XRF spectra can be enhanced by large factors.

Surface-enhanced x-ray fluorescence (SEn-XRF) spectroscopy is a form of surface-enhanced spectroscopy that was conceived as a means of obtaining greater sensitivity in x-ray fluorescence (XRF) spectroscopy. As such, SEn-XRF spectroscopy joins the ranks of such other, longer-wavelength surface-enhanced spectroscopies as those based on surface-enhanced Raman scattering (SERS), surface-enhanced resonance Raman scattering (SERRS), and surface-enhanced infrared Raman absorption (SEIRA), which have been described in previous NASA Tech Briefs articles.

XRF spectroscopy has been used in analytical chemistry for determining the elemental compositions of small samples. XRF spectroscopy is rapid and quantitative and has been applied to a variety of metal and mineralogical samples. The main drawback of XRF spectroscopy as practiced heretofore is that sensitivity has not been as high as required for some applications.

Figure 1. This Atomic-Force-Microscope Image shows a portion of the surface of an aluminum substrate that was roughened by a 1-minute etch in concentrated hydrochloric acid.
In SEn-XRF as in the other surface-enhanced spectroscopies, one exploits several interacting near-field phenomena, occurring on nanotextured surfaces, that give rise to local concentrations of incident far-field illumination. In this case, the far-field illumination comes from an x-ray source. Depending on the chemical composition and the geometry of a given nanotextured surface, these phenomena could include the lightning-rod effect (concentration of electric fields at the sharpest points on needlelike surface features), surface plasmon resonances, and grazing incidence geometric effects. In the far field, the observable effect of these phenomena is an increase in the intensity of the spectrum of interest — in this case, the x-ray fluorescence spectrum of chemical elements of interest that may be present within a surface layer at distances no more than a few nanometers from the surface.

Figure 2. Silicon X-Ray Fluorescence Peaks at 1.74 keV were measured on aluminum surfaces coated with 340-nm-thick layers of silicone oil. The peak from the nanotextured surface was about 2.5 times as high as that from the nanotextured surface. Taking account of calculations showing that the enhancement occurs only within the 5-nm-thick sublayer closest to the surface, it was estimated that if the surfaces were coated to only 5-nm thickness, the factor of enhancement would be at least 170.
In experiments, SEn-XRF was demonstrated on aluminum substrates, the surfaces of some of which had been randomly nanotextured (see Figure 1) by briefly etching them in hydrochloric acid. Thin layers containing elements of interest (Si, I, K, Hg, and Pb) were deposited on the substrate, variously, from dilute salt solutions (in the case of K, Hg, and Pb), by vapor sublimation (in the case of I), or in a thin film of silicone oil (in the case of Si). XRF spectra of the thus-coated substrate surfaces were obtained by use of a commercial XRF microprobe instrument. In some cases, the XRF spectra of elements of interest on the nanotextured substrates were found to be enhanced significantly over the spectra from the corresponding substrates that had not been nanotextured (for example, see Figure 2).

This work was done by Mark Anderson of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category.

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Refer to NPO-44351, volume and number of this NASA Tech Briefs issue, and the page number.

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