Au/Ni bilayers with three thickness combinations were deposited on SiO2/Si substrates. After annealing at 1050 °C for 1 min in forming gas (mixture of Ar and H2), scattered spots (Supporting Information File 1, Figure S1) can be found on the surface. The enlarged insets present the circular feature of those spots and their height distributions indicate that circular areas are below the substrate surface. Hence, they will be referred to as cavities below. The enlarged view of the morphologies of the circular spots and the structure details outside the cavities are shown in Figure 1. Flower-like structures, called nanoflowers below, and particles with smooth surfaces can be observed in 5Au15Ni and 10Au10Ni. However, only nanoflowers are found in 15Au5Ni. The nanoflowers exhibit different morphologies as shown in Figure 1d. The length and number of their branches decrease with increasing distance from the border of the cavity in 10Au10Ni. The EDS result of one nanoflower is also shown in Figure 1e. The core of the nanoflower is mainly composed of Au, Ni, and Si while its branch parts show a much faster increase in O concentration than that of Si, indicating the possible formation of SiOx branches. To further detail the composition, EDS results were measured on the tilted morphology showing larger areas of the branch part (Supporting Information File 1, Figure S2). A similar concentration of O and Si corresponding to the substrate agrees well with Figure 1e. However, a much higher O concentration than that of Si corresponding to the branch part proves the possibility of SiOx branches again. Also, both Au and Ni show negligible concentrations, which means that EDS measured only substrate and branch parts. In accordance with previous works [3,4], the nanoflowers can be identified as heterostructures with a core particle and surrounding SiOx nanowires. The core particle is made of segregated Ni silicide and Au, which can be proved by their heterogeneous distributions in Figure 1e. Similar results of the other two samples are summarized in Supporting Information File 1, Figure S3 and Figure S4.

Figure 1: Morphology around decomposed areas (a–c). Distribution and composition of nanoflowers and particles outside the decomposition cavity in 10Au10Ni, respectively (d, e). Images (a–c) show 5Au15Ni, 10Au10Ni and 15Au5Ni, respectively. The dotted circles in (a, b) show the boundary of nanoflowers and particles. Images 1–4 in (d) show areas increasingly further away from the border of the cavity in 10Au10Ni, as marked in (b). The scale bar in (c) is also valid for (a, b), and the scales of the four images in (d) are the same. The scale bars of the inset in (e) are 200 nm.

The formation of the circular cavities can be attributed to the decomposition of the SiO2 layer at high temperature in reducing atmosphere. It has been reported after the annealing of Au thin films deposited on SiO2/Si substrates with different thicknesses of the SiO2 layer [3,4,36]. The active oxidation of Si also occurs once the Si substrate is exposed [2,3,37], which can be proved by the calculated oxygen partial pressure (Supporting Information File 1) and the much greater average depths of cavities (more than 600 nm) compared with the thickness of the SiO2 layer (300 nm). Besides, the number of visible spots increases with Au thickness as indicated by the numbers in Supporting Information File 1, Figure S1. Metallic elements, such as Au and Ni, can diffuse to the Si/SiO2 interface and enhance the decomposition rate there [38-40]. Hence, increasing decomposed areas with the thicker Au layer means that Au enhances the decomposition of SiO2 more than Ni.

Completely different structures can be observed inside the decomposed areas, as shown in Figure 2. There are mainly two shapes of microstructures, namely particles and lines. The particles present bright and dark parts. The bright areas should be rich in Au based on the material contrast, and the EDS results also indicate the high Au content in Figure 2. The dark areas consist of more Si and Ni in 5Au15Ni and 10Au10Ni but less Ni in 15Au5Ni, which has the lowest Ni concentration (Supporting Information File 1, Figure S5). The line structures show epitaxial self-assembly growth and their EDS results show a comparatively high content of Ni apart from Si, which may partially come from the substrate. Considering previous works in which the line structures were absent when only Au thin films were deposited [3,4], the existence of such epitaxial line structures should be highly related to the addition of Ni by depositing Au/Ni bilayers. This can also be proved by the plateau of Ni in EDS results of line structures. A number of works about self-assembled epitaxial Ni silicide have been published [41-46], and some works pointed out that the Ni2Si phase formed first, followed by NiSi and NiSi2 after annealing [47-49]. Generally, NiSi2 forms above 600 °C [42-45,48]. Therefore, the self-assembled epitaxial line structures in this work are supposed to be NiSi2.

Figure 2: (a–c) Morphology inside the decomposition cavities. (d) Composition of the particle and (e) the line in 10Au10Ni. Images (a–c) correspond to 5Au15Ni, 10Au10Ni and 15Au5Ni, respectively. The scale bar in (a) is also valid for (b, c), and the scale bar of the insets in (d) is 200 nm.

XRD patterns are shown in Figure 3. Most reflexes show clear deviations from the reported positions of Ni silicide (Supporting Information File 1, Figure S6). The absence of Ni silicide reflexes may be attributed to the low concentrations of the Ni silicide phases, because the line structures are only observed inside the cavities, which only account for a very small percentage of the whole sample surface. The reflex positions of pure elemental Au and Ni are marked in Figure 3. Both Au and Ni are mixed to a great extent after annealing, which is confirmed by the main peak shifts between the positions of pure Au and Ni. However, the mixing is incomplete because there are still small peaks of pure Au and Ni. The partial mixing can also be evidenced by the multiple reflexes between the positions of pure Au and Ni since only one main reflex should be observed when the two elements are completely mixed [20,23,25]. The annealing temperatures are above the miscibility gap [23,50]. Thus, the partial mixing comes from the phase separation of Au and Ni during cooling [25].

Figure 3: XRD patterns of the dewetted systems after annealing at 1050 °C. The standard data of Au (PDF 03-065-2870) and Ni (PDF 03-065-0380) are listed.

According to the results presented above, in Figure 4, we propose the following processes to explain the formation of nanoflowers with changing size in their branches outside the decomposed areas as well as the particles and epitaxial line structures inside the decomposed areas. Similar to previous works [3,4], dewetting of the Au/Ni bilayers and diffusion of Au and Ni atoms from the bilayers to the SiO2/Si interface begin at high temperatures. Simultaneously, decomposition is initiated at the SiO2/Si interface, and it can be strengthened by the diffused Au and Ni atoms to finally form the decomposition cavities. The active oxidation of Si also happens once the Si substrate is exposed [2,3,37]. Both decomposition and active oxidation can produce volatile SiO gas as the Si vapor source for the formation of SiOx NWs based on VLS mechanism [2,26,27,51,52]. Several NWs nucleate and then grow around particles because they are large enough to provide several nucleation sites [2,3], leading to the shape of flowers. Since SiO vapor can be only formed in the cavities, there is a non-uniform distribution of SiO gas concentration around the cavities. Namely, the closer to the cavities, the higher the concentration of the SiO gas, as shown in Figure 4b. This inhomogeneous distribution of the growth source leads to the different growth rates of nanoflowers in the area around the cavities. Basically, higher source concentrations enable higher growth speeds. This is why the particles close to the cavities grow into nanoflowers with much longer branches, whereas further away only small SiOx NWs or even no NWs are formed (Figure 1d). Similar uneven distributions of the Si source have been reported [2]. There are more cavities in 15Au5Ni than in the other two samples (Supporting Information File 1, Figure S1), meaning more SiO gas is produced. Thus, the inhomogeneity of the Si source is reduced and particles far away can also grow into nanoflowers (Supporting Information File 1, Figure S4). A much weaker inhomogeneity of the Si source has also been observed in the case of Au single layer when using similar annealing parameters [3], which further proves the higher ability of Au, compared to Ni, to enhance the SiO2 decomposition.

Figure 4: Formation mechanisms at elevated temperatures. (a) As-deposited bilayers and Au/Ni diffusion along nanochannels (dashed lines) in the SiO2 layer. All dashed lines pointing to the common point at the SiO2/Si interface aim to highlight the enhancement of SiO2 decomposition by the thin films. (b) SiO concentration gradient outside the decomposition cavity and cross sections of nanoflowers with changing branch length. (c) Cross sections of particles and epitaxial line structures inside the decomposition cavity.

The cavities keep growing laterally after piercing vertically the SiO2 layer and exposing the Si substrate [39,53,54]. Then, structures around the border of cavities will drop inside and get in contact with the exposed Si substrate, as marked in Figure 4c. The outer SiOx NWs can be decomposed by the Si substrate, and the core particle consisting of Au and Ni can get in direct contact with the substrate. Thus, Au–Si droplets and Ni silicide can form due to the easy interdiffusion of Au, Ni, and Si. Au/Si phase separation occurs during cooling [3,55], and Ni silicide may remain stable down to room temperature [41-46], finally forming particles with two contrasts. Besides, Ni may also diffuse into the Si substrate, leading to the formation of the Ni silicide, and a cross-sectional view of Ni silicide is given in Figure 4c based on reported works [44,48,56]. The elongation process of the symmetric NiSi2 clusters is mainly governed by the growth kinetics [44,57,58].