S1 Estimation of atom evaporation by annealing
A very thin Au film (3 nm) is expected to form only small part of the dewetted CoAu
particles. As the desorption of Au is significant at elevated temperature, a very thin Au film (3
nm) is expected to desorb completely. The evaporation rate (Re) of atoms can be estimated by
Langmuir equation:
1
𝑅𝑒 = (3.513 × 1022 )(𝑀𝑊 𝑇)−2 𝑃𝑒𝑞,𝑇
where MW is the molar mass of the atom (g/mol), T is the annealing temperature (K) and P eq,T
is the equilibrium vapor pressure at T. Table S1 shows the calculation for the atoms
evaporation process during annealing at 800°C for 2 hours. As can be seen, significant amount
of Au atoms has evaporated from 3 nm Au with only 6.8 % volume left while for the same
thickness, the % volume remaining for Co is still as high as 96.8. It is therefore expected that
in the case of CoAu alloy with 3 nm Au, the Au component has almost completely evaporated
after the annealing. This is supported by the observation in Fig. 10(b) where all the particles
seem to be composed mainly of Co.
Table S1. Calculation of atoms evaporated during annealing at 800°C for 2 hours for 3 nm Au
and 3 nm Co. Sample size is 0.5 cm x 0.5 cm.
Material
ρ (g/cm3)
MW (g/mol)
Peq (Torr)
Re
(atoms/cm2.s)
# Atoms
evaporated
# Atoms
available
% Volume
remaining
Au
19.3
197
3 x 10-8
2.3 x 1012
4.1 x 1015
4.4 x 1015
6.8
Co
8.9
59
~ 1 x 10-9
1.4 x 1011
2.5 x 1014
6.8 x 1015
96.3
S2 Estimation of particle density
Since interparticle spacing scales with film thickness, we can also estimate the particle
density as:
25 2
𝐷 = ( ) 𝐷0
ℎ
where D is particle density, a is average interparticle spacing, h is initial film thickness, and
D0 is particle density for film thickness of 25 nm, which is determined from our experiment.
Table S2 shows the calculated particle densities of CoAu alloy, which roughly match the
estimated values. For CoAu alloy with distinguishable Co-rich and Au-rich particles, the
particle density can be calculated separately. For CoAu alloy with larger Co composition (22
nm Co and 15 nm Co), the particle density is calculated by taking the overall particles and the
estimation value is obtained by adding up the particle density of Co and Au for the respective
thicknesses.
Table S2 Measured particle density of CoAu alloy. Estimated value is given for comparison.