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Metastable phase formation in the Au-Si system via ultrafast nanocalorimetry
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Image of FIG. 1.
FIG. 1.

Schematic diagram of solid δ1 and liquid Au-Si alloy on gold substrates. Liquid Au-Si shows the 2D surface freezing with the same lattice parameters as δ1 and the ordering within the liquid due to both the crystalline surface and the Aus/(Au-Si)L interface.

Image of FIG. 2.
FIG. 2.

(a) Schematic of the nanocalorimetry device showing the sample placement. (b) Caloric curves shown with the δ1 melting peak at 305 ± 5 °C. Two caloric curves from day 0 (black) and day 2 (red) are overlapped and are almost identical. The melting temperature is within 0.5 °C while the heat of fusion is within 3% for the day 0 and day 2 results indicating stability of δ1 under vacuum. The caloric curves are averages of 50 pulses for each experiment. The baseline of this data set is taken after Si deposition before Au is deposited.

Image of FIG. 3.
FIG. 3.

(a) Diffraction patterns of single crystal δ1 oriented with the electron beam parallel to [100], [001], [101], [011], [111], and [201] directions. (b)An example of a diffraction pattern for the Au core and δ1 shell structure(diagrammed) with a δ1 [001] orientation showing the epitaxial relation between the δ1 shell on a Au core that is most often observed: Au[111]//4 × δ1[100].

Image of FIG. 4.
FIG. 4.

The heat flow during heating (red) and cooling (blue) from measurement plotted as the time-derivative of the differential voltage, dV)/dt, versus temperature of Au-Si alloys. (Bottom) Au-rich, single-phase sample: Only one set of melting and solidification peaks are observed corresponding to δ1; (top) Si-rich, two-phase sample: Au-Si eutectic phase melting and solidification peaks can be found in addition to that from δ1. The supercooling of δ1 is ∼30 °C, whereas the supercooling of Au-Si eutectic is much larger ∼220 °C. The cooling rate is variable but on the order of 104 °C/s in both cases.

Image of FIG. 5.
FIG. 5.

Three dimensional (3D) Cp(T, t) caloric maps during (a) Au deposition with the sample initially Si-rich and (b) Si deposition with the sample initially Au-rich. Eutectic forms first in (a) followed by δ1 (Si-rich to Au-rich), whereas δ1 forms first in (b) followed by the eutectic (Au-rich to Si-rich). Both δ1 and eutectic are present in Si-rich samples beyond the thickness range shown in (b). Inset in (a) shows TEM lattice images of a 5nm Aus (2.34 Å) nanoparticle embedded in a Sis (3.16 Å) matrix at the early stages of eutectic melting which exhibits melting point depression. Inset in (b) is a Cp(T) plot showing solidification (exothermic) of liquid after δ1 melts which occurs in the presence of Au(s) and Si(s).

Image of FIG. 6.
FIG. 6.

(a) Schematic of the wettability of δ1 on Au and Si and TEM micrographs showing high wettability of δ1 on Au and low wettability of δ1 on Si. (b) Plot of the fraction of the total melting enthalpy of δ1 and the eutectic vs. composition for experiments from Fig. 5 with composition changing from 0-95 Si at. % and the reverse 0-95 Au at. %. The median of the range of composition where δ1 initially forms is 73% ± 6% Au.


Generic image for table
Table I.

Comparison of lattice constants.

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Table II.

Thermodynamic properties.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Metastable phase formation in the Au-Si system via ultrafast nanocalorimetry