Recently, the research group of Academician Zhang Ze and Researcher Wang of Zhejiang University cooperated with the professor of Georgia Institute of Technology and researcher Du Kui of Institute of Metals, Chinese Academy of Sciences, etc. Based on the general characteristics of plastic deformation of materials, a "top-down" mechanical thinning method was proposed to prepare single-element metal two-dimensional materials, and Au thin films with single atomic thickness were successfully obtained by in-situ mechanical stretching of Au bimorph/polycrystalline nanostructures. Related research results were published in ACS nano, titled "Self-supporting two-dimensional gold thin films produced by extreme mechanical thinning".
Paper link:
https://doi.org/ 10. 102 1/acsnano.0c06697
In the process of plastic deformation of materials, strain concentration or strain localization is easy to occur at defects and interfaces. On the micro-nano scale, this strain localization will promote the rapid local necking of materials and lead to mechanical thinning. Based on this deformation feature, researchers have designed various gold twin/polycrystalline nanostructures. Under the condition of no substrate, no alloying and no chemical treatment, the mechanical thinning at the grain boundary can be significantly realized only through plastic deformation, and the two-dimensional Au film is induced to nucleate at the grain boundary first, and then the crystal nucleus grows continuously through the gradual expansion of the film/substrate interface (figure 1). Microstructure analysis and theoretical model show that this two-dimensional Au film has a simple hexagonal structure (Figure 2).
Figure 1. Stress-induced (a-f) nucleation, growth and fracture of 2D- gold thin films. Interfacial growth process of 2D- Au thin films at (g-j) atomic scale. Schematic diagram.
Figure 2. Simple hexagonal atomic structure of 2D- gold thin films. Interface dislocations coordinate the lattice mismatch and orientation difference at the interface between SH thin films and face-centered cubic substrates.
In-situ high-resolution characterization further clarified the dynamic mechanism of nucleation and growth of two-dimensional Au thin films at atomic scale. The results show that the stress-driven defect evolution process, including the slip and climb of interface dislocations and the diffusion and migration of interface atoms, is the key mechanism for the formation of two-dimensional Au thin films (Figure 3). A large number of experiments show that lattice defects in metal nanostructures, such as grain boundaries, twin grain boundaries, dislocations and cracks, can be used as preferential nucleation sites for two-dimensional metal films. Through the above mechanism, we successfully realized the mechanical preparation of various two-dimensional metal films (such as Pt and Ag). ) at room temperature.
Figure 3. (a-d) Stress-induced 2D- gold films nucleate from * * * lattice twin boundaries (ITB) and then grow. (e-I)2D- Au thin films grow rapidly with the slip and climb of interface dislocations. (j) Atomic structure at the interface between 2D-gold film and substrate.
The comparative experiments under stress-free and high-dose electron irradiation show that electron beam irradiation plays a relatively limited auxiliary role in the preparation of two-dimensional Au thin films. Electron beam irradiation can activate the limited dislocation motion to some extent, promote the diffusion and migration of atoms/vacancies in the film or interface, and thus promote the formation and growth of two-dimensional Au films (Figure 4), but it will only have a significant impact when the electron beam dose is high. Nevertheless, electron beam irradiation also provides an effective method to prepare two-dimensional materials of single element metal.
Figure 4. Electron beam irradiation is helpful to the preparation of 2D- gold thin films, which can activate the limited dislocation motion and promote the diffusion and migration of atoms/vacancies on the film surface.
The successful preparation of two-dimensional metal films by combining mechanical means and defect engineering provides new enlightenment for the preparation of other types of two-dimensional materials by "top-down" method. The research results of this paper are supported by the National Natural Science Foundation.
* Thanks to the team of paper authors for their strong support to this article.