Scalable Fabrication of Metallic Nanostructures Based on Polymer Surface Buckling Enabled Exfoliation: From 2D Nanosheets to Helical Tubes

Abstract

Unlike the traditional 2D materials that possess intrinsic layered structures like graphene, monolayer metals are not energetically favored in nature because of the excessive dangling bonds on their surface. Instead of seeking monolayer metals, chemists turned to geometric 2D metallic nanosheets, which had a thickness ranging from 1 nm to 100 nm and were proven useful in modern nanotechnology as a new kind of nanomaterials. Albeit the long-perceived importance, synthesis of freestanding 2D metallic nanosheets, or the so-called 2D metals, however, has been restricted to elemental metals with a very limited in-plane size (< 10 µm). During my PhD study, I developed a low-cost, scalable, and facile method to synthesize 2D metals through polymer surface buckling enabled exfoliation (PSBEE). The outcome of my research demonstrates that the PSBEE method is versatile and capable of fabricating complex nanostructures, such as helical tubes and scrolls.

In the second chapter, I will discuss my work on the PSBEE method, which enables the fabrication of large-area chemically complex alloy nanosheets based on a 3D printed hydrogel plate. The 2D metallic nanosheets so obtained could be as chemically complex as high entropy alloys while possessing in-plane dimensions at the scale of bulk metals (> 1 cm). Based on PSBEE, I successfully synthesized a variety of 2D metals, such as 2D high entropy alloy and 2D metallic glass, with controllable geometries and morphologies. Moreover, my approach can be readily extended to non-metals and composites, thereby opening a large window to the fabrication of a wide range of 2D materials of technologic importance.

Mass production of two dimensional (2D) materials is pivotal to their widespread engineering applications. However, unlike graphene, h-BN, and MoS₂, there still lacks a mass production method for 2D metals. In the third chapter, I describe the development of a low cost, high yield mass production method. Compared to the traditional synthesis methods for 2D metals, the refined PSBEE method is desirable in terms of its high yield (30% -100%), superior production rate (10¹-10³ mg/h) and low energy consumption rate (10⁷-10⁹ J/g). Most importantly, the refined PSBEE method also enables fabrication of freestanding 2D metallic nanosheets with a thermodynamically stable or metastable atomic structure, a large in-plane size and unrestricted chemical makeup (from elemental metals to chemically complex alloys), which is unprecedented in the field of 2D materials.

The rapid development of the state-of-the-art nanotechnology is driven by the emerging novel nanofabrication methods, such as self-rolling of 2D materials or nanosheets. Nonetheless, the traditional chemical etching based “roll-up” technologies suffer from a low fabrication efficiency and generally produce only scroll-like structures. In the fourth chapter, I will describe the development of a versatile, ultrafast and etching free method to synthesize self-rolled metallic nanostructures through PSBEE, which enables rapid exfoliation and self-rolling of metallic nanosheets at a rate about one to two orders of magnitude faster than other methods. Furthermore, we observe a scroll-helix-scroll transition through twisting of the self-rolled nanosheets. Through extensive finite element simulations and experiments, we reveal the thermodynamics underpinning these configurational transitions.

Date of Award	12 Aug 2021
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yong YANG (Supervisor) & Yangyang LI (Co-supervisor)