Nanoscale Surface Analysis of High Entropy Alloy

Abstract

Nanoscale surface analysis of 1 micrometer thick high entropy alloys (HEAs) was carried out using nano-IR for hyperspectral imaging and single point spectroscopy in the 700-1700 1/cm spectral range. Nano-IR is based on the detection of scattered light from an oscillating metal coated nano-tip in one of the arms of the Fourier transform infrared spectrometer and has a resolution defined by the tip radius of the probe, 20 nm, regardless of the excitation wavelength. HEA CuPdAgPtAu showed an absorption and reflection increase at 900-1100 1/cm band, which is consistent with Drude-Lorenz modeling of permittivity, however, could also signify oxide formation as tested by X-ray photoelectron spectroscopy of CuPdAgPtAu and CrFeCoNiCuMo. Realization of polarization analysis for nano-IR nano-spectroscopy in the plane perpendicular to the sample's surface is discussed and modeled. The currently available modality of surface analysis with the excitation-detection mode of the p-pol. antenna can be extended to full 3D analysis of the orientational dependencies of local absorbance and refractive index.

Figures (11)

• Figure 1: (a) Optical image of a side-view microtome cut Au-HEA sample on black Kapton. Thickness of the microtome slice $960\pm 40$ nm (see SEM inset). Resolution of optical micro-image 0.7 $\mu$m. (b) Topography (height) from $8\times 8~\mu$m$^2$ in $200\times 100$ pixels, 9 ms/pixel. (c) IR optical signal (spectrally integrated); see Fig. \ref{['f-AR']} for the reflectance and absorbance maps and averaged spectra along A-A' image cross section.
• Figure 2: Spectral mapping: 50 spectra recorded across 5 $\mu$m distance; spectral resolution: 12.5 cm$^-1$, measurement time 42 minutes. (a) Nano-FTIR reflectivity. (b) Nano-FTIR absorption. (c) Reflectivity averaged over three materials: black Kapton (top in (a,b)), Au-HEA (middle), microtome polymer matrix (bottom).
• Figure 3: Maps of nano-FTIR reflectance, absorbance, and averaged absorbance for: black Kapton (left), Au-HEA (middle), microtome polymer matrix (right). The black-Kapton bands are referenced well to polyimide with recognisable Si-O-Si (or Si-OH) bands related to silica filler.
• Figure 4: Topography (height) and IR optical maps: $5\time 5~\mu$m$^2$, $150\times 150$ pixels, 9 ms/pixel. Nano-FTIR reflectance and absorbance at six random locations (normalised to Si). Sample: cover glass coated with Au-HEA. Experiment: 10 averaged interferograms per measurement point; 1024 pixels per interferogram, 10 ms per pixel, spectral resolution 12.5 cm$^{-1}$.
• Figure 5: X-ray photo-electron spectrum (XPS) analysis of two HEA samples on black Kapton: (a) Au-HEA CuPdAgPtAu and (b) Fe-HEA CrFeCoNiCuMo. Carbon was not taken into account for elemental atomic concentrations in (a); it is not shown in (b). Au-HEA and Fe-HEA were Ar plasma cleaned for 2 min and 4 min, respectively before quantification measurement was taken.