It is well-understood that during the liquid-to-solid phase transition of alloys, elements segregate in the bulk phase with the formation of microstructures. In contrast, we show here that in a Bi–Ga alloy system, highly ordered nanopatterns emerge preferentially at the alloy surfaces during solidification. We observed a variety of transition, hybrid and crystal-defect-like patterns, in addition to lamellar and rod-like structures. Combining experiments and molecular dynamics simulations, we investigated the influence of the superficial Bi and Ga2O3 layers during surface solidification and elucidated the pattern-formation mechanisms, which involve surface-catalysed heterogeneous nucleation. We further demonstrated the dynamic nature and robustness of the phenomenon under different solidification conditions and for various alloy systems. The surface patterns we observed enable high-spatial-resolution nanoscale-infrared and surface-enhanced Raman mapping, which reveal promising potential for surface- and nanoscale-based applications.
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This work was supported by the Australian Research Council (ARC) Laureate Fellowship grant (FL180100053) and the ARC Centre of Excellence FLEET (CE170100039). N.M. and S.P.R. are supported by the ARC Centre of Excellence Grant (CE170100026). This work was also supported by computational resources provided by the Australian Government through the National Computational Infrastructure National Facility and the Pawsey Supercomputer Centre.
The authors declare no competing interests.
Peer review information Nature Nanotechnology thanks April S. Brown and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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a, Schematic of EBiGa selective etching using NaOH aqueous solution (1 mol L-1). b,c, AFM topography of a lamellar surface pattern before b and after etching (c, 30 min). d,e, Height profiles of the lamellar surface patterns along the dashed lines in b and c, respectively. The positions of the Bi and Ga structures are indicated. f-h, Free-standing Bi nanoribbons and nanoplates obtained after etching for 5 hrs. i, j, Examples of fully exposed Bi surface patterns showing the Bi structure thickness. Scale bars: b,c, 500 nm; f, 5 μm; g,h, 1 μm; i,j, 2 μm (left) and 500 nm (right).
Extended Data Fig. 2 X-ray photoelectron spectroscopy (XPS) spectra of the solid EBiGa sample surface.
a, XPS spectrum of the Ga 2p region. b, XPS spectrum of the Bi 4f region. c, XPS spectrum of the O 1s region. The XPS analysis suggests that the dominant phases of Ga and Bi at the sample surface are Ga2O3 and metallic Bi, respectively. Such selective Ga oxidation is a result of its much greater Gibbs free energy of oxide formation (from Ga to Ga2O3, ΔGf°(Ga2O3) = -998.3 kJ mol-1) than that of Bi (from Bi to Bi2O3, ΔGf°(Bi2O3) = -493.7 kJ mol-1).
a-c, AMF topographies. d-f, Height profiles along the dashed lines in a-c. The circles indicate the position of the Bi structures. Scale bars: 2 μm.
Extended Data Fig. 4 Bi-Bi bonding and Bi atom configuration of Bi-Ga clusters from ab initio molecular dynamics simulations.
a, The Bi(s)9 cluster with a subsurface Bi and 9 surface Bi atoms. b, The Bi(c)10 cluster with a centre Bi and 10 surface Bi atoms. The internal Bi is indicated with a yellow cross. In both cases, the Bi atoms on the cluster surface form persistent zig-zag motifs which can be observed at different calculation times.
a-c, Back-scattered electron (BSE) SEM images of particulate Bi surface patterns solidified under 10 °C min-1 (a) and 50 °C min-1 (b, c). d-e, Magnified views of a-c showing the patterning and ordering of the surface Bi particles. Scale bars: a-c, 10 μm; d-f, 1 μm.
Extended Data Fig. 6 Demonstration of the controlled surface solidification process of a liquid EBiGa film.
a, Liquid EBiGa before solidification. b, Solidification triggered by touching the liquid metal surface with a Ga-crystal-coated tip. c, Solidification process during which the SSF propagates across the sample. d, Fully solidified sample. Scale bars: 2 mm. The EBiGa liquid metal sample was spread on a silicon substrate and kept at 25 °C. Due to such small supercooling ∆T (about 5 °C), the sample would not solidify automatically. A Ga-crystal-coated tip was then brought in touch with the liquid metal surface to overcome the nucleation barrier so that the solidification process could take place (Supplementary Video 5). The snapshots from the recorded video of the solidification process show that the solidification front propagates out from the ‘artificial’ nucleation site (the tip location), gradually covering the entire sample surface. The formation of the solidification front is supposed to be a result of localised nucleation at the alloy surface in the early stage of solidification. Of the two processes (nucleation and crystal growth) in solidification, the first process, nucleation is a statistic event. As discussed in our main text, the liquid metal surface favours nucleation. Once localised nucleation occurs on the surface, the growth of nuclei, which is faster in comparison to the nucleation process, creates a solidification front propagating outward from the nucleation sites on the surface.
a, The Bi-Ga binary phase diagram. The Bi-Ga mixing ratios that have been investigated in this study are marked with red boxes with the labels indicating different sample groups. b, Surface patterns formed on a hypoeutectic sample (Bi0.001Ga0.999). c, Surface patterns formed on a eutectic sample (Bi0.0022Ga0.9978). d-g, Surface patterns formed on hypereutectic samples: d,e, Bi0.085Ga0.915; f, Bi0.50Ga0.50, and g, Bi0.99Ga0.01. In the Bi0.085Ga0.915 sample, both few-micrometre (d) and sub-micrometre (e) Bi particles were observed to form on the surface. Scale bars: b-d, 2 μm; e, 500 nm; f,g, 5 μm.
Extended Data Fig. 8 Surface solidification in other dilute binary alloy systems with different melting temperatures.
a, Ag0.01Ga0.99 (melting temperature TM ≈ 29.8 °C). b, Pb0.01Ga0.99 (TM ≈ 29.8 °C). c, Ag0.01Bi0.99 (TM ≈ 271.4 °C). d, Au0.001Bi0.999 (TM ≈ 271.4 °C). e, Bi0.01Cu0.99 (TM ≈ 1083 °C). In each case, the first panel is the EDX spectrum, the second panel is the BSE-SEM image, the third and fourth panels are the EDX mappings of the binary alloy phases. Scale bars: a, 1 μm; b,c, 5 μm; d,e, 2 μm.
Extended Data Fig. 9 Surface solidification patterns of binary alloys with equal (atomic) mixing ratio of the constitute metals.
a,b, Zn0.50Sn0.50. c,d, Bi0.50Zn0.50. b and d are magnified images of a and c, respectively. In each case, the first panel is the EDX spectrum, the second panel is the BSE-SEM image, the third and fourth panels are the EDX mappings of the binary alloy phases. Scale bars: a,c, 50 μm; b,d, 5 μm.
a, EDX spectra with a magnified view at the region of 2 to 4 keV showing the Bi and Ag peaks. b, BSE-SEM image and the EDX mapping of Ag, Bi, and Ga. Scale bar: 2 μm.
Supplementary Figs. 1–9 and Supplementary Discussions 1–3.
Phase separation and pattern formation behind the propagating surface solidification front (SSF).
Melting and re-solidification of a BiGa droplet showing the existence of the oxide layer during the surface phase transition process.
Ab initio molecular dynamics simulation examples of BiGa nanoclusters with different seeding structures.
Classical molecular dynamics simulations of the diffusion of 48 Bi atoms in 4800 Ga atoms with a Ga/vacuum interface and a Ga/Ga2O3 interface.
Triggering surface solidification with a Ga-crystal-coated tip.
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Tang, J., Lambie, S., Meftahi, N. et al. Unique surface patterns emerging during solidification of liquid metal alloys. Nat. Nanotechnol. 16, 431–439 (2021). https://doi.org/10.1038/s41565-020-00835-7
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