXAFS) are used to obtain structural and chemical details about surface thin films. Surface MALDI, also known as MALDI-ToF MS (see Section 5.4.2), offers high mass resolution for analyzing surface films and molecular layers using the m/z of various ions generated from the sample surface (mixed with an... 

Having seen the power (and limitation) of nexafs spectroscopy in the preceding sections, one can readily envision the enhanced utility of nexafs spectroscopy as a characterization tool that would result from the addition of high spatial resolution capabilities. Since the spectroscopic sensitivity to specific moieties and functional groups can in many or even most cases be exceeded by ir, nmr, and Raman spectroscopies, nexafs microscopy will have to exceed the spatial resolution of these other spectroscopy techniques in order to be truly useful. To date, nexafs microscopy has surpassed a spatial resolution of 50 nm both in transmission to measure bulk properties (75-77) and in a reflection geometry to study surfaces (78,79). This level of spatial resolution is at least an order of magnitude better than what can be accomplished with complementary compositional analysis techniques. Future developments in nexafs microscopy might achieve a spatial resolution of a few nanometers (80,81). In addition, nexafs microscopy has exceptional surface sensitivity of about 10 nm, a sensitivity that could be improved to about 1 nm with photoemission electron microscopes (peem s) that incorporate a bandpass filter (80-82). 

In this chapter we will confine ourselves to a general description of two main characterization methods as X-ray photoelectron spectroscopy (XPS) and Near Edge X-ray Absorption Fine Stmcture spectroscopy (NEXAFS) and their... 

Functional macromolecular materials, such as those considered in this book are suitable for being investigated by X-ray photoelecfron spectroscopy (XPS) also induced by synchrotron radiation and near edge X-ray absorption spectroscopy (NEXAFS). 

NEXAFS spectroscopy is ideally suited for the study of these systems for several reasons as a start, it probes the local unoccupied electronic states of the atoms absorbing X-rays, providing information on the chemical states. Moreover, it is capable of giving insight about the spatial orientation of the probed states, e. g. for molecules, the molecular orientation. What is more, as a synchrotron-based spectroscopy, NEXAFS effectively probes the molecular system in a light-induced state, which is crucial for all electro-optics applications mentioned above [23]. 

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