Nano-engineering and nano-manufacturing in 2D materials: marvels of nanotechnology

Two-dimensional materials have attracted significant interest and investigation since the marvellous discovery of graphene. Due to their unique physical, mechanical and optical properties, van der Waals (vdW) materials possess extraordinary potential for application in future optoelectronics devices. Nano-engineering and nano-manufacturing in the atomically thin regime has further opened multifarious avenues to explore novel physical properties. Among them, moiré heterostructures, strain engineering and substrate manipulation have created numerous exotic and topological phenomena such as unconventional superconductivity, orbital magnetism, flexible nanoelectronics and highly efficient photovoltaics. This review comprehensively summarizes the three most influential techniques of nano-engineering in 2D materials. The latest development in the marvels of moiré structures in vdW materials is discussed; in addition, topological structures in layered materials and substrate engineering on the nanoscale are thoroughly scrutinized to highlight their significance in micro- and nano-devices. Finally, we conclude with remarks on challenges and possible future directions in the rapidly expanding field of nanotechnology and nanomaterial.

Paraffin-Enabled Compressive Folding of Two-Dimensional Materials with Controllable Broadening of the Electronic Band Gap

The capability to manipulate the size of the electronic band gap is of importance to semiconductor technology. Among these, a wide direct band gap is particularly helpful in optoelectronic devices due to the efficient utilization of blue and ultraviolet light. Here, we reported a paraffin-enabled compressive folding (PCF) strategy to widen the band gap of two-dimensional (2D) materials. Due to the large thermal expansion coefficient of paraffin, folded 2D materials can be achieved via thermal engineering of the paraffin-assisted transfer process. It can controllably introduce 0.2-1.3% compressive strain onto folded structures depending on the temperature differences and transfer the folding product to both rigid and soft substrates. Exemplified by MoS₂, its folded multilayers demonstrated blue-shifts at direct gap transition peaks, six times stronger photoluminescence intensity, almost double mobility, and 20 times higher photoresponsivity over unfolded MoS₂. This PCF strategy can attain controllable widening band gap of 2D materials, which will open up novel applications in optoelectronics.

Wrinkling and failure behavior of single-layer MoS2 sheets under in-plane shear

In this paper, the wrinkling and failure behavior of single layer MoS₂ (SLMoS₂) sheets under in-plane shear is investigated using molecular simulations and the nonlocal model. Wrinkling and failure features, such as the stress-strain relation, the amplitude and the half-wavelength, are comprehensively explored. The effects of size, temperature and pre-existing cracks on the wrinkling and failure behavior are then taken into consideration. It is found that the whole process can be divided into three stages, i.e., the pre-buckling stage, the buckling stage and the failure stage. The classical continuum model is found to be limited in quantitatively analyzing the wrinkling behavior due to the lack of size effect. The nonlocal parameter, a key parameter to characterize the size effect, is first reported. What is more, compared with edge cracks, SLMoS₂ sheets are more sensitive to pre-existing centre cracks. This work can provide a better understanding of the wrinkling and failure properties of SLMoS₂ sheets under shear loads, and should be helpful for developing various flexible electronic devices.

Wrinkling of two-dimensional materials: methods, properties and applications

Recently, two-dimensional (2D) materials, including graphene, its derivatives, metal films, MXenes and transition metal dichalcogenides (TMDs), have been widely studied because of their tunable electronic structures and special electrical and optical properties. However, during the fabrication of these 2D materials with atomic thickness, formation of wrinkles or folds is unavoidable to enable their stable existence. Meaningfully, it is found that wrinkled structures simultaneously impose positive changes on the 2D materials. Specifically, the architecture of wrinkled structures in 2D materials additionally induces excellent properties, which are of great importance for their practical applications. In this review, we provide an overview of categories of 2D materials, which contains formation and fabrication methods of wrinkled patterns and relevant mechanisms, as well as the induced mechanical, electrical, thermal and optical properties. Furthermore, these properties are modifiable by controlling the surface topography or even by dynamically stretching the 2D materials. Wrinkling offers a platform for 2D materials to be applied in some promising fields such as field emitters, energy containers and suppliers, field effect transistors, hydrophobic surfaces, sensors for flexible electronics and artificial intelligence. Finally, the opportunities and challenges of wrinkled 2D materials in the near future are discussed.

The unexpected effect of vacancies and wrinkling on the electronic properties of MoS2 layers

We report a combined experimental/theoretical approach to study the connection of S-vacancies and wrinkling on MoS₂ layers, and how this feature produces significant changes in the electronic structure and reactivity of this 2D material.