Nanomanufacturing is industrial manufacturing with nanoscale precision, low cost and reliability, including materials, structures, equipment and systems. Laser-induced ultrafast periodic surface structures can create nanostructures smaller than the wavelength of light, which has attracted the attention of the scientific and technical community.

Formation of high-density nanowells in ultrafast laser irradiation of thin-film metallic glass

Mathilde Prudan, Jafar Iabbaden, Florent Bourcard, Stephanie Reynaud, Yaya Lefkir, Alejandro Borroto, Jean-Francois Pierson, Florence Garrelier, Jean-Philippe Colombier*

Nano-Micro Letters (2022) 14:103

https://doi.org/10.1007/s40820-022-00850-4

Highlights

1. Ultrafast laser modification of nanotopography: a large number of 20 nm nanopores are created on the surface, which can be used to store chemical and biologically active substances and prevent crack propagation.

2. Ultrafast laser structural optimization: transformation of metallic glasses into composites of monoclinic zirconia crystallites embedded in amorphous metallic glasses.

3. A flexible one-step laser irradiation process without direct mechanical contact for surface functionalization of thin-film metallic glasses.

Introduction

The research team of Prof. Jean-Philippe Colombier of the Hubert Curie Laboratories of the French National Center for Scientific Research (CNRS) has proposed an efficient method for fabricating nanopore arrays using a one-step laser process, which is useful for storing and detecting chemical or biological elements and has broad application prospects. Biocompatible metallic glass films are made from selected Zr₆5Cu3₅ compositions that exhibit excellent mechanical and glass forming properties. Dense arrays of nanoholes spontaneously form in spots of ultrafast laser irradiation up to 20 nm in size. The resulting horn shape is ideal for protecting chemical or biological materials within a nanopore. This also indicates that the distribution of cavitation nanopores can be controlled by the density and size of the initial nanogap of the columnar structure of the film deposited by magnetron sputtering. The resulting surface structures were analyzed using SEM and STEM as well as EDS and EELS to elucidate the properties of the created nanopores and study the resulting crystal structure after laser irradiation.

Graphic guide

I Initial material surface characteristics and laser irradiation strategy

Thin-film metallic glass based on Zr₆₅Cu₃₅ was obtained by magnetron sputtering of zirconium and copper targets in an argon atmosphere. The composition of the thin film is controlled by controlling the discharge current applied to the target. On fig. 1a–b show SEM images of the surface and cross section of the samples, respectively. The observed columnar morphology is typical of sputtered samples. The width and smoothness of the surface of the columnar relief can be adjusted by adjusting the operating pressure of argon and lowering the temperature. On fig. 1c shows a schematic of an experiment with a fast laser.

Fig. Fig. 1. SEM images of the surface (a) and cross section (b) of unirradiated Zr-Cu thin-film metallic glass obtained by magnetron sputtering. (c) Schematic of the experimental setup used to generate single or double femtosecond pulses with adjustable delay and polarization.

II Topological structure and chemical characteristics of nanopores

Before laser irradiation, Thin Film Metallic Glass (TFMG) exhibits a very low RMS roughness of less than 2 nm, as measured by atomic force microscopy (AFM). High roughness contributes to the appearance of large periodic structures with many irregularities and branches. On fig. Figure 2a shows SEM images of the microstructural evolution of the TFMG surface after different laser irradiation times. The nanopore density varies nonmonotonically depending on the number of pulses. On fig. Figure 2b shows the evolution of the number of nanoholes and the evolution of the average distance between nanoholes for different numbers of laser pulses. Starting from a density of 550 nanopores per µm² at 1 irradiation, it increases to 680 µm⁻² (from 2 to 10 laser irradiations), beyond this threshold, the density of nanoholes stabilizes at about 650 µm⁻² (50 laser irradiations of irradiation). In accordance with the change in the concentration of nanopores, the distance between them ranges from 20 to 30 nm.

Fig. 2. (a) At a flux of 0.06 J cm⁻², at various laser irradiation times (1, 2, 3, 4, 5, 10, 20, 30, 40, and 50), SEM image of the nanopore microstructure obtained in femtosecond laser region. (b) Changes in the density of nanopores and the average distance between nanopores with increasing number of irradiations.

To fully characterize these nanopores, two FIB slices were taken from the unirradiated area (Fig. 3e) and the irradiated area exposed to 50 pulses (0.06 J cm⁻² fluence) (Fig. 3j). Figures 3a-d show HAADF-STEM images with different magnifications of selected FIB slices in the non-irradiated area. The bottom of the image shows a sample that has been coated with platinum and carbon layers. By comparing Figures 1b and 3a, it was found that the contrast lines seen on the STEM images correspond to the voids between the columns formed during the deposition. This initial columnar morphology is clearly visible on the sample surface in the STEM image, consistent with the very low initial surface roughness measured by AFM. The enlarged image in figure 3d shows its amorphous structure.

The second FIB flake selected from the irradiated area (Fig. 3f) showed nanopores evenly distributed throughout the flake, and the nanopores were close to the sample surface (about 50 nm from the surface). The nanopore diameter ranges from 10 to 30 nm. Some contrast changes can be observed in STEM images at the surface of the sample and around the nanopores, as seen in non-irradiated areas. Also, in this case, the high-resolution STEM image (HR-STEM) (Fig. 3i) is characterized by the atomic lattice surrounding the nanopore. This is typical for the crystal structure around the hole and on the surface of the irradiated area.

Fig. 3. (a-d) HAADF-STEM images of FIB flakes selected from non-irradiated areas showing the initial columnar film morphology; (e) SEM images of selected areas of FIB flakes from non-irradiated samples. (f–i) HAADF-STEM images of FIB flakes sampled from irradiated textured areas. (h) shows an "open" nanopore. (i) shows the atomic features of the crystal structure surrounding the nanopore. (j) SEM image of a selected area of ​​the FIB slice in the irradiated sample.

In addition to STEM analysis, EDS elemental analysis is performed in certain areas. On fig. Figure 4a shows images of the distribution of EMF elements on a non-irradiated sample in the bright field (BF) and HAADF modes. The Fourier transform of the enlarged region confirmed the absence of a crystal structure. The distribution of the EMF elements of the selected area is shown in fig. 4b, Zr is uniformly distributed over the entire sample in the FIB flake, while no such distribution is observed for Cu and O.

In fig. 4c–e show images of the distribution of EMF elements on samples irradiated with an ultrafast laser. The EMF analysis was carried out in two different areas (open and closed nanopores). Both analyzes gave similar results. On fig. 4c shows images of the distribution of EMF elements of the irradiated sample in the bright field (BF) and HAADF modes. On fig. 4d shows a map of the distribution of EMF elements of the same region, where four figures show the distribution of Zr, Cu, O and the overlay of all these elements. On fig. Figure 4e shows maps of the relative intensity of the Kα and Lα peaks for the elements Zr and Cu, respectively. The concentration of zirconium in the sample is relatively uniform, except for the center of the nanopore. A deficiency of Cu was observed on the surface and in the region around the nanopores. On the contrary, these copper-poor regions are particularly rich in oxygen. This layer, rich in Zr and O, corresponds to the observed contrast areas of light and dark in HAADF-STEM images.

Fig. 4 (a) STEM image of an unirradiated sample in bright field (BF) and HAADF modes, inset: 2D-FT showing amorphous structure. (b) Elementary EDS map of the STEM area shown in (a). (c) STEM image of the nanopore region of the irradiated sample in bright field (BF) and HAADF modes, inset: 2D-FT showing the crystal structure. (d) Elementary EDS map of the STEM area shown in (c). (e) Plot of the relative intensity of the Kα and Lα peaks of Zr and Cu.

III Periodic nanostructures irradiated with a two-pulse laser

Double-pulse laser irradiation of Zr-Cu thin-film metallic glass (TFMG) is shown in fig. 5, where the SEM photographs show the evolution of the surface of the irradiated area as a function of the time delay between two pulses.

Fig. Fig. 5. SEM images of samples irradiated with 50 double laser pulses with horizontal collinear polarization.

In fig. 6a–d show STEM images obtained at various magnifications in the HAADF-STEM mode. The HSFL depth is approximately 15 to 20 nm, which explains the low contrast observed in the SEM analysis as well as the structural unevenness of the original TFMG columnar profile. Nanopores are visible and randomly distributed across the FIB sheet. Their morphology somewhat differs from that obtained with a single pulse. The nanopores appear smaller than in single pulse experiments and are mostly "closed" below the sample surface, suggesting that the cavitation process is more limited. A high-contrast layer on the sample surface is visible throughout the FIB flake. This also occurs around the nanopore under single-pulse laser action. The layer is about 10 nm thick and contains nanocrystals, and the atomic lattice in various orientations can be seen in Figure 6d. The EELS and EDS element distribution maps (Fig. 6f and g) show that the dark contrast layer on the surface and in the area around the nanopores mainly contains zirconium and a high concentration of oxygen with a very low content of copper.

Fig. 6. (a-d) HAADF-STEM images of FIB slices extracted from irradiated textured areas. (d) shows the atomic features of the crystal structure surrounding the nanopore. (e) SEM image of the FIB flake extraction area. (f) EELS element distribution map. (g) EDS element distribution map.

In order to confirm the mechanism of the predominant appearance of nanopores in interstices and the formation of monoclinic zirconia, a simulation of laser irradiation on the surface of Zr-Cu metal glass containing preformed interstices was carried out. On fig. Figure 7a shows the formation of nanocavities and the kinetics of their nucleation after laser irradiation. The simulation results show that in the void that existed before irradiation, nanopores with a diameter of about 15 nm are obtained, and nanopores that appear below the irradiated surface have a diameter of about 40 nm, which is similar to the properties of experimentally obtained nanopores. Two maps of temperature (Fig. 7b) and pressure (Fig. 7c) around the nanopore were chosen. The pressure around a typical nanopore reaches the order of GPa, and the average temperature is about 3000 K. Preferential oxidation of zirconium and diffusion of copper atoms occur before rapid solidification. Upon cooling, zirconium nanocrystals stabilize in the monoclinic phase.

Fig. 7(a) Molecular dynamics simulation showing the formation of nanocavities on the surface of Zr-Cu metallic glass under the action of fast laser radiation. Evolution of temperature (b) and pressure (c) around the nanopore 165 ps after the start of irradiation simulation.

IV Conclusion

In this article, experimental and theoretical ultrafast laser irradiation of thin-film metallic glasses based on Zr₆₅Cu₃₅ shows that the generated nanopores are uniformly distributed over the film surface and can initiate the growth of zirconium dioxide nanocrystals. Thanks to single-pulse and double-pulse laser irradiation, these structural and topographical changes occur simultaneously in a reproducible and controlled manner. These results demonstrate the controllability of the entire fine-working process for fabricating Zr-Cu thin-film metallic glasses with single-column morphology, thereby facilitating the formation of double structures consisting of embedded nano-holes and nano-periodic bands with controlled concentration and uniformity. These uniformly distributed nanopores pave the way for applications based on nanoscale liquid storage and improved mechanical properties of TFMG.