Method to Determine the Distribution of Substituted or Intercalated Ions in Transition-Metal Dichalcogenides: Fe_xVSe₂ and Fe_1–xV_xSe₂

Abstract

Multiple techniques are combined to determine the amount of intercalation and/or substitution in transition-metal dichalcogenides. Kiessig fringes in the X-ray reflectivity pattern are used to calculate thickness. Laue oscillations in the specular diffraction pattern of the crystallographically aligned samples determine the number of unit cells in the coherently diffracting domains. The amount of impurity phase(s) is estimated by the difference between the thickness of the films and the size of the domains. If the difference is small relative to the total film thickness, the composition of the crystalline phase can be determined from X-ray fluorescence measurements. The number of unit cells possible can be calculated from the amount of each element determined by X-ray fluorescence measurements using in-plane lattice parameters, and the amount of the anion should agree with the number of unit cells determined from the Laue oscillations. The total amount of the metal relative to that required for the number of unit cells from the Laue oscillations provides a direct determination of the relative amount of intercalation and/or substitution in the crystalline dichalcogenide. The utility of this approach is illustrated in Fe x V1–y Se2 samples. The relative amount of intercalation versus substitution was determined independently using electron microscopy and Rietveld refinement of diffraction patterns and is consistent with this new approach.

Expanding the Composition Range of Transition Metal Dichalcogenide Alloys, Fex(FeyV1–ySe2)

“…We used data from multiple techniques to experimentally calculate the values of x and y in our Fe x (Fe y V 1– y Se 2 ) samples, as lattice parameters alone are insufficient for this determination. Compositions from XRF data, thickness measurements from XRR data, analysis of Laue oscillations in specular XRD patterns, analysis of cross-sectional STEM-EDS data, and Rietveld refinements of diffraction patterns were used to quantify the amounts of substitution, intercalation, and potential impurity phases . The specular XRD patterns for samples 2 – 4 and 6 – 8 each contain Laue oscillations on either side of the 001 Bragg reflection (Figure ), from which the average number of coherently diffracting dichalcogenide unit cells in each sample can be determined .…”

“…Compositions from XRF data, thickness measurements from XRR data, analysis of Laue oscillations in specular XRD patterns, analysis of cross-sectional STEM-EDS data, and Rietveld refinements of diffraction patterns were used to quantify the amounts of substitution, intercalation, and potential impurity phases. 31 The specular XRD patterns for samples 2−4 and 6−8 each contain Laue oscillations on either The total percentage of Fe in each sample was normalized as Fe/(Fe + V). The values for the number of intercalated atoms are estimated from the amount of excess metal atoms above a (V + Fe)/Se ratio of 1:2 and assume that no secondary impurity phases are present.…”

“…Determining the values of x and y in the resulting compounds required a combination of several techniques (X-ray reflectivity, X-ray diffraction, X-ray fluorescence, and analysis of Laue oscillations). The accuracy of this approach was verified by measuring the ratio of x and y directly using scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy of focused ion beam-prepared cross sections . The ratio of x and y and the atoms’ chemical states in the samples were further probed via X-ray photoelectron spectroscopy.…”

“…The accuracy of this approach was verified by measuring the ratio of x and y directly using scanning transmission electron microscopy−energy-dispersive X-ray spectroscopy of focused ion beam-prepared cross sections. 31 The ratio of x and y and the atoms' chemical states in the samples were further probed via X-ray photoelectron spectroscopy.…”