Two-dimensional (2D) nanomaterials with atomic and nano-scale thickness (< 100 nm) have attracted tremendous research interests for their outstanding physical and chemical properties, which have a great potential of use in a wide range of applications. To this end, mechanical properties of 2D nanomaterials play a pivotal role as they strongly affect the stability and performance of emerging devices made of the 2D nanomaterials. In general, the mechanical properties of 2D nanomaterials differ from those of their bulk counterparts. Due to their ultrathin thickness and low flexural rigidity, the 2D nanomaterials could be folded, self-rolled and/or unfolded in order to release their internal residual stress in the presence or absence of external forces (i.e., elasto-capillary forces). These behaviours are interesting and worthy of a systematic study.

My PhD study is mainly focused on the investigation of the nanomechanical behaviour of 2D inorganic nanomaterials. First, I synthesized large-sized 2D inorganic nanomaterials with the method of polymer surface buckling enabled exfoliation (PSBEE) that was developed previously in my group. In the second chapter, I investigated the mechanical properties of large-sized 2D titanium nanomaterials which possess a unique heterogeneous nanostructure containing nano-sized titanium, titanium oxide and MXene-like phases. Interestingly, these 2D titanium exhibit both superb mechanical strength (6-13 GPa) and remarkable ductility (30%-40%) at room temperature, outperforming all other titanium-based materials reported so far. I demonstrated that the 2D titanium also showed good performance in triboelectric sensing and can be used to fabricate self-powered on-skin conformal triboelectric sensors with good mechanical reliability.

In the third chapter, I developed a method to construct 2D titanium-based origami structures through self-rolling. I showed that there are two types of origami structures (folding versus rolling) that can be constructed by controlling the in-plane size and thickness of the 2D titanium, which has never been reported before in the origami literature. On one hand, the 2D titanium-based origami can be formed by folding the 2D titanium when the origami size is above a critical value; on the other hand, the origami can be formed by rolling the 2D titanium when the origami size is below. Interestingly, we show that the micro-origami structures so obtained can be unfolded rapidly by surface tension at a liquid-air interface, thus indicative of potential applications in controlled encapsulation and drug release.

In the fourth chapter, I developed a liquid-based approach to manipulate 2D inorganic nanomaterials in the light of balance between the elastic and capillary force acting on them when the 2D inorganic nanomaterials are floating on a liquid surface. By systematically altering the surface tension of the liquid, one can easily unfold and/or re-fold the 2D nanomaterials, and this behavior of unfolding/refolding can be predicted very well by a continuum model I built. As inspired by these result, I successfully formed small structures with various 2D inorganic nanomaterials on the liquid surface (e.g. origami structures, heterogeneous scrolls, optical resonators) and subsequently transferred them out of the liquid surface onto other surfaces for the manufacturing of electronic devices. The outcome of the combined research, which includes experiments, modeling, and simulations, indicates that the controlled nanomechanical behavior provides an avenue for structuring and manipulation of 2D nanomaterials, facilitating their applications in future small-scale devices.