Fabrication principle

The multi-COAD method is illustrated in Fig. 1a. It combines multiple targets co-sputtering15,16,17 with oblique angle deposition2,18,19. In conventional deposition of metallic glasses, a single alloy target is typically used for sputtering. However, in many cases, the deposited composition differs significantly from that of the alloy target20, rendering the control of composition difficult. We demonstrated recently that co-sputtering using multiple elemental targets allows fine tuning of composition17. The high effective cooling rate on the order of 109 K s−1 (ref. 21) during the deposition process enables the formation of glassy films in a wide composition range including the vast group of marginal metallic glass formers. This offers the opportunity to optimize chemistry tailored for desired NMG functions. In contrast to the usual oblique angle deposition18,19, which is also known as glancing angle deposition and has been utilized to produce nanoarchitectures of simple chemistry22,23, multi-COAD comprises three sputtering targets arranged in a confocal geometry. The substrate is orientated with a glancing angle, α, with respect to the targets. This angle can be manipulated independently relative to each sputtering target by changing either the substrate orientation or the tilt angle of the sputtering guns. Variation of the azimuthal angle, ϕ, primarily not only warrants the control of NMG uniformity but can also be utilized to achieve complex 3D shapes. Deposition with multi-COAD proceeds in a carousel fashion24, where the substrate holder rotates at constant speed (see the green arrow in Fig. 1a). After receiving the flux from target A during holder rotation, the substrate conforms to target B and subsequently to target C. The compositional homogeneity and glassy structure is controlled by varying independently the deposition rate from each target and the rotation speed of the carousel substrate holder24.

Figure 1: The scheme of multiple targets COAD. (a) Configuration of the deposition system. The sputtering guns are arranged in a confocal geometry, but the tilt angle of each gun can be changed independently. The angle-α between the normal of the substrate and the incoming flux is called glancing angle. Variation of azimuthal angle ϕ controls the structure uniformity and also allows the creation of complex structures. During deposition, the substrate spins along the direction of the green arrow. (b) Morphology of a template with ∼100 nm gold nanoparticles prepared by dewetting an ultrathin gold film. (c) Illustration of shadowed growth. Scale bar, 500 nm (b). Full size image

The first step in the bottom-up synthesis of NMGs via multi-COAD is the preparation of a template to catalyse the directional growth of the nanostructures. Various methods are established for template synthesis with either ordered or disordered nano- or micropatterns. Among those are optical or electron-beam lithography, nanocolloid self-assembly, embossing and block co-polymer micelle nanolithography25. Since our focus is to demonstrate the feasibility and universality of multi-COAD for the versatile synthesis of metallic glass nanoarchitectures, we fabricate for convenience the template by dewetting gold thin films on silicon wafer to yield Au nanoparticles26. Figure 1b shows such a template with Au nanoparticles of a mean size of ∼100 nm. The mean particle size can be controlled through the initial gold film thickness (Supplementary Fig. 1). When the template is exposed to vapour flux at a glancing angle, the nanoparticles act as seeds onto which the incoming atoms nucleate and grow in a columnar manner (Fig. 1c) owing to the amplified shadowing effect2.

Wafer-scale synthesis of metallic glass nanostructures

To demonstrate multi-COAD, we choose ZrCuAl alloy system. Owing to its low formability and high oxidation rate, ZrCuAl NMGs have not been realized using nanomoulding technique. To demonstrate the versatility of our approach, using elemental sputtering targets, we synthesize NMGs with a composition that is far away from the best glass-forming alloy in this system. Figure 2a shows the ZrCuAl NMGs on a 50-mm-diameter Si wafer templated with ∼50 nm gold nanoparticles. Energy dispersive X-ray analysis at different locations of the wafer (Fig. 2a) confirms the homogeneous chemistry across the entire wafer (Fig. 2b and Supplementary Table 1). The composition of the NMGs is Zr 44 Cu 35 Al 21 , which is nearly identical to the set composition of Zr 45 Cu 35 Al 20 through deposition rate. This indicates that the composition of the NMG structures fabricated by multi-COAD can be controlled accurately. Figure 2c displays the X-ray diffraction (XRD) spectrum of the as-grown NMGs. The halo diffraction pattern centred ∼36° confirms the amorphous structure of the NMGs. The weak Bragg peaks in the spectrum originate from the gold nanoparticles used as template.

Figure 2: Grown metallic glass nanostructures. (a) Appearance of as-grown metallic glass nanostructures on a 50-mm-diameter Si wafer. (b) Energy dispersive X-ray analyses at different locations as labelled by the numbers indicate a homogeneous composition across the wafer. (c) XRD spectrum on the as-grown nanostructures. The halo pattern suggests the amorphous nature of the samples. The sharp Bragg peak originates from the gold template. (d) Top view and (e) side view of the morphologies of the metallic glass nanostructures at locations labelled by the numbers in a. (f) Free-standing nanorods. (g) Close-up of individual rods. Scale bars, 1 μm (d,e), 2 μm (f) and 200 nm (g). Full size image

Figure 2d,e displays the NMG morphologies observed at different locations of the wafer, top and side views, respectively. The NMGs are rods with uniform lateral dimensions. Originating from the oblique deposition angle, the rods are tilted by ∼20° relative to the substrate normal (Fig. 2e), which is dependent on the azimuthal angle-ϕ, the glancing angle-α and deposition rate, all of which are tunable19. Figure 2f presents the free-standing nanorods. The NMGs, as shown in Fig. 2g, exhibit surfaces that are decorated with small features. The surface of multi-COAD-synthesized NMGs can be controlled by the deposition conditions, such as sputtering gas pressure20. This enhanced surface area is beneficial for applications, such as surface-enhanced Raman scattering catalysis1. Usually, NMGs produced by thermoplastic moulding have smooth surfaces8,10. For an increased surface area, additional processing steps are needed, which, however, sacrifice the glassy structures27,28. The multi-COAD-grown hierarchical nanostructures suggest that further processing steps to enhance surface area are not required, so that the advantages of the amorphous structure can be fully retained.

Along the horizontal direction in Fig. 2a, such as from 1 to 5, the rods exhibit similar lengths. A gradual decrease in rod length can be seen along the vertical direction, for instance, from location 6 to 9. This length variation relative to the distance from the sputtering target is a consequence of the deposition rate difference at a constant azimuthal angle-ϕ19. Variation of ϕ can lead to improved length uniformity19. The high number density of NMGs is set by the template choice, here, the dewetted gold nanoparticles for seeding (Fig. 2e). Generally, the spacing and arraying of the rods can be controlled by tailoring the seeding templates as demonstrated with other materials29.

Metallic glass nanoarchitecture of complex 3D shape

The unique thermoplastic formability of metallic glasses has been explored to shape complex geometries on the macroscale, of which many were predominately unachievable with other metals30. In contrast on the nanoscale, only simple shapes, such as dots, rods or trenches, can be fabricated using moulding8,9,10,31. Complex nanoscale shapes of large aspect ratio are unachievable due to wettability requirements of a metallic glass with mould and the unavailability of complex moulds. During multi-COAD, the variation of ϕ, which is usually used to improve uniformity, enables us to manipulate the shape of the NMGs in three dimensions19. Figure 3a shows zigzag structures made from ZrCuAl growing on templates of various seed sizes. The zigzag structures are fabricated by periodically changing the template orientation (for example, ϕ=180°) around its normal. The zigzag-shaped NMGs can be grown to a large aspect ratio. In Fig. 3a, an aspect ratio of ∼20 is obtained for the NMGs grown on the smallest seeds. Even larger aspect ratios can be achieved by a prolonged deposition time for each segment or increased number of segments. The angles between the segments of the zigzag structures can be altered through the variation of glancing angle, α32,33. As shown in Fig. 3b, the lateral dimension of the structures is dependent on the size of seeding particles in the template, suggesting that it is possible to modulate the dimension of the NMGs by varying the seed size. Although the smallest seed size is ∼50 nm in this work, smaller dimensions can be achieved using smaller seeds2,29. These grown zigzag NMGs of high aspect ratios are only one example of the geometrical versatility of multi-COAD.

Figure 3: Seed size controlled metallic glass nanostructures of complex shapes. (a) Zigzag nanostructures made from ZrCuAl metallic glass on templates of different nanoparticles. (b) Plot of the lateral dimensions of the nanostructures as function of template size. Scale bar, 500 nm (a). Full size image

Hybrid metallic glass nanostructures

To further demonstrate the versatility of multi-COAD, we fabricate hybrid nanoarchitectures (Fig. 4). The bottom-up approach offers the opportunity to create complex hybrid nanoarchitectures if the composition of the vapour flux is changed while growing. Figure 4a presents a hybrid structure comprising two metallic glasses, ZrCuAl and NiNbSn, grown on 100 nm seeding particles. Because of their low thermoplastic processability, both alloy systems are difficult to work with using the nanomoulding technique. With multi-COAD, however, we can vary the vapour chemistry from ZrCuAl to NiNbSn every time the template is reorientated around its normal. As a consequence, each segment contains different alloy materials (Fig. 4a). The inset of Fig. 4a displays the XRD spectra of ZrCuAl and ZrCuAl/NiNbSn NMG structures, respectively. The dual glassy diffuse peaks as indicated by the vertical lines in the spectrum of ZrCuAl/NiNbSn NMG confirm that the hybrid structures are composed of amorphous ZrCuAl and amorphous NiNbSn. As shown in Fig. 4b–e, despite the change in vapour chemistry during growth, we are still able to maintain the shape and lateral dimension of the structures as shown in Fig. 3a. To our knowledge, this is the first demonstration of metallic glass hybrid nanostructures, which allow integration of multiple components with different functionalities as demanded in sensing and detecting devices34.

Figure 4: Hybrid nanostructures comprising two metallic glasses. (a) The overview of large area showing the morphologies of the hybrid nanostructures, the shadowed growth and dispersed individual zigzag rods. The inset shows the XRD spectra of nanostructures consisting of single ZrCuAl glass phase and hybrid ZrCuAl/NiNbSn glass phases. The peaks in the spectra are due to gold from the templates. (b–e) Hybrid nanostructures of different lateral dimensions. Scale bars, 2 μm (a) and 500 nm (b-e). Full size image

Nanoporous metallic glass membrane

In addition to the 1D nanostructures, multi-COAD can also synthesize 2D structures. Metallic glass nanoporous membranes are desired owing to their extraordinary strength and elasticity5,7. Fabrication of metallic glass nanoporous membranes, however, is difficult with existing methods. In contrast, our bottom-up synthesis through multi-COAD enables us to make metallic glass membranes (Fig. 5a) without the requirement for material removal as in thermoplastic moulding7,30. The nanoporous membrane shown in Fig. 5 is Mg 65 Cu 25 Y 10 and was formed using the anodic alumina oxide as a template. The pores exhibit a mean size of ∼100 nm (Fig. 5b), the same size as the holes in the template (Fig. 5c). This indicates accurate replication of the template patterns using multi-COAD, and the porosity can be controlled by varying the feature sizes in the template. This process is distinct from typical deposition where flux impings parallel to substrate normal, which can fill the pores and result in a continuous film. We emphasize that hybrid membranes composed of different materials can be readily fabricated through multi-COAD by changing vapour chemistry during growth, the same strategy as in Fig. 4, and would, for example, yield different surface chemistries on the two sides of the membrane.