Thin films of PtAl2 and AuAl2 by solid-state reactive synthesis

Abstract

The intermetallic compounds AuAl 2 and PtAl 2 are colored purple and yellow respectively. In the past they have been prepared by bulk melting techniques or by co-deposition in a magnetron sputterer. Here, however, we investigate films of AuAl 2 , PtAl 2 and (Au,Pt)Al 2 prepared by sequential physical vapor deposition of the elements, followed by in situ solid-state reaction. The microstructure, dielectric functions, optical properties and thermal stability of the resulting films are characterized and compared to those prepared by bulk melting or co-deposition. The (Au,Pt)Al 2 films show a color gamut that stretches from purple to brassy yellow depending on composition and microstructure. High temperature synchrotron X-ray diffraction experiments show that the (Au,Pt)Al 2 phase is metastable, decomposing when heated above 420 °C. In contrast, the pure AuAl 2 or PtAl 2 phases are stable to about 580 °C before they oxidize or decompose. The alternative possibility of producing the purple-to-yellow color gamut by depositing optical stacks of very thin films of AuAl 2 and PtAl 2 is also assessed. Either scheme will provide a range of colors lying between those of the binary compound endpoints. Calculations predict that deposition of AuAl 2 onto PtAl 2 will produce more intense colors than vice versa, an unexpected finding that is worth further investigation.

High Temperature Spectrally Selective Solar Absorbers Using Plasmonic AuAl2:AlN Nanoparticle Composites

“…As mentioned, XRD analysis suggested that annealing of the solar absorber above 220 °C causes diffusion of Al atoms from the substrate into the Au/AlN cermet where the Al reacts with the Au nanoparticles to form AuAl­ 2 . This is in approximate agreement with previous studies which have found the formation of well‐crystallized AuAl 2 to occur after annealing at 250 °C or greater …”

“…This is probably linked to the mobility of both Al and Au on the substrate and is in agreement with our previous experience that poorly crystallized or defective AuAl 2 can be deposited by magnetron sputtering at room temperature . Formation of the well‐crystallized AuAl 2 that is detectable by XRD will occur once the deposit is annealed at higher temperatures …”

“…[22,32] Formation of the well-crystallized AuAl 2 that is detectable by XRD will occur once the deposit is annealed at higher temperatures. [33] The distribution of pores is supported spectrally and was well fitted using Maxwell-Garnett theory. Figure 4 shows the evolution of the extinction coefficient k with a consequential addition of the Au and the AuAl 2 NPs into the Al-doped AlN layer.…”

“…This is in approximate agreement with previous studies which have found the formation of well-crystallized AuAl 2 to occur after annealing at 250 C or greater. [33] 400-510 C. This is another "stabilization region." A moderate change in the ellipsometric parameters occurs, indicating a completion of the reaction of Al with Au.…”

Launching low-energy surface plasmons in purple gold (AuAl2)

“…However, these materials do not possess surface plasmon resonances in the low-energy half of the visible spectrum (1.6 eV -2.4 eV), which is quite rare. Among metals, only AuAl2 and its analogs, including AuGa2, AuIn2 [16], and PtAl2 [24,25] support SP resonances in this range, which makes an investigation of their plasmonic properties of interest.…”