Rapid dry exfoliation method for tuneable production of molybdenum disulphide quantum dots or large micron-dimension sheets

Two-dimensional (2D) materials offer outstanding mechanical, electronic and optical properties that enabled considerable developments in a variety of applications. As such, the synthesis methods for 2D materials have gained much research interest. The preparation and synthesis of 2D materials play a significant role in its quality, hence its properties and suitability for any application. Furthermore, the cost-effective, fast, large-scale industrial production has been always a top priority and a pivotal key to the success of any application employing 2D materials. The move towards rapid large production, however, often presents trade-offs to the quality of the final product. As such, a need arises to address the problem without compromising either quality or quantity at each other's expense. A class of 2D materials, known as transitional metal dichalcogenides, has taken the focus of research in 2D materials in the last ten years and even more recently; due to their natural abundance and interesting properties offered, when thinned down to 2D crystals. Molybdenum disulphide (MoS_2), in particular, has been extensively researched; due to its bulk form stability at room conditions, and the well-studied stability of its thinned form, allowing better control over its use, while having promising electronic and optical applications due to a suitable a bandgap, in addition to its catalytic properties, thus was found to be of use in biosensing and energy applications as well. Many techniques have been developed for producing 2D MoS_2, whether through bottom-up synthesis or top-down exfoliation from bulk. Through the review of synthesis techniques of 2D MoS_2, none of the present approaches gave a solution that does not compromise one aspect; dictating the use of one method over the other for each application, which are often performed on small lab scale with little potential for scaling-up. The synthesis of large micron-dimension single-layer sheets of these materials remains a challenge, especially if an alternative to the slow, expensive and complex bottom-up approaches such as chemical vapor deposition or molecular beam epitaxy is desired. The top-down conventional mechanical exfoliation tape method, which was first used by Geim and Novoselov in 2004 for isolating 2D graphene from graphite and ignited a spark for the 2D materials field, remains a golden standard for pristine quality 2D materials sheets synthesis. A simple but low yield multistep method that required a lot of skill with little potential for scaling-up, however, it is still considered the standard for high quality large sheets production; due to being one of the least invasive techniques. Other mechanical exfoliation methods have been devised to address the shortcoming of the very low yield, but they usually lacked scale-up potential and introduced the use of additives for the transfer and post processing. Another approach researched in parallel was the use of liquid solvents for exfoliation that has progressed through the years starting from the harsh chemically aided exfoliation to the less invasive ultrasonication assisted exfoliation, with a promising potential for scaling-up. The liquid exfoliation techniques had a limited success in obtaining large sheets, although not comparable to the very large sheets obtained from complex bottom-up approaches but could reach the size ranges from mechanical exfoliation tape methods but with compromises made to quality in exchange for scalability. Liquid exfoliation techniques excelled in the production of smaller quantum dots, though. They offered larger yields and better processing time with relatively less complex equipment used and less skill employed. On the other hand, the use of solvents or any additives, even after extensive post processing, still affected the quality of the produced 2D material, hence dictating the suitability for a specific application over the other. The objective of this work is mainly to review the current synthesis techniques for 2D materials and specifically MoS_2 as the most in use example of transitional metal dichalcogenides class of 2D materials. Then to evaluate each method advantages and highlight its drawbacks to ultimately choose a synthesis route and design a platform that addresses most of the shortcomings with little or no compromise to any aspect of the produced 2D material. A novel and a unique method to rapidly exfoliate MoS_2 is presented. Tuneability is offered to produce small nanometer-dimension quantum dots of a desired size range. Moreover, the platform flexibility allowed to produce large micron-dimension as well. Both products were predominantly monolayers to few layers and the exfoliation process was conducted in dry conditions with no use of liquids or additives. The platform employs nanometer-amplitude MHz-order surface vibrations in the form of surface acoustic waves. To produce quantum dots, the bulk material is subjected to massive surface acceleration -on the order of 10^8 m/s^2- to be repeatedly impacted, ejected and collide with miniature enclosure inner walls, in order to laterally break reducing its dimensions as well as thinned down to single or few layers. On the other hand, sheets are produced by suppressing the iterative impacts cycles through reducing the enclosure height to almost zero through the use adhesive tape in place of the miniature enclosure, which serves to fix upper layers of the bulk material while the lowermost one is subjected to a shearing force from the travelling surface acoustic wave, thus progressively thinning the material into sheets while preserving their lateral dimension. A dry stable exfoliated powder product with limited restacking problems is obtained that can stored as a stock and readily used. Depending on the intended application, further suspending it before use in an easily removable solvent such as water-ethanol binary mixture can be performed for finer size separation. The platform is applicable to larger particle size bulk material feed instead of the used commercially available 6 µm powder for demonstration, especially if optimization towards the production of sheets rather than quantum dots is intended. Furthermore, quantum dots are produced in a fast millisecond scale process in a miniaturized platform with potential for scaling-up through massive parallelization. In conclusion, a fast, additive-free and dry exfoliation platform is presented that potentially presents a simple yet scalable micromechanical exfoliation method towards viable commercial production of 2D transition metal dichalcogenides